c/EPA
United States Industrial Environmental Research EPA-600/7-78-170
Environmental Protection Laboratory August 1978
Agency Research Triangle Park NC 27711
Proceedings:
Symposium on New
Concepts for Fine
Particle Control
Interagency
Energy/Environment
R&D Program Report
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U S Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
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The nine series are
1 Environmental Health Effects Research
2 Environmental Protection Technology
3 Ecological Research
4. Environmental Monitoring :
5 Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8 "Special" Reports
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This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects, assessments of, and development of, control technologies for energy
systems, and integrated assessments of a wide range of energy-related environ-
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REVIEW NOTICE
This report has been reviewed by the participating Federal Agencies, and approved
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This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161
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EPA-600/7-78-170
August 1978
Proceedings:
Symposium on New Concepts
for Fine Particle Control
Teoman Ariman, Compiler
University of Notre Dame
Notre Dame. Indiana 46556
Grants: R805148 (EPA) and ENG. 77-02016 (NSF)
Program Element No. EHE624
Project Officers:
Dennis C. Drehmel Morris S. Ojalvo
Industrial Environmental Research Laboratory and Solid and Particulate Processing Program
Office of Energy, Minerals, and Industry National Science Foundation
Research Triangle Park, NC 27711 Washington, DC 20550
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY National Science Foundation
Office of Research and Development Solid and Particulate Processing Program
Washington, DC 20460 Washington. DC 20550
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TABLE OF CONTENTS
Page
List of Participants ...................... 3
Technical Program ...................... °
Welcome Address - Joseph C. Hogan ............... 12
Opening Address - Teoman Ariman ............... ^
State of the Art and Objectives of the Workshop - T. Ariman . , ^
ACOUSTICS IN PARTICULATE-GAS SEPARATION
Pulse-Jet Acoustic Dust .................... ^3
Acoustic and Turbulent Agglomeration of Sodium Aerosols -
Experimental Results - William C. Hinds and Eugene F. Mallove. . 35
New Application of Acoustic Agglomeration in Particulate
Emission Control - David T. Shaw and David Wegrzyn ....... "4
Effects of Electric and Acoustic Fields on Particle
Collision Rates in Aerocollodial Suspensions - Paul D. Scholz. . 86
HIGH GRADIENT MAGNETIC FIELD IN PARTICULATE-GAS SEPARATION
Engineering Aspects of Dry Magnetic Separation - Robin R. Oder . 108
Magnetic Separation of Particulate Air Pollutants -
Dennis C. Drehmel and C. H. Gooding ...............
Research Needs and Opportunities in High Gradient Magnetic
Separation of Particulate Gas Systems - Y.A.Liu and C.G.Lin. . . 170
High Gradient Magnetic Separation: Review of Single Wire
Models - F. J. Friedlander ................... 183
GENERAL LECTURE
The Influence of Electrostatic Forces for Particle Collection
in Fibrous Filters - Friedrich LBffler ............. 206
ELECTROSTATIC FILTRATION
Electrofluidized Bed for Industrial-Scale Air Pollution
Control - James R. Melcher ................... 237
Approximate Equations for Predicting Electrostatic Particle
Collection - Douglas W. Cooper ................. 260
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Page
Electrostatic Filtration and the Apitron - Design and Field
Performance - Dennis J. Helfritch and Teoman Ariman ........ 286
Electrostatic Capture of Fine Particles in Fiber Beds -
Donald L. Reid .......................... 305
ALL OTHER NEW METHODS
Fine Particulate and Gaseous Emissions Control Experience with
the TRW Charged Droplet Scrubber - R. R. Koppang .......... 320
Research on New Equipment for Cotton Dust Collection -
Albert Baril,Jr. , Devron P. Thibodeaux and Robert B. Reif .....
PANEL DISCUSSION
Remarks - Dennis C. Drehmel
Remarks - Morris S. Ojalvo
Closing Remarks - Teoman Ariman
Dielectrophoretic Air Filtration - Progress and Problems -
J. K. Thompson, R. C. Clark and G. H. Fielding ........... 36l
Fine Particulate Control Using Foam Scrubbing -
Tom E. Ctvrtnicek, H. H. S. Yu, C. M. Moscowitz and
Geddes H. Ramsey .......................... 373
The Anomalous Behavior of Asbestos Fibers - James W. Gentry .... 399
GENERAL LECTURE
High Temperature Filtration-Technical Prospects and as Means
for Check on Theories - Michel M. Benarie ............. Ul3
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PARTICIPANTS
Ronald B. Adams
Division of Industrial Energy Conservation
ERDA
20 Massachusetts Ave., N.W.
Washington, DC 20545
Ali Aktay
Inland Steel Company
Gary, IN
Jeffrey C. Alexander
88 Beacon St., Apt. 13
Somerville, MA 02143
Teoman Ariman
Department of Aeospace and Mechanical
Engineering
University of Notre Dame
Notre Dame, IN 46556
Albert Baril, Jr.
Cotton Textile Processing Laboratory
Southern Regional Research Center, ARS
1100 Robt. E. Lee Blvd.
New Orleans, LA 70179
Michael Benarie
Institut National de Recherche
Chimique Appliquee
16, Rue Jules Cesar 75012
Paris, France
Charles E. Billings
740 Boylston Street
Chestnut Hill, MA 02167
Warren L. Buck
Components Technology Division
Argonne National Laboratory
9700 S. Cass Avenue
Argonne, IL 60439
Kurt Carlsson
AB Svenska Flaklfabriken
Fack
S-35187 Vaxjo,Sweden
William Charles
Magnetic Engineering Associates
247 Third Street
Cambridge, MA 02142
Douglas W. Cooper
Dept. of Environmental Health Sciences
Harvard University
665 Huntington Avenue
Boston, MA 02115
T. W. Ctvrtnicek
Monsanto Research Laboratory
Dayton Division
1515 Nicholas Road
Dayton, OH 45407
H. P. Dibbs
Air Pollution Control Directorate
Environment Canada
Ottawa, Ont.
Canada K1A 1C8
Dennis C. Drehmel
Industrial Environmental Research
Laboratory
Environmental Protection Agency
Research Triangle Park, NC 27711
George H. Fielding
Combustion and Fuels Branch
Naval Research Laboratory
Washington, DC 20375
Edward R. Frederick
Technical Operations Manager
Air Pollution Control Association
P.O. Box 2861
Pittsburgh, PA 15230
F. J. Friedlander
Professor
School of Electrical Engineering
Purdue University
West Lafayette, IN 47907
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James B. Galeski
Senior Chemical Engineer
Midwest Research Institute
425 Volker Blvd.
Kansas City, MO 64132
James W. Gentry
Department of Chemical Engineering
University of Maryland
College Park, MD 20742
James A. Gieseke ft
Aerosol and Emissions Technology Group
Battelle - Columbus
505 King Avenue
Columbus, OH 43201
Charles H. Gooding
Research Triangle Institute
Research Triangle Park, NC 27709
Eugene E. Grassel
Donaldson Co., Inc.
P.O. Box 1299
Minneapolis, MN 55440
R. Duncan Hay
Magnetic Engineering Associates
247 Third St.
Cambridge, MA 02142
Dennis J. Helfritch
APITRON, American Precision Industries
1 Executive Park, Suite 500
Charlotte, NC 28287
William C. Hinds
Department of Environmental Health Sciences
Harvard University
665 Huntington Avenue
Boston, MA 02115
David R. Kelland
Massachusetts Institute of Technology
NW14-3113
Cambridge, MA 02139
Richard R. Koppang
Environmental Products Development
TRW Inc., Energy Systems Group
1 Space Park
Redondo Beach, CA 90278
Tuncer M. Kuzay
Reactor Analysis and Safety Division
Argonne National Laboratory
9700 S. Cass Avenue
Argonne, IL 60439
George E. Lamb
Textile Research Institute
P.O. Box 625
Princeton, NJ 08540
William F. Lawson
Morgantown Energy Research Center -
USERDA
P.O. Box 880
Morgantown, WV 26505
C. J. Lin
Department of Chemical Engineering
Auburn University
Auburn, AL 36830
Benjamin Y. H. Liu
Particle Technology Laboratory
Department of Mechanical Engineering
University of Minnesota
Minneapolis, MN 55455
Y. A. Liu
Department of Chemical Engineering
Auburn University
Auburn, AL 36830
Friedrich Loffler
Institut fur Mechanische Verfahrenstechnik
Universitat Karlsruhe
7500 Karlsruhe 1, Germany
E. Maxwell
Francis Bitter National Magnet Laboratory
Massachusetts Institute of Technology
170 Albany St.
Cambridge, MA 02139
Andrew R. McFarland
Department of Civil Engineering
Texas A&M University
College Station, TX 77843
James R. Melcher
Department of Electrical Engineering
and Computer Science
Rm 36-13
Massachusetts Institute of Technology
Cambridge, MA 02139
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Grady B. Nichols
Environmental Engineering Division
Southern Research Institute
2000 Ninth Ave. South
Birmingham, AL 35205
John A. Oberteuffer
Sala Magnetics, Inc.
247 Third St.
Cambridge, MA 02142
Morris S. Ojalvo
Solid and Particulate Processing Program
Engineering Division
National Science Foundation
Washington, DC 20550
Robin R. Oder
Research and Engineering
Bechtel Corporation
P.O. Box 3965
San Francisco, CA 94119
Gaylord W. Penney
Department of Electrical Engineering
Carnegie-Mellon University
Schenley Park
Pittsburgh, PA 15213
Leon Petrakis
Gulf Research and Development Company
P.O. Drawer 2038
Pittsburgh, PA 15230
Michael J. Pilat
Air Resources Engineering
Department of Civil Engineering
University of Washington
Seattle, WA 98195
Donald L. Reid
Battelle-Northwest
P.O. Box 999
Richland, WA 99352
Robert R. Reif
Engineering Physics and Electronics Section
Battelle-Columbus
505 King Avenue
Columbus, OH 43201
Paul D. Scholz
Division of Energy Engineering
College of Engineering
The University of Iowa
Iowa City, IA 52242
David A. Schulz
U.S. EPA
230 S. Dearborn
Chicago, IL 60604
David S. Scott
Department of Mechanical Engineering
University of Toronto
Toronto, Ont.
Canada M5S 1A4
Michael Shackletown
Acurex Aerotherm
485 Clyde Avenue
Mountain View, CA 94040
David T. Shaw
State University of New York at Buffalo
4232 Ridge Lea Rd.
Buffalo, NY 14226
Norman Surprenant
GCA/Technology Division
Burlington Road
Bedford, MA 01730
William Swift
Department of Chemical Engineering
Argonne National Laboratory
9700 S. Cass Avenue
Argonne, IL 60439
James Wegrzyn
State University of New York at Buffalo
4232 Ridge Lea Road
Buffalo, NY 14226
K. T. Yang
Department of Aerospace and Mechanical
Engineering
University of Notre Dame
Notre Dame, IN 46556
Hsu-Chi Yeh
Inhalation Toxicology Research Institute
Lovelace Foundation for Medical Education
and Research
P.O. Box 5890
Albuquerque, NM 87115
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WORKSHOP PROGRAM
WEDNESDAY, APRIL 20, 1977
8:00 am Registration
8:30 am Welcome
JOSEPH C. HOGAN
Dean, College of Engineering
University of Notre Dame
Notre Dame, Indiana
8:40 am Opening Remarks
TEOMAN ARIMAN
University of Notre Dame
Notre Dame, Indiana
9:00 am Acoustics in Particulate-Gas Separation
Chairman:
K. T. YANG
University of Notre Dame
Notre Dame, Indiana
"Pulse-Jet Acoustic Dust Conditioning and
Some New Potential Applications"
DAVID S. SCOTT
University of Toronto
Toronto, Canada
"Acoustic and Turbulent Agglomeration of
Sodium Aerosols - Experimental Results"
WILLIAM C. HINDS and EUGENE F. MALLOVE
Harvard University
Cambridge, Massachusetts
10:30 am Coffee Break
10:45 am "New Applications of Acoustic Agglomeration
in Particulate Emission Control"
DAVID T. SHAW and JAMES WEGRZYN
State University of New York at Buffalo
Buffalo, New York
"Effects of Electric and Acoustic Fields on
Particle Collission Rate's in Aerocollodial
Suspensions"
PAUL D. SCHOLZ
The University of Iowa
Iowa City, Iowa
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12:15 pm Lunch
1:30 pm High Gradient Magnetic Field in Particulate-
Gas Separation
Chairman:
Benjamin Y. H. Liu
University of Minnesota
Minneapolis, Minnesota
"Engineering Aspects of Dry Magnetic Separation"
ROBIN R. ODER
Research and Engineering
Bechtel Corporation
San Francisco, California
"Magnetic Separation of Particulate Air Pollutants"
DENNIS C. DREHMEL
Environmental Protection Agency
and
C. H, GOODING
Research Triangle Institute
Research Triangle Park, North Carolina
3:00 pm Refreshments Break
3:15 pm "Research Needs and Opportunities in High Gradient
Magnetic Separation of Particulate Gas Systems"
Y. A. LIU and C. G. LIN
Auburn University
Auburn, Alabama
"High Gradient Magnetic Separation: Review of
Single Wire Models:
F. J. FRIEDLANDER
Purdue University
West Lafayette, Indiana
THURSDAY, APRIL 21, 1977
8:00 am General Lecture
Chairman:
MORRIS S, OJALVO
National Science Foundation
Washington, D. C.
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"The Influence of Electrostatic Forces for
Particle Collection in Fibrous Filters"
FRIEDRICH LOFFLER
University of Karlsruhe
Karlsruhe, W. Germany
9:00 am Electrostatic Filtration
"Electrofluidized Beds for Industrial-Scale
Air Pollution Control"
JAMES R. MELCHER
Massachusetts Institute of Technology
Cambridge, Massachusetts
"Approximate Equations for Predicting
Electrostatic Particle Collection:
DOUGLAS W, COOPER
Harvard University
Cambridge, Massachusetts
10:30 am Coffee Break
10:45 am "Electrostatic Filtration and the Apitron-Design
and Field Performance"
DENNIS J. HELFRITCH
Apitron
Buffalo, New York
and
TEOMAN ARIMAN
University of Notre Dame
Notre Dame, Indiana
"Electrostatic Capture of Fine Particles in
Fiber Beds"
DONALD L. REID
Battelle Northwest
Richland, Washington
12:15 pm Lunch
1:30 pm All Other New Methods
Chairman
DENNIS C. DREHMEL
Environmental Protection Agency
Research Triangle Park, North Carolina
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"Fine Particulate and Gaseous Emissions
Control Experience with the TRW Charged
Droplet Scrubber"
R. R. KOPPANG
TRW, Inc.
Redondo Beach, California
"Research on New Equipment for Cotton Dust
Collection"
ALBERT BARIL, JR
DEVRON P. THIBODEAUX
Southern Regional Research Center
Department of Agriculture
New Orleans, Louisiana
and
ROBERT B. REIF
Battelle Columbus Laboratories
Columbus, Ohio
3:00 pm Refreshments Break
3:15 pm "Dielectrophoretic Air Filtration - Progress and
Problems"
GEORGE H. FIELDING
Chemistry Division
Naval Research Laboratory
Washington, D. C.
"Fine Particulate Control Using Foam Scrubbing"
TOM E. CTVRTNICEK
Monsanto Research Corporation
Dayton, Ohio
"The Anomalous Behavior of Asbestos Fibers"
JAMES W. GENTRY
University of Maryland
College Park, Maryland
6:00 pm Social Hour
7:00 pm Banquet
Speaker:
TOM PAGNA
South Bend, Indiana
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10
FRIDAY, APRIL 22, 1977
8:30 am General Lecture
Chairman:
ANDREW R. McFARLAND
Texas A&M University
College Station, Texas
"High Temperature Filtration - Technical Prospects
and as Means for Check on Theories"
MICHEL M. BENARIE
National Institute for Applied Chemistry
Paris, France
9:30 am Panel Discussion
"New Directions in Particulate-Gas Separation
Research"
Moderator:
BENJAMIN Y. H. LIU
University of Minnesota
Minneapolis, Minnesota
Panel Members:
TEOMAN ARIMAN
MICHAEL M. BENARIE
DENNIS C. DREHMEL
ROBIN R. ODER
DAVID S. SCOTT
10:30 am Coffee Break
10:45 am Continuation of the Panel Discussion
12:15 pro Closing Remarks
TEOMAN ARIMAN
12:30 pm Lunch
1:45- Guided Tour of the Campus
2:30 pm
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11
ACKNOWLEDGEMENT
The editor of this proceedings gratefully acknowledges at every phase of
the Workshop, the interest and continuous support and advise of Dr. Morris S.
Ojalvo, Director of the Particulate and Multiphase Processes Program of the
National Science Foundation and Dr. Dennis C. Drehmel of the Industrial
Environmental Research Laboratory of the Environmental Protection Agency.
Special thanks are due to Mrs. Shirley Wills for her enormous efforts in the
transcription of the taped sessions of the workshop and her expert typing
of most of the proceedings. I would also like to acknowledge the diligent
work by two of our students at Notre Dame, Mr, Greg Muleski and Mr. Carl Aumen
in the preparation of the open discussions and panel session and in editing
of the proceedings.
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12
WELCOME ADDRESS
Joseph C. Hogan
Dean, College of Engineering
University of Notre Dame
Notre Dame, Indiana 46556
Let me welcome you to the University of Notre Dame. I'll not go through
the Chamber of Commerce sales pitch because engineering schools are known by
the faculty they have and I hope you are familiar with members of our faculty.
You are more than welcome to go over to the Engineering Building, but you
will have to tread through a good deal of mud to get there. This is due to
the eight million dollar addition which is just being started and should be
completed by December of next year. If any of you would like to see that
particular construction, I'm sure K. T. or Teo will be glad to make arrange-
ments for this or anything else you'd care to see while you're here.
We are honored to have such a distinguished group here. It is farsighted
for NSF and EPA to act as co-sponsors. Often a field moves forward in tiny
increments because the papers and work are presented at conferences at random
times. I think that getting the top people in the field together at one
time is without question the best way to move the field forward. I want to
congratulate NSF and EPA again for their farsightedness and for sponsoring
this program. I hope it will prove to be an interesting and productive two
and a half days. Thank you.
OPENING ADDRESS
Teoman Ariman
Workshop Director
Department of Aerospace and Mechanical Engineering
University of Notre Dame
Notre Dame, Indiana 46556
Welcome to the Workshop entitled "Novel Concepts, Methods and Advanced
Technology in Particulate-Gas Separation." As the Dean said, the workshop
is sponsored by NSF and EPA and we would like to express our appreciation,
especially to Dr. Morris Ojalvo of NSF and Dr. Dennis Drehmel of EPA, for
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13
help with the workshop and continuous support. We would also like to thank
the members of the planning committee, Professor Benjamin Liu of the
University of Minnesota and Professor K. T. Yang, Chairman of the Aerospace
and Mechanical Engineering Department here at Notre Dame. We sincerely hope
that everything went well last night and that the transportation and hotel
accommodations are satisfactory. If there are any complaints, please see
Dr. Yang or Dean Hogan. If there are any compliments, please see me.
I would like to briefly introduce the workshop. As you know, a
number of emission control systems have been used for years—fabric filters,
electrostatic scrubbers, electrostatic precipitators, etc. In years past,
energy has been quite cheap. But times have changed and we face an energy
crisis unless we do something. Furthermore, we face the challenge of
controlling fine particulate emissions. Taken together, these two demand
that we search out new technologies, and in the past few years, there has been
substantial growth in this area. The thrust of this workshop is the bring-
ing together of people and their technologies. We wish to discuss not only
collection efficiency but also energy efficiency. With the close work of the
planning committee and the NSF and EPA, it was decided to invite three or
four people in each area - a theoretical analyst, an experimentalist and a
design/applications specialist. By mixing people from universities, govern-
ment laboratories, private laboratories, and industry, we hope to have a
healthy exchange of ideas. I sincerely hope that the workshop reaches that
goal.
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14
STATE OF THE ART AND OBJECTIVES OF THE WORKSHOP
Teoman Ariman
Director of the Workshop
Department of Aerospace and Mechanical Engineering
University of Notre Dame
Notre Dame, Indiana 46556
Traditionally, developments in particulate control equipment have been
slow in starting, as evidenced by the fact that today's basic equipment
differs little from that in use 100 years ago. One major reason for the
lack of progress is that successful innovation requires field trials, where
the cost of failure to the trial plant is large. Therefore, the risk must
be small, and innovative can be only a small departure from the traditional
design. Moreover, most industrial applications of air pollution control
equipment are considered as non-productive economic burdens on the processes
to which they are applied. As a result, control devices so far has received
minimum attention, and incentive for research and development to improve
their performance is lacking.
Fabric filtration systems have been employed in industry for over a
century with relatively few technological modifications [1,2]. Indeed,
there have been in recent years only a few advances of note: introduction
of fabrics capable of withstanding higher temperatures and development of
pneumatic cleaning techniques. For the most part, there was in the past
little incentive for improvement. The collection efficiency of the systems
was mostly satisfactory and the units were inexpensive to purchase. However,
with the recent substantial increase in energy costs attention should now
be given to operating expenses (e.g., energy consumption). As a result, the
filtration systems of yesteryear may not be the best approach for future
applications.
Recently a new method has been developed for the particulate-gas
separation through the fabric filtration. Some initial analytical and/or
experimental investigations have been performed (e.g., Zebel [3,4], Butterworth
[5], Ariman [6-9] and Penney [10]) for the particulate-gas separation by fiber
and fabric filters in an electrostatic field. It has been observed that
the application of an external electrostatic field improves the efficiency
of fine particle collection considerably. More recently Ariman has begun
to investigate the pressure drop across a flat fabric filter due to the
collection of electrically charged dust particles. There was no electro-
static field placed across the filter. Two industrial dust samples, fly
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15
ash and limestone, and Nomex 14 oz. fabric filter samples were utilized in
the bench scale tests. It was found that under the same operating conditions
the pressure drop across the dust layer and filter was reduced by 1/2 to 2/3
as compared to the conventional noncharged dust particles case. An electron-
microscopic study of the dust cakes for charged and noncharged cases
indicate that charged dust particles deposit in a relatively porous layer
resulting in a low pressure drop across the filter. The resulting more
porous dust layer also facilitates cleaning of the filter, since it bends
to fall off readily, leaving the cloth relatively clean. It was also found
that there was considerable improvement in the collection efficiency.
It appears that this new method has a great potential in particulate-gas
separation by fabric filters and may bring enormous changes in the con-
ventional filtration area. However, the research performed to date in this
field has only just scratched the surface and a number of unknown aspects
of this highly promising area need immediate attention of researchers
[12-34]. For example the electrical behavior of aerosols, electrostatic
charge on fiber and fabric filters, particulate cake structure, collection
efficiency and pressure drop and a number of other unknown aspects of the
field need to be investigated.
Another new method in the particulate-gas separation has recently been
initiated by Drehmel and Gooding [35]. The method is directed toward a
combination of magnetic and filtration collection mechanisms. In its
simplest practical form, the high gradient magnetic separator (HGMS) consists
of a canister packed with the fibers of a ferromagnetic material (such as
stainless steel wool) and magnetized by a strong external magnetic field [35].
The strong magnetic forces produced near the edges of the fibers are cabable
of trapping fine, weakly magnetic particles with high efficiency.
The packing density of the separation zone can be adjusted to obtain
a proper balance between conventional filtration phenomena and the HGMS
effect. Reducing the packing density lowers the pressure drop through the
separator but requires a higher magnetic field for efficient particle
collection. The overall particle collection efficiency is theoretically a
function of the applied magnetic field, or filter mesh parameters (fiber
diameter and magnetization, packing density, and length of mesh in direction
of flow), of particle parameters (magnetic susceptibility and diameter) and
of fluid parameters (superficial velocity and viscosity).
To date the high gradient magnetic separation has been applied extensively
to particle liquid separations [36-39]. The bench-scale experimental work
performed by Drehmel and Gooding [35] appears to be the first known attempt
to apply HGMS to particle-gas separations. It is known that conventional
magnetic separation is generally useful for the separation of relatively
large particles of strongly magnetic materials such as iron and magnetite.
HGMS, by contrast, makes possible the efficient separation of micron-sized
particles of weakly paramagnetic materials from liquids at high process
rates. It can be shown through generalized theory that application can also
be made to particle removal from gas streams. Magnetic forces can be cost
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16
effective if the control device is properly designed. One of the most
important feature of the recent work by Drehmel and Gooding [35] is the
dramatic improvement of collection efficiency when a low-strength magnetic
field was applied. However, at present substantial analytical and experi-
mental research efforts are needed to identify other particle-gas
separation problems to which high gradient magnetic separation might be
applicable, to develop analytical models based on the experimental investi-
gations and to provide economic estimates for specific promising applications
such as particulate emission control from basic oxygen surfaces.
Another recently developed new method in particulate-gas separation
is based on acoustic coagulation. Recently there has been a substantial
growth in research dealing with various aspects of acoustic-aerosol
interactions. In the last few years considerable attention has been devoted
to the area of acoustic propagation and attenuation in aerosols [45].
Theories have been developed for both small amplitude [41-42] and large
amplitude [43-44] sound waves, including the processes of mass, momentum
and energy transfer between droplets and the surrounding gas-vapor mixture.
It is well known that acoustic streaming can substantially increase the
rates of convective transport of mass and energy and chemical reactions at
solid-fluid and fluid-fluid interfaces, provided the sound pressure level is
sufficiently high [45]. The aforementioned investigations have indicated
[46] that the acoustic streaming mechanism may be of importance in droplet
evaporation, particle drying, acoustic coagulation, and droplet or particle
combustion, especially in those cases where the droplets or particles are
large enough so that the acoustic boundary layer thickness is much smaller
than the droplet radius. In that case very significant increases in heat
and mass transfer are possible [47].
Initial experimental investigations were directed to the use of shock
waves to coagulate fine particles [48-50]. They also included the use of
pulsating combustion burners for providing the shock or more correctly
saw-tooth waves, i.e., sound waves with periodic discontinuities for
agglomeration [51]. The pulse output approximating a saw-tooth wave with
a step wave front has successfully agglomerated ZnOo aerosols to bring
about a five-fold increase in mean particle size [51j. Since no special
agglomeration chamber was used, the experiment was equivalent to placing the
pulse-jet in a suitable elbow of the duct transporting the dusty gas in an
industrial plant. Recently Scott and Rennick have experimentally investigated
several aspects of acoustic-aerosol interactions in finite-amplitude,
mutually perpendicular, standing and progressive acoustic fields [52-55].
Acoustically generated turbulence and the role it may play in aerosol
agglomeration has been more recently investigated [55-56].
Another recent experimental program has been initiated by Shaw [57] and
aims to provide information on the experimental investigation of acoustic
coagulation in the flowing coagulation chamber, the evaluation of sinusoidal
standing wave acoustic system versus saw-tooth traveling wave acoustic
system and on the search and testing of large acoustic generators.
As indicated here although in recent years there has been a substantial
growth in research dealing with various aspects of acoustic-aerosol
interaction, a number of vital investigation areas need to be identified in
this promising field.
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17
New concepts and methods in the particulate-gas separation are not
restricted to the stated three major areas. For example in the recently
developed Charged-Droplet Scrubber (CDS), the scrubbing droplets, usually
water, carry a high electrical charge which is induced during or after
formation [58-59]. The droplets may move under the influence of electric
fields, and the particulate may also be electrically charged. More
investigation may be needed for the various charged droplet scrubbing
mechanisms toward the control of fine particulate (0.1 - 1.0 vim in diameter).
In another research effort, an experimental investigation has been
conducted by Scholz [60] on the effects of sonic and electric fields on the
collision rate between suspended submicron sized particles. Recently a
number of new concepts have been considered for the collection of submicron
particles, specifically cotton dust in the Southern Region Research Center
of the Department of Agriculture [61]. However, these developments are at
the initial stage and a number of unknown aspects of these efforts may need
immediate attention of investigators.
As indicated above,all three new methods and the other new techniques
appear in the urgent need of a number of investigations. In order to identify
new research areas associated with these methods as well as to stimulate
future research activities in these fields, the research workshop on the
novel concepts, methods and advanced technology in particulate-gas
separation was held with the sponsorship of the National Science Foundation
and Environmental Protection Agency at the University of Notre Dame.
A technical committee consisting of Drs. T. Ariman, B. Y. H. Liu and
K. T. Yang, was in charge of the technical content of the research workshop.
Drs. Ariman and Yang have been associate professor and professor and chairman
respectively at the Department of Aerospace and Mechanical Engineering of the
University of Notre Dame, while Dr. Liu has been a professor of the
Department of Mechanical Engineering and the director of the Particle
Technology Laboratory at the University of Minnesota.
Three groups of researchers were invited by the technical committee;
1. theoretical analysts; 2. experimentalists; 3. design and application
specialists. The interaction between the three groups through presentations
and intensive discussions is emphasized particularly, because very often a
researcher in one group can benefit from the other groups. It was quite
gratifying that practically all participants attended the workshop in its
entirety and took an active role in every aspect of it.
The workshop took place on April 20, 21 and 22, 1977 at the University
of Notre Dame Campus. The number of participants was limited to 56 scientists
and engineers from universities, government and other research organizations,
and industry. Two general lectures by internationally known scientists, one
by Dr. M. Benarie of France and another by Professor F. Lbffler of W. Germany,
and 17 invited lectures by the prominent experts were presented. The time
allowed for general and invited lectures were 45 and 25 minutes respectively.
Each presentation was immediately followed by an in-depth discussion of 15
minutes for general and 20 minutes for invited lectures. The latter part
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18
generally began with a designated discusser for each presentation and then
a general discussion open to every participant was conducted. Each
designated discusser made a considerable preparation for a constructive in-
depth discussion of each presentation and submitted his written discussion
well before the workshop date. In order to stimulate the discussions, the
copies of the manuscripts were distributed to the relevant discussers prior
to the workshop. On the morning of Friday, April 20, a panel discussion on
"New Directions in Particulate-Gas Separation Research" was conducted. The
formal presentations by the members of the panel and concise discussions and
comments by participants were quite productive in identifying new research
topics in the particulate-gas separation area. The entire workshop sessions
were tape-recorded and the tapes were extensively utilized in the preparation
of these proceedings.
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REFERENCES
[1] Billings, C. E. and Wilder, J. Handbook of Fabric Filter Technology.
Volumes I-IV, GCA-TR-70-17-G, prepared for NAPCA (EPA) (1970).
[2] Air Pollution Control. Part 1, Edited by W. Strauss, 337, Pergamon
Press, New York (1966).
[3] Zebel, G..'J. of Colloid Sci.. 20, 522 (1965).
[4] Zebel, G., Staub. (English Translation), 26, 18 (1966).
[5] Butterworth, E., Manufacturing Chemist, 65, Feb., (1964).
[6] Ariman, T., Rao, K. S., Yang, K. T. and Hosbein, R. L., "Collection
of Dust by Fabric Filtration in an Electrostatic Field," Proceedings
of the Second Annual Environmental Engineering and Science Conference,
p. 555 (1973).
[7] Ariman, T. and Tang, L., "Collection of Aerosol Particles in an Electro-
static Field," invited lecture, presented at the Symposium on Surface
and Colloid Chemistry in Air Pollution Control, August (1973) and
appeared in the Atmospheric Environment, 10, 205 (1976).
[8] Ariman, T. and Lane, M. J., "On Experimental Determination of Collection
Efficiency and Pressure Drop in Fabric Filters in an Electrostatic
Field," to be published.
[9] Rao, S. K. and Ariman, T., "Collection of Dust by Fiber Filters in an
Electrostatic Field," Survey Article, to be published.
[10] Gaylord, W. Penny, U.S. Patent No. 3, 910, 779, on an Electrostatic
Dust Filter, October 9, 1975.
[11] Liu, B. Y. H. and Pui, D. Y. H., J. Aerosol Science. .5, 465 (1974).
[12] Liu, B. Y. H. and Pui, D. Y. H., J. of Colloid and Interface Science.
49 . 305 (1974).
[13] Liu, B. Y. H. and Pui, D. Y. H., "On Unipolar Diffusion Charging of
Aerosols in the Continuum Regime," presented at the International
Conference on Colloids and Surfaces, San Juan, Puerto Rico, June, 1976.
Particle Technology Laboratory Pub. No. 311 (1976).
[14] Fuchs, N. A., The Mechanics of Aerosols. Pergamon Press, Oxford (1966).
[15] Davies, C. N., Air Filtration. Academic Press, New York (1973).
[16] Frederick, E. R., Chem Eng.. 68, 107 (1961).
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20
[17] Kraemer, H. F., and Johnstone, H. F., Ind. Engr. Chem. 47, 2426,
(1955). —
[18] Dawkins, G.S., Technical Report No. 15, Engineering Experiment Station,
University of Illinois (1958).
[19] Natanson, G. L., Proc. of the Academy of Science. U.S.S.R., Physical
Chemistry Section, 112, 95, (1957).
[20] Lundgren, D.A. and K. T. Whitby, I and E. C. Fund (1965).
[21] Yoshioka, N., Emi, H., Hattori, M., and Tamori, I., Kagaka Kugaku.
Chem. Ene. Japan. 32, 815 (1968).
[22] Rossano, A. J., Jr., and Silverman, L., AEC NYO-1594. Air Cleaning
Laboratory, Harvard School of Public Health.
[23] Goyer, G. C., Gruen, R., and LaMer, V. K., J. Phys. Chem.. 58, 137,(1954)
[24] Silverman, L., Conners, E. W., Jr., and D. M. Anderson, AEC NYO 4610,
Air Cleaning Laboratory, Harvard School of Public Health (1956).
[25] Rivers, R. D., ASHRAEJI. k_t 37, (1962).
[26] Whitby, K. T., and Liu, B.Y. H., Aerosol Science. Academic Press (1966).
[27] Gillespie, T., J. Colloid, Sci., 10, 299 (1955).
[28] Thomas, J. W., and Woodfin, E. J., Applic. and Industry. (AIEE) (1959).
[29] Sweitzer, D., Electrets, Literature Search No.308, Jet Propulsion
Laboratory, Cal. Inst. of Tech., (1961).
[30] Dennis, R., Kristal, E., and Silverman, L., AEC NYO-4614, Air Cleaning
Laboratory, Harvard School of Public Health (1958).
[31] Anderson, D. M., and Silverman, L., AEC NYO-4615, Air Cleaning
Laboratory, Harvard School of Public Health (1958)
[32] Lapple, C. E., Advances in Chem. Engr.. j}, Academic Press.
[33] Havlicek, V., Int. J. of Air Water Pollution. 4., 225 (1961).
[34] Silverman, L., Billings, C. E., and Dennis, R., AEC NYO-1592, Air
Cleaning Laboratory, Harvard of Public Health.
[35] Drehrael, D. C. and Gooding, C. H., "High Gradient Magnetic Particulate
Collection," presented at the 82nd National Meeting of the AIChE,
Atlantic City, 1976.
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21
[36] Kolm, H., Oberteuffer, J. and Kelland, D., Scientific American. 233.
47 (1975).
[37] Oberteuffer, J., IEEE Transactions on Magnetics. MAG-10 223 (1974).
[38] Melville, D., Paul F. and Roath, S., IEEE Transactions on Magnetics,
MAG-11, 1701 (1975).
[39] Sala, Magnetics, Inc., "Water Reclamation Using SALA-HGMS Magnetic
Separation Equipment and Filtration Processes, Sala Magnetics, Inc.
Cambridge, Mass., (1975).
[40] Scittm D, S, (Editor), Acoustic-Aerosol Interactions, Proceedings of a
Technical Discussion Session held during 82 National meeting of the
AIChE, Atlantic, City, 1976.
[41] Davidson, G. A., J. of Atmospheric Sci.. 32, 8201 (1975).
[42] Cole, J. E., and Dobbins, R. A., J. of Atmospheric Sci.. 27. 426, (1970).
[43] Davidson, G. A., and Scott, D. S., J. of Acoustical Society of America,
53, 1717 (1974).
[44] Davidson, G. A., J. of Sound and Vibration. _38_, 475 (1975).
[45] Folger, H. S., (Editor), Sonochemical Engr., AIChE Chem. Engr. Progress
Symposium Sci., 67, (1971).
[46] Lyman, F. A., "Aspect of Sound Attentuation in Aerosols," Research
Progress Report, Acoustic-Aerosol Interactions. Edited by D. S. Scott,
16, (1976).
[47] Lyman, F. A., "Attentuation of High-Intensity Sound in a Droplet - Laden
Gas," Technical Report. MAE-5 192-T1 (1976).
[48] Mednikov, E. P., "Acoustic Coagulation and Precipitation of Aerosols,"
Translation from Russian by Consultants Bureau, New York, -(1965).
[49] Gulyaev, A. I. and Kuznetsov, V., "Coagulation of Aerosols in Response
to Periodic Shock Waves," Translation from Russian by Consultants
Bureau, New York (1965).
[50] Jahn, R., "Improvements Relating to Sound Generator," U.S. Patent #849,
504, (1960).
[51] Last, A. J., "Acoustic Conditioning - Sound Sources," Research Progress
Report, Acoustic - Aerosol Interactions, Edited by, D. S. Scott, 13,
(1976).
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[52] Scott, D. S., J. Sound Vib.. 43, 607, (1975).
[53] Scott, D. S., and Rennick, D. F., J. Aerosol Sci., 5_, 301 (1974).
[54] Rennick, D. F., and Scott, D. S., Proc. 6th Inter. Symp. on Non-Linear
Acoustics, Moscow, 174 (1975).
[55] Scott, D. S., and Rennick, D. F., Abstracts 7th Inter. Symp. on Non-
Linear Acoustics, 43, (1976).
[56] Scott, D. S., "Acoustic Dust Conditioning, Acoustically Generated
Turbulent and F - A Sound Propagation Through Aerosols," Research
Progress Report, Acoustic-Aerosol Interactions, Edited by D, S.Scott,
29, (1976).
[57] Shaw, D. T., "Research Activity Summary on Acoustic Coagulation and
Acoustic Generators," Research Progress Report, Acoustic-Aerosol
Interactions. Edited by D. S. Scott, 31, (1976).
[58] Lear, C. W., Knieve, W. F., and Cohen, E., J. of APCA. 25,184, (1975).
[59] Koppang, R. R., "Pollution Control Equipment and Technology - A New
Hybrid EP Concept, The TRW Changed Droplet Scrubber," presented at the
4th International Pollution Exposition and Congress, Cleveland, (1975).
[60] Scholz, P.O., "Experimental Study of Sonic and Electric Field Induced
Coagulation in Aerocolloidal Suspensions," Research Progress Report,
Acoustics-Aerosol Interactions, Edited by D. S. Scott 25, (1976).
[61] Deluca, L. B., Weller, H., and Claassen, B., Personal Communications,
Oct., (1976).
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PULSE-JET ACOUSTIC DUST CONDITIONING AND SOME
NEW POTENTIAL APPLICATIONS
by
David S. Scott
Mechanical Engineering, University of Toronto
I INTRODUCTION
Acoustic conditioning of fine-particle aerosols is a
process by which the mean size of the particles is increased
and their number density decreased through exposure to finite-
amplitude acoustic fields. This change in the particulate
size distribution is important - for it can allow an increase
in the collection efficiency of a downstream dust collector
and/or a reduction in the overall particulate separation cost
because, with few exceptions, collecting devices retain higher
mass fractions as the particulate size is increased. The
increased coagulation rate which achieves this rapid change in
the size distribution is the result of manyfold increase in the
particle-particle collision frequency — which in turn results
from dynamic acousto-aerosol interactions. Experimental evidence,
and straightforward physical arguments, indicate that the process
can be particularly effective on high dust load fine particle
aerosols.
Although the principle of acoustic dust conditioning (ADC)
has been known for sometime and has been shown to be technically
effective, the process has enjoyed little industrial application.
In my view, this lack of industrial application has been the
consequence — primarily — of economic limitations. The first
of these is high operating costs due to high specific power
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requirements, and the second is high capital costs.
Studies performed at the Ontario Research Foundation and
the University of Toronto have examinedanew approach to ADC
— in which the following two complimentary principles were
explored.
(i) Firstly, the conventional standing sinusoidal
acoustic field, used in essentially all earlier acoustic dust
conditioning work, was dropped in favour of progressive saw-tooth
fields. This change in acoustic configuration allowed major
simplifications in the agglomeration chamber yielding both
capital and operating cost savings.
(ii) Secondly, the normal techniques of sound generation
— such as sirens and mechanically activated pistons — were
discarded in favour of a resonant pulse-jet acoustic source.
This not only allowed a direct conversion of fuel to sound and
the use of a simple no-moving-part device, but also readily
produced high intensity progressive saw-tooth acoustic fields.
The results of this research/development program are
thoroughly reviewed in Reference 1. In summary, the practical
results of this program showed that significant — 0(10) — advances
can likely be achieved by this new process in all three areas of
(a) capital, (b) maintenance, and (c) operating costs. At the
same time, the mean particle size was increased 5-7 fold. But
as noted, the details of this research are well covered in
Reference 1 and it would be inappropriate to review these here.
II THE NEXT STAGE OF ADC DEVELOPMENT
Since the work described in Reference 1 was performed,
the acoustic-aerosol research with which I have been involved
has primarily concentrated on more fundamental acousto-aerosol
phenomena. Insofar as the applied aspects of pulse-jet acoustic
dust conditioning we have, as they say in the performing arts,
been between shows. And this gives us time to pause and take
note of where we are. To date, it seems to me that research
on pulse-jet dust conditioning has indicated economic and
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technical feasibility in principle. The next stage in the
systematic development of these concepts must examine promising
conditioner/collector systems in the context of likely applica-
tions. It is this next stage of the development, to which I
address the remainder of this paper.
But by way of philosophical introduction, I preface my
remarks by saying that it is this stage which is, perhaps, the
most critical. The history of acoustic agglomeration has been
marred by overzealous researchers who have felt the process to
be a likely panacea for all fine particle collection requirements,
as well as by ambitious entrepreneurs anxious for solution to
specific problems on short-time-scale development programs. My
own view is that ADC will never be a panacea-type solution to
fine particle removal from industrial sources. But if well
conceived development programs are carried out with selected
conditioner/collector systems, and with specific applications in
mind (and over development time scales that can allow careful and
systematic evaluation of the relevant parameters) then it seems
reasonable that in specific technical sceneries, ADC in combination
with an appropriate collector, could be more effective than any
alternate approach. Moreover, once we have identified single
application in which this situation exists, the value of careful
development will have been illustrated. And by that impetus the
process development can be expanded to further applications.
But care — and perhaps some luck — is required in the choice
of both the conditioner/collector configurations as well as the
problem to which they will be applied. And those funding this
development must not be mislead into thinking that conclusive
results can be obtained in a matter of months.
With this background, I will briefly outline two development
projects with which I am associated, and which have been initiated
within the past few weeks. Clearly, I can only describe the work
we hope to do rather than present any results.
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2.1 ADC/Wet Scrubbers in the Pulp and Paper Industry
In Reference 1, wet scrubbers are identified as perhaps
the most promising of collector systems for use in combination
with ADC. To illustrate by example; in the case of a particular
venturi,scrubber, 3,150 bhp was required to collect 90% of the
0.2 ym particles while only 60 bhp was required to collect 90%
of the 0.5 ym particles. The implication for ADC is that by
affecting a 2.5 fold increase in mean particle size (we have
previously noted 5~7 fold increases) a 50 fold reduction in
scrubber power requirements would be realized. While this
illustration refers to a particular venturi scrubber, all
scrubbers exhibit sharp transitions in collection efficiency
as a function of particulate size within the size range
0.1-1.0 ym.
Of course wet scrubbing can also be used to simultaneously
remove noisome (noxious/odorous) gases. And there is some
evidence that the presence of a finite-amplitude acoustic field
can also improve noisome gas scrubbing effectiveness. In this
circumstance, the acoustic field might be considered to act both
as a conditioner for particulate collection and as a catalyst
for noisome gas removal. Although the latter potential benefit
is more speculative, it could be important in applications within
the Pulp and Paper Industry and so has been included in our
program.
A possible deleterious effect on scrubber performance
would occur if the sound were to rapidly agglomerate the liquid
droplets in the scrubber itself, thereby markedly reducing the
surface volume ratio of the scavenging liquids. Whereas it is
possible to visualize a situation in which this occurs — it is
also apparent that configurations can be designed to minimize
this effect if it should prove important, as proper design un-
doubtably involves some balance between maximizing the droplet
"swept" volume and minimizing droplet agglomeration.
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The research program will proceed as follows.
(1) Visits will be made to various pulp and paper mills
for the purpose of familiarization with the practice and
potential of wet scrubber gas-cleaning. This study will
include an appraisal of (a) existing scrubber installations,
(b) potential new installations both in new mills and as
replacements for existing non-scrubber equipment, (c) potential
for pulse-jet conditioning of existing scrubbers for the
purpose of upgraded performance and/or reduced annual costs,
(d) identification of the mill environment in gas-cleaning
locations for the purpose of evaluating noise isolation
requirements .
(2) With the benefit of (1), the experimental program
will be finalized. In particular, key parameters requiring
examination will be identified and a desired data point grid
constructed.
(3) A parametric evaluation of the effectiveness of
pulse-jet acoustic conditioning on particulate collection by a
medium energy venturi scrubber will be performed. Flow rates
between 1000-3000 cfm will be studied. A ZnO fume will be
generated on-line and used as the test aerosol. By this means,
particulate size distributions and densities can be readily
achieved which properly model typical pulp and paper aerosol
effluents.
(4) The influence of intense acoustic fields on gas
scrubbing performance in the above facility and at flow rates
of 1000-3000 cfm will be studied. The test noisome gas will
be S02 and the test scrubbing liquid an Na2C03 solution.
(5) If practical from an experimental point of view,
the pulse-jet acoustic field will be subdivided into a dirty
gas stream and a clean gas stream in order to obtain data for
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extrapolating to the ultimate flow rates which may be treated
by the pulse-jet. (It rs expected that the pulse-jet will be
able to condition a much greater flow volume than the 3000 cfm
generated in this program).
?kOL&
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particulate laden, gases cannot pass through gas turbines
without massive destruction to the turbine blades resulting in
unacceptably short lifetimes. And so we are faced with a
particulate-gas separation problem at high temperatures and
pressures that is critical not only for environmental consider-
ations but also for the success of the process itself.
At the temperatures 0(400°K) and pressures 0(10 atm.)
which are anticipated, most conventional particulate removal
techniques are unsuitable. Mechanical, or inertial, separation
by cyclones seems to be one of the few systems that can, in
principle, operate at these temperatures and pressures.
Unfortunately, the performance characteristics of cyclones are
not sufficient to meet the gas cleanliness standards set by
turbine manufacturers — although there remains some uncertainty
regarding what these standards should be. It now appears that
the ultimate criteria may require that there be no more than
0.5 mg/m3 of particulate matter above 2 vim diameter.
Results of the FBC program at ANL have shown that the mass
loading and size distribution of the particulate matter exiting
from the second of two inertial separators is not far from that
which would be acceptable as turbine inlet conditions. Moreover,
a fairly straightforward evaluation indicates that if the size
distribution of particulate matter to the entering the cyclone
was increased by perhaps less than a factor of 5, the performance
of the cylcone would be improved such that the effluent would
meet turbine inlet criteria. For these reasons the group at
ANL have decided to initiate a research program to investigate
the effects of using ADC in this application. And it is in this
regard that I have become associated with these activities.
The program at ANL will evaluate effectiveness of ADC in
a bench scale fluidized-bed combustor. It is proposed that the
sound source be a resonant pulse-jet and that progressive saw-
tooth waves constitute the acoustic field configuration.
There are several promising aspects of this approach.
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For instance the heat of combustion from the pulse-jet simply
adds to the overall process heat. Thus the effective power
requirements will, in principle, be zero - although the cost
per heating unit for pulse-jet fuel may be expected to be some-
what higher than that of coal. Furthermore, the pulse-jet
will run at higher power levels due to the elevated pressures
and should, in principle, be a more effective sound generator
than if it were running at ambient conditions.
Conceivably, there might also be deleterious ADC effects.
For instance, very fine submicron particles could be agglomer-
ated into a size range below that which would be effectively
removed by the cyclone, but within a range which could do damage
to turbines. Evaluations of the mass loading ratio in the
very fine particle spectrum indicate that this should not be a
problem — but it will clearly be an aspect which shall be monitored
during the experimental program.
Turning to the experimental program itself, one of the main
difficulties in this initial stage of process evaluation shall
be "scaling". Put simply, a pulse-jet is a device which cannot
be scaled up or down without changing the acoustic characteristic
- for a pulse-jet is a quarter wavelength device. Ultimately,
it is expected that single pulse-jets will generate sufficient
sound to treat between 5,000 to 10,000 cfm. But the current
test plant at ANL yields approximately 81 scfm, or on the order
of 20 acfm.
In response to these scaling demands, the current intention
is to design a pulse-jet that will exhaust into a resonant-manifold,
The manifold shall be capable of splitting the off-gases into a
"waste" stream and two "process streams". Two process streams
are required in order to examine the affects of ADC on both the
primary and secondary cyclone performance. By means of the
resonant-manifold a large total acoustic power output (Watts)
can be reduced in total magnitidue to match the amount of flue
gas being treated. At the same time, this approach will allow the
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acoustic field to be varied in intensity from approximately
165 db downwards, in order to parametrically evaluate sound
intensity requirements. It is my feeling that by exhausting
the pulse-jet into a resonant-manifold, and then drawing off
the acoustic power as required for the much smaller scale
dust treatment streams, we will both eliminate the scaling
difficulties as well as allow a much greater degree of
parametric flexibility, than would be the case if we exhaust
the pulse-jet directly into the particulate laden gas. Of
course the negative of this approach is that it requires the
design, construction and debugging of a portable pulse-jet/
resonant-manifold system.
Although the experimental program I have just described
is still very much in the planning stage, and alternative
programs may, in fact, be followed — the entire concept of
applying ADC to high pressure, high temperature, particulate
removal sytems is, I think, very exciting.
Ill CONCLUDING COMMENTS
It is my view that ADC has reached the stage where economic
and technical feasibility have been shown to be feasible in
principle. We must now carefully select certain applications
where ADC may have clear advantages over alternate systems,
and then perform careful unhurried evaluations of its capabilities
in these technical sceneries. Perhaps one of the two applications
I have reviewed in this paper — and which will be evaluated over
the next 1~2 years - will be the application that gives us the
first real illustration of an ADC/collector system meeting the
required criteria better than any alternative process. I hope
other investigators, and laboratories, seek other unique applica-
tions where their special requirements might yield an "application
breakthrough".
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REFERENCE
1. Scott, D.S., "A New Approach to the Acoustic Conditioning
of Industrial Aerosol Emissions", J. Sound and Vj-biation,
Vol. 43, no. 4, pps. 607-619, 1975.
OPEN DISCUSSION
Billings: I think there is a misconception in accordance with what we might
expect. We would all agree that agglomeration of some sort, whether mechanical,
thermal, or turbulent,is desirable to improve the mechanical collector. I
think zinc oxide, as an aerosol, is a questionable aerosol to use.
What you said was that you went down in concentration and up in apparent
particle size. Correspondingly, the aerodynamic diameter parameter would go
down. Although this may be 2.0 to 7.0 microns, the fact of the matter is that
the momentum relaxation time is about 0.1 of the original value. This is what
you should be measuring.
Scott: I agree with what you are saying. One thing that was not brought out,
and for obvious reasons, is the kinematic viscosity of the gas. Aerodynamic
diameter is a characteristic parameter of the aerosol and the momentum
relaxation time involves both the characteristics of the gas and the particles.
I would like to clarify that all the sizes talked about are aerodynamic
sizes and are a measure of the momentum relaxation time or of aerodynamic
diameter. Results of size distribution were obtained from a cascade impactor.
The means of measuring size is a means of predicting improved performance
since collectors,particularly cyclone collectors, are inertia collectors.
Melcher: What are the resonance times requires for gas treatment to go from
0.5 to 2.0 microns?
Scott: Approximately between 1.5 and 2.5 seconds. Resonance times must be
a factor of concentration. Acoustic vibration constants are inversely pro-
portional to the concentration. So you should put the stuff in front of a
conventional device where the concentration of particles is high. This is a
necessary requirement for acoustic coagulation.
Scott: Is it possible to amend the resonance times by using an adverse density
gradient?
Shaw: If you have an amount of the fine particles,your acoustic agglomeration
is small so the count is large. With a large standard deviation you need
large particles to act as collectors for the small particles. It is a
complicated function of standard deviation, mass concentration and frequency.
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That is why I am skeptical about the recent report by the EPA. It is based
on the wrong equation, wrong data and, of course, has the wrong conclusion.
Another point I'd like to bring up is that because of economic
comparison on acoustic agglomerators with conventional devices, comparisons
can only be made for specific applications. A general statement is too
dangerous. It can be too optimistic or too pessimistic.
Something else worthy of note is on the acoustic device, for instance,
sirens; the last reference I could find was published in 1949. Bell
Laboratories were to build a siren to put on the Empire State Building for
air raid warnings. Acoustic devices must be studied using up-to-date
information on acoustic agglomoration.
Finally, I have a question. By using a pulse generator, isn't a
combustion product used which may get into the system?
Scott: To further elaborate on resonance times, no studies were done on
parametric times for the resonance frequency. We are now studying an oil
mist aerosol at the University of Toronto (UT) and have found that the change
from uncoagulated to fully coagulated particles with no further change is
very rapid. Well, under 1 second. The density was above 1 grain/ft3. So
we can assume that resonance times are not a problem.
From my work at Argonne National Laboratories, we found that by using
a spiral configuration it would be easier to modify resonance times simply by
changing the length of the spiral. Parametric evaluations would be easier
to conduct.
From my work through the Ontario Research Foundation, we found that .
economics depend mainly on the costs attendant to acoustic isolation. Total
capital, operating and maintenance costs ran $.50 to $1.50 per cubic foot.
So it is economically feasible as compared to current devices, but it is a
cost which must be added on since it is a conditioning. It is within or
under costs of current devices.
Concerning the pulse jet, the products of combustion did go into the
gas stream. This represented less than 1% of the total. In doing preliminary
design work at Argonne on resonant manifolds, I think it will be possible to
drop the products of combustion which get into the gas stream down to zero.
A resonant manifold system can be used in almost any application.
•j
Koppang: What is the actual energy input in watts/ftj?
Scott: I don't recall right off hand, but it is in the paper.
Baril: Because there are maximum noise levels in the U.S., we must be concerned
that the noise from the system isn't too loud. Is this a problem?
Scott: Not really. The pressure levels achieved are not larger than those
from an internal combustion engine. Since mufflers can be built for cars, it
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34
should be no problem to build one for the system. This will be even easier
since an internal combustion engine operates at a range of pressure levels,
whereas the acoustic device has a resonant frequency and the muffler can be
tuned to that frequency.
B. Liu: The agglomeration that you observed at a bend was irrespective of
the pipe. That would mean that the ortho-kinetic agglomeration is really
not very effective. Rather it is based on something that is acted upon
through turbulence and the acoustics.
Scott: Your explanation is very good. Ortho-kinetic interaction is import-
and because you can describe it physically and everyone can believe it. I
don't think it is the important mechanism.
One of the things that has come out of our research at UT is that an
acoustic field itself can generate a turbulent field, even if no turbulence
existed (we think). If you have a turbulent field with a flow process, then
it is clear that an acoustic field adds to that turbulence. I think there
is a whole set of messy processes occurring here that increases particle-to-
particle collisions. And that's where I'd like to stop until we get some more
fundamental work in.
It is interesting, diagnostically, to look at this question though.
Because we are now using microfilms, hot wires and LD-8's (which go very well
with the fact that we are using particles) the diagnostic procedures should
improve the information gathered.
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35
ACOUSTIC AND TURBULENT AGGLOMERATION OF SODIUM AEROSOLS
W. Hinds, E. P. Mallove, and M. W. First
Department of Environmental Health Sciences, Harvard
University, School of Public Health
ABSTRACT
Tests of turbulent and acoustic agglomeration of captive
sodium fire aerosols at concentrations of 0.1 to 20 gm/mJ were
conducted in 90 m3 and 0.65 m3 vessels to evaluate these mech-
anisms for direct application air cleaning systems. Aerosol
mass concentration decay with time was monitored by sequential
filter samples. Turbulence was generated mechanically with a
51 cm diameter centrifugal fan impeller and a reverberant acous-
tic field was created with an electronic siren. The effectiveness
of each method over a range of particle concentrations and power
densities was evaluated by an agglomeration index, a measurement
of particle growth based on sedimentation characteristics. Both
turbulent and acoustic treatment markedly enhanced sedimentation
rate compared to undisturbed settling. The effectiveness of
both methods increased with increasing aersol mass concentra-
tion and increasing power input per unit volume of aerosol. The
agglomeration index reached 20 for turbulent agglomeration at an
aerosol mass concentration of 3 gm/m3 and 7 for acoustic agglo-
meration at 14 gm/m3 when using an acoustic intensity of 115 dB.
Turbulent agglomeration was more effective than acoustic agglo-
meration for the same mass concentration and power density con-
ditions.
INTRODUCTION
A study is being conducted in the Harvard Air Cleaning La-
boratory to examine the feasibility of using direct application
air cleaning as a treatment method for high concentrations of
sodium fire aerosol that would likely fill a reactor contain-
ment vessel following a release inside a liquid metal fast bree-
der reactor. Most particulate air cleaning systems are dynamic,
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36
flow-through devices where particulate matter is continuously-
removed from a stream of dust laden gas as it passes through.the
device. A less common, approach can be applied when the dust
laden gas is confined within a vessel. This approach treats the
entire gas volume simultaneously, as a batch process, to reduce
airborne particulate concentrations as rapidly as possible.
Leakage from the containment vessel is expected to be small,
(no more than 0.1$ of the containment volume per day). Because
it is continuous, significant public health benefits to the sur-
rounding community would accrue from a rapid reduction in air-
borne particulate concentration within the containment vessel.
To reach the design goal, reducing by a factor of 10 the two-
hour integrated concentration, it is necessary to reduce the
airborne concentration within a matter of minutes by 90% or more.
Although rapid clean up can be accomplished by a conventional
air cleaning loop, such a system would have to be very large
and expensive to treat a containment volume of 7x10^ m3 in a
few minutes. A direct application air cleaning approach, on
the other hand, has considerable potential for this situation
because it treats the entire containment volume simultaneously
without the necessity of having to pass large volumes of gas
through air cleaning devices.
Three approaches have been evaluated experimentally: sca-
venging by inert powder injection, enhancement of coagulation by
mechanically generated turbulence, and enhancement of coagula-
tion by high intensity sound. The latter two methods are simi-
lar and are the subject of this report. Inert powder injections
have been described in Reference 3- Both turbulence and high in-
tensity sound enhanced coagulation by applying energy to the gas,
throughout its volume, to create relative motion between particles
Because of the complex geometry of a containment vessel, in-
cluding its machinery, the use of a tuned sonic coagulation sys-
tem was not considered feasible and consequently no attempt was
made to evaluate such a system. Instead, random traveling waves
in a reverberant enclosure were used. Mechanically generated
turbulence was produced with a centrifugal blower wheel. The
experiments provided some insight into coagulation mechanisms
and the effectiveness of two different methods operating under
similar conditions. Effectiveness was evaluated by monitoring
the change in aerosol mass concentration as a function of treat-
ment time in 0.65 and 90 m3 chambers. Results show the effect of
initial mass concentration and applied power density on coagula-
tion rate.
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37
EXPERIMENTAL PROCEDURES
1. Aerosol Generation for Turublent Agglomeration Tests
Turbulent agglomeration tests were conducted in a 90 m3
sealed chamber, 4 meters in height. Sodium aerosol was genera-
ted by burning the metal in an electrically heated steel pot.
The sodium pool, 260 cm2 in area, produced a dense white aero-
sol that filled the chamber within 10 minutes after initiation
of a fire. A one pound sodium pool fire would burn for 20-30
minutes. Decay in mass concentration with time was obtained
with open-faced MSA 1106B high efficiency filter samples taken
near mid-chamber height. Ten to 15 samples were taken over a
3 to 6 hour period at 18.4 1pm for 0.5 to 3 minutes. Less than
1% of the aerosol was removed by sampling.
Undisturbed,mass concentration decay tests were conduc-
ted in the 90 m3 chamber to serve as a baseline against which to
measure the effectiveness of turbulent enhancement. The results
shown in Figure 1 were obtained from eight baseline tests fol-
lowing the burning of 1 Ib. sodium. Initial chamber relative
humidity ranged from 1*4 to 22%. Analysis of the unhindered set-
tling data showed that this aerosol has the decay characteris-
tics equivalent to that for lJ urn monodisperse particle under
stirred settling. The actual mass median diameter (MMD) and
geometric standard deviation (GSD) of the aerosol cloud was
determined with the aid of eight-stage Andersen impactors and
an aerosol centrifuge [1]. The size distributions, 20 minutes
after initiation of a 1 Ib. sodium fire for all of the generated
aerosols were found to be nearly the same with a MMD of 2.1 urn
and a GSD of 1.8. Measurements taken at later times indicated
an equilibrium between sedimentation and growth with MMD stabi-
lizing at about 3 ym.
Measurements were made of particle density and dynamic
shape factor from the aerodynamic equivalent diameters deter-
mined by aerosol centrifuge and geometric parameters measured in
the scanning electron microscopy [2]. Untreated aerosols aged
from 34 to 376 minutes gave an average particle density of .78
gm/cm3 and average dynamic shape factor of 1.37 [2].
Quantitative chemical analysis indicated that the aero-
sol particles were porous sodium carbonate with some water of
hydration [3]. During the undisturbed settling tests, aerosol
deposition per unit area of chamber wall was 0.10 to 0.01 of the
amount that settled on the floor.
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38
S
W)
c
o
•H
-p
-------
39
2. Turbulent Agglomeration Tests
A 7.5 HP, 3,000 cfm centrifugal blower (New York Blower
Co., type GI, size 20) was placed in the center of the 90 m3
chamber, to induce turbulence. The blower was started shortly
after peak aerosol concentration was reached. The time decay
history of five such tests with peak aerosol concentration near
2 gm/m3 is presented in Figure 2.
The blower was run with and without the scroll casing in
place to distinguish the effect of centrifugal deposition from
the effect of turbulence enhanced agglomeration and sedimenta-
tion. Operation with the scroll casing off takes away a major
surface for deposition, though some cleaning credit must still
be taken because of particle impaction on the fan blades. Re-
moving the casing did not greatly change the decay profile from
casing-on performance, as can be seen in Figure 2. The primary
aerosol decay mechanism was, therefore, by enhanced agglomeration
and sedimentation onto the chamber floor. Analysis of samples
of wall, floor, and blade deposits indicated that aerosol depo-
sition occurs, roughly, 28% on walls, 65% on floor, and 7% on the
fan blades. Numerous large spherical particles (d > 100 ym) were
found deposited on horizontal surfaces following each turbulence
test.
Substantially faster decay rates were obtained with tur-
bulent agglomeration when the peak aerosol mass concentration in
the chamber was above 2 gm/m3. One such tests in shown in Figure
3.
By using different pulley combinations on the blower, it
was possible to generate different levels of turbulence in the
chamber. Power densities, determined from measurements of fan
speed and electricity consumption, are discussed in a later sec-
tion. Assuming an electrical to mechanical energy conversion
efficiency of 0.80, power .densities in the range of 20-70 watts
per cubic meters of chamber volume were utilized.
3. Acoustic Agglomeration Tests
Efforts to determine the effect of acoustic agglomera-
tion on sodium combustion aerosols began with the search for a
suitable and convenient source of high intensity sound. The
first sound generating devices employed in this research were
pulse jet engines [4]. A special pulse jet engine constructed
by Ontario Research Foundation (Canada) was unable to provide a
sound field of sufficient intensity for acoustic agglomeration
experiments in the 90 m3 chamber. These tests were described in
Reference 3.
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40
FIGURE 2
TURBULENT AGGLOMERATION AEROSOL CLEARANCE TESTS
U.O
CO
g
60
d
o
•H
•P
0]
k
•P
c
CO
O
C
O
0
CO
01
cd
S
o
CO
O
1.0
O Blower Casing On
O Blower Casing On
A Blower Casing On
D Blower Casing Off
0 Blower Casing Off
Baseline vx
Concentration >»
Profile Without -
Induced Turbulence
(From Figure 1)
0.1
0.01
50 100 150 200 250
Time After Blower Start (Minutes)
300
-------
41
S
o
c
o
•H
-p
cd
fc
-p
c
-------
42
For later tests, a Federal CJ-24 electronic siren (a
type employed in emergency vehicles) was used to generate in-
tense sound fields in a steel chamber having a volume of 0.65 m3
and an average internal height of 75 cm (Figure 4). A smaller
chamber was needed because the acoustic power radiated by a sin-
gle CJ-2*I siren will not produce adequate sound levels though-
out a 90 m3 chamber. The CJ-24 "siren" is not a gas-dynamic
rotating siren, by the usual definition, but is a very powerful
loud speaker powered by a PA-20A amplifier and signal'generator.
When 60 watts of electrical power were used to energize this
siren, sound pressure levels of 1*15 dB were measured at the sam-
pling port shown in Figure 4. The frequency spectrum was broad
band with a peak in the 600 to 1,200 Hz range. Variations in
intensity did not exceed 3 dB throughout the 0.65 m3 chamber.
The siren was used in the "yelp" signal mode, a complex wave
form in the audible range which has a superimposed oscillation
of approximately 5 Hz. The acoustic power of the siren was cal-
culated from data supplied by the manufacturer. The acoustic
power output of the siren for lower sound pressure levels was
calculated by assuming a reverberant field in the chamber with
constant wall losses. Acoustic tests were performed at 1^5 dB,
132 dB, and 125 dB, spanning a power density range from 10 watt/
mo to 0.1 watt/m3. Sound pressure levels were measured with a
General Radio octave band analyzer in the flat response mode.
In a typical test, between 50 and 100 gm of sodium metal
was heated rapidly to burning temperature in a steel pot and
allowed to burn for 10 minutes filling the sealed chamber with a
dense sodium aerosol. Open-faced glass fiber filter samples
were obtained at mid-chamber height to determine the mass con-
centration over time for unperturbed and siren-on conditions.
Each filter sample was restricted to 9.2 liters (I.H% of the
chamber volume) to minimize depletion of the aerosol by sam-
pling. Up to 11 samples were taken giving a maximum concentra-
tion decrease due to sampling of 1*1$. Because unperturbed and
siren-on tests were sampled at roughly the same time intervals,
changes in concentration due to sampling were identical for each.
Figure 5 shows mass concentration decay results for un-
perturbed sedimentation in the small chamber and sedimentation
enhanced by acoustic agglomeration. The curves for the three
sonic tests indicate moderate enhancement of sedimentation.
RESULTS
Data for undistrubed settling and turbulent agglomeration
tests in the 90 m3 chamber are summarized in Table 1. The basic
parameters which were extracted from the concentration decay
graphs for each test are initial aerosol mass concentration
-------
FIGURE
0.65 M3 CHAMBER FOR ACOUSTIC AGGLOMERATION EXPERIMENTS
Power
Supply
Federal
Elec-
tronic
Siren
Light for Illumi-
nating chamber
Signal Generator
l-r
122 cm
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44
S
bO
C
O
•H
-p
0)
!H
•P
C
(U
O
c
O
u
oJ
a
o
n
O
^
OJ
FIGURE 5
ACOUSTIC AGGLOMERATION AND UNDISTURBED SETTLING
IN A 0.65 M3 CHAMBER
Arrows Indicate Time
Siren Turned On
Unperturbed Settling
Siren On
1CLS7
0.5-
0.1
10 20 30 UO 50 60 70
Time After Peak Aerosol Mass Concentration (Minutes)
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45
TABLE 1 - SUMMARY OF UNDISTURBED SETTLING AND TURBULENT AGGLO-
MERATION TESTS IN 90 M3 VESSEL
Initial
Aerosol
Mass
Conc'n
(gm/m3)
6.5
3.8
2.4
1.6
1.1
0.084
Time for
C/C0=e-l
(min. )
47
105
162
175
290
592
Mass
Fraction
Removed
in
5 min.
Undisturbed
0.10
0.047
0.031
0.028
0.017
0.008
Mono-
disperse
Equivalent
Size ,d*
(ym)
Settling Tests
9.2
6.2
5-0
4.8
3.7
2.6
Agglomer-
ation
Index
IA
4.4
3.0
?.4
2.3
1.8
1.2
Power
Density
watts/m3
0
0
0
0
0
0
Turbulent Agglomeration Tests
3.4
3-1
2.0
2.0
1.9
1.7
1.6
1.3
1.2
0.94
0.85
3.4
2.1
7.5
2.3
3-8
16
15
12.5
25
13
25.5
0.76
0.91
0.49
0.89
0.74
0.27
0.29
0.33
0.18
0.32
0.18
33.8
43.7
23.1
41.7
32.5
15.8
16.3
17.9
12.7
17.6
12.5
16.1
20.8
11.0
19.9
15.5
7.5
7.8
8.5
6.0
8.4
6.0
27
27
21
27
71
18
18
27
18
18
27
-------
46
(C0) and time (te) for the mass concentration to reach (C0/e).
For undisturbed settling, the initial mass concentration was
considered to be the peak concentration reached. For the tur-
bulence tests, initial concentration was defined as the measured
concentration at the time turbulence was initiated, usually
within a few minutes of peak chamber concentration. Data from
undisturbed settling and acoustic agglomeration tests in the
0.65 m3 chamber are similarly presented in Table 2 and defined
in like manner.
Most mass concentration decay profiles gave straight lines
when plotted semi-logarithmically, indicating exponential decay
functions. Stirred settling conditions occur in the chambers
due to thermal convective motion and energy from the agglomera-
tion mechanisms themselves. 'It is well known that stirred set-
tling of monodisperse aerosols produces exponential mass con-
centration decay. Even though the sodium aerosol is not mono-
disperse, it is extremely useful to approximate aerosol settling
characteristics by the exponential decay of a monodisperse aero-
sol with 1/e decay time, te, measured directly from the concen-
tration data. The mass fraction removed, f(t), in any standard
time interval (e.g., 5 minutes, as in Tables 1 and 2) is readily
determined from the relation,
f(t) = l-e~t/te
(1)
More si'gnificantly, the particle diameter of an "aerodynamically
equivalent monodisperse aerosol" with the same settling charac-
teristics, djf, can be derived from the decay profiles. We have,
for a stirred settling monidisperse aerosol of diameter, djf, in
a chamber of height, h,
Pd#2gt
c(t)/co '
where g = gravitation acceleration
n = gas viscosity
p = particle density
X = aerodynamic shape factor
Equating the bracketed quantity in Equation 2 with t/te gives
d* as,
-------
TABLE 2 - SUMMARY OP UNDISTURBED SETTLING AND ACOUSTIC AGGLOMER-
ATION TESTS IN 0.65 M3 VESSEL
Initial
Aerosol
Mass
Cone Tn
(gm/m3)
Mass Mono-
Fraction disperse Agglomer-
Time for Removed Equivalent ation
C/CQ=e-l in Size,d* Index
(min. ) 5 min. (ym) IA
Power
Density
watts/m3
Undisturbed Settling Tests
21.
16.
8.
6.
5
5
2
8
13
11
20
23
0
0
0
0
• 32
.37
.22
.20
Acoustic
14.
10.
9.
7.
1».
2
5
0
6
2
3.3
1.9
5.3
7.0
28.5
0
0
0
0
0
.78
.6/1
.61
.51
.16
7
8
6
5
.6
.3
.1
.7
3
4
2
2
.6
.0
.9
.7
0
0
0
0
Agglomeration Tests
15
12
11
10
5
.1
.if
.9
.4
.1
7
5
5
5
2
.2
.9
.7
.0
.4
9.1
9.1
9.1
0.46
0.091
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48
Values for d^ computed for each test are given in Tables 1 and 2.
A useful parameter for characterizing both undisturbed and
enhanced agglomeration tests is the "agglomeration index", IA,
defined here as the ratio of dx to the initial aerosol MMD.
Eight-stage Andersen impactor data indicate that within the
first 20 minutes after initiation of a sodium pool fire, the MMD
of the aerosol Is about 2.1 ym. Using this reference MMD to
form the agglomeration index, we have plotted IA versus initial
mass concentration for undisturbed settling tests and for turbu-
lent and acoustic agglomeration tests as shown in Figures 6 and
7. No distinction is made in these figures between tests at
different power densities. Because of the limited range of our
power densities, strong statements about their relation to IA
are not justified.
Figures 6 and 7 also present trend lines fitted to the data.
In each case, the trend line is constrained to pass through IA=
1.0 at zero mass concentration, a constraint which has an obvi-
ous and strong physical interpretation. As expected from basic
coagulation, theory [5] each of the four trend lines shows increas-
ing agglomeration indices with increasing mass concentration.
Enhanced agglomeration rate leads naturally to an increased
average particle size due to agglomeration in a given time peri-
od. By adding turbulent or acoustic energy to the aerosol,
relative motion between particles is enhanced and agglomeration
speeded up. For the range of power densities used, turbulence
yields a higher agglomeration index than acoustic for the same
initial mass concentration.
For the turbulence experiments and the acoustic tests, the
following results of multiple regressions of agglomeration index
with respect to initial mass concentration and power denisty
were obtained. The point IA=! at C0=0 was included in each
case:
IA = 0.302 + 4.85 C0 + 0.093 P turbulent agglomeration
IA = 1-05 + 0.441 Co + 0.029 P acoustic agglomeration
where Co = initial mass concentration in gm/m3
P = power density in watts/m3
Significance tests of the regression equation coefficients showed
that both concentration coefficients were significant (P<0.02)
and both power density coefficients were not significant. It is
apparent that turbulent agglomeration is more effective than
acoustic agglomeration for a given initial mass concentration
and that turbulent agglomeration is enhanced to a greater extent
-------
'FIGURE 6
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50
FIGURE 7
AGGLOMERATION INDEX AS A FUNCTION OF MASS CONCNETRATION FOR ACOUSTIC
AGGLOMERATION AND UNDISTURBED SETTLING TESTS IN 0.65 M3 VESSEL
X
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51
for a fixed fractional increase in initial mass concentration
than is acoustic agglomeration.
DISCUSSION AND CONCLUSION
The range of agglomeration indices found in our acoustic
experiments is consistent with analogous measurements made by
other researchers. Although no previous work compares directly
with our research because sound sources were usually employed
only for brief periods (seconds) on flowing rather than fixed
volume systems, Scott [4] and'jVolk and Moroz [5], obtained ag-
glomeration indices from 3 to 9 for sound pressure levels from
120 dB to 165 dB. Their mass concentrations were in the range
0.5 to 2.5 gm/m3 and sound was continued from 2.5 to 40 sec-
onds .
By using the concept of "scale of turbulence" noted by
Fuchs [6], it is possible to gain some insight into why turbu-
lent agglomeration does not show significant increases in effec
tiveness for significant increases in power density. The in-
ternal scale of turbulence, X0, used by Levich and referred to
by Puchs, is defined for a turbulent medium by,
where v is the kinematic viscosity of the medium and e is the
power density per unit mass of the medium. For the range of
power densities employed here the scale of turbulence, Xo, ranges
from 90 ym to 120 urn; a range unlikely to be associated with
large changes in agglomeration index.
Turbulent motion and high intensity reverberant acoustic
fields induce rapid agglomeration of sodium aerosols. The _ high-
er the initial aerosol mass concentration, the more effective
are these agglomeration mechanisms which produce enhanced sedi-
mentation. For undisturbed settling, sedimentation is similarly
enhanced by increasing initial mass concentration. For the same
energy expenditure turbulence appears to be a more effective ag-
glomerating mechanism than acoustic energy at a fixed mass con-
centration. Increasing the applied power density of turbulence
or sound produces smaller increases in agglomeration index than
does a similar fractional increase in initial mass concentration.
Both mechanisms show promise for direct application air clean-
ing systems. Because of the relative ease of generating turbu-
lence versus generating a large-scale high intensity sound field,
turbulent agglomeration may be preferred.
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52
ACKNOWLEDGEMENT
Support of this research was provided by the U.S. Energy
Research and Development Administration under Contract No. E(ll-
D-2801.
REFERENCES
[1] Tillery, M. I., "A Concentric Aerosol Spectrometer", Am.
Ind. Hyg. Assn. J. , 34:62, (197*1).
[2] Hinds, W., Mallove, E., and First, M. W., "Density and Shape
Factor of Sodium Aerosol", Final Report, COO-2803-5, Febru-
ary 1977, ERDA Contract No. E(ll-l)-2803.
[3] Mallove, E., Hinds, W., and First, M. W., "Direct In-Vessel
Application Experiments at Harvard Air Cleaning Laboratory",
Annual Report, February 1977, ERDA Contract No. E(ll-l)-
2801.
[4] Scott, D. S., "A New Approach to the Acoustic Conditioning
of Industrial Aerosol Emissions", Journal of Sound and Vi-
bration, Vol 43, No. 4, 1975, 607-6T9~!~
[5] Volk, Michael, Jr., and Moroz, William J., "Sonic Agglomera-
tion of Aerosol Particles", Water. Air, and Soil Pollution,
Vol. 5, 1976, 319-334. —
[6] Fuchs. N. A., The Mechanics of Aerosols. New York, Pergamon
Press, 1964. ~
WRITTEN DISCUSSION
J.A. Gieseke, L.D. Reed, and K.W. Lee
Battelle, 505 King Avenue
Columbus Laboratories
Columbus, Ohio 43201
Aerosol behavior within Liquid Metal Fast Breeder Reactor (LMFBR) con-
tainments is of critical importance since most of the radioactive species are
expected to be associated with particulate forms, and the mass of radiologically
significant material leaked to the ambient atmosphere is directly related to
the aerosol concentration airborne within the containment. Mathematical models
describing the behavior of aerosols in closed environments, besides providing
a direct means of assessing the importance of specific assumptions regarding
accident sequences, also serve as the basic tool with which to predict the con-
sequences of various postulated accident stiuations. Consequently, considerable
efforts have been recently directed toward the development of accurate and
physically realistic theoretical aerosol behavior models.
These models have accounted for various mechanisms affecting agglomeration
rates of airborne particles matter as well as particle removal rates from closed
systems. In all cases, spatial variations within containments have been neglected
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53
and a well-mixed control volume has been assumed.
The most.recent of these, the HAARM-2 code developed at Battelle's Columbus
Laboratories ?, is an extension of the HAA-3B code developed at Atomics
International . The HAARM-2 model differs from previous models in that it
allows temporal variation of containment gas temperature, pressure, and tem-
perature gradient normal to the containment walls. Also, settling velocities
which are dependent upon the morphological properties of.individual agglomerates
are corrected using an experimental dynamic shape factor . In addition, wall
deposition by thermophoresis is included as an aerosol deposition mechanism
and a calculated particle-particle collision efficiency is employed. Though
not included previously in any model, terms describing agglomeration as caused
by gas turbulence have been added to the HAARM-2 code so that calculations in
support of this discussion could be made.
GENERAL THEORY
The theory used to develop the HAARM-2 code is based on solution of the
governing intftgro-differential equation which describes the rate of change of
particle concentration due to various agglomeration and removal mechanisms.
This equation may be written in the following form:
- n(x,t) = K [1/2
O
-n(x,t) /
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54
4 3
rirr = volume of particle with radius r
4 3
' = volume of particle with radius r1
n(x,t) = the size distribution function
R(x) = the removal rate of particles produced by gravitational
settling to the floor, diffusion to the walls
(wall plating) , and leakage
S(x,t) = represents the source rate of particles input to the vessel.
The first integral in Equation (1) represents the formation rate of
particles between the sizes x and x+dx as a result of collisions between
particles of volumes £ and x-£. Similarly, the second integral represents
the disappearance rate of particles in the size range between x and x+dx
due to collisions with all other particles.
The functional form of the collision kernel (x>£) depends upon the
coagulation mechanisms present in a given system. In an enclosed containment
vessel, possible mechanisms causing relative motion between particles, and
thus coagulation, include Brownian motion of the particles, gravitational
settling, and turbulent gas motion. In most analyses where more than one of
these mechanisms is present, they are assumed to be separable and additive
such that
•Kx,£) = B(x,O + G(x,O + T(x,£) (2)
the agglomeration and removal terms will be described in more detail.
AGGLOMERATION TERMS
Brownian Agglomeration
Current aerosol models use the Brownian collision parameter B in a form
which can be written as
K B(x,C) = (r
0
r+a+be .r'+a+be
r
(3)
where a, b, and X are constants defining the Cunningham correction factor
which accounts for the low Knudsen number effects present for small particles.
Gravitational Agglomeration
The collision parameter for gravitational agglomeration is based on
consideration of the relative sedimentation rates for different sized particles
and is dependent on the collection of small particles from the volume swept
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55
out by a large, rapidly settling particle.
parameter is given by
The gravitational agglomeration
,5) -{| ro(r) - r'a(r')| • (r + r')3
(r + r1)2] + b(r + r1)2 -| re"Xra(r)-r' e~XrIa(r')|} (4)
where
e(x,£) - the particle-particle collision efficiency
a(r) = the particle shape factor
p = the density of the particle material.
Turbulent Agglomeration
Particles are caused to collide in a turbulent gas by being suspended in ,
a shear flow and by inertial effects cuasing relative motion between .particles .
The shear and inertial components of the turbulent collision parameter are
termed T.. and T?, respectively. These should appear as T = (T. + T» ) , but
have been assumed additive as T - T- + T_ to permit analytical integration.
The expression for the shear induced component of the turbulent collision
parameter is
1/2 ,
(r+r')J, (5)
and the inertial component of the turbulent collision parameter is
1/4
ATT 1/2T
P
9n
15n
|(r) (r2 + ar + bre"Xr) -a (r1)
) |
(6)
where e is the rate of turbulent energy dissipation per unit mass
(cm2/sec3), and p is the gas density.
Removal Terms
In Equation (1) , R(x) represents the removal terms. HAAM-2 accounts for
any of the four possible mechanisms : (1) removal at the vessel floor due to
gravitational settling, (2)Brownian diffusion to the walls (wall plating),
(3) thermophoretic deposition on the walls, (4) leakage out of the vessel.
Mathematically formulated
r(x) = G0 r(r + a + be Xr) + PD ~4 (r + a + be Xr) +
K R t.
+ T.
R
(7)
-------
56
where
2ga(r)p
G = —jjr - , the gravitational settling constant
K
, the plating coefficient
R^ = leak rate from the vessel
A
TR = ~-^U , thermophoretic deposition coefficient.
further, h is the vessel height, A the wall area, A the distance from the wall
over which a particle concentration gradient exists, U the thermophoretic
velocity, and V the vessel volume. In the HAARM-2 code the assumption has
been made that the final term in the slip correction factor is negligible and
hence for both gravitational settling and agglomeration the constant is taken
as zero.
The method of moments was used to solve the governing integro-differential
equation, Equation (1), and the solution technique has been described
previously ' ' . Basic to this method is the assumption that the particle
size distribution may be represented by a log-normal distribution at all times
and the parameters describing the particle population (number concentration,
mean size, and geometric standard deviation) are derived at the end of
selected time intervals in terms of the moments.
RESULTS
A series of calculations were performed to compare the HAARM-2 aerosol
behavior model with the experimental measurements of Hinds, Mailove, and First.
Of particular interest was the comparison of the predictions and experimental
results for the case of turbulent induced augmentation of the agglomeration rate
since this is the first such comparison ever possible. Of course, the base
cases with no added turbulence are of importance in establishing a reference
point for comparison with the turbulent cases.
The base cases with no added turbulence were calculated in three ways.
First, the instantaneous release of sodium oxide aerosol was assumed to occur
at the end of the sodium burning period of 20 minutes. The characteristics
of the aerosol at this time were taken as aerodynamic mean diameter = 2.1pm,
effective3density = 0.78 g/cm , shape factor = 1.37 and mass concentration
=2.0 g/m . The HAARM-2 code employs mass equivalent particle diameters and
makes its own corrections to particle mobility accounting for effective
density and shape factor combined. Therefore, an initial mass equivalent
radius, r, of 1.4 pm was used (calculated by correcting the impactor determined
aerodynamic diameter for density and shape factor), a geometric standard
deviation, a , of 1.8 was assumed for the distribution, the material density,
p , was taken" as that for hydrated Ha2CO_ or 1.5 g/cm , and a constant correct-
iBn for density, a, of 0.33 was assumed. These values are consistent with
those reported by Hinds, Mallove and First for the particles at the end of the
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57
burning period.
A second similar case also assumed an instantaneous source and identical
conditions except that the value used for the aerodynamic mean radius was 1.5
ym as reported by Hinds, Mallove and First for times after the end of the fire
which corrected for effective density and shape gives the value of r =2.1.
A third case followed the procedure usually used to predict aerosol
behavior following a sodium fire. A constant particle source rate was
assumed over the 20-minute burning period. In this case the source particles
were-assumed to have a mass mean radius of 0.5 urn, a material density of 2.27
g/cm , and a geometric standard deviation of 2. The effective density for
the particles was calculated within the program as a function of the number of
particles forming an agglomerate.
The results of HAARM-2 code calculations for these three cases are shown
in Figure 1. It is seen that the predicted airborne mass concentrations are
consistently higher than the experimental values. This is consistent with the
nature of the HAARM-2 code which was intended to provide conservative
estimates for nuclear reactor safety analyses and, therefore, employs procedures
which always lead to slight overestimates of airborne concentration. Neverthe-
less, the aerosol behavior model shows reasonable agreement with these most
useful data.
To compare the HAARM-2 code calculations with the experimental results
when the blower was operated, the terms for turbulent agglomeration were added
to the HAARM-2 code. To simplify the integration for this comparison it was
assumed that the particle-particle collision efficiency, e, which appears in
Equations (5) and (6) was a constant rather than a function of particle size.
This constant is taken as unity in the turbulent agglomeration terms but is
included as a function of particle size in the gravitational agglomeration
term. The calculations to show turbulent agglomeration effects were based on
a value-for e , the turbulent energy dissipation rate, equivalent to 45
watts/m whicS is the mid-point in the range of 20-70 watts/m reported by Hinds,
Mallove, and First for their experiments._ Other assumptions were the same as
for the baseline case calculations using r = 2.1 ym.
The results of the calculation for turbulent agglomeration are shown in
Figure 2. It is seen that the prediction is on the low wide but adjacent
to the range of experimental results. This is reasonable agreement but will
probably be improved when the HAARM-2 code is modified to include a size
dependent collision efficiency, e, in the turbulent agglomeration terms
and some accounting is made for overestimates resulting from the additive
procedure which was used for agglomeration rates in the form T=Tj+T2. The
agreement between theory and experiment is very encouraging and snows promise
of future improvement.
CONCLUSIONS
The data reported by Hinds, Mallove, and First are very useful as
additional confirmation of sodium aerosol behavior in enclosed vessels and in
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58
PREDICTED AND MEASURED AEROSOL BEHAVIOR
0.01
Time dependent calculation within HAARM-2_
code, value ranged from 1.0 to 0.47
50 100 150 200 250 300 350
Time After Initiation of Fire, minutes
FIGURE 1. PREDICTED AND MEASURED AEROSOL BEHAVIOR
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59
Predicted, no turbulence
Baseline data
Data with turbulence added
from Hinds, Mallove and
First
Predicted with
turbulence
0.01
0 50 100 150 200 250 300
Time After Blower Start, minutes
FIGURE 2. COMPARISON OF PREDICTED AND MEASURED
TURBULENT AGGLOMERATION EFFECTS
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60
particular provide a most useful basis for evaluating the effects of turbu-
lence on agglomeration rates. The agreement between the HAARM-2 code
calculations and experimental results for the baseline conditions without
significant turbulence is very good with the HAARM-2 code, as expected,
giving slight overestimates of airborne concentration. The agreement
between HAARM-2 code predictions and experimental results for cases with
significant turbulent agglomeration is quite good but some improvement can
be expected from scheduled code modifications.
ACKNOWLEDGEMENT
The research on which this discussion is based was performed for the U.S.
Nuclear Regulatory Commission under Contract No. AT(49-24)-0293, Task 7.
REFERENCES
[1] Reed, L.D., and Gieseke, J.A., "HAARM-2 Users Manual", BMI-X-665
(October 31. 1975).
[2] Hubner, R.S., Vauehan, E.U., and Baurmash, L.. HAA-3 User Report. AI-
AEC-13038 (1973).
[3] Kops, J., Dibbets, G., and van de Vate, J.F., J. Aerosol Sci., £, 329 (1975).
[4] Saffman, P.G., and Turner, J.S., J. Fluid Mechanics, 1, 16 (1956).
[5] Cohen, R.E., and Vaughan, E.U., J. Colloid Interf, Sci., 35, 612 (1971).
OPEN DISCUSSION
Shaw: I was wondering, Jim and Bill, how the turbulence - both with acoustics
and without acoustics - affect your cutting into the wall.
Hinds: We saw an effect on the deposition on the walls from turbulence,
however, the predominant deposition was still on the floor. About 65% of the
material ended up on the floor. Half of the remaining 35% was on the walls,
and the rest was trapped directed on the blades of the device.
Shaw: Do you see any significant increase because of the turbulence?
Hinds: We had between 1% and 10% without turbulence on the walls, and then
went up several fold because particles large enough to get in are driven onto
the wall by turbulence on the chamber.
Cooper: The different coefficients for the concentration for the two different
cases might suggest that there were somewhat different particle size
distributions. So that, although the work shows the turbulent agglomeration
can be significant, perhaps you have to be careful in making a comparison between
the turbulent agglomeration in the big room and the acoustic agglomeration of
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61
the smaller chamber. Especially when these concentration coefficients are so
different.
Hinds: We measured the size distribution in both cases. In the case of the
small chamber, the burn took place very rapidly, about 2 minutes versus 15
minutes in the other one.
We waited until the fire ended before we made any measurements or
treatment. At that time we had roughly the same size distribution. But it is
a younger aerosol; the aerodynamic sizes were roughly the same.
Benarie: How did you monitor or control humidity in the chamber? Because you
are working with a very humidity - sensitive aerosol, the conditions of a few
percent change in humidity can cause a sizeable change in turbulence. Your
experiments end up to what I am going to say on Friday morning.
In fiber filtration, we generally do not consider that air in the
gates entering the filter develops a turbulence or anything like a turbulence.
I will show you how this occurs. The turbulence contributes a large measure
of agglomeration and leads to a better performance of filtration because parts
of the filter are already agglomerated. So in developing filtration mechanisms
which are most formed in uniform and laminar flow, a turbulent agglomeration
should also be considered.
Hinds: The effect of humidity is important to a hygroscopic aerosol. In the
runs that I described earlier, the humidity ranged from 16% to 24%. We found
that in that range, based on our baseline runs, we got very reproducible
results. When we went to 35% to 50% humidity, these results were quite
different; by a factor of two in mass concentration.
Scott: I am very interested in your results. I noted that the highest sound
pressure levels were 145 dB and I am intrigued that you caught anything there.
You obviously caught something, but you will find that there is a marked increase
in agglomeration ratio up towards 155 dB.
I think it would be interesting if you would introduce some other runs.
It would be interesting if they were done at the same time and in the same
chamber. It would also be interesting if you had three conditions. 1) Pure
turbulence 2) Pure acoustic 3) A combination of turbulent and acoustic. Our
preliminary results show that the third condition is where things really start
to happen.
My last comment refers to power. I think you said that "turbulent
agglomeration, for the power requirements, would be satisfactory without the
acoustics." But we hope that this would only be used in the event of a
catastrophe. At that point power would not be a criteria at all and should
have no relevance. Would you please comment on this?
Hinds: On power requirements, your statement is true. However, in the case
of an emergency, power may not be available. But slight changes in efficiency
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62
are really irrelevant in this case.
On sound pressure levels, we were aware that these were on the low end
of the scale and we weren't able to reach the higher pressures. But we felt
that if we could get above 140 dB we would have something that was worthwhile.
We did reduce it down to 125 and didn't observe a great deal of difference.
The combination of systems is worth note and we are looking into it.
Lamb: What is the feasibility of artifically increasing the aerosol
concentration? I'm thinking of a case in an emergency where you would blow a
harmless aerosol which would scrub.
Hinds: If we could artifically increase the concentration we would be much
better off. This is worth considering given the shape of those curves. In
terms of the particular situation with the reactor, we have many thousands of
pounds of non-radioactive sodium in the thing. All you have to do is open a
valve, let sodium burn and you have got a lot higher concentration. But if
you have an accident with a lot of sodium around, you just don't want to burn
more. So this may not be very realistic.
Baril: Could you tell us a little bit about your sampling techniques. Where
in the chamber did you take your samples?
Hinds: We sampled about mid-height in the chamber through a 1 1/2" diameter
opening at about 20 liters/min. The sampling tube is 4 inches long and has a
filter. We have a valve that we could shut off to isolate the chamber. We
sampled directly under the filter. The samples were then weighed within a
matter of seconds after removal to prevent any inclusion of moisture. The
samplings were brief, lasting between one and three minutes, and spaced about
10 to 15 minutes apart. All were done at one height in the chamber.
Initially, when we burn the sodium, we see stratification. The hot
sodium rises to the top, but there is enough convective energy from the burning
sodium to mix it well. When the tests were taken, there'was no stratification.
Oder: Will increased humidity enhance sedimentation, in which case, it is
very easy to boil water without fear of loss of sodium?
Hinds: Yes. You could inject steam. However, if you have metallic sodium
present this would be dangerous because of the evolution of hydrogen gas. So
you have a tradeoff of blowing the thing up and getting the air back out.
Loftier: We sometimes use acoustics in liquids for agglomeration. I would like
to know if there is any maximum or optimum power input or perhaps a final size
of agglomerate which you can obtain if you put more energy into the system?
Hinds: The mechanisms involved in the scale of turbulence is such that it
would tend to scale surfaces on the order of about 100 microns. It would also
tend to break up particles that were long on that order of dimension and you
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63
would end up with a kind of spherical shape.
As for turbulence in the break-up of agglomerates in acoustics, I
think David Scott would be more qualified to respond.
Scott: If you do break up particles in an aerosol, it is clear that it is the
big ones that you are breaking up. It also makes sense, almost intuitively,
that if you have got some long stringy thing that you are going to break up,
it would break approximately in the middle, rather than chip little bits off
the end. So it seems that you could bring the agglomerate to too large a
size and break it into approximately equal size particles and you would kick
up the number density which is desirable.
If you are talking about a solid particle in a liquid, that means you
would be propagating through a nearly incompressible fluid. For a given
acoustic field and intensity, the wavelengths are totally different. The
amplitude, which is the critical thing, would be much smaller than in a gas.
These things occur because you do have a dynamic motion. In a liquid, the
dynamic motion would be much less than in a gas unless it was a bubbly liquid
where you have sort of an analog thing too.
Loffler: A most important thing would be the stress which acts on the agglomerate
and the forces, for example, adhesion forces. In the case of turbulent flow
you will not get shear stress for particles in the range of one to five
microns which will break the agglomerates. However, in acoustics it may be
possible.
Hinds: I would tend to agree. The situation of mechanically generating
turbulence gives a more intense turbulence than you would find in a normal
turbulent flow.
Yang: Another note on the relation between the frequency of excitation and
the size that you have. In turbulence in the case of a liquid in a tunnel
you have an additional mechanism, a Reynolds stability. You can stretch the
filaments and they will break up. It depends on the size and adhesion force.
-------
NEW APPLICATIONS OF ACOUSTIC AGGLOMERATORS IN PARTICIPATE
EMISSION CONTROL
D. T. Shaw and James Wegrzyn
Laboratory for Power and Environmental Studies, State
University of New York at Buffalo
INTRODUCTION
In order to meet the national goal of energy self-independence,
expanded research and development on coal-fired and nuclear power
generating plants have become a national priority. Numerous new
programs have been established by various Federal agencies to seek
answers to problems associated with how to generate electric power
with low-grade (high sulfur) coal, to improve power plant efficiency,
to increase our capacity in nuclear power generation, and above all
to accomplish all these with minimum environmental degradation.
These new programs lead to the new engineering problems such as the
control or suppression of airborne dust particles in very unconven-
tional situations - the combination of high temperature, pressure,
or corrosive environment - in which the conventional air-pollution
abatement equipment cannot function effectively. In many of these
situations it appears that acoustic agglomerators possess certain
characteristics which seem very attractive for dust-particle control
in several of these new programs. These will be discussed first in
the following sections, followed by the descriptions of some large
sonic generators which may be suitable for these new applications.
Acoustic agglomeration is not a new idea. As early as 1931,
Patterson and Cadwood [1] studied the effect of sound on aerosol
dispersion. A great many papers appeared in the literature of the
1950's and these papers were summarized in the well-known book on
sonic agglomeration by Mednikov, published in 1963 [2]. Since then
however, there has been an almost complete vacuum in which little
interest has been shown on the sonic agglomeration of aerosol par-
ticles. Shirokova reviewed acoustic agglomeration in 1973 with
virtually no new experimental data. A series of papers were pub-
lished in the USSR concerning the theoretical treatment of aerosol
behavior in an acoustic field [3-10]. A group at the University of
Ontario proposed the idea of using an acoustic field to enhance
dust collection by filters [11]. At about the same time Scott and
his associates worked on the idea of using a progressive wave for
industrial aerosol emission control [12-15]. Their work shows that
when properly designed, acoustic agglomerators can be economically
competitive with more conventional devices for special appications
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65
in which conventional devices are not effective.
It is important to emphasize here that sonic agglomerators
cannot compete in applications where the conventional devices
already have been successfully used. Thus, in a recent report
by the Midwest Research Institute [16], it is found that sonic
ajglomerators are too expensive for conventional pollution control
because of the high power consumption in the generation of strong
acoustic fields.
However, the sonic agglomerators have some characteristics-
which make them uniquely suitable for certain special applications.
These include the acoustic conditioning of fine particulate emis-
sions (particle diameter between 0.2-2ym) the suppression of radio-
active sodium-fire aerosol in hypothetical Liquid Metal Fast Breeder
Reactor (LMFBR) accidents, and the control of particulate emission
under special conditions where conventional abatement techniques
are not applicable. The objectives of this paper are to describe
these new acoustic applications and to identify the predominant
sonic agglomeration mechanisms for each application,
LMFBR SODIUM-FIRE AEROSOLS
In the development of the Liquid Metal Fast Breeder Reactor
(LMFBR), a major safety problem concerns the suppression of the
radioactive sodium-oxide aerosol particulates produced in a hypo-
thetical core-disruptive accident. Assuming that a small fraction
of the airborne sodium aerosol leaks into the environment, radio-
logical analysis of the site boundary does show that inhalation
of aerosol particles containing plutonium oxides may be a major
concern. This is different from the problem associated with the
Light Water Reactors (WR) in which the airborne thyroid doses
from radioiodine is of principle concern. To minimize the leaking
of these radioactive aerosols into the environment through small
cracks, pipe fittings and so on, the effective suppression of
these aerosol particles inside the containment building in case
of such an accident is probably the highest priority in the devel-
opment of LMFBR emergency air cleaning systems.
This so-called sodium-fire aerosol is produced from the combus-
tion of liquid-metal sodium coolant in air. Sodium combustion can
take place either rapdily through, for example, a ruptured pressur-
ized pipe; or from a slow spill of sodium on to the reactor floor.
In the former case it is called a spray fire, and in the later case
a pool fire. A review covering both types of these fires can be
found in the literature [17].
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66
The sodium fire aerosol which consists primarily of sodium
peroxides, have diameters predominantly in the range of 2-20um.
Owing to the constant thermal convective stirring, these relatively
large particles stay airborne for a long period of time, say two
or three davs, with a very high total mass-loading (in the range
of 1-50 g/m4). In the recent report entitled, "Evaluation of Air
Cleaning Concepts for Emergency Use in the LMFBR Plants", twenty-
four different air cleaning systems were investigated and these
are listed in Table 1. Among these, direct in-containment acoustic
agglomeration is rated to be number one in terms of effectiveness,
and number four in terms of fabrication, as shown in Table 2.
TABLE 1
System
Number
SR-1
SR-2
SR-3
SR-4
SR-5
SR-6
SR-7
SR-8
SR-9
SR-10
SR-11
SR-12
SR-1 3
Description
Prefilter-HEPA
Deep Bed Graded Media
Sand Bed
Bag Filter
Cyclone
Cyclone plus HEPA
Wet Scrubber
Fluidized Bed
Acoustic plus Cyclone
Mechanical Separator
Settling Chamber
Electrostatic Precipator
Steam Conditioning
System
Number
SD-14
SD-15
SD-16
SD-17
SD-18
SC-19
SC-20
CF-21A
CF-21B
CF-22
CF-23
CF-24
Description
Liquid Sprays
Powder Discharge
Foam Encapsulation
Acoustic Agglomerator
Direct Electro. Precip.
Comb. Acoustic plus Cyclone
Comb. Powder plus Filter
Large HEPA, Charcoal HEPA
Small HEPA, Charcoal, HEPA
Wet Scrubber, HEPA, Char.,HEPA
Elec.Precip.,HEPA, Char.,HEPA
Sand Bed, HEPA, Char., HEPA
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67
TABLE 2
RANKING OF TWENTY EACS CANDIDATES BY TOTAL SCORE AND BY
CRITERIA GROUPS - SINGLE CONTAINMENT PLANT
Overall
Rank
9
14
11
SD-17
SD-15
Sc-19
Total Ranking by Criteria
System
In-con tainment
Acoustic
In-Containment
Powder
In-Cont. Acoustic
Score A B
309.5 1 7
296.4 2 16
304.6 3 8
C
15
11
16
D
16
18
rs
Group
E
16
14
11
F
4
6
9
+Recirc.Cyc
13
16
SD-14 Liquid Spray
SC-20 Powder + Reef re.
HEPA
296.8
288.7
4
5
10
18
19
14
11
17
13
3
1
14
* A- Effectiveness, B- Reliability, C- Compatibility, D- Credibility,
E- Flexibility, F- Fabrication
This suggests that if R § D effort can improve the credibility,
reliability and compatibility of this concept,.acoustic agglom-
eration may be one of the top contenders for such emergency
application.
CONTROL OF FINE-PARTICULATE EMISSIONS FOR STATIONARY SOURCES
The use of mass weight is an acceptable practice in particu-
late emission control since the current regulations are completely
based on emission mass. Missing from the current air quality
criteria is the particles sizes, which is probably the most impor-
tant parameter in the discussion of environmental effects of
particulate emissions. Large particles (diameter larger than 2pm)
constituted the major mass fraction of most emissions sources.
Although they represent a significant public welfare problem by
contributing to the soiling and unsightly smoke plumes, these large
particles are not a major health problem since inherently they tend
to deposit in the upper airways of the human respiratory system.
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68
It is the fine particles (diameter less than 2ym) that usually
penetrate very deeply into the pulmonary airways and cause perman-
ent health effects. Furthermore, these fine particles, when
released into the atmosphere, stay airborne for an extended period;
in contrast to the large particles, most of which are quickly
brought back to the earth's surface by either direct fallout or
by precipitation scavaging.
Large particles can be effectively controlled by conventional
equipment such as cyclones, filters, scrubbers, electrostatic pre-
cipitators, etc. However, these devices have relatively poor effi-
ciencies for the control of fine particles with diameters ranging
from 0.2-2um.
Since acoustic agglomeration has as its principle advantage the
ability to control particles of any size without a minimum in the
collection efficiency, it is potentially attractive for use as a
preconditioner of the conventional emission control devices for the
effective removal of fine particles. In other words, acoustic
agglomerators offer distinct advantages in hybrid-unit applications
in which the acoustic agglomerator is used upstream from the con-
ventional devices. In this way acoustic agglomerators can be
used to precondition the size distribution of the particles which
permits a more effective emission control of fine particles. The
purpose of the acoustic preconditioner is to shift the maximum of
the dust particle size distribution to a larger particle size,
leading to a more effective removal of the submicron particles by
electrostatic precipitation.
«o«ti ° """"."""P""" for the production of the
"- ^ -»•< «.„,».
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69
EMISSION CONTROL OF A FLUIDIZED
STEAM TURBINE COMBINED CYCLE
BED BOILER WITH A GAS TURBINE-
To meet the requirement of producing adequate amounts of elec-
tric power with a minimum degradation of the environment, it has
been increasingly important to find a way of burning the high-sulfur
coal and at the same time, to remove SO- from the flue gas before
release into the atmosphere. Among the concepts being developed,
the idea of fluidized-bed combustion is rapdily gaining favor. In
this case the high-sulfur coal is injected and burns in the bed
at a temperature of 1450-1750°F. The S02 released during the burn-
ing of the coal is reacted with lime and in the presence of excess
oxygen in the bed forms calcium sulphate. Fresh, crushed high-
calcium limestone (dolomite limestone) is injected continuously
into the bed and the sulphate lime is removed continuously.
Several concepts of the fluidized in-bed combustors have been
considered for power plant applications [19-22]. Among these the
ideas of using a pressurized, fluidized bed combustor in a combined
gas-turbine and steam-turbine cycle is particularly attractive, as
demonstrated in Fig. 1. Steam for operating a turbine is raised by
Slack
\ Disposal/Utilization
or Regeneration
(from each Lied)
'•e.g. spent tfolomi'.e (CaSO.j-M5G.
CaC03- MgO)
H33t Recovery Unit
(Flue Gas)
Boiler Fe&l-
W3ler Pump
Figure 1 -Pressurized fluuiizr] lied toiler - power pknt sclitwlSc
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70
both the steam tubes immersed in the fluidized bed and by the heat
converter boiler of the gas turbine. The heat of combustion is
removed from the bed to run directly to the gas-turbine. Typically
such a fluidized bed uses crushed coal of about 1/4" diameter. Most
of the fly-a-sh particulate particles are rather large, say lOOym;
however, some smaller particles are also produced which must be
removed from the flue gas to avoid the attack of gas-turbine blades
by such corrosive particulates as sodium salts.
Cyclones are used to effectively remove large particles. For
particles in the range of 2-20um diameter non-conventional devices
are suitable for this application. Filters are difficult to use
because of the high temperature, corrosive environment and because
of the inherent high-pressure drop across the filters. The electro-
static precipitator is not applicable because under the high pressure
(10-20 Atm in the fluidized-bed boiler) it is difficult to produce
the corona discharge without generating excessive electric arcs.
Again, the use of an acoustic agglomerator, operating either in
standing-wave or traveling-wave mode, appears very attractive.
SOLID-SEED EXTRACTIONS FROM MHD GENERATORS
Because of the increased cost and reduced availability of
fossil fuels, a strong incentive now exists to substantially in-
crease the electric power plant efficiency which is currently run-
ning between 33-40% depending on whether it is a nuclear or conven-
tional fossil-fuel plant. Since 1960 new plants have not done
appreciably better in terms of the power plant efficiency. This
comes primarily from inherent upper temperature limits of about
1050°F imposed on the steam-turbine cycle because of some inherent
thermal and material problems. Alloy steels lose strength rapidly
above this temperature. Furthermore, water begins to decompose
at this temperature and it attacks and weakens the boiler tube
walls. Consequently unless a new thermodynamic unit is developed
we have reached a point of diminishing returns in our attempt to
further increase the thermal efficiency of central power plants.
Magnetohydrodynamic (MHD) topping units have been developed
as a way to increase the peak temperature and the efficiency of
electric power stations [23-25]. A schematic diagram is shown in
Fig. 2 to illustrate the important components that have to be
developed for a commercial unit of this type [23]. One of the key
technical problems in the development of such a coal-fired MHD
powered generator involved the separation and the efficient removal
of the seed material, which is usually either sodium or potassium,
before releasing the flue gas into the atmosphere. Again, because
of the high temperature and the excessively corrosive environment,
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71
T I I
it
Fig.2 Schematic diagram of important MHD generator components
(Taken from ref. [23])
conventional devices are difficult to use in this situation. A
recent study shows that cyclones can be used to remove most of the
large particles (diameter larger than lOym). New techniques must
be found to collect particles in smaller sizes. Particles are
mostly spherical because of the very high operating temperature at
which MHD generators perform, typically between 2000-2500°C. Figure
3 shows typical agglomerates collected from cyclones from a MHD
generator [23] .
Fig. 3 Typical agglomerates collected from cyclones from a MHD generator
(Reference [23])
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72
THEORETICAL ASPECTS
As discussed in the preceding section, the interest in
various applications of acoustic agglomerators concerns partic'les
in the range from 0.2- to about 20pm. Particles smaller than 0.2ym
usually coagulate quickly into a larger size; while particles larger
than 20um can be effectively removed by mechanical devices such as
cyclones, scrubbers, etc. For particles of this size region acoustic
agglomeration offers some unique characteristics as will be explained
later. Figure 4 shows the variations of the relative amplitudes of
the oscillating particles to gas with respect to particle radius for
various acoustic frequencies. On the top of the figure the oscilla-
tion of the particles is also plotted in terms of the parameter T/T.
2 3 4 6 8 i Z 3
Particle radius
6 e 10 20
Fig. 4 Variation of relative particle amplitudes vs. particle radius
for various particle frequencies
It is seen that the particles vibrate virtually with the same
amplitude as the carrier gas at low frequencies. As the sonic
frequencies increase the inertia of the particle becomes increas-
ingly evident until eventually a frequency is reached in which the
particles remain virtually stationary despite the intense vibration
of the carrier gas. In the intermediate frequencies the particles
precipitate in the vibrations to varying degrees, depending upon the
sizes. For example, if the frequency of the acoustic field is
20 Kc/sec, particles with diameters smaller than 2um vibrate with
nearly full amplitude (Xp/Xg = 1), while particles with diameter
larger than 4um are virtually stable (Xp/Xg= 0). This differential
motion of vibrating particles leads to the agglomeration of particles
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73
Since the inertia of a particle can be conveniently expressed
in terms of T, which can be expressed as:
.!a* a)
where p is the particle density, n is the dynamic viscosity of the
carrierp gas and a is the particle radius. T = 2iT/u= 1/f where w
and f are the angular velocity and frequency of the acoustic field,
respectively. T/T characterizes the relative effectiveness of the
inertial force during one oscillating cycle. When T/T » 1 the
inertia is too large for the particle to respond to the acoustis
oscillation. Thus, the particle can be considered to be relatively
stationary. When T/T « 1.0, the inertia is negligible and the
particle is oscillating together with the carrier gas. The degree
to which a particle is carried along with the oscillating motion is
specified by an entrainment factor, y which is defined by:
X , _ , u
u =/= [1 + 4/ (t/T)Y = / (2)
8 g
where Xp and Xg are the oscillating amplitudes of particles and the
carrier gas, respectively. Equation (2) is plotted in Fig. 4.
Owing to the differential motions, smaller particles are agglom
erated into larger ones and the time rate of small particle concen-
tration can be written as:
dt
Where n_ is the small particle concentration. Integrating Eq.(3)
and using the initial conditions n-- at t = 0, we have
--
-K t
n2 ' n20 e
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74
where Kfl is defined as the acpustic agglomeration constant given
by
K& = a e TT(a]L + a2)2(2Xp)n1f (5)
where a is a filling-up factor, n is the concentration of large
particles which act as collectors of small particles, Xp is defined
in Eq.(2), al and a2 are the radius of large and small particles,
respectively. e is defined as the particles collision efficiency.
To understand the acoustic agglomeration mechanisms it is necessary
to describe briefly the theoretical predictions of the values of e.
The particles of diameter range 0.2-20ym, the calculation of e may
be divided into two subregions. The first region is for particles
of diameter range from 0.2 to about 2um. For these particles the
predominant mechanism for acoustic agglomeration is the so-called
ortho-kinetic interaction. Basically, orthokinetic interaction
occurs between two particles of different sizes which are moving
at different velocities according to Eq.(2). Such differential
motion causes the two particles to collide with each other, leading
to agglomeration into one larger particle. For'this interaction the
particle efficiency e can be expressed as [26,27]:
e = K* (Ke + 0.25)-2 (fi)
where 2u a
Typical results of Eq. (6) are shown in Fig. 5. For particles of
diameter range from 2-20 the predominant mechanism for acoustic
agglomeration is the so-called hydrodynamic interaction. In this
case particles interact with each other due to the hydrodynamic
interaction forces resulting from the disturbance of the flow field
of one particle to another. It is important to point out that
the orthokinetic interaction is not important in this region because
particles larger than 20ym seldom exist in any of the applications
as discussed in this paper. For this mechanism the particle colli-
sion efficiency e can be determined by using the following expression
for
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75
. _ 1.587 . 32.73 .
where B = + —^— +
344
I ,1.56
a (20/a )
e e
a -10 a -10
exp - — - sin — -
15 63
(7)
(2u
p • and p are constants such that y = 0 when 1 = 0 and y = 1/4
•L £ C C
when 1=1 and are determined by iteration for each a .
10*
Fig. 5 6 (ratio of'acoustic agglomeration rate to the agglomeration rate
without acoustic field) vs. particle-radius ratio for various
acoustic frequencies (in kHz) for large particle radius (urn)
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76
Typical results of Eq.(7) are shown in Fig. 6. For both types
of interactions a very large increase in the agglomeration rate
as compared with the conventional gravitational coagulation can
be expected. However, it is important to point out that the
theoretical prediction in both Figs. 5 and 6 are likely to be too
optimistic since in both figures the filling-up factor a (as shown
in Eq.5) is assumed to be one. In practical cases a is less than
10'
10
8
6
4 68io'
6*,o°
Fig. 6 6 vs. small particle radius for various acoustic frequencies
(a = 5y, j = lw/cm2)
one. The actual value depends mostly on the acoustic turbulent
strength. No information, neither experimental nor theoretical,
is available on a at the present time. It should be noted also
that for control of small particles (diameters less than 2ym) a
high frequency acoustic field is much more effective; while for
large particles (diameter between 2-20 m) , a low frequency field
is much more effective.
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77
The rate of reduction, as indicated in Eq.(3) depends on the
square of particle concentration. Thus the higher the particle
concentration the more effective is the acoustic agglomeration.
For the LMFBR applications, typical mass loading is high (range
from l-30g/m ), thus acoustic agglomeration is expected to be
very effective. For other applications in which the particle
number concentrations are not as high as in the case of MHD and
fluidized bed boilers, it may be necessary to add a mist to bring
the particle loading to a satisfactory level. However, the mist
must be chemically and thermally stable under the high temperature
and high pressure environment, such that the mist droplets will
be removed completely by the cyclone after leaving the acoustic
agglomerate. The required acoustic intensity depends on the
requirements on particle agglomeration time, since the acoustic
coagulation constant K is directly proportional to the square
root of acoustic intensity.
ACKNOWLEDGEMENT
This work was supported in part by EPRI and NSF.
REFERENCES
[]] Patterson and W. Cawood, Nature 127, 667 1931.
[2] Mednikov, E.P., Acoustic Coagulation and Precipitation of
Aerosols, English transl. by Consultants Bureau, New York.
[3]
[4]
[5]
[6]
[7]
[8]
Shirokova, N.L., Physical Principles of Ultrasonic Technology
2 (L.D.Rozenberg, ed.)477-539.Aerosol Coagaulation, Plenum
Press, 1973.
Timoshenko, V.I.,
Timoshenko, V.I.,
Timoshenko, V.I.,
Appl. Acoustics, 1, 173, 1968.
Appl. Acoustics, 1, 183, 1968.
Appl. Acoustics, 1, 192, 1968.
Timoshenko, V.I., Appl. Acoustics, 1, 200, 1968.
Shirokova, N. L., Interaction of Aerosol Particles in a Sound
Field (Candidate's Dissertation), Akust.Inst.Akad,Nauk USSR,
Moscow 1968.
[9] Mednikov, E. P., Dokl.Akad.Nauk.SSSR, 183 (2): 382, 1968.
[10] Mednikov, E. P., Akust. Zh., 14 (4): 582, 1968,
-------
78
[11] Mguyen, S. and Beeckmans, J.M., J.Aerosol Sci, 5, 133-143.
1974.
[12] Scott, D.S., Proceedings Symposium on Control of Fine^-Particu-.
late Emissions from Industrial Sources, 597^621. San Francis.co
1974.
[13] Davidson,?.A. and Scott, D.S., J. Acoust.Soc, of Amer. 53,
1717-1729, 1973.
[14] Davidson, F.A., J. Sound and Vibration 38, 475-495, 1975,
[15] Davidson, F.A. and Scott, D.S., J. Aero.Sci,5, 55-69, 1974,
[16] Hegarty, R. and Shannon, L.J., Evaluation of Sonics for Fine
Particle Control, EPA-600/2-76-001, January 1976.
[17] MacPherson, R.E., Report No. ORNL-TM-1937, 1968,
[18] Milliard, R.K., McCormack, J.D., Postma, A.K., Report No.
TC-536, Hanford Engineering Devel. Lab, 1975,
[19] Jonke, A.A., Swift, W.M. and Vogel, G. J., Transactions AIME,
Vol. 258, 159-167, 1975.
[20] Ehrlich, S. and McCurdy, W.A. .Proceedings 9th Intersociety
Energy Conversion Eng.Conf., San Francisco, 1974.
[21] Wright, S.J., Proceedings 3rd Int1.Conf.Fluidized Bed Combus-
tion, EPA-650/2-73-053, EPA, 1973.
[22] Highley, J. Ed, Combustion of Coal in Fluidized Beds, Proceed-
ings, Coal Research Establishment, May, 1968.
[2-3] Jackson, W.D., ERDA, Zygielbaum, P.S. EPRI, et al,, MHD Power
Generation 769176, Status Report, llth Intersoc. Energy
Conversion Eng. Conf., Sept., 1976.
[24] Office of Coal Research Report #71, prepared by MHD Power
Generation Study Group, 1972.
[25] Dicks, J.B. et al., Proceedings, llth Intersoc. Energy Con-
version Eng. Conf. 769177, Sept., 1976.
[26] Fuchs, N.A., The Mechanics of Aerosols, trans, by R.E. Daisley
and M. Fuchs, Pergamon Press, New York, 1964.
[27] Hidy, P.M. and Brock, J.R., Dynamics of Aerocolloidal Systems,
Vol. 1, Pergamon Press, 1970.
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79
WRITTEN DISCUSSION
W.M. Swift
Department of Chemical Engineering
Argonne National Laboratory
9700 South Cass Avenue
Argonne, Illinois 60439
Argonne National Laboratory's interest in controlling particulate emissions
with acoustic agglomerators arises from its participation in a research and
development program designed to develop the concept of fluidized bed com-
bustion (FBC) and to eventually demonstrate on a plant scale that the process
is economical and meets pollution standards for stationary power sources.
Simply stated, fluidized bed combustion is a process in which coal is burned
with excess air in a fluidized bed of limestone or dolomite at temperatures
of 800 to 950°C. The heat of combustion is used to generate steam for process
use or to generate electrical power. Fluidized bed combustion, which results
in both reduced SO- emissions (by reaction of the SO- with CaO in the fluidized
bed) and reduced NO emissions (by reason of the low combustion temperatures),
is potentially bothxenergy efficient and economically attractive.
In advanced concepts of FBC, the combustor is operated at elevated pressure
(e.g., 10 atm). Energy is extracted from the process by raising steam in
boiler tubes and by expanding the flue gas through a gas turbine to take
advantage of the potentially higher thermal efficiencies offered by a gas
turbine-steam turbine combined cycle. Use of a gas turbine, however, requires
that the particulate loading of the high temperature-high pressure flue gas
from the pressurized FBC (PFBC) be reduced to very low levels. Estimates of
acceptable dust loading for turbine operation range from 0.05 to 0.0002 grains/
scf.
Until recently, the problem of high temperature-high pressure flue gas
cleaning (particularly as it relates to PFBC) was not adequately addressed.
At present, however, a fairly intensive effort is being made to develop
viable techniques for sufficient cleaning of flue gas from a pressurized fluid-
ized bed combustor to allow its use in reliable gas turbine operation.
Currently, ANL is conducting basic supportive studies in fluidized bed
combustion. One task in our program is to test and evaluate novel concepts
for high temperature-high pressure flue gas cleaning in our process develop-
ment facility, which includes a 6-in.-dia. fluidized bed combustor which can
be operated at pressures up to 10 atm. Initially, a review was made of the
existing and developing particle-gas separation technologies to identify
concepts which would be amenable to the high temperature-high pressure require-
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80
ments of pressurized FBC. One area identified for investigation was
acoustic agglomeration. As a result, we entered into a working relation-
ship with Dr. David Scott of the University of Toronto, who is also present-
ing a paper on acoustic agglomeration at this workshop. It is our intention
to investigate the effect of acoustic dust conditioning on the efficiency of
particulate removal devices in the flue gas system of the ANL experimental
combustor.
Although the agglomerating influence of an acoustic field on an aerosol
has been known for slightly over 100 years, the development of acoustic
agglomeration (AA) as an aid in the control of particulate aerosols has passed
through cycles varying from enthusiasm to almost total disinterest. When the
electrostatic precipitator won the performance/cost war in about 1952,
interest in acoustic agglomerators in the U.S. essentially went to zero. At
the same time, however, interest in AA was generated in the U.S.S.R., Hungary,
Poland, and Japan. This interest in foreign countries has continued to the
present. As a result, the U.S. has been left behind in both developing this
technology and maintaining the necessary scientific interest.
Some of the factors which led to the premature disinterest in AA were:
(1) a poor understanding of the aerosol mechanics of AA, (2) less than
optimum configurations in the designs originally developed for commercial
application, (3) a demonstrated superiority of electrostatic precipitators,
(4) concern over the high power requirements for the generation of the acoustic
fieldl, and (5) potentially high capital cost for acoustic agglomeration
systems.
In the paper by Shaw and Wegrzyn, the argument is made that sonic
agglomerators cannot compete in applications where conventional devices
already have been successfully used, but that they have some characteristics
which make them uniquely suitable for certain special applications. The
phrase "uniquely suitable" must not be interpreted to imply a priori that
sonic agglomeration is necessarily the most suitable control technique for
these special applications. In the example of the LMFBR sodium-fire aerosol,
although the acoustic control technique was ranked first in effectiveness,
it was ranked ninth in overall preference due to other considerations. Also
although acoustic agglomeration appears attractive in controlling emissions
from a fluidized-bed boiler to protect a gas turbine in a gas turbine-steam
turbine combined cycle, a number of alternative (and generally more highly
regarded) control technologies are being investigated. Granular-bed filters
and ceramic filters (either fabric or porous sintered) are the more commonly
mentioned alternatives being considered for this application.
It is interesting to note in the ranking of control methods by criteria
for the control of LMFBR sodium-fire aerosols (Table 2 in paper by Shaw and
Wegrzyn), one of the problems facing acoustic agglomeration was that of
credibility. This same problem of credibility exists in the application of
acoustic dust conditioning to PFBC applications. The problem of credibility
arises mainly from the lack of experimental correlations or experimentally
verified theory which can be used to predict either performance or cost in a
given application.
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81
The credibility problem is really not too surprising in view of the
complexity of the equations which govern the aerosol mechanics and the f-act
that the experimental data on acoustic agglomeration are for the most part
of no use for evaluating theory. If AA is to gain credibility, both the
performance and the energy requirements have to be established (either by
experiment or from reliable theory) for the particular application being
considered and a performance-cost comparison made against competing
technologies.
A good argument for determining energy requirements can be made in the
case of particulate control from pressurized fluidized-bed combustors. For
atmospheric fluidized bed combustion, an overall plant efficiency of 36-39
percent is estimated. In atmospheric operation, the flue gas can be adequately
cleaned using conventional technology. First generation pressurized FBC
systems will have an estimated plant efficiency of 40-43 percent. The small
but significant gain in plant efficiency for pressurized systems is at the cost
of considerably more complex development problems (of which cleaning of high
temperature-high pressure flue gas is one). If cleaning of the high temper-
ature-high pressure flue gas to acceptable particle levels for turbine
operation by AA or any other method is excessively energy-intensive, the PFBC
loses much of its advantage over the atmospheric concept.
With reference to the theoretical aspects of acoustic agglomeration as
presented by Shaw and Wegrzyn, the following comments are offered. First,
the authors are to be commended for their efforts to identify the controlling
mechanisms in AA and to experimentally verify the respective governing
equations for these mechanisms. It should be emphasized, however, that, the
agglomeration mechanisms considered by Shaw and Wegrzyn (i.e., orthokinetic
vibration and hydrodynamic attraction) are but two of approximately nine mechan-
isms known to have some effect on the agglomeration kinetics. The problem is
sufficiently complex that the mechanisms of AA may depend upon the nature of
the aerosol, the acoustic field, and the system geometry. Thus, the con-
clusions of a particular study may be seemingly contradicted by the results
of other experiments when both sets of data are equally valid but depict
different mechanisms under different circumstances. The statements, there-
fore, by Shaw and Wegrzyn that the predominant mechanism is acoustic agglom-
eration for particles of diameter range 0.2 to 2 pm is ortho-kinetic inter-
action and the predominant mechanism for particles of diameter range 2 to
20 ym is hydrodynamic interaction should not be accepted without justification
as general observations which can be applied to all acoustic-aerosol inter-
actions.
REFERENCES
[1]
Problem." 0
(in preparation).
LENCES
R. Razgaitis, "An Analysis of the High-Temperature Particulate Collection
Problem," Argonne National Laboratory, Argonne, 111., Report No.
ANL-77-14 (in preparation).
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82
OPEN DISCUSSION
Shaw: I agree with what Bill said about the nine mechanisms he mentioned.
The other seven are mostly associated with the turbulence. The parakinetic
is also what I term as the filling up of the space. So I agree with what
you are saying on the nine mechanisms. However, I don't think you can
really identify the way we are approaching the problem on the mechanism
beyond these two. We have already identified the linear case; we are
also looking at the affect of turbulence overall. You have to define some
parameter in terms of acoustic turbulence which I think will cover other
mechanisms such as the parakinetic and so on, because there is no way to
separate the parakinetic from the turbulence under high intensity. They are
interrelated.
We have done separate things in a nonlinear case, theoretically and a
few experiments under high intensity; but we are far away from matching the
theory and experiment.
Yang: Bill, is the work that you are just finishing available?
Swift: No, it is in a state of being published, both as an Argonne report
and an NTIS document.
Oder: Could you comment on the strength of coagules that are formed by the
agglomeration process? I gather that they are brought together by turbulent
acoustic forces and they settle out in rather undisturbed environments.
Suppose, instead, that you try to extract these particulates under turbulent
conditions, what is it that holds them together?
Shaw: That is a very good question. So far we have only done experiments
with particles of a very small size, primarily for the application of fine
particulate control which is supported by EPRI. We have not done any work on
the high concentration yet. We are expecting a contract from ERDA on the
high intensity agglomeration of large particles for sodium aerosol. The
small particle sticking is not a problem. That comes when large particles,
sticking together, break apart. In that case, some kind of salient condition
must exist so that they stick together. This is something that we are just
still speculating, we have no experimental data to show. The breaking up by
the acoustics will be important only when you have very large particles.
Oder: I am sorry, I still didn't hear what it is that makes them stick
together.
Shaw: Surface molecular forces. Just like when you have a particle on an
impactor, it is the same force holding a particle to an impactors plate as
when the particles are sticking together.
Helfritch: You mentioned that, most often, the use of agglomeration techniques
applies to cyclones. There are two questions that have been brought up
about the success of that application. The apparent specific gravity of the
agglomerates would be quite low. Secondly, they may break up. I was wonder-
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83
ing if it had actually been tried in a cyclone.
Shaw: No, we have not tried it. The only reason mentioning cyclones is
that it can only be used in applications like the Argonne system where they
have a cyclone and want to work it with something else. Whenever you have
a situation where the pressure drops and pumping is a problem, the cyclone
seems to be the answer. It requires very little pumping power.
Helfritch: You mentioned that a high pressure drop across the filter is
part of the reason for projecting that technique. However, I would submit
that the pressure drop across the filter is typically on the order of pressure
drop across the cyclones. I cannot accept that argument.
Shaw: If that is the case, then I am wrong. When I talked to Argonne
people, they told me that they cannot use filters. My impression is that the
problem with filters are high temperatures, the other is a pressure drop
problem.
Helfritch: Finally related to that is the fact that high temperature, and
pressures adversely affect performances of cyclones. That is because of the
viscosity relationship.
Swift: By acoustically treating the aerosol you can increase the efficiency
of the cyclone over that of the untreated aerosol even though you have high
temperatures and pressures.
Cooper: The problem with the sodium aerosol is that it clogs the filter in
a way that is very unusual and very difficult. So you are not talking about
the usual few inches of pressure drop and any reasonable phase velocity;
you are talking about much higher than that. Second, there is a difference
in degrading the performance of the cyclone by changing its collection
efficiency cut point by the higher temperature and pressure. And indeed,
the degradation of the filter is where it just falls apart.
Grassel: It seems to me that the work that is going on now, is just repeating
work that was done a number of years ago by the Bureau of Mines and other
places during the war. Yet it does not address itself to the practical
problem of implementing sonics as a method of treating aerosols. It does not
address itself to the problem of true output, sound level and size. You
are going to have to give up one of those for the others; sound level is what
you are attempting to compromise. You are attempting to use sound level
as a method of accomplishing size and true output. This represents itself
in power. As the power goes in, you have two practical problems of application:
containment of the sound level and dissipating it.
The power necessary to agglomerate particles is very low. The power
necessary to generate a field of sufficient intensity to collect those particles
or to agglomerate those particles is very high.
Shaw: What you are saying is the power attenuation in the particle is small.
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84
Grassel: But the power put into the system is very large.
Shaw: You must obtain a level of 160 dB, That is precisely the reason
why we think a standing wave kind of thing would be good for fine particle
control.
In response to your comments, we are doing something like the whistle
experiments in the 1950's. We have to go through this phase to reactivate
the field. The thing we are doing different nowadays is that the particle
sizing techniques are much more improved than the wartime. I do not agree
that others have done the same thing. No one before has measured size
distribution change in an acoustic field. The only other related work has
been done by a Russian who sampled the size by putting in something to collect
the particles by sedimentation.
What we are doing in our laboratory is trying to bring us to large
scale testing to give us credibility and demonstrate to people that it will
work. If you do it right it will work. The problem is how well it will
work. This is the thing we are driving at. We have to go through the large
scale testing which is nothing new. I emphasize that the state of our
siren is 1949. But we are applying the new sizing technology to study this
problem.
Benarie: Let me take just a minute for this problem of the cyclone. Perhaps
it would simplify concepts if I explained that we have vortex agglomeration .
for both cases: (1) a cyclone with a hardware around the vortex and (2)
when there is no hardware around it. It is a turbulent agglomeration.
Shaw: In no way do we think the acoustic cyclone works the best. In fact,
from the data, we feel that some kind of acoustic thing working together
with a scrubber, with a very fine mist, for certain applications is best.
With high temperatures you have trouble maintaining that mist.
Scott: All the work that was done at the Ontario Research Foundation was
with a cyclone. So the improvements noted, from 35 and 40% up to 75 and 80%,
were done in cyclones. It is clear that we want to go that way again.
Second, on the power, that was what the whole project was about. It was to
determine the cost of the power. Our evaluation has shown that if you use
your fuel evaluation for the pulse jet and take the cost of the fuel, then
the power requirements are way down for scrubbers. We are talking about 25c/
ACFM/year for power. That is for a pulse jet, which in that case was designed
with an aerodynamic valve. I think I simply must not let the thing go without
saying that cyclones have been used. Secondly, we have concentrated on power.
That was really the whole topic of our work. Lastly, as far as the Bureau
of Mines is concerned, I do not think we are repeating very much. I would
be happy to see those areas where you think we are. I think we are doing
things that are quite different.
Cooper: Generally, surprisingly little power is required to remove particles
from the gas stream if you just take the force on to particles times the
distance they are to be moved. In some sense we ought to keep that in mind.
-------
That is one of the advantages of electrostatic and magnetic separation as
opposed to other methods which operate on the gas which often has very much
more mass than the particles. There is an inherent advantage to those
systems which apply the force directly to the particle rather than through
the medium of the gas to the particle.
85
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86
EFFECTS OF ELECTRIC AND ACOUSTIC FIELDS ON PARTICLE COLLISION RATES IN
AEROCOLLOIDAL SUSPENSIONS
P. D. Scholz
Division of Energy Engineering, University of Iowa, Iowa City, Iowa
ABSTRACT
An experimental investigation is described that involves the use of an
a.c. electric field and a traveling sound wave field to increase the colli-
sion rate between submicron particles suspended in air. A brief discussion
of the theoretical collision rates in the free molecule regime is also in-
cluded for the cases where each field is applied separately. Velocity pro-
files and values of the turbulent intensity in the continuous flow coagulation
tube are presented and discussed. In addition, the research plan is briefly
outlined.
I. INTRODUCTION
The work reported here is based on an experimental project that is under-
way to investigate the effects of an a.c. electric field and a traveling wave
sound field on the collision rates between suspended particles with radii
less than about 0.3 um. We have chosen to work with particles in the sub-
micron range because of the compelling need to find ways to effectively re-
move these particles from man-made suspensions. Although we are not dealing
with the direct removal of the particles from the parent gas, the work dis-
cussed here involves the use of two external fields to "condition" the sus-
pension by inducing particle growth of the fine particulate matter through
agglomeration or coalescence. Following the "conditioning" process, the
suspension is envisioned to be passed through a downstream and conventional
removal device where the resulting agglomerates are to be removed. The
induced growth is achieved by using the external fields to selectively in-
crease the relative motion and therefore, the probability of collisions be-
tween the suspended particles.
The motion of the particles under the influence of an a.c. electric field
and a traveling sound wave may be demonstrated in the following way. Con-
sider a charged particle with a charge*q and mobility B suspended in a gas.
The velocity of the particle in the presence of an a.c. electric field is
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87
v = q B E sin u t,
where ID is the angular frequency of the field and E is the electric field
vector. Here we assume that the product of the angular frequency and the
particle relaxation time, T, is much less than unity, i.e., u T « 1. Now
consider that a continuous train of traveling planar sound waves passes
through the suspension. The velocity of the particle in this case is [l]
U
— [sin <
(1 +
where u is the angular frequency of the wave, U is the magnitude of the
oscillating velocity of the gas molecules, ty is the phase lag between the
particle and the oscillating gas molecules, and ft is a unit vector in the
direction of propagation. The magnitude of the particle velocity,
and the phase lag, <|>, are dependent on the product u> T, where T is dependent
on the size and mass of the particle. For u T « 1, the particle oscillates
at nearly the same speed as the gas molecules and with very little phase dif-
ference. On the other hand, for u T » 1, the particle is virtually sta-
tionary. (For two different particles, one with uaT « 1 and one with
u>ar » 1, there is a large difference in the oscillating speeds. Such a
differential motion is the basis for orthokinetic coagulation.)
By superimposing the two fields, the resultant motion of the particle is
a function of the two field frequencies and magnitudes, the directions of the
two fields, and of the particle parameters, which include charge, size, and
mass. It is this resultant motion of many particles submerged in these two
fields which can lead to an increase in the particle-particle collision rate
that is of interest here.
It should be noted that, whereas acoustic coagulation generally requires
high frequency sources (i.e. , on the order of 10-30 kHz) to set up the dif-
ferential motion in a suspension of fine particles, the present two-field
scheme utilizes full entrainment of all the particles and hence lower sound
frequencies. This feature has some sound transducer and power supply advan-
tages. However, this scheme also requires that the particles be charged,
which necessitates the1 addition of some type of charging stage in the system.
It is also noted that a traveling wave sound field, as opposed to a standing
wave field, is considered here following the work of Scott [2].
For this case of combining an electric field and a sound field to pro-
mote an increase in particle-particle collisions, there is no known theoreti-
cal expression that relates the collision rate to the field and aerosol
-------
parameters. However, past work involving derivations of the collision rates
for electric field enhanced coagulation and for acoustically induced coagula-
tion can be used as a basis for considering a new model for the case where
both fields are applied.
A. Theoretical Background
Using the particle Knudsen number to identify the usual aerosol size
regimes (see Hidy and Brock [3], for example), suspensions of particles at
standard conditions with radii in the approximate range 0.01 < R < 1 pm are
classified as transition regime aerosols. (Continuum regime aerosols have
particle radii larger than about 1 ym and free molecule regime aerosols have
particle radii less than about 0.01 vim.) The particle radii of interest here
are in the transition regime, and herein lies a problem for treating the
collision dynamics of these particles. Whereas, continuum mechanics is used
in treating the dynamics of continuum aerosols and kinetic theory is used in
the free molecule regime, the transition regime is without a single, unified
theoretical base. Extensions of continuum regime theories and of free
molecule theories into the transition regime for the thermal collision rate
are available [3,4]. Experimental results show that in some cases these ex-
tensions are reasonably accurate [4], However, extensions do not exist for
collision rates involving electric or acoustic fields. Expressions are
available for the collision rate as a function of the field quantities for a
sound field in the continuum regime (see Mednikov [1]) and for an electric
field in the free molecule regime (see Brock and Hidy [5]). Rather than try
patching together existing theories and then extending the results into the
transition regime, we have taken the approach of using a free molecule theory
to construct an expression for the collision rate due to these external
fields. This approach, which is discussed in the next major section, empha-
sizes the diffusional processes and also provides a framework for dealing
with the basic collisional dynamics of the suspended particles in the pre-
sence of the two fields. There is evidence that diffusional effects are
significant in sonic agglomeration of particles with radii below about 1 ym
[6]. This fact is reason enough to justify the development and examination
of this free molecule approach.
B. Summary of Past Experimental Work
There is very little experimental data available on the collision
rates of suspended submicron sized particles subjected to electric or acous-
tic fields. In the transition regime, Mednikov [1] reports some experimental
results of acoustic coagulation in DOF fogs with mean particle diameters of
about 0.28 urn. The coagulation constant was found to be directly proportion-
al to the product of the sound pressure and the time that the sound field was
applied to the fog. Details of the experiment are vague and the accuracy of
the results is unknown. The work in the past has involved aerosols that were
not very monodisperse, and hence the reported collision rates are not very size
sensitive. In addition, many of the earlier studies involved time decay
measurements in a closed chamber and were subject to significant diffusion
loss effects to the chamber walls as the coagulation process evolved and the
samples were extracted for size analysis [6].. Currently, there are several
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89
experimental investigations underway that should contribute significantly to
the understanding of aerosol-acoustic field coupling. Much of this current
work is possible because of significant improvements in the equipment avail-
able to generate and analyze aerosols. The experimental investigation re-
ported here is a product of this improved technology.
C. Paper Organization
The next section, which is Section II, is a summary of a first order
derivation using kinetic theory to determine the particle-particle collision
rate as a function of the aerosol parameters and the field frequency and
magnitude for an a.c. electric field. The derivation of the collision rate
for a traveling sound wave is also summarized. The expressions for these
rates, along with the known expressions discussed earlier, are to be tested
during the experimental investigation, which is underway and which is de-
scribed in the third section of the paper. Section III also contains the
results of velocity and turbulence measurements taken in the coagulation tube.
In addition, the experimental plan is briefly discussed. Section IV consists
of a summary of the paper.
II. COLLISION RATES IN THE FREE MOLECULE REGIME
As mentioned before, an expression for the collision rate between parti-
cles in an aerocolloidal suspension subjected to a combined electric field
and traveling sound field does not exist. It is our desire to construct a
simplified model to describe the particle-particle collisions in this case.
Because diffusional effects are important in the collisional processes in-
volving submicron particles [6], it was decided to approach the problem using
a free molecule regime model. However, before solving the more complicated
combined field problem, it was decided to derive the expressions for the col-
lision rates for each field separately to better understand the effect that
each field has on particle collisions. These derivations have been completed
recently and the following is a summary of that work. The derived rates are
to be checked with experimentally determined rates during the experimental
investigation which is underway and which is discussed in Section III of this
paper,
A. General Approach to Deriving the Collision Rate in the Free
Molecule Regime
In the free molecule regime, we treat the aerocolloidal system as a
dilute mixture of gas molecules and two separate, but homogeneously mixed
monodisperse aerosol species with particle radii R^ and Rj and concentrations
n- and n*, respectively. For each species, the particles are considered to
be spherical and the number density of either particle species is much less
than the number density of the gas molecules, while the mass per particle of
either species is greater than the mass of a gas molecule. (This is equiva-
lent to a very low mass loading.) In this case the three governing Boltzmann
equations (i.e., one for each type of aerosol species and one for the gas
molecules) are uncoupled and the equations for each aerosol species are iden-
tical, except for a difference in subscripts. An approximation technique is
-------
90
then used to solve one of the aerosol equations for the particle distribution
function, say flf after assuming the form of the distribution function for the
suspending gas. (The second species distribution function, fj, is the same,
except for a change of subscripts.) An expression for the collision rate is
then determined by evaluating the collision integral of the products of the
two particle distribution functions for a specified particle-particle force
law. This approach has been discussed elsewhere (see Scholz and Barber [8] ,
for example) and will not be presented here.
B. Collision Rate for an Aerosol with an a.c. Electric Field
In this case we assume that the dilute mixture is spatially uniform and
that the particles in each species have an average charge per particle of
qitand q j , respectively. The acceleration due to the externally applied
electric field is
r
-r = m~ (E COS
where mr is the particle mass of each aerosol species and E cos o)et is the
magnitude of the z directed, time varying electric field. In addition, the
gas is assumed to obey a Maxwellian velocity distribution function.
Following the procedure outlined by Scholz and Barber [8], the Boltz
mann equation for the itn aerosol species may be solved by an approximation
scheme using an expansion with a weighting function equal to some zero-order
approximation of f^. In this work the zero-order function is taken as the
Maxwellian distribution function. In addition, the particle-gas collisions
are assumed to obey an inverse fifth power central force law (i.e., the
Maxwell force law). Using this approach, the approximate particle distribu-
tion function for a three term expansion is
fi ~ Do
\ B2[sin2(o>et -
where
- tan
1
2ir]T' -
and where (fi)^ is the Maxwellian function for the i**1 aerosol species. In
these expressions n^ and m^ are the aerosol particle concentration and mass
for species i, n and m are similar quantities for the gas molecules, v
is the particle speed in the z direction (and is a variable) , k is the
Boltzmann constant, T is the absolute temperature of the suspension, AI is a
collisional constant for the particle-gas collisions and equals 0.422, K is
the inverse fifth power force constant and DQ is an arbitrary constant. As
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91
mentioned earlier, the expression for f. is identical except the i subscripts
are replaced by j.
It should be noted that this expression for f^ differs from the dis-
tribution function used by Brock and Hidy [5] to determine an expression for
the electric field induced collision rate. In that work, they expressed the
effect of the electric field as an equivalent drift term. This term was
then included in the argument of the exponential velocity term of a Max-
wellian velocity distribution function used to construct fi- The approach
here is more fundamental for we have accounted directly for the effects of
gas-particle collisions in the collision integral when we solved the Boltz-
mann equation for f..
As mentioned previously, the collision rate is determined by evaluat-
ing the collision integral of the products of the i and j particle distribu-
tion functions for a specified particle-particle force law. Following Hidy
and Brock [3], the specified force law for the particle-particle collisions
is based on a hard sphere collision model with a Debye-Huckel shielded elec-
trostatic potential acting between the colliding and charged spheres. The
collision rate, LE, for the special case where the two species are the same
size (i.e., R^ = R..) and have the same, but opposite average charge per
particle is
where L is the self-thermal collision rate and is
R2n2 /kT//2
In this expression n is the fraction of particles that stick upon contact
and J2 is the charged particle interaction parameter and is
q2 exp(- 2R /A )
fl - 1 +
16ir e
where e is the dielectric constant of the aerosol, and
is the Debye radius. This case where R± = Rj and q± = -qj corresponds to a
quasi-neutral, monodisperse aerosol and might represent an initially neutral,
monodisperse aerosol that is passed through a bipolar charger in which com-
plete charging takes place.
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92
C. Collision Rate for an Aerosol with a Traveling Sound Wave
In this case we consider planar sound waves propagating in the posi-
tive z direction with a speed of c and an angular frequency of u>a. The gas
is assumed to obey the Wang-Chang and Uhlenbeck [9] distribution function for
small amplitude sound waves propagating in a monatomic gas. That is,
v 5v
1 + a 1-^ + —^- I sinui
oo \ c 3(;2 / a
M
In this expression a00 is an arbitrary constant, the g subscript refers to
the gas, and (fg)M is a Maxwellian distribution for the gas. The arbitrary
constant, aQO, is evaluated here by setting the z component of the average
speed of the gas at the piston face (i.e. , at z = 0) equal to the oscillating
piston speed, which, in the absence of attenuation effects, is also the mag-
nitude_of the oscillating gas speed, Ug. That is, vgz(z = 0) = Ugsinu)at,
where vgz represents the z component of the gas speed averaged over
velocity space. Although the Wang-Chang and Uhlenbeck distriubtion function
for the gas is limited to small amplitude oscillations, the function does
appear to have the correct structure since the moments of the distribution
function yield reasonable expressions for the density, pressure, and temper-
ature of the gas. The structure of fg is important to this development of
determining f^ since the effect of the sound wave on the suspended particles
is through collisions of the particles with the gas molecules; that is,
through the collision integral in the Boltzmann equation for the ic^ parti-
cles. (This indirect coupling occurs because the mass loading for the parti-
cles has been taken to be very low.) As in the electric field case, the
particle-gas collisions are modeled with an inverse fifth power force law.
Again, using an approximation technique with a three term expansion to solve
the Boltzmann equation, the particle distribution function is
fi ~
7V" + 5V
izJ [
f
cos(ocosB -
where
a = 1 -
,2 2 2C,c[£ + atan(acos6 - x)]
t- ~A , JL
2A
2 2
2 2 2C.c[C + atan(ocos9-
C — A JL
2A'
22 2
UA CC + 0r )
0) C
a
fir2 25 A2
oc - -T— A
2 2~~
c - 3A
C-A
2
21 ? 2S 2
c - 3A) -
, and
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93
2
In these equations 6 = ioa(t - z/c), A = kT/m.p DI is an arbitrary constant,
and as before (f^^ is the Maxwellian function for the ith aerosol species.
A2 is another collisional constant for the particle-gas collisions and equals
0.436 and Dj^ is an arbitrary constant, as is the phase angle x- The other
quantities were identified earlier. As before, the expression for fj is
identical to f^ with a change of subscripts from i to j.
One property of this particle distribution function is that it exhib-
its a drift phenomenon due to the expUcosS) term. This stems from the sin9
factor in the gas distribution function, which reflects itself through the
collision integral of the Boltzmann equation. This factor subsequently be-
comes a coefficient in the set of partial differential equations that are
solved for the expansion coefficients of f..
As before, the collision rate is determined by evaluating the colli-
sion integral for i-j particle collisions. We again assume that each species
is charged. Using the hard sphere Debye-Hiickel shielded potential model to
account for the interaction of the charged particles during collisions, the
collision rate, LA, for the case where the two species are the same size
(i.e., R.£ = R.J) and have the same, but opposite average charge per particle
is J
7 2 2
LA = D,(0.23776 ^r - 0.99420) [exp(2?cos0)][cos (ocosG)] L_,
A 1 A2 T
where all of the terms have been introduced before. (This collision rate
may also be used for uncharged, hard sphere particles by setting q^_ = 0 in
the charged particle interaction parameter, Q.)
D. The Behavior of LE/LT and L./L-
A plot of the maximum ratio of the self-collision rates, LE/LT
and LA/LT» is given in Figures 1 and 2 for R£ = 10~2 ym sized particles. The
arbitrary constant, D2,, in the electric field case is eliminated by setting
the ratio LE/LT to unity at zero electric field. The constant, D2, in the
acoustic case is eliminated by normalizing all the values of the ratio L^/Lf
with the value of the ratio evaluated for Ug = 0, which is taken to corre-
spond to the state where collisions result from thermal motion only. The
maximum values are plotted, instead of the time averaged values, because in
both cases all points in the suspension will experience the total fluctua-
tions of each field.
The curves in Figure 1 indicate that at a frequency of lO^Hz the
self-collision rates increase when the magnitude of the electric field in-
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94
FREQUENCY OF EACH
FIELD IS 103 Hz
102
I
MAGNITUDE OF THE A-C ELECTRIC FIELD (v/ro)
1 - 1 - 1 — I I I I I I _ l l l l l
104
J
10'
10
MAGNITUDE OF THE ACOUSTIC PISTON SPEED (m/sec)
10'
FIG (1) SELF-COLLISION RATES AS A FUNCTION OF FIELD
MAGNITUDES FOR Rj = 10'2 pm PARTICLES
18
16
I-
<
_l
(t
o
k-
_l
UJ
1 4
1.2
1 0 -
10J
L
LE/LT FOR E = 104 v/m
LA/LT FOR Ug = 1 m/sec
_L
FREQUENCY OF THE A-C ELECTRIC FIELD (Hz)
-I - 1 - 1 I I I I I I - 1 _ I _ I I
io5
l l I i
10< io3
FREQUENCY OF THE ACOUSTIC WAVE (Hz)
10"
FIG (2) SELF -COLLISION RATES AS A FUNCTION OF FREQUENCY
FOR R, = 10
'z
PARTICLES
-------
creases above about 10 V/m or when the piston speed increases above about
0.5 m/sec. At room temperature the mean thermal speed of 0.01 urn sized
particles is about 1 m/sec and, for a particle charge of one electron, the
maximum speed of the particles in an electric field of strength lO^V/m is
about 6 m/sec. Then, as expected, the values of the collision rates in-
crease above the thermal rates only when the directed motion due to the ex-
ternal fields is on the order of, or larger than the mean thermal speed of
the particles. The curves in Figure 2 show that as the field frequencies
increase, the ratios of the collision rates decrease and approach unity.
This effect is due to the inertial lag of the particles which increases as
the frequency of oscillations increases.
At this point, the simple kinetic model based on a dilute model of
hard sphere particles suspended in a gas with the two external fields seems
to be qualitatively correct. (The collision rates for the case of unlike
particle collisions (that is, for i $ j) are currently being computed.) The
next step is to construct a similar model and include both fields simultane-
ously. In the meantime, the rates discussed above are going to be quanti-
tatively checked during an experimental investigation involving self-colli-
sions of monodisperse aerosols. The experimental program is discussed next.
III. EXPERIMENTAL INVESTIGATION
A. Introduction
The objective of the experimental investigation is to determine the
collision rate and growth characteristics of submicron airborne suspensions
subjected to an a.c. electric field, to an acoustic field, and to a combina-
tion of these two external fields. During the study, liquid and solid parti-
cles are to be used and the following parameters are to be varied: particle
composition, concentration, and size; average electrical charge per particle;
and the magnitude and frequency of each field. Monodisperse aerosols are to
be studied primarily, but some simple polydisperse aerosols are to be
examined later in the project. At the present time no coagulation studies
have been carried out in the system. However, the experimental system is
nearly ready for some initial self-collision rate measurements. The experi-
mental system is described in the next section, followed by a discussion of
the velocity and turbulence measurements recently completed. The last sec-
tion describes the experimental program.
B. Experimental System
The experiments are to be carried out in a continuous flow coagula-
tion system which is shown schematically in Figure 3. The aerosol is to be
generated by a combination of an atomizer, particle conditioner, and an
electric mobility classifier. This subsystem is capable of producing highly
monodisperse particles in the size range from about 0.01 to 0.3 ym. The
aerosol generating subsystem is discussed in more detail in a later paragraph.
The generated aerosol is mixed with filtered air in the mixing chamber to in-
crease the volume flow rate. From the mixing chamber the aerosol enters the
95
-------
VACUUM
PUMP
vo
CTN
ELECTRIC AEROSOL
SIZE ANALYZER
r- ELECTRIC
\ FIELD SUPPLY
MASS FLOW
TRANSDUCER
r-ACOUSTICAL
\ DRIVER SUPPLY
AEROSOL
ENTRY SECTION
FROM FILTERED
AIR SUPPLY
AEROSOL
MIXING CHAMBER
PARTICLE I
NEUTRALIZER-^LJ
T
ELECTROSTATIC
CLASSIFIER
WITH ELECTRIC
AEROSOL DETECTOR
COLLISON
ATOMIZER
EVAPORATOR/CONDENSER
CONDITIONER
MASS FLOW
TRANSDUCER
FROM
FILTERED AIR
SUPPLY
FIG (3) EXPERIMENTAL SCHEMATIC OF THE CONTINUOUS FLOW COAGULATION SYSTEM
-------
97
horizontal coagulation tube through an inlet section. The continuous flow
coagulation tube is shown in more detail in Figure 4. The coagulation tube
is about 4.9 m long and is made of 6 inch (inside diameter) Plexiglas tubing.
The tube has two ports for isokinetic sampling probes, two ring electrodes
wrapped around the outside of the middle section, plus an electro-mechanical,
acoustical driver mounted at the end with the aerosol inlet section. The
acoustical driver is to generate a continuous wave of traveling sound waves
which travel down the coagulation tube and which are absorbed by a fiberglass
mat located at the far end. The acoustical driver is a James B. Lansing pro-
fessional series compression driver, Model 2440, fitted with a specially
machined, air tight, exponential horn which makes a smooth transition from
the 2 inch diameter of the driver throat to the inside diameter of the coagu-
lation tube. A thin membrane of plastic wrap is to be used to keep the
aerosol particles out of the driver. The acoustical driver is to be driven
by an audio power amplifier with an output up to 60 watts. A signal genera-
tor is to be used as the input to the amplifier. The ring electrodes, which
are spaced about 150 cm apart, are used to generate the axial electric field.
The a.c. electric power supply consists of an inhouse built high voltage
amplifier (0 to 6 kV) driven by the output from a signal generator over the
frequency range of about 30-400 Hz. Two moveable, isokinetic sampling probes
are used (one at a time) to extract a continuous sample from various points
across the diameter of the coagulation tube at the entrance and exit planes
of the test section. The distance between the probes is about 183 cm, which
is the length of the test section within the coagulation tube. The extracted
sample is diluted with clean air and passed through an electric aerosol size
analyzer to determine the particle size distribution. (More detail on the
probe design and the analyzer is provided below.) The aerosol is continu-
ously pumped through the coagulation tube and the pressure can be varied
about ± 5 inches of water using a vacuum pump. As mentioned earlier, this
feature of a continuous flow minimizes transient effects that have been known
to cause erroneous results in past studies that depended on time decay
measurements to determine collision rates.
The coagulation tube is a critical component for it is important that
the flow in the tube be uniform and relatively free of turbulence. As
shown in Figure 4, the flow enters the tube through the entry section which
introduces the flow uniformly and axially so as to minimize turbulent ef-
fects. The distance from the entry section to the inlet of the test section
(i.e., the location of the upstream isokinetic probe) is 60.96 cm and the
distance from the inlet to the outlet of the test section is 182.9 cm. For
a flow rate of 44 1pm and a tube diameter of 15.24 cm, the Reynolds number
is about 430. Assuming that the flow is totally uniform at the entrance of
the coagulation tube, the thickness of the laminar boundary layer at the
test section Inlet is estimated to be about 1.01 cm and at the outlet
the thickness is 4.04 cm. Therefore, the flow is not fully developed in
the test section and there should be a central core of nearly uniform axial
flow with a uniform speed slightly greater than the average speed given by
the flow rate divided by the cross-sectional area of the tube. (.Velocity
and turbulence measurements have been taken across the inlet and outlet
cross-sections using a hot film anemometer inserted through the sampling
tube ports and the results are to be discussed in a later paragraph.)
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98
AEROSOL -
DILUTED 1 10
AT 44
•ACOUSTICAL DRIVER
MASS FLOW
TRANSDUCER
TO a-c
ELECTRIC
POWER
SUPPLY
•AEROSOL ENTRY SECTION
FINE MESH SCREEN
HONEYCOMB STRUCTURE
MOVEABLE ISOKINETIC SAMPLING PROBE
PARTICLE
IVIEUTRALIZER
UNDILUTED AEROSOL
AT 0.4 ipm ( TO
ELECTRIC AEROSOL SIZE
ANALYZER OR STAGED
MEMBRANE FILTER SET)
6 in (ID) PLEXIGLASS TUBING
TO VACUUM
PUMP AT
~ 44 1pm
MOVEABLE ISOKINETIC PROBE
( IN RETRACTED POSITION)
•ACOUSTICAL ABSORBER
FIG (4) SCHEMATIC OF ENTRY SECTION AND COAGULATION TUBE
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99
The aerosol is to be sampled and analyzed at different positions
across the coagulation tube diameter at the inlet and outlet stations of the
test section. The samples are to be taken with the use of isokinetic probes
(inserted one at a time so as to minimize flow disturbances). The diameter
of the probes is 1.45 cm which gives a probe flow rate of 0.4 1pm at the
average flow velocity in the tube. A dilution system (shown in Figure 3) is
used to increase the total flow rate to the analyzer to a value of 4 1pm.
This corresponds to a concentration dilution of about 1:10.
The aerosol generating equipment is commercially available from
Thermo-Systems Incorporated of St. Paul, Minnesota, and is based on the sub-
micron aerosol generating system developed by Liu and associates at the
University of Minnesota. (The system is described and discussed by Liu and
Pui in Reference [10].) The system consists of an atomizer, an evaporator-
condenser conditioner, and electrostatic classifier with an electric aerosol
detector, and a neutralizer. The atomizer can generate a moderately mono-
disperse aerosol from non-volatile liquids (such as DOP) dissolved in ethyl
alcohol and from aqueous solutions such as methylene blue. Suspensions of
pre-sized latex spheres and of carbon (non-spherical) and iron oxide (sper-
ical) particles with a mean particle size in the range from 0.01 to about 0.3
ym may also be produced by mixing these particles in very pure demineralized
water, or in alcohol. (All of these particles are to be studied during the
coagulation tests to be described later.) The evaporator-condenser is in-
cluded to convert the moderately monodisperse aerosols produced by the atom-
izer from liquids (like DOP) into a more monodisperse suspension by an
evaporation process followed by a condensation process. (The evaporator-
condenser is not to be used-for the solid particle suspensions.) The elec-
trostatic classifier serves as the final stage to select particles, on the
basis of their electric mobility, to make up a truly monodisperse aerosol.
An electric aerosol detector is included so that the particle concentration
of the generated monodisperse aerosol may be determined when desired. (As
discussed later, the concentration measurements are required in order to
calibrate the electric aerosol analyzer used with the isokinetic sampling
probes.) A Kr-85 neutralizer is also shown in Figure 3 and is to be used
when a neutral aerosol is desired for the coagulation tests. Liu and Pui
[10] indicate that this system is capable of producing uniform particles from
0.01 to 0.5 ym at concentrations up to 106 cm~3. (The manufacturer indicates
that concentrations up to 107 are possible with DOP aerosols.) According to
Liu and Piu, the particle diameter of the aerosols generated by this system
may be determined to within an accuracy of about 2% and the concentration
to within about 5%. The relative standard deviation of the size distribution
for aerosols produced by this system ranges from about 0.04 to 0.08.
The particle size analyzer is an Electric Aerosol Size Analyzer (EAA),
Model 3030, manufactured by Thermo - Systems Incorporated and is an instrument
developed by Whitby and Liu at the University of Minnesota. The analyzer
first electrically charges the incoming aerosol particles and then sizes them
on the basis of their electrical mobility in the presence of an electrostatic
field. The performance of the analyzer is presented in a paper by Liu and
Pui [11]. The analyzer has been purchased with two additional voltage divid-
er chips: one to divide the 0.02 to 0.22 ym size range into ten arithmetic
-------
100
size intervals of 0.02 ym each, and one to divide the 0.20 to 0.40 ym size
range into ten arithmetic size intervals of 0.02 ym each. (The standard
voltage divider chip divides the 0.0032 to 1.0 ym size range into ten geo-
metrical size intervals that vary in size from 0.0024 ym to 0.438 ym.) This
modification is included in order to improve the size resolution of the in-
strument. Using the aerosol generating system described above, the EAA is
to be calibrated by connecting the analyzer directly to the output of the
aerosol generator. It is expected that through a careful calibration pro-
cedure, the size analyzer may be used to measure unknown concentrations in
the individual channels to within an accuracy of about 5%, which is the con-
centration accuracy of the submicron aerosol generating equipment discussed
above.
A staged membrane filter set is also to be used in conjunction with
the isokinetic sampling probe to extract and collect particle samples for
analysis of particle shape, size uniformity, and cluster configuration. The
membrane filters are to be arranged in order of decreasing pore size from
0.8 to 0.025 ym. Photomicrographs of the collected samples are to be used to
check the EAA results and to determine cluster configuration during the
coagulation studies using solid particles. (In addition, an attempt will be
made to borrow a portable diffusion battery and a condensation nuclei counter
during the investigation to provide an additional check on the EAA and on the
filter- photomicrograph data.)
C. Velocity Profiles and Turbulence in the Coagulation Tube
A hot film anemometer has been used to analyze the flow through the
coagulation tube. Filtered air without any particle matter was used during
the tests. The turbulence structure was initially unsatisfactory and the
flow was not uniform. Following the addition of a screen in the inlet sec-
tion and a honeycomb structure across the coagulation tube at the inlet
section, the flow was found to be uniform and the turbulence fairly low.
Velocity profiles at the upstream and downstream locations of the isokinetic
probes (i.e., at each end of the test section) are shown in Figure 5. The
profiles were taken using a probe mounted on a 90° support sting that could
be rotated and translated in order to survey the cross-sectional area of the
coagulation tube at each location. The probe sting passed through the iso-
kinetic probe port and "0" rings were used to ensure that there was no leak-
age from the coagulation tube. The probe used for the measurements was a
Disa fiber film probe, Model 55R11. The hot film was operated in the con-
stant temperature mode by a Disa anemometer, Model 55D01. Using the square
wave technique, the frequency response of the hot film anemometer was about
11 kHz at a mean flow speed of 4 cm/sec. The anemometer output was not
linearized and the system was calibrated using the fully developed flow in a
1.50 cm diameter tube with an L/D ratio of 80. The flow rate through the
calibration tube was measured using a Sierra Instruments mass flow transducer,
Model 500-3, which was calibrated to within an accuracy of 1%. A signal of
510 mV corresponded to a speed of 4 cm/sec. It was found that the system
could not resolve speeds less than about 0.8 cm/sec. The calibration curve
was repeatable to within 3% and the velocity profiles within 7%.
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3 E
VELOCITY PROFILES AT UPSTREAM PROBE PORT
VELOCITY PROFILES AT DOWNSTREAM PROBE PORT
FIG (5) VELOCITY PROFILES AT UPSTREAM AND
DOWNSTREAM ISOKINETIC PROBE PORTS.
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The velocity profiles in Figure 5 show the existence of a central
core where the flow is nearly uniform. The diameter of the core at the up-
stream port is about 9 cm and at the downstream port the diameter is about
6.5 cm. The profiles are stable and steady. Flow visualization tests with
smoke indicated that the streamlines are straight and that there is no rota-
tion of the flow.
Frequency correlated turbulence measurements were also taken using
a Disa analog correlator, Model 55D70, with a signal generator, an active
low-pass filter, and an rms voltmeter. The rms values of turbulence for the
0-11 kHz frequency range averaged about 3.8% of the mean flow speed values
over the core region of the flow within the coagulation tube. The greatest
turbulent intensity levels were detected in the lower frequency ranges. Some
of the intensity peaks could be correlated at specific wave numbers with
characteristic dimensions of the coagulation tube diameter and length and the
diameter and length of the straws used in the honeycomb structure inserted at
the entrance of the coagulation tube. The velocity profiles and turbulence
measurements indicate that the air flow in the central region of the coagula-
tion tube is relatively uniform and quiescent. This condition is desirable
in order to isolate the effects due to the external fields that are to be
investigated later. In this case, the effects of velocity gradients and
turbulence should be negligible.
D. Experimental Program
The remainder of the investigation is envisioned to be conducted in
three phases. The first phase involves using monodisperse DOF aerosols to
calibrate and check out the system. During this time, the wall losses due
to diffusion and gravitational settling are to be investigated using the
isokinetic probe sampling system. In addition, the thermal self-collision
rate is to be determined and checked with earlier work [3, 4]. It should be
noted that the results of this thermal self-collision work will extend the
range of the experimentally determined collision rates from a Knudsen number
of about 1 to 6. In addition, the effect of particle charge is to be exam-
ined by conducting tests without the nuetralizer that is located between the
aerosol generator and the mixing chamber.
The second phase involves determining the collision rate and growth
characteristics of DOP aerosols subjected to the external fields. The mag-
nitude of the a.c. electric field is to be varied from about 100 V/m to about
4kV/m and the frequency from about 30 to about 400 Hz. The power delivered
to the acoustical driver is to be varied from 0 to about 60 watts at fre-
quencies ranging from about 300 'to 10 Hz. The self-collision rates using
a single monodisperse DOP aerosol are to be determined for each field and for
the two fields together. It should be noted that regardless of which field
is applied, the number of collisions per unit time between particles of the
same size is expected to exceed the number due to thermal motion alone. This
increase is due to the superposition of the thermal motion and the motion in-
duced by either field. For the very fine particles (i.e., particles with
radii less than 0.1 ym) the thermal motion is relatively large. When both
fields are applied simultaneously, the number of collisions per unit time
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between particles of the same size exceeds the thermal rate because of the
differential motion induced by both fields. Of course, the thermal motion
will also contribute to the collision rate for the smaller particles in this
case. In addition to the use of mohodisperse suspensions, some simple bimodal
suspensions are also to be generated and studied. During these tests the
charge on the particles is to be varied by removing the neutralizer located
downstream of the aerosol generating subsystem and by inserting a corona
charging stage in the aerosol mixing chamber. In all of these tests, photo-
micrographs of particle samples are to be used to check the EAA results for
size uniformity. The samples are to be withdrawn through the isokinetic
probes and collected by the staged membrane filter set.
The third phase involves repeating the second phase for different
types of monodisperse particles. The different materials include latex, NaCl,
carbon, titanium oxide, and iron oxide. Photomicrography of samples withdrawn
through the probes is to be used to monitor particle size and shape, including
cluster configurations in the case of the solid particle tests.
IV. SUMMARY
An experimental investigation has been described that involves the use of
an a.c. electric field and a traveling wave sound field to increase the col-
lision rate between particles in an aerocolloidal suspension. In addition,
a simple free molecule regime theory for the collision rates due to each
field applied separately has been discussed. Experimentally determined
velocity profiles and average levels of turbulence in the continuous flow
coagulation tube have also been reported. The plan for completing the in-
vestigation has been outlined.
The investigation is a basic study of the collision rates between parti-
cles with radii less than about 0.3 urn suspended in air and subjected to
electric and acoustic fields. The results of the study should contribute to
a better understanding of particle collisions and growth characteristics
under the influence of electric and acoustic fields. In addition, it is an-
ticipated that the results will show that these fields can provide an effec-
tive means for preconditioning man-made particulate suspensions so that the
very fine particles may be removed in a subsequent and conventional particle
removal device.
ACKNOWLEDGMENTS
The author is appreciative of the work of P. H. Paul who made the hot
film anemometer measurements and L, W, Byrd who designed the inlet section
for the coagulation tube. This study is supported by a research grant from
the Solid and Particulate Processing Program of the National Science Founda-
tion.
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REFERENCES
[1] Mednikov, E. P., Acoustic Coagulation and Precipitation of Aerosols,
translated from Russian and published by Consultants Bureau, New York,
1965.
[2] Scott, D. S., J. of Sound and Vibration, Vol. 43, No. 4, p. 607, 1975.
[3] Hidy, G. M. and Brock, J. R., The Dynamics of Aerocolloidal Systems,
Pergamon Press, New York, 1970.
[4] Chatterjee, A., Kerker, M. and Cooke, D. D., J. Colloid and Interface
Sci., Vol. 53, No. 1, p. 71, 1975.
[5] Brock, J. R., and Hidy, G. M., J. Applied Physics, Vol. 36, No. 6,
p. 1857, 1965.
[6] Shaw, D. T. and Tu, K., "Acoustic Coagulation and Precipitation of
Aerosols," Paper No. 15d, Aerosol Science and Technology Symposium,
AIChE National Meeting, Atlantic City, New Jersey, August 29, 1976.
[7] Fuchs, N. A. and Sutugin, A. G., J. Colloid Sci., Vol. 20, p. 492, 1965.
«
[8] Scholz, P. D. and Barber, D. R., J. Colloid and Interface Sci., Vol. 46,
No. 2, p. 220, 1974.
[9] Wang Chang, C. S. and Uhlenbeck, G. E., "On the Propagation of Sound in
Monotomic Gases," Engineering Research Institute Report for Project
M999, University of Michigan, Ann Arbor, October, 1952.
[10] Liu, B. Y. H. and Pui, D. Y. H., J. Colloid and Interface Sci., Vo. 47,
No. 1, p. 155, 1974.
[11] Liu, B. Y. H. and Pui, D. Y. H., J. Aerosol Sci., Vol. 6, p. 249, 1975.
WRITTEN DISCUSSION
Andrew R. McFarland
Department of Civil Engineering
Texas A&M University
College Station, Texas 77843
The experimental research efforts proposed by Scholz should permit the
achievement of a new level of refinement in data quality. With reference to
Scholz's Figure 4, the apparatus which is to be employed is as good as the
state-of-the-art has to offer and the flow schematic seems to be well thought
out. It may be anticipated that a high quality aerosol will result through
uniformization of a Collision Atomizer spray with both an evaporator/
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105
condenser and an electrostatic classifier. It is planned to use a specially
equipped electrostatic aerosol analyzer which is fitted to divide the range of
0.2 - 0.4 micrometers into ten arithmetic intervals, however this resolution
may demand more of the instrument than it is capable of providing.
With respect to the analytical modeling which is presented, p. 4, the
effort to date has been directed towards the free molecular regime with
separate consideration of the electric and acoustic fields. The proposed
next step, that of modeling the effects of the combined fields, should provide
an interesting extension to the understanding of aerosol technology.
The predictions which have been made of the augmentation to thermal
collision rates effected by acoustic and electric effects (Figures 1 and 2)
show that for particles of 10 micrometers radius the expectation is that
the rates would be approximately doubled — at least for the operational
conditions which were examined. Considering^that it is necessary to have
on the order of 10 collisions to cause a 10 micrometer droplet growth to
one micrometer, a substantial augmentation is needed.
OPEN DISCUSSION
Scholz: I have a couple of comments to make regarding the taxation of the
electrostatic aerosol size analyses. I cannot say at this time what the
detection range would be. I had an impression that the system is capable
of producing aerosols with concentration up to 10& particles per cubic
centimeter. I do not know how good that value is in terms of different
sizes and different materials. At this point all I can say is that we are
expecting to get good results.
For the comment on the particle size under 100 ym not being terribly
important; I would like to indicate that I used the 100 ym particle size
as a demonstration to show the significance of the thermal motion. Our
approach is to try to understand the interaction of forces with the
particles and to try to get a good handle on the basic mechanism taking place
within the aerosol. It is from this point of view that we used the 100 ym
size particle as a sample.
Baril: We have some work going on in our laboratory which is comparable to
your study. I was wondering if you are familiar with Professor Massudu's
work on hammering particles with traveling waves. I gather that you are
going to use this electric field to enhance the traveling wave. We have
done extensive work in this area. If you would increase the number of rings
you could have a much better effect. In fact you can transport the particles
down the tube without the acoustical energy. You might even be able to take
the air energy out and just use the electric field. If you change the size
of the rings as you go down, it will make the particles come to the
center of the system.
Scholz: I am not familiar with any of this. However, I am very much
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interested in learning more about your research in this area.
Helfritch: Paul, is the inverse force law the right one to use between a
charged particle and a non-charged gas molecule?
Scholz: I do not exactly know the answer. In kinetic theory, the inverse
power force law is the usual one and seems to give qualitative effects, if
you are interested in qualitative interaction. I used it basically as a
standard model. I do not know how bad it is in terms of a neutral versus
a charged particle interactionalwhether it is second order or primary, I
do not know.
Reif: Back in the 1940's, Pohl did quite a bit of work with sonic dispersion
of fog. He reportedly had a system that worked on ships which would
disperse fog up to 300 meters. Are you familiar with this work and if so,
whatever became of it.
Scholz: I am not familiar with the work, but there may be some people here
who are.
Baril: I might add something to that, Bob. There was also some work done
at Orly airport by Dr. Bouche with sirens. If I remember correctly, this
all fell by the wayside. It turned out that the amount of energy and money
put into these systems was astronomical.
Shaw: I can comment on that. It was proven that if you use seeding techniques
it is much more economical for that particular application than acoustic
sirens. Some redesign work was done by Calspan Corporation.
Liu: I am a little mystified with the use of the Debye length for this
calculation. It would seem to me that it is applicable primarily to the
plasmas and colloidial suspensions where the bipolar ions are around the
particles. When you are dealing with a gaseous medium, how does the Debye
length come into play?
Scholz: It is my understanding that as long as the Debye length is greater
than the mean distance between the particles, then the Debye particle theory
may be applied. The other necessary condition is that the energy in the
electrostatic field of the particle at one Debye length must be less than
the thermal energy that the particle has. Both of these conditions are
satisfied in our application.
In terms of polarization effects and so forth, I don't know. I am only
saying that it is a model. In terms of kinetic theory, the coulomb inter-
particle force law has an infinite cross section. In order to treat the
system you have to cut it off at some point. Kinetic theory is based on
two-particle interaction. It is necessary to cut off the collision inter-
action length at some point. We used the Debye length.
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Liu: The Debye length depends on the concentration of the charge carriers
and the particles.
Scholz: The concentration, dielectric strength, the thermal characteristics
are also important.
Hidy and Brock have a section in their book on the application of the
Debye Potential Theory in aerosols and have used it.
Cooper: In the experimental set-up, the plexiglass, unless you are careful,
is going to have .areas of localized charge. When you apply the voltage on
the rings, you will get stress charging which can create more problems. The
use of isokinetic samplers can also cause problems since it is crucial at
which position they should be isokinetic. The right angle bends are going
to cause some difficulties. Precisely when the particle relaxation time
times its velocity is comparable to the dimension of the sampler. Those
bends look as though they are going to give substantial impaction on the
inside surface.
Scholz: In reference to the plexiglass, at this time we have only used low
field intensities, and we have not noticed any localized effects. It is
possible that changing to higher intensity might cause some problems.
Cooper: You may notice localized areas of intensities because the fields may
be as high as a thousand volts per centimeter.
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ENGINEERING ASPECTS OF DRY MAGNETIC SEPARATION
R. R. Oder*
Bechtel Corporation, San Francisco, CA
ABSTRACT
Engineering aspects of batch-type and continuously operating
magnetic gas-particulate separation methods are compared. The comparison
is made in conceptual applications to desulfurization of dry fine-
pulverized pretreated coal. With pretreatment, it is claimed that
magnetic separability of mineral sulfur forms occurring in coal can be
considerably enhanced so that a variety of magnetic methods could be
expected to be applicable. In processing fine pulverized coal, batch-
operated HGMS-type separators offer the advantage of efficient desul-
furization of high coal-value recovery but can incur practical difficulties
associated with packed bed operation. By contrast, continuously operating
smooth-bore magnetic separators have the advantages and simplicity of
continuous operation but are not as effective in desulfurization in high
gas-velocity applications. Trade-offs between separator performance and
total process requirements will be very important in future developments
of magnetic methods for gas-particulate separations. New applications
of magnetism can be anticipated in process industries employing entrained
and fluidized bed operations.
INTRODUCTION
Two widely different approaches have been considered for
development in the application of magnetic methods to gas-particulate
separations. Basically, these two approaches provide the researcher
with a choice between batch-operated magnetic filtration (modified
HGMS-type separators) or continuously operating smooth-bore beam-splitting-
type devices (e.g., multipole magnetic separators). These magnetic
Presently with Gulf Research & Development Company, Pittsburgh, Pennsylvania
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separators can be expected to have widely different capabilities in gas-
particle separations and to exhibit significantly different process
design requirements in high velocity and high throughput applications to
gas clean-up.
The batch-type magnetic separator envisaged for gas-particulate
separation is similar to the HGMS-type separator recently developed for
wet minerals beneficiation. An example of the HGMS-type separator is
illustrated in Figure 1[1]. This device utilizes flow through a relatively
open and porous magnetized bed which is composed of filimentary strands
of strongly magnetic material such as ferritic stainless steel wool,
wire mesh, or expanded metal lath. The magnetized bed elements provide
a volume distribution of very short range but intense magnetic capture
forces which can be made specific to particle size. These batch-operated
devices have proven commercially effective in wet separations of feebly
paramagnetic minerals with size distributions occurring into the submicron
range [2]. —
CLEANEO
j [^EFFLUENT
MAGNETIC PARTICLES-
Figure 1. VERTICAL SECTION THROUGH CENTER Figure 2. QUADRUPLE MAGNETIC SEPARATOR
OF HIGH GRADIENT MAGNETIC (Re(- BO)
SEPARATOR
The second approach to dry magnetic separation of gas-entrained
particulates employs continuously operating smooth-bore magnetic beam-
splitting devices utilizing multipole magnets of the type presently
employed in particle physics applications [3,4], These devices, which
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have not yet been applied outside of the laboratory, offer the potential
simplicity of continuous smooth-bore operation. The magnetic forces
they generate, however, are weaker and less specific to fine-size
materials and they are inherently power-intensive devices. An example
of a quadrupole magnetic separator is shown in Figure 2[3].
HGMS-type structures are probably closer to commercial develop-
ment in dry handling of fine pulverized materials than are continuously
operated magnetic devices. The latter type separators, however, are
deserving of consideration because of several significant technical
reasons. They require no gas flow interruption, are relatively immune
to abrasive wear, and are low-pressure drop devices. In gas-particle
transport, entraining velocities can be expected to be very high because
of the large density difference between the suspended particles and the
fluidizing gas. Even though modified HGMS-type devices are superior to
multipole structures from the standpoint of fine particulates removal,
they may be limited in application because of severe problems of flow
distribution and pressure drop associated with high velocity flow
through the magnetized filter bed. In applications of 50 000 to 100 000 cfm
flows, pressure drops of a few hundred inches of water can result in
drive-fan power requirements in excess of megawatts and may well over-
shadow magnet capital and operating costs. Multipole magnet structures
appear to offer considerable potential for control of entrained and
fluidized bed reactor operations.
The understanding of two-phase flow (solids and entraining
fluid) through consolidated porous media such as HGMS-type packed filter
beds is incomplete at present. Each potential application must be
treated individually with due consideration for the magnetic particle
density, mean size, and size distribution and for the size and shape of
individual magnetic filter bed elements. Preliminary engineering
evaluations indicate that quadrupole-type smooth-bore magnetic sepa-
rators may possibly compete with batch-operated filter-type magnetic
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Ill
separators in high velocity separations of strongly magnetic micron-size
materials. To be competitive in separations applications, however, it
is believed that multipole structures will have to be developed with
pole-face field strengths of the order of 100 000 Oe at one foot pole-
opening.
A comparison of these magnetic separation methods will be
given in a coriceptual application to the removal of dry mineral sulfur
from fine-pulverized coal.
MAGNETIC DESULFURIZATION OF DRY COAL
Coal has traditionally been cleaned to control mineral matter
content. Cleaning for sulfur control is now becoming increasingly
important because of governmental regulations of sulfur oxide emission
in coal combustion flue gases.
Sulfur occurs in coal both as a discrete mineral form (pre-
dominately iron disulfide [FeS2]) and as "organic sulfur" which is
believed to be chemically bound to the coal structure. Because of the
need to remove both sulfur forms, chemical methods are being developed
to augment more conventional physical cleaning methods which are generally
restricted to removal of coarse (>35 mesh) mineral sulfur only. Recent
studies have shown, however, that chemical cleaning per se can be
expected to be expensive [5]. For this reason, there is renewed emphasis
on upgrading state-of-the-art physical cleaning technologies and on
developing new approaches to combined physical and chemical cleaning
which have the capability of preparing a moderate cost clean fuel
without requiring coal conversion.
Coal washability studies [6]- have shown that mineral sulfur
occurs in U.S. coals in a great variety of particle sizes. Most of the
important Eastern utility grade coals have significant amounts of mineral
sulfur with particle sizes occurring down into the micron range. Any
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comprehensive new coal cleaning method developed for these coals, then,
will of necessity be required to process coals which have been pulverized
to sizes generally finer than 100 mesh (75 microns). There is no such
commercial coal cleaning method in use today.
Laboratory investigations have demonstrated the scientific and
technical feasibility for magnetic desulfurization of coal [7]. Those
investigations show that the mineral sulfur occurring in coal is para-
magnetic and is magnetically separable from mineral-free coal which is
diamagnetic. That work was not pursued, however, because the laboratory
magnetic separators employed could not be adapted economically to commercial-
size applications. With the recent development of HGMS-type devices,
however, there is now a magnetic separation method available for adaptation
to desulfurization of coal with the potential for economical removal of
micron-size mineral contaminants in commercial-scale installations.
Because of this potential, the concept of magnetic desul-
furization of coal is again being pursued in several laboratory investi-
gations. The results reported here were derived in an engineering
evaluation of dry magnetic methods for desulfurization of pneumatically
conveyed fine-pulverized coals.
The most direct use of magnetic methods in dry coal desul-
furization would be in power plants which burn fine-pulverized coal.
The magnetic separator would be "engineered" to fit in line between the
pulverizer and the coal burner inlet. A preliminary evaluation of this
concept [8], however, has indicated that the use of magnetic separators
in this application may not be practicable because of severe limitations
imposed by the feeble paramagnetism of the mineral sulfur and the use of
very high carrier gas velocities (5 000 to 6 000 ft/min). Estimates
indicate that capital and operating costs for this direct approach to
magnetic desulfurization may be similar to those of stack gas scrubbing.
These costs are largely associated with the high magnetic field strength
required to capture paramagnetic minerals and with power requirements
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for auxiliary equipment. Because of this unfavorable cost estimate, the
engineering evaluation presented here was restricted to the use of
magnetic separators in the processing of dry coals which have been
pretreated to make the mineral matter strongly magnetic.
MAGNEX PROCESS
Several methods have been proposed for converting mineral
matter in coal to strongly magnetic forms [9]. One new method is the
Magnex process which exposes dry pulverized coal to gaseous iron
pentacrbonyl [FeCCO)-] under conditions whereby the impurity mineral
matter becomes strongly magnetic and can be separated from the coal by
magnetic separation methods [10].
Under the Magnex reaction conditions, gaseous iron pentacarbonyl
is substantially decomposed into free iron and gaseous carbon monoxide.
It is claimed that the liberated iron reacts with the surface of the
pyrite mineral to form a strongly ferrimagnetic pyrrhotite-like sulfurous
mineral. The reaction stoichiometry is assumed to be:
17°-200°C> l-FeS • (^FeS] + 5xCO (1)
FeS2 + xFe(CO)5 -> [(l-y)FeS2] •
iron iron "iron-rich disulfide" free
d-isulfide pentacarbonyl carbon
(pyrite) monoxide
If x moles of iron pentacarbonyl react with 1 mole of granular
and completely liberated iron pyrite, it is assumed that a fraction of
y moles of the outside of the pyrite grain react to give y-Hx moles of an
iron-sulfide of sulfur to iron atomic ratio of 2y/(y4x). If one demands
that this ratio correspond to that of magnetic monoclinic pyrrhotite
(^Fe S ), then the mole fraction y is determined by the mole quantity of
7 8
iron pentacarbonyl used, x (see iron-sulfide phase diagram[ll]). This
relationship is shown in Table I where percent conversion of iron pyrite
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to monoclinic pyrrhotite is shown for a coal containing about 1.2 wt.%
pyritic sulfur and where, for simplicity, it has been assumed that free
iron is deposited only on the surface of the iron pyrites.
In the magnex process, it is conjectured that some of the free
iron also coats the non-sulfurous mineral matter, making it amenable to
magnetic separation as well. For this case, the conversions shown in
Table I are upper limits expected for reactions with pyrites only. It
is claimed that free iron does not adhere to the mineral-free coal
structure.
Laboratory quality monoclinic pyrrhotite (Fe^.) is strongly
ferrimagnetic with a spontaneous magnetic moment of 13 emu/gm at 200°C
which saturates at =4 kOe [1]. This moment is about 14 000 times
greater than the paramagnetic moment of pure pyrite at 200°C in a 4 kOe
magnetic field. Indeed, only about 0.7% of the pyrite need be converted
for the magnetic moment of the "iron-rich disulfide" to predominate that
of the converted mineral. The resulting impurity saturation magnetic
moment calculated for assumed selected reaction of iron pentacarbonyl
with iron-disulfide is shown in Table I. Magnex has recommended 32 Ib
iron pentacarbonyl per ton coal for most Eastern coals, so that with
this concentration, one can, under ideal conditions, anticipate a
magnetic moment of 8 emu/gm which saturates in a 4 kOe magnetic field.
TABLE I
"Ideal" Pyrite Conversion by Iron-Pentacarbonyl
for a Coal Containing 1.2 wt.% Pyritic Sulfur — Dry Basis
Iron Pentacarbonyl Calculated Calculated
Concentration % Conversion Magnetic Moment
Ib Fe(CO),/T drv coal J^F^S^ (emu/gm) @200°C
0 0 o
20 36 5
32 59 8
40 73 9
55 100 13
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IRON CARBONYL PROCESS
A process flow diagram for an iron-pentacarbonyl process is
shown in Figure 3. This layout is similar to the Magnex process [10]
except that here fine-pulverized coal is specifically employed and iron
pentacarbonyl is consumed. In this process layout, carbon monoxide
liberated in the reaction is flared to produce carbon dioxide which, in
turn, is used for penumatic transport purposes. In a commercial process,
the liberated carbon monoxide might be passed through an iron reservoir
for generation of process iron pentacarbonyl. In this manner, iron
rather than iron pentacarbonyl would be consumed. It is to be emphasized
that the hypothetical process described here differs from the Magnex
process primarily in that the coal is fine-pulverized. When coarser
coals are treated with the Magnex process, pneumatic transport is not
necessary and simple magnetic separators adaptable from other areas of
minerals beneficiation can be employed [13]. The hypothetical process
given here for desulfurizing fine-pulverized coals is chosen specifi-
cally to illustrate the comparison of advanced magnetic separation
systems such as might be developed for handling gas particulate sepa-
rations in applications where the particle grain size is smaller than
that presently considered practicable for physical coal beneficiation.
These applications are significant and can include steel furnace off-gas
cleanup as well as coal desulfurization. Extensions of the principles
illustrated here may also find applications in electromagnetic control
of entrained and fluidized bed reactors.
The fine-pulverized coal is pneumatically conveyed to the
cyclone collector as shown in the upper left corner of Figure 3[5],
where it is uncoupled from the gas and dropped through a lock hopper
into a fluidized bed reactor where the coal is reacted with gaseous iron
pentacarbonyl. The dry treated coal is passed through a second lock
hopper at the bottom of the reactor and is pneumatically conveyed to a
magnetic separator where clean coal and magnetic refuse are separated.
Lock hoppering is required because of the toxic properties of iron
pentacarbonyl and carbon monoxide.
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PULVERIZED
COAL
FLUE GAS TO COAL PREPARATION
Figure 3 IRON CARBONYL PROCESS
FLOW DIAGRAM
CO BURNER
Pulverized coal is pneumatically conveyed in this process with
inert flue gas which is primarily carbon dioxide. Under normal operation
conditions, between 2 and 5 Ib O>2 gas would be used to convey the coal
at velocities betwen 2 000 and 5 000 ft/min. The use of these conditions
would be prohibitive in the application of almost any type magnetic
separator because: (a) very high velocity applications will require
excessive pressure drop if a magnetized bed concept is employed, and
(b) dilute solids suspensions will require large magnetized cross sections
in order to be able to handle the full process flow. In order to circumvent
these difficulties, a cyclone separator and bypass fans have been used
to uncouple the coal dust from the main volume of entraining gas flow [8].
For the examples to follow, pulverized coal is conceptually blown through
the magnetic separator at velocities ranging between the normal settling
velocities for the dust and transport velocities of 5 000 to 6 000 ft/min.
Gas loadings of four pounds of coal per pound of carbon dioxide are
employed. The use of these conditions allows considerable latitude in
the choice of magnet size.
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Conceptual applications of batch-operated and smooth-bore
magnetic separators have been compared for coals pretreated by the iron
pentacarbonyl process described above. In the comparison, a high volatile A.
bituminous Pittsburgh Seam coal containing 1.2 wt.% iron pyrite was the
coal (100 mesh top size and 50 micron mean particle size) that had been
pulverized to 70 to 80% finer than 200 mesh (power plant grind). With
this grind, mineral sulfur is assumed virtually completely liberated for
this coal.
A. Batch-Operated HGMS-Type Separators
Magnetic capture of pneumatrically conveyed fine-pulverized
"iron-rich disulfide" in HGMS-type separators has been calculated with
the use of a mathematical model adopted from that developed to describe
the capture of paramagnetic minerals from highly dispersed wet-minerals
processes (kaolin clay) [14]. The model has been modified for this
application to describe the capture of strongly magnetic materials from
turbulent flows. The fractional removal of "liberated" magnetic particles
is described by Equation (2) where C_ is the ingress and C is the egress
concentration of these materials.
Kl , 1
R =
1-e" CNR/24 (1+K2)
y(d/D)2e (1-e )L
9nv
NR is the Reynolds number defined by,
N =f (4)
K. n
and C is the drag coefficient defined by the relationship,
drag force = l/8p Cird2v2. (5)
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118
At Reynolds numbers generally below 1/2, the drag force is given by
Stokes Law,
Stokes Law drag force = Sirridv. (6)
The term KZ of Equation (2) is the ratio of particle weight to
viscous drag,
(7)
and is generally unimportant for the flow conditions considered here.
The mathematical modeling of Equations (2)-(7) is for "ideal"
unloaded filters and infers that retention in the magnetic filter is
dominated by forces which hold the magnetic particle on the capture
element. Capture on "loaded" filter elements can be handled empirically
by a variety of methods including adjustments to the capture surface
strand diameter D. In the above equations, M (gauss) is the magnetic
capture surface magnetization, p is the magnetic particle density
(gm/cc), y is the "iron-rich disulfide" magnetic moment (emu/gin), d is
the magnetic particle diameter (cm), D is the capture surface strand
element diameter (cm), e is the bed fractional void volume, L is the bed
depth (cm), n is the fluid viscosity (poise), and v is the superficial
velocity through the filter bed reckoned for 100% void volume, pf is
the fluid density (gm/cc), and g is the acceleration due to gravity
2
(cm/sec ). The fluidizing gas is carbon dioxide at 200°C where p. =
-3 -It.
1.13 x 10 gm/cc and r\ = 2.27 x 10 poise.
The magnetized bed configuration visualized for the conceptual
application discussed here is a three-dimensional diamond-shaped structure
composed basically of one-inch segments of 1 mm diameter No. 430 or
No. 446 ferritic stainless steel. This material and its three-dimensional
structure and element size have been chosen on the basis of commercial
availability (demister tower packing), corrosive resistance under process
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119
conditions, immunity to abrasive wear, and effective magnetic removal of
mean-size magnetic particles [8]. Coals pulverized to power plant
fineness contain particles ranging downward from about 150 microns with
a mean particle size of 50 microns (see Table II). A choice of thinner
capture surface material, such as, for example, 100 micron diameter
steel wool, would have provided for better removal of the finest size
components of the coal dust, but its use would have involved more serious
abrasive wear and would have led to increased pressure drop (in excess
of 100 inches of water).
Because of the strong ferrimagnetism of the "iron-rich disulfide,"
a magnetic field of only 2 kOe has been employed. The magnetic field
strength dependence of both the capture surface magnetization and of the
calculated mineral magnetization are shown schematically in Figure 4.
SATURATION .
MAGNETIC
MOVEMENT
IEMU/GM)
V s 430 STAINLESS STEEL
12 ' EXPANDED METAL
0 2 « 6
Figure 4 MAGNETIC PROPERTIES OF STAINLESS STEEL, AND Fe7S8
The ideal capture model of Equation (2) predicts virtually
100% removal of the coarse "iron-rich disulfide" particulates occurring
in coals pulverized to power plant fineness. The broad performance
characteristic inferred for HGMS removal of "iron-rich disulfide" from
pulverized coal is shown in Table II.
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120
TABLE II
Calculated HGMS Removal of 7 MJ emu/gm
"Iron-Rich Disulfide" from Pulverized Coal
Particle
Diameter
(U)
150
100
50
10
3
1
Particle
Size
Distribution
(% Finer Than)
100
83
50
18
7
3
Mineral Matter Removed
Velocity (ft/min)
200 1600
100
100
100
100
100
99
100
100
100
100
99
45
On the basis of this simple model, it is theorized that HGMS-
type filtration can achieve efficient removals of strongly magnetic
mineral contaminants even at velocities which are large fractions of the
entrained gas velocity. The calculated capture for fine particle components
less than five micron diameter are shown in Figure 5, where it can be
seen that 100% removal is predicted for particulates down to 3 micron
diameter at flow velocities up to 1600 ft/min. Ninety-five percent
removal of iron-rich particulates down to 4 micron diameter is predicted
for flows of 6600 ft/min.
3 2
PARTICLE DIAMETER (MICRONS)
Figure 5 CALCULATED HGMS CAPTURE OF Fe?Sg
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121
The calculated capabilities discussed above suggest that HGMS
has the potential for 90+% removal of mineral sulfur from fine-pulverized
iron carbonyl treated coals in direct in-line application between the
pulverizer and the burner. It is estimated that the HGMS-type separator
can achieve this mineral sulfur removal for the reference coal with a
95+% recovery of mineral-free coal. This type process performance is
not inconsistent with the experience of HGMS wet minerals beneficiation
in the clay industry. Estimated add-on cost for desulfurization alone
with the iron-pentacarbonyl process described here (including batch-
operated HGMS separators) is less than $7.00 per ton of clean coal and
compares favorably with current costs for high level commercial physical
coal cleaning [15]. Conventional utility coal washing, however, is
generally limited to the processing of coals coarser than 35 mesh.
1. Magnet Structure
The pillbox-type HGMS magnetic structure shown in Figure 1 is
not well suited to high velocity processing. The top and bottom of the
HGMS structure of Figure 1 form the north and south poles of the magnet
and are solid iron pole pieces. In the wet application, slurries enter
and exit the magnetized bed through a multiplicity of small diameter
holes bored through the pole pieces. The entrance and exit pipes also
promote flow distribution in the relatively dense mineral slurry.
Complete utilization of the entire magnetized volume is essential to the
economic viability of the HGMS concept, so that good flow distribution
and low pressure drop are essential features of the design of entrance
and exit to the magnetized bed.
It is believed that neither good flow distribution nor low
pressure drop can be achieved with the magnet structure shown in Figure 1
in high velocity gas-particulate separations. Use of an iron-clad
Bitter solenoid [16] with poles removed or of a cross-flow H-frame
structure recently proposed by Allen [17] would provide better means for
flow distribution and pressure drop control than does the pillbox magnet
structure of Figure 1. The Bitter-type magnet, with iron poles removed,
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122
would allow for ease of access to the magnetized volume but would have
marked disadvantages of requiring higher currents to achieve fields
equal to those developed by the magnet structure of Figure 1 and would
have the added occupational hazard of large dipole fringing fields
outside the magnet in the working environment.
The magnet conceptualized for the gas-particulate separator is
designed around the H-frame magnet structure proposed by Allen and shown
in Figure 6. This magnet is essentially identical to the pillbox magnet
of Figure 1, except that the iron on two of the outside edges has been
removed, and the energizing coils have been separated in the center and
folded up in a saddle shape so as to permit transverse access to the
magnetized volume. Performance characteristics for this type magnet are
essentially identical to those of the Bitter-type magnet structure, but
dipolar fringing fields are low.
SADDLE SHAPED
UPPER COIL
TRANSVERSE FLOW
SADDLE SHAPED
LOWER COIL
FigureG H-FRAME MAGNET
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123
The H-frame magnetic separator is different from the pillbox
structure in another important aspect. In the pillbox structure, both
filtration flow and flushing flow occur along the same length which is
parallel to the magnetic field direction in Figure 1. Flushing can be
difficult when the pillbox separator employs a deep filter bed. The
Allen patent teaches filtration flow along the long pole length trans-
verse to the short pole gap opening and flushing flow, with the magnet
de-energized, along the short length parallel to the pole opening. In
this fashion, flushing time is expected to be less, for equal bed volume,
than for the pillbox separator. Reduction of dead time in filter cleaning
is important in improving the batch-operated filter on-line time.
With the use of transverse flow the relative orientation of
magnetized capture surface element, magnetic field direction, and flow
velocity is different from that employed with the pillbox magnet structure.
It is believed, however, that the magnetic capture surface designed for
this study permits good capture of strongly magnetic materials at low
pressure drop in high velocity turbulent flow applications.
2. Process Characteristics
Magnet size to handle process capacity is determined with use
of Eq. (8).
C = 0.72 S EvAe (8)
c
In Eq. (8), C is the process capacity (8 000 tons coal per day), S is
the pounds of entrained coal per cubic foot of flue gas (0.28), E is the
on-line factor for the batch-operated magnet, v is the flow velocity in
the filter bed (ft/min), A is the cross-sectional area of the filter bed
2
(ft ) for a magnet operating with on-line factor E, and e is the filter
fractional void volume.
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124
8 7
FRACTIONAL VOID VOLUME, e. V = 1600 FT/MIN
Figure 7 EFFECT OF BED PACKING DENSITY ON MAGNET SIZE AND PRESSURE
DROP AT 1600 FT/MIN. GAS FLOW
Calculations of magnet size requirements (magnetized cross-
sectional area) and pressure drop across the filter bed are shown in
Figures 7 and 8 as functions of the bed fractional void volume and
velocity of flow through the bed, respectively. The calculation of
magnet size involves use of the on-line factor shown in Figures 7 and 8.
Details of on-line factor calculations can be found in the literature [18],
As is apparent in the figures, significant reductions in magnet size
requirements can be achieved with use of densely packed filter beds, but
only at the expense of high pressure drop. By way of example, a single
2 kOe HGMS-type magnetic separator operating at 60% on-line factor could
handle the entire 8 000-ton-per-day process stream with an 8.3-ft deep
2
bed of 45 ft cross-sectional area when packed to 90% void volume. The
device would develop a calculated pressure drop of 138-inch water gauge.
At a flow rate of 39 000 cfm, the system would require a 1400 hp auxiliary
fan of 1.05 megawatts peak power consumption. This power requirement is
large compared with the 140 kilowatts estimated for magnet operation.
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125
High velocity gas entrained flow of particulates through
porous media is a very complex phenomenon, and magnetic capture in such
beds will probably have to be studied and understood on an individual
basis. The calculations presented illustrate the important point that
the magnetic separator cannot be isolated from the overall process for
planning and estimating purposes because choice of auxiliary process
operations and conditions may well be more important than magnet optimization.
NORMALIZED MAGNETIZED CROSS SECTION
200
400
1400
1600
1800
600 800 1000 1200
FLOW VELOCITY (FT/MIN I,C = 09
F,9Ure 8 EFFECT OF FLOW VELOCITY ON MAGNET SIZE AND PRESSURE DROP
AT 90% VOID VOLUME
2000
While the example presented is a severe one, it is encouraging to know
that the use of HGMS hypothesized for this dry application, even with
its expensive ancillary cyclones and fans, still appears to be economical
in the coal processing context. Estimates of the costs of HGMS coal
cleaning with the Magnex process [5] indicate that magnetic separation
will be a small component of the capital and operating costs, and that
this type of coal cleaning can be expected to be accomplished at add-on
fuel costs for desulfurization of about $7.00 per ton of clean coal
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126
produced. This estimated cost is competitive with that known for stack
gas desulfurization and with those estimated for more elaborate chemical
cleaning and coal conversion technologies.
B. Quadrupole Magnetic Separators
Quadrupole magnets are structures which produce a uniform
magnetic field gradient throughout a relatively large working volume.
Figure 9 shows a cross-section transverse to the length of a quadrupole
magnet. The magnetic field lines emerging from the north poles are
deflected to the south poles in such a manner that the lines of isofield
intensity are concentric circles about the symmetry axis [19]. The
magnetic field strength is zero on the axis and increases linearly to a
maximum value on the pole tip which is typically 10 to 12 kOe for modern
iron-return electromagnet quatrupole structures.
MAOMTK OMTOMHT f KIT
Figurt 9 CROSS SECTION "A-A" OF FIGURE 10
WITHOUT BEAM SPLITTER
Figur* 10 CROSS SECTION "B B" OF FIGURE 9
INCLUDING BEAM SPLITTER
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127
Figure 10 shows a vertical section through the symmetry axis
of a quadrupole structure in which a "beam-splitter" pipe has been
inserted for the purpose of separating the clean coal product from the
magnetic fraction. The coal enters the quadrupole field region in the
annular space of inner and outer radii R... and R.,, respectively. The
inside radius R is the minimum distance from the symmetry axis for
which the magnetic field strength is sufficient to saturate the magne-
tization of the "iron-rich disulfide."
Hs ' Ho
The outer radius, R,,, is chosen so that N magnets with annular entrance
2 2
areas, ir(R^ - R ), can handle the entire process flow
C(lb/min) =
where W is the carbon dioxide to coal ratio, PC02 is the C02 gas density
(lbs/ft3), and V is the gas flow velocity (ft/min).
In flowing through the quadrupole field region, the magnetic
component will be deflected radially outward toward the magnetic poles
and will exit through a take-off pipe on the outside diameter of the
entrance pipe. A small fraction of the clean coal inadvertently exists
with the magnetic component, but the major portion of the clean coal
emerges from the pipe of radius R at the bottom of the quadrupole beam
splitter.
The equation of motion along the vertical axis is
= v + PfS (11)
dt V0 + 9n U '
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128
where p is the particle density, r is the particle radius, g is the
acceleration due to gravity, and n is the fluid viscosity. Buoyancy
forces have been neglected because of the great difference between
particle and gas densities. Stokes Law has been assumed for the viscous
drag force, and particle-particle interactions have been ignored. The
residence time, T, in the magnet structure of length L is given by the
relationship
9n
With similar assumptions, the radial component of the equation
of motion is given in Eq. (13), and the time-dependent radial motion is
given in Eq. (14) where the particle is assumed to have entered the
field region on initial radius
with zero radial velocity.
3H •
V = % 3R - 67rnrR
R(t) = R + - (e -a - i) + -* t
a
yH
In Eq. (14), Y = — where y is the "iron-rich disulfide" ferrimagnetic
moment,
and H(R) is equal to H R/FL^.
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129
Assuming that all particles which strike the inner wall at
radius R^ are captured, the equation of motion, Eq. (14), and the residence
time, T, can be combined to implicitly define a quadrupole-structure
length, L (r.p.V.), such that all particles of radius greater than r
will be collected where p is the percent of such particles in the annular
region between R. and R^.
KT
Vr*p'V =
where
W
K = 5- (17)
^CO11' (VV *
and
V
In these equations, T is the solution to the equation f(T) = 0 where
2
f (T) = e'aT + aT - a[ ^- {l-/l-[l-(Hs/Ho)2]P/100} + I/a] (19)
The appropriate parameters are: pCQ = 1.129 x 10 gm/cc, nco = 229 x 10
poise, y = 6.9 emu/gm, HQ = 10 000 Oe, W * .25 Ib C02/lb coal, and p ^4.5 gm/cc
for "iron-rich disulfide."
Figure 11 shows the calculated length requirements for a
quadrupole magnetic separator which achieves 100% removal of 10 micron
diameter Fe,SQ from pneumatically conveyed pulverized coal. Performance
7 8
at this level is equivalent to 90% removal of all mineral sulfur from
the coal.
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130
It can be seen that a quadrupole structure is not as efficient
in removal of fine-sized particles as is HGMS. This is basically because
of the low magnetic field gradient, 656 gauss/cm, produced by the
quadrupole. Unfortunately, the quadrupole magnet requires full field
strength operation to produce this field gradient even when processing
strongly magnetic materials. With use of the above idealized equations,
it can be seen that a quadrupole of about 16.5 ft length by 1 ft diameter
opening will be required for 100% removal of 20-micron diameter magnetic
particulates from a 8000 ton coal per day process stream flowing at
velocities of 1600 ft/min.
1000
100% REMOVAL OF PYRITES TO 10/J
PIPE DIAMETER= 1 FT
01
100
200
600
300 400 500
FLOW VELOCITY (FT/MIN )
Figure 11 QUADRUPLE LENGTH REQUIREMENTS
700
800
It is interesting to note the significant reduction in quadru-
pole lengths that could be achieved if increased field strengths could
be utilized. With pole tip fields of the order of 100 kOe, quadrupole
magnetic separators would begin to be competitive with HGMS structures
in this application.
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131
1. Magnetic Structures
Quadrupole magnet costs and power consumptions have been
calculated from design bases developed at the Lawrence Berkeley Labo-
ratory [20]. It is of interest to note that there is a power/field-
gradient trade-off implicit in the design of quadrupole magnet structures.
In Figure 9, the surface shape of the "ideal" pole for the
quadrupole structure is shown by the dotted lines. To make the "real"
quadrupole, shown by the solid line, the iron of the "ideal" pole pieces
has been shaved back in order to make room for the electromagnet windings.
The windings are shown as circles in the figure. Because of the relatively
small space made available in this shaving operation, the conductor
cross-section per winding is limited so that quadrupoles are inherently
power-consuming devices.
A conservative design basis has been chosen whereby moderate
pole piece field strengths can be developed without excessive power
requirements (Modification III, pages 8 and 9, Reference [20]).
2. Process Characteristics
It has been noted that smooth bore magnetic separators will be
inherently low pressure drop devices. By way of comparison, it can be
calculated that the power requirement of all fans to supply the quadrupole
magnet installation to handle 8000 T/D coal is only about 100 hp as com-
pared with 1400 hp for HGMS desulfurization. While quadrupole particulate
removal performance may not equal that of the batch-operated device, it
well could prove to be the case that ancillary process requirements
might dictate cost/benefit-type compromise in determining overall process
optimization.
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132
CONCLUSIONS
A technical and cost comparison for HGMS and quadrupole magnetic
separators is shown in Table III. The HGMS installation is estimated to
be about a factor of 5 less expensive than the bank of 33 quatrupoles
and consumes about one-seventh the power. If auxiliary fan requirements
are now included, however, the HGMS installation cost estimate increases
to about one-fourth that of the quadrupoles, and the total power consumption
estimate increases to about one-half that of the quadrupole installation.
Including fan requirements, operating costs estimated for the HGMS
installation are only about one-half that of the conventional iron-
return-electromagnet quadrupole structure (power assumed at 33 mills/kW-hr).
Table III
Conceptual Level Comparison of HGMS and Quadrupole
Magnetic Separators for Dry Coal Desulfurization
HGMS Quadrupole
100% Removal Size (u-esd)
% Removal - All Pyrites
% Coal Recovery
V (fpm)
No. Magnets
Duty Cycle (%)
Field Strength (Oe)
Total Cost ($)
All Magnets
Canisters
Matrices
Power Supply (including
15% uncertainty)
$/ton Clean Coal
Installed Capital Cost
Actual Process CFM
3
96
98
1600
4
50
2000
327 000
175 000
97 000
28 000
627 000
0.03
$1.22/CFM
20
82
97
1 600
33
100
10 000
3 123 000
19 000
o
201 000
3 343 000
0.13
$6.6/CF*
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133
It is now interesting to again recall the results of Figure. 11
which suggest that if superconducting or pulsed field quadrupole magnets
could be developed to significantly increase pole tip field strength,
then quadrupole units would be more competitive with HGMS on a cost-
effective basis. While power consumption could be expected to be sig-
nificantly lower for a superconducting magnet installation, for example,
capital costs cannot be assessed at this time because there is no history
of commercial superconducting magnet installations. A factor of 10
increase in field strength will result roughly in a factor of 10 reduction
in pole length, but the economy of scale in superconducting installations
is now known. If capital costs were reduced by a factor of four, operating
costs for the two approaches would be similar. The point is that one
cannot properly estimate magnetic separator costs alone for comparative
purposes but must estimate the separator costs within the process context.
The mathematical model of capture in the HGMS device that has
been employed in this study is a straightforward modification of a model
successfully developed for describing the capture of weakly paramagnetic
materials in the wet beneficiation of kaolin clay. The modelling reported
here has, accordingly, assumed a modest surface loading of 30 wt%. While
this assumption may be appropriate to applications such as coal desulfuriza-
tion, where nonmagnetic particulates, in particular fine clay components,
are inadvertently mechanically captured, it may be overly conservative
in applications such as steel furnace off-gas scrubbing where particulate
concentration is very low, but where the percentage of magnetic particulates
is high.
In the magnetic capture of very fine size and strongly magnetic
materials, the continued capture of fine particulates may even be enhanced
by the history of fine particulate capture—up to a point. There are
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134
local surface imperfections on the capture material which produce highly
localized capture forces distributed over the entire surface of the
magnetized bed which are believed to be selective to micron and sub-
micron size particulates. Very fine size and strongly magnetic par-
ticulates which are captured at these force centers may have a tendency
to create new sites for further fine particulate capture. Turbulent
flow aids this process in bringing the particulates to the capture
surface from all directions.
Paramagnetic particles, on the contrary, are believed to be
captured preferentially on the leading and trailing edges of the magnetic
capture material in the regions of stagnation points for laminar flow [14].
Magnetic filter performance degrades continuously with time in the
processing of these materials (kaolin clay) because of the extremely
weak forces for capture of paramagnetic materials. This loss of efficiency
is due to particle buildup on the capture surface. In effect, the
capture of paramagnetic material destroys surface area for collection.
By way of contrast, the capture of very fine size but strongly magnetic
materials creates rather than destroys new capture sites. This being
the case, one would expect HGMS filter performance to be relatively
uninfluenced by time in the capture of strongly magnetic materials until
such time as a breakthrough occurs due to turbulent breakup of the
captured coagules.
The magnetic desulfurization of coal described here illustrates
an extreme and difficult application of magnetic separation technology.
In this example, particle size distribution is very broad, the flow
velocity is very high, and restrictive pressure drip requirements are
imposed by process considerations. The performance characteristics
calculated for both HGMS and quadrupole separation indicate an attractive
possibility for the use of magnetic methods in gas-particulate separations
for strongly magnetic particulates. Some very interesting magnetic
separator development lies ahead.
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135
ACKNOWLEDGMENTS
The results reported here are a continuation of work originally
supported by the U.S. Bureau of Mines. It is a pleasure to acknowledge
technical assistance provided by Messrs. F. A. Karlson, J. Vicory, and
John McCaig, and to thank Messrs. G. H. Dyer and E. L. Ekholm for making
Bechtel resources available for this investigation.
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136
REFERENCES
[1] "Proceedings of the High Gradient Magnetic Separation Symposium,"
Massachusetts Institute of Technology, Francis Bitter National
Magnet Laboratory, Cambridge, MA, June 22, 1973.
[2] Oder, R. R. and C. R. Price, "Brightness Beneficiation of Kaolin
Clays by Magnetic Treatment," TAPPI 56. No. 10, October 1973,
pp. 75-78.
[3] Aubrey, W. M., Jr., et al., "Magnetic Separator Method and Apparatus,"
U.S. Patent No. 3,608,718, issued September 28, 1971.
[4] Good, J. A. and E. Cohen, "A Superconducting Magnet System for a Very
High Intensity Magnetic Mineral Separator," IEEE Trans, on Magnetics.
MAG-12. No. 5, September 1976, pp. 503-506.
[5] Oder, R. R., et al., "Technical and Cost Comparisons for Chemical
Coal Cleaning Processes," American Mining Congress Journal. 63,
No. 8, pp. 42-49, August, 1977.
[6] McCartney, J. T., et al., "Pyrite Size Distribution and Coal-Pyrite
Particle Association in Steam Coals," Report of Investigation 7231,
U.S. Department of Interior, Bureau of Mines, February 1969.
[7] Kester, W. M., et al., "Reduction of Sulfur from Steam Coal by
Magnetic Methods," Mining Cong. J. 53. 1967, pp. 70-75.
[8] Karlson, F. V., "High Gradient Magnetic Desulfurization of Pulverized
Coal," Bechtel Corporation, May 1976, unpublished.
[9] Ergun, S. and E. H. Bean, "Magnetic Separation of Pyrite from Coal,"
Report of Investigation 7181, U.S. Department of Interior, Bureau of
Mines, September 1968.
[10] Kindig, J. K. and R. L. Turner, "Process for Improving Coal,"
U.S. Patent No. 3,938,966, issued February 17, 1976. See also:
J. K. Kindig and R. L. Turner, "Dry Chemical Process to Magnetize
Pyrite and Ash for Removal from Coal," Preprint No. 76-F-306,
presented at SME-AIME Fall Meeting, Denver, CO, September 1976.
[11] Power, L. F. and H. A. Fine, "The Iron-Sulfur System," Mins. Sci. &
Engrg. 8. No. 2, April 1974, p. 107.
[12] Besnus, M. J. and A. J. P. Meyer, "New Experimental Data on the
Magnetism of Natural Pyrrhotite," Proceedings of International Con-
ference on Magnetism, Nottingham, England, 1964, pp. 507-511. See
also: F. K. Latgering, "On the Ferrimagnetism of Some Sulfides and
Oxides," Philips Research Reports, Vol. 11, 1956, pp. 190-249.
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137
References (cont'd)
[13] Porter, C.R. and D.N. Goens, "Magnex Pilot Plant Evaluation — A
Dry Chemical Process for the Removal of Pyrite and Ash from Coal,"
Preprint No. 77-F-352, presented at SHE Fall Meeting, St. Louis, MO,
October 1977.
[14] Oder, R.R. and C.R. Price, "HGMS: Mathematical Modeling of Commercial
Practice," AIP Conference Proceedings, No. 29, Magnetism and Materials,
1975, pp. 641-643. See also: Oder, R.R., U.S. Patent No. 3,985,646,
"Method for Magnetic Beneficiation of Particle Dispersion," issued
October 12, 1976.
[15] "Coal Preparation for Combustion and Conversion," prepared for the
Electric Power Research Institute, Research Project 466-1, Final Report,
January 1, 1977. Gibbs & Hill, Inc., 393 Seventh Avenue, New York, New
York.
[16] Bitter, F., "The Design of Powerful Electromagnets, Part 1. The Use of
Iron, RSI 7. p. 479, 1936.
[17] Allen, J.W., "Magnetic Separator," U.S. Patent No. 3,819, 515, issued
June 25, 1974.
[18] Oder, R.R., "Magnetic Desulfurization of Liquefied Coals: Conceptual
Process Design and Cost Estimation," IEEE Trans, on Magnetics MAG-12,
No. 5, September 1976, p. 535.
[19] Aubrey, W.M. and R.M. Funk, "The Quadrupole Magnetic Separator — A New
Concept in Magnetic Separation Equipment," presented at the 32nd Annual
Mining Symposium, Duluth, MN, January 1971.
[20] Hubbard, E.L., "Design of Strong Focusing Magnets for Linear Accelerators,"
Engineering Note, File No. CV-7, Radiation Laboratory, University of
California at Berkeley, November 15, 1953.
WRITTEN DISCUSSION
E. Maxwell
Francis Bitter National Magnet Laboratory,*
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
Dr. Oder has presented an interesting scheme for processing dry coal by
HGMS. I would like to stress however, that there is really no experimental data
at this time to support the implicit assumption that the separation of pyrite from
coal suspensions in high velocity air streams will be as straightforward as
the separation from water slurries. We have done a little work on this problem and
can report that the problem is more complex. Coal which can be successfully
processed in water slurries by HGMS does not respond when simply propelled through
a matrix in high air stream. Agglomeration and stickness appear to be major
problems. It may be that pretreatment of the coal to enhance the magnetic sus-
ceptibility of the pyrite may help overcome this difficulty but this is yet
to be established.
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Successful dry magnetic separation of coal was reported ten years ago
by Kester et al.;1 they were most successful with an Upper Freeport coal,
removing 80% of the pyritic sulfur. They used a Frantz Isodynamic Separator
which is not a high gradient device but rather a laboratory instrument for
making a split between a magnetic and a non-magnetic fraction. The mineral
particles slide down a tilted inclined plane. The magnetic force competes
with the gravitational force to separate the particles into separate streams.
The particles move rather slowly under the influence of these two forces and
are not air propelled. It is essentially an open gradient device and there-
fore in the same general category as the quadrupole separator but substitutes
gravity feed for air propulsion. At the Auburn Conference on coal desulfuri-
zation by HGSM, H.H. Murray2 reported on some HGMS separation of dry coal again
using gravity feed.
Whatever the other problems of magnetically desulfurizing coal may be,
the conversion of pyrite to pyrhhotite is a forward step if it can be carried
out economically. It would be useful to have more data on the reaction ki-
netics of the conversion process. The magnetization of the FeSx sulfides is
a sensitive function of the composition parameter x, as illustrated in the
figure. In his calculation of the magnetization of the converted pyrite Dr.
Oder assumes the end product is simply Fe?S8. Actually a continuous range of
compositions is possible; the other stoichiometries have lower magnetization.
Incidentally, the composition Te.^ ^S^ does not correspond to Fe?S .
The temperature dependence of the magnetization is quite complex for some
of the sulfides, as shown in the work of Lotgering.3 From his investigations
it appears that rapid quenching of the pyrhhotite from 200°C. to ambient
temperature should result in a significant increase in the magnetization. The
separation should therefore be carried out in a cooled air stream and not at
the Reaction temperature.
Dr. Oder mentions that because of the toxic properties of iron pentacarbonyl
a lock hopper is required to contain it. How good must the lock hopper be? What
is the experience in handling the relatively large quantities of similar toxic
materials under conditions simulating power plant operation? Power plant
operators are generally reluctant to undertake operations which require chemical
engineering expertise because theirs is a different kind of operation.
Dr. Oder points out that the power consumption of the fans required to
deliver the air for propelling the coal is a significant factor in the overall
cost. Is it worth considering alternate schemes for carrying out the beneficiation?
For example, one might visualize a process in which the separation was carried
out in a separator by gravity feed and the coal was then pelletized. In that
case the processing could be done near the mine and the power plant would be saved
the task of disposing of the reject material and paying for its freight.
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139
REFERENCES
[1] W.M.-Dester et al., Min. Cong. Jour. 53_, 70 (1967).
[2] H.H. Murray, IEEE Trans. Magn., MAG-12, 498 (1976).
[3] F.K. Lotgering, Philips Res. Repts., :U, 140 (1952).
*Supported by the National Science Foundation.
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140
20
o
I I I I I I t>? I I
I I I I
1.05
110
X
1.15
20
-------
141
OPEN DISCUSSION
Oder: Dr. Maxwell mentions, for reasons other than what I had pointed out,
that the straight use of the magnetic separator in the processing will not
occur. That is true. I don't know whether it has to do with agglomeration
or not. Our point simply is that, just from an economic standpoint, you
need a 20 kilo horsepower magnet to handle the process strain. Those are
expensive to make. From that standpoint alone we do not think it is
economical, and suggest looking at alternative approaches such as Hazen .
There are chemical and thermal methods that do the conversions well. With
regard to the agglomeration problem it is my hunch that turbulent flow through
that device actually aids magnetic separation. I can not prove it, but it
aids it in two regards. First is the stripping that was mentioned. This is
a very difficult gas - patticle separation. Because you have got a surface
for capture and therefore you can capture magnetic particles. You can also
mechanically entrain non-magnetic particles. This is to your disadvantage
with HGMS technology because you eat up captured surface. If you have got
a turbulent drag force that carries those non-magnetic particles out of there,
then you don't hinder your captured surface characteristic which is good.
The other point is that when you capture a strongly magnetic particle
on a captured surface, if that particle is itself equivalent in magnetization
to the capture surface, then you create more capture surface. The turbulent
flow will bring particles in to the new sight from all directions. Therefore
the probability of capture is high.
Again the feeling is that processes like Hazen are good. They have
done this at Debang. They have a big scale unit that is running. They also
have a pilot unit that has been running for four months. In that four
month period, they have brought non-compliance coal into compliance and they
have done this basically so far with coarsely pulvarized coal (only going to
14 mesh). They select their coal so the pyrites have a coarse size
distribution. They have been clever in that regard. They choose Upper Freeport
coals or particular coals that are pyrite, they respond well, and don't
have to be pulverized. My point is that, if you are going to look at mid-
western and eastern coals, you are going to have to face the problem of
pulverizing it. To do that you are going to have the technology to handle
pulverized coals. I don't think it will work unless you convert to pyrites.
Maxwell: On the role of turbulence, I think if you manage to capture the
pyrhhotite, since it is more magnetic,you have a chance to strip any coal
attached to it by turbulence.
I don't think captured pyrhhotite particles will act very effectively as
nuclation centers simply because the magnetization of pyrhhotite is not that
large. We have seen, for example, studies of captured magnetite and hematite
particles. Magnatite particles have a magnetization saturation of about one-
third that of iron and act as a nucleation center. Drum separators are an
example. Hematite on the other hand, which is probably down in order of
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142
magnitude from magnetite, is perhaps probably more comparable to pyrhhotite
and doesn't behave in this way. It collects until it shields the captured
particles from the rest of the other particles then you no longer capture
particles. So I don't really think that pyrhhotite particle would increase
your effective capture.
Friedlander: At higher velocities we have more problems, strangely -enough,
with liquid systems and we get erratic behavior. You can make matters worse
by increasing velocity to the region of unsteady flow.
Baril: You are proposing to do a process that people demonstrated could be
done electrostatically many years ago. That process has not been successful
or applied. The reason is that not all coals are the same. The objective
is to remove the sulfur not just the pyrite. Sulfur occurs both in the
form of pyrite and organically formed sulfur which is not released by grinding
Incidently all pyritic sulfur is not released by grinding in some coals.
Generally you grind out to 200 mesh to relieve most of it in most coals. If
you don't remove all the sulfur, you have to process most coals through some
other operation to meet the emission standards. So actually you have not
accomplished anything more than has been accomplished 10 or 15 years ago.
Oder: That is not true. We have studied the chemical processes in great
detail. We studied the TRW process and the whole schlemiel. The idea is to
look at the limit on the organic sulfur level that you think you must remove
to get a significant upgrading of our Eastern reserves. You take a look at
that and the cost of chemical cleaning, projected at $20 per ton, this implies
that you must come back to physical cleaning and do a better job of it,
because you are talking about dollars per ton. You have to draw a line in
there somewhere. There are some coals that can be cleaned well physically,
some chemically and some converted through gasification and liquefaction.
But there is no single technology that is going to handle it all.
Baril: Electrostatic techniques require less power than magnetic techniques.
There are existing large scale separators in operation like the Wabush in
Canada. They are removing magnetite from sand and upgrading iron work; it is
commercial. What is the advantage of going to magnetic if there is another
process which is perhaps cheaper? It may be more expensive for initial
installation costs; I do not know. But the operation costs would be lower.
Liu: On behalf of all the magnetic people, I would like to defend your
criticism about comparison of magnetics and other approaches. There is various
documentation I can show you that your argument is not necessarily true. Over
17% of the annual coal production in the U.S. is low enough in organic sulfur and
less than 0.7% of that can't be cleaned magnetically. It costs less than all
the chemical or physical processes.
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143
MAGNETIC SEPARATION OF PARTICULATE AIR POLLUTANTS
D. C. Drehmel
U. S. Environmental Protection Agency
Industrial Environmental Research Laboratory
Research Triangle Park, N. C. 27711
and
C. H. Gooding
Research Triangle Institute
P. 0. Box 12194
Research Triangle Park, N. C. 27709
INTRODUCTION
In the last several years, particulate control technology has been
developed to the extent that methods are now available to control large
particle emissions from most industrial sources with an efficiency of
greater than 99 percent. Emphasis on particulate control has now shifted
toward the fine particle size range, particularly to particles which
have diameters between 0.3 and 3 micrometers. These particles are of
primary interest because they tend to remain in the atmosphere for long
periods of time where they may cause atmospheric haze and be transported
to cause a long range adverse health impact. They also are the particles
which readily enter and deposit in the human respiratory system.
The three major conventional technologies for the control of fine particu-
late emissions are electrostatic precipitation, wet scrubbing, and
fabric filtration. The cost of applying these control methods and the
fractional collection efficiency obtained vary according to the specific
application. Recent EPA research in particulate emission control has
followed two complementary courses: (a) study of the fine particle
collection characteristics of the conventional technologies and (b)
evaluation of viable alternatives to the conventional technologies. Both
of these courses have as their goals the enhancement of fine particulate
collection efficiency and the reduction of the capital and operating
costs of industrial emission control.
In the last decade research and commercial applications have demon-
strated that high gradient magnetic separation (HGMS) is an effective
and economical method of removing small paramagnetic particles from
selected liquid streams. In this study, published theoretical and
experimental reports of HGMS were utilized to evaluate the potential
success of the process in removing fine, paramagnetic particles from gas
streams. Industrial sources of fine, paramagnetic particles were
identified, and the steelmaking basic oxygen furnace (BOF) was selected
as the primary candidate for initial HGMS evaluation. A bench-scale
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144
apparatus was constructed and HGMS experiments were conducted using
redispersed BOF dust. With existing theoretical equations as a guide-
line, the experimental data were analyzed and favorable operating
conditions were identified.
THEORETICAL ASPECTS OF HGMS
To evaluate the usefulness and limitations of high gradient magnetic
separation, the theoretical basis of the concept must be examined.
Engineers have long sought a simple but accurate theoretical model to
describe conventional filtration. The mathematics of most useful models
quickly becomes unwieldy, necessitating simplifying assumptions. Never-
theless, the solutions often yield valuable information about the
phenomenological effects of key operating variables.
The recent development of HGMS models has followed much the same course.
Watson has published two papers [1,2] describing the single-fiber model
depicted in Figure 1. The radius of the cylindrical ferromagnetic wire
is a, and the radius of the particle is R. For r»a, the velocity of
the particle is V and parallel to but in the negative x direction.
0 R
Figure 1. Coordinate system for HGMS single-fiber model.
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145
For r»a, the magnetic field strength is Hg and parallel to but in the
positive x direction. Near the wire the field is concentrated, and a
net magnetic force acts on the particle, changing its trajectory.
Watson developed the expression for the magnetic force and incorporated
it in the equations of motion to describe the particle trajectory.
Watson presented an argument that the inertial force terms in the re-
sulting equations are negligible in comparison to magnetic and viscous
force terms [1], and finally obtained for the radial component of motion
+ Cos 201
(1)
and for the angular component
where r = -£-, normalized radial position,
a a
dimensionless
a = radius of wire, m
t = time, s
VQ = fluid velocity, m/s
0 = angular position
V =
2 x MB R
m
n — magnetic velocity, m/s
y n a
where x = magnetic susceptibility of particle, dimensionless
M = saturation magnetization of wire, A/m
B. = magnetic flux density, T
-------
146
R = particle radius, m
n = fluid viscosity, Ns/m2
HQ = magnetic field strength, A/m.
The fluid velocity is assumed to be normal to the wire at some point
upstream where the x-coordinate is large compared to the wire diameter
The solution of equations (1) and (2) can then be used to identify
particle trajectories which result in the particle striking the wire as
illustrated in Figure 2. The coordinates are normalized by the wire
radius: the initial coordinate corresponding to the maximum y whose
trajectory results in collection,
particles with initial coordinates
, is defined as RC. All
such that - R
•
-------
147
To calculate the collection efficiency for an HGMS apparatus containing
many randomly oriented cylindrical fibers, Watson [1] first extrapolated
from the single fiber to a filter element of differential thickness and
unit cross-sectional area as follows:
FAdL = total volume of wire,
FdL = volume of wire/unit cross section of filter
enclosure,
= length of wire/unit cross section of filter
enclosure,
(2R a) = total capture cross section/unit cross
c section of filter enclosure.
where F = volumetric packing density of mesh, dimensionless
2
A = cross-sectional area of the filter enclosure, m
L = length of the filter mesh, m
2FRrdL
Thus the quantity —^— represents the fraction of particles captured
in a differential length of the filter. Watson assumed that 1n a filter
of randomly oriented filters, only two-thirds are oriented correctly to
capture particles since fibers parallel to the magnetic field are
ineffective. He also presented an argument showing that the wires in a
differential thickness affect particles independently provided the
packing density does not exceed a critical value. Critical values of F
for selected values of RC are shown in Table I.
TABLE I. MAXIMUM VALUES OF PACKING DENSITY FOR
INDEPENDENT WIRE BEHAVIOR [1]
Vm
Vo
400
100
40
10
4
Rc (Case I)
9.45
5.83
4.15
2.39
1.55
Maximum F
0.0092
0.025
0.049
0.182
0.65
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148
The collection efficiency expression was then integrated over the length
of the filter to obtain
p .—^ . ... i 4FRcL
where P = penetration
Nout= number of particles out
Nin = number of particles in
The quantity (1-P) represents the fractional mass efficiency of the
filter for any given size of particles, provided that all of the particles
of that size are of equal mass density.
Other theories of HGMS have been advanced by Oberteuffer [3] and by
Cummings, et al. [4]. Oberteuffer developed a force balance model
without resorting directly to the equation of motion. His result is
parametrically similar to Watson's but does not yield direct quantita-
tive efficiency results. The major difference between Watson's theory
and that of Cummings, et al. lies in the calculation of the magnetic
force on the particle. In the present treatment (after Watson [1]) the
magnetic field strength in the particle was assumed to be very nearly
equal to the field strength outside the particle. In the strictest sense
this implies that the potential energy of the particle is zero; however,
U is easily shown that the result is the same as the more rigorous
solution obtained by Cummings, et al. if the magnetic permeability of
the fluid stream and the particle are nearly the same (as will be the
case of paramagnetic particles in a gas stream). For the collection of
ferromagnetic particles this simplifying assumption would be inaccurate.
RESULTS OF PREVIOUS HGMS APPLICATIONS
High gradient magnetic separation has been applied extensively to particle/
liquid separations [3,5,6,7,8,9,10,11]. The primary commercial application
has been in the removal of weakly magnetic color bodies less than 2 ym
in diameter from kaolin clay. Pilot-scale and laboratory tests have
been conducted on industrial waste process water from steel mills and
electroplating operations, on nuclear reactor coolants, and on oils and
hydraulic fluids (to remove wear particles). Preliminary experimentation
has shown promise of application of HGMS to the desulfurization of coal
and to the reduction of solids and dissolved orthophosphate from municipal
waste. The latter application was preceded by seeding the waste stream
with a magnetic material that became chemically or physically associated
with the pollutant. Another recent novel experimental application was
to the separation of red cells from whole blood.
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149
Data have been reported from two studies in sufficient detail to allow
comparison to the theoretical predictions of equation (3). In the first
case Melville, et al. [7] studied the high gradient magnetic separation
of red cells from whole blood. Red blood cells are flexible, biconcave,
discoid bodies with well-defined dimensions of 8.5 ± 0.4 ym diameter and
2.3 ±0.1 urn thickness. The filter contained 25 ym diameter cylindrical
stainless steel wire. Flow velocities varied from 1.6 x 10"4 m/s to
5.8 x 10'4 m/s, and the magnetic field ranged from 0.6T to 2.4T. In all
runs the parameter
was less than 0.2. The experimental data and
Case I theoretical prediction are shown in Figure 3. The equivalent
spherical diameter assumed here for the purpose of modeling was 6.3 urn.
For any given efficiency, the experimentally required value of
Vm
-tp- is about 1 to 1.5 orders of magnitude higher than the theoretical
o
value. Melville attributed this result to a postulated reduction in the
actual value of Rc which occurred when the bare wire was covered with
red blood cells within the first few seconds of filtration.
l.0r
o o oo o
03 00
o
THEORETICAL
UJ
z
Ul
OL
Figure 3. HGMS theory vs. experimental data:
cells from whole blood.
removal of red
-------
150
Oberteuffer [8] reported data for the removal of 10 urn particles of
cupric oxide from a slurry also containing diamagnetic aluminum oxide.
Flow velocities ranged from 0.022 m/s to 0.22 m/s and the magnetic field
ranged from 0.5T to 10T. Steel wool fiber packing was used. Luborsky
and Drummpnd [12] reported additional details of the experiment and
included in their analysis the assumption that the fibers could be
approximated by an idealized-ribbon geometry (with width, S, and thick-
ness, 2a).' If only the small edges of this type fiber produce a signifi-
cant magnetic force on the particles, then for a randomly packed filter
only one-third of the fibers on the average would be ortented correctly.
An argument analogous to that presented before then results in the
theoretical expression
P = exp -
(4)
The experimental data are shown in Figure 4. Theoretical penetrations
calculated from equation (4) with Case I values of RC fall
1.0 r-
<
P o..h
u
ui
Q.
001
o
o
THEORETICAL
-
3S
2B,
'=02
O 0.075,0.12,0.24
0.1
1.0
%
vo
Figure 4. HGMS theory vs. experimental data:
particles from a water slurry.
10
removal of cupric oxide
100
-------
151
roughly two orders of magnitude to the left of the data. Some of the
data corresponding to 2B$ > 0.2 are plotted separately and tend to
o
show a slightly enhanced collection. Luborsky and Drummond [6] were
able to correlate these data with three modifications to Watson's model:
(1) accounting for the effect on flow potential of particle buildup in
an assumed geometric pattern (the field potential was assumed to be
unaffected); (2) introduction of an empirically determined factor to
account for the fraction of wires that are active in capturing particles
(in addition to the one-third orientation factor); (3) introduction of
an empirically determined factor to account for mechanical filtration of
particles.
POTENTIAL APPLICATIONS OF HGMS IN PARTICULATE EMISSION CONTROL
Probably the most important parameter to be considered in the evaluation
of potential application of HGMS is the particle susceptibility. Suscepti-
bility of heterogeneous dust particles cannot be predicted accurately
because it is strongly dependent on the distribution of magnetic species
in the dust. The composition of industrial dust emissions obviously
varies for different processes. Within a single process category the
composition will vary over a more narrow range with respect to raw feed
and operating variations. In Table II typical compositions of paramagnetic
compounds are listed for several industrial processes. Blast furnace
TABLE II. POTENTIAL APPLICATIONS OF HGMS IN PARTICULATE EMISSION CONTROL
Industrial Process References Typical Dust Composition,
mass basis
Blast Furnace [13] 35-50% Fe, 12% FeO, 0.5-0.9% Mn
Basic Oxygen Furnace [13, 14] 90% Fe203, 1.5% FeO, 4% Mn304
Open Hearth Furnace [13, 14] 85-90% Fe203> 1-4 FeO, 0.5% MnO
Electric Arc Furnace [13, 14] 20-55% Fe203, 4-10% FeO, 0.5% MnO
Silico-manganese Furnace [13, 14] 4-7% FeO, 30-35% MnO
Ferro-manganese Furnace [13, 15] 6% FeO, 34% MnO
Ferro-chrome Furnace [13, 15] 7-11% FeO, 3% MnO, 29% total Cr
as Cr203
Coal-fired Boiler [14] 2-36% Fe203
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152
dust, which has a high reduced iron content, should be relatively easy
to collect. Since the dust would likely have residual magnetism,
cleaning of the filter mesh could be a problem. Basic oxygen and
electric-arc steelmaking furnaces are estimated to be prime candidates
for HGMS control. Open hearth furnace dust has similar characteristics,
but most existing open hearth furnaces are well-controlled, and the
majority of new steelmaking installations will more likely be basic
oxygen or electric-arc furnaces. In the ferroalloy industry, processes
involving combinations of iron, manganese, and chromium have reasonable
potential for HGMS application although the dust composition of these
processes is highly variable. Coal-fired boilers are unlikely to be
amenable to HGMS application, because the composition of magnetic
species is relatively low.
Particle size also varies greatly in different types of dust. HGMS
theory indicates that collection efficiency decreases with particle
size, but experimentation will be required to determine the fractional
efficiency for particular dusts. Gas temperature may also be an Im-
portant factor. Particle magnetic susceptibility is reduced and gas
viscosity is increased at increased temperatures. Both of these trends
theoretically reduce HGMS efficiency, but the effect could be minor
provided the efficiency at lower temperatures is sufficiently high.
DISCUSSION OF EXPERIMENTAL FEATURES
The bench-scale experiments reported in this paper were conducted with
an 8.9 cm (3.5 in.) diameter HGMS system collecting dusts from a basic
oxygen furnace. An operating temperature of 38°C (100°F) was used.
Apparatus and procedures were discussed in detail in a previous paper
[17]. The ranges of important operating parameters are summarized in
Table III. Total mass concentrations on the order of 0.01 to 0.1 g/m3
were produced by the EOF dust redispersion apparatus. Figure 5 shows
the typical particle size distribution at the inlet of the magnetic
separator as determined by Andersen impactor runs. The distribution is
skewed more toward larger sizes than are typical primary emissions from
a basic oxygen furnace, indicating that a significant mass percentage of
the dust was still in an agglomerated form.
One long term run was made to study the variation in sampling results
with time and the transient effects of mesh loading. The results are
shown in Figure 6. The penetration (1-fractional collection efficiency)
varied without any pattern for the first 8 hours of the test and then
began to rise. After 10.5 hours the superficial gas velocity could no
longer be maintained at its original value because of the increased
pressure drop, and the run was stopped after 11.5 hours. At the end of
the run the filter mesh was carefully removed and weighed. The clean
weight was 120 g and a total of 81.8 g of dust had been collected.
Assuming the total mass collection efficiency was constant over the 11.5
hour run (neglecting the small decrease in the last 4 hours), these
-------
TABLE III. RANGES OF OPERATING PARAMETERS
Filter Mesh Number
Filter length, cm
Packing density
Packing orientation
Magnetic field, T
Gas velocity, m/s
Pressure drop, kPa (in. H20)
4
35.6
0.0088
random
0.094-0.308
6.2-8.7
1.2-2.0
(4.6-7.8)
5
20.3
0.0088
spiral
0.094-0.308
5.9-9.3
0.7-1.3
(2.8-5.3)
6
20.3
0.0174
spiral
0.094-0.214
5.9-10.2
1.4-3.2
(5.4-12.8)
7
20.3
0.0132
spiral
0.094-0.214
6.6-10.6
1.2-2.9
(4.7-11.5)
8
20.3
0.0132
spiral
0.094
6.1-7.0
1.2-1.4
(4.6-5.4)
U)
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154
10.0
8.0
ao
40
0 ID
3 o
o
5 0.6
0.4
Q2
Ql
J ' '
J_
_L
_L
J L
J L
_L
-L
_L
_L
0.10.2 05 I 2 5 10 20 30 40 50 60 70 80 90 95 98 99
CUMULATIVE MASS PERCENT GREATER THAN INDICATED SIZE
99.8993
Figure 5. Size distribution of redispersed BOF dust.
-------
0.08
007
Q06
0.05
0.04
z
LU
Q.
0.03
002
0.01
MESHES
BQ=Q094T
V0 = 6.lm/s
0.7-1.0 /xm
A
H
a:
ui
tr
tn
I
8
10
II
234567
TIME, hr £
Figure 6. Transient loading effects and random variations in HGMS collection efficiency and pressure drop.
-------
156
results indicate that the filter collected approximately 50 percent of
its own weight in dust before the collection efficiency was appreciably
affected. However, during this time the pressure drop across the filter
also increased by about 50 percent.
COMPARISON TO THEORETICAL MODEL
All of the filter meshes used in this study were made from a steel wool
material similar to that used by Oberteuffer, but it was rolled into a
spiral configuration in which essentially all of the fibers were per-
pendicular to the field and gas flow. Reasoning analogous to that stated
before implies that half the fibers would have been active in collection.
Using Watson's model the theoretical expression for penetration becomes
= exp -
Appropriate theoretical values of R. were determined from the case
M0MS c
where -sn — = 2. With the low fields used in this work the
magnetization of the steel wool varied with the applied background field
in a nonlinear fashion. Table IV reports the estimated value of fiber
magnetization as calculated by Magnetic Engineering Associates, a con-
sultant to the study. The calculated values of the near field correction
parameter using estimates of both saturation magnetization and actual
magnetization are tabulated.
TABLE IV. FIBER MAGNETIZATION AND NEAR FIELD CORRECTION PARAMETER
Bo'T
0.094
0.214
0.308
Estimated M
M, A/m
3.2 x
6.8 x
8.4 x
e = 106 A/m
105
105
105
yn M
0
2Bn
0
2.2
2.0
1.7
p M
0 S
2B
0
6.7
2.9
2.0
-------
157
At the time these experiments were conducted, equipment to measure the
particle magnetization was not available. Based on the reported data of
typical BOF dust composition the volume susceptibility was estimated to
be 4 x 10~3 in dimensionless SI units. Recently, access to a Faraday
apparatus was arranged and three BOF dust samples were processed with
the results shown in Figure 7. The particulate is much more strongly
magnetic than anticipated. At 0.094T, the apparent volume susceptibility
with an assumed particle specific gravity of 3.5 is approximately 1.5
dimensionless SI units.
Typical experimental data from this study are presented 1n Figure 8 with
comparison to the corresponding theoretical prediction. To calculate
the abscissa value for the data, the estimated values M = 3.2 x 105 A/m
and x = 1-5 were utilized. In general this data set as well as the
remaining data agree more closely with the theoretical prediction than
the liquid system data of Melville and Oberteuffer. This feature is
perhaps due to enhancement of collection efficiency associated with the
near-field correction. The near field contribution is apparently important
due to the combination of small particle size, low magnetic field and
high fluid velocity. Table V summarizes the differences in the values
of these variables for the three experiments. In view of the significant
difference in the ranges of operating parameters as well as the extra-
polation from liquid to gas systems, discrepancies between this work and
the earlier experiments is not surprising.
Certain other features of the data should also be noted. Changes in the
abscissa of Figure 8 represent changes in two parameters—gas velocity
and particle size. Most of the scatter is believed to be associated
with the measurement of particle size and concentration. Each data
point represents the average of three or more upstream and downstream
counts with an optical analyzer and sampling/dilution system. The
individual penetration measurements for a given particle size during a
given run varied by as much as 50 percent from the average. These
variations were generally larger for 1 to 3 ym particles which appear as
data clustered to the right of Figure 8. The ability to measure low
penetration values for these large particles was also limited somewhat
by low particle counts. Thus the apparent tendency for the data to
V
level out toward larger -n—values could reflect a sampling limitation
o
rather than an actual physical trend. It should also be noted that the
nominal particle diameters indicated by the optical analyzer are based
on calibration with monodisperse latex spheres. Since the dust is known
to have contained agglomerated particles (probably of various shapes),
the relationship among the actual physical particle size and shape,
effective particle size for interaction with the wires, and particle
size indicated by the optical analyzer is not clearly defined.
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158
35
30
25
-E?
I 20
%
O
15
10
O SAMPLE #1
A SAMPLE #2
D SAMPLE #3
0.2
0.4
0.6
0.8
1.0
APPLIED MAGNETIC FIELD. OERSTEDS
Figure 7. Experimental measurements of particle magnetization.
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159
1.0
THEORETICAL
0.1
Z
tc
in
2
0.01 _
0.001
0.01
0.1
1.0
10
Figure 8. HGMS theory vs. experimental data: mesh No. 5,
BQ = 0.094T.
-------
ON
o
TABLE V. SUMMARY OF DIFFERENCES IN THE PRESENT AND PREVIOUS WORK
Parameter
Particles
Particles diameter, ym
Particle susceptibility, SI units
Fiber type, geometry and size
Packing density
Magnetic field, T
Fluid
2
Fluid viscosity, Ns/m
Fluid velocity, m/s
2Bo
Packing parameter
Melville, et al. [7]
red blood cells
6.3
3.0 x 10"6
25 ym diameter cylindrical
stainless steel
0.02
0.6-2.4T
diluted blood
1 x 10'3
(1.6-5.8) x 10"4
0.11-1.6
On/i A 17
. U4-U. 1 /
Oberteuffer, [8, 12]
cupric oxide
10
2.4 x 10"4
20 ym x 200 ym ribbons
stainless steel
0.05
0.5-10T
water
1 x ID'3
(0.022-0.22) x 10"2
0.37-73
.075-1 .5
3S
This Work
EOF dust
0.3 - 3.0
1.5
20 ym x 200 ym
ribbons stain-
less steel
0.0088-0.0174
0. 094-0. 308T
air
2 x 10"5
5.9-10.6
0.17-30
.0-6.7
3T=5'2
^ = 4.5-8.8
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161
Attempts to compare the effects of individual parameters to the
theoretical predictions were thwarted primarily by the relatively narrow
range of the individual parameter variations and the scatter in the
experimental data. The nonlinear relationship of R and —y—also
o
complicates individual parameter analysis. The strongest evidence of
some physical mechanism not accounted for in Watson's theory was seen in
analysis of penetration-vs-velocity data with other parameters constant.
V
The theory predicts for ^- <0.2:
o
P = exp
where k is a constant; i.e., as velocity increases, penetration in-
creases. In several cases the data tend to show penetration decreasing
with increasing velocity or perhaps a minimum penetration at a mid-range
velocity. Figure 9 shows one of these cases. This behavior conceivably
reflects a complicated interaction of high gradient magnetic separation,
inertial impaction (which is neglected in Watson's development), and
reentrainment.
With respect to the magnitude of observed collection efficiencies, the
data in Figure 8 indicate penetrations on the order of 0.01 for particles
in the 1 to 3 vim range and 0.15 for particles in the 0.3 to 0.5 ym range
with a relatively low applied magnetic field. In other runs when the
magnetic field or the filter mesh density was increased penetration of
the 0.3 to 0.5 pm particles was easily reduced to 0.05 and below.
CONTINUING WORK
Basically the objective of this research is to demonstrate on a small
scale that high gradient magnetic separation may be an effective and
economic technique for the control of fine particle emissions from
selected industrial sources.
To satisfy this objective, experiments are continuing on a pilot-scale
HGMS apparatus of approximately 0.7 m3/s (1500 CFM) capacity. At least
two levels of the following parameters will be studied: superficial gas
velocity, gas temperature, magnetic flux density, particulate source,
and particulate size.
Particle collection efficiency will be correlated as a function of
operating parameters either by an extension of Watson's theoretical
model or if necessary, by a semi-empirical approach. The primary purpose of
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162
z
o
1-
ct
H
UJ
Z
LJ
0.
0.11
0.10
0.09
008
O07
Q06
005
.004
0.03
002
001
0
-
. ° \ °
\
\
\
V
0 \
\
o \
^^
^^^
^^
^^^* ^^^
o
H*
••
1 till
8 9
VELOCITY, m/s
10
II
Figure 9. Anomolous effect of velocity on penetration: mesh No. 7,
BQ = 0.094T, 0.3-0.5 ym.
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163
the correlation will be to develop a reliable basis for designing a
larger scale device with a high probability of success. A secondary
goal, which is desirable but may prove to be beyond the scope of the
present research, is to develop a model of sufficient completeness and
accuracy to allow elementary optimization techniques to be incorporated
in the scale-up design (i.e., to provide a basis for determining the
incremental cost effect associated with adjustment of the major operating
parameters at constant collection efficiency so that each of these
parameters can be set at its optimum value.) Although Watson's model
has been useful so far, it may not be sufficient without modification
for at least three reasons:
1) The experimental data indicate a velocity effect that is not
predicted by the model;
2) At low fields the fibers are not magnetically saturated as the
model assumes;
3) The particles are ferromagnetic rather than paramagnetic as
the model assumes.
CONCLUSIONS
The following conclusions have been drawn from the preliminary experiments
and theoretical analysis conducted to date:
(1) Fundamental theory predicts the potential success of the
concept.
(2) Preliminary experiments support the potential capability of at
least 90 percent removal of all submicrpn particulate from BOF
furnaces. Collection of submicron particulate might actually
be enhanced at low magnetic fields.
(3) No inherent health and safety hazards or any other unfavorable
environmental impacts have been associated with HGMS application.
(4) No prohibitive design and reliability problems have been
identified.
(5) HGMS is potentially applicable to several sources of fine
particulate; retrofit capability should be enhanced by the
high gas velocities demonstrated in preliminary experiments.
ACKNOWLEDGMENTS
The authors are indebted to Dr. Larry K. Monteith of North Carolina
State University for major assistance in the mathematical model in-
terpretation.
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164
REFERENCES
1. Watson, J. H. P., Magnetic Filtration, J. Appl. Phys., Vo. 44, No
9, September 1973, pp. 4209-4213.
2. Watson, J. H. P., Theory of Capture of Particles in Magnetic High-
Intensity Filters, IEEE Transactions on Magnetics, Vol. MAG-11,
No. 5, September 1975, pp. 1597-1599.
3. Oberteuffer, J. A., Magnetic Separation: A Review of Principles,
Devices, and Applications, IEEE Transactions on Magnetics, Vol.
MAG-10, June 1974, pp. 223-238.
4. Cummings, D. L., D. A. Himmelblau, J. A. Oberteuffer, and 6. J.
Powers, Capture of Small Paramagnetic Particles by Magnetic Forces
from Low Speed Fluid Flows, AIChE Journal, Vol. 22, No. 3, May
1976, pp. 569-575.
5. Kolm, H., J. Oberteuffer, and D. Kelland, High Gradient Magnetic
Separation, Scientific American, Vol. 233, No. 5, November 1975,
pp. 47-53.
6. lammartino, N. R., New Tasks for Magnetism, Chemical Engineering,
Vol. 81, No. 1, January 7, 1974, pp. 50-52.
7. Melville, D., F. Paul, and S. Roath, High Gradient Magnetic Separation
of Red Cells from Whole Blood, IEEE Transactions on Magnetics, Vol.
MAG-11, No. 6, November 1975, pp. 1701-1704.
8. Oberteuffer, J. A., High Gradient Magnetic Separation, IEEE Transactions
on Magnetics, Vol. MAG-9, No. 3, September 1973, pp. 303-306.
9. Kelland, D. R., High Gradient Magnetic Separation Applied to Mineral
Beneficiation, IEEE Transactions on Magnetics, Vol. MAG-9, No. 3,
September 1973, pp. 307-310.
10. Trindade, S. G., and H. H. Kolm, Magnetic Desulfurization of Coal,
IEEE Transactions on Magnetics, Vol. MAG-9, No. 3, September 1973,
pp • «31 U™o Io •
11. deLatour, C., Magnetic Separation in Water Pollution Control, IEEE
Transactions on Magnetics, Vol. MAG-9, No. 3, September 1973, pp.
»3 I H~*3 I D»
12. Luborsky, F. E. and B. J. Drummond, High Gradient Magnetic Separation:
Theory Versus Experiment, IEEE Transactions on Magnetics, Vol. MAG-
11, No. 6, November 1975, pp. 1696-1700.
13. Katari, V., 6. Issacs and T. W. Devitt, Trace Pollutant Emissions from the
Processing of Metallic Ores. EPA-650/.2-74-115, October 1974':
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165
14. Hedley, W. H., S. M. Mehta, C. M. Moscowitz, A. D. Snyder, H. H. S.
Yu and D. L. Zanders, Sources and Characterization of Fine Particulate
Test Dusts EPA-650/2-74-117, November 1974.
15. Dealy, J. 0. and A. M. Killin, Engineering and Cost Study of The
Ferroalloy Industry. EPA-450/2-74-008, May 1974.
16. Drehmel, D. C. and C. H. Gooding, High Gradient Magnetic Particulate
Collection, 82nd AIChE Nat. Meeting, Atlantic City, August 29-
September 1, 1976.
WRITTEN DISCUSSION
Duncan R. Hay
Magnetic Engineering Associates, 247 Third St.
Cambridge, Massachusetts 02142
Because this technique is based on intrinsic magnetic susceptabilitv its
application must be limited to such materials that have magnetic properties.
On a practical level the separation of fine particulates in a large volume
gas stream may only be possible with materials of high magnetic susceptability.
The experimental results of the Research Triangle Institute (RTI) do support
the validity of the Watson model. It is especially interesting to note, that by
using the measured (rather than assumed) particle susceptability a much better
fit is found to the theoretical model.
During the aerodynamic and magnetic field characteristics of the spaces
between the stainless steel fibers is perhaps the most difficult analytical
task,. The usual model assumes a uniform spacing proportional to the matrix
density and fiber size. This ignores the actual situation where there is a
large random spacial variation, from touching to many fiber diameters separa-
tion. It is probable that the ratio of widely spaced to closely spaced
fibers is decreased with increasing matrix density. This will have a pro-
found effect on the capture of smaller size particles where the viscous
forces are proportionately larger than for large particles. Investigation
into the specific relationship of capture radius with respect to particle
velocity, the geometric spacing and magnetic field distribution of adjacent
fibers is needed.
Predicting the performance of a high gradient mangetic separator for air
carried particulates from the results of water mixed experiments has not
been successful. In the Stokes relationship there is orders of magnitude
difference in the viscosity and density ratio between air and water. Thus
any scaling would be quite sensitive to the actual experimental conditions.
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166
A steel wool matrix would probably show a poorer correlation than an expanded
metal one because of greater displacement effect of high density water on
individual fibers. The literature contains some analyses of particle
trajectories in a low density medium. However none has considered local density
variations in a high velocity air stream passing through a fiber array and how
this may affect the path of very small particles in the superimposed magnetic
field gradients.
The high magnetic gradient filtration of air carried ferric oxide par-
ticulate from steel making processes ia a very appropriate application being
investigated by RTI. It is also being evaluated by other groups for desulp-
hurization of an air powdered coal mixture.
Another potential application lies in the filtration of petroleum gas
streams containing finely divided iron and sulphur compounds. A rather unusual
problem is found in high temperature utility steam turbines. Where iron oxide
particles exfoliate from the pipe walls and are carried with the stream into
the turbine causing blade erosion or in the extreme, blade rupture. It is
possible to concentrate the particles with inertial and magnetic gradients
and separate this from the main steam flow into a high gradient magnetic
separator.
In all of these applications the capability of operating up to 500C is a
distinct advantage of the Magnetic Precipitator. (This designation has been
suggested by Henry Kolm and seems very appropriate to describe the action of
high magnetic gradient devices for air/particulate mixtures.) Of course the
saturation field for the matrix material will decrease with increasing
temperature, (to zero at the curie point) which could result in a requirement
to increase the magnetizing field to compensate.
The design of magnetic precipitator for large volume process rates most
conveniently takes the form of several large solenoids in parallel. The
collected magnetics are cyclically flushed out of each unit in turn by
turning off the magnet. A conventional bag house or spray collector is needed
to receive the blow down. Because the buildup time for the magnetics is long
for the usual low loading rates, flushing may be needed at 5 or even 20 minute
intervals and experiment has shown flushing is complete in less than 5 seconds
at process flow rates. Because of this, the flush may be dumped in a large
volume receiving tank and the particulate recovery filter operated at perhaps
5% of the system flow rate. Otherwise the flushing filter requirements would
dominate the whole installation which is completely unacceptable. The size
and number of solenoid magnetic matrix units would depend on the particular
application, with consideration for flow rate, operating cost and capital costs.
The alternative designs use a moving matrix, either rotating or oscillating,
where collection of the particulates in the magnetic field is continually
passes through a field free region for flushing. The two principle differences
compared to the solenoid geometry are decreased magnetic efficiency and the
need for moving seals to maintain the pressure difference across the matrix.
These requirements must be evaluated against the need to valve or direct a large
volume air flow from flushing to a receiver for each solenoid. Treating the
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167
flushed mags is the same for each design.
REVIEW OF EXPERIMENTAL RESULTS
The experimental results on EOF dust obtained by RTI in their laboratory
work using a SALS HGMS system clearly show a velocity dependent penetration
fields up to 3 kilogauss. The effect is most pronounced for particles smaller
than about 0.7 urn and is field dependent with a minimum in the 1400 to 1700 ft/min
velocity range. For particles of 1 to 2 y (and larger) diameter magnetic
saturation takes place at 2 kilogauss with almost 100% filtration efficiency.
A logical explanation for this velocity dependency suggests that at lower
flow rates the particle. flow is laminar and stream lines do not come close
enough to the fibers for capture in the magnetic gradient. At the highest
flow rates the fluid drag is stronger than the magnetic attraction and
particles are reentrained.
A long term run to measure the maximum loading capacity of the matrix
resulted in confirming the usual assumption that the accumulation can equal
the matrix weight. Insufficient data was taken on the effect of matrix length
to report but penetration appears to be inversely linear with length.
There is a strong inverse dependency of matrix density and penetration
for particles of less than 0.5y the minimum penetration decreases from 8 to
2% for the smaller particles to the reduction of average and of the maximum
distance between matrix fibers, but is obtained at a two fold increase in the
pressure drop across the matrix. Since pumping power is about 10 times more
costly than magnet power it appears there will be a trade off between a maximum
achievable reduction in the smaller particulate effluent and an economically
acceptable performance.
One aspect of experiments with air entrained particulates is finding a
satisfactory technique for introducing properly dispersed material. So far,
the use of a fluidized bed has been used to break up agglomerations of particles,
but the maximum size which can be dispersed in near 30 ym and the total mass
flow which can be entrained is perhaps 50 times smaller than desired loading
per cubic foot of air. Other particulate dispersal methods should be developed
to give a much broader control over experimental particle loading rates.
OPEN DISCUSSION
Kelland: When the particulate is on the side of the fiber, the gradient is in
the reverse direction. Not only does the viscous force take the particle off
but the magnetic force tends to push it away from the fiber also. However,
there were some experiments done 5 or 6 years ago at MIT by Dr. John Oberteuffer
in which expanded metal matrices were used in three different orientations
to measure collection efficiency. Almost no difference in collection efficiency
was found. How that translates to the practical devices of today I am not
sure.
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168
Gooding: Charlie pointed this out also; that his fibers were not round
but were usually more flattened. The orientation of the flatness may have
an effect on separation efficiency.
Friedlander: I disagree with that and have pictures to prove otherwise!
The capture effect change is not true because the magnetic force tries to
hold them there. What happens at higher velocities, however, I do not know.
Olson: Mr. Gooding has followed Mr. Watson's analysis very well, however, if
one considers a particle trajectory model a drier separation occurs. In
particular, the conditions that Gooding and Drehmel considered (Stoke's
number between .1 and 30) is a region on the low side of transition; from
a noninertial region where the particle inertia is not important to a region
where the particle inertia is very important. So, in response, as
Gooding suggested, there will be inertial impaction in filters over much of
the velocity and particle sizes he considered.
The same would also be expected of the more conventional high gradient
magnetic separator Robin Oder considered. In addition, in the Gooding-
Drehmel work, the Stokes drag turns out not to be a very good approximation
for the viscous forces on the particle. They are in the transition region
where the Stokes theory for the flow around a particle breaks down. So
there will perhaps be a different dependence upon the relative particle fluid
velocity rather than a linear one.
Gooding: I would just say that I agree with what you said. This is something
we have put some thinking into. But we have not gone through to try to improve
the model.
Friedlander: It seems to me that the performance goes in the opposite direction
than what Maxwell said a little while ago about much worse performance at higher
velocities. If I read your graph correctly, improved performance comes
with higher velocities.
Gooding: This is a single component system where you are not trying to separate
two different solid particulates in an airstream. It is a much simpler problem
than the problem where you have two components that you are trying to separate.
I would not compare them at all.
Friedlander: Without taking away from Watson, I think one has to say that
Zebel did the basic thing. We should give credit to Zebel.
Oder: I think we need a knock-down drag-out session just on magnetic separation.
Now you know why I chose the example I did, I knew the magnetic component you
would be looking at. I jimmied up the Hazen process to come out with moments
in the same ballpark. We are not that far apart and your flow velocities are
of the order of 1000 ft/min. If you can convert coal minerals to a very strong
magnetic component, you get some indirect proof that it should work.
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169
Gooding: I think your velocity dependence looks like inertial deposition.
I think there are two things going on, inertial deposition and reentrainment.
I have played with the mathematics a little bit. We may find that there is
a minimum or optimum velocity.
Oder: Did you ever run with your magnet turned off?
Gooding: Yes. The figure shows no field, and you see the larger particles
are getting as high as 50% collected. There is a fair amount of uncertainty
in this data. This is one of the reaons we have not tried to do any more
sophisticated modeling. Referring back to this slide, we had indications
that some of the larger particles were agglomerated as they impact on the
wire, which would mean that the zero field numbers are suspect. We have
improved that aspect of our experimental apparatus by building a scaled-up
version. We are also taking data as a baseline at zero field.
When you are dealing with a ferro-magnetic particle, another force that
you really should consider is one that you do not normally calculate in the
analysis. If you have a ferro-magnetic particle, particularly a very small
one and it gets impacted on a ferro-magnetic surface which is larger, the
image force due to the moment in the ferro-magnetic wall surface should be
considered. In other words, the particle itself is a source of magnetic field
gradient which makes it stick to the wall. This operates only at distances
comparable to the diameter of the particle. It is something which should be
taken into account when you are dealing with ferro-magnetic particles. This
may have some relation to what you seek in your zero field measurements. Be-
cause, even though you turn your field off, if this is steel wool or something
like that, it does have a small amount of residual magnetism. It will pick
up ferro-magnetic particles. I believe you are correct. In our present
experiments we have designed it so that all our zero magnetic field data is
taken before we magnetize the matrix.
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170
RESEARCH NEEDS AND OPPORTUNITIES IN HIGH GRADIENT MAGNETIC SEPARATION OF
PARTICULATE-GAS SYSTEMS
Y. A. Liu and C. J. Lin
Magnetochemical Engineering Laboratory
Department of Chemical Engineering, Auburn University, Auburn, Alabama
36830
ABSTRACT
This paper suggests the major needs and opportunities in high gradient
magnetic separation (HGMS) of particulate-gas systems according to three
categories: (1) process and engineering research oriented towards effective
applications of existing devices; (2) fundamental research aimed at develop-
ing a better understanding of the mechanisms and principles of the separation;
and (3) scientific and engineering development of new separation methods and
devices.
INTRODUCTION
High gradient magnetic separation (HGMS) is a new technique that provides
a practical means for separating micron-size, feebly magnetic materials on a
large scale and at much faster flow rates than are possible in ordinary fil-
tration. The technique is also applicable to separating nonmagnetic materials
which can be made to associate with magnetic seeding materials. HGMS has
already been applied industrially for the wet cleaning of feebly magnetic con-
taminants from kaolin clay with very low costs and outstanding technical per-
formance {15, 16, 24, 28, 29, 34-36, 38}. It is also proving applicable to
solving many important problems related to energy production (e.g., coal
destilfurization), environmental control (e.g., particulate collection and
wastewater treatment), resource recovery (e.g., mineral beneficiation), and
other minerals and chemical processing problems. The significance of the
HGMS, and its latest engineering and commercial developments have been ade-
quately discussed elsewhere {16, 28, 29, 35, 36, 38}.
Because of the necessary costs for drying and dewatering the products in
most wet applications, it is generally believed that the most economical
application of the HGMS is for the dry separation of many particulate-gas
systems, especially those particulates of less than 2 to 3 microns in diameter.
Existing experimental data have already demonstrated the technical feasibility
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171
of utilizing the HGMS for the collection of the basic oxygen furnace (EOF)
dust {10}, and for the removal of pyritic sulfur and ash from dry pulverized
coal {32}. These reported results and other unpublished data known to the
authors have suggested that the HGMS appears to hold much promise as an
effective alternative to particulate-gas separation, and further research
and development work is clearly needed.
The purpose of this paper is to briefly suggest the major needs and
opportunities in the HGMS of particulate-gas systems. For convenience, the
discussion will be divided into three categories: (1) process and engineering
research oriented towards applications of existing devices; (2) fundamental
research aimed at developing a better understanding of the mechanisms and
principles of the separation; and (3) scientific and engineering development
of new separation methods and devices.
APPLICATIONS-ORIENTED PROCESS AND ENGINEERING RESEARCH
Applications-oriented process and engineering research is needed for demon-
strating that the HGMS is an effective and economical method for separating
particulate-gas systems. This type of research may include the development of
magnetic process for particulate-gas separation which has been previously
considered to be impossible or uneconomical; and the adaption of the magnetic
approach to improve the performance of existing particulate-gas separation
devices. Obviously, there are numerous areas or opportunities for applying
the existing HGMS devices to particulate-gas separation. Three practical exam-
ples of such applications related to energy production, environmental control
and resource recovery can be cited here to illustrate the broad range of
research areas or opportunities available.
Among the many areas in which the HGMS may have potential applications to
particulate-gas separation, the magnetic removal of pyritic sulfur and ash
from dry pulverized coal is believed to be particularly important and promising
{19, 21, 24-27, 32, 37, 38}. Substantial reduction in both pyritic sulfur and
ash contents in the HGMS of several Illinois Basin coals has already been re-
ported in a recent study by Murray {32}. An interesting observation from this
study is that the combined use of several finely divided filamentary matrix
materials such as steel wool, steel filings, etc., can significantly enhance
the magnetic removal of both pyritic sulfur and ash. This observation confirms
the previously recognized importance of the matrix characteristics in providing
strong high-gradient magnetic fields {16, 29, 34, 35, 38}. Clearly, research
and development work in optimum matrix design and composition will continue
to contribute significantly to advancing the state of the HGMS technology,
especially in applying to particulate-gas separation. Another important area
which has not received much attention is the optimum use of the latest fluidi-
zation technology in conjunction with the high gradient magnetic desulfurization
of dry pulverized coal. Identifying the problems associated with fluidizing
pulverized coal, determining the conditions for achieving good fluidization,
and characterizing the performance of the separation are only some of the work
needed.
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172
Because of the national interest in increasing the utilization of coal,
more financial support for process and engineering research related to the
magnetic desulfurization of dry pulverized coal is being provided by several
governmental and funding agencies. However, the results from a recent survey
of the supported research projects on the subject conducted by the authors
have suggested that the current level of research support does not seem to
reflect the proper appreciation of the significance of the HGMS, and the po-
tential and economics of the magnetic cleaning of dry pulverized coal. The
latter has been adequately discussed in the literature {21,29, 32, 37, 38}.
Here, it is sufficient to mention that the new HGMS technology has provided
a possible commercial method for economically separating the sulfur-bearing
and ash-forming minerals from dry pulverized coal on a large scale. It has
been estimated that a total of 100 million short tons of U. S. coals per
year, low enough in organic sulfur, can be magnetically cleaned as an environ-
mentally acceptable, low-sulfur low-ash fuel. This amounts to over 17% of
the total U. S. production per year {25-27}. Further, recent analyses have
clearly indicated that the costs of high gradient magnetic desulfurization,
especially of dry pulverized coal, appear to be attractive when compared to
those of other pyrite removal processes currently under active developments
{23, 37}. All of these aspects will suggest that increased research support
for the work on the magnetic desulfurization of dry pulverized coal is most
likely to produce the greatest dividends.
of ann?vi™e?LWS™<;b¥ DreV and. Goodl'ng {10} has il^strated the potential
of applying the HGMS to particulate-gas separation in the important area of
fhp'S!!?! *£°ntr?,1' J" Part1cular> the technical feasibility of utilizing
the HGMS for the collection of the basic oxygen furnace (BOF) dust has been
demonstrated. An important observation from this work is the dramatic improve-
ment of particulate collection efficiency resulted from applying a relatively
low-intensity magnetic field. Total mass collection efficiencies of 99 9 per-
SlnrXi "S wre/™nd with * 3.08 kilogauss field, which compared quite
favorably with the typical efficiencies of 50 percent or less observed without
applying the magnetic field. However, it was also found from this study that
the existing theory of HGMS failed to interpret quantitatively the effect of
some separation variables such as particle size, field intensity, etc. Des-
S™t Jhe Prellminary. nature of this investigation, it is apparent that the
HGMS holds much promise of application to the important problem of fine parti-
rSl2Ha?12S1!n-COntr!!! e|Pfc1a"y 1n the iron, steel, ferroalloy, and other
related industries. This follows because a majority of the particulate matters
in these industries are sufficiently magnetic to be separated by applying a
high gradient magnetic field. The primary need at this point is to identify
clearly several potential areas of application in these industries by a close
interdTsciplinary cooperation between the particulate control practitioners and
the magnetic separation technologists. Further, it is anticipated that many
technical problems will arise in applying the existing HGMS devices to some of
the selected areas. This necessitates some proper process and engineering
modifications of the current HGMS technology. For instance, effective means
must be developed to allow the use of the present HGMS devices in separating
the high-temperature particulate-gas mixtures such as these off gases from
steelmaking furnaces; and to prevent the chemically corrosive components such
as chlorine existing in some parti cu late-gas mixtures from corroding the
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173
separator matrix. The major objective of such process and engineering work
should be, of course, on demonstrating that the desired particulate-gas
separation can be carried out effectively by the HGMS at a low enough cost
to be economically attractive when compared to those of the competing fine
particulate control technologies (1, 11, 12, 31 40}.
Applications-oriented research has also been initiated in this laboratory
and elsewhere on utilizing the HGMS to recover the resuable components from
the fine airborne dusts emitted from industrial operations. This type of re-
search attempts to extend the potential applicability of the HGMS for separa-
ting particulate-gas systems in environmental control to an increasingly im-
portant area of resource recovery. As an illustration, it is known that fly
ash from coal-fired power plants contains a large fraction of reusable, fine
maonetic particulates such as magnetites. Existing analysis has already
suggested that they can be economically recovered by the HGMS, thus reducing
the load on the more expensive electrostatic precipitators {29}. Preliminary
work in this laboratory and unpublished prior work in the coal mining industry
have also indicated that the coal ash derived magnetite performs better in
the heavy-media coal cleaning process than does mined magnetite {26}. In
view of the large amount of fly ash generated from coal-fired power plants in
the U.S., and the increasing cost of the commercial-grade mined magnetite being
used in coal preparation, further research toward the development of an econo-
mical ash-derived-magnetite coal cleaning process is clearly justified. It
is also apparent that there are many other areas or opportunities related to
resource recovery, in which the use of HGMS for separating particulate-gas
systems plays a major role; and they are too diverse to be surveyed here.
Regardless of the area of application, process and engineering research
oriented towards the effective and economical utilization of the existing HGMS
devices for particulate-gas separation should normally include the following
steps: (1) examination of the evidence of significant magnetic removal of
particulates from their carrier gas; (2) study of the quantitative effects of
operating conditions such as field intensity, residence time, etc. on the grade
and recovery of the separation; (3) study of the separator capacity and scale-
up; (4) conceptual process design and cost estimation; and (5) comparison of
costs and performance with those of the competing technologies. Here, the
recent experience in carrying out most of these steps in the development of
HGMS processes for coal desulfurization and kaolin beneficiation reported
in the literature {21-28, 33, 34, 37-39} is believed to be helpful to those
involved in the process and engineering research on the HGMS of particulate-
gas systems.
FUNDAMENTAL RESEARCH
The key to successful utilization of HGMS devices for particulate-gas
separation is the clear understanding of the mechanisms and principles invol-
ved in relation to specific applications. Recent experience gained in the
pilot-scale and commercial applications of HGMS to coal desulfurization and
kaolin beneficiation {21, 22, 27, 33, 37-39} has demonstrated the importance
of utilizing quantitative separation models in conjunction with the separation
testing. In particular, the successful experimental verification of two
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simplified mathematical models due to Bean {5}, and Oder and Price {39} in
such reported applications has allowed one to quantitatively predict the trade-
off of some operating parameters such as field intensity, residence time, etc
so as to optimize the magnetic desulfurization of coal and beneficiation of
kaolin {22, 39}. None of these two simplified models, however, could satis-
factorily predict the phenomena of mechanical entrapment and matrix loading
(concentration breakthrough) observed experimentally.
Similar research related to the above fundamental aspects of HGMS applied
to particulate-gas separation is, unfortunately, being pursued much less vigor-
ously. This may be exemplified by the considerable uncertainties in the theo-
retical understanding of the quantitative effects of separation variables in
the recent study of high gradient magnetic particulate collection by Drehmel
and Gooding {10}. Clearly, in order to permit a more scientific approach to
future applications of HGMS to particulate-gas systems, much work is needed
to develop the necessary conceptual understanding and the proper modeling
framework. From a detailed review of the recent development and current status
of the quantitative modeling of HGMS described elsewhere {28}, it may be sug-
gested that such a modeling framework should at least take into account of
the phenomena of mechanical entrapment and matrix loading, as well as the
characteristics of the particulates to be separated. A proper approach ac-
counting for such factors also necessitates the conventional matrix-type
HGMS device to be modeled as a magnetic adsorption column as suggested by
Kaiser, et al. {17}. An example of a typical mathematical model based on this
approach can be found from the work in this laboratory {28, 33}, in which'the
successful application of the modeling results to quantitatively predict the
experimental data from coal desulfurization is also demonstrated. It is be-
lieved that the same modeling framework can be properly extended for predicting
the performance of HGMS of particulate-gas systems.
The success of the preceding attempts in developing quantitative models
to satisfactorily predict the overall performance of the separation such as
grade, recovery, etc., can be greatly enhanced by simultaneously conducting
experimental studies on some synthetic and well-defined particulate-gas
systems. Such experiments may be carried out, for example, by using air-
entrained monodispersed particles with uniformly spaced wires as collecting
fibers in the separator matrix. Here, a number of similar experimental studies
of the HGMS of particles in water slurries reported in the literature {6-9,
14, 18} may serve as good references. The experimental results thus obtained
should provide some helpful insight into the mechanisms of particle capture
and buildup, etc., in particulate-gas systems.
EQUIPMENT DESIGN AND DEVELOPMENT
Research in this category includes the improved design and scale-up of the
existing HGMS devices, as well as the development of devices utilizing new
separation concepts. The importance of this type of research can be readily
recognized by noting the significant effort currently being devoted to develo-
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ping new separation methods and devices, including those suitable for dry
separation of part1culate-gas systems {3, 4, 13, 15, 16, 36, 42}, Further,
the commercial breakthrough in kaolin beneficiation resulted from introdu-
cing a new level of HGMS unit in 1969 has also made it clear that expanded
research activity in equipment design and development will offer the grea-
test opportunities in advancing the technology of HGMS of particulate-gas
systems.
A number of research needs and opportunities in equipment design and deve-
lopment can be briefly suggested by examing the status of several recent
technological innovations in HGMS, and their potential applicability in par-
ticulate-gas separation. First, it has been generally agreed that improved
or new design, and scale-up of the original production prototype HGMS unit
have been made during the past few years; and stainless steel wool is widely
accepted as a common magnetic matrix. What has been largely neglected here
is the systematic research aimed at developing alternative matrix collectors,
especially for use in particulate-gas separation. Next, for certain separa-
tion problems in which the feed stream contains a large fraction of magnetic
particles, technical and economical constraints often necessitate the use of
continuous HGMS devices to increase the separation throughput. For example,
both moving-matrix Carousel separator {36} and open-gradient stream splitter
{3, 4} are now being developed for such separations. Conceptually speaking,
the stream splitter is especially attractive because it is a truly continuous
device in which the magnetic particles, rather than being collected on a ma-
trix, are deflected to one side of the splitter by the magnetic field; wher-
eas the Carousel separator contains moving parts, and sometimes does not op-
erate in a truly continuous fashion. Because of the lack of the gradient-
producing collection matrix in the stream splitter, it has not been proven
practical for separations of fine paramagnetic particles at present. Thus,
it has been suggested that stream splitting devices are best used in conjunc-
tion with the high-intensity magnetic fields generated by superconducting
magnets {13}, which appears to hold much promise of application to separating
particulate-gas systems. Here, the open space existing in the superconducting
stream splitting device allows for the use of the latest fluidization techno-
logy to enhance the particulate-gas separation. Clearly, further work is need-
ed to quantitively characterize such an approach. Alternatively, there is
also a need to develop a high-intensity, high-gradient stream splitting device,
without using the superconducting magnet. The recent work by Allen and lannice-
11i {2} illustrates one promising approach to this development, and this
new design has yet to be tested in actual separations. Finally, perhaps the
most important need in equipment design and development is an increase in
research support, as significant advances can be made in particulate-gas se-
paration with the development of the new generation of HGMS technology.
ACKNOWLEDGEMENT
The authors wish to gratefully acknowledge the financial support for their
work on magnetic separation provided by the National Science Foundation, the
Gulf Oil Foundation, the Board of Trustees and the Engineering Experiment
Station of Auburn University, and the U. S. Energy Research and Development
Administration- Oak Ridge National Laboratory.
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{19} Kester, W. M., "The Effect of High-Intensity Magnetic Cleaning on Pul-
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{20} Lawson, W. F., Jr., "The Dynamics of Paramagnetic Particles Near a Mag-
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{21} Lin, C. J., Liu, Y. A., Vives, D. L., Oak, M. J., and Crow, G. E., "Sul-
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{22} Lin, C. J., Liu, Y. A., Vives, D. L., Oak, M, J,, Crow, G. E., and Huffman,
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{24} Liu, Y. A., Symposium Chairman and Proceedings Editor, "Proceedings of
Magnetic Desulfurization of Coal Symposium: A Symposium on the Theory
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25, 1976; published by the IEEE Magnetics Society as a Special Issue on
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(25) Liu, Y. A., and Lin, C. J., "Assessment of Sulfur and Ash Removal from
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12, No. 5, p. 538 (1976).
(26) Liu, Y. A., and Lin, C. J., "Applications of Magnetic Separation Method
to Coal Preparation", Invited Chapter, in "Coal Preparation", 4th Edition,
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{27} Liu, Y. A., and Lin, C. J., "Status and Problems in the Development of
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{28} Liu, Y. A., Lin, C. J., Vives, D. L., Oak, M. J., and Crow, G. E., "Theory
and Applications of High Gradient Magnetic Separation: A Review", Invi-
ted paper, International Magnetics Conference, Los Angeles, June (1977).
{29} Luborsky, F. E., "High-Field High-Gradient Magnetic Separation: A Re-
view", 21st National Conference on Magnetism and Magnetic Materials,
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{30} Luborsky, F. E., and Drummond, B. J., "High Gradient Magnetic Separation:
Theory Versus Experiment", IEEE Transactions on Magnetics, Vol. MAG-11,
No. 6, 1696 (1975).
{31} Marchello, J. M., and Kelly, J. J., Editors, "Gas Cleaning and Air Quality
Control", Marcel Dekker, Inc., New York, New York (1975).
(32} Murray, H. H., "Magnetic Desulfurization of Some Illinois Basin Coals",
ACS National Meeting, Fuel Chemistry Division, New Orleans, Preprints,
Vol. 22, No. 2, p. 106 (1977).
{33} Oak, M. J., "Modeling and Experimental Studies of High Gradient Magnetic
Separation with Applications to Coal Desulfurization", M.S. Thesis, Auburn
University, Auburn, Alabama (1977).
{34} Oberteuffer, J. A., and D. R. Kelland, Editors, "Proceedings of High Gra-
dient Magnetic Separation Symposium", Massachusetts Institute of Techno-
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{35} Oberteuffer, J. A., "Magnetic Separation: A Review of Principles, Devices,
and Applications", IEEE Transactions on Magnetics, Vol. MAG-10, No, 2,
p. 223 (1974).
{36} Oberteuffer, J. A., "Engineering Development of High Gradient Magnetic
Separations", IEEE Transactions on Magnetics, Vol, MAG-12, No. 5, p. 444
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{37} Oder, R. R., "HGMS Desulfurization of Pulverized Dry Coal", Technical
Report, Job No. 90371, Scientific Development, Bechtel Corporation,
December (1975).
{38} Oder, R. R., "High Gradient Magnetic Separation: Theory and Applications",'
IEEE Transactions on Magnetics, Vol. MAG-12, No. 5, p. 428 (1976).
[39} Oder, R. R., and Price, C. R., "HGMS: Mathematical Modeling of Commercial
Practice", 21st Annual Conference on Magnetism and Magnetic Materials,
Philadelphia, Pennsylvania, December (1976).
{40} Oglesby, S., Jr., and Nichols, G. B., "Electrostatic Precipitators", Chap-
ter 4, in "Gas Cleaning and Air Quality Control", Marchello, J. M., and
Kelly, J. J., Editors, Marcel Dekker, Inc., New York, New York (1975).
{41} Riley, P. W., and Watson, J. H. P., "The Use of Paramagnetic Matrices for
Magnetic Separation", Paper presented at the Conference on Advances in
Magnetic Materials and their applications, London, England, September
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{42} Stekly, Z. J. J., "A Superconducting High Intensity Magnetic Separator",
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don, England, April (1975).
{43} Watson, J. H. P., and Hocking, D., "The Beneficiation of Clay Using a
Super-conducting Magnetic Separator", IEEE Transactions on Magnetics,
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{44} Watson, J. H. P., "Magnetic Separation at High Magnetic Fields", Pro-
ceedings of 6th International Cryogenic Engineering Conference, p. 223
(1976).
WRITTEN DISCUSSION
David R. Kelland
Francis Bitter, National Magnet Laboratory
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
Most of the work done on applying High Gradient Magnetic Separation
(HGMS) to particulate-gas systems apparently has been in the area of coal
desulfurization. For the purpose of this paper as stated in the title it
would be worthwhile reviewing the work done so far. Also, the level of
success achieved should be mentioned and the conditions under which the ex-
periments have been carried out should be made clear. Further, a careful
distinction must be made between particulate-gas systems, in this case dry
coal cleaning and water slurry separations.
CoaVwater slurries have been the subject of a considerable amount of
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research which has been going for several years. It can be said that for
a given coal in which the inorganic matter has been liberated by grinding or
perhaps other means, most of inorganic sulfur minerals can be removed by
HGMS along with some part of the ash-forming mineral matter. The same cannot
be said yet for dry separations.
Several attempts have been made to improve the magnetic properties of the
impurities in coal so that these might be removed by liberation and subsequent
magnetic separation. Siddiqui steam-treated Pakistani coal and noted that a
small part of the coal could be picked up by a permanent magnet. This
fraction had a higher sulfur content than the non-magnetic fraction. Yurovskii
and Remesnikov used shorter treatment times on a Soviet coal and reported a
sulfur reduction in a fraction beneficiated in an induced roll separator.
Adamov added magnetite to coal ground to minus 3mm and achieved some ash
reduction by magnetic separation.
Kester, Leonard and Wilson at W. Virginia University found that from 54 to
of the inorganic sulfur could be removed from untreated coal. The size
range was 48 x 200 mesh and the best results were obtained on Upper Freeport
and Pittsburgh seam coals. Erratic results were obtained with a 48 x 0 size
fraction. The separations were performed using a Frantz Isodynamic Separator.
Ergun and Bean at the Bureau of Mines did an extensive project on pretreat-
ing and beneficiating coals. Separating untreated coals in an induced roll
separator, they report reductions in the pyritic sulfur of from 7 to 60%
depending on the coal. Three passes were made through the separator. A -300
micron sample of Pittsburgh coal was pretreated with a heated gas in a
fluidized bed and then subjected to separation in an Isodynamic Separator.
Reduction in pyritic sulfur increased from 27% before pretreatment to 32 to
45% afterwards.
This work seems to indicate that some of the pyritic sulfur can be removed
from some coals provided that it can be liberated at a large enough particle
size. All of these studies have used gravity feed to the separator and have
been concerned at least with dry separations if not separations of material
from a gas stream. Power plants blow fine coal (often -200 mesh) into their
furnaces. If the pyritic sulfur is liberated, it might be interesting to see
whether magnetic separation could be used on a relatively high velocity gas
stream carrying coal to reduce the sulfur content.
Studies at MIT last year on an Upper Freeport coal blown through a high
gradient separator showed that this kind of separation is more difficult to
perform than a corresponding coal-water slurry separation. In fact, some of
the best wet separation results we know of were obtained with this coal before
the particulate-gas separations were attempted. Total sulfur reductions
reached 57% compared to about 40% obtained by Trindade on Brazilian coal at
MIT in 1973 and up to 45% by Murray on Illinois and Indiana coals recently.
It is important to distinguish between total sulfur reduction and inorganic
sulfur reduction since sulfur is found in both forms in coal. The organic
sulfur is generally not removed by physical means such as magnetic separation.
Murray's test result in dry separation of 40% inorganic sulfur reduction would
correspond to a 20% total sulfur reduction if half the total sulfur occurred
as inorganic, say pyrite.
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Sulfur reduction by physical separation depends on the type of coal,
its history, grinding, liberation, etc., so it is very difficult to compare
any of these figures. But the problem of dry separation of impurities from
coal is far from solved and represents a research need and opportunity.
I cannot be optimistic enough to agree with the authors overstatement
that "HGMS has provided a possible commercial method, for economically
separating the sulfur-bearing and ash-forming minerals from dry pulverized
coal on a large scale." There are technical problems remaining in dry
separation and it is not clear what kind or how much of pretreatment might
be necessary nor how the HGMS process might be applied here.
This is a fertile field for research and development and the need for such
should not be obscured by statements which imply that the problem has been
solved and is waiting for commercial development. This is one area still in
need of research and that research ought to be done by a number of people or
research groups. These should include magneticians, fluid flow experts,
chemists, and other experts in surface phenomena, electro-statoc effects,
etc.
There is a reference here to the "commercial applications of HGMS to coal
desulfurization." We are not aware of any such commercial applications and
should comment that the authors' use of "pilot-scale" often refers to
laboratory experiments.
The application of modeling to HGMS has been made by several people.
Reference should be made to work reported at Intermag meetings of the IEEE.
In the case of coal, Trindade at MIT used a model for water-coal separations
in 1973 and Clarkson has extended the fluid model to an extended range of
Reynolds numbers and has taken inertia and the boundary layer near matrix
fibers into account.
Beyond the generalities offered here about research needs, there are
a number of specific topics that could and should be mentioned.
Agglomeration of particles may be a major problem in particulate-gas
systems especially when a separation is to be performed and an effort has
been made to liberate one particle species from another. Obviously, in
collection systems this may be desirable but the agglomeration of material
needs to be understood in any case.
The effect of fines on particle agglomeration needs to be known. Methods
of grinding to reduce fines may be employed and moisture levels varied to
change the effect of the fines content in the particulate system. Electro-
static charges on particle surfaces have an effect on agglomeration and are
certainly affected by moisture. The carrier gas, its moisture content and
temperature are all considerations for separation studies.
For matrix separations the flow characteristics of gas and particles past
matrix materials should be studied over a reasonable range of Reynolds
numbers. A number of studies of this sort have been done for fluids based
on work such as Zebel's on aerosols but they have generally considered the
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fluid incompressible.
Loading of matrices in a high gradient separator will have to be studied.
There are now available both batch-type and continuous separators, the latter
offering a possible solution to short duty cycles in the former when they
occur. Since neither coal loss nor inefficient collection of impurities is
desirable in coal cleaning a clean separation is the goal. Besides agglomera-
tion mentioned before, mechanial trapping, collection efficiency, and
changes in the flow resistance, all functions of loading, affect the quality
of separation.
The authors point to the work of Drehmel and Gooding as an illustration
of the application of HGMS to particulate-gas separation. This is a very
interesting piece of work and should be continued as a vigorous program. A
point not made here though is that in the case of BOF dust all of the
material wants to be trapped out of the gas stream. I would call that
collection rather than separation. The difference between the two is signi-
ficant. Mechanical trapping aids collection and therefore enhances the
magnetic effect. In particle separations on the other hand mechanical
trapping can be a serious problem whether in gas streams or liquids.
It is stated by the authors that an "observation" is made in Murray's
work that confirms the enhancement of high gradient magnetic separation by
the use of combined matrix materials. In fact, Murray reports only the use
of Frantz screens in his experiments. Many people have patented matrix
designs using carpet tacks, steel wool, ribbon screens, grooved plates, etc.
In our experience, however, steel wool or stainless steel wool (not necessarily
the magnetic 400 series) remains an excellent matrix material. Expanded
metal is nearly as effective and is used in larger pilot plant moving matrix
HGMS devices. Steel shot, filings and grooved plates have the disadvantage
of occupying part of the magnetized volume of the HGMS device and magnetized
volume is what one pays for.
Matrices consisting of combinations of material types have been used in
batch and moving matrix HGMS devices. We found this type of matrix to be
very useful in treating iron ores in a program carried both at our laboratory
and on location in Minnesota. It could almost go without saying that matrix
design constitutes a development used and that companies engaged in machine
production are not unaware of this need.
OPEN DISCUSSION
Liu: I think I should quickly mention that our research has been totally
funded by the RANN Division of NSF. They have been very helpful supporting
magnetic separation, as Dr. Oder is obviously aware. I agree with most of
the comments made about magnetic separation. Much of the data was not
produced in our paper for this workshop because approval has not been
granted by the Electric Power Research Institute.
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HIGH GRADIENT MAGNETIC SEPARATION: REVIEW OF SINGLE WIRE MODELS
F. J. Friedlaender
School of Electrical Engineering, Purdue University
West Lafayette, Indiana 4790?
INTRODUCTION
High Gradient Magnetic Separation (HGMS) has become a subject of consi-
derable interest since its introduction into commercial beneficiation pro-
cesses and the hope that it may be a means of removing certain pollutants
encountered in industrial processes and energy generation. Most systems
employing HGMS which are in use so far have been wet systems but a discussion
of such systems may be useful in suggesting possibilities, as well as diffi-
culties, with the use of air or other gases instead of water. Here our main
concern will be with the collection process on single fibers which can be
considered the basic "building block" of the more complicated systems in
actual use.
In HGMS processes a collection of ferromagnetic fibers is commonly placed
in a magnetic field of sufficient strength to saturate the fibers. Hence,
each properly oriented fiber becomes essentially a line dipole which produces
rapidly changing fields with position in its vicinity. The resultant field
gradients produce a magnetic force on magnetized particles in the vicinity of
the fiber. Under appropriate conditions, particles will be swept toward the sur-
face of the fiber and wil 1 be held there as long as the field is present. The major
competing forces on the particles in such systems are usually the magnetic forces
which sweep the particles toward the fibers and the forces due to the fluid which
carr ies the particles. In a properly designed system, t he magnet i zed particles
will be retained on the fibers which act essentially as a filter for these particles.
Here we shall review the various factors that are of importance in the
collection process and discuss the mechanism of particle capture on single
fibers. In most practical HGMS filters, presently built, water carries the
particles which are to be separated out through a steel wool filter. The
ability of the magnetized fibers of the steel wool to hold the magnetized
particles will determine the effectiveness of the filter. Single fiber
experiments to study the collection process are usually carried out with
steel or nickel wires acting as collecting fibers. The reader will be
referred to the appropriate literature for derivations of various quanti-
tative models. The emphasis here will be on the presentation of detailed
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experimental results of particle buildup on single collecting fibers (nickel
wire) for several geometrical arrangements of the fiber relative to the flow
and the applied magnetic field.
FACTORS INFLUENCING PARTICLE CAPTURE
It is convenient to consider separately each of the elements entering
into filter operation. Such an approach provides considerable simplification
though sometimes with some sacrifice of rigor. To begin with there are the
particles which are to be removed from the slurry. It is obvious that these
particles must be capable of being magnetized. If the particles are ferro-
or ferr Imagnetic they will either possess a net magnetic moment or can be
magnetized by a magnetic field. Usually the moment will be large, and, once
induced by a field, may be permanent. Such particles may be conveniently
removed by more conventional methods and HGMS may pose certain problems. For
instance, attempts to remove the collected particles by flushing out the filter
after turning off the field may not be fully successful since the particles
may, in effect, become permanent magnets. Paramagnetic particles, on the
other hand, have no permanent magnetic moment and a field is required to in-
duce such a moment which is proportional to the product of the applied field
and the susceptibility of the particle. It is obvious that weakly paramag-
netic particles require larger fields. Diamagnetic substances have not been
removed by HGMS since the very weak magnetic moment induced in them opposes
the applied field so that weak, repulsive rather than attractive forces
result in the usual collection region.
The magnitude of the force, considered for paramagnetic particles only
in the following, is proportional to the product of the magnitude of the .
induced magnetic moment, the gradient field and the volume of the particle. '
For larger particles the relationship becomes more complicated, partly because
the field gradient can no longer be considered uniform over the entire parti-
cle volume.3 But major interest usually centers on small particle sizes since
it was the (successful) intent of HGMS to obtain sufficiently large gradient
fields to make separation of micron and submicron size particles possible.
At those small sizes, the major competing force is the fluid drag which tries
to carry the particles along. Obviously, below some critical size the fluid
force will always be larger than the magnetic force and no collection will
take place. There are no particular difficulties in calculating the magnetic
forces although different approaches are possible and are still the subject
of inquiry. Gravitational forces and particle inertial effects may play an
increasingly important role in the case of larger particles. At the other
end of the scale, very small particles could be subject to thermal motion as
well as other effects. Electrical forces play a role in the capture process
by influencing particle to particle, as well as fiber to particle, inter-
actions. Finally, magnetic interactions between particles can become a
factor for particles with large moments.
The fiber serves both as the collection center and as a means of produ-
cing the high gradient field. It has to have appropriate mechanical and
chemical properties (I.e., itshouldnot be corroded by the water usually used
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185
as the fluid that carries the particles). Secondly, it should be ferromag-
netic with a large magnetic moment so that high gradients can be achieved.
The shape of the fiber cross-section is the other factor that determines the
magnitude of the gradient. Hence, small diameter wires or steel fibers with
relatively sharp edges are used to obtain large gradients. Steel with appro-
priate magnetic and corrosion-resistant properties is probably the best fiber
material for many HGMS applications, but nickel can be used and is sometimes
more readily available in a variety of sizes, as needed for experimental
purposes.
The particle carrier, for instance the water used to make the slurry in
conventional HGMS systems in use today, plays a vital role in the process
since it carries the particle past the collecting fibers. A determination
of the flow fields and how they affect the collection process is one of the
major problems facing investigators.
Generally, the fluid flow problem has been considered under various,
simplifying assumptions. At very small Reynolds' numbers, a viscous flow,
also called creeping flow^S, has been assumed by several investigators and
has been solved by a method due to Lamb. This solution has been claimed to
give a better approximation of actual flow conditions close to the cylindrical
fiber than other often used solutions. At somewhat larger Reynolds' numbers
the assumption of potential flow is made, but is probably reasonable only
some distance from the fiber. Boundary layer effects near the fiber surface
have been postulated 6 to take into account the hydrodynamic forces on
particles near the fiber surface. When considering the particle capture
process, the calculation of forces on particles building up in layers at the
fiber surface has been based on various assumptions6'7'8'^. In some cases,
predictions of particle buildup seem to follow observed patterns, mothers
they are off by several orders of magnitude. The actual flow conditions such
as a stagnant region and the formation of eddies and an unsteady flow region,
all downstream behind the cylindrical fiber seem to make calculations more
difficult, especially at larger Reynolds' numbers which occur at higher flow
velocities. Experimental results will be presented to point out such diffi-
culties. One of the references quoted above deals with the capture of parti-
cles By a dielectric fiber1* due to electric rather than magnetic forces. When
the particles are electrically neutral but possess an electric dipole moment
induced by an applied electric field, then the capture mechanism becomes
analogous to that of magnetic dipoles captured on a ferromagnetic fiber.
Since such analogies may be useful--capture by electric fields is receiving
a great deal of attention by workers studying the behavior of aerosols--
a further discussion of this analogy is in order.
Generally, the induced electric dipole moment can be assumed to depend
linearly on the applied field. The same assumption can be made for magnetic
dipoles over a limited range of applied magnetic fields. A paramagnetic parti
cle will have a dipole moment proportional to applied field for most practi-
cally obtainable values of field. However, magnetic fibers will saturate
with sufficiently large fields whereas dielectric fibers will only show
saturation phenomena if they are made of a ferroelectric substance.
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186
Electric fields are necessarily limited in magnitude to below electric
breakdown gradients, but there is no such limitation on magnetic fields.
(However, the region in which corona discharges take place may be used in
electric separation processes.) Hence, magnetic gradients are limited only
by geometrical considerations and by the magnetic properties of the collecting
fibers.
EXPERIMENTAL RESULTS
Collection of paramagnetic particles on a nickel wire has been studied
by photographic means and through the use of a hot wire anemometer10»'1»12.
Here photographic collection data will be presented for the three idealized
cases that are usually discussed in the literature.
Case A has the collecting fiber,flow, and the field all mutually perpen-
dicular.
Case B has field and flow parallel to each other and perpendicular to
the fiber axis.
Finally, the third arrangment, case C, has the flow along the axis of
the fiber with the field again perpendicular, as it must be if the fiber-
field combination is to produce a large magnetic field gradient. All three
cases are shown in Figure 1.
The details of the experimental method are given in references 9-11.
All experiments described here were carried out with a nickel wire as the
collecting fiber C»TrMs = 6900 Gauss for nickel) and paramagnetic particles
of manganese pyrophosphate being collected from the slurry. The manganese
pyrophosphate, J^PzOy- (3HzO) is reported to have a magnetic susceptibility
of 186 x 10~6 emu/oested/gram. Particle sizes ranged from about 1 vim to
100 vim with most of the particles in the 1-10 ym range, but an appreciable
fraction of the total particle volume is made up of large particles.
Photographs for case A and case B have been obtained by taking pictures
of the collecting wire "end on." Case C shows the wire with the attractive
regions on the sides and one repulsive region facing the camera with the flow parallel
to the wire. In all pictures the flow is from top to the bottom except in Figure 10.
Figure 2 shows the buildup of particles on a 25 pm diameter nickel
collecting wire, for case A geometry. Parameters are 4.5 cm/s slurry
velocity and a 12.5 kOe applied field. The concentration of Mn2P207'(3H20)
is 0.0264 g/1. The time shown below each picture is measured from the instant
at which the slurry first reached the wire (A flow of deionized water was
carried past the collecting wire until a diverter valve changed the flow to
introduce the slurry. Flow velocities were controlled by means of an orifice
downstream far from the collection region).
Figure 3 shows the buildup again for case A using a 25 vim diameter wire
and 0.0264 g/1 concentration of Mn2P207'(3H20), with a flow velocity of
-------
187
1
case B
case C
Figure 1. The three cases of flow and magnetic field orientation
relative to the collection wire.
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188
OQ urn I2"*u sec
Figure 2. Buildup for case A--.0264 g/1 Mn2P207*(3HoO^ field
12.5 kOe, flow velocity 4.5 cm/s, 25 ym diameter
nickel collecting wire.
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189
100 >r
Figure 3. Case A—0.26^ g/1 Mr^PzOy (3^0), Field 7kOe,
flow velocity 10 cm/s, 25 ym diameter collecting
wire.
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190
10 cm/s and a field of 7 kOe. Figures 4, 5, 6 show the buildup on a 125 urn
diameter nickel wire for case B for increasing velocities 3.7 cm/s in Figure
4, 17.8 cm/s in Figure 5 and 35.0 cm/s in Figure 6, all for a 12.5 kOe
applied field. In Figures 7 and 8 a 17-8 cm/s flow velocity was used, with
a^3.0 kOe field in Figure 7 and a 1.0 kOe field in Figure 8. It was more
difficult to obtain photographs which were adequate for reproduction for case
B with 25 ym diameter nickel wire. Figure 9 shows case B with 8 cm/s flow
velocity and a 12.5 kOe field, and slurry concentration as before. Finally,
in Figure 10 collection on a 250 ym diameter wire for case C is shown.
Theoretical predictions of buildup for cases A and B were given in
references 6 and 11 and are reproduced here in Figures 11 and 12 to show
the good agreement with experiment for the lower values of velocity. As can
be seen by examining some of the data taken at higher velocities'?, less
reliable data is obtained at these velocities. Downstream collection "behind"
the collecting fiber, for case B, appears to indicate the appearance of a
stagnant region that can be expected to occur at the lower velocities. At
the highest velocity (35 cm/s) collection becomes unreliable. Further dis-
cussion on this point will be found in reference 12. Case C has been discussed
in detail in references 13 and 14.
DISCUSSION AND CONCLUSION
In summary it can be said that experimental results and theoretical
predictions give substantial agreement in regions in which the assumption of
potential flow holds, possibly modified to allow for the existence of a
boundary layer near the surface of the collecting fiber. Lamb's solution
(creeping flow) may give better predictions in some cases, but further
studies are needed to settle this claim. Calculations have usually been made-
for the separation of a single component in a slurry, often assumed to consist
of uniform-sized particles (spherical), with no mention of agglomeration.
Experiments have been carried out, essentially duplicating these assump-
tions, except that particle size or shape has not often been controlled and,
again, agglomeration is not mentioned but may play a role. In regions in
which the flow may be other than "smooth," i.e., unsteady flow with eddies
"behind" the wire'5, or generally higher velocity (and, thus, larger Reynolds'
numbers) flow, collection data indicate that problems arise which still await
a solution. The difficulties with unsteady or turbulant flow, both from a
theoretical and from an experimental point of view may not, however, be a
matter of major concern since regions in which such flow occurs do not seem
to be advantageous in HGMS. Mixed systems, containing particles with different
magnetic susceptibilities with the more magnetic particles to be separated
from the slurry need to be investigated both theoretically and experimentally
in single fiber systems. Questions of particle to fiber and particle to parti-
cle adhesion and how they differ in liquid and gaseous carrier systems need to
be answered. Considerable differences between the two systems (wet and dry)
can be expected and have apparently been observed in the operation of entire
systems^. Instabilities may occur in the particle mot ion'7 and experimental
observation of particle trajectories is needed to test the theoretical
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191
t = 1066 sec
'
Figure k. Case B—0.264 g/1 Mr^PzOy(3H20), flow velocity
2.7 cm/s, field 12.5 kOe, 125 m diameter collecting
wi re.
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192
333 >
*
Figure 5. Case B—.0264 g/1 Mn2P20y (3H20), field 12.5 kOe,
flow velocity 17-8 cm/s, 125 ym diameter collecting
wire.
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193
717 sec
Figure 6. Case B— .026** g/1 Mn2P2°7* (3H20), field 12-5 k°e>
flow velocity 35.0 cm/s, 125 ym diameter collecting
wi re.
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194
•
1U6 s«e
1039 sec
1868 sec
100 an
2632 sec
Figure 7- Case B—.026^ g/1 Mr^O;'(3H20), field 3 kOe,
flow velocity 17.8 cm/s, 125 ym diameter collecting
wi re.
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195
•
Figure 8. Case B—.0264 g/1 I
flow velocity 17-8 cm/s,
wire.
' (3H20), field 1 kOe,
125 nm diameter collecting
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196
, ' -
838 sec
Figure 9. Case B—.0264 g/1 Mn2P207' (3H-0), field 12.5 kOe,
flow velocity 8 cm/s, 25 ym diameter collecting
wi re.
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197
Figure 10. Case C — .0664 g/1 Mn2P207' (3H20), field 12.5 kOe,
flow velocity 10 cm/s, 250 ym diameter collecting
wire.
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198
THEORETICAL
FROM
PHOTOGRAPH
Figure 11. The theoretical buildup for case A compared to buildup
shown in photographs.
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199
THEORETICAL
FROM PHOTOGRAPH
LOWER SECTION IS SHOWN
SCALE OF UPPER SECTION
THEORETICAL
FROM PHOTOGRAPH
Figure 12. The theoretical buildup from Luborsky compared to
buildup as shown on photographs for case B.
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200
predictions. Since many systems in use today have collecting fibers arranged
in other than the idealized geometries of cases A, B, or C some thought has to
be given to the effect of more complicated geometries obtained essentially by
a linear combination of two or more of the ideal models.
ACKNOWLEDGMENTS
This work was supported in part by a grant from the National Science
Foundation. The photographs of particle buildup on a single wire were taken
from a thesis by Carl Cowen.'8
REFERENCES
[1] Oberteuffer, J. A., "Magnetic Separation: A Review of Principle, Devices,
and Applications," IEEE Trans. Magn., Vol. MAG-10, p. 223, June 1974.
[2] Watson, J. H. 0., "Magnetic Filtration," J. Appl. Phys., Vol. 44, No. 9,
p. 4209, Sept. 1973.
[3] Aharoni, A., "Traction Force on Paramagnetic Particles in Magnetic
Separators," IEEE Trans. Magn., Vol. MAG-12, p. 234, May 1976.
[4] Zebel, G., "Deposition of Aerosol Flowing Past a Cylindrical Fiber in a
Uniform Electric Field," Journal of Colloid Science," Vol. 20, p. 522,
1965.
[5] Cummings, D. L., Prieve, D. C., and Powers, G. J., "The Motion of Small
Paramagnetic Particles in a High Gradient Magnetic Separation," IEEE
Trans. Magn., Vol. MAG-12, pp. 471-473, Sept. 1976.
[6] Luborsky, F. E., and Drummond, B. J., "Build-up of Particles on Fibers
in a High-Field-High Gradient Separator," IEEE Trans. Magn., Vol. MAG-
12, No. 5, p. 463, Sept. 1976.
[71 Luborsky, F. E., and Drummond, B. J., "High Gradient Magnetic Separa-
tion: Theory vs. Experiment," IEEE Trans. Magn., Vol. MAG-11, No. 6,
p. 1697, Nov. 1975.
[8] Stekley, Z. J. J., and Minervini, J. V., "Shape Effect of the Matrix on
the Capture Cross Section of Particles in High Gradient Magnetic Separa-
tion," IEEE Trans. Magn., Vol. MAG-12, No. 5, p. 474, Sept. 1976.
[9l Cowen, C. C., Friedlaender, F. J., and Jaluria, Rajiv, "Single Wire Model
of High Gradient Magnetic Separation Processes II," IEEE Trans. Magn.,
Vol. MAG-12, No. 5, p. 466, Sept. 1976.
[10] Cowen, C. C., Friedlaender, F. J., and Jaluria, Rajiv, "High Gradient
Magnetic Field Particle Capture on a Single Wire," IEEE Trans. Magn.,
Vol. MAG-11, No. 5, p. 1600, Sept. 1975-
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201
[11] Cowen, C. C., Friedlaender, F. J., and Jaluria, Rajiv, "Single Wire Model
of High Gradient Magnetic Separation Processes II," IEEE Trans. Magn.,
Vol. MAG-12, No. 6, p. 898, Nov. 1976.
[12] Cowen, C. C., and Friedlaender, F. J., "Single Wire Model of High Gradient
Magnetic Separation Processes III," IEEE Trans. Magn., Vol. MAG-13, No. 5,
Sept. 1977-
[13] Birss, R. R., Gerber, R., and Parker, M. R., "Theory and Design of Axially
Ordered Filters for High Intensity Magnetic Separation," IEEE Trans.
Magn., Vol. MAG-12, No. 6, p. 892, Nov. 1976.
[1ft] Uchiyama, S., Rondo, S., Takayasu, M., and Eguchi, I., "Performance of
Parallel Stream Type Magnetic Filter for HGMS," IEEE Trans., Magn.,
Vol. MAG-12, No. 5, pp. 895-897, Nov. 1976.
[15] Batchelor, G. K., "An Introduction to Fluid Dynamics," Cambridge Univer-
sity Press, Cambridge, England, 1970.
[16] Luborsky, F. E., Private Communication.
[17] Lawson, W. F., Jr., Simons, W. H., and Treat, R. P., "The Dynamics of a
Particle Attracted by a Magnetized Wire," J. Appl. Phys., to be
pub)I shed.
[18] Cowen, C. C., "High Gradient Magnetic Separation: Single Wire Studies,"
Ph.D. Thesis, Purdue University, December 1976.
WRITTEN DISCUSSION
W. F. Lawson, Jr.
Morgantown Energy Research Center
U.S. ERDA
P.O. Box 880
Morgantown, W.V. 26505
As Professor Friedlaender points out, most HGMS experiments have been
concerned with the separation of particles using a liquid carrier media. In
viscous fluids, particle inertia and the Earth's gravitational force can
generally be disregarded when describing the particle dynamics. This is the
low Stokes number regime where Watson's theory1 is applicable. The agreement
betwee.n gross filter efficiencies calculated using Watson's theory and re-
sults of actual filter experiments is generally unsatisfactory. Luborsky
states in his review2 that from the mass of material captured in randomly
packed magnetic filters it was clear that Watson's theory would have to be
extended to account for particulate accumulation on individual fibers. Thus,
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202
the need arose to examine the particulate buildup on single wires. Subsequently,
theories were developed based on trajectory models and concurrently the photo-
graphy of particulate buildup on single wires in an aqueous medium was begun
at Professor Friedlaender's lab. The agreement between these theories and
experimental observations has not been entirely satisfactory and, as noted in
the article under discussion, only the slower flows substantiated the
theoretical predictions.
The problem with the theories may be the result of the complexity of the
physical situation. Professor Friedlaender correctly indicates many of the
difficulties, in particular those associated with the description of the fluid
flow. Different flow fields around the cylindrical fiber have been used in
single fiber theories and have yielded quite different quantitative results.
In addition it appears that there are conditions of practical interest for
which the relative velocity between the particle and the fluid medium become
large. For such non-Stokesian particles one should of course use the empirical
formula for the fluid drag in the equation of particle motion.
It is perhaps worth emphasizing again that the above theories are based
on a particle trajectory model. Although Himmelblau3 has observed some HGMS
trajectories, neither these observations nor other experiments provide satis-
factory particle dynamical data to check this basic model. As Kolm^ has
advocated and Professor Friedlaender suggests, careful observations of the
particle dynamics are needed to check the trajectory models and to provide a
better understanding of the basic processes involved in particle capture.
What of particulate-gas separations using HGMS? Professor Friedlaender
has noted some of the potentially different aspects of dry separation as com-
pared to wet HGMS. Drehmel and Gooding5 have demonstrated the efficiency of
dry magnetic separation in cleaning simulated iron furnace emissions. However,
in many instances there was not even qualitative agreement with fiber efficiency
predictions based on Watson's theory. This is not surprising since Stokes
numbers for their experiment ranged from about 2.5 to 250 consistently above
the threshold marking the onset of inertial effects. A general treatment of the
effects of particle inertia and the Earth's gravitational force when vectorially
combined with the magnetic and fluid forces is given in Reference 6, but
these results are still experimentally untested. Calculations of single fiber
capture distances and particle trajectories and more details on the qualitative
differences between the inertial and noninertial cases of particle dynamics
can be found there. The two factors which make inertial effects manifest in
dry HGMS are the reduced viscosity of the medium and the typically higher
flow velocities used in gas filtration. The reduced fluid mass density can
cause the Earth's gravitational pull to be more significant than with denser
liquids.
Moreover, it is not yet clear that a satisfactory filter efficiency can
be derived from single fiber trajectory calculations. In fact, some pre-
liminary work on multiple fiber filter theory indicates that the effect of
neighboring fibers in large arrays can be quite significant. Shown in Figure
1 are two pairs of capture efficiency curves for a long fiber presented as a
-------
1.0
.8
C5
a.
«t
C_J
INTERIOR CELL OF
MULT I FIBER ARRAY
SINGLE FIBER
0 L_
.0
FIGURE 1
J_
,5
1.0
1 .5
LOG TT-
2.0
2.5
3.0
Capture efffency of an interior cell of a multifiber filter based on 97% and 33%
void filter fractions using flow and magnetic field descriptions of a squar^
multifiber array (solid lines) and an isolated single fiber (dashed lines).
3.5
ro
O
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204
function of Watson's parameter V^V^. The capture efficiency is defined here
as the fraction of particles starting from the boundary of an interior cell
which is captured by the enclosed fiber. The geometry is Professor
Friedlaender's case B. The pair of curves labelled 99% are (a) the capture
efficiency of single interior cell of an infinite square array of parallel
fibers (the solid line) and (b) the capture efficiency of a single isolated
fiber (i.e., a single fiber in a non-interacting array), calculated by the
method of Zebel8 (the dashed line). The efficiencies and fiber spacing in
the arrays are based upon a 99% void fraction in the filter. The pair of
curves labelled 97% are the analogous cases of (a) and (b) above for 97%
void fraction. In analyzing the information contained in these curves, one
should be aware that the flow has been treated as potential flow, the viscous
force is assumed to be Stokes drag, and that the square matrix geometry
analyzed is probably not the most practical design for a filter. However, the
magnitude of the difference between the isolated fiber and the multifiber
efficiencies with even loosely-packed arrays certainly indicates a need for
caution in applying single fiber results to predict raultifiber efficiencies.
We agree with the lists of research needs in fundamental HGMS work
compiled by Professor Friedlaender in these proceedings and by KolnP. It is
our belief, however, that priorities must be established in the investigation
of high gradient magnetic capture mechanisms. First and foremost, the
dynamics of the single fiber particle trajectory model in both inertial and
non-inertial regimes should be checked experimentally. The trajectory model
has been the cornerstone of most recent buildup and filter efficiency
calculations and its validity has not been directly investigated. Next, in
view of new multifiber data presented here, the nature of particle capture in
geometrically - arrayed multifiber filters should be studied. Information
provided in these areas could enable designers to make more intelligent
decisions balancing engineering and economic considerations in future commer-
cializations of HGMS.
REFERENCES
[1] J. H. P. Watson, J. Appl. Phys., 44, 9, p. 4209 (1973).
[2] F. E. Luborsky, AIP Conf. Proc., 29_, p. 633 (1976).
[3] D. A. Himmelblau, M.S. Thesis (MIT, Cambridge, 1974) (unpublished).
[4] H. H. Kolm, IEEE Trans. Magn., MAG-12. 5, p. 450 (1976).
[5] D. C. Drehmel and C. H. Gooding, "High Gradient Magnetic Particulate
Collection," 82nd AIChE Nat. Meeting, Atlantic City, Aug. 29 - Sept. 1,
1976.
[6] W. F. Lawson, Jr., W. H. Simons, and R. P. Treat, J. Appl. Phys., 48
p. 3213 (1977).
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205
[7] W. H. Simons and R. P. Treat, private communications. Drs. Simons and
Treat provided the multifiber capture efficiency data shown in Figure 1.
A full account of their work will be published at a later date.
[8] G. Zebel, J. Colloid Sci., 20, p. 522 (1975).
OPEN DISCUSSION
Ariman: If you go back through all the HGMS presentations, I have a couple of
things to say. First of all, the workshop presentations and discussions so
far have been very exciting. In the morning sessions we did have very
interesting comments from specialists in different fields. For example, in
the morning, acoustics was the main area of presentations. We had stimulating
discussions from researchers in the acoustics,filtration and HGMS areas. But
when we cover the HGMS area it was quite interesting to see that, all of the
comments during the discussion presentation were made by HGMS workers.
I would like to make the following comments for the interest of the
participants who are experts in the HGMS. It seems to me there is a strong
emphasis for the basic reference in this field to Zebel's work. I think, it
is an excellent work. Quite a few of you have been doing a lot of work using
his model. It is a single fiber, single particle model. Professor Friedlander's
statement that Zebel did not only consider dielectric particles and conducting
fibers, but he looked also at both conducting and nonconducting particles.
For one of the cases, he obtains a closed form solution for the particle
trajectory equation. But for the other case he has a numerical solution for
the particle trajectory.
He also looks at both viscous and non-viscous flow cases. He has published
a total of four articles on this subject. Two were coauthored by one of his
associates, Walkenhorst. They also did some experimental work. But they talk
of a very special type of fiber filter. In other words, this fiber had a
special formation and a lot of distance between the fibers. That single
fiber model will hopefully be suitable for that particular filter. However, I
really do not know how good that single wire model would be for the very
complicated filter which is used in the HGMS process.
On the other hand we have modified and extended the single fiber model to
include the interference effects between the fibers. In the model there are
an infinite number of rows and infinite number of fibers on each row. We found
out that when the distance between the fibers is somewhat small, there was
quite a bit of difference between Zebel's model and our model. Furthermore,
a relatively recent publication, Ionia and his co-authors did both analytical
and experimental work and their experimental results compare very well with our
analytical model findings. Furthermore they concluded that when porosity of
the filter increases, there is a decrease in 'the collection efficiency. Those
are the results we obtained . We found the same result through our analytical
model prior to their publication.
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206
THE INFLUENCE OF ELECTROSTATIC FORCES AND OF THE PROBABILITY
OF ADHESION FOR PARTICLE COLLECTION IN FIBROUS FILTERS
F. Loffler
Institut fur Mechanische Verfahrenstechnik,
Universitat Karlsruhe
INTRODUCTION
Fibre filters have long been used industrially to sepa-
rate solid or liquid particles from gases . The number of pub-
lications on this topic is correspondingly large. The answer
to some important questions are nevertheless obscure to the
present day. This is particularly true for the technically
interesting particle size range above 0.5 - 1 ym and for
Reynolds' numbers larger than 0.1. Exceptionally striking
differences between theoretical calculations and experiments
have been observed in these regions.
Generally speaking the distances between the fibres in
these filters are large compared with the particles. The
particles are therefore not deposited by a screening action
but must be moved to the fibre surface and retained there. It
has already been shown | 1 | that the deposition efficiency is
the product of the collision efficiency n times the probabi-
lity of adhesion h.
= n • h (1)
One important reason for the above mentioned discrepancy
is the fact that the significance of the probability of ad-
hesion has been rather neglected, i.e. it was assumed that
all particles striking a fibre would also stick there. However,
this is frequently not so. Many theoretical papers on trans-
port processes, on the other hand, do not take sufficient
account of physical realities and simplify the problem unduly.
In order to elucidate some of the open questions on trans-
port and adhesion processes we have carried out a three-part
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207
research programme: theoretical and experimental studies of
single fibres, model filters and industrial filters. Some of
the results will be discussed below.
THEORETICAL MODEL
The deposition efficiency is calculated on the basis of
a model cylinder on which the incident flow is at right
angles.
U0,M,P
Fig.(l) - Model deposition on a fibre
The Brownian motion can be neglected in the particle size
range D > 0.5 ym under consideration here. In this case the
other p&ssible transport mechanisms include: inertia, gravi-
ty and electrostatic forces. Let the collision efficiency in
these cases be defined as (cf. Fig. 1):
(2)
where y is the origin of the coordinate of the limiting
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208
particle trajectory in the undisturbed flow in front of the
cylinder, i.e. the trajectory of particles which still just
touch the fibre. All particles of size D which arrive in the
band 2yQ will strike the fibre.
In order to calculate the collision efficiency it is
necessary to find the limiting particle trajectory by solving
the equation of motion of the particles. The general formula-
tion is: the sum of all the forces shall equal zero:
Fd + F. + Fg + p^
where Fd = the force of resistance, F. = the force of inertia,
Fg = force of gravity, Fel = the electrostatic force.
In the general case it follows from equation (3) that
nTES = f (*, Re, W, Fr, Nel) (4)
p D 2-UQ
where ijj = ^ P p the inertia parameter (Stokes'' number) ,
,_ _ VDf'
the Reynolds number, W = ' - the density ratio,
Froude's number and Nel an electric charge para-
meter.
Nel is tne ratio of the electrostatic force at a certain
point to the drag force. Various modifications of the elec-
trostatic effect are in principle possible, depending on
whether the particles, the fibres, or both of them are charged
and whether external fields are applied. Assuming that both the
particles and the fibres are charged, i.e. that Coulomb forces
act,
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209
N . = N
Q ' * - (5)
where Q is the fibre charge per unit length, q = particle
charge, e = influence constant =8.86 • 1CT12 Coul/V-m.
The differential equation of the particle trajectory can
only be solved numerically. The required condition of contact
is that the particle surface must touch the surface of the
fibre.
Most authors simplify the calculation according to equation
(3) by entering only a single transport effect and estimating
the overall effect by superimposing various partial ones. Par-
ticles are also frequently treated as mass points, and their
geometrical size is additively superimposed as what is some-
times called the interception effect. The following results
will show that both these simplifications lead to errors.
The solution of the equation of motion involves a con-
siderable problem: the velocity field for the flow round the
fibre is required. There are no closed solutions for the
Navier-Stokes equations for the region of Reynolds' numbers
0.1 < Re < 100 which is relevant for many practical, industrial
cases. Suneja and Lee |2| worked out a numerical solution for
discrete Re numbers (1, 10, 50). All that is otherwise
available are approximate solutions, e.g. Lambs' solution for
the viscous flow round a single cylinder, or that of Kuwabara
for a fibre system who neglected the forces of inertia com-
pletely in the flow; this solution therefore holds strictly
only for Re = 0.
For Re > 100 the equations for potential flow are often
used for single cylinders. Any decision on how closely these
various approximations reproduce the true flow round single
fibres, even layers of fibres, must be arrived at by careful
experiments.
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210
SOME NUMERICAL RESULTS FOR THEORETICAL SINGLE FIBRE COLLISION
EFFICIENCIES
A. No electrostatic forces
(Nel = 0)
We calculated particle trajectories for Re > 50 with
the potential flow field plus an approximate equation for the
boundary layer flow |3|. For Re < 1 we used Lambs flow field.
XT 2 t 6 810 2 4 6810° 2 4 6810 2 4 6810
18uDf
Fig. (2) - Theoretical single fibre collision
efficiencies nT (Inertia effect only)
Fig. 2 shows collision efficiencies nT calculated for an
inertia effect only-i.e.(without gravity) for a particle den-
sity p = 0,86 g/cm . In the high Reynolds number-range only
the curves for Re = 50 and 500 are plotted. The curves for
Re = 1, 10, 50 represent results of Suneja and Lee J2|. We
see that their and our results for Re = 50 are in fairly good
agreement.
For Re = 500 we found good corrrsponda nee with the results
of Langmuir and Blodgett in the range of fy > 2. Langmuir and
Blodgett used only the potential flow field and their results
-------
211
are not plotted in the figure. For ijj < 2 our values are some
what smaller, which is due to the influence of the boundary
layer flow.
It is possible to fit our numerical results for 50 < Re
500 with good accuracy by the following equations:
nT = n., (*» Re) + n2 (*, Re, p/pp) (6)
2
f.,(Re)i|i + f2(Re) i|i + f3(Re)
1/2
" -
f (Re) = - O.O133 In Re + O.9 31
fJ(Re) = + 0.0353 In Re - 0.36
f^ (Re) = - 0.0537 In Re + 0.398
The curves of Suneja and Lee show that for \l> < 1 nT de-
creases with decreasing Reynolds number, whilst for ip > 1 we
observed the reverse tendency. This is because to keep ty
constant we need a bigger particle size D with decreasing
Re i.e. with decreasing velocity U . This^results in a higher
interception effect; therefore nT increases.
The same effect we found also in our calculations in the
range Re < 1 using Lambs flow field.
Fig. 3 shows the results for the Reynolds-numbers 0.1, 0.2
and 0.66. There are two groups of curves. The dotted lines
give the results which have been calculated by assuming the
particles to be mass points ("without interception effect )
whilst the full lines give the results when the geometrical
size of the particles was allowed for.
-------
212
1.2
1.1
1.0
as
0.8
0.7
| 0.6
-0.5
0.4
0.3
0.2
0.1
Inertia effect
Pp'2.164g/cm3
without
interception
Pp' 0.86 und
2.164 g/cm3
0.1 0,2 0,4 OJ50.81
4 6 8 10 20
100
Fig. (3) - Theoretical single fibre collision
efficiencies ru, (Inertia effect,
,'T
without gravity,Lamb's flow field)
From this example it will be seen that the negleot of the
particle dimensions leads to considerable errors, particu-
larly in the range i|> > 1 . The correct contact condition must
be used from the very start.
The influence of gravity was calculated for a vertical
flow downwards, i.e. for a flow in the same direction as
gravity is acting.
Fig. 4 shows curves plotted for different Reynolds-numbers
and for two particle densities p = 2.164 g/cm3 and 0.86 g/cm3.
The dotted lines have been calcinated as before and the particle
dimension is neglected.
We see that for the correct contact condition we have high
values for the collision efficiencies r\ . The increasing rimc
with decreasing Reynolds number or decreasing particle density
is again due to the fact that for constant ty we have bigger
particle sizes D for decreasing Re or p .
-------
213
Fig. (4) - Theoretical single fibre collision
efficiencies TITS
(Inertia effect , with gravity ,
Lamb's flow field)
In our opinion this effect of gravity has not been con-
sidered sufficiently till now.
B,
With electrostatic forces
We must expect considerable effects in the case of
coulomb forces. For this case we calculated the influence of
electrostatic forces by inserting them into the equation of
motion and solving the complete equation numerically. Most
important are the results for the range Re < 1 which will be
discussed in the following.
Fig. 5 shows some calculated particle trajectories round a
cylindrical fibre for different particle sizes in the diameter
range 1 - 10 ym.
-------
214
Lamb's flow field
Fig. (5) - Theoretical particle trajectories
(Lamb's flow field. Re = 0.63)
The upper two examples show trajectories which just pass
the fibre. All particles of the same size starting at a point
1/1OOO D closer to the axis of symmetry will touch the fibre
surface. This touching trajectory is called the limiting tra-
jectory.
The top picture shows the case without electrostatic forces,
Here collision is only due to inertia. Small particles pass
round the fibre, i.e. they have small collision efficiencies.
The 10 urn-particles are only little influenced by curved flow
around the fibre.
-------
215
In the centre we see the corresponding particle trajecto-
ries for charged particles and fibres. These have a product of
the charges Q • q = 10~28 Coul2/cm. This value is approximately
equal to our experimental values.
The enormous influence of the electrostatic forces can be
seen clearly especially for the smaller particles. Their colli-
sion efficiencies are increased. Obviously there exists a mini-
mum collision efficiency at a particle size of about 5 urn. The
path of the 1O ym-particle is nearly unaffected by the electro-
static forces.
The focussing of the particles on the axis of symmetry be-
hind the fibre is a most interesting effect. This nonhomogenous
distribution of particles behind the fibre has some influence
of course on the deposition in the following layers of fibres.
This is in contrast with any theory which assumes homogenous
particle distribution.
In the lower picture we see the touching and thus limiting
trajectories. The larger particles which exhibit little influ-
ence by the electrostatic forces strike the fiber on its front
side. The smaller ones first pass the fibre and are then attrac-
ted to its far side.
From the limiting particle trajectories we calculate the
theoretical collision efficiencies, shown in fig. 6.
In fig. 6 the single fibre collision efficiency HTES nas
been plotted, as a complete solution of equation (3), against
the inertia parameter ty for a particle density pp = 2.164 g/cm
and for Re = 0.2. The parameter of the curves is the charge pa-
rameter NQg. The above, correct contact condition was assumed
for the curves marked "with interception effect", i.e. the-.geo-
metrical size of the particles was allowed for, while they were
regarded as mass points in the curves "without interception
effect".
The curve for Ngg = O holds when there is no charge. As
NQg rises, the collision efficiency increases quickly for
small ijj values. However, the curves for Ngg pass through a
minimum at about ij; = 1 and then approach the curve for Ngq = 0
asymptotically. This means that above ty = 1 the influence of
-------
216
the Coulomb force becomes less than that of the force of
inertia. The total collision efficiency in the region of the
minimum is less than that produced by electrostatic attraction
alone, i.e. inertia and the force of gravity counteract the
electrostatic force. If the force of inertia and the force
of gravity were to be neglected, the electrostatic attraction
by itself would give a collision efficiency independent of ijj.
This shape of the curves shows that partial solution for
the equations of motion obtained by neglecting individual
forces cannot be superimposed. That means that all the effec-
tive forces must be entered in the equation of motion, which
should really be a matter of course but has generally not been
done in the past.
In order to cover the additional influence of electrostatic
forces as against the forces of inertia and gravity, the
difference between HTES an^ tne collision efficiencies was
formed for the case HTS witn no charge.
nE nTES ~ nTS
(7)
This difference can be approximated by
= 1 .22(2 - In Re) . N
E ( |jj_ , 1 .5 + 1 " Qq
2/Re
This equation determines for the first time the dependence
of the additional collision efficiency TIE on the parameter fy,
Re and N. Past publications always gave. TIE no more than HE =
const f(N_), which is fundamentally incorrect.
-------
217
t.6
Fig. (6) - Theoretical single fibre
collision efficiencies
(Inertia with gravity with
electrostatic forces. Lamb's
flow field)
DEPOSITION EFFICIENCY MEASUREMENTS
Comprehensive experiments with model filters were carried
out as a check of the collision theories |5|.
Fig. 7 shows a schematic diagram of the apparatus.
The model filters consisted of several layers of parallel
fibres. The fibre diameter was Df = 38 ym. The distances bet-
ween the fibres varied between 1T5 ym and 1600 ym in the direc-
tion of the flow and between 115 ym and 275 ym at right angles
-------
218
to this direction so that the percentages of the fibre volume
ranged between 0.3 and 8 %.
Monodisperse NaCl particles and paraffin wax particles in
the range of about 1 to 10 ym were used.
Kr-85
p-ray source
filter
rotor
exhaust
Fig. (7) - Schematic diagram of experimental
apparatus
For the production of the NaCl-particles we used a
spinning-top-Generator as shown in Fig. 7. The geometric stan-
dard deviation a was for all size distribution < 1.12. The
Kr 85-3 ray source served for reducing the electrostatic
charge of the particles in certain experimental runs.
Paraffin wax particles were produced by a Sinclair-La Mer-
Generator. These particles are almost spheres and practically
without electrostatic charge; their size distributions have
geometric standard deviations 1.09 < o < 1.12.
Particle size distribution and concentration are measured
continously and directly in the undisturbed flow by means of
a scattered light instrument. This instrument developed in our
-------
219
institute does not require the extraction of a sample and is
also capable of measuring high particle concentrations up to
106 Part./cm3.
The velocity of the incident flow was varied between 25 cm/s
and 200 cm/s. This results in a range from Re = 0.63 to Re = 5.2,
1.4
1.3
12
1.1
1.0
09
0.8
07
06
05
0.4
03
0.2
0.1
Symbol
0.0 01 02 05 1
5 10
18 M OF
Fig. (8) - Single fibre deposition
efficiencies for NaCl
particles, particles and
fibres charged
These measurements determine the deposition efficiency
i.e. the product of the collision efficiency n and the proba-
bility of adhesion h. In Fig. 8 the experimental single, fibre
deposition efficiencies
-------
220
The shape of the curves is very reminiscent of the theo-
retical curves in Fig. 6. The strong maximum in the region
ty < 1 for particle sizes Dp < 3 ym is very remarkable. The
deposition efficiencies measured in this region can only be
explained by the effect of Coulomb forces. This is also shown
by the theoretical curve for Q-q = const = 7.8 • 109 eQ2/cm and
for Re = 0.63 which has been entered for comparison as sequen-
ces of two dots and a dash.
9 2
The charge product 7.8-10 eQ /cm was confirmed by
measuring the charge on particles and fibres; e.g. for 2.7 ym
particles we measured about 1100 e charges, the fibre charge
per unit length was in the order of magnitude of 106 - 10'e^cm.
The fact that electrostatic forces significantly affect the
curves, especially for particles smaller than 3 ym, was also
demonstrated by changing the charges on particles and fibres.
Fig. (9) - Single fibre deposition efficiencies
for NaCl particles.
(Particles and fibres discharged)
-------
221
In Fig. 9 some single fibre deposition efficiencies have
been plotted against the inertia parameter iji for three different
Reynolds-numbers. The corresponding velocities were in the
range 25 cm/s to 100 cm/s. Compared with the results in Fig. 8
we find a drastic change in deposition especially for values
of ty < 1, i.e. for fine particles < 3 ym. But presumably in
this range particles and fibres were not completely discharged;
in the range ijj < 0.2 the experimental values of deposition
efficiencies are somewhat higher than the theoretical collision
efficiencies. We suppose this to be an effect of remaining
charges.
Particles of 5 ym and more were about equally deposited
whether they were charged or not. This observation again con-
firms the theoretical prediction of a decrease in the influence
of the electrostatic charge as the effect of inertia increases.
In the region i|j > 1 it is striking that the deposition
efficiencies become smaller with increasing Re. This contra-
dicts theoretical expectations. We explain it by an increase
in the elastic rebound of the crystalline NaCl particles
striking the fibre which is equivalent to a drop in the pro-
bability of adhesion. This process will be dealt with in the
next section.
A completely different behaviour from that of the NaCl
particles is shown by the softer paraffin wax particles.
These electrostatically neutral particles were deposited much
better than the NaCl particles at large inertia parameter ip.
As we see in Fig. 10 the deposition efficiency decreased
steeply with dropping i|>. The Reynolds number had practically
no observable influence between Re = 0.63 to Re = 2.63. This,
incidentally, is also in agreement with the theoretical pre-
diction when the size of the particles is taken into account.
A decrease in the deposition efficiency was only measured
for Re = 5.26, corresponding to Uo = 2 m/s, and for smaller
Reynold's numbers at ip > 2-3, and here, too, the explanation
is a decrease in the probability of adhesion.
With regard to the influence of the filter fibre volume
fraction packing density a it may be noted that no dependence
was found for a < 3 %. The fibres behaved like single fibres.
-------
222
A slight rise in the deposition efficiency was only measured
at a = 5.4 %.
o
I-
0
1 6810
o
PflRRFFIN
PRRTICLES
('(
1C
00
[ O 0
lo.c
o
8
fY
c
<
(1
v\
-, it
•o ji
u
1
d
,
()°
"^ ^)
^
'' DA
,,/<
1 0
l'» /
/
>1!/
L
3«
R
n/
/ >
!
'y
y f
^
P
x
/
E
«5i
M
/
c >
c
/
=0.6
•=.&/REzS.6
/
5
o-'
io-' 10° lo1
INERTIfl PBRflMETER-PSI
10'
Fig.(10) - Single fibre depo-
sition efficiencies
for paraffin wax
particles. Fibre Vo-
lume fraction
a = 0.017
Fig.(11) -
Comparison of
single fibre de-
position efficien-
cies and theories
for paraffin wax
particles
In Fig. 11 the experimental values of single fibre depo-
sition efficiences for paraffin wax particles for all fibre
volume fractions and all Reynolds-numbers have been plotted.
The full lines represent theoretical calculations of Suneja
and Lee |2| when they are recalculated for Re = 0.6 and
Re = 5.6.
-------
223
When a particle strikes a fibre surface it can only stick
if, firstly it does not rebound and, secondly, if it is not
subsequently detached.
Earlier studies |8| have shown that once particles are
deposited on a fibre they stick very tenaciously. What is
crucial therefore is the impact process when the particle
strikes the fibre which is an elastic-plastic impact invol-
ving elastic forces of resilience which may cause the particle
to rebound.
The speed at which the start of rebounding may be expec-
ted can be estimated by means of an energy balance |3,9|.
From this we obtain with the assumption that the deforma-
tion at the contact point is inealstic and that the main ad-
hesion mechanism is due to van der Waals forces:
u° = ^ ^7^ = Lifshitz-van der Waals energy
h* = h/2 ir where h = Planck's constant
z = minimum distance of surfaces on
0 contact * 4-10~° cm
H = micro-hardness of the softer material
p = particle density
If we assume the deformation to be an elastic one we also ob-
tain a Uc a (k-DD)~T porportionality. The coefficient of
restitution involves the material properties but it is sub-
stantially unknown for microscopic particles. We are there-
fore currently working on a project for determining the im-
pact number.
-------
224
From estimates for k and other material parameters we ob-
tained, for example, for Dp = 10 ym, U = 2-15 cm/s and for
= 5 ym, U = 4 - 10 cm/s.
D
However, these estimates do not supply the distribution
of the probability of adhesion but merely the start of the
rebound. More accurate information on the probability of ad-
hesion must be obtained experimentally. We therefore deve-
loped a method for the direct determination of the probabi-
lity of adhesion by the high-speed cinematograph |1,9 of
particles striking a single fibre. We regard this direct me-
thod as more accurate and reliable than the comparison of
measured deposition efficiencies with theoretical collision
efficiencies as suggested recently by Stenhouse et al. 1 10 1.
Btta -Source
Elutnotor
Lead Sheat
/ Xenon -lamp Da
irk Field Condenser/ !
Housing
Test Fiber, adjustable
big
Fig. (12) - Scheme of apparatus for determining
probability of adhesion
Fig. 12 shows a scheme of the apparatus by which we ob-
served the particle trajectories.
In the apparatus a free jet, loaded with particles, im-
pinges vertically downwards upon a fibre. The observed space
-------
225
is illuminated with a 250 W xenon lamp which is pulsed to three
times the power during the periods of film exposure.
In order to reduce the electrostatic charge on the particle
impinging on the fibre, from a 0 ray source, ions are genera-
ted, which accumulate on the particles. An electric field is
applied at right angles to the direction of flow and this
defelcts the charged particles sufficiently to stop them from
colliding with the fibre.
The particle trajectories for determining the adhesion pro-
bability are recorded by the arrangement shown in Fig. 12 in
which the high-speed film camera is at right angles to the
axis of the fibre (max. speed 30.000 frames/second). The par-
ticle velocity can be controlled between 3 cm/s and several m/s,
and particles as small as 3 ym can be recorded.
IP
Polyamidfibre 20 pm
Quartz - particles
• Glass - spheres
70 80 cm/s 100
U0
Fig. (13) - Probability of adhesion for quartz
particles and glass spheres on a 2O ym
polyamide fibre
-------
226
Muhr's theoretical curve for Re = 0.66 was calculated with
Lamb's flow field. The comparison with the readings and with
the Suneja and Lee curve for Re = 0.6 suggests that at Re = 0.66
Lamb's flow field evidently provides unduly small collision
efficiencies.
Below ty = 0.2 the readings are higher than would be expec-
ted from the pure effect of inertia. This can, however, be ex-
plained with the Stenhouse theory |6| through elecstrostatic
forces between uncharged particles and charged fibres.
Nguyen and Beeckmans |7| derived an empirical equation from
measurements, but this provides much smaller deposition effi-
ciencies than our measurements with paraffin wax particles.
These authors also found a strong dependence on the Reynolds
number and the percentage of the fibre volume. Presumably the
discrepancy arises from the fact that Nguyen et al. based the
fitting of their equation mainly on measurements with crystal-
line particles. In these cases the material property results
in differences in the probability of adhesion - rather as with
our NaCl measurements. We therefore consider the application'
of this empirical equation to other practical cases to be
highly problematical.
In the current state of knowledge we regard Suneja and
Lee's theory for the calculation of collision efficiencies in
the transition region of Reynolds' numbers O.5 < Re <50 as very
suitable as long as there are no electrostatic forces. In the
case of Coulomb forces their additional effect can be estima-
ted with equation (8) according to Muhr.
PROBABILITY OF ADHESION
It has already been pointed out in the introduction that
particles transported to the surface of the fibre must also be
retained there. That this is by no means always the case is
particularly clearly shown by the experiments with NaCl par-
ticles but also by those with paraffin wax particles. The
literature contains numerous references to inadequate adhesion,
e.g. the experiments of Gillespie and of Whitby are typical.
-------
227
100
h/"/.
60
40
20
n
\
\
^
— A
•
V.
\ .'
X.
•
dp = 5,1 pm
dp = 7pm
A A
— — — """" "•
•»
•
^H*
>1^
•
roruiii
Polyiai
n - Oil
rud- Fibre 20pm
0 10 20 30 tO SO 60 70
U0 fan Is
(14) - Probability of adhesion for droplets
of paraffin oil
By way of example the probability of adhesion h has been
plotted in Fig. 13 against the speed of incident flow UQ for
various kinds and sizes of particles and types of fibres.
Rebounding starts at about 5-15 cm/s. As the speed in-
creases, the probability of adhesion drops quickly. In accor-
dance with theoretical expectations the probability of adhesion
is less for D = 10 ym than for D = 5 ym, and the particles also
rebound more Itrongly on the hardir glass fibre than on the poly-
amide fibre. The values for the start of rebounding and the trend
with respect to particle size and fibre hardness correspond fair-
ly well with theoretical estimates.
The lower adhesion probability with glass spheres is presum-
ably an effect of the contact geometry. Irregular quartz par-
ticles may have multi-point or area contacts while only single-
point contacts are possible with the very smooth glass spheres.
The readings for the probability of adhesion for oil drop-
lets shown in Fig. 14 demonstrate that 7 pm droplets are more
prone to rebound than 5 ym droplets. It may be surprising that
-------
228
at 30 cm/s as many as 35 to 50% should rebound. So far we ex-
plain it by the fact that in this velocity region the droplets
react as if they were elastic. At higher velocities the plastic
deformation of the droplets and therefore the contact area are
larger which results in a higher probability of adhesion. Still,
this behaviour will have to be examined in more detail. Current-
ly we are also examining the probability of adhesion of paraf-
fin wax particles. Here, too, first results indicate rebounding
which had already been recognisable, at any rate at higher velo-
cities and with larger particles, during the deposition experi-
ments on model filters.
Our own studies with model filters and real filters, just
like the work of other authors, have shown that the problem of
the probability of adhesion is not only important for single
fibres but also for layers of fibres. With model filters |5|
and NaCl particles very similar distributions of the probabili-
ty of adhesion were found as for quartz particles on single
fibres, even when the percentage of fibres by volume was 8%.
The reason must be that in many cases the acceleration zones
for particles between the fibres are often smaller than the
mean fibre distances and that the particles, after a first
rebound, will therefore strike subsequent fibres at approxi-
mately the same velocity. Raczynski et al |11|, for instance,
have reported on studies of the deposition of quartz particles
in glass sphere filters. These authors, too, found probabili-
ties of adhesion of less than 1, even for 1 pm particles.
So far no final description of the probability of adhesion
is possible because the investigations have not yet been con-
cluded. Nevertheless existing results already explain many of
the phenomena observed in the laboratory and in practice, and
they also yield useful suggestions for the development or
preparation of filters. Some questions are still left open but
the need to allow more thoroughly for the probability of ad-
hesion to solve practical problems is nevertheless evident.
-------
229
FINAL OBSERVATION
The theoretical and experimental investigations presented
here assume simple models: a single fibre and a model filter
consisting of parallel fibres. In reality filter practice is
quite different, at least with respect to filter structure.
Nevertheless we selected this way not only because it corresponds
to the procedure in the classic filter theory but also because
it was possible to achieve very informative detail on the basic
phenomena with the influences separated.
In a third step, of course, investigations of real filters
are necessary and these we have started already. The aim of this
new work is to find out the relationship between model ex-
periments and real filters. In doing so the material properties
of the particles are to be allowed for, especially with respect
to adhesion properties.
-------
230
REFERENCES
|1| Loffler, F., H. Umhauer: Eine optische Methode zur Bestim-
mung der Teilchenabscheidung an Filterfasern.
Staub - Reinhaltung der Luft 31, 51-55 (1971)
|2| Suneja, S.K., C.H. Lee: Aerosol filtration by fibrous fil-
ters at intermediale Reynolds numbers.
Atm. Environment 8, 10-81-1094 (1974)
|3| Loffler, F., W. Muhr: Die Abscheidung von Feststoffteilchen
und Tropfen an Kreiszylindern infolge von Trag-
heitskraften.
Chemie-Ing.-Tech. £4, 510-514 (1972)
|4| Muhr, W.: Theoretische und experimentelle Untersuchungen
der Partikelabscheidung in Faserfiltern durch Feld-
und Tragheitskrafte.
Dissertation Universitat Karlsruhe, 1976
|5| Loffler, F., W. Muhr: A study of the deposition of partic-
les in the 1-10 microns range in model filters.
Filtration - Separation V1_, 172-178 (1974)
|6| Stenhouse, J.I.T.: The influence of electrostatic forces
in fibrous filtration.
Filtration - Separation 11, 25 - 26 (1975)
|7| Nguyen, C., J.M. Beeckmans: Single fibre capture efficien-
cies of aerosol particles in real and model fil-
ters in the intertial interceptive domain.
J. Aerosol Sci. 6, 205-212 (1975)
|8| Loffler, F.: Blow-off of particles collected on filter
fibres.
Filtration - Separation £, 688-696 (1972)
|9| Loffler, F.: Adhesion probability in fibre filters.
Clean Air fit, 75-78 (1974)
|10|Stenhouse, J.I.T., D.C. Freshwater: Particle adhesion in
fibrous air filters.
Trans.Instn.Chem.Engrs. 54, 95-99 (1976)
-------
231
[11] Raczynski, B., J.C. Guichard: Penetration des aerosols dans les lits
fixes de billes de verre. Froc. 1st World Filtration Congress, Paris,
14-17, Mai 1974.
WRITTEN DISCUSSION
Hsu-Chi-Yeh, Ph.D.
Inhalation Toxicology Research Institute
Lovelace Biomedical and Environmental Research Institute, Inc.
P.O. Box 5890
Albuquerque, New Mexico 87115
The effects of electrostatic forces and the coefficient of adhesion on
particle deposition in fibrous filters are among those areas which are still
not well understood in filtration theory. Professor Lb'effler's paper will
surely contribute to further understanding of these two interesting areas of
filtration and will serve to stimulate future studies.
Two parameters are important in the design of fibrous filters: the
efficiency of collection and the pressure drop across the filter. Practical
fibrous filters usually have high porosity, with the inter-fiber distance
large as compared to the fiber diameter, and their fiber packing density is
normally less than 0.1. Fibrous filters can be grouped into two classes :
very fine filters of high efficiency with fiber diameter on the order of 20
ym or less and moderate to low efficiency filters with larger diameter fibers
of up to about 200 ym. High efficiency filters are normally used to remove
submicron particles and are operated with relatively low gas velocity of a
few centimeters per second. Therefore, the Reynolds number (Re) is normally
less than 1. For moderate to low efficiency filters which may be used as
pre-filters in ventilating systems, higher gas velocities of about 1-3 m/sec
may be used and the Re could go up to about 50. For filters operated at low
flow velocity, i.e., Re < 1, those deposition mechanisms due to diffusion,
inertial impaction and interception are understood, and various modern
filtration theories ' can be used to predict the collection efficiency with
good agreement with experimental data when inhomogerieity of a real filter is
taken into account from the measurement of the pressure drop across the filter.
Still not well understood in filtration by fibrous filters are deposition
mechanisms such as electrostatic attraction, gravity effect, acoustical
loading effect, the phenomenon of particle rebound at higher flow velocity,
etc.
-------
232
The first step in solving the basic problem of filtration is the calculat-
ion of the flow field associated with filters. Because of the complicated
structures of a real filter, a filter model has to be used. Numerous,such
modeljs and their calculated flow fields, such as an isolated cylinder , a cell
1 ' , a staggered-array model and a fan model ,-have been reported.
I dl/tT.TO ^VlA 0^4 r*r»n^>nA •*«•*»«««* .^J —. 1 ._ *..«. J 1_._ V_l_ 1 Ti • *\ O _1_
model'
Figure 1 shows the staggered-array model used by Yeh and Figures 2-3 show
the flow field calculated by solving full Navier-Stokes equations numerically
for two Reynolds numbers, 0.4 and 30, with fiber packing density of 0.1. The
results of these flow field calculations indicate that both the Lamb's flow
and potential flow describe poorly for a system of cylinders. They also suggest
that Kuwabara's flow field is a good approximation to the staggered-array
model for Re less than 1, even though the Kuwabara's flow was derived from
Re=0. The same conclusion has been obtained experimentally by Kirsh and Fuchs .
It has been shown that the cell model by Kuwabara and the fan model both
describe adequately the dependence of pressure drop upon fiber packing density
and face velocity for Re < 1. For Re > 1, there is still no analytical solution
of flow field in either a system of cylinders or an isolated cylinder.
Fig. (1) Staggered-Array Model
Many current theories of aerosol filtration by fibrous filters have been
based on or modified from the Kuwabara-Happel flow field for the cell model.
Various theories which take into account two or more deposition mechanisms
simultaneously have been developed for calculating the single fiber efficiency
of a filter. These are diffusion with interception ,, inertia with intercept-
ion2, 3, 10 and diffusion plus inertia plus interception . It would be interesting
to comparec. results of inertia with interception presented here with other
theories, but the theoretical treatment of inertia with or without interception
-------
233
2.5
2
• Kuwabara's CeU Model,
A Happel's CeU Kodel,
Q Klrsh & Fuehs1 Fan llodel,
V Splelnan & Coren's I!odel,
O Lai* "8 Flow,
O Potential Flow,
1
' 0.05999, 0.2076
0.1,205 , 0.6835
• 0.05999, 0.2076
• 0.05999, 0.2076
• 0.05999, 0.2076
• 0.05999
0.05999
T
RE = 0.4
w rotcmiai rxow, T- v*vsf7r
Theoretical Results for Staggered-Array Model
Figure 2: Comparison of Streamlines for Various Theories
(Re =0.4 and Fiber Packing Density a = 0.1)
KUWABARAS FLOW, ^=0.107
0 I
10 II
Figure 3: Streamlines for Re = 30 and Fiber Packing Density a = 0.1
-------
234
in his paper is not very clear. Also, some parameters such as interception
parameters and fiber packing density are needed to explore other theories
but are not listed, making comparison impossible. In addition, there is
some doubt about the validity of his Figure 2. If it is for the inertial effect
only without interception, then the single fiber efficiency curve should not
go beyond the maximum of 1. If the figure represents inertia with inter-
ception, then the method for obtaining the curves which represent the results
of Suneja and Lee should be mentioned, for their original curves for Re = 1,
10, and 60 (not 50 as quoted in the paper) represent results of inertia with-
out interception.
The application of electrostatic forces in fibrous filter filtration is a
novel way to enhance filter efficiency. As mentioned in Professor LBffler's
paper, there are several forms of electrostatic forces depending upon whether
the particles, the fibers, or both of them are charged or whether an external
electric field is applied. In the paper, Professor LOffler showed some in-
teresting effects of coulomb force on filtration (i.e., both particles and fibers
are charged), such as nonhomogenous distribution of particles behind the fiber.
On the other hand, it also raises some questions which need to be answered.
In the case of charged particles with fibers either charged or uncharged, the
electrostatic force (or electric charge parameter) will change with time as
charged particles continue to be collected on the fiber. What the effects of
this would be are as yet unanswered. According to Fig. 6, his theory predicted
that, for Stokes number < 1, the physical size of the particle is immaterial
(i.e., independent of the interception parameter) and this needs to be checked
experimentally using aerosols of different densities. In experiments where
both particles and fibers were discharged, he showed that for paraffin wax
particles (Fig. 10 in the paper), the Reynolds number had practically no
influence between Re = 0.63 to Re - 2.63, while for NaCl particles Fig. 9),
the Reynolds number clearly showed influence on deposition efficiency, even
for Stokes number < 1 where rebound is not a problem. The reason for this
discrepancy is unknown. Also it would be interesting to compare Figs. 9
and 10 of the data with the theoretical prediction shown in Fig. 3 and 4.
The coefficient of adhesion (sometimes called accommodation coefficient)
is not only important in high velocity filtration.it is also important in
sampling and/or sizing devices such as impactors. Professor LOffler's experi-
mental approach, direct observation of particle rebound using a high-speed
cinematography technique, would also be useful in investigating rebound
problems in other devices. The probability of adhesion in filtration is an
area where further studies are needed.
-------
235
REFERENCES
[ 11 Stenhouse, J. I. T., P. J. Lloyd and R. E. Buxton, "The retention of
large particles (> 2 ym) in fibrous filters," Amer. Ind. Hyg. Assoc. J.
37J7): 432-436, 1976.
[21 Stechkina, I. B., A. A. Kirsh and N. A. Fuchs, "Investigations of fibrous
filters for aerosols. Calculation of aerosol deposition in model filters
in the region of maximum particle breakthrough," Kolloidn. Zh. 31_: 121-
126, 1969.
[3] Yeh, H. C. and B. Y. H. Liu, "Aerosol filtration by fibrous filters - I.
Theoretical." J. Aerosol Science 5: 191-204, 1974.
[4] Lamb, H., Hydrodynamics. 6th ed., p. 77, Cambridge University Press,
London, 1932.
[5] Kuwabara, S., "The forces experienced by randomly distributed parallel
circular cylinders or spheres in a viscous flow at small Reynolds numbers,"
J. Phys. Soc. Jpn. 14: 527-532, 1959.
[6] Happel, J., "Viscous flow relative to arrays of cylinders," A.I.Ch.E. J.
5.: 174-177, 1959.
[7] Yeh, H. C., "A fundamental study of aerosol filtration by fibrous filters,"
Ph.D. Thesis, University of Minnesota, Minneapolis, 1972.
[81 Kirsch, A. A. and N. A. Fuchs, "The fluid flow in a system of parallel
cylinders perpendicular to the flow direction at small Reynolds numbers,"
J. Phys. Soc. Jpn. 22(5): 1251-1255, 1967.
[9] Stechkina, I. B. and N. A. Fuchs, "Studies on fibrous aerosol filters - I.
Calculation of diffusional deposition of aerosols in fibrous filters,"
Ann. Occup. Hyg. £: 59-64, 1966.
[10] Harrop, J. A. and J. I. T. Stenhouse, "The theoretical prediction of inertial
impaction efficiencies in fibrous filters," Chem. Eng. Sci. 24: 1475-1481,
1969.
[11] Suneja, S. K. and C. H. Lee, "Aerosol filtration by fibrous filters at inter-
mediate Reynolds numbers (< 100)," Atmos. Environ. 8: 1081-1094, 1974.
OPEN DISCUSSION
Lbffler: Thank you for the interesting comments,Dr. Yeh. You mentioned that
the Kuwabara flow field in the experiment of Fuchs, confirmed the validity of
Kuwabara flow field. We should not forget that Kuwarbara flow field assumed
that the Reynolds number is very near to zero. If you deal with flow fields
with Reynolds numbers between 4 and 30, I doubt if this is possible. Because
we suppose that these Kuwabara flow fields are tested by Fuchs in a very low
-------
236
Reynolds number range (about .001) and is only tested by a measure of direct
force. The direct force with a velocity and pressure distribution in the
very near neighborhood of the fibers is important. As for particle
collection we need the velocity distributions in the whole flow field.
Perhaps I should point out that I talked a lot about theories. This
is in no good accordance with the time we spent for experiments. We have
spent a very short time, about 1 1/2 years, with the theories, and
about 10 years doing experiments. That means we believe the theories are
very helpful but we can not find a solution to the problem of filtration
only from the theoretical point of view. We need experiments, especially
since in industrial filters the situation is quite different from the
theoretical models. That is why we do no.t like to emphasize the theoretical
views. We should look more into the material behavior.
You mentioned there is a difference between the influence of Reynolds
number to collection efficiency between parafin wax particles and filter.
That is just the point. What I meant was the material behavior. For
sodium chloride particles, we are sure to have an elastic rebound. Because
we could recalculate adhesion probability, we could theorize the same
behavior from the probability of adhesion. We measured the beginning of
rebound of a particle with a velocity in the range of 5 to 7 centimeters/
second. In our experiments with sodium chloride particles, and multifilter
the lowest velocity has been 25 centimeters/second. It is in a range where
adhesion probability is about 80%. This is our explanation for the
difference between sodium chloride particles and wax particles.
One very important change is the electrostatic behavior of particles at
the fiber. From the theoretical point of view, I can only expect there
must be something going on during the build-up. We do not have a
theoretical solution. We have tried to find experimental solutions!
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237
ELECTROFLUIDIZED BEDS FOR INDUSTRIAL SCALE AIR POLLUTION CONTROL
J. R. Melcher
Department of Electrical Engineering and Computer Science
Massachusetts Institute of Technology
ABSTRACT
A review is given of the fundamental and practical development of the
electrofluidized bed, beginning with application to the control of submicron
particualte. Models and corroborating experiments establish fundamental
particle collection coefficients, extend two-phase fluidized bed theory to
account for bubble bypassing and describe the performance with ac excitations.
Practical developments in the control of oil ash and asphaltic fume are de-
scribed. Bed particle scale electromechanics are elucidated by recently
developed models that are the basis for current efforts to apply the bed to
controlling coal ash.
INTRODUCTION
As it is applied to air pollution control, an electrofluidized bed (EFB)
is a shallow bed of particles, fluidized by the polluted gas, with an electric
field applied by means of electrodes. Typically, the gas-entrained pollutant
particles are electrically charged by ion impact prior to entering the EFB.
Stressed as they are by the electric field, the bed particles act as collec-
tion sites for the pollutant. By design, they do not carry a significant net
charge. The fluidized, usually semi-insulating, particles are subjected to
an electric field that is imposed either transverse to the flow (the cross-
flow configuration) or, by means of screen electrodes and the distributor
plate, co-linear with the flow (co-flow configuration).
The following sections present an overview of fundamental and developmen-
tal efforts aimed at applying the EFB to large-scale air pollution control.
FUNDAMENTALS OF SUBMICRON PARTICULATE COLLECTION
A major incentive for using the EFB is the extremely short gas residence
time required for collection of submicron particulate. A remarkably good
model for plug-flow beds pictures the bed particles as being much like the
electrodes of an ESP, with the electric field terminating over one side of
each particle and originating on the other. Instead of the Deutsch model
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238
used for turbulent flow ESP's [1], the "local mixing model" is invoked with
the assumption that on the average the fluid mechanics supplies the full sur-
face of a bed particle with the gas-entrained particulate. The electric field
simply brings the particulate from the local volume to the bed particle sur-
face. According to .this model [2], a bed having unfluidized height I , bed
particles of mean radius R, a superficial gas velocity U and an imposed elec-
tric field E collects particles of mobility b with the efficiency
n = 1 - exp (-
u _ Sir o /bE\
M = ~0~ - IT \TTJ
cj ^ U
(1)
where c is a coefficient of order unity.
This model is valid provided that bubbling and reentrainment are not sub-
stantial. The first of these conditions is promoted by the several screens
used to impose the electric field in the co-flow configuration. These tend
to prevent the growth of bubbles. The second is not a problem for liquid
aerosols such as OOP (used in fundamental studies).
With the objective of testing this model in a way that would separate
out less fundamental (but perhaps essential) phenomena, extensive efficiency
experiments have been carried out using an apparatus much like that shown in
Fig. 1. For the data shown in Fig. 2, the cross-flow configuration shown in
Fig. ' is replaced by a co-flow bed. Efficiencies for OOP aerosols having
two different mobilities are represented, the superficial velocity varies
from 0.8 to 2.5 m/sec. For all of the measurements, i = 6 cm and the bed
particles are sand, R = 1 mm. The solid curve is Eq. T.1) with c = 1.
Fig 1 - Facility for funda-
mental room temperature
tests using di-octyl phthalate
aerosol.
FLUIDIZED
,' BED
-------
239
0
05 bE/U —
Co-flow efficiency as a function of bE/U for two different mobilities
and sizes of participate.
-------
240
The residence-time advantage of the EFB is emphasized by recognizing that
for an ESP having electrode spacing s, the Deutsch equation for the efficiency
takes the form of Eq. (1), but with the collection coefficient K, replaced by
AbE/sU. Because 3irc/8 is essentially unity, this means that the unfluidized
height of an EFB required to give the same efficiency as an ESP of length I
is fcQ = fc(R/s). Typically, bed particles have a radius R on the order of
1 mm, so that for an electrode spacing of 10 cm the length of the EFB (not
counting the charter) is theoretically 100 times less than that of the EFB.
Initial tests were aimed at establishing this residence- time advantage of
the EFB. Subsequently reported results [2] (typified by Fig. 2) corroborated
the model underlying Eq. (1) in the submicron range where the electrically
induced collection is dominant. Note from Fig. 2 that without an applied
electric field, the bed is between 20 and 30% efficient as a collector. With
bubble bypassing minimized and the bed relatively well controlled in a plug-
flow mode, these tests are the best available basis for establishing the para-
meter c. For highly fluidized beds, it is found that c = 0.9 with variations
of about 15%. For packed or almost packed plug-flow beds, c is about unity
and varies with test conditions by 25%. The local mixing model for dilute bed
particles predicts c = 1. These tests were performed with sand at relative
humidities of 50 - 90%, so that the particles were semi-insulating and hence
the collection field placed under control [1,3].
Unless baffled to promote bubble breakup, gas fluidized beds do not expand
uniformly as the fluidizing velocity increases and the resulting effects of
nonuniformity in the gas-solid distribution is not accounted for in the plug-
flow model. This is especially true in the cross-flow beds where the screen
electrodes are not used. Models have been developed and correlated with ex-
periments carried out in an apparatus such as that of Fig. 1. These combine
the particle-scale collection model tested in the plug-flow experiments with
two-phase models that have been developed for the transfer characteristics of
conventional fluidized beds. It is important to recognize that the structure
of beds can be strongly influenced by an applied electric field. In what will
be termed the "bed electromechanics," particles tend to form "chains" or
"strings" while gas flow causes a fluttering motion instead of the typical
bubbling one [3]. These effects have important implications for bed mixing
and particulate elutriation. However, high submicron collection efficiencies
can be obtained at relatively low electric field Intensities where electro-
mechanical effects are not of significance. Thus, in this work, the Davidson
model is adopted for a conventional bubble having the 'diameter Dk (in which
the particle density is low) interacting with the dense phase. Part of the
gas passes through a dense region at about the minimum fluidization velocity
Umf Wh1le the reminder bypasses the system in the form of bubbles. There is
a continual exchange of particles between phases accounted for by the Davidson
model.
Two possible extremes can be used to model the penetration of particulate
through the bed as a whole. The dense phase can again be viewed as evolving
uniformly in the flow direction (the plug-flow model). Because of bubble agi-
tation a complete mixing model is also plausible. Both models have been carried
through 1n this study, with the dense phase plug-flow model clearly found to
be more representative of what happens.
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241
The plug-flow bubbling model shows how parameters not represented in the
collection coefficient Kn come into play. The predicted efficiency is
(2)
lip IIin
where m, and n^ are the roots of
and
_ 9 (VVf) !W_
C< 2 Db U'Umf
The collection coefficient K, is the same as defined with Eq. (1), U and A-
are respectively the superficial velocity and fluidized bed height under tne
conditions of interest while U f and i f are these quantities under conditions
of minimum fluldization. The mist Important parameter is D., the bubble dia-
meter. The quantities necessary to evaluate these expressions are of course
averages. The bed height 1s a fluctuating quantity and even in the shallow
beds of interest here, bubbles grow significantly as they pass through the
bed.
If the rate of gas interchange between the bubbles and the dense phase is
large, which according to Davidson's model means that c * », the efficiency
predicted by Eq. (2) reduces to that for the single-phase plug-flow model, Eq.
(1). The most Important aspect of the bubbling model is its prediction of ef-
ficiencies less than 100% no matter how large the collection parameter. In
the limit K, •*• », the efficiency approaches the limiting value
,*!-[,-Y].^
(3)
Using experiments to evaluate c , it is found that over a wide range of fluidi-
zation states, this parameter is mainly a function of the bubble diameter Db.
In connection with cross-flow experiments without baffles, the two-phase
model has been compared with extensive tests aimed at showing the dependence
of collection efficiency on electric field intensity, bed particle size,
static bed height, particle mobility and superficial gas velocity, with em-
phasis on the degree of bypassing through the bubble phase. The bed cross-
section was 10 x 10 cm. In order to fix the electrical characteristics of
the bed, the relative humidity was controlled at 902. The fluidized particles
were glass beads of 0.5 mm and 1 mm diameter as well as sand with mean diameters
of 0.8 and 2 mm. The distributor plate was a 30 mesh plastic screen sandwiched
between two insulating perforated plates with large holes.
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242
Efficiency measurements over a wide range of parameters are summarized
in Fig. 3, where the parameter is K, from Eq. (1). The solid curve is Eq.
(1), and hence the limiting efficiency if bubbling is ignorable. A correction
is made on the efficiency for mechanical effects. Unfluidized bed heights of
1Q = 4,6 and 8 cm are represented by the data.
That the experimental efficiencies for these bubbling beds do not correlate
with the parameter K, is evident from Fig. 3. In effect, the bed height be-
comes an additional parameter in correlating experiments carried out under
different conditions. The data for the 6 cm beds is summarized in Fig. 4,
where the ratio of experimentally observed to theoretically predicted efficien-
cies is shown as a function of the collection coefficient. The theoretical
efficiency is given by Eq. (2).
The tendency for the model to predict the observations best at high values
of K, is also seen in results for the other two bed heights. Correlation with
the model over the full range of Kj is best for the deepest bed.
Details of this work on effects of bubble bypassing are given in a forth-
coming article [3].
100-
80-
Fig. 3 - Composite of efficiencies for cross-flow beds as a function of col-
lection parameter. Widely varying states of fluidization are represented.
-------
EXP
.4
0
i i i
u
If
KEY
0.35
6
•
.= 6 CM
0.75
7
a
1.0
8.8
H
1.35
9
0
1.75
II
a
3.0
20
a
243
J L
5 10
20
K
Fig. 4 - Measured efficiency normalized to theoretical efficiency as a function
of collection parameter.
SUBMICRON PARTICULATES COLLECTION WITH AC EXCITATIONS
Savings from a reduced need for power conditioning equipment is an obvious
advantage of being able to use alternating voltages to excite the EFB. But,
perhaps a more important motive comes from the difficulty encountered when
conventional ESP's are used to collect highly resistive materials. Poor per-
formance is linked to the dc energization of the ESP. The collection of highly
resistive ash results in the buildup of net charge and an attendant interference
with the imposed precipitating field. Such effects are not expected in alter-
nating field devices. To be effective with an ESP required to collect submicron
particulate, alternating fields must have an extremely low frequency (on the or-
der of 1 Hz). (Laminar flow ac ESP performance is worked out in detail by Alexan-
der [5].) Can the EFB operate with 60 Hz excitations? More fundamentally, what
are the implications of the frequency dependence of the efficiency for the basic
collection process?
With beds of varying particle size and states of fluidization, and with applied
fields of different amplitudes, experiments have been carried out using the ap-
paratus of Fig. 2. Efficiencies were measured as a function of frequency. Both
cross-flow and co-flow configurations were tested over the frequency range of
30 - 5000 Hz.
A correlation of the cross-flow data for the former experiments 1s given in
Fig. 5. In order to emphasize the particle-scale collection process to the ex-
clusion of bubble bypassing or bed scale mixing, the beds were tested at low
overall efficiencies, 1n a range where the efficiency 1s proportional to the
single particle rate of collection r. Thus,
-------
244
- r
nf
-
rdc " rnf
ndc * nnf
(4)
where <> indicates the time-average with ac excitation, while nf and dc denote
no field and with dc excitation respectively. It is this normalized efficiency
that is plotted in correlating the data of Fig. 5. To correlate the data, the
angular excitation frequency u is normalized to an angular cutoff frequency
<*>e • \/9TT/8(cs bE /R), where E is the peak applied electric field and s is the
velocity gradient adjacent to the equator of a given particle (with the local
flow approaching the particle along a polar axis). This parameter can be ap-
proximated by s = 3U f(l-Kb)/R (2-3K+3K5 - 2K6) where K = R/R is the ratio
of particle radius to the radius of a unit cell representing the particle voidage.
The solid curves represent asymptotic predictions predicated on the period of
the applied field either being very low or very high compared to the time re-
quired for particulate to traverse a unit cell.
effic1enc* as a fu"ction of norma-
n« »ndj"9f.fluency theoretical asymptotes. Solid
lines-cross-flow theory, dashed lines-co-flow theory. Rightmost of these
curves for u>e/W|n = 1, leftmost for u>e/u>m = 0.75.
Details of the asymptotic models and specifics of the experiments will be
available in the literature in the near future [6]. It seems clear that such
frequency response measurements are a way of probing the mass transfer process
at the bed particle scale. From a practical point of view, it is the fact
that the 60 Hz efficiency is well below the critical frequency in all of the
tests that is most significant. In this low frequency range, the efficiency
m ^ - - - - —.. v- • • v • w*i • t ^^ M\> 11 w^ i u i iy
is as predicted by the models for the dc excitations with E
dc
As
-------
245
the field reverses, collection occurs first on one side and then on the other
side of a collection site. The effective electric field is therefore the
average of E .jcos ut| over a full period.
Experiments, correlated with theoretical models, have been carried forward
to understand how ac excitations can be used in the corona, ion-impact charging
section. The general disposition of the charger just below the distributor
plate is as in Fig. 1. At some penalty of power required, the charging can
be accomplished at 60 Hz. This makes it possible to operate the entire EFB
system, the bed and the charger, from a transformer. As an example, Fig. 6
shows the efficiency with the bed in this entirely ac excited type of operation.
Details are given elsewhere [5].
Fig. 6 - Efficiency with 60 Hz on EFB _?
and charger. R = 0.5 mm, b = 1.2 x 10
(m/sec)/(volt/m). Fluidized: i
U = 1.3 m/sec. Packed: «-. = 7.5
= 10
cm,
cm,
U = 0.5 m/sec.
PACKED
a FLUIDIZED
COLLECTION OF OIL ASH
UR
2 3
IN UNITS OF
10* V-SEC/M
A natural application of the EFB for large-scale gas cleanup is to the
products of oil combustion. Experiments have been conducted on a 100 cfm by-
pass on flue gas prior to the economizer in the M.I.T. Central Utility Plant.
Loss of heat in the connecting pipes cooled the gas to about 350° F. Tests
were conducted in both the co-flow and cross-flow configurations using 1 mm
glass beads and 0.8 and 2 mm sand.
The oil ash was entirely in the submicron range with loadings of the order
of 0.01 to 0.06 grains/scf. Unlike the OOP, the oil ash is a dry material.
Could the EFB be used to collect an essentially dry material and what residence
time for the particles was consistent with retaining a reasonable efficiency?
A typical test of efficiency as a function of time is shown in Fig. 7. For
this case, a medium state of fluidization prevailed and the configuration was
co-flow. The open circles represent collection with no charging and no applied
field. Because the bed was at a sufficiently high temperature that the humidity
did not render the particles semi-insulating, there were appreciable "micro-
fields" associated with patches of charge on the particles created by frlctlonal
effects. Thus, the open half circles which represent data for collection with
charging but without a field applied to the bed, show a considerably higher
-------
246
efficiency than found in experiments such as represented by Fig. 2. The solid
data points are with both charger and applied field on.
uu
80 '
60 (
40 (
20
0
1 • r— i—i r-
1* *••••• t ^ 0
3 9 o 9 99 9 9
3ooo°°oo0
°Oo°O 99(1
°oo-
1/ '
5 10
Time (hour)
2Z 24
? -
Efficiency as a function of time for co-flow EFB on bypass from M.I.T.
Utilities while burning low sulfur oil/ Open data points are for no-
charging and no bed field. Half circles are with charging but no field
Closed circles are for charger and field on.
It is clear that the bed can operate for substantial periods of time at
this mass loading. Either using a once through or a reprocessing system for
the sand, the EFB appears to be an economically feasible approach to a utilities
scale operation. However, there are questions that must be confronted by a
development. The M.I.T. plant is required to use low sulfur oil and considerable
excess air, so for the tests conducted the combustion products were extremely
light and free of carbon. What is the effect on bed electrical losses of col-
lecting combustion products of high sulfur oil? Losses with the tests reported
here were insignificant.
A sketch of a co-flow EFB incorporated into a stack installation 1s shown
in F1g. 8. The top view shows, to scale, the cross-section of a stack, with
the EFB constructed around its base. Flue gas is fed into the annular EFB
through eight ducts, passed through the bed and then into the stack through
ducts in the side walls. The dimensions are for a 106 cfm unit. A typical
pressure drop for 90 - 95% efficiency would be 10 cm H20.
CONTROL OF ASPHALTIC FUMES IN REPROCESSING OF ASPHALTIC HIGHWAY
Recycling of asphaltic pavement is being developed by the Warren Brothers
Co. To be economical, the process should be carried out in adapted conventional
plants. A major impediment to the conversion of existing plants is the forma-
tion of fine partlculate smoke when crushed pavement is processed through the
conventional aggregate drier. In the drier, the residual asphalt In the old
crushed pavement begins to crack and release hydrocarbon vapor which subsequently
condenses into droplets.
Devices currently available for collection of this submicron material are
ill suited. The asphaltic by-products are not easily removed from the plates
of an ESP. Fabric filters that will catch the submicron particulate subsequently
-------
247
EFB
Fig. 8 - A 10° cfm EFB
constructed around the
base of a stack.
SAND IN
are fouled. Wet wall electrostatic precipitators, charged droplet scrubbers
and high energy Venturi scrubbers all create a water pollution problem. The
EFB is well suited to this application, not only because of its high efficiency
in the submicron range, but because the material used to collect the particulate
is easily processed through the EFB and conveniently incorporated into the
asphalt hot mix product. In fact, in this and other related operations oiling
of the sand is a desirable step in the plant process.
A prototype EFB of cross-sectional area 1 ft has been tested on both a
small-scale prototype asphalt recycling system and on a bypass to a full-scale
asphalt plant operating in the recycle mode (Warren Brothers plant #135, Rich-
mond, Va.). In both of the tests reported here, a bag-house was used ahead
of the EFB to catch large dust particles from the drier. The submicron material
easily penetrated this bag-house. Tests were performed on a batch basis using
sand of 2 mm median diameter. The charger was of the plate wire type. Tem-
peratures ranged up to 500° F.
-------
248
The results of three separate tests of the collection efficiency of the
EFB on the small-scale recycling system are reported in Table 1. Experimental
results are shown along with the theoretical efficiencies predicted by the
plug-flow model and by the two-phase bubbling model.
EFB OPERATING PARAMETERS
Particulate
Loadi ng
(mg/sm3)
U
(m/sec)
*o
(cm)
E
(kv/m)
EFFICIENCY (%)
Experimental Theoretical
Plug-flow
Model
Bubbling
Model
167
82
44
1.7
1.8
1.8
10
10
13
465
250
500
96 99
93 94
94 99
96
91
92
Table 1 Summary of the collection performance of the EFB; comparison of experimen-
tal and predicted efficiencies.
After removal of dust by the bag-filter, the mass median diameter of the smoke
ranges from 0.3 to 0.5 ym, with a geometric standard deviation of about 2.
An important parameter is the amount of time that sand can be used before
it must be replaced because of a drop in collection efficiency. The mechanism
for this drop seems to be channeling of the gas through spouts in the bed, oc-
curing after the sand becomes viscous due to excessive oil collection. In
Table 2, tests under similar operating conditions are compared. In.the two
cases for which the oil loading is 0.7%, the measured efficiency for the oily
sand is essentially the same as for the clean sand. In the case of 1.2% oil
loading, a drop in efficiency is observed.
EFB OPERATING PARAMETERS EFFICIENCY (%)
U E JL Fresh Bed Oil
(m/sec) (kv/m) (cm) Particulate Experi-
Loading mental
(mg/sm )
1.8 500 13 44 94
1.8 500 13 44 94
1.7 465 10 167 96
Coated Bed
Particulate Oil *
Loading Loading
107
200
233
0.7%
0.7%
1.2%
Experi-
mental
94
95
85
Oil loading is defined as the ratio of the weight of collected oil to the bed
total weight.
Table 2 Comparison of collection performance of fresh and oil coated beds.
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249
Pressure drop usually accounts for the largest share of EFB operating
costs. In these tests, the pressure drop across the EFB, including the dis-
tributor plate, was less than 18 cm O. With the bed operating properly,
this drop is essentially that across the distributor plate plus what drop
is required to support the bed. As spouting occurs because of bed overloading,
there is an associated loss of pressure drop that can be taken as an external
indication of loss in efficiency.
Results from the Richmond tests are shown in Tablets, where 400~cfm is fil-
tered with particulate loadings varying between 4 mg/m and 41 mg/m . Parti-
culate was observed to have a mass median diameter of 0.35 urn with a geometric
standard deviation of 1.4
Efficiency Measuring EFB Operating Parameters Efficiency
Device
E(kv/m) U(m/sec) H
Mass Monitor
Andersen Impactor
400
400
1.9
2.1
10
11
99.3
99.1
Table 3 Summary of collection performance in Richmond.
This work will be reported in more detail in a forthcoming article [8].
ELECTROMECHANICS OF ELECTROFLUIDIZED BEDS
In the application of the EFB to submicron particulates control, it is
possible to separate the electrical interaction between the bed particles and
the charged particulate from the electrical interaction between bed particles.
It is the latter that is termed the bed "electromechanics." The propensity
of the field for altering the state of fluidization by inducing interparticle
stringing is discussed in previous publications [3].
If the bed is to be applied to collection of both submicron and super-
micron particles, it is necessary to understand the implications of the sur-
prisingly large electrical effects at the bed particle scale. To what extent
can these forces be relied upon to prevent loss of large particles from the
bed? Because the field can turn a bubble into a "fritter," the turbulence
within the bed is considerably reduced by the field. Each bed particle is
subjected to a reduced amount of agitation because the "wake mixing" associated
with the classical bubble is reduced or eliminated. To what extent are these
effects useful in achieving EFB operations in a mode where bed particles are
agglomerates of the particulate, the self-agglomerative mode?
Two types of fundamental experiments have been carried out with the objec-
tive of having a physical picture of the interparticle force. In the first
of these, the EFB (in a cross-flow configuration) is in the annul us between
the cylinders of a Couette viscometer, as shown in Fig. 9. The shear stress
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250
SUPPORT
SYSTEM
INNER
CYLINDER
x FLUIOIZED
PARTICLES
OUTER
CYLINDER
PLASTIC
200
100
50
jgzo
LJ
QL
w 10
CC
<
UJ g.
I 5
1
I ,1 I I
2 5 10 20
VOLTAGE (KV)
Cross-flow EFB in viscometer. Outer cylinder rotates and inner one is
fixed. Voltage applied between inner and outer cylinders results in
shear stress at inner cylinder as shown.
tends to a common asymptote at high field strengths over a range of shear
rates. In this high field regime, the electrical force is large enough that
the bed is electropacked. Each particle chain transmits its force through
friction to the electrodes, so the shear stress can be used as a measure of
the interparticle force.
For the particles typical of the EFB applied to air pollution control,
polarization forces (associated with the permittivity of the particles) are
too small to be responsible for the observed effects. Also, such observations
as the dependence of the electromechanical effects on applied voltage negate
such polarization forces as the basis for what is observed. A theoretical
model has been developed in which the force between particles is accounted for
in terms of the concentration of the electric field near the contact caps
between particles. In this region there is a strong constriction of the
current as it passes between particles. The correlation between theory and
experiment supports the contention that it is this field concentration that
is responsible for the large electromechanical effects and their dependence
on the applied voltage. This work is reported in more detail in a forth-
coming article [9].
-------
251
Further experiments allow deduction of this interpartjcle force from bed
characteristics. In the first of two experiments in this category, the pres-
sure drop at incipient fluidization is measured as a function of the applied
voltage. In the second, the minimum voltage required to suspend a bed against
an upper screen is measured as a function of the gas-flow rate. Both experi-
ments are performed using the semi-insulating particles typical of the sand
at high relative humidities with the cross-flow configuration. Typical re-
sults for this latter experiment are shown in F1g. 10, which shows the super-
ficial velocity at which particles fall from a pinned position as a function
Of voltage. The inset shows an idealization of the curves, identifying two
regimes. In the low-field regime, the velocity 1s above U -, and so there is
enough pressure drop to support the bed as a whole. In this regime, failure
is because particles at the bottom interface are not held in equilibrium. In
the high-field regime, the electric field makes it possible to support the
bed as a whole and failure involves the entire bed. Correlations between the
bed measurements and a theory based on interparticle forces due to current
constriction and a contact cap determined in size by electrical breakdown are
not only successful in predicting the dependence of the parameters, but in
giving quantitative predictions as well. This work will be reported in the
literature [10].
Fig. 10 - Air velocity required to support a cross-flow electropacked bed against
an upper stream as a function of the applied voltage.
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252
COLLECTION OF COAL ASH
Work is being carried forward by Alexander (Ph.D. research) to apply the
EFB to the control of coal ash. Experiments are on a 100 cfm facility that
ranges to 600° F in temperature. Two approaches are being investigated. In
the first, the bed particles are coal, which is used as a collection site
for ash in a process that builds up particles of about twice the original mass
before the coal and ash particle is removed from the system. Combustible ad-
ditives are used to promote stable agglomeration. A picture of one of these
composite products from the EFB is shown in Fig. 11. A typical collection
efficiency for the bed is shown in Fig. 12, where the advantage of using the
EFB for the submicron collection is apparent. Note that a fluidized bed with-
out electrification can be used for removal of particles above about 3 ym. In
the system being developed, two beds are used. In the first, the large particles
are removed and in the second, charging and application of a field results in
good efficiency for collection of the fines.
In the system using coal as the seed material, the output of the bed is
to be injected into the combustor where the ash.cracks away and the coal and
additive are burned. Provided that the combustor is one of relatively high
ash retention, this approach can be shown to lead to good overall efficiency
with the ash removed from the combustor. The fluidized bed combustor is a
natural candidate for this mode of operation.
The second system under development starts the collection process by "seeding"
the bed with foreign particles. As the agglomerates are built up, they are
removed, treated, and a fraction broken into particles suitable for reinjection
into the bed. Thus, the bed is operated in a self-agglomerative mode. Detailed
studies are being made of the interplay between parameters contributing to this
process.
* %**
F19-
- Composite ash and coal particles built up in the EFB. Coal particle
alright (2 mm diameter) is seed for agglomerates shown at left. The
third particle from the left has been crushed.
-------
100
80 -
- 60
-------
254
to the general application of the EFB to air pollution control have been
indicated to be held allowable. A third relating to the coal ash collection
is pending.
REFERENCES
[1] White, H.J., Industrial Electrostatic Precipitation, Addison-Wesley, Pub.
Co., Reading, Mass., p. 164,1963.
[2] Zahedi, K. and Melcher, J.R., "Electrofluidized Beds in the Filtration
of Submicron Aerosols," APCA Journal, Vol. 26, p. 345, 1976.
[3] Johnson, T.W. and Melcher, J.R., "Electromechanics of Electrofluidized
Beds," I&EC Fund., Vol. 14, p. 146, 1975.
[4] Zahedi, K. and Melcher, J.R., "Collection of Submicron Particulate in
Bubbling Electrofluidized Beds," I&EC Fund.(in publication).
[5] Alexander, J.C., "Frequency Characteristics of Electrofluidized Beds in
the Collection of Submicron Particulate," M.S. thesis, Oct., 1975.
[6] Alexander, J.C. and Melcher, J.R., "Alternating Field Electrofluidized
Beds in the Collection-of Submicron Aerosols," I&EC Fund, (in publication).
[7] Final Report to ESEERCO on Electrofluidized Beds in the Filtration of
Submicron Particulate, Continuum Electrotnechanics Group. M.I.T., T976".
[8] Zieve, P.B., Zahedi, K., Melcher, J.R., and Denton, J.F., "Electrofluidized
Beds in the Filtration of Smoke Emissions from an Asphaltic Pavement Re-
cycling Process," Env. Sci. & Tech., submitted for publication.
[9] Dietz, P.W. and Melcher, J.R., "Momentum Transfer in Electrofluidized Beds," AIChE
Symposium Series Book on Air, submitted for publication.
[10] Dietz, P.W. and Melcher, J.R., "Interparticle Electrical Forces in Packed
and Fluidized Beds," I&EC Fund., submitted for publication.
WRITTEN DISCUSSION
Robert B. Rief
Battello Colombus Laboratories, 505 King Avenue
Columbus, Ohio 43201
My experience with electrofluidized beds differs somewhat from Jim
Melcher's experience in that in my work the beds were used to supply powder
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255
to electrostatic coating and printing processes which required moving
particles larger than 10 microns through or out of the beds. Particles finer
than 10 microns and especially finer than 1 micron were avoided because the
surface molecular forces or Van der Waal forces cause the fine particles to
stick to the larger bed particles.
I have been very interested in Jim Melcher's work and have been following
it for some time. Although a number of questions can be raised on the basis
of the information mentioned in this paper, these questions have been answered
in papers which have been published earlier. I strongly recommend these
papers for further reading to anyone interested in electrofluidized beds.
I find a problem with only one part of the paper. The statement is made
that "Typically, bed particles have a radius R on the order of 1 mm, so that
for an electrode spacing of 10 cm the length of the EFB (not counting the
charger) is theoretically 100 times less than that of the ESP." The factor
"100" was obtained by equating the K coefficients in the Deutsch equation and
in the efficiency equation of the electrofluidized bed. A factor of 2 was
omitted from the K factor of the Deutsch equation. Thus, the length of the
equivalent ESP should be 200 times rather than 100 times longer than the
length of the EFB.
Since I have no further comment about this work, I shall spend the rest
of the alloted time in discussing another new air cleaning system which is
called an electroinertial air cleaner.
Inertial air cleaners consisting of a bank of straight flow tubes, each
tube having a swirl means to cause dust to be centrifugally thrown outwardly.
toward the tube wall, are well known for engine air cleaning application:
Dust near the tube wall is drawn from the main stream by a scavenger fan and
cleaned air emerges from the core zone of the tube through a small diameter
take-off tube extending into the downstream end of the flow tube. However,
the centrifugal separator action is relatively ineffective on particles smaller
than 5 microns; therefore, electrostatic separator action was added in electro-
inertial devices to enhance overall collection efficiency. In one arrangement,
a fine wire at a potential of approximately 15-20 kV is extended through a
1-1/2-inch-diameter tube on the tube centerline with the tube wall at ground
potential to establish a flow of negative ions. The resultant negative charges
on the particles and the radial electrical field from the ionizer wire to the
tube enhance outward migration especially of the smaller particles thereby
improving overall collection efficiency.
Unfortunately, the electrostatic separator action causes some particles
to precipitate and adhere rather strongly to the tube side wall. Therefore,
the tube usually must be periodically rapped or vibrated in the radial and/or
axial direction in order to dislodge the particles sufficiently to permit the
scavenger air to carry them away. A rapper equipped tube is shown in Figure 1.
Under some circumstances the particles are jarred with such force as to be
reentrained into the clean air stream. Even when properly applied, rapping or
vibrating requires special shock mounting of the tube; mechanical wear on the
mounts is a problem. Therefore, the use of rappers as a dislodging expedient
-------
a-.
FIGURE 1. ELECTROINERTIAL UNIT WITH RAPPER
-------
257
Steel Tube
Dirty Air
Exit
Clean Air I
Tube
FIGURE 2. ELECTROINERTIAL UNIT WITH
GLASS LINED TUBE
-------
258
is not entirely satisfactory.
While working with a unit of this type under Contract No. DAAE07-72-C-
0293*, a glass lining'was tried in the tube as shown in Figure 2. During trials
without vibrating the tube,AC fine test dust was passed through a 1.5-inch-
diameter by 6-inch unit at a flow rate of 40 cfm including 4 cfm of scavenger
air. No measurable amount of dust accumulated in the tube. The fact that no
dust accumulated in the glass-lined tube represents a major advance in the
technology. The overall efficiency was about 92 percent. For comparison, with
a vibrated unlined tube with comparable voltage on the wire, the overall
efficiency is generally about 99 percent. As an inertial unit, with no applied
voltage, the same size unit would remove about 80 percent of the dust. The
electrical contribution with the glass-lined tube was significant—increasing
the efficiency from 80 percent to 92 percent.
The low material retention in the glass-lined tube is believed to be
unique to "leaky" insulators such as glass. In the precipitator section, large
numbers of ions are generated but only part of the ions are effective in
charging the dust. Once the dust is deposited on the tube, excess ions bombard
the dust and inhibit its release from conductive surfaces. However, the excess
ions accumulate on insulators and prevent the deposition of the dust. If the
insulator is too good, the potential will rise on the surface and the corona
from the wire will be shut off causing the efficiency to drop. "Leaky"
insulators, such as glass, let some current pass so that the charging process
continues but some charge accumulates on the surface. This surface charge,
which is the same polarity as the charge on the dust, repels the dust as it
approaches and prevents it from making good contact with the surface. As a
result,the surface molecular forces, which act over very short distances and
cause fine particles to adhere tenaciously when good contact is made, are not
effective. The loosely held dust thus moves along the charged surface of the
tube and out through the scavenger system.
Recently U.S.patent4,010,Oil was issued on this device and further work
has been done for the USDA on other modifications of the basic electroinertial
design for removal of fine cotton dust.
USA TACOM.
OPEN DISCUSSION
Oder: The question has to do with bubbles and the suppression of bubbles
with the electrofluidized bed. What is the fundamental mechanism for the
creation of bubbles and why should the electrostatic field suppress that?
Melcher: The basic reason for the bubbles, is instability in fluid. Usually
that is how it is approached in the literature of fluidized beds. It is
unstable, and the nonlinear equilibrium that results is the bubble. The
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259
other question, how is it that the electrostatic field seems to fill in the
bubble? Again this is the sort of dependency in this self precipitation
time constant. Particles do not have net charge, they do not go off the
wall, and they do not annihilate each other. But they are polarized and that
now has a new time constant. That time constant is now in the steady state
configuration in competition with the turbulence and, at a certain point, is
short enough that it overcomes the inherent mixing time constant and they
form strings. These strings are held together by this capped force which
we are talking about. I can use an electric field to support a static bed
very deep in spite of the fact that a field that large will not support
water at an altitude of a fraction of a millimeter. The force between the
particles is a very.very strong one and it can be transmitted to the walls.
That is why you get the overshoot. It can change the effective shear stresses
as well. Students in industry are now working with S02 on the type of
approach.
Oder: Let me further my question. You are not really getting rid of a
fundamental instability. You have really put a gas distributor in there.
In the bed where you have this string of particles do you have mass bed
mixing or is the bed stable?
Melcher: The bed is indeed mixing. I have a movie with me if any one wants
to see it. With the field on, the bed mixing is much slower. The way in
which the particles are treated by the bubbles is different because the
bubble goes through and you get strong mixing in the trail. What the field
does is eliminate that trail, it eliminates a lot of the agitation in the
bed.
Koppang: I was wondering what your mechanisms are for cleaning your bed.
Melcher: I appreciate your question. Two advantages of the thing are,
first of all, the savings you get in the pressure drop and next, it flows
like a fluid. You put a hole in this thing and it runs out. You can run
this thing on a continuous basis. Generally you let it run down through a
hole and you put a little in. It is a very slow rate at which you do that.
They use more oil and sand than we can supply. For certain kinds of
applications I can see for fly ash you might agglomerate and recycle the
whole thing.
On the fly ash we have two approaches. One is to self agglomerate
the fly ash on itself. The other is to agglomerate on coal. In either case
we want to recover the heat value of additive of the coal injected into the
combustor. A combustor like a fluidized bed combustor has a high ash
retention, so that taking all the ash out of the combustor is the grand plan.
Also I might mention we are working on recycling seed for MHD generators,
where we have a local pollution problem in our MHD experiment. The sodzum
carbonate that we put in as seed turned out a submicron particulate all
over the campus and an EPA warning. We've gotten ourselves a 70 inch pressure
drop scrubber which gets 70% of the stuff. We are trying to collect it our-
selves since it is valuable. It is a nice application; we can put it right
back into the combustor.
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260
APPROXIMATE EQUATIONS FOR PREDICTING ELECTROSTATIC
PARTICLE COLLECTION
Douglas W. Cooper
Department of Environmental Health. Sciences, Harvard University
ABSTRACT
A new approach to calculating approximate single-collector collection
efficiencies is presented that eliminates the need to calculate particle
trajectories, for particles of negligible inertia. The general case of a
central force, F = k/rm (m i 0), is solved for spherical and cylindrical
collectors having collection efficiencies either much greater than or much
less than one. For those values of m for which efficiencies have already
been evaluated by others (using trajectory integrations), the results agree
quite well, except for the low-efficiency case of a charged particle and an
uncharged collector, where the central force approximation breaks down.
The approach taken here emphasizes the significance of the aerosol con-
centration profile, n(r). It shows that increased collection efficiency will
result from improved mixing of the aerosol in the vicinity of the collectors,
perhaps through induced turbulence. The collection efficiencies are ex-
pressed in terms of K = ws/vo> the ratio of the particle radial velocity at
the collector surface (due to the central force) to the mean gas velocity far
upstream from the collector. The collection efficiencies are as follows:
Collector K « 1 K » 1
cylinder n K (TT K)1/m
sphere 4K (4K)2/'m
The results are exact for coulombic attraction (charged particle, charged
collector, point and/or line charge approximation), if inertia is negligible
and if interception is not important enough to produce critical trajectories
which strike the collector upstream from the rear stagnation point. Also
discussed are the effects of other particles, other collectors, and other
fields, with respect to the collection efficiency of a single collector.
INTRODUCTION
Recent emphasis on the control of respirable dust, airborne particles
with aerodynamic diameters s 3 um, has led to increased interest in using
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261
electrostatic forces to enhance the collection in filters and scrubbers. To
predict the enhanced collection efficiency of these control devices exactly
is very difficult. Often one can determine whether or not the addition of
electrostatic forces to a conventional scrubber or filter would be a major
improvement through the use of approximate equations for collection
efficiency.
GOALS
The goals of this work are to make it easier to calculate approximate
collection efficiencies of control devices (such as scrubbers and filters)
which employ electrostatics to augment their usual collection mechanisms
and to clarify the role of electrostatics in such devices. The alternative
to such simple calculations is often the rather difficult determination of
particle trajectories for the variety of conditions and particle sizes of
interest, an expensive endeavor.
ANALYSIS
A. Particle Motion
Denote the particle velocity by u and the gas velocity by v. The
well-known equation of motion for a particle governed by Stokes1 law with
the Cunningham slip correction is
m du/dt = -3nu d (u - v)/C + F (1)
in which m is particle mass (g), t is time (s), u is gas viscosity (poise),
dp is particle diameter, C is the Cunningham slip correction (approximately
1 + 0.16 um/dp at NTP), and F is the vector sum of any external forces
acting on the particle (dynes). Fuchs[l] discussed this equation's range of
application in detail. It is generally applicable to particles with diameters
from 0.1 urn to 10 urn.
Equation (1) can be simplified by introducing particle mobility, B, given
by
B = C/3TT|_id (2)
for spherical particles. The mobility is the terminal velocity of the particle
with respect to the gas per unit applied force. The mobility is also the ratio
of the particle aerodynamic characteristic time, T, to the particle mass:
B=T/m (3)
where, for spheres,
T = Cp d 2/18u (4)
P P
(p is the particle density).
P
Equation (1) then becomes
-------
262
T du/dt = -(u-v) + BF. (5)
HThe first simplification we make is to eliminate the inertial term
T du/dt. We distinguish between the "control device" and the "collectors"
(fibers, droplets, etc. ) within the device. Let the mean flow velocity (or
the flow far upstream from the collectors) be vo and let the collector's
smallest dimension be L. Then T du/dt becomes negligible when
vQT/L«l (6a)
BFT/L«1 (6b)
under which conditions
u = v*+ B? (7)
Inequality (6a) means that the particle inertia is insufficient to cause it
to deviate appreciably from the streamlines in the vicinity of the collector.
Inequality (6b) means the particle inertia is insufficient to prevent the
particle from almost instantaneously achieving its terminal velocity with
respect to the fluid due to the force F. Equation (7) indicates that under
these assumptions the particle motion is the motion^of the gas plus the
particle terminal velocity with respect to the gas, w = FB.
B. Particle Collection
Figure 1 shows a collector of arbitrary shape in an aerosol flow of
free-stream velocity VQ and free-stream concentration n . The number of
particles collected per unit time at steady-state will be tRe integral over
the collector surface of the particle velocity normal to the surface times
the concentration:
s s
N = -d> n u • d (8)
• l°'
where Ag is the surface area (cm ), u is the particle velocity (cm/s ),
and ng is the particle concentration at tine surface (cm~3). In steady state,
the number of particles removed. per unit time from the volume (spherical
or cylindrical) within R2 is also N,
N = - n - d (9)
Evaluation of the preceding integrals requires concentrations, velocities,
and surface areas. One simplification is that at the surface of the col-
lector the normal component of velocity for the gas must be zero, so
Equation (8) becomes
Ns = - £ ng ws- dAg (10)
As
-------
263
>
Fig. 1 Collector of arbitrary shape in aerosol flow of free-stream
velocity v and concentration n .
'
C. Collection Efficiency - Coulombic Attraction Only
Coulombic attraction is the attraction between one point charge (the)
particle) and another (the collector) or between a point charge and a
line charge (a cylindrical collector). The charges are opposite in sign
and are treated as being located at the centers of the particle and the
collector. The force between the point charges decreases with the inverse
square of the distance (r'2); the force between the particle and the line
charge decreases inversely with distance (r'1)- This force will often
predominate over the other electrostatic forces. Equation (1) becomes
easy to evaluate because, as Levin demonstrated (see Fuchs [1]): for
forces, such as gravity or coulombic attraction, for which the divergence
is zero; that is, for
V.F=O <">
the concentration of particles in an inertialess aerosol subjected to that
force is constant along particle trajectories. Thus, the surface con-
centration at the collector will be the same as the free stream concentration
8 O
which means
-------
264
N = n w A (13)
oss l '
for a spherical or cylindrical collection surface, where A is the surface
area. s
If Ac is the cross-sectional area of the collector in a plane perpen-
dicular to the mean flow, v , then the collection efficiency of a single
collector is defined as
oVoAc <14)
For a cylindrical collector and couloumbic attraction alone, the collection
efficiency becomes, from equations (13) and (14)
Tl = TT W /v (15)
and the collection efficiency is
n = 4wg/vo (16)
for a sphere. Usually, efficiency is expressed in terms of the ratio, K,
of the electrostatic force on the particle at the collector surface (F ) to
the drag force (Fj^) associated with a velocity vo, but in fact
F w /B w
K _ s _ s s_ ^ ,J7j
D vo vo
Note that the coulombic force decreas.es as rapidly as the collection area
increases; thus, the collection rate, N, is independent of size, even
though the collection efficiency is dependent on size. If particulate capture
by interception can be correctly modeled as an increase in the effective
collector size (increased outward from the surface by a dimension equal to
the particle radius), interception will not increase the rate of capture,
unless ws(r=R-lr ) at the collector rear is smaller than v(r=R+r ) there.
f p.
It is also significant that the rate of capture nowsA8» is independent of
the flow profile, and so is the efficiency.
Further, if a surface such as RI in Figure 1 can be drawn surrounding
an irregular particle, and if that surface is completely intersected by
particle trajectories (collection efficiency much greater than one should
suffice), the collection rate will also be independent of shape, for coulombic
attraction, which agrees with a result of Levin (see Fuchs [1]).
D. Collection Efficiencies - Central Forces Other than Couloumbic
For the other electrostatic forces, it is not generally true that the
divergence is zero. Thus, particle concentrations are not constant along
trajectories. Instead, for a central force
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265
F=k/rm
in steady-state with no gas flow, the rate of particle passage through a
(spherical or cylindrical) surface surrounding the collector would be con-
stant at each distance r (cm):
N = n(r)w(r)A(r)
If the concentration is nQ at rSRQ, then
m A(R )
- " (20)
For the case in which the velocity v is much less than w , equation
(20) should hold near the collector. Thus for a spherical collector:
m R _ _ m-2
and for a cylindrical collector
n(r) _ , _ r_vm-l (22)
no Ro
assuming w » v . Often collection is put in terms of the free-stream
streamline farthest from the flow centerline that is cleaned, a distance R*
such that
N = n v IT R*2 <23a)
o o
or
N = n v 2R*L (23b)
00 C
for a sphere or a cylinder (of length Lc), respectively. The assumption
ws> > v implies that R*> > R,where R is the collector radius, which
makes plausible the assumption
n(R*) '= n(RQ) = nQ . (24)
The concentration at R* is approximately the free-stream concentration.
For a sphere, this implies
N = n v n R*2 = n(R)w(R)A(R) (25a)
o o ,
2 = nw
,
m-£ R, 7
r)HTTR2) (25b)
R
-------
266
Thus,
R* = (4Bk/v )
I/
m
o'
(26)
One obtains the collection rate from equation (23a) and the collection
efficiency from
TI = (R*/R)2 = (4Bk/Rmvo)2/m (27)
which is equivalent to
T] = (4w /vJZ/m = (4K)2/m
s o
(28)
This is a general expression for strong central force collection efficiency
for a sphere, derived without reference to a specific flow or a specific
force ( m / 0 »
For a cylinder, a similar analysis yields
Tl = (R*/R) = (TTBk/Rmvo)1/m = (nK)1/m (29)
a general expression for strong central-force collection efficiency for a
cylinder. This approach is believed to be original.
In the other limit, w « v , the collection velocity is much less than the
free -stream velocity. Because the collection, velocity is low compared with
convection (vo), the concentration near the collector surface should be quite
nearly the free-stream concentration:
•
ns = V (30)
in which case the collection rate becomes:
•
N = n w A . {->i\
oss 1-5 -U
The collection efficiency for a spherical collector will be
r, = 4wg/vo = 4K (32)
and for a cylindrical collector
= TTW/V = nK (33)
O
Equations (32) and (33) may also be correct for w ~v when the gas is
sufficiently turbulent to mix the particles well in the vtcin°ty of the collector,
because the equations are based on approximating n(R) by n . Turbulence
might, thus, raise collection. °
Intermediate cases (K = w /v ~ 1) should have efficiencies intermediate
between the K» 1 and the K %< 1 cases studied above, because the
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267
concentration at the collector surface should be between no and the n(R)
calculated for K» 1.
Approximate efficiency formulas have been obtained here without recourse
to trajectory calculations and without assuming particular flow profiles,
assumptions having been made about concentration/ profiles instead. The
equations apply to any force such that F = k/rm.
RESULTS: COMPARISON WITH OTHER EFFICIENCY CALCULATIONS
Table 1 is based partly on a similar table by Whitby and Liu (2). The
first column has the collector shape. The second indicates which force is
being isolated; the subscript Q means the collector is charged and the
subscript q means the particle is charged. The third column shows the
exact or approximate form of the radial dependence of the central force.
The fifth column has the efficiency calculated by the approximations above
for K range shown in the fourth column. The last three columns give the
efficiencies obtained by others, the flow conditions they assumed, and the
reference,
Where there are other determinations with which to compare them, the
approximations made here agree rather well (except for Foq at K« 1,
where the central force expression does not remain valid).
It can be noted, in general, that as K increases, the efficiency goes
from being linear in K to being sublinear in K, except for coulombic attrac-
tion, which means that as steps are taken to raise K, a law of diminishing
returns sets in, TI increasing less rapidly with K.
DISCUSSION
This simplified approach for the calculation of the single collector elec-
trostatic collection efficiency neglects differences in flow profile, collection
by particle interception, and particle inertia. These aspects, and others,
are discussed next.
A. Flow Profiles
Kraemer and Johnstone [3] and others [4, 8] have found that for coul-
ombic attraction without interception far inertialess particles, the collection
efficiency is independent of whether the flow profile is potential or viscous.
Our analysis shows coulombic collection of inertialess particles (even with
interception) to be independent of the flow profile whenever the critical
particle trajectory strikes the collector directly in the rear (rather than
when part of the collector is not intersected by particle trajectories).
For non-coulombic attraction, the results of Kraemer and Johnstone
and others indicate some dependence of collection efficiency on flow profile.
Results [3] of calculations for collection of uncharged particles on a charged
spherical collector were nearly indistinguishable for potential and viscous
flows. For charged spherical particles, and an uncharged spherical
-------
Table 1. Efficiency Determination Results
IS)
c^
CO
Collector
Force
_ Collection Efficiency Determinations
SJroxl- ?ther Efflclency Determinations
matlon Efficiency Flow
Ref.
Sphere
Qq
~2
all
UK
potential
QO
IK
Cylinder
0q
[33
potential [3]
potential and viscous [3]
"QQ
QO
0q
all
all
4K_
irK
2.9(K)°'35 potential
viscous
viscous
nK
irK
UK)1/*"
ii
irK
(_JfK_)
v2-ln Re;
(6,rK)1/3
potential
all(r_,
p .
potential
all(rp/R«l)'
viscous*
potential"
viscous
( experiment)
K« 1 n K
2.3(K)1/2
...I3J
[3]
_.C5J
C5]
•Natanson's condition was that capture could occur at the rear of the cylinder:
-BF(rp+R)/v(rp+R)^l at 6=0.
[5]
C5]
[6]
C6]
[7]
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269
collector, the Kraemer and Johnstone [3] calculations showed that the
collection efficiencies were two to three times higher for potential flow than
for viscous flow ( the streamlines are farther from the collector in vis-
cous flow), for K in the range 10'4 to 10. Other profiles might pro-
duce greater differences in efficiency for the same K, especially flows
at sufficiently high collector Reynolds number to produce a turbulent wake
behind the collector.
B. Particle Interception
Calculations [3] for spherical collectors and spherical particles
showed interception to be negligible for K> > rp/R and dominant for
rp/R»K, for inertialess particles.
C. Particle Inertia and Impaction
Particle inertia can lead to impaction of the particles, the particles
may also delay significantly in reaching the electrostatic terminal velocity.
Grover and Beard [9] calculated coulombic collection efficiencies for
charged cloud drops and charged particles. They did this by trajectory
determinations, using flow fields which were calculated by solving the
Navier-Stokes equations, rather than using the viscous or potential flow
approximations. (They found that the critical trajectories for inertialess
particles did indeed strike the collector directly in the rear). The expression
n = 4K (34)
was found by them to be correct within a factor of two for collector Reynolds
numbers less than 100 and for inertial parameters
Stk = vQT/R<0.2 (35)
for the range of particle and droplet charge levels studied. This confirms
the restriction on the impaction parameter:
Stk«l . (36)
The same work indicated that as Stk becomes much larger than 1, the
collection efficiency became dominated by impaction.
That raises another interesting point. The particle collection rate for
the collector with combined impaction and electrostatic attraction will be
very approximately the sum of the two collection rates:
N = n w A + n v nT(Stk)A (37)
s s s o o ' 1 c
where r\. (Stk) is the collection efficiency due to impaction alone. The
collection efficiency becomes
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270
1 = rTTST- + Vstk) (38)
o o c
The electrostatic efficiency will decrease as vo increases, and the impac-
tion efficiency will increase. Although this means there is a minimum col-
lection efficiency at some non-zero vo, it is misleading. The particle
collection rate should increase monotonically as vo is increased, even
when electrostatic attraction dominates impaction. For the same gas volume
flow rate and the same total collector area, this would indicate total col-
lection can be increased by making the entire collection zone of the control
device have a smaller cross-sectional area perpendicular to the mean flow
and a longer dimension parallel to the mean flow. This should not be pursued
to extremes, however, because the pressure drop across the colle'ctor will
also increase as the geometry is changed in this way.
D. Determining Whether Electrostatic Forces are Important
Collection efficiencies are not generally additive, but the effect of two
or more collection mechanisms should be greater than the largest of any of
them and less than their sum. The removal of particles from the gas means
creating a net particle velocity perpendicular to the mean flow sufficient to
cause the particles to strike and be held by the collection surface. The
total collection surface will be the collector surfaces plus the inner surface
of the device's collection zone, that volume that contains the collectors. A
first approximation for comparing the magnitudes of the collection mechanisms
is to estimate the effective velocity at the surface of a typical collector due
to each of the mechanisms; these approximate velocities are as follows:
- impaction: v Stk
- interception: v r /R
op
- gravitation: g T
- diffusion: D/R (D is particle diffusivity, cm2/s)
- electrostatics: w
s
If the estimated electrostatic velocity, w is similar to or larger
than the largest of the other estimated velocities, then it should be included
in the efficiency estimate.
The magnitudes of the particle migration velocity w due to electro-
static forces will clearly depend upon details of particle charge and size
and collector charge and size. Figure 2 shows these velocities [13] as a
function of particle size under the following assumptions: the particles,
if charged, are charged to saturation in an electric field of 10 kV/cm; the
collector, if charged, produces a field of 10 kV/cm at its surface and is a
100 (am spherical collection (not grounded). The particles are unit density
spheres of the indicated diameter, For the space charge case, the particles
were assumed to be charged as above, monodisperse at the size indicated,
and having a concentration of lg/m3. The migration velocities calculated in
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271
this way ranged from ~10 cm/s to ~10 cm/s for particle diameters
from 0. 1 to 3 |_im. In decreasing order the velocities were as follows
(at 1 urn diameter): charged particle and charged collector (coulomb
attraction), charged collector and uncharged particle, charged particle
and uncharged collector, space charge repulsion.
O
10s
I02
I01
E
u
t 10°
O
I
i '°" \ x
-'
10
1C'4
"---..X
/
^
• I COULOMB FORCE-CHARGED PARTICLE IN
A FIELD.
A 2 CHARGED PARTICLE WITH UNCHARGED
COLLECTOR.
D 3. IMAGE FORCE-CHARGED COLLECTOR WITH
UNCHARGED PARTICLE.
* 4. CHARGED PARTICLE IN SPACE CHARGE FIELD
I 1
3 10 30
PARTICLE DIAMETER , Jim
Fig. 2 Migration velocities (particle velocity at collector surface wg)
calculated under conditions indicated in text.
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272
E. Other Fields Superimposed
To a first approximation, the addition of a gravitational field should
produce an increase in collection rate due to a gravitational collection rate
Ng = n Ahvg= ^h8 (39)
the product of the mean concentration, the horizontal area of the collectors
and duct, and the gravitational settling velocity of the particles. The situa-
tion is actually more complicated, especially where the flow is either pre-
dominantly upward or downward; gravitation can then act as though it changes
VQ, allowing the particles more or less time to be in the vicinity of collectors.
An external electric field will not only produce a net migration velocity
equal to the product of the particle charge (q ), the field (E) and the particle
mobility (B), but the field will also polarize tfie collectors, which will in
turn modify the field in their vicinity. Zebel [10] gives the following equa-
tion for the radial component of field in the vicinity of a cylinder of dielectric
constant e placed in a field perpendicular to its axis, a field having a
strength Ec:
~) EQ cos 9 (40)
r
The angle 0 is measured from the direction of E . The directional nature
of the field produced means that the integration °
2TT
•
J s D r c \^^/
o
is complicated. To a first approximation, the collection efficiency should be
between what it would be for a field of strength Eo alone (integrating only
over the region where q E cos 0 < 0)
o P
and twice this, what it would be for a field of strength 2E , the maximum
value of E , achieved where the collectors are conductors, e -»°°.
r c
Nielsen [8] studied the combination of coulombic attraction, gravitation,
and external electric field for the collection of inertialess particles on
circular cylinders. He found the collection efficiency to be the same
"under many conditions for a wide variety of flow profiles, which includes
potential, Oseen, and stationary-vortex flows." As one expects from the
constancy of particle concentration along trajectories, Nielsen found the
deposition density on the cylinder to be uniform for the case of coulombic
attraction alone. Combining coulombic and external electrical fields did not
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273
necessarily improve collection. Addition of gravitation and an external
electric field gave maximum collection when they were perpendicular and
minimum when they were aligned.
If collectors are inserted into a region having an electric field, the
presence of the collectors will alter that field. If the field is maintained by
applying a constant voltage difference, for example across parallel plates,
then the field in the gaps amid the collectors will be larger than it would be
without the collectors. If the voltage difference V is applied across two
parallel plates or grids, the spacing of which is H, then the electric field
will be V/H if there are no collectors between the plates. If cylindrical
.collectors are-placed between the grids, with the collector axes parallel
to the plane of the grids, then the electric field in the gaps amid the col-
lectors becomes [11]:
E = (V/H)/(l-a/4vT)
in which expression a is the cylindrical collector volume fraction. Thus
the field is increased in the gaps as the volume fraction of collectors
increases.
If, on the other hand, the external field is due to fixed charges (rather
than fixed voltages), the effect becomes the opposite: the more collector
volume between the charges creating the field, the less field in the gaps
amid the collectors. This shielding is similar to that of a "Faraday cage
and can be quite nearly complete when the collectors are conductive.
-F. Cooperative Effects: Many Particles
If the particles are all of like sign, they will repel each other, a
phenomenon called "electrostatic scattering" or "space charge repulsion.
It can be shown that the force on an individual particle near a conductive
collector is just the same as coulombic attraction would be if the collector
had charge located at its center equal to the product of the collector volume
and the aerosol charge per unit volume. Thus, the efficiencies of collection
for spheres and cylinders are 4K and TiK, respectively.
If there are only charged particles in a circular duct, and the particles
are all of the same sign in charge, then the electric field at the inner
surface of the duct is given by the volume integral
E = [ n>q dV]/2TTR,L (43)
E = — R /2 (44)
P d
where q is the mean particle charge and R^ the duct radius. The
depositiBn rate at the inner surface of the duct will be
N = nq~~BEA/2 = nq B~Sq 2nR,L /2 (45)
^p d p p d d
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274
or
N =n qp B2nRd Ld/2 (46)
The presence of particles of opposite charge, collectors of opposite
charge, or even collectors which are uncharged will all serve to lessen
collection due to space charge repulsion at the duct inner surface. Correct
calculation of the space charge field within a filter or scrubber requires
taking into account in detail the geometry, particle charge concentration,
collector volume fraction, collector charge, and collector dielectric constant.
This effect may be important when, for example, non-conductive fibers
collect unipolarly charged aerosol particles.
If the particles are of equally positive and negative charge, then
they will coagulate more readily, but the net field at the inner surface of
the duct will have a zero contribution from particle charge.
G. Cooperative Effects: Many Collectors
In general, if there are many collectors (droplets, fibers, etc. ), the
collection efficiency of the assembly is
E = 1-exp ( WA/Q) (47)
where
WA = £(w A ).
^ s si
that is, WA is the sum of the products of the collection areas and the
migration velocities. Q is the volume flow rate through the collector. The
presence of other collectors will change somewhat the gas flow in the
vicinity of a given collector, so that its collection efficiency will be some-
what different from that calculated assuming potential or viscous flow. As
we have seen, the collection efficiencies for electrostatic effects have not
been strongly influenced by details of the flow profile. If the collectors are
charged to the same sign, this will enhance the space charge repulsion
effect. If they are of mixed charge, that will decrease space charge but
enhance collection on the collectors, because the electrical fields will be
somewhat greater than those calculated by considering the isolated
collector..
H. Cooperative Effects: Loss of Collectors
In filters, the collection surfaces are held fixed with respect to the gas
stream to be cleaned. In scrubbers, both collectors and particles move
with the gas. Melcher and Sachar [12] analyzed many different charged
particle-charged droplet configurations. The rates of change of con-
centration of particles and collecting droplets were put in terms of
characteristic times {times for approximately 1/e decrease in concentration).
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275
For particles,
t = 'l/4nq2Bn <«)
cL
is appropriate for particle neutralization due to agglomeration in a bi-polar
aerosol or for particle concentration decrease, due to electrostatic
scattering, for a unipolar aerosol. For droplet neutralization (if bipolar)
or scattering (if unipolar) a similar time can be calculated:
tR = 1/4, Q2Bdnd (50)
where the subscript d indicates these are the mobility and number con-
centration of the droplets. In order for the droplet charge or concentration
not to be greatly diminished before collection is completed, tR should be
large in comparison to the time characteristic of coulombic collection
t = l/4TTqQBNd.
Similarly, if the charge on the particles is bi-polar, then for nearly complete
collection, ta must be greater than t , which in turn must be much greater
than the residence time of the particles in the collection zone.
CONCLUSION
It has been shown for inertialess particles undergoing collection due to
central forces that the collection efficiency for a single spherical collector
is (neglecting interception):
n= (4K)2/m (52)
for K » 1. where K is the ratio of the particle terminal velocity at the
collector surface, due to a force which varies radially as 1/r , and v i
the free -stream velocity. The corresponding equation for a cylindrical
n^K)17"1 (53)
When K < < 1, the approximate collection efficiencies are
„ '• « (»«
for a spherical collector and
T, = TTK <55>
for a cylindrical collector. These approximation^ should hold for any flow
profile and any central force of the form F - k/r .
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276
REFERENCES
[1] Fuchs, N A., Mechanics of Aerosols. Pergamon Press, New York
and London (1964).
[2] Whitby, K T. and Liu, B.Y.H. , "The Electrical Behaviour of Aero-
sols", Aerosol Science. C.N. Davies, ed. , Academic Press, New
York and London (1966).
[3] Kraemer, H.F. and Johnstone, H.F. "Collection of Aerosol Particles
in Presence of Electrostatic Fields", Indus. Eng. Chem. 47_, 2476
(1955). Correction in Indus. Eng. Chem. . 48, 812 (1956).
[4] Nielsen, K.A. and Hill, J.C., "Collection on Spheres with Electrical
Forces", Presentedat the Sixty-Eighth Annual Meeting, A.I.Ch.E.,
Los Angeles, November 1975.
[5] Natanson, G. , "Deposition of Aerosols by Electrostatic Attraction
Upon a Cylinder Around Which They are Flowing", Dokl. Akad, Nauk. USSR
11£, 696-699, (1957).
[6] Lundgren, .D. A. and Whitby, K.T., "Effect of Particle Electrostatic
Charge on Filtration by Fibrous Filters", Indus. Eng. Chem. Proc.
Design fc Dev. . 4_, 345-349 (1965).
[7] Yoshioka, N., et al. , Chem. Eng. (Japan), 33., 1013 (1969).
[8] Nielsen, K.A., "Collection of Inertialess Particles on Circular
Cylinders with Electrical Forces and Gravitation", Presented at the
Sixty-Ninth Annual Meeting, A.I.Ch.E., Chicago, November 1976.
[9] Grover, S.N. and Beard, K.V., "A Numerical Determination of the
Efficiency with Which Electrically Charged Cloud Drops and Small
Raindrops Collide with Electrically Charged Spherical Particles of
Various Densities", J. Atmos. Sci. . ^£;2156-2165 (1975).
[10] Zebel, G. , "Deposition of Aerosol Flowing Past a Cylindrical Fibre
in a Uniform Electric Field", J. Colloid Sci. . 20. 522 (1965).
[11] Havlicek, V., "The Improvement of the Efficiency of Fibrous Die-
lectric Filters by Application of an External Electric Field", Int.
J. Air & Water Pollut. 4, 225-236 (J961).
]12] Melcher, J.R. and Sachar, K S. , "Charged Droplet Scrubbing of
Submicron Particulate", Draft Final Report to EPA for Contract
No. 68-02-0250, July 1974.
[13] Cooper, D. W. , "Fine Particle Control by Electrostatic Augmentation
of Existing Methods, " Presented at the Sixty-Eighth Annual Meeting,
A.P.C.A., Boston, June 1975.
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277
ACKNOWLEDGEMENT
Support by the National Institutes of Health through NIH Grant 5-S07-RR-
05446-15 is acknowledged.
WRITTEN DISCUSSION
Kenneth Andrew Nielsen
Union Carbide Corporation
South Charleston, West Virginia 25303
The particle flux method used by Dr. Cooper, which was developed
independently by Chalmers [14] and Dukhin and Deryagin [15], and the stream
function method for determining particle trajectories [4, 8, 16, 17], which
was first used by Whipple and Chalmers [18], are two powerful techniques for
analyzing particle collection behavior when particle inertia can be ignored.
Both methods can only be applied, however, to solenoidal particle fields
resulting from incompressible flows and solenoidal forces. Such forces include
the gravitational force and coulombic and external electric field forces on a
charged particle. These electrical forces are given by QpE, where Qp is
the particle charge and E is the electric field intensity present in the
absence of the charged particle and, from Maxwell's equations, E is solenoidal.
If the flow, force, and hence particle fields are solenoidal, then for two-
dimensional systems stream functions for the fields exist. The stream function
for particle trajectories, which provides a complete description of particle
motion, is then given by superposition of the flow and force stream functions.
In the particle flux method, the solenoidal property of the particle field
makes it possible to calculate the particle flux at the collector surface
without having to solve for the particle concentration profile in the flow,
because particle concentrations are constant along the particle trajectories.
This gives limited but useful information about the particle motion, namely,
the collection efficiency. The most useful application of this method is in
obtaining results for three-dimensional systems, for which the stream function
method can not be used. Whenever both attractive and repulsive forces are
present at the surface of the collector (such as for a combination of coulombic
and external electric field forces), however, the method must be applied with
caution to avoid erroneous results. From just a knowledge of the force
distribution at the collector, it is not possible to distinguish between the
flux of particles coming from upstream and the flux of particles on closed
trajectories originating elsewhere on the collector. This determination
requires a knowledge of the flow field and the types of limiting trajectories
that occur. Although the method was originally proposed for point particles,
it can also be applied to collection with interception, using the force
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278
distribution at the surface surrounding the collector that is defined by
the centers of particles at capture [17].
The results found by Dr. Cooper for the coulombic force case, given by
collection efficiencies of -4Kq for spheres and -irKqq for cylinders, where
Kqq is the coulombic force parameter (negative for attraction), are remark-
able in their generality. Furthermore, the result for spheres has been
verified experimentally [3] for measurements over a wide range of parameter
values (10""-* < -Kqq < 10). That these results hold for all steady incom-
pressible flows was shown by Nielsen and Hill [4,16] for spheres and Natanson
[5] and Nielsen [8] for circular cylinders. That both results are insensitive
to both collector shape and flow profile was shown by Levin [18] and Nielsen
[17]. In addition, Nielsen also noted that the rate of particle collection
on fibers should be unaffected by accumulation of previously captured particles
(provided net fiber charge is not significantly affected). Coulombic force
influence on particle dendrite formation was also examined.
Using both the stream function and particle flux methods, Nielsen [8, 17]
has obtained particle trajectories and collection efficiencies for the
external electric field force on a charged particle for cases in which the
force acts alone or in combination with the coulombic force and gravitation.
Collector geometries used are spheres, prolate and oblate spheroids, disks,
circular and elliptical cylinders, ribbons, and collectors of arbitrary shape
as well. Flow fields considered are potential, Stokes, Oseen, stationary-
vortex, cellular, and arbitrary steady incompressible flows. Results were
found for arbitrary external electric field, flow and collector orientations
and include the effects of particle interception. In general, the results
show that for any given collector shape and given flow and field orientations,
for any combination of the forces the collection efficiency is the same for
all steady incompressible flows when interception effects are negligible. In
cases in which the coulombic force is sufficiently dominant, the collection
efficiency is given by -4Kqq/Vo for sphere-type collectors and -irKQq/Vo
for cylinder-type collectors independent of the collector shape, of the flow
field, and of the orientations of the collector and external field.
VQ = (1 + 2Kexcos9+ Kex2)1/2 is the ratio of the particle approach speed
far upstream to the free-stream velocity, where Kgx is the external electric
field force parameter (see Table 3) and 9 is the angle between the flow and
external field. For the external electric field force alone, the efficiency
is given by nmaxKex/vo> where Hmax* the collection efficiency for infinite
field strength, is a function only of collector geometry, dielectric constant,
and orientation of the collector with respect to the external field.
The problem of calculating collection efficiencies for nonsolenoidal
electrical image forces is much more difficult than for the coulombic force
case since the particle concentration profile is not uniform and changes with
flow field. When analytical descriptions of a flow field are available it is
a simple task to obtain efficiencies via numerical integration of particle
trajectories, but the simple flow fields normally used are not very represen-
tative of real flows and, hence, the results are often less than satisfying.
Dr. Cooper's approximate analyses represent the first attempt to use the
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279
particle flux method to obtain results that reflect an "average" influence
of flow field. Indeed, if such results were available for the whole range
of image force strengths they would be very useful.
In Table 1, Dr. Cooper has assembled results of previous studies for
comparison with his high and low efficiency asymptotic results. Unfortunately,
Table 1 contains several inconsistencies, misinterpretations, and omissions
of pertinent results. The apparent difference in results obtained by Kraemer
and Johnstone [3] and Natanson [5] for force FQQ for cylinders comes from
parameter definitions that differ by a factor of two. For force FQq for
spheres, Nielsen [4] and Kraemer and Johnstone obtained identical results for
the viscous flow case and in their analyses used a force expression that
goes to 2r~5 for large r; hence their parameter definitions differ from
Cooper's by a factor of two. Natanson1s [5] results for force FQQ for
cylinders were derived for the low and not high efficiency case. In order to
avoid confusion for the reader and to include additional results,a revised
version of Table 1 is given in Table 2, with results reported on the basis of
the electrical force parameter definitions appearing in Table 3, which have
been used by Kraemer and Johnstone and Nielsen.
Compared to the coulombic force, the electrical image force (FQQ) between
a neutral particle and a charged collector is a weak, short range force.
When both forces occur, the image force can be neglected (for spherical
collectors) if the particle-to-collector charge ratio is much greater than
the volume ratio. However, for droplet collectors charged to near the
Rayleigh limit by electrical atomization the image force can be important,
but efficiencies of micron sized particles are much less than needed for high
efficiency results to be valid. As indicated in Table 2, a number of
investigators have obtained results for the charged-collector image force
case. In addition, for intermediate parameter values for spheres, results
which do not follow a power law are available for potential and Stokes flows
[3, 2, 16]. For the high efficiency results (K»l), the limiting trajectory
lies far from the collector, where flow fields all degenerate to uniform flow.
Hence these results are independent of flow field and, similarly, independent
of collector geometry. The low efficiencyresults (K«l), which agree with
those of Dr. Cooper, are said to be valid for all flows, but this results
from ignoring radial variations of the force near the collector and hence
making the particle field artifically solenoidal. In the exact case results
should vary with velocity gradient at the surface; that Kraemer and Johnstone's
numerical results for potential and Stokes flows do not merge even at very
low parameter values supports this.
The electrical image force (FQ ) between a charged particle and a neutral
collector has the unique property of becoming infinite at the collector sur-
face, although it drops off very rapidly with distance. It is negligible
compared to coulombic interaction (for spheres) when particle charge is much
less than collector charge. For micron sized particles collected on droplets,
parameter values are typically much less than one. However, the force will
always exert some influence on trajectories passing very near the collector
surface. The high efficiency limit results are independent of flow field,
-------
Table 2. Comparison of Collection Efficiency Results
Parameter Cooper's Other Studies
to! lector horce r Dependence* Range Results Efficiency
Sphere FQq r"2 all -4K -4K
F ~~5 ix >> i /Aif\2/5 /15npl/\2/5
'00 ' * -1 I™; \ 3 K;
K « 1 4K 4K
rl 9 /R 1 R 9 /K
r U *•»••» 1 /OI/\^'3 / •!• ^1» I/ \ ^ / ^
Oq 2 \2 " 3 * ' ^ 4 ^J
,- .002iK5.1 - (20. 2K)'353
= 2r~
.002£K£.l - (2.50K)1/2
for r » 1
K« 1 8K (^K)17-*
(2K)l/2
Cylinder FQQ r"1 all -nK -i^K
F¥»~ I/ ^^ 1 /_ I/ \ ^/ ** / *5 AlX \ I/ ^
OQ ' i\^^i \T^<^/ \^r^/
.01 11 fc I/ ">•> 1 f*eV\i/c- f/ll/\*-tt-
i-Qq ir-i; K ^>^i ^K; ^K;
= r"2 - - (5.29K)1/2
,forr»l K < .03 - (2.25K)1/2
.001
-------
Table 3. Electrical Force Parameter Definitions (K)
281
SPHERE CYLINDER
Coulombic Force (F_ )*
Charged-Collector Image Force
. **
o c o
Charged-Particle Image Force (FQ )
2
n /] if DD^II \ • f- f /
CQr
External Electric Field Force
(all collector geometries)
Q collector charge
w
Qn particle charge
collector radius
particle radius
p collector charge per unit length UQ free stream velocity
fluid dielectric constant
particle dielectric constant
collector dielectric constant
C Cunningham-Millikan slip correction factor
M. viscosity
uniform external electric
field intensity
* For collectors of different geometry R is an arbitrary collector reference
length (an equivalent radius, for instance) upon which the definition of
collection efficiency depends.
** For cylinders multiply by 2 to get Kraemer and Johnstone's definition.
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282
but not necessarily independent of collector shape. As noted by Dr. Cooper,
it is not valid to use the large distance force approximation near the
collector. The literature low efficiency results reflect the actual force
near the surface but, as Davies [23] has pointed out, approximations made in
their derivations could produce serious limitations on their usefulness.
Nevertheless, for cylinders the one half power law for viscous flow, which
appears valid over the entire range of parameter values, has received experi-
mental [6,7] and numerical [22] confirmation. For spheres the results for
Stokes flow show little deviation from a one half power law [3,4].
When particle inertia is present, second-order effects can create very
complex particle motions, and analytical solutions are generally not avail-
able. For spherical collectors, however, collection efficiencies for the
combined influences of particle inertia and electrical forces have been ob-
tained by Nielsen and Hill [4, 23] through numerical integration of particle
trajectories using potential and Stokes flows. The electrical forces con-
sidered are the coulombic force, both electrical image forces, and external
electric field forces, with the external field parallel to the flow. Results
for perpendicular fields and for prolate spheroidal collectors are given by
Mukherjee and Hill [24]. These results show that characterizations made on
the basis of inertialess analyses (such as flow independence of coulombic
force collection) can not be extended into the inertial realm. Although
maximum collection is favored by large particle inertia when electrical forces
are absent, collection with electrical forces often reaches a maximum for
negligible particle inertia. In general, the steeper the velocity gradient
at the collector, the greater the effect inertia has. The statement by
Dr. Cooper on the additivity of purely inertial and purely electrical collec-
tion efficiencies (Equation 38) simply is not borne out by these studies
even in approximate form.
Although analyses of particle collection on single, isolated droplets and
fibers are very similar, the dynamic characteristics of droplets in wet
scrubbers and fibers in filters are very different. In wet scrubbers, drop-
lets follow the bulk flow and only affect a localized region. Shadow or
distortion effects caused by the particle field interacting successively with
several droplets is minimal, and isolated collector results may be a good
approximation. Whereas, in filters fibers continually sweep the bulk flow,
and upstream fibers can cast particle shadows on downstream fibers or other-
wise distort the particle field such that simultaneous analysis of multiple
fibers is really required. This is a difficult task, however, for which new
insights into particle collection behavior, such as found in Dr. Cooper's
approximate analyses, may be required.
REFERENCES
[14] Chalmers, J. A., "The Capture of Ions by Ice Particles," Quant. J.
Royal Meteorol. Soc.. ^3, 324 (1947).
[15] Dukhin, S. S. and Deryagin, B. V., "Method of Calculating the Precipi
tation of Disperse Particles from a Stream on an Obstacle (Russ.)",
Kolloid Zh.. 20, 326 (1958).
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283
[16] Nielsen, K. A. and Hill, J.C., "Collection of Inertialess Particles
on Spheres with Electrical Forces," Indus. Eng. Chem Fundamentals',
15, 149 (1976).
[17] Nielsen, K. A., "Turbulent Mixing with Chemical Reaction and Collection
of Inertialess Particles on Cylinders and Spheroids with Electrical
Forces and Gravitation," Ph.D. Thesis, Iowa State University, Ames,
Iowa, 1977.
[18] Levin, L. M., "Electrostatic Precipitation of Aerosol Particles from a
Flow Upon Large Bodies," Izv. Akad. Nauk SSSR (Ser. Geofiz.), ]_,
1073, (1959).
[19] Smirnov, L. P. and Deryagin, B. V., "Inertialess Electrostatic Deposi-
tion of Aerosol Particles on a Sphere Surrounded by a Viscous Stream
(Russ.)," Kolloid Zh.. 2£, 400 (1967).
[20] Cochet, R., "Evolution d'une Gouttelette d'eau Chargee dans un Nuage a
Temperature Positive," Annales de Geophysique, 8^, 33 (1952).
[21] Knutson, E. 0., "Approximate Formulas for Electrostatic Collection of
Aerosol Particles by Spheroids," J. Colloid Interface Sci., 54, 453,
(1976).
[22] Stenhouse, J. I. T., "The Influence of Electrostatic Forces in Fibrous
Filtration," Filtration & Separation. 11, 25 (1974).
[23] Davies, C. N., Air Filtration, Academic Press, New York, 1973, p. 85.
[24] Mukherjee, S., Nielsen, K. A., and Hill, J. C., "Effects of Electrical
Forces on Particle Collection by Spheres and Spheroids," Presented at
the Ninth Annual Meeting of the Fine Particle Society, Menlo Park,
California, August 25-26, 1977.
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284
OPEN DISCUSSION
Friedlander: We have heard a criticism with high gradient magnetic separation
theory. It may be interesting to point out that there is no central force
in the HGMS theory and that complicates matters tremendously. This is
very clearly demonstrated. Calculations can be made rather readily in the
case of the electric field. In the case of the magnetic field, there are
always dipolar interactions which never have central forces. That is what
makes matters so much worse. On the other hand, in the situations so far
reported, the collection agent is usually stationary. That will change as
one goes to gaseous streams.
Cooper: A quick comment. One of the ways you can pursue it, at least to
an approximate level is to find the directional force and the area of the
particle collectors which indeed collect. You then have some idea of what
the velocity is on the surface of that area. You have to make an assumption
about the concentration profile.
Loffler: I agree completely with your remarks and that you expect from
theory to get some ideas on the most important inputs and so on. That is
the same point of our theoretical work.
There is one point on your assumptions and simplifications that may cause
a problem. I agree as long as particles are below .1 ym but as I have shown
earlier they may range between .1 and 1 ym. You should think of the
supposition you used to simplify your equation. Another thing is that all
these comments also refer to the interception effect, and I must insist that
interception is not an impact force mechanism. Interception is only the con-
tact between particle and collector. You might speak of interception if the
mass densities of particles in the fluid are equivalent. Then, you have some
effect, but it is not an impact force mechanism. Sometimes it is important,
especially in your case where the particle is collected in the rear.
Melcher: On the question of impaction and interception, the paper by Brogan
& Beer doing for the spherical case what Professor Loffler has done for the
cylindrical case, may be of interest. Essentially you want to take collection
efficiency as a function of velocity. For fairly weak electrical forces
you are going to have decreasing collection efficiency. Then you will have
the typical impaction curve and you will get a combined thing. It will come
off about 1 or a little higher if you want to take interception into account.
If you have a very strong electrical force, you will eventually have the
collection efficiency asymptotically approach the impaction collection
efficiency. It is a little misleading, however, because if you look at
the collection rate, it turns out to be equal to the concentration on the
surface times the velocity at the surface times collecting area of the sur-
face plust the impaction contribution.
The rate of collection will remain almost independent of velocity.
There is a Reynolds number correction or if we get too high, there is a problem
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285
of collecting particles with great inertia. The last thing we have is the
freestream concentration which will not change, The velocity will go up.
As it turns out, with the same collectors and volume flow, you want to
arrange the geometry that as long as you can handle the pressure drop, the
velocity is high. That suggests that going from a stubby collection area to
one that is small and long, both having the same volume flow. Indeed then
you can get the impaction.
The other point is that, after a while, the particles get so massive
that you just cannot pull them in with electrical forces. Impaction does
work well.
Liu: I understand your assumption, the divergence of the force will be equal
to zero, which leads to the conclusion that the concentration of particles
along the trajectory remains constant.
Cooper: That is for the first part of the paper where I deal with special
cases of Coulombic force, gravitational force and uniform electric field. We
stopped that development at that point, then took away that assumption and
dealt with central forces in general. The rest of the paper deals with
a generalized central force.
Liu: The first part of your paper would also be applicable to the magnetic
situation, because you have a uniform particle concentration along the
trajectory even for the case of dipole magnetic particles. Because the
divergence of the field will be zero.
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286
ELECTROSTATIC FILTRATION AND THE APITRON/
DESIGN AND FIELD PERFORMANCE
Dennis J. Helfritch
American Precision Industries Inc.
and
Teoman Ariman
Notre Dame University
ABSTRACT
The electrostatic augmentation of fabric filtration has been examined
for many years. Most often these studies have been directed toward the
effects of electrostatics on the efficiency of particle collection, and it has
generally been shown that efficiency increases with the addition of electro-
statics. It is shown in this paper that electrostatic augmentation also
gives rise to substantial increases in filtration rate per unit fabric area.
An industrial fabric filter utilizing electrostatics is described, and its
performance is demonstrated on several typical applications.
INTRODUCTION
The effects of electrostatics on the fabric filtration process have been
investigated and discussed for decades. In 1930, N. J. Hansen discovered
that the filtering efficiency of a wool filter pad was much improved when
powdered with ground resin because the resin possessed a substantial
static charge (1). This discovery was used to improve the particulate
collection efficiency of military gas masks. Since that time, many studies
have been made concerning the effects of charged particles, charged
fibers, externally generated electrostatic fields, or combinations of these
factors.
Frederick (2) studied the interactions between naturally charged dusts
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287
and fabrics. Lundgren and Whitby (3) investigated the effect of particle
charge on the filtration efficiency of uncharged fibers. Walkenhorst and
Zebel (4) applied an electrostatic field across a fabric filter. Inculet and
Castle (5) describe a device in which particles are charged and a field is
applied across the filter. The work of Lundgren and Whitby (3), shown in
Figure 1, illustrates the magnitude of the influence of electrostatics on
fabric filter collection efficiency.
PARTICLE
CHARGE
10 30 100
FILTRATION RATE, CFM/SQ.FT.
300
Figure (1) - Particulate penetration through a filter as
a function of particle charge and filtration
rate (from Ref. 3)
Mathematical models for the efficiency of fabric filters have been pre-
sented by several investigators. Natanson (6) gives equations for the
collision efficiency between a charged particle and a dielectric fiber.
Zebel adds an external electrostatic field to the same problem and presents
solutions to the governing equations of particle motion. Ariman and Tang
(8) extend Zebel's results by including the effects of neighboring fibers.
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288
Virtually all of these investigations, both experimental and theoretical,
indicate that a substantial increase in collection efficiency can be realized
when particles are charged or fields applied. This increased collection
efficiency is the result of induced polarity of the fibers or particles, which
effectively increases the collision cross section between a fiber and an
approaching particle. Experimentally, this effect appears to obey the
equation (1, 3)
fl = single fiber collision efficiency = OC (K)^
OC = 2. 3 (ref. 1) K = image force parameter
1.5 (ref. 3)
= ( gc - D/( £c + DQ2
Cc = dielectric constant fiber /A = gas viscosity
Qp = particle charge Dp = particle diameter
D£ = fiber diameter *yo - gas velocity
It is obvious from the results so far reported that electrostatic augmenta-
tion of fabric filters yields improvement in collection efficiency, but very
little information concerning the effects of electrostatics on filter resis-
tance is available. It will be shown in this paper that electrostatics can
have a profound effect on the flow resistance of fabric filters. For example,
it will be shown that by simply charging particles, the filtration rate per
unit fabric area of a given fabric filter can be increased by a factor of four.
This implies that with the aid of electrostatics it might reasonably be
expected that fabric filters can be reduced in size by a factor of four,
while achieving better collection efficiency.
BASIC STUDIES
The results of recent bench scale tests can best be used to illustrate the
variation of filter resistance with particle charge. At constant face velo-
city, suspended dusts were filtered by initially clean fabrics. The dusts
were charged prior to deposition by means of passage through a corona
discharge. Degree of charge was controlled by the magnitude of the field
strength, and pressure drop was recorded 'after a specified weight of dust
had been deposited.
Filter resistance, or pressure drop, decreases as the degree of particle
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289
charging increases, as is shown in Figure 2.
NOMEX
15 FPM FACE VELOCITY
A LIMESTONE
D SILICA
2 4 6 8 10 12 14 16
VOLTAGE GRADIENT, KV/IN.
Figure (2) - Pressure drop vs. charging voltage
gradient for the deposition of lime-
stone and silica dusts on -Nomex felt
Each point of this figure represents the pressure drop resulting from the
deposition of 25 grams of dust at a 15 foot per minute face velocity. In
particular, for. the silica dust, it can be seen that filter resistance is
decreased by a factor of 8. Although there are quantitative differences in
the magnitude of the effect of charging dusts prior to filtration, the effect
is qualitatively identical for all dusts tested.
Fabric type exerts some influence on the electrostatic filtration process,
but the relative differences among fabrics are small, as is shown in Figure
3. Charging dust particles always results in a substantial reduction of
pressure drop, regardless of fabric type.
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290
LIMESTONE
15 FPM FACE VELOCITY
O ORLON
D POLYESTER
A NOMEX
0 2 4 6 8 10 12 14 16
VOLTAGE GRADIENT, KVA/IN.
Figure (3) - Pressure drop vs. charging voltage
gradient for the deposition of lime-
stone dust on three felted fabrics
The variation of filter resistance with face velocity is shown in Figure
4. Each point of this figure represents the pressure drop resulting from
the deposition of 25 grams of dust at the face velocity given. Pressure
drop increases with face velocity as expected, and it can be seen that
particle charging results in a two-fold increase in face velocity at an equiv-
alent pressure drop.
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291
o.
o
(£.
O
10
9
8
7
- 6
LIMESTONE/NOMEX
O- 0 KV/IN.
A — 7 KV/IN.
o-IO KV/IN.
•~I3 KV/IN.
4 6 8 10 12 14 16 18
FACE VELOCITY, FPM
Figure (4) - Pressure drop vs. deposition face
velocity for limestone dust on Nomex
felt as a function of charging voltage
gradient
FILTER DESIGN
In order to apply electrostatics to the fabric filtration process, efforts
were initiated in 1971 for the design of a practical device. Various con-
figurations were tried, and the geometry of Figure 5 was ultimately
chosen as most acceptable. This device is designated the electrostatic
fabric filter or "Apitron". During normal operation, incoming airflow
enters the precipitator section in the area below the walkway. It is
deflected downward and distributed among the tubes by means of an
inclined, expanded metal grating. The flow then passes upward, through
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292
the tubes, where the particulate is charged and where most is deposited.
Flow continues upward past the tubes, into and through the bags, where
the final filtration of the charged particulate takes place. Clean air exits
the unit at the exhaust, located at the top of the baghousing section.
BAG HOOKS ABOVE
11
CORONA WIRE HOOK
ON H.V. G^ID. 5.PRIN
TENSIONED 8,
INSULATED AT TOP
INSULATOR
. /CORONA /
\ WIRt /
"\ /
\ ™
IGH VOLTAGE
TO GRID
Figure (5) - The Apitron industrial scale electro-
statically augmented fabric filter
Periodically, the deposit of particulate is cleaned from the tube and
fabric. Six bags and tubes are cleaned of deposited dust at one time. This
cleaning is initiated when an electrical pulse from the control box opens one
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293
of the1 twelve diaphragm valves for 1/10 second. The blow pipe connected
to the valve is then pressurized and compressed air jets downward from
six nozzles, each directly above a tube and concentric to a corona wire.
The jet of air flowing downward through the tube entrains and mixes with
a secondary airflow, and the tube is swept clean of deposited dust by the
mixture of high velocity air. The secondary airflow, passing from the out-
side to the inside of the bag, snaps the bag inward and dislodges dust
deposit from it. Typically, a single bag and tube would be cleaned in this
manner once every five minutes.
PERFORMANCE
When no high voltage is applied, the fabric filter behaves as a conven-
tional continuous cleaning, pulse type baghouse. As voltage is applied,
filter resistance decreases as shown in Figure 6.
10 15 20 25 30
KILOVOLTS
35 40 45 50
Figure (6) - Pressure drop vs. charging voltage
for the Apitron collecting silica dust
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294
This decrease in resistance continues as voltage is increased because
particles become more highly charged and because more particulate is
deposited on the metal tube. This behavior is similar to that shown in
Figure 2 for the bench scale tests.
Figure 7 shows a recorder tracing of pressure drop during the collec-
tion of silica at a 15 CFM/Sq.Ft. filtration rate.
20 30 40
MINUTES
Figure (7) - Pressure drop vs. time,
showing the effect of a
high voltage shut down
The applied voltage was shut down at the 11 minute point and restored at
23 minutes. Under normal conditions, with applied voltage and particle
charging, the pressure drop was 1 inch W. G. The pressure drop increased
to 6 inches W.G. after the voltage was shut down and returned to 1 inch
following high voltage restoration. Discontinuities in the tracing were
caused by bag cleanings, which continued during the voltage shutdown, and
it can be seen that each cleaning caused a decrease in the pressure drop
as expected.
Field testing of the Apitron unit was initiated in April, 1976. All tests are
carried out on a 1000 CFM, trailer-mounted pilot plant unit, shown in
Figure 8. A slipstream of process gas is withdrawn from exhaust ductwork
and is then filtered by the pilot plant unit. Generally, this operation is
carried out in parallel with existing emission control systems, often bag-
houses, and comparative performance can be easily defined. Each field
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295
test is run for two weeks or longer, and stable operation is achieved
before data is taken. Occasionally, dilution air or water spray is added
to the pilot plant inlet in order to reduce temperature or to affect some
degree of gas conditioning.
AMERICAN
PRECISION
Figure (8) - Apitron mobile pilot plant at asphalt
batching test location
The results are shown in Table 1, and Figures 9 and 10 are examples
of typical data. The performance of parallel operating baghouses is shown,
and it can be seen that air cloth ratios from four to seven times higher than
that of conventional equipment can be achieved. In addition, collection effi-
ciency is generally improved, resulting in a 1/2 to 1/10 reduction of parti -
culate emissions on a weight basis.
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VO
TABLE 1
Application
Asphalt Plant Rock Dryer
Gypsum Rock Dryer
Phosphate Conveyors
Electric Steel Furnace
Asbestos Wallboard Finishing
Cement Kiln
Air Cloth
Ratio *
CFM/
Sq. Ft.
12.5
13.5
13.0
8.0
15.0
8.0
Inlet
Loading
Grain /
ACF
32
10
7
.13
.32
3.5
Ou tie t
Loading
Grain /
ACF
.002
.001
.0008
.0008
.0004
.004
Parti culate
MMD
Microns
>40. 0
7.0
12.0
2.5
Fibrous 'N-3.
2.8
0
* Measured at a 3-inch W. G.
Pressure Drop
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297
STEEL FURNACE FUME
.13 GR./CU. FT. INLET
H.V. OFF A H.V. ON 0
.0016 GR/CU.FT. .0008 GR./CU. FT.
OUTLET OUTLET
10
5:
•
z
o
on
o
UJ 4
o: n
o
CO
UJ
£2
EXISTING
BAGHOUSE
2 4 6 8 10 12
FILTRATION RATE, CFM/SQ. FT.
Figure (9) - Results of pilot plant testing on
fume from an electric steel
furnace, showing Apitron per-
formance with and without electro-
static augmentation
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298
ASBESTOS- CEMENT
.32 GR./CU. FT. INLET
.0004 GR/CU. FT. OUTLET
5 10 15
FILTRATION RATE, CFM/SQ. FT.
Figure (10) - Results of pilot plant testing on dust
from the ventilation of an asbestos
cement wallboard trimming operation
CONCLUSIONS
Two basic conclusions can be drawn from the given results: high
collection efficiencies can be expected from the Apitron, and high air
cloth ratios can generally be achieved. As previously discussed,
improved collection efficiency results from the electrostatic augmentation
of the filtration process because of polarization effects between charged
particles and fibers.
A more surprising result is that of the demonstrated increases in filtra-
tion capacity per square foot of fabric when charged particulate is collected.
This occurs because the particulate deposits on the fabric in a more open,
loosely packed structure, and this difference in structure can be observed
under magnification. The structure is more permeable and, hence,
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299
allows greater filtration rates.
For a given flow rate of gas, the filtration capacity, or air cloth ratio,
directly determines the overall dimensions of a fabric filter because it
specifies the area of fabric needed. The method and frequency of removal
of the deposited particulate from the fabric can effect air cloth ratio
requirements. For example, a fabric filter utilizing a shake cleaning
mechanism might require an air cloth ratio of 2, while a jet pulse fabric
filter would yield adequate performance on the same application at an air
cloth ratio of 5. With the addition of electrostatics, the air cloth ratio
could be increased to 14. Typically, an air cloth ratio is selected such
that the pressure loss of the fabric filter will fall between three and six
inches of water. Figure 11 shows the air cloth ratio capabilities of three
types of fabric filters and the resulting floor space requirements.
FLOOR SPACE REQUIREMENTS
VS.
AIR CLOTH RATIO
|/
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300
The exact air cloth ratio required depends on the application, but, in
general, when the ratio is in the high range for one type of filter, it will
also be in the high range for the other two types.
It has been seen that the electrostatic augmentation of the fabric filtra-
tion process yields improved collection efficiency and reduced space
requirements. The Apitron system can be regarded as a new category
of particulate control device, offering performance unattainable with more
conventional equipment.
REFERENCES
(1) Davies, C. N., "Air Filtration", Academic Press, London, 1973;
pp. 106-108.
(2) Frederick, E. R., Chemical Engineering, June 26, 1961; pp. 107-
114.
(3) Lundgren, D. A., and Whitby, K. T. , I & EC Process Design,
Vol. 4, October, 1965; pp. 345-349.
(4) Walkenhorst, W. and Zebel, G., Staub-Reinhalt, Luft, Vol. 24,
1964; pp. 444-448.
(5) Inculet, I. I. and Castle, G. S. P., ASHRAE Journal, March,
1971; pp. 47-52.
(6) Natanson, G., Dokl, Akad, Nauk USSR, Vol. 112, 1957; pp. 696-
699.
(7) Zebel, G., Journal of Colloid Science, Vol. 20, 1965; pp. 552-543.
(8) Ariman, T. and Tang, L., Atmospheric Environment, Vol. 10,
1976; pp. 205-210.
WRITTEN DISCUSSION
T.M. Kuzay
Argonne National Laboratory
Argonne, Illinois 60439
The authors are to be congratulated for bringing to our attention a field
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301
tested filtration technique with much reduced pressure drop feature compared to
a conventional bag-house design.
A central theme of the paper hinges on the gains achievable in lowering
the overall pressure drop in the practical bag-house type filtration systems
by the application of electric fields to such systems. This reduction is
achieved with equal or better collection efficiencies when compared with a
similar system of the non-charged case. Qualitatively it is asserted in the
paper that the smaller pressure drop feature is due to dust cake being more
porous in electrostatically charged systems and the increased efficiency is
possible because collision cross section between the particle and the collector
(fiber) is larger due to induced polarity.
Ramifications of the paper are enhanced further when the nation is turning
to recognize the energy shortage and the conservation as an effective measure.
The smaller pressure drop in a filtering system for the same flow rate and dust
loading, but with equal or better collection efficiency, affords a direct reduc-
tion in the fan power necessary to drive the system. For the proposed system,
however, economics should also be studied to correlate the cost aspect of the
increased hardware and the maintenance of the fields. I presume the work being
preliminary in nature will be expanded and encouraged to cover such matters in
the future.
As noted earlier, data for the paper have been obtained essentially from a
field tested pilot plant and to a limited extent from a bench scale model.
Therefore, only qualitative discussions are possible. To increase accuracy
and for comprehensive data, bench top models need be built. In this report,
for example, a comparison of Figure 2 and 4 shows that at the same face velocity
(15 FPM) and for the same cloth and dust variety (NOMEX/LIMESTONE) the data
points shown in Figure 4 could be shifted to higher absolute pressure drop
values resulting in less favorable pressure drop ratio between the non-charged
and the charged filtration cases.
Examination of Figures 2,3 and 4 shows also that the pressure drop in the
charged filtration system levels off after approximately 10 kV/in applied
voltage gradient for all fabric and dust varities presently tested. Is the
mechanism for such an applied potential gradient threshold well understood?
Existence of such a "saturation" point is important in the system optimization
for power reduction. Furthermore, if such a threshold, indeed, exists, then
this value might be considered for normalizing the data.
The data presented in Figure 9 raises questions in two respects and the
author's comments are welcome. First, is the given dust loading figure at
outlet the same for both systems? Second, is indeed the single data point
shown for the bag-house a valid data point? Or, were there some extraneous
effects such as humidity difference between the two gas streams as the paper
mentioned as a general comment.
Figure 10 is perhaps a more direct comparison of the improvements possible
with a charged system over a similar non-charged system. It appears that even
the non-charged APITRON has been a favorable design over an existing
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302
conventional bag-house system in terms of its pressure drop characteristics.
Consequently, Figure 9 should be interpreted with this difference in mind.
In conclusion, while I find the authors'presentation most interesting and
far reaching in its energy saving potential in engineering applications, I
would like to emphasize the necessity for more comprehensive and well-controlled
research in fundamental assessment and understanding of the system presented.
OPEN DISCUSSION
Helfritch: I would like to reply to Dr. Kuzay's comments.
First of all, I think we are referring to Fig. 2 and A in the manuscript.
These figures show that at the same face velocity with the same flow and dust,
the data points shown in Fig. A should be shifted to a higher absolute pressure
drop. Those two sets of data were not taken under identical conditions. The
one parameter that was different between the two was the relative humidity.
This is also a factor that I have not talked about, but is being looked at.
I think that accounts for the difference you see in pressure drop at given
filtration rates.
Examination of Figures 2, 3 and 4 show also the pressure drop levels off
at approximately 10 k volts/inch. I do not know if there is a threshold' but
it is possible. We could not usually go much beyond the voltages I have
shown because that is the spark level point. If we were able to carry the
voltage gradient out farther, I do not know whether we would reach some sort
of a threshold or not. At the inlet the grains per cubic foot would be the
same for both systems. What is shown in Fig. 10 is the outlet concentration.
Liu: Could you clarify the voltage gradient? Is this the voltage of the
wire divided by the distance of the wire from the cylinder? Or is this
actually the voltage of the wire?
Helfritch: It is not a good representation. The actual voltage gradient is
not linear at all. It is just the average voltage.
Nichols: You showed a variation in pressure drop with applied voltage. Have
you conducted any tests where you operated at a lower voltage than the corona
inception?
Helfritch: No.
Nichols: There is a potential for the formation of dipoles that may lead to the
same type of mechanism. If that is the case, you may want to put a pipe down
the middle of your bag rather than having an auxiliary charger if only the
dipoles are necessary. Secondly, have you made any attempt to determine the
relative collection in the precipitator versus what it is in the bag filter?
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303
Halfritch: On occasion we have done that and it varies quite a bit with the
dust. The best I can say it is between 20 and 80 percent, depending on
conditions and the type of dust we are using. It is not a very efficient
precipitator. We run very high velocities through those pipes.
Nichols: I wonder if it is the charging of particles or whether it is the
formation of dipoles that is important for the reduction in the pressure drop.
Helfritch: No, it is the charging of the particles.
Nichols: But it still could be forming a dipole that leads to the low pressure
drop. They would not, necessarily, have to be charged, they could form dipoles.
Helfritch: There is no field to do that.
Nichols: How about from the previously deposited material?
Helfritch: But that requires a charged material.
Nichols: The charge is on there when you charge the particle, If all you
need is to form a dipole, all you would need to do is form a field within the
filter by having metal fabric in the bag with a pipe in the middle, If you
form the dipoles at the slow face velocities, you would still have time for
these to align and give the low pressure drop. I do not know whether it is the
fact the particles are charged or the fact that they have formed the dipoles
that leads to the low pressure drop.
Lamb: I have seen similar effects but we do not control the charge on the
particles. We do not charge the particles, but apply a field in the plane of
the fabric by just putting electrodes in the fabric itself.
Penny: Do you have figures on the resistivity of the dust at which this
comparison was made? Precipitators are so dependent on this resistivity.
Helfritch: We have never measured resistivity. We have no equipment for- that.
We have noticed a strong effect that we interpret as high resistivity problems.
We have also noticed strong corona quenching effects when we go to very small
particles. In these cases, we generally have to run at lower voltages and
currents than otherwise. However, even in these cases, particularly in Table
1 for the case of the cement kiln and the electric furnace fume, we were way
down in voltage and current but we still got improvements in performance. I
attribute low back voltage and current to resistivity of the dust.
Cooper: It may happen that changing the resistivity of the dust cake would
change the particle size distribution. This might be expected if you have 20
to 80 percent collection in the electrical case beforehand. If you had a
coarser dust that would be consistent with a lower pressure drop per weight
per unit area on the filter. It also might suggest to some degree the different
structure in the cake.
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304
Helfritch: Again I allude to the bench scale-testing where none of the
particulate was collected in the precipitator section, Dust particles were
deposited in the charged state. Basically in our APITRON we are getting sort
of the same thing.
Reif: With the configuration you are using, I do not think there would be
any evidence to suspect that there are polarization effects present. Any
particle polarized upstream probably would "unpolarize" by the time it got
into the filter.
Nichols: The space charge would still have an electric field in the bag.
Helfritch: You are talking about a non-ionizing field; you are saying you
produce a dipole before you produce corona. You do not have a charged
particle so you do not have a space charge field.
Nichols: But you can have a field set up where you can induce dipoles to
determine whether it is a charge or dipole that cause this effect. It is
possible, but you did not do it.
In Figure 6, the dotted portion of the curve should look steeper, I
would expect that there would almost be a vertical drop in pressure before the
corona, unless there is something else going on.
Looking at Figures 2, 3 and 4, there is an area where there is no
corona generated.
Grassel: Does this affect the cake structure? Does it affect the depth of
deposit on the fabric filter and does it affect the strength of the
agglomerate that comes off?
Helfritch: It does affect the cake structure. The depth of particulates
penetrate into the filter would probably have to be reduced because most of the
pressure drop you see across a felted filter is due to particles that have
penetrated the filter.
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305
ELECTROSTATIC CAPTURE OF FINE PARTICLES IN FIBER BEDS
D. L. Reid
Applied Engineering & Development Section, Battelle-Northwest
ABSTRACT
Experiments at Battelle-Northwest led to the discovery that charged
particles could be efficiently collected by a bed of knitted fibers having
a very high bed volume -- fiber volume ratio and very low pressure drop.
The described concept depends on unipolarly charged particles being
deposited on fibers of like charge. Results are presented for the system
efficiency for submicron particles as a function of velocity, particle
resistivity and bed depth and porosity.
INTRODUCTION
The capture of charged fine particles in highly porous fiber beds was
discovered by A. K. Postma and W. K. Winegardner near the end of a study on
the collection of charged submicron particles by oppositely charged spray
drops. To demonstrate the enhancement of collection by the charged spray
drops, the negatively charged particles were passed through a dry system
which consisted of a spray chamber and, fortuitously, a polypropylene fiber
demister. Surprisingly, a rather dramatic increase in the collection effi-
ciency was observed rather than an expected lower efficiency. This obser-
vation inaugurated an investigation which eventually reduced the concept to
practice and ultimately a patent.
Since the Demister® pad produced little or no attenuation of submicron
particles without field charging, some phenomenon other than natural depo-
sition processes was involved. Subsequent tests revealed that the collec-
tion mechanism depended on the development of an induced space charge within
a bed of electrically non-conducting fibers. This is simply accomplished by
passing the aerosol through a negative corona field and then through the
fiber bed. The deposition of charge on the fibers instantaneously creates
a negative space charge field within the bed. Then if a highly electrically
resistive fiber is used and the particle is electrically non-conducting, the
bed field charge is maintained at a magnitude sufficient to produce a high
removal efficiency.
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306
THEORY OF CONCEPT
The mathematical modeling of the concept, by D. L. Lessor, is too
extensive for this presentation so only a brief explanation is presented.
Since the electrical field in the bed is induced by the deposition of
charge, efficient operation of the system depends on the development of
electrical fields of like polarity sufficient to overcome the coulombic
repulsion force of a single fiber within times shorter than the residence
time of the particle within the bed.
As the charged aerosol enters the bed of like charge, the interaction
between one particle and the fiber nearest to it is a repulsion force.
However, the charge distribution on the whole bed produced an electric
field force that drives the charge from the bed. This bed scale field
apparently has a force sufficient to divert the charged particlefrom the
normal flow lines and onto the fibers. Thus, the fiber bed concept is
effective when the bed scale field force is significantly greater in mag-
nitude than the repulsion force of a single fiber. The following calcu-
lation compares the magnitude of the bed scale field in the gas flow direc
tion (z), E , with the radial electric field, E . caused by the nearest
fiber. r
Consider a fiber filter of thickness 2h in the direction of the gas
flow and infinite in the transverse direction. If the fiber length per
unit of pad volume, £v, is uniform and the fibers carry uniform charge per
unit length, q£, the macroscopic charge density, t,, is
This charge density serves as a source for the bed scale electric ffeld
whose local average over several fiber spacings has a component E given by
0 C. 2
E7 = -^r- (for -I
where e is the permitivity of the bed, and z is measured from the midplane
of the bed. Positive values are in the direction of the gas flow.
The radial electric field from the nearest charged fiber is
E . J!s-
r 2nre
where r is the distance from the center of the fiber.
Then the ratio of these forces at the fiber surface is
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307
rr - 2nVfz = 2B^
o
where rf is the fiber radius and B is nrf £v, the fraction of the bed
occupied by fibers.
This ratio is much larger than unity for the conditions used in the
fiber bed concept. For example, with an occupied bed fraction of 0.05 and
a ratio of bed thickness to fiber radius of 500, then Ez/Er is 50.
A computer program has been developed which calculates steady-state
conditions for simultaneous charged aerosol deposition, charge conduction
on the bed, charge leakage off the bed edges, and effect on deposition by
the field from the retained charge. This modeling work indicates that
* electrically resistive beds retaining substantial charge provide
enhanced filtration efficiency and
* the bed scale electric field effect is a dominant colle'ction
mechanism for submicron aerosols.
EXPERIMENTAL PROGRAM
Based on initial observations variables thought to influence system
efficiency were (1) particle-size, concentration, resistivity and charge
level, (2) gas-velocity, temperature and composition, and (3) fiber bed-
resistivity, thickness, porosity and dust surface coverage. A complete
block experimental program involving all variables would require more than
a practical number of tests for proof of the concept. Therefore, the
initial program was designed to look principally at the bounding condi-
tions over some reasonable operating range. Table I lists the test condi-
tions for the parameters of interest.
TABLE I. System Test Conditions
Parameter Range
Particle Size 0.06 - 3 ym
Aerosol Concentration 10 - 250 mg/M3
Particle Resistivity 108 - 1013 ohm-cm
Particle Charge Level Near Saturation
Gas Velocity 0.25-2 m/sec
Gas Temperature 20 - 130 C
Gas Composition Air and Combustion Products
Fib.er Resistivity Conducting and Nonconducting
Bed Thickness 7.5 - 30 centimeters
Bed Porosity Vpid Fraction - 0.94 - 0.98
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308
TEST SYSTEM
Figure 1 is a schematic drawing of the system which consists of a 7
foot long 24 inch diameter pipe, a short corona charging section with an
effective cross-sectional charging area equal to that of the duct, a 3 foot
long section between the charger and fiber bed to preclude any direct field
charge effects from the charge section, and a bed frame for the knitted
fiber mats. The inlets and outlets to the bed were 24 inches diameter.
Since the bed was hand packed, the bed face dimension was made 28 inches by
28 inches to extend the experimental run time before edge leakage became
the predominant source of downstream particles. The frame was made with
lucite to accommodate up to a 12 inch thick bed and was electrically iso-
lated from the rest of the system as was the charging section. The system
was built in sections for future alteration of configurations and distances
between principal components and was held together with electrically insu-
lated external straps. A dispersion plate was placed in the 12 inch diam-
eter inlet at the transition point to produce the normal velocity profile
at the three principal sampling points at positions 1, 2 and 3.
FIBER BED VOID FRACTION
The void fraction of the fiber bed was defined as that volume of the bed
not occupied by the fiber volume and was determined from the bulk weight of
the fiber mats, density of the fiber per unit length and the fiber diameter.
The knitted mats were supported within the frame by a grid of teflon coated
wires having about a 4 inch wire spacing. The 15 cm thick bed was packed to
^0.96 void fraction (VF). The 7.5 cm bed had an ^0.94 VF. For the 30 cm
thick bed, the first 15 cm had a VF of MJ.975 and the last 15 cm ^0.96. The
lower fiber density in the first half was used to increase depth of penetra-
tion and thus increase the load to bed pressure drop ratio. This accomplished
both goals, but appeared to be detrimental with respect to efficiencies in the
later stages of a run series as well as length of operating time before a
decrease in efficiency was observed. Visually, the depth of penetration
appeared to be directly related to particle resistivity for the submicron
aerosols — increasing with increasing resistivity. Particle size was not a
primary variable. Interest was in the 0.1 - 0.3 ym size range except for
fly ash which was collected downstream of a two-stage ESP.
PARTICLE GENERATION AND CHARACTERISTICS
Four particles were used in this series of experiments to highlight any
change in efficiency with particle resistivity — namely, amonium chloride,
magnesium oxide, sodium oxide, and fly ash. The fly ash, obtained from a
plant burning low-sulfur Western coal, was resuspended by aspiration. The
NH4C1 particle was generated by air sparging of HC1 and NfyOH solutions with
interception of the gas streams at the inlet to the system. The particle
generation rate was controlled by varying both the concentration of the
solutions and the sparge air flow rate. The MgO and Na20 particles were
generated by metal vaporization and oxidation. The metal was heated induc-
tively in a boat enclosed in a tube. An inert gas forced the metal vapors
-------
WIND TUNNEL SCHEMATIC
AEROSOL DISPERSION
GENERATOR PLATE
SAMPLING POINTS
EXHAUST
SUPPORTS CHARGE SECTION FIBER BED
FIGURE 1
to
O
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310
out the end of the tube where ignition to the oxide occurred. Since a
technique was available for varying the particle size, some tests were
made with MgO particles having an AMD of less than 0.1 urn.
An eight-stage Anderson sampler was inserted upstream of the charge
section to obtain the aerodynamic mean diameter for each generation in
which the conditions were altered.
TEST PROCEDURE
After establishing the desired particle generation rate mass samples
were collected on membrane or fiber glass filters at three positions:
upstream of the charger and upstream and downstream of the bed. The high
particle and field charges upstream of the bed produced odd results that
defied interpretation and precluded the use of much of the data for deter-
mining bed efficiency. Consequently, the results reported are for the
system efficiency. Later tests have shown good agreement between bed and
system efficiencies -- apparently the result of cutting the current flow in
the charge section by one-half.
Generally, operation was continuous until the end of the day or until
apparent breakthrough (whichever came first) except during adjustment of
generation rate and flow. Usually, two consecutive measurements were made
before changing the flow and/or concentration. Low aerosol concentration
runs generally required two days of operation. When this occurred, the
system was shut down over night. Prior to particle generation the second
day, the charger was turned on to impose a field charge on the bed and a
reentrainment test made at 1.5 m/sec flow. There were no apparent degrading
effects even though the bed charge was at zero over night. For a series of
measurements, the flow rate was varied from low to high and back td lo»\» or
in the reverse order.
RESULTS AND DISCUSSION
Typical results obtained with the 15 cm and 30 cm thick beds whose
void fractions were described earlier, are shown in Figure 2. The decon-
tamination factors are plotted as a function of velocity. The data
approximate the expected relationship with the log of the Df varying
linearly with a power of the velocity. For the 15 cm bed all data were
considered to be from the same population for the least squares fit repre-
sented by the line. The coefficient of determination was 0.9 and all data
fell within the 2a error limits.
For the 30 cm bed, the log-log plot of the data suggested a real
although small difference in the system efficiency for NfyCl particles.
The NH4C1 data were segregated from Na20 and MgO for the linear regres-
sion analysis. The average efficiency for NH4C1 was lower than that of
MgO or Na20 by about 2 to 5 percent over the range of 0.5 - 1.5 m/sec face
velocity. This is the first data that might confirm the model's prediction
that efficiency should decrease with decreasing resistivity. Since the
-------
99.8
99.5
LU
* 99
^j-
o
"Z.
n 98
o
i
1 95
0
o 90
o
80
50
- 500
-uT 200
i— i
r—t
•of 100
o
t—
0
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312
bulk of the data does not indicate a like trend, final judgment is reserved
until confirmed. Also bearing on the reservation is the lack of any dis-
cernable difference in the bed field charge, inferred from current measure-
ments, between the four aerosols used. Thus, it appears that something
more subtle than particle resistivity was responsible for the lower effi-
ciency observed and the threshold for this effect would occur at a lower
resistivity than the 108 ohm-cm of NfyCl. The anomalously low efficiencies
for NfyCl and MgO at 0.5 m/sec face velocity were the last samples taken in
each series when the operating log noted edge leakage and were not included
in the analysis.
8 cm Thick Fiber Bed
Since the efficiency appeared to be influenced by fiber density as well
as thickness, a run series was made with the NfyCl aerosol using a 7.5 cm
thick bed packed to a void fraction of ^0.94. In this test series it was
planned to obtain successive measurements at one velocity until break-
through at the bed edges became apparent. In an effort to increase the
number of consecutive measurements at the high aerosol concentrations used,
a folded fiber mat was inserted at the bed-frame interface to bring the
fiber density at that point nearer to that in the rest of the bed. This
produced a marked improvement in total run time before edge breakthrough
was indicated at a much higher pressure drop than in previous tests. Five
successive measurements were made for all flowrates except 0.5 m/sec where
twenty measurements were made. The particle size for this series of tests
hovered around 0.25 pm AMD and a mass concentration near 75 mg/M3.
A typical equation developed from the least squares fit of the effi-
ciency versus velocity was
ES = 102.8 - 10.28 V
where
Ej = system efficiency in percent and,
V = bed face velocity in m/sec and
2
r = 0.986, correlation coefficient.
The plot of the decontamination factor vs. velocity (Figure 3) shows the
average values for each velocity along with the 95 percent confidence
levels. Since the average efficiency (99.95 percent) at 0.25 m/sec flow
was higher than the measurement sensitivity, the lower value of 99.93 per-
cent was used for curve fitting. The slope of the line was highly influ-
enced by the high Df at the lowest velocity tested. This was the first
indication that the log of the decontamination factor may not be linearly
related to some power of the velocity for all conditions. More tests at
lower velocities will be required to define the relationship. But first
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313
DECONTAMINATION FACTOR
7.6 cm POLYPROPYLENE BED
0.946 VOID FRACTION
o
UJ
o
99.9
99.8
99.5
99
98
CO
95
90
80
1000
500
100
50
o
o
10
SENSITIVITY LIMIT
NH4CI-0.25MmAMD
Dp - (16.24) V"
BEDAP /
Es = SYSTEM EFFICIENCY
® = NUMBER OF DATA POINTS
1.0
o
CM
O
a:
o
LU
O£
CO
UJ
Of.
Q_
OQ
I
O
0.1
0.05
0.1 1.0 3.0
SUPERFICIAL GAS VELOCITY, m/sec
FIGURE 3
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314
some measurement method other than mass, if available, will be required to
provide adequate sensitivity at the lower velocities.
Comparatively, this thinner <^8 cm bed with a lower void fraction
exhibited an efficiency about equal to the thicker beds at 0.5 m/sec flow
and at 1.5 m/sec flow was about 5 percent higher and lower than the 15 and
30 cm thick beds.
Loading Run
Continuous loading runs were made to examine loading and pressure drop
characteristics of the bed. Generally the system was operated continuously
during the day and shut down over night. After being shut down, the corona
charger was turned on before air flow was re-established and aerosol gen-
eration effected. A typical run made with fly ash having an AMD of 2 vm
and particle resistivity near 1013 ohm-cm is shown in Figure 4 to highlight
the higher efficiency obtained with aerosols having AMD's greater than 1 ym.
Also included in the figure are the load and pressure drop as a function of
time. Since the load-pressure drop relationship is a function of the par-
ticle resistivity and bed void fraction the onset of an increase in the
pressure drop and the rate of change may be different than that observed
for fly ash.
The lower efficiencies observed for the first few samples after restart-
ing the system the first five times are as yet unexplained. Since this was
not observed in other loading runs, it is thought to be peculiar to this
series of tests. Comparable efficiencies for the 8 cm thick bed at a face
velocity of 1.5 m/sec were >99 percent for the 2 um AMD fly ash and -v,90
percent for the submicron aerosols. This higher efficiency may be due, in
part, to particle attenuation by natural processes such as impaction and
interception. The long period of operation before an increase in the bed
AP was observed reflects the high void volume as well as the high load
capacity that can be obtained especially with beds of higher void fractions.
SUMMARY AND CONCLUSIONS
The efficiency and design flexibility of the fiber bed for the collec-
tion of submicron particles suggest many practical applications for rela-
tively economical particle control that is not attainable with presently
available equipment. The deposition of a negatively charged particle on a
negatively charged fiber is apparently dependent upon the magnitude of the
ratio of the bed scale electrical field to the electrical field of the
individual fiber — both created by the deposition of charge carried by the
aerosol. Other principal observations were as follows:
* Velocity was the prime variable with the efficiency decreasing
approximately linearly with increasing velocity.
" Particle resistivities over the range tested produced no consistent
trend in collection efficiency. However, some degradation in
-------
POLYPROPYLENE FIBER BED
7.5 cm THICK - 0.94 VOID FRACTION
FLYASH - 2pm AMD - 10*3 OHM - CM RESISTIVITY
1.5 m/sec FACE VELOCITY
0
100
99
98
97
°-° - ^
oo°o°0
0 o
L
INDICATES OVERNIGHT STANDBY
' ill J 1 L.
4 8 12 16 20 24 28 32 36 40 44 48 52 56
TIME IN HOURS
0.40
o 0.36
CM
^ 0.32
D_
< 0.28
o
J I I L
J L
J L
FIGURE 4
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316
efficiency might be expected at some lower resistivity -- the thres-
hold, of which, is yet unknown.
* Particle size had no apparent effect on efficiency for AMD's less
than 1 urn. A significant increase in the efficiency was shown for
aerosols of 2-3 urn AMD — ascribed to both electrostatic effects and
natural deposition processes.
* Fiber electrical resistivity is essential for efficient particle
control in the submicron range. Any fiber that maintains a high
electrical resistance during operation should be applicable.
* Bed thickness had a pronounced effect on collection efficiency espe-
cially at the higher flow rates. However, small changes in the
fiber volume density produced significant changes in the efficiency.
Apparently the bed thickness and fiber density can be tailored for
the aerosol condition.
* Dust concentrations between 10-250 mg/M3 were used without any sig-
nificant effect on bed efficiency. As might be expected, data
reproducibility was better at the higher concentrations.
" Particle charge level was not varied intentionally. For these
initial investigations, a near saturation charge was desirable.
The fiber beds had no affinity for uncharged particles at the
higher velocities and showed <\-8 percent retention at a 0.25 m/sec
flow.
The test results substantiate the influence of the bed scale electrical
field forces for collecting submicron particles. The computed Ez/Er ratio
for the beds tested ranged from 20 - 70 without major changes in the
efficiency.
OPEN DISCUSSION
Melcher: I would like to set the trend of the discussion if I could. I use
a rule of thumb in dealing with electrostatics and that is that there is no
such thing as a repulsion force. The reason I say there is no such thing as
a repulsion is you really ought to account for where the charge is.
You have the filter, with charged particulate passing through it, the
question arises: how is it that the collection sight collects anything? Let
us look at a calculation, call the half spacings, S, my contention would be that
one way to look at it is the particulate gets into the matrix, then, in a
van de Graffe sense, with the gas as the belt, and this (the collectors) as the
resistic load generating its own electric field. It is that electric field that
then allows it to be collected on sight, As a calculation, let us say we
have Pmas a density per unit volume, that would mean if we have a charge on
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317
each of the particles in there, we would have a charge per unit volume which
would be q.
Gauss' law says that coming out of this region is an electric field which
is equal to S times the charge density, or eQE actually. So if you look
at a particle out in front we find that it is experiencing this sort of
polarizing field. Not only does it have a charge, q, but it is also in an
ambient field. That means the particle with charge q is generating a field
that would equal R -5^ — , where R is the radius of this spherical particle.
So the situation is such that the particle experiences the ambient field and out
of the particle comes its own field. Certainly if plus particles are going
to get collected on that surface, we have to have E lines coming in. So you
see how you might solve the problem. Just look at it as a conducting sphere
in a uniform field carrying that charge. You would find that it is a matter
of whether there are any E lines coming in. It depends on if C^~*~ R) is
less than the E field coming in, which is -^a- where 3 is the
concentration factor. You can then cancel out the charge and e , Let us also
o
view the packing as being determined so we get the particle size as much as
possible by the spacing between particles, aR. In other words I am normalizing
the spacing to the size of the particle. In that case the number density is
^=lparticle ^ We make thg condition t^j- the particles will collect if
a r
, < (3S)(4Tr)
3
Ra
What this tells you is that the thickness of the filter must be bigger than
the size of the fiber particles, which is a very good approximation. In order
for it to compete with an electrostatic precipitator, you want this self-
generated electric field to be larger than what you have in the ESP. It will
do much better in resonance time than ESP. To me it is an electro-packed
bed with a self -generating field, where this is a big resistor that drains
off current. One thing that bothers me about the experimental work is how
much it can be related to something else when in fact you are not held
accountable for where the images are. If the load is on the edges then the
field is not actually one dimensional, it is actually quite complicated and
will depend on the size. If you had screens clearly the field would be
longitudinal and you would have the thing well under control. I would worry
how relevant it is to scale-up.
There is a patent by Cole in which he packs between screens particles which
are semi- insulating. He sends the current through and collects in this mode.
What he has done is to apply the field rather than to self- generate it.
That would be my view of it.
Reid: I did not do the mathematical modeling. This was done by D. L. Lessor
This is true if you have a uniform charge throughout the bed and a uniform
fiber length per unit volume. At this position in the bed plane, essentially
your charge effects are zero. However, this is not the condition encountered
in the fiber bed. If you look at it with respect to this and you look at the
ratio of electrical force in the bed versus the electrical force at the surface
of the fiber, you end up with a relation of ± . These are just physical
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318
dimension of the system but it does give an approximate relationship of the
bed scale field forces with respect to the force at the fiber surface itself,
The system, as we have run it, varied from ratios of 3 to 10, We
interpret the repulsive forces as acting on the particle itself and diverting
them from the normal flow lines that you would expect in the system. That
would mean a diversion of submicron particles from their normal flow lines
to allow them to deposit on the fiber itself. The magnitude of the bed scale
field effect overcame the repulsion forces of the fiber and allowed it to
deposit.
Melcher: This, by the way, is the configuration in which Whipple & Chalmers
say en isolated particle will not collect. This is that "backward configuration,"
One that we say collects just as well. But one had better put something into
his model that adds turbulence, or has something about the wake in it,
Reid: He has a very extensive mathematical model development of this system.
It is worthy of a paper in itself and I hope it will be published in the not
too distant future.
On the upstream face of the 12 inch bed, it appears that it is a completely
closed structure. It should have an extremely high pressure drop in this
system. Under these conditions the pressure drop at the time we terminated
this test is about 1.2 inches of water. The particles deposited completely
around the surface area of the fiber not in any one particular orientation,
front or rear. The initial modeling work predicted the particles would all
deposit on the rear face of the fibers. He had to revise his mathematical
modeling on the basis of his observations. The particles deposited around
the fibers.
Cooper: The thing that intrigued me most was that if you shut it down, waited
overnight, turned the gas on the next day with a corona charger, you'could
get the thing back up to the condition where it was catching charged particles.
That suggests to me that something other than the particles themselves was
bringing the charge to the fibers.
There are two possible solutions: 1) You are some how getting it from the
ions and 2) Maybe we are seeing some sort of tribo effect in the plastic. That
fact really puzzled me.
Reid: If we run on uncharged particles for the bed we see a positive leakage
current, not a negative one. That suggests that the bed is producing a positive
charge. It is very weak charge we are producing under the hydro-electric
effect, principally since we have such an open structure.
It is true that all we had to do is turn on the charged section, although
we could not use filtered air, and all air has particles in it. So whether
we are looking at ion deposition or charged condensation nuclei count particles
that are deposited on the fiber, we do not know. We produced approximately
an order of magnitude less field force in the bed than when we were generating
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319
particles. We do not know whether it is ion deposition or deposition of
condensation nuclei that have been charged. We know the system is about 95%
efficient for condensation nuclei particles.
Cooper: I am still puzzled on how a negatively charged particle is deposited
on a negatively charged bed. The strong coulombic repulsion, it would seem,
would not allow a deposition.
Reid: The key is the ratio of the field forces.
Liu: I would like to suggest another point. Your device is very similar to
a space charge precipitator,except you have localized, immobilized centers
on which the space charge can be held stationary. So your figures depicting
repulsion are still valid except that in half the electric field you have
additional condensation centers which would act as impaction centers for the
particle. For the case of the space charge precipitator, the particle would
have to be driven all the way to the driving surface before it could be
collected. It is a combination of Melcher's fluidized bed device plus the
space charge precipitator.
Reid: That's pretty much right.
Friedlander: I am a little confused. Where is the energy coming from?
Melcher: It is a self-induced space charge. It is a gaseous van de Graffe.
Reid: The particles are the source of the energy. There is an imposed charge
put on the particles upstream of the system.
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320
FINE PARTICULATE AND GASEOUS EMISSIONS CONTROL EXPERIENCE WITH
THE TRW CHARGED DROPLET SCRUBBER
R. R. Koppang
TRW Inc., Energy Systems Group
ABSTRACT
A unique, hybrid electrostatic precipitator for the collec-
tion of particulate and gaseous emissions has been developed and
commercialized by TRW. The TRW Charged Droplet Scrubber (CDS)
combines high energy wet scrubbing collection for micron size
particulate with electrostatic precipitation for submicron
particulate collection. The gas cleaning process steps of
water droplet atomization, acceleration, particulate collection
and demisting are performed through applied principles of
electrostatics within a gas treatment equipment length of eight
feet. Other CDS operating advantages include: no moving parts
in the gas stream; low sensitivity to particulate physical
properties; and low liquid-to-gas ratios. Particulate collection
efficiencies exceeding 99% and 96% have been accomplished in the
micron and submicron region, respectively. HC1 gaseous emissions
control from 90 to 99% have been obtained in pilot scale work with
minor modification of the equipment internals. S02 flue gas
desulfurization also appears promising.
INTRODUCTION
Recent, more stringent air pollution controls require high
collecting efficiencies of submicron particulate to meet stack
opacity regulations. The simultaneous control of gaseous and
particulate emissions with one unit operation is highly
desirable. Such collection must be accomplished with equipment
which minimizes capital costs and significantly reduces energy
consumption.
The CDS employs a unique engineering concept which provides
for more diverse and economic applications of electrostatic
precipitators in particulate emission control. Both large and
small gas flow rates can be economically controlled with
modular preassembled CDS equipment.
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321
The CDS is a hybrid particulate collector using two
parallel mechanisms; high energy water scrubbing and electro-
static mechanisms. These mechanisms are further augmented by
the gas conditioning effects of droplet evaporation. Larger
particulate is captured through high energy collisions with
optimally sized water droplets. The benefits of the small
equipment size associated with wet scrubbers and the low energy
requirements of electrostatic atomization and precipitation are
thus obtained.
The CDS is less sensitive than ESP's to particulate
physical properties. The electrostatically sprayed water
droplets create highly concentrated space charge fields and
the evaporative losses are sufficient to improve the stability
of high voltage breakdown. The walls of the plates used to
collect charged particulate and demisted charged water droplets
are evenly wetted and the material continuously flushed as a
slurry. Its small size minimizes plant modifications, where
space is usually limited, and permits factory assembled equip-
ment to 100,000 cfm capacity. Low liquid to gas ratios, less
than 1 gal/1000 cfm, minimize waste water treatment and the
operating and capital costs associated therewith, and frequent-
ly allow waste water to be returned to other points in the
factory production processes without secondary treatment. In
many applications the CDS discharge gas is not saturated thus
allowing less expensive fan and duct materials and minimizing
the frequency of steam plumes.
These attributes of the CDS have found wide appeal among
users in a number of basic industries, such as pulp and paper
or iron and steel, as well as several specialty applications.
Described below are some of the more unusual operating
principles. This is followed by a discussion of recent develop-
ment and commercial experience with equipment configured for
improved performance on submicron particulate and gaseous
emissions. A previous paper, Reference 1, reported on
earlier pilot and commercial experience on a first generation
design.
OPERATING PRINCIPLES
The CDS operating principles are briefly summarized below.
More detailed explanations may be found in References 1 and 2.
The CDS accomplishes the major process steps for particle
collection simultaneously within a small treatment zone using
electrostatic principles for droplet atomization, charging,
acceleration, particulate collection and demisting.
In the use of electrostatic atomization the CDS is unique to
chemical process equipment design. Electrostatic spraying
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322
results in well dispersed water droplets which, when collected,
evenly distribute water films on collector plates. Within the
CDS water distribution and atomization are accomplished by an
array of metering tubes arranged along a conducting, water
carrying, high voltage energized electrode, Figure 1. Local,
high voltage gradients are created at the tips of the tubes
located equidistant between grounded parallel collecting
electrodes (plates).
During the engineering development of the CDS various types
of electrostatic atomizer designs, hollow tubes and rods, were
considered. Currently a tube design has been selected which
provides sufficient water flow rate for good droplet spatial
distribution, optimal mean droplet size for high collecting
efficiency, good plate water film coverage, and acceptable
evaporation rates. High speed (1 second) photographs of the
electrostatic spraying phenomenon for a voltage gradient of
5 kv/cm equivalent to corona breakdown in air are shown in
Figures 2 and 3. Without high voltage, the water jets issuing
from the tubes do not become atomized because of the low feed
pressure (under 10" we). As voltage is increased above
2-3 kv/cm, the water column becomes highly unstable bursting
into a convoluted filament as it searches for a minimum gradient
within the space charge field. A highly filtered DC (±5%) power
supply is required to obtain stable electrostatic spraying at
maximum operating voltage. Electrode operating rms voltages
(35-55 kv) are typical of conventional dry electrostatic precip-
itators having similar electrode geometry.
Figure 2 is a time sequence photograph (15 exposure) which
recorded the position of the water column as it adjusted to the
droplet space charge; from this photograph the excellent space
distribution of water droplets can be noted. The Figure 3
photograph recorded the early stages of disintegration of a
portion of filament that separated from the main column; charged
droplets are sprayed off the ends of the column and from kinks
in the middle. Filament break-up processes, including local
surface separation where surface charge forces exceed tension
forces (Rayleigh instability), can also be observed in both
pictures. Typical droplet sizes were in the range from 120 to
180p m model (geometric) diameter, and 300 to 400u m mass-mean
diameter. Droplet size is influenced by nozzle tip geometry and
water flow rate. The maximum velocity component seen for both
120p droplets and 180p droplets was about 30 m/sec.
Altering the physical properties of the spray liquid by
adding a surfactant to reduce surface tension or soluble salt
to increase conductivity have only secondary effects on spray
patterns.
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INSULATOR
SCRUBBED GAS
DISCHARGE
TO ATMOSPHERE
D-C
POWER
DUST LADEN
GAS FLOW
ELECTRODE
(HIGH POS. POTENTIAL)
CHARGED
DROPLET
SPRAY
COLLECTOR
PLATE
(NEG. POTENTIAL)
WATER/DUST
SLURRY
CARRY-OFF
Figure 1 Charged Droplet Scrubber Operating Principle and General
Component Arrangement for a Single Electrostatic
Spraying Stage
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Figure 2 Droplet Formation by electro-
hydrodynamics spraying,
expsoures at 1/15 second
intervals
Figure 3 Droplet formation, filament
breakup
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325
For example, increasing conductivity from nominally 300 to
10,000 micromhos/cm requires a reduction in electrode voltage
of 2-4 kv, but does not substantially change the characteristics
of the spray. Apparently, the improved conductivity decreases
the relaxation time for droplet charging.
Particulate collection in the CDS is accomplished by both
direct collision of high energy water droplets and electrostatic
precipitation. Direct collision and agglomeration is the
dominate mechanism (efficiency) for particulate greater than one
micron, depending upon particle density and wetability. Water
droplets, instantaneously charged and accelerated, reach terminal
velocities approaching 100 fps with a velocity trajectory which
is substantially cross flow to the particulate velocity. The
resulting high relative velocity (cross flow mass transfer) and
optimally generated water droplet size (100-200 microns) result
in target collecting efficiencies as high as 99+% for 2-3u sized
particle.
Two electrostatic particulate charging mechanisms are used
to achieve submicron particulate collection. Charge transfer
to the particulate is accomplished through gas ion current
generated at the tube tips by corona discharge (conventional
ESP) and by grazing encounters of the particulate with water
droplets. In the latter case, charge transfer is the result of
field distortions during the droplet-particle encounter which
result in a droplet surface electrical breakdown. Such surface
electrical breakdown can effectively occur at surface to
particle distance of up to 2y for a O.lp particle (e.g.: a
single encounter will transfer sufficient charge to precipitate
a particle at nominal operating conditions). The optimum
relationship between particle and droplet size, Figure 4, has
been estimated from theoretical considerations. The optimum
droplet size is about proportional to the particle size, but
always a factor of ten larger.
EQUIPMENT DESIGN
The previous section described the equipment configuration
and operating parameters for a single chamber (module) and
electrostatic spraying stage. CDS overall efficiencies are a
function of process conditions and equipment design. Signifi-
cant process conditions are particle size and composition.
Physical properties of importance are specific gravity,
wetability, and to a lesser extent, high electrical resistivity.
In considering overall equipment efficiency for a specified
process the two most significant CDS design parameters are:
Number of stages operated (Figure 5) and the plate spacing - gas
flow velocity relationship, (Figure 6). Both of these parameters
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0.8
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0.1
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S, = .5 MICRONS
0.4 0.6 0.8 1.0 2.0
PARTICLE RADIUS (MICRONS)
DROPLET RADIUS
S] = 100 MICRON
S =60 MICRON
.SQ = 30 MICRON.
4.0
6.0 8.0 10.0
Figure 4 Theoretical Droplet-Particle Removal Effectiveness
Relative to a 30 micron Radius Droplet
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NUMBER OF STAGES
Fiaure 5 Effect of Series Electrostatic Spraying Stages or. Overall
Collecting Efficiencies
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SPECIFIC COLLECTING AREA, ft /cfm
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Figure 6 Effect of Plate Spacing, Residence Time, and Voltaae
on 3-Stage Collection Efficiency for 1.8p Mass Mean'
Mineral Dust
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329
are fundamentally bounded by the collector plate treatment
height of eight feet, which was selected to meet the attractive
commercial design requirements of preassembled, truck shippable
structures and access for maintenance from a single work plat-
form. Depending upon the characteristics of the process gas,
which in turn affects the final plate spacings, this dimension
allows either three or four electrical spray stages in series
and, primarily, sets the baseline collecting efficiency. Greater
efficiencies are achieved by concurrently selecting a plate
spacing and gas velocity.
The collector plates can be arranged on 5 inch to 8 inch
centers in increments of 1 inch to make the first efficiency
adjustment. Operating voltages are set for spark rates of
100 to 250 arcs/min. Greater spark rate frequencies allow
momentary field collapse and gas bypass. The possible benefits
of higher voltage are consequently limited by the controlling
spark rate criteria.
Gas flow capacity for a single module is governed by the
plate spacing, the plate width and the gas velocity. Again
considering the limitations imposed by contemplated truck
transportation, the collector plate width has been fixed at
10 feet. This selected design dimension results in a nominal
modular flow capacity of 2400 cfm at a base design plate
spacing of 8 inches and a gas velocity of approximately
6 feet per second. All pilot CDS (1000 cfm nominal rating)
incorporate the basic CDS electrical configurations consequently,
there is no electrical scaling required to achieve full scale
equipment design from pilot test data. In full scale equipment
the flow capacity is controlled by preselected groups of modules
arranged in parallel to conform to the standard equipment model
sizes.
RECENT DEVELOPMENT AND OPERATING EXPERIENCE
The operating experience (pilot and commercial) with the
first generation three stage CDS has been previously reported,
References 1 and 3. This work identified a need for more
economical equipment for independent and simultaneous fine
particulate and noxious gases emissions control. Fortunately,
these requirements need not be mutually exclusive (if the system
is wet). Indeed, similar changes in equipment configuration and
operation are required.
• Higher liquid-to-gas ratios (longer liquid surface
residence times).
• Extended, wetted surfaces for absorption/space
charge collection.
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330
The first change to the baseline three stage design was to
add a fourth stage within the same envelope. This is possible
because the requirement for higher specific plates areas
dictates closer plate spacing which, in turn, allows closer
stage spacing without space charge interactions. A four stage
commercial pilot is shown in Figure 7, with its auxiliary
equipment.
Simulation tests with submicron iron oxide fumes substanti-
ated improved collection efficiencies of the four stage unit.
The following pilot and commercial applications experience
has subsequently been acquired on fine particle control.
• Pilot scale urea prilling tower.
• Commercial scale flue gases containing varying amounts
of S02-
FINE PARTICLE CONTROL
Typically, the most difficult emission control requirement
is to meet stack discharge opacities when substantial fractions
of micron sized particles are present.
Urea prilling is a direct contact evaporative process for
crystallizing urea droplets. Drying is accomplished by a long
gravity free-fall between the injection point and the collection
point. Rising drying air entrains sub-micron urea particulate
and emits fume to the atmosphere. Aerodynamic mass mean
particle sizes are in the range'of .6 to 1.2 microns, (Figure 8)
Dust loads are typically in the range of .012 and .038 gr/dscf,
substantially out of compliance as opacities exceed 60%. The
urea is very hydroscopic and soluble in water.
A four stage pilot scale TRW CDS was installed at a typical
site for manufacturing urea by the prilling process.
The pilot CDS was essentially identical in its internal design
to a full scale, commercial unit, thus, no performance scale-up
is necessary. Previous experience indicated that placing the
collector plates on 6 inch centers would achieve the desired
performance requirements. The performance was varied by
adjusting the gas velocity between the collector plates.
Breeching between the CDS and the process was accomplished
through a length of about 150 feet and 12 inch pipe. No attempt
was made to extract an isokinetic sample since the micron
sized particulate should not stratify. The EPA Method 5
procedure was used to measure inlet and discharge particulate
concentrations.
The test results have been generalized in a plot of
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331
Feed Water
Isolation
Ports for Measuring
/'Stratification Effects
41 x 9' Collector Plate
Electrostatic Spraying
"tages
Low Ripple DC
Power Supply
Figure 7 Pilot CDS, Full Scale Four-Stage Module Use for Fine
Particulate and Gaseous Emissions Development Program
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INLET SAMPLES
© 1-1
V 1-2
1-3
1-4
1-5
CDS
OUTLET, .0031 GR/SGF
TEST NO. 13. 6/13/76
0.1
2 5 10 20 30 40 50 60 70 80 90 95 98
COMMULATIVE WEIGHT FRACTION LESS THAN CUT DIAMETER
Figure 8 Urea Prilling Tower Particle Size Distribution
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333
efficiency versus specific plate area (velocity), Figure 9.
Efficiencies can be varied from 70% at a specific plate area and
gas velocity of about .063 and 8.8 fps, respectively to 95% by
derating velocity. The most cost-effective operating point on
the curve is near its knee at about a .08 specific plate area.
The pilot CDS stack discharge was substantially clear at this
operating point. A downdraft configuration was also tested at
the design efficiency to determine if performance improvements
could be achieved. About a 2 to 3% efficiency increase at a
lower electrode waterflow rate (about 25% reduction) was
measured. The higher efficiencies are attributed to the fact
that the lower water flow rates result in larger residence times
of the water columns in the electrostatic field. This results in
a smaller mean droplet size and improved collection efficiency of
submicron particulate (see Figure 4). This configuration may
also offer some significant economies for ground level installa-
tions by reducing the amount of duct-work required, particularly
for an induced draft fan configuration. Higher electrode
operating voltages (38 to 40 kv) improved efficiency at the
design gas flow velocity. Utilities requirements at the design
point were:
• Electrical, .4 watts/acfm
• Water, .46 gpm/1000 acfm
Process gas flange-to-flange pressure drops were nominally
0.5 inches we and recycled water conductivity varied from 200 to
2400 micromhos/cm.
GASEOUS EMISSION CONTROL
The disadvantages of a wet system (disposal of a sludge) can
be turned to advantage if simultaneous, selective absorption of
noxious gases can be effected. A two-part development program
has been completed to evaluate the feasibility of using the CDS
to absorb the chemically active (HCl) and inactive (SO2) gases.
Chemically active systems require high mass transfer rates
through the gas film boundary layer, inactive systems require
improved mass transfer rates into the liquid surface. Typical
process conditions are combustion flue gases containing 400 to
2000 ppm noxious gases such as S02-
For the reactive HCl system investigation a modified pilot
scale CDS (four electrostatic spray stages and plates on 5 inch
centers) was used. A hydrogen chloride high pressure cylinder
was connected to a trace gas injector installed within a
mixing chamber. The hydrogen chloride gas was distributed
through three horizontal tube runs containing 15 equally spaced
orifices. Sampling ports were located in the vicinity of the
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334
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W
•°6 -07 .08 .09 .10 .11 12
Specific Collecting Area, A/Q, ft2/acfm
987 6 5 4.5
Collecting Velocity, fps
.13
PROCESS SPECIFICATION
Dust Load, gr/dscf
Temperature, °F
Humidity, %
Particle Size - mean, y
- % < ly
Particle Composition
CDS DESIGN AND OPERATION
Plate Spacing, inch
Gas Velocity, fps
Transformer - Voltage, KV
- Power, va/cfm
Number of Stages
0.012 - 0.038
Ambient
100
0.7 - 1.2
30 - 60
Urea
50 - 55
0.4 - 0.8
4
Figure 9 CDS Pilot Performance Urea Prilling Tower
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335
inlet to the CDS where stratification of the injected gas was
anticipated to be small. The CDS discharge sampling points were
located at ground level just before the CDS induced draft fan.
The primary procedure for determining hydrogen chloride
absorption rates was to sample simultaneously at the CDS inlet
and outlet. The sample was dry gas metered and drawn through a
train consisting of three fritted glass impingers in a sodium
hydroxide solution. This solution was then analyzed for chloride
by titration of a known silver nitrate solution. The correlation
between metered to sampled inlet hydrogen chloride concentrations
was usually within ±20%.
Tests were initially run to determine if sufficient wet
contacting and absorption could be accomplished using electro-
static water spraying and plate water films. The tests were run
at two space velocities (3.5 and 5 fps) resulting in liquid to
gas ratios of 1.26 and .9, respectively. The absorption is
substantially removed (order of magnitude) from equilibrium
saturation suggesting that the system is mass transfer limited.
The data indicate that the absorption efficiencies are sensitive
to inlet HCl concentration with the higher efficiencies being
accomplished at the lower inlet loads. Figure 10. This would
suggest that mass transfer rate is limited in part by the water
droplet film. Higher liquid rates improve efficiency about 26%,
in proportion to the increase in liquid to gas ratio.
To improve collection efficiencies by increasing the absorp-
tivity of the liquid, a weak sodium hydroxide solution was
applied through the plate wall wash system. The liquid to gas
ratio for the plate wall wash is a factor of three higher than
the electrode water so the comingled electrode spray water does
not substantially dilute the concentration of sodium hydroxide.
Total wetted surface area is not affected by this change so
improvement in absorption must be attributed to improved liquid
film mass transfer. This configuration allows recirculating the
sodium hydroxide liquor through the wall wash system until a
substantial fraction of the NaOH is consumed. Further efficiency
improvements of up to 95% were achieved.
Interfacial CDS velocities are similar to those used in
packed bed absorbers allowing a simple modification to affect a
low pressure drop series equipment arrangement.
The current pilot and commercial configurations of the CDS
have about 2 feet of open space between the gas distribution
baffles and outlet flange within the lower section. This void
space can be converted to a prescrubber by the addition of a
non-plugging type of packing. In this case, the upper stage
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336
99,6
99.0
0°
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U2 95.0
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90.0
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70.0
60.0
50.0
2000
SYM
V
A
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Q
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VELOCIiY
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2
1
0
0
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A
Oo.5%NaOH
0.90
1000 500 300 200
INLET CONCENTRATION, ppmV
100
PROCESS SPECIFICATION
HC1 Concentrations, ppmV
Temperature, °F
Humidity, %
180 - 1000
80 - 185
20 - 30
CDS DESIGN AND OPERATION
Plate Spacing, inches
Gas Velocity, fps
Transformer - Voltage, kv
- Power, watt/cfm
Number of Stages
3.5 - 5
50
0.9 - 1.2
4
Figure 10 HCl Absorption Efficiencies for Several
Equipment Configuration and Operating Variables
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337
COKC-OVEN GAS SCRUBBING
KILN FLUE GAS SCRUBBING
Pscy MM HG
Figure 11 Preliminary Data Showing Unusual S02-Absorbing Capability
of CDS
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338
electrostatic spraying discharge water should be separated from
the post-particulate scrubbing section to minimize the suspended
solids clean-up for the recirculated sodium hydroxide solution.
Liquid to gas ratios must be increased to about four, with the
packed bed being operated at a L/G of 3. The total system
pressure drop with the maximum packing depth of 2 feet was
1.5 inches we. These test results, Figure 10, demonstrate that
HC1 absorption efficiencies of 99+% are achievable at 5 fps space
velocity and HCl input of 1000 ppmV.
SO, has an extremely low solubility in water when an
absorBant such as an alkali salt is absent. Routine testing of
CDS controlling particulate emissions from various combustion
process off gases containing S02 have indicated absorption
efficiencies which substantially exceed saturation by an order
of magnitude (Figure 11). These data suggest that electro-
static phenomena may be enhancing the reactivity at the liquid
interface. Hypothesized chemistry includes the formation of
activated states of S02 and 02 in the corona fields. Research
and development to understand and exploit this chemistry is
currently in progress.
REFERENCES
1. R. R. Koppang, "POLLUTION CONTROL EQUIPMENT AND
TECHNOLOGY, Broader Availability of Electrostatic
Precipitators - A New Hybrid EP Concept, The TRW Charged
Droplet Scrubber", TRW Inc., Energy Systems Group, 1975.
2. c. W. Lear, "Charged Droplet Scrubber for Fine Particle
Control: Laboratory Study", TRW Systems Group,
EPA-600/2-76-249a.
3. W. F. Krieve and J. M. Bell, "Charged Droplet Scrubber
for Fine Particle Control: Pilot Demonstration", TRW
Defense and Space Systems Group, EPA-600/2-76-249b.
WRITTEN DISCUSSION
Michael J. Pilat
Department of Civil Engineering
University of Washington
Seattle, Washington
The paper presents the description and performance of a space charged
scrubber-precipator named the "TRW Charged Droplet Scrubber." My comments
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339
concerning the paper are mainly in the form of questions; these questions
being posed in order to better understand the scrubber system. My questions
are as follows:
1. On page 3 a voltage gradient of 2.3 kV/cm is mentioned as necessary
to achieve electrostatic spraying. Then on page 6 is it mentioned that in-
creasing the water conductivity from 300 to 10,000 micromhos/cm requires a
reduction in the electrical voltage of 2 to A kV. Why is this voltage re-
duction necessary?
2. In Fig. 4 what are the definitions of P^) , A^) , P(SQ), and A(SQ)?
3. In Fig. 5 what are the units and definitions of A/Q?
4. The water to gas flowrate ratio of 0.46 gal/1000 acf seems somewhat
low (page 14). Is this the water requirement for one stage, or is the water
usage not including water recycling?
5. The electrical utility requirement is listed as 0.4 watts per acfm.
Does this electrical usage include power required for water pumping, the fan,
and the high-voltage power supplies?
6. Is the particle collection efficiency (either overall or at a- specif ic
particle size such as at 0.5 micron diameter) of the CDS sensitive to the inlet
particle mass concentration? The collection efficiency data presented (Fig.
5, Fig. 6, and Fig. 9) only mentions the area prilling concentration in the
0.012 to 0.038 gr/scf.
For a comparison with the TRW CDS system, let us look at the U. of W.
Electrostatic Scrubber (UWES) . A schematic illustration of the UWES is provided
in Fig. 1. The UW Electrostatic Scrubber involves the use of electrostatically
charged water droplets to collect air pollutant particles electrostatically
charged to a polarity opposite from the droplets. The particles are electro-
statically charged (negative polarity) in the corona section. From the
crona section the gases and charged particles flow into a scrubber chamber into
which electrostatically charged water droplets (positive polarity) are sprayed.
The gases and some entrained water droplets flow out of the spray chamber into
a most eliminator consisting of a positively charged corona section in which
the positively charged water droplets are removed from the gaseous stream.
The general layout of the UW Electrostatic Scrubber pilot plant (Mark 2P
model) is shown in Fig. 2. The system (in the direction of gas flow) includes
a gas cooling tower, an inlet test duct with sampling port, a particle charging
corona section (corona no. 1), a charged water spray tower (tower no. 1),
a particle charging corona section (corona no. 2), a charged water spray tower
(tower no. 2), a positively charged corona section to collect the positively
charged water droplets, an outlet test duct with sampling port, and a fan.
The pilot plant is housed in a 40 foot long trailer and can be easily trans-
ported to emission sources.
In Fig. 3 the particle collection efficiency as a function of particle size
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340
GAS INLET
GAS OUTLET
CORONA
(PARTICLE CHARGING)
CHARGED WATER SPRAYS
(COLLECTION OF CHARGED PARTICLES
BY OPPOSITELY CHARGED WATER DROPLETS)
MIST ELIMINATOR
Fig. (1) UW Electrostatic Scrubber
is presented for emissions from an electric arc steel furnace. Note that the
specific collecting are.a of the UWES is .05 ft2/acfm which is somewhat less
than the .08 to .22 ft /acfm presented by Koppang for the CDS system. Of
course this illustrates the difference in the particle collection operating
mechanisms utilized, the UWES using charged water droplets to collect the
particles and the CDS having a greater collection plate area. Fig. 4 and
Fig. 5 present data obtained at a pulverized coal-fired power plant. Fig. 4
illustrates the increased collection efficiency obtained at the higher water
charging voltage. Fig. 5 presents the effect of water to gas flowrate ratio
on the collection efficiency.
For additional information on the UWES system, see the references listed
below:
[1] Pilat, M.J., S.A. Jaasund, and L.E. Sparks (1974) "Collection of Aerosol
Particles by Electrostatic Droplet Spray Scrubbers" Envir. Sci. & Tech.
8 340-348.
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INCOMING
GASES
INLET TEST DUCT
SPRAY TOWER NO. 2
CORONA NO. 1
EXHAUST
GASES
SECTION A-A
CROSS SECTIONAL VIEW OP
THREE PASS HORIZONTAL SECTION
OUTLET TEST DUCT
MIST
ELIMINATOR
SPRAY TOWER NO. Z
SPRAY TOWER NO. 1
CORONA NO. 2
ELEVATION VIEW
FAN
Fig. 2 General Layout of Electrostatic Scrubber Pilot Plant (Mark 2P Model)
UJ
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342
[2] Pilat, M.J. (1975) "Collection of Aerosol Particles by Electrostatic
Droplet Spray Scrubber" APCA Journal 25 176-178.
[3] Pilat, M.J. and D.F. Meyer (1976) "University of Washington Electrostatic
Spray Scrubber Evaluation" Final Report on Grant No. R803278, EPA Report
No. EPA-600/2-76/100 (NTIS No. PB 252653/AS).
Particle
Collection
Efficiency
100
95
90
85
80
75
I IMIIII| I I ||| | | i
- Electric Arc Steel Furnace
Test 23
s
/
Test 22
Gas Velocities (ft/sec)_
Corona section 4.8
Spray towers 2.4
SCA =0.05 ft2/acfm
Test
No.
22
23
Water to Gas
Ratio
(gal/1000acfO
17.2
16.8
Overall Mass
Eff.
98.6
96.4
Corona
Voltage
(kV)
70
70
Liquid
Voltage
(kV)
10
10
' MHIllll I Mil I I I I I lilililil i i.i..ml MM! i III, III,I,[7
.1
•2 .3 .4 .5 .6 .-8 1.0 2.
Particle Diameter (microns)
3. 4. 5. 6. 8. 10.
Fig. (3) Collection Efficiencies With Similar Operating Conditions
At Electric Arc Steel Furnace
-------
100
95
Particle 90
Collection
Efficiency
85
80
— Cool-Fired Boiler
Test No. I
Overall Eff. = 99.555
SCA = 0.05 ft2/acfm
IIMM
'Cool-Fired Boiler Test No. 2
Overall Eff. = 98.1%
Notes: I. Water droplet charging voltage
= 20 kV on test no. I and 15 kV -
on test no. 2.
2. All other pilot plant operating —
parameters essentially unchanged
Illllllll I Illllllll Mill I I I I I Illllll'
343
0.1
.2 .3 A .5 .6 .8 I. 2.
Particle Diameter (microns)
3. 4. 5. 6 8. 10.
Fig. 4 Influence of Water Droplet Charging Voltage on Particle Collection
Efficiency
100
95
Porlicle 90
Collection
Efficiency
(%)
85
80
75
TTTT M 11 11 l i TTT I I I I I I I I I I III iTTnT I TTT| I I I I | T
I I I I I I I I
" *
Coal-Fired Boiler Test No.I
Overall Eff. = 99.556
Outlet Cone. = .002 grains/scf
Cool- Fired Boiier Test No. 3
Overall Eff. = 96.1%
1 1 1 1 1 1 1 1 1
,
1 1 1 1 1 1 i I i 1 1 1 1 1 1 1 1 1 1 1
r
Notes: I. Water to gas ratio (gal./ ~
1,000 acf ) = 5 82 in test ~
no.I and 2.36 in test
no. 3
2. All other pilot plant
operating parameters
essentially unchanged
SCA = .05 ft2/acfm
LUiu.lillll I I l I. Mill
0.1
.2 .3 rt .5 .6' .8 I. 2.
Particle Diameter (microns)
3. 4. 5. 6. 8. 10.
'Fig. 5 Influence of Water to Gas Flow Rate Ratio on Particle Collection
Efficiency
-------
344
RESEARCH ON NEW EQUIPMENT FOR DUST COLLECTION-7'
A. Baril, Jr., D. P. Thibodeaux, and B. J. Claassen
U. S. Dept. of Agriculture, ARS, SRRC, New Orleans, La.-'
R. B. Reifl/
Battelle Memorial Institute, Columbus, Ohio
INTRODUCTION
OSHA cotton dust standards and the possible occurrence of
byssinosis in cotton textile workers has focused attention on
the dust problem in cotton textile mills and related opera-
tions where cotton and cottonseed are processed. One of the
most urgent problems facing cotton textile mills is that of
removing resplrable dust from the work environment and main-
taining the dust level within prevailing OSHA standards with-
out a disturbing economic unbalance and with no increase in
energy consumption (I-1!).
A wide variety of systems is currently being used to
clean the air in textile mills that process lint cotton. Most
of these are custom designed, and use proprietory equipment
such as bag filters, rotary drum filters, condensers, V cells
etc. (5), but many systems have Just evolved as a result of
additions and changes needed at the moment. The better sys-
tems have several stages, including a waste separator that
removes lint and large trash particles, a dust separator
that can reduce dust content of the air to one mg/m3 by some
form of filtration, and a fine dust separator capable of re-
moving respirable dust (less than 15 aim). The effective con-
trol and removal of dust in this size range is quite diffi-
cult and expensive. Presently, there are few, if any, air
handling systems in the textile industry capable of econo-
mically and efficiently collecting and removing respirable
dust. For the past several years, our laboratory has spon-
sored and carried out research to such a system. The first
two devices described were developed by Battelle Columbus
-------
345
Laboratories under contract to U.S.D.A., and the third one
was developed at the Southern Regional Research Center in
New Orleans.
Wet Wall Electroinertial Unit
The first device, called a wet wall electroinertial unit
(WWEU) air cleaner, was developed to meet the requirements of
high operating efficiency, low maintenance, low operating
cost, and simplicity of design. To achieve high efficiency
with low pressure loss, an electrostatic precipitation system
was the best candidate. A wet wall was desirable to flush the
precipitated material away and to minimize maintenance.
Inertial effects were added to assist in moving material
toward the wall and to improve the cleaning action of the
water on the wall.
air miet
• 3 B75in ID i 10m
Lucite luDe
• Insuiotor
Water inlet
These features were best incorporated in the design shown
in Figure 1. The unit consists of a concentric wire-in-tube
precipitator. The charging
wire is located axially in a
stainless steel tube in a ver-
tical orientation. Air enters
tangentially at the upper end
of the tube (through a duct
with a flattened cross section)
and acquires a rotating move-
ment. When high voltage is
applied to the wire, an ioni-
zing corona discharge charges
the dust in the air. As the
air flows through the tube,
the charged dust is driven to
the wall by the radial elec-
trical field. Water from the
upper water inlet flows down
the wall and flushes the pre-
cipitated dust into the water
outlet at the lower end of the
tube. The rotational movement
of the air also induces rota-
tional flow of the water,
which assists in uniformly
wetting the surface of the
tube. Cleaned air is expelled
3B7m ID t 26in
Stainless steel lube
30m 00 x 60in
Bross tube
0008m Diameter
• Corono wire
. 'nsulatar
Waier outlet
exhaust
from the tube's lower end.
Fig. 1. Wet Wall Electroinertial
Unit
-------
346
Tests of the WWEU were conducted with 4 in. and 8 in.
diameter units at flow rates ranging from 100 to 800
ft-Vmin. Particle collection efficiency of these units was
measured with atmospheric dust, AC fine test dustz/ (mass
mean diameter (MMD) = 12 jam), artificial cotton dust (MMD =
4.0 jam), and ootton dust drawn from the processing area of
a card (MMD = 3.0jum). Efficiency data with atmospheric
dust was obtained by isokinetic-sampling downstream of the
unit with an optical particle counter. Sample preparation
and testing procedures with the AC test dust and artificial
cotton dust have been described previously (6). Essentially,
known quantities of these dusts were fed to the WWEU and all
particles that escaped were captured on an absolute filter
installed downstream of the unit. Preliminary results are
also included on tests with an 8 in. WWEU that cleaned dust
laden air drawn from a full scale cotton card operating at
20 Ib/hr.
Tests were conducted with the 4 in. diameter WWEU to
determine the effects of tube length, air flow rate, and
voltage polarity on efficiency of particle collection.
Tube lengths of 1, 2, and 4 ft were tried. Shearing of
water droplets from the wall at higher velocities made the
practical upper limit on flow rate about 200 ftVmin. Re-
sults of the efficiency tests with the 2 ft unit are inclu-
ded in Table I. Runs made at zero potential and zero cur-
rent with the rather coarse AC dust show efficiencies in the
BQ% range due merely to centrifugal forces. Increasing the
voltage to -30 kV raises the efficiency to 992. Which
diminishes to 98.5$ when +33 kV is applied. Tests with the
fine artificial cotton dust Indicate a somewhat lower effi-
ciency (98.1 and 96.15? respectively) for 100 and 200
ftVmin.
Table II contains the data for tests with the 8 in. dia-
meter, 66 in. long WWEU. These data show that the 8 in.
unit can be scaled up from the 4 in. WWEU, maintaining the
same efficiency levels for a higher flow rate. While scal-
ing up the capacity of the units, the velocity and field
strength were maintained at the same values. In this case,
particle collection efficiencies are still better than 95?.
Differences in efficiencies with the artificial and real
cotton dust for zero voltage are no doubt caused by the
relative fineness of real cotton dust, which diminishes the
effects of centrifugal forces. More recent studies with the
8 in. unit to clean air from an actual textile card show
that the unit operates quite satisfactorily under realistic
test conditions.
-------
347
TABLE I
TOTAL MASS EFFICIENCY OF THE 4 IN. WET
WALL ELECTROINERTIAL UNITS./".'
Test Circulation Poten- Cur-
Dust Rate tial rent
ftVmin
AC fine
AC fine
AC fine
AC fine
AC fine
Cotton dust
Card trash
Cotton dust
100
200
100
200
100
100
200
200
kV
0
0
-30
-30
+ 33
-30
-30
-30
mA
0
0
2.4
2.1
— —
2.2
2.2
2.1
Dust
Feed
Rate
g/min
1.0
2.0
0.2
0.2
1.0
0.83
0.67
0.62
Pres-
sure
Drop
in.
H20
1.25
3.75
1.25
1.25
1.25
1.25
3.75
3.75
Efficiency
%
81.2
87.6
99.0
99.0
98.5
98.1
99.7
96.1
a/ Length of WWEU was 2 ft.
b/ Water flow was 0.25 gal/min.
TABLE II
TOTAL MASS EFFICIENCY OF 8 IN. WET
WALL ELECTROINERTIAL UNITa/
Test Dust
AC fine dust
Card trash
Card trash
Artificial
cotton dust
Artificial
cotton dust
Card output^./
Card output!!/
Card output*!/
Poten-
tial
KV
-60
-60
0
-60
0
0
-58
+60
Cur-
rent
ma
^mm
-7
0
-7
0
0
-6
+5-8
Water
Flow
g/min
0.46
0.46
0.46
0.46
0.46
0.64
0.65
0.65
Air
Flow
rt -vmin
800
800
800
800
800
800
800
800
Efficiency
*
97.6
98.4
99.8
99.4
80.2
30.3
99.9
96.9
a/ Length of WWEU was 66 in.
b/ Tested at SRRC on output of V-cell filter.
-------
348
High Speed Rotary Drum Air Cleaner
The air cleaning systems of most textile mills utilize a
rotary drum (low speed) filter that allows the unit to con-
tinuously self-strip the accumulated filter cake. This pro-
cedure allows for a low pressure drop in the air circulating
system but leads to inefficient dust removal, particularly at
the respirable dust range. Investigation of these cleaners
with the intention of increasing their efficiency led to ap-
plication of the principle that movement of a collection
surface normal to the direction of the air flow through it
increases the effective interception cross section of the
collector surface. If the collector surface moves at a
velocity n times that of the air stream, the number of par-
ticles impacted on the surface is increased by a factor of
between n and n + 1.
Air Inlrt
12" x 12 x 17"
Luclte enclosure
Plow orifice
RLower Intake
I
The apparatus
shown in Figure 2
was constructed to
evaluate this con-
cept. A hollow
aluminum cylinder
, was mounted coaxi-
eSSeSSeS I I I]ally on a drive
rrHshaft on one end
and was connected
via a large bearing
OOOOOOOO
lOOOO OOOO
Pig. 2
High Speed Rotary
Drum Air Cleaner
lolorv drun tO Si 3
meter suction pipe
on the other end.
The surface of the
aluminum cylinder
was perforated
, . with holes arrang-
ed in equally spaced rows parallel to the axis of the cy-
linder. Filter materials were wrapped around the drum to
cover the holes, and the drum was rotated to increase the
effective cross section of the fibers. In operation, air
was drawn in through the holes and out through the 3 in.
diameter suction pipe by a high pressure blower as the cy-
linder was rotated. The efficiency of the rotary drum unit
was evaluated for several configurations, such as hole size,
open area, rotational speed, flow rate, and cylinder dia-
meter. The drums were evaluated with and without filter
media on the surface, and some of the drums were evaluated
with electrostatic assist.
-------
349
Initially, the efficiency of the rotary drum unit was
evaluated by using a particle monitor to measure the parti-
culate concentration in the air in the room outside the
drum and in the air at a point Inside the drum. During
this work, some variation was observed depending on the
position of the sampling probe inside the drum. These data
generally reflect relative dust loading differences in the
portion of the air flow sampled and do not indicate overall
efficiency for the total air flow. The true efficiency of
the unit was measured with AC fine test dust with a fiber-
glass filter installed downstream of the rotary drum to col-
lect all of the dust that passed through it.
The first drum tested was a 4.5 in. OD x 10 in. long
cylinder with a 5/32 in. wall. Sixty 1 in. diameter holes
were drilled in ten equally spaced rows, with six equally
spaced holes in each row to provide a total open area of
0.327 ft2. The partial efficiency was remarkably high
with the sampling probe inside the drum at a position on
the axis of the drum and 3 in. from the bearing. As shown
in Table III, from 58? to 93? of the atmospheric dust was
removed from the air passing through this area when the
drum was rotated at a surface speed of 3000 ft/min with
air flow rate of 65 ftVmin. The efficiency increased
with drum speed up to about 3,000 ft/min; no further im-
provement was attained at 4,000 ft/mln. The efficiency
decreased as the air flow rate was increased to 200 and
300 ft^/min. Particle counts outside the drum showed
increased dust concentration In the air around the drum.
With a layer of polyester filter material covering
the drum, the partial efficiency of the system was general-
ly higher than that of the plain drum in the smaller
particle size ranges. The efficiency in the larger parti-
cle size ranges is quite good O90JS), but is not signifi-
cantly better than for the plain drum. Covering the drum
with a PC (Pneumafil Corp.) second stage filter gave
almost the same efficiencies as the aplin drum, whereas
covering with a PC first stage filter resulted in a de-
crease in efficiency of the plain drum.
Two types of electrostatic systems were evaluated in
conjunction with the rotary drum with 1 in. holes. In the
first system, electrical fields were applied to the drum
from a 1/4 in. hardware cloth cylindrical grid, spaced
1 in. from the drum. With potentials up to 20 kV on the
grid, the efficiency of particle removal with the plain
-------
350
TABLE III
PARTIAL EFFICIENCY OF ROTARY DRUM WITH 1-IN. DIAMETER HOLES
Drum
Speed
ft/min
Flow
Rat e,
tVmin
Cover
Efficiency, percent
ToT5 > 5.0
micron microns
ft
1,000 65 None H.l
2,000 65 None H2.3
3,000 65 None 58.2
4,000 65 None 51.21
1,000 200 None 3.1
2,000 200 None 9.3
3,000 200 None 16.1
0 65 E2B-P01B 16.9
1,000 65 E2B-P01B 63.8
2,000 65 E2B-P01B 75.7
3,000 65 E2B-P01B 82.6
0 65 PC, first stage 1.1
1,000 65 PC, first stage 29.3
2,000 65 PC, first stage 41.6
3,000 65 PC, first stage 51.8
0 65 PC, second stage 3.0
1,000 65 PC, second stage 48.9
2,000 65 PC, second stage 59.3
3,000 65 PC, second stage 61.1
23.5
85.9
93.1
93.5
12.7
55.4
60.8
49.1
95.4
95.4
94.6
3.9
81.4
83.0
86.9
32.7
90.6
9.3.5
93.4
drum increased to the level achieved with filters covering
the drum. With the external field and filters on the drum,
the increase in removal of submicron particles was about
10% to 2058.
In the second electrostatic system, a series of 0.005
in. diameter charging wires were spaced at about 2 in. in-
tervals parallel to and 1 in. from the drum. Data in
Table IV show that the partial efficiency was higher with
auxiliary electrostatic charging. With the drum station-
ary, the charging system alone removed 56!? to 73% of the
atmospheric dust at 65 ftVmin; however, when the flow
rate was increased to 200 the efficiency decreased
although not as much as without the electrostatic assist.
-------
351
At low flow rates, the good performance of the E2B-P01B fil-
ter was improved considerably by adding the electrostatic
assist.
TABLE IV
PARTIAL EFFICIENCY OF ROTARY DRUM (1-IN. -DIAMETER
HOLES WITH AUXILIARY ELECTROSTATIC SYSTEM)
Drum
Speed
Efficiency
Flow
Rate
ftVmin
Cover
Poten
tial
> 0.5
micron
> 5.0
microns
ft/min
0
0
1,000
2,000
3,000
4,000
1,000
2,000
3,000
0
0
1,000
2,000
3,000
kV
65
200
65
65
65
65
200
200
200
65
65
65
65
65
None
None
None
None
None
None
None
None
None
E2B-P01B
E2B-P01B
E2B-P01B
E2B-P01B
E2B-P01B
-20
-20
-20
-20
-20
-20
-20
-20
-20
-10
-15
-15
-15
-15
55.9
11.7
62
76
85
45,
49
75.6
49.0
14,
40,
84,
89,
73.0
52.8
82.7
90.1
96.1
95.1
66.6
79.6
77.3
20
46
,6
,9
92.3
96.3
96.3
96.4
The second drum tested (45 in. OD x 10 in.long with
5/32 in. wall) had 897 holes 1/4 in diameter drilled in 39
equally spaced rows with 23 equally spaced holes in each
row to provide a total open area of 0.305 ft2. This drum
was considerably more efficient than the drum with 1 in.
holes for removing atmospheric dust at lower speeds.
A third cylinder (same dimensions in the first 2) was
made with additional 1 in. diameter holes to provide more
open air: 91 holes were arranged in 13 rows of 7 holes
each to provide an open area of 7 holes each to provide an
open area of 0.496 ft2. A flow rate of 65 ft3/min produced
a gas velocity of about 130 ft/min through the holes in
this drum, as compared with velocities of 200 to 210 ft/min
-------
352
with the first two. In general, performance of the drum with
91 1-in. diameter holes was erratic. Efficiency usually im-
proved when surface speed was increased and worsened when air
flow was increased. However, the overall performance was
inferior to that of the drums with fewer or smaller holes.
Tests with AC fine dust were run with the drum enclosed
in a 12 x 12 by 17-in. Lucite box with a 4 in. diameter air
inlet in the center of the top of the box, as shown in
Figure 2. A 1 in. diameter spherical deflector disk was
mounted about 6 in. below the air inlet to distribute the
incoming air around the box. AC fine dust was fed into a
4 in. diameter pipe connected to the inlet of the box by
means of a small rotary platform feeder. Efficiency was
determined by collecting material that passed through the
drum on a 1/2 in. PF 105 Fiberglas filter downstream of the
drum. Data from these tests run on the drum with both the
1 in. and the 1/1 in. holes is shown in Table V. For the
drum with the 1 in. holes, the mass efficiencies agree
quite well with the partial efficiencies for atmospheric
dust. The mass efficiencies for the drum with the 1/4 in.
holes do not agree with the partial efficiencies obtained
with the optical counter since there is no evidence of a
maximum at the low flow rates.
TABLE V
MASS EFFICIENCY OF THE ROTARY DRUM WITH AC DUST
Drum
Speed
ft/min
3,000
3,000
3,000
325
805
1,530
3,000
Hole
Diam. ,
in.
1.0
1.0
1.0
0.25
0.25
0.25
0.25
Flow
Rate,
ftVmin
65
65
65
65
65
65
65
Cover
None
None
PO-1B
None
None
None
None
Poten-
tial,
kV
0
-20
0
0
0
0
0
Efficiency
i
90.0
93.0
98.0
57
72.3
80.8
90.3
The various measurements and observations made while
working with the rotary drum Indicate that separation of
particulates occurs externally and internally. Some par-
ticulates are rejected from the air stream at the surface
of the drum;
-------
353
as a result, the partlculate concentration in the air
around the drum increases. At the same time, particulate
concentration inside the drum varies at different points.
Lowest particle concentrations generally occur along the
axial center of the drum. At first the air stream was
sampled along the axial center of the drum to avoid partl-
culates generated by the bearing or introduced by air
leaks through the unsealed bearing on the exhaust end of
the unit. However, as work progressed, evidence indicated
that particulate concentration in the air inside the drum
was higher in off-axis positions. A functional design thus
must provide means for removing the dust-laden air outside
the drum and for separating the clean air from the dust-
laden air Inside the drum.
Fluid Electrode Precipltator
In general, the best way to remove fine dust from an
air stream is to use conventional electrostatic precipita-
tors. Although the efficiencies of these devices are quite
high, there are inherent drawbacks that do not make them
acceptable in continuously operating air systems. One of
the major drawbacks to self-cleaning electrostatic precipl-
tators is that they must be shut down in order to clean
themselves. The development of the fluid electrode preci-
pitator is an attempt to alleviate this problem. The fluid
electrode precipitator illustrated in Figure 3 consists of
an array of electrodes in a casing to charge, direct, and
collect fine dust. Tungsten wires serve as discharge elec-
trodes for all units tested, auxiliary driving electrodes
are charged metal plates or screens that can be inserted
between the fluid electrodes, and the collection electrodes
are formed by vertical tubes that release a laminar flow of
water falling through the height of the casing.
The precipitator operation is quite simple. The fall-
Ing columns of grounded water act as cylinders in cross
flow and create vorticies for enhanced particle collection.
Auxiliary electrodes and discharge electrodes can be posi-
tioned in arrays to direct charged dust into the grounded
fluid.
The use of free-falling fluid results in a system that
is less complex than rigid surfaces with wetting devices.
Shear forces due to air flow over a wetted surface can
severely disrupt the thin layer of fluid flow. A cylin-
drical fluid column is a stable shape in cross flow, and
-------
354
Upper water Inlet
is deflected very little by drag forces at velocities up to
1000 ft/min. The fluid is deflected by electrostatic at-
traction, but this can be compensated for by auxiliary
electrode placement.
The fluid electrode precipi-
tator concept was applied to
experimental model units, and
collection efficiency of pre-
cipitator array configurations
was measured in a cotton dust
test facility. This apparatus
consists of a 9 x 9 in.
straight tunnel 9 ft. long,
containing traversable samp-
ling probes located upstream
and downstream of the preci-
pitator test section. Flow
straighteners near these
locations allow isokinetic
sampling through probe tips
of various inlet sizes. Dust
analysis based on projected
area diameter was performed
with an optical particle
counter. A mass monitor was
used to analyze dust mass
concentration.
Pig. 3.
Fluid Electrode
Precipitator
Two series of tests, one
at low capacity and one at high, were performed with cotton
dust from different sources. For low capacity tests., air
was drawn through a model-size cotton carding machine run-
ning at 2 Ib/hr. The air was prefiltered to remove lint
and large particles and was then passed through the test
tunnel. In the high capacity tests, fine dust released by
mechanically tapping a V-cell nonwoven filter was used.
The filter was already loaded with lint and dust from two
production cotton carding machines. The air drawn through
the filter and passed through the test tunnel contained
fine dust that was similar to the prefiltered air from the
model card.
Samples of dust from each of the two dust sources were
collected for testing. They were dispersed in an electro-
lyte and analyzed for size on a volume basis. Model card
dust had a mass mean diameter of 3.6>i with a geometric stan-
dard deviation of 1.6u. The cotton dust released by tapping
-------
355
the loaded filter varied from 4.5 to 6.7ju mass mean diameter
with a geometric standard deviation of about 2. All test
concentrations were in the range of 0.5 to 1.5 mg/m3, which
is typical of cotton mill fine dust concentration.
Table VI contains the operational data and collection
efficiency for a typical configuration tested.
TABLE VI
EFFICIENCY OF TYPICAL FLUID ELECTRODE
PRECIPITATOR CONFIGURATION
Velocity
ft/min
250
350
500
Field
Strength
kV/cm
6.2
5.9
4.9
Dust Supply
Concentration
mg/m-i
0.8
0.5
0.8
Count
Efficiency
1.5u-10u
%
75
81
60
Mass
Efficiency
%
86
93
78
In the design, development, and testing of model fluid
electrode devices, some qualitative findings are notable
concerning fine cotton dust collection or other dust col-
lection. The establishment and control of fluid electrodes
requires careful balancing of flow rates and use of smooth
tubes to assure laminar flow in the fluid electrodes. In
practice, fluid can be piped from a central reservoir
through a branching network to fluid electrode locations.
Units of practical size would require Intermediate flow
tubes at approximately 1 ft intervals of vertical fall to
catch fluid, decelerate it, and return laminar conditions.
Once properly adjusted, an array of fluid electrodes
can operate to continuously remove dust, with low mainte-
nance. Dust entrained in the fluid can continue to circu-
late through the system until its accumulation warrants a
fluid cleaning cycle. Fluid cleaning would not require
precipitator shutdown. Applied to textile mills, con-
tinuous fine cotton dust removal of 85$ would greatly reduce
levels of fine airborne dust. For other industrial dusts,
possibly with collection fluids other than water, similar
applications could be developed.
-------
356
Further research is being conducted to apply findings to
large prototype units, to increase efficiencies with deeper
or more optimally selected electrode arrays, and to extend
principles to other industrial situations requiring con-
tinuous fine dust collection.
REFERENCES
[1] National Institute for Occupational Safety and Health,
"Criteria for a recommended standard... Occupational
exposure to cotton dust," U. S. Department of Health,
Education, and Welfare, NIOSH 75-118, 1974.
[2] Anderson, C. D., "Federal regulations and citations
for cotton dust," Conference Proceedings on The Engi-
neering Control of Cotton Dust, sponsored by Clemson
University, July 29. 1972. PP. 21-26.
[3] Wakelyn, P. J., "Composition of cotton dust: possible
agents causing byssinosis," Conference Proceedings on
the Engineering Control of Cotton Dust, sponsored by
Clemson University. July 29, 1972. pp. 77-99.
[4] Merchant, J. A., et al., "Dose response studies in
cotton textile workers," J. Occup. Med.. vol. 15-3,
pp. 222-230, March 1973.
[5] Barr, H. S., Hocutt, R. H., and Smith, J. B., "Cotton
dust controls in yarn manufacturing," U. S. Department
of Health, Education, and Welfare, Cincinnati, Ohio,
Report No. HSM 99-72-M, March 197M.
[6] Thibodeaux, D. P., Baril, A., Jr., and Reif, R., "A
Wet-Wall Electroinertial Precipitator: A Highly
Efficient Air Cleaner for Cotton Dust," Conf. Rec.
1976 Annual Meeting IEEE Ind. Appl. Soc., pp.333-339.
I/ Presented at the Dust Workshop, Notre Dame University,
Notre Dame, Ind., April 20-21, 1977.
2_/ One of the facilities of the Southern Region, Agricul-
tural Research Service, U.S. Department of Agriculture.
3_/ Employee of Battelle Columbus, Columbus, Ohio.
4/ Names of companies or commercial products are given
solely for the purpose of providing specific informa-
tion; their mention does not imply recommendation or
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357
endorsement by the U. S. Department of Agriculture over
others not mentioned.
WRITTEN DISCUSSION
Dennis J. Helfritch
Apitron Division, American Precision Industries
Buffalo, New York, New York 14225
The authors should be congratulated for the innovative designs which are
described in their paper. These new devices have been designed for the
removal of respirable dusts in the textile industry, but could equally well
be applied to scores of other applications.
A general comment concerns the reported efficiencies of the three novel
devices. The authors state»that the indoor dust content of textile mills
can be reduced to one mg./m , but do-not indicate the dust collector effi-
ciency required to achieve one mg./m . Therefore, one does not know if the
devices described are satisfactory for this service. The most efficient
conventional device, the baghouse, is used extensively in the asbestos
industry and typically achieves 99.99% efficiency on fibrous dusts less
than 15 microns. I do not know why the devices achieving 95%-99% efficiency
as described in the paper are considered improvements.
A few specific comments, regarding data and hardware can be made at
this point. Two devices utilize corona wires, and I would think these
wires would require a method for cleaning deposits which are almost sure
to accumulate over periods of time. The Wet-Wall Electronertial unit is
described as having low operating costs, but the corona input is approximately
500 watts/1000 CFM. Including perhaps a 1-inch pressure drop and water
pumping, it seems that power consumption is relatively high.
According to Table III, in some cases the efficiency of the High Speed
Rotary Drum actually decreases when the 1-inch diameter holes are covered
with fabric. According to Table VI, the efficiency of the Fluid Electrode
Prescipitator increases when velocity is increased from 250 to 350 FPM
and field strength is decreased from 6.2 to 5.9 KV/cm. The data in Tables
III and VI is contrary to general principles and should be examined more
closely.
AUTHORS' WRITTEN RESPONSE
The level of "one mg/m3" quoted from the paper refers to total dust of
all sizes. The efficiencies of present cleaning systems used in textile
mills to produce these levels can be greater than 90% for total mass removal
of lint, trash, and dust. However, these systems have very poor efficiencies
(less than 50%) for capturing respirable dust (less than 15 ym).
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358
Regarding the question of possibly using the baghouse, it is our understand-
ing that the textile industry has had some experience here and that the peculiar
nature of cotton lint, trash, and dust is such that baghouses are not satis-
factory. These deficiencies are particularly in the areas of automatically
cleaning and maintaining the bags and in the tremendous areas required by the
high air capacity of textile mills. Our experience with cotton dust is that it
is significantly different in character from asbestos dust and no valid analogies
between the two can be drawn.
The question about cleaning of trash collected on the corona wires is well
taken. Our experience with running the two devices in question is that this
has not been a problem. From the standpoint of the W.W.E. unit the vibration
which the wire experiences in operation tends to keep it clean. As regards the
power requirements, our understanding is that a conventional precipitator could
require 50 to 500 watts/1000 CFM with the air velocity below 500 ft/mia. The
W.W.E. unit operates at 500 watts/1000 CFM but handles air velocities between
2000 and 3000 ft/min. 2Another advantage of the W.W.E. unit is that its collection
surface is about 10, ft /1000 CFM, whereas conventional precipitators require
from 50 to 1000 ft /1000 CFM. These factors (higher operating velocities and
smaller collection areas) put the W.W.E. in a very favorable position compared
to a conventional precipitator or other air cleaner.
With regards to the Reviewer's comments concerning anomolies in the efficiency
of the rotary drum as given in Table III, it is true that in some instances the
efficiency of the drum did decrease when the 1-inch diameter holes were covered
with fabric. This has been explained by the fact that apparently the particle
counter was registering particles that were being knocked off of the back side
of the filter fabric. Concerning the apparent problems posed in interpreting
Table VI, it should be clarified that this data is not presented for all the
same experimental configurations, thus explaining the reason for the seeming
inconsistency in the way the efficiency followed the principles of physics.
This data was merely presented as a sample of the types of efficiencies obtained
with the fluid electrode precipitator.
OPEN DISCUSSION
Baril: As far as we know we do not have to clean the corona wire. We have
not had any problem on the wet wall electro-inertial unit. The rotating drum
was looked at as an air cleaner , also as an electro-inertial unit.
Reif: For the rotary drum the dust goes in the bottom of the box, most of it
does not enter the holes in the drum. The comment about the fiber filter not
seeming to improve the effectiveness of the device is true. These types of
fiber filters are used currently in the textile industry which do not have very
high efficiencies. That is one reason why we were looking at them. If you
look at them with the atmospheric dust type test that we were using, their
efficiencies are just a few percent by themselves. The base velocities are
about 200 ft/min. So they are very poor filters actually. The first stage
filters were designed to pull out long cotton fibers, the second stage filters
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are generally of higher quality but still are not particularly good filters.
Baril: Concerning ourselves with table 2, the artificial cotton dust at
zero potential and zero current has an efficiency percent of about 80.2%.
Then right under it is a card output (this is at zero current), its efficiency
is 30.3%. The content of larger particles is different and you are seeing
the effect of electro-inertial input.
Billings: I do know you had card type dusts and this analysis is a serious
problem. I am concerned what you will do with this collector and how you
will distribute the dust, and what you are talking about in terms of the
actual staple, yarn and textile weaving. Are you basically going to have a
ventilation system which sucks all of the air out of one end of the building,
cleans it and redistributes it? Are you going to to have local exhaust
ventilation on every dust source? How are these going to be configured in
attempting to meet the standard?
Baril: You have asked a question that people in the air handling textile
industry suggest that the best thing they could do is get out of that industry
and service some other industry.
This is a very serious problem and the people who service the textile
industry are few and far between (one is Albert Baril). To answer this question
let us go back to a question asked by Dennis Helfritch. He said you have units
and this and that, we readily agree that you have a lot of units and they work
very well. We could go to a bag unit which is very efficient. EPA has looked
at some of this. The estimate comes back that the baghouse must be about the
size of the textile mill. This is one of the problems.
Right at the moment, we do not know what we are going to do. The government
is going to set some standards. The Department of Agriculture is going to try
like mad to get the standard high. If we get that, we are not in as much
trouble as you propose. If we do go to the 200 bin it will take us from 5
to 7 years to get there. If we set a standard that will bring it down to 200 in
5 years, who is going to buy a piece of machinery that will bring it down to
500 in a year?
What it looks like right now is that a lot of the weave rooms have air
which comes in at a high level and escapes completely through the floor and
is taken out and cleaned or it sweeps across the room and does this. To
increase that system to handle what the bin could possibly take, I then think
we might have to have terrific exchanges of air. We would have to bring this
air down to someplace where we could actually clean it. This incurs a cost
exceeding $2 billion. This was worked out by TRI Princeton. Right now this
section is really up in arms. One possible way to go is to build individual
containment units around many machines. They have already been successful in
doing this on the other end of the textile industry; that is cards, enclosing
them and capturing them.
We also enter a second problem which we have not addressed at all
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and that is capturing the dust so you can clean it. You do have leakage into
the system and you can only capture a percentage of the total dust, and by
cleaning, putting it back into the room, you are also leaking some of the un-
wanted particles into it. The capture system has to be worked on too.
We have been working on an entirely, completely new system for textile
mills which puts all the systems together in an integrated machine. In other
words, you start off at one end by putting in cotton puffs and you get yarn out
of the other end of the machine. This machine could be totally enclosed. We
are doing research on that line.
Billings: It is important to mention one other federal agency which comes
into the discussion and that is the FDA. If you use an electro-static precipitator
that involves any kind of ionization you are going to make ozone. There are
already ozone standards with regard to this kind of technology.
Baril: We have been monitoring the ozone levels.
Benarie: I have some information about rotary filters that are used, for
instance, under a British patent on submarines. There are three possibilities
for building rotary filters. The mechanism of the rotary filter is quite easy
to explain. The impaction mechanism on the filter, on the individual fibers,
instead of being impacted individually by the vector of the air stream is
impacted by the vector of the whole movement within the airstream which can be
10 times as large, at say 3000 rpm. The fibers can be oriented either
radially or longitudinally and the stream can be from inside toward the
outside or in reverse. Now the reverse system seems much better because the
cake is thrown away automatically by centrifugal force. Rotary air filters
are made which are used in the electro-chemical industry and are quite good.
I have some more information about it if you are interested.
Baril: We built a model that the British have and it was designed primarily
to remove radioactive particles from the stream. We built 3 units and
approached it 3 ways. All three were good, but we felt the fluid electrode
was the best. We have not given up on that. We still have it and work on it.
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DIELECTROPHORETIC AIR FILTRATION: PROGRESS AND PROBLEMS
J. K. Thompson, R. C. Clark, and G. H. Fielding
U. S. Naval Research Laboratory (Code 6180), Washington, D.C. 20375
ABSTRACT
Dielectrophoretic air filters under development at the Naval Research
Laboratory, as well as those previously reported or currently manufactured,
are discussed with a view to establishing successful design principles.
INTRODUCTION
Our burgeoning air and gas filtration technology concentrates largely on
the removal of particulate material from the gaseous discharges of industrial
processes. This is legally necessary and highly desirable from an environ-
mental health point of view. There is, however, another air filtration chal-
lenge: that of cleaning the air drawn into, and circulated within, occupied
buildings, vehicles and other enclosures. The aerosols of principal concern
are of several types: (1) the airborne pathogens that can cause cross-in-
fection in homes, hospitals, schools, theaters, etc.; (2) the miscellaneous
airborne debris that affects the operation of delicate electrical and elec-
tronic equipment of many kinds; (3) the common dirt and dust that deposits
on surfaces and requires control as a matter of esthetics and good housekeep-
ing; (4) the particulates formed in motor vehicle operation; and (5) the
industrial airborne refuse which is not removed at its sources. Other common
aerosols may in time be thought sufficiently objectionable to require removal
from the ventilation air of buildings. Such aerosols include the sulfate/
sulfuric acid originating in the burning of sulfur-containing fuels (1), the
terpene-derived blue haze produced by vegetation (2), and the metal-containing
particles produced by vegetation (3).
The means presently available for high-efficiency filtration of ventila-
tion air appear to have sufficient disadvantages to severely limit their use.
Such means include HEPA filters, very high quality glass fiber media, and
electrostatic precipitators. Their disadvantages are either high cost or
high pressure losses, or both.
We believe that a promising means of handling present and future prob-
lems in ventilation air filtration is the technique of dielectrophoresis.
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Equipment of this type is inherently simple, low in cost, and very compact.
Since a dielectrophoretic air filter is a field-operated rather than a current-
operated device, its power consumption is near zero. Additionally, there is
no corona discharge, ozone production is zero, and sparking between electrodes
can be eliminated.
A brief historical review of dielectrophoresis and dielectrophoretic air
filtration is desirable, both to give credit where due and to assist others in
avoiding duplication of ineffective or non-optimized designs.
The word dielectrophoresis was coined by Pohl to describe the accelerated
separation of solids from liquids in nonuniform electric fields (4). Me have
continued to use the same word to describe the accelerated separation (filtra-
tion) of solid or liquid particles from air in nonuniform electric fields.
Such an unusual descriptor helps nonspecialists to understand that we are not
talking about electrostatic precipitation.
Let us first describe and discuss briefly the dielectrophoretic process.
uielectrophoresis is the movement of a particle in a nonuniform electric field
the direction of movement usually being toward the most intense part of the
field. This mechanism is conveniently applied in air filtration by passing the
air through a thin layer of air filter material which is sandwiched between
metal screen electrodes. The air moves through the sandwich perpendicular to
its plane, and experiences no resistance other than that of the filter medium
itself, which may be a commercial glass fiber mat or a slab of open-pore
plastic foam. Since every glass fiber or every element of the foam locally
distorts the applied electric field, the mat or slab contains an enormous num-
ber of field innomogeneities. Since tne glass fiber and the plastic foam have
dielectric constants greater than that of air, the field is more intense close
to a fiber or foam element than remote from it. Therefore airborne particles
within the filter medium experience a dielectrophoretic force. This force is
approximately described by:
Thus, the dielectrophoretic force F is proportional to the square of the ap-
plied field E and, to the cube of the particle radius r; the dielectric con-
stants of the medium, KX, and of the particles, K2, are also involved.
NRL STUDIES
The NRL studies have employed a uniform geometry comprising a flat glass
fiber mat, U.64 cm thick, separating two steel wire screen electrodes. Vari-
ables have been the composition of the mat (three as-received commercial air
filter media), voltage (0 to 7kv, equivalent to 0 to 11 kv/cm), air velocity
and aerosol (OOP of 0.3 and U.8 micrometer diameter, and a fine fly ash). The
dielectrophoretic effect is illustrated oy the Dielectrophoretic Augmentation
Factor (DAF). This is the ratio of the aerosol percent penetration (100% -
percent aerosol retention) at zero voltage to the percent penetration at the
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voltage of interest. Tables 1 and 2 present data for one filter medium (the
Farr Go's HP-100) at several voltages and air speeds, and for two OOP particle
sizes. The manufacturer's rated air speed for the HP-100 medium is 20 cm/sec
(40 fpm), at which speed the initial resistance is 1 cm wg (0.40 iwg). The
DAF's at 7 kv and 20 cm/sec are 21 for 0.3 micrometer OOP and 28 for 0.8 micro-
meter OOP.
TABLE 1
DAF VS AIR SPEED AND VOLTAGE FOR HP-100 FILTER MEDIUM;
AEROSOL 0.3 MICROMETER OOP
Air Speed Applied Voltage, Kv
cm/sec 2 3.5 _5_ _J_
3 8 19 95 330
5 3 13 39 120
8 3 11 28 100
13 2 6 13 42
18 259 27
26 2 4 6 14
3* 2 I \ I
46 1236
TABLE 2
DAF VS AIR SPEED AND VOLTAGE FOR HP-100 FILTER MEDIUM;
AEROSOL 0.8 MICROMETER OOP
Air Speed Applied Voltage, Kv
cm/sec 2 3.5 —5_
3 30 110 300 1100
5 6 30 95 360
8 4 18 50 170
13 3 10 20 50
18 2 6 13 35
26 2 4 8 18
37 235 11
46 1237
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364
It should be noted that the dielectrophoretic force does not operate alone
in a filter medium. There are also the ever-present aerodynamic forces- and
in some cases, forces due to net electric charges on the particles, the filter
medium, or both. Thus, a complete analysis of the performance of dielectro-
phoretic air filters has not yet been made. It is clear that the relationship
between the dielectrophoretic force and the applied field squared is likely to
hold regardless of aerodynamic factors. However, since both the aerodynamic
forces and the dielectrophoretic forces are affected by both particle size and
filter element size, the total interaction may be difficult to predict.
We offer the hypothesis that the dielectrophoretic force is effective only
at shorter range than the aerodynamic forces and that therefore, the former is
operative only when there is pronounced evidence of aerodynamic effects in the
absence of the electric field.
The interplay between the aerodynamic and the dielectrophoretic forces
is complex. For example, aerosol particles of a given size which are moving
through a filter medium deposit inertially on a small filter element more read-
ily than on a large one. However, the electric field gradient is stronger
adjacent to a large filter element than to a small one. There would seem to
be a conflicting requirement here: small filter elements for inertial deposi-
tion of particles, and large elements for dielectrophoretic deposition. In
fact, however, small elements seem to be desirable for dielectrophoretic
deposition also; only with small elements do the aerodynamic effects allow
particles to pass close enough to the elements for the dielectrophoretic foam
to act effectively.
OTHER UIELECTROPHORETIC AIR FILTERS
As far as we know, the first dielectrophoretic air filter, the "Electro-
Polar , was constructed by Sproull and others at the Western Precipitation
Corporation in Los Angeles during the early 1950's (5). This was a full-scale
device with an air flow rate of 3500 cfm. The two filter media used were typ-
ical commercial glass filter materials of good quality. It was tested by the
Harvard University Air Cleaning Laboratory for the Atomic Energy Commission.
The reported results were mediocre (6,7), although in our view the performance
was better than reported. The dc voltage used for the Electro-Polar device
was 15 kv for a one-inch medium. This provided a gradient of 5.9 kv/cm.
Influential in the choice of voltages was the desire to avoid both corona
discharge on the expanded-metal electrodes and a spark discharge through
the filter medium. 3
A report by Thomas and Woodfin in 1959 described a dielectrophoretic air
filter of unusual design (8). The spaces between a series of charged and
grounded metal plates, oriented parallel to the air flow as in an electro-
static precipitator, were packed with glass fiber. A filter one inch deep
in the direction of the air flow showed "great improvement" when the voltage
was applied. The performance of a filter 14.5 inches deep at a voltage gra-
dient of 9.3 kv/cm was improved by a factor of over five at air velocities
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of 400-500 fpm. No further mention of the Thomas-Woodfin device is found in
the literature.
Havlicek's valuable theoretical dielectrophoresis paper of 1961 reported
on the effectiveness of electric fields parallel and perpendicular to the di-
rection of air flow (9). The former was found to be preferable on the basis
that it allows the maximum field gradient to occur at points where the rela-
tive particle-to-fiber velocity is at a minimum. Thus the two effects act to-
gether to maximize particle deposition. In fields perpendicular to air flow,
deposition is correspondingly minimized. Havlicek's theorem indicates that
the geometry of Thomas and Woodfin's filter was not such as to optimize fil-
ter efficiency.
Rivers' 1962 paper provides a useful theory review and, in addition, de-
scribes a unique air filter which may have some dielectrophoretic properties
(10). The medium is a multilayer paper; the electric field causes a separa-
tion of the layers. The modest improvement due to electrification has been
attributed to separation of the paper layers, probably attributable to elec-
trostatic repulsion. Rivers' device also places the electric field perpendic-
ular to the air flow direction.
The next example in our historical review is that of Walkenhorst and
Zebel in 1964 (11). Their filter medium unit was ten layers of nylon hosiery
fabric between screen electrodes; several of the units were placed in series.
Using aerosols of crushed coal or quartz, good filtration performance was ob-
tained in an electric field. Remarkably, a similar filter, exposed to the NRL
OOP aerosols, showed no significant particle deposition.
Finally, a currently manufactured air filter of dielectrophoretic de-
sign is the "Fiberstatic", by Filterlab of Houston, Texas. The design of
this device is a high voltage screen electrode having a 0.6 cm polyurethane
foam mat on each side, all sandwiched between two grounded screen electrodes.
In a test with a OOP aerosol no appreciable filtration efficiency was
observed.
My colleagues and I would now like to discuss the historical cases we
have cited in order to assist future development of dielectrophoretic air
filters. We consider that the Electro-Polar of Sproull, et al was the best
of those we have reviewed, as well as being the earliest. Why, then, did not
the Electro-Polar perform in consonance with the potential indicated by the
NRL data. Perhaps it was an idea whose time had not yet come. More signif-
icantly, the problem may have been the unavailability of glass filter mats
thinner than 1.3 cm (0.5 in). The thicker media available required a higher
voltage to attain an efficient voltage gradient, but the higher voltage in-
creased the probability of the undesirable corona discharge. Forced to use
a low voltage gradient, the Electro-Polar team then were hurt by the square-
law dependence of dielectrophoretic force on voltage gradient. Another fac-
tor may have affected the operating voltage chosen. This was the increased
probability of a spark discharge through the filter medium as the voltage
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366
was raised. Spark discharges produce holes in the filter mat, and may ignite
flammable materials deposited in the filter. It is possible that the sharp
edges of the expanded metal lath electrodes were likely sources of both corona
and spark discharges. This problem has not yet been fully solved, but an NRL
development may eliminate this difficulty. In any case, steps should be taken
in dielectrophoretic filter design to allow operation at the highest practi-
cable voltage gradient, taking advantage of the square-law relationship.
Turning attention to the Rivers and Thomas/Woodfin devices, we believe
that they demonstrate the validity of Havlicek's axiom that dielectrophoretic
filters should be designed for parallel alignment of air flow and electric
field. This should be considered an established principle of dielectrophoretic
filter construction
Walkenhorst and Zebel's filter, with the stacked nylon hosiery medium,
has the property of good performance with solid aerosols of crushed coal or
quartz, and negligible performance with the NRL OOP aerosols. Further, the
relatively coarse nylon filaments (10 to 15 micrometers) would not be expected
to be well suited as dielectrophoretic filter fibers. We suggest reconciling
the Walkenhorst/Zebel and NRL data as follows. The nylon medium per se is in
fact ill suited for dielectrophoresis because of its large fiber diameter.
However, as a few solid aerosol particles deposit on the fiber, probably by an
aerodynamic mechanism, the sharp points and edges of the deposited solid parti-
cles serve to produce electric field inhomogeneities. Dielectrophoretic action
then increases progressively. With OOP aerosols, some OOP droplets may also
deposit aerodynamically, but then yield no sharp points or edges. In fact,
they simply spread smoothly over the nylon fibers, increasing their diameter
slightly. If the above reasoning is correct, widely differing performance can
be expected from a large-fiber or large-element dielectrophoretic filter, de-
pending on whether the test aerosol is a liquid or certain solids. It is pos-
sible, also, that the satisfactory performance of Walkenhorst/Zebel filter with
the coal or quartz aerosols was in part dependent on particles larger than the
0.3 and 0.8 micrometer OOP.
While we have no test data on the Fiberstatic filter with a solid aerosol,
it is plausible that this filter performs well in the manufacturer's hands, and
that the test aerosol is a solid. Thus, the test pattern would correspond to
that of the Walkenhorst/Zebel filter. This could be expected since the ele-
ments of the reticulated polyurethane foam are also very large compared with
the glass filaments of the NRL filters.
CONCLUSIONS
We believe that dielectrophoretic air filtration has valuable character-
istics that will contribute toward very wide application in the future. How-
ever, even the "first generation" NRL filters require careful consideration of
the basic phenomena to insure satisfactory performance. Consideration of the
design of prior and current dielectrophoretic air filters is helpful in pro-
ducing new designs.
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367
REFERENCES
(1) R.E. Weiss et al, Science 195:979-980 (11 March 1977).
(2) R.A. Rasmussen and F.W. Went, Proc. Natl. Acad. Sci. USA 53:215 (1964).
(3) W. Beauford et al, Science 195:571-572 (11 February 1977).
(4) H.A. Pohl, J. Appl. Phys. 22:869-871 (1951).
(5) Persona] communications from W.A. Sproull.
(6) C.E. Billings, R. Dennis, and L. Silverman, Harvard Air Cleaning Labora-
tory, Boston, MA, Report NYO-1592 (1954).
(7) L. Silverman, Indust. Hyg. Quarterly:183-192 (Sept. 1954).
(8) J.W. Thomas and E.J. Woodfin, AIEE Trans. Pt. II 78:276-278 (1959).
(9) V. Havlicek, Int. J.Air and Water Poll. 4:225-236 (1961).
(10) R.D. Rivers, ASHRAE Jnl:37-40 (Feb. 1962).
(11) W. Walkenhorst and G. Zebel, Staub. 24:444-448 (1964).
WRITTEN DISCUSSION
E.R. Frederick
Air Pollution Control Association, P.O. Box 2861
Pittsburgh, Pennsylvania 15230
In accepting this assignment reluctantly, I reminded Professor Ariman that
my experience has been confined mostly to the practical aspects of particulate
control by fabric filters without electrical augmentation. While quite some-
time ago, limited studies were conducted with artificial electrification of
an experimental fabric filter-1-, most of my work has been restricted to consider-
ations of the effects of natural charging in both the media and particulates.
But because of the very outstanding benefit realized by augmenting the charge
on the medium and my personal views concerning critical importance of electro-
static involvement in the filtration process, I can be completely sympathetic
to the Thompson et al observations. Hopefully, studies such as this will
receive further attention.
I would like to take this opportunity to ask the following questions that,
perhaps, I should have had answered from my own earlier evaluations if only
they had been made under more controlled conditions at that time.
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368
1. How was the aerosol generated and conducted to the filter and was it
charged in the process?
2. What,for example, is the ultimate fate of the collected product?
3. What form does it take as it deposits or forms on the collecting
surface(s)?
4. How much longer can the electrified field filter operate than the non-
electrified variety?
5. Since we might expect DOP to coalesce, particularly in the field, does
this occur and to what extent?
6. If the DOP mist coalesces on the collecting fibers, does the oil form
on - and then run from - the surface without affecting, adversely, the filter's
performance for extended periods of service?
7. If this is a realistic appraisal of the process, can it be applied to
other mists or mist-like emissions? And if so,
8. What are the limitations, considering the flammability and explosive-
ness of some mist-like solvent emissions?
The above questions are based primarily on my suspicion that, like effective
natural charging, electrical augmentation may lead to improved aggregation or
agglomeration of the collected product. Most often, when this is accomplished
ideally, the nature of the deposited cake is most favorable for optimum col-
lectability, for most effective cleanability and for highest efficiency. With
a condensable liquid mist, this effect should be easy to detect.
Professor Penney has provided significant substantiation of some of our
experiences through his fundamental studies by relating the combined effects
of particle charge and fields with the deposited product. He has shown that
"impact" charged particles form a chain-like deposit on a fabric without the
use of any high voltage on the particulate or on the collecting surface. In
another study, again without external potential impressed on the filter fabric,
corona-charged particles also deposited in a chain-like aggregated manner on
one and only one of the fibers in a composite, two fiber filters.
Figure 1 is a photomicrograph showing a blend of two different (wool and
acrylic) fibers about 3 ym in diameter. The fact that one fiber remains
clean while the adjacent fibers and only these electropositive fibers collect
the negatively charged particles as a porous aggregate is extremely significant.
If this is the kind of open deposit that can develop because of electrostatic
forces, it would seem likely that the exceptional performance of the NRL
dielectrophoretic filter could also be attributed to the formation of such
a super porous deposit.
In the simulated filtration tests that we carry out without charge aug-
mentation, natural electrostatic charges, when suitably balanced, offer a most
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FIGURE t AGGREGATED DEPOSIT OF PRECHARGED PARTICUL*TE ON A FILTER FIBER
O
FILTRATION CONDITIONS: A/C = 5.4
DELTA P LIMIT = 6 IN, W..C.
TEMP. = 150 °F
CLEANING - HORIZONTAL SHAKE
RUN NO.. 41 - FABRIC 16 [_SP. DA.. (55) fl. SILICONEJ. PERM. = 36 CFM/FT2@ 0.5 IN. WX..
RUN NO.. 40 - FABRIC 15 CFll_ DA.. (?)J. PERM. = 39 CFM/FT2 S O.S IN.. W..C.
RUN NO. 42 - FABRIC 18 QSP. P-E.. NAPPEDJ PERM. = 38 CFM/FT2® 0.5 IN.. W..C.
20
40 60
TOTAL COLLECTED SOLIDS, G
80
103
FIGURE 2 EXPERIMENTAL FILTRATION OF ELECTRIC FURNACE DUST
-------
370
desirable means for achieving the highest level of filtration performance.
The evidence implies that when a particulate will form a porous deposit or
aggregate, this most desirable change in the cake can be achieved during the
filtration process by a favorable balance of the electrostatic properties
of the particulate and of the collecting fabric. While most participates can
form porous deposits, not all dusts aggregate so easily that any but the more
active (electrostatically) fabrics can promote the change. Extraordinary
persuasion seems possible by using media of the most suitable electrostatic
polarity, charge intensity and, sometimes, of perferred charge dissipation
rate.
On page 12 of the Helfritch paper "Electrical Filtration and the Apitron/
Design and Field Performance," results are given for pilot tests on electric
furnace (steel) dust. The curves indicate that electrical augmentation
increases filtration performance (rate and efficiency) very significantly. The
curves obtained in our non-electrified filtration tests also carried out on
electric furnace (steel) dust have a similar appearance indicating, possibly,
the same kind of improvement in deposited cake. Thus, Figure 2 shows a plot of
the quantity of electric furnace solids collected as a function of bag pressure
drop (limited in the tests to 6 inches w.c.) for three oolvester fabrics used
in the writer's filtration study of an electric furnace (steel) dust. The
conditions in the three evaluations were essentially identical and the fabrics,
although of different construction, were of about the same permeability.
Significant variations were noted in electrostatic properties among the three
media with the filament Dacron fabric (#15) being quite electropositive, the
napped polyester fabric (#18) being most electronegative with highest intensity
and the spun Dacron fabric (#16) of intermediate polarity and intensity.
Professor Penney?s reproduction of nodules or "dingle-berries" in experi-
mental filter tests also seem to reaffirm the fact that charges play a very
important role in aggregate formation. The nodules were produced on high cover
(fuzzy), not filament (smooth), fabrics with a strong electric field. In
subsequent tests, it was found that the nodules formed more quickly in a region
where there was little field compared to the strong field area. Nodule for-
mation can be induced by electrostatic forces and their retention to the fiber
seems to be due to this same force.
It is clear that not all of the facts concerning charge related particulate
aggregation are known, but it does seem evident that the porous deposit - low
flow phenomenon resistant type of cake formation could account for the super
performance of the NRL filter.
Whatever the specific mechanism by which electric charges and fields
influence the collection process, it is apparent that if the advantages of
electrical effects are to be generally utilized, certain specific properties of
filter media and particulates, not now considered significant in commerical
fabric filtration circles, need to receive more serious consideration. Cer-
tainly, the electrical resistivity of both the particulate and the medium have
to be considered worthy of determination routinely. Quite likely, oversight of
this property in the past could have been a factor contributing to the poor or
non-reproducibility of data noted in Thompson's review of "Other Dielectro-
-------
371
phoretic Air Filters."
That high resistivity is a problem limiting the charging of certain fly
ashes and, thereby, restricting their collection by electrostatic precipi-
tation, is well established. It is also clear that the resistivity of other
participates vary from relatively low (108 £2 cm) to extremely high
(> 1014 n cm) values. Since particulates and fabrics of high resistivity
cannot be charged, except naturally, it is quite possible that some incon-
sistencies in performance and the non-reproducibility of results indicated
with the various dielectrophoretic devices reported by Thompson et al can be
attributed to such variations.
Our experience with many filter fabrics provided by a number of different
suppliers indicates conservatively, that up to 20 percent of the commercial
media offered to the baghouse users carry residual finishes which reduce the
resistivity of the fabric to 1010 fl/Q or less. It is not too difficult to
imagine, then, that different lots of supposedly the same product have
performed differently in dielectrophoretic trials, just as they function
differently in non-electrified filtration operations.
Product differences among manufacturers introduce another variable and
represent a problem that is more the rule than the exception. When to this
uncertainty is added the strong competition to cut prices and, in some instances,
a policy of interchanging fibers of a generic type under one specification, it
becomes evident that reproducibility in the performance of filter media cannot
be achieved without identical duplication.
REFERENCES
[1] Frederick, Edward R.,"Some Effects of Electrostatic Charges in Fabric
Filtration," J. Air Poll. Control Assn., 24, 1167 (1974).
[2] Penney, G.W., "Collection of Electrically Charged Particles in Filters,"
J. Air Poll. Control Assn., 26, 58 (1976).
OPEN DISCUSSION
Penney: There was quite a flurry of activity among manufacturers on this about
25 years ago. They were trying two^stage precipitators. Nothing was ever
published about it. There was a big difference whether you could charge the
corona dust, then you could get fairly high efficiency. But you were in
trouble if the efficiency was not high enough because you would have space
charge troubles.
There was the remark about a filter giving a high efficiency on quartz
or coal dust and not on DOP. That is to be expected. My experience has
told me that it is difficult to disperse the quartz without having the particles
strongly charged.
The same thing happened with Rivers when he presented the paper at Penn
-------
372
State. He noticed that his artifically dispersed dust gave a high efficiency,
but the atmospheric air which has had a long time to get naturally discharged
showed a low efficiency. It seemed a question of whether or not your test
particles are charged or not.
Frederick: Do you think that the fact the particles were solid rather than
oil spheres contributed to their collection efficiency?
Penney: I do not know. I think it is more than likely to be the dispersion
of particles.
Friedlander: This paper is quite interesting; it shows the similarities and
differences from the HGMS. First of all Zebel's paper is not quoted by the
authors. This paper shows very clearly that it is easy in the electric case
to get confused between the dipolar and the charged cases. You can not keep
them apart. In the magnetic case, there is no confusion because there are
no monopolar magnetic charges available.
In this case you have the problem of electric dipolar. Magnetics people
do not have magnetic monopoles because we do not have monopoles. We need
large currents of low power if we use superconducting currents. Many appli-
cations would be well suited for permanent magnets. The ferro-electric
does not really get high enough electric radiation.
I think that the electro-ferretic people can learn from HGMS. Because
all the things discussed in this paper have been thoroughly investigated
in HGMS. Now you could go back and see where you could use this information
the best.
-------
373
COLLECTION OF FINE PARTICULATES USING A FOAM SCRUBBER
Tom E. Ctvrtnicek. H. H. S. Yu*, and C. M. Moscowitz
Monsanto Research Corporation, Dayton, Ohio
Geddes H. Ramsey, EPA, Industrial Environmental Research
Laboratory, Research Triangle Park, North Carolina
ABSTRACT
This paper addresses the subject of application of foam scrub-
bing to fine particulate control. Diffusion and sedimentation
appear to be the primary mechanisms of particle collection. Fun-
damental theory for these two mechanisms is discussed.
Principles of foam scrubbing were experimentally verified for
particles between 0.18 and 1.0 vim diameter on a bench (2.5 acfm)
and small pilot scale (500 acfm). The results of these experi-
ments are presented. In either scale the experimental results
agree well with the theory. Comparisons of economics indicate
that the foam scrubber can be competitive with conventional col-
lection devices including electrostatic precipitators, fabric
filters, and high energy scrubbers in both capital investment
and operating costs.
ACKNOWLEDGEMENT
This work was supported by the EPA under contract 68-02-1453.
Appreciation is expressed for EPA's continued interest in the
foam scrubber.
INTRODUCTION
The basic principle of the foam scrubber is illustrated in Fig-
ures 1 and 2. Gas stream contaminated with the particulate matter
is forced through a screen wetted with a surfactant solution. As
the gas passes through the wet screen openings, foam is generated
and the gas is enclosed in a multitude of small bubbles, Figure 2.
The distance which a particle must travel to reach a collection
surface is now greatly reduced by the bubble enclosure. The foam
*Present address: APT, San Diego, California.
-------
374
•SCREEN WIRE
V
AEROSOL » » » »
Figure 1. The basic principle Figure 2. Aerosol enclosure
of foam scrubbing in a foam bubble
proceeds through the main chamber of the scrubber where the par-
ticles are given time to migrate to collide with the bubble wall.
As the foam is broken, particles retained on the bubble walls are
collected in the surfactant liquor.
Monsanto Research Corporation (MRC) initiated work on foam
scrubbing in the latter half of 1974 under a contract with IERL-RTP,
EPA. The contract involved theoretical evaluation, a bench-scale
verification, and a small pilot scale demonstration of the foam
scrubber. Results of this work form the basis of this paper.
The next -section deals with theoretical aspects of particle col-
lection by foam. The results obtained on a bench-scale (2.5 cfm)
and small pilot scale (500 acfm) foam scrubber units are then pre-
sented. Finally, the economics of the foam scrubber are presented.
In this study, a scrubber system using wet screen foam gen-
eration was considered (see Figure 1). It should be noted that
other methods of foam generation may result in conclusions differ-
ent from those stated in this paper. Methods of foam generation
other than the wet screen are not a subject of this paper.
THEORETICAL ASPECTS OF PARTICLE COLLECTION BY FOAM
Several governing equations could be applied to describe the
process of foam scrubbing. These include the equation of contin-
uity, momentum (Navier-Stokes), energy, and diffusion. As is
often the case in practical situations, exact descriptions of
the phenomena using these equations are characterized by rigorous
formulas too difficult to solve mathematically. As a result,
many simplifications must be applied.
Let us first consider the idealized case of bubble formation as
shown in Figure 2. Aerosol with constant particle concentration,
C0, rushes into a foam bubble as it is being formed. The bubble
containing the aerosol detaches at time t = 0. As long as the cross-
sectional areas of the foam generator and the foam scrubber are sim-
ilar, the mass of aerosol stream will move with essentially the same
velocity before and after the formation of a bubble. It is therefore
reasonable to assume that the aerosol inside the bubble and the
foam bubble surface progress with the same velocity through the
-------
375
scrubber. Consequently, the impaction forces active in this sys-
tem may be assumed to be negligible.
Now let us consider diffusion of particles enclosed in the
foam bubble. Assuming an isothermal system (i.e., gas tempera-
ture - bubble film temperature) and a very small, spherical bubble,
if the motion of gas is negligible inside the bubble after the
bubble has been formed, then from the set of governing equations,
Pick's Second law applies:
|£ = D72C (1)
d L
Assuming uniform aerosol concentration and transformation of
Equation 1 into spherical coordinates, we obtain:
3t I, 3r
_ _ (
n> J
The diffusion coefficient, D, for small particles may be ex-
pressed by1:
D ' 60- [ X + A(r-> + Q< (r-> e~brP/A~| (3)
PL p p J
_22 2
where k1 = Boltzmann's constant (1.38 x 10 kg m/s °K)
T = temperature
y = viscosity of gas
r = radius of diffusing particle
X = mean free path of the gas
A,Q',b = empirical constants
The solution of Equation 2 will give the particle concentration
distribution inside a bubble as a function of time and radius for
various particle sizes, assuming that the particulate cloud is
monodisperse and, at given concentration, the coagulation of par-
ticles is negligible.
The partial differential equation, Equation 2, can be solved
by the separation-of-variables method. To solve Equation 2 the
initial and boundary conditions must be summarized:
-------
376
(a) C(r,0) = C initial concentration at
t = 0 is constant (4)
(b) C(R,t) = 0 on the surface of the
sphere at t > 0 (5)
(c) C(r,t) 7* », r —»• 0 (6)
The general solution will have a form as follows:
C(r,t) = (cie-"2Dt + C2ea2Dt)
fc 1
N
(7)
i i
+ cv
where a, d, Ci, Cs, Ci* = constants
J = Bessel functions
n
By use of boundary conditions, Equations 4, 5, and 6, and lengthy
derivation and transformation, Equation 7 becomes:
,mi, ,
n=l
This equation gives the instantaneous particle concentration
inside the bubble.
The instantaneous rate of loss of particles to the bubble
surface is now given by:
q(R,t) = -D(4uR2) J£
7.
>-(^) Dt
K (9)
E-(^)
R
e
r=R n=l
The total loss of particles in time t from time t = 0 is:
= /q(R,
t)dt =
8R3C
_ _
(10)
-------
377
Hence, the particle collection efficiency, E (%), in time t may
be determined as follows:
E =
Q
4/3nR3C
100 = —
1 - e
x 100
(ID
noLo:
n=l
'- 6"
Inspection of Equation 11 shows that bubble diameter, R, is of
primary importance to collection efficiency. It appears as a
square term in the denominator of the exponential term. For
illustration, particle collection efficiency, Equation 11 was
plotted for bubble size of 1.0 mm diameter and three particle
sizes, 0.01, 0.1, and 1 ym diameter, as a function of foam resi-
dence time, Figure 3. Collection efficiency by diffusion only of
0.01 um particles within a reasonable practical residence time
(say 5 seconds) is very high as compared with a 1.0 ym particle.
PRESSURE - 760 mm Hg
TEMPERATURE-23°C
BUBBLE DIAMETER-1.0 mm
40
RESIDENCE TIME, s
50
60
Figure 3. Theoretical particle collection efficiency
-------
378
It is obvious that some of the enclosed particles could reach
the bubble surface by sedimentation. The number of particles
deposited per unit time inside the bubbles by sedimentation can
be expressed as UR2)CVS, where Vs = particle settling velocity.
For simplicity, it is assumed that the concentration of particles,
C, inside the bubble is uniform. Using this expression we can
formulate the coefficient of absorption by sedimentation (ratio
of the rate of particles deposited to the total number of particles
in the bubble), as, as follows:
TrR2CVo 3V
s s
ft ~ "i"^^™™^^""^^^"^^^ S «w^^«^ / T f\ \
S (4/3irR3)C 4R
The settling velocity can be expressed by1:
d£p«9 / «7» 01 -bd /2A'
Vs =
where p = particle density
Combination of Equations 12 and 13 gives:
-bd /2X
p
Typical sedimentation velocities of particles (Equation 13) and
coefficients of absorption by sedimentation (Equation 14) are
listed in Table 1 for a bubble diameter of 1 mm and a particle
specific gravity of 1 g/cm3.
-------
379
Table 1. TYPICAL SEDIMENTATION VELOCITY AND
COEFFICIENT OF SEDIMENTATION
(for R = 0.0005 m, p = lg/cm3)
Vg, m/s
V 1/s
d (ym)
0.01
6.5 x 10~C
0.0001
0.1
8.8 x 10~7
0.0013
1.0
3.5 x
0.052
10~5
Evidently, for particles larger that 1 ym, sedimentation
would significantly enhance the collection of particles during
the foam scrubbing process. For 1 ym particles, the table indi-
cates that about 5.2% of the remaining particles could be re-
moved by sedimentation mechanism every second, assuming uniform
particle concentration within the foam bubble. Because of the
diffusion process, the particle concentration profile, Equation 7
would not be uniform throughout the bubble (i.e., leaner near
the wall) and, therefore, the actual sedimentation coefficient
would be somewhat less than indicated above.
The combined particle collection efficiency (including dif-
fusion and sedimentation) was determined in a step-by-step cal-
culation. Although the method may not be absolute, it provides
a tool for estimating foam scrubber collection efficiency.
The computational results are presented in Figure 4, a through
i, for various bubble diameters, particle sizes, and particle den-
sities. For convenience, the diffusion collection curves are also
shown in these figures. As evidenced by the collection curves,
the enhancement of collection efficiency by the sedimentation
mechanism is far more important than the diffusion mechanism
for the larger particles. However, for the particles smaller
than 0.1 vim, the sedimentation mechanism of particle collection
is negligible.
It was mentioned earlier that the impaction mechanism of col-
lection in the foam scrubber may be assumed negligible. To eval-
uate the impaction one would need to analyze flow development condi-
tions within the foam bubbles applying the Navier-Stokes equation.
This is not an easy task.
-------
380
BUBBLEDIAMETER-0.5nm
PARTICLE DENSITY - Ig/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE 23 ° C
d -l.Oum
P _T^-
DIFFUSION AND SEDIMENTATION
DIFFUSION ONLY
• CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
j_
30 « SO
RESIDENCE TIME, s
(a)
3 «
p • 0.01pm •
10 20
BUBBLE DIAMETER- 0.5 mm
PARTICLE DENSITY-2 o/cm'
PRESSURE-ATMOSPHERIC
TEMPERATURE - 23 ° C
DIFFUSION AND
"~ SEDIMENTATION
DIFFUSION ONLY
• CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
30 40 50
RESIDENCE TIME, s
(b)
BUBBLE DIAMETER- 0.5 mm
PARTICLE DENSITY- 3g/cm3
PRESSURE - ATMOSPHERIC
TEMPERATURE-23° C
0 10 20
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
• CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
-i 1 L_ i
30 « 50 a
RESIDENCE TIME, i
(c)
BUBBLE DIAMETER- 1.0mm
PARTICLE DENSITY- Jo/cm3
PRESSURE - ATMOSPHERIC
TEMPERATURE - 23 »C
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
• CURVE IDENTICAL WITH CURVE FOR
DIFFUSION AND SEDIMENTATION
» 40 50
RESIDENCE TIME, s
BUBBLE DIAMETER-1.0 mm
PARTICLE DENSITY- Ig/cm'
PRESSURE - ATMOSPHERIC
TEMPERATURE - 23 ° C
DIFFUSION AND
SEDIMENTATION
DIFFUSION ONLY
CURVE IDENTICAL WITH CURVE
FOR DIFFUSION AND SEDIMENTATION
30 « 50
RESIDENCE TIME. 5
(d)
_- BUBBLE DIAMETER-1.0mm
PARTICLE DENSITY - 3o/cm3
PRESSURE-ATMOSPHERIC
TEMPERATURE -23°C
IffUSION AND SEDIMENTATION
DIFFUSION ONLY
• CURVE IDENTICAL WITH CURVE
FOR DIFFUSION AND SEDIMENTATION
30 40 SO
RESIDENCE TIME, s
(e)
(f)
Figure 4. Theoretical collection efficiency (cont'd next page)
-------
381
DIFFUSION ONLY
• CURVE IDENTICAL WITH ClKVE fOB
t MtiM DIFFUSIONANp_SEpl«NHltON
KSIKNCITINE. 1
(g)
ffiSIHICI 1IW. 1
(h)
8UB8LE OIAWETER -2.0mm
PARTICLE DENSITY - la/cm'
PBESSUW-ATMOSPHfHIC
DIFFUSION *f*0
SEDIMtflTATION
DIFFUSION WHY
(i)
Figure 4. Theoretical collection efficiency (concluded)
However, as the first approximation, the order of magnitude
of relaxation time for gas flow inside the bubbles to reach a
steady state can be estimated from an analogous case of a gas
bubble rising through a liquid, Figure 5. This will provide
some indication of significance of the unsteady state on the
overall collection process. Let us determine how fast the steady
flow profile can be attained in the bubble illustrated in Figure 5
LIQUID
••••• ^^
o o
O LJ-J
UJ _J
Q; CD
— CO
O ID
CO
V,
(AT t » 0)
GAS BUBBLE
Figure 5. Flow profile of a rising bubble
-------
382
From the result of the analysis of Couette flow between the
annulus with one moving wall, the value required for the flow to
reach 95% of final velocity profile from the start was investi-
gated by H. S. Yu and E. M Sparrow:2
| = 0.32 Re (15)
where x = entrance length
R = channel width, or in this
case, radius of bubble
Re = Reynolds number
The time required for flow to reach 95% of final profile is
therefore:
t = - = 0.32R(Re)/V (16)
- Vb b
For a bubble 1 mm in diameter, rate of bubble rise of 0.30 m/s,
and a Reynolds number of 100, Equation 16 gives t = 0.05 second.
That means the gas flow in a 1 mm bubble would reach the steady
state almost immediately, say in less than 0.1 second. The motion
of gas, however, would persist as long as the bubbles are rising
through the liquid layer. By the same reasoning, as soon as the
foam bubble is out of the liquid, the internal gas motion would sub-
side immediately in the same relaxation time (<0.1 s) due to
absence of surface film motion.
For in the case of a screen foam generator, initial disturbance
of gas movement from the foam generation process would be expected
to subside in the same short period of time. This would tend to
indicate that due to insignificant relative motion between moving
bubbles and gas, and short flow relaxation time, contribution
of impaction would appear insignificant. This supports the
assumption previously made.
This concludes the discussion of the major mechanisms that
may be involved in particle collection by the foam scrubber.
These should be reevaluated for some specific conditions of
scrubber operation when other mechanisms might come into play.
-------
383
EXPERIMENTAL EVALUATION OF FOAM SCRUBBER
Theoretical evaluations were most helpful in designing the
experimental test unit shown in Figure 6. Many experiments were
performed using this unit to learn more about foam generation,
foam destruction, foam flow characteristics, foam stability, foam
bubble size, and particle collection efficiency. It would be
difficult to cover all of these tests in this presentation and
the reader is referred to the references for additional details.3/4
Four surfactants were evaluated in our experiments. They are
listed in Table 2. Most of the particle collection experiments
were performed using Tergitol surfactant.
The foam destruction chamber was built to enable testing of
various foam destruction techniques. Specifically, five foam
destruction techniques were investigated. They included liquid
spray, thermal destruction, ultrasonic destruction, compressed
air spray, and high speed rotating disk.
The high speed rotating disk was found to be the most effec-
tive device for foam destruction because of low energy require-
ments and minimum deterioration of surfactant liquor qualities
for a possible recycle. Consequently, this device was selected
and used in particle collection efficiency evaluations.
Two types of aerosol particles were also tested. They were
OOP (dioctyl phthalate) and polypropylene glycol. Relative to
water the two particle types possess different wettability pro-
perties, the dioctyl phthalate being essentially nonwettable and
the propylene glycol being wettable and miscible with water in
all proportions. The experimental aerosols were generated by
means of a condensation aerosol generator previously described.5
Monitoring of particles was done by different particle counters
including a nuclei condensation counter, an optical counter
(Royco), and an electrical mobility analyzer. These instru-
ments have a combined capability to measure aerosol particles
in the size range from 0.003 ym to about 30 ym and were selected
depending on the particle size range investigated at the time.
Collection efficiency measurements were made with different
scrubber residence times. The scrubber residence times were
varied by changing the scrubber length. At the same time the
air flow velocity through the scrubber was maintained constant.
This arrangement permitted performance of collection measure-
ments with identical foam characteristics. The results of
collection efficiency experiments are summarized in graphical
form in Figures 7 and 8.
-------
384
ROTAMETER
HUMIDIFI CATION
CHAMBER
COMPRESSED
AIR REGULATOR
—01
EXHAUST
FOAM
DESTRUCTION
CHAMBER
Figure 6. Experimental foam scrubber
DRAIN
Table 2
SURFACTANT TYPES
Surfactant
Tergitol TMN
Sterox NH
Alkanol DW
Aerosol OT
(751)
Manufacturer
Union Carbide
Corp. ,
Plastics Div.
Monsanto Co. ,
Inorganic
Chemicals Dlv.
E.I. du Pont
de Nemours &
Co., Inc.
American
Cyanamid Co. ,
Industrial
Chemicals, Div.
Composition
Trimethyl nonyl
polyethylene
glycol ether
Alkyl phenol
ethylene
oxide adduct
Sodium alkyl-
aryl sulfonate
Dioctyl ester
of sodium sul-
fosuccinic acid
Chemical
type
Nonionic
Nonionic
Anionic
An ionic
Concen-
tration, %
90
99.5
28
75
Comments
Wetting and re-
wetting agent
General purpose
detergent
(melting point
30-35°C)
General use
detergent; du
Pont says it is
in tight supply
at the present
time
Fruity pungent
odor
-------
385
< wo-
I
a.
>-
¥
60
Figure 7.
20
FOAM CHARACTERISTICS
250 MESH SCREEN , ,
AIRFLOW 1.137x10, m3/s
WATER FLOW 5 83 cnT / s
BUBBLE SIZED 81 mm
RELATIVE HUMIDITY
45* -5* PARTICLE SIZE
o • 0 18 -0.32 jjm
o • 0.32-0.56pm
A A 0.56 -1.00 jjm
10
20
30
40
50
60
70
80
RESIDENCE TIME, s
Foam scrubber collection efficiency of polypropylene
glycol 425 using 2% Tergitol foam with rotating disk
for foam destruction
in
<£
CO
8
tx.
•<
Q_
100
80
* 60
o
iZ 40
o
y 20
o
o
Figure 8.
FOAM CHARACTERISTICS
2* SOLUTION , ,
AIRFLOW 1.137x10, m/s
WATER FLOW 5.83 cm3 Is
TERGITOL-
PARTICLE SIZE
o • 0.18 - 0.32 jjm
o • 0.32 -0.56 urn
« • 0.56 -1.00 jum
10
20
30
40
50
60
70
80
RESIDENCE TIME, S
Collection of OOP aerosol using foam scrubbing with
rotating disk for foam destruction
-------
386
Because the collection of fine particles in foam takes place
mainly by diffusion and sedimentation, the collection efficiency
is a strong function of scrubber residence time. Both Figures 7
and 8 demonstrate curves asymptotically approaching 100% collec-
tion with an increase of scrubber residence time, which agrees
with the theoretically predicted curves reasonably well (see
Figures 4, a through i).
Bubble diameter in all cases was about 0.8 mm. Somewhat
higher collection efficiencies than those predicted have been
observed using polypropylene glycol (wettable) aerosol especially
at the lower scrubber residence times (Figure 7). This indicates
that the compatibility between particles and the foam (as for
example suggested by the aerosol wettability) is an important factor
to consider in optimization of foam scrubber design and operation.
Figure 8 shows several additional data points obtained with the
OOP aerosol and different surfactants including Tergitol, Sterox,
and Aerosol OT. These points further support the factor of
aerosol/foam compatibility since some changes in collection
efficiency were noted due to the change of surfactant. Even
though the collection efficiencies were found to be generally
lower than those observed using Tergitol foam, proper surfactant
selection might improve OOP collection and approach the effici-
ency that was observed for polypropylene glycol aerosol.
A comparison of the collection data presented in Figures 7
and 8 suggests that there might be somewhat different collection
rate controlling factors in the cases of OOP and polypropylene
glycol aerosols. While the small size particles showed better
collection efficiency than large size particles using DOP
aerosols, this trend was found to be reversed for particles of
polypropylene glycol.
Figure 7 also includes two sets of data measured with dif-
ferent humidity in the scrubber. Two relative humidity levels,
5% and 45%, were investigated using glycol aerosol and Tergitol
foam. More humid input air increased the particle collection by
about 1 to 2% for residence times of 13 and 26 seconds. Since
the accuracy of the particle concentration measurement is be-
lieved to be about 10 to 20% it is not certain whether the effect
of humidity is significant or not.
The effect of bubble size on collection efficiency was also
evaluated. The 60 mesh screen was used to generate coarser foam
with the other operating parameters kept constant. The size of
cells for the 60 mesh screen was photographically determined to
be about 3.9 mm. The observed collection rates are presented in
Figure 9 . The collection rates are significantly less than the
-------
387
values determined for the small-celled foam (0.83 mm). The DOP
aerosol showed a collection rate of about 40% with a particle
size between 0.56 and 1.0 mm and 80 second residence time (75%
collection was observed with the small-celled foam). The glycol
achieved about 84% collection under similar conditions (98% with
small-celled foam). Two data sets are also displayed using a
large-celled 1% Alkanol foam. While this foam improved collec-
tion of DOP aerosol, the collection of polypropylene glycol
aerosol was essentially unchanged.
The operating conditions which have been used in our experi-
ments give an air (or foam) velocity in the scrubber of about
0.15 m/s (0.5 ft/s). It is postulated that a higher average
velocity might influence the collection rate, especially at the
point of foam generation where higher turbulence could enhance
the effect of collection by impaction. Consequently, the ve-
locity was increased by a factor of three to about 0.45 m/s
(1.5 ft/s). At the same time the scrubber was lengthened to
5.5 m (18 ft) so that a 13 second residence time remained un-
changed. The collection results for both DOP and the polyglycol
were essentially the same as for the 13 second residence time at
the lower air velocity.
This supports the theoretical evaluations presented earlier
which indicated that the collection rates during the foam forma-
tion will be negligible and that the majority of particles will
have to be collected by diffusion and sedimentation mechanisms.
There does, however, appear to be a point where collection
efficiency is enhanced by capture of particles in the foam genera-
tion step. This occurs at high liquid-to-gas ratios causing the
foam scrubber to behave similar to a wet scrubber. Figure 10
shows the effect of distilled water sprayed on the 250 mesh
screen in relation to collection efficiency of DOP particles in
the size range between 0.7 and 1.0 ym. No collection of parti-
cles is seen for liquid-to-gas ratios below 5,130 cm3/m3 (38.4
gal./I,000 ft3). Thus, the collection efficiencies for foam
scrubbing reported above primarily represent the mechanisms of
diffusion and sedimentation.
Experience from bench scale operation was used to scale-up
the foam scrubber to a small pilot unit with a capacity of 500
acfm. The objectives of this scale-up may be summarized as
follows: (1) demonstrate the foam scrubber on a larger scale,
(2) test it using an industrial aerosol particle, (3) gain ex-
perience in scale-up, and (4) verify a potential for multiple
surfactant liquor recycles. It may be said that all these ob-
jectives were met with a considerable degree of success.
-------
388
(/>
s 100
o
o
0.
60
40
Us
20 -
o
o
2 % TERGITOL FOAM EXCEPT WHERE SHOWN OTHERWISE
Figure 9.
POLYPROPYLENE GLYCOL AEROSOL {-5
DOP AEROSOL!^*
PARTICLE SIZE
• 0.18-0.32jum
• 0.32-0.56ym
* 0.56-1.00jjm
_L
_L
10 20 30 40 50
RESIDENCE TIME, s
60
70
Foam scrubber collection efficiency of polypropylene
glycol 425 and OOP using large-celled (3.9 mm) foam
/l
<
oo
100
g 80
oc
<
>-
CJ
60
g 40
Ll_
u_
UJ
I 20
S o
BENCH SCALE
OPERATION
20
40
60
GALLONS / 1000 ft
80 100
Figure 10.
2.0
12.0
14.0
4.0 6.0 8.0 10.0
WATER ROW RATE, I03 cm3/m3
Collection efficiency values for distilled water
sprayed on the screen versus water flow rate, OOP
particle size: 0.7 - 1.0 \im
-------
389
The scrubber was scaled-up and operated with no difficulties
for several weeks. Collection efficiencies were measured using
an industrial aerosol, a flyash obtained from a stoker coal-fired,
power plant, Nucla, Colorado. The schematic of the small pilot
scale foam scrubber is shown in Figure 11.
As shown in Figures 3 and 4, the particle collection effi-
ciencies follow curves that steadily increase with time and
asymptotically approach 100%. After a certain time, the gain in
particle collection efficiency that would result with a further
increase of residence time becomes rather negligible, and gets
within the range of experimental error. To operate the pilot scrubber
in this region would greatly reduce the possibilities for observing
the effect of influential variables on the scrubber collection
efficiency. It was decided that high scrubber collection efficiencies
obtainable with longer scrubber residence times would be sacrificed
for the purpose of having a more flexible pilot test facility, and
the scrubber was thus designed for short residence time.
The scrubber residence time was actually measured and
determined to be 20.5 seconds. Other operating conditions are
listed below:
gas glow 0.260 m3/s (550 acfm)
liquid flow 6.81 x 10 4m3/s (10.8 gpm)
scrubber AP 1.84 kPa (7.4 in. H20)
Collection efficiencies for submicron particles were measured
and are illustrated in Figure 12. They ranged between 50.0% and
75% by count for a particle size range of 0.056 pm to 1.0 ym.
Corresponding theoretical collection efficiencies for particles
in the size range between 0.1 pm and 1.0 ym and for the residence
time of about 20 seconds are 32% to 67% by count (see Figure 4h).
The data in Figure 12 are average collection efficiencies
over the period of 20 hours . The surfactant concentration during
the 20-hour experiment was kept at 0.25% by weight. This is a
significant improvement over the bench-scale phase where about 2%
surfactant solutions were needed to generate satisfactory foams.
In addition, the surfactant liquor requirements were reduced from
5.17 x 10~3 m3 per m3 of gas (38.4 gpm per 1,000 cfm) to 2.64 x
10~3 m3 per m3 of gas (19.6 gpm per 1,000 cfm).
During the 20-hour run, the surfactant liquor was recircu-
lated. As a consequence of the recirculation, the concentration
of particles in the scrubbing liquor steadily increased to a
level of 2 kg/m3. No difficulties were observed in generating
foam with the surfactant liquor containing this concentration of
solids.
-------
10 PMfCL MANOMETER
PIEXICIASS SECTION
—EC-SCRUBBER
00
VO
O
Figure 11. Pilot-scale foam scrubber process layout.
-------
391
SHADED AREA - OUTLET CONCENTRATIONS
SHADED + UNSHADED AREAS • INLET CONCENTRATIONS
0.08 0.1 0.2 0.3 0.4 0.6 0.8 1.0
PARTICLE DIAMETER, dp, pm
Figure 12.
Average particle collection efficiency data (12 runs)
Percent collection shown inside the unshaded area.
Scrubber residence time 20.5 seconds.
-------
392
FOAM SCRUBBER ECONOMICS
Based on the experience from operating the foam scrubber
on bench and small pilot scales, an economic evaluation of this
device was performed. The economics were compared with those for
an electrostatic precipitator, a high energy wet scrubber, and
a fabric filter.
Figures 13 and 14 show capital and operating costs for the
three conventional devices as well as the foam scrubber.5
CONCLUSIONS
Darticlfrnn" ^ sedimentation aPP*ar to be the most important
particle collection mechanisms active in a foam scrubber. Theo-
retical evaluations of these mechanisms indicate that over 90%
collection efficiencies are possible in foams with bubbles 1 mm
of ^eo S af ab?Ut 4° t0 6° S6COnd residence time. ThI remits
of theoretical evaluations seem to agree well with the results
obtained by experiments in a bench and a small pilot scale foam
are comoar^ "^ ^ °^ati^ costs of foam scribing
M^in?P i 4- r0? the conventional particle collectors in-
cluding electrostatic precipitators, fabric filters, and hiah
™^6"- At Present, however, the foam slubber
be1called commercially available and fully provln
o?%hr?ntS *" K^ tO test the Performance^'
of the foam scrubber in industrial applications.
REFERENCES
1. Fuchs, N. A. , The Mechanics of Aerosols, A Pergamon Press
Book, The Macmillan Company, New York 1964.
2. Yu, H. S., and E. M. Sparrow. Flow Development in a Channel
Having a Longitudinally Moving Wall. Journal of Applied
Mechanics. 37 (2) :498-507, June 1970. FP^ea
3. Ctvrtnicek, T. E, T. F. Walburg, C. M. Moscowitz, and H. H. S
Yu, Monsanto Research Corporation, Dayton, Ohio, Application
'
,
t0 Fine' ^tide Control, Phase I, Contract
, May 1976, MRC-DA-556, EPA-600/2-76-125.
4. Ctvrtnicek, T. E. , C. M. Moscowitz, S. J. Rusek, L. N. Cash,
Monsanto Research Corporation, Dayton, Ohio, Application of
Foam Scrubbing to Fine Particle Control, Phase II, Contract
No. 68-02-1453, December 1976.
5. Liu, B. Y. H., K. T. Whitby, and H. H. S. Yu. A Condensation
Aerosol Generator for Producing Monodispersal Aerosols in the
Size Range of 0.036 to 1.3 Microns. (Presented at the 6th
Inst. Conf. on Condensation Nuclei. Paris. May 1966.)
J. de Recherches Atmos. (Paris.) No. 3:397-406 (1966).
-------
393
• CAPACITY. ACFM
i
FOAM SCRUBBER
RESIDENCE
aECTROSTATIC
PRECIPITATOR
| HIGH ENERGY
L-WET SCRUBBER
FABRIC FILTER
HIGH ENERGY WET SCRUBBER
ELECTROSTATIC PRECIPITATOR
FOAM SCRUBBER
1.0
10.0
100
1000
CAPACITY, m'/s
Figure 13. Capital cost for particulate control devices
105
s
o
«c
85
0.
o
T-|—I J
CAPACITY, ACFM
10
106
, FOAM SCRUBBER - SURFACTANT MAKEUP (-2%)
(40s)
CASE A
; FABRIC
- FILTER
HIGH ENERGY WET SCRUBBER
FOAM SCRUBBER -NO
SURFACTANT MAKEUP
(40$
- ELECTROSTATIC
PRECIPITATOR
10.0 -
i.o
CAPACITY, nr/s
Figure 14 . Operating cost for particulate control devices
-------
394
WRITTEN DISCUSSION
George E.R. Lamb
Textile Research Institute
Princeton, New Jersey 08540
The main problem of the technique, according to the data shown in Figure
12 of the paper, is the low collection efficiency. If the method is to be
acceptable for pollution control applications, the efficiency will have to
rise to the order of 99%. We can estimate the bubble size required to achieve
this by using a relationship found at TRI in connection with fabric filtration
We have found that capture efficiency for a particle passing through a pore
was a function of the log of r /s where r is the Brownian radius of a
particle, and s is the pore diameter.
The Brownian radius is given by
yr
r
e
where n is the viscosity of air
1 the mean free path in air
r the particle radius
t the residence time
Table I gives values of r for various particle sizes for a residence time
of 10 seconds. Figure 1 shows values of r /s where s is 0.5, 1 and 2 mm, where
s is now the bubble diameter. Capture efficiency values were obtained from
Figure 4a,d and g at 10 second residence times and all points in Figure 1
seem to fall on a common curve. The curve indicates that for 99% efficiency
re/s must be about 0.3.
Table II gives bubble diameters (in pm) that will satisfy this requirement
at 10, 20 and 100 second residence times. Since the capture of one-pm
particles is aided by sedimentation, we may consider the bubbles for 0.5 urn
particles to be the largest permissible and we see that at 100 second residence
time, thesg are 1/4 mm. The formation of bubbles having a surface tension of
70 ergs/cm requires pressure drops shown in Table III. We see that formation
of 1/4 mm bubbles only requires a theoretical pressure drop of about 7 inches
of water and is thus within tolerable limits. The values in Table III were
calculated on the assumption that the bubbles are already joined to neighbor-
ing bubbles as they form, i.e., that each bubble wall serves as the common
wall to two adjacent bubbles. If the bubbles are formed individually, the
pressure drops in Table III must be doubled, since a bubble has two surfaces.
-------
395
TABLE I
BROWNIAN RADIUS OF VARIOUS PARTICLES
Particle radius r
(urn)
1
0.
0.
0.
0.
5
1
05
01
Brownian radius, r
(vim)
16
23
66
114
482
TABLE II
BUBBLE DIAMETERS GIVING 99% CAPTURE EFFICIENCY
Particle
radius
(pm)
1
0.5
0.1
0.05
0.01
Residence time,
seconds
10
51
78
220
380
1600
20
72
110
311
540
2260
100
160
250
700
1200
5000
TABLE III
PRESSURE DROP REQUIRED FOR FORMATION OF
BUBBLES OF 70 ERGS/CM2
Bubble Diameter
(mm)
1
0.5
0.25
AP
(mm H20)
42
84
168
-------
00
s
o u
-------
397
The paper makes no mention of the effects of temperatures which must be
limited to somewhere short of 100°C, and presumably limits application of the
technique.
OPEN DISCUSSION
Cooper: I was struck with the lack of time dependence of the low collection
efficiency values, for instance, in the DOP aerosols,page 16. One of the
things being suggested is that there is the occurrence of another generation
mechanism. We have the formation of very fine sodium chloride particles
by the breaking of the bubbles. Maybe the device is better than we think
because when the bubbles break, they are putting some of the material back
into the air. I was wondering if you had a comment on that.
Ctvrtnicek: You mentioned that the collection efficiencies were high on the
500 cfm unit. I tried to explain that the unit was purposefully designed to
have low collection efficiencies, that we wanted to be on that steep end of
the curve. Our results were somewhat better than the theoretical collection
efficiencies. We extended the time for deposition and still could not reach
900 gauss collection efficiency. Whatever we achieved was not achieved through
any extraordinary effort to optimize the device. We were primarily after a
demonstration of the model. We never optimized bubble generation, or the
geometric structure. For example, in the bench scale, we were generating
foam destruction on the order of 10 particles per cubic centimeter. When
we went to the pilot scale, we cut that down by a factor of 10. I believe
that it is very likely that, by proper geometry of the disc and impaction
surfaces,this can be reduced even further. The same goes for the foam
generation. We really did not try to optimize it at all. Once we got a
decent foam we could use to make experiments, we just performed the experiments.
There were no optimization efforts at all.
Lawson: You mentioned that the collection efficiency might be enhanced if
you had smaller bubbles. Unfortunately, one of the undesirable effects,
with the understanding that there has been no real effort to optimize the
situation, is that if you go to smaller bubbles, you probably need even more
surfactant to get the total bubble volume that is required. That is a possibility
because the wall surface of the bubble will probably not go down nearly as
fast as the size of the bubble. You will end up making smaller bubbles
requiring even more surfactant. So surfactant recovery will really be
important in that case.
Ctvrtnicek: That is a good point. However, when you can believe in recycles,
surfactant really does become a factor. In operating this unit we used semi-
horizontal and vertical modes. I believed that the device worked much better
-------
398
in the vertical mode because you were continuously getting a drainage of
the foam. As the foam becomes drier, it is easier to destroy. You can
reduce the energies for destruction and make it even more effective.
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399
"THE ANOMALOUS BEHAVIOR OF ASBESTOS FIBERS"
J. W. Gentry
Department of Chemical Engineering, University of Maryland
INTRODUCTION
Asbestos is a term used to include a number of fibrous silcate
minerals including amosite, crocidolite, chrysotile, tremolite,
and anthophylite. Because of its inflammability it is widely
used for insulation in clothing and construction, for brake-
linings, and as a binder with cement and asphalt.
Recently there has been evidence showing a 50-300% increase
in cancers of the digestive system among insulation workers [1].
A significant increase of lung cancers [2] especially mesothe-
lioma among workers in the asbestos mining has been found. In
this case a significant synergistic effect was demonstrated for
smokers. Finally, an examination of cancerous tissue indicated
the pressence [3] of longer fibers (fiber length > 5y); although
no similar studies have been carried out for healthy tissue.
This latter effect provides some insight into the reasons for
increased interest into asbestos fibers. Probably the major
emphasis to date has been in the detection of fibers. First the
concentrations are very low, in the order of 105 to 10b particles/
liter for rainwater. This corresponds to a weight fraction of
107 - 106.
Typical concentrations of^fibers in air at rural locations are
of the order of 10 fibers/m . Such concentrations require a
lengthy sampling time which has so far precluded automated
instrumentation [5]. Furthermore, as is discussed, below only a
fraction of the fibers can be asbestos. In recent years the
separation of fibers has become recognized as an important prob-
lem. For example, are only long fibers carcinogenic? The point
is important in that the general population is typically exposed
only to short fibers whereas industrial workers are exposed to
a wide spectrum of particle sizes. There is, also, the need to
separate fibers from spherical particles in industrial regions.
Typically fibers provide only a small fraction of the total
-------
400
fibers.
One point that should be emphasized is the difference between
the mineral fibers. Amosite and crocidolite are rod-like, they
appear rigid, and their shape can be approximated by a prolate
spheroid. Chrysotile which is the most widely used fiber con-
sists of hollow, bent tubes which appear to be flexible.
Anthophylite appears to be mostly plate-like resembling shingles.
For most measurements amosite or crocidolite are recommended.
GENERAL PROPERTIES
Before the specific properties of non-spherical particles are
discussed, it is well to outline the steps required for a
systematic development of spherical particles. Condensation and
evaporation are, of course, less important than for spherical
particles, since non-spherical particles are solids. Similarly
the problem of coagulation has been less studied, primarily
because of the much more complicated expressions and the
stochastic nature of the collision process. Other than these
differences the problems are similar. First, the aerosols must
be generated. The question is what is the analog of a monodisperse
distribution of spherical particles. Next the particles must be
measured. The methods of measuring non-spherical particles by
microscopy, by aerodynamic behavior and by optical methods are
related to those of spherical particles. In this lecture,
emphasis will be focused primarily on the aerodynamic behavior
because it is the most thoroughly investigated. Finally there
are the separation processes - electrostatic precipitation,
filtration, gravitational settling in an elutriator all of which
are just beginning to be studied.
GENERATION
The ideal generator would produce a uniform, monodisperse cloud
of fibers with a known and controllable charge distribution. A
monodisperse fiber would be one having the same shape and the
same dimensions. For the approximation to this fiber one would
have an amosite fiber which can be approximated by a prolate
spheroid. The monodisperse distribution would then be amosite
fibers having the same length and diameter. Unfortunately we do
not have the analog of latex particles or a Sinclair-La Mer
generator for non-spherical particles.
Many of the current asbestos generators which work by scraping
-------
401
a packed cake of ground asbestos fibers are completely unaccept-
able for experimental studies of the properties of fibers in
that the distribution is very broad consisting primarily of
aggregate rather than the necessary individual fibers.
The most successful generator [6] currently available uses a
vibrating bed with the flow rates through the bed at sub-fluid-
ization velocities. The asbestos fibers are ground and dried
prior to placing in the generator. Mixing the asbestos with
small metal spheres (Nickel) proved unsuccessful in that the
fibers produced by the generator were coated with a thin layer
of nickel dust. The generator described above has been proven
to give a narrow particle size distribution with the diameter
of the fibers ranging from 0.2-0.4y and the lengths from 1 to
10y. The mean value of the aspect ratio (the ratio of major
to minor axis) was approximately 10.
2
The generator can produce fibers in the concentration 10
104 fibers/cm*. Examination of the fibers indicate that
approximately 15% of the weight of the fibers is in agglomerates
but approximately 85% of the weight is in individual fibers.
The particles are mostly shorter than 5y and are relatively highly
charged in comparison to spherical particles.
It has been suggested that non-spherical particles can be gen-
erated using solutions which yield non-spherical crystals on
evaporation [7]. Also, certain types of spores are oblong and
could be used to model non-spherical particles. Neither of
these suggestions have been successfully developed.
DETECTION
The only currently reliable method of determining the size
and number of asbestos particles is electron microscopy. The
key points that have been raised regarding microscopy of non-
spherical particles can be summarized as follows:
1. Not all fiberous particles are asbestos.
2. Electron microscopy gives only a 2 dimensional pro-
jection. In order to unambiguously characterize the fiber it
should be examined from three dimensions.
3. Nuclepore filters are the best current technology for
collecting a sample.
-------
402
The first result was demonstrated by Spurny and co-workers in
their measurement of air in the mountains near the source of
the Ruhr. They examined the elemental composition of each
apparent fiber (particles with an aspect ratio > 3). Any fiber
which contained Fe, Si, and Mg was classified as a possible
asbestos fibers. These experiments showed that no more than 5%
of the sampled particles could be asbestos. Similar results
were found for air sampled from Munster a city of 200,000 with-
out heavy industry.
In analyzing asbestos fibers, the prevailing assumption is
that fibers lie flat on the surface, so that the particle di-
mensions can be obtained directly from the projection. Doubt
has been raised regarding this point, and studies with spores
(1 < B < 3) have shown some particles aligned with the surface.
Because of the non-random orientation of particles, Cauchy's
theorem cannot be applied to obtain a particle size and the
particle must be viewed from three angles. The work required
is painstaking and tedious. The hypothesis of the fibers
lying flat has not been adequately tested.
Three methods have been examined for collecting samples -
filtration through membrane or nuclepore filters, electrostatic
precipitation, and thermal precipitation. The results of a
comparative study are discussed extensively in a recent review
article [5]. Two conclusions were drawn from the article -
nuclepore filters are superior to membrane filters in the
preparation of the sample and the filters require considerably
less time per experiment than do the thermal or electrostatic
precipitator.
AERODYNAMIC DIAMETER
Perhaps, the most extensively studied aspect of non-spherical
particles is their behavior in free fall. In comparing experi-
mental measurements with theoretical calculations - use is made
of the implied assumption that the particles may be treated as
prolate or oblate spheroids.
In discussing results for non-spherical particles it is
necessary to define the aerodynamic diameter and the dynamic shape
factor for a non-spherical particle.
The aerodynamic diameter D of a particle is defined as a
ae
-------
403
sphere of unit density having the same settling velocity as the
particle at low Reynolds number. The aerodynamic diameter is
related to the volume equivalent diameter D and the dynamic
shape factor K by equating the settling velocities giving:
v 1 pCDae _ 1 p De K _
s 18 nF(Dae,vs) 18 n F(De,vs)
where the volume equivalent diameter is the diameter of a
hypothetical sphere having the same volume as the particle.
The function F(D,V ) is related to the drag coefficient (CD)
and Reynolds numbe? (Re) by
T)(*
F(D,V) = CD If
For the important limiting case of continuum mechanics and
creeping flow the aerodynamic diameter is related to the
equivalent diameter by
D2
K - £--§-
P°Dae
For spheroids and cylinders, the theoretical shape factors
have been found for motion parallel to the polar axis K, , and
perpendicular to the polar axis K22. For a prolate spheroid
these results are
Kll
B -1 'B -1
where B is the aspect ratio (the ratio of major to minor axis).
Similar expressions exist for cylinders and oblabe spheroids
[8].
Several key points should be stressed regarding the shape
factors for non-spherical particles. First the shape factors
are a function of the aspect ratio alone. Secondly, the ratio
of K_2/K is bounded between 1 and 2. The function K22 is a
slowly increasing function of B.
-------
404
The effective or experimental value of the shape factor can
be calculated by
F.2TT
66 [K.. + K0J f(0) sin9
where f(6) is the angular distribution of the particle orientation.
It is precisely the difficulties with the form of f(6) which is
the subject of much uncertainty and which accounts for the curious
behavior of non-spherical particles that have been observed
experimentally. These effects can be summarized in the table below.
Table 1
1. Random-Orientation Assumption
2. Cans Diffusion Solution
a. Gallily-Cohen Simulations
b. Comparison with Random-Orientation Assumption
3. Collection Efficiency Measurements
a. Spiral Centrifuge
b. Inertial Impactor
c. Filters
d. Elutriators
The basic result of all experimental measurements have been that
the particles tend to acquire a preferential orientation, usually
in the optimum position for penetration, in any flow field where
there are inhomogenieties in the flow field.
For the special case of free-fall of particles in a stationary
medium, the shape factor can be computed if the functional form
of f (0) is known. By suitable experiments, experimental value of
the aerodynamic diameter can be measured and compared with theory.
Assuming creeping flow and a continuum, one has
D
ae
18nV
cosa
P
o
The product V cosa is the vertical component of the settling
velocity where a is the angle of descent of particles. As is
known, non-spherical particles do not descend vertically, but
at an angle a [9] which can be related to the ratio of the shape
-------
405
factors for movement parallel and perpendicular to the major axis
and the angle of orientation 0. The angle a is defined by
tana =
- —
(K22/Kn+tan 6)
In a recent note [10] , comparisons were made of calculations
for the aerodynamic diameter using the assumption of random
orientation, the assumption of an averaged orientation, and a
stochastic simulation where the particle was subject to fre-
quent changes in orientation with the probability chosen to
correspond to Cans solution [12] . It was found that the average
value of the aerodynamic diameters were similar for all three
calculations, but that the scatter of measurements or the
standard deviation of the aerodynamic diameter was less for the
numerical simulations than found for the assumption of random
orientation. It should be stressed that the two calculations
are based upon different assumptions, and that the simulation
gives a lesser degree of randomness manifested by a lower
standard deviation than does the assumption of random orientation.
The discrepancy indicated here emphasizes the need for experi-
mental measurements to check the assumption of random orientation
(which is the prevailing assumption) and the need for looking
at variances of stochastic processes rather than mean quantities,
Quanitative experiments in which the aerodynamic properties of
non- spherical particles have been examined include sedimentation
experiments in silicone oil, experiments with the spiral centri-
fuge, experiments with an inertial impactor, and experiments
with nuclepore filters.
The spiral centrifuge is widely used for particle separation
and analysis of polydisperse aerosols. The particles are removed
by centripetal force as a function of their aerodynamic diameters,
Particles having a smaller aerodynamic diameter are deposited
on a metal strip lining the walls of the spiral nearer the gas
inlet then larger particles. The advantage of the instrument
is that there are distinct regions of deposition depending on
the aerodynamic diameters.
Experimental measurements (primarily by Stober [13] and co-
workers) have found the following important results:
-------
406
1. The aerodynamic behavior of clumps of monodisperse
latex spheres differ depending on whether they are ball-like or
chain-like clusters. Specifically
D a N ' for chain aggregates
cl6
and D a N ' for clusters.
ae
2. Experiments with aerosols generated from an exploding
wire in which the particles showed a high degree of agglomeration
could be correlated with similar expressions as the latex
particles [14]. Two separate regions were found with R for the
lower values of N correlating according to the chain-liice clusters
3. Experiments with asbestos fibers (amosite and
crocidolite which both provide rod-like fibers) showed that the
aerodynamic diameter could be correlated with expressions of the
form [13]
Dae a al Dl (L/D)b
where D is the diameter of the fiber, a was found to be approxi-
mately 2 and the exponent b ranged between 0.1 and 0.2. It
should be noted that there was considerable scatter in the data
and that the L/D ratio used in this study was approximately 10.
In summary the spiral centrifuge can be used to separate non-
spherical particles, however the separation is strongly related
to the diameter and a weak function of the aspect ratio indicating
that other methods will probably be necessary in order to deter-
mine the length. It should be noted that the deposition length
in the spiral centrifuge is a calibrated quantity.
OPTICAL METHODS
The optical properties of fibers are not only effected by
the physical properties of the fiber but the angle of orientation
as well. Although the problem of back-scattering from an arbitrary
prolate spheroid has been solved for a conducting spheroid at
direct incidence [15], the problem for a general orientation has
not been. Further as pointed out in the previous section the
-------
407
orientation of the fibers are generally not known.
Several investigators have measured the fiber properties with
a Royco. Early experiments reported good agreement with micro-
scopic [16] examination for large fibers, but recent experiments
show that the distribution of the "apparent [17] optical diameter"
is significantly broader than the distribution of the diameter,
length, or aerodynamic diameter determined from measurements of
the individual particles. The location of the maximum for the
aerodynamic diameter and the "optical" diameter almost overlapped.
By modifying the Royco so that higher velocities could be
obtained Gentry and Spurny demonstrated an apparent narrowing of
the distribution. This effect was attributed to the particles
acquiring a less random orientation. Their experiments suggest
that forcing the particles to acquire a preferred orientation
may be the only method for measuring the particle distribution
by light scattering.
Three methods have been suggested for imposing a preferred
orientation on the particles. These methods are electrostatic
forces, magnetic fields, and by increasing the flow rate.
Generally speaking, all the basic problems - the rate unipolar
charging of the particles, the distribution of charges on the
particles, the mobility of particles in an electric field and
the charge distribution are unsolved. Preliminary measurements
of doublets of latex particles [18] in an electrical field show
an alignment with the particles aligned parallel to the field.
Qualitative experiments indicate that fibers tend to be more
heavily charged than spherical particles.
The use of magnetic field to align particles has been suggested
by Timbrell [19] as a means of particle separation. The method
has been tested in quiescent liquids. However, the use of the
method for particles in air has not been tested as yet. There
are several drawbacks in the proposed method which can be in-
dicated as follows:
1. The fibers may take orientations parallel or per-
pendicular to the field, including both orientations.
2. The preferred orientation can not be predicted in the
absence of experiment, and
-------
408
3. When fibers are cleaved along the major axis, the
preferred orientation may be opposite of the parent fiber.
It should be pointed out that for asbestos fibers no treatment
is necessary and that it has been shown that non-magnetic fibers
can be treated so that they can be aligned in the field.
There is little doubt that the presence of a small gradients
can induce alignment in fibers. The results are striking, as
demonstrated by the following set of photographs taken for a
series of nuclepore filters in series. The filters range through
the sizes 8.0y to 0.2y. The length and diameters of the fibers
were measured.
What is striking about this measurement is that the particles
deposited on the last stage penetrated a filter with openings of
0.6y diameter. Since the average dimension of the fibers was a
length of 2.3y and a diameter of 0.4y, only if the fibers were
aligned with the gas stream lines could they penetrate to the
backing filter.
Similar results have been found for a cascade impactor where
almost 80% (by mass) of the particles penetrate to the backing
filter although measurements of the aerodynamic diameter (cal-
culated from electron micrographs of the fibers) indicate most
of the particles should be deposited on the 4th or 5th stage.
SUMMARY
The characterization of asbestos fibers (or non-spherical
particles) provide a number of significant differences to the
characterization of spherical particles.
For efficiency measurements of spherical particles it is
necessary to measure only the relative intensities of mono-
disperse particles. For fibers there are no monodisperse particles,
and the amount of light scattered depends on orientation as well
as the length and diameter.
For spherical particles the distribution can commonly be
characterized by a log-normal distribution. For non-spherical
particles there are additional dimensions that must be accounted
for in the distribution. In addition average parameters such as
the optical diameter can not be characterized by a simple dis-
-------
409
tribution.
There, however, have been significant discoveries that indi-
cate the directions of the future. These include
(i) The development of a generator capable of giving
reproducible distributions of individual fibers,
(ii) The demonstration that only a small fraction of
fibers in ambient air are asbestos,
(iii) The demonstration that the tendency of fibers to
align with the gas stream lines in the presence of small grad-
ients is sufficiently great that inertial impactors are useless,
and
(iv) The proof that electrical, magnetic, or differences
in flow gradients can impose a preferred orientation on the
fibers.
REFERENCES
[1] Hallenbeck, W. H. and C. S. Hesse, Reviews on Environmental
Health, II, (1977).
[2] Selikoff, I., Environmental Health Perspect., £: 299-305
(1974).
[3] Borow, M. , A. Conston, L. Livornese, N. Schalat, Chest, 64_,
641-646 (1973).
[4] Hesse, C. S. and W. H. Hallenbeck, Atmospheric Environment,
(1977).
[5] Spurny, K. R., J. W. Gentry, W. Stober, "Sampling and Analysis
of Fibrous Aerosol Particles", Ed. D. Shaw (1977).
[6] Spurny, K. R., C. Boose, and D. Hochrainer, Staub-Reinhalt
Luft, 3^, 440 (1975).
[7] Kerker, M., in Assessment of Airborne Particles, ed. by T. E.
Mercer et al., Charles Thomas (1972).
[8] Fuchs, N. A., The Mechanics of Aerosols, Pergamon Press (1964)
-------
410
[9] Cans, R. , S. Ber. Bayer. Akad, Wiss.y 41, 191 (1911).
[10] Gentry, J. W., "Comments on a Paper by Gallily and Cohen",
to be published, J. Colloid and Interface Sci., (1977).
[11] Gallily, I. and A. Cohen, J. Colloid and Interface Sci., 56,
443 (1976). —
[12] Cans, R., Ann. Phys. (Leipzig), 86, 628-656 (1928).
[13] Stober, w., H. Flachsbart, D. Hochrainer, Staub, 30, 277,
(1970). —
[14] Kops, J. A. M., G. Dibbets, L. Hermans, J.F. v.d. Vate, J.
Aerosol Sci., £, 329 (1975).
[15] Schultz, F., "Scattering by a Prolate Spheroid", ERI, Univ.
of Michigan (1950) .
[16] Addingley, C. H., Ann. Occup. Hyg., j), 73 (1966).
[17] Gentry, J. and K. Spurny, "The Effect of Orientation on the
Optical and Collection Properties of Fibers", GAF
Jahreskongress, Bad Soden (1976).
[18] Boose, C. and D. Hochrainer, "Orientation of Elongated
Particles in an Electric Field", GAF Jahreskongress, Bad
Soden (1974).
[19] Timbrell, V., Ann. Occup. Hyg., 18, 299 (1975).
-------
411
OPEN DISCUSSION
Gentry: The reason I presented more problems than solutions, is because
there are more problems than solutions. Secondly, i. > 3 and should be 6.
D
Friedlander: At the bottom of page 9, about the use of magnetic fields, no
statement is made about what the magnetic and dielectric fields are.
Gentry: What has been suggested here is using the property that the
particles will fall at different angles as a method of separation. A series
of studies have been carried out that investigated this type of thing.
First by dropping particles without a magnetic field and then with a magnetic
field to align them. The particles were aligned in aqueous solutions which
solidified. We looked at the solidified mass and found that the particles
aligned either parallel or perpendicular to the field, depending on which
asbestos was used. It was not merely a matter of whether the magnetic field
was on or off. For example, by using permalite from Uganda and permalite
from Uganda and from South Africa, there were different preferential
orientations.
Oder: I have been involved in a lot of mineralogical studies with asbestos,
and generally speaking, asbestos is associated with iron ore processing. The
magnetic properties of asbestos are usually associated with the impurities
attached to the surface. When I looked at your slides, the thing that
struck me was that the crystals seemed to be very clean. Nothing was on
the surface at all.
Gentry: Let me point out that the asbestos, which is pure (at a commercial
grade) to start with, is not of the same magnitude of particle generation
as that which is usually dealt with. Our generator output is about 13
milligrams to .13 milligrams per cubic meter. This is probably why the
particles are so clean.
Oder: Since you have cleaned off the fibers, the fibers are not as they
would occur in nature, but as attached to magnetic particles. The magnetism
left over would probably be diamagnetism for fibers that would have very high
substitutional iron or paramagnetic depending on the iron composition.
Friedlander: I would go further than that and say that just by taking a
chemical analysis, you can not tell. An example is that the alpha and
gamma phases of the iron would be quite different. Or maybe for iron oxide,
one case would be parallel and the other ferromagnetic. So just knowing the
composition does not tell you the answer either.
Gentry: If you use chrysotile, it will cleave along the major axis. The
fibrils will then have a different magnetic orientation.
Friedlander: The paper says: "The presence of small gradients will produce
-------
412
alignment of fibers." What gradients are you talking about? Flow field?
Magnetic field?
Gentry: Gradients of flow field.
Billings: Isn't asbestos of several different types? Aren't some quartz
and some serpentine or magnesium?
Gentry: In our study, we defined three types of asbestos. They are
chrysotile, crocidolite and amosite. Other people have carried out studies
with tremolite and other asbestos types, but all basically consist of iron,
magnesium and silicon. Silicon dioxide is not considered asbestos.
Oder: Isn't there a dependence between biological activity and fiber length?
Gentry: It is speculated that way and studies have been done. It has
always been associated with longer particles. It is an interesting point
now that carcinogens are important. People have suggested that only particles
over 5 microns have any carcinogenic effect. The size dependence is deter-
mined by autopsy. If you look at a lung with a lesion, you will only find
particles of over 5 microns.
Billings: I would like to comment on your measurement technique. There is
a tendency for particles to bounce on the impactor stages and if you are
not using some type of grease, there is a high probability that these
particles just bounced all the way through. Once they bounce, they will go
all the way to the back-up filter, because you do not have an efficiency
going from zero to 100%. If goes from zero up to 90% then goes back down.
Gentry: I am fairly certain that for the most part there was a backing
filter of petroleum jelly on each stage. This was used as a substrate
for collecting particles and acts somewhat as an impactor.
The 90 to 95% of the particles that look like asbestos were predominately
calcium sulfate. The analysis of chemicals was obtained by x-ray phoresence.
Lbffler: Do the particles coming out of the fluidized bed carry any
charge? What is their behavior?
Gentry: They are charged. We put the particles through a decharger to
study them.
-------
413
HIGH TEMPERATURE FILTRATION - TECHNICAL PROSPECTS AND AS MEANS
FOR CHECK ON THEORIES.
Michel M. Benarie
Institut National de Recherche Chimique Appliquee,B.P. 1
Vert-le-Petit,France.
ABSTRACT
Adding temperature as a variable to the classic test parame-
ters ( particle size and face velocity ) demonstrated that the
flow field in fibrous filters.even at low fiber Reynold's number,
shows departures from laminarity.Two phenomena might be involved:
a. unsteady flow at capillary entrances and branchings and b. se-
condary flow.
These turbulence-like phenomena improve the filter performance
in the sense of lowering pressure drop and increasing efficiency.
It is surmised that increasing the turbulence within the mat
could result in improved industrial filters.
O.INTRODUCTION
Filtration through fibrous media at elevated temperatures is
a well established practice in the cement and other industries.
From the environmental point of view this process has the advan-
tage that buoyancy of the hot gases is preserved and provides
better dispersal.Nuclear technology also uses high temperature
filtration.Considering its actual and potential importance,it is
amazing that but so few papers were published about filtration at
higher temperatures : First et al.(1),Billings et al.(2),Spaite
et al.(3) .Thring and Strauss (4),Pich and Binek (5),Strauss and
Lancaster (6),Dyment (?).But when you get down to experimentation
and notice that tests which normally take half an hour need a
whole day or more at high temperature then the amazement stops.
Some of the difficulties encountered are: the nature of the aero-
sol and its dispersal; problems in attaining thermal equilibrium
within the apparatus; difficulties in attaining gas-tightness;
delay of cooling of the apparatus to enable the change of the
-------
414
filter in view of weighing...)•It goes without saying,that when
"high temperature" is not 50O°C for research purposes but just
10O°C for technical assessment as described in Sect. 3.2, the ex-
perimental obstacles become less forbidding.
Besides its technical importance, tests at elevated tempera-
tures allow deeper comprehension of the real behavior of fibrous
filters.Given the well known inaccuracies of the filter testing,
it is not surprising to see many a theory confirmed or at least
not being contradicted by experiments made in the three-dimensio-
nal efficiency-particle size-face velocity space.But with one
more dimension added,namely temperature,the challenge on the mo-
del-concept becomes harder.Dimensional analysis with respect to
temperature gives a hitherto unsuspected insight in the physics
of filtration.Discrepancies which seemed previously incidental,
become apparent and this way our understanding about what happens
might be improved.
Two aspects of the behavior of filters at elevated tempera-
tures are of concern here : their pressure drop and their effi-
cacity.The subject has been reviewed more or less recently in a
very comrehensive way by several authors : Chen (8),Rich (9),
Spurny (1O) and Davies (11).Thus we assume that the theory of
aerosol filtration by fibrous filters as well as the basic ter-
minology are well known and therefore no details will be given
here.
1.PRESSURE DROP AT ELEVATED TEMPERATURES
There are four lines of basic approach for the understanding
of the pressure drop of filters:
a.The channel theory, b.the drag theory, c.the fiber drag
and ineraction ( e.g. the Kuwabara(l2) flow field ) and d.the
geometrical theory of pore structure (13).
Even the hydrodynamically most elaborate of these theo-
ries as those of Happel (14), Fuchs and Stechkina (15) or Spiel-
man and Goren (16) and which at room temperature are experimen-
tally well verified,show one common feature : the pressure drop
is directly proporttnal to the dynamic viscosity of the carrier
gas.The reason is the more or less implicit use of Stokes1 lami-
marity criterion,i.e. that the streamlines have up and downstream
symmetry.
-------
415
Our experiments show against all theoretical expectation that
at variable temperature the pressure drop does not follow exactly
the increase of the gas viscosity: Figs 1 and 2 show that the
pressure drop of the media tested either in air or in argon in-
creases less steeply than does the dynamic viscosity.For all ex-
perimental detail,see Quetier (17).
11
1 III
-
(vnVM H 0 . pale..
'f ii Tn n^^' s ft^4o
fs
4* _
/$r taic.
^* /S+
Awow^r x X
j^r ^f "~~ —
s*t
to*/*
\ \ 1 i ~"
1&0
160
144
120
100
40
T 300° 400° 500° 600° K
60
g. (1 ) - Increase of the pres-
sure drop with tempe- SO
rature of a sintered
I 111
_ _
calc.
"T 6 Kfa , ""•
1^.U A *+^
*W^Tfn n^U ^^x
— xx^ —
^
x
_ ^^^ _
If.** i
Vs 10 CW/S
calc.
x/e*P--
x^X
x^*
>^ _
*'
>X/*5c-m/t -
»• ' I / [
glass filter,average
pore diameter 40 - 9O
micron,at constant
face velocity,in air
and argon.
T 300°
400° 500° 600«K
Fig. (2) - Increase of the
pressure drop with
temperature of a sin-
tered glass filter
of average pore dia~
meter of 2O - 4O mic-
ron,as a function of
face velocity.
-------
416
Two different mechanisms are susceptible to be involved in
such an effect:
a. For short tubes the pressure drop is increased by a kine-
tic energy term caused by the end effect involved in establishing
the steady laminar flow:
+ a?T (2)
While the first term of Eqs.(l) or (2) indicates the linear
relationship between A^and ny as already mentioned, the second
term yields Af prop T~ because of o /T = const.
b. An other mechanism which could be made responsible for
the pressure drop deficiency effect is the secondary flow arising
in curved pipes. From White's (18) experiments it may be computed
that pressure drop caused by secondary flow alone would increase
1/4
approximately as T .
Although we lack experimental evidence to decide if one or
the other, or both mechanisms concurrently might be involved, an
estimation could be tempted to assess their relative importance.
Gaseous viscosity may fairly well approximated by the formu-
la
where b is a constant generally comprised between O.6 and 1.5.
Eq. (3) is valid over the range from 2O°C to 4OO°C with an error
not beyond 2%. The value of b e.g. for air is O.768. For ex-
perimental checks ( e.g. as in the Figs 1 and 2 ) we always use
the exact experimental values of gas viscosity. For the purpose
of dimensional analysis alone we assumed that the viscosity of
air increases with the 3/4 power of temperature.
Considering that e.g. Figs. 1 and 2 show at 6OO°K about 6%
less pressure drop than required by the pure viscosity term alone,
the solution of the following equations ( 4a and 4b) may yield
x, the fraction of energy dissipated by one or the other of the
mechanisms.
-------
417
" • WO \ .^_ j _ f-^ i ^p* t "XI ' * I \ T~ f . / k
1/4 (4b)
The exponent y has the value of -1 for the end effect
mechanism and 1/4 for the secondary flow mechanism. As for 60O°K,
T/To«2, from 4a and 4b one obtains about 57% in the first and
63% in the second case. Without any pretention to accuracy,one
may state that both phenomena seem to play a rather important
role in the pressure drop of fibrous media.They may accessorily
explain.why so often the deposition on the lee side of the fibers
can take place — for which laminar-flow theories offer no ready
explanation.
Empirical pressure drop relationships give obviously a nearer
fit to experimental data.Whitby (19) introduced an empirical func-
tion for the dependency of the drag coefficient on the Reynolds'
number.Kimura and linoya (2o) fitted the following expression to
a great number of experimental data:
with
The temperature dependency of the pressure drop in the Kimura-
linoya equation is:
* = A + Bf1/8 + CT3/4 (6)
In order to fit experimental facts, an empirical term wi
steep variation than T ' had to be introduced. Eq. (6)
with
less steep variation than T ' had to be ntroduce. q. (6) fits
also quite well our experiments at all temperatures and two dif-
ferent test gases ( air and hydrogen) .Fig. 3 shows the example
of a carbon fiber mat tested in air.
The exponent in the Kimura-Iinoya equation allows a further
estimate : all over the range 1 $ T/TQ 4 2 , the function
(T/T0)~1'8 may well bfe approximated by (T/TO) --HT/TO)~ .
This could be taken as an indication that about one third of the
energy dissipation by non-laminar mechanisms could be attributed
to secondary flow and the remaining two thirds by the unsteadi-
ness at capillary entrances or ramifications.
-------
418
2500 -
2000 -
1500
1000
Fig. (3) - Experimental fit obtained by
Eq.(?) in the case of carbon
fiber mats of variable thick-
ness,tested with air.
-------
419
2. A RAPID THEORETICAL REVIEW OF THE DEPENDENCY OF FILTER
EFFICIENCY ON TEMPERATURE.
This is a short brush-up of ideas already developed in the
quoted basic references.
The collection efficiency E of a filter mat is given by
the rather well known expression ("log-penetration law") :
(7)
where it is assumed that ZE , the collection efficiency of a
single fiber is obtained by adding up single fiber efficiencies
due to separate mechanisms :
ZE = Ed + Er + E. ... (8)
E = single fiber efficiency by diffusion
E = single fiber efficiency by direct interception
E. = single fiber efficiency by inertial impaction.
We do not take into consideration electrical forces because
the main experiments were made with earthed semiconducting carbon
fiber mats. Deposition by gravitational forces was neglected as
not significant.
Single fiber efficiency by diffusion was calculated by many
authors on basis of slighltly different assumptions , but all the
results may be brought to the form :
Ed = B ( r)X (9)
with x either -1/2 or -2/3 or -1. Anyway, E as a function of
a positive power of T , must increase in the same sense .Follow-
ing Eq,(7),the overall efficiency must grow in the same direction.
As for E. .Whitby's (19) approximation represents fairly
well all previous experimental results :
EA = exp [-(log St)2] (10)
where St, the Stokes' number or inertial parameter is defined
by:
-------
420
St -.-?**0" *4
15/r? df
When the temperature increases , E. decreases, provided that
St<1. Thus by Eq.(?) the overall efficiency varies also in the
same direction.
According to well established concepts of the majority of
the authors quoted, E is independent or only very slightly de-
pendent on the temperature.
The experimental and technical interest of filter tests at
variable temperature follows from these basic theoretical consi-
derations.In order to improve a filter systematically,a model
must fit its real behavior.Theory allows to compute individual
fiber efficiencies,but unfortunately ( or fortunately ) we do
not have just one or two theories,but rather a dozen able to yield
quite divergent numerical results.Tests at variable temperature
provide a ready means to get insight in the prevailing mechanism.
To assess fiber efficiencies experimentally, the tests have
to be run at constant temperature with particles of uniform size
classes.This v/as done and published by about a score of authors.
Before the automatic particle spectrum measurements were known,
to gain insight in the filtration mechanism at constant tempera-
ture of a few filter mats was already a complex research project.
Because of the time,expenses and care involved.it was out of ques-
tion to study filtration mechanism of industrial filters this
way.
As it will be shown below,tests executed at variable tempe-
ratures and not necessarily over a very wide range,represent an
easy means to get insight into the filtration mechanism even with
mechanically dispersed particles of non-uniform size.The tests
thus become easier and more rapid.
3. FILTER EFFICIENCY AND TEMPERATURE: EXPERIMENTAL.
3.1. Particle size dependency of the single fiber efficiency.
The filter mats tested were of carbon fibers, df = ,
£= O.969 and L = 3.9 mm .The test aerosol was obtained by atomi-
zing and drying of a NaCl solution; the filtration velocity in
all tests was 1OO cm s~ .The particle counting and sizing was
-------
421
done by an automatic "scintillation" ( flame photometric ) ana-
lyzer manufactured by Sartorius ( Gb'ttingen ) and developed by
Binek (21).The apparent fiber efficiencies were computed from
the experimentally measured mat efficiency by Eq.(7).
The results obtained at 2O°C, at 1OO°C ( = 393°K) and at 2OO°C
{ = 493°K) are represented in the Figs.4 ,5 and 6, which also
show ( dashes ) the theoretical single fiber efficiencies calcu-
lated by one of the most sophisticated theories : stepwise cal-
culation of particle trajectories following Dawson (22) using
the Spielman-Goren (16) flow filed for interception and inertia;
the diffusion term after Stechkina and Fuchs (23) as exposed in
detail by Davies (11) p. 74 ff.
ncT
\Theor«ttc.al
\
bmtflt
Fig. (4) - Single fiber efficiencies as function
of the particle size,carbon fiber mat at
2O°C;face velocity 1OO cm s~ .
-------
422
Fig.(5)
Fig.(6)
L*>I a.f
\ K
\
\
200°C
Single fiber efficiencies as a function of the particle size;
carbon fiber mat,face velocity 1OO cm s~ .
-------
423
Looking, as usual, only at the results obtained at room tem-
perature ,Fig .4 , theory and experiment are in rather good fit.Both
curves have the same overall trend and the fact that there is
no exact coincidence should not bother us too much, as some of
the constants of the theories are still empirical and depend on
fiber orientation and other factors.
Up to this point,the theory seems adequately confirmed.The
situation changes when looking at the results obtained at higher
temperatures.Figs. 5 and 6.The'minimum requested by the theory
fails to show up.At first sight,we would be inclined not to trust
the experiment as it often happens.But another representation
of the same results ( Fig. 7) does not allow this easy explanation.
Up to the particle diameter of O.13/tm, one would assert that the
theory is well confirmed because neither computation,nor the ex-
periment can have any claim to very high precision.Conjecturing
a somewhat different flow fi,eld and computational formula, theore-
tical single fiber efficiencies differing by a factor of five are
often obtained ( see Davies.l.c. p.64).Due to this reason,we may
1
-10
a
u
-J
V
V
_Q
.,*
*
vn
8
c
a
.0
--
- '
-
L0« V
«r
•--
»' i
•m
FKtei*.
00' 1
.07
--
:
<>•«
L--
«• 2
n-.~. s
flo* 3^
.15
0' 10
^.•m
<>• 2
ThMv.
00* 31
._.
0» 40
Dr-
,__
o* a<
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90* a
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xf \
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;
»• 2
)
00- I
rs^
)
i
~~
0' It
*m
3
:
--
»• 2
-
TH.OV.
-
-
00*
Fig.(7) - Single fiber efficiencies with particle size and tem-
perature as parameter;carbon fiber mat,face velocity
1OO cm s~ .
-------
424
disregard on the first sight that from 0.2 yum diameter on, the
experimental fiber efficiencies are two to five times higher
than the theoretical ones. More significant is the trend : at
0.2 and 0.4 yum,there appears a clear rise in the experimental
efficiency, unpredicted by the theory. At 0.75yu.m,the computed
fiber efficiency shows already the decrease requested by the
inertial mechanism,while the experimental results show a clear
minimum for the intermediate temperature.
These findings, considered in conjunction with our former
hypothesis that the pressure drop cannot be fully accounted for
by the laminarity concept, seem to indicate that the flow field
at the basis of the computations does not adequately take into
account non-laminar conditions which seem to exist within the
filter.
Extrapolating from this point on, we might presume that a
filter in which turbulence is prevalent, will show less pressure
drop and a better efficiency than a purely laminar one.Therefore,
in order to develop new concepts in fibrous filtration.unsteady,
turbulent or secondary-flow conditions should be1 sought.
3.2. Tests with polysdispersed particles.
The filters employed in these tests were the sintered glass
ones already mentioned and described in the captions of Figs. 1
and 2. The test aerosol was a mechanical dispersion of silica
dust, approximately log-normally distributed between 1 and 15m
with the mass-median diameter of 1.5 m and a geometric standard
deviation of 4 .
The X signs in Fig. 8 show a case of decrease of the overall
efficiency with temperature ( slope approximately -3/4 ) .consequent-
ly the prevailing effect of inertial impaction.The signs 0 per-
tain to a "filter" made up of a paralell bundle of glass fibers,
all arranged somewhat like a brush aligned in the sense of the
flow.In that way,impaction was kept at minimum and the particles
could be stopped only by diffusion. As expected.the overall ef-
ficiency is rising with a slope but slightly different from 7/8.
The signs + in Fig. 8 show a case where both mechanisms
seem to be involved,with a minimum of the efficiency near 4OO°K.
-------
425
90
3-
'°
(U70
"u
it 6o
9)
s0
o
a*
40
30
i
i
300° 400° 500° 600° K
Fig. (8) - Tests with polydispersed silica dust,
-------
426
LIST OF SYMBOLS.
d, fiber diameter
d particle diameter
D capillary diameter
Ed single fiber efficiency by diffusion
E. single fiber efficiency by inertial impaction
Er single fiber efficiency by direct interception
ET overall collection efficiency of the filter
g acceleration of the gravity
L thickness of the filter mat
Ap pressure drop
Q volume flow
St Stokes' number ( see Eq.1O)
T absolute temperature
v ( flow ) velocity
VQ face velocity ( velocity of the upstream flow )
£ porosity of the filter mat
^ density ( gaseous...)
^ density of the particle
rn dynamic gas viscosity
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REFERENCES
(1) First, M.W..Graham,J.B..Butler,G,M..Walworth,C.B. and Warren,
R.P.,Ind. and Engr.Chem.,Vol.48,p. 696, 1956.
(2) Billings, C.E..Silverman, L.,Dennis,R. and Levenbaum.L.H.,
J. Air Poll. Contr. Assoc. Vol.5, p.159, 1955 ; Vol.8, p.53,
1958 ; Vol.10, p.318, 196O.
(3) Spaite.W..Stephen,D.G. and Rose,A.H.,J. Air Poll. Contr. Assoc.
Vol.10, p. 215, 196O.
(4) Thring.M.W. and Strauss W..Trans.Inst.Chem.Engr.,Vol.41 , p.
519, 1963.
(5) Pich.J. and Binek,B.,"Aerosols,Physical Chemistry and Appli-
cation" ,Proc. 1st Nat. Conf. on Aerosols.Czehoslovak Akad.Sci.
Prague pp. 257-264, 1965.
(6) Strauss.W. and Lagcaster,B.W..Atm.Env.,Vol.2, p.135, 1968.
(?) Dyment.J.."Assessment of Air Filters at Elevated Temperatures
and Pressures" - Filtration Society's Conference .London-, Sept.
23-25, 1969.
(8) Chen,C.Y.."Filtration of Aerosols by Fibrous Media".Chem.Rev.
Vol. 55, p. 595, 1955.
(9) Pich, J.,"Theory of Aerosol Filtration by Fibrous and Membrane
Filters".Aerosol Science,C.N. Davies,ed..Academic Press,Lon-
don , pp. 223-285, 1966.
(OO) Spumy, K. ."Grundlagen der Filtrationstheorie in aerodisper-
sen Systemen und die realen Ultrafilter".Fortschr.-Ber.VDI-Z.
Ser. 3 Nr. 17, 1967.
(11) Davies, C.N., "Air Filtration".Academic Press,London-New York,
1973.
(12) Kuwabara, S.,"The Forces Experienced by Randomly Distributed
paralell Circular Cylinders or Spheres in Viscous Flow at
Small Reynolds Numbers",J.Phys. Soc. Japan Vol.14,p. 527,1959.
(13) Piekar, H.W. and Clarenburg, L.,"Aerosol filters - the tor-
tuosity factor in fibrous filters".Chem.Eng.Sci.Vol.22,p.765,
1967.
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(14) Happel, J. "Viscous flow relative to arrays of cylinders",
Am. Inst. Chem. Engr. J.,Vol. 5, p. 174, 1959.
(15) Fuchs, N.A. and Stechkina, I.B. "A note on the theory of
fibrous aerosol filters", Ann. Occup. Hyg. Vol. 6, p.27,1963.
(16) Spielman, L. and Goren, S.L. " Model for pressure drop and
filtration efficiency in fibrous media", Env. Sci. & Techn.,
Vol. 2, p. 279, 1968.
(17) Quetier, J.P. " Etude experimental des effets de la tem-
perature sur les mecanismes de la filtrationVThesis,Paris
1977.
(18) White, C.M.,Proc. Roy. Soc. Vol. A 123, 1929.
(19) Whitby, K.T. " Calculation of the clean fractional effici-
ency of low media density filters" Am. Soc. Heating,Refrig.
Air Cond. Engrs. J., p. 56, Sept. 1965.
(2O) Kimura, W. and lionya, Chem. Eng.(Tokyo),Vol. 23, p.792,1959.
(21) Binek.B. "Szintillationsspektrometer-Analysator fur Aerosol-
teilchen",Staub Vol.2O, p.184, 1960.
(22) Dawson, S.V. "Theory of collection of airborne particles
by fibrous filters" Sc.D. Thesis,Harvard School of Public
Health, 1969.
(23) Stechkina, I.B. and Fuchs, N.A. "Studies on fibrous aerosol
filters.I.Calculation of diffusional deposition of aerosols
in fibrous filters," Ann. Occup. Hyg. Vol.9, p. 59, 1966.
WRITTEN DISCUSSION
K.T. Yang, Chairman
Department of Aerospace and Mechanical Engineering
University of Notre Dame
Notre Dame, Indiana 46556
Dr. Benarie should be congratulated for a most stimulating talk on a very
timely subject. His qualitative analysis on the temperature effects on both
the pressure drop and collection efficiency should be very useful in inter-
preting experimental data for high temperature filtration.
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My comments are primarily directed toward the following issue: Do the
same physical mechanisms at low temperature for the pressure drop and col-
lection efficiency behavior operate also at high temperature? If so, a
variable property analysis is all that is necessary to predict the behaviors
at high temperatures, provided that the physical mechanisms can be properly
modeled at low temperatures. Professor Benarie seems to think that this is not
the case,and other effects,such as capillary entrances, secondary flows and
the like are operative even at moderately high temperatures. However, I would
like to suggest that uncertainties in low temperature behaviors in pressure
drop and collection efficiency far out-weighs the uncertainties in the variable
property analysis to predict the high temperature behaviors. In other words,
if we had an adequate quantitative theory for low temperature filtration
through fibrous media, the variable property analysis may be quite adequate for
high temperatures, with possible exception at very high temperatures.
One evidence of my speculation is the correlation of the pressure drop data
shown in Fig. 3 of Dr. Benarie's paper, which, as I undertsand, covers a wide
range of temperature and is based on a variable property analysis on viscosity
and density based on the Kimura-Iinoya equation for the pressure drop at room
temperatures.
A second evidence can be observed in Figs. 4,5 and 6 in Dr. Benarie's paper,
which show the experimental fiber efficiencies over the size range at three
different temperatures and one face velocity. These are the solid curves in
the figures. Also shown are the best theoretical predictions at low tempera-
tures, modified with variable properties at high temperatures, and these are
the dotted curved in the figures. If we had, a good quantitative theory for
single fiber collection efficiency at room temperatures, then such a theory
would give up a prediction close to the solid curve instead of the dotted
curve in Fig. 4. Now suppose that we use this curve as a base and extend it
by a variable property analysis to 100°C, we would obtain a curve in Fig. 5
which would fit the experimental data even better than the experimental best-
fit curve. Furthermore, this curve would also exhibit a minimum around O.OSum.
If the same procedure is extended to 200°C, the case of Fig. 6, the resulting
curve would be much closer to the experimental data than the theoretical curve
indicated, even though in this case, there is a consistent deviation from the
experimental data points, which seems to indicate that additional physical
mechanisms are indeed at play at this temperature level.
Based on these observations, it certainly appears that more studies, either
analytical or empirical, are needed to provide a better predictive theory for
low temperature filtration before any meaningful predictions can be made at
elevated temperatures.
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OPEN DISCUSSION
Benarie: My point was that these experiments should shed some light on
the theories of the dependence, of filtration on temperature. They were
analyzed in three dimensions rather than two. I also pointed out that my
suspicion is that some turbulence of significant duration is important.
None of the actual filtration theories takes into consideration these facets
of filtration. The next step should be to introduce these mechanisms, into
filtration techniques in general, then we shall see a temperature dependence.
Billings: It seems to me that there is a fair amount of confusion among
people in filtration about what happens in filtration. First of all, the
phenomena themselves are not very well understood. They are not very well
documented and not very well analyzed. The first thing desired is a
phenomonological model. Before you can have a phenomonological model, though,
you have to have observations. What we observe is macroscopic. We go in
and measure the concentration coming into a black box. Then we measure
the concentration coining out of the black box. We get back to a theory on
individual fibers and individual fiber efficiency.
If I then ask one of the noted professors to explain this, they would
sit down and start with a continuum theory, assume a potential flow, and
imagine a streamline with a particle on it, and a fiber in the path of the
particle. By doing a trajectory calculation, they can figure out where it
intercepts the fiber. A lot of assumptions were made up to this point,
the biggest being the matter of the particle-fiber interaction. If you
look carefully at the underlying assumptions that have gotten us to this
point, you will see that there are no direct measurements, by eye,of the
interaction between the particle and the fiber. It all goes back to
Dr. Friedlander's experiments in liquids - that is not a gas-solid case, it is
a liquid-solid case. As you well know, the forces that come into play in
liquids between two surfaces are orders of magnitude different from the
forces that come into play in gas-solid interaction.
From our phenomonological model we can derive an analytic model. Given
the analytical model, what does one do? You say that the particles will be
stopped by the obstacle. Now we have a fiber efficiency and can build an
analytic model. The first assumption is the agglomeration coefficient. I
have given this approach to the problem many times before. It is a macroscopic
basis that is dealt with. But it must be approached on a microscopic basis.
The third one is a molecular basis. What you have to do is ask how effective
that interaction is and what is the influence of temperature on that inter-
action. The same would be true of the viscosity. Instead of using the
horizontal approximation, we have to look at several vertical coefficients
on the equation of state at the molecular level.
Benarie: By past experiments it has been shown that filtration theories
are not the crux of the problem. Perhaps all we have to do is introduce
some new concepts. We ignore what was previously introduced.
Liu: Most of the theories developed are for a fiber of uniform size. If
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you want to translate the single fiber efficiency to a multi-fiber filter,
you assume the filter to be composed of uniform fibers. In the real filters,
the fibers are not uniform. So if you try to compare theory with experiment,
you must try to find a particle size that will give you a good fit to the
data based on the pressure drop. A different particle size may be required
to fit the collection efficiency data. The question to answer is: How do
you determine the correct particle size to use? That determines how much
you can respect your theory.
Benarie: In this case, it was a very uniform fiber size. It was determined
that it was almost one size. I would like to add that,in the theory, it
has been found that a number of different fibers sizes are acceptable.
Friedlander: It is interesting to point out, even though we are using
liquid systems, that we are not only having problems correlating the types
of existing models in the unsteady flow or turbulent flow region, but
there is also a problem at the downstream end, especially at the lower
velocities. So there seems to be problems at both ends of the model.
Lbffler: I am afraid I did not explain clearly enough my approach. My
approach is not to consider the theory but to go into the details to analyze
the fundamental phenomena.
One comment to this problem of Dr. Benarie: I can assume that in his
experiments, the difference between experimental and theoretical values
should be an effect of electrostatic forces. This is because you used
sodium chloride particles produced by atomizing. All these particles have
charges. The tendency of this electrostatic effect of increasing 10 or 20%
is minimal. This would mean that increasing the viscosity would create an
effective decrease.
Benarie: Therefore we do first the electro-conducting filter. Your explanation
does not correlate with the pressure drop. I consider the pressure drop
easier to measure and more indicative of turbulence than the deficiencies that
are being criticized,because electrostatic effects, adhesion and impurities
can be changed by temperature. I need not grade my arguments,but if I woud, I
would give myself a 9 out of 10 for keeping pressure drop and coefficients,
Oder: I think Dr. Benarie gets a W for candor.
Cooper: One thing that strikes me is that the experimental results are
substantially higher than the theoretical, above a few tenths of a micron.
Which would probably suggest that we are not having accommodation or
rebound on the fiber. One of the things I wanted to know was what would be
the Reynolds numbers of the fibers in the flow. Could we expect a wake?
Benarie: Do not be surprised if the results are 2 or 5 times different from
the theory. Because if you take the models ,97 of them,
and compared the different flow fields within the same theoriesj discrepancies
among theories are present. So a person chooses the theories they desire to
make the best fit.
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The Reynolds number will be discussed in the panel discussion. If you
have a tube, what is the Reynolds number; the thickness of the wall or the
tube diameter? Why is the fiber Reynolds number so important? There is not
a great difference. With a porosity of 90% there is a factor of about 2
we have to deal with. The right Reynolds number is around unity. Alone, the
Reynolds number can not explain the turbulent development. But we can cover
that in the panel discussion.
Billings: My point is that you are assuming that the Sutherland approximation
to a Kuwabara approximation to the flow field is not going to lead to a
set of problems. I think we need to look at the accommodation coefficient for
the molecular material on the solid. It is at the molecular level that the
effects come in. The viscosity arises because of the accommodation coefficient
of the solid for the gas. That is a non-stick assumption on the solid which
is probably not true. I think you have to look for temperature dependence
on the equation of state to look for the change in the viscosity.
Benarie: We analyzed the viscosity effect and about six different possible
effects. The non-slip coefficients explain only about 1% of the observed
effects.
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PANEL DISCUSSION
Subject: "New Directions in Particulate-Gas Separation Research"
Moderator: Benjamin Y. H. Liu
Panel Members: Teoman Ariman, Michael M. Benarie, Dennis C. Drehmel,
Robin R. Oder and David S. Scott.
B. Liu: First of all I would like to congratulate Dr. Ariman for the excellent
job in the organization and guidance of this workshop. The purpose of the panel
discussion is to look at the new directions in particulate-gas separation research.
I think the panel is well represented with experts in the various research areas.
It will be a ten minute presentation by each member of the panel. Then a
general discussion with the participation of everyone will be held.
I would like to begin the discussion by giving a few of my general comments,
informal questions, and suggestions and finally some of my opinions.
First of all with regard to magnetic separation, I really don't know much
about magnetic separation but one thing that has bothered me is that no one in
this conference has dealt with the problem of energy needed in order to grind a
particle down to the size to separate out the mineral impurities and the sulfur.
So I would very much like to see Dr. Oder address the question of the energy
that you have to put in the system to grind a particle to two or three microns
in order to separate out impurities. With regard to electrostatic separation I
would like to say that, if my memory serves me right, a few years ago electro-
statics was the whipping boy for the blame of the experimentalist whenever he
could not make an experiment agree with the theory in aerosol studies. Usually
the first thing you point out is electrostatics. So I am very happy to see that
now at least we are putting electrostatics to good use and I think the electro-
static is no longer a black magic as it once was. It has begun really to become
a science. People are beginning to understand and study it and know methods of
measuring the charge and neutralize the charge and handle the electrostatic
problem much better than they did before. So,I am very happy to see that ad-
vances have been made in the electrostatic area.
Now for the case of the acoustic interaction I think Dr. Shaw had pointed
out clearly that the problem seems to be that we don't have a good handle on the
theoretical coagulation coefficient for particles in the acoustic field. We
know that it is very expensive to perform full-scale testing of these ideas. There-
fore, sound theoretical understanding of the basic phenomena involved is very,
very important.
So at this point I would like to put out a pitch to EPA and NSF for the
importance of the fundamental research. I think the reason that we have to devote
attention to the basic research is that by understanding fundamental phenomena
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at a microscopic level we can avoid the very expensive and costly mistake that
we could make by using a larger scale apparatus. So I am very glad to see
that the NSF and EPA are joining hands in sponsoring this workshop. Obviously
EPA has its own mission to meet so therefore they cannot sponsor too much of
the fundamental research whereas NSF certainly is in the business of support-
ing basic research. I would like to see in the future that more fundamental
university research programs be supported because it is very important in the
long run. Furthermore the fundamental study may turn out to be also the most
economical way of solving some of these problems.
So these are some of my very brief comments. At this point I would like
to turn to Dr. Benarie.
Benarie: I begin this discussion with an answer to a question. You see quite
clearly that my purpose of presenting some research in three-dimensional channel
filtration was to rock the boat a little bit. I continue rocking the boat with
the following.
One of the most important theories of pressure drop is the channel theory.
We do not have a channel theory of efficiency; we speak only about fibers. I
would like to draw attention to the fact that there are holes between the fibers.
Because the air passes through the holes, the particles are captured by the
fibers. I find that by drawing just the holes in front of the channels, and,
instead of looking at the fibers, by looking at the channels of air coming in,
you immediately have turbulent agglomeration. We have no. where taken into
account the turbulent agglomeration within the filter.
Next, we have here the effect of secondary flow, because it disturbs us,
we don't take it into account. Thirdly, at all beginnings and ends of channels
we have an end effect which is turbulent-like; this also is not taken into
account.
I suggest that what we need theoretically is a channel theory of sections.
My second suggestion is that we try filters with different shaped fibers to try
to enhance in-filter turbulence and take a step forward in meaningful turbulent
filtration by mass. Currently we do most filtration through fibrous media
which has not straight but curved sides. More advanced technology would allow
us to make any micro-fiber shape. A turbulent pressure drop is less energy-
wasting than a laminar pressure drop. Perhaps we can get more efficient and
more energy conservative fiber shapes. A number of fiber capsules with rough
and smooth fibers are available to us for full testing. So exterior fiber
geometry still has the potential to bring a renaissance to this old art of
fibrous filtration.
Drehmel: I guess like Dr. Billings I have to start with the microscopic level
and work backward. We of course at EPA are mission oriented. We have to pro-
vide demonstrated technology to control particulate emission from stationary
sources. And unlike some of the other goals that we might pursue such as
understanding the theory better we quite often get ourselves into the position
of knowing a piece of hardware works and then asking ourselves, what happened.
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In order for us to make some sense of what is happening with the equipment
and also to understand how it will fit into the total picture of particulate
control equipment,we do get involved with theory. But our goal is to provide
technology. So let us spend my few minutes looking at where problems are, and
some of the solutions we have taken so far.
So far we have three types of technology that are available: filtration,
scrubbing, and electrostatic precipitators. We know that under certain circum-
stances at least an efficiency of 99.9% exits. We have heard people mention today
that filtration efficiency was 99.99%. Collecting the particles isn't necessarily
the problem. It is the circumstances under which you have to catch these
particles that give us some difficulty. The precipitators have problems with
resistivities that are too high or that are too low, the pressure drop in
scrubbers is too high and the filters are too big. And so research to this
point has taken the attack one of trying to improve standard technology and
also to go out and find new types of technology that could be brought to bear.
The types of research we have looked at are pre-charge chambers or 2 stage pre-
cipitators in order to get around resistivity problems. We looked at condition-
ing with agents such as SO, or a sodium carbonate. We have looked at baghouse
problems where we have size limitation. We are looking at high velocity types
of filtration where we go to air cloth ratios of 3 to 10. We like to look at
an air cloth ratio of 1000. We also would like to look at media that were not
limited to less hostile environments. We would like to look at media that
could withstand high temperatures, we would like to go to 800°C, not just a
couple hundred degrees. We would also like to be able to deal with environments
that have acid gases, sulpheric acid, HF, we have found a great deal of work
on that.
Since the point has been brought up about filter geometry, we have funded
research looking at smooth fibers vs. trilobal* fibers. And we found in fact
that the trilobal fibers worked much better. For scrubbers to get around the
pressure drop problem, we have looked at the device called the flux force con-
densation scrubber. We bring to bear condensation on the particulate which
helps remove particulate both by growing it and also by bringing to bear
diffusional and thermal mechanisms.
So this is where some of our research has been oriented on conventional
devices and trying to take advantage of new mechanisms. We have funded research
in several areas, we have looked at sonics and spent some money on magnetic,
but primarily we have looked at the combination of electrostatic and other
forces. And for example we had funded some research at MIT to look at systems
with charged droplets and particulates. That is where the time constant theories
of Dr. Melcher came from. We asked Doug Cooper when he was still at GCA to
look at evaluating electrostatically augmenting devices. And you have also
heard about these other projects with Battelle Northwest and TRW. We have also
talked to George Fielding to do some work with us. We are currently funding
Mike Pilat to look at his charged droplet scrubber. We felt that electrostatics
provides a very effective force in the control technology of removing fine
particles.
As to where we have research needs in the future, rather than talking
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about some of the theoretical research needs (I am sure it will be brought up
more by the other members of the panel) I would like to take a different attack
and say that primarily we have talked about particulate-gas separation from a
conventional point of view. We have talked about them in terms of a stationary
source with temperatures or pressures that we are used to looking at. We find
that the suspended particulate, where we have problems in not meeting particulate
standards, is primarily due to sources that are not enclosed sources. They are
what we would call either fugitive emissions or fugitive dust emissions.
Fugitive emissions would be the type of thing where a source is not completely
contained, not properly hooded. Because of the process we have had some
difficulty with an open electric hearth, for instance, where we have some
emission around the site or sintering machines which are not completely enclosed.
We would also have fugitive dust emissions where dust has been deposited from an
industrial process which is now a roadway in a plant or it is part of an open
mine face, or it is where work is being done in an ore deposit. Even more
troublesome are the types of things where we have asbestos from brake linings
or we have particulate from the use of tires. We find that all these things
contribute to this suspended particulate and we find a
this area. We need to have research oriented to the control of particulate
from non-enclosed sources. Also we just heard that we are going to use more
coal. One of the ways we go about using more coal is to gasify it or burn in a
fluidized bed combustor. In either case you can make these processes much more
efficient if you can use a gas turbine. A gas turbine operates at high pressure
and it would be necessary to remove the particulate in advance to the gas turbine.
We are talking about particulate control at 1500°F (800°C) and 10 atmosphere
pressure. Again we suddenly find that there is no technology available. We
have various types of cyclones that can be used or have been talked about. They
do not provide control technology below 1 micron. In order to provide particulate
control for both the turbine and environmental protection we will need research
in this area. So I would see that-much of the applied research for the near
future will be in line with conventional devices. We will work with conventional
devices to make them solve the problems but innovative research has a large
application to areas where we have no technology; to the high temperature, high
pressure control problems and to the fugitive dust emission area.
Oder: Coming from an engineering firm, I came prepared to address many of the
topics already treated by Dennis. So now that he has given his presentation,
I am free to come back to magnetic separation and other areas.
Before doing that, however, I would encourage the funding agencies to
fund scientists and engineers so as to bring these people into the heart of the
problem together. The scientist, from the engineer perspective, is apt to be
paranoid and to think that the engineer is limited in his vision and is a
negative influence. The scientist is concerned that the program may be stopped
at an early stage because of peripheral constraint that the scientist is not
even interested in. I think that this viewpoint is wrong. I also think that
overconcern with engineering economics can be a negative influence. So I would
like to see a program that studies feasibility from a creative perspective at
a very early stage in any developmental program and magnetic separation is one
area that certainly could use that kind of insight.
Now let me turn to magnetic separation. Gas-particulate separation in
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this area is new. Basically, the only experimental evidence has been gathered
by Drehmel and Gooding. So other than their work, there is conjecture and
a great deal of that. The technology of HGMS that they are studying has been
successfully developed commerically for wet mineral beneficiation. However,
in the area of dry separations it is in its infancy.
Now I want to also remind you that the magnetic technology is severely
limited in its applicability. It doesn't have broad general applicability. It
is usable only in systems which can be magnetized. The stronger the magnetic
material the more suited it is to magnetic separation. Now with that idea in
mind I would also suggest to you that when looking for applications of the
technology in gas-particulate separations you must address the problems
associated with high velocity processing. In order to be able to afford HGMS
technology high material throughputs are required. So whether it be in areas
of removing mineral matter from coal or whether it be removal of suspended
particulate, such as in the case for the steel making furnace off gases, you
need to have large capacity systems to even think of the technology in terms
of air-particulate separation.
There is a plus going for magnetic separaston which hasn't been mentioned
here at all. In those areas where the materials are strongly magnetic and
where magnetism is iron based, the technology can be employed, in principle,
up to 1400°F, which is the cure temperature of iron. So in principle you are
talking about potential applications up to that temperature. So if one has
applications where magnetic forces will work, he is not limited to low temperature
applications only.
Now with regard to the question about the cost for pulverizing; first of
all, I did not want the emphasis of my talk to be on coal cleaning. Rather
I wanted coal cleaning to serve as an example of an area where one could
potentially make strongly magnetic materials so that an application of magnetic
methods of gas-particle separation similar to the steel furnace application
could be studied. But in that particular context, the costs breakdown the
following way: pulverizing including crushing is about $2 or $2.40 a ton based
upon a conservative design. Coal separation might be done for less. The capital
cost for the desulfurization circuit considered here including all reactors and
including the magnetic separator, is about $105 per ton per hour of clean coal
product. Operating costs are estimated at $7 per annual ton of clean coal for
desulfurization only. And if one wants to store the clean coal, then the cost
for compacting the pulverizing coal is about $4 per ton. So, the costs for
pulverizing are expensive, but I don't think that is going to be a major con-
sideration in limiting the application to dry processing of 200 mesh coal.
Now let me say something about fibers. The magnetic separation technology
has perhaps been studied more extensively from the standpoint of choice of
collection material than has the electric filtration and that surprises me. It
is known that the shape of fiber, the local surface curvature and the deployment
of the fiber in the packed bed is crucial and extremely important to magnetic
separation. Kaolin processor have learned how to selectively remove particles
on the basis of their size. The bulk strand diameter is important in collect-
ing large particles over 8 microns in size and fine size surface imperfections
on the strand surface are important to the removal of materials in the 0.25
micron size range. This can be confirmed by etching the imperfections away,
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I will now list areas in gas-particulate separation that I think are very
interesting.
I would like to see more information about the strength of the coagula
that are formed utilizing the agglomeration technologies. The idea is that
electric or magnetic separations might be aided with use of appropriate seed
material. For example, nonmagnetic particulates could be seeded with magnetic
particulates to form a coagulum to be removed magnetically. What is the
strength of that coagulum? What strength turbulent forces would break it up?
With regard to fluidized beds I feel there is something very fundamental that
hasn't really been touched on and that is the stability of fluidized beds. I
do feel naively, but cannot prove to you, that it is possible to introduce
electric and magnetic forces to promote stability in fluidized bed processing.
By stability I mean no bubbles, no mixing in the bed whatsoever. Particles
which are absolutely fixed in space would be useful as filter elements.
I am curious to know if anyone has tried the use of magnetic forces to
suppress arcing in electrostatic precipitators? One thinks of capacitor
suppression of electric arcing in an automobile ignition system and of magnetic
methods for suppression of arcs in submarine battery systems. These devices
are magnetic in nature and they blow the unwanted arc away. Just a curious
point, there might be some magnetic way to suppress that arc in electrostatic
precipitators.
The last comment that catches my imagination is the use of foaming
technology to capture fly ash from coal combustion products in flue gases. I
wouldn't suggest the same foaming agents that have been discussed here. But if
you could precede the precipitators with some kind of a foaming agent that was
liquid at the operating temperature (a few hundred degrees Fahrenheit) and you
could blow large bubbles (a quarter of an inch or so in size) then you could
encapsulate many millions of micron size particles per bubble. Then by captur-
ing and destroying those bubbles, you would have a very nice way of capturing
particulates which are otherwise very difficult to control. That is my list of
suggestions.
Scott: I obviously would be glad to talk about acoustic dust conditioning but
I think I will not. Rather I will title my comments something like "Notes of
a Workshop Watcher". And I want to discuss two themes under the "workshop
watching idea".
First is the theme of technology transfer. I think at this meeting we
have a set of people who are working to better understand the fundamental
mechanisms of those processes with which we deal in particulate-gas separation.
Needless to say, to do that right they are isolating various sub-mechanisms
and so, necessarily they can examine only a few. It is, therefore, very
difficult to thoroughly determine the macro-mechanism of any given process.
But it is clear that this fundamental micro-mechanism research is the touch-
stone of all the work we are doing. And I agree entirely with Professor Liu
that this must be encouraged because it is the only way we can really achieve
breakthroughs in the second level.
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The second level is when people perform large scale evaluations, or
overall evaluations, of total processes. Things like power requirements,
resident times, capital costs, collector efficiencies, etc. are examined. I
guess the comment I would make here is that these have to be done as completely
and honestly as possible, using similar criteria in each research group -
particularly on collection efficiency. I think there is obviously an area of
research that we haven't talked about very much here, and that is the develop-
ment of techniques for particle sampling, in particular, very fine particle
sampling. And maybe I could suggest that Grady Nichols or Michael Pilat (who
at least in times past, I think, have worked on fine particle sizing) might
make a comment later on.
The next step is to commercial operation. This is where technology
transfer becomes more difficult. Let us presume that we have a process which
has been developed and works well. Somehow it has to be used. And the fact
is, that the best way for it to be used is by our free enterprise system; that
is, you have to have manufacturers which are picking these things up and carry-
ing them on. But then we are into proprietary interests, practical marketing
constraints, etc., which sometimes are not at all scientific but are nonetheless
very, very real. The technology transfer from fundamental research to second
level, or more applied research is a process which works. It works from con-
ferences like this. But once you get the process into equipment manufacturing
- with sales, maintenance, and many other forces at play - then I am not very
sure how it works. That is why I introduce the question. I am really suggest-
ing that in the next hour, there might be some comments on technology transfer
barriers. Maybe it is not as serious as I think it is. But I think the
difficulty of commercializing the university (as distinct from industrial)
scientists' ideas is very real.
My second theme under this "Notes of a Workshop Watcher" discussion, is
a theme on the interrelation of processes for accomplishing gas-particulate
separation. This comes from two observations of the last couple of days. The
first observation is many approaches and perhaps the most promising approaches -
how one judges the most promising approaches is difficult - are hybrid, or they
involve physical phenomena of several different types. My own field is acoustics
which, of course, is a conditioning process and thus demands that there be
some other collector. The point is that there are many systems. I think it is
unlikely that any single process as it was defined at this conference will find
its way to wide industrial application - unpollinated by other ideas and
processes.
A further observation related to the interrelation of processes is that
new particulate-gas separation applications are popping up like mushrooms after
a warm spring rain. I think Dr. Drehmel has mentioned this, and others have
seen it. But it is something that I have certainly been struck by during this
workshop. And these new applications are coming with obviously a set of new
requirements. I think a lot of processes are hybrid and will continue to be
hybrid. Moreover, applications emerging which will yield requirements we
cannot anticipate. All leads to a comment about the credibility of a process.
I think that none of the processes we have been discussing, are yet credible.
I think, in fact, that we don't even know how to judge them so that they could
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be defined as credible. For example, we don't even know yet what the dust load
requirements at the inlet of a gas turbine are! We don't know yet what the
cotton processing requirements will be. So how can we say that processes de-
signed to satisfy these as yet undefined requirements are credible or not? I
think the question is not the credibility of the process, rather it is the
credibility of the study.
Finally, I come back and wrap all this up. First of all, we need funda-
mental work which is credible. Secondly, we need results - especially those
that analyze the macro-processes to be as complete as possible. In that sense
it is as important to identify the faults of the process as it is to identify
the good parts. Perceived negatives may turn out to be strengths after the
applications are clarified. Thirdly, we have to identify all the criteria.
I have mentioned several: power, dust load, size distribution. If Jim Melcher
is going to be at the next conference you know for sure you have to evaluate
residence times for him to give you his ear.
My last comment with regard to all of this is that the pride in one's
work is obviously essentialy for good work. I suppose it is only natural that
those who are working on a particular process have more faith in that process
than do their peers working on other processes. We are all at fault, sometimes,
in this. We've got to be careful that this natural pride does not change into
something that sounds more like shrill advocacy. I think intense,well -
articulated discussion on the interpretation of the physics of processes is very
productive. Yet I think we must avoid a posture of shrill salesmanship for
specific processes. As such,it could be very counterproductive for all of us.
Ariman: I think I would like to concentrate on mostly electrostatic filtration
in this short presentation. I am sure you noticed that there are several
interesting things happening and many more difficult problems remain to be
solved in electrostatic filtration. When you talk about electrostatic filtration
you have to differentiate two areas. Namely electrostatic fibrous filtration
and electrostatic fabric filtration. If you noticed in this workshop and
literature we really are concentrating on a mathematical modeling of electro-
static filtration by fibrous filters. Professor LHffler, Dr. Zebel and number
of other investigators have been looking into the problem of a single fiber
and a single particle and are trying to make a mathematical model for it.
Hopefully maybe for specially built fibrous filters this model could be used.
However, when you look at the fibrous filters you use in your air conditioner,
furnace, etc., we are really facing a much more complicated situation than a
model with a single fiber and a single particle. On one hand, we have a very
basic and also quite mathematical single-fiber model. On the other hand, we
have almost thing but a black box as Dr. Billings indicated.
Furthermore, we have been doing a lot of testing in particulate collection
with fabric filters. Then, by mostly using the experimental data we have been
developing empirical or semi-empirical formulations for the collection efficiency
and pressure drop in fabric filtration. It seems to me the time has come,
particularly when an external electric field does exist, to take a systematic
approach to the very difficult problems of fibrous filtration, in the following
way (1) a single fiber model (2) a group fibers in the same row (3) fibers in
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a number in parallel rows (4) a staggered array model of fibers (5) consideration
of fibers in perpendicular directions (6) a three-dimensional model which would
be the most realistic. I readily accept that the stated models and particularly
the last two are quite complicated but I feel that this proposed way could be a
reasonable approach to the analysis of real fibrous filters. Hopefully, with
some modifications and depending on the series of it for fibrous filters, a
corresponding approach could be utilized for fabric filters.
I would also like to see experimental work hand in hand with the analytical
investigations. It is certainly very attractice to carry out pure theoretical
work in fibrous or fabric filtration with fourth-order, sixth-order, ordinary,
partial, linear, non-linear differential equations. We also have powerful
computers which give us a lot of interesting numbers. We also have a capable
draftsman to make nice figures; so we can get some excellent publications out
of all these in reputable journals. However, I do not think this pure
theoretical approach with very little relation to the real life of filtration
is a healthy one. Therefore, we should make sure that whatever mathematical
model we propose, we provide a very valuable support to it and verification of
the model with a suitable experimental program.
On the other hand yesterday, I am sure you noticed a remarkable decrease
in the pressure drop in the electrostatic filtration by APITRON of the API
and our bench scale test equipment at Notre Dame. However, some of my colleagues
tell me "well, look, you are trying to convince us that you save some energy by
reducing the pressure drop, but how about the additional energy you are putting
into the system by charging the particles or charging the filter or both? What
happens to that?" Well, if I may have a couple of transparencies, let me show
you something. Based on our bench scale experimental results, and the full
scale field tests by Dr. Helfritch in industrial dust filtration, we obtain a
substantial decrease in the pressure drop. First of all, why is this happening?
We studied the dust cake under the microscope and we have found out that the
dust cake differs quite a bit from the noncharged cake. The charged cake looks
much more porous than the noncharged case. When you do have more porous dust
cake, you will have less resistance against the flow and you don't need too much
power to pull air or gas through the system. Hopefully, we will be able to
produce some three dimensional pictures and publish them for a better under-
standing.
Now if I go back to a power situation for the bench scale testing, we have
up to 80% decrease in the pressure drop. In our bench scale experiment we have
usually a corona formation by a DC charger around 5.5 or 6 kilovolt per inch
(kv/in.). In our experiments we obtained the best results between the 7 or 8
kv/in. In the graph of power, you can see that it is less than half a watt.
Now I am not speculating that will be the case in the full scale application,
but at least our bench scale experiment shows that we use almost all the power
for the exhaust fan system and almost nothing for electric energy. Therefore
energy savings up to 80% could be realized. In 1976 approximately one
billion dollars was spent for the power required in fabric filtration in
this country. Therefore, it appears that it is very worthwhile to conduct
further research in the electrostatic filtration. Now the second aspect I would
like to discuss is the effect of the fabric structure. We all know when energy
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ASBESTOS
8 -
No Charge
WOOL (16 oz.)
5 10
VELOCITY (ft./min.)
ORLON (16 oz.)
No Charge
5 10
VELOCITY (ft./min.)
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443
UNCHARGED ASBESTOS CEMENT DUST
DEPOSITED ON FABRIC - 5X MAGNIFICATION
CHARGED ASBESTOS CEMENT DUST
DEPOSITED ON FABRIC - 5X MAGNIFICATION
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444
FLY ASH
8h-
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T= 81° F
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15
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445
was very cheap nobody cared about possible savings through a lower pressure.
The second transparency shows the effect of fabric type for coal fly ash dust
samples and you can see that there is a considerable effect of fabric type on
pressure drop. So I think experimentally we should look into the fabric
structure in filtration with or without electrostatics. I should point out
that Dr. Drehmel and his associates were the first to study the effect of
fabric structure in the filtration process.
I would like to mention one more important aspect of filtration, humidity.
If we do not control the relative humidity it seems to me there is quite a bit
of difference in experimental efficiency and especially pressure drop results.
In the full size outdoor experiments or testing in the field we have very little
way to control the relative humidity in the air. It may rain a day just prior
to an experiment or you may run your experiment after the rain and there may
be some substantial changes in the relative humidity. Until my visit in 1973
to Dr. Drehmel in EPA, I was not too aware of the importance of the relative
humidity factor. Incidentally, a large number of researchers have been con-
ducting bench scale or full scale test results in the laboratory or in the
field without too much attention to the relative humidity. However, it is seen
that the relative humidity affects the pressure drop a great deal for a number
of dust samples we have tested with our bench scale equipment. It appears that
a reaglomeration of particles prior to the filtration phase takes place at the
higher humidity level. As a result it can be seen under the microscope that we
usually have more porous dust cakes than we do have at lower humidity levels.
As indicated by Professor LOffler, this reaglomeration is due to the change in
adhesive forces between the particles themselves and between particles and
filter surface.
This time instead of bench scale testing we used miniature single bag
dust collector designed by Dr. Helfritch of American Precision Industries as
in the figure. There is just one single bag with jet pulse cleaning. With
this single bag equipment we were able to measure the pressure drop for a
number of dust samples with different fabric bags at different humidity levels.
Let me show you a couple of results. In this figure you see the pressure drop
versus velocity for the three different humidity levels at 24%, 46%, 84%, for furnace
fume dust. Again you see a linear decrease in the pressure drop with an
increase in the relative humidity. Now, this really shows that we should be
aware of the fact that if we do not control the humidity conditions in our
tests, it might be quite different to compare test results. Secondly, it is
possible to condition the gas by water spray not only to cool off the hot gas
but also to provide a decrease in pressure drop.
B. Liu: Before we begin the general discussion from the audience, I think
Dr. Robin Oder would like to make additional statements regarding magnetic
separation.
Oder: Yes, I want to address the question of power consumption in magnetic
separation. There seems to be a reluctance to accept the magnetic technology
on the pretense that large amounts of energy are required. I want to make the
suggestion that you make that comparison fairly. For example, in the use of
electrostatic precipitators, one might estimate how much energy is consumed
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446
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30
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447
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448
in terms of kilowatt hours per ton of material that you capture.
Now, the only commercial technology in high gradient magnetic separation
is in the processing of kaolin clay. As I showed you Wednesday, the separations
are 500 kilowatt magnets. That sounds excessive, but if one puts the power
consumption on a per ton basis, it is not so bad. Those machines operate in
a variety of modes depending upon what the clay processor desires. These
magnets operate in a range of 9 to 100 kilowatt hours per ton of material pro-
cessed. If you put that in dollars and cents and use 2.4c per kilowatt hour
which is an industrial rate, that is 21 cents to $2 per ton. That is not a
lot of money. So I think that if you are worried about power consumption for
magnetics, then put these estimates on a comparative basis with your technology.
Now the point to pulverizing is that liberated mineral impurities can be
removed from coal with magnetic separators or any other technology for that
matter. It is felt that by pulverizing from 70 to 80% through 200 mesh, which
is called power plant grind, one can remove mineral sulfur from about 80 to
90% of our eastern coal. So extraordinary pulverizing to 1 micron size prob-
ably will not be required in physical cleaning. Such extensive grinding is for
the future, for selected coals. So you do not have an extraordinary cost for
pulverizing coal in that particular case.
B. Liu: I would like to ask if any panel member would like to comment on the
remarks by any other member.
Benarie: Yes, just a brief comment. Measuring costs of taking out substances
is not a good measure of particulate separation because as everyone knows if
I take out one kilo of substance, it means 99.999% purities cost a lot more
than 50%. Secondly, your price is to be compared with flue gas desulfurization.
As you know the EPA made a very important 400 page report about the cost compari-
son of 6 possible methods of flue gas desulfurization. After analyzing
al], the cost is between 0.6 and 2.4 cents per kilowatt depending on the
method.
Oder: Yes, that is an operating cost figure. Costs for flue gas separation
have been estimated at 16 dollars per ton of coal fed into the plant, That
is expensive. And furthermore, you have to realize that flue gas desulfuri-
zation is tied to the operating factor of the plant. When the plant is down,
you are still paying for the installation. But if you are not running, you
are not burning clean fuel. I think that is a significant point.
Drehmel: We are entering the area of comparison between different types of
control devices and I think it is a very difficult area to consider. Not only
can we use the types of measures already mentioned but of course we also use
dollars per kilowatt. People want to know what is going into capital equipment.
We can also talk about how much it is going to cost the consumer. That is
just the economic basis of the comparison. We have talked about power. We
might want to talk about resident time or total space and floor space. The
ways in which you could compare a device are so diverse from one application
to the next that the only really fair way to compare is to take an actual
circumstance and say, "I've got this type of aerosol with this type of gas and
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these types of retrofit problems." Is it a retrofit or I can redesign with
some new installation ? What can I do under these circumstances? Look at
the actual design. As soon as you start talking in generalities, about watts
per CFM, dollars per CFM, you are going to get into trouble.
Liu: I would like to open this up to the floor now.
Baril: Do you envision that this cleaning of the coal takes place at the
sight of burning?
Oder: I really didn't intend for that to be coal. I was trying to get into
gas-particulate but we have done an extensive study for the Bureau of Mines
where we considered the chemical cleaning of coal. This is short of coal
conversion such as gasification. This would include, however, physical con-
cleaning and some modest amount of hydro-thermal-chemical operations such as
ditioning of fine coal dust. Dry coal cleaning employing pulverizing might
best be developed at the power plant location unless economical methods are
developed for compacting dry pulverized coal. With this technology coal
cleaning could be located remote from the power plant site.
Baril: My second question is, unless you could do it and do it very well to
meet the standards, you would not replace the necessity of having to clean the
exhaust from a coal burning plant.
Oder: It has been estimated that about 21% of the mid-Western and Eastern
U.S. bituminous coal reserves can be brought into compliance at the 1.2 Ibs
S02/mm Btu level utilizing physical cleaning technologies with the capability
of removing 95% of the mineral sulfur in the coal. Chemical cleaning, with
the ability of removing 40% of the organic sulfur, is estimated to have the
potential for bringing about 38% of these-reserves into compliance. The re-
maining coals might be brought into compliance by a combination of technologies
employing physical & chemical coal cleaning, coal blending and flue gas desul-
furization. The extent to which this mix is employed will depend critically
upon component economics and upon sulfur emissions standards which must be
met. The proper mix of technologies will most likely be determined on a case
by case basis.
Billings: It seems to me the next question would be: What is the iron ore
benefit to that process? You have half of the weight percent which is iron
that you are going to get out of it and you ought to be able to sell that for
$10 to $20 per ton or something like that.
Oder: We have not credited any of the processes.
Billings: Is it a major credit?
Oder: Actually the sulfur could be a major credit in Brazil. The first
studies by Sergio Trindade in coal cleaning with magnetic separators looked
at coals from Brazil. Brazil has a lot of dirty coal but doesn't have much
sulfur. Trindade's emphasis was not environmental, but rather to pursue
recovery of pyrites because of their sulfur content. The next major appli-
cation of magnetic separation, interestingly enough, could occur in processing
of rich iron ores. And these ores are mined and beneficiated for the purpose
of iron ore recovery.
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Billings: I don't know what ore goes for but it has to be $10 to $20 a ton.
You start adding that up to a hundred million tons a year of iron ore for the
1985 capacity in the U.S., then you start looking at 5 million tons of coal
and it sounds like you could meet the iron ore requirement from this
beneficiated material.
Oder: It could be, but we didn't consider it.
Billings: Well, if you got a hundred million tons of iron out of that process,
it would make it a very attractive way.
Oder: Iron and sulfur could, in principle, be recovered as values in physical
coal cleaning. However, I feel that annual recoveries on the order of 100
million tons of iron are out of the question since the 1973 market for coal
supplied to electric utilities was itself only 380 million tons. If 21% of
this utility coal had been cleaned completely of mineral sulfur using an
iron carbonyl process (32 Ibs iron carbonyl/ton of coal), then I estimate
about one million tons of iron at most would have been available as iron-rich
disulfides. While this is not insignificant, it falls far short of the 100
million tons per year quoted by you.
Walston: I agree with what Robin has said - but just to keep us honest and I
have vested interest in coal - if we can take out the pyrite, the main goal
in the U.S. is S0» reduction. But we still will have a particular problem
which I am sure everyone is aware of. I just wish to make that comment and
also that a couple of times during this workshop my name has been mentioned
in connection with some HGMS model work. I want to mention that Professors
Richard Treed and William Simons were full collaborative investigators in that.
Melcher: Just a suggestion regarding a question that came up about compari-
sons among different clean-up techniques. It is a pretty simple matter to
improve the efficiency or pressure drop by using something akin to effective-
ness factor of a filter that was defined by Chen back in the mid '40's where
you used like a negative log of the penetration divided by pressure drop.
When you use augmentation of some sort, rather than using pressure drop perhaps
you should use total power input. That gives you a clear indication of what
you have done. You can always improve efficiency by putting in more layers;
of course, it doesn't help you in the pressure drop. And there has been a
lot of talk about improving the process this way or that way and it seems
reasonable to look at it in terms of the power input.
Oder: That is just the message I was trying to give on the comparison
between the magnetic filter bed and the quadrapole. They are diversive
approaches to the same problem. One has excellent technical ability from the
standpoint of separation but poor performance from the standpoint of pressure
drop and total power requirement. The other one has moderate to poor technical
performance, high power requirements but no pressure drop problem. I am say-
ing that you have to look at the total device within the process context.
Melcher: In the magnetic separation in both the two types, the bed and the
quadrapole design, you end up loosing some of the coal into the slip stream
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of the coal off the quadrapole or you deposit some in your filter bed. I
think Billings reported deficiencies with magnet off as high as 50%. You can't
tolerate a 50% loss to your coal if you are going to run that data. I can see
it as a clean-up device where you would like to get 50%, but if you are going
to separate, you don't want that to occur. How much do you think you can
tolerate in terms of loss of coal, for instance?
Oder: I will tell you what can be achieved in commercial practice for kaolin,
then I will tell you what we estimated for coal. Again I emphasize that coal
is a calculation and an estimate where problems associated with fine clay
inclusion, have been ignored. For the processing of kaolin clay which is a
wet mineral beneficiation, where 90% of the particles are finer than 2 microns
in size a mineral impurity is removed that has a bimodal distribution, 1/4
micron and 8 micron in particle size. The magnetic process operates in a
97% kaolin clay yield. That is one of the main reasons that the technology
has won out over such alternative technologies as differential flocculation and
flotation. Now the coal that we used for the Bureau of Mines survey is the
same coal that has been considered in the paper presented here. It is a coal
that is chosen on the basis of several levels (1) the organic sulfur level was
low enough for the conceptual study such that if you removed most of the
pyrites sulfur you would make a coal that comes into compliance (that seems to
be significant) and (2) we chose a coal that cannot be cleaned with a present
day cleaning circuit. We chose a coal that had a particle size distribution
that corresponds very closely to 200 mesh grind. The feeling was if we grind
to 200 mesh, then we will do a very good job of liberation. Given that size
distribution, we can estimate the liberation in 200 mesh grind.
On the basis of that estimate, the HGMS technology in the example that I
gave should receive about a 95% Btu recovery. The quadrapole I think had maybe
80 to 85% Btu recovery. It is in the paper. There is obviously a liberation
problem; you will have to pulverize the coal to eliminate the mineral matter.
Melcher: You are saying then that perhaps something like 10% of your coal is
lost.
Oder: Look, no one has done this. I am guessing that it is 10%. If you have
problems of coalescense of the particles like I have been listening to here,
the loss could be a great deal more. If one pyrite particle (even though it
is ground to liberation) has 2 or 3 coal particles struck to it because of
electrical attraction, when you take out the pyrite, you might take out a lot
of coal. The estimate is based strictly on a simplified, theoretical calculation.
Nichols: I thought I would make one comment about your magnetic quenching
sparking in your electrostatic circuitry. Anyone who is going to do research
in this area should consider the fact that the energy is stored in the electrode
system rather than at the power supply. So your magnetic quencher has to be
distributed throughout the whole corona wire conduction electrode system. The
majority of energy that goes into this part is stored energy that is in the
distributive capacitance of the system.
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Oder: Yes, I wouldn't know how to do it. I would suggest either couple it
inductively or put in a permanent magnet.
Nichols: Yes, that is one area they might do research in. It has to be in
a box.
Friedlander: First, last night I gave it some thought and I guess this exist-
ing system has some merit to it, because it quenches quickly, it localizes the
arc, but I also would like to consider any method that you always quench after
you stop the arc. In magnetic method you use nothing to quench unless you stop
it. So you still have a problem there. I want to get back to something. My
feeling is quite strong about the many technologies of the agglomerations
sticking together. It is really important in two ways, (1) To avoid where you
don't want it and (2) To use it in seeding. Two years ago I read about water-
based technology of seeding water with iron oxide to remove bacteria viruses.
Apparently it works very well and I have not heard much more about it. The air
system,! haven't heard anything about it yet except here. There are some
problems that can be solved that way. But I think a lot of work has to be done
to understand the system and the technology to make it work. The magnetic
technology is a peculiar one. There are not that many particles around today.
But what about seeding magnetic particles, then getting that magnetic particle
to marry another magnetic particle? It is not the usual agglomeration problems
where you have to get a lot of them together. If you don't have high densities,
you only need one magnetic particle to remove particles so you only have to
marry two particles and not a large number of particles. The time constant may
not be that bad.
Liu: I would like to change the topic just slightly. I think I would like to
ask Don Reid about his electrostatic augmented filters. I am just wondering:
eventually the filter will clog up and you have to replace it, don't you. I
just wonder what type of application you think your equipment would have.
Reid: We haven't done any extensive studies of the cleaning techniques. Again,
we do know that we can clean it back to the original pressure drops, which
indicates that we have a clean bed. It has a high loading capacity. We have
operated some of these filters for 60 hours and only had 0.1 inch increase in
pressure drop. Of course the length of operation time before cleaning is going
to be a function of concentration of the aerosol.
Liu: So you are intending to use this as a clean up filter. You wash it after
it gets clogged.
Reid: You have to clean it. It is not one you just throw away.
Liu: I see.
Billings: I wonder if Professor Loffler would give us the benefit of his think-
ing about the strengths of aggregates or agglomerate particles. What the forces
are, what the orders of magnitude are, that kind of thing. Would that be a
fair question?
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453
L'dffler: I think it would be a topic for another meeting if you talk about
agglomeration processes between various particles. I could only give a very
brief review of the main mechanisms. We could list all the forces which are
possible, we can go back to non-material binding, such as Van der Waals forces,
electrostatic forces, or any other dipole interaction. We have worked with a
material which is like a liquid (water or something like water) where there
are chemical forces, surface tension forces and you might have some solid which
is crystallized. Those are some of the mechanisms. As far as magnitudes: The
highest of course is ? solid which might also be an effect of surface diffusion.
The next range is the liquid which also acts over large distances. The
magnitude below that you get the van der Waals forces. The order below that,
electrostatic forces. As long as the particles are in direct contact the
distance must be in the range of several angstrom units. But electrostatic
forces are long range forces. To give you an idea about the orders of magnitude:
For a 1 ym particle, the van der Waals adhesion force, to a surface would be
one million times the gravity. That should give you an estimate on the amount
of energy needed to deagglomerate. This means that it is normally impossible to
deagglomerate very fine particles only by shear forces in a gas flow. It is
easy through the use of impact forces. This way you can deagglomerate particles.
We found that in deagglomeration, impact onto a hard surface, you need velocities
approaching 200 meters/second.
Oder: Professor Friedlander asked if anything was occurring in the
seeding technology for wet processing with magnetic separators
and the answer is "yes". It bears on what you are saying. There is a great
deal proceeding from the standpoint of industrial development for processing
the effluent water from municipal and industrial complexes and it is a technology
that utilizes the seeding technology. Coagules are formed in a normal
flocculation chamber where you change the pH so as to make the particles
coagulate and form on the surface of magnetic iron oxide particles which are
about 20 microns in diameter. Now what has been found is that the magnetic
capture force for these coagules in high velocity flow through that packed bed
is so great that it is shear force that is breaking up the coagules. The process
rate through the magnet is limited by the breakup of the coagules and not by
the availability of a magnetic force to hold the seed material on the capture
surface. In that case we have a seed material that is rough in shape, about
20 microns in diameter, and that can capture four or five times its volume in
entrained materials. Studies have been made of capture of iron oxide particles,
oil agglomerates and even bacterial systems which are the products of tertiary
and secondary sewage processing. You can capture and hole these up to a point,
but as the flow velocity increases eventually viscous forces will break the
coagules.
Billings: I think that there seems to be a general concern, both on the gas
side and the liquid side, to attempt to understand something that is pretty
fundamental. That is, what is the force of adhesion between two fibers, similar
and dissimilar, and how does that compare what those forces are? First, where
do they come from? Second, how does one speculate about the order of magnitude?
Then you can begin to compare those forces between the particles with the other
forces that come into play, like the fluid mechanical forces. The forces of
adhesion between particles in gases at normal temperatures and pressures are
enormous. They are very much higher than in liquids. It becomes very difficult
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to go from one fluid to the other because the adhesion forces dominate in the
gas side.
Oder: Is that because of double layering in the liquid? Do you have shielding
of the charge in some parts?
Billings: From an engineer's point of view, the adhesion force of the particles
in a gas without the Van der Waals, without electrical, without sintering, with-
out actual contacts is primarily the surface tension of water. It is
essentially a humidity effect. It is demonstrated very easily by varying the
humidity and examining the force between the particle and the surface. The
point is that tension force is higher than anything else. The adhesion force .
is of the order of 5 dynes. The weight of the particle is on the order of 10
dynes. The drag force = SiiyV^ is of the order of 10"^ times the velocity. So
the velocity has to be of the order of 10^ meters/sec in order to blow that
particle off the surface.
Oder: Question. Do you know of, or does any one know of, dispersing materials
to be used in dry systems? Wet particulates can be stabilized in fluids with
detergents effectively. One adds polyphosphates so that the particulates be-
come stabilized by charge repulsion. Are there equivalent materials for use
with dry material to promote stability in separation in particles?
Liu: An answer to that is no. A number of chemical warfare people have tried
for many years to stabilize the aerosols so that they would not flocculate and
coagulate. I think the efforts have all been negative. You do not have an
equivalent double layer situation in a gas system. When they come together,
particles simply stick. I think one of the biggest problems with gas separation
is you have to deal with very high concentrations of particulate in order to
reduce the volume of gas in the process. That is conducive to coagulation.
Coagulation has to be looked at very carefully before one can say anything about
the economic feasibility in magnetic separation.
Scott: I just want to return to the electrostatic filter. Question: It is not
my field, but what is the physics or mechanism that allows this particular pro-
cess to occur with much lower pressure drops. I'm particularly intrigued by
one of the slides where you had low pressure drop, then you turned off the
electric field and the pressure drop went up; then, magically, you turned on
the electric field and I think it dropped again. I wonder if I misinterpreted
that or could someone give me a little bit more physical interpretation of that
part.
Ariman: Dennis, would you please make some comments on the last part of the
question?
Helfritch: When the electric field is turned off, we do know we have briefly
a precipitated section that the aerosol has to pass through before it is filtered
by the fabric. Some of it is taken out. Second, when dust deposits on the
fabric in the charged state, it deposits in a different way, more porously
and openly. When the charge is turned off, the dust then can deposit in its
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normal fashion, which is a fairly tightly packed structure with a lower
permeability. We clean the fabric periodically. The entire unit is cleaned
every 5 minutes.
Ariman: Let me add a couple of things to his comments. We do have a more
porous dust cake when we have an electric charge on the particles. In the
pictures you will see just the particles are charged. We do have charges on
particles, and it looks like they keep their charge at least for a while de-
pending on the filter. In bench scale experiments once one particle lands on
the filter the next one because of the charge of the first particle lands some-
where else on the filter. However, the second one would land next to the first
particle in the case of non-charged particles. Therefore for the charged case
there is a nonuniform distribution of the landing of particles on that particular
filter. As I indicated earlier there are little hills, valleys, and bridges.
If you are not careful in carrying the dust cake to the microscope, you can damage
the cake structure. It is very fluffy and porous. We hope to shed more light
into it in the near future.
Liu: I would like to suggest one of the more important mechanisms might be
due to the poly dispersity of the dust. If you do not have an electrostatic
charge, then the fine particles would have a tendency of filling the pores be-
cause this is where it can be collected by interception. Electrostatically
charged aerosols tend to be collected at the entrance of those pores, probably
keeping those pores open so that the fluid can go through.
Cooper: One of the things that struck me was decrease in the flow resistance
of the dust cake as humidity increased and adhesive forces which might tend to
pack it tighter and electrical forces which may decrease by humidity. Those
results are surprising. The humidity is allowing us to build up a more open
cake simply because once we get a particle striking another, it stays in a
forward position rather than going into a backward position.
Ariman: First of all, let me clarify one thing. The results provided here
were not for the charged cakes. They were obtained by the use of single
miniature bag testing devices. We really tried to compare our bench scale
testing which uses flat fabric with a regular single bag, but in a small scale.
So we did not have any charge. Also we did have greater complexity when we
had electric charge because of the interaction between humidity and the electric
field. Now I'll go back to your first question - how come the humidity was
causing a decrease in the pressure drop? It is my understanding, that we do
have a change in the adhesive forces between the particles when relative
humidity varies. Also the same thing applies for the forces between the
particles and the fiber filter surface. It looks like there is an increase
in agglomeration process of the particles before the landing on the filter.
Instead of having very tightly packed dust cakes we have quite porous ones.
Penny: There is one factor that wasn't mentioned in connection with humidity
and that is the question of resistivity. For the information of the porous
cake the surface has to be in equipotential. If the surface is equipotential
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456
then the field will concentrate on any projection. It turns out that requires
a certain amount of conductivity for the surface to be in equipotential. So it
seems to me that when we mention humidity, we ought to mention the kind of
material and how its resistivity varies from humidity. In other words, it is
not sufficient to have charged particles in that cake - you really want that
charge conducted. If you do it right you build up these particles into almost
like fiber like clouds. But that requires high enough conductivity.
Ariman: I agree with Professor Penney. Resistivity is another area that I was
going to suggest as equally important with relative humidity. I think we should
look into the resistivity very, very closely and for bench scale testing, we
should find a way to measure the resistivity of each dust particle we have been
using. We should also try to incorporate resistivity into the mathematical
model of filtration in an acceptable way.
Loffler: What is needed is to go into more detail, not in theory but in
experiment, to get more information on the material qualities. All the effects
that were observed rely on very specific materials and conditions. For example,
we did measurements on low humidity using a very similar apparatus as you did
and found that in this case the particles formed a cake. There was a continuous
variation in the permeability of the cake. That means the cake was compressed.
There was an increasing pressure drop across this cake and this could be
theorized by going for a short moment with a higher velocity and with a lower
velocity. You would obtain a lower permeability. I would speculate that in
your case one of the possible reasons is with increasing humidity you get a
liquid which is at the contact point and is without a higher adhesion force iri
stronger agglomerates. This type of cake perhaps does not reduce permeability.
Dr. Friedlander: Basically this research turns out to strongly involve the
property of solid materials. On the other hand, those of us at the materials
research laboratories, are told that any work that considers such things can
not be considered to have anything to do with money that is given to the study
of materials which is also contradictory because we end up right at this point
finding that where the lack of knowledge is extremely strong is in the under-
standing of the solid materials involved. The NSF says that this is not so.
But yet it is interesting to find that the interaction between these two may be
extremely important. It should be encouraged rather than discouraged.
Liu: Dr. Ojalvo, I would like to hear your comments on that.
Ojalvo: I certainly agree and it takes workshops like this to bring this sort
of thing out. Eventually this reaches the people at the top of the NSF ranking
and decision-making and it has been argued strongly enough, forcefully enough,
and a number of times. It is good this sort of work can be brought together.
Billings: It seems like Dr. Friedlander's comments should be modified in the
following way. That we really aren't interested in the bulk properties of
material. To a great extent, these are inconsequential. We really are interested
in interfacial property. And that has nothing to do or very little to do with
the material science area.
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457
Friedlander: Surface studies are the big area - Purdue University does surface
study and get well paid. This is precisely the interaction of surface study on
the solid material, the oxide. And here we find a different viewpoint but
exactly the same property, when you get down to real basics, we are talking
about the same thing they are. Maybe they know a lot more than we do except
maybe it doesn't apply to this function. We are saying the same thing but for
a different situation.
Lamb: We have reviewed reports by Lo'ffler on a phenomenon which he has re-
ported reducing the pressure drops by a factor of six. The important thing
is that he describes this and nobody knows why it is happening. I am somewhat
discouraged that he lists several papers on the theory of fiber filtration that
always come to deal with a single fiber being struck by a single particle.
They always deal with conditions that can be described as fictitious. The
situation is one in which we are coming to know very precisely whatever happens.
The filter is always clean and we never see what happens when the second
particle comes along and is faced with a different situation. Perhaps it is
time that we devoted efforts to this very messy situation.
Ariman: I agree with you, Dr. Lamb, that is one of the major points that I
have tried to stress in my presentation. The time has come to go from this
single fiber, single particle approach step-by-step to the real filtration with
close coordination of experimental work.
Liu: The time is getting late. Some people have to leave for the airport so
I think we should close at this point. So before we do that I would like to
ask Dr. Ojalvo and Dr. Drehmel if they have some closing remarks.
Drehmel: It seems to me that one thing that we really haven't addressed is
how good our theories are. Do we believe them and what happens when that
second particle comes in? I agree with Dr. Lamb - there are a lot of other
theories that can be proposed. So far as comments on this workshop are
concerned, I think that we owe a great deal of thanks to Dr. Ariman for organiz-
ing this conference and getting the facilities for us. I think the attention to
details that he has given us on this workshop has just been fantastic.
Ojalvo: I would like first to make some general remarks about the National
Science Foundation (NSF) and then conclude with information about my program
and its relationship to this workshop.
The basic purpose of NSF is to initiate and support scientific research
(including social science and engineering) and programs with scientific research
potential - such as science education. To accomplish this purpose NSF is
organized into seven directorates, each headed by and Assistant Director, under
the Director of the NSF. The first three directorates are concerned with basic
research - Mathematical and Physical Sciences and Engineering; Astronomical,
Atmospheric, Earth and Ocean Sciences; and Biological, Behavioral and Social
Sciences. The other four are: Science Education; Research Applied to National
Needs (RANN); Scientific, Technological and International Affairs; and finally
a Directorate for Administration. The first three directorates (basic research)
are the heart of NSF and account for 70% to 75% of its total budget.
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There are five divisions in the Mathematical and Physical Sciences and
Engineering Directorate - Physics; Chemistry; Mathematical and Computer Sciences;
Materials Research; and Engineering. The Engineering Division consists of three
sections: Electrical Sciences and Analysis; Engineering Mechanics; and the
section with which I am associated - Engineering Chemistry and Energetics.
The program which I direct, Solid and Particulate Processing, is one of five
in this section. The other four are Heat Transfer; Engineering Energetics;
Thermodynamics and Mass Transfer; and Chemical Processes.
The four major areas in my program are: Particle and Particle System
Characterization; Particle Generation and Size Modification; Interfacial and
Colloidal Phenomena; and finally, the area of interest in this workshop -
Particle Processing and Separation.
The program is quite new having been initiated two years ago. One of the
first activities was to hold workshops. There was much interest from mineral
engineers, so the first workshop was on Mineral Processing. Then, the Fine
Particle Society conducted a workshop on Particle Technology. One of the
conclusions of that workshop was that particulate - gas separation was an area
that could be explored in a separate workshop by itself. So here we are doing
just that!
I want to congratulate Dr. Ariman for conducting a very fine and productive
workshop. I feel that quite a bit has been accomplished. I would like to
have the Proceedings include all the papers, invited lectures, discussions and
comments presented here so that we at NSF (more specifically the Solid and
Particulate Processing Program) and others in the technical community can use
it as guidance to focus our efforts on the promising research opportunities in
particulate - gas separation.
Ariman: Thank you, Dr. Drehmel and Dr. Ojalvo, for the nice comments you have
made concerning the workshop. First of all, I would like to thank our
distinguished lecturers, Professor Loffler and Dr. Benarie. They were kind
enough to come from Europe on such a short notice. I also would like to thank
the lecturers and the designated discussers for their fine contributions and
certainly the important contributions of all the other participants through
open discussions.
It is proper also to express my appreciation and thanks for the great help
to the members of the organizing committee, Professors Ben Liu and K. T. Yang.
Also special thanks to our sponsors NSF, EPA and host institution University of
Notre Dame. Without their help, there was no possibility of having this
workshop. Very special thanks to Dr. Ojalvo of NSF and Dr. Drehmel of the EPA,
for their continuous support from the beginning and for their wise advice and
suggestions. So, on behalf of the organizing committee and the University of
Notre Dame, we are very grateful to them. I hope it was a useful workshop
for every participant and I would like to congratulate all participants .for
their very valuable contributions.
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TECHNICAL REPORT DATA
(Pleate read Inunctions on the reverse before completing)
1. REPORT NO.
EPA-600/7-78-170
2.
3. RECIPIENT'S ACCESSION NO.
AND SUBTITLE proceedings: Symposium on New
Concepts for Fine Particle Control
5. REPORT DATE
August 1978
5. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Teoman Ariman, Compiler
B. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME AND ADDRESS
University of Notre Dame
Notre Dame, Indiana 46556
10. PROGRAM ELEMENT NO.
EHE624
11. CONTRACT/GRANT NO.
EPA Grant R805148; NSF
Grant ENG. 77-02016
12 SPONSORING AGENCY NAME AND ADDRESS
SPA, Office of Research and Development(*)
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD (
Proceedings; 1/77-7/78
(COVERED
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES Project officers are: Dennis C. Drehmel (EPA), Mail Drop 61, 919
541-2925, and Morris S. Ojalvo (NSF). (*) The National Science Foundation is co-
sponsor.
i6. ABSTRACT rpj^ rep0rt documents presentations made during a symposium on novel
concepts, methods, and advanced technology in particulate/gas separation. The sym-
posium , held at the University of Notre Dame and sponsored by the National Science
Foundation and the Environmental Protection Agency, was held both to identify new
research areas and to stimulate future research activities. The presentations
included two general lectures by internationally known scientists (Dr. M. Benarie of
France and Professor F. Loffler of West Germany) and 17 invited lectures by other
prominent experts.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATl Field/Group
Pollution
Dust
Flue Gases
Measurement
Separation
Air Cleaners
Pollution Control
Stationary Sources
Particulate
13B
11G
2 IB
14B
13A
9. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (This Report I
Unclassified
21. NO. OF PAGES
459
20 SECURITY CLASS fllils page)
Unclassified
22. PRICE
EPA Form 2220-1 (9 73)
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