EPA-650/2-74-112
OCTOBER 1974
Environmental Protection Technology Series
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EPA-650/2-74-112
EPA FINE PARTICLE
SCRUBBER SYMPOSIUM
(SAN DIEGO, 5/28-30/74)
by
A. P. T. , Inc.
P.O. Box 71
Riverside, California 92502
Contract No. 68-02-1328
Task 2
ROAP No. 21ADL-034
Program Element No. 1AB012
EPA Project Officer: Dennis C. Drehmel
Control Systems Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
October 1974
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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CONTENTS
Technical Program iv
Foreword vi
Symposium Objectives vii
Papers
"Fine Participates--The Misunderstood Air
Pollutant" 1
"Engineering Design of Fine Particle Scrubbers" 12
"Submicron Particulate Scrubbing with a
Two Phase Jet Scrubber" 33
"Performance of a,Steam-Ejector Scrubber" 54
"Performance of Wet Scrubbers on Liquid
and Solid Particulate Matter" 68
"Rotating Concentric Homogeneous Turbulence
Gas Scrubber" 82
"Mean Drop Size in a Full Scale Venturi
Scrubber Via Transmissometer" 91
"Fine Particle Collection Efficiency Related
to Pressure Drop, Scrubbant and Particle
Properties and Contact Mechanism" ' 109
"Effects of Water Injection Arrangement on
the Performance of a Venturi Scrubber" 128
"Fine Particulate Removal and SO,,
Absorption with a Two-Stage Wet Scrubber" 144
"Flux Force/Condensation Scrubbing" 161
"Flux Force Condensation Aspirative Wet
Scrubbing of Sub-Micron Particles" 194
"Entrainment Separators for Scrubbers" 207
"Future Needs for Fine Particle Scrubber
Capabilities" 229
Panel Discussion 231
Closing Comments 240
List of People Attending the E.P.A. Symposium 242
111
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TECHNICAL PROGRAM
WEDNESDAY, MAY 29, 1974:
Morning Session Chairman, John K. Burchard
9:00 - 9:05 Welcome to Symposium
Seymour Calvert
9:05 - 9:20 Symposium Objectives
Alfred B. Craig
9:20 - 9:50 Keynote Paper - "Fine Participates —
The Misunderstood Air Pollutant"
Richard E. Harrington
9:50 -10:00 Discussion
10:00-10:30 "Engineering Design of Fine Particle Scrubbers'
Seymour Calvert
10:45-11:15 "Submicron Particulate Scrubbing with a Two
Phase Jet Scrubber"
H. E. Gardenier
11:15-11:45 "Performance of a Steam-Ejector Scrubber"
L. E. Sparks, J. D. McCain, and W. B. Smith
11:45-12:15 Discussion
Afternoon Session Chairman, Gary J. Foley
1:45 - 2:15 "Performance of Wet Scrubbers on Liquid and
Solid Particulate Matter"
2:15 - 2:45 "Rotating Concentric Homogeneous Turbulence
Gas Scrubber"
William C. Leith
2:45 - 3:00 Discussion
3:00 - 5:00 Panel Discussion
Moderator - Leslie E. Sparks
Panel Members -
Harold H. Haaland
Charles W. Lear
Robert C. Lorentz
J. R. Martin
Jack E. Phelan
Alexander Weir, Jr.
iv
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THURSDAY, MAY 30, 1974:
Morning Session Chairman, Charles E. Lapple
9:00 - 9:30 "Mean Drop Size in a. Full Scale Venturi
Scrubber Via Transmissometer"
R. H. Boll, L. R. Flais, P. W. Maurer, and
W. L. Thompson
9:30 -10:00 "Fine Particle Collection Efficiency Related
to Pressure Drop, Scrubbant and Particle
Properties and Contact Mechanism"
Howard E. Hesketh
10:00-10:30 Discussion
10:45-11:15 "Effects of Water Injection Arrangement on
the Performance of a Venturi Scrubber"
S. W. Behie and J. M. Beeckmans
11:15-11:45 "Fine Particulate Removal and SCL Absorption
with a Two-Stage Wet Scrubber"
J. I. Accortt, A. L. Plumley and J. R. Martin
11:45-12:15 Discussion
Afternoon Session Chairman, James H. Abbott
1:45 - 2:15 "Flux Force/Condensation Scrubbing"
Seymour Calvert and Nikhil C. Jhaveri
2:15 - 2:45 "Flux Force Condensation Aspirative Wet Scrub-
bing of Sub-Micron Particles"
Stanley R. Rich and T. G. Pantazelos
2:45 - 3:15 "Entrainment Separators for Scrubbers"
Seymour Calvert, I. Jashnani and S. Yung
3:45 - 4:15 "Future Needs for Fine Particle Scrubber
Capabilities"
Michael J. Pilat
4:15 - 4:45 "Expression of Air Pollution Control Association
Interest"
Harold M. Englund
4:45 - 5:00 Closing Remarks
Dennis C. Drehmel
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FOREWORD
Fine particles have come to be recognized as being much
more significant air pollutants than particles larger than
several microns diameter. The removal of fine particles from
effluent gas stream is, unfortunately, more difficult than that
of large particles. Because wet scrubbers are one of the major
types of air pollution control equipment which can collect
fine particles, it is important to define and exploit their
potentialities.
To meet this need a symposium on the subject of fine
particle scrubbing was sponsored by A.P.T., Inc. and the U.S.
Environmental Protection Agency. It emphasized the collection
of fine particles in any type of wet collector possible includ-
ing hybrid devices. The objective was to stimulate and generate
new and novel ideas for fine particulate control and promote
interchange of ideas among scrubber experts.
Invited papers which contained significant new data and
a panel discussion exploring new concepts were presented during
two days of technical presentations and discussions. These pro-
ceedings are being furnished to the attendees, who included the
users and the developers of new scrubber technology.
The Symposium organization committee members were:
SEYMOUR CALVERT, General Chairman - President,
A.P.T., Inc., Riverside, California
JAMES H. ABBOTT, Program Committee - Chief, Par-
ticulate Technology Section, Environmental
Protection Agency, Research Triangle Park, North
Carolina
DENNIS C. DREHMEL, Program Committee - Control
Systems Laboratory, Environmental Protection Agency,
Research Triangle Park, North Carolina
PHYLLIS Z. CALVERT, Symposium Coordinator - A.P.T.,
Inc., Riverside, California
vi
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SYMPOSIUM OBJECTIVES
by
Alfred B. Craig
Environmental Protection Agency
National Environmental Research Centei
Office of Research and Development
Control Systems Laboratory
Research Triangle Park, North Carolina 27711
vn
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"Symposium Objectives"
by
Alfred B. Craig
The Control Systems Laboratory (CSL) of the Environmental
Protection Agency has been developing improved technology for
the control of particulate emissions from stationary sources
for nearly ten years. Starting about three years ago, emphasis
has been gradually shifted to the study of fine particulate
which we define as solid or liquid particles less than about
three microns in diameter. Rationale for this shift in emphasis
will be covered in Mr. Harrington's Keynote Paper, the next on
our program.
In an effort to increase interest in fine particulate
control technology within private industry and academic
circles, CSL has established the following series of symposia
covering all facets of this subject:
1. SEMINAR ON ELECTROSTATICS AND FINE PARTICLES
National Environmental Research Center
Research Triangle Park, North Carolina
September 6-7, 1973
2. SYMPOSIUM ON THE USE OF FABRIC FILTERS FOR THE
CONTROL OF SUBMICRON PARTICULATES
Boston, Massachusetts
April 8-10, 1974
3. FINE PARTICLE SCRUBBER SYMPOSIUM
San Diego, California
May 28-30, 1974
4. SYMPOSIUM ON ELECTROSTATIC PRECIPITATORS FOR THE
CONTROL OF FINE PARTICLES
Pensacola, Florida
September 30-October 2, 1974
5, GENERATION AND COLLECTION OF FINE PARTICLE SYMPOSIUM
(Tentative)
Spring, 1975
viii
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The basic objectives of each of these symposia have
been to:
1. Bring together in one location many of the
leading authorities in the subject field of
technology;
2. Present a comprehensive series of technical
papers covering the broader areas of the
subject technology;
3. Establish a forum for in-depth discussions of
all facets of the control of fine particulates
by the subject technology;
4. Stimulate new ideas for the development of new
or improved techniques for control of fine
particulates.
A review of the list of attendees at this meeting and
the program arranged by Dr. Calvert indicates that we
should quite ably meet all of these objectives at this
symposium covering the use of wet scrubbers for the control
of fine particulates for stationary sources.
IX
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FINE PARTICULATES--THE MISUNDERSTOOD AIR POLLUTANT
by
R. E, Harrington, Director
Air Pollution Control Division
Office of Research and Development
Environmental Protection Agency
ABSTRACT
This paper examines the basis of concern for particulate
as an air pollutant. It concludes that while the data base
for quantitative assessment of health and welfare affects is
inadequate, there is sufficient evidence to show that fine
particulate is a serious air pollution problem that must be
controlled. Scrubbers have unique potential for dealing with
this problem. Conventional scrubbers, however, have limita-
tions for efficient capture of fine particulate and therefore
must be augmented to realize their full potential.
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FINE PARTICULATES--THE MISUNDERSTOOD AIR POLLUTANT
by
R. E. Harrington, Director
Air Pollution Control Division
Office of Research and Development
Environmental Protection Agency
In the late 1950s and early '60s, there began a public
awakening to the problem of air pollution. This increasing
concern over the pollution of our air was stirred by the
public awareness of the extremes to which water pollution had
progressed in many of our rivers and lakes; by the increasing
interests of air pollution as detectable by visible smog and
harsh chemical odors; by the alarming rate of increase in the
number of cities that were being added to the list of smog
plagued cities and communities; and by specific incidents
where ill health or even death could be traced to specific
episodes of air pollution. We were in a period of our history
in which our National attitude was characterized by an in-
creasing awareness and concern over a wide range of domestic
matters such as civil rights, increased recreational oppor-
tunities, a shorter work week, crime and ecology.
It isn't surprising then that the very real but little
understood problem of air pollution control and the broader
problem of environmental pollution control became a National
issue in the middle and late '60s. It became a major issue in
the political arena and finally culminated in the Clean Air Act
of 1967 and its major amendment of 1970 which further strength-
ened the Act. In December of 1970 the Environmental Protection
Agency was established. Equally important during this period,
numerous ecology groups were either established or escalated
to a new level of activity.
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In retrospect, I believe that most would agree that this
sequence of events has been beneficial. While history will
probably show that there have been extremes of inaction,
action, and over-reaction, these very extremes have served
to help focus on the specifics of the problem and define areas
where more information, data, technology or control are needed.
Upon analysis, it is clear that the single major problem
limiting our rate and efficiency of attack on the air pollution
control problem is the paucity of good data in almost all areas
including the nature and source of pollutants, effects of
pollutants, ability to control pollutants, and the economics
of control. While considerable progress has been made over
the past 5 years we have only scratched the surface.
Typical of the problem of inadequate information is the
problem of particulate control. The initial approach to the
particulate control problem was first to define any finely
divided solid or liquid material emitted into the air as
particulate. Second, assume that if we controlled the mass
emission of particulate emissions to a specified level, it
would provide adequate protection of public health and welfare.
Third, we may observe that for the "average" particulate
emission stream there is existing control technology to attain
the prescribed degree of mass emission control. Missing from
this rationale is any consideration of quality, compositional
or receptor - particulate characteristics interface factors.
Upon closer examination it becomes obvious that these factors
are of major importance.
Perhaps the most important single factor is particle size.
Large particles (over 5 microns) constitute the major mass
fraction of most sources of emission and it is true that
economical control equipment exists for their efficient control.
Further, while these larger particles frequently represent a
significant welfare problem by contributing to soiling, unsightly
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smoke plumes and light obscurance they are not a major health
problem in that they tend to settle relatively quickly, and
are more easily scrubbed from the atmosphere by rain and they
are not able to penetrate the respiratory system. By far,
the most important form of particulate air pollution is the
fine particulate.
There are several reasons why fine particulates^ defined
as solid or liquid airborne aerosols less than three microns
in diameter, are one of the most important forms of air
pollutants. First, fine particulates usually occur as a
fraction of a distribution of larger particulates. While
coarser particulates are easily collected by conventional
control equipment, fine particulate pass through with much
lower efficiencies of collection. Further, the finer particu-
lates emitted into the atmosphere remain airborne for extended
periods. Second, their greater ability to obstruct light
causes the limited visibility typical of air pollution haze
and smog. Third, fine particulates are a health hazard, since
in contrast to coarser particles, they can bypass the body's
respiratory filters and penetrate deeply into the lungs. The
chemical and physical characteristics of fine particulate can
further aggrevate their health impact. Because of their high
surface area, some fine particles have been identified as
transport vehicles for gaseous pollutants, both adsorbed and
reacted, and hence can produce synergistic effects deleterious
to human health. Since many fine particulates are metallic
materials, some are chemically and catalytically highly active.
Fine particulate air pollutants may be classified into
two major classes based on their origin. These are (1) primary
fine particulates which are emitted or immediately condensed
as fine particulates from a specified source and (2) secondary
fine particulates which are the products of atmospheric
reactions. Water is excepted as an air pollutant except as it
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may occur as man-made ice fog.
Primary participates typically result from physical or
chemical processes, which may include condensation of gaseous
products or products of chemical reactions. High temperature
processes such as metallurgical operations and combustion of
fossil fuels are major sources. Metallurgical operations
represent major sources of metal fumes unique to the process,
such as lead, zinc, arsenic, mercury, copper or iron oxide
fumes. Combustion processes produce a spectrum of materials
found as ash components of the fuels. Combustion of residual
oil, for example, produces quantities of vanadium, chromium,
nickel, iron, copper and other highly reactive and catalytic
metals. Primary sources of fine particulates represent the
principle source of these metallic constituents in the air.
It has been theorized that these highly active and catalytic
materials play a key role in the formation of secondary particu-
lates by acting as catalysts in chemical and photochemical
smog-forming processes.
Some processes emit solid and liquid hydrocarbon emissions
such as organic condensibles, tars and carbon particles capable
of sorption of more volatile constituents or gases. These
emissions constitute another type of primary fine particulates.
The processes from which these emissions come include pyrolysis,
incomplete combustion, vaporization of lubricating or process
oils, and many chemical processes such as textile, refinery,
petrochemical and plastics production operations. Forest fires
as well as controlled agricultural and slash burning also are
sources of this category of fine particulates.
Secondary particula_te_s_ result from atmospheric reactions
between gaseous pollutants. Photochemical reactions requiring
sunlight as a stimulus have been long known. Although they
have been studied for several years, these reactions have been
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found to be complex and difficult to model. Some of these
reactions result in condensible, solid or liquid state
components or products that react readily with water to
produce particulates. Since these secondary particulates
are usually the product of gaseous reactions, they are seldom
if ever the source of metallic particulates. Being anionic in
nature, they can and probably frequently combine with metallic
particulates to form salts.
There are no good data on the relative amounts of primary
and secondary fine particulates in the atmosphere. Since both
are almost exclusively the product of man-made pollution, they
would be expected to vary significantly from site to site
depending upon such factors as industry composition, fuel-use
patterns, and climate and weather. It seems clear, however,
that since both primary and secondary particulates are the
results of emissions from human activity, the key to their
control is to prevent their release or the release of their
precursors to the atmosphere.
lias is of Concern for Fine Particulates
As is frequently the case with non-infectious pollutants
and toxicants, the health effects case against fine particulates
is not clear cut. First it must be remembered that fine particu-
lates are not a single pollutant but a large category of
pollutants with a common set of size, transport and behavioral
characteristics. Once dispersed, fine particulates behave,
depending upon- their size, like something between a coarse
particle and a gas. They remain suspended, diffuse, are subject
to brownian motion, exhibit little inertial characteristics,
follow fluid flow around obstacles, and like gas molecules can
penetrate deeply into the respiratory system.
The moderate amount of information that is available con-
cerning the deposition of particles in man and lower animals
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is based upon mathematical models and to a limited extent,
experimental data obtained from man. Particles larger than
5 microns are deposited in the nasal cavity or nasopharynx.
Increasing numbers of smaller particles are deposited in the
lungs. Contrary to the often quoted position that particles
less than about 0.2 microns enter the respiratory system and
are subsequently exhaled, over 501 of the number of particles
between 0.01 and 0.1 microns that penetrate into the pulmonary
will be deposited. This ability of particulates to penetrate
into the respiratory system and be captured is principally a
function of their size and is almost completely independent of
the chemical properties of the particle.
The health effects of fine particulates that have pene-
trated the respiratory system and been captured, on the other
hand, is almost completely dependent on their chemical or
toxic nature. It is, therefore, not possible to generalize
on health effects of fine particulates;,. rather the health
effects of specific materials must be considered. Here the
data become sparce and it becomes necessary to draw on our
knowledge of toxic characteristics of specific substances
gained through other information sources and our understanding
of physiological mechanisms that work to dispose of collected
materials.
The principal means through which air pollutants exert
an effect on health is through inhalation and direct effects
on the respiratory system. They may result in short term
irritant effects or longer term damage such as silicosis or
asbestoses. In either case the respiratory system is directly
impaired.
A second mechanism involves the respiratory system in-
directly as a significant route of entry for non-respiratory
toxicants. In this case, substances which are deposited in the
respiratory system are translocated to the gastro-intestinal
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system by muco-cilliary transport and swallowed. They may
then exert a primary toxic effect or be absorbed and trans-
located to other tissues where an adverse health effect might
be elicited.
In addition to the chemical and toxic nature of the
particulate, they have potential for causing adverse health
effects dependent upon the solubility of the particle in the
transport mucus; if highly soluble, particulates may cause
toxic inflammation.
Particles deposited deep in the lungs may be cleared
very slowly. In this case, the clearing is d'ependent upon
particle solubility. The preponderance of evidence indicates
that non-soluble particles remain deep in the lungs for long
periods--weeks, months, even years. Thus, the carcinogenic
hazard of long-lived radioactive metals and airborne chemicals;
hydrocarbons are of special concern.
Because certain metals may be soluble in respiratory
secretion, the toxic properties of these substances may be
manifested in the lung or airways or may be translocated to
other organs. Vanadium is one example of such a metal. Un-
fortunately, however, the minimum time concentration exposure
of vanadium and other metals is not adequately known. Needless
to say, the combined effects of multiple pollutants is also not
adequately known.
Because of the present paucity of knowledge concerning
the health effects of specific pollutants and combinations of
pollutants, many years will be required to develop a data base
to quantify the health effects problem of fine particulates.
This quantitative understanding will come as our data base is
enlarged through continued and expanded programs such as the
EPA Community Health Environmental Surveillance Study (CHESS)
program and studies of selected cities in the United States.
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Sufficient information does exist, however, to conclude that
fine particulates must be controlled if public health is to
be protected. It is therefore essential that the research
strategies for fine particulate control include a program of
control technology development that will assure the ability
of industry to prevent the release of primary particulates
and the gaseous precursors of secondary fine particulates.
The need for control of fine particulate has only recently
been recognized. We do not adequately understand it importance
or effect on public health and welfare. We do not *know what
pollutants are most important or insidious or to what extent
they should be controlled. We do recognize, however, that
control is necessary.
Since this is a new problem the technology requirements
for measurement and control are not yet 'assessed and developed.
It is essential that we learn what level of control can be
^
achieved with existing control technology, where existing sys-
tems can be applied, and what new technologies are needed.
New, advanced, more economic methods are needed to fill the
technological gaps.
Most conventional scrubbers, when used in the conventional
way, have a limited capability for controlling fine particulate.
This is because most conventional scrubbers depend on some form
of inertial collection of particulate as their primary mechanism
of capture. Because of this, collection efficiency decreases
rapidly as particle size is decreased to the point where inertial
forces become insignificantly small. As a result it becomes
necessary to greatly increase the energy input into a scrubber
to significantly improve its ability to collect smaller particu-
late. Even with large energy inputs their collection efficien-
cies in the sub-micron ran^e is not good.
Scrubbers on the other hand have unique characteristics
useful to fine particulate control. Since the particulate
collection media is liquid the captured particle is trapped
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in such a way as to avoid reentrainment and can be easily
removed from the collection device. Scrubbers also can be
used with potentially explosive gases and high temperature
gases where cooling of the gas is acceptable.
Beyond their normal mode of operation* scrubbers offer
considerable potential for modification for use in fine
particulate control. Scrubbers designed to make maximum use
of filtration or electrostatic mechanisms may greatly extend
their capability into the small particle region. Using the
scrubber as*a condensation device has already been demonstrated
to significantly increase their fine particulate collection
efficiency. Hybrids of these and other techniques such as
sonics, foams, and fluidized beds may provide opportunities
for needed break-throughs that will permit efficient and
economic use of scrubbers for fine particulate control.
The principal reason why we do not today have all the
instrumentation we would like for measuring particle size;
adequate control devices for collection of fine particulate
and adequate information on health effects is that prior to
the past year or so, there has been no need for collecting
fine particulates. The small amount of product lost, ash un-
collected, or haze residual in discharge plumes was unimportant.
It has now become important.
I am convinces that starting with a proper understanding
of the problem and its priority we can develop the necessary
technologies and data to deal with the problem. Many possible
and some promising techniques have already suggested themselves.
I am convinced that given a little time and the necessary re-
sources the scientific community will rise to the challenge.
Key to achieving the objectives of defining and controlling
the fine particulate air pollution problem is good information
exchange. Seldom in modern times have major developments been
made by one man or woman working by himself. Communication and
10
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the resultant exchange and cross pollination of ideas have
been the key ingredient responsible for the exponential
progress that has been made in the field of technology in the
past 100 years of man's history. Symposia such as this with
their papers, debates, questions and answers, negotiations
and general dialog is a major form of this essential ingredient,
communication.
This symposium has an impressive agenda, list of authors,
participants and attendees. I urge you to take maximum ad-
vantage of this forum to learn, teach, test ideas and establish
continuing working relationships and lines of communication
and cooperation so that we can quickly, efficiently, and
economically and profitably solve the problem of fine particu-
lat control.
n
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'ENGINEERING DESIGN OF FINE PARTICLE SCRUBBERS'
by
Dr. Seymour Calvert, President
A.P.T., Inc.
ABSTRACT
A concise, quantitative picture of the state of the art
of particle scrubbing is presented in the form of performance
prediction methods. A new relationship between the particle
diameter collected at 50% efficiency and scrubber pressure
drop for several of the most common scrubber types is a
design tool of great utility. Scrubber capability for the
collection of sub-micron particles by diffusion is described
in a graph for several scrubber types.
12
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"Engineering Design of Fine Particle Scrubbers"
by D^. Seymour Calvert
President, A. P. T., Inc.
Wet scrubbers of appropriate type have the ability to
collect fine particles (i.e., those smaller than 2.0 ym
diameter) with high efficiency under the right conditions.
This paper is an outline of the general capabilities of
scrubbers and the circumstances under which they will perform
at various levels of efficiency on fine particle collection,
We will begin by noting the general types of scrubbers and
the basic mechanisms by which particles can be separated
from the gas phase.
Scrubbers may be classified according to their geometry,
or their "unit mechanisms", or other characteristics. We
prefer the first two, as given in the "Scrubber Handbook"
(1972) and as summarized in Table I. Hote that the unit
mechanisms are the simple particle collection elements which
account for the scrubber's capability.
Within any of the unit mechanisms the particles may be
separated from the gas by one or more of the following particle
deposition phenomena:
1. Inertial impaction
2. Interception
3. Brownian diffusion
4. Turbulent diffusion
5. Gravitational force
6. Electrostatic flux force
7. Diffusiophoretic flux force
8. Thermophoretic flux force
9. Magnetic flux force
10. Photophoretic flux force
13
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TABLE I
Scrubber Classifications
Geometric Type
Unit Mechanism for Particle
Collection
Plate
Massive packing
Fibrous packing
Pre-Formed Spray
Gas Atomized Spray
Centrifugal
Baffle and Secondary Flow
Impingement and Entrainment
Mechanically aided
Moving Bed
Combinations
Jet impingement, bubbles
Sheets (curved or plane),
jet impingement
Cylinders
Drops
Drops, cylinders, sheets
Sheets
Sheets
Sheets, drops; cylinders,
jets
Drops, cylinders, sheets
Bubbles, sheets
The understanding and analysis of any scrubber can be
reached by determining which combination(s) of unit mechanism
and particle deposition phenomenon are involved. Once the
basic elements of the scrubber are determined and their per-
formance capabilities defined by mathematical equations or
charts, the performance of the scrubber can be predicted.
We will now turn to the discussion of the^ capabilities of
scrubbers in present use; but first we must explain the way
in which we will describe scrubber performance.
Difficulty of separation
The "cut diameter" method, first described in the "Scrubber
Handbook" and further discussed by Calvert et al. (1974), will
be used. This method is based on the idea that the most sig-
nificant single parameter to define both the difficulty of
14
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separating particles from gas and the performance of a
scrubber is the particle diameter for which collection
efficiency is 0.5 (50%).
For inertial impaction, the most common particle
separation process in presently used scrubbers, aerodynamic
diameter defines the particle properties of importance.
d = d (p C')Y2, common units = um(g/cm3)V2=ymA (1)
pa p p
When other separation mechanisms are important, other particle
properties may be more significant but this will occur gener-
ally when "d " is less than a micron.
P
When a range of sizes is involved, the overall collection
efficiency will depend on the amount of each size present and
on the efficiency of collection for that size. We can take
these into account if the difficulty of separation is defined
as the aerodynamic diameter at which collection efficiency
(or penetration) must be 50%, in order that the necessary
overall efficiency for the entire size distribution be attained.
This particle size is the required "separation cut diameter",
"d '' and it is related to the required overall penetration,
KC
Pt, and the size distribution parameters.
The number and weight size distribution data for most
industrial particulate emissions follow the log probability
law. Hence, the two well established parameters of the log-
normal law adequately describe the size distributions of
particulate matter. They are the geometric mean weight diameter
"d " and the geometric standard deviation "a "
Penetration for many types of inertial collection equip-
ment can be expressed as:
Pt = exp (-Aa dpa B) (2)
We use the simplifying assumption that this relationship
can be based on actual diameter, d . This will not introduce
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much error and it will conservatively utilize too low an
efficiency for particles smaller than a micron or so.
Thus:
Pt = exp (-Adp B) (3)
Packed towers, centrifugal scrubbers, and sieve plate
columns follow the above relationship. For the packed tower
and sieve plate column "B" has a value of 2. For centrifugal
scrubbers "B" is about 0.67. Venturi scrubbers also follow
the above relationship and B ~ 2 when the throat impaction
parameter is between 1 and 10.
The overall (integrated) penetration, P t, of any device
on a dust of any type of size distribution will be:
w
Pt = f (£!) Pt (4)
o
The right-hand side of the above equation is the inte-
gral of the product of each weight fraction of dust times
the penetration on that fraction. If equation (4) is solved
for a log-normal size distribution and collection as given
by equation (3), the resulting equation can be solved to
yield Figures 1 and 2.
Figure 1 is a plot of "Ft" vs. (d 5Q/d )B with "B In
(0 )" as a parameter. For a required "Pt" one can find the
value of d~r when "d ", "a ", and "B" are given. For con-
venience, Figure 2 is presented as a plot of "Ft" vs.
(d Cr>/d ) with a as the parameter when B = 2.
pSO' pg' g
To illustrate the use of the separation cut diameter,
assume that 1\ penetration is needed for dust with d = 10 ym,
r &
p = 3g/cm3 and
-------
that is good; so "dRp" is the maximum cut diameter acceptable.
Some scrubbers, such as Venturis, are only approximately fit-
2
ted by relating penetration to exp (d ) and more accurate
plots can be prepared by using more representative performance
equations. To avoid confusion these will not be given here,
although they are presented in the "Scrubber Handbook".
Scrubber Performance
Collection efficiencies have been reported in the form
of "grade efficiency" curves, which are plots of particle
collection efficiency versus particle diameter for "typical"
scrubbers. Unfortunately, there can be great variation in
performance, depending on operating conditions and scrubber
geometry so that one would need a grade efficiency curve for
each important set of parameters .
The cut diameter approach proves to be a much more com-
pact way to characterize scrubber performance. We have
applied it to a number of the important types of scrubbers
and present performance graphs for them. It has the great
virtue of being a single-number criterion with a wide range
of quantitive validity. Capability is defined by "perform-
ance cut diameter", "dpc"> which is the aerodynamic particle
diameter at which the scrubber gives 50% collection efficiency.
Once a scrubber type, size, and operating conditions are
chosen by matching the "separation" and "performance" cut
diameters, (i.e., d = dp ) a more accurate efficiency-
diameter relationship can be developed and a more accurate
computation of overall penetration can be made. The reason
this step is necessary is that the relationship between overall
penetration and separation cut diameter is shown in Figures 1
and 2 is only correct for packed beds and similar devices and
is an approximation for others.
Spray Chamber Performance
A spray chamber consists essentially of a round or
rectangular chamber into which water is introduced through
17
-------
one or more sprays. Drop size depends upon liquid pressure
and the type of nozzle used. Some solutions of the equa-
tions for inertial collection in a counter-current spray
chamber are plotted in Figure 3 as "dpc", vs. column height,
with drop diameter, air velocity and water to air ratio as
parameters. Standard air and water properties have been
used, so the figures can only be applied to cases where the
gas and liquid properties approximate those. In the cross-
flow case the water is sprayed at the top of the spray chamber
while the gas flows horizontally. The predictions for
inertial collection in a cross-current spray chamber are
plotted in Figure 4.
The lack of uniformity in liquid distribution and the
fraction of liquid hitting the walls will vary from one
spray chamber to another and will introduce empirical cor-
rection factor. For small scrubbers the correction factor
might be on the order of 0.2 to account for spray running
down the walls, (i.e., use 0.2 x Qj/QG actual.)
Venturi
Venturi scrubbers employ gradually converging, then
diverging sections, although geometry does not seem to have
an important effect on performance. Usually liquid enters
the venturi upstream of the throat through nozzles. Alter-
nately, the liquid may flow along the converging section
walls until reaching the throat. At the throat, the liquid
is shattered into droplets by the high velocity gas.
Venturi performance is shown in terms of its predicted
aerodynamic cut size against gas velocity, with liquid to
gas ratio as parameter, and with constant pressure drop
lines indicated in Figure 5, based on mean drop diameter.
A value of 0.25 for the empirical factor "f" has been used
as is appropriate for hydrophobic particles and medium to
high liquid to gas ratios. For hydrophyllic particles a value
of 0.4 to 0.5 for "f" should be used.
18
-------
Once one has computed the required separation cut
diameter for a given application, he can find the approxi-
mate operating region from Figure 5 for hydrophobic particles.
This does not tell the whole story however, because the
penetration depends not only on the collection efficiency of
a. single drop but also on the extent to which the gas is
swept by drops. In other words, the drop holdup [volume
fraction drops) in the throat is a significant factor in
determining particle penetration and it cannot be accounted
for by a simple power relationship with particle diameter.
Penetration reaches a limiting value as particle size
increases even if the collection efficiency of one drop
for that size particle approaches 100% when there are not
enough drops to completely sweep the gas stream.
Plate and Packed Columns
Particle separation in sieve (perforated) plates can
be defined mathematically by starting from the basic mechanisms
of particle collection in bubbles and jet impaction and then
correlating experimental data. Some examples of the perform-
ance predictions are given in Figure 6, a plot of aerodynamic
cut diameter for a sieve plate as a function of air velocity
through the holes, u, , the froth density, F, and hole diameter,
d, . Predictions are given for wettable particles, froth
densities of 0.4 and 0.65, and for standard air and water
properties. Note that cut diameter is inversely proportional
to froth density. Froth density must be predicted from
relationships for sieve plate behavior.
Packings
Particle collection in packed columns can be described
in terms of gas flow through curved passages and performance
for a variety of packing shapes can be correlated simply by
the packing diameter. Aerodynamic cut diameter is predicted
as a function of packing diameter, d , bed depth, Z, and bed
19
-------
porosity, e, for three different superficial air velocities
and plotted in Figure 7. Any effect of liquid rate is
neglected; this is on the conservative sice since the avail-
able data indicate that efficiency increases with "Q. "
LI
SCRUBBER ENERGY
The energy required for particle scrubbing is mainly a
function of the gas pressure drop, except for pre-formed
sprays and mechanically aided scrubbers. Previously we have
been shown that there is an empirical relationship between
particle penetration and power input to the scrubber for a
given scrubber and a specific particle size distribution
(Lapple and Karaack (1955) and Semerau (I960)). However, this
"power law" did not provide a way to predict performance vs.
power input for any size dust, without first determining the
relationship experimentally.
A new relationship, between "dpp" and scrubber pressure
drop, has been developed by the author and is presented here.
Figure 8 is a plot of performance cut diameter, dp_, versus
gas pressure drop for sieve plates, venturi (and similar),
impingement plates, and packed columns. Predictions were
made by means of design methods given in the "Scrubber
Handbook".
1. Sieve plate penetration and pressure drop predictions
for one plate are plotted as lines la and Ib for perforation
diameters of 0.5 cm and 0.3 cm, respectively, and F = 0.4.
Cut diameters for other froth densities can be computed from
the relationship that they are inversely proportional to "F".
Cut diameters for two and three plates in series would be 84$
and 80% of those for one plate at any given pressure drop.
Note that these predictions are for wettable particles and
that both froth density and pressure drop are dependent.on
plate design and operation.
20
-------
2. Venturi penetration and pressure drop data are
given for f = 0.25 and £ = 0.5 in lines 2a and 2b, respect-
ively. The predictions are for a liquid to gas ratio,
QT/Qr " 1 £/m3, corresponding to about the minimum pressure
Li Li
drop for a given penetration. Data recently obtained by
A.P.T. for a large coal-fired power plant scrubber fit a
value of f = 0,5.
3. Impingement plate data used for line #3 were
predicted for one plate. Cut diameters for 2 and 3 plates
in series are 88% and 83% of those shown in line #3.
4. Packed column performance as shown by line #4 is
representative of columns from 1 to 3 meters high and
packing of 2.5 cm nominal diameter.
To estimate the penetration for particle diameters
other than the cut size, under a given set of operating
conditions, one can use the approximation of equation (3)
with B = 2.0. Alternatively, one could use more precise data
or predictions for a given scrubber. Figure 9 is a plot of
the ratio of particle aerodynamic diameter to cut diameter
versus penetration for that size particle (d ) , on log-
pa
probability paper. One line is for equation (3) and the other
is based on data for a venturi scrubber.
Performance Limit for Inertial Impaction
The limit of what one can expect of a scrubber utilizing
inertial impaction is clearly indicated by Figure 8. If a
cut diameter of 1.0 pmA, or smaller is required, the necessary
pressure drop is in the medium to high energy range. High
efficiency on particles smaller than 0.5 ymA diameter would
require extremely high pressure drop if inertial impaction
were the only mechanism active.
High efficiency scrubbing of sub-micron particles at
moderate pressure drop is possible, but it required either
the application of some particle separation force which is
21
-------
not dependent on gas velocity or the growth of particles so
that they can be collected easily. Particle separation
phenomena which offer promise and have been proven to some
extent are the "flux forces" due to diffusiophoresis, thermo-
phoresis, and electrophoresis. Brownian diffusion is also
useful when particles are smaller than about 0.1 ym diameter.
Particle growth can be accomplished through:
1. Coagulation (agglomeration)
2. Chemical reaction
3. Condensation on particles
4. Ultrasonic vibrations
5. Electrostatic attraction
Diffusional Collection
Particle collection by Brownian diffusion can be de-
scribed by relationships for mass transfer and it is possible
to outline the magnitude of efficiency which can be attained
with typical scrubbers. The general relationship which
describes particle deposition in any control device in which
turbulent mixing eliminates any concentration gradient normal
to the flow outside the boundary layer and in which the dep-
osition velocity is constant is:
Pt = exp - 1-^5—-J (5)
The particle deposition velocity for Brownian diffusion,
u_D, can be estimated from penetration theory as:
UBD = 1.12
For packed columns^ the penetration time, 0, can be taken
as the time required for the gas to travel one packing diam-
eter. For plate scrubbers which involve bubbles rising
through liquid, the penetration time for a circulating bubble
22
-------
is about that for the bubble to rise one diameter, as shown
by Taheri and Calvert (1968). For spray scrubbers the pene-
tration time is that for the gas to travel one drop diameter.
Predictions of particle penetration due to Brownian
diffusion only were made by means of equations (5) and (6)
for a typical sieve plate and packed columns. A prediction
for a venturi scrubber was made by means of "Scrubber Hand-
book" equation (5.2.6-17), for gas phase controlled mass
transfer.
The results are plotted on Figure 11 as collection
efficiency vs. particle diameter. It can be seen that high
efficiency collection of 0.01 ym diameter particles is
readily attainable with a three plate scrubber, typical of
a moderately effective device for mass transfer. Collection
efficiency for particles a few tenths micron diameter is
poor, as is well known.
Particle separation by flux force mechanisms is not
amenable to such simple treatment as Brownian diffusion
because of the variation of deposition velocity with heat
and mass transfer rates within the scrubber. Since this
topic is being covered by another presentation in this
symposium, it will not be discussed further in this paper.
Summary and Conclusions
Wet scrubbers can collect fine particles with high
efficiency under the proper circumstances. When particle
collection is due to inerti-al impaction only, high efficiency
on sub-micron particles requires the expenditure of high
pressure drop. Other particle separation phenomena such
as Brownian diffusion, diffusiophoresis, thermophoresis,
and electrophoresis can give high efficiency at low pressure
drop. Particle growth by any of several mechanisms can
provide the means for subsequent high efficiency collection
23
-------
by inertial impaction.
The new relationship between particle cut diameter
and scrubber pressure drop for collection by inertial
impaction, which is presented here, provides a simple
means for estimating scrubber performance. More refined
predictions can be made by means of the performance cut
diameter method.
REFERENCES
S. Calvert, J. Goldshmid, D. Leith, and D. Mehta. "Scrubber
Handbook", A.P.T., Inc., Riverside, California. EPA Contract
No. CPA-70-95. August 1972. PB-213-016.
S. Calvert, J. Goldshmid, and D. Leith, "Scrubber Performance
for Particle Collection", A.I.Ch.E. Symposium Series 70
(137):357 (1974)
C. W. Lapple, and H. J. Kamack. "Performance of Wet Dust
Scrubbers". Chem. Eng. Prog. 5U3) :110-121, March 1955.
K. T. Semrau. J. Air Pollution Control Assoc. 10, 200
(1960) .
M. Taheri, and S. Calvert. "Removal of Small Particles From
Air by Foam in a Sieve-Plate Column". J. Air Pollution
Control Assoc. 18:240-245, 1968.
24
-------
NOMENCLATURE
Latin
A a constant in eq. (3)
A, = total collection surface area in scrubber cm2
B = a constant in eq. (3)
C1 = Cunningham "slip" correction factor, dimensionless
d, = bubble diameter, cm
d = packing diameter (nominal), cm
d, = drop diameter, cm
d, = sieve plate hole diameter, cm
d = particle diameter ym or cm
d rn ~ diameter of particle collected with 50% efficiency urn
d = aerodynamic particle diameter umA
pa
dpc = performance cut diameter (aerodynamic), ymA
d = geometric mean particle diameter, urn
sr o
dR(_, = required separation cut diameter (aerodynamic) , umA
D = particle diffusivity, cmz/sec
E = efficiency, fraction or %
f = empirical constant for sprays, dimensionless
F = foam density, g/cm3
h = height of scrubber, cm
Pt = penetration = 1 - E3 fraction or %
P~t = average (integrated over particle size distributipn)
penetration, fraction or I
A? = pressure drop, cm W.C. or atm.
25
-------
Qp = gas volumetric flow rate, m3/sec
Qj = liquid volume flow, m3/sec or I/sec
UBD = Particle deposition velocity for Brownian diffusion,
cm/sec
Up = gas velocity relative to duct, cm/sec
u, = gas velocity through sieve plate hole, cm/sec
UPD = Particle deposition velocity, cm/sec
w = weight of particles, g
W.C. = water column = pressure as measured by water manometer,
cm
,Z = height of packing or column, m or cm
Greek
e = fraction void volume space
p. = liquid density, g/cm3
3
p = particle density, g/cm
a = geometric standard deviation of particle size dis-
5
tribution
8 = penetration time, sec.
26
-------
a:
h-
UJ
z
LJ
CL
Q
UJ
UJ
0.001
0.001
0.01
0.
1.0
FIGURE 1. INTEGRATED (OVERALL) PENETRATION AS A
FUNCTION OF CUT DIAMETER, PARTICLE
PARAMETERS AND COLLECTOR CHARACTERISTIC.
27
-------
1.0
lol
«•
g
H
tr
I-
LL)
Z
LJ
a.
a:
UJ
g
Pt=EXP -Ad
O.I
.01
.001
1 I
0.001
0.01
O.I
1.0
FIGURE 2
OVERALL PENETRATION AS A FUNCTION OF
CUT DIAMETER AND PARTICLE PARAMETERS
FOR COMMON SCRUBBER CHARCTERISTIC,
B= 2.
28
-------
IOO
o
CL 10
T3
Q /Q =l//m3
3 L G
u =0.6m/sec
b
O.I
1.0
10
(O
CURVE NO. i
DROP OIA, |J.m 200
2
500
3
1,000
FIGURE 3
PERFORMANCE CUT DIAMETER PREDICTIONS FOR
TYPICAL VERTICAL COUNTERCURRENT SPRAY.
100
O
Q.
-a
10
Q./Q =l//m3
L G
O.I
1.0
10
h(m)
(B)
FIGURE 4
PERFORMANCE CUT DIAMETER PREDICTIONS
FOR TYPICAL CROSS-CURRENT SPRAY
29
-------
3.0
2.0
I.O
.5
f=0.25
0.2
.4 .5
O
Q.
0L/QG (l/m3)
FIGURE 5
PERFORMANCE CUT DIAMETER PREDICTIONS FOR
VENTURI SCRUBBER.
0.5
30
UQ= HOLE VELOCITY m/sec
FIGURI: 6
PERFORI.1ANCE CUT DIAMETER PREDICTION
FOR TYPICAL SIEVIZ PLATE CONDITIONS.
-------
o
a
T3
4
3
o
Q.
•o
Of 2
UJ
1'°
O
(J
O
U
0.5
0.2
O.I
Z(m)
(A3
FIGURE 7
PERFORMANCE CUT DFAMETER PREDICTIONS FOR
TYPICAL PACKED BED CONDITIONS.
la
Ib Sieve, F=0.4,dh = 0.3cm.
2a Ventun,F=0.25
2b Venturi,F=0.5
3 Impingement Plate
4 Packed Column, dc= 2.5 cm
10
20 30 40 50
100
2OO 300
PRESSURF DROP, cm. W.C
FIGURE 8
REPRESENTATIVE CUT DIAMETERS AS A FUNCTION
OF PRESSURE DROP FOR SEVERAL SCRUBBER TYPES.
31
-------
10
a
Q.
1.0
For Venturi
0.1
x- / 2
' ' For Pt = exp - A d Dt
10
50
90
99
COLLECTION EFFICIENCY FOR dDn {%)
pa
FIGURE 9
RATIO OF PARTICLE DIAMETER TO CUT DIAMETER AS A
FUNCTION OF COLLECTION EFFICIENCY.
100
0.01
PARTICLE DIAMETER, //m
FIGURE 10
PREDICTED PARTICLE COLLECTION BY DIFFUSION
IN PLATES,PACKING, AND VENTURI SCRUBBERS.
32
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SUBMICRON PARTICULATE SCRUBBING
WITH A TWO PHASE JET SCRUBBER
by
H. E. Gardenier
Aronetics, Inc.
Houston, Texas
ABSTRACT
The two-phase jet scrubbing system utilizes waste
thermal energy to provide the operating power for a two-
phase jet which simultaneously cleans and induces the
necessary draft. Test results have shown excellent clean-
ing performance and a substantially reduced operating
cost.
33
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"Stibmicron Particulate Scrubbing With a Two
Phase Jet Scrubber"
by
H. E. Gardenier
Aronetics, Inc.
Houston, Texas
There are dozens of different types of devices which
fall into the general classification of wet scrubber. How-
ever, only a very limited number of types of equipment are
capable of efficiently removing submicron particulate. Much
has been written regarding the actual mechanism by which
solid particulate is entrained in a scrubbing fluid. Experts
disagree regarding the predominant physical phenomena. In
our view the primary mechanism is inertial impaction; conden-
sation and Brownian movement are secondary effects which may
improve the overall scrubber efficiency. For certain cases
the solid particulate size approaches the molecule size of
the carrier gas; as a result it becomes a very significant
problem to provide contact between scrubber fluid droplets
and the solid particulate. Providing the impact energy in
an economical manner is indeed a challenging task.
If a scrubber is to have reasonable water requirements
and yet provide a high probability of contact between the
liquid droplets and the solid particulate there must be a
very fine atomization of the scrubbing liquid. This provides
a high population density of droplets and a good probability
of collision between particles and droplets. The inertial
impaction energy must come from a difference in velocity
between the droplet and the solid particulate. Either high
velocity particulate must be introduced into a finely atom-
ized scrubbing fluid, or a high velocity, finely atomized
scrubbing fluid must be introduced into the particulate laden
34
-------
gas stream. Assuming that fine atomization has been achieved,
and the droplets are uniformly distributed throughout the gas
stream, then the controlling parameter in cleaning efficiency
and the determining factor in the size of particulate that
can be removed, becomes the differential velocity between the
droplet and the particulate.
Several mechanisms are commercially available to achieve
some degree of the desired results and the most familiar is
the venturi scrubber. While there are many variations of the
venturi scrubber, the general concept is to accelerate the
particle laden gas through a restriction in the ducting where
water is injected into the gas stream. The velocity of the
gas stream provides the dual function of atomizing the scrub-
bing fluid and providing a differential velocity between the
particulate and the liquid droplets. By utilizing very high
power fans to accelerate the gas stream, it is possible to
generate a gas velocity at the venturi .throat of as much as
500 ft. per second. The pressure drop, and therefore the
fan horsepower requirement, is directly proportional to the
gas velocity squared, and the liquid to gas ratio (the term
L/G used herein is the conventional scrubber terminology of
liquid, in gallons, over gas flow, in thousands of standard
cubic feet). It is obvious that velocities are possible only
at a substantial pressure drop which results in a high level
of energy expended.
Alternate Scrubber Methods
Another conventional approach to achieving a velocity
differential between scrubbing liquid droplets and particulate
matter is a high pressure water jet scrubber. In this type
of device the liquid is injected at a high pressure into the
slow moving gas stream. A proper nozzle configuration will
provide satisfactory atomization of the scrubbing liquid and
will also induce a modest draft which assists in drawing the
35
-------
gas through the cleaning device. Power requirements for the
fan ?re either eliminated completely or substantially reduced
when compared to the venturi type scrubber. However, refer-
ring to Figure 1 we find again that a differential velocity
of over 500 feet per second is quite difficult to obtain in
this type scrubber. The limiting factors are the practical
considerations of pump output pressure and reliability of
equipment operating at these pressure levels. In an effort
to overcome this limitation several devices have been developed
which use high pressure gas or steam to provide the motive
force to shear and accelerate the water droplets. A number
of patents have been issued on devices of this type and some
are in operation with varying degrees of success.
TWO-PHASE JET SCRUBBER
During the past five years our company has worked on the
development and application of the type of scrubber which
overcomes some of the inherent disadvantages of many scrubbers
used for the removal of submicron particulate. We have dis-
covered that a pressurized, heated, liquid when passed through
a properly designed nozzle will produce a two-phase mixture
of vapor and liquid droplets that is an excellent cleaning
medium. The droplets can be accelerated to extremely high
velocity as a result of the expansion force created by a
portion of the liquid being converted to vapor. The general
configuration of this type of scrubber is shown in Figure 2.
As with the water jet scrubber, the proper arrangement of
components allows a draft to be induced which eliminates or
drastically reduces fan power requirements. The two-phase
jet scrubber produces water droplet velocities which vary
with the temperature of the scrubbing fluid as shown in
Figure 3.
It is our experience that jet velocities in the range
of 1000 feet per second are quite satisfactory for particulate
36
-------
removal in the size range down to 0.1 microns. When these
data are presented, we are frequently asked two questions:
(1) Is this a supersonic velocity? [2] What noise level
is generated from this type of device? The answer to the
first question is yes; the velocity in the region immediately
downstream of the nozzle is probably substantially supersonic
since there is considerable evidence that sonic velocity in a
two-phase mixture may be as low as 350 feet per second. How-
ever, we believe that the inertlal impaction is the control-
ling mechanism in cleaning and that the existence, or absence
of the shock phenomena associated with supersonic flow is
not an advantage in the cleaning effectiveness. Thus, the
velocity in feet per second is the controlling parameter
rather than the Mach Number, or relationship of velocity to
the local speed of sound. The answer to the second question
is that the device operates at a very low noise level. The
sound frequency and level is similar to that generated by a
garden hose nozzle.
With respect to the draft producing capacity of this
type of scrubber system, it should be evident that this is
a pure momentum transfer mechanism; therefore, the amount of
draft is a function of the amount of fluid passed through the
nozzle and the degree to which this fluid is accelerated.
The velbcity is a function of initial water temperature, as
shown in the previous figure. The effect of water flow rate
on system pressure rise is shown in Figure 4 for water at a
temperature of 400°F. It should be emphasized that this is
a rise in pressure across the scrubber and should not be
confused with the pressure drop which is associated with the
venturi type scrubber. In most applications the pressure
rise produced by the two-phase jet is sufficient to overcome
the pressure drop in other components of a complete system.
37
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APPLICATION OF TWO-PHASE SCRUBBING
The most direct application of the two-phase scrubbing
system is in the control of emissions from processes which
generate high temperature gas, laden with submicron particu-
late. Typical examples are the various metallurgical furnaces
and processes. Figure 9 shows schematically the general
arrangement of components for one type of application where
the exhaust gas is at an elevated temperature. If the process
off-gas is at a temperature of 800°F or above, an economizer
type of heat exchanger may be used to transfer thermal energy
from the gas to pressurized hot water which is delivered to
the heat exchanger by a pump. Water exiting the heat exchanger
is delivered directly to the nozzle in its liquid state.
For most applications the water temperature is approxi-
mately 400°F and the water pressure is approximately 350 psi,
or high enough to ensure that the fluid remains in the liquid
state until it has passed the nozzle throat. A properly
dimensioned mixing section must be provided for intimate
contact between the accelerated water droplets and the particle
laden gas. The final component in the scrubbing system train
is a. separator which will remove the dirty water droplets and
allow the clean gas to be discharged. Water drained from the
separator is passed to water treatment equipment which may be
used to remove substances scrubbed from the gas and to prepare
the scrubbing liquid for recycling.
If the process off-gas is at a temperature level above
1200°F, an additional option becomes available in the selection
of components. The economizer type of heat exchanger may still
be used to deliver heated water directly to the nozzle, or a
steam boiler may be used as an intermediate step in the heating
of water. If sufficient energy is contained in the gas it is
possible that a quantity of steam may be produced which is
38
-------
greater than the demands of the scrubber. This steam is then
available for other possible plant applications. Other ele-
ments of the system remain essentially the same with the
exception of the addition of a method to transfer thermal
energy from steam to water. The choice between the steam
boiler and the economizer type of system for the very high
temperature gases is dictated by local conditions at the site
in question.
A number of industrial processes produce a combustible
gas which has sufficient heating value to be used as a fuel.
Typical examples are carbide furnaces and basic oxygen furn-
aces with a sealed hood. For this type of application the
process off-gas is cleaned and cooled as quickly as practical
to reduce the explosion hazard. The two-phase nozzle, mixing
section, and separator are similar in configuration to those
used in other types of applications. A portion of the fuel
gas is drawn off to supply an externally fired water heater
or boiler which generates the heated fluid required for the
two-phase jet scrubber.
This general type of arrangement may also be practical in
some cases where the fuel must be supplied from a purchased
source. It can be shown that the total system operating cost,
utilizing purchased fuel as opposed to utilizing purchased
electricity for a venturi scrubber system, is approximately
equal if the venturi pressure drop is approximately 60 inches
of water. For greater venturi pressure drop requirements,
the cost of purchased electrical energy is increasingly more
expensive when compared to fuel costs to achieve the same
cleaning results in a two-phase jet scrubber.
Cleaning^ Performance
Cleaning performance results have been obtained during
the last three years with plant installations operated by
customer personnel. The point of equipment operated by cust-
omer personnel should be emphasized since there is a substantial
39
-------
difference between our trained technical personnel operating
the equipment at peak performance, as opposed to customer
personnel operating it as they see fit. As a point of inter-
est, the latest system provided by our company has been in
operation for three months, and no Aronetics personnel have
been at the plant site. These tests have been conducted on
a variety of difficult emissions such as solid particulate
emitted from submerged arc furnaces. Particle size distri-
butions obtained from this type of photograph are shown in
Table I. The large percentage of particles less than 0.1
microns should be particularly noted. The cleaning perform-
ance results are shown in terms of outlet grain loading in
Table II. These results meet or exceed current projected
requirements regarding solid particulate emissions. It
should also be noted that opacity requirements are met in
all cases with these levels of outlet grain loading.
FIELD EXPERIENCE
It would be a fortunate experience indeed if new equip-
ment were placed in the field without operational difficulties
of some kind. The engineering prototype of the two-phase jet
system did encounter certain difficulties which required
modifications to the original installation. It is perhaps
of some value to document the problems encountered and the
solutions that were implemented.
The original selection of a heat exchanger, on first cost
economic considerations, was a fire tube heat exchanger arranged
in a vertical orientation with hot gas entering at the top and
hot water exiting from the top of the heat exchanger. While
this heat exchanger operated for a sufficient period of time
to establish the suitability of the scrubber system concept, it
was replaced after a few months of operation because of recur-
ring mechanical and thermodynamic problems. The replacement
40
-------
heat exchanger is of the conventional water tube design
utilizing bare tubes arranged perpendicular to the gas flow
path. While this type of unit is somewhat more expensive
than the fire tube design, it is much more satisfactory.
When the operation of our original engineering prototype
was begun, we were astounded to discover that the gas weight
flow through the system was SOI less than the value used in
the equipment design. While this would have been a favorable
outcome for many emission control systems it is not an accept-
able situation for the two-phase jet system since it is operated
by thermal energy, and the thermal energy available is a
direct function of the gas weight flow as well as the gas
temperature. In carefully evaluating the design criteria we
believe that it was clearly established that there is a
fundamental error involved in the measurement of a hot gas
flow by the pitotstatic probe method. Results obtained with
this method of data acquisition apparently have a consistent
bias which indicates a higher gas flow than actually exists.
Other experimenters apparently have encountered this
same phenomena; however, we are not aware of a satisfactory
explanation at this time. As a result, we have abandoned
this method of determining gas flow and use other information
which is normally available or can be calculated from the
process data. A final and accurate determination of the gas
weight flow can be made using a thermal balance method after
the installation is completed.
It did not take many days of operation with the original
engineering prototype to discover that this equipment would
be subject to the characteristic deposition at the wet-dry
interface that is encountered in many other scrubber systems.
The solution to this problem is the same as that proven satis-
factory in other installations. Namely, proper irrigation of
the wall at the wet-dry interface.
41
-------
While the original equipment was designed for particulate
removal, it was quickly discovered that the scrubber was quite
capable of removing certain gaseous components such as HCL and
S02. The resulting slurry, naturally, has a very low pH and
will quickly destroy separators constructed of unsuitable
material. After replacing one set of separators we have become
quite conscious of the necessity to identify all components
of the emission in order to avoid unnecessary surprises.
Finally, as a result of operating experience we learned
that it was necessary to face the water quality problem in
conjunction with the air quality problem. With properly
designed, and operated, equipment it has been shown to be
entirely practical to treat scrubber slurry to the extent that
it can be either reused or discharged in the public waterways
in a condition satisfactory to meet the recently published
Environmental Protection Agency effluent guidelines. It is
our opinion that wet scrubber manufacturers will be required
in the future to provide their customers with adequate service
in the water treatment aspects of the entire pollution control
problem.
Along with the unpleasant surprises encountered in initial
field experience with new equipment, one would hope to encounter
some things that exceed the initial expectations. The most
important objective of the entire effort was to satisfactorily
remove submicron solid particulate from a gas stream. This
aspect of the equipment performance exceeded our expectations
as indicated in the summary presented in Table II. As previousl)
mentioned in this paper the soundlessness of the equipment in
operation was a very pleasant surprise. No erosion of any
kind has been experienced in the Aronetics equipment operating
at this time.
One of the important initial objectives of the system,
in addition to satisfactory cleaning performance, was to
42
-------
operate at a substantially lower cost for power than other
systems with a comparable cleaning performance. Personnel
of the Chromium Mining and Smelting Corp. undertook to perform
a complete power consumption cost analysis for the two-phase
jet system, as installed at their Memphis plant, compared to
other types of equipment installed at other plants. We are
indebted to Chromium Mining and Smelting Corp. for providing
the data presented in Table III.
It is our understanding that all power costs associated
with operation of the two-phase jet system was charged to
this system. This means that the cost shown covers all water
transfer pumps, slurry pumps, air compressors, mixers or
other devices which consume electrical energy. The actual
power costs were divided by the furnace input power to estab-
lish the air pollution control power cost per megawatt of
furnace power on an annual basis. In each case this number
was multiplied by 40 to adjust each plant to the size of the
Memphis installation. As noted in the right hand column the
closest competing system had an annual power cost of $152,000
in excess of costs incurred with the two-phase jet system.
The amount of water required for a scrubber system is
also a finite operating cost. However, the cost of water
varies so dramatically from one location to another that it
is difficult to assign a dollar value to the water rate. How-
ever, a general trend can be indicated in the L/G ratio to
achieve 99% cleaning efficiency with various particle sizes.
The trend of these data is shown in Figure 6. It should be
kept in mind that the normal venturi L/G is in the range of
10-15 gallons of water per 1000 cubic feet of gas. It is clear
that the smaller particle sizes and higher cleaning efficiencies
require larger quantities of water.
43
-------
CONCLUSIONS
Certain types of wet scrubber systems have demonstrated
satisfactory cleaning performance in the submicron particle
size range. It is believed that inertial impaction is the
principal cleaning mechanism in scrubbers of this type. Be-
cause of the high energy demands required to operate these
scrubbers it is important to consider alternate methods to
achieve the desired results. A suitable alternate, the two-
phase jet scrubber, has been developed. This system utilizes
waste thermal energy to produce a two-phase flow capable of
simultaneous cleaning and draft induction. Test results show
excellent cleaning performance with particulate as small as
0.10 micron.
44
-------
in
4000
100 200 300 400
Water Jet Velocity, ft/sec
500
FIGURE 1. EFFECT'OF'WATER PRESSURE ON WATER JET
VELOCITY FOR A TYPICAL JET SCRUBBER
-------
gas
in
gas and
liquid out
FIGURE 2. GENERALIZED TWO-PHASE JET
SCRUBBER SCHEMATIC
46
-------
450
400
•2 350
2
CD
Q.
1
K 300
Nozzle Efficiency 90 Per Cent
- 100 Per Cent
250
0 200 400 600 800 1000 1200 1400
Water Jet Velocity— ft/sec
FIGURE 3. EFFECT OF WATER TEMPERATURE ON
JET VELOCITY
-------
oo
8
7
•i s
(0
QC
CO 4
to
CD
^ 3
Water Temperature — 400°F
48 12 16
Pressure Rise, Inches of Water
20
FIGURE 4. EFFECT OF LIQUID TO GAS RATIO ON
SYSTEM PRESSURE RISE
-------
two-phase
/\e\ nozzle
mixing
//section
u
—heat exchanger
separator-^
make-up
water
pump
|J stack
hot gas
V
waste water
treatment
.two-phase HL
(jet nozzle /mixing section ^ ^
/hot water
separator-
heater
—boiler
i water in
hot gas
pump
\vasfe ivafer
treatment
FIGURE 5. PROCESS SCHEMATIC FOR HIGH TEMPERATURE
EXHAUST GAS
49
-------
in
O
5 • —
2 --
0.2 0.4 0.6
Particle Size, Microns
0.8
1.0
FIGURE 6. APPROXIMATE WATER RATE REQUIRED
FOR 99% CLEANING EFFIENCY FOR
THE TWO PHASE JET SYSTEM
-------
PARTICLE SIZE DISTRIBUTION OF FERRO- SILICON DUST
80% Ferro-Silicon
50% Ferro-Silicon
Finer Than 0.1 Micron
Finer Than 0.2 Micron
Finer Than 0.3 Micron
Finer Than 0.5 Micron
86.57
91.32
97.57
100.00
89.97
92.47
96.63
100.00
TABLE 1. PARTICLE SIZE DISTRIBUTION FOR EMISSIONS FROM
FERRO-SILICON FURNACES
-------
tn
IsJ
Process
Steel Making
Basic Oxygen Furnace
80% Ferrosilicon
Submerged-Arc Furnace
50% Ferrosilicon
Submerged-Arc Furnace
Silico Manganese
Submerged-Arc Furnace
Charge Chrome
Submerged-Arc Furnace
Cement Making Kiln
Carbon Black Dryer
Particle Size
Distribution
50% Less Than 0.1 Microns
16% Less Than 0.04 Microns
100% Less Than 0.5 Microns
86% Less Than 0.1 Microns
100% Less Than 0.5 Microns
90% Less Than 0.1 Microns
85% Less Than 0.5 Microns
43% Less Than 0.3 Microns
Outlet Grain
Loading, Grains/SCF
0.015
0.02
0.02
0.007
0.01
0.008
0.005
TABLE 2. TYPICAL CLEANING PERFORMANCE RESULTS
-------
Wl
OJ
Operating Electrical Power for Ferro-Alloy
Air Pollution Control Systems
Tjpt
Systim
Annettes®
System
Scrubber
Scrubber
Scrubber
Scrubber
Bag House
Bag House
Bag House
Bag House
Bag House
Bag House
Mint Siio
Mt|i»«m
40 M.W.
• Plant
18M.W.
Plant
27 M.W.
Furnace
27 M.W.
Furnace
28 M.W.
Furnace
17M.W.
Furnace
20 M.W.
19 M.W.
Furnace
35M.W.
18 M.W.
Furnace
20 M.W.
Furnace
Pollution Control
HP. p*r MtfMitt
14
85
106
106
95
276
90
155
95
72.5
109
Ptrctnt of
Furiuct Powtr
7.2%
6.4%
7.9%
7.9%
7.7%
20.6%
6.8%
77-5%
7.7%
5.4%
8.7%
Annual Pown
Coit for Air
Pollution Control M.W.
$970
$5,500
$6,890
$6,890
$6,795
$77,940
$5,900
$70,000
$6,795
$4,772
$7,085
AiMull Powtr
Cost for i
40 M.W. Plint
$36,400
$220,000
$276,000
$276,000
$248,000
$717,600
$235,000
$400,000
$248,000
$188,400
$283,000
Amount of
Powtr Sivint
With Aronttici*
$784,000
$246,000
$246.000
$272,000
$687,000
$799,000
$364,000
$232,000
$752,000
$247,000
Power costs based on rate
Data courtesy of Chromium
of $0.01 per kilowatt hour
Mining and Smelting Corp.
TABLE 3. POWER COST FOR FERRO-ALLOY CONTROL
-------
PERFORMANCE OF A STEAM-EJECTOR SCRUBBER
by
L, E. Sparks
Control Systems Laboratory
and
J. D. McCain and W. B. Smith
Southern Research Institute
ABSTRACT
The results of fractional and overall mass efficiency
tests of a steam-ejector scrubber for controlling particulate
emissions from an open hearth furnace under several operating
conditions are presented. Particulate mass concentrations
were determined by conventional (Method 5) techniques and
cascade impactors for sizes from about 0.3 um to 5 ym. Number
concentrations were measured for sizes smaller than about 1 ym
using optical and diffusional methods.
The measured efficiencies based on total particulate mass
concentrations with the scrubber operating under near optimum
conditions ranged from 99.84$ to 99.9%. The measured
fractional efficiencies ranged from a maximum of 99.991 for
particles having diameters of 1 vim to values of 97 and 99.9%
for particles having sizes of 0.1 urn, 5 urn, respectively.
54
-------
"Performance of a Steam-Ejector Scrubber"
by
L. E. Sparks
Control Systems Laboratory
and
J. D. McCain and W. B. Smith
Southern Research Institute
As part of a program to develop technology to control
fine particulate emissions from stationary sources, the
Control Systems Laboratory of the Environmental Protection
Agency (EPA) seeks out and evaluates novel devices to deter-
mine their potential for collecting fine particles. Emission
tests are conducted on those devices which a preliminary
evaluation indicates have significant potential for collecting
fine particles. These emission tests are designed to deter-
mine both the overall mass efficiency and the efficiency as a
function of particle diameter of the device. The results of
the emission tests of a steam ejector scrubber are presented
in this paper. The results reported herein are based on tests
performed by Southern Research Institute for EPA (McCain and
Smith, 1974) and do not constitute endorsement or recommenda-
tion for use by either EPA or Southern Research Institute.
Description of Open Hearth Facility
At the time tests were conducted four of five open hearth
furnaces at this plant were operating continuously 24 hours
per day. Each furnace produces three 300 ton batchs of steel
per day with the production time scheduled for each of the
four furnaces staggered by about two hours for logistical pur-
poses, although the actual timing for any one furnace varied
somewhat from this schedule. The operations for any one batch
were: (1) charging of the furnace with scrap metal, requiring
55
-------
about four hours, (2) addition of iron directly from a blast
furnace, requiring about thirty minutes, (3) the refining
phase [oxygen lance), requiring about three hours, and
finally (4) furnace tapping and pouring, requiring about
thirty minutes. The actual emission rate and size distribu-
tion of the particulate are quite variable throughout the
cycle. This variability causes some difficulty in both
measurement and interpretation of data as is described in
the discussion section of this report.
The waste process gases, at temperatures of about 1500°F
from the four furnaces, are carried through a series of flues,
flow controllers, and ducts to three waste heat boilers, each
of which supplies steam to drive seven of the scrubber modules
shown in Figure 1. The scrubbers are arranged in a semi-
circle around the boilers. The gas temperature leaving the
boilers is about 530°F. The draft for the entire furnace and
scrubber system is provided by the steam ejectors in the
scrubber modules. Because three boiler/scrubber systems are
used to control the emissions from four furnaces, each system
treats the emissions from more than one furnace. The system
on which the measurements were made was fed primarily by
furnaces 3 and 4 with approximately 671 of the gas handled by
the system coming from furnace No. 4. The fact that more than
one furnace supplied the system being measured also added to
the difficulty in interpreting some of the results and made it
impractical to attempt to isolate certain portions of the over-
all furnace cycle for analysis.
Description of ttie Scrubber
The scrubber tested was a steam ejector scrubber. A
diagram of the scrubber is shown in Figure 1. The atomizer
chamber is a prescrubber to remove large particles and cool
56
-------
the gas. Fine particle collection occurs in the mixing tube
following injection of both steam and water drops. The drops
are removed by cyclone entrainment separators.
Measurement Techniques
A total of four measurement techniques were used during
the tests. These were: (1) diffusional techniques using
condensation nuclei counters and diffusion batteries for deter-
mining concentration and size distribution on a number basis
for particles having diameters less than approximately 0.2 urn,
(2) optical techniques to determine concentrations and size
distribution for particles having diameters between approxi-
mately 0.3 pm and 1.5 ym, (3) inertial techniques using cascade
impactors for determining concentrations and size distributions
on a mass basis for particles having diameters between approxi-
mately 0.25 urn and 5 ym, (4) standard mass train measurements
for determining total inlet and outlet mass loadings.
The useful concentration ranges of both the optical
counter and the condensation nuclei counters are such that
extensive dilution of the gas streams being sampled was re-
quired. Dilution factors of about 65:1 were used for the
outlet measurements and about 500:1 for the inlet measurements.
In order to insure that condensation effects were minimal and
that the particles were dry as measured, the diluent air was
dried and filtered, and diffusional driers were utilized in
the lines carrying the diluted samples to the various instru-
ments.
Because of the size and complexity of the optical and
diffusional measuring systems, and the fact that only one set
of equipment exists for measurements of this type, it was not
possible to obtain simultaneous inlet and outlet data with
these methods. The system was first installed at the outlet
sampling location, the scrubber was tuned, and all the outlet
57
-------
data were obtained. The equipment was then moved to the
inlet and the necessary inlet data were obtained.
For the purposes of calculating the efficiency of the
scrubber, the assumption was made that the inlet data, as
obtained above, were a valid representation of that which
would have been obtained during the time the outlet measure-
ments were made. Accuracy in the diffusional measurements
was limited by process variations and the efficiencies derived
from these data are rather uncertain. However, the trends in
the fractional efficiencies derived from the data are probably
real and the fractions of the influent material that penetrate
the scrubber are probably correct to within a factor of two
to three.
The optical data are presented on the basis of equivalent
polystyrene latex sizes and the indicated sizes can differ
from the true sizes by factors as large as two to three. Data
obtained using this method were primarily intended as a means
of real time monitoring of process changes and the results of
changes in the scrubber operation, but also serve as rough
checks on the data obtained with the cascade impactors. The
sampling system used for obtaining the optical and diffusional
data is illustrated diagrammatically in Figure 2.
Inertial sizing was accomplished using Brink cascade
impactors for inlet measurements and Andersen impactors for
outlet measurements. Sampling was done at near isokinetic
rates. Errors due to deviations from isokinetic sampling
should be of little consequence for particles having aerody-
namic diameters smaller than 5 ym or physical diameters smaller
than 2 ym for and assumed density of 5.2 gm/cm3. Further,
because the sampling was at near isokinetic rates, the calculated
collection efficiencies for larger particles are probably
reasonably close to the true values. Because of the relatively
small duct dimensions as compared to the sizes of the impactors,
single point sampling was used in the ducts with the inlet
58
-------
impactors at flue gas temperature O515°F). The outlet
impactors were heated to about 40° above flue gas temperature
to insure that no condensation took place within the impactor.
Such condensation might cause operational difficulties or lead
to incorrect sizing.
Because of the wide disparity in the inlet and outlet mass
loadings (inlet ^1-2 grains/cf, and outlet ^0.001 grain/cf)
complete simultaneity in the inlet and outlet sampling was not
possible. Outlet samples were generally of about 6 hours
duration while inlet samples were of about 6 minutes duration.
Because of the very low outlet loading and the consequent
length of the outlet sampling time, it was found to be im-
practical to attempt to isolate individual portions of the
overall furnace cycle for analysis. Since the inlet sampling
could not correspond directly with the outlet sampling, an
inlet mass loading history for one complete furnace cycle
was synthesized for each size interval .covered by the inlet
impaction stages. Examples of these synthesized histories are
shown in Figure 3.
Results
The tests took place on December 4 through December 11,
1973, with December 4 primarily used for instrumentation setup,
checkout, and preliminary measurements. Optimization of the
scrubber operating parameters was accomplished on December 5
using the optical and condensation nuclei counters. The
results of these tests are given in Table I, which includes
the three primary operating variables (cyclone accelerator
position, steam pressure at the ejection nozzle inlet, and gas
flow rate). Direct comparisons of data between some of the
test conditions are not meaningful because of variations in the
open hearth process. This is especially true of tests that are
separated by periods of more than a few minutes. The optimum
59
-------
conditions appeared to be accelerator position 3, steam
pressure 250 Ibs and 11,000 scfm flow rate. Other conditions
tested were accelerator position 2, 250 Ibs steam pressure and
15,000 scfm; and accelerator position 3, 300 Ibs steam pressure,
and 13,000 scfm.
Diffusional data for efficiencies below 0.3 ym were ob-
tained only under the apparent optimum condition. A brief
test using the condensation nuclei and optical techniques
with the atomizer water turned off indicated a definite increase
in the concentration of submicron particles with the atomizer
water off. Insufficient data were obtained to fully quantify
the effect. Impactor data were obtained under both optimum
and non-optimum operating conditions.
Figure 4 shows typical inlet and outlet size distribu-
tions as obtained by optical and diffusional methods during
oxygen lance, the most stable process during the heat cycle.
Figure 5 shows the fractional efficiencies calculated from
these data together with a set of typical results from the
impactor measurements. Fractional efficiencies based on the
impactor data only are shown in Figure 6. The results of EPA
method 5 mass train tests are shown in Table II.
Conclusions
The collection efficiency of the steam-ejector air clean-
ing system is quite high. As measured using conventional
(Method 5) techniques the efficiency was 99.90 and 99.84% for
two days of testing. Measured fractional efficiencies were
about 901 at 0.01 ym, about 70% at 0.05 ym, 851 at 0.1 urn,
99.9% at 0.5 ym, 99.99% at 1 ym, and 99.6% at 5 ym. The
minimum in the fractional efficiency at about 0.05 ym is prob-
ably real, but the actual value is somewhat uncertain because
of difficulties in making diffusional measurements in the time
variable open hearth process. The manufacturer's estimate of
60
-------
the energy requirements for achieving the efficiencies given
above are approximately 8250 BTU/1000 SCF to 12,750 BTU/10QO
SCF for system back pressures ranging from one to six inches
of water.
References
J. D. McCain and W. B. Smith (1974). Report to Environ-
mental Protection Agency NTIS No. PB232-436/AS
Acknowledgements
The work reported herein was conducted under Task No. 11
of EPA Contract 68-02-1308.
61
-------
TABLE I
Optimization of Scrubber Performance
Particle
Acceleration
Time
1130
1140
1150
1225
1210
1240
1255
1300
1310
1330
1340
O\ 1730
10 1740
1750
Position
2
2
2
1
1
1
0
0
0
2
2
3
3
3
Steam
Pressure
psig
250
300
350
250
300
350
250
300
350
250
300
350
300
250
Gas Flow
Ibs/min
1320
1453
1526
1404
1498
1581
1510
1612
1702
1096
1182
1163
1048
916
Particles/on1* Particles/cm'*
Dia.iO,06 kirn
0.72 x 10'
0.69
0.90
0.94
1.S1
2.24
0.93
1.56
1.47
0.78
0.78
1.38
1.1
0.79
Dia.^0.45 um
2.0 x 10'
2.0
2.2
> 2.3
> 2.3
> 2.3
> 2.4
> 2.4
> 2.4
> 2.2
> 2.2
1.35
1.24
1.17
Particles/cm1*
Dia.il. 0 um
45
14
200
2100
2400
2400
> 2400
> 2400
1200
1.6
1.9
6.7
< 2
< 2
Particles/cm1'
Dia.il. 6 um
2
2
11
105
123
248
588
235
214
11
11
< 2
< 2
< 2
1800
250
1283
0.85
> 2.1
15
Concentration of particles larger than the stated size in the scrubber effluent gas stream.
Table II
Mass Train Results Steam Ejector Scrubber
Date
Test No.
Flow/DSCFM
Grains/DSCF Inlet
Grains/DSCF Outlet
Penetration I
12/10
I
9110
0.597
0.0003
0.050
12/10
2
10476
0.255
0.0003
0.20
12/10
3
11036
0.525
0.0005
0.10
12/11
1
8849
0.312
0.0007
0.22
12/11
2
10940
0.921
0.0007
0.076
-------
Outlet sampling
locations
Mixing tube
Injection water-
Steam nozzel
intet
Particle
accelerator
Cyclones
Atomizer water-
Cyclone
slurry
Inlet duct
Inlet sampling
locations
Flue gas from waste
heat boiler. Fed by
open hearth furnance
Atomizer slurry
FIGURE 1. THE STEAM EJECTOR SCRUBBER SYSTEM.
63
-------
Flowmeters
Cyclone Pump
Particulate
Sample Line
Diffusional Dryer
(Optional)
Pressure
Balancing
Line
Recirculated
Clean Dilution
Air
Filter
Pump
Bleed
FIGURE 2. OPTICAL AND DIFFUSIONAL SIZING SYSTEM
64
-------
W1
o
M
a
Q
O
a
I
Ex
U
W
g
2 -
I -
O 0
Top Start
charge
i i
End Start End hot metal
charg« hot start lance
metal
I
Top
FIGURE 3. TIME HISTORY OF THE PARTICULATE LOADING
AT THE INLET OF THE SCRUBBER. THE TIME
PERIOD SHOWN IS ABOUT EIGHT HOURS.
Ola
-------
U.VI
O.I
1
5
10
0
5
jji
U
£ 50
tt.
gt
90
99
99.9
99.99
0
:- ++ ^ * ** **:
D
•
-
0 0
" 0
o
'
.
•
"
-
O DIFF
a OPT
+ IMPACT
•
t i iiitiil i i iiijiil i i iiiiii
99.99
99.9
99
95 .^
90 .
u
U
u
C
kl
50 |
u
o
u
10
5
1
O.I
A rtl
01 • O.I 1.0 10.0
PARTICLE DIAMETER, ;im
FIGURE 4.FRACTIONAL EFFICIENCY OF THE SCRUBBER
-------
OOOI
0.01
f£ O.I
tu
LJ
a.
1.0
IO.O
0.01
0
*
0
o
i
J
o
o
+
•
o
+
ACC.
POS.
3
2
3
STEAM
PRESSURE
250
250
300
GAS
FLOW
11000
15000
I3OOO
i i i
j—i—i i i 11 n 90
99.99
99.9
99
O.I 1.0
PARTICLE DIAMETER,
!0.0
o
z
U)
o
u.
o
o
o
u
FIGURE 5. FRACTIONAL EFFICIENCY CALCULATED USING
IMPACTOR DATA ONLY
67
-------
PERFORMANCE OF WET SCRUBBERS ON
LIQUID AND SOLID PARTICULATE MATTER
by
John S. Eckert and Ralph F. Strigle, Jr,
Norton Company
Akron, Ohio
ABSTRACT
A general discussion of packed scrubbers for particle
collection is presented. Data on liquid entrainment separa-
tion, ammonium chloride fume collection, and clay particle
collection are given.
68
-------
Performance of Wet Scrubbers on
Liquid and Solid Particulate Matter
by
John S. Eckert and Ralph F. Strigle, Jr,
Norton Company
Akron, Ohio
Packed wet scrubbers are very good devices for the econ-
omical removal of particulates down to a nominal size of
5 urn. Below 5 ym size the removal efficiency of these
devices will either begin to fall off or it will be necessary
to operate them at higher than normal pressure drops or some-
thing over 0.25 in. of water head per foot of packed depth.
It is possible to maintain a high efficiency scrubbing action
down to about 3 \im particle size, however by this time the
pressure drop will be up to 0.75 to 1.0 in. water per foot
and severe misting of the irrigation liquid will set in.
This is about the upper limit at which a packed wet scrubber
can be operated in countercurrent flow. Efficient removal
of any smaller particulates will require a different operating
technique because this is the limit of loading capability for
the countercurrent flow operated packed bed.
Higher gas mass velocities are needed for the removal
of smaller size particles which means much more energy will
be needed to provide separations. The designer has several
choices at this point:
1) Venturi Scrubbing
2) Flooded Bed Scrubbing
3) Fabric Filter Separation
4) Cocurrent Packed Bed Scrubbing
There are possibly others, but the above four are the usual
approaches.
Venturis are commonly used, however they are only effect-
ive at very high pumping cost for either the gas or the liquid
69
-------
and usually both. Venturis are therefore high consumers of
energy though, if properly designed, they should remove
particulate matter efficiently down through the 1.0 ym range
or less and have been known to be effective on some particulates
as small as 0.1 urn.
The flooded bed scrubber is reputed to be as efficient
as the venturi with a minimum amount of liquid being required.
They are not capable of handling wide gas loading variations
and are subject to rather high maintenance costs. Flooded
beds are usually operated as multistage devices to achieve
good efficiencies. There is a good possibility that a sieve
tray or a valve tray would be as efficient as a flooded bed
if operated at the same pressure drop per tray as the flooded
-bed is per bed. They usually end up as operating with about
the same overall pressure drop as the venturi and therefore
requiring about the same gas pumping energy but with a great
saving in liquid requirement, especially if there is some
reason for not wanting to recirculate the discharge liquid
from the scrubber.
Wet fabric filters are supplied in a wide variety of
designs, the most successful of which is probably the Brink.
Very good claims are made for this type of separator going
down to effective removal efficiency well below 0.1 urn.
Various manufacturers claim operating pressure drops from
10 to 40 inches of water. There is one weakness about this
type of separator in that it is only good with liquids. The
presence of any solids in the gas stream will very quickly
blind the filter and require a shutdown of the equipment for
either a change of filters or a backwash. Those which operate
on the moving belt principle for purposes of cleaning will not
effect high submicron removal efficiencies.
The cocurrent packed bed will approximate the venturi in
efficiency with the same amount of energy input for the gas.
70
-------
It has the advantage that it will give increasing efficiency
with increasing pressure drop for particles from 3 ym diameter
down to 0.1 urn diameter. They do tend to "scale up" somewhat
more rapidly than either the venturi or the flooded bed but
have a much better operating range than either the venturi or
the flooded bed unless the venturi has been provided with a
variable area throat to maintain high gas velocity under low
loading conditions.
It should be kept in mind that this paper is intended to
deal with wet scrubbers so that which has been said above has
been on the assumption that all of these various devices will
be used to remove particulates from a gas stream when it is
necessary to use a liquid as a means of removal or a liquid
particulate is being removed. All of this leaves two things
in common. First, they depend on wetting the particulate,
thus making it a part of the liquid continuum as a means of
removing it from the gas stream. Second, they depend on
inertial and velocity effects to force a particulate to follow
a different path than the gas, thereby achieving an impinge-
ment of the particulate on a wetted surface resulting in
capture of the particulate by the liquid.
The packed bed will be specifically discussed because it
is this type of removal equipment which has received the
greatest amount of study by the author from both a laboratory
and applications point of view. The application of packings
may be divided into two parts, i.e., that of entrainment
separation and that of wet washed scrubbers.
Nonirrigated entrainment separators, such as knitted
mesh, the Brink entrainment separator, or beds of commercial
size packings are all commonly used. If solids are present
then some provision must be made to clean the separator after
it has become fouled, as will be indicated by breakthrough
of entrained material and/or excessive pressure drop. Both
71
-------
mesh type separators and the packed bed are commonly installed
with "on stream" backwash provisions but the fabric separator
must be used exclusively for the separations of liquids or
must be shutdown for cleaning.
Entrainment Separation
The testing of commercial size packings for entrainment
separation was done in the laboratory with a conventional
20 in. diameter tower. Pressure drop was measured across the
bed as indicated to show when the liquid loading in the bed
was approaching flood point. A material balance was taken
on the system by measuring the amount of evaporation of water
as calculated by the relative humidity of the room air and
the exhaust air from the system. Air mass movement was measured
by means of an accurately calibrated anemometer. Total weight
of water fed to the system was measured and total weight of
water drained from the bottom. All measurements were taken
at regular intervals until the system had come to a constant
operating condition at each rate studied. Efficiency of the
entrainment separator was then calculated as follows:
Ib/min FUO out the bottom
Efficiency = 100 lb/min ^Q to £og nozzie - lb/min ,H20 evaporated
The nozzle was a conventional low capacity full cone spray type.
At 150 psig on this nozzle, a fog was developed which carried
particle sizes ranging from 3 urn to about 50 ym as determined
by settling rates in still air. In all likelihood the finer
particle sizes represented all of the breakthrough during the
highest efficiency performance of the packings.
High removal efficiencies were obtained with velocities
as high as 14 ft/sec (Fig. 1) and as the velocity of the air
exceeded this figure, the efficiency fell off quite rapidly
72
-------
because the air velocity was sufficiently high to reentrain
the smaller droplets of water falling from the wetted surface
of the packing. It may be said that the separator is flooded
under this condition of operation.
Increasing bed depth (Fig. 2) does not greatly improve
efficiency. This is consistent with the findings of Jackson
and Calvert (Ref. 1). It is normally not advisable to use
beds of less than ten or twelve packing sizes in depth because
of the possibility of an occasional unusual void allowing
breakthrough. The beds illustrated in Fig. 1 were carefully
placed to avoid any unusually large void space, therefore
they are applied here at and below recommended safe depths.
It is important to keep in mind that gases whose densities
are different from that of air will flood at a higher velocity
at lesser gas density and lower velocity at higher gas density.
Also, the ability of the separator to remove fine particulates
at these different gas densities may vary, however this has
never been investigated. It is felt though that this effect
will be small except at very large changes in density such
as gases having densities of the magnitude of less than one-
tenth that of air or more than ten times that of air.
Larger packings tend to be less efficient in particulate
removal than smaller ones (Fig. 3). The higher capacity pack-
ings though can be operated at higher velocities than the
lower capacity ones and still maintain high removal efficiency
(Fig. 4). The higher capacity packings show a somewhat higher
particulate removal efficiency in the same size range than
the lower capacity ones.
The removal efficiency of this type of separator appears
to be rather good until consideration is given as to just what
2 to 5% breakthrough of particulates represents. Undoubtedly,
the entrainment separator, as illustrated by the small improve-
ment with increased bed depth, is acting as a size separator
73
-------
of the participates, i.e., it is allowing most of the very
small droplets to go through.
Depending on their method of generation, solid particu-
lates may or may not be associated with an electric charge.
It is because of their departure from sphericity and the
tendency to carry an electrical charge that two intriguing
problems present themselves. First, what is the particle
size of a rod shaped particle 0.1 ym in diameter by 3 ym
long or a platelet with an average diameter of 3 ym which
is 0.1 ym thick? More interesting yet is that if either of
these is electrically charged, how will the charge be distri-
buted? Will it make them behave as spheres? Second, probably
because of the shape factor enigma posed above, in most fumes
where a solid particulate is present, removal efficiency is
only predictable with wet scrubbing by first running an actual
test with the material.
Tests were run with a cocurrent (vertical downward flow)
packed test tower using a bed of 1 inch plastic Intalox
saddles with 3 feet of depth. Ammonium chloride smoke gener-
ated in the gas phase was used as the particulate. This
smoke based on microscopic examination of material picked
up on a Gelman filter paper represents particles ranging in
size from less than 0.1 ym to a few as high as 3 ym (Ref. 2).
This tower, operating at an overall pressure drop of 7-1/2 in.
of water, a-water rate of 960 lbs/fti-hr and a gas rate of
3300 lb.s/ft2-hr, was capable of removing 60% of the ammonium
chloride smoke based on the amount of solids picked up in a
Gelman filter at the inlet versus the amount of solids in the
filter on the- outlet. All samples were taken under isokinetic
conditions.
The tower was then tested using a No. 4 dry ball clay.
This clay was dispersed into the air stream at a fixed rate.
74
-------
It was first dispersed with a high pressure air jet and then
passed with the air stream through two centrifugal blowers
operating in series. All other operating conditions were the
same except that the air rate was now increased to 5000 Ibs/ft2-hr.
The clay being insoluble in water made it possible to get
an accurate particle size analysis both in and out as well as
a distribution count of particle size in both the in and out
gases. Particle size count was taken with a Coulter counter
(Table I). A removal efficiency of over 801 of particles
1 ym in size was unexpectedly good. From a weight standpoint,
the scrubber has removed 981 of the particulate matter in the
gas stream which in this case had an average particle size of
0.5 pm.
One acceptable method for the removal of submicron
particulates with a wet scrubber is to use a technique known
as nucleation. This process (Ref. 3) consists of injecting
a condensable gas, usually steam, into the gas stream carrying
the submicron particulate in such a manner that the steam will
condense. This is usually accomplished by cooling the gas
stream with a fog nozzle if it is 'not already cool enough.
As the temperature of the gas falls below the dewpoint, the
particles act as nuclei upon which the steam can condense,
thereby increasing their effective diameter to such an extent
that they can easily be scrubbed out of the inert gas stream.
The process has one weakness in that a dwell time of at least
3 or 4 seconds is required for sufficient nucleation to take
place. This sets a requirement for sufficient volume to exist
in the scrubbing system to allow nucleation to take place
before the air enters the scrubber.
Fig. 5 which has been given by the courtesy of the Mine
Safety Appliance Company shows the origin of various particu-
lates which may be ordinarily encountered in the atmosphere
75
-------
and their usual size ranges. Lines defining five size ranges
or zones have been added to the original table to indicate
what type of packed wet scrubber may be used in each zone.
Zone A covers particulates from 75 ym on up in size which
may most economically be scrubbed with efficient removal using
a cross flow packed bed. If a little soluble gas is accompany-
ing the particulate or if there is a large amount of solid
matter present, then the regular cross flow scrubber can be
made a bit more sophisticated by converting it to a horizontal
cocurrent flow device.
Countercurrent flow will be found to be effective in
Zone B or for handling particulate matter down to a little
below 5 ym in diameter (this range may be extended to as low
as 2 ym if the particles are very elongated and a bed depth
of ten feet or more may be used).
It is advisable to use a cocurrent flow for particulates
in Zone C or down to 0.5 ym in size. Cocurrent flow is also
advisable for larger particle sizes where there is a very
heavy loading of solids in the gas stream in that they tend
to foul much less than countercurrent or cross flow scrubbers.
The operating range of the cocurrent flow scrubber can some-
times be extended to handle particles as small as 0.1 urn by
going to bed depths of as much as 6 or 8 feet if the particles
are of the elongated type.
Zone D is ordinarily not considered a good region in which
to operate packed wet scrubbers if removal efficiencies in
excess of 50% are expected. Nucleation techniques must be used
in this region to move it to Zone C if wet scrubbing techniques
are to be used.
Zone E is where the packed wet scrubber really comes into
its very best field of application in that removal efficiencies
of 100% may be realized if necessary. In this region particu-
lates come under the influence of diffusional kinetic energy
76
-------
and behave as true gases. This type of equipment is then
designed using the classic techniques for gas absorbers
employing mass transfer coefficients and vapor pressure
drive.
In summary, it might be said that wet packed scrubbers
are very effective in the removal of particulate matter from
an air stream in particle sizes up to 0.01 m and then in
particle sizes from 0.5 m up to whatever size of particulate
can be carried in the gas stream. They are ordinarily operated
at a temperature below the boiling point of water and are best
applied when the material to be removed is of an odorous,
corrosive or moist nature.
REFERENCES
1. S. Jackson § S. Calvert, A.I.Ch.E. Journal, Vol. 12,
No. 6, p. 1075.
2. H. P. Hunger, Present Status of Air-Pollution Research,
Mech. Eng., 73:405-411 (1951).
3. B. W. Lancaster, W. Strauss, Industrial Eng. Fundamentals,
Vol. 10, No. 3, p. 362, (1971).
77
-------
TABLE I
PERFORMANCE CHARACTERISTICS OF COCURRENT SCRUBBER ON CLAY
WITH AVERAGE PARTICLE SIZE OF 0.5pm
IN
OUT
-•4
00
Particle
Size
lOu &
5
2
1
0.5 -
0.25-0
0.10-0
over
10u
5u
2vi
IM
• 5u
.25y
Ut.
g
.07
.03
.09
.22
.26
.18
.15
1.00
** No. Of
Particles
2
2
8
2
2
1
1
.8 x
.8 x
.4 x
.6 x
.46 x
.37 x
.12 x
10 7
10 7
10 8
1010
1011
1012
101'
*S ample
Wt. g
.00
.00
.03
.14
.40
.28
.15
*Actual
Wt. g
.00
.00
.0006
.0028
.0080
.0056
.0030
0.02
**No. Of
Particles
5.6
3.3
7.58
4.31
2.24
0
0
x
x
X
X
X
10s
10 8
109
10"
10"
% Of
Particles
Removed
100.
100.
99.3
98.7
97.0
96.9
98.0
ft Overall Wt.% Removal Was 98
,'.Wt. Out = (Wt.% OUT) (1.00 - 0.98)
** No. of Particles = [Wt.%/25 = Vol.]/Particle Size3
1 g Sample Assumed Spg. = 2.5
-------
DE ENTRAINING EFFICIENCY
<
o
90
2
UJ
o
ft:
IL
85
so
PLASTIC F^LL
i
•<
GIN. B
—
E"D
.-—•-•
o
(X)
a.
GAS VELOCITY FT/SEC.
FIG i
DE ENTRAINING EFFICIENCY
I
u
UJ
(J
o:
iLl
Q.
95
UN. PLASTIC
PALL
6 IN. BED
10 IN- BED
__ "1 •-- _ 4.
' ~~ ^. ---*^
\
3
s
o
X
±
DL
<
GAS VELOCITY
F\G 2
FT /SEC
79
-------
EFFECT OF PACKING SIZE
PERCENT REMOVAL-
as
Q/-Y
oo F
as ~
-SO
j
™ • •
'LA5TIC PA
S/01H.S
}i M d
(IM- Ol
1-1/2 IN.
1
l^» *W a^^
. """' *** ^^
LL RINGS11
IZE
ZE
SIZE
0 1
X ~— :^-
5
GAS VELOCITY FT/SEC.
PIG 3
EFFECT OF PACKING TYPE
L
UJ
o:
H
UJ
O
It
LU
CL
90
85
IN. PACKINGS
PLASTIC
1O
15
GAS VELOCITY
F\G, 4
FT /SEC.
80
-------
SIZES OF AIR-BORNE CONTAMINANTS
E
H20-NH3
02/
H?0-8-00-(
N2 COZ
!
i
i
-
D
I
C
B
AEROSOLS
NORMAL IMPURITIES - IN QUIE
OUTDOOR A R
i *fS
T
Fnr,
6.7.B
1
METALLURGICAL DUST AND FUMES
4,9,10
TOBACCO
MOSAIC
VIRUS
16
I
1
TOSACC
NECROS
VIRUS
17
i ill
SHELTER DUST AND FUWS
5,11
AMMONIUM
CHLORIDE
FUMES
> 1
A
MIST
4
RAIN DROPS
4
FOUNDR' DUST
9
ALKALI FUMES
ZINC OIIDE FUMES |
0 VIRUS 1
IS PROTEIN
9
|t
FLOUR MILL
DUST, SPRA
ZINC OUST
i.ll
H
ED
.ROUND
4
LIMESTONE
.9
SULPHIDE ORE. PULPS
FOR FLOTATION
ELECTROPLATING
MIST
SULFURIC ACID MIST I
C£«ENT OUST
13
CONDENSED
ZINC DUST
5,11
I
INSECTICIDE DUSTS
14.15
PULVERIZED COAL
4,9
PLANT
SPORES
4
BACTERIA
4. IB
CARSON BLACK
19.20
DIAMETER Of GAS
MOLECULES
1
RANSE OF SIZES
'
SMALL RANGE - AVERAGE
*
) DOUBTFUL VALUES
1
0001 0.0004 Q.001 0.005 0
E
TOBACCO S"OKE
1 OIL SUTKE
MAGNESIUM OIIOE SMOKE 1
POLLENS
21
SNEEZES
Fit KU I
9 |
!
ILVER IODIDE
24
NUCLEI
B
PIGWHTS
ENAMELS) (FLATS)
9
1
SEA SALT
NUCLEI
01 O.OS 0.1 0
D
1
SPRAY ORI
MILK
4 9
CFEREN
SIZE
S 1
C
:E
0
1 SANO TAILING;
1 4
WASHED FCX/KDRY
SAND
HUMAN HAIR DIAMTTER
25
1
1
1 _
VISIBLE TO EVE
SCREEN 400 J25 200 100 6548JS 28 1
KCU LI 1 refill
s 10
B
SO
100
1
500 1.000 5.000 10,000
A
NOTE: THT NifMlirftS RfPRfttNT
FTC. 5
81
-------
ROTATING CONCENTRIC HOMOGENEOUS TURBULENCE
GAS SCRUBBER
by
William C. Leith
Cominco Engineering
Trail, B.C. Canada V1R 4L4
ABSTRACT
A device which utilizes the Couette (secondary) flow
patterns, which are developed in a fluid between rotating
concentric cylinders, is proposed for use in particle collection.
The gas being cleaned is dispersed in the scrubbing liquid within
the annular space between cylinders. The device was untested at
the time of presentation of the paper.
82
-------
Rotating Concentric Homogeneous Turbulence
Gas Scrubber
by
William C. Leith
Cominco Engineering
Trail, B.C. Canada V1R 4L4
Introduction
A new concept in fine particle scrubbing is presented
which utilizes a dirty gas dispersed through an annulus of
scrubber liquid constrained between two rotating concentric
perforated cylinders, based on the classical theory of
couette motion discovered in 1890 (1). Taylor circular
couette motion caused by centrifugal forces imposes an adjust-
able residence time for uniform mixing prior to separation by
wetting, absorption, solution, agglomeration and settling of
gaseous, liquid, and solid particulates in the annulus of
scrubber liquid. Gas scrubbers usually operate with a short
residence time, less than 2 seconds, for the dirty gas to
impinge and then to penetrate through a fog, mist, spray,
jet, or thin liquid curtain of scrubber liquid, whereas this
concept permits an adjustable residence time (theoretically)
of 5 to 10 seconds for fine particle scrubbing.
Two recent patents (2)(3) in pollution control have
demonstrated the practical application of "homogeneous
turbulence" with specific regimes of Taylor circular couette
motion with a multiple array of single row or double row
secondary flow cellular vortices. G. I. Taylor (4) reported
the theoretical conditions for the occurrence of multiple
arrays of secondary flow cellular vortices in couette motion;
and Coles (5) described the specific regimes of Taylor circular
couette motion which establish some practical criteria for
adjustable residence times.
83
-------
Gas scrubbers usually operate with a decreased diffusion
gradient due to build up of adhering layers of particulates
on contact surfaces, whereas this concept should maintain a
near-optimum diffusion gradient by an intermittent cleaning
cycle.
COMINCO "DOYLE" SCRUBBER
A successful wet impingement scrubber was developed by
Cominco as reported by Doyle and Brooks (6), as the final step
in certain gas treatment processes to remove essentially all
particulate matter from metallurgical and fertilizer plant
waste gases prior to their discharge up the stack. The Cominco
"Doyle" scrubber depends on impingement of the dirty gas and
then penetration through a spray curtain of scrubber liquid,
with much success in numerous commercial installations with
particulates greater than 5 microns.
However, to meet even today's rigid specifications (with
more stringent ones to come), the author suggests that the
residence time for the original Cominco Doyle Scrubber, esti-
mated at less than 2 seconds, is too short. Therefore, the
addition of "homogeneous turbulence" effects by installation
of new hardware in the inlet chamber of a Cominco "Doyle"
scrubber, could increase the theoretical residence time to
5 or even 10 seconds - in the author's opinion - which would
enhance the fine scrubbing characteristics for particulates
less than 5 microns down to 0.1 microns.
Collection Efficiency by Weight
Field tests of collection efficiency by weight related to
particle size for a Cominco Doyle scrubber, which uses simple
jet impingement effects, can be represented as a straight line
on log/log paper, see Figure 1.
Predicted collection efficiency for a rotating concentric
"homogeneous turbulence" gas scrubber can be calculated using
84
-------
Brownian movement where the increased residence time
accentuates the increase of Brownian displacement with smaller
particle size.
TYPES OF COUETTE MOTION
In general, couette motion refers to motion imparted to
a liquid contained in the annulus between two rotating con-
centric cylinders which move relatively to one another with
a uniform angular velocity.
1. Simple Couette Motion, often called shear flow, occurs
when the cylinders rotate quite slowly within the ranges
of stable laminar flow. There is a linear velocity dis-
tribution when the inner cylinder is stationary and the
outer cylinder rotates slowly within Rayleigh's theory
of laminar stability. When both cylinders rotate slowly
in opposite directions, there is a stationary liquid
plane at mid-annulus, with a linear velocity distribution
in each half-annulus of opposite sign.
2. Taylor Circular Couette Motion includes a multiple array
of secondary flow cellular vortices in the annulus be-
tween the rotating cylinders with several particular
regimes. A single row of helical vortices occurs when
the outer cylinder rotates in the same direction and at
about half of the angular velocity of the inner cylinder.
A double row of helical vortices occurs when the outer
cylinder rotates in the opposite direction and about at
half of the angular velocity of the inner cylinder. A
spiral band ot cellular vortices in a travelling wave can
sweep upwards or downwards, when the outer cylinder rotates
in the opposite direction and about three times the angular
velocity of the inner cylinder.
Figure 2 is a sectional elevation of the rotating con-
centric "homogeneous turbulence" gas scrubber showing the
annulus of scrubber liquid between the inner and outer perforated
85
-------
cylinders. Figure 3 shows the multiple array of single row
secondary flow cellular vortices when the outer cylinder
rotates in the same direction and with about half of the
angular velocity as the inner cylinder. Figure 4 shows the
multiple array of double row secondary flow cellular vortices
when the outer cylinder rotates in the opposite direction and
with about half of the angular velocity as the inner cylinder.
Figure 5 shows the specific regimes of Taylor circular
couette motion which are relevant to three basic cycles for
a pair of gas scrubbers:
CYCLE A
Large (more than 50 microns in diameter) gaseous, liquid,
and solid particulates are removed mostly by high velocity
impingement of the dirty carrier gas into the scrubber liquid
within the inner perforated cylinder with agglomeration,
thermal precipitation, and settling of the wetted particulates;
and slightly by diffusion gradients (concentration, tempera-
ture and mixing) with a short adjustable residence time for
uniform mixing prior to separation by wetting, absorption,
solution, agglomeration, and settling of the gaseous, liquid,
and solid particulates in the multiple array of single row
secondary flow cellular vortices in the annulus of scrubber
liquid between the inner and the outer perforated cylinders;
when the outer cylinder rotates in the same direction and with
about half of the angular velocity as the inner cylinder.
CYCLE B
Small (under 5 microns in diameter) gaseous, liquid and
solid particulates are removed slightly by impingement of the
dirty carrier gas into the scrubber liquid within the inner
perforated cylinder with agglomeration, thermal precipitation,
and settling of the wetted particulates; and mostly by diffusion
gradients (concentration, temperature, and mixing) with a longer
adjustable residence time for uniform mixing prior to separa-
tion by wetting, absorption, solution, agglomeration, and
86
-------
settling of the gaseous, liquid, and solid particulates in
the multiple array of double row secondary flow cellular
vortices in the annulus of scrubber liquid between the inner
and outer perforated cylinders; when the outer cylinder
rotates in the opposite direction and with about half of the
angular velocity as the inner cylinder.
CYCLE C
An intermittent cleaning action removes the adhering
layers of gaseous, liquid, and solid particulates on the
inner and outer perforated cylinders, in a downward direction
into the bottom sludge, by a spiral band of turbulence in a
travelling wave in the annulus of scrubber liquid between
the inner and outer perforated cylinders; when the outer
cylinder rotates in the opposite direction and with about
three times the angular velocity of the inner cylinder.
CONCLUSION
The rotating concentric "homogeneous turbulence" gas
scrubber is an improvement for fine particle scrubbing, which
utilizes Taylor circular couette motion caused be centrifugal
forces, to impose an adjustable residence time for uniform
mixing prior to separation by scrubbing.
REFERENCES
1. M. M. Couette, Annals Chim. Phys.
Series VI, Vol. 21, 1890, p. 433
2. W. C. Leith, Rotating Concentric "homogeneous turbulence"
Fabric Bag Gas Cleaner Method,
U.S. Patent 3,785,123 issued Jan. 15, 1974
3. W. C. Leith, Rotating Concentric "homogeneous turbulence"
Electrostatic Precipitation Gas Cleaner
Method, U.S. Patent 3,785,117 issued
Jan. 15, 1974
4. G. I. Taylor, Phil Trans. Royal Soc., London
Series A, Vol. 223, 1923 p. 289
5. D. Coles, Jour Fluid Mech. 1965
Vol. 21, part 3 pages 385-425
6. H. Doyle § A. F. Brooks, The Doyle Scrubber
Ind. and Chem. Eng. Dec., 1951
87
-------
COLLECTION
EFFICIENCY
BY % WEIGHT
JOO —
00
00
80 —
7o -
to .
«o.
4o-
-,».
ae.
to-
DOUBLE ROW
OF VORTICES
COMINCO DOYLE
JET IMPINGEMENT
SCRUBBER
—I , ,—I I I I 111—
o.f 4.O
PARTICLE SIZE, MICRONS
T 1 1—J I I I I
(o
FIGURE 1
PREDICTED COLLECTION EFFICIENCY
FOR A ROTATING CONCENTRIC " HOMOGENEOUS
TURBULANCE" GAS SCRUBBER
-------
CLEAN GAS
OUTLET ...
DIRTY CAS
INLET
SLUDGE
PERFORATED
CYLINDERS
SCRUBBER LIQUID
FIGURE 2
ROTATING CONCENTRIC " HOMOGENEOUS TURBULANCE"
GAS SCRUBBER
FIGURE 3
SINGLE ROW OF VORTICES
FIGURE 4
DOUBLE ROW OF VORTICES
89
-------
Cycle C
Spiral
Turbulence
Inner Cylinder
4 angular velocity
•Cycle B
Double Row of
Secondary Flow
Cellular Vortices
Laminar Flow
Cycle A
Number of
Secondary Flov\
Cellular Vortices
Stable Flow
-i r
angular (-)
velocity
Outer
Cylinder
(+)angutar
velocity
Outer
Cylinder
FIGURE 5
SPECIFIC REGIMES OF TAYLOR CIRCULAR COUETTE MOTION
90
-------
MEAN DROP SIZE IN A FULL SCALE
VENTURI SCRUBBER VIA TRANSMISSOMETER
by
R. H. Boll, L.R. Flais, P.W. Maurer and W.L. Thompson
Babcock § Wilcox Company
P. 0. Box 815
Alliance, Ohio 44601
ABSTRACT
This paper describes the construction of and the data
obtained from a light transmissometer capable of making mean
drop size measurements within about ± 151. The experimental
venturi had a throat flow cross-section of 12 inches by 14
inches and overall length in the flow direction was 15 feet.
It was found that the Nukiyama-Tanasawa equation gave accurate
estimates of Sauter mean drop size only for a throat velocity
of 150 ft/sec.
91
-------
"Mean Drop Size in a Full Scale
Venturi Scrubber via Transmissometer"
by
R. H. Boll, L.R. Flais, P.W. Maurer and W. L. Thompson
Babcock § Wilcox Company
P. 0. Box 835
Alliance, Ohio 44601
Venturi scrubbers are well known for the ability to collect
very small particles. Their main practical advantages are low
first cost and minimal maintenance. Their main disadvantage is
relatively high pressure drop. Their performance theory, upon
which optimized designs can be based, has been developed by
123
several workers ' ' . However, the theory invariably starts
from presumed knowledge of mean drop size, which is an important
basic variable. Moreover, predictions of mean drop size usually
4 5
invoke the empirical Nukiyama-Tanasawa correlation * . Un-
fortunately, compared with commercial venturi scrubber practice,
this correlation was developed using gas velocities that were
too high, liquor-to-gas ratios that were too low, and equipment
scale that was much too small - so, its use represents an
extrapolation. More recent drop size studies, which have been
reviewed before , tend to confirm the N-T equation, but not in
a convincing manner over the full ranges of all the variables.
The purpose of the present study was to help fill this gap
by measuring mean drop sizes in a commercial-scale venturi using
practical ranges of operating conditions. Since there is no
standard method for doing this, we first had to develop an
adequate tool.
APPARATUS
Those familiar with the problem of characterizing sprays
know that it is easy to make a measurement, but very difficult
to make an accurate one. In the present test setup, the prob-
lem is compounded by the large volume of spray liquid (up to
200 gpm) and the high gas velocities in the region where
92
-------
measurements must be made (up to 300 ft/sec). These circum-
stances, combined with the expected variations in liquid
distribution across the venturi, made some kind of optical
technique virtually mandatory. We chose light transmission
because both its theory and practice have been adequately
developed
T r an s m i s s ome t e_r__Th e o r y
The attenuation of a beam of monochromatic light in
f\ 7 R
traversing a suspension of drops is given by * * :
T °° 1
- m I = li f RK.D2NdD (1)
0 {
where: K = K (a, m) = theoretical extinction coefficient
12 13
calculated from the Mie theory ' , dimensionless,
R = R (a9) = receiver coefficient accounting for the
fact that practical light receivers accept some
scattered light as if it had not been scattered,
dimensionless.
D = Drop diameter, cm
N = N(D) = particle-size-distribution function, drops
per cm of diameter per cm of suspension.
I/I = Transmission ratio, light transmission
signal with drops in the light beam
divided by light transmission signal
with no drops in the light beam,
dimensionless.
ji = Light path length through the suspension
of drops, cm.
a = TTD/X = drop-size parameter, dimensionless.
X = wavelength of light, cm
m = ratio of refractive index of drops to that of
surrounding medium, dimensionless
9 = half-angle of cone of acceptance of light receiver,
degrees.
Equation (1) has been verified experimentally for the following
conditions ' ;
1. Actual divergence angle of light beam less than 1°.
2. Acceptance angle of receiver, 9, less than 1.5°.
3. Diameter of light beam less than the diameter of the
receiver.
93
-------
4. Transmission ratio (!/!_) greater than about 0.1,
Moreover, if the divergence of the light beam is less than
about 0.3° and the receiver and light source are reasonably
well aligned (say, within 0.3°) then the R function is given
bv: 2 2
1 + J * (cx9) + J/ (a6)
R = - 0 - i - (2)
2
where Jn and J. are Bessel functions of zero and first order,
710
respectively ' . Values of K (a, ra) are tabulated in a
number of places ' ' . Thus, if one but observes the
restrictions, all of the physical quantities required to apply
Equation (1) are readily available - even without empirical
calibration.
To apply Equation (1), it is convenient to rearrange
it as follows:
* C I (4)
0
where:
. I RK ND2dD
t r ND'dD
A - NI)3dD
ND^dD
I
1.5 RK.
p (8)
P ^32
94
-------
That is, KK is a mean value of the actual light extinction
coefficient (dimensionless) , U,2 is the Sauter mean drop
diameter (cm), C is the mass concentration of drops in the
light beam (gm/cm ), and $ is the specific extinction cross-
section of the drops (cm /gm), As a practical matter, the
value of KK't is given within 10% by:
Kirt = RKt (D32) (9)
provided only that the drop-size distribution function, N, is
q
of reasonable shape . Thus, for given optics and wavelength,
$ becomes a function of B",- alone.
Figure 1 shows $ as a function of EL, for water drops in
o *>£
air (m = 1.33), X = 7000 A, G = 0.35°. Thus, using Equation
(4) and knowing the light-path length and the liquid concentra-
tion, a measurement of the light-transmission ratio, (I/I.)
gives the Sauter mean drop size. We shall discuss how one can
know the liquid concentration, C , in a venturi a little later.
Optical Construction of Transmissometer
Figure 2 indicates the light-source construction. A strip-
filament lamp is used to maximize beam power for a given
divergence angle. Achromatic lenses are used so that they do
not have to be repositioned upon changing the operating wave-
length. The wavelength (actually the mean wavelength) is
determined by the interference filter. In measuring drops, we
O
used X = 7000 A with a band width (at half intensity points) of
o
± 300 A. The two lenses, C, focus an image of the filament upon
the pinhole. The latter is placed at the focal point of lens,
K, so that its 1.09 mm diameter and the 149 mm focal length fix
the half angle of divergence of the beam at 0.21°. The beam
diameter is determined by stop, J, to be about 12.7 mm, or
1/2 inch.
So that the light from the collimated beam can be disting-
uished from scattered ambient light, the beam light is chopped
95
-------
at 400 Hz by a "tuning fork" chopper, F, in Figure 2. Tem-
poral variations in lamp intensity are monitored by photocell,
L, which "sees" some of the light that gets through the pin-
hole via mirrors, I and G. Stop, H, serves to block back-
scattered light from the drops being illuminated (not shown).
The receiver optics involve a 2.11 mm diameter pinhole
placed at the focal point of the 172 mm focal length lens
produces a half angle of acceptance of 0.35°. In the present
setup, the distance between the collimating lens of the source
and the receiver lens is about 47 inches, making the diameter
of the light beam at the receiver equal to 21.4 mm, or about
10 mm less than the receiver lens diameter. The optical axes
of the source and receiver were lined up by adjusting the
receiver pinhole position within the plane perpendicular to
the optical axis so as to center the filament image within the
receiver pinhole. It is estimated that this alignment was
within 0.02° .
The light source and receiver are mounted together on a
common light pipe. The latter is 2.3 inches in outside
diameter and about 5 feet long, the receiver occupying the
last foot on the far end. The light pipe serves to maintain
alignment between source and receiver and to define a path
length for the light through the droplet cloud in the venturi.
This is accomplished by providing a 2.91 inch slot in the
center of the light pipe and inserting the light pipe through
the venturi transverse to the gas flow.
Transmissometer Electronics
The silicon photocells are Solar Systems Type SS-300-2.
These have the characteristic of producing a current that is
linearly related to incident radiant power provided that the
voltage at the photocell output terminals is maintained close
to zero. In the present instance, this is achieved by connect-
ing them to the input terminals of a high-gain operational
amplifier, Burr-Brown Model 3104A/12C. The amplifier output
96
-------
is fed back through a resistor, making the amplifier output
voltage linearly related to the radiant power falling on the
photocell. These operational amplifiers and their associated
circuitry are located within the source and receiver housings
to minimize pickup and noise.
During "dark" periods, when the chopper vanes in the
source are closed, the output signals from the preamplifiers
are sampled and proportional dc signals are negatively fed
back to the inputs of the Burr-Brown amplifiers so as to
effectively cancel the preamplifier outputs. This eliminates
spurious contributions to the preamplifier signals from stray
light and/or preamplifier imbalance.
During "light" periods, when the chopper vanes are open,
the preamplifier output signals are again sampled and converted
to corresponding dc signals. The latter are fed to a Philbrick
Model SPLRA/N log-ratio amplifier. This amplifier produces an
output signal that is linearly related to (- Jin I/I..). Gain
and zero adjust circuitry is provided to convert this into an
actual output indication that is equal to (- Iog10 I/In)•
Power supplies and timing circuits complete the electronics
package, which is housed in a portable 18" x 12" x 12" box.
The read-out meter linearity was found to be within 0.007 log,,,
units, an accuracy within about 1.5% of reading.
Proof Tests of Transmissometer
A few measurements were made on plastic beads to check the
overall accuracy of the transmissometer's optics, transducers,
and electronics. The data of Figure 3 and Table 1 are typical
of the results. These were obtained using uniformly sized
plastic beads supplied by the Dow Chemical Co.
The data of Figure 3 were obtained by pipetting one-mi
portions of a stock solution of beads into a 5 cm x. 5 cm x 10 cm
high transmission cell initially filled with distilled water.
The concentration of the stock solution was measured gravimet-
rically by evaporating a portion to dryness. It will be noted
97
-------
that the data are very nearly linear (in agreement with
Equation (4)) up to about I/In = 0.1. Increasing departure
from linearity occurs beyond this point, possibly because
of slight polychromaticity of the "monochromatic" light,
possibly because of the effects of multiple scattering. Thus,
linearity of phntotubes and electronics is confirmed.
Table 1 is a comparison of experimental and theoretical
values of the specific extinction cross sections, 4>. The
theoretical values of K were obtained from the tabulations
1/1 *7 *}
of Chu et al. by plotting Kt versus (m - 1) a/(m + 2),
which removes most of the effect of relative refractive index,
m. It will be noted that the experimental 41 values are 0.8%
above 7.0% below the theoretical values. The probable causes
of these slight discrepancies would include:
1. Very slight nonlinearity of the data plots (Figure 3)
even below I/IQ = 0.1 (at most 3% of the log I/IQ
reading).
2. Slight inaccuracy of the X values due to filter
tolerances (at most 5%).
3. Slight inaccuracy of the K values because of the
uniformly sized particles and the use of smoothed
plots rather than precise calculation.
In the measurement of drops in a venturi, items 2 and 3 become
unimportant because the larger drops have a K value that is
independent of X, and the dispersion of sizes tends to average
out the minor variations of Kt with the size parameter (a).
Thus, for measuring drop sizes, the data imply that the trans -
missometer is probably accurate within the validity of
Equations (4) and (9), i.e., within 101, assuming Cm and £ are
precisely known.
Venturi Construction
The flow cross-sectional area of the venturi throat is
12 inches by 14 inches. This gives a total gas flow of about
20,000 cfm at a throat velocity of 300 ft/sec. In a commercial
venturi, the 12-inch dimension would become several feet. Thus,
98
-------
the laboratory venturi amounts to a 1-ft slice taken out of a
full-scale commercial venturi. The other dimensions of the
venturi are shown in Figure 4. The liquor injection nozzles
are Spraying Systems "Vee Jet", No. 3/4U80400. The trans-
missometer can be moved in and out so that the slot in the
light pipes samples different transverse positions in the
venturi diffuser.
Transmissometer Position in Venturi
Wherever the liquor drops and the gas are traveling
together at the same velocity, the liquid concentration (gm/cm )
must be numerically equal to the ratio of liquid and gas flow
rates. If the liquid is going slower than the gas, as it is
when it is first injected, then the liquid concentration must
be higher than the flow ratio. The converse is true in the
lower part of the diffuser, where the drop velocity exceeds the
gas velocity. To be able to know the liquid concentration, C ,
in Equation (4), then, it is essential to place the transmiss-
ometer at the point in the venturi where the liquid and gas
velocities are substantially equal. Fortunately, this position
is not too greatly affected by variations in venturi operating
conditions and drop size.
To locate the proper transmissometer position, we used
the mathematical model previously developed to calculate drop-
velocity profiles for wide ranges of liquid-to-gas ratios,
throat velocity, and drop size. Results are summarized in
Table 2. It can be seen that the point where drop and gas
velocities are equal varies from about 4.5 to 61 inches below
the throat. However, at a point 24 inches below the throat, the
relative velocities generally differ by less than +_ 15% for
individual drops that are, respectively equal to, half, and
twice the N-T size. On the other hand, since actual drop popu-
lations contain sizes that are both larger and smaller than the
99
-------
mean, the average error should be less than this extreme - say
+_ 101. Further considering the ^ 10% accuracy of approximating
RlC , Equation (9), results in an estimated overall accuracy of
about +_ 154 .
Procedure
At each set of venturi operating conditions (throat velocity
and L/G), transmissometer readings were taken at five positions
across the diffuser. At each position, the liquor flow was
temporarily cut off by shutting down the recirculating pump.
This permitted a transmissometer reading to be obtained with no
drops in the light path so as to permit correction for the few
drops that invariably got onto the lenses of the light source
and receiver. The correction (in log units) was subtracted from
the reading with water flow on. The correction was maintained
less than 0.3 log units by periodic cleaning the lenses.
The five corrected readings (- log I/Ig) were averaged to
obtain the 4* value according to Equation (4) . The value of C
was taken to be equal to the liquor-to-gas ratio. The value
of X. was taken to be the slot length, 2.91 inches. The value
of n_2 was, then, obtained from the $ value via Figure 1.
RESULTS
Mean drop size data from the transmissometer are plotted
as solid points in Figure 5. For comparison, the predictions
of the Nukiyama-Tanasawa equation are given by the curves
terminating in similarly shaped open points. It will be noted
that the N-T equation is accurate only for the data at a throat
velocity of about ISO ft/sec (109 ft/sec at the point of liquid
injection). Otherwise, it underestimates the effect of gas
velocity rather seriously, predicting drop sizes about 48% too
large at a throat velocity of 300 ft/sec and about 25% too low
at a throat velocity of 100 ft/sec.
An empirical equation that correlates the present data
fairly well is:
IT - 283.000 + 793
°32 1.602
100
-------
where: L/G = Liquor-to-gas ratio, gal/1000 ft .
v. = Gas vleocity at the point of liquid injection,
ft/sec.
I),- = Sauther mean drop size, microns.
The predictions of this equation are indicated in Figure 7 by
the "star" shaped points. It will be of interest to note that
the present gas velocity exponent of -1.602 agrees well with
Wetzel and Marshal's exponent of -1.68 obtained from spraying
wax in a small venturi. (Further comparison is, however, in-
appropriate because they included an orifice-diameter effect
and omitted L/G).
Figure 6 shows how liquid distribution can be affected by
venturi operating conditions. The plotted distribution function
is just the local value of (- log I/In) divided by the average
value. Thus, assuming IL- does not vary across the venturi,
the distribution function shows how liquid concentration, C ,
varies. It will be noted that, at the higher throat velocity,
there is a distinct tendency for injected liquid to fail to
penetrate all the way to the venturi center line. Moreover,
even at the lower throat velocity, there is a distinct tendency
for the point value ef liquid concentration to vary by a factor
of two or three in traversing the venturi. Indeed, this kind
of variation is typical of the thirty runs made in the present
series. Thus, even liquid distribution that appears uniform to
the eye may contain enough transverse maldistribution to explain
theoretical overprediction of particulate collection .
CONCLUSION
A transmissometer has been constructed that is well suited
to measuring mean drop sizes in full-scale venturi scrubbers.
Its accuracy in measuring D,2 i-5 probably within +_ 15%. Its
application has shown that the Nukiyama-Tanasawa equation gives
values of mean drop size that are accurate within about 50% for
L/G's and throat velocities of commercial interest. However,
at least in the one venturi geometry so far studied, the N-T
101
-------
equation consistently overestimates mean drop size at high gas
velocities and underestimates it at low gas velocities. An
empirical equation (Equation (10)) involving gas velocity to
the -1.602 power is much more successful. Additional data for
other venturi configurations would be highly desirable.
The transmissometer also provides a practical quantitative
means for studying liquid distribution in Venturis. Indications
so far are that this distribution is far less uniform than one
might suppose from a visual observation. Indeed, it seems
likely that maldistribution is the main reason that theoretical
assessment of particle collection typically overestimates
actual performance.
102
-------
REFERENCES
1. R. H. Boll, "Particle Collection and Pressure Drop in Venturi Scrubbers,"
Ind. Eng. Chem. Fundamentals 12j40 (1975).
2. S. Calvert, D. Lundgren, and D. S. Mehta, "Venturi Scrubber Performance,"
J. A.P.C.A. 22;529 (1972).
3. J. A. Gieseke, "Pressure Loss and Dust Collection in Venturi Scrubbers,"
PhD Thesis, University of Washington, Seattle, Washington, 1963.
4. S. Nukiyama and Y. Tanasawa, "Experiments on Atomization of liquids in an
Air Stream," Trans. Soc. Mech. Engrs. (Japan) 4, _5, £ (1938-1940); Translated
by E. Hope, Defense Research Board, Dept. of National Defense (Canada),
March 18, 1950.
5. W. R. Marshall, Jr. and S. J. Friedman, "Drying," Section 13 in "Chemical
Engineer's Handbook," 3rd ed., J. H. Perry, Ed., McGraw-Hill, New York,
New York, 1950.
6. R. 0. Gumprecht, "Particle Size Measurement by Light Scattering,"
Doctoral Dissertation, University of i-iichigan, Ann Arbor, 19527
7. R. 0. Gumprecht and C. M. Sliepcevich, "Scattering-of Light by Large
Spherical Particles," J. Phys. Chen., 57^90 (1953).
8. R. 0. Gumprecht and C. M. Sliepcevich, "Measurement of Particle Size
in Polydispersed Systems by l-'esns of Light Transmission Measurements
Combined with Differential Settling," J. Phys. Chem., 57:95 (19S3).
9. R. H. Boll, "A Rapid Technique for Determining Specific burface in
Liquid-Liquid Sprays," Doctoral Dissertation, University of Michigan,
Ann Arbor, 1955.
10. R. H. Boll and C. M. Sliepcevich, "Evaluation of Errors of Optical Origin
Arising in the Size Analysis of a Dispersion by Light Transmission,"
J. Opt. Soc. Am., 46^:200 (1956).
11, J. R. Hodkinson, "The Optical Measurement of Aerosols," Chapter X in
"Aerosol Science," C. N. Davies, Ed., Academic Press, New York, N.Y., 1966.
12. J. A. Stratton, "Electromagnetic Theory," McGraw-Hill, New York, 1941.
13. H. C. VanDeHulst, "Light Scattering by Small Particles," John Wiley § Sons,
New York, 1957.
103
-------
REFERENCES (Cont'-d)
14. C. M. Chu, G. Ur. Clark, and S. W. Churchill, "Tables of Angular
Distribution Coefficients for Light Scattering by Spheres," University
of Michigan, Engr. Res. Inst., Ann Arbor, Michigan,*1957.
15. R. B. Penndorf, "New Tables of Total Mie Scattering Coefficients for
Spherical Particles of Real Refractive Indices (1-33 <. n ±1.50),"
J. Opt. Soc. Ara., 47: 1010 (1957).
16. W. R. iMarshal, Jr., "Atomization and Spray Drying,1' Chem. Engr. Prog.
Monograph Series Xo. 2, 50 (1954).
TABLE 1 TYPICAL BE^D TEST RESULTS
X air, A 7,000 6,000
Bead Si:e, Microns 0.714 0.714
Ppt giryon3 LOS 1-05
m 1.193 L193
a air 3.20 3.73
a water 4.25 4.96
06,
« 1.12 I-30
1.00 1.00
1.23 1.65
24,600 33,000
, on2/gm 24,800 30.700
1.008 0.930
.
9exp. theor.
104
-------
TA5LE 2 RELAii'vt: LIQUID AI\"D GA3 VELOCITIES
Liquid-to Throat Mean Drop Intersection of Gas- Liquid Velocity Relative
Gas Ratio _ Velocity Diaireter Liquid Velocity Curves to Gas Velocity at
:al/1000 ft5 (ft/sec) (iricrons) (in. below bottom of throat) Point 24" Below Throat. I
Nukiyana-Tanasawa Drop Size
4.5 100 217 24.27 -1
4.5 200 120 21.83 +5
4.5 300 86 13.19 +6
4.5 400 68 8.78 +7
9.0 100 241 26.59 -1
9.0 200 145 19.74 +2
9.0 300 110 17.60 +4
9.0 400 93 15.37 +4
18 100 311 35.02 -7
18 200 214 28.60 -4
18 300 180 28.51 -3
18 400 162 28.47 -3
1/2 Nukiyana-Tanasava Drop Size
4.5 100 108 8.86 +7
4.5 200 60 4.76 +7
4.5 300 43 4.50 +5
4.5 400 34 4.45 +5
9.0 100 121 13.32 +6
9.0 200 72 8.73 +7
9.0 300 55 6.59 +7
9.0 400 46 4.58 +6
18 100 155 15.68 +5
18 200 107 13.45 +5
18 300 90 13.14 +6
18 400 81 13.27 +5
1\dce Nukiy ana- Tanas ava Drop Size
4.5 100 434 48.08 -12
4.5 200 241 33.06 -5
4.5 300 172 26.37 -2
4.5 400 137 22.23 0
9.0 100 483 50.52 -14
9.0 200 290 39.34 -9
9.0 500 221 32.79 -6
9.0 400 186 30.87 -3
18 100 621 61.18 -19
18 200 428 52.49 -16
18 300 360 50.65 -13
18 400 324 48.58 -12
105
-------
FIGURE 1 SPECIFIC EXTINCTION CROSS SECTION FOR WATER DROPS IN AIR
SILICON tHOTOCELL
OHHATED IN SMCMT
ClMCMl MOO(
FIGURE 2 SOURCE OPTICS
106
-------
t 3
I 2
_° 1 I
J? 10
o
-I
09
08
o:
06
0.5
0.4
0.3
02
O.I
01 3 34567 8 9 10 II 12 13 It 15 X10~-
£cm. gm/cm2
] li;tllil: 3 TRANSMISSION !>ATA FOR 0.7I4-HICRON BEADS
LIQUID NOZZLES.
4 PER SIDE
fRANSMISSOMETER
FIGURE 4 VENTUR1 CONFICllkATION
107
-------
g
t.
100 FT/SEC THROAT VELOCITY
150 FT/SEC
300 FT/SEC
NOTE: SOLID POINTS ARE
PRESENT DATA OPEN
POINTS ARE FROM N-T
EQUATION. "STAR"
POINTS ARE FROM EQ 10.
10 12 14
LAG. GAL/1000 FT3
FIGURL 5 DROP SIZE RESULTS
2
g
5 as
a
§
A 300 FT/SEC * Sift GAL/1000 FT3. RUN 2
I 200 FT/SEC & 5.67 CAS/1000 FT3. RUN 8
6W 3W CENTER 3£ 6E
DISTANCE WEST OR EAST OF VENTURI CENTER (INCHES)
FIGURE 6 LIQUID DISTRIBUTION FUNCTION ACROSS VENTURI
108
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FINE PARTICLE COLLECTION EFFICIENCY RELATED TO PRESSURE DROP,
SCRUBBANT AND PARTICLE PROPERTIES AND CONTACT MECHANISM
by
Howard E. Hesketh, P. E.
Southern Illinois University
ABSTRACT
Collection efficiencies are shown for control of fine
particles in venturi scrubbers (1) as a function of pressure
drop and (2) as a function of throat area and liquid to gas
ratio. A relationship of pressure drop to throat area, gas
density, throat velocity and liquid to gas ratio is given
and is used to provide a method for estimating efficiency
knowing only these scrubber design parameters. The effect
of charged particles and of surface active agents on col-
lection efficiency are discussed briefly.
109
-------
NOMENCLATURE FOR TERMS NOT OTHERWISE DEFINED
ACFM = actual cubic feet per minute of gas
C /C. = ratio of mass concentration out to concentration in
by weight, 1-E
AP = pressure drop across Venturi, inches water
E = collection efficiency fraction by weight, 1-C /C.
exp = signifies e (natural log base) to the exponent
indicated by the quantity in brackets after exp
fps = feet per second
gr = grain, 1/7000 of a pound
ID = inside diameter
L = liquid to gas ratio, gal/1000 SCF
scfd = standard cubic feet of dry gas
scfm = standard cubic feet per minute at 70 F and 1
atmosphere
u = microns or micro meters
CONVERSION FACTORS
English to Metric Units
1 scfm
1 gpm
1 ft3
1 gal
1 gal/1000 ft!
1 grain/ft3
1 ft/sec
1 in
1 pound
=1.6 Nm3/hr
= 0.227 m3/hr
= 0.0283 m3
= 3.785JI
= 0.134 Vm3
=2.29 g/m3
= 0.3048 m/sec
= 2.54 cm
= 454 grams
110
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INTRODUCTION
It is extremely difficult to obtain accurate and re-
liable data related to the collection efficiency and operating
pressure drop of venturi scrubbers. Those who are in the
position of wishing to purchase one of these high efficiency
wet scrubbing devices are usually confused by the differing
unit sizes and operating conditions recommended by the various
suppliers of venturi scrubbers. It is significant that a
proper device be chosen because an underdesigned unit will
result in failure to meet emission control requirements and
an oversized unit could be much more costly than necessary
to operate.
This paper presents results of industrial and laboratory
studies and provides a method of more accurately estimating
collection efficiency and pressure drop. Even though these
data are the combination of an enormous amount of measurements
and even though a new parameter (throat, area) is included to
make the prediction equations more accurate, it is anticipated
that further refinements and improvements will be possible.
Some of these prediction equations were presented in original
form by Hesketh (1) at the U.S.-U.S.S.R. Symposiums on Control
of Fine Particulate Emissions in January 1974.
PREVIOUS STUDIES
The recent Scrubber Handbook by Calvert (2) very
thoroughly summarizes the studies and data related to effi-
ciency and pressure drop in venturi scrubbers and other wet
scrubbing systems. The study presents a method for estimating
pressure drop through venturi scrubbers as a function of
throat gas velocity and liquid to gas ratio (Equation
5.3.6-10, page 5-122). This theoretical equation was de-
rived assuming that all energy is used to accelerate the
111
-------
liquid droplets to the throat velocity of the gas. The
equation in metric units is:
AP1 = 1.03 x 10~3 (v^.)2 L1 (1)
where: AP1 = pressure drop, cm H~O
v? = gas throat velocity, cm/sec
L1 = liquid to gas ratio,
Calvert also presents a collection efficiency equation
(Equation 5.3.5-6, p. 5-122). Several assumptions related
to collection mechanism, slip, drag, size of the atomized
droplet collectors and uniformity of the atomized liquid
are used to develop the collection efficiency equation in
terms of particle penetration, Pt. Penetration is defined
as "one minus efficiency" and can be stated as 1-E or C /C. ,
where C is mass concentration out of scrubber and C . is
o i
concentration in. Calvert 's Equation 5.3.6-5 is:
(-2 L'v 'p d
Pt = exp 2—=-_£ F(K , f)| (2)
L 55 yg Pfc
where: d. = diameter of atomized liquid droplet, cm
p. = density of liquid, g/cm3
U = viscosity of gas, g/(cm sec)
The expression F(K ., f) is used to relate the effects of
the inertial impaction parameter evaluated at the throat
gas velocity, K ,, and the "unknown factor, f".
The factor f is believed to relate non-uniformity of
the atomized liquid and the resulting difference in the li-
quid and gas velocities. Ranges of f are suggested to be
about 0.1 to 0.3 for hydrophobic aerosols and higher for
hydrophilic aerosols. An average value of 0.25 is used for
112
-------
f in the graphs presented in the handbook.
Information related to pneumatic two fluid atomization
and specifically as to whether atomization produces cloud
type or drop type droplets is reported in previous studies
(1,3,4). Study (1) relates this information to pressure
drop in a 1,500 cfm pilot plant venturi scrubbing coal fly
ash. These pressure drop data vs. throat gas velocity are
presented here as Figure 1. The incomplete-complete atomi-
zation predictions are based on acceleration observations
reported in the study.
These later data show that the pressure drop predic-
tions obtained from throat velocity measurements may be
subject to error at low velocities if an expression as
Equation (1) is applied for all ranges of velocities.
Utilization of available energy in atomization is poor
(0.53% reported by Marshall (5) for drop-type atomization
and 6.1% calculated for cloud-type atomization), but during
the plateau shown on the curve in Figure 1, energy is being
utilized to complete the atomization. On either side of this
region, an increase in energy goes into creating system
turbulence which is recorded as increased pressure drop.
This 1,500 cfm system is a fixed throat venturi and was
operated so as to produce cloud-type atomization. The data
are for before the throat injection of the scrubbing water.
VENTURI SCRUBBER PRESSURE DROP
Pressure drop data were obtained from many fixed throat
venturi scrubbers. These systems include 600 cfm labora-
tory units, 1,500 cfm pilot plant systems and commercial
facilities as large as 300,000 cfm capacities. Various gases
and particulate matter were present ranging from industrial
emissions to combustion gases.
113
-------
Evaluation and correlation of all data enabled the
following equations to be developed by Hesketh (1) and which
are restated here:
v2 A0.133
AP = _t—2 (0.56 + 0.125 L + 2.3 x lO'V) (3)
507
v2 p A0'133 L°'78
AP = -£—2 (4)
1270
where: AP = venturi pressure drop, inches water gauge
L = liquid to gas ratio, gal/1000 ACF
p * gas density downstream from venturi throat, lb/ft3
v, = throat velocity of gas, ft/sec
A = throat cross-section area, ft2
Table I lists industrial venturi scrubber applications
and gives the operating conditions. These data span a
variety of scrubber sizes, pressure drops, throat velocities
and liquid to gas ratios. Listed in this table are the
pressure drops calculated using Equations (3) and (4) with
the proper design and operating parameters and gas density.
These AP data show good correlation with the measured values.
Figure 2 shows that these equations also apply to 1,500 cfm
venturi scrubbers.
Equations (3) and (4) are applicable to small 1,500 cfm
venturi scrubbers. The full curve in Figure 2 (at L = 20)
is measured data and Equation (3) fits this curve. The
family of curves shown at other liquid to gas ratios were
calculated using Equation (3).
These equations introduce two parameters (A and p )
compared with Equation (1) which should be accounted for in
this work. Throat area (or diameter) is a direct expression
of system turbulence as revealed by its relation to Reynolds
114
-------
TABLE I
VENTURI SCRUBBER DATA
PRESSURE DROP,
inches H-,0
EXAMPLE
NUMBER
1
2
3
4
5
6
E 7
in
8
9
10
THROAT GAS
LIQUID TO GAS
SATURATED GAS, VELOCITY v^. RATIO L,
ACFM
274,000
50,000
50,000
185,000
60,000
225,000
280,000
90,000
41,400
54,000
TEMP , ~F
126
164
164
177
105
135
125-187
125-187
130
325
ft/sec gal/1000 ACF
100
100
175
200
100
360
350
350
350
230
20
40
14
15
6
14
6-15 (b)
6-15 (c)
16.9
15
DUST CONCENTRATION THROAT CfiLC.
grains/SCF
in
/
8
8
3
0.61
4
5
5
8
/
out
/
0.
0.
0.
0.
0.
0.
0.
0.
/
AREA, A, MEASURED
1
04(a)
05
05
005
005
005
03
'ft
45.7
8.33
4.76
15.42
10.00
10.42
13.33
4.29
1.97
3.91
6.9
10
20
35
5
60
60
60
55
28
EQ.
(3)
9.1
12.3
14.4
29.3
2.6
57.9
/
/
52.0
25.8
BY
EQ.
(4)
9.5
13.0
16.3
32.8
3.0
65.5
/
/
57.4
28.9
NOTES: (a) Guaranteed outlet concentration, no test made.
(b) Calculate L = 10.5 using Eq. (4)
(c) Calculate L = 12.8 using Eq. (4)
APPLICATIONS ARE: 1-Cyclone Boiler, 2-Lime Kiln, 3-Lime Kiln, 4-Black Liquor Recovery
Boiler, 5-Fly Ash Sinter Furnace, 6-Blast Furnace, 7-Blast Furnace, 8-Blast
Furnace, 9-Foundry Cupola, 10-TPA Processing.
-------
number. This parameter may help solve the problem experienced
by use of the "unknown factor f" that appears in Equation (2).
Venturi pressure drop is directly effected by gas density,
and though small, it should be accounted for. Caution must
be exercised to be sure the system is operated at non-scaling
and non-plugging conditions, the pressure lines are properly
cleared and the pressure drop readings are completely valid.
The data used for Equations (3) and (4) were obtained
from units operated so as to produce cloud type atomization
and are for venturi scrubbers that have liquid injected be-
fore the throat. When the same amount of liquid is injected
at the throat, data available so far indicate that the AP
is somewhat higher (up to 10%).
No factor for liquid properties is included in Equations
(3) and (4). Most of the data for these equations were ob-
tained using river water, but some chemical scrubbants were
used. Further data are needed to determine the effects on
AP of variations in liquid density, viscosity, composition
and particle wettability.
Fine Particles and Collection Efficiencies
Fine particles in this work are considered to be all
particles 5 microns or smaller in size. It is considered
that the venturi scrubber is 100% efficient for the removal
of particles larger than 5y. The penetration, which is one
minus collection efficiency of particles, is expressed as
C /C. where C and C. are respectively the weight concentra-
tion of less than 5 micron particles out and into the venturi
scrubber.
The value for C. is established by multiplying the inlet
dust loading by the weight fraction of particles that are 5
microns or less in size. These values are obtained from
literature such as Reference (2) assuming log mean particle
116
-------
size distribution and are listed in Table II. For example,
the amount of foundry coupola dust 5y and smaller in size is
assumed to be 0.14 times the scrubber inlet dust loading.
Note that inlet dust loadings may be subject ,to con-
siderable variation and therefore, in some cases, an average
inlet concentration must be used. The blast furnace data in
example numbers 7 and 8 are averages as the reported inlet
loadings range from 3-7 grains/scf.
The data in Table I are relisted in Table II and the
measured ratios of C /C. for <5u particles are calculated and
listed for the various open throated venturi scrubbers. A
very distinct trend can be seen when these ratios are plotted
against venturi pressure drop as in Figure 4. These data
are marked + in Figure 4.
Some unpublished Swedish removal efficiency data for
venturi scrubbers and non-charged particles were obtained.
These data, which represent many hundreds of industrial data
points, give removal efficiency as a function of particle
size for various venturi scrubber pressure drops. Assuming
that typical fine industrial particles have a geometric size
distribution, a mean diameter by weight of about 1 micron and
an approximate standard deviation of 3.9, it is possible to
estimate overall collection efficiencies using the Swedish
data. These efficiencies were calculated by graphical inte-
gration and are shown as C /Ci by the "dots" in Figure 3. The
line drawn through these dots agrees well with the other
measured efficiency data for collection of fine particles.
Fine Particle Collection Equations
Actual fine particle collection efficiencies would depend
on numerous parameters as shown in Equation (2). Liquid, gas
and particle properties need to be considered as well as equip-
ment design and operating conditions. However, for approxi-
mating fine particle collection efficiencies, Figure 4 shows
117
-------
that good estimates should be possible if accurate pressure
drop data are obtained.
Collection efficiency (E) of particles <5y in size ex-
pressed as penetration (1-E) is approximately related to
pressure drop by the line in Figure 3 which is given as:
Co/Ci = 3-47 Ap~1*43 <5>
where: AP = pressure drop, inches of water
Combining this equation with Equation (4) which has been
shown to be an accurate method for predicting pressure drop,
gives:
9.52 x 10*
C /Ci = (6)
v 2.86 1.43A0.190L1.12
t g
Equation (6) is good for the collection of fine, non-charged
particles.
Equation (6) efficiency predictions agree better with
the Table II dusts than for the particulates formed as fumes.
The fumes may be charged because of their method of forma-
tion. The scatter of data that are present are compounded
when Equations (4) and (5) are combined in Equation (6) and
further, accurate test data will be useful to show how these
relationships should be corrected, if necessary. Diffusio-
phoresis and Stephan flow factors are not considered in these
equations. As mors data become available, it may be possible
to add modifications to account for these effects.
With the relationship given as Equation (6), it is pos-
sible to predict approximate collection efficiencies of fine
particles by venturi scrubbers by specifying only the design
parameters of throat velocity (v , ft/sec), throat area
(A, ft ), gas density (p , Ib/ft ) and liquid to gas ratio
(L, gal/1000 ACF).
118
-------
TABLE II
VD
VENTURI SCRUBBER COLLECTION EFFICIENCIES
FOR <5 MICRON PARTICLES
DUST CONCENTRATION,
FOR <5y
EXAMPLE
NO.
2
3
4
5
6
7
3
9
10
WT. FRACTION DUST
ORIGINALLY <5u
0.17
0.17
0.60
0.10
0.45
0.45
0.45
0.14
0.30
grains/SCF
"Cr
1.36
1.36
1.80
0.06
1.80
2.25
2.25
1.12
/
Co
0.10
0.04
0.05
0.05
0.005
0.005
0.005
0.03
/
MEASURED
0,074
0.029
0.028
0.833
0.0028
0.0022
0.0022
0.027
/
CALCULATED
BY EQ. (6)
0.087
0.062
0.021
0.706
0.009
0.012
0.012
0.010
0.031
PREDICTED
BY EQ. (8)
.0009
.0018
.0012
.0032
.0028
.0026
.0022
.0023
-------
It should be pointed out that a simplified relation-
ship is available for predicting changes in the ratio C /C^.
The study in Reference (1) reveals that:
Co/C.a A'0'092 IT1'39 (7)
The data in Tables I and II are plotted in Figure 4 to show
how these data relate to the equation (7) proportionality.
The three points at the bottom of this figure are the blast
furnace data, example numbers 6, 8 and 7, in that order, from
left to right.
Charged Particle Collection
Quite frequently, venturi scrubbers have been installed
following electrostatic precipitators for one reason or
another. The fine particles that pass through precipitators
that are operating are electrically charged. These charged
particles are more efficiently collected by venturi scrubbers
and some of this data is now being obtained from these in-
stallations.
Reference (1) presents a fine particle collection ef-
ficiency prediction equation for venturi scrubbers. Much of
these data were obtained using charged particles and, as a
result, the equation is applicable to collection of fine
charged particles. This equation is restated below as Equa-
tion (8) :
Co/Ci = 3*45 x 10 vt A l^-) (8)
The efficiencies predicted by Equation (8) for non-charged,
non-metallic dusts are in error by being too high compared
with the measured or Equation (6) values. However, the ef-
ficiencies predicted by Equation (8) for metallic fumes are
in good agreement with the measured values. The metallic
fumes probably are charged because of their method of
120
-------
formation. Equation (8) has not been tested and more work is
needed to determine the reliability.
Use of Wetting Agents in Scrubbers
Wetting agents or surfactants reduce the surface tension
of the liquid and if properly used, improve particle collec-
tion by not only making the atomization occur easily but also
by enhancing particle wettability. Several non-ionic, low
foaming surfactants were studied and the optimum results
were obtained using 0.1% by weight Rohm and Haus Triton CF-10
which reduced the water surface tension to about 10 dyne/cm
at room temperature. The results of this study are also re-
ported in Reference (1) along with the equation:
C/C. » 8.42 x 10-V3-87A0'157 fe)1'93 O)
01 t VAP/ (for water +
wetting agent)
This equation has the same reservations as Equation (8) and
needs to be tested further.
During the study, the use of the wetting agent CF-10 did
decrease outlet dust loadings by about 50%. Outlet dust load-
ings ranged from 0.001 to 0.0093 grains/scf and inlet dust was
2.5 grains/scf.
SUMMARY AND CONCLUSIONS
It appears that fine particle collection efficiency in a
venturi scrubber is closely related to scrubber throat pres-
sure drop. A large amount of industrial data was extrapolated
to establish the efficiency for <5y dust and from this,
Equation (5) was developed:
C /C. = 3.47 AP~1>43 (5)
o i
Using both industrial and research pilot data, new equa-
tions are developed for accurately predicting venturi scrubber
throat pressure drop for open throat, non-plugging and
121
-------
non-scaling systems where the liquid is injected before the
throat. These equations (3) and (4) include the important
parameters of venturi throat area and gas density as well as
the throat gas velocity (at the saturated gas temperature)
and the liquid to gas ratio. No equation is given for pre-
dicting pressure drop when the water is injected at the throat,
but it appears that this causes about a 10% higher Ap.
The combination of the efficiency and pressure drop equa-
tions makes it possible to predict fine particle collection
efficiency knowing only the design parameters of throat velo-
city and area, gas density and liquid to gas ratio. Equation
(6) is developed for this purpose and appears good for non-
charged particles. Charged particles have a greater collec-
tion efficiency and Equation (7) is given for this. More
work is needed to establish the reliability of these equa-
tions .
Wetting agents can also improve collection efficiency
and preliminary studies show that dust loadings can be re-
duced by as much as 50%. It is important to use non-foaming
surfactants.
REFERENCES
(1) Hesketh, Howard E., "Atomization and Cloud Behavior in
Wet Scrubbers", U.S.-U.S.S.R. Symposium on Control
of Fine Particulate Emissions, Jan. 15-18, 1974.
(2) Calvert, Seymour, et al., "Scrubber Handbook, Wet Scrub-
ber System Study, Volume 1", Prepared for EPA
Control Systems Division, August 1972.
(3) Behie, S. W. and J. M. Beeckmans, "Trajectory and Dis-
persion of Transverse Jets of Water in a Turbulent
Air Stream", prepared for AIChE Meeting, Tulsa, OK,
March 1974.
(4) Hesketh, Howard E., "Atomization and Cloud Behavior in
Venturi Scrubbing, JAPCA, Vol. 23, No. 7, p. 600
(1973).
(5) Marshall, W. R., Jr., "Atomization and Spray Drying",
Chem. Eng. Progress Monograph Series, No. 2, Vol.
50 (1954).
122
-------
FIGURE l
VENTURI SCRUBBING OF COAL FLY ASH AFTER ELECTROSTATIC PRECIPITATOR
25
20
IS
10
Anticipate
Incomplete--
Atom izat ion
Anticipate
• Complete
Atomization
rt
O
tsJ
O
Liquid to Gas Ratio L »
20.0-23.4 gal/1000 ACFM
1500 cfm Scrubber
so
100
120
140 160 180
Throat Gas Velocity, fp=
200
220
240
-------
60
50
Correct
Curve
40
to
0)
30
•o
01
I*
to
0)
£
20
10
Calculated using Eq. 3
Calculated using Eq. 4
10 20 30 40 50
Calculated Pressure Drop, inches of water
FIGURE 2
60
70
COMPARISON OF INDUSTRIAL VENTURI SCRUBBER
MEASURED PRESSURE DROP AND PREDICTED PRESSURE
DROP CALCULATED BY EQUATIONS [3] and [4]
124
-------
FIGURE 3
PRESSURE DROP VERSUS GAS VELOCITY FOR CO-CURRENT WET SCRUBBERS
25
1 c
*••>
10
Parameters = L in gal/1000 ACFM
Scrubber cross-section area = 0.125
a
ct
R
8*
a:
NJ
O
80
100
120
140 160 180
Throat Gas Velocity, fps
200
220
240
260
-------
1.0
.7
.5
.3
.2
.1
.07
.05
FIGURE 4
VENTOR1 SCRUBBER FINE PARTICLE COLLECTION
EFFICIENCY VS PRESSURE DROP
§
VI
4J
.03
.02
.01 -
.007
.005
.003
.002
.001
•*•
3 5 7 10 20 30 50 70 100
Pressure Drop, Inches of water
126
-------
FIGURE 5
FINE PARTICLE COLLECTION EFFICIENCY
IN A VENTURI SCRUBBER AS A FUNCTION
OF THROAT AREA AND LIQUID TO GAS RATIO
0.08 -
0.06 _
0.04 -
0.02 -
(A
0.092
127
-------
EFFECTS OF WATER INJECTION ARRANGEMENT ON THE
PERFORMANCE OF A VENTURI SCRUBBER
by
S. W. Behie and J. M. Beeckmans
ABSTRACT
The effect of three different arrangements of two water
manifold injection systems on the performance of a venturi
scrubber was studied. Velocities ranging from 23.4 to 49.7
m/sec were used with aerosol diameters of 0.8, 1.6, 2.9
and 5.0 microns. Scrubber efficiency was found to be
larger than predicted theoretically with the larger par-
ticles, and smaller than predicted theoretically with th3
smaller particles. The spacing, diameter, and location of
the injection orifices were found to have an effect on
scrubber efficiency.
128
-------
INTRODUCTION
Although venturi scrubbers have been used to combat
particulate emissions in various industries for almost thirty
years, it has only been recently that advances in basic
knowledge of scrubber operation have been made. These ad-
vances have been made possible by the use of monodispersed
aerosol particles which has allowed some reliable basic data
on venturi scrubber efficiency to be obtained . (Mathe-
matical models of venturi scrubbers can only be properly tested
with data of this nature.) In addition insight has been
gained into the atomization process occurring within the
4-7
scrubber throat . Since aerosol capture is due mainly
to inertial effects requiring good aerosol-droplet contacting,
one would expect that a requisite for good scrubber perfor-
mance would be complete initial coverage of the scrubber by
the atomized droplets. In fact the increase in aerosol cap-
ture efficiencies of large units over'smaller units of simi-
lar design has been explained on the basis of better initial
8 9
coverage of the larger unit by the atomized spray ' . One
would expect that investigators not reporting this trend
(Semra'u et al. , Collins , Basse' , Mellor and Stevens ,
Johnstone and Roberts , West , Walker and Hall ) ensured
good coverage of the throat in both units. Of course, in-
creased pressure drops (power consumption) goes hand in hand
with improved initial coverage.
The relative performance of venturi scrubbers for dif-
ferent water injection methods has been reported. In a few
9 17
cases slight differences in performance were measured '
for the three quite different water injection methods used.
18
From experiments conducted on a much larger unit , the same
scrubber performance was reported using three different water
injection methods. Despite the fact that the initial throat
coverage as measured photographically was poor (between 40-50%),
129
-------
efficiencies greater than 90% (2.3 transfer units) were re-
ported. The highly turbulent flow pattern of the gas, pro-
moting intimate mixing, was stated as the reason for this be-
havior. Although the importance of turbulence has been sup-
ported by other authors ' ' , one would expect its ef-
fect to be felt only in situations of incomplete contacting
(poor initial throat coverage). For a state approaching
complete mixing the intimate contacting between the aerosol
and water droplets precludes any need to transport the aerosol
particles through turbulent diffusion to the water droplets.
At this time we are reporting results obtained with a
pilot plant scale venturi scrubber using experimental
techniques similar to those recently reported . Three dif-
ferent water injection arrangements of two different mani-
fold systems were studied for throat velocities ranging from
23.4 to 49.7 m/sec. In each case monodispersed uranine-
methylene blue particles having diameters of 0.8, 1.6, 2.9
and 5.0 microns, and density 1.42 g/cm3 were used.
APPARATUS
The experimental apparatus used in this study was quite
similar to that used in our previous recently published
study , with a few notable differences. It consisted of a
venturi scrubber, followed by a cyclonic demister. Aerosol
particles were generated by a spinning disk aerosol genera-
tor, and passed into a 20 cm diameter pipe, 1.5 m in length.
Sampling was isokinetic, at the end of the pipe, just prior
to entry to the scrubber. The gas passed from the cyclone
into an involute chamber, which served to remove the vorti-
city induced by the cyclone. A 1.5 m length of 20 cm pipe
followed the involute chamber, and the aerosol was sampled
isokinetically at the end of this pipe. In runs.made to
measure the demister efficiency/ the aerosol was introduced
downstream from the scrubber. The plexiglass test section,
130
-------
127 cm long, had throat dimensions of 19.0 cm width by 6.45
cm depth, and was 7.62 cm in length. The total converger
angle was 25° while that of the diffuser was 7°. In this
study, the system was on the suction side of a centrifugal
blower, thus eliminating the high temperatures produced in
the previous system operating on the pressure side of a
centrifugal blower. Except for minor changes, the remaining
experimental set-up and aerosol sampling system were the same.
Water was injected into the throat of the venturi through
several arrangements of two different manifold systems.
Figure 1 shows a section through the throat of the scrubber,
in planes normal and parallel to the gas flow, while de-
scriptions of the manifolds are given in Table I.
THEORETICAL CALCULATION OF AEROSOL CAPTURE AND PRESSURE DROP
The basic calculation techniques used in the theoreti-
cal model have recently been described elsewhere , and are
1 20
similar in nature to the methods of Calvert and of Boll
The equations of particle motion and of inertial impaction
were integrated numerically for specific cases, taking into
account scrubber geometry. This model assumes that a state
of perfect mixing exists between gas and water droplets in
planes normal to the flow direction, as well as the fact that
no water reaches the scrubber surfaces. As such one would ex-
pect theoretical results predicted by this model to be higher
than the corresponding experimental results, since maldistri-
bution in the droplet flux would result in paths through the
scrubber in which the flux is low, or in which the relative
velocity between gas and spray is low, or both. Two dif-
ferent expressions were used for the drag coefficient, one
21
being the Ingebo relation , the other an analytical re-
22
presentation for the standard drag curve
The major factor contributing to the overall pressure
losses encountered in a venturi scrubber is the energy expended
131'
-------
in accelerating the water droplets. The pressure loss due
to this factor over a differential time interval dt equals
where p,, is the mass flux of droplets, D , V , p the droplet
F p p p
diameter, velocity, and density, V, p, the air velocity and
density and C_, the instantaneous drag coefficient.
Integration with respect to time over the throat and dif-
fuser of the scrubber gives the overall pressure loss due to
momentum transfer to the spray. During those time increments
over which the water droplets, because of their inertia, move
faster than the air, momentum transfer is to the air, thus
representing a recovery term. Other factors such as friction
losses through system and the atomization process cause pres-
sure losses which are quite small in comparison and are dis-
cussed in detail elsewhere . Frictional losses were taken
into account by assuming 88% recovery of the converger pres-
20
sure loss in the dry system. Boll describes a fundamental
approach to frictional losses in venturi scrubber.
RESULTS AND DISCUSSION
Efficiencies in transfer units [}IT = - Zn(l-n)], for the
scrubber-cyclone combination and for the cyclone separately,
plotted linearly against water loading; Figure 2 shows a
typical set of results for a specific aerosol particle size,
throat velocity and manifold arrangement. Since the ef-
ficiencies of serial units, when expressed in transfer units,
are additive, the transfer unit efficiency of the scrubber
is obtained as the difference between the two curves shown in
Figure 2. Similarly, the difference between the slopes of
the two curves equals the slope of the curve of transfer unit
132
-------
efficiency of the scrubber against water loading. The
experimental results are summarized in this form in Table II,
the values in the table being computed by a linear regression
analysis on the data points. Table III shows ratios of ex-
perimental to calculated slopes, using both the Ingebo and
the standard drag expressions; these results are for ar-
rangement O. The droplet sizes used in these calculations
23
were based on the Nukiyama-Tanasawa equation . It should
be noted that the latter equation predicts an effect of water
loading on droplet size, and as a result the theoretical
curves were not completely linear; however the curvature was
so slight as to be barely detectable visually, and a mean
value was used.
Several pertinent conclusions may be drawn from a peru-
sal of Table III. Perhaps the most interesting is that
the theory in some circumstances underestimates scrubber
efficiency substantially, particularly with large aerosol
particles. Calculations indicate that with particles 3 \im
or larger, particle size has little effect on theoretical
scrubber efficiency; presumably this is because the relative
velocity between aerosol particles and spray droplets in
the throat, where most of the aerosol capture occurs, is so
large that the single particle capture efficiency approaches
unity. Downstream from this region, where the relative velo-
city is reduced to the point where particle size can affect
single particle capture efficiency, the droplets move nearly
as rapidly as the gas and the time of effective scrubbing
by droplets in this region is very short. The experimental
results are clearly at variance with this view, since scrubber
efficiency increased considerably with the 5.0 urn particles
over the 2.9 ym particles, under all conditions. The reasons
for this phenomenon are far from clear, but they must be re-
lated to the mechanics of the jets, which have been shown to
behave very differently from the concept of isolated droplets
133
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evenly dispersed in a gas having a uniform velocity distribu-
tion throughout the cross-section ' . These studies have
shown that droplet acceleration rates in the sprays are much
slower than would be expected on the basis of particle size
and mean air velocity, and that a considerable resistance
occurs to transport of momentum from the air to the bulk of
the droplets at the interior of the spray. Under these con-
ditions, slower acceleration of droplets would occur, over a
longer period of time, which would probably result in increased
aerosol capture with the larger particle sizes. With small
aerosols the slower droplet acceleration may result in such
low single particle impaction efficiencies that overall
aerosol capture efficiency in the scrubber would be adversely
affected. This was in fact observed, particularly at the
lowest velocities, but inefficient throat coverage by the
spray may also have been partly responsible. It should be
noted that in studies on aerosol capture by glass beads in a
vertical pneumatic transport line, which is in many respects
similar to the venturi scrubber, no anomalous effects were
24
found and theory and experiments were in basic agreement
The major difference between the two situations appears to be
that the water is injected in coherent jets in the scrubber,
whereas in the pneumatic transport line the glass beads were
injected more or less evenly over a column cross-section.
A conclusion which may be drawn from Table II is that
scrubber efficiency was higher in nearly all cases with mani-
fold 1 (0.5 mm orifices) than with manifold 2 (2.0 mm orifices).
The reason is thought to lie in better throat coverage with
the more closely spaced orifices in manifold 1, but other
factors may be involved* such as the nature of the atomization
(Manifold 1 resulted primarily in drop-type atomization, whereas
manifold 2 gave rise mostly to cloud-type atomization). In
general injection of water from one side only of the scrubber
(arrangement 1) resulted in the highest efficiencies. The
134
-------
reduced number of injection holes implied a corresponding
increase in the injection velocity of the water, and a higher
water injection pressure. The energy required to inject the
water was small/ however, compared with the energy losses by
the gas phase in the scrubber.
Typical pressure drop results (expressed in the number of
converging section heads) are shown in Figure 3 for the three
arrangements of both manifolds at a mean air velocity of
49.7 m/s. There were little consistent differences in pres-
sure drop for the different arrangements. Good agreement with
theory is noted. Table IV shows a comparison of theoretical
and experimental pressure drop data (for all velocities) in
terms of slopes. The experimental slopes were computed by
regression analysis on the data. The reverse trend of the
experimental pressure drop data at the lowest velocity was
due to increased reentrainment of water from the top diffuser
surface; at this velocity more water penetrated directly to
the diffuser surfaces.
It should be noted that both theoretical and experimental
pressure drop results expressed even in this manner are func-
tions of throat velocity. This fact has been overlooked by
8 10
some investigators ' when comparing published pressure drop
data from several sources.
135
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REFERENCES
1. S. Calvert, D. Lundgren, D. S. Mehta, "Venturi Scrubber
Performance". J.A.P.C.A. , 22_, 529 (1972).
2. T. L. Go, "Particle Capture Mechanism Studies by Cloud-
Type Atomization in a Venturi Scrubber". M.S. Thesis,
School of Engineering and Technology, Southern Illinois
University (1971).
3. S. W. Behie and J. M. Beeckmans, "On the Efficiency of a
Venturi Scrubber". Canadian Journal of Chemical
Engineering, 51, 430 (1973).
4. A. I. Akbrut and L. I. Kropp, "Determination of the
Average Size of Droplets to Design a Scrubber With
a Venturi Tube". Teploenergetika, 19, 118 (1972).
5. H. E. Hesketh, A. J. Engel, and S. Calvert, "Atomization-
A New Type for Better Gas Scrubbing". Atmospheric
Environment, 4_, 639 (1970).
6. H. E. Hesketh, "Atomization and Cloud Behavior in Venturi
Scrubbing". J.A.P.C.A., 2_3, 600 (1973) .
7. S. W. Behie and J. M. Beeckmans, "Trajectory and Dispersion
of Transverse Jets of Water in a Turbulent Air Stream".
Presented at the 76th National Meeting of A.l.Ch.E.,
Tulsa, Oklahoma (March 1974).
8. J. A. Brink and C. E. Contant, "Experiments on an Indus-
trial Venturi Scrubber". Ind. Eng. Chem., 50, 1157
(1958).
9. F. 0. Ekman and H. F. Johnstone, "Collection of Aerosols
in a Venturi Scrubber". Ind. Eng. Chem., 43, 1358
(1951).
10. K. T. Semrau, C. W. Marynowski, K. E. Lunde, and C. E.
Lapple, "Influence of Power Input on Efficiency of
Dust Scrubbers". Ind. Eng. Chem., 50, 1615 (1958).
11. T. T. Collins, "The Scrubbing of Sulphate Recovery Furnace
Stack Gases". Paper Ind. and Paper World, 28 680
(1947).
12. B. Basse, "Venturi Scrubber for Cleaning Cupola Gases".
J.A.P.C.A., 6, 218 (1957).
136
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13. D. Mellor and P. G. Stevens, "Use of a Venturi Scrubber".
Australian Pulp and Paper Tech., 9_, 222 (1955).
14. H. F. Johnstone and M. H. Roberts, "Deposition of Aerosol
Particles from Moving Gas Streams". Ind. Eng. Chem.,
41, 11, 2417 (1949).
15. P. H. West, "Chemical and Heat Recovery with Venturi Scrub-
bers at Thilmany". Tappi, 3£, 399 (1955).
16. A. B. Walker and R. M. Hall, "Operating Experience with a
Flooded Disc Scrubber - A New Variable Throat Orifice
Contactor". .T.A.P.C.A. , 18, 319 (1968).
17. C. E. Lapple and H. J. Kamack, "Performance of Wet Dust
Scrubbers". Chem. Eng. Prog., 51, 110 (1955).
18. M, Taheri and G. F. Haines, "Optimization of Factors Af-
fecting Scrubber Performance". J.A.P.C.A., 19, 427
(1969).
19. M. Taheri, S. A. Beg, and M. Beizaie, "The Effect of Scale-
Up on the Performance of High Energy Scrubbers".
J.A.P.C.A., 2^, 963 (1973).
20. R. H. Boll, "Particle Collection and Pressure Drop in
Venturi Scrubbers". Ind. Eng. Chem. Fundamentals,
12_f 40 (1973).
21. R. D. Ingebo, "Drag Coefficients for Droplets and Solid
Spheres in Clouds Accelerating in Air Streams".
N.A.C.A., Technical Note 3762 (1956).
22. S» A. Morsi and A. J. Alexander, "Collision Efficiency of
a Particle with a Sphere and a Cylinder". Presented
at the First International Conference on the Pneumatic
Transport of Solids in Pipes, Cambridge, England,
Paper B27 (September 1971).
23. S. Nukiyama and Y. Tanasawa, "An Experiment on the Atomi-
zation of Liquid by Means of an Air Stream". Trans.
Soc. Mech. Engrs. (Japan), 4_, 86 (1938).
24. P. Knettig and J. M. Beeckmans, "Inertial Capture of
Aerosol Particles by Swarms of Accelerating Spheres".
Journal of Aerosol Science, 5_, (1974). (In press).
25. S. W. Behie, "Aerosol Capture, Jet Dispersion and Pressure
Drop Studies on Venturi Scrubbers". Ph.D. Thesis,
Faculty of Engineering Science,' The University of
Western Ontario (1974).
137
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TABLE I
SCHEDULE OF WATER INJECTION ARRANGEMENTS
MANIFOLD 1
Arrangement
MANIFOLD 2
Arrangement
(0.5
0
1
2
(2.0
0
1
2
NUMBER OF UPPER
ORIFICES
mm ORIFICES)
14
0
7
mm ORIFICES)
8
0
4
NUMBER OF LOWER
ORIFICES
15
15
8
7
7
3
TABLE IV
PRESSURE DROP DATA
THROAT VELOCITY (m/s)
23.4
34.1
41.2
49.7
SYSTEM SLOPE (M3/KG)
EXPERIMENTAL
0.55
0.45
0.85
1.25
THEORETICAL
0.25
0.85
1.00
1.15
138
-------
TABLE II
EXPERIMENTAL RESULTS IN TERMS OF SLOPE
(TRANSFER UNITS/WATER LOADING, M3/KG)
MANIFOLD L MANIFOLD 2
THROAT VELOCITY
-------
TABLE III
COMPARISON OF EXPERIMENTAL AND THEORETICAL RESULTS
THROAT
VEL(m/s)
23.4
23.4
23.4
34.0
34.0
34.0
34.0
41.2
41.2
41.2
41.2
49.7
49.7
49.7
49.7
I
AEROSOL
DIA. ( m)
5.0
2.9
1.6
5.0
2.9
1.6
0.8
5.0
2.9
1.6
0.8
5.0
2.9
1.6
0.8
SLOPE RATIO (EXPERIMENTAL/THEORETICAL)
MANIFOLD 1
(INGEBO DRAG)
1.24
0.49
0.50
2.07
1.04
0.56
0.40
3.7
1.6
0.82
0.53
5.2
2.7
1.5
0.67
(STANDARD)
2.03
0.84
0.97
3.2
1.7
0.97
0.84
5.7
2.5
1.33
1.24
7.9
4.2
2.4
1.8
MANIFOLD 2
(INGEBO)
0.59
0.22
0.13
1.23
0.70
0.30
0.51
1.8
1.1
0.37
0.42
2.9
1.4
0.86
0.44
(STANDARD)
0.98
0.38
0.26
1.9
1.1
0.53
1.1
2.8
1.67
0.61
0.99
4.1
2.1
1.4
1.2
140
-------
B
i i i i i i i i i i i 1^1 r
* r ' ' ' ' ' i i i i r
B
SECTION A-A
r
UA
SECTION B-B
FIGURE 1 SECTIONS THROUGH THE VENTURI SCRUBBER THROAT (SCHEMATIC)
141
-------
CO
Z
OS
UJ
O
OS
UJ
CQ
4.0-
O OVERALL SYSTEM EFFICIENCY
• WET SEPARATOR EFFICIENCY
O
O
3.0-
2.0-
(9
O
0
•
O
O
MANIFOLD 2 , ARRANGEMENT O
THROAT VELOCITY =41.2 M/S
5.O MICRON AEROSOL
•
*
o.o-
O.O O.I O.2 O.3
WATER TO AIR LOADING RATIO (KG/M3)
FIGURE 2 TYPICAL EXPERIMENTAL SCRUBBER EFFICIENCY RESULTS
0:4
-------
t/5
Q
UJ
X
O
Z
O
C£
UJ
Z
O
u
ce
UJ
CO
O.6
0.5-
O.3 •
o.o
THEORET1CAL CURVE
THROAT VELOCITY = 49.7 M/S
/
OO
O.O4
O.O8
O.I 2
O.16
O.2O
WATER TO AIR LOADING RATIO ( KG/M3 )
FIGURE 3 PRESSURE LOSS ACROSS THE SCRUBBER AS A FUNCTION
OF WATER LOADING AT 49.7 m/sec THROAT VELOCITY
-------
FINE PARTICULATE REMOVAL
3RPTION WITH t
WET SCRUBBER
AND S02 ABSORPTION WITH A TWO-STAGE
by
J. I. Accortt, A. L. Plumley and J.R. Martin
Combustion Engineering, Inc.
Windsor, Connecticut
ABSTRACT
This paper includes results from pilot plant studies and
early field demonstration units and a discussion of the appli-
cation of the limestone wet scrubbing process on a low sulfur
sub-bituminous coal. This latter application required the
development of a hybrid two-stage scrubber to enable collection
of the fine particulate matter as well as removal of a
significant amount of the sulfur dioxide.
144
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FINE PARTICULATE REMOVAL AND S02 ABSORPTION
WITH A TWO-STAGE WET SCRUBBER
by
J. I. Accortt, A. L. Plumley and J. R. Martin
Combustion Engineering
Windsor, Connecticut
INTRODUCTION
In 1965 Combustion Engineering, Inc. initiated studies
of limestone wet-scrubbing of SO- from utility boiler flue
f 11
gas and the simultaneous wet removal of fly ash. '
This paper will deal specifically with that part of
C-E's wet scrubbing development program pertaining to the
collection of fly ash. Included are results from pilot plant
studies and early field demonstration units constructed in
1968; a review of the measurement techniques employed by C-E
in obtaining the mass dust loadings and particulate matter
sizing information; and a discussion of the application of the
limestone wet scrubbing process on a low sulfur sub-bituminous
coal. This latter application required the development of the
fine particulate matter as well as removal of a significant
amount of the sulfur dioxide. These particulate and S02
removal systems are called Air Quality Control Systems (AQCS)
at C-E.
BACKGROUND
Wet scrubbers had also been used as dust collectors in
many industrial applications. Unfortunately, most of the
experiences involved trial and error investigation with little
engineering design. These early experiences were hampered by
the general lack of good measuring techniques which caused
"judgment" to be the major design tool of the wet scrubber
industry.
145
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Combustion Engineering's initial survey indicated there
were twenty-six manufacturers of commercially available wet
scrubbers. Based on a review of literature and contacts with
manufacturers, we established our criteria for a wet scrubber
as follows:
A. 99% particulate collection
B. 90% or better S02 removal
C. Low pressure drop (six inches or less)
D. A liquid to gas ratio of approximately ten gallons
per thousand acfm.
E. Low plugging potential
In light of the research and development program we have con-
ducted during the last eight years, we have come to reevaluate
these initial criteria. Several have proven unattainable, and
most have been determined to be oversimplified. Establishing
design criteria today requires significantly more quantifica-
tion of the process variables and the desired function of the
overall process.
The types of scrubbers that C-E studied included four
general categories: (a) impingement, (b) spray tower,
(c) venturi, and (d) marble bed. After a nine month study
conducted in 1965, it was concluded that the marble bed scrubber
offered the best overall performance relative to the design
criteria.
FIELD DEMONSTRATION UNITS
C-E's early pilot plant efforts led to the building of
the first full-size limestone wet scrubbing demonstration
units at Kansas Power and Light Co. Lawrence 4 and Union
Electric Co. Meramec Station 2.*- »3)4J
Emissions tests conducted on these units between 1969 and
1972 demonstrated their capability of removing 98.5 to 99% of
the dust entering the scrubber system. Actual dust loadings
on the inlet to the scrubber varied between 4.5 and 6.3 gr/SCF
146
-------
while the outlet dust loading measured in the range of
0.05 - 0.07 gr/SCF.
Particulate sizing measurements were made using the
Bahco technique. Table IA displays representative data from
the Kansas Power and Light Co. Lawrence 4 unit. These measure-
ments were considered valid since no accurate means of deter-
mining size distribution below one micron were available.
Additional dust samples taken with a calibrated cyclone
indicated that the scrubber was removing approximately 501
of particulate matter one micron or less in size.
Chemical analyses of the outlet dust loadings further
revealed that approximately 50% of the material being emitted
from the unit was calcium sulfate,. This was interpreted to
mean either that finely calcined limestone was reacting with
sulfur trioxide in the boiler and passing through the scrubber
without being wetted, or that a significant amount of liquid
carryover through the mist eliminator was occurring. This
carryover was at or near saturation with calcium sulfate and
therefore could produce solid calcium sulfate particulate
matter after the liquid portion was evaporated in the stack
gas reheater.
Both explanations gave us reason to believe that future
non-furnace injection systems with more efficient mist elimi-
nator components would produce lower particulate matter emission
rates. Further, the particulate matter sizing data obtained at
Kansas Power and Light Co. indicated good removal of fine dust
particles (i.e., 2 microns or less).
PILOT PROGRAM
A pilot plant test program was conducted to establish the
system requirements for a wet scrubbing system to remove 99%
of the fly ash from the low-sulfur western coal and lower the
S02 emission levels to a point acceptable to state and federal
regulatory agencies. The results of this test program conducted
147
-------
in the summer of 1970 were quite encouraging. '
A. A single marble bed scrubber was able to produce
an outlet particulate matter emission of 0.03 gr/SCF
(Inlet loading equaled 2.0 - 3.0 gr/SCF).
B. A two-stage marble bed scrubber was able to further
reduce the dust loading to 0.02 gr/SCF.
At about the same time, C-E entered into negotiations
with a midwestern utility to supply a wet scrubber system for
a new 700-Mw unit of C-E design which would burn low-sulfur
sub-bituminous western coal. Utilizing the information devel-
oped from our pilot plant work on low-sulfur western coal and
the particulate matter emission data obtained on full size
operating systems, C-E entered into a contract to supply a
system which would remove 50% of the SC>2 and 99% of the
particulate matter to achieve emission levels of 200 ppm SO-
and 0.04 gr/SCF of particulate matter, ' A new pilot program
tailored to this project was completed in 1971 with the system
design illustrated in Fig. 1.
As the next step in the program to develop a successful
system, it was decided by the customer and Combustion Engineer-
ing to erect a large prototype test facility to test the system
parameters, verify system operation, and train operating
personnel. •* A significant amount of experimental work was
also required to quantify particulate matter collection and
sizing. This effort is discussed in a separate section of the
paper.
FIELD PROTOTYPE TESTING
In 1973, C-E and the utility conducted a joint ten month
test program of the prototype marble bed assembly system on a
f 81
unit burning sub-bituminous western coal. ' It was determined
that the original system could not produce an outlet particulate
loading required by the regulatory agency of 0.04 gr/SCF. Initial
test work showed outlet particulate loadings to be 0.07 to
148
-------
0.08 gr/SCF. Inlet dust loadings averaged 2.0 gr/SCF. A slip-
stream of flue gas was taken from the air heater outlet duct
in an isokinetic manner at a rate of 12,000 CFM @ 135 F
(design). Analysis of the coal ash, at the AQCS inlet, re-
vealed a typical proportion of fines coming to the system.
Fines are defined in this case as particles below two microns
in size.
Comparison of particle size distribution (Table IA) as
determined by a Bahco centrifugal classifier showed that the
midwestern and western coals had comparable size distribution,
despite variation in collection efficiency of the wet scrubber.
At this point, an in-stack inertial impactor was used to obtain
a size distribution of the ash. The results (Table IB) showed
that nearly 851 of the particulate matter with a size less
than 1.5 microns was actually 0.3 microns or less. This
significant amount of "superfine" particulate matter required
a major revision in the scrubber system to meet the particu-
late emission level of 0.04 gr/SCF.
Concurrent with the field prototype operation, Combustion
Engineering was pursuing the development of other types of
scrubber internals. One of these projects had led to the
conclusion that a gas atomizing scrubber in which slurry atom-
ized into droplets by the gas stream and accelerated to high
velocity between parallel rows of rods could be coupled with
a marble bed scrubber in series to form a two-stage system.
A decision was made to revise the field prototype system to
include a first stage scrubber,
DESIGNING OF FIRST STAGE SCRUBBER
The basic objectives in the design of the first stage
were as follows:
1. To develop a high energy device (gas side) which
would insure intimate contact between gas-borne
particulate matter and scrubbing liquid so that fines
would agglomerate and be removed in the marble bed
scrubber.
149
-------
2. Utilize the same 10% alkaline slurry scrubbing
liquid in both the first stage scrubber and the
marble bed scrubber.
3. Provide relatively constant removal efficiencies
over a wide range of load variations.
4. Develop a geometry that would demonstrate trouble-
free operation for the following considerations;
a. Minimize and control wet/dry interface.
b. Select materials of construction for a
corrosive/erosive environment.
c. Develop pluggage free nozzles.
d. Insure deposit free scrubber internal surfaces.
A first stage scrubber was developed after initial labora-
tory testing on a 1200-cfm pilot plant and subsequent operation
of a 12,000-cfm prototype in the field under actual conditions.
We have logged over 5000 hours of operation with a first stage
scrubber in service and results from an operational as well as
performance standpoint exceeded our expectations.
Figure 1 shows how the first stage is coupled to the
system. Gas comes to the first stage from the air heater at
approximately 350 F. An inlet gas nozzle extends into the
expansion chamber. Above this point a steam soot blower is
located. This blower is actuated periodically and sweeps the
entire inlet nozzle while moving the entire length of the
scrubber. The gas expands into the chamber where it is
irrigated with slurry.
TWO-STAGE TESTING
Operation of the field prototype as a two-stage system
has proceeded for six months. In this mode the gas atomizing
scrubber was followed by a marble bed scrubber. The system was
evaluated in terms of AP requirements to meet guarantee emission
rates of S02 as well as particulate matter. Materials of con-
struction, design features and liquid requirements were studied
150
-------
on the first stage. In summary, we found that a dust loading
of less than 0.04 gr/SCF on the outlet could be obtained
continuously with a system AP of 13.5 inches and a total L/G
of 25.
This was later verified in a thirty day continuous run
to demonstrate overall system availability and performance.
Subsequently, the system was modified by removing the marble
bed and the first stage was operated alone as the SO- and dust
contactor. This was done in order to determine performance
criteria of the gas atomizing scrubber alone. The variables
studied were L/G and AP while maintaining gas flow and boiler
load conditions constant. The circulating liquid to the
system was 101 slurry, the solids in the liquid being fly ash
and sulfur salts.
During these periods of operation, system availability
from the AQCS standpoint was 994. Routine equipment mainten-
ance was the only reason for short duration shutdowns. Sulfur
dioxide removal during these tests varied from 55 to 751.
Aside from the major consideration of determining che
required energy necessary to accomplish removal of the "super-
fines", we also established the following: Liquid feed to the
first stage was needed in quantities exceeding the amount
necessary for dust collection to keep the system deposit free.
Specially designed nozzles, uniquely located, were required to
operate the first stage scrubber.
Performance data obtained with the first stage and marble
bed scrubber is illustrated in Fig. 2. The curve shows system
AP as a function of outlet dust emissions. It can be seen
that with at least a 10-inch AP across the first stage and the
marble bed, the required 0.04 gr/SCF was obtained. Figure 3
shows performance of the first stage scrubber alone. To obtain
the required emission an 8-inch AP across the first stage was
necessary at high L/G, while at low L/G a 10-inch AP was
required.
151
-------
PARTICIPATE MATTER SAMPLING
A number of forms of sampling equipment have been devel-
oped to meet specific requirements in sampling particulate
matter emissions from various processes. Stack sampling
systems generally consist of sampling nozzle, a probe, a
particle collector, a condenser to remove excess moisture, a
flow measuring device, a gas exhauster or pump, a temperature
measuring device and a device for regulating gas flow.
Both in-stack and external samplers have been used during
C-E investigations. For sampling the inlet to the scrubbing
( Q)
system either the BCURA cyclone-filter probe*- J or the ASME
alundum filter in-stack systems have been employed. Outlet
samples have been obtained utilizing the EPA external
cyclone-filter set^ ' or the ASME system.
In-stack inertial impactors are extremely valuable in
determining particle size information at the emission source.
The Mark III University of Washington Source Test Cascade
impactor consists of seven impaction stages followed by a
filter. The Casella cascade impactor consists of a system of
four air jets impinging, in series, on glass discs^ . In
the laboratory the centrifugal classifier (Banco) has class-
ically been used for determination of terminal velocity
distribution of particles.
CONTINUOUS MONITORING OF PARTICULATE EMISSION
In conjunction with manual emission tests performed at a
field prototype test facility, C-E also tested a commercial
opacity meter in order to determine the meter's usefulness as
a continuous monitor of solid particulate emission of AQCS stack
gases. As shown in Fig. 4, the meter's output in terms of opti-
cal density correlated well with the EPA method of source test-
ing. The meter's accuracy was ± 4% of full scale for a 95%
confidence limit. Equally important, the instrument's sensi-
tivity was high enough to reflect changes in the liquid to gas
ratio (L/G) and changes in the differential pressure (AP) of
152
-------
the rod scrubber.
Basically, the meter consists of a light source and
photocell mounted on one side of the stack with a reflector
f 12)
mounted diametrically opposed. J The light beam emitted
by the source traverses the stack twice. Dust loading, or
particulate density is measured by measuring the attenuation
of the light beam as it traverses the stack. Since attenuation
is directly proportional to optical density, optical density
is linearly related to dust loading, assuming a fixed particle
size distribution and path length. As possible future appli-
cation, this instrument could be used to control the pressure
drop required in the two-stage scrubbing system concept.
CONCLUSIONS
Based upon data obtained on our prototype unit handling
12,000 CFM of gas with an inlet dust loading averaging
2.0 gr/SCF and a particle size distribution showing a sub-
stantial amount of fines (smaller than 1V5 microns), the marble
bed scrubber at design pressure drops and L/G could only
collect 96.10% of the total dust. With the addition of a
first stage collection device, namely a gas atomizing type
collector, overall removal efficiency was increased to as high
as 99.20%. At this time Combustion Engineering is not prepared
to classify this specific sub-bituminous western coal as
typical; however, this case represents a specific application
of the wet scrubbing system and demonstrates how C-E has
designed hardware to handle the particular problems of this
application.
153
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REFERENCES
1. Plumley, A.L., Whiddon, O.D. , Shutleo, F.W., and Jonakin, J. ,
"Removal of S02 and Dust from Stack Gases," American Power
Conference Proceedings, Vol 29, Apr 25-27, 1967.
2. Martin, J.R., Taylor, W.C., and Plumley, A.L., "The C-E Air
Pollution System - Operating Experience - By-product Utilization,"
1970 Industrial Coal Conference, University of Kentucky, Apr
8-9, 1970.
3. Plumley, A.L. and Gogineni, M.R. /'Research and Development in
Wet Scrubber Systems," Second International Lime/Limestone Sym-
posium, New Orleans, La, Nov 8-12, 1971.
4. Jonakin, J. and Martin, J.R., "Applications of the C-E Air
Pollution Control System," Ibid.
S. Stengel, M.P., Internal C-E Report.
6. Kettner, J.E., Butcher, R.M., Singer, J.G. ,"Sherburne County
Generating Plant, Design and Environmental Considerations,"
American Power Conference Proceedings, Vol 35, May 8-10, 1973.
7. Gogineni, H.R., Martin, J.R., and Maurin, P.G., "Status of
C-E's Air Quality Control Systems," Flue Gas Desulfurization
Symposium, New Orleans, La, May 14-17, 1973.
8. Powell, E. M., Singer, J.G., Maurin, P.G., and Martin, J.R.,
"Designing from Experience with 960 Mw of Air Quality Control
Systems," Air Pollution Control Association Meeting, Denver, Colo,
June 9-13, 1974.
9. Hawksley, P.G.W., Badzioch, S., and Blackett, J.H., "Measure-
ment of Solids in Flue Gases," British Coal Utilization Research
Association, Surrey, England, 1961.
154
-------
REFERENCES (CONTINUED)
10. "Standards of Performance for New Stationary Sources,"
Federal Register, Vol 36, No. 247, Dec 23, 1971.
11. "Casella Impactor Instruction Leaflet 3013/TE ,"BGI, Inc.,
Waltham, Mass.
12. Hudson, A.E. and Mau, E.E., Introduction to Stack Sampling
for Particulates, Lear Siegler, Inc., 1973.
155
-------
INLET DUST SAMPLES
PARTICLE SIZE ANALYSIS
TABLE IA BAHCO ANALYSIS
Western Sub-bituminous Fly Ash
Micron Size % Collection
+ 29
+ 13.4
+ 5.4
+ 2.6
+ 1.65
+ 0
22.5
18.4
29.5
16.9
7.2
5.5
Midwestern Bituminous Fly Ash
Micron Size % Collection
+ 27
+ 15.5
+ 6.2
+ 3.0
+ 1.9
+ 0
18.7
13.6
32.3
21.2
7.5
6.7
TABLE IB - FLY ASH FROM SUB-BITUMINOUS WESTERN COAL
Field Samples
Impactor
Micron Size
+25
+11
+ 5
+ 2
+ 1.3
+0.55
+0.31
-0.3
* Collection
BCURA Sample Analyzed By
Bahco
Micron Size
Collection
+29
+ 12
+ 4
50
+ 1.
+ 0
.8
2.3
45
,3
,7
16.
18.
31.6
21.2
7.3
4.9
156
-------
FIG. 1: MODIFIED TEST FLOW DIAGRAM
POST 8/73
RIVER
MAKE-UP
WATER
THICKENER
UNDERFLOW
PUMP
ASH POND
RETURN PUMP
-------
FIG. 2: FIRST STAGE AND MARBLE BED SCRUBBER PERFORMANCE CURVE
0.06
in
oo
6 8 10 12 14
SYSTEM DIFFERENTIAL AP-IN. H20
18
-------
FIG. 3: FIRST STAGE PERFORMANCE CURVE
0.06
tn
<£>
8 10 12 14
ROD SECTION AP-IN. H20
6
18
-------
O.I 4
0.12
UPPER 95%
CONFIDENCE
LIMIT /
LOWER 95%
CONFIDENCE
LIMIT
0.00 O.OI O.O2 0.03 O.O4 0.05 0.06
GRAINS/STD. CU, FT. (E.P.A. GRAVIMETRIC)
CORRELATION COEFF. = 0.953
FIG. 4: CORRELATION OF OPTICAL DENSITY AS A FUNCTION OF
GRAIN LOADING.
160
-------
"FLUX FORCE/CONDENSATION SCRUBBING"
by
Seymour Calvert
Nikhil C. Jhaveri
A.P.T., Inc.
P. 0. Box 71, Riverside, California
ABSTRACT
Considerations for the engineering design of flux
force/condensation (FF/C) scrubbers are reviewed. Fine
particulate removal in multiple sieve plate FF/C scrubbers
is predicted, using mathematical design models. Results
of experimental studies of two multiple sieve plate scrubbers
for the removal of submicron particles are given. The
published experimental data on FF/C scrubber performance
are summarized. A preliminary analyses of the economics of
FF/C scrubbers, as compared to the conventional high energy
scrubbers, define the most favorable operating conditions
for the application of FF/C scrubbers.
161
-------
"Flux Force/Condensation Scrubbing"
by
Seymour Calvert
Nikhil C. Jhaveri
A.P.T., Inc.
P.O. Box 71, Riverside, California
Wet scrubbers are widely used for the control of air
pollution mainly because of their low initial cost and their
ability to remove particulate as well as gaseous contaminants.
A major drawback of present day scrubbers is the large energy
expenditure required to achieve high removal efficiencies for
fine particles in the size range of 0.1 to 2 microns (pm) in
diameter. This is due to the decreased effectiveness of the
inertial and diffusional collection mechanisms for particles
in this size range.
Flux force and vapor condensation effects have the poten-
tial to cause high removal efficiencies for fine particles in
low energy scrubbers. These effects can result from the
cooling of a hot, humid gas by contact with cold liquid, the
condensation of injected steam, or other means.
Flux force effects on particles have been known for many
years and the background is reviewed and discussed in depth
by authors such as Waldmann and Schmitt (1966), Goldsmith and
May (1966), Hidy and Brock (1970) and Calvert et al. (1972).
The present discussion is limited to thermophoresis and diffusio-
phoresis (which we define to include both diffusiophoretic and
Stefan flow forces). Accordingly, we consider only those FF/C
scrubbers where particle removal from the gas is aided by a
temperature gradient, a vapor concentration gradient, vapor
condensation, particle growth due to vapor condensation or
combinations of the four.
The work reported in this paper includes both theoretical
and experimental studies. Engineering design considerations
for FF/C scrubbers are reviewed and the mathematical model for
162
-------
a plate scrubber developed by Calvert et al. (1973) is extended
to predict performance of multistage sieve plate FF/C scrubbers.
Experimental studies of a bench scale three plate FF/C scrubber
and a pilot scale five plate FF/C scrubber are reported.
Results of the bench scale study are compared with theoretical
predictions made with mathematical design models. A summary
of available experimental data on FF/C scrubbing is presented.
Economics of FF/C scrubbing is analyzed and compared with the
conventional high energy venturi scrubbers on the basis of
equivalent performance for fine particulate removal.
FF/C SCRUBBER DESIGN
ENGINEERING DESIGN CONSIDERATIONS
Theoretical and experimentally derived design analyses
for FF/C scrubbing have been presented by several investigators,
such as Rozen and Kostin (1967) , Sparks and Pilat (1970) and
Davis and Truitt (1972) . An extensive study of design procedures
was recently reported by Calvert et al.'(1973). They used the
unit mechanism approach to develop design equations for spray,
sieve plate, impinging jet, wetted wall column and packed bed
scrubbers for the removal of fine particles.
The relevant deposition velocities are those due to the
flux forces, inertial, gravitational and Brownian diffusional
effects. Since the flux force deposition velocities are
functions of the temperature and vapor pressure gradients, the
magnitude of these gradients at various distances (or residing
times) along the gas path through the scrubber must be deter-
mined. To account for particle growth, information on the
critical saturation ratio for nucleation is needed and this
depends on the particle properties and the vapor composition
as a function of distance or time inside the scrubber.
The inertial and gravitational deposition velocities are
functions of the particle size and density which in turn change
163
-------
uue to any vapor condensation on the particle. Vapor composi-
tion depends on heat and mass transfer between gas/particles
and gas/liquid, which change in magnitude as the gas proceeds
through the scrubber. Because of the rapid changes in condi-
tions and the competition between the particles and the liquid
surface for the condensing vapor, any realistic design method
must consider the point-to-point conditions.
PREDICTIONS FOR MULTISTAGE SIEVE PLATE SCRUBBER
A mathematical model of an FF/C sieve plate scrubber,
developed and experimentally verified by Calvert et al. (1973)
was extended to predict the performance of multistage scrubbers
during this study. The scrubber performance was predicted for
two cases; when particles are wettable so that particle growth
occurs, and when particle surface properties prevent particle
growth. For both the cases, the bottom plate performance was
predicted by using the model of Calvert et al. (1973) and the
inlet particle size was assumed to be d = 0.75 ymA. The
model incorporates the following phenomena:
1, Heat transfer between bubbles and liquid.
2. Heat and mass transfer between bubbles and particles.
3. Particle deposition by:
A. Impaction during bubble formation
B. Diffusiophoresis
C. Thermophoresis
D. Centrifugation during bubble rise.
For the case of wettable particles, predictions for the
subsequent plates were based on the assumption that no addition-
al particle growth or flux force deposition occurs on these
plates. Deposition on these plates was predicted for inertial
impaction at the point of bubble formation only. This assumption
is valid as most of the vapor condenses out on the bottom plate
164
-------
and the particle growth permits high removal due to inertial
impaction on the subsequent plates. The penetrations were
calculated from the following equation:
Pt = exp (-40 F2 K ) (1)
The predicted penetrations for 1, 2 and 3 plates in series
are plotted against q' (g vapor condensed/g dry air) on
Figure 1. The penetration is highly dependent on the initial
particle concentration, as indicated by the comparison of the
solid lines, for n. = 107 particles/cm3 with the broken lines,
for ni = 2xl05 particles/cm3.
For the case where no particle growth occurs, the predic-
tion was based on the following assumptions:
1. Steam is introduced under each plate so that the gas
under each plate is saturated.
2. The penetration on each plate is the same if the amount
of steam condensed is the same.
Figure 2 is the plot of predicted penetration versus q'
for 1,2,3 and 4 identical sieve plates in series. The amount
of vapor condensed is the total for the number of plates shown.
From the above predictions we can conclude that:
1. For a single plate scrubber there is little effect of
particle concentration or critical saturation ratio.
2. Particle growth leads to better performance in multiplate
scrubbers than the no-growth situation.
3. If no growth occurs, better steam utilization results from
the introduction of steam under each plate rather than
only the bottom plate.
EXPERIMENTAL
An experimental study of a single sieve plate FF/C scrubber
was reported by Calvert et al. (1973). This research showed
that diffusiophoresis was the major collection mechanism for
165
-------
the single plate scrubber. Particle growth by condensation
could not be utilized significantly on a single plate as the
conditions for removing grown particles by impaction did not
exist on the plate. The purpose of the present investigation
was to determine the FF/C scrubber performance at higher q1
values and provide conditions for collecting grown particles
by impaction by using a multistage scrubber.
BENCH SCALE FF/C SCRUBBER
APPARATUS
A schematic flow diagram of the experimental apparatus
is shown in Figure 3. The major components were a three
sieve plate FF/C scrubber and the aerosol generator. The
sieve plates were made from 1,6 mm aluminum sheet with the
overall diameter of 10.2 cm. Each plate had 60 perforations
of 3.2 mm (1/8") diameter, adding up to 5.9% free area on the
plate. On each plate the flow was radially inward to a
central downcomer, 2.54 cm in diameter.
Aerosol was produced by dispersing reagent grade iron
oxide (Fe_0_) powder from an aqueous suspension. Freshly
prepared aqueous suspension of 4% Fe,0, and 0.051 Na.P-07«10 H-0
was used for each experimental run. The sodium pyrophosphate
was added to the suspension as a dispersing agent. The sus-
pension was continuously stirred during the run to further
prevent settling. The suspension was dispersed by a compressed
air atomizer. The aerosol was dried by heating and then passing
it over concentrated sulfuric acid. An impactor with a cut
diameter of approximately 2 ym removed the larger particles
from the aerosol stream.
EXPERIMENTAL PROCEDURE
The experiment was started by adjusting the water, aerosol
generator and ambient air flow rates to the desired values.
After attaining steady foams on the sieve plates, heat and
166
-------
filtered steam were added to the air stream to attain the
desired values of temperature and humidity. After a steady
state was reached, which took from thirty to sixty minutes,
the particulate sampling was started. The experimental
conditions were very stable once the steady state was reached.
For all the experimental runs reported, the inlet gas tempera-
ture varied within ^0.5°C during the experimental period.
Two heated sample probes were located in the scrubber
inlet and outlet gas pipes, respectively. A third heated probe
was located above the bottom plate inside the scrubber. The
particulate size distributions were measured with an eight
stage non-viable Andersen Sampler. The sampler was placed in
a temperature controlled oven and was followed by a Gelman
type "E" glass fiber filter to capture particles penetrating
the bottom stage of the impactor. The size distributions were
determined gravimetrically. Particle loadings in the scrubber
inlet and outlet gas streams were measured gravimetrically
with Gelman type "E" glass fiber filters placed in the oven.
The samples were pulled simultaneously from the two gas streams.
The sampling period was one hour and the samples weighing less
than 2.S rag were discarded.
RESULTS AND DISCUSSIONS
Operating conditions and performance of the three sieve
plate column are shown in Table 1. The dry air and water flow
rates remained constant for all experimental runs. The per-
formance of the column was studied for different gas inlet
temperatures. The inlet gas was maintained saturated with water
vapor and the scrubber performance was measured for a wide range
of q1 values, from 0.07 to 0.53 (g vapor condensed/g dry air),
During the experimental runs, the particle size distribution
was found to be log-normally distributed in the following
167
-------
range:
d = 0.75 jM3.ll ymA; a = 1.61 + 0.22
r *> o
Overall penetration values for the experimental runs are
plotted against q (g vapor condensed/g particles in) on
Figure 4 and against q' (g vapor condensed/g dry air) on
Figure 5. For comparison, the performance of only the bottom
plate of the three plate column is also plotted on these figures
The results verified the following predictions from the mathe-
matical model described earlier:
1. For the same amount of vapor condensed in the scrubber,
the three plate column gives lower penetrations compared
to a single plate column. Note that most of the conden-
sation occurred on the bottom plate of the scrubber.
Thus, higher performance of the three plate column can
be attributed to the particle growth effect.
2. A single plate does not give high efficiencies even when
the amount of vapor condensed is high. The major contri-
bution of the bottom plate is to provide for particle
growth during bubble rise on the plate. Thus, the bottom
plate may be designed so as to minimize the pressure drop.
During the experimental runs the foam density, F, was
measured to be 0.4 when the entering gas was almost dry. The
foam density increased to about 0.75 when the entering gas
was saturated with water vapor and the foam appearance changed
to irregular bubble shapes.
Kotrappa and Wilkinson (1972) report that the density of
iron oxide aerosol particles range from 2.2 to 2.6 g/cm3, when
the aerodynamic diameters are in the size range of 0.3 to 3.5
microns. If a particle density of 2.5 g/cm3 is assumed, the
iron oxide particle number concentrations at the scrubber inlet
was in the range of 10s to 106 particles/cm3 during the experi-
mental runs.
168
-------
The experimental results are compared with predictions
from the mathematical model on Figure 6. Comparison of the
single plate performance showed a good agreement up to q'=0.1.
For higher values of q', the model predicts a better perform-
ance. This is possibly due tc the change in foam character-
istics in this range. Experimental results for the three
plate scrubber show a higher performance than predicted when
no particle growth takes place; and a lower performance than
predicted when particles grow. Besides the effect of foam
characteristics, the discrepancy is interpreted to indicate
the existence of more nuclei than accountable as iron oxide
particles.
The results compare better with predictions for 107 or
more particles/cm3, indicating that there may have been
competing nuclei, other than iron oxide, present in the air
stream. These may have been inadvertently introduced with
the steam or formed by nucleation of -sulfuric acid vapor
during the aerosol drying process. The gas temperature as it
passed over the concentrated sulfuric acid bath was somewhat
less than 90°C. At this temperature the sulfuric acid vapor
pressure is 0.12 mm Hg. For a comparable sulfuric acid vapor
pressure, Amelin (1967) reported the formation of sulfuric
acid nuclei due to homogeneous condensation, in the order of
1010 nuclei per cm3. For the gas resident time in the apparatus,
coagulation due to Brownian diffusion would reduce this number
concentration to about 108 nuclei per cm3. This would then
clearly account for the discrepancy with the predictions from
the mathematical model.
PILOT SCALE FF/C SCRUBBER
APPARATUS
The same general set up of experimental apparatus as
described for the bench scale studies was used in a pilot scale
169
-------
study. The major components were a five plate FF/C scrubber
and the aerosol generator. All the plates were identical and
were made from 0.3 m x 0.3 m x 1.6 mm 316 stainless steel
sheet. The perforations were 3.2 mm (1/8") in diameter, adding
up to 94 free area on the plates.
The aerosol was produced by dry dispersing black iron
oxide powder. The powder, after drying, was sieved through
16 mesh screen. It was then fed to the inlet of a high pressure
blower through a screwfeeder arrangement. The radial blades
of the blower were modified to increase recirculation and shear
on the aerosol particles within the blower. A multiple round
jets impactor with a cut diameter of 4 ymA was used to remove
large particles from the dispersed aerosol. Eight polonium
210 ionizing units were used to neutralize the electrical
charges on the particles.
EXPERIMENTAL PROCEDURE
The scrubber operating procedure was the same as described
for the three plate bench scale scrubber. Andersen samplers,
followed by Gelman type "E" glass fiber filter papers were used
to determine the particle size distributions and loadings in
the scrubber inlet and outlet gas streams. The outlet sample
flow rates were between 2 to 3 times the inlet rates, due to
the low Ft values. When q' was greater than 0.2, the outlet
was sampled for 250 minutes while the inlet was sampled inter-
mittently for the first 10 minutes of the hour, for a total of
50 minutes.
RESULTS AND DISCUSSION
Operating conditions and performance of the five plate
column are shown in Table 2. The particle penetrations for
these runs, as a function of particle diameter (aerodynamic)
are plotted on Figures 7, 8, 9 and 10. The particle penetra-
tion, as a function of the condensation ratio, is plotted on
Figure 11 for 0.6 ymA and 1.0 ymA particles.
170
-------
The scrubber performance was far superior to the perform-
ance of the bench scale scrubber. Again assuming the particle
density of 2.5 g/c'm3 , the particle number concentration enter-
ing the scrubber ranged from SxlO5 to SxlO6 particles/cm3,
during the experimental runs. These results compare well with
the predictions from the mathematical design model for wettable
particles; indicating higher penetrations than predicted for
2xl05 particles/cm3 and lower penetrations than predicted for
107 particles/cm3.
The results clearly indicate the significant effect of
condensation ratio, q', on the scrubber performance. The
particle penetrations decreased with an increase in q'. Also,
the particle penetration decreased with an increase in the
inlet particle size.
REVIEW OF FF/C SCRUBBING STUDIES
A summary of the high points of the available data on
FF/C scrubbing is presented in Table 3, "FF/C Scrubbing
Performance Data Sources" and Figure 12, "Particle Penetration
versus Condensation or Injection Ratio". Some noteworthy
points shown in Table 3 and Figure 12 are as follows:
1. Particle penetration depends heavily on the amount of
vater vapor condensed per unit mass of dry gas (q1).
The condensation ratio, q', can be shown theoretically
to be sufficient to define particle deposition rate,
without regard to particle concentration, if there is no
condensation on the particles.
2. Particle penetration also depends significantly on particle
concentration. By referring to the concentration data
given in Table 3, one can see that there is a clear trend
of penetration decreasing as particle number concentration
decreases. This effect can be shown theoretically to
accompany condensation on particles and their growth at
the expense of the water vapor concentration in the gas.
171
-------
Because the particles use some of the steam, there is a
lower diffusiophoretic sweep velocity to deposit them.
Also, the fewer the particles which share a given quantity
of condensation, the larger they will grow and the easier
they are to collect.
3. The data for references 5 and 7 are the only ones in
Figure 12 for penetration versus steam injection ratio
rather than condensation ratio. The exceptionally low
penetration shown for soluble materials such as Na-CO,
and Na-SO. may be partly due to their being able to grow
by condensing water vapor when the relative humidity is
less than 100%.
FF/C SCRUBBING COSTS
Costs for FF/C scrubbing are highly dependent on the
amount of vapor which is condensed, especially if steam (or
the fuel to evaporate water) has to be purchased. In order to
provide some general guides as to economically attractive
operating conditions, the major operating costs for FF/C
scrubbers have been compared to those for high energy scrubbers.
Depreciation costs are not included in these comparisons because
they will be roughly in the same cost range and they will
usually be overshadowed by power and utility costs. Likewise,
any costs for waste treatment would be nearly the same.
A base case of 1,420 m3/minute (50,000 C.F.M.) of dry gas
was chosen for illustration. Fan power costs for 200 cm W.C.
(80"W.C.), 400 cm W.C., and SOO cm W.C. pressure drop scrubbers
were computed for an overall fan and motor efficiency of 50%
and power costs of 1^/KWH. Hourly costs for these conditions
are plotted (dashed lines) on Figure 13 a graph of hourly
operating cost vs. the condensation ratio. Since no vapor is
condensed in the high energy scrubber, the dashed lines are
horizontal.
172
-------
If steam has to be purchased or generated from purchased
fuel, it might cost somewhere around $1.32/MKg ($0.60/1,000 Ib.)
One line is shown on Figure 13 for costs due to steam alone.
Cooling water will also be required to condense the steam and
it will cost anywhere from 0.26«f/MKg (H/M gal) to 4
-------
to FF/C scrubbing would, therefore, be about ($9.50/$2 . SO) ,
or 3.8 times more expensive than FF/C.
2. Cooling water costs as high as 2.64
-------
Distribution of the condensing vapor over several stages is
preferable because of the enhanced growth which can occur
after the particle concentration has been reduced.
Economic considerations define the most favorable area
of application for FF/C scrubbing as those situations in which
the enthalpy of vaporization is available from the gas to be
cleaned. The purchase 6-f steam can be justified when high
collection efficiency on fine particles is needed, or an
existing scrubber has to be upgraded for the removal of sub-
micron fume. The FF/C scrubber efficiency is relatively
unaffected by particle size. Thus, FF/C scrubbers become
comparatively more economical as the particle size of the
pollutants decrease.
ACKNOWLEDGEMENT
The work upon which this publication is based was
performed pursuant to Contract No. 68-02-1082 with the
Environmental Protection Agency.
The authors wish to express their appreciation for
excellent technical coordination and for very helpful
assistance to Dr. Leslie E. Sparks, E.P.A., Project Officer.
175
-------
REFERENCES
Amelin, A. G. "Theory of Fog Condensation." Second Edition.
Jerusalem, Israel Program for Scientific Translations, p. 33-39.
1967.
Calvert, S., J. Goldshmid, D. Leith, and D. Mehta. "Scrubber
Handbook." A.P.T., Inc. Riverside, Calfiornia. EPA Contract No.
CPA-70-95. NTIS # PB 213 016. August 1972.
Calvert, S., J. Goldshmid, D. Leith, and N. Jhaveri. "Feasibility
of Flux Force/Condensation Scrubbing for Fine Particulate Collection."
A.P.T., Inc. Riverside, California. EPA Contract No. 68-02-0256.
NTIS # PB 227 307. October 1973.
Davis, R. J., and J. Truitt. "Using Condensing Steam for Air Cleaning."
Instruments and Control Systems. P. 68-70, 1972.
Fahnoe, F., A. E. Lindroos , and R. J. Abelson. "Aerosol Build-up
Techniques." Ind. Eng. Chem. 45^ 1336, 1951.
Goldsmith, P., and F. G. May. "Diffusiophoresis and Thermophoresis
in Water Vapor Systems." In: Aerosol Science, Davies, C.N. (ed.)
New York, Academic Press, p.163-194, 1966.
Hidy, G. M., and J. R. Brock. "The Dynamics of Aerocolloidal Systems".
New York, Pergamon Press, 379 p., 1970.
Kotrappa, P., and C. J. Wilkinson. "Densities in Relation to Size
of Spherical Aerosols Produced by Nebulization and Heat Degradation."
A.I.H.A. Journal. 33(7): 449-453, July 1972.
Lancaster, B. W., and W. Strauss. "A Study of Steam Injection Into
Wet Scrubbers." Ind Eng Chem Fundamentals. 10(3):362-369, March 1971.
Litvinov, A. T. "Influence of Condensation on the Effectiveness of
Capture of Fine Particles During Cleaning of Gases by the Wet Method."
Zhurnal Prikladnoi Khimii. 40(2):353-361, February 1967.
Prakash, C. B. and F. E. Murray. "Particle Conditioning by Steam
Condensation." Preprint of paper presented 1973.
Rozen, A. M., and V.M. Kostin. "Collection of Finely Dispersed
Aerosols in Plate Columns by Condensation Enlargement." Inter Chem
Eng. 7:464-467, July 1967.
Schauer, P. J. "Condensation Techniques for Dust Detection and
Collection." cited by A. Bralove in Chapt 40 of Air Pollution.
L. C. McCabe. McGraw-Hill Co. 1952.
176
-------
Sparks, L. E,, and M. J. Pilat. "Effect of Diffusiophoresis on
Particle Collection by Wet Scrubbers". Atmospheric Environment.
4:1-10, 1970.
Stinchombe, R. A. and P. Goldsmith. "Removal of Iodine From the
Atmosphere by Condensing Steam. J Nuc Energy. 20, 261, 1966.
Terebenin, A. N. and A. P. Bykov. "Aerosol Sedimentation Mechanisms
in Water Vapor Diffusion Fields". Zhurn Priklad Khinu 4j, 1012,
1972.
Walsmann, L., and K. H. Schmitt. "Thermophoresis and Diffusiophoresis
of Aerosols". In: Aerosol Science, Davies( C. N. (ed.) New York,
Academic Press, p 137-161, 1966.
177
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NOMENCLATURE
c = particle mass concentration, g particulate/cm3 gas
d = diameter, cm or ym
d = mass mean diameter, ym or pro (g/cm3)V*
F = foam density, volume fraction
K = particle inertial impaction parameter
P Jpa vh X +°'9
9 »G dh
n = particle number concentration, no. /cm3
Pt = penetration (one minus efficiency), fraction or
percent
Ft = overall penetration
q = vapor condensed per unit mass of inlet particles,
mass fraction
q' = condensation ratio, g vapor condensed/g dry gas
r = radius, cm or urn
S = saturation ratio, vapor partial pressure/vapor
pressure at the gas temperature
T = temperature, °K or °C
u = deposition velocity, cm/sec
v - velocity, cm/sec
y = gas humidity, g vapor/g dry gas
a = geometric standard deviation
p = viscosity, g/cm-sec
Subscripts
a = aerodynamic
G = gas
h = plate perforation
i = inlet
L = liquid
o = outlet
Subscripts (continued)
p = particle
pC = particle, centrifugation
pD = particle, dif fusiophoresis
pT = particle, thermophoresis
178
-------
Table 1. THREE PLATE FF/C SCRUBBER; EXPERIMENTAL CONDITIONS AND RESULTS
All plates are identical, 60 perforations of 0.32 cm diameter
Dry air flow rate = 0.4 (mVmin) @ 21°C, 1 atm.
Inlet water flow rate = 2.54 (liters/min)
Inlet air saturated at T
Run
No.
1
2
3
4
5
6
7
8
9
10
Gas Conditions
Ti
51
51
60
60
60
71
71
71
80
80
T
'1,2
33.3
33.3
38.8
38.8
38.8
--
--
58.2
58.2
T
4,3
30
30
31
31
31
33
33
33
39
39
T
o
27.2
27.2
28.4
28.4
28.4
27.8
27.8
27.8
29.5
29.5
q'
0.07
0.07
0.13
0.13
0.13
0.27
0.27
0.27
0.5
0.5
Water Temperatures
T. .
Li
27
27
27
27
27
26.5
26.5
26. S
27.5
27. S
T
1L1,2
27
27
28.7
28.7
28.7
29.5
29.5
29.5
35.2
35.2
TLo
31
31
36.9
36.9
36.9
40
40
40
58
58
Pressure Drop
[cm H^O)
Pl
4.7
4.7
4.9
4.9
4.9
5.0
5.0
S.O
P2
4.1
4.1
4.4
4.4
4.4
4.2
4.2
4.2
5.0
5.0
P3
5.1
5.1
5.6
5.6
5.6
5.7
5.7
5.7
6.0
6.0
cxlO8
Cg/cm3)
13.4
14.0
15.0
13.5
14.4
15.8
18.1
13.2
14.8
18.2
^1
(*)
81.8
84.8
7 C . G
76.1
75.5
64.8
59 ,6
64.1
50.0
47 .0
Ft
Kt2 x
Pt3
C*f
61
59.3
45.1
44.4
49.6
40.0
34.7
34.2
33.1
36.8
Pt
(*)
49.9
50.3
31.9
33.8
37.4
25.9
20.7
21.8
16.6
17.3
NOTE: All temperatures in degrees centigrade.
q' = g vapor condensed/g dry air.
c = particle mass concentration
Numbered subscript refer to the plate number, bottom plate is denoted as
plate 1. A combination of numbers in a subscript such as (1,2) refer to
conditions in between the two plates.
-------
Table 2. PILOT SCALE FF/C SCRUBBER; OPERATING CONDITIONS
Plate configuration - Five identical plates with 3.2 mm round perforations
Free area » 9%; Plate active area
9.29x10-* m*
Dust used - Pure black iron oxide/ - 16 mesh
Cold water introduced on top plate/ flow rate = 0.64 liters/sec.
Run
No.
1
2
3
4
5
6
7
8
9
Gas Inlet Conditions
Flow
/dsm3\
(min j
5.17
5.3
5.3
5.14
4.73
4.85
5.06
3 .08
2.83
Temp .
(°c)
20
25
23
42
43
55
60
68.2
71.2
Moisture
(% Vol.)
1.2
1.3
1.4
8.2
8.5
16.0
18.2
27.8
33.4
q'
xlO
-
-
-
0.4
0.49
1.04
1.27
? .32
2.81
Liquid
Temp .
(°C)
In
18
22
-
19.5
13.8
13.5
11
15.3
27
Out
-
-
-
28.0
22.9
33
42.5
45,0
57.5
Particulate
Load xlO3
(g/dsm3)
In
45.1
43.1
46.3
44,0
28.8
58.9
.15.7
28.1
46.0
Out
9.4
7.1
10.5
7.33
1.63
0.94
0.35
0.42
0.74
d
per
in
1.27
1.37
0.79
1.21
1.12
0.97
1.21
1.16
Out
i-ilter
0.92
1.06
0.68
0.73
0.74
0.8
0.6
0.55
%
In
Samp
1 .8
1.8
2 .0
1.7
1 .8
1 .6
1.8
1.6
Out
les
1.4
1.5
1.5
1.6
1.6
1.7
1.4
1.4
Scrubber
Press .
drop
(cm W.C. )
34 .8
32 .5
32 .8
34 .8
32 .3
32 .5
33 .8
34.2
31.9
00
o
NOTE: All flow rates expressed at 0"C, 1 atm,
q' = vapor condansed, g/g dry air
d = aerodynamic mass mean diameter,
geometric standard deviation.
-------
Table 3. FF/C SCRUBBING PERFORMANCE DATA SOURCES
Ref.
#
1
2
3
4
5
6
7
8
9
10
11
Investigator(s)
Calvert, Goldshmid,
Leith, Jhaveri
(1973)
Calvert, Jhaveri
(present investiga-
tion)
Fahnoe, Lindroos,
and Abelson (1951)
Goldsmith and May
(1966)
Lancaster and
Strauss (1971)
Litvinov
(1967)
Prakash and
Murray (1973)
Rozen and Kostin
(1967)
Schauer
(1952)
Stinchombe and
Goldsmith (1966)
Terebenin and
Bykov (1972)
Scrubber
Type
Sieve plate
(1 cold plate)
Sieve plate
(3 cold plates)
(A) Cyclone, or
(B) Peabody
(1 plate)
Tubular
Condenser
Steam Nozzle +
Spray + Cyclone
Venturi + 2
sieve plates
Steam nozzle +
Dry Duct ,
Sieve plate with
alternate hot and
cold plates
Steam nozzle +
Peabody(5 plates)
Tubular
Condenser
Vertical Wetted
Planes
d
Pg
(vim)
0.7
0.4
<2.0
?
1.0
1.7
0.3
0.3
0.1
0.05
Particle
Material
D.B.P.
Ferric
Oxide
NaCl
Nichrome
5 others
ZnO
Apatite
ZnO, CaCO,,
Na2C03,
Na2S04
Oil
D.O.P.
Iodine
Tin Fume
n
#/cm3
SxlO5
105-106
103
?
105
-10s
~105
10 =
-106
2xl07
103
-10"
SxlO7
Note:
d = Mass median diameter, urn
pg '
n = Number concentration of particles, #/cm3
181
-------
1.0
z
o
I-
o
<
o:
it.
O.I
o:
K
Ld
UJ
Q.
.01
NO.
-PLATES=1
PARTICLES GROW
AT S>I.O
. l\l
0.01 O.I
q1 (g VAPOR CONDENSED/g DRY AIR)
PREDICTED PENETRATION FOR
SIEVE PLATES
FIGURE 1
0.5
182
-------
1.0
o
o 5
o:
u.
E .2
UJ
LJ
Q.
.1
0.01 0.05 O.I
q'(g VAPOR CONDENSED/g DRY AIR)
PREDICTED PENETRATION FOR 4 SIEVE PLATES
NO PARTICLE GROWTH
0.5
FIGURE 2
-------
WATER AIR
AIR GAS WATER
i ;
i
i
i
i
i)
f
V
r
>
r
^
r
\
t
\
i
t
».
[1
i
,
1
1
I)
i
1
•i
i
i
-
7
'
3
'
9
r
r
5
v»
"-*
7
i
8
I
9
1
|i
-*
f|
6
1 1
1 1
\ '
I
i
,
1
i
1
,
1
1
i
-
4
r
5
>
0
»
*-
VENT
!T2
F
r
S
s
I
10 1 k
To
S^
i
H
r
i
i
(
t
f
LEGEND:
I VALVES
2 PRESSURE REGULATOR
3 ROTAMETERS
4 WATER COOLER
5 AIR PREFILTER
6 AIR BLOWER
7 AIR COOLER
8 VENTURI METER
9 AIR HEATER
10 "ABSOLUTE" FILTERS
TO DRAIN
I I PARTICLE CHARGE NEUTRALIZER
I 2 GAS MIXING SECTION
13 BOILER
14 STEAM ENTRAPMENT SEPARATOR
15 PRESSURE INDICATOR
16 AIR FILTER
17 PRESSURE INDICATOR
18 TWO-FLUID ATOMIZER
19 AEROSOL DRYER
20 IMPACTOR
21 SIEVE-PLATE SCRUBBER
EXPERIMENTAL APPARATUS
FIGURE 3
184
-------
00
On
1.0
z
o
O 0.5
<
cc
U.
O 0.3
a:
LJ
Q.
O.I
I03
O THREE-PLATE COLUMN
• BOTTOM PLATE OF THE THREE-PLATE COLUMN
5xl03 I04
q'(g VAPOR CONDENSED/g PARTICLES IN)
5xl04
RELATIONSHIP BETWEEN PENETRATION AND VAPOR
CONSUMPTION IN THE THREE-PLATE COLUMN
FIGURE H
-------
1.0
0.2
LU
LU
Q.
O.I
O THREE-PLATE COLUMN
Q BOTTOM PLATE OF THE THREE-PLATE COLUMN
.10 .20 .30 .40 .50
q'(g VAPOR CONDENSED/g DRY AIR )
PERFORMANCE OF THE
THREE PLATE SCRUBBER
FIGURE 5
1.0
0.5
z
o
o
I °2
O
!5 o.,
ui
z
Ul
O.O5
0.02
NO GROWTH
O EXPERIMENTAL RESULTS
O.2
0.5
q1 Cg VAPOR CONDENSED /g DRY AIR)
COMPARISON OF EXPERIMENTAL
RESULTS WITH PREDICTIONS
FOR A THREE PLATE SCRUBBER
FIfiURE 5
186
-------
LO
cr
i-
UJ
z
LJ
a. 0.2
Ld
h-
o:
q' =
O RUN #2
D RUN
o
O.I
0.3
0.5
.0
2.0
(/imA)
FIGURE 7 - PARTICLE PENETRATION VERSUS
AERODYNAMIC DIAMETER,
187
-------
o
I—t
E-
U
2
2
O
E-
W
2
W
G.
W
_j
U
i—i
S
<
a.
0.5
0.3
0.2
0.1
0.05
0.03
0.015
3.0
FIGURE 8 - PARTICLE PENETRATION VERSUS
AERODYNAMIC DIAMETER
188
-------
H
u
o
I—I
H
uq
pq
0.06
0.04
0.02
0.01
0.005
0.003
s^:
"T---
-^— - -,-i-- _
i;Hi iif-Fi5
SH S^
•gsg=:
11411:1
HHEE
5HH?
::rtr
;.:;-jz:r
IS
:r±^
iS
ET^
E£3
;;;;;;;
iglEE
^PFFS^ffl
^:i|:3'
0.3 0.5 1.0
V
2.0 3.0
FIGURE 9 - PARTICLE PENETRATION VERSUS
AERODYNAMIC DIAMETER
189
-------
0.06
0.04
os
PL,
o
I—I
<
H
PJ
•J
U
0.02
0.01
0.005
0.002
I
im
i
0.3
0.5
pa
1.0
(ymA)
2.0 3.0
FIGURE 10 - PATICLE PENETRATION VERSUS
AERODYNAMIC DIAMETER
190
-------
1.0
§
2
W
o,
W
p-3
U
i—i
H
OS
2 o.oi _~-T7- -
o.ooi
o.oi o.i
q' (g vapor condensed/g dry air)
1.0
FIGURE 11
191
-------
1.0
M3
M
o
h-
o
<
o:
u.
O.I
o:
H
LJ
z
u
Q.
0.01
0.01
NOTE: NUMBERS ON CURVES
CORRESPOND TO REFERENCES
IN TABLE 3
8, np=l
7,Na2C03
Na2S04
LOW VELOCITY x
\
J L
4, HIGH VELOCITY
j__i—i i i i I—
0.05 O.I 0.5 1.0 5.0
CONDENSATION OR INJECTION RATIO (g H20/g DRY GAS)
FIGURE 12
-------
STEAM (a) 8 1.32/M Kg
PLUS WATER o O.8 */ M Ka
STEAM 8 U32/M Kg
OOcm.W.C
(Pt=l7%fa>d
, ,^500cm.W.C. FAN POWER
OOcm.W.C. FAN
pa
POWER
olf)
COOLING WATER
2.64 tf/M Kg
200 cm./W.C. FAN POWER
WATERO O.8 rf/M Kg
PLUS O 25 cm FAN
COOLING WATER (3 0.8 */M Kg
I I I
O.I 0.2 0.3 0.4 0.5
q' (g H2O CONDENSED/g DRY GAS)
NOTES:
I. COSTS ARE FOR A 1,420 m3/min. (50,000 CFM) SCRUBBER
2. PARTICLE DIAMETER. dpa , IS AERODYNAMIC (^m{g/cm3)'/2)
3. POWER COST IS I t /KWH
4. FAN PLUS MOTOR EFFICIENCY IS 50%
OPERATING COSTS COMPARISON
FIGURE 13
193
-------
FLUX FORCE CONDENSATION ASPIRATIVE WET
SCRUBBING OF SUB-MICRON PARTICLES
by
Stanley R. Rich and Theophanes G. Pantazelos
RP Industries, Inc.
Hudson, Mass. 01749
ABSTRACT
A novel aspirative wet scrubber system makes use of
condensation forces to enhance fine particulate collection.
A pressure gain of 2 to 4 inches, w.c. is generated by the
system and total water pump power requirements for multi-
stage systems are 2 horsepower per thousand s.c.f.m. of gas
throughput. No fans or blowers are utilized. When scrubbing
hot combustion products carrying 0.1 to 6.0 micron metal oxide
particles from a kiln, total collection efficiency was 99.4%.
Collection of fly ash, in the particle size range 0.1 to 10
microns, has been measured at 99.51 efficiency.
194
-------
"Flux Force Condensation Aspirative Wet
Scrubbing of Sub-Micron Particles"
by
Stanley R. Rich and Theophanes G. Pantazelos
RP Industries, Inc.
Hudson, Mass. 01749
Condensation force enhancement of sub-micron particle
collection in wet scrubbers has been the subject of a number
of theoretical and small scale experimental studies in recent
years. Sparks and Pilat (1970) have studied the improvement
of sub-micron particle collection in wet scrubbers due to
thermo and diffusiophoresis. S. Calvert et al. (1973) con-
tinued the studies of fine particle collection as enhanced by
condensation forces. In addition, the Calvert et al. report
includes a small-scale actual verification of the basic theory.
While condensation force scrubbing does indeed improve
fine particle collection, the costs involved, as discussed by
Calvert, are substantial. It was the objective of development
activities at RP Industries, Inc. to reduce greatly the cost
of condensation force scrubbing. It was a further objective
to reduce significantly both the capital and operating costs
of the equipment required for fine particulate removal. An
additional objective was the development of large-scale wet
scrubbing equipment that makes use of aspirative principles in
order to reduce power consumption, and avoid the employment of
fans or blowers, thus reducing maintenance requirements and
enhancing system simplicity. The Dynactor is the trade name
given to a patented aspirative gas/liquid/solids contactor/
scrubber developed at RP Industries and it is the aspirative
system discussed in this paper.
The scrubbing liquid, usually water, is introduced into
a specially designed nozzle system at a pressure of up to
250 p.s.i.g. (according to application). The resulting high
195
-------
velocity shower of thin films and fine particles entrains air
or gas as the shower diffuses into a reaction column. The
result is establishment of a partial vacuum which is coupled
to the source of air or gas by means of a radial impedance
transformation section which acts to transform high pressure,
low velocity gas into low pressure, high velocity gas with
minimum losses. The principle involved can be described as
the macroscopic application of diffusion pump techniques.
Conventional aspirators, using venturi eductor principles,
require between 75 and 100 g.p.m. of motive liquid to aspirate
each 1,000 c.f.m. of gas. Power requirements are thus reduced
by more than an order of magnitude compared with earlier
aspirative venturi eductors. Water usage is reduced by a factor
of 15 to 20. Scrubbing systems up to 50,000 c.f.m. in through-
put capacity are manufactured and marketed at the present time
and it is expected that aspirative system capacities up to
1,000,000 c.f.m. will be available by the end of 1974.
DESCRIPTION OF EQUIPMENT
Figure 1 is a schematic diagram of a two stage system.
Both stages are usually identical to one another. Scrubbing
liquid is pumped from each reservoir/separator, as shown, to
a specially designed nozzle system in each stage, at a pressure
of 200 pounds per square inch. While this pressure may be
varied, depending upon application, from 150 to 250 p.s.i.g., a
typical delivery pressure is 200 p.s.i.g. Under these conditions
the liquid is atomized into thin films and droplets of average
thickness or diameter less than 500 microns.
This liquid discharge causing air or gas to be aspirated
then continues to travel down the reaction column, where
deliberately induced turbulence causes intimate contact to be
maintained between gas and liquid. This results in physical
and chemical equilibria to be produced by the time the mixed
fluid exits from the reaction column into the separation reser-
voir.
196
-------
By utilizing diffusion rather than Bernoulli principles,
up to 4,800 standard volumes of gas is aspirated per volume
of motive liquid. In gas scrubbing, 2,000 volumes of gas are
aspirated per volume of liquid. In comparison, venturi
eductors typically aspirate not more than 100 volumes of gas
per volume of motive liquid. Two-stage aspirative systems
typically consume a total of 2 horsepower per 1,000 a.c.f.m.
(one hp. per stage per 1,000 a.c.f.m.).
The radial impedance transformation section is employed
to couple efficiently the sources of air or gas at outside
ambient pressure to the partial vacuum that obtains within
the reaction column. Ambient gas at low velocity and higher
pressure accelerates smoothly and continuously within the
radial impedance transformation section. Thus, the potential
energy of the influent gas is converted to kinetic energy at
high efficiency.
The mixed fluid is separated into its liquid and gaseous
components in the reservoir/separator sections of the system,
as shown in Figure 1, A system of fixed baffles is utilized
to ensure liquid/gas separation, which minimizes liquid carry-
over. If influent gas carries small particles along with it,
such particles are wetted and captured by the liquid throughout
the entire length of the reaction chamber. By contrast,
venturi wet scrubbers make effective contact between gas and
liquid only in the constricted throat region. Contact time,
therefore, is about 20 times longer than in venturi devices.
Table 1 shows the power requirements of a variety of
scrubbing systems, including one, two, three, and four stages
of each type of system. It is clear that the operation of a
four stage aspirative system is less costly than operation of
a single stage of many conventional wet scrubbing systems.
Scrubbing efficiency increases geometrically with the number
197
-------
of scrubbing stages that operate in series with one another.
For fine particle scrubbing, second, third, and fourth stages,
regardless of system species, are often required to achieve
high percentages of fine particle removal.
For example, if the influent gas is at a temperature above
ambient, the first stage acts both to collect larger particles
and to evaporate water. In the first stage, smaller particles
are driven away from the water droplets by negative thermo-and
diffusiophoresis, resulting in a decrease in the efficiency of
fine particle collection. Moisture from the saturated warm air
effluent from the first stage condenses on the scrubbing liquid
in the second stage. The resulting positive thermo-and diffusio-
phoresis acts to increase significantly the efficiency of
collection of fine particles. Thus, a two stage system permits
the employment of condensation forces to enhance the scrubbing
of fine particles - without the addition of any additional
power.
The addition of a water spray to pre-condition the influent
gas permits positive thermo-and diffusiophoresis to be utilized
in multiple stages. The power required to spray small quanti-
ties of water into the influent gas stream is less than 1/10
horsepower per 1,000 a.c.f.m. of influent hot gas. Usually the
temperature of the preconditioned, saturated gas remains above
the temperature of the scrubbing water in both stages. The
saturated air comes into contact with the cold water in each
stage, which results in condensation of water vapor and this
materially increases the efficiency of capture of sub-micron
particles.
If a cold influent gas is to be scrubbed, steam can be
added both to raise the temperature of the gas and to ensure
water saturation. Under these conditions, additional energy is
required to generate the steam, resulting in an increase in the
cost of scrubbing fine particles. Generally speaking, the in-
crease in power requirements is approximately equal to the
198
-------
power consumption of the aspirative system itself. For
example, the 50,000 s.c.f.m. system power requirements are
100 horsepower and an additional 100 horsepower is required
to generate the steam necessary.
DESCRIPTION OF TESTS
TEST #1
A two stage aspirative system was connected to the effluent
from a cyclone whose input was hot gases issuing from a kiln.
The kiln was fired by natural gas. The function of the process
was the drying of a thickened metal oxide suspension in water
in order to produce dry metal oxide powder. The system arrange-
ment is shown in Figure 2. The kiln feeds a cyclone, in which
most of the metal oxide powder above 6 microns is recovered.
The gaseous output from the cyclone, containing metal oxide
powder particles below 6 microns, is aspirated into the system.
Figure 3 is a particle distribution curve of the metal oxide
in the output of the cyclone.
A preconditioning water spray is introduced and the temp-
erature of the gas which was 400°F at the output of the
cyclone dropped to 150°F after preconditioning. Thus, conden-
sation-force scrubbing was obtained in both stages.
Both input and output grain loading were measured isokinet-
ically at 8 traverse points by a heated filter (600°F). The
influent grain loading was measured at 1.95 grains per cubic
foot. At the same time, the output showed a grain loading of
0.012 giains per cubic foot. The measurement time was 48 min-
utes - 6 minutes at each station. The total collection
efficiency was 99.4%.
TEST #2
The efficiency of particle removal was measured in the
collection of fly ash, in the particle size range 0.1 to 10
microns. In this case., the arrangement was constructed as
199
-------
shown in Figure 4. A simulated stack gas was created by a
"salamander" powered by a #2 fuel oil. The temperature of the
output of the salamander at 800 a.c.f.m. was 600°F. A powder
feeder introduced fly ash into the hot gaseous effluent from
the heat source at a grain loading of 0.74 grains per cubic
foot. A preconditioning spray of water, four feet from the
entrance of the system, reduced the gas temperature to 1SO°F.
The fly ash grain loading at the output was measured at 0.004
grains per cubic foot. The measurements were made isokinetically
as in the tests on metal oxide, described above. The overall
particle capture efficiency was 99.51.
TEST *3
Ambient air, dry bulb temperature 80°F, relative humidity
304, was aspirated into the system at 1,000 a.c.f.m. Fly ash
was added, using the same powder feeder as in Test #2, above.
Again, the particle size range was 0.1 to 10 microns. There
was no preconditioning either by water or steam. Input grain
loading was measured at 0.82 grains per cubic foot. Output
grain loading was measured at 0.02 grains per cubic foot. The
efficiency of particle collection was 98.91.
DISCUSSION
All single-stage wet scrubbers display a decrease in the
collection of fine particles if the input gas is significantly
hotter than the scrubbing liquid because of negative thermo-and
diffusiophoresis. To overcome negative thermo-and diffusio^
phoresis, it is desirable to employ a preconditioning procedure
to saturate the hot, dry, influent gas with water vapor.
As pointed out by Calvert, et al. (1973), added steam will
successfully precondition hot, dry, influent gas. The cost for
steam alone in one example is cited as 10.7< per 1,000 s.c.f.m.
For a 50,000 s.c.f.m. scrubbing system the cost would be 35
-------
(8000 hours). This cost is about equal to that of furnishing
power to a 1,400 horsepower blower at l.Stf per kwh.
If the inlet gas is hot, steam need not be added and a
preferred alternative is the use of two or more wet scrubber
stages. In the first stage particles larger than several
microns are collected efficiently and the second, third and
fourth stages function as condensation-force scrubbers.
Generally, two aspirative stages suffice for most particu-
late scrubbing applications. At one horsepower per 1,000
a.c.f.m. per scrubbing section, a two-stage, 50,000 a.c.f.m.
system will operate at a cost of $1.50 per hour, $12,000. per
year.
The foregoing tests have established that the scrubbing
efficiency of the tested system compares favorably with the
particle scrubbing efficiency of conventional high energy
venturi scrubbers but at much lower operating cost. Further,
maintenance and other problems surrounding the use of powerful
blowers and fans are not present in aspirative systems. The
only moving parts are the pumps which furnish the scrubbing
liquor to the system at 200 p.s.i.g. and at flow rates of
5 g.p.m. per 1,000 c.f.m. Makeup liquid is usually introduced
into the system at 10% of the recirculation of the scrubbing
liquid. For the scrubbing of 50,000 a.c.f.m., 25 to 50 g.p.m.
of water usually is required both to replenish evaporated
scrubbing liquor as well as to flush captured contaminants out
of the system.
REFERENCES
1. Sparks, L.E., and M. J. Pilat. Effect of Diffusiophoresis
on Particle Collection by Wet Scrubbers, Atmospheric Environ-
ment. 4: 1-10, 1970.
2. Feasibility of Flux Force/Condensation Scrubbing for Fine
Particulate Collection, S. Calvert, J. Goldshmid, D. Leith,
and N. C. Jhaveri, E.P.A.-650/2-73-036 (Oct., 1973).
201
-------
50,OOO A.C.F.M.
SCRUBBER
SINGLE STAGE
ASPIRATIVE
VENTURI
TRAY
EDUCTOR (VENTURI)
TWO STAGES
ASPIRATIVE
VENTURI
TRAY
EDUCTOR (VENTURI)
THREE STAGES
ASPIRATIVE
VENTURI
TRAY
EDUCTOR (VENTURI)
FOUR STAGES
ASPIRATIVE
VENTURI
TRAY
EDUCTOR (VENTURI)
PRESSURE
DROP
" W.C.
1" GAIN
44
15
1" GAIN
2" GAIN
88
30
2" GAIN
3" GAIN
132"
45"
3" GAIN
4" GAIN
176"
60"
4" GAIN
HORSEPOWER
FAN
0
535
183
O
O
1070
366
O
0
1605
549
0
0
2140
732
0
PUMP
50
21
2.5
225
100
42
5
450
150
63
7.5
675
200
84
10
900
$ COST/YEAR
POWER (8000 HRS/YR.)
(1.5C/K.W.Hr.)
6,000
66,700
22,250
27 , 000
12,000
133,400
44 , 500
. 54,000
18,000
200,000
66,750
81 , 000
24 , 000
266,800
89 , 000
108,000
TABLE 1
POWER REQUIREMENTS AND COSTS
202
-------
DYNACTOR SCHEMATIC
PLENUM
CHAMBER
RADIAL
IMPEDANCE
TRANSFORMATION
SECTION
BAFFLE
PUMP
LIQUID
OUT
LIQUID
MAKE
UP
RESERVOWSEPMATOft
FIGURE I
203
-------
METAL OXIDE
SUSPENSION
bi
FUEL
KILN
GAS CARRYING
METAL OXIDE V
PARTICLES
550 "F
CYCLONES I
Vj l~.
METAL OXIDE
PARTICLES
OVER 6 MICRONS
to
o
Ji.
OUTPUT
GAS CARRYING
METAL OXIDE
PARTICLES
UNDER 6
MICRONS
400°F.
3AS GAS AT 150 'F,
MOISTURE-SATURATED
CARRYING METAL
L
J L
V"
r
MICRONS
RECONDITIONI
1C
TWO STAGE DYNACTOR SYSTEM
FIGURE 2 - CONDENSATION-FORCE WET SCRUBBING OF FINE METAL OXIDE PARTICLES
-------
FIGURE 3
NJ
o
t/i
I
a
w
3
I
1
Ou
PARTICLE DISTRIBUTION OF METAL
OXIDE IN OUTPUT OF CYCLONES
PARTICLE SIZE, MICRONS
-------
POWDER
FEEDER
AIR
SALAMANDER
GAS,
800 a.c.f.m.
600 »F
FUEL OIL
CLEAN GAS
OUTPUT
DYNACTORS
STAGE 2
STAGE 1
FLY ASH
PRECONDITIONING
WATER SPRAY
GAS & FLY ASH, MOISTURE
SATURATED, ISO'F
FIGURE 4 - CONDENSATION FORCE DYNACTOR SCRUBBING OF FLY ASH
-------
ENTRAINMENT SEPARATORS FOR SCRUBBERS
by
Seymour Calvert
Indrakumar L. Jashnani
Shuichow Yung
A.P.T., inc.
P. O. Box 71, Riverside, California
ABSTRACT
Liquid entrainment separation was studied experi-
mentally for packed bed, knitted mesh, zigzag baffle, tube
bank, and cyclone type apparatus. Primary collection
efficiency data compare well with theoretical predictions
but re-entrainment of the collected liquid did not agree
with theoretical models. Separation efficiency, drop size,
and pressure drop data are presented.
207
-------
"Entrainment Separators for Scrubbers"
by
Seymour Calvert
Indrakumar L. Jashnani
Shuichow Yung
Gas emerging from wet scrubbers and other types of
equipment such as evaporators, boilers, and stills carries
with it small drops of liquid. In the past little attention
was paid to entrainment separator design but the increased
use of scrubbers in power plants and other sources have
shown that problems with entrainment separators can be very
significant. The entrainment separator is no longer viewed
as a minor part of the scrubber body but represents a signi-
ficant investment and operating cost.
Drops formed from bursting bubbles have a bimodel dis-
tribution with mass median diameter for small drops around
40 ym and for large drops above 1,000 ym. The drops formed
by tearing of liquid sheets and ligaments and by splashing
of liquid drops are 200 ym and larger. The mist formed by
the condensation of saturated vapor contains sub-micron
drops.
The entrained drops generated by scrubbers are quite
large, so that separators utilizing gravitational and in-
ertial forces are suitable. Devices such as packed beds,
fibrous beds, cyclones, louvers or zigzag baffles or chevrons,
and banks of rods are used for entrainment separation. Once
the drops are captured (this is what we call "primary collec-
tion") , the resulting coalesced fluid must be removed from
the separator without being re-entrained by the gas. Thus
the overall performance includes the effects of both primary
collection and reentrainment.
A literature search revealed that equations to predict
primary collection efficiency and pressure drops are avail-
able for most of the entrainment separators except for the
208
-------
zigzag baffle type. Some of the problems the present day
entrainment separators face are absence of reliable data and
design equations, plugging, reentrainment and drainage. The
purpose of the present study is to define performance as a
function of gas and liquid flow rates and other parameters.
Theoretical Predictions
Theoretical equations for determining the primary col-
lection for most entrainment separator types are summarized
by Calvert et al. (1972) . However, theoretical equations
for zigzag baffles had not previously been developed. A
model has been derived in this study for the prediction of
primary collection efficiency in baffle type separators, based
on inertial mechanisms:
u ,, *
(1)
_ , /ut n w 6 \
E = 1 - exp - ^- b fcan ej
where, E = fractional collection efficiency
b = distance between baffles normal to gas flow, cm
u. = drop terminal velocity, cm/sec
UG = superficial gas velocity, cm/sec
n = number of rows of baffles
w = width of the baffle, cm
9 = angle of baffle from flow direction, radian
It was also necessary to develop a mathematical model
to predict pressure drop in a baffled separator. The fol-
lowing derivation is based on the drag coefficients for plates
inclined to the flow.
Page and Johansen (1927) present drag coefficients as a
function of angle incidence, 6, for plates inclined to the
flow.
AP = I f^i^-) P. U^ (2)
209
-------
where, A^ = projected frontal area, cm2
Aj, = total flow cross sectional area, cm2
fD = drag coefficient for a plate inclined at angle "6"
9 30° 45° 60° 90°
FD 0.65 0.85 0.78 1.07
For the baffles after the first row, the angle of incidence
to the flow is doubled and the fraction of the flow cross
section covered by the projected drag area also increases.
Mathematical models for reentrainment in horizontal
and vertical baffle separators and in cyclones were also de-
rived in this study. However, the predicted reentrainment
velocities are much lower than the experimental.
EXPERIMENTAL PILOT PLANT
'Experimental studies were made in a pilot plant with
gas flow capacity of 85 m3/min (3,000 cfm). The test sections
are large enough to have minimal wall effects for packed
separators and provide a fairly long collection element when
cross-flow effects are important. The apparatus includes
an air prefilter, blower, air heater, spray section, test
section, observation section, feed and catch tanks, control
panel and measuring devices.
Detailed descriptions of the separators which were
studied are as follows:
1. Mesh - Model 4CA (ACS Industries). Type - layered, (with
crimping in alternate directions). Density = 0.144 g/cm3.
Wire diameter = 0.028 cm. Percent voids = 98.2%. Mesh
surface area = 2.8 cm2/cm3. Thickness = 10 cm. Material
of construction = AISI 304.
2. Packed Bed - Packing - 2.5 cm Pall rings. Specific sur-
face = 1.9 cm2/cm3. Density = 0.088 g/cm3. Material of
construction = Polypropylene plastic. Bed length = 30 cm.
210
-------
3. Zigzag Baffle Section - Baffle dimension = 7.5 cm width
x 61 cm high x 0.16 cm thick. No. of rows = 6. Spacing
between rows = 2.5 cm. Angle between baffle surface and
air flow direction = 30°. Spacing between baffles in a
row =7.3 cm. See Figure 1.
4. Bank of Tubes - Number of rows = 6. External diameter =
1.9 cm. Length = 61 cm. Number of tubes in a row = 8.
The tubes were equispaced. See Figure 1.
5- Cyclone - The cyclone is a cylinder 61 cm diameter x 243
cm overall height. The cyclone inlet is 30.5 cm high
and 15 cm wide, giving a maximum inlet velocity of 3,000
cm/sec. Higher velocities were studied by using a vane
in the inlet. The design is described by Stearman and
Williamson (1972) and is a straight cylinder with flat
bottom.
Entrainment separators numbers 1, 2, 3 and 5 were selec-
ted because they are the most common separators in industrial
use. Banks of streamlined struts are reported to operate
at low pressure drop, with high efficiency and a high reen-
trainment velocity. Although this type of separator is not
in common use, it appeared promising enough to study.
The range of major variables studied is given below:
Drop diameter = 82 to 1,600 ym.
Air velocity = 100 to 750 cm/sec in all except 400 to
500 cm/sec for the cyclones.
Liquid flow rate = 103 to 4x10"* cm3/niin
Air flow rate = 8-85 m3/min
Sampling Procedure
Drop size determination. The drop size in the experi-
ments varied between 40-2,000 um. Filter papers coated with
1% potassium ferricynide and ferrous ammonium sulphate were
used to determine the drop diameters. The grain size of the
chemicals normally limit the lower diameter by this method
211
-------
to 5 to 10 m. A correlation given by Chilton (1952) is
used to convert blot diameter on filter paper to actual
drop diameter.
In a few runs, isokinetic sampling through a filter
holder containing treated filter was done to measure entrain-
ment drop diameter. This method is used when the collection
efficiency of a filter paper held in air would be too low
for small drops.
The determination of entrainment loading involved taking
an isokinetic sample through a nozzle which was heated to
evaporate the entrainment. Humidity was measured by dry and
wet bulb thermometers at the end of the nozzle. Fifteen (15)
samples at 4 cm interval were measured along the vertical
height of the duct to get average value of entrainment. When
the liquid loading was large an impactor with about an 8.0
Vim cut diameter was used before the nozzle and the liquid
caught in the impactor was added to that determined by the
evaporation technique.
EXPERIMENTAL RESULTS
Collection efficiency versus gas velocity for the baffle
type entrainment separator is presented in Figure 2. A
theoretical curve for the collection efficiency of mist with
d = 90 ym and geometric standard deviation of 1.35 is shown
by a solid line in Figure 2.
Experimental data for the collection of mist with
d = 380 ym in zigzag baffles is compared with other data
from the literature in Figure 3. Results reported by Bell
and Strauss (1973) for zigzag baffles are plotted in Figure
3 and it can be seen that the collection efficiencies of
the present study were much higher. This is due to Bell's
smaller number of rows (4), non-staggered baffles, smaller
width of baffles (6.2 cm) and large distance between baffles
(8.8 cm).
212
-------
The solid line in Figure 3 represents Houghton and
Radford's (1939) data for equipment comparable to ours. Their
experiments were conducted with two droplet distributions
in the inlet. These results are comparable with the present
results due to similarities in the design.
Packed bed efficiency data are presented as points for
several drop sizes in Figure 4 and it can be seen that at
velocities lower than 600 cm/sec there was no penetration.
The solid line was predicted for primary collection by means
of the method given in the "Scrubber Handbook". The data
agree well with the theory.
The overall efficiency data for a tube bank and for
knitted mesh are also shown in Figure 4. The efficiency in
all the runs was nearly 100%. Penetration of drops (with
mass median diameter 84 Vm) was observed at velocities below
240 cm/sec in the tube bank.
Experimental collection efficiency in the cyclone with
and without an inlet vane is shown in Figure 5. The col-
lection efficiency was 100% throughout the experimental range.
The gas velocity without inlet vane was varied from 800 cm/sec
to 2,300 cm/sec and with inlet vane was varied from 1,600
cm/sec to 3,700 cm/sec.
Pressure Drop
Experimental dry pressure drop (air flow only) for
baffles agree well with the theory. The dry pressure drop
versus gas velocity relationships for packing, baffles, tube
bank, and mesh are shown in Figure 6. It should be noted
that the efficiencies of different separators at a given
velocity are not the same.
The effect of liquid load on pressure drop was found to
be negligible except in the knitted mesh. For 0 < L/A < 1,
the pressure drop in the mesh was 1.5 APd and for
0 < L/A < 5, the pressure drop was 2.3 APdry- Here, L/A is
213
-------
the superficial liquid velocity, cm/min. A similar effect
of liquid load on pressure drop in mesh was also found by
York (1954).
The pressure drop data of Houghton and Radford (1939)
compare well with the present results. The higher pressure
drop obtained by Houghton and Radford may be due to no spacing
between the rows and lips on the fourth and fifth row of
baffles. For the tube banks they give comparable results at
velocities lower than 500 cm/sec. At higher velocities, the
present study gives higher pressure drops, which may be due
to smaller spacing between the rows and to cylindrical tubes
rather than streamlined tubes. Houghton and Radford's pres-
sure drop data for knitted mesh lie within our results for
L/A = 0 and 1 < L/A < 5.
The dry pressure drop is plotted against geometric
average gas velocity in the cyclone inlet and outlet in Figure
7. As seen in the figure the data correlate well along a
straight line. Shepherd and Lapple's (1940) pressure drop
correlation for cyclones with inlet vane is plotted for com-
parison. Their prediction is 2.7 times higher than the
present pressure drops.
Reentrainment
Table 1 gives the conditions observed for onset of
reentrainment and shows that the velocity for onset of re-
entrainment decreases with increase in the liquid load. The
mass median outlet drop diameter increases with gas velocity,
as is shown in Figure 8.
The effect of liquid load, at a constant gas velocity,
was to decrease the mass median drop diameter of the re-
entrainment. At 600 cm/sec gas velocity, increasing the li-
quid flow rate from 7.6 Jl/min to 13.3 A/min, the mass median
drop diameter decreased from 265 ym to 185 ym. Bell and
Strauss measured the drop size distribution of entrainment
214
-------
coming out of baffle type separator. Their results are
comparable with the drop size distribution found in the
present study.
The minimum outlet drop sizes found for the separators
tested are shown in Figure 9. Outlet drop size distributions
for 82 ym drops entering baffles at 600 cm/sec air velocity
and a water flow rate of 3.3 Jl/min (0.86 GPM) was d = 430
P9
ym and a = 1.7, and at 5 Jl/min (1.33 GPM), d = 620 urn
and a = 1.7. For the smaller size distribution, 0.01% of
the drops are smaller than 60 ym diameter.
Figure 10 shows the performance of baffle type separator.
The shaded region shows the onset of reentrainment. The re-
entrainment observed in all the experiments was 0.5-1%. The
theoretical model is based on ideal conditions and therefore
predicts reentrainment at higher velocities.
In Figure 11, experimental data for the onset of reen-
trainment are plotted for various entrainment separators.
The ordinate represents the actual liquid to gas volumetric
ratio approaching the entrainment separator. The onset of
reentrainment velocity decreases as the liquid to gas ratio
is increased.
It should be pointed out that the design of the liquid
drainage path is quite important. An overdesign will lead to
flow of gas where liquid is to be collected and reentrainment
will result. An underdesign will result in liquid creeping
in the entrainment separator floor leading to reentrainment.
CONCLUSIONS
1. The theoretical models presented for calculating primary
efficiency and pressure drop for baffle section show good
agreement with experimental data.
2. The pressure drop in cyclone varies as a square of geo-
metric average velocity in the cyclone inlet and outlet.
215
-------
3. The effect of liquid load on pressure drop is negligible
in the packed bed, baffles, tube bank and cyclone.
4. Collection efficiency in the experimental range is nearly
100% with 0.5% to 1% reentrainment at higher gas velocities,
5. The mass median drop size for reentrainment varies be-
tween 25-650 um.
6. The minimum drop size present in the reentrainment is
40 urn.
Table 1. EFFECT OF LIQUID LOAD ON REENTRAINMENT IN
DIFFERENT ENTRAINMENT SEPARATORS.
Test Section
Tube Bank
Tube Bank
Mesh
Mesh
Zigzag Baffles
Zigzag Baffles
Average Mass
Median
Diameter
90 urn
350 ym
170 um
260 pm
700 um
700 ym
Average Liquid
Load
cm3/sec
4xl02
2.7xl02
4xl02
l.SxlO2
2.7xlOz
4xl02
2.7xl02
Onset of
Reentrain-
ment
150
350
180
450
150
300
216
-------
ACKNOWLEDGEMENT
The work upon which this publication is based was per-
formed pursuant to Contract No. 68-02-0637 with the
Environmental Protection Agency.
The authors wish to express their appreciation for
excellent technical coordination for very helpful assistance
to Dr. Leslie E. Sparks, E.P.A., Project Officer.
BIBLIOGRAPHY
1. S. Calvert, J. Goldshmid, D. Leith and D. Mehta.
"Scrubber Handbook", Volume I. Prepared for E.P.A.
under Contract No. CPA 70-95. (1972)
2. A. Page and F. C. Johansen, Proc. Royal Soc. (London),
116A: 1970 (1927).
3. L. Y. Zhivaikin. I.C.E., 2, 337 (1962).
4. J. D. Anderson, R. E. Bellinger and D. E. Lamb. A.I.Ch.E,
Journal 10, 640 (1964).
5. F. Stearman and G. J. Williamson. "Spray Elimination in
Processes for Air Pollution Control", Nonhebel, 2nd ed.,
CRC Press, Cleveland (1972).
6. H. Chilton. Trans. I.C.E., 30, 235 (1952).
7. C. G. Bell and W. Strauss. Journal of the A.P.C.A. 23,
pp. 967-9, November 1973.
8. H. G. Houghton and w. H, Radford. Trans. Am. Inst. of
Ch. E. 3_5r 427 (1939) .
9. 0. H. York, Chem. Engr. Prog., 50, 421 (1954).
10. C. B. Shepherd and C. E. Lapple, I.E.G. Chem. 31, 1246
(1940).
217
-------
FIGURE 1
SEPARATOR SKETCHES
DOOOOOOO
ooooooo o
pooooooo
TUBE BANK
(3 ROWS SHOWN)
30.5 cm
\\\\l
ZIGZAG BAFFLES
(3 ROWS SHOWN)
218
-------
100 -
•
THE SOLID LINE IS PREDICTED FOR
d = 90 urn and o = 1.35.
Pg g
I
I
200
400
600
800
GAS VELOCITY, cm/sec
FIGURE 2
COLLECTION EFFICIENCY VERSUS GAS VELOCITY
IN ZIGZAG BAFFLES WITH n = 6 and 6 = 30° .
219
-------
100
SS 80
•*
O
il
U_
UJ
O 4O
I-
o
UJ
_l
_J
O
o 20
PRESENT RESULTS
HOUGHTON AND
RADFORD DATA
n=6 0=30
BELL & STRAUS
DATA FOR 2 "v"
BAFFLES IN
SERIES
\
_L
200 400 600
AIR VELOCITY, cm/sec
FIGURE 3
COMPARISON OF PRESENT STUDY WITH LITERATURE
DATA FOR HORIZONTAL GAS FLOW IN ZIGZAG BAFFLES.
800
220
-------
100
9O
80
i—r
PACKED BED
> 100
o
LU
^ 90
u_
UJ
80
II
•^TTiF-a^O—C^iKMxini>^—
D
TUBE BANK
_j i i
0
J L
o
LU
8 I0°
90
KNITTED MESH
l l
i
100 200 30O 400 500 600 700 800 900
GAS VELOCITY, cm/sec
EXPERIMENTAL COLLECTION EFFICIENCY AS A
FUNCTION OF GAS VELOCITY IN A PACKED BED, TUBE
BANK AND KNITTED MESH WITH HORIZONTAL GAS
FLOW.
INLET DROP DIAMETER,
V 84
0 380
D 1,225
O >l,225
FIGURE 4
221
-------
100
to
(O
u
2
UJ
o
Li_
U_
UJ
o
UJ
o
CJ
50
INLET
O 30.5 x 15.2 cm (NO VANE)
• 30.5 x 7.6 cm (WITH VANE)
0 1,000 2,000 3,000 4,000
GAS VELOCITY IN CYCLONE INLET, cm/sec
FIGURE 5
EXPERIMENTAL COLLECTION EFFICIENCY IN CYCLONE.
-------
0.05 L
0.03
-2
-3
10 ~ 10
GAS VELOCITY, cm/sec
FIGURE 6
DRY PRESSURE DROP VERSUS GAS VELOCITY IN
DIFFERENT SEPARATORS USED IN PILOT PLANT,
223
-------
50
O
CM
E
o
10
Q_
oc 50
Q
LU
cr
ID
CO
CO
LU
tr
Q_
a:
o
.0
0.5
SHEPHERD
LAPPLE
EXPERIMENTAL
INLET WIDTH, cm
A 15.2 (NO VANE)
0 11.4
D 7.6
O 3.8
500 1,000
5,000 10,000
GEOMETRIC AVERAGE GAS VELOCITY
IN THE CYCLONE INLET AND OUTLET
cm/sec
FIGURE 7
COMPARISON OF EXPERIMENTAL PRESSURE DROP
DATA AND PREDICTED PRESSURE DROP FOR
CYCLONE WITH INLET VANE BY SHEPHERD AND
LAPPLE (1940).
224
-------
1,000
800
oc
LU
o
O_
CC
O
<
o
600
400
200
aoo
GAS
400 600
VELOCITY, cm/sec
FIGURE 8
MASS MEDIAN OUTLET DROP DIAMETER FOR
HORIZONTAL GAS FLOW IN KNITTED MESH.
INLET d =82 pm.
f &
800
225
-------
£UU
E
oT I5°
LU
LJ
^ 100
0
cr
a
MINIMUM
Ol
0 0
*-*.
I 1 I 1
° A MESH
O STRUT
0 D PACKED BED
0 BAFFLES
O
O
o ^ o 9 o^
o o g og o $ o
D O cpO ^
1 1 1 1
) 200 400 600 800
GAS VELOCITY, cm/sec
FIGURE 9
MINIMUM OUTLET DIAMETER VERSUS GAS VELOCITY
-------
x 10-3
O
ro
e
\
ro
E
a:
co
<
o
o
i-
Q
^
o
IxlO
-4
0
0
IxlO
O
A SOME REENTRAINMENT (
-------
10
-3
ro
2
\
ro
(T
CO
-------
Future Needs for Fine Particle Scrubber Capabilities
by
Michael J. Pilat
Water and Air Resources Division
Department of Civil Engineering
University of Washington
Seattle, Washington 98195
The topic of future needs for fine particle scrubber
capabilities can be addressed by posing two questions:
1. Are scrubbers needed for the control of fine
particle emissions?
2. What are the research needs concerning fine
particle scrubbers?
There are a number of cases where the use of wet scrubbers
for fine particle control may be advantageous compared to
electrostatic precipitators or filters. Some of the reasons
for using wet scrubbers include:
1. Treatment of wet, corrosive, and/or explosive gases.
2. Simultaneous collection of particulate and gaseous
pollutants.
Our overall objective concerning the development of fine
particle scrubber technology should be to use theoretical and
experimental studies to relate the scrubber particle efficiency
as a function of particle size to the scrubber design and oper-
ating parameters. The scrubber design parameters include the
geometry, dimensions, and locations of water sprays, inlets,
and drains. The scrubber operating parameters include the gas
pressure drop, water pressure drop, gas residence time, water/
gas flow rate ratio, distance droplet travels with respect to
the gas, depth of bubble froth, bubble size, distribution,
droplet size distribution, gas velocity, water temperature,
gas temperature, water vapor content, particle electrostatic
charge/mass ratio, water electrostatic charge/mass ratio, and
particle solubility in water.
229
-------
In addition to pilot plant and full-scale scrubber
studies, research is needed concerning scrubber particle
collection mechanisms on a micro-scale, such as with single
droplets (spray scrubbers) and single bubbles (sieve plates).
The micro-scale studies should consider all the possible
particle collecting mechanisms including inertial impaction,
diffusiophoresis, thermophoresis, electrophoresis, magneto-
phoresis, and Brownian diffusion. Also research is needed on
particle-liquid interfacial phenomena (how particles reach when
they impact upon the liquid surface; effects of wetting agents,
electrostatic charges, particle solubility, etc.).
Good test methods and instrumentation are needed to measure
the particle properties and the scrubber operating parameters.
Parameters such as bubble size distribution, droplet size
distribution, water electrostatic charge/mass ratio, particle
electrostatic charge/mass ratio, etc. may have a significant
effect upon the performance of fine particle scrubbers, yet
these parameters are seldomly measured.
Scrubbers capable of collecting 99+1 of fine (0.02 to 2
micron diameter) particles and having outlet particle concentra-
tions in the 0.001 to 0.0001 grain/acf (2290 to 229 micrograms/
m3) range should be developed. Although such high particle
collection efficiencies may not be needed in some cases, it
would assist the construction and operation of reliable lower
efficiency (say 95%) fine particle collection scrubbers if the
99+$ fine particle collection efficiency scrubber technology
was available,
REFERENCES
Calvert, S. J. Goldshmid, P. Leith, arid D. Mehta. "Research and
Development Plan", Wet Scrubber System Study, Final Report and
Bibliography. Vol. II, pp. 56-65 (July 1972)..
Calvert, S., J. Goldshmid, D. Leith, and N. Jhaveri, "Future
Research Recommendations", Feasibility of Flux Force/Condensa-
tion Scrubbing for Fine Particle Collection, pp^133-137.~~
Engelmann, R. J., "Priorities in Scavenging Research", Precipita-
tion Scavenging, pp. 6-1-, USAEC, NTIS No. CONF-700601, (Dec.
1970).
230
-------
PANEL DISCUSSION
Moderator - Dr. Leslie E. Sparks
Panel Members -
Harold M. Haaland
Charles W. Lear
Robert C. Lorentz
James R. Martin
Jack E. Phelan
Alexander Weir, Jr.
Panel members gave brief introductory statements and then
the floor was opened for questions from the audience. Those
introductory statements which were available in written form
at the time of printing are given below.
Harold M. Haaland
Western Precipitation Division
Joy Manufacturing Company
When we discuss the future of the collection of fine par-
ticulate from industrial gas streams I believe the first thing
we must do is to unshackle our minds from the hardware concept
of scrubbers, fabric filters, and electrostatic precipitators
and concentrate on explaining the physical forces and mechanisms
doing the work of separation.
The problem of air pollution by fine particulate is as old
as time itself with naturally occurring fine particulate from
volcanic action, sea water evaporation, and hydrocarbon release
from forests. The industrial problem became great as soon as
man created a high demand for metals at the start of the Indus-
trial Revolution and smelters the world over volatilized and
condensed as fine particulate tons of many metals. By 1896 we
find Malvern W. lies writing on "Methods for the Collection of
Metallurgic Dust and Fume" in which he differentiates between
common dust and "infinitely finer" fume. In that article he
discusses deposition in long flues, filtering through porous
media, use of water as drops, jets, and/or steam, use of static
231
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electricity, filtration through cloth and other methods. Then,
and now, the real problems in evolving satisfactory engineering
and hardware for industry were not in the area of ideas as to
how to solve the problem; but rather how to solve the problem
reasonably economically, using materials and techniques readily
available.
The sound equipment designer must always maintain his
position abreast of the state of the art and the better ones
always maintain at least one probing effort out into the blue.
The net result, in our area, is that most understanding and
explanation of fundamentals has been preceded by empirically
established equipment and a secondary effect has been to dis-
courage fundamental research in industry as simply trying to
explain what has already been done as opposed to what needs
doing. This, of course, is poor reasoning as the cost of
empirical experimentation and the price of failure go up as-
tronomically in proportion to the degree of extrapolation from
a sound fundamental data base.
On another tack, if we want to look for what has been done
in the past as a guide to the future in very high efficiency
particulate collection we must look at process gas cleaning
rather than gas cleaning for air pollution purposes. This is
due to the rigid requirements of some forms of process gas
cleaning and the inherent payout on the equipment involved as
compared to air pollution control work which has tended to only
meet whatever regulations existed at any point in time.
Of these processes the cleaning of smelter gases for treat-
ment in acid making plants and the treatment of blast furnace
gases for use as auxiliary fuel probably represent the major
flows of gases reduced to very low levels of particulate content.
Other processes such as the cleaning of recirculating air in a
long-submerged submarine can illustrate how far we can go but
represent still more decades of cost increase and equipment
complexity.
232
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Both of the major gas stream examples illustrate the
melding of process and equipment over a period of time to
achieve the desired cleanliness goal. Both started with hot,
dry dust collection equipment alone which served for a time
but became obsolete as a demand for cleaner process gas evolved;
and both switched to multiple collectors of varying types in
series. Both are characterized in practice by being measured
on final results largely by ad hoc plant methods based on
visual and qualitative means.
The core elements to be derived from their experience and
to serve as a guide to the future in fine particulate cleaning
are:
(1) There is tremendous need for accurate, reliable instru-
mentation and techniques for the complete characterization
of fine particulate in a quantitative way both physically
and chemically.
(2) To achieve really fine cleaning of fine particulate from
industrial gases a system approach must be used melding
both approach process and cleaning equipment to get the
optimum mix for any given application. It is doubtful that
any one system will meet the needs of industry across the
board.
(3) Even where a single piece of collection equipment appears
capable of doing the cleaning job alternates should be
considered to improve the long term life and reliability
of the system; i.e. dry collectors before scrubbers to
improve abrasive wear conditions, change chemical reaction
conditions, or avoid plugging by dust surges.
(4) In many applications the cost of these next few steps to-
ward ultra-cleaning are going to be extremely high; another
order of magnitude over present costs. This is mandated by
the avoidance of even the smallest of leakage paths, the
widespread use of wet methods in various combinations with
attendant corrosion and wet handling problems, and the
sheer size of add-on equipment.
233
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In conclusion, then, we can probably do the job that needs
to be done but the tremendous costs likely to be imposed to
achieve this end will demand the best the scientific and en-
gineering communities can produce to accurately define the
needs, the sources, and the measurement techniques in the fine
particulate area.
Charles W. Lear
TRW Systems Group
Charged droplet scrubbing, like other scrubbing methods,
is a means of removing particulate and fumes from dirty air
through interactions of droplets of scrubbing liquor with the
particles of dirt. Besides the normal impact and diffusional
mechanisms, charged droplet scrubbing also includes electrical
interactions. As the name implies, the scrubbing droplets,
usually water, may carry an electrical charge and may move under
the influence of electric fields. The particulate may also
carry a charge other than its naturally occurring electric
charge. The enhancement in droplet collection efficiency due
to the electrical interactions make charged droplet scrubbing
an attractive method for some applications involving particulate
in the 0.1 to 1.0 micron size range.
In the TRW concept of charged droplet scrubbing, the scrub-
bing water is raised from ground to high voltage (about 40 kv)
by flowing through a long electrical resistance path, which
isolates with the electrical resistance of the water itself.
The water is introduced into a hollow electrode which contains
a series of hollow, elongated spray tubes. Emerging at the
tips of these spray tubes, the water sees a high electric field
force. Droplets are formed by the joint action of electrical
and surface tension forces, in a classical electrohydrodynamic
spraying process. Once formed, these droplets are highly
charged, almost to the local breakdown limit or stability limit.
234
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They move swiftly through the scrubbing volume under the in-
fluence of the electric field applied between the electrode
and the collecting walls.
Because of the high droplet velocities (around 30 m/sec)
induced by the ambient electric field, there is a large relative
motion between droplets and particulate. This large relative
motion enables the small particles to overcome aerodynamic
forces which would normally sweep them around the droplet with
the flow stream. Under inertial forces, they are able to ap-
proach the droplet more closely and interact. One such mode
of interaction would be the familiar impact and agglomeration
mode (electrically induced impact scrubbing).
If the particle passes close to the droplet but is not
captured, it may still interact through a charge transfer from
droplet to particle. The particle may become sufficiently
highly charged to be precipitated within the scrubber. This
mechanism is known as induced charging, and is important in
the sub-micron size range.
In company research projects and in the present EPA funded
program, mass removal efficiencies on the order of 60 percent
per stage have been demonstrated on sub-micron particulates.
Typical particulate distributions are newly dispersed iron
oxide, or zinc oxide, each with estimated mass-mean-diameters
on the order of one micron. A typical Charged-Droplet-Scrubber
will consist of three stages with 95 percent efficiency or
better.
Jack E. Phelan
Nalco Chemical Company
Case Histories on Application of Wet Scrubber Additives
Nalco Chemical Company is intensely involved in water
treatment applications to the industries of this country. It
is this background which has brought us into the wet gas scrubber
field. We will now outline several case histories directly re-
lated to wet gas scrubbing.
235
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Case History #1
The steel industry has found that the addition of No. 6
fuel oil or tar into the tuyeres of the blast furnace can par-
tially replace some of the coke. However, this practice re-
quires very close control, and slight deviations in application
of oil produce unburned carbon, call it lamp black, carried out
in the blast furnace gases. These gases are first passed through
dry dust catchers and then through wet venturi type gas scrub-
bers. The solids picked up in the wet gas scrubber are sent
to thickeners where they are precipitated and the water either
sent to discharge or recycled. The solids are dewatered by
vacuum filtration and reused in the sinter plant or land filled.
Recently we experienced in several plants, severe dis-
charge problems of floating black particles going to the re-
ceiving streams. Severe foam conditions also existed on top of
the clarifying equipment. This problem caused the plants to
cut back on their fuel injection.
Initially we recommended better firing practices for the
fuel. Then we screened a number of experimental chemical pro-
ducts and found that two products working together had the
ability of wetting the carbon black and keeping the Venturis
clean without interfering with the settling rate of the clari-
fication system. This program was an immediate success and
has been in operation approximately nine months. It has also
allowed the plant to increase fuel oil injection.
Further work yielded a single product to provide the fol-
lowing benefits in a variety of wet scrubber applications.
a) no lamp black problem on clarifiers
b) improved removal of particulate matter in gases going
to combustion areas
c) increased suspended solids in water going to thickener,
indicating good deposit control
d) cleaner operation of burner equipment utilizing blast
furnace gas
236
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Case History #2
A large midwest gray iron foundry was experiencing severe
fouling of their packed wet gas scrubber equipment. This was
resulting in down time cleaning, once every three weeks, plus
giving the plant opacity problems. The plant stacks were
reading 40-45% and citations were being given. A major expendi-
ture of around $1,000,000 was being worked up to correct this
condition.
A trial run was made on one of the scrubber systems during
the last week of a three week cycle to see what effect a dirty
system would make. After one week, the system was shut down
to inspect. The normally very dirty rings were only slightly
fouled and the opacity had improved.
After a full three week program we found the equipment
practically as clean as when we started and also found the
opacity had decreased to the range of 20 to 25%, meeting the
standards required.
Four times in the next six months we were called by the
plant to check out increasing opacity readings. Each time we
found the mechanical feeding equipment had been shut off and
after placing it back in service, had the opacity readings go
back to the range of 20 to 25%. Permanent installation of
feedings equipment is now installed so that supervisory per-
sonnel can monitor the equipment daily.
Case History 13
A large Eastern Steel Mill has a wet scrubbing B.O.F.
operation and has been successfully fulfilling their emission
standard readings. Their opacity readings are constantly
around ten percent and their grains per standard cubic foot
are within regulatory legislation.
However, during the warmer months they show a small amount
of fine red coloring in the emission stack. In a current
evaluation at this plant our findings to date show us a decrease
in particulate matter ranging from 20 to 25%. Further work is
being done in this area.
237
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Conclusions
The normally very efficient wet scrubbers manufactured
by many corporations, can show improved performance by control-
led chemical treatment of the wet scrubber water. These im-
provements can be -
a) dispersion of solids
b) improved wetting of particulate
c) maintaining clean nozzles and flooded surfaces
d) control of deposits
e) preventing corrosion and clogging with corrosion products
f) de-tackifying "sticky" dispersions
Data from successful applications shows that chemical
treatment can mean the difference between compliance with regu-
lations on air emissions or citation for failure to comply.
But success also requires close surveillance of the water
chemistry of the system for such basic control parameters as
pH, TDS, suspended solids, hardness, alkalinity and additive
dosage.
Dr. Alexander Weir, Jr.
Southern California Edison Company
Theee has been considerable discussion of the removal of
particles under 2 microns in the symposium, but since the first
of this year our company at the Mojave Generating Station had
removed about 83 tons of particles under 2 microns with one
scrubber. When the cost of the scrubber was prorated over this
removal, it appeared that we spent about $100,000 a ton removing
these particles and one might question whether the cost was
worth the benefit. The scrubber was operated primarily to remove
sulfur dioxide and the inlet concentration of sulfur dioxide to
the scrubber (around 200 ppm) was lower than the exit concen-
tration of sulfur dioxide achieved in the EPA scrubber program
at Shawnee.
238
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There is a contention that the presence of fine particles
resulted in a reduction of visibility but in the Los Angeles
Basin that the visibility reduction is probably due primarily
to the formation of photochemical smog, most of which was due
to emissions of the NOX and hydrocarbon vapor from automobiles.
Whatever the automobile's contribution of NO , it is generally
X
accepted that in Los Angeles visibility reduction is not due
to emissions from power plants.
Our experience supports the view that scaling up particu-
late removal data from pilot plants to full size scrubbers is
in fact very difficult.
239
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Closing Comments
by
D. C. Drehmel
Fine particles are those which penetrate into the respiratory
system where they may act as a carcinogen, may contribute to
blockage of the alveoli, or may accommodate pulmonary pathogens.
Mr. Harrington observed that fine particles are not a single
pollutant but a large category of pollutants. It is therefore
difficult to agree on a general definition of fine particles as a
pollutant. However, it has been shown that particles with diameters
smaller than 5 micrometers do penetrate past the nasal cavity and
nasopharynx into the lungs and that particles with diameters as
small as 0.01 micrometers are deposited in the lungs. Other
problems associated with submicron particles are that they are slow
in settling out of the atmosphere and they create haze and adverse
weather modification.
Scrubbers have a significant role to play in the control of
fine particle emissions from stationary sources. As already noted
by Dr. Pilat, scrubbers will be necessary for simultaneous collection
of gaseous and particulate emissions and for the collection of
particulate in the presence of wet, corrosive or explosive gases.
Furthermore, scrubbers may be necessary for the control of particles
which have properties unsuitable for the operation of alternative
control devices; for example, a particulate with poor cake release
properties in a fabric filter.
The ability of conventional scrubbers to control fine particles
was discussed by several authors (Beeckmans, Calvert, Eckert, and
Hesketh). For particles in the 1 to 5 (and above) micrometer range,
impaction is the important mechanism and in the 0.01 to 0.1 micrometer
range, diffusion is the important mechanism. Control of particulate
emissions in these regions is highly efficient when the effect of
240
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the appropriate collection mechanism is optimized. The use of
special designs or new concepts to capture fine particles was the
subject of most of the authors (Gardenier, Jhaveri, Sparks, Leith,
and Rich). Even in the particle size region where both the
diffusion and impaction mechanisms are least effective, these
authors have shown that scrubbers can be highly effective in
collecting fine particles.
It was the intent of the Control Systems Laboratory of EPA
in sponsoring this symposium to stimulate the development of new
concepts for the control of fine particulate emissions. New
concepts discussed at the symposium verify not only that the
development of scrubbers is important to the solution of the air
quality problem but also that new scrubber concepts are a first
step to controlling fine particulate emissions in all size ranges.
241
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LIST OF PEOPLE ATTENDING THE E.P.A.SYMPOSIUM
James Abercrombie
Stanislaus County APCD
820 Scenic Drive
Modesto, California 95350
J. I. Accortt
Combustion Engineering, Inc.
1000 Prospect Hill Road
Windsor, Connecticut 06095
Larry W. Anderson
Aerotherm
485 Clyde Avenue
Mt. View, California 94042
David Ball
Battelle Columbus Labs
505 King Avenue
Columbus, Ohio 43201
J. M. Beeckmans
Faculty of Engineering
University of Western Ontario
London, Ontario, Canada
R. E. Besalke
A.P. Green Refractories
Green Blvd.
Mexico, Mo. 65265
William B. Bispeck
B. F. Goodrich Chemical Co.
6100 Oak Tree Blvd.
Cleveland, Ohio 44131
T. R. Blackwood
Monsanto Research Corp.
Station B, Box 8
Dayton, Ohio 45407
R. H. Boll
Babcock § Wilcox
P. 0. Box 835
Alliance, Ohio 44601
John 0. Burckle
U. S. Environmental Progection Agency
5555 Ridge Avenue
Cincinnati, Ohio 45268
Seymour Calvert
A.P.T., Inc.
P. 0. Box 71
Riverside, California 92502
Robert M. Christiansen
Stearns Roger, Inc.
655 S. Monroe Way
Denver, Colorado 80209
Hung Ben Chu
L.A. Dept. of Water § Power
111 Hope Street
Los Angeles, California 90051
Howard P. Clark
U.S. Borax § Chemical Corp.
Boron, California 93516
A. B. Craig
Environmental Protection Agency
Research Triangle Park
North Carolina 27711
Frank G. Dagerman
Monsanto Company
6251 Paramount Blvd.
Long Beach, Calif. 90805
H. Daggett
Joy Manufacturing
P.O. Box 2744 Terminal Annex
Los Angeles, California 90054
John Davidson
Energy Policy Project
1776 Massachusetts Ave. , N.W.
Washington, D.C. 20036
Timothy W. Devitt
Pedco Environmental
Suite 13 Atkinson Square
Cincinnati, Ohio 45246
Thomas M. Distler
Lawrence Livermore Lab
Livermore, California 94550
D. C. Drehmel
U.S. Environmental Protection
Agency
Research Triangle Park
North Carolina 27711
J. E. Drummond
Maxwell Labs
9244 Balboa Avenue
San Diego, California 92123
242
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John S. Eckert
Norton Company
P. 0. Box 350
Akron, Ohio 44309
Robert W. Edson
Occidental Chemical Co.
P.O. Box 198
Lathrop, California 95330
H. M. Englund
Air Pollution Control Assoc.
4400 Fifth Avenue
Pittsburgh, PA 15213
David S. Ensor
Meteorology Research, Inc.
464 W. tfoodbury Road
Altadena, CA 91001
M. R. Eriksson
Air Resources Board
1709 llth Street
Sacramento, CA 95814
John D. Ferrell
Steams-Roger, Inc.
Box 5888
Denver, Colorado 80217
C. E. Fisher
Anaconda Aluminum
Box 10
Columbia Falls, Montana 59912
Richard Fitterer
Glen Odell Consulting Engineers
520 S.K. 6th
Portland, Oregon 97204
Gary J. Foley
U.S. Environmental Protection Agency
7509 Harps Mill Road
Raleigh, N.C. 27609
Malcolm Fraser
Intertechnology Corporation
Box 340
Warrenton, Virginia
R. Fribourghouse
Celesco Industries
3333 Harbor Blvd.
Costa Mesa, California 92626
H. E. Gardenier
Aronetics Incorporated
P. 0. Box 13030
Houston, Texas 77019
D. C. Gehri
Atomics International
P. 0. Box 309
Canoga Park, California 91304
Richard Gerstle
Pedco
Suite 13 Atkinson Square
Cincinnati, Ohio 45246
Richard Glassbrook
Bechtel Power Corporation
12400 E. Imperial Highway
Norwalk, California 90650
Morris Goldberg
Environmental Protection Agency
Region IX
100 California Street
San Francisco, California 94111
John P. Gooch
Southern Research Institute
2000 9th Avenue, S.
Birmingham, Alabama 35205
Alan R. Goodley
Calif. Air Resources Board
1709 llth Street
Sacramento, California 95814
John N. Goulias
Mo. Air Conservation Commission
1304 St. Mary's Blvd.
Jefferson City, Mo. 65101
Eugene E. Grassell
Donaldson Co., Inc.
P, 0. Box 1299
Minneapolis, Minn. 55440
243
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Harold H. Haaland
Joy Manufacturing Co.
P. 0. Box 2744
Los Angeles, California 90051
Harlan N. Head
Bechtel Corporation
P. 0. Box 3965
San Francisco, California 94119
Larry Herndon
A.P. Green Refractories
Green Blvd.
Mexico, Missouri 65265
J. R. Herrig
Weiser Company
4100 Ardmore
South Gate, California 90280
Howard E. Hesketh
Southern Illinois University
School of Engineering
Carbondale, Illinois 62901
B. F. Hightower
General Atomic Company
P. 0. Box 81608
San Diego, California 92138
Nick Hild
Motorola, Inc.
5005 E. McDowell Road
Phoenix, Arizona 85008
Delbert Horton
Radian Corporation
Box 99408
Austin, Texas 78766
C. K. Hsieh
Southern California Edison
P. 0. Box 800
Rosemead, California 91770
F. Dale Huillet
Scott Paper Company
2600 Block Federal
Everett, Washington 98206
Thomas L. Hurst
Kerr McGee Corporation
P. 0. Box 25861
Oklahoma City, Oklahoma 73125
M. L. Jackson
University of Idaho
JEB 125
Moscow, Idaho 83843
I. L. Jashnani
A.P.T., Inc.
P.O. Box 71
Riverside, CA 92502
Raymond J. Jaworowski
Air Correction Division of UOP
P.O. Box 1107
Darien, Conn. 06820
N. C. Jhaveri
A.P.T., Inc.
P.O. Box 71
Riverside, California 92502
K. J. Kaminski
B.F. Goodrich Chemical Co.
6100 Oak Tree Blvd.
Cleveland, Ohio 44131
Albert J. Klee
Solid § Hazardous Waste Research
Laboratory
National Environmental Research
Center
Cincinnati, Ohio 45268
Shou S. Kwong
Bechtel Power Corporation
12400 E. Imperial Highway
Norwalk, California 90650
Michael R. Lake
San Diego County APCD
1600 Pacific Highway
San Diego, California 92101
Brian Lancaster
Westinghouse R 5 D
Churchill Boro, PA 15235
A. W. Langeland
UOP-ACD
P.O. Box 1107
Darien, Connecticut 06820
244
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C. E. Lapple
Stanford Research Institute
333 Ravenswood Avenue
Menlo Park, California 94025
Ron Lawter
Arthur D. Little, Inc.
Acorn Park
Cambridge, Massachusetts 02140
Charles Lear
TRW Systems'Group
1 Space Park
Redondo Beach, CA 90278
W. C. Leith
Cominco Engineering
Trail, British Columbia
Canada
E. D. Lemon
U. S. Borax § Chemical Corp.
3075 Wilshire
Los Angeles, CA 90010
Benjamin Linsky
West Virginia University
College of Engineering
Morgantown, W. Virginia 26506
R. C. Lorentz
U.S. Environmental Protection
Agency
Durham, North Carolina 27711
David S. McCaffrey
Exxon Research § Eng. Co.
P.O. Box 101
Florham Park, New Jersey 07932
R. K. McCluskey
Southern California Edison
P. 0. Box 800
Rosemead, California 91770
Andrew R. McFarland
University of Notre Dame
Dept. of Civil Engineering
Notre Dame, Indiana 46556
R. W. Mcllvaine
The Mcllvaine Company
2970 Maria Drive
Northbrook, Illinois 60062
B. G. McKinney
Tenn. Valley Authority
524 Power Building
Chattanooga, Tenn. 37401
James L. Ma
Western Precipitation Division
Joy Manufacturing Company
P.O. Box 2744 Terminal Annex
Los Angeles, California 90051
Reynold A. Mack
3 M Company
Box 33331
St. Paul, Minnesota 55133
Lee Markley
Lever Brothers Company
6300 E. Sheila Street
Los Angeles, California 90022
James R. Martin
Combustion Engineering Inc.
1000 Prospect Hill Road
Dept. 532-19W
Windsor, Conn. 06092
Josep Massari
Monsanto
6251 Paramount Blvd.
Long Beach, California
I . T . Mayr
Air Products
Box 538
Allentown, PA 18105
Chem,, Inc.
M. S. Mehta
Mikropul, Div. of U.S. Filter
10 Chatham Road
Summit, N. Jersey 07901
Peter Milovsoroff
Air Pollution Systems
Box 88811
Tukwila, Washington 98188
245
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Fred Moreno
Aerotherm
485 Clyde Avenue
Mountain View, CA 94042
Marc Myton
Celesco Industries
3333 Harbor Blvd.
Costa Mesa, California 92626
Thomas E. Nelson
Teledyne Wah Chang
P.O. Box 460
Albany, Oregon 97321
William M. Neuner
Lever Brothers Co.
390 Park Avenue
New York, N.Y. 10022
Ralph R. Nielsen
Nalco Chemical Co.
6216 W. 66th Place
Chicago, Illinois 60638
Henry H. Osborn
Air Preheater Company
Wellsville, N. Y. 14895
Robert Parkinson
San Diego County APCD
1600 Pacific Highway
San Diego, California 92101
Robert R. Patrick
Union Carbide Corporation
P. 0. Box 8361
South Charleston, W. Virginia 25303
John C. Paulsen
U.S. Borax § Chemical Corp.
Boron, California 93516
Tom Paxson
Kern County APCD
P. 0. Box 997
Bakersfield, California 93309
Thomas F. Payne
Anaconda Aluminum Company
P. 0. Box 10
Columbia Falls, Montana 59912
Wesley W. Pepper
L.A. Dept. of Water 5 Power
P. 0. Box 111, Room 667
Los Angeles, California 90051
John Petering
Air Products § Chemicals, Inc.
P. 0. Box 538
Allentown, PA 18105
Jack E. Phelan
Nalco Chemical Company
180 N. Michigan Avenue
Chicago, Illinois 60601
T. M. Phillips
Pacific Power 5 Light Company
920 S.W. Sixth
Portland, Oregon 97204
R. W. Piekarz
Minerals Division
Eagle Picher Ind. Inc.
P. 0. Box 1869
Reno, Nevada 89505
Michael Pilat
Dept. of Civil Engineering
University of Washington
Seattle, Washington 98195
A. Plumley
Combustion Engineering
Dept. 683-3
1000 Prospect Hill Road
Windsor, Connecticut 06095
Stanley R. Rich
R. P. Industries, Inc.
15 Kane Industrial Drive
Hudson, Mass. 01749
Michael D. Roach
MW WAPA
2585 State Street
Salem, Oregon
C. Robertson
Yolo-Solano APCD
P.O. Box 1006
Woodland, California 95695
246
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Bill Schofield
Air Products § Chemical:
Corp. Engineering
Allentown, PA 18105
M. P. Schrag
Midwest Research Institute
425 Volker Blvd.
Kansas City, Missouri 64110
James J. Schwab
Air Pollution Systems, Inc.
P. 0. Box 88811
Tukwilla, Washington 98188
Larry J. Shannon
Midwest Research Institute
425 Volker Blvd.
Kansas City, Missouri 64110
James E. Short
Albuquerque Environmental
Health Department
Box 1293
Albuquerque, New Mexico 87103
H. Paul Sidhu
Air Pollution Control District
3660 Eagle Street #B
San Diego, California 92103
A. V. Slack
SAS Corporation
RFD 1
Sheffield, Alabama 35660
Glen A. Smith
ITT Rayonier
409 E. Harvard Street
Shelton, Washington 98584
Leslie E. Sparks
Control Systems Laboratory
Environmental Protection Agency
Research Triangle Park, N.C. 27711
M. L. Spector
Air Products § Chemicals, Inc.
P. 0. Box 538
Allentown, PA 18105
Dr. R. Spink
Dept. of Chemical Engineering
University of Waterloo
Waterloo, Ontario, Canada
Mike Stanek
Occidental Chemical Co.
P. 0. Box 198
Lathrop, California 95330
Leslie M. Steffensen
Georgia Pacific Corporation
P. 0. Box 1618
Eugene, Oregon 97401
D. Stelman
Atomics International
P. 0. Box 309
Canoga Park, California 91304
William A. Summers
Babcock § Wilcox Company
20 S. Van Buren Avenue
Berberton, Ohio 44203
G. C. Teste
Aluminum Co. of Canada
448 Villeneuve Street
Arvida, Quebec, Canada
Louis Theodore
Air Pollution Training Institute
Environmental Protection Agency
Research Triangle Park, N.C. 27711
Terry L. Thoem
EPA, Region VIII
1860 Lincoln Street
Denver, Colorado 80203
Gordon A. Turl
Fresno County APCD
1246 "L" Street
Fresno, California 93721
Joseph A. Vaillancourt
Hercules, Inc.
910 Market Street
Wilmington, Delaware 19899
247
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Ted VanDecar
Puget Power
Box 868
Bellevue, Washington 98009
Gene Vandegrift
Midwest Research Institute
425 Volker Blvd.
Kansas City, Mo. 64110
Alexander Weir
Southern California Edison
2244 Walnut Grove
Rosemead, California 91770
Roger Murray Wells
Radian Corporation
P. 0. Box 9948
Austin, Texas 78766
Frank L. Worley, Jr.
Chemical Engineering Department
University of Houston
3800 Cullen
Houston, Texas 77004
James S. Wu
E.P.A., Region IV
1421 Peachtree Street, N.E.
Atlanta, Georgia 30309
Shuichow Yung
A.P.T., Inc.
P. 0. Box 71
Riverside, California 92502
248
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TECHNICAUREPORT DATA
(Please read Inaructions on the reverse before completing)
i. REPORT NO.
EPA-650/2-74-112
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE ANDSUBTITLE
EPA Fine Particle Scrubber Symposium
(San Diego, 5/28-30/74)
5. REPORT DATE
October 1974
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Dennis C. Drehmel (Project Officer)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Miscellaneous
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-034
11. CONTRACT/GRANT NO.
68-02-1328 (Task 2)
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
NERC-RTP, Control Systems Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Proceedings! 5/28-30/74
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16 ABSTRACTThese proceedings contain the 14 papers presented during the symposium,
which emphasized the collection of fine particles (solid or liquid particles smaller
than about 3 microns) by any type of wet collector, including hybrid devices. The
objective of the symposium was to stimulate and generate new and novel ideas for
controlling fine particulate emitted from stationary sources and promote interchange
of ideas among scrubber experts. The consensus of the symposium was that scrub-
bers have a significant role to play in controlling fine particulate. Several papers
discussed the ability of conventional scrubbers to control fine particles. Most of the
papers dealt with the use of special designs or new concepts to capture fine particles.
Symposium attendees concluded that not only is scrubber development important to
the solution of the air quality problem, but that new scrubber concepts are a first
step to controlling particulate emissions in all size ranges.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Scrubbers
Design Criteria
Air Pollution Control
Stationary Sources
Fine Particulate
Engineering Design
13B
07A
14A
8. DISTRIBUTION STATEMENT
Unlimited
19, SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
258
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
249
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