IITRI Report No. C6186-5
(Final Report)
DEVELOPMENT OP PARTICULATE EMISSIONS
CONTROL TECHNIQUES
FOR SPARK IGNITION ENGINES
Environmental Protection Agency
Air Pollution Control Office
Division of Motor Vehicle R & D
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PROPERTY OF
EPA LIBRARY
RTP.NC
IITRI Report No. C6186-5
(Final Report)
:DEVELOPMENT OF PARTICULATE EMISSIONS
. CONTROL TECHNIQUES
FOR SPARK IGNITION ENGINES
Environmental Protection Agency
Air Pollution Control Office
Division of Motor Vehicle R & D
. Ann;Arbor, Michigan 48103
-------
IITRI Report No. C6186-5
(Final Report)
DEVELOPMENT OF PARTICULATE EMISSIONS CONTROL
TECHNIQUES FOR SPARK IGNITION ENGINES
June 22, 1969 to November 30, 1970
Prepared by
Sudesh K. Sood
and
Richard Karuhn
Submitted by
lIT Research Institute
10 West 35 Street
Chicago, Illinois 60616
for
Environmental Protection Agency
Air Pollution Control Office
Division of Motor Vehicle R & D
Ann Arbor, Michigan 48103
Attention: Mr. Charles Gray, Jr.
Project Officer
February, 1971
Copy No.
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FOREWORD
This is the Final Report on IITRI Project C6186, entitled,
"Development of Particulate Emissions Control Techniques for
Spark Ignition Engines." The research program was sponsored by
the Air Pollution Control Office (formerly the National Air
Pollution Control Administration) of the Environmental Protection
Agency under Contract No. CPA-22-69-134. Personnel contributing
to the project were Ted Rymarz, Don Werle, Brent Boldt, Henry
Karplus, John Stockham and Meryl Jackson.
This report covers the period from June 22, 1969 to
November 30, 1970. All data are recorded in IITRI Logbooks
C19521, C19678 and C20199.
Respectfully submitted,
lIT Research Institute
~~e~~ ~.~Utr~.
Sudesh K. Sood
Associate Engineer
~;('~
Richard Karuhn
Experimentalist
Approved by:
JrL~
John Stockham
Manager, Fine Particles
Research
SKS:db
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ABSTRACT
DEVELOPMENT OF PARTICULATE EMISSIONS CONTROL
TECHNIQUES FOR SPARK IGNITION ENGINES
This report describes the experimental results of two
techniques for the removal of particulate contaminants from
spark ig'nition eng'ine exhausts.
The first technique was based on the thermal deposition
of lead aerosol particles in the size rang'e 0.1 - O. 8j-L in a
packed bed.. The effect, of gas-packing' temperature differential,
packing material, packing shape and size, contamination build-
up, and gas velocity on collection efficiency of the bed was
studied. Experimental results show that collection efficiency
of the packed bed device depends primarily on the gas-packing'
temperature differential. At a gas velocity of 15.5 cm/sec the
collection efficiency of the device exceeds 95% at temperature
differential greater than 200°C. Increasing the gas velocity
to 130 cm/sec lowered the collection efficiency of the bed by
10-15%.
The second technique was based on the use of sonic waves
to increase the collisions between the aerosol particles and
the relatively coarse particles of a fluidized bed, and hence
increase tne collection efficiency. The effect of sound
frequency, gas velocitYt and power input to the sound driver
units was studied. Experimental results showed that there was
no significant effect of sound frequ.ency, in the range 250-
2700 HZ, on collection efficiency of the fluidized bed. Col-
lection efficiency of the fluidized bed was found to increase
sharply with power input to the sound driver units when standing'
sound waves were used. The use of travelling sound waves did
not enhance the collection efficiency of the fluidized bed
significantly.
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Foreword
Abstract
TABLE OF CONTENTS
I.
INTRODUCTION
A.
EXP£RIM~NTAL PROCEDURES
Aerosol Generation
II.
l~
2.
Generation of Carbonaceous Aerosols
Generation of Lead Chloride Aerosols
B. Analytical Techniques
III. Thermal Packed Bed Device
A.
B.
Q.
D.
IV.
Description of Experimental Setup for Thermal
Deposition Studies
Determination of the Collection Efficiency of
the Thermal Packed Bed Device
Experimental Data and Discussion of Results
1. Effect of Particle Size on Collection
Efficiency
2. Effect of Surface Area and Shape of the
Packing on Collection Efficiency
3. :Effect of Heat Capacity of the Packing
Material on Collection Efficiency
4. Effect of Gas Velocity on Collection
Efficiency
5. Effect of Aerosol Concentration on
Collection Efficiency
6. Effect of Contamination Build-Up on
Collection Efficiency
7. Effect of Gas-Packing Temperature Dif-
ference on Collection ~fficiency
Compatibility of the Thermal Packed Bed Device
with Automotive Systems
SONIC FLUIDIZ~D BED DEVICE
A.
Collection Efficiency of Fluidized Bed Without
Sonic Enhancement
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10
10
10
10
15
20
20
22
24
28
28
31
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36
39
42
43
48
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TABLE OF CONTENTS (Cont'd)
B.
Enchancement of Collection Efficiency of
Fluidized Beds with Sonic Techniques
C.
1. Theory
2. Determination of Acoustic Particle
Velocity
Effect of Traveling' Sound Waves' on Collection
Efficiency of the Fluidized Bed
1. Effect of Sound Frequency on Collection
Efficiency ,
2. Effect of Particle Velocity"on Collection
Efficiency
Effect of Standing- Sound Waves on Collection
Efficiency of the Fluidized Bed
Compatibility of the Sonic Fluidized Bed
Device with Automotive Systems
D.
E.
V.
CONCLUSIONS
VI.
RECOMMENDATIONS FOR FUTURE WORK
References
APPENDIX A
Analytical Techniques
1. Reflectometer
2. Polarographic Technique
3. Colorimetric Technique
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52
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60
62
62
62
71
75
77
78
A-2
A-2
A-3
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Fig-.
No.
1
2
3
10
11
12
LIST OF FIGURES
Simplified Airflow Past a Filament
Collection Diameter of a Filament
4
5
Schematic DiagTam for Generation and Deposition of
Carbon Particles in a Packed Bed
Schematic Diag-ram of Lead Chloride Aerosol Generator
Photomicrog-raph of Lead Chloride Particles at
Different Furnace Temperatures
Particle Size Distributions of Lead Chloride
Aerosols at Different Furnace Temperatures
Effect. of Furnace Temperature on Mean Frequency
Diameter of Aerosol Particles
Effect of Furnace Temperature on Mass Mean Diameter
of Aerosol Particles
Apparatus for Study of Thermal Deposition of Lead
Chloride Aerosol in a Cold Packed Bed
6
7
8
9
13
Filters from Thermal System Tests
Effect of the Shape of the Packing- on Collection
Efficiency of PbC12 Particles by the-Thermal Packed
Bed
Effect of Surface Area on Collection Efficiency of
PbC12 Particles by the Thermal Packed Bed
Effect of Bed Material on Collection Efficiency of
the Thermal Bed
Effect of Heat Capacity of Packing- Material on
Heating- Rate of the Packed Bed Device
Collection Efficiency of the Thermal Packed Bed
Device as a Function of Time
Effect of Aerosol Flow Velocity on Collection
Efficiency of the Thermal Bed
Effect of Aerosol Concentration on Collection
Efficiency of the Thermal Bed
Effect of Carbon Contamination on Collection
Efficiency of the Thermal Bed
Carbon Particles Coming- Out of the Contaminated Bed
Effect of Gas-Packing' Temperature Difference on
Thermal Deposition Coefficient
Packed Beds For Collection of Submicron Particles
by Thermal Precipitation
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15
16
17
18
19
20
21
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6
7
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14
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18
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25
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Fig.
No.
22
23
24
25
26
27
28
29
30
31
II
III
LIST OF FIGURES (Cont'd)
Effect of Superficial Velocity on Collection
Efficiency of Fluidized Bed Without Sonic
Enhancement
Experimental Setup with Travelling Waves
Difference in Sound Pressure Levels when
Measured with and without the Probe Tube
32
Sound Pressure Levels as a Function of Frequency
at the Retaining Screen, 10, 20 and 26 cm. above
the screen (No Bed Present)
Effect of Flow Velocity and Frequency of Sound
on V /A
P
Effect of Sound Frequency on Collection Efficiency
of the Fluidized Bed with Travelling Waves
Effect of Particle Velocity on Collection Efficiency
of the Fluidized Bed with Travelling' Waves
Experimental Setup to Study the Effect of Standing
Sound Waves
Experimental Setup with Standing Waves
Effect of Power Input to the Speakers on the
Collection Efficiency of the Fluidized Bed with
Standing Sound Waves
Effect of Particle Velocity on Collection Efficiency
of the Sonic Fluidized Bed Device
Sonic Ag'glomeration in a Fluidized Bed
Calibration Curve for the Polarog-raph
Calibration Curve for Lead Chloride vs Absorbence
at Wavelength of 520 m~
33
34
35
1
LIST OF TABLES
Settling' Velocity of 2.0 Specific Gravity Particles
in Air 3
A Typical Particle Size Distribution of Lead Aerosols 16
Collection Efficiency Data on the Thermal Packed Bed
Device
Effect of Packing' Material on the Total Heat Capacity
of the Bed
Experimental Results with Traveling Sound Waves
IV
V
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DEVELOPMENT OF PARTICULATE EMISSIONS CONTROL
TECHNIQUES FOR SPARK IGNITION ENGINES
I.
INTRODUCTION
The operation of millions of motor vehicles in the United
States produces significant exhaust particulate emissions. The
majority of the current motor vehicles operate with fuel
containing lead antiknock compounds; these vehicles emit exhaust
particulates containing lead. Vehicles burning gasoline
containing 3 ml of TEL (Tetra Ethyl Lead) motor mix per gallon
have been observed to emit from 1 to 500 ~g of lead compounds
per liter of exhaust. It is also estimated that these vehicles
emit about 0.1-0.2 grams per mile of lead compounds. The
concentration of lead compounds in the exhaust gas depends on
the operating speed of the car (Ref. 1,2). The size of lead
bearing particulates in automobile exhausts has been found to
be less than 1 ~ by many investigators (Ref. 3,4,5). The exhaust
flow in vehicle exhaust systems ranges from about 9 to 95 standard
liters per second (approximately 20 to 200 scfm), while the
exhaust gas temperature depends on position in the exhaust system
and engine operating conditions and can range from 150 to 1000°C
(Ref. 6). Automotive exhaust particulates containing lead
,
compounds have also been reported to cause adverse effects on
vegetation, crops and human health (Ref. 7,8,9).
Of the myriad of particulate collection equipment commercially
available none are capable of high collection efficiency on
submicron sized particles when space and cost limitations imposed
by the proposed use are applied. Careful analysis of the problem
from a systems engineering approach leads to the conclusion that
the potential development of a suitable device is likely to result
only from intense technical efforts in a relatively narrow area
of technology.
A review of state-of-the-art techniques for particulate
removal reveals that the many types of devices available for
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4.
5.
6.
of particles from gas streams all depend on either one
of the following principles:
Mechanical; filters or screens
Inertial forces; centrifuges, cyclones, scrubbers
Electrostatic forces; electrostatic precipitators or
charged screens
Gravitational forces; settling chambers
Magnetic forces; magnetic separators
Thermal forces; thermal precipitators
removal
or more
1.
2.
3.
The separation of particles from a gas stream implies
relative motion between the particles and the gas. Hence, the
relative magnitudes of the separating force and the viscous drag
on the particles is an important parameter in most separation
mechanisms. With large particles, 100 ~ in diameter or larger,
the mass of the particles is high and the force of gravity can
be used to achieve separation in a settling chamber. As particle
size decreases, the settling velocity also decreases and other
techniques must be employed to achieve separation.
With particles in the size range 5 to 100 ~, the mass to
surface area ratio is high enough so that inertial forces can be
effectively employed to achieve separation. Cyclones, centrifuges,
impactors, and scrubbers are efficient devices in this size range.
Although there are differences in these devices, they all depend
on the inertia of the aerosol particles. Cyclones direct the
gas into a spiral path and achieve separation by the centrifugal
force on the particle. Centrifuges differ from cyclones only in
that a mechanically driven fan or rotor is used to produce the
rotational velocity. Impactors achieve collection by interposing
a target in the path of the dirty gas stream, causing those
aerosol particles with sufficient inertia to impact on the target.
Scrubbers depend upon the inertia of the aerosol particles and
the scrubbing droplets to bring about collision and consequent
capture of the aerosol particles.
Particles less than 5 ~ in diameter have a low mass to drag
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ratio. This is easily apparent from Table I which presents the
settling velocity of particles in air.
Table I
SETTLING VELOCITY OF 2.0
SPECIFIC GRAVITY PARTICLES IN AIR
0.01
0.1
1.0
5.0
10.0
50.0
100
Settling Velocity,
em/see
1.5 x 10-5
1.8 x 10-4
7 x 10-3
0.2
0.6
20
45
Diameter,
Microns
The problems encountered in attempting to separate 1 ~
diameter particles from a gas stream can be illustrated by
calculating some of the geometric parameters of a hypothetical
centrifugal separator. At low relative velocities the particle
will move through the medium at a rate directly proportional to
the force acting upon it. Thus, in a centrifugal field, in
differential form:
dR=V F
dt s c
(1)
where
R
V
s
F
c
t
= the radius of curvature of the stream path
= the settling velocity at one gravity of force
= the centrifugal force, gravities
= time
the centrifugal force, Fe is
V2
g R
c
(2)
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where gc is the gravitational constant, V is the linear velocity
and the residence time is
L
V
( 3)
where L is the path length, the integrated relationship is then
V 2 ~ -- V VL
t;.,R = Vs
gcR V S gcR
(4)
If we assume that a relative motion of one tenth of a foot is
required (equivalent to assuming a stream one-tenth
in width), a linear velocity of 100 feet per second
R, of 0.5 feet then
of a foot
and a radius,
7 x 10-3
0.1 = 30.5
100 L
x 32.2 x .5
L = 70 ft
That is, a path length of 70 ft would be required or more than
22 revolutions in a one-foot diameter cyclone. This would be
difficult to achieve and would require considerable energy input.
Also, to accomodate 146 cfm the cross sectional area of the flow
path would have to be
146 2
A = 100 x 60 = .025 ft
or 0.25 ft high and 0.1 ft wide. For 22 revolutions the cyclone
would have to be at least 22 x .25 or 5.5 ft tall. These figures
clearly indicate that cyclone type devices are not applicable to
the present problem.
Another possible geometric configuration for using centrifugal
forces is one in which the gas flows radially against the
centrifugal force. If we assume a cylinder of 0.5 ft in radius,
then the centrifugal force at 6000 rpm would be
= ro2Rq = (100 x 2IT)2
Fc gc 32.2 x g x 0.5 = 6100 gravities
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The maximum face velocity which would prevent passage of 1 micron
particles would be
7 x 10-3
Vmax = 6100 x 30.5 = 1.4 ft/sec
For a flow of 146 cfm the length required would be
146
L = 60 x x 1.4 = .55 ft
This size is feasible, that is a cylinder 1 ft in diameter
and 0.55 ft long could be accomodated in most motor vehicles.
However, operation at 6000 rpm for long periods of time would
require expensive construction to assure maintenance free
operation. Also, the device would be prohibitively large if it
were designed to be efficient on 0.1 micron particles which are
known to be present in high concentration in engine exhausts.
Small particles can also be collected by impingement. The
efficiency of impingement increases when the size of the target
decreases. This principle has been effectively developed for
certain applications (Ref. 10).
Consider a wire filament in a plane perpendicular to an
airstream containing aerosol droplets of various sizes. The
airstream will flow past the filament as shown diagrammatically
in Figure 1.
Particles of low inertia may be carried along the flow lines
and never reach the filament, but at a critical size the particle
inertia will be large enough for it to cross the flow lines and
impact on the filament.
It will be seen that d', the effective filament diameter,
is defined by the flow pattern for capture of particles not
crossing the flow lines. Thus d', shown in Figure I, is the
effective fiber diameter for particles smaller than or the same
size as particles just notcrossing flow lines; dD increases with
particles size until it reaches a maximum of (df + dp) where df
is filament diameter and dp is particle diameter, as shown in
Figure 2.
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Figure 1
SIMPLIFIED AIRFLOW PAST A FILAMENT
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-~7J_--
1-\7--
df -\- dp
Figure 2
COLLECTION DIAMETER OF A FILAMENT
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The ratio d'/df is defined as the efficiency of impaction,
Er' The impaction of aerosol particles by differently shaped
objects placed in their paths has been studied by Wong et al
(Ref. 11). They experimentally determined that the collection
efficiency is a function of the inertia parameter, W, and that
for circular cylinders the collection efficiency is 90% when W
is about 4.0. The inertia parameter, W, is defined by:
2
CppVdp
W = 18 d
Tl c
(5)
where
C is the Cunningham correction factor
Pp is the density of the aerosol particles, gm/cm3
V is the velocity of the particles relative to
the collector, cm/sec
Tl is the viscosity of the aerosol carrier gas, poise
dp is the diameter of the aerosol particle, cm
dc is the diameter of the collecting cylinder, cm
The experimental results of Wong et al agree well with
theoretical values of Equation 5, calculated by other workers.
However, even this technique is difficult to apply to low
mass submicron sized particles. rf a filament diameter of 0.01
cm is assumed to be the smallest practical diameter, it is
calculated that a rotational speed of at least 10,000 rpm is
required on a brush of at least 18.5 cm radius. These conditions
are judged to be too severe for maintenance free operation for
long periods of time.
Particles in the submicron size range can be effectively
separated by filtration and by electrostatic precipitation.
However, the fine filters required for submicron sized particles
plug easily and have a relatively high resistance to flow even
when clean. Filters for this application would require periodic
changing and would be expensive.
Electrostatic precipitators although possessing the
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capability of removing submicron particles can do so at the cost
of high space and initial cost requirements. Electrostatic
precipitators for collecting fly ash operate with retention times
of from 1 to 5 seconds (Ref. 12). Thus, to clean 146 scfm of
gas would require a minimum precipitator volume of 2.45 cubic
feet and this only if the gas were cooled to ambient conditions.
The necessary power conditioning equipment can be expected to be
expensive and periodic maintenance would be required since
efficiency would decrease if electrically resistive deposits
accumulated in the precipitator. For these reasons, the
electrostatic precipitator is not considered the best approach
to the present problem.
This research effort was undertaken to establish the
feasibility of developing two high efficiency collection devices
for the removal of particulate matter from internal combustion
engine exhausts. The first approach is thermal precipitation
which makes use of the phenomenon of particle migration and
deposition in a temperature gradient. The second technique is
based on sonic agglomeration in a fluidized bed.
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II.
EXPERIMENTAL PROCEDURES
Since it was not feasible to use automobile exhausts directly
in the present study, aerosols were generated having a particle
size range, concentrations, and other characteristics approximating
the lead based constituents found in automobile exhausts.
A.
Aerosol Generation
1.
Generation of Carbonaceous Aerosols
Initially, carbonaceous aerosols were generated by incomplete
combustion of benzene in a natural gas flame to test the thermal
packed bed device. Benzene was allowed to flow through a
capillary orifice by gravity feed and drip on the gas flame.
The resulting aerosol, consisting of submicron particles, was
directed into the packed bed as shown in Figure 3. The generation
of carbonaceous aerosols was stopped after preliminary tests, and
subsequent experiments were performed with lead chloride aerosols,
as suggested by the project officer to obtain data more
representative of motor vehicle exhaust.
2.
Generation of Lead Chloride Aerosols
The experimental procedure for generating submicron aerosols
by vaporization-condensation of lead chloride was similar to that
described by Espenscheid et aI, for producing sodium chloride
(Ref. 13) and silver chloride aerosols (Ref. 14). The technique
essentially consists of the passage of a carrier gas through a
furnace cOQtaining the metal halide. The carrier gas picks up
vapor of the material from which aerosol is to be formed in the
furnace. The gas-vapor mixture cools after leaving the furnace
and condensation of the vapor takes place to form aerosol
particles.
A schematic diagram of the experimental setup for producing
PbC12 aerosols by vaporization-condensation technique is shown in
Figure 4. A stream of dry filtered air at a flow rate of 20
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.....
.....
I:J
~ BLOWER I J !
I p , I
! / I pRIFICE
--DAMPER-, ! /'
__~-L1c-----._-~ -. -- -- ..-----------
\ \ (~jS+-_~__L ~
\ \ i' SAMPLING PROBE
\ PACKED. '--or. C (2)
~ \ \, F=--~-~ MANOMETER
~....~ - \ ""-, ~~T.C. (1)
\ t1 \ I I IT- -- -. -
:. -=- ~ "-.C" - -- CO :.::c ---F-~ =-=---:----.:_~nUi' - 11 :- -; ji:
. --- -----J ....
DISK-I ,------.-- -, -_. - i
VALVE SAMPLING PLUG VALVE
n PROBE
U BURNER
NATURAL GAS ==1 I
BENZENE
RESERVOIR
VENT
-------~
-------
,
Diluting \
",FlaSk ~I
t " . ---/ I
FURNACE. - I IJ il r
~~IiECORDER . i
i i I
j! I
I I rl
. u t PINCH VALVE
i ViNT
MANOMETER
Diluting
Air
......
N
Carrier
Air
~otarneter
Filter
fAir
I
I
1- ROTAMETER
l~
> Aerosol
-I
TEMPERATURE
CONTROLLER
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liters/minute was directed through a Vycor combustion tube placed
in a high temperature furnace. The temperature of the boat
containing PbCl2 was monitored by a chromel-alumel thermocouple
connected to the inside of the boat. The thermocouple output
was fed into a Wheelco model A4897 proportional temperature
controller* and a Mosley model 680 recorder**. A stainless steel
jacket surrounding the Vycor tube promoted uniform temperature
distribution and avoided rapid temperature changes in the boat.
The temperature of the melt in the boat was maintained within
+ 10°C of the desired value.
The gas-vapor mixture cooled on leaving the furnace forming
lead chloride aerosol by self nucleation. Aerosol coming out
of the generator was very concentrated and was diluted with a
stream of dry filtered air in a large mixing chamber. The
diameter and concentration of lead chloride particles was easily
controlled by adjusting the furnace temperature and the flow of
diluting air. This technique for generating PbCl2 aerosols
showed excellent reproducibility.
The aerosol from the dilution chamber was sampled with an
electrostatic precipitator. The particles were deposited on
electron-microscope grids coated with a thin film of carbon.
The grids were examined with a Hitachi (Model HS-6) electron
microscope and photomicrographs of the deposited particles were
taken.
The temperature of the furnace was varied at a constant
carrier gas flow rate of 20 liters/minute to study the effect of
furnace temperature on the particle size distribution of the
resulting aerosols. Typical electron photomicrographs of lead
chloride particles are shown in Figure 5. Size distributions of
the particles were determined from the electron photomicrographs.
*Manufactured by Wheelco Instruments, Div. of Barber Colman
Co., Rockford, Illinois.
**Manufactured by Mos+ey Div., Hewlett Packard, Pasadena,
California.
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1Q
I
r
10
I
I
P
J
~
, .
.' ~
.. ~ , .
o
,0
J
o
,
. -, -
, .
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.. '..
" ,,' ,
~. ",. ,
. '" -t:#' ,r -
--".~ ".
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.f>
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o
10
I Furnace: 390°C
Carrier Flow: 20 l/min
MMD. 0. 216J..l.
10,000 Magnification
1 cm= IJ..l.'
)
I
!
In
_J
t"
18
,0
,
10
~.
o
o
o
o
o
o
-~-
'.
. .
.
Furnace: 450°C
Carrier Flow: 20 l/min
MMD. 0.4871-1
5,000 Magnification
1 cm = 2.01-1
Figure 5:
PhotomJ..crogTaph of Lead Chloride Particles
at Different Furnace Temperatures.
-------
Table II shows a typical particle size distribution obtained
during the calibration of the aerosol generator. Figure 6 shows
the size distribution of aerosol particles at four furnace temp-
eratures on log' probability paper.
The effect of furnace temperature on frequency mean diameter
and mass mean diameter has been plotted on Figures 7 and 8
respectively. Figure 8 was used to g'enerate aerosols of known
mass mean diameters during subsequent experiments with the packed
bed and the fluidized bed.
B.
AnalvticalTechniques
The concentration of carbonaceous aerosols was determined
using a reflectometer. The concentration of PbC12 aerosols was
determined using polarographic and colorimetric techniques. The
experimental procedures and calibration curves are g'iven in
Appendix I.
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Table II
A TYPICAL PARTICLE SIZE DISTRIBUTION OF LEAD AEROSOLS
Furnace Temp. = 468°C
Carrier Gas Flow = 20 lit/min
Dilution Flow = 16 lit/min
% Equal to or
Number Cumulative Greater Than
Size, ~ Count Count Indicated Size
0.025-0.049 30 30 76
0.049-0.098 58 88 29.6
0.098-0.196 26 114 8.8
0.196-0.295 9 123 1.6
0.295-0.394 1 124 0.8
n+l 125
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100
70
50
40
30
N
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1-1
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Percent Particles Greater than the Stated Size
""''',,-
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. T -
Furnace-
AT =
Furnace
eT -
Furnace-
. T -
Furnace-
550°C
5l8°C
468°C
393°C
10
'"
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95
99
99.9
Figure 6 - Particl,: Size Distributions of Lead'Chloride Aerosols at Different
-------
0.25
0.20
::L
...
Sol
Q)
+J 0.15
Q)
e <-
10
-rot
Q-
...... :>.
00 ()
~
Q)
::s
tJI
Q) 0.10
Sol
~
~
10
Q)
~
0.0
0.03
-
-
---
320
350
-
I
/
/
/
I I
400 450
Furnace Temperature, °C
I
500
I
550
.
600
-------
0.5
.... ::i.
\0 -..
501 0.4
GJ
4J
~
IU
--ri
Q
-s;: 0.3
IU
GJ
~
(I)
-(I)
IU
X
0.2
.,------- -----.
350
400 450
Furnace Temperature, °C
500
550
600
-------
III. THERMAL PACKED BED DEVICE
The motion of small particles suspended in a gas stream
under the influence of thermal gradients has been the subject
of many investigations (Ref. 15,16,17). Particles move from hot
gas streams to cold surfaces under the influence of thermal
forces. A particle in a thermal gradient can be expected to be
hotter on one side than on the other. Gas molecules striking
the hot side, on the average, will rebound at a higher temperature
and hence, a higher velocity than those striking the cold side.
This imbalance creates a force on the particle directed toward
the colder end of the temperature gradient. This phenomenon is
well known and is used in commercially available thermal
precipitators used to collect submicron sized aerosols with
virtually 100 percent efficiency. A more familiar example of
the same phenomenon is the cold wall in a room which always gets
dirtier sooner than the warmer walls.
Although the thermal precipitation effect is well known and
apparently has been shown to be effective to some degree for the
present application, the principle has not yet been applied to
an optimum degree. Best results can be expected when thermal
gradients are maximized and migration distance minimized. A
packed bed of high heat capacity material appears to have these
desired properties.
Description of Experimental Setup for Thermal Deposition
Studies
A.
The laboratory experimental setup of the apparatus for
thermal deposition studies in a packed bed is shown in Figure 9.
The packed bed device, 28 cm high and 12.1 cm diameter, was
constructed from 11 gauge, 304 stainless steel to eliminate
corrosion of the inside walls. A coarse retaining screen was
welded to the bottom of the shell. The cylindrical shell was
packed with the desired packing material by pouring the material
from the top and tapping the sides of the bed to obtain uniform
lIT RESEARCH INSTITUTE
-------
I FURNACE
'"
......
Carrier
Air
Rotameter
Filter
fAir
ROT Al1ETER
Figure 9:
'f
4
Manometer
t .' - ~
'
Room
Ai\
L0-
Sampling
Filters
( ~
Aerosol
--
~ I
Gas- -g CJ
I
~ 0
t Pinch Burner I >-
valve Sampling
Filters
Vent
Apparatus for Study of Thermal Deposition of Lead
-------
packing density within the bed. The packing height was kept
constant for all tests and the weight of different packing
materials to fill the bed to a depth of 7.6 cm above the retaining
screen was determined. The bed was insulated on the outside with
a 1.5 cm layer of pipe insulation to prevent particle deposition
on the walls and to minimize the heat losses.
The unit was designed to allow simultaneous determinations
of the upstream gas temperature (Tl)' apparent bed temperature
(T2), and the effluent gas temperature (T3). The temperature
profiles Tl and T2 with time were monitored simultaneously with
a Bristol multipoint recorder*. The downstream effluent
temperature (T3) was recorded using a Mosley 620 recorder**.
A small portion of the aerosol stream leaving the dilution
flask was fed into the simulated exhaust pipe where it was mixed
with room air. The blower on top of the packed bed was turned
on along with a burner which heated the room air stream. The
flow rate of air containing PbC12 particles through the bed was
controlled by adjusting a damper downstream of the bed. The
flow rate was determined by monitoring the pressure drop across
a 2.5 cm diameter calibrated orifice. This way the superficial
velocity through the bed could be varied from 15 cm/sec to
130 cm/sec. The pressure drop across the packing was also
recorded.
B.
Determination of the Collection Efficiency of the Thermal
Packed Bed Device
The collection efficiency of the packed bed device was
determined by simultaneous sampling of the gas stream both
upstream and downstream of the bed. Identical sampling lines
were constructed from 6 rom stainless steel tubing and connected
to sampling probes inserted both upstream and downstream of the
*Manufactured by the Bristol Co., Waterbury, Connecticut
**Manufactured by Hewlett Packard, Pasadena, California
liT RESEARCH INSTITUTE
-------
packed bed. The sampling probes could be moved in the radial
direction to get a complete concentration traverse. Initial
tests showed that aerosol concentration did not change with
radial position of the probe. All subsequent sampling was
performed by positioning the probes at the axis of the bed.
However, the sampling was not isokinetic. Since isokinetic
sampling is unimportant for submicron particles, anisokinetic
sampling did not introduce any significant error in concentration
measurements. The other ends of the sampling lines were
connected to sampling banks and all exposed portions of the lines
were kept at 300°C with electrical heating tapes to prevent
particle loss due to thermal deposition on the inside walls of
the sampling lines. Each sampling bank contained three Millipore
filter holders with 2.5 cm diameter Gelman Type A fiber glass
filters. The filter holders were connected to solenoid valves
to permit simultaneous sampling of aerosol streams on both ends
of the bed. The sampling banks were connected to a vacuum pump
through calibrated flow restriction orifices. This sampling
procedure allowed three sets of samples to be taken during a test
run. The downstream orifice was selected to sample at a higher
flow rate than the upstream orifice because of low concentrations
to be expected at the downstream of the bed. A typical test run
with the packed bed proceeded as follows:
The aerosol generator furnace temperature was preset on the
temperature control unit to give the desired aerosol size using
Figure 8. The carrier and dilution air flow rates were adjusted
to 20 and 16 lpm respectively.
At steady furnace temperature, the aerosol stream from the
dilution chamber was connected to the bed and mixed with heated
room air in the simulated exhaust pipe. The damper was adjusted
to give the desired gas velocity through the bed and the
temperature recorders for monitoring Tl' T2 and T3 were engaged.
The upstream gas temperature, Tl' was brought to the desired
value by adjusting the gas burner. The bed temperature, T2' at
liT RESEARCH INSTITUTE
-------
the onset of the test run was low and the gas-packing temperature
differential (Tl-T2) was large. The bed heated up with time on
the passage of hot air containing lead chloride particles, thus
reducing the gas-packing temperature difference.
Aerosol samples were collected both upstream and downstream
of the bed for one minute by activating a set of solenoid valves.
Temperatures Tl' T2 and T3 were marked on the charts. The same
procedure was repeated at two higher bed temperatures to obtain
three sets of samples during a test run. At the completion of
a test run, the aerosol stream was disconnected, the burner was
shut off, and the bed allowed to cool.
Thre.e sets of samples obtained at different bT values were
then analyzed for lead using either the polarographic or the
colorimetric technique (see Appendix I). The concentration of
PbC12 in the gas stream was calculated from the analytical
results and the volume of aerosol sampled. The collection
. .
E, of the thermal packed bed device was calculated
efficiency,
from:
Co - Cl
E = C x 100%
o
(6)
where,
Co = upstream aerosol concentration, ~g PbC12/liter
Cl = downstream aerosol concentration, ~g PbC12/liter
C.
Experimental Data and Discussion of Results
Initially tests were conducted using carbonaceous aerosols
and a bed packed with 8 rom glass helices to a depth of 5.7 cm.
Filter samples were collected on both the upstream and the
downstream of the bed at a gas-packing temperature difference
of 600°C.
The reflectivity of the filters shown in Figure 10 was
measured with a reflectometer. The collection efficiency was
calculated from the reflectance measurements using Equation 4-A
(See Appendix I). The collection efficiency of the thermal
II T RES EAR CHI N S T IT UTI!
-------
.--
I
,0
j
!o
,r
~.
[
r
.........,
C
C
i .c
I I
I !
I
I[
[
'e
I
I
IC
I
,[
C
'e
c
t
A
B
Test 1 - A - Upstream sample, 2% Reflectivity
B - Downstream sample, 31% Reflectivity
1---
"I
,0'
-..~'
-.
I '
~
'I
. I
fl
Ilt~!
::'
'~.,
I
I
l
.:1;,'" .
A
B
[I
U;
I
Test 2 - A - Upstream sample, 2% Reflectivity
B - Downstream sample, 31% Reflectivity
Ie
IC
I
Figure 10 - Filters from Thermal System Tests
.0.
-------
packed bed device for carbonaceous particles was found to be
68%. This value of collection efficiency is not very accurate
since the Beer-Lambert Law does not apply to very dark surfaces.
Subsequent experimental work was done using PbCl2 aerosols as
the test material.
A series of experimental tests were performed with the
thermal packed bed to study the effect of particle size, size and
shape of the packing, heat capacity of the packing material, gas
velocity through the bed, concentration of the aerosol,
contamination buildup on the packing material, and the gas-packing
temperature difference on the collection efficiency of the bed.
All experimental data obtained using PbCl2 particles as test
aerosols are presented in Table III.
During the course of an experiment 6T changes not only with
time but also with position within the bed because the upstream
side of the packing is heated at a faster rate than the downstream
side. Moreover, a thermocouple inserted into the bed does not
accurately measure the packing surface temperature. Although
the relationship between thermocouple reading to the bed surface
temperature is not known, the thermocouple reading can be used as
a parameter since a definite relationship between the two values
must exist. The gas-packing temperature difference is a measure
of the thermal force acting between the aerosol particles and the
packing surface. Therefore, all collection efficiency data have
been correlated with the apparent gas-packing temperature
difference 6T, (TI-T2).
In Table III, the temperatures TI' T2 and T3 refer to the
upstream, bed and downstream temperatures, respectively. Since
obtaining a sufficiently large aerosol sample for analysis
required sampling for periods from several seconds to one minute,
these temperatures changed slightly during the sampling period.
Therefore, the values indicated in Tab~e III represent time average
values of the temperatures during the sampling period.
The velocity through the bed, VB' has been calculated as a
lIT RESEARCH INSTITUTE
-------
Table III
COLLECTION EFFICIENCY DATA ON THE THERMAL PACKED BED DEVICE
Tl! T2' T3' LIT 1-2 MMD VB l\Pbed Co "
Test No. ~ ~ ~ ~ -4L- cm/sec cm H20 ILq/liter EFF Packinq
2-14-70 27 27 27 0 0.45 15.5 0.73 29.4 41 Helices
2-14 27 27 27 0 0.45 15.5 0.73 16.0 37 Helices
2-14 27 27 27 0 0.45 15.5 0.73 14.6 34 Helices
2-19 220 96 34 124 0.48 26.2 0.73 29.3 87 Helices
234 156 42 78 0.48 27.4 0.73 61.0 66 Helices
266 208 67 58 0'.48 27.0 0.73 17.1 69 Helices
2-20 234 170 30 167 0.30 20.7 1.04 2.4 >95 Helices
226 138 42 88 0.30 20.7 1.02 2.2 71 ~s
226 178 61 48 0.30 24.9 .97 2.7 66 c s
2-27 227 103 45 125 0.30 20.6 ;97 3.2 66 Helfces
227 170 73 58 0.30 22.4 .97 2.4 57 Helices
222 211 103 58 0.30 16.1 ..97 2.4 5 He1.ices
3-2 191 55 32 147 0.35 25.7 1.2 1.2 95 He,lices
218 134 59 84 0.35 27.7 .1.5 1.2 90 ~lices
226 205 128 20 0.35 26.4 1.3 1.2 12 lices
3-3 231 107 39 124 0.54 28.8 1.5 12.2 66 elices
222 182 63 40 0.54 27.( 1.5 17.1 50 Helices
238 218 113 20 0.54 25.6 1.5 14.6 42 Helices
3-6 237 80 33 57 0.47 23.4 1.8 7.3 87 Helices
266 167 50 99 0.50 24.5 1.8 14.6 54 Helices
274 228 88 46 0.50 27.0 1.8 14.6 53 Helices
3-10 231 70 34 161 0.36 25.4 2.5 1.2 >95 Helices
225 150 43 75 0.36 25.4 2.5 1.6 85 Helices
230 215 77 153 0.36 22.3 2.5 1.2 68 Helices
3-30 226 33 30 193 0.33 15.5 0.71 1.2 95 6mm Glass
Spheres
226 41 30 185 0.33 15.5 0.71 1.6 85 j
218 50 30 168 0.33 15.5 0.71 1.2 69
4-14 207 41 30 166 0.45 15.5 0.71 14.6 66
223 131 66 92 0.45 15.5 0.71 9.8 67
218 200 126 18 0.45 15.5 0.71 29. 33
7-15 211 25 27 186 0.45 15.5 0.63 24.4 38
222 100 57 122 0.45 15.5 0.63 24. 40
215 160 83 55 0.45 15.5 0.68 26. 30
6-23-70/1 27 27 27 0 0.35 15.5 0.56 35 27 6mm Ceramic
Spheres
6-23-70/2 218 40 30 178 0.35 15.5 0.56 51 95
222 142 42 80 0.35 15.5 0.56 39 34
211 195 105 16 0.35 15.5 0.56 41.5 24
6-25-70/1 225 80 30 145 0.35 15.5 0.56 74 77
6-25-70/3 231 46 26 185 0.35 15.5 0.56 9 >95
222 101 31 121 0.35 15.5 0.56 25 56
8-4 25 25 25 0 0.28 15.5 0.56 1 26
8-5-70/1 237 33 33 234 0.28 15.5 0.56 1 >95
218 109 99 109 0.28 15.5 0.56 1 50
226 146 156 80 0.28 15.5 0.56 1 30
8-5-70/2 218 59 35 159 0.28 15.5 0.56 1 72
222 128 61 94 0.28 15.5 0.56 1 46
222 170 101 52 0.28 15.5 0.56 1 27
8-6-70/1 236 36 35 200 0.35 15.5 0.56 21 97 6mm Steel
Spheres
226 41 35 185 0.35 15.5 0.56 24 84
231 76 35 155 0.35 15.5 0.56 27 83
8-6-70/2 227 99 56 128 0.35 15.5 0.56 22 75
227 151 81 76 0.35 15.5 0.56 23 36
227 185 108 42 0.35 15.5 0.56 10 30
8-13-70/1 25 25 25 0 0.35 15.5 0.56 156 27
8-13-70/2 226 135 25 91 0.35 15.5 0.56 91 34
218 165 25 53 0.35 15.5 0.56 100 24
8-13-70/3 223 34 25 189 0.35 15.5 0.56 130 84
218 55 25 163 0.35 15.5 0.56 96
215 101 25 114 0.35 15.5 0.56 118 44
8-17 218 57 39 161 0.35 89 6 5 73
229 143 74 86 0.35 89 6 7 42
8-20 215 66 40 149 0.35 89 6 87 81
218 154 81 64 0.35 89 6 103
222 194 124 28 0.35 89 6 56 28
8-28 222 183 118 39 0.35 89 6 76 35
9-3-70/1 25 25 25 0 0.43 129 10.2 40.5 34
9-3-70/2 223 97 56 12(\ OA3 129 10.2 140 59
9-8-70/1 221 76 40 145 0.43 129 10.2 74 51
243 165 100 78 0.43 129 10.2 59 24
9-9 211 41 95 170 0.43 129 10~2 46 59
264 47 34 217 0.43 129 10.2 22 87
9-11 198 28 25 170 0.33 15.5 0.5 102 98 Contaminate
6 mm Steel
Spheres
222 63 34 159 0.33 15.5 0.5 102 93 1
215 100 49 115 0.33 15.5 0.5 110 70
231 143 75 88 0.33 15.5 0.5 96 37
213 174 100 39 0.33 15.5 0.5 150 37
-------
superficial velocity as if the packing occupied no volume, and
with the assumption that the gas was at temperature Tl'
1.
Effect of Particle Size on Collection Efficiency
The deposition of submicron particles under the influence
of a thermal force is believed to be independent of particle
size. This was confirmed during initial tests when aerosol
samples collected on the upstream and downstream of the bed were
examined with an electron microscope. While the concentration
of particles in the downstream samples was much smaller than the
upstream samples, the two size distributions were found to be
nearly identical. This suggests that particles in the size range
0.1-0.8 ~ were collected by the packed bed device with nearly
the same efficiency. Further tests were conducted using aerosols
having 0.30-0.55 ~ mass mean diameters.
2.
Effect of Surface Area and Shape of the Packinq on
Collection Efficiency
The effect of surface area and shape of the packing on
collection efficiency of the thermal packed bed device was
determined using 8 rom diameter glass helices and 6 rom diameter
glass beads. Experiments were performed by loading the bed with
either packing to a depth of 7.6 cm. The total surface area of
the packings in the bed was calculated to be 2.94 x 104 cm2 and
0.54 x 104 cm2 for the helices and beads respectively. Tests
were conducted at a constant gas velocity of 15.5 cm/sec using
PbC12 aerosols in the size range 0.30-0.55 ~ MMD.
The effect of packing shape and surface area on collection
efficiency is shown in Figures 11 and 12. Figure 11 shows that
the collection efficiency of helices is higher than that of glass
beads. The figure also shows that the collection efficiency of
helices increases more rapidly with increasing 6T compared to
glass beads. This can be explained as follows:
The helices, which pack more compactly compared to spherical
lIT RESEARCH INSTITUTE
-------
100
90
tV 80
\0
70
60
:>1
u
s:: 50
Q)
-.-I
U
..-t 4
~
~
~
~ 30
20
10
Bed Depth = 7.6 Cm
Gas Velocity = 15 - 30 em/see
.
.
Glass Helices
6mm Glass Balls
.
~. ./
///
.
50
--,
100
t:"T °C
150
200
Figure 11 - Effect of the Shape of the Packing On Collection Efficiency of PbC12 ~articles
-------
100
~
..
>t 90
u
~ 80i
~ 70
~
B 60
-ri
W +'
a u 50-
w
rl
.....
8 40
+'
~ 30
u
~ 20
Po.
10
Bed Depth = 7.6 em
Gas Velocity = 15.5 Cm/Sec
--" ------.-----....
.- _." -- --...------ -- -_.----
-m--
-----8- ---------.----- ----
f'::,T = 200°C
~
---- -
------
- ..- ...-.-.- --
-,,--"-
------ - --ioo ° C
----- b T :::
---~
------~
~
~---
--------,-c-
0.5
.....---- -'-'-'-~ - -- --- - -- - -j---- -- - n'- - - -.
-.-----~_. -- -- -...- -'-'------r--~'-'---- ----.,--
1 1.5 2
Surface Area of the Packing, Sp x 104 em2
2.5
3
Figure 12- Effect of Surface Area on Collection Efficiency of PbC12 Particles
-------
beads, have a higher collection efficiency due to the increased
filtration effect in the bed. Moreover, in the case of helices
the particle migration distance is very much smaller than that
for spheres, causing a higher collection efficiency by thermal
deposition. Figure 12 shows the effect of surface area on
collection efficiency at different values of ~T. At low value
of ~T (100°C) the collection efficiency of the bed increases
rapidly with increasing surface area compared to that at higher
value of ~T (200°C). It seems that at high gas-packing temperature
differences, the temperature gradient at the packing surface
becomes the controlling particle deposition mechanism, and surface
area of the packing has a relatively small effect.
3.
Effect of Heat Capacity of the Packinq Material on
Collection Efficiency
Effect of heat capacity of the packing on collection
efficiency of the thermal packed bed device was determined using
6 rom glass, ceramic, and chrome-steel balls having heat capacities
of 0.16, 0.26 and 0.11 gm calories/gmOC respectively. The bed
height and gas velocity were kept constant at 7.6 cm and 15.5
cm/sec. Experiments were performed using aerosols in the size
range 0.30-0.45 ~ MMD at a concentration of approximately
12-25 ~g PbC12/liter. Figure 13 shows that the collection
efficiency of the bed is independent of the packing material at
small values of ~T «80°C). This would be expected since at low
gas-packing temperature differences thermal forces are negligible
and the particle capture is due to aerodynamic forces only. At
constant bed height, gas flow rate and packing shape, the
aerodynamic capture of the particles would be independent of the
heat capacity of the material. At higher values of 6T, the
collection efficiency of the bed seems to depend on the nature
of the packing material. However, there is no systematic change
in collection efficiency with heat capacity of the packing material.
The bed packed with 6 rom glass beads, which have an intermediate
liT RESEARCH INSTITUTE
-------
Bed Depth = 7.6 em
Aerosol Velocity = 15.5 Cm/Sec
. 6 MM Steel C = 0.11 ca1/gmOC
p
. 6 MM Ceramic C = o. 26 "
p
. 6 MM Glass C = 0.16 "
p
100 I
901
J
UJ 701
. ~ 1
60""
~ I
; 50.J
~ 40!
q.~l
~... ,
~ 30t-
20-
10 I
h
.
.. --..
.
. .
---~.__.~._--,..-....-._--------
, , I
200
240
40
60
80
100
120
/j"T °C
140
160
180
Fig. 13 - Effect of Bed Material on Collection Efficiency of the Thermal Bed
-------
value of heat capacity between steel and ceramic balls, has the
lowest collection efficiency for all values of 6T >80°C. The
higher collection efficiency with ceramic packing is attributed
to the rough surface characteristics of the ceramic balls
compared to steel and glass balls.
Figure 14 shows the effect of packing material on the
heating characteristics of the bed packed to a constant height
of 7.6 cm. Even though the packings occupied a constant volume
in the bed their mass was different due to variations in density.
Therefore, the total heat capacity of the bed varied
significantly for the three materials used as shown in Table IV.
Table IV
EFFECT OF PACKING MATERIAL
ON THE TOTAL HEAT CAPACITY OF THE BED
Bed Diameter = 12.1 cm, Bed Height = 7.6 cm
Gas Velocity = 15.5 cm/sec
Mass of Heat Total
Packing Capacity, Heat Capacity
In The Bed, gm Of The Bed
Packinq Material qm Calories/qmOC qm Calories/oC
6 rom Glass 1360 0.16 217.6
Spheres
6 rom Ceramic 1176 0.26 305.8
Spheres
6 rom Steel 8607 0.11 946.8
Spheres
From the heating curves shown in Figure 14, it is evident
that it is possible to maintain a high gas-packing temperature
difference for a longer period of time by using a high heat
capacity bed. Figure 15 shows the change in collection
efficiency of the packed bed device with time. Initially, when
the bed is cold (or gas-packing temperature difference is high),
the collection efficiency of the device is high for all packings.
liT RESEARCH INSTITUTE
-------
w
~
u
o
.. 216
Q)
~ 180
1;; 144
~
8. 108
m 72
E-4
36
Bed Height. 7.6 em
Bed Velocity - 15.5 em/see
--End of Heating cycle for Glass Packing
/
/
Glass Spheres (T2)
- Ceramic
End of Heating cycle for Ceramic Packing
--
......-
o
10
20
Spheres (T2)
~ Upstream Temperature (Tl)
~ ~ End of Heating cycle for Steel
Packing
Carbon Contaminated
Spheres (T2)
Clean Steel
Spheres (T2)
Steel
'---------
- -
---
-- ---~
30 40
Time, Minutes
60
70
50
Figure 14: Effect of Heat Capacity of Packing Material
-------
Bed Height = 7.6 em
Gas Velocity = 15.5 cm(Sec
Upstream Gas Temperature (Tl) ~ 200°C
.
6 rom ceramic spheres
.
6 rom glass spheres
. 6 rom steel spheres
100
90
:>t
0
t: 80
Q)
-1"/
UJ 0
111 '1"/ 70
~
~
riI
~ 60.
50
40
\
\. \.
~\ 6''',.
\. '~
D "" . "'--.
"'- -------
---- "--' '0 -i'--- -.t'
.
3°1
201
I
J-~--~.~
o 4 8 12
-_. ........ _. _. .
. I
I - -.--. -'-,--.-.. .-.. -.---'- ---"--'----r-'--
16
20
24
28
32
36
Time, Minutes
Figure 15:
Collection Efficiency of the Thermal Packed Bed Device as a
-------
The collection efficiency of beds packed with ceramic or glass
spheres, however, decreases rapidly with time compared to the
bed packed with steel spheres. Figure 15 also shows that beds
packed with steel spheres will operate with efficiencies greater
than 90% for four minutes compared to 1.5 minutes for beds
containing glass or ceramic packings.
4.
Effect of Gas Velocity on Collection Efficiency
The effect of superficial gas velocity on the collection
efficiency of the thermal packed bed device was studied using
6 rom steel balls as the packing material. Figure 16 shows that
there is no apparent change in collection efficiency with
increase in gas velocity from 15.5 to 89 cm/sec. However, at a
gas velocity of 130 cm/sec the collection of the bed decreased
by 10-15%. This is attributed to re-entrainment of deposited
particles at high gas velocities.
5.
Effect of Aerosol Concentration on Collection Efficiency
Figure 17 shows the effect of aerosol concentration on the
collection efficiency of the thermal packed bed device. Varying
the concentration from 1-125 ~ PbC12/liter has no effect on the
collection efficiency of the bed packed with 6 rom steel balls to
a depth of 7.6 cm and gas velocity of 15.5 cm/sec. The
collection efficiency is independent of aerosol concentration
over all values of gas-packing temperature difference. Therefore,
one may write
dC
- = -Kc
dt
(7)
where
C = aerosol concentration,
flg PbC12/liter
t = time, second
K = thermal deposition coefficient, second-l
/IT RESEARCH INSTITUTE
-------
Packing Material = 6 rom Steel Balls
Bed Depth = 7.6 em
Concentration Range = 1 - 125 ~g/liter
. 15.5 em/See
.89 em/See
.. 130 em/See
100 ----
90 -----
80
w
-...]
70
60
:>,
u
~ 50
OJ
.,.j
U
'r-! 40.
4-1
4-1
fiI
~ 30
20
10
, I I , I .... I I 240'
20 40 60 80 100 120 140 160 180 200 220
~T "C
-------
w
(X)
,
100 -I
90 ~
I
80 ~
f
I
,
70 ~
i
1
1
I
!
30 ~
:: j
i
>t
()
~ 60
Q)
.r-i
o
~ 50
4-1
ILl 40
~
Packing Material = 6 rom Steel Balls
Bed Depth = 7.6 em
Aerosol Velocity = 15.5 em/See
o
@ 1 ~g PbC12/1iter
@ 25 ~g PbC12/1iter
@ 125 ~g PbC12/1iter
~
o
/
,/'
l";)
.~,.//
./-
.....9//
o
~'
~/ GJ
_A-e-----/- G
.---- -- 0
01-- --- -20-------40------6'0 .--- -80----1-00--
~T °C
8
//
", (:)
Q
/"
.,/
140
I '-----r-------,~
200
- -T.-~W---- .,.-~--'-
120
160
180
Figure 17 - Effect of Aerosol Concentration on Collection Efficiency of the Thermal Bed
220
-------
The rate equation can be integrated to give
C
In - = -Kt
Co
(8)
In the thermal packed bed device, K would be a function of
6T essentially, because aerosol concentration, particle size,
heat capacity of the packing and gas velocity have a negligible
effect. At very high gas velocities, the particle deposition
would decrease due to re-entrainment.
6.
Effect of Contamination Build-Up on Collection
Efficiency
The bed packed with 6 rom steel balls was contaminated with
carbon soot from an oil lamp placed below the bed. The blower
was left on to get an even coating of carbonaceous particles
throughout the bed until visual examination showed that the bed
had been contaminated quite heavily.
The heating characteristic of the contaminated bed has been
plotted in Figure 14 which shows that the contaminated bed heats
more slowly than the clean bed. The effect of carbon contamination
on collection efficiency of the thermal packed bed device is
shown in Figure 18. The collection efficiency of contaminated
bed is hi~her than that of a clean bed. The contaminated bed
is expected to have a higher surface area and rough packing
surface. These effects would tend to increase the aerodynamic
filtration efficiency of the bed.
Samples collected upstream and downstream of the contaminated
bed were examined to see if any re-entrainment of contamination
particles was taking place. There was no difference in the
reflectance of the upstream and downstream samples. This
confirmed that there was no significant dislodging of carbon
particles. The filter samples were also examined with an optical
microscope which showed there were very few carbon particles on
the downstream samples. Photomicrographs shown in Figure 19 show
liT RESEARCH INSTITUTE
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~
o
100
90
80
70
60
>,
o
s::
Q)
""
U.
-rot
14-1
14-1
~
~
50
40
20
10
20
Bed Depth = 7.6 Om
Aerosol Velocity = 15.5 Cm/Sec
o
o
Carbon Contaminated 6 MM Steel Balls
Clean 6 MM Steel Balls
."
41)--"T----'6b
140
260
160
ldo
120
8'0
180
t:,TOC
Figure 18- Effect of Carbon Contamination on Collection Efficiency of the The~mal Bed
220
-------
Figure 19 - Carbon Particles Coming Out of the Contaminated Bed
-------
two such particles. It must be noted that the number of carbon
particles disloQged wa~ small and their size was greater than
10~. It is possible that large carbon particles dislodging from
the bed would carry some lead particles that had deposited in the
bed.
7.
Effect of Gas-Packinq Temperature Difference on
Collection Efficiency
The gas-packing temperature difference, 6T, is a measure of
the thermal force which causes particles to move from a hot gas
stream to the relatively cold packing surface. All the figures
showing the effect of 6T on.the collection efficiency have a
typical S shape. At small values of 6T «80°C), the thermal
deposition is insignificant compared to aerodynamic capture of
the particles, i.e.., at low values of 6T the packed bed
essentially behaves as a coarse filter. However, as the gas-
packing temperature difference increases thermal deposition
starts contributing to the deposition processes and at high values
of 6T (>200°C) the packed bed attains an efficiency of nearly
100%. The overall efficiency Eo' of the packed bed device can
be expressed as the sum of two efficiencies as
Eo = EA + ET
(9 )
where
EA=
collection efficiency of the bed due to aerodynamic
effects only::;: collection efficiency of the bed at
6T =00c. .. .
collection efficiency of the bed due to thermal forces.
ET =
It should be noted that EA would be independent of gas-
packing temperature difference and Err would primarily depend on
the gas-packing temperature difference.
From Equation 8 we get
ET = Co-C ::;: I - e -K7 .
Co
(10)
liT RESEARCH INSTITUTE
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where
K = thermal deposition coefficient, second-l
~ = particle residence time in the bed, second
7 has a value of
spheres at a gas
9 and 10, we get
0.2 seconds for a bed packed to 7.6 cm with 6 mm
velocity of 15.5 cm/second. Combining Equations
E
o
- E = 1 -
A
-K1'
e
(11)
The values of thermal deposition coefficient K have been
calculated using equation 11 from the experimental values of
Eo' EA' l' and 6T. Figure 20 shows the effect of 6T on thermal
disposition coefficient K. The deposition coefficient is seen
to increase very rapidly with the gas-packing temperature
difference and approaches an asymptotic value at 6T >200°C which
corresponds to virtually a 100% collection efficiency.
D.
Compatibility of the Thermal Packed Bed Device with
Automotive Systems
Experimental results in the preceeding section show that the
most important variable affecting the collection efficiency of
the thermal packed bed device is the gas-packing temperature
difference. It is essential to maintain temperature differentials
in excess of 200°C to operate the device with nearly 100%
efficiency. Aerosol concentration and particle size have been
found to have no significant effect on the performance of the
device. Experimental results also show that the thermal packed
bed device can be operated with high collection efficiency at gas
velocities in the range 15 - 100 cm/sec. These experimental
findings show that the mechanism of thermal deposition can be
exploited by allowing the hot exhaust gases to flow past a cold
packed bed maintaining a high temperature gradient.
It is essential that the thermal packed bed device be located
downstream of the manifold where the exhaust gas temperature
difference would be sufficient to insure virtually 100% collection.
liT RESEARCH INSTITUTE
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10
7
5
4
3
2
Packing Material = 6 rom Steel
Bed Depth = 7.6'Cm
ID
- .7
r-I
'
.
0 ..5 0
CD
U) .~
-
~
.. ..3
~
~ /
CD .2 / 0
ori I
0 /
ori .
~ /
~
CD I
8 I
.8 ;'
~
0
ori .)
+J .01
.~
U)
~ .05
.04
r-I
fU .03
e
~
CD
~ .02
40
8,0
6T °C
200
Figure 20 '- Effect of Gas-Packing Temperature Difference
on rh~rmal Deposition Coefficient
.44
-------
Moreover, the vapor pressure of lead compounds at 300°C is low
(0.001 rom Hg) and most of the lead in exhaust gas is in the
particulate form. Under these conditions one can calculate the
size of the packed bed device needed to process automobile exhaust
as follows:
Exhaust rate (at standard conditions) = 9-95 l/sec (Ref. 1)
Operating temperature = 250°C
Exhaust rate (at operating conditions) = 17-182 l/sec
It is reasonable to assume that the maximum permissible
superficial gas velocity through the bed is 100 em/sec. Then Db'
the diameter of the thermal packed bed device, can be calculated
from
D =V40
b TN
s
(12)
where 0 is the exhaust rate in cm3/sec and Vs is the superficial
gas velocity through the bed in em/sec. Substituting for 0 and
Vs in Equation 12 we get
Db ="""\ /4 x 182,000 = 48 em
V Tr x 100
Therefore, a packed bed at least 48 em in diameter would be needed
to process the maximum exhaust emissions from an automobile engine.
At the other extreme, where the exhaust rate is low (17 l/sec),
the gas velocity through a 48 em diameter bed would be
approximately 9.5 em/sec. The thermal packed bed device should
be very effective at a low gas velocity of 9.5 em/see, since
the residence time,~, of exhaust gas in the bed would be large.
The product K7 in Equation 11 would also be large, increasing the
collection efficiency of the device. Therefore, a 48 em diameter
bed could be packed to a height of about 8 em with a suitable
packing material to obtain a high collection efficiency with
moderate pressure drop. The packing material should have a high
surface area to volume ratio and the heat capacity of the bed
would have to be quite large to maintain a high gas-packing
lIT RESEARCH INSTITUTE
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temperature differential for a long period. It is possible to
use metal turnings or a fine packing material with high surface
to volume ratio. The optimum size and shape of the packing
material would have to be ultimately balanced against the
maximum permissible pressure drop in the bed, which increases
rapidly with increase in surface to volume ratio of the packing.
Moreover, since the bed vrould heat with time on the passage of
hot exhaust gases, a technique needs to be devised to permit the
operation of the device maintaining a large gas-packing
temperature differential all the time. One method for
accomplishing this desired result in an automotive application
is shown schematically in Figure 21.
Two packed beds would be used intermittently to clean the
gas. As the exhaust gas flows through bed B, bed A is cooled
by a flow of relatively cold outside air. When the temperature
sensor above bed B indicates that the bed is too warm to operate
efficiently the butterfly valve changes to channel the flow to
the second bed A.
An alternate approach would be to cool the bed internally.
If a suitable technique could be devised for internal cooling of
the bed, then only one packed bed would be required to collect
particles from auto exhausts with a high collection efficiency.
In time, the beds could accumulate enough carbon and other
deposits to impede the flow, and maintenance could be required.
However, this may involve simply dumping the packing and replacing
with fresh packing, probably some cheap material such as sand or
gravel.
II T RES EAR CHI N S T I T',U.T E
-------
A
(cocliflg)
cold
Ai !-"
Vene.
r I
Clean
Gas
r
Packed. Bed
Packed Bed
- - --
-- --
- - .- -
- --
---r
Hot Dirty 80S
>
Figure 21
Packed Beds FDr Collection of Submicron
Particles by Thermal Precipitation
47
B
(Co ld)
-------
IV.
SONIC FLUIDIZED BED DEVICE
Aerosols in the submicron size rang~ which cannot be
conveniently collected can sometimes be coagulated to form
agglomer~tes which are more easily separat~ Coagulation can
occur naturally by Brownian motion or can be induced by sonic
waves or turbulence. Brownian coagulation has been shown to
proceed according to the Smoluchowski equation
dn = - Kn2
dt
(13)
where
3
n = aerosol concentration, particles/em
t = time, second
K = Coagulatfon constant, cm3/second
The Smoluchowski equation can be integrated to
1 1
---=Kt
n no
where n is the final concentration and no is the original
concentration of aerosol. In order to II grow II 0.1 micron particles
to 1000 times the mass, the equivalent of a one micron particle,
long times are required. The coagulation constant is in the
order of 7 x 10-10 (Ref. 16). Therefore,
( 14)
1000 - 1- = 7 x 10-10 t
no no
999 x 1010
t =
no x 7
=
1.42 x 1012
n .
o
Thus, unless the original particle concentration were in order
of 1012 particles per cm3, Brownian coagulation cannot be
expected to achieve sufficient growth of the aerosol particles
in short enough time to enable collection by inertial devices.
High frequency sound waves have been demonstrated to
accelerate coagulation of aerosols. For example, Stokes (Ref. 18)
II' IESEARCH INSTITUTE
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reported that carbon black smoke ~mich had a primary particle
size of 0.05 micron could be coagulated with sound frequencies
of 4000 cycles per second. Sonic agglomeration follo~'s the
relationship (Ref. !9,20).
n = n exp - Kt
o
(15)
where n is the particle count, no is the original particle
count, and K is the coagulation coefficient. Again to II grow II
the particles by a factor of 1000
1
IOOO = exp - Kt
- Kt = - 6.9
and for a holding time of 1 second a K of 6.9 would be required.
Brandt (Ref. 19) reports a K of 1.28 at a sonic flux of 0.1
watts/cm2. Sipce K varies as the square root of the power flux,
a power flux of 4.7 watts per square centimeter would be
required to achieve the necessary coagulation rate. This ~'ould
be equivalent to approximately 6 horsepower for one square foot
of agglomeration chamber~ This power level would have to be
reduced for a practical device, possibly by increasing the number
of coagulation cepters (Ref. 21), i.e., large particles ~mich
sweep the smaller particles under the influence of sonic vibration.
This phenomenon has been demonstrated and reported in the
literature (Ref. 22). Water sprays were shown to increase the
coagulation rate of carbon smokes in a sonic field. Thus, it is
reasonable to expect that in a dense system such as a fluidized
bed (Ref. 16) where the distance between particles is small,
high deposition rates of the submicron particles onto the larger
fluidized particles would occur. The sonic waves cannot be
expected to penetrate deeply into the bed. However, absorption
of sonic energy is a direct result of particle motion which is
the desired effect.
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The second technique studied to control particle emissions
from auto exhausts was the fluidized bed. The mechanism of
aerosol removal in fluidized beds is related to the high velocity
gradients in a gas flow around the particles in the bed. The
aerosol particles do not follow the flow stream lines because
of their inertia and impact on the fluidized bed particles,
adjering to them. The velocity gradients between the bed and
aerosol particles can be enhanced by superimposing a sound field.
Moreover, the ratio of the amplitude of vibration of submicron
aerosol particles to the amplitude of the sound motion to which
they are subjected is very nearly unity. Therefore, in the
presence of sound waves, the effective particle size along the
axis of vibration becomes larger, increasing the probability of
collision between the bed and the aerosol parti~les.
A.
Collection Efficiency of Fluidized Bed Without Sonic
Enhancement
The effectiveness of a 3.8 cm diameter fluidized bed for
removing submicron lead particles from an air stream was
investigated at superficial gas velocities of 15-60 cm/secondo
The aerosol stream from the generator was diluted to give
approximately 50 ~g PbC12/1iter. The diluted aerosol stream was
used to fluidize the bed. Simultaneous aerosol samples were
collected both upstream and downstream of the bed..
Initial tests, with 74-149 ~ sand particles as the fluidized
material, showed that there was considerable attrition of the
sand particles on fluidization resulting in carry-over of the
fine material. Therefore, it was decided to fluidize 210-500 ~
glass beads and study the effect of superficial gas velocity on
the collection efficiency of the bed.
The experimental results on collection efficiency of the
fluidized bed device without any sonic enhancement is shown in
Figure 22. The collection efficiency decreases rapidly with
increasing superficial velocity. This is because at low flow
liT RESEARCH INSTITUTE
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Packed bed
100
(ZI~
8
90
:>t 80
o
~
Q)
'r-! 70
o
.....
4-f
~ 60 -
~ 50
.r-!
+J
o
Q) 40
~
~
~ 8 30
+J
~ 20
o .
$.!
8. 10
o
o
o - 210-50011 glass beads, dp::=0.2311 MMD
~ - 74-14911 sand, dp=0.2311 MMD
0 - 210-50011 glass beads, dp=0.3611 MMD 0
10 20 30 40 50 60
Superficial Velocity, V , em/sec.
s
Figure 22 - Effect of Superficial Velocity on Collection Effic:i,ency of Fluidized Bed
Without Sonic Enhancement
-------
velocities, the bed essentially behaves as a coarse filter more
than compensating for its large pore size by its thickness. At
higher flow velocities the bed starts IIboilingll and its
filtration effectiveness is considerably lessened. Figure 22
also shows that there is no significant effect of particle size
in the range 0.2-0.4 ~ on collection efficiency.
B. - Enhancement of Collection Efficiencv of Fluidized Beds
with Sonic Techniques
1. Theory
Sonic energy has been noted to enhance the filtering
efficiency of packed beds, (Ref. 23), but questions remain on
the optimum values of frequency and amplitude and the relative
merits of sonic energy as compared to larger or deeper beds. In
the application in mind at this time, i.e., incre~se in collection
efficiency of particulntes from automobile exhaust by a fluidized
bed ¥ith sound, sonic energy is very desirable because of the
space and mass limitations.
The hypothesis is that the effectiveness of sound energy in
increasing the collection of aerosol particles depends on the
oscillatory motion of the aerosol particles in the sound field.
The fact that the pressure also fluctuates is believed to have
no bearing on the problem. If the hypothesis as stated is
correct, we expect to find that the sonic energy must be such
that the velocity amplitude of the sound wave must be at least
of the same order as the steady flow velocity through the bed.
This velocity will then appear as something like a threshold and
there ¥ill be no effect of sound for all amplitudes significantly
below this level. Also, if the displacement amplitude of the
particles in the sound ¥ave exceeds the depth of the bed further
increases in amplitude "rill have smaller effects.
In order to test the hypothesis, experiments ¥ere conducted
to determine the acoustic particle velocity in the bed, using the
experimental set-up shown in Figure 23. Direct measurements of
the velocity amplitude ¥ere not possible, therefore, it ¥as
liT RESEARCH INSTITUTE
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CJ
lTI
W
L,~]
r:J
L-J
c---J
c::J
CJ
c-J
Cl
CJ
CJ
CJ
L-:=J
CJ
~_J
[ J
CJ
CJ
c=J
-------
necessary to measure pressure and standing wave ratios and compute
the amplitude transmitted into the bed. precision of measurement
of the relative quantities was impaired by the large fluctuations
in the bed properties due to breaking up of individual gas
bubbles during fluidization.
The velocity amplitude, V, is related to the acoustic
pressure, P, (as measured with a microphone) in a traveling wave
when there are no reflections through the acoustic impedance, Z,
of the medium by the ratio:
plv = Z = pC
(16)
where p is the density and C the sound velocity in the medium.
The velocity amplitude in the bed was computed from. the pressure
amplitude of the wave above it. The following observations
permitted computations to be simplified.
(a) With no flow through the bed, the bed behaved like a
rigid wall and the pressure minima were exactly one quarter
wavelength from the surface of the bed.
(b) When air flow was introduced, the pressure level at
the minima increased but their position was not greatly affected.
(c) The att~nuation of the pressure amplitude in the bed
itself was sufficiently large so that reflections from the lower
end of the bed could be neglected. This observation was further
corraborated by the additional observation that the phase change
within the bed was uniform with distance in contrast to the
step-like phase transitions at the minima of the standing "rave
above the bed.
The following argument is used to compute the velocity
amplitude, Vt, transmitted into the bed from the pressure maxima,
Pm' and the pressure minima, Pn' in the standing wave in front
of the bed. The standing wave in front of the bed is made up of
the incident wave Pi and the reflected wave, Pro The corresponding
velocity amplitudes, V, are designated with the same subscripts.
Then:
liT RESEARCH INSTITUTE
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p./v. = -P Iv = Z (17)
1 1 r r a
pt/Vt = Z (18)
b
P. + P = P and p, - Pr = P (19)
1 r m 1 n
Vt = V. + V (20)
1 r
P. P P
=---1: r n
- - -
Z Z Z
a a a
where Za is the impedance of the air above the bed andZb is the
impedance of the bed itself. Thus, the velocity amplitude of
the sound wave traveling into the bed immediately below the
surface of the bed is Vto = pn/za. At greater depths, the
velocity amplitude deminishes through the attentuation factor,
a:
-ax
Vtx = Vto e
( 21)
In derivation of Equation 21, variation of velocity
amplitude with bed depth x has been assumed to be identical to
the variation of the pressure amplitude by neglecting additional
reflections from the base of the bed. The high attenuation in
the bed, in nearly all cases, minimizes errors due to this
assumption.
2.
Determination of Acoustic Particle Velocity
The parameter of greatest interest is the acoustic particle
velocity, Vp' in the sound wave. The parameter cannot be measured
directly. It is only possible to measure the acoustic pressure
and infer the particle velocity from the impedance of the medium.
The impedance of the medium was determined from the standing
wave pattern in the tube above the bed itself. A 3 mm diameter
probe tube permitted measurement of the sound pressure at any
point in the bed. The probe tube attenuation was determined by
lIT R E.S EAR CHI N S TIT UTE
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comparison with the output of a microphone mounted in the ~all
of the tube containing the bed at different frequencies from
Figure 24.
The velocity amplitude ~as subsequently related to the
current in the loud speaker voice coils eliminating the need for
a microphone, probe tube and auxilliary equipment during actual'
experiments ,,'ith lead aerosols.
The microphone output was found to vary by less than i 3dB
at any frequency at different locations above the screen as
shown in Figure 25. The effect of sound frequency and gas
velocity on particle velocity, vp' was calculated from the
maximum and minimum sound pressure levels in front of the bed at
a constant current input to the voice coils of the speakers 0
The ratio of particle velocity to current input, A, to the
speakers has been plotted in Figure 26 for traveling sound waves.
In the case of standing waves, the particle velocity was
computed from the relationship
vp max = Pmax/pC
( 22)
where Vp max is the maximum particle velocity. It must be
noted that in order to attain maximum particle velocity, the
bed must be positioned at a pressure minimum of the sound ,,'ave.
At room temperature, the density of air is 1.2 x 10-3 gm/cm3 and
the velocity of sound is 3.44 x 104 cm/sec. Therefore, the
acoustic impedance, pC, of air at room temperature is 41.3 gm/cm2
second. Equation 22 then becomes
v = P /41.3
p max max
( 23)
The maximum particle velocity was calculated using Equation
23 from the maximum sound pressure level, Pmax' (measured by a
microphone) at different current inputs to the sound driver
units. At a gas velocity of 29 cm/sec and frequency of 660 Hz,
Pmax was found to be 146 dB and 156 dB at current inputs of
liT RESEARCH INSTITUTE
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40
35
WITHOUT THE PROBE TUBE
30
U1
....:J
~25
Z
1&1
«1)20
z
o
D..
~15 ,
0:
10 :
5
2
20
6
8
..
WITH THE PROBE TUBE
11 1 8 9 1
roo
2 8 .. 6 11 7 8 9,obo
FREQUENCY IN CYCLES PER SECOND
Figure 24
Difference in Sound Pressure Levels when
Measured with and without the Probe Tube
2
6
11 1 l
-------
4
35
3
oj
C 25-
Z
IJJ
(1)2
Z
U1 0 "
00 n.
~ 15
II::
1 -!" .l1
2
20
8
.,
6
B
2
., 667891
1000
FREQUENCY IN CYCLES PER SECOND
Figure 25
Sound Pressure Levels as a Function of
Freq~ency at the Retaining Screen, 10, 20 and
26 em. above the screen (No Bed Present)
2
20000
B
IS
-------
~ 100
Q)
~
X 80
'0
~
o
tJ
Q)
(/)
'S- 60
tJ
..
>~
180.
0 250 HZ
160 0 666 HZ .
o 1200 HZ
f:::,. 2700 HZ
140
120
.40
20
G
10
~.~._--~. I r
20 30
Vs' em/second
I
60
I
40
I
50
Figure 261- Effect of Flow Velocity and Frequency of Sound on V IA
P
-------
0.315 and 1 ampere respectively.
23, we get:
Using these values in Equation
v
p max = 305 cm
A sec x ampere
( 24)
C.
Effect of Travelinq Sound Waves on Collection Efficiency
of the Fluidized Bed
Figure 23 shows the experimental set-up to determine the
effect of traveling sound waves on the collection efficiency of
the fluidized bed. The 3.8 cm diameter bed was filled to a
depth of 5 cm with 210-500 ~ glass beads. A small stream of
aerosol from the aerosol generator was mixed with a stream of
dry filtered air in a chamber. The aerosol concentration and
flow rate could be varied by adjusting the two streams and the
mixed stream was used to fluidize the bed. Simultaneous aerosol
samples were obtained both upstream and downstream. of the fluidized
bed as shown in Figure 23.
A speaker bank was assembled by connecting four University
Model ID-60 driver units* in parallel and mounted on top of the
fluidized bed so that the sound waves traveled down the
fluidization chamber. The speaker bank was connected to a
frequency oscillator type N01210-C** through a 400 watt amplifier
and an ammeter. A 1.2 meter tube packed with fiberglass was
connected to the bottom of the fluidized bed. The packed tube
served as a sound absorber to give near ideal traveling sound
waves within the fluidized bed device.
A series of experiments were performed to study the effect
of sound frequency and power input to the speakers on the
collection efficiency of the bed. The experimental data on the
collection efficiency of the fluidized bed device with traveling
sound waves is given in Table V.
* Manufactured by Altec Electronics, White Plaines, N.Y.
**Manufactured by General Radio Co., Concord, Massachusetts
liT RESEARCH INSTITUTE ",
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Table V
EXPERIMENTAL RESULTS WITH TRAVELING SOUND WAVES
Aerosol Size, dp :::: 0.34 !l MMD
Bed Height :::: 5 cm
Bed Diameter :::: 308 cm
Bed Material :::: 210-500 !l glass beads
Gas Velocity, V :::: 29 cm/sec
s
Current to, Aerosol %
Concentration,
Sound Drivers, Frequency, !l g PbC12/1it V /A V Collection
Amperes HZ ---E- p Efficien2Y
0.3 2700 44.3 94.5 28.4 32.6
1.0 2700 50.6 94.5 94.5 1304
1.5 2700 35.7 94.5 141.8 25.1
2.0 2700 43.5 94.5 189 23.2
2.5 2700 55.2 94.5 236 30.4
3 2700 24.9 94.5 283 39.3
0.3 1200 66.6 70 21 2108
1.0 1200 46.4 70 70 30.4
1.5 1200 49.9 70 105 37.5
2.0 1200 77.4 70 140 40.8
2.5 1200 60.1 70 175 32.8
3 1200 61.4 70 210 29.6
0.3 660 83.7 50.5 15.1 38.1
1.5 660 65.6 50.5 75 40.5
3.0 660 50.8 50.5 150 42.5
0.3 250 24.9 40.0 120 43.5
1.5 250 81.4 40.0 60 37.9
3.0 250 81.4 40.0 120 31.8
lIT RESEARCH INSTITUT.E
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1.
Effect of Sound Frequency on Collection Efficiency
The effect of sound frequency, in the range 250-2700 HZ, on
the collection efficiency of the fluidized bed ¥ith traveling
sound ¥aves is shown in Figure 27. This frequency range
corresponds to high values of sound pressure levels in the bed
as shown in Figure 24 and should be more effective than other
frequencies. Figure 27 shows that there is no systematic change
in collection efficiency of the bed with frequency at any power
input to the sound driver unit. Some energy is lost through
mechanical friction, and eddy currents in the electromagnetically
driven diaphragm type sound driver units. Moreover, some energy
is also absorbed by the bed. Since all these energy losses vary
with frequency, it is not surprising that there is no pattern
on the effect of frequency on collection efficiency of the bed.
2.
Effect of Particle Velocity on Collection Efficiency
Particle velocity, V was determined using Figure 26 from
p
known values of sound frequency, gas velocity and current input
to the speakers. Figure 28 shows the effect of particle
velocity on collection efficiency of the bed. Since Vp is a
function of frequency, gas velocity and current to the speakers,
the collection efficiency should be a function of the particle
velocity alone even though the other parameters ¥ere varied.
Figure 28 shows that even at high particle velocities of 200-
300 cm/sec there is no significant increase in the collection
efficiency of the bed. Collection efficiency values of 30-40%
indicate that traveling sound waves are very ineffective and do
not enhance the collection efficiency of the fluidized bed.
D.
Effect of Standinq Sound Waves on Collection Efficiency of
the Fluidized Bed
The experimental set-up to study the effect of standing
sound waves on collection efficiency of the fluidized bed is
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100
90
o 0.3 amps
1:1 3.0 amps
o 1.5 amps
o 2.0 amps
o 2.5 amps
~ 1. 5 amps
~ 5
u
~ ~ 0
t.J Q) ~
'r-! 4 []
u 0 0
'r-!
\i-I
~ &. ~
fiI 3
~
0
A
8
8
1
p
-,
200
~'o'----- -1050---- 1:>00'---2000---
Sound Frequency, HZ
25do
2700
- -~. ------
Figure 27 - Effect of Sound Frequency on Collection Efficiency of the Fluidized
-------
(j\
,j::o.
100 j
90 I
j
I
80 1
I
:: 1
>t 50 i
u I
!:: .
Q) I 0
.~ 40 i
.~ [J
~ I
~ i
fiI 30 ~
~ j
201
1
10J
~~~~;~- -~o -- ~-~
220
'--1" ---- ',"-- - ---- T- --- ---j"----"- .. ! --"'----,-- - -roo "'''---T ---r--- ,-- --- ---
300
o 250 HZ
o 660 HZ
6. 1200 HZ
o 2700 HZ
[]
8 0
A
0 A
8.
o 0
(.)
'--
o
o
A
A
o
.-..-' j- -- --.
80
100
120
140
160
180
200
Figure 28-
Particle Velocity, V , em/sec.
p
Effect of Particle Velocity on Collection Efficiency
with Travelling Waves
240
260
280
o
. -.- ---------
-------
shown in Figure 29. The aerosol generation, sampling and flow
system were the same as in the case of tr.aveling waves. The
fiber glass packed sound absorbing tube. ,,!as removed and the
fluidized bed device was modified to get standing wave patterns
in the bed.
The system consisted essentially of_three nested glass tubes
sealed with rubber rings and provided with a screen to support
the fluidized bed. The sound source, consisting of four loud
speaker driver units (University ID-60) , was mounted above the
bed and connected to the fluidized bed unit with a short section
of brass pipe as shown in Figure 30.
The nested tubes were required to tune the system which was
essentially a closed pipe resonator. Ideally, maximum sound
amplitude should occur at multiples of quarter wave lengths from
the reflecting bottom of the tubes and the distance from the horn
diaphragm to the bed should also be a multiple of quarter wave
lengths for optimum effect of sound.. The nested tube arrangement
allowed adjustment of both the horn to bed and the reflector to
bed distances.
The position of the bed was adjusted with respect to the
sound driver units and the bottom of the reflector tube by noting
the intensity of scattered light from the aerosol downstream of
the bed. At the best location of the bed, the concentration of
PbC12 aerosol particles downstream of the bed was small. This
position of the bed coincides with the minimum pressure and
maximum velocity level in the soundwave.
Figure 31 shows the effect of current input to the speakers
on the collection efficiency at a frequency of 660 Hz and gas
velocity of 29 cm/sec. It is seen that the collection efficiency
of the fluidized bed increases rapidly with power input when
standing sound waves were used.
The collection efficiency of the bed exceeds 90% at a
current of 5.6 Amperes, which corresponds to an electrical power
input of 125 watts, to the sound driver units. Because some
lIT RESEARCH INSTITUTE
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Rotameter
Flow Restriction
Excess Aerosol
Vent
~
0'\
0'\
Furnace
o Rotameter
Air
Filter
t
t
Recorder
Air
Filter
Sampler
o t
Manometer
Temperature
Controller
Spea ker
Bank
Downstream
Sampler
-." ,/ .
.-.
Bed
! I
L-'
Amplifier
Oscillator
Ammeter
-------
CJ
0\
-..J
(. J
~
'J
Ll
[J
.~
!J
;-J
LJ
CJ
LJ
IJ
r:::::J
LJ
~
c.::::J
c-=J
r-:J
-------
())
00
:>t
u
t:
Q)
..-j 40
u
-.-j
4-j
~ 30
~
100
Frequency - 660 HZ
Aerosol Size - 0.34~ MMD
Bed Material - 2l0-500~ Glass
Gas Velocity - 29 em/Sec.
Beads
90
80
70
60 -
.
50
.
.
.
20
10
---,-
1
-~
I
6
3
4
5
2
Current to Speakers, Ampere
Fig-ure 31 - Effect of Power Input to the Speakers on the Collection Efficiency of the Fluidized
-------
energy is lost ,due to mechanical friction and eddy current
dissipation in the sound driver units, and by absorption in the
bed itself, only a fraction of the electrical power input to the
sound driver units would be effective as sound energy in the bed.
It is reasonable to assume that approximately 5% of the po"Ter
input to the speakers was converted to useful sonic energy.
Then, the lIactive sonic fluxll, W, in the 3.8 em diameter
fluidized bed is:
W - 125 x 0.05 = 0.5 watts/cm2
- 11 x (3.8)2
4
(25)
Therefore, it would be necessary to have a sonic flux of at
least 0.5 watts/cm2 in the bed for the sonic fluidized bed device
to operate with an efficiency of 90%. It was not possible to
conduct experiments at higher power inputs, with the driver
units employed in the study, because of maximum power input
limitations imposed by the design of the units.
The effect of particle velocity on collection efficiency of
the sonic fluidized bed device is shown in Figure 32, both for
traveling and standing sound waves. It is evident from the
figure that at low values of particle velocity the collection
efficiency is not affected by the particle velocity. The
collection efficiency increases with particle velocity "~en the
latter exceeds 500 em/sec. This corroborates our original
assumption that acoustic particle velocity must be significantly
greater than the steady gas velocity through the bed. The
higher values of collection efficiency of the bed with standing
waves compared to traveling waves can be explained as follows:
From Figure 26, at gas velocity of 29 em/see and frequency
of 666 Hz we get,
/ em
(Vp A)traveling waves = 50 see x amp
( 26)
For standing waves at gas velocity of 29
frequency of 660 Hz we get (see Equation 24) ,
liT RESEARCH INSTITUTE
em/see and
-------
~
t)
~ 90
-rf
t)
-rf
~ 80
f1I
s::
.S 70
+'
t)
Q)
...:J .~60
o 0
()
~ 50
Q)
t)
~
~40 ,
10
. Travelling Waves
o Standing Waves
Gas Velocity = 29
. :
..
.
30 .- ..
.. 0
20 0
.
10
o
200
cm/sec.
400
o
600
800
o
1000
Particle Velocity V I cm/sec.
p
1200
1400
1600
o
1800
Figure 32 - Effect of Particle Velocity on Collection Efficiency of the Sonic Fluidized
-------
/ cm
(Vp max A) standing waves = 305 sec x amp
Dividing Equation 24 by 26 we get,
Vp max/Vp = 350~ ~ 6
( 27)
Equation 27 shows that ~hen standing waves are used and the
bed is located at a pressure minimum of the sound waves, we c~n
get about six times the particle velocity as compared to
traveling waves at the same current input to the voice coils.
Thus, the difference in acoustic particle velocity and steady
gas velocity is large in the case of standing waves, accounting
for the higher values of collection efficiency.
E.
Compatibility of the Sonic Fluidized Bed Device with
Automotive Systems
Experimental results in the preceeding section show that a
sonic fluidized bed device can be used to clean automotive
exhaust gases of their particulate matter by using standing
sound waves. It is, however, imperative that the sonic flux be
at least 0.5 watts/cm2 to attain collection efficiencies in the
order of 90%. Since sonic agglomeration has been demonstrated
to be effective on particles from 0.05 ~ to more than 8 ~ in
diameter, most of the range of particle sizes present in engine
exhaust should be susceptible to this treatment. Since fluidized
bed densities are generally 400 - 650 Kg/M3, a 15 cm deep bed
would have a pressure drop of approximately 5.9 cm of water.
For a V-8 engine with a maximum exhaust of 95 standard liters/sec
at 250°C the flow rate would be
523
95 x 273 = 182 liters/sec
A 15 cm deep bed of 500 ~ spheres can be kept fluidized at a
velocity of approximately 300 cm/sec. Therefore, the bed would
have to be approximately 607 cm2 in cross-sectional area or 28 cm
liT RESEARCH INSTITUTE
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in diameter.
of velocities.
The bed can be fluidized in a relatively low range
However, at lower exhaust rates retention times
would be longer and possibly enough coagulation could occur-..even
if the bed were not fluidized. Furthermore, at low gas
velocities the aerodynamic filtration efficiency of the bed
would increase because of the small distances between the bed
particles. Moreover, the fluidized bed device could be designed
to have a tapered cross-section, such that, a portion of the bed
would always fluidize at all flow rates.
The sound pressure levels in 5 cm diameter exhaust pipe
of an automobile vary from 160 to 170 dB, that is, 1 to 10
watts/cm2. Assuming, an average value of 5.5 watts/cm2, the
total sonic power in the exhaust pipe of an automobile is
5.5 x {x (5)2 = 108 watts
This magnitude of sound when spread over the cross-section of
a 28 cm diameter fluidized bed device would give a sonic flux
of 0.18 watts/cm2, or approximately 1/3 of the sonic flux
(0.5 watts/cm2) required for a 90% efficient device. Therefore,
the natural sound level in auto exhaust pipes is not sufficient
and an auxiliary acoustic unit would be needed to generate the
additional 200 watts of acoustic energy. One method of
accomplishing this desired result in an automotive application
is shown schematically in Figure 33.
As shown in Figure 33, the dirty exhaust gas would pass
through a relatively shallow fluidized bed. A sound generator
,,'ould be mounted above the bed, projecting the sound "'aves into
the bed. The submicron particles in the exhaust gas would
coagulate on the surface of the fluidized large particles under
the influence of the sonic vibrations.
Submicron particles once brought in contact with the course
material would adhere to it. Some attrition could be expected
but material dislodged from the large particles ,,'ould be in the
form of large agglomerates, submicron particles once agglomerated
'IT RESEARCH INSTITUTE
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'"
~'"
-".. """"-"
.........."
""'-..,.
-----~-_.
Sound
Absorbent
Material
",
'.
"
"
Sonic
Generator
-----
""
"'" .
r--'
(
>
Clean Gas
"
".
'......,.~-
~ /,';, " ,/ / / / / /
""" / ..' /. /
. , Flu~d~zed Bed.
/ . /
/ ' / /
Screen
'. .
",- .
- - --
~.,.
'-'.
'.' ._---...---
Dirty Gas
~ -
,
"
,I
",
"".
",
Figure 33
Sonic Agglomeration in a Fluidized Bed
-------
require very high shearing forces to bring about deagglomeration.
The agglomerated material dislodged from the bed could easily be
separated with a state-of-the-art inertial separator such as a
cyclone.
In the device envisioned there would be energy dissipation
in the form of sonic vibrations. These will be within the
audible range and consideration of sonic damping will be
mandatory. Since we are relying on the high absorption and
conversion of sonic energy by particulate beds into particle
motion, it is anticipated that the damping will be achieved by
adequate shielding with sound absorbant material which could-even
be particulate in nature.
The entire system would be expected to occupy less than one
fifteenth cubic meters and weigh approximately 20 Kg with all the
manifolding and sonic generator. The sonic generator would
probably be an air driven siren which would be supplied by a
belt driven air compressor.
An additional feature of this system is thAt the fluidized
bed could well be chosen for its catalytic properties and aid in
oxidizing unburned hydrocarbons and carbon monoxide.
It is difficult to estimate the cost of a workable device
at this stage since no accurate design data exists, however, an
educated guess would be somewhat less than $100 at the retail
level.
Maintenance should be minimal. If the bed should "coke up"
after long operation the bed could be dumped and refilled with
approximately 10 Kg of a material which would probably cost no
more than 20-30 cents per Kg, a cost of approximately $2.50.
liT RESEARCH INSTITUTE
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v.
CONCLUSIONS
The following conclusions are based on the results of our
studies.
1.
2.
The thermal packed bed device has high collection effi-
ciency (>95%) at gas-packing temperature differences
exceeding 200°C.
Large gas-packing temperature differences in the bed
can be maintained for a longer period of time with a
high heat capacity packing material.
Collection efficiency of the thermal packed bed device
does not change significantly with particle size and
concentration in the range found in automobile
exhausts.
3.
4.
Contamination buildup in the bed seems to have no sig-
nificant affect on the collection efficiency of the
bed at moderate gas velocities through the bed.
Increasing gas velocity from 15.5 to 130 cm/sec lowers
the collection efficiency of the bed by only 10 to
15%.
Collection efficiency of the sonic fluidized bed
device does not change appreciably with either the
power input to the sound driver units or sound fre-
quency when travelling waves are used.
Collection efficiency of the sonic fluidized bed
device approaches 90% using standing sound waves at
electrical power inputs to the sound driver units
exceeding 125 watts.
Standing waves increase the aerosol particle velocity
by a factor of 6 compared to travelling waves for the
same power input to the sound driver units at a con-
stant frequency.
5.
6.
7.
8.
From the above conclusions, it is evident that the thermal
packed bed device can be employed to control particle emissions
from spark ignition engines under most operating conditions.
liT RESEARCH I'NSTITUTE
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The sonic fluidized bed device is capable of attaining
collection efficiencies in excess of 90% with the use of stand-
ing waves and sound flux in excess of 0.5 watts/cm2. The
natural sound intensity in an automobile is of the order of
108 watts which is not sufficient when spread over the cross-
sectional area of a 28 cm diameter bed needed to process
automobile exhaust. Therefore, an auxiliary sound generator
capable of supplying 200 watts of active acoustic power into
the bed would be needed. This magnitude of auxiliary acoustic
power and the corresponding sound absorbing mufflers would add
both to the capital equipment and operating cost.
liT RESEARCH INSTITUTE
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VI.
RECOMMENDATIONS FOR FUTURE WORK
Further research should be carried out on the thermal
packed bed device. The packed bed should be scaled up and
optimized to process automobile exhausts under varying driving
conditions with collection efficiencies close to that obtained
with the laboratory
should be evaluated
if needed.
Moreover, it is necessary to have gas-packing temperature
difference greater than 200°C for the device to have collection
efficiency of 95% and greater. Techniques for cooling of the
bed to obtain optimum gas-packing temperature difference need
further development.
We are optimistic that the experimental data on the labor-
atory model can be used successfully to construct prototypes of
the packed bed device for additional tests with exhausts from
spark-ignition engines.
model. Prototypes of the optimized device
on test cars and further refinements made,
liT RESEARCH INSTITUTE
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10.
11.
12.
REFERENCES
1.
"Particulate Lead Compounds in Automobile Exhaust Gas,"
Hirschler, D. A., et al, Ind. Eng. Chern., 49,1131-1142
(1957) .
2.
"Concentration of Fine Particles
Mueller, P. K., et al, Symposium
ment Methods, ASTM Special Tech.
77 (1964).
and Lead in Car Exhausts I"
on Air Pollution Measure-
Publication No. 352, 60-
3.
"Automobile Exhaust Particulates -- Source and Variation,"
McKee, H. C. and McMahon, W. A., J. Air Pollution Control
Assn., 10, 457-461 (1960).
4.
"Nature of Lead in Automobile Exhaust Gas," Hirschler, D.
A. and Gilbert, L. F., Archives of Environmental Health,
Symposium on Lead, Feb. 1964.
5.
"Characterization of Particulate Lead in Vehicle Exhaust-
Experimental Techniques," Habibi, K., Environmental Sci.
Tech., .4, No.3, 239-248 (1970).
6.
"Development of the Molten Carbonate Process to Remove
Lead and Other Particulates from Spark Ignition Engine
Exhausts," Natl. Air Poll. Control Admin., Contract No.
CPA 70-3, Final Rept. by Atomic Intl., N. Amer. Rockwell,
1970.
7.
"Studies on the Carcinogenicity of Gasoline Exhaust, II
Hoffman, D., et al, J. Air Poll. Control Assn., 15, 162-
165, (1965).
8.
"Blood Lead of Persons Living Near Freeways," Thomas, H. V. I
et al, Archives of Environmental Health, 15, 695-702,
(1967) . -
9.
"Lead in Soils and Plants -- Its Relationship to Traffic
Volume and Proximity to Highways," Motto, H. L., et al,
Environmental Sci. and Tech., .4, 231-237 (1970).
"Rotating Brush Aerosol Separator," Naval Air Systerns Com-
Mand, Contract No. N00019-68-C-0459, Final Rept. by lIT
Res. Inst., 1969.
"Impaction of Dust and Smoke Particles on Surface and Body
Collectors," Ranz, W. and Wong, J., Ind. Eng. Chern., 44,
p. 1371 (1952).
Encyclopedia of Chemical Process Equipment, W. J. Mead,
Editor, Reinhold Pub. Corp., N. Y., p. 731 (1964).
liT RESEARCH INSTITUTE
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13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
"Aerosols Consisting of Spherical Particles of Sodium
Chloride," Matij evie, E., et aI, J. Colloid S ci., 18,
91-93 (1963).
"Aerosol Studies by Light Scattering: IV - Preparation and
Particle Size Distribution of Aerosols Consisting of Con-
centric Spheres," Espenscheid, W. F., et aI, J. ColI. Sci.,
20,501 (1965).
"Particle Deposition from Turbulent Streams by Means of
Thermal Force," Byers, R. L. and Calvert, S., AIChE, 63rd
Natl. Meet., Preprint 27C, St. Louis, Feb., 1968.
"The Mechanis of Aerosols," Fuchs, N. A., the MacMillan Co.,
N. Y., 1964.
"On the Theory of Thermal Forces Acting on Aerosol Parti-
cles," Brock, J. R., J. ColI. Sci., 17, 768-780 (1962).
"Sonic Agglomeration of Carbon Black Aerosols,"
C. A., Chern. Eng. Prog., 46, 423-432 (1950).
Stokes,
"Uber das Verhalten von Schwebstoffen in Schwingenden Gasen
bei Schall -- Und Ul traschallfrequenzen," Brandt, 0.,
Kolloid Zeit., 76, 272-278 (1936).
"Effect of the Magnitude of Acoustic Exposure on Acoustic
Coagulation of Aerosols," Podoshevnikov, B. F., Zh. Prikl.
Khimi, 34, 2664-2668 (1961).
"Acoustic Coagulation and Precipitation of Aerosols,"
Mednikov, E. P., Acad. of Sci., USSR Press, p. 118, 1963.
"Application of Sonic Energy in the Process Industries,"
Stokes, C. A. and Vivian, I. E., Chern. Eng. Prog. Symposi-
um Series 1, 47(1), 11-21 (1951).
"Enhanced Deposition in an Aerosol Filter in the Presence
of Low Frequency Sound," Beeckmans, J. M. and Keillor, S.
A., J. ColI. Sci., 30, 387-390 (1969).
"Systematic Polarographic Metal Analysis," Lingane,
Ind. and Eng. Chern., 15, 583-585 (1943).
J., J.
"Polarographic Determination of Lead in Blood," Nylander,
A. L. and Holmquist, C. E., AMA Arch. Ind. Hyg. and Occu-
pational Health, 10, 183-191 (1954).
"Some Experiences with Polarographic Methods in Controlling
a Lead Hazard in Brass Foundries," Weber, H. J., J. Ind.
Hyg. and Toxicology, 29, 158-161 (1947).
liT RESEARCH INSTITUTE
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27.
28.
29.
~O.
"Methods for Determining x"ead in Air and in Biological
Materials," Amer. Pub. Health Assn., 2nd Ed., 38-39 (1955).
"A Polarographic Method for Lead and Zinc in Paints,"
Abraham, M. B. and Huffman, R. 5., Ind. and Eng. Chern.,
~, 656-666 (1940).
"Official Methods of Analysis of the Association of Offi-
cial AgricQltural Chemists," Assn. of Official Agricul-
tural Chemists, Washington, D. C., 10th ed., 367-74 (1965).
"Colorimetric Determination of Traces of Metals, Vol. III, II
Sandell, E. B., 563-571, Inter Scienge Publishers, Inc.,
N. Y., 1959.
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APPENDIX A
ANALYTICAL TECHNIQUES
liT RESEARCH INSTITUT!
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ANALYTICAL TECHNIQUES
The following analytical techniques were used in this study:
1.
Reflectometer
Initiqlly, when carbon particles were used as the test aero-
sol, a reflectometer was used to determine the collection effi-
ciency of the thermal bed. Aerosol was sampled both upstream
and downstream of the packed bed on white fiberglass filters.
The reflectivity of t~e filters was measured using a reflectom-
eter. If it is assumed that the amount of light reflected from
the filter follows Beer-Lambert Law, then
-L -
10 -
exp (-AC)
( l-A)
where I is the transmitted or reflected light, IO is the incident
light, A is a constant, and C is the concentration of light
absorbing particles.
11
ln -
10
=
-ACl
( 2-A)
and
I
2
ln -
10
=
-AC2
( 3 -A)
where the sUbscripts 1
samples, respect!vely.
for percent collection
and 2 refer to the upstream
From Equations 2-A and 3-A
efficiency, E, was derived.
and downstream
an expression
II T RE SEA R CHI N S TH UTE
-------
E
=
Cl-C 2 In (I 1/10) -In (12/10) .
Cl x 100 = In (11/10) x 100
(4-A)
Equation 4-A has been used to calculate the collection effi-
ciency of the thermal packed bed device from the reflectometer
readings.
2.
Polaroqraphic Technique
Polarographic techniques have been used widely to determine
microgram quantities of lead (Refs. 24, 25, 26, 27, 28). The
polarograph is a device that produces a diffusion current which
is proportional to the lead ion concentration in a supporting
electrolyte. The following technique was used to calculate lead
chloride concentration in the aerosol stream using a Sargent
Model XXI Polarograph.* The polarograph was calibrated using
standard solutions of PbC12 to get the calibration curve shown
in Figure 34.
Aerosol samples, both up- and downstream of the packed bed,
were collected on Gelman type A glass fiber filters. The fil-
ters were placed in 100 ml beakers and boiled with distilled
water for 1 hr. The liquid was allowed to concentrate to about
15 ml and then the samples were cooled to room temperature. The
concentrated mother liquor was washed into a graduated cylinder
and diluted to the 25 ml mark. The entire contents of the cyl-
inder were poured into the polarographic cell and 10 ml of sup-
porting electrolyte was added. The supporting electrolyte con-
sisted of 0.1 M KCl and 0.01% gelatin. The cell was purged with
a stream of dry, high purity, nitrogen for 10 min to remove the
dissolved oxygen. After purging, the cell was sealed to prevent
air from entering the cell. Mercury drops from the dropping
mercury electrode were allowed to enter the cell containing the
*
Manufactured by E. H. Sargent & Co., Chicago, Illinois.
liT RESEARCH INSTITUTE
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I
i
6
Damping = 2
Volts Additive = 1.50
Volts Opposed = 0.50
Dropping Hg Rate = 20
drops/minute
5
4 /
/'
(/)
~ ..-
//
~ 0 3 /
~ /
U
..-1
~
,
+J
s::
Q)
~ 2
~
::J
t)
s::
0
..-1
(/)
::J
4-1
4-1 1
..-1
~
.1
I
.3 .4 .S .6 .7 .8 .9 1.0
Concentration expressed as Mg PbC12/25
34: Calibration Curve for the Polarograph
1. 1 1.2
C.C.
.2
Figure
-------
solution at the rate of one drop every 3 sec.
The diffusion
current corresponding to lead ions was determined from the
polarographic trace and the corresponding PbC12 concentration
obtained from the calibration curve on Figure 34:". The PbC12
concentration was then calculated from the aerosol sampling rate,
sampling time, and the polarograph results.
While the polarograph method is accurate, it does not lend
itself to rapid analysis of a large number of samples. An
alternate time saving and just as accurate colorometric method
described in Section 3 was used to analyze samples after the
month of July.
3.
Colorimetric Technique
The colorimetric method using dithizone permits detection
and measurement of lead concentration as low as 0.1 ~t lead with
sufficient accuracy (Ref. 29).
The following procedure was used to analyze filter samples.
in the present study. Immediately after conducting a test run,
upstream and downstream filter samples were placed in small
wide-mouth jars. The jars were sealed to eliminate any lead con-
tamination of the filters. The filters were removed from the
jars and placed in 150 ml beakers. The beakers were then heated
with 100 ml of 1% nitric acid solution in water to extract the
lead from the filter papers. The heating was done at 80°C for
1 hr to ensure total dissolution of PbC12 particles. The
beakers were cooled to room temperature and the samples were
.t ~ ,
filtered into 200 ml volumeteric flasks through Millipore
membrane with pore opening of 0.45 j.L. The filter and beaker
were rinsed with 1% nitric acid and the total volume of the
filtrate was brought up to 200 ml mark with addition of 1%
acid solution. The reagents were prepared by Sandell's tech-
nique (Ref. 30) as follows:
liT RESEARCH INSTITUTE
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0.6
/
~/
/
/
0.5
~
Q)
o
s::
Q)
.Q
~
o
fI) 0.3
~
0.4
I
I
I
0.2
.-
/"
//
/
0.1
---r-
50
10
20
60
Figure 35- Calibration
30 40
Concentration of PbC12' ~g
Curve for Lead Chloride vs Absorbence
-------
Dithizone:
Reagent grade.
Stock solution, 0.005% in CC1*
Solution for analysis prepare~ by diluting
to 0.001% at the time of analysis.
stock
Buffer:
NH3-CN-SO solution
to 98 ml ~f concentrated reagent NH40H were added
0.75 9 of KCN and 0.375 g NA2S03' The mixture
was diluted to a final volume of 250 cc.
Analysis
Wavelength:
5 20 It\U.
Procedure:
Add 10 ml of 0.001% dithizone to 10 ml of the
buffer in a 60 ml separatory funnel. An appropri-
ate size aliquot (usually 2 cc) of PbC12 extract
is added next. The mixture is extractea for
~ min. The CC~4 la~er is irnrnediately.dra~n off
1nto the color1metr1c tube for analys1s w1th a
Spectronic 20 spectrometer.**
The absorbance given by the spectrometer is a measure of
lead concentration in the solution. A calibration curve shown
in Figure 35 was prepared using standard solutions of lead
chloride. The aerosol concentration was easily calculated
the calibration curve and the volume of aerosol sampled.
from
*
Dithizone is unstable and must be kept in a refrigerator until
diluted for the day's analysis.
**
Manufactured by Beckman Instruments.
liT RESEARCH INSTITUTE
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DISTRIBUTION LIST
Recipient
National Air Pollution Control Administration
Consumer Protection & Environmental Health
Service
Division of Motor Vehicle R & D
5 Research Drive
Ann Arbor, Michigan 48103
Attn: Mr. Charles Gray, Project Officer
Division of General Services
Environmental Health Service, PHS
Parklawn Building, Room 4A-44
5600 Fishers Lane
Rockville, Maryland 20852
Attn: Martin K. Trusty, Contracting
Negotiated Contracts Section
Officer
IIT Research Institute
M.J. Klein/Division Files
IIT Research Institute
S.L. Blum
IIT Research Institute
G.E. Burkholder/Main Files
IIT Research Institute
R. Karuhn
IIT Research Institute
S.K. Sood
IIT Research Institute
J.D. Stockham/Section Files
IIT Research Institute
P. Caputo
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