United States Industrial Environmental Research EPA-600/7-79-232a
Environmental Protection Laboratory October 1979
Agency Research Triangle Park NC 27711
Assessment of Diesel
Particulate Control:
Filters, Scrubbers, and
Precipitators
Interagency
Energy/Environment
R&D Program Report
-------
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EPA-600/7-79-232a
October 1979
Assessment of Diesel Particulate Control
Filters, Scrubbers, and Precipitators
by
M.G. Faulkner, E.B. Dismukes, J.R. McDonald,
D.H. Pontius, and A.H. Dean
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-2610
Task No. 14
Program Element No. EHE624A
EPA Project Officer: Dennis C. Drehmel
Industrial Environmental Research Laboratory
Office of Environmental Engineering and Technology
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
-------
DISCLAMIER
This report has been reviewed by the Industrial Environ-
mental Research Laboratory, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina 27711, and approved for
publication. Approval does not signify that the contents neces-
sarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial
products contribute endorsement or recommendation for use.
11
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EXECUTIVE SUMMARY
Introduction
As a result of the Federal Government's requirement that
the average fuel mileage of U.S. automobile manufacturers be
at least 27.5 mi/gal, by 1985, there is increased interest in
the widespread use of diesel engines for light-duty vehicles
because of the diesel's superior fuel economy. Unfortunately,
the diesel engine produces a high level of combustion by-products
that are potentially hazardous, and therefore the Environmental
Protection Agency has proposed emission standards which would
require up to 70% particulate removal efficiencies for some
current models.
Several approaches for controlling particulate emissions
are under investigation by automobile researchers, including
fuel modification, combustion modification, and aftertreatment
of the exhaust in a control device. The purpose of this report
is to examine the potential applicability to diesel particulate
control of stationary source particulate control technology
employing electrostatic precipitation, filtration, and wet scrub-
bing as collection processes. The removal of particulate by
catalyzed or uncatalyzed oxidation is another aftertreatment
concept that is being investigated elsewhere, but which may
ultimately need to be considered as a disposal strategy in con-
nection with particulate removal by a filtration or electrostatic
precipitation device.
A consideration of diesel particulate characteristics yields
some insight into the difficulty of designing a practical control
device. The size distribution of the material has a mass median
diameter (mmd) of about 2 x 10~7 m (0.2 ym), which is the par-
ticle size at which conventional control concepts are least ef-
fective. If suitable collection is achieved, the bulk density
of the soot is only about 120 kg/m3, which creates a storage
and disposal problem. For example, if 90% particulate control
is achieved on an Oldsmobile 350 CID diesel, about 0.019 m3 (5 gal.)
of material would be accumulated in 4,800 km (3,000 mi) if no
compaction were achieved. The approach used in this study has
been to identify and select for screening studies control con-
cepts which, on the basis of existing knowledge of the fundamental
collection processes, offer potential for effective particulate
111
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capture without adversely influencing the engine performance.
Those concepts that continue to appear promising after the screen-
ing data have been evaluated can then be further investigated
with more emphasis on particulate storage and disposal.
Filtration
Filtration is a complex process involving several particle
collection mechanisms for which theoretical models are not entirely
adequate. Filtration theory attempts to predict overall particle
removal in a filter based on an understanding of the interaction
of particles with a single filter element, which may consist
of a fiber, a granule, or a previously collected particle. The
currently available filtration theory was employed to estimate
the performance of various filter designs. The approach used
was to select a reasonable filter volume and path length and
then to examine the performance of filters varying in both volume
and path length. Both fiber bed and granular bed filters were
considered, and the porosity of each was fixed at a reasonable
value. The fiber and granule diameter were fixed, and the prop-
erties of the gas and aerosol were fixed at a base-case condition
of 200°C, with 0.14 m3/s (300 ft3/nun) of exhaust gas transport-
ing 7 x 10~5 kg/m3 (0.03 gr/ft3) of particulate with an mmd of
0.2 ym.
The analysis resulted in the selection for further study
of a fibrous filter with uniform 10-ym diameter fibers used as
packing at a porosity of 99%. Fibrous filter packing of this
type is currently available and is considered typical with respect
to filtration and pressure drop characteristics. Stainless steel
is a possible material for the fibers. A sketch of this proposed
prototype is shown in Figure E-l. The estimated pressure drop
through this device is 732 Pa (2.9 in. of water) and the estimated
maximum lifetime is 20,900 km (13,000 miles) at an emission rate
of 5 x 10"1* kg/mi and a bulk density of 120 kg/m3. This is the
maximum possible lifetime based on the simplistic approach of
estimating how long it would take to fill the void volume with
collected particles, and it ignores the increase in pressure
drop as porosity decreases. The lifetime expected in actual
practice would therefore always be less than the lifetime calcu-
lated from the void-filling approach. Efficiency of collection
is estimated as 82% using size distribution data obtained by
Southern Research Institute from an Oldsmobile 350 CID diesel.
Uncertainties in the calculations which were used in the
development of the above design, beyond the accuracy of the pre-
dictive equations themselves, include the effects of 1) condensa-
tion of hydrocarbons or water, especially during cold starts,
2) reentrainment and bounce of collected particles, and 3) dust
load. Uncertainties regarding the effects of dust load are such
that empirical verification of the practicality of the above
configuration is required.
iv
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EXHAUST GAS
OUTLET BELOW
FILTER
RECTANGULAR
FIBROUS FILTER
ELEMENT
EXHAUST GAS
INLET ABOVE
FILTER
0.10m
Figure £-1. Prototype fibrous filter.
-------
Several authors have reported on the use of filtration as
a means of collecting diesel particulate. Springer and Stahman
tested a total of 48 combinations of devices and identified a
best system for particulate removal. The system consisted of
two alumina-coated, steel wool-packed filters and initially re-
duced the exhaust particulate by 64%. However, the collection
efficiency decreased rapidly with distance, accompanied by a
sharp increase in system backpressure. The high backpressure
of the system had no great effect on the fuel economy of the
test vehicle, but the acceleration rate, already a weak point
on diesels, was reduced by 20%.
Sullivan et al. examined six different filter materials
in a study concerned with emission of underground diesel engines.
Although good collection efficiencies were reported, relatively
high backpressures were experienced, and the rate of increase
of backpressure was too high for automotive use. General Motors
has also tested a number of filter materials including both paper
and metal mesh filters. The paper filters exhibited high collec-
tion efficiencies, but high backpressures were experienced.
The metal mesh filters showed efficiency degradation and, in
some cases, evidence of filter destruction from incineration
was observed.
The Eikosha Company in Japan has been conducting develop-
mental work on a particulate collection device called an Aut-
Ainer intended for use on both gasoline and diesel vehicles.
Figure E-2 shows one of these. The initial concept for this
device was to provide for the collection of emissions by con-
densation growth with collection on a mesh material. The origi-
nal system consisted of a number of expansion chambers followed
by regions filled with metal mesh to serve as a collection medium.
A ram air cooling tube was also provided down the center on the
device. This device has been carried through a number of stages
of development using an empirical approach. At the current stage
of development, the device has a collection efficiency of about
70% when clean. However, it is necessary to clean it every 2,000 km
(1,200 mi). Without cleaning, the device will act as an agglomerator
For this reason, the developers are investigating the use of
a post-collection device. One scheme is shown in Figure E-2.
After the gases pass through the mesh material, they swirl through
the cyclone dumping the spilled particles into a collection bag.
This portion of the system still needs a lot of work, but the
approach seems promising.
The review of published work on filtration devices has not
revealed data on collection efficiency as a function of particle
size or on the applicability of existing filtration theory to
the various devices which have been tested. It is therefore
not possible to directly relate the proposed fibrous filter proto-
type discussed previously with the prior studies without additional
exper imentation.
vi
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EXHAUST
AIR COOLING
SOOT
COLLECTOR
Figure E-2. Aut-Ainer filter with cyclone soot collector.
vn
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Electrostatic Precipitation
The electrostatic precipitation process consists of the
fundamental steps of particle charging, particle collection,
and the removal of the collected material from the collection
and discharge electrodes. The particle charging process is ac-
complished through the creation of an electric field and a corona
current by applying a large potential difference between a small-
radius electrode and a much larger electrode, where the two elec-
trodes are separated by a region of space containing an insulat-
ing gas. Particle charging is essential to the precipitator
process because the electrical force that causes a particle to
migrate toward the collection electrode is directly proportional
to the charge on the particle. The most significant factors
influencing particle charging are particle diameter, applied
electric field, current density, and exposure time. Particle
collection rates for a given value of particle charge are a
function of particle size, the electric field in the region of
the collection electrode, gas flow rate, gas viscosity, and
electrode geometry. Removal of the collected material is usually
accomplished by mechanical vibrations of the collection and
discharge electrodes, but irrigated electrode systems are also
in use which employ sprays of liquid to clean the electrodes.
The electrical resistivity of the particulate matter for dry
applications strongly influences the collection efficiency which
can be achieved with a given electrode geometry.
An electrostatic control device for diesel particulate has
the potential advantage of providing high efficiency collection
of small particles with low backpressure and low energy consump-
tion. However, there are two serious problems, namely particle
reentrainment and current leakage through conductive films on
high voltage insulators, which must be overcome in order to allow
a practical electrostatic device to be employed. Removal and
disposal of the collected particulate matter must also be accomp-
lished in a practical manner.
As a result of the effectiveness of electrostatic precipita-
tion in small particle collection, two prototype devices have
been proposed for construction and evaluation. Both devices
separate the particle charging and particle collection step as
a result of the special requirements needed for diesel particu-
late collection. Figure E-3 is a conceptual sketch indicating
the basic features of a device conceived by Southern Research
Institute, which employs a periodic wet flushing scheme to clean
the collected particulate thoroughly from all internal surfaces
of the system. A vertically oriented, cylindrical geometry
appears best suited to such a cleaning method, and optimum from
the standpoint of structural strength. A two-stage device was
selected to provide a maximum collecting surface with the space
viii
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TO SPRAY
NOZZLES
INSULATOR
CABLE TO HIGH
VOLTAGE SUPPLY
SPRAY NOZZLES
INLET
CHARGING
SECTION
u
0.20 m DIA
COLLECTING
SECTION
0.30 m LONG
(12 in.)
VJJ//( / /' / 777 /77V
CABLE TO HIGH
VOLTAGE SUPPLY
STAR-SHAPED
ELECTRODES
(DETAIL)
SUPPORT SPIDER FOR
GROUNDED CYLINDERS
SUPPORT SPIDER FOR
HIGH VOLTAGE
CYLINDERS
INSULATOR
SUPPORT SPIDER
OUTLET
REMOVABLE
FIBER FILTER
STORAGE TANK
Figure E-3. First prototype electrostatic precipitator for collecting
diesel particulate.
IX
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limitations of a vehicular installation. The exhaust gas from
the engine enters the precipitator tangentially so as to avoid
immediate impaction on the high voltage insulators. The cyclonic
motion of the gas in the first stage (upper half) of the pre-
cipitator has little effect on particulate collection by impac-
tion, since most of the particles are very fine; however, the
circular path allows for adequate charging time for the particles
before they enter the collecting stage.
Particle charging is achieved by means of an electrical
corona discharge from flat, star-shaped electrodes mounted on
the axial rod extending downward from the insulator at the top
of the device. A corona ball on the end of the rod suppresses
discharges to the grounded plates in the collecting stage. This
structure is preferred over a more conventional fine-wire corona
discharge electrode because of its ruggedness.
The collecting stage consists of a set of concentric cylinders.
In sequence of decreasing diameter, the odd-numbered cylinders
are connected to electrical ground, and the even-numbered cylinders
are connected to the output of a high voltage power supply.
The high voltage cylinders are connected together by a metal
bus bar and nested between the grounded insulators. Insulating
spacers between cylinders are avoided in order to minimize leak-
age resistance due to fouling by low resistivity material. Three
stand-off insulators are used to support the entire array of
high voltage cylinders.
The precipitator is to be cleaned periodically by spraying
a nonvolatile liquid through the nozzles at the top of the de-
vice. The liquid is pulled through a filter at the bottom of
the precipitator and then pumped into a storage tank for the
next cleaning cycle. The period between flushing operations
would probably be governed by the length of time required for
the particulate buildup on the insulators to develop a signifi-
cant current leakage path between high voltage components and
ground. Provisions would be required to bypass the precipitator
during the cleaning operation, which might take 30 to 60 s.
The 0.20-m diameter and 0.15-m long charging region and
the collection area of this device allow an estimation of col-
lection efficiency to be made using mathematical relationships
that describe the particle charging and collection processes.
If favorable electrical conditions can be maintained, the collec-
tion efficiency of 0.2-um diameter particles from an exhaust
gas stream of 0.14 m3/s is estimated to be 80%.
A second electrostatic device that has been proposed by
Castle for diesel exhaust is a radial flow device, which uses
dielectric filter material in the collection region. This de-
vice, shown in Figure E-4, has the following features:
-------
HIGH
VOLTAGE
COLLECTOR
0.64 m (25 in.)
0.61 m (24 in.)
<> J
0.15 m (6 in. )-
0.13 m (5.25 in.)
0.08 m (3.25 in.)-
0.03 m (1.25 in.)
V
PERFORATED
METAL
SCREENS
,—— -
.— —
S'
«-»i
^ ••
^^,
^N
^*X
N
4 a\ i \
\ 1 1
V..
^
Ni
\
L.
r j i
FILTER
MATERIAL
CONCENTRIC PRECIPITATOR
WITH DIELECTRIC COLLECTING
MEDIA
Figure E-4. Second prototype electrostatic precipitator for collecting
diesel paniculate.
-------
a) two stage operation, thus minimizing ozone generation
and power consumption,
b) adaption of high velocity gas throughput in small diam-
eter ducts to low radial velocities for collection,
thereby reducing reentrainment,
c) high efficiency for the submicron particle size range,
d) utilization of a combination of mechanical collection
forces as well as coulombic, dielectrophoretic, and
image electrical forces,
e) convenient geometry for using electrified media in the
collection stage and ready adaptation to removable car-
tridge form.
It is believed that the most effective form of collector
would involve a gradation using three collection zones. The
first would be a mechanical impaction collector utilizing the
high velocity jets of gas produced by the inner perforated screen
that changes the gas flow from the axial to the radial direction.
The second zone could be a relatively coarse fibrous bed of
collection media with superimposed electrostatic field. The
final zone would be finer graded bed of fibrous media with a
superimposed electrostatic field.
The efficiency of this device is harder to predict. If
the effects of the dielectric material are neglected and the
particles are assumed to have acquired the same charge as in
the wire-cylinder example earlier in this section (admittedly
a bad assumption), then using the metal collection plate area
of 1.2 m2 an efficiency of 70% can be calculated. The effect
of the dielectric material will be to increase the collection
efficiency. Figure E-5 shows the results of using various geo-
metries of dielectric material in a similar device. Note that
the collection efficiency for the device was improved with any
dielectric inserted and that the most efficient case was for
a fibrous bed similar to that proposed here.
This device does have two possible design drawbacks. The
first of these is the susceptibility to conductive contamination
on the insulators. This will probably be worse on the ionizer
and may necessitate the use of a different type of ionizer.
The second is the increase in backpressure that will result when
the dielectric fiber bed becomes loaded with particles. The
magnitude of this problem can only be determined experimentally.
Wet Scrubber
The collection of particulate in a wet scrubber is accomp-
lished through various mechanisms, including inertial impaction,
xii
-------
ui
TV. GEOMETRY E
I^siI I I
u
o
u
UJ
99.9 8
0.2 0.4 0.6 0.8 1.1 2.0 4.0
PARTICLE SIZE, urn
B
NOTHING IN COLLECTION
REGION
TEN ACRYLIC RODS
TWENTY-FOUR
FLAT ACRYLIC
PLATES IN TWO
CONCENTRIC
ARRAYS
INSULATED COPPER
WIRE MAT
FIBROUS MEDIA
Figure E-5. Collection efficiency of a concentric precipitator with
dielectric collection media.49
Xlll
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gravitational collection, diffusion, electrostatic collection,
and thermophoresis and diffusiophoresis. Mathematical descrip-
tions of these mechanisms were employed to calculate particle
collection efficiencies in various scrubber designs that might
be employed for collection of diesel particluate. The general
conclusion derived from these calculations was that wet scrubbers
are not suitable for removing particulate from diesel exhaust.
Under the constraint of a 2,500-Pa pressure drop (10 in.
of water), scrubbers are not efficient enough in removing the
fine particles present in diesel smoke. A possible exception
is the charged droplet scrubber, but it is not clear that this
device would be preferable to some other type of electrostatic
device.
The above conclusions are reinforced by consideration of
the rate of water loss by evaporation in a wet scrubber. Diesel
exhaust entering a scrubber at 200°C at a rate of 0.14 m3/s
(300 ftVmin) will contain about 3% by volume water vapor as
the result of fuel combustion in the engine. This rate corre-
sponds to the flow of 1.97 g/s of water vapor. On leaving the
scrubber at 50°C with water vapor at the saturation level (con-
centration, 12.17% by volume), the gas stream will carry water
away at the rate of 8.83 g/s. Thus, water will be lost at the
rate of 6.86 g/s. This is equivalent to about 6.86 x 10~6 m3/s.
For comparison, the rate of fuel consumption may be calculated
by assuming that the exhaust flow rate corresponds to a highway
speed of 1.61 km/s (60 mi/h) and that the fuel consumption rate
is 2.52 x 10~6 m3/s (25 mi/gal, or 0.04 gal./min). The conclusion,
therefore, is that water would be consumed at 2.7 times the rate
of consumption of diesel fuel.
Conclusion
From the preceeding discussion, it is possible to draw some
conclusions regarding the applicability of scrubbers, electro-
static devices, and filters to the problem of controlling diesel
particulate emissions. The first of these, scrubbers, can be
eliminated from further consideration due to the large size
required to obtain adequate efficiencies at reasonable backpres-
sures, and to the high rate of water consumption.
Second, electrostatic devices merit further consideration
for the important reasons that they present very little back-
pressure to the system and that they maintain a relatively high
collection efficiency over the 0.1- to 0.8-ym particle size range
over which, according to theory, a dip in filter efficiecy occurs.
To offset this, they have the severe disadvantage of dysfunction
due to conductive particle contamination. Further research will
have to be performed to design a system which can eliminate this
difficulty. A lesser problem is that of disposal of the collected
xiv
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particulate. A method must be developed whereby the device may
be cleaned and the collected particulate properly and safely
disposed of as a matter of routine maintenance.
The third mechanism, filtration, demonstrates the same
problem of disposal of the collected particulate. Another prob-
lem area is the possibility of high backpressure. Although it
is possible to design a device of appropriately low backpressure
when new, only experimentation will determine the amount of
particulate the filter can collect without elevating the back-
pressure beyond allowable limits and adversely effecting engine
performance. The problem of surface seal over can be avoided
by utilizing a filter with a large face area such as the proposed
prototype. The advantage of filtration is that it is a mechani-
cally simpler concept. Filters, while not necessarily simple
to design, are simpler to build and maintain in the field than
electrostatic devices and as a consequence should be cheaper
and more convenient in use.
Both electrostatic devices and filtration devices show
promise of becoming workable solutions to the diesel exhaust
problem. However, more research is needed on both types of
devices. Prototypes need to be fabricated and tested on actual
diesel exhaust streams. Determinations of collection efficiency
as a function of particle size would be useful in the study of
the collection mechanisms involved, which should in turn lead
to the development of improved collection devices. Testing of
prototype devices will also yield additional insights into the
particulate problems that affect each device.
Additional study is also needed on the effect of gas stream
temperature on the hydrocarbon portion of the particulate. A
study by Black and High indicates that most of the condensation
and subsequent adsorption of vaporous hydrocarbons occurs in
the last few feet of the vehicle's tail pipe. This implies that
a temperature reduction in the exhaust system would increase
the amount of hydrocarbons which condense and can therefore be
collected as particulate. Supportive evidence has been given
by Masuda, who reports that Eikosha has seen a 20% by volume
increase in collected particulate occur when the cooling air
flow on the Aut-Ainer filter was increased.
xv
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CONTENTS
Executive Summary iii
Figures xvii
Tables xix
Acknowledgments xxi
1. Introduction 1
2. Characteristics of Diesel Emissions 6
3. Filtration 12
Filtration Fundamentals 12
Previous Uses of Filtration for Diesel Exhaust 27
Filter Characteristics for Diesel Particulate
Emissions 33
Prototype Filter for Diesel Exhaust 43
4. Electrostatic Devices 49
Fundamental Steps in the Electrostatic
Precipitation Process 49
Limiting Factors Affecting Precipitator
Per f ormance 58
Previous Uses of Electrostatic Devices for
Diesel Exhaust 61
Application of Electrostatic Fundamentals to
Diesel Exhaust 63
Description of Prototype Electrostatic Devices 65
5. Wet Scrubbers 71
Description of Scrubber Processes for Collection
of Particulate Matter 71
Discussion of the Literature on Wet Scrubbers 78
Calculation of Scrubber Efficiencies for
Particulate Control 79
Further Considerations 92
6. Conclusion 95
7. References 97
Appendices
I. Particle Size Measurements of Automotive
Diesel Emissions 102
II. Vapor Pressure Versus Temperature Data on Organic
Compounds Listed by Menster and Sharky for
Diesel Exhaust 114
Bibliography 119
xvi
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FIGURES
Number Page
E-l Prototype fibrous filter v
E-2 Aut-Ainer filter with cyclone soot collector vii
E-3 First prototype electrostatic precipitator for
collecting diesel particulate ix
E-4 Second prototype electrostatic precipitator for
collecting diesel particulate xi
E-5 Collection efficiency of a concentric precipitator
with dielectric collection media xiii
1 Summary of diesel engine variables effecting soot
particulate formation 2
2 Size distribution of diesel particulate 7
3 System efficiency and exhaust restrictions as
functions of distance traveled 29
4 Aut-Ainer filter with cyclone soot collector 30
5 Filter efficiency versus volume 38
6 Pressure drop versus filter volume 40
7 Maximum lifetime (mi) versus filter volume 41
8 Prototype fibrous filter 45
9 Fibrous filter efficiency versus particle diameter.... 48
10 Region near small-radius electrode 51
11 Electric field configuration for wire-plate geometry.. 51
12 Electric field configuration during field charging.... 53
xvn
-------
FIGURES (concluded)
Number^ Pag<
13 Electric field configuration and ion distribution
for particle charging in an applied field after
saturation charge is reached 54
14 Electric field configuration and ion distribution
for particle charging with no applied field 54
15 Electrostatic particulate remover 62
16 First prototype electrostatic precipitator for
collecting diesel particulate 66
17 Second prototype electrostatic precipitator for
collecting diesel particulate 68
18 Collection efficiency of a concentric precipitator
with dielectric collection media 70
19 Diagram of sieve plate bubble scrubber 91
XVlll
-------
TABLES
Number Page
1 Rate of Fall of Spherical Particles in Still Air 3
2 Proposed Emission Standards for Light-Duty Vehicles... 4
3 Particulate Emission Rates from Light-Duty Diesels
Tested Over the EPA Recommended Test Procedure 4
4 Physical Characteristics of Diesel Particulate 6
5 Chemical and Physical Properties of Diesel Fuel 8
6 A Summary of Compounds Extracted from Diesel
Particulate Matter 9
7 Carcinogenic Compounds Found in Diesel Exhaust
Particulate Emissions 10
8 Values of H for Use in Equation 3.10 18
9 Performance Results of Filter Materials 28
10 Summary of Filtration Materials Tested by
General Motors 31
11 Estimated Filter Performance for Base-Case
Conditions 34
12 Diesel Particulate Emission Properties: Base
Conditions Used for Filter Design 35
13 Prototype Filter Sensitivity Analysis 46
14 Required Collection Area for a Desired Collection
Efficiency for an Electrostatic Precipitator 65
15 Operating and Design Parameters of Scrubbers 76
16 Efficiency of Particulate Removal in a Venturi
at 30°C 83
xix
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TABLES (concluded)
Number Page
17 Efficiency of Particulate Removal in a Venturi
at 50°C 84
18 Minimum Orifice Velocity and Maximum Superficial
Velocity in a Seive Plate Bubble Scrubber 88
19 Scrubber Parameters and Single-stage Efficiency in
a Sieve Plate Bubble Scrubber at 30°C 89
20 Scrubber Parameters and Single-stage Efficiency
in a Sieve Plate Bubble Scrubber at 50°C 89
21 Column Diameters of a Sieve Plate Bubble Scrubber 90
22 Pressure Drop Values in a Single Stage of a Sieve
Plate Bubble Scrubber 90
23 Summary of Calculations for a Sieve Plate Bubble
Scrubber 93
xx
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ACKNOWLEDGEMENTS
In addition to the authors of this report, certain other
members of the staff of Southern Research Institute made signi-
ficant contributions to the report. These staff members are:
Grady B. Nichols, Associate Director of Engineering and
Applied Sciences. (Mr. Nichols made a trip to Japan
to investigate the use of filters in that country for
the control of diesel particulate.)
William E. Farthing, Research Physicist.
Ralph B. Spafford, Research Chemist.
In addition to staff members of the Institute, other con-
tributors who served as consultants in various areas were as
follows:
David H. Leith, Tom Kalinowski, and Mike Ellenbacker
of Leith Environmental Research Corporation, Newton,
Massachusetts 02164 — particulate control by filtration.
G.S.P. Castle, University of Western Ontario, London,
Ontario (Canada) N6A5B9 — electrostatic precipitation.
M.J. Pilat, B.D. Wright, and J.M. Wilder, University
of Washington, Seattle, Washington 98195 — wet scrubbing,
xxi
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SECTION 1
INTRODUCTION
The Federal Government has issued a regulation that requires
the average fuel mileage for each auto manufacturer to be at
least 27.5 mi/gal, by the year 1985. The diesel engine, accord-
ing to various estimates, yields 25 to 75% better fuel mileage
than does a gasoline engine of the same displacement. For this
reason, the automobile manufacturers are showing an interest
in widespread use of diesel engines in light-duty vehicles.
General Motors, in particular, has indicated that it may have
to depend on the diesel to meet the 1985 requirement.
Unfortunately, one characteristic of the diesel engine is
a high level of combustion by-products. Light-duty diesels
produce particulate at rates ranging from 0.25 g/mi (Volkswagen
Rabbit) to 0.80 g/mi (General Motors 350).* These amounts are
50 to 100 times as much particulate as is produced by engines
using unleaded gasoline. The source of this particulate lies
in the combustion process. In a diesel engine, fuel is sprayed
into hot compressed air in the cylinder where it vaporizes and
ignites spontaneously. Most of the fuel is burned in this pro-
cess, but a small amount is cracked, polymerized, or partially
oxidized into a large array of compounds including elemental
carbon. The variables which affect this process are summarized
in Figure 1. In the exhaust cycle, the products of combustion
enter the relatively cooler exhaust system where they are subject
to agglomeration and condensation. Beyond the exhaust system,
the exhaust enters the outside environment where it is diluted
and further cooled.
The particulate that results from diesel combustion con-
sists largely of agglomerates of very small particles of carbon
(on the order of 0.01 pm in diameter),3 which have condensed
hydrocarbons adsorbed onto them. These particles have a mass
median diameter (mmd) of 0.3 ym, and 70% have diameters smaller
than 1 vim1* (Reference 4 is included in this report as Appendix I) .
* EPA has defined particulate as everything collected on a 52°C
(125°F) filter after dilution with ambient air in a dilution
tunnel.
-------
C PARTICULATE
DIESEL EXHAUST
AMOUNT OF
INCOMPLETELY MIXED
FUEL ft ITS
EQUIVALENCE RATIO
FUEL INTRODUCTION •
AFTER IGNITION
IGNITION DELAY
RATE OF MIXING -
t
FLAME TEMPERATURE
SOOT BURNUP
TIME
AVAILABLE
UNMIXED FRACTION
OF FUEL ACCUMULATED
DURING DELAY
OVERALL FUEL/AIR RATIO
RATE OF INJECTION.
FUEL DISTRIBUTION
PREMIXING DURING
DELAY
COMPRESSION RATIO
HEAT LOSS TO WALLS
AIR VELOCITY &
TURBULENCE
RATE & TIMING
OF HEAT RELEASE
HEAT LOSS TO WALLS
COMPRESSION
RATIO
OVERALL FUEL/AIR RATIO
MIXING OF COMBUSTION
PRODUCTS
ENGINE SPEED
RATE ft DURATION
OF INJECTION
FUEL INJECTION PRESSURE
CETANE NO.
OF FUEL
COMPRESSION
RATIO
INJECTION
INJECTION NOZZLE
CONFIGURATION OF
FUEL SPRAYS
FUEL ATOMIZATION
ft EVAPORATION
FUEL PENETRATION
ft IMPINGEMENT
AIR VELOCITY a
TURBULENCE
INJECTION NOZZLE
NO. OF HOLES
OIA. OF HOLES
LENGTH OF HOLES
FUEL INJECTION
PRESSURE
COMBUSTION CHAMBER
DESIGN
ENGINE SPEED
INLET PORT DESIGN
NOTE UNDERLINED VARIANCES ARE
DIRECTLY CONTROLLABLE
INCREASE IN VARIABLE INCREASES
EXHAUST SMOKE
INCREASE IN VARIABLE REDUCES
EXHAUST SMOKE
Figure 1. Summary of diesel engine variables effecting soot
paniculate formation.?
-------
Particles of this size remain airborne for extended periods of
time as shown in Table 1. According to this table, a 0.1-ym
particle will take 5 days to fall 40 cm from the exhaust pipe
to the ground in still air. Of course, the air will be quite
turbulent, which means that the particles may be transported
to greater heights, so that they remain airborne longer. Alter-
natively, the particles may impact onto the ground or onto other
particles, thus reducing the time in suspension. In general,
however, the suspension time may be measured in days. since
these particles fall into the respirable size range, the suspen-
sion time is directly related to the exposure time for human
beings.
TABLE 1. RATE OF FALL OF SPHERICAL PARTICLES IN STILL AIR5
Particle diameter Rate of fall
(urn) (cm/sec)
100 30
10 0.3
1 0.003
0.1 0.00009
Laboratory tests on the hydrocarbons derived from diesel
particulate show a high level of toxicity. Some of the compounds,
notably the polynuclear aromatics such as benzo(a)pyrene, are
known to be carcinogenic. Others have been found to be mutagenic
in the Ames Salmonella test but have not yet been proven carcino-
genic. However, mutagenicity in the Ames test has been shown
to have 85 to 90% correlation to carcinogenicity in animal tests.6
Still other compounds that are known to be noncarcinogenic and
nonmutagenic, such as perylene, have been transformed to muta-
genic substances by exposure to low concentration of NO (about
1 ppm) in the presence of traces of HNO as is found in common
photochemical smog.7 The transformation of a normally nonmuta-
genic pollutant into a mutagen by another pollutant indicates
the complexity of the air pollution problem and the danger of
attempting to isolate the effects of any one pollution element.
The Environmental Protection Agency has proposed standards
for regulating diesel emission.8 These standards are given in
Table 2. The particulate emission characteristics for several
diesel automobiles are shown in Table 3. From this table, it
can be seen that several manufacturers are not affected by the
1981 particulate standard, although all of the General Motors
vehicles are affected. However, all manufacturers are affected
by the 1983 standard.
-------
TABLE 2. PROPOSED EMISSION STANDARDS FOR LIGHT-DUTY VEHICLES8
Type of emission
Hydrocarbons (total)
Carbon monoxide
Nitrogen oxides
Particulates
1981*
0.41
3.4
1.0
0.6
1983*
0.41
3.4
1.0
0.2
* Emission rates are in g/mi.
TABLE 3. PARTICIPATE EMISSION RATES FROM LIGHT-DUTY DIESELS
TESTED OVER THE EPA RECOMMENDED TEST PROCEDURE8
Grams per
Type of vehicle mile
Typical gasoline-powered vehicle (catalyst equipped) 0.008
VW Rabbit 0.23
Peugeot 504 0.29
VW Dasher 0.32
Mercedes 300SD 0.45
IHC Scout (with Nissan diesel) 0.47
Mercedes 240D 0.53
Chevrolet pick-up (with Oldsmobile 350 diesel) 0.59-0.61
Dodge pick-up (with Mitsubishi diesel) 0.81
Oldsmobile 260 0.73-1.02
Mercedes 300D 0.83
Oldsmobile 350 0.84
-------
In order to meet these standards, automobile researchers
are considering a wide variety of particulate control methods.
Many of these are outside the scope of this report but will be
briefly mentioned here for completeness. The first of these
is fuel modification. General Motors has shown a 27% reduction
in particulate emission on the 1975 Federal Test Procedure by
using special blends of fuel in their 350 CID Oldsmobile. General
Motors has also demonstrated a 17% particulate reduction by using
additives in standard fuels. A third modification is the use
of a water-in-fuel emulsion. The theory is that, on injection,
the water explodes into super-heated steam which shatters the
fuel droplet into a finer mist, thereby providing a better fuel-
air mixture. General Motors has shown a 7% particulate reduc-
tion using water-in-fuel emulsions.9
Another direction of investigation involves improving the
combustion mechanism in the engine. Several investigators are
examining the possibility of redesigning the combustion chamber
for greater efficiency. With current designs, however, lower
particulate emissions have been achieved by turbocharging, which
increases the available oxygen in the combustion chamber and
promotes better fuel-air mixing by increasing turbulence. An
example of this approach is the Mercedes 300SD, which emits
0.45 g/mi of particulate, compared to 0.83 g/mi for the Mercedes
300D.9
A third direction for reduction of particulate emissions
is aftertreatment of the exhaust gases. Aftertreatment offers
the possibility of much greater effectiveness than fuel or engine
modifications. However, it has the disadvantages of requiring
separate maintenance and allowing relative ease of circumvention.
One form of aftertreatment that will not be discussed in depth
in this report is catalysis. Although devices for reducing the
gaseous output of diesel engines are commercially available,
removal of particulate by catalysis is a largely unexplored area
being investigated elsewhere.
Some other devices for exhaust aftertreatment are scrubbers,
electrostatic devices, and filters. These are devices which
have found widespread application for controlling emissions of
stationary sources. Their extension to control of mobile diesel
particulate is the subject of this report. The following chapters
discuss the theory of operation of each of these devices and
the applicability of each of these devices to the diesel control
problem. In addition, a survey of previous attempts to use these
devices on mobile diesel sources is presented along with designs
of prototype devices which may be fabricated and tested.
-------
SECTION 2
CHARACTERISTICS OF DIESEL EMISSIONS
The particulate emissions from diesel combustion have the
form of branched chains of particles about 0.01 ym in diameter.3
These agglomerates have an mmd of 0.3 ym with 70% smaller than
1 ym.1* Figure 2 illustrates the size distribution data measured
at temperatures from 270 to 430°C on an Oldsmobile diesel by
Southern Research Institute. These data show good agreement
with size distribution data measured on other vehicles.3'6'10
Although the density of the soot particles, which are primarily
elemental carbon, is about 2,000 kg/m , the open structure of
the diesel particulate gives rise to a very low bulk density
of 120 kg/m .ll If this bulk density is used with the emission
data from Table 3, it can be seen that the output volume per
1,000 mi ranges from 2.1 x 10~3 m3 (0.55 gal.) for the Volkswagen
Rabbit to 6.9 x 10~3 m3 (1.8 gal.) for the Oldsmobile 350. At
highway speed, this particulate is delivered in exhaust gas at
a rate of about 0.14 m3/s (300 ft3/min) with a mass loading of
7 x 10~5 kg/m3 (0.03 gr/ft3). The physical data for diesel
particulate are summarized in Table 4. The properties of diesel
fuel are given in Table 5.
TABLE 4. PHYSICAL CHARACTERISTICS OF DIESEL PARTICULATE
Parameter Magnitude Reference
Individual particle size 0.01 ym 3
Agglomerated particle size
mmd 0.3 ym 4
% smaller than 1 ym 70
Exhaust temperature
Manifold 190-275°C 10
Muffler 164-210°C 10
Bulk density 120 kg/m3 11
Gas flow rate (estimated for
200°C) 0.14 m3/s 4
Mass loading 7 x 10~5 kg/m3 11
-------
99-99
1
p—
^
^
Ld
H
5
g
99. Bf
99.5-;
99-!
98 j
35-.
.
»
»
V
L
»
90ir
80^
704
604
5Oi
4O1
L
•
•
»
M
»
sol
104
5-j
t
»
•
»
•
•
?I
j» ^
0.5f
O.E±
O«l*
O«o5$
n.ni X — •
.i1
I
m.a
•i IMPACTOR
_• ONLY
.*•*
^w^
J»*
•
•
INTEGRATION OF
0 (0.5-10)
•
•
•
0«»
•
4 1 — 1 1 1 1114 1 1 -I 1 1 1 1 l-l 1— J — 1 1 I 1 1 U
10"5 1O"1 10° 101
PARTICLE DIAMETER (MICROMETERS)
Figure 2. Size distribution of diesel paniculate.*
-------
TABLE 5. CHEMICAL AND PHYSICAL PROPERTIES OF DIESEL FUEL11
Flash point, °F minimum 125
Pour point, °F maximum -5
Water and sediment, percent by volume, maximum 0.05
Carbon residue on 10% residuum, percent maximum 0.35
Distillation temperature, °F
90% point, maximum 650
End point, maximum 700
Distillation recovery, percent by volume, minimum 97.0
Viscosity at 100°F, SSU
Minimum 32
Maximum 45
Sulfur, percent by weight, maximum 0.75
Cetane Number, minimum 45
Gravity, degrees API 38
The characteristic oiliness of diesel particulate is due
to condensed hydrocarbons adsorbed onto the agglomerated soot.
An analysis of the collected particulate indicates that 33 to
50% is composed of organic substances.11 Table 6 lists some
of the compounds extracted from diesel particulate. The class
of polynuclear aromatics, in particular, is associated with
health problems, especially with lung cancer. Table 7 shows
the relative carcinogenicity of some of these polynuclear aro-
matics. Detailed vapor pressure data for these and other com-
pounds found in diesel exhausts are found in Appendix II.
The concentration of benzo(a)pyrene, one of the more potent
carcinogens of the polynuclear aromatics, is often used as an
indication of the potential health danger of exhaust products.
Although the benzo(a)pyrene concentration of light duty diesel
engines is equivalent to that of noncatalyst gasoline engines,
it is an order of magnitude greater than the levels found in
catalyst-equipped gasoline engines. In addition, there is some
evidence that the high particulate level of diesel exhaust can
increase the pulmonary retention time of substances such as
benzo(a)pyrene, thereby increasing the chance of absorption by
the body.
8
-------
TABLE 6. A SUMMARY OF COMPOUNDS EXTRACTED FROM DIESEL
PARTICULATE MATTER12
Acidic Compounds
Paraffinic Compounds Aromatic Compounds
Phenol
Cresol
Dinitro-o-cresol
Benzoic acid
o,m,p-Phenylphenols
Basic Compounds
Pyridine
Aniline
Benzacridene
o-Toluidine
Transitional Compounds
Dioxane
Methoxyphenanthrene
Pentane
Hexane
Octane
n-Tetradecane
n-Pentadecane
n-Hexadecane
n-Octadecane
n-Nonadecane
n-Eicosane
n-Heneicosane
Cyclohexane
Methylcylohexane
Oxygenated Compounds
Hydroquinone
9,10-Anthraquinone
Furfuryl alcohol
Cyclohexanol
2-Hexanone
Methylacetate
Crotonaldehyde
2-Pentanone
Benzene
Toluene
Diphenyl
Anthracene
Styrene
Xylene
Pyrene
Chrysene
Benzo(a)pyrene
Benzo(j)fluoranthene
Benzo(b)fluoranthene
Tr imethylnaphthalene
Ethylfluorene
Dimethylphenanthrene
-------
TABLE 7. CARCINOGENIC COMPOUNDS FOUND IN DIESEL EXHAUST
PARTICULATE EMISSIONS13
Carcinogenic compound with
Formula corresponding formula
Carcinogenicity*
Molecular
weight
CisHia Chrysene
Benzo (c) phenanthrene
Benz (a) anthracene
C2oHi2 Benzo(a)pyrene
Benzo (b) fluoranthene
Benzo (j ) fluoranthene
CaoHm Benz ( j) aceanthrylene
C2oHi 6 7,12-Dimethylbenz (a) anthracene
CaiHiH Dibenzo(a,g) fluorene
C2oHi3N Dibenzo(c,g)carbazole
CaiHie 3-Methylcholanthrene
C22H16 Indeno(l,2,3-cd)pyrene
Dibenz (a, h) anthracene
Dibenz) a/ j) anthracene
Dibenz (a,c) anthracene
C2iHi3N Dibenz (a, h) acridine
Dibenz (a, j) acridine
Ca^Hut Dibenzo(a,h)pyrene
Dibenzo(a,i)pyrene
Dibenzo(a/l)pyrene
228.0936
252.0936
254.1092
256.1248
266.1092
267.1045
268.1248
276.0936
278.1092
279.1045
302.1092
* The carcinogenicities are given in "Particulate Polycyclic Organic
Matter," National Academy of Science/ Washington, D.C., 1972,
according to the following code:
± uncertain or weakly carcinogenic
+ carcinogenic
-H-, -HH-, i M-I , strongly carcinogenic.
10
-------
Consideration of the particle characteristics yields some
insight into the problems associated with adapting control de-
vices for stationary sources to mobile sources. The ultrafine
particle size places the particulates in a region where collec-
tion devices are least efficient. Increasing the efficiency
by increasing the device size is limited by the space available.
On the other hand, increasing the efficiency by increasing the
air stream blockage is limited by the allowable backpressure
on the engine. Assuming a suitable efficiency is achieved, the
next problem is storage of the collected particulate. At 90%
collection, a treatment device for the Oldsmobile 350 diesel
will have to store 0.019 m3 (5 gal.) of particulate in 4,800 km
(3,000 mi), assuming no compaction. The collected particulate
is then subject to reentrainment by the relatively high 0.14-m3/s
(300-ft3/min) flow rate.
11
-------
SECTION 3
FILTRATION
FILTRATION FUNDAMENTALS
Introduction
Filtration is a commonly used particle removal process with
applications ranging from dust removal from household heating
and ventilation systems to high efficiency removal of radioactive
particles in the nuclear industry. At a fundamental level, filtra-
tion is a complex process involving several particle collection
mechanisms for which theoretical models are not entirely ade-
quate. Filter design often relies on emiprical studies.
The following sections review the fundamental particle col-
lection mechanisms which affect particle removal in filters,
either separately or in combination depending on the character-
istics of the aerosol, filter, and application. Available^ quanti-
tative methods for predicting particle collection in fibrous
filters and granular bed filters are then reviewed.
Fundamental Particle Collection Mechanisms
Filtration theory attempts to predict overall particle re-
moval in a filter based on an understanding of the interaction
of particles with a single filter element, i.e., a fiber, granule
or previously collected particles. This requires an understanding
of the flow field around single collectors including the manner
in which the single collectors influence each other in the assembly,
and the mechanisms by which particles are transferred from the
gas stream to the collector element.
The single element efficiency, r\, is defined as the number
of particles collected by a filter element divided by the number
of particles whose centers pass through the element's projected
area as the particles approach it. The major mechanisms by which
particles collect on a filter element are listed below and de-
scribed in the following sections:
1. diffusion
2. interception
12
-------
3. inertial impaction
4. gravity settling
5. electrostatic deposition
6. thermophoresis
7. flux forces
A single element efficiency due to any one mechanism, m»
can be estimated for each of the fundamental collection mech-
anisms; however, it is frequently the case that one mechanism
does not dominate. In this case it is necessary to combine two
or more mechanisms when predicting the overall single element
efficiency, n. As a first approximation, it is often assumed
that particles not collected by one mechanism may be collected
by others such that
n = 1 - IT (1 - n ) (3.1)
where ni is the computed single element efficiency for each
collection mechanism.
Diffusion—
For particles in the submicron size range, Brownian motion
enhances the probability of the particle hitting a collector
element as the particle passes the collector. Diffusional depo-
sition has been found to be inversely proportional to the dimen-
sionless Peclet number, Pe, which is the ratio of transport by
convective forces to transport by diffusional forces:
Vd
Pe = j^-£ (3.2)
P
where: V = superficial velocity (m/s)
d = collector diameter (m)
Dp = particle diffusivity (m2/s)
Particle diffusivity, D , can be determined from:
C kT
DP = <3'3>
13
-------
where: C = Cunningham slip correction factor (dimensionless)
c
k = Boltzmann constant (joule/K)
T = absolute temperature (K)
M = gas viscosity (Pa-s)
d = particle diameter (m)
As indicated by the inverse relationship with Peclet number,
the single element efficiency factor for diffusional deposition,
T]Q, increases with decreasing velocity, particle diameter, and
collector diameter.
Interception —
When a streamline passes within one particle radius of a
collector element, a particle on that streamline can be removed
by grazing contact with the collector surface. This removal
process is dependent on particle size and is characterized by
the dimensionless interception number, R:
R =
c
where d = particle diameter (m)
d = collector diameter (m)
G
Interception is not directly dependent on velocity and can be
effective even when diffusional, inertial, or other mechanisms
are ineffective.
Inertial Impaction —
As the gas flows past a collector element, the streamlines
curve around the collector. Near the collector surface, inertia
will cause particles to deviate from the streamlines. Particles
with sufficient inertia can cross streamlines and hit the col-
lector. Inertial impaction has been found to be proportional
to the dimessionless impaction parameter \)j:
p d 2VC
18ydc
where: p = particle density (kg/m3)
d = particle diameter (m)
V = superficial velocity (m/s)
14
-------
C = Cunningham correction factor
c
y = gas viscosity (Pa-s)
d = collector diameter (m)
The impaction parameter is the ratio of particle stopping dis-
tance (a measure of the deviation from the streamline) to the
collector diameter. Single element impaction collection effi-
ciency, rii* is directly proportional to velocity and the square
of particle diameter, and inversely proportional to collector
diameter.
Gravity Settling—
Due to gravitational force, particles will have settling
velocities which can cause deviations from flow streamlines
around collectors. The single element efficiency for gravity
settling, T\Q, is proportional to the ratio of the particle settl-
ing velocity to the superficial gas velocity. Thus, the effect
of gravity settling increases as the bulk gas velocity decreases,
in effect allowing greater residence time in the filter for
settling to occur. Terminal particle velocity is directly pro-
portional to particle diameter squared and density; gravity
settling will thus be enhanced when larger, denser particles
are present.
Electrostatic Deposition—
The presence of charges on the particles or the collector
elements can enhance particle collection. Theories for predict-
ion of electrostatic deposition require estimates of particle
and collector charge and dielectric properties which are not
generally available in practice. Theoretical derivations of
other collection mechanisms generally neglect electrostatic
effects, and experiments are conducted so as to minimize electro-
static effects. Thus, in the absence of an applied electrical
field, it is assumed that filter efficiencies predicted by other
mechanisms will tend to underestimate overall efficiency to the
extent electrostatic forces exist and electrostatic collection
occurs.
Thermophoresis—
If a temperature gradient exists such that particle-laden
gas entering a filter is hotter than the filter elements them-
selves, particle deviation towards the collector elements will
occur. This is caused by more energetic molecular bombardment
on the hotter side of the particle than on the side away from
the colder collector surface. This tends to kick the particle
15
-------
toward the collector surface. The thermal force causing particle
deviation is proportional to the thermal gradient; therefore,
if filters equilibrate to gas temperatures, thermophoresis does
not occur.
Molecular Flux Forces—
Near condensing water droplets, the flux of gas molecules
toward the droplet surface can cause particle deviation toward
the droplet and collection on the droplet. Because droplets
are larger than the particles, and are more easily removed in
some cases, overall particle removal efficiency may be enhanced.
Fibrous Filters
Collection Efficiency—
Prediction of overall efficiency of a bed of fibers requires
estimation of the single fiber efficiency resulting from the
mechanisms described previously. Overall efficiency of a fibrous
filter can be predicted by the following equation:
( 4 L (1 - e) „ )
EPT = 1 - exp { - ? £—<• n [ (3.6)
where: EFT = overall efficiency of fibrous filter (fraction)
L = filter thickness (m)
d,, = fiber diameter (m)
r
e = filter porosity (fraction)
n = single fiber efficiency (fraction)
The fiber diameter is the cross-sectional diameter for a
filter of uniform fibers. For filters of nonuniform fibers,
Chen15 suggests d =(o^)s2/aF", where (cfp)s2 is the surface average
fiber diameter and d is the arithmetic average fiber diameter.
The porosity of fibrous filters is large, generally greater
than 0.90, with 0.99 taken here as a typical value. Porosity
is defined as:
-S (3.7)
PF
16
-------
where: a = solidity (fraction)
= fiber volume
total filter volume
PB = bulk density of filter (kg/m3)
Pp = density of fiber (kg/m3)
The single fiber efficiency factor, r\F , is for the dominant
collection mechanism or combination of collection mechanisms
calculated by equation 3.1. Theoretical and empirical methods
are available to provide quantitative estimates of the single
fiber efficiency resulting from diffusion, interception, impac-
tion, and gravity settling.
The single fiber efficiency factor for particle collection
by diffusion, HQ, in a fibrous filter is estimated by Fuchs and
Strechkina: 1 6
nD = [-% in(l-e) - 0.5] Pe (3'8)
where: e = filter porosity (fraction)
Pe = Peclet number
The single fiber efficiency factor for interception is given
by Davies17 as:
n_ = (1+R) ln(l+R) - %(1+R) + JjU+R)-1 (3.9)
R
where: R = interception parameter
= dp/dF
The single fiber efficiency factor for impact ion in fibrous
filters, nj, has been expressed by Davies17 as a function of
the impact ion parameter i|» as:
n-j. = H\|> (3.10)
where: H = complicated function of porosity and d /d (See
Table 8) p F
\|> = impact ion parameter
The values of H and, thus, rij increase as the fiber diameter
decreases and as porosity decreases. Also, the impact ion param-
eter, i|j, increases as the fiber diameter decreases and as gas
velocity and particle diameter increase.
17
-------
TABLE 8. VALUES OF H FOR USE IN EQUATION 3.10
a* dp(ym)
0.02 20
10
4
2
0.01 20
10
4
2
VdFf
0.01
0.02
0.05
0.10
0.01
0.02
0.05
0.10
H
0.0011
0.0041
0.0255
0.0926
0.0009
0.0034
0.0208
0.0755
* a = 1 - e
t
Particle diameter, d = 0.20 pro
P
The single fiber efficiency for gravity settling, r\ , in
fibrous filters can be estimated by:
(d + dp) V g
where: d = particle diameter (m)
P
d_ = fiber diameter (m)
F
V__ = particle settling velocity (m/s)
i
2 C
P
18y
i
P d2 C g
= P P c
g = acceleration due to gravity (m/s2)
p = particle density (kg/m3)
P
y = gas viscosity (Pa-s)
C = Cunningham correction factor (dimensionless)
C
V = superficial gas velocity (m/s)
18
-------
Generally, the effect of gravity settling is small because when
a particle is large enough to have a significant settling velocity
it will be collected by impaction. However, at low superficial
gas velocities where impaction is limited, gravity settling can
be significant for particles too large to be removed by diffusion.
Pressure Drop —
1**
For clean, unloaded fibrous filters, Davies* has suggested
the following expression for prediction of pressure drop:
Ap ,
where
AP
y
L
V
dFe
= total pressure drop (Pa)
= gas viscosity (Pa-s)
= filter thickness (m)
= superficial velocity (m/s)
a = 1 - e (fraction)
F(a) = 64CX1 >s (l+56a3) for a < 0.02
F(a) = VOa1 >s (l+52a3) for a > 0.02
For fibrous filters of low solidity, a < 0.02, equation
3.12 can be approximated by:
AP -
(3.13)
Fe
Pressure drop is seen to be directly proportional to velocity.
At practical operating conditions for fibrous filters, the flow
around fibers is always laminar. Also, pressure drop increases
rapidly as fiber diameter is decreased (at a constant a) indi-
cating that attempts to increase filter efficiency by using
smaller fibers will result in a pressure drop penalty.
Other Important Factors —
Temperature — Fundamental particle collection mechanisms
are influenced by operating temperature. Although recent in-
terest in high temperature filtration has increased experimenta-
tion in that area, filter performance at temperatures above
19
-------
ambient conditions can be estimated by inspection of the tempera-
ture dependent factors in the filtration equations. The base-
case conditions indicate that exhaust gas entering the filter
would be 200°C (392°F).
Also, it should be noted that the volume of exhaust gas
to be treated is a function of temperature. For example, the
volume of gas at 20°C is only 62% of the volume at 200°C. Thus,
as temperature and gas volume increase, the superficial gas
velocity entering a filter of fixed cross-sectional area will
also increase. Higher superficial gas velocity improves particle
collection by inertial impaction and reduces collection by gravity
settling and diffusion.
The collection efficiency of a filter due to inertial impac-
tion is proportional to the Stokes number. The temperature de-
pendence of the Stokes number is contained in the ratio of the
Cunningham correction factor to gas viscosity, C /p.
c
Gas viscosity increases as temperature increases. Viscosity
can be adequately predicted as a function of temperature, e.g.,
the viscosity of air is 1.81 x 10~s Pa-s at 20°c and
2.56 x 10~5 Pa-s at 200°C.
The Cunningham slip correction factor, which is significant
for submicron particles, increases as the mean free path of the
gas increases. At pressures near atmospheric, increases in ab-
solute temperature will produce proportional increases in the
mean free path of air molecules, e.g., Xair is 0.069 pm at 20°C
and 0.117 ym at 200°C. For a 0.2-pm diameter particle, the Cun-
ningham correction factor will increase from 1.92 at 20°C to
2.65 at 200°C.
The increase in Cc counterbalances the decrease in C /p
:o viscosity increase. Thus, at 200°C the ratio Cc/y is
97.2% of its value at 20°C. This indicates that collection of
0.2-pm diameter particles by inertial impaction at 200°C will
be only slightly reduced when compared to filter performance
at 20°C.
Collecton efficiency by gravity settling is also propor-
tional to the ratio CC/M. Therefore, gravity settling can be
expected to be slightly diminished at 200°C compared to perform-
ance at 20°C.
Collection efficiency by interception is shown to be only
a function of the dimensionless interception parameter, d_/d ,
which is not affected by temperature. Other expressions which
have been proposed for interception show a weak dependence on
temperature through the inclusion of Reynolds number, which
results in a decrease in H as temperature increases.18
20
-------
Due to the strong influence of temperature on particle dif-
fusivity, Dp, particle collection efficiency by diffusion in-
creases as temperature increases. Particle diffusivity is pro-
portional to the ratio (CCT)/M, which increases by 63% between
20°C and 200°C.
Thus, considering the only slightly diminished collection
by inertial impaction, gravity settling, and interception and
the significantly improved collection by diffusion, the base-
case temperature of 200°C appears to improve collection effi-
ciency somewhat, compared to operation at room temperature, at
least for particles small enough to be collected by diffusion.
Filter compression—Equations 3.12 and 3.13 predict a
linear change in pressure drop with velocity. However, fibrous
filters tend to compress under high velocity, so that filter
solidity, a, may increase with increasing velocity. The result
is a nonlinear increase in pressure drop as filtration velocity
increases.
Dust loading—The accumulation of dust within a filter has
two basic effects: 1) a decrease in filter porosity, e, and
2) a change in fiber diameter, dp. These changes influence col-
lection efficiency and pressure drop as dust loading occurs,
and the equations for prediction of clean filter efficiency and
pressure drop are no longer appropriate.
Although loaded filters often show increased efficiency,
which is generally attributed to the presence of new collector
sites, there is presently no adequate theory for the prediction
of the efficiency of loaded filters. We will use clean bed per-
formance models to estimate overall filter collection efficiency
as a first, conservative estimate.
The change in the distribution of fiber diameters due to
loading is difficult to estimate. However, if the particles
and fibers are about the same diameter, the effect on pressure
drop may be negligible. Davies17 reviewed the theoretical litera-
ture and concluded that "the increase in pressure drop is almost
independent of fiber radius." Therefore, the effect of dust
loading on pressure drop is primarily due to the change in filter
porosity.
The change in filter porosity with dust loading can occur
either uniformly throughout the fiber filter or nonuniformly
as a dust cake on the surface of the filter. Uniform deposition
throughout the filter is more desirable in terms of filter storage
capacity, i.e., useful life, and pressure drop buildup; however,
one cannot be sure that surface cake will not form. There is
no adequate general theory to explain pressure drop increase
with dust load.
21
-------
Condensation effects—Condensation of water or hydrocarbons
within the filter can have advantages and disadvantages. The
flux forces described previously can enhance particle collection
on condensing droplets. The coating of fibers with condensate
may increase adhesion of particles, thereby reducing reentrain-
ment and bounce (discussed below). Also, condensation of hydro-
carbons and subsequent droplet removal would reduce overall hydro-
carbon emission. However, these positive effects may be negated
if clogging of the filter occurs too rapidly, resulting in filter
blinding or excessive pressure drop. Experimentation will be
required to determine the significance of condensation on filter
performance.
Fault processes—The particle collection models discussed
assume that contact between particles and fibers results in com-
plete particle retention. However, particles may fail to stick
on contact, i.e., bounce, or be reentrained after contact, either
as individual particles or as agglomerates. Such fault processes
can reduce overall filter efficiency below that predicted by
fundamental collection mechanisms. Important factors are par-
ticle and fiber size, shape and surface properties, and gas
velocity and temperature.
Reentrainment—Theoretical and experimental studies have
shown that, once a particle is collected on a fiber, a velocity
much higher than the deposition velocity is needed to blow the
particle off the fiber. Thus, it is reasonable to assume that
once a particle is collected in a filter it will not reentrain
unless specific efforts to clean the filter are made.
Once collected, particles act as collection sites and ad-
ditional particles strike them. Over time, therefore, agglo-
merates of particles build up; the collection efficiency of a
fiber bed may increase above its initial value due to the presence
of collected particles.
However, under high loading conditions, the particle agglom-
erates may become sufficiently large so that the drag force
exerted by the filtration velocity itself is sufficient to break
agglomerates loose. Stenhouse20 has stated that the amount of
agglomerate blow-off is strongly dependent on the level of the
particle-fiber and particle-particle adhesion forces. If the
adhesion forces are high, collection efficiency will increase
with dust loading; if the forces are low, collection efficiency
may decrease as agglomerates are reentrained.
It is difficult to make definite statements about dust pene-
tration as a function of loading. Davies17 has noted the lack
of experimental data in this area. Penetration may stay con-
stant, increase, or decrease, depending on the nature of the
22
-------
dust and fiber and the filtration conditions. Since it is not
possible to predict filter performance in advance, the perfor-
mance of a particular filter design under high loading conditions
should be determined experimentally.
Bounce—Classical filtration theory assumes that all par-
ticles which hit a collecting body stick and are not removed.
However, when particles with high inertia strike a collecting
body, they may bounce off and thus not be collected. In order
for a particle to stick to a fiber, the energy of adhesion must
be greater than the elastic energy available from the impact.
An accurate analysis of the probability of particle adhesion
requires a detailed calculation of the energies involved. These
calculations require knowledge of the elastic-inelastic proper-
ties of the fiber and particle and are beyond the scope of this
report. However, general trends can be presented here.
The energy of adhesion between particle and fiber is prin-
cipally due to the van der Waal's attractive force, which is
inversely proportional to the particle diameter. The energy
needed to overcome adhesion is provided by the particle kinetic
energy, which is proportional to the square of particle velocity
and cube of the particle diameter. Thus, as particle inertia
increases, the ratio of kinetic to binding energy will increase
rapidly, until at some point particle bounce will become signi-
ficant.
For a given particle size, collection due to inertia will
be a strong function of velocity. At low velocities, the in-
ertial collection will be negligible; as velocity is increased,
the amount of inertial impaction will increase until the theo-
retical single fiber collection efficiency from impaction reaches
100%. At some point, however, the velocity will be high enough
so that particle bounce becomes significant, and further increases
in velocity will cause decreases in collection efficiency.
Experiments by Loffler21 and Stenhouse22 have shown that,
for typical combinations of particles and fibers, particle bounce
may become significant when the inertial impaction parameter
approaches 10. Thus, when designing a fiber filter, this value
of impaction parameter can be used to give an indication of
operating regions where particle bounce may become significant
and degrade performance. Exact calculation of adhesion proba-
bilities is difficult and filled with uncertainties, and experi-
ments are necessary to obtain good quantitative information.
23
-------
Granular Filters
Collection Efficiency—
The methodology for prediction of the total efficiency for
a bed of granules is analogous to that for fibrous filters.
The major differences between fibrous filters and granular filters
are the collector element geometry and filter porosity. Assuming
spherical collectors packed in a bed with porosity, e, the fol-
lowing equation is appropriate for prediction of total bed effi-
ciency: '2I*
- exo -1 5
- exp 1-1.5
(3.14)
g
where: EfiT = total bed efficiency (fraction)
L = bed depth (m)
d = granule diameter (m)
e = bed porosity (fraction)
n = single granule efficiency (fraction)
The granule diameter used in equation 3.14 is the physical diam-
eter, assuming uniform, spherical collectors.
The porosity of granular beds generally ranges from 0.26
for closest packing of spheres to 0.44 for loose, random pack-
ing.25 A typical porosity value for fixed bed granular filters
is 0.40, and this value will be used here to characterize a
prototype granular filter.
As for fibrous filters, the single granule efficiency fac-
tor, n, represents the dominant collection mechanism or combina-
tion of collection mechanisms for a single collector located
in an assembly of collectors. Empirical and theoretical methods
have been proposed for estimating the single granule efficiency
corresponding to each of the fundamental collection mechanisms.
The equations for estimating n presented below have been selected
as reasonable for calculation purposes, but are by no means the
only possible choices.
The single granule efficiency for particle collection by
diffusion, nD» in a granular bed is estimated by:
5.224 „ ~2/3
Pe (3.15)
24
-------
where: e = bed porosity (fraction)
Pe = Peclet number (dimensionless)
This expression for nD results from an approximation proposed
by Tardos et al.2u for conditions where Re < 10, Pe > 1000, and
0.35 < e < 0.70, based on the hydrodynamic model of Neale and
Nader.
The single granule efficiency for particle collection by
interception, nR/ is estimated by:
3 / l - a \ R2 ,-, ,,.
( 6)
_ _
R Z l - 9(a)1/3 + a - l/5(a)2
where a = bed solidity = 1 - e (fraction)
d
R = interception parameter = - (dimensionless)
Equation 3.16 is an approximate solution proposed by Lee and
Gieseke26 based on the Kuwabara Flow Field where solidity, ot,
approaches 5/8. For porosity, e, of 0.4 (a = 0.6), equation
3.16, reduces to:
nl = 61'6 2 at
There are no generally accepted models for prediction of
the single granule efficiency factor for particle collection
by impaction, rij, and an empirical relationship must be used.
27
Schmidt et al.27 suggests the following empirical expression
on an analysis by Jackson and Calvert:
= KjStk (3.18)
where: Kj = ;L.5(l_e)
Kj = 3.49 6 e = 0.4
Stk = Stokes number
P d 2VC
- P
9yd
9
Pp = particle density (kg/m3)
dp = particle diameter (m)
V = superficial velocity (m/s)
y = gas viscosity (Pa-s)
25
-------
d = granule diameter (m)
9
C = Cunningham correction factor (dimensionless)
It should be noted that the Stokes number, Stk, is essentially
the same as the impaction parameter, \|>, described previously
except that, by defining convention, the impaction parameter
is smaller than the Stokes number of a factor of 2.
The single granule efficiency factor for particle collec-
tion by gravity settling, n^, is:
n = -^ K n
G V KG U
where: V = particle terminal settling velocity (m/s)
t S
V = superficial velocity (m/s)
KG = fraction of projected area of single collector
available for particle capture
Paretsky et al. give KG = 0.0377 for triangular packing and 0.0871
for square packing and suggest using an average value of 0.0624
for KG- Paretsky found that gravity settling decreases pene-
tration when gas flow is downward through a packed bed and in-
creases penetration for upward flow.
Pressure drop—
The pressure drop across a clean bed of granules can be
predicted by the Ergun equation:^"5
ISOyVL
V
(1-e) *
e3
1.75p V2L
+
dg
~
-------
Other Important Factors—
Temperature—Because the fundamental collection mechanisms
in granular filters are the same as in fibrous filters, the
effects of temperature should be similar. Operation of a granular
filter at 200°C should not present any problems with respect
to fundamental collection mechanisms. However, recent tests
of granular (sand) bed filters at 150°C for removal of fly ash
suggest that adhesion properties may be reduced, thus increasing
the significance of fault processes.30
Dust loading—Accumulation of dust within a granular filter
will reduce the porosity of the bed and provide new sites for
additional particle collection. Pressure drop and collection
should increase with dust loading in a granular bed. However,
there appear to be granule sizes and gas velocity conditions
at which saturation of the bed is possible, i.e., efficiency
falls to zero at a high dust loading within the bed.31
As is true for fibrous filters, there are no adequate,
general theories for prediction of collection efficiency and
pressure drop in loaded granular bed filters. Loaded filter
performance requires experimental verification.
Condensation effects—The effects of condensation within
a granular bed are similar to those described for fibrous filters.
Fault processes—As mentioned, bed saturation at high load-
ing and adhesion problems at high temperature may reduce filter
performance through reentrainment and bounce. Goren32 found re-
entrainment to decrease efficiency for 1- to 3-ym diameter par-
ticles at superficial velocities greater than 0.35 m/s in a bed
of 2-mm diameter alumina spheres. Lee et al.30 found 0.22 m/s
to be the approximate velocity at which reentrainment became
significant in a bed of 40-50 mesh sand at 150°C.
In general, granular material, such as sand or alumina,
should be suitable for the base-case application. For best re-
sults, granule diameter should be 1 to 2 mm. Smaller granules
would provide better initial particle collection but would have
poorer dust storage characteristics and high pressure drop.
PREVIOUS USES OF FILTRATION FOR DIESEL EXHAUST
Several authors have reported on the use of filtration to
trap diesel particulate. Springer and Stahman10 utilized avail-
able lead-trap technology in their study. From a total of 48
combinations of devices, a best system for particulate removal
was identified. This system, which consisted of two alumina-
coated, steel wool-packed filters, initially reduced the exhaust
27
-------
particulate by 64%. However, as shown in Figure 3, the collec-
tion efficiency decreased rapidly with distance travelled, ac-
companied by a sharp increase in the system backpressure. The
high backpressure of this system had no great effect on the fuel
economy of the test vehicle but the acceleration rate, already
a weak point on diesels, was reduced by 20%.
Sullivan, Tissier, Hermance, and Bragg33 examined six dif-
ferent filter materials in a study concerned with emission of
underground diesel engines. Their results are summarized in
Table 9. Although the collection efficiencies were quite good,
the backpressures were very high. The first five materials
formed a cake of particulate on the face of the filter, effec-
tively sealing off the filter. The stainless steel-fiberglass
material appeared to collect particulate matter throughout the
entire filter without forming a cake. This resulted in a much
slower rise in backpressure than the other materials tested,
although it is still too rapid for automotive use.
TABLE 9. PERFORMANCE RESULTS OF FILTER MATERIALS33
^ ~~
• .
Time required
to backpressure
Collection of 2.5 x 103 Pa,
Filter material efficiency (Hours)
Teflon felt
Nomex felt
Woven fiberglass - graphite
treated
Fiberglas batt
Fiberglas Aerocor
Knitted stainless steel fiberglas
matrix
99.9
99 +
98 +
98 +
97 +
80
0.
0.
0.
3
1
12
25
25
25
The Eikosha Company3" in Japan has been conducting develop-
mental work on a particulate collection device called an Aut-
Ainer intended for use on both gasoline and diesel vehicles.
Figure 4 shows one of these. The initial concept for this device
was to collect emissions by condensation growth on a mesh ma-
terial. The original system consisted of a number of expansion
chambers followed by regions filled with metal mesh to serve
as a collection medium. A ram air cooling tube was also provided
down the center on the device. This device has been carried
through a number of stages of development using fundamentally
an experimental approach.
At the current stage of development, the device develops
a collection efficiency of about 70% for diesel particulate when
the system is clean. However, it is necessary to provide for
28
-------
3?
O
0
ui
O
O
o
70
60
50
40
30
20
10
0
DISTANCES BASED ON A-IR
EFFICIENCIES BASED ON 1975 FTP
PRESSURES BASED ON 88.5 KM/HR
O)
I
E
340
300
260
220
180
140
100
°j
c
182
139
96
54
£
45.3
34.7
24.0
13.3
ut
ee
D
%
LU
CC
ct
CQ
1-
V)
X
LU
8
DISTANCE, KM
Figure 3. System efficiency and exhaust restrictions as functions
of distance traveled. 10
29
-------
EXHAUST
AIR COOLING
SOOT
COLLECTOR
Figure 4. Aut-Ainer filter with cyclone soot collector.
30
-------
cleaning at intervals of 2,000 kilometers of operation. In the
absence of cleaning, the collection sites become covered with
the very low density soot particles after which reentrainment
occurs. Therefore, the device initially acts as a collection
device until reentrainment occurs, at which time the character-
istic behavior changes to that of an agglomerator.
The recognition of this fact led the developers to investi-
gate adding a post-collection device as a means for collecting
this reentrained material. Two methods are currently under in-
vestigation, each of which involves the use of a collector ope-
rating on a side stream consisting of about 10 to 15% of the
total flow through the system. The method shown in Figure 4
uses a cyclone to divert the particulate to a collection bag.
The other method uses a rotating particle catching wheel which
passes through a backflow of air where the particulate is blown
into the collection bag. This portion of the system still needs
a lot of work, but the approach does show promise. Further study
is needed to devise a more reliable method for diverting the
reentrained material into the post-collection device.
General Motors Corporation9 has tested a number of filter
materials on diesel automobiles. One material tested was pleated
paper. Each paper cartridge has a total volume of about 0.011 m3
(3 gal.) with a surface area of 3.2 m2 (5,000 in.2). Collec-
tion efficiency was in the 80 to 90% range. However, after
1,100 km (700 mi) of Federal Test Procedure driving on an Olds-
mobile test vehicle, the system backpressure had risen to
2.0 x 101* Pa (80 in. of water) from the 5.0 x 103 Pa (20 in.
of water) from the standard exhaust system. Also, on several
occasions the paper element caught fire.
General Motors also tested metal mesh filters, which showed
efficiencies as high as 60%. Results of a 19,300-km (12,000-mi)
test under very mild driving conditions showed efficiency de-
gradation from 55% to 36% during the test. There was evidence
that particulate blow-off and incineration had occurred during
the course of the test. In other tests, filter destruction from
incineration was observed. Table 10 summarizes the tests of
filtration materials performed at General Motors.
TABLE 10. SUMMARY OF FILTRATION MATERIALS
TESTED BY GENERAL MOTORS9
Trap material Efficiency, % Remarks
Opel 2.1 Liter Engine
Corrugated foil fecralloy 36 High temperature resis-
tance - good oxide adhesion
Chopped fecralloy 29
(continued)
31
-------
TABLE
10 (continued)
Trap material
Chromium alloy ribbon
Glass fiber fabric
Fiberfrax fiber fabric
Alumina fiber material
Catalyst Beads
- Quadralobe
- Trilobe
- Low density spheres
- Large Quadralobe
- Porous ceramic
- Small bead
- Production catalyst
Catalyst beads
Efficiency, %
37
34
65
46
61
32
46
62
39
43
50
41
47
52
<10
Remarks
Low efficiency - high tem-
perature resistance
Quick loadup
Good efficiency - rapid
loadup
Fair efficiency - fast load
up
Good efficiency
Low endurance
2- and 4-in. diameter car-
tridge - good efficiency
Catalytic converter - good
efficiency
Rapid loadup time
Moderate efficiency
Moderate efficiency
Moderate efficiency
Moderate efficiency
Moderate efficiency
Moderate efficiency
Low efficiency
Ceramic monolith extruded
Low density -
Torturous path ceramic
Ceramic bobbin
52
49
39
42
Low efficiency
Low efficiency
Fair efficiency
Fair efficiency
Thermal failure
High temperature resistance
Low capacity
(continued)
32
-------
TABLE 10 (concluded)
Trap material
Efficiency, %
Remarks
Alumina coated metal mesh
Metal mesh
63
28
44
Good efficiency - oxidation
resistance to 1500°F
Poor efficiency
Effectiveness at start -
poor; fair after buildup
Olds 5.7 Liter Engine
Catalyst beads
-Quadralobe-
Production catalyst
Ceramic monolith extruded
-low density-
Metal wool
Corrugated fecralloy
Alumina coated mesh
Paper element
Fiberglas element
56
40
30
60
30
65
90
76
FTP-4 Series Converters
FTP-4 Series Converters
Fine grade promising
High efficiency trap material
Excellent efficiency -
not heat resistant
Temperature resistance to
750°C
FILTER CHARACTERISTICS FOR DIESEL PARTICULATE EMISSIONS
Introduction
The preceeding reports were based on the experimental ap-
proach to filter design utilizing existing filter materials.
In contrast to that, this discussion uses the equations described
previously for prediction of collection efficiency and pressure
drop in clean fibrous and granular filters to estimate the per-
formance of various filter designs.
Characteristics of the filters investigated in this section
are given in Table 11. The approach used is to select a rea-
sonable filter volume and path length and then to examine the
performance of filters varying in both volume and path length.
Both fiber bed and granular bed filters are considered. Porosity
33
-------
TABLE 11. ESTIMATED FILTER PERFORMANCE FOR BASE-CASE CONDITIONS
u>
Confi-
gura-
tion Volume
(m3)
1
2
3
4
5
6
7
8
9
0.0113
0.0113
0.0113
0.0225
0.0225
0.0225
0.0450
0.0450
0.0450
Length
(m)
0.10
0.30
0.90
0.10
0.30
0.90
0.10
0.30
0.90
Velocity
(m/s)
1.
3.
11.
0.
1.
5.
0.
0.
2.
26
78
3
630
89
67
315
944
83
Mass removal
efficiency (%)
F*
76.1
90.6
94.6
78.5
93.5
97.6
82.2
95.0
99.3
Gf
84.0
97.1
100
81.3
95.8
100
79.8
96.5
99.9
Pressure
F*
1,860
16,700
150,000
927
8,350
75,100
464
4,170
37,500
Drop
G
4
74
1,590
1
24
450
8
137
(Pa)
t
,370
,000
,000
,700
,000
,000
732
,740
,000
Maximum
lifetime (mi)
F*
3,500
3,000
2,800
6,800
5,700
5,500
13,000
11,300
10,800
Gf
1,300
1,100
1,100
2,700
2,300
2,200
5,400
4,500
4,300
*F - fibrous filter: d.= 1 x 10 5 m (10 ym) , e = 0.99.
r
G - granular filter: d = 1 x 10~3 m (1 mm), e = 0.40.
-------
of both fiber and granular bed filters is fixed at a "reasonable"
value, respectively, for all cases. Fiber and granule diameters
are also fixed. All variables concerning the properties of the
gas stream and aerosol were fixed at the values listed in Table
12 for all cases. A specific filter configuration, which appears
acceptable on the basis of predicted initial efficiency, pressure
drop, and lifetime (mileage), is selected for more detailed in-
vestigation later.
TABLE 12. DIESEL PARTICULATE EMISSION PROPERTIES:
BASE CONDITIONS USED FOR FILTER DESIGN
Gas stream conditions
Temperature
Flow rate
Viscosity
Density
Particle Properties
Concentration
Density
Bulk density
Emission rate
Size distribution
200°C
0.14 m3/s (300 ft3/min)
2.3 x 10~5 Pa-s
0.74 kg/m3 (7.4 x 10~" g/cm3)
7 x 10~5 kg/m3 (0.03 gr/ft3)
1,100 kg/m3 (1.1 g/cm3)
120 kg/m3 (0.12 g/cm3)
0.5 g/mi
mmd = 0.2 ym,
70% smaller than 1 ym by mass
It is most important to point out the limitations of the
analysis. Filtration fundamentals are not understood well enough
to permit confident calculations of filter efficiency and pres-
sure drop, even for beds of clean, new fibers or granules.
Comparatively little work, either experimental or theoretical,
has been done on filters loaded with collected dust so that the
performance of dust-conditioned filters is not well known or
understood.
In general, it is to be expected that the collection effi-
ciency of a conditioned filter will exceed that of a clean filter,
However, no acceptable theory to allow calculation of filter
efficiency with increasing filter loading currently exists.
The approach used here is to identify from theory the efficiency
of a clean bed and assume that this is the lowest efficiency
expected in practice.
35
-------
The pressure drop across a conditioned filter should exceed
that across a clean filter. Ideally, it should be possible to
predict the increase in pressure drop with filter loading. How-
ever, no acceptable theory currently exists to allow making this
calculation. The best that current understanding allows is cal-
culation of the pressure drop across a clean filter, and the
assumption that subsequent pressure drop will be higher once
the filter is conditioned.
The presence of water vapor and condensible hydrocarbons
in the exhaust gas suggests that particle collection by flux
forces such as condensation/particle growth, diffusiophoresis,
and Stephan flow may occur if the temperature of the gas stream
is sufficiently low. Also, adhesion to the filter material may
be increased. These effects might increase collection efficiency
for small diesel exhaust particles. However, condensation might
also affect the performance of the filter adversely by filling
it with materials other than the particles it is designed to
collect or by interfering with in situ cleaning mechanisms,
thereby decreasing the lifetime. Such uncertainties point out
the necessity of thorough empirical testing over all operating
parameters before the design can be optimized.
Filter Configuration
Size—
A fundamental constraint on filter selection is the allow-
able volume of the filter container. The initial assumption
is that the filter can be approximately the size of a muffler,
assumed to be 1.0 m long by 0.30 m in diameter for total volume
of 0.0225 m3. To investigate the effect of filter size, filter
volumes half that and twice that of our typical muffler size
are also used.
Within a given filter volume, the gas flow can be oriented
many ways, e.g., parallel or perpendicular to the long dimension
or distributed through baffles or an annulus. Thus, in addition
to filter volume, a characteristic length, L, or filter depth
was selected. The lengths considered are 0.10, 0.30, and 0.90 m.
Selection of filter volume and length fixes the cross-sec-
tional area, A, available for filtration as:
A = v/L (3.21)
where: A = area (m2)
v = volume (m3)
L = length (m)
36
-------
Therefore, for given volumetric flow rate, Q (m3/s), through
the filter, the superficial gas velocity, V (m/s), can be calcu-
lated as:
V = Q/A = QL/v (3.22)
The velocities which result for the base case volumetric flow
rate of 0.14 m3/s (300 ft3/rnin) and the assumed filter volumes
and lengths are shown in Table 11.
Filter Media—
The fibrous filter calculations are based on a filter pack-
ing of uniform 10-ym diameter fibers at a porosity of 99%
(a = 0.01). Fibrous filter packing of this type is currently
available and is considered typical with respect to filtration
and pressure drop characteristics. The fibers could be stainless
steel.
The granular bed filter calculations are based on a filter
packing of uniform 1-mm diameter spheres at a porosity of 40%
(e = 0.40). Sand or ceramic granules in this size range are
available and are considered typical granular filter media.
Estimated Filter Performance
Collection Efficiency—
The estimated mass removal efficiencies of the representa-
tive fibrous and granular filters over the range of filter con-
figurations are listed in Table 11 and shown in Figure 5. The
mass removal efficiencies are calculated from estimated filter
fractional efficiency, i.e., efficiency as a function of particle
size, and the base case particle mass size distribution. Filter
efficiency was determined for four particle sizes (0.01 urn, 0.074 ym,
0.52 ym, and 6.5 ym), each of which is the midpoint of a quartile
on the particle mass size distribution, i.e., each size represents
25% of the mass entering the filter. The overall mass removal
efficiency listed in Table 11 is, thus, the average of the effici-
encies found at each of the four quartile sizes.
In general, the fractional efficiency calculations show
99 to 100% collection of the smallest and largest quartile par-
ticle sizes, the former by diffusion as the dominant collection
mechanism and the latter by inertial impaction. At the middle
of the particle size distribution, characterized by particle
sizes of 0.074 ym and 0.52 ym, collection efficiencies were
generally less. The lowest calculated fractional efficiency
was 46.1% for 0.074 ym particles in the granular filter under
configuration 1.
37
-------
o
o
il
u.
UJ
100
90
80
70
L = 0.10m
O—O FIBROUS FILTER
GRANULAR FILTER
60
I
I
I
0.01 0.02 0.03 0.04
FILTER VOLUME, m3
Figure 5. Filter efficiency versus volume.
0.05
38
-------
Because the base-case particle mass size distribution is
extrapolated from two points, it is important to note the in-
fluence of particle size on predicted efficiency. If more of
the particle mass is associated with particle sizes between 0.07
to 0.5 ym than indicated by the base-case distribution, the
overall mass removal efficiencies can be expected to be less
than the values listed in Table 11.
By inspection of Figure 5, it appears that the typical
fibrous and granular filters give similar overall clean filter
efficiency at a given characteristic length. At a bed depth
of 0.10 m, overall mass removal efficiencies of roughly 80 per-
cent are predicted. Increased depth enhances particle removal,
but it should be noted that these predicted efficiencies are
for unloaded filters. As filter load increases, efficiency will
generally increase assuming reentrainment does not become signi-
ficant. It is likely that filters showing nearly perfect initial
collection efficiency will quickly load and blind, providing
reduced lifetime.
Pressure Drop—
Pressure drop is plotted against filter volume in Figure 6
for both granular bed filters and fiber bed filters. Assuming
a maximum allowable pressure drop of 2,500 Pa (10 in. of water)
for proper engine performance, the pressure drop data presented
in Table 11 reveal impracticable pressure drops for all configura-
tions with filter lengths greater than or equal to 0.30 m and
associated superficial velocity greater than roughly 1.0 m/s.
The fiber bed pressure drops listed in Table 11 are opti-
mistic, since a fiber bed will compress under flow and thus de-
crease in porosity. This effect probably only becomes signifi-
cant at velocities greater than 1.0 m/s, however, and the pressure
drops at these velocities are excessive even without considering
compression.
Only configurations 4 and 7 show reasonable pressure drops
for both fibrous and granular filters, and configuration 1 is
possible for a fibrous filter only on the basis of pressure drop.
Overall mass efficiencies for these configurations are approxi-
mately 80%. For all remaining configurations (showing initial
efficiencies greater than 90%), the pressure drops are excessive.
Lifetime—
Filter lifetime is plotted against filter volume for both
fiber bed filters and granular bed filters in Figure 7. The
useful lifetime of a filter measured in terms of mileage between
changes is impossible to predict accurately due to the uncer-
tainties of pressure drop increase in loaded filters. However,
39
-------
£
100,000
80,000
60,000
40,000
20,000
10,000
8,000
6,000
cc
O
DC
I
LLJ
DC
O.
4,000
2,000
1,000
800
600
400
200
100
L = 0.9 m
O—O FIBROUS FILTER
A--A GRANULAR FILTER
I
I
I
1
0.01 0.02 0.03 0.04
FILTER VOLUME, m3
Figure 6. Pressure drop versus filter volume.
0.05
40
-------
UJ
5
14,000
12,000
10,000
8,000
T
s
i
X 6,000
4,000
2,000
FIBROUS FILTER
GRANULAR FILTER
L = 0.90 m
I
I
I
I
0.01
0.04
0.02 0.03
FILTER VOLUME, m3
Figure 7. Maximum lifetime (mi) versus filter volume.
0.05
41
-------
a simplistic approach to predict maximum possible lifetime in-
volves estimating how long it would take to fill the void volume
of a given filter with collected particles. This approach, of
course, ignores the increase in pressure drop as porosity de-
creases; pressure drop may become unacceptably high long before
bed voids are completely filled with collected particles. The
lifetimes calculated using the void-filling approach are, there-
fore, much longer than would be expected in practice.
Using the simple model of filling void volume, maximum pos-
sible lifetime (as mileage) can be predicted by:
vep
lifetime (mi) = — — - (3.23)
where: v = filter volume (m3)
e = filter porosity (fraction)
W = particle mass emission rate (kg/mi)
E = overall filter efficiency
PB = bulk packing density of collected dust (kg/m3)
Using the base case emission rate of 5 x ID"1* kg/mi and bulk
density of 120 kg/m3, the maximum lifetime of each configuration
can be estimated as shown in Table 11.
Assuming a maximum possible lifetime of at least 12,000 km
(7,500 mi) (approximately one oil change), the granular filter
is deficient in terms of storage capacity of collected dust for
all configurations shown in Table 11. For fibrous filters, only
configuration 7 meets the 12,000-km (7,500-mi) criterion, al-
though configuration 4 may be acceptable if slightly different
emission rates or bulk densities are used.
Summary
Granular filters can offer high efficiency and utilize cheap
materials such as sand, which has good thermal and chemical resis-
tance. However, because of inherently lower porosity, granular
filters have a greater pressure drop, less dust storage capacity,
and a greater weight than a fibrous filter of the same volume.
From the data presented in Table 11, the allowable pressure
drop of 2,500 Pa and desired lifetime of 12,000 km (7,500 mi),
only configuration 7 for a fibrous filter appears practicable;
its efficiency is 82%. Configuration 7 is a large filter (0.045 m3)
oriented or baffled so as to provide a large cross-sectional
area (0.45 m2) for filtration across a depth of 0.1 m.
42
-------
The major uncertainties in these calculations, beyond the
accuracy of the predictive equations themselves, include the
effects of 1) condensation of hydrocarbons or water, especially
during cold starts, 2) reentrainment and bounce of "collected"
particles which reduce overall efficiency, and 3) dust load.
As discussed throughout this study, uncertainties regarding the
effects of dust load make empirical verification of filter practi-
cability necessary.
Also, the theoretical filter performance curves presented
are based on the data set given in Table 12 to describe emissions
from a diesel engine. Actual emissions deviate about these data
as the engine is used in practice; to the extent that these
aerosols are dissimilar to that described in Table 12, the analy-
sis presented in this report is incomplete. Conclusions and
recommendations based on this report must, therefore, be regarded
as tentative. A prototype design based on this analysis must
be considered preliminary.
PROTOTYPE FILTER FOR DIESEL EXHAUST
Description of Prototype Filter
Based on the screening of alternative configurations de-
veloped earlier in this section, a fibrous filter approximately
the size of a conventional muffler or larger would provide accept-
able particle removal efficiency and pressure drop for the base-
case diesel emission conditions. In particular, configuration
7 would provide 82% efficiency at a pressure drop of 460 Pa (less
than 2 in. of water) when new. Configuration 7, hereinafter
referred to as the prototype fibrous filter, has a total volume
of 0.045 m3 with a nominal thickness, L, of 0.10 m and a cross-
sectional area of 0.45 m2 at an exhaust gas volumetric flowrate
of 0.14 m3/s (300 ft3/min at 200°C).
This size device is considered a probable maximum size which
provides large safety margins with respect to allowable pressure
drop and desirable dust storage capacity. As shown in Figures 5,
6 and 7, total filter volume could be reduced to roughly 0.025 m3
without violating the arbitrary lifetime criterion of 12,000 km
(7,500 mi) while maintaining an efficiency of approximately 80%.
Allowance of extra filter volume at this level of screening is
appropriate considering the uncertainties concerning the effects
of dust loading on filter pressure drop and lifetime.
There are many possible orientations of the fibrous filter
media with respect to gas flow which maintain the required thick-
ness and total cross-sectional area. The gas flow could be
radially outward or inward with an annular filter element in
a cylindrical container. A baffle arrangement could provide
43
-------
the flow splitting required for a series of rectangular or oval
filter panels within the filter container. The simplest design
is a single rectangular filter element within a container oriented
perpendicular or parallel to the exhaust pipe, the latter being
shown in Figure 8.
The design shown in Figure 8 appears simple to construct.
A single filter element could be inserted into the container
from one end. Of course, specific applications may require other
designs which are better left to automotive engineers.
Sensitivity Analysis
The equations used to calculate clean filter efficiency
and pressure drop require values for superficial velocity, filter
thickness, particle diameter, fiber diameter, filter porosity,
and physical constants for gas stream conditions. The following
sections discuss the impact on predicted filter performance of
variations in these parameters. The results of the sensitivity
analysis are listed in Table 13. Case 1 is the prototype design.
Filter Thickness—
Cases 2 and 3 in Table 13 show the effect of reducing the
prototype filter thickness to 0.05 m and increasing it to 0.20 m,
respectively, while maintaining all other parameters at the de-
sign condition. Efficiency decreases to 73% for the 0.05-m thick
filter and increases to 91% for the 0.20-m thick filter. The
pressure drop is directly proportional to filter thickness and,
thus, doubles when the filter thickness is doubled.
Volumetric Flow Rate—
The base case specifies an exhaust gas volumetric flow rate
of 0.14 m3/s (300 ftVmin) at 200°C. Cases 4 and 5 in Table 13
show the effect of reducing the flow rate to 0.094 m3/s (200 ft3/min)
and increasing it to 0.212 m3/s (450 ft3/min), respectively. For
a fixed filter geometry, the superficial gas velocity is propor-
tional to exhaust gas flow rate. Efficiency improves slightly
at the lower flow rate, but exhaust gas flow rates between 0.094
and 0.212 m3/s appear to have little effect on the efficiency
of the prototype filter. Clean filter pressure drop is propor-
tional to the exhaust gas flow rate.
Fiber Diameter—
Cases 6 and 7 in Table 13 show the significance of fiber
diameter. For the prototype filter design and exhaust gas flow
rate, significant improvement in efficiency can be attained by
reducing fiber diameter to 5 urn. Overall mass removal efficiency
for the base-case aerosol increases to 99.7%, but the pressure
drop penalty is great.
44
-------
0.10 m
EXHAUST GAS
OUTLET BELOW
FILTER
RECTANGULAR
FIBROUS FILTER
ELEMENT
EXHAUST GAS
INLET ABOVE
FILTER
Figure 8. Prototype fibrous filter.
-------
TABLE 13. PROTOTYPE FILTER SENSITIVITY ANALYSIS
Case
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Q =
d =
*
Qq V
(m3/s) (m/s)
0.142 0.315
0.142
0.142
0.094 0.210
0.212 0.472
0.142 0.315
0.142
0.142
0.142
0.142
exhaust gas flow
thickness. .
L dp dp
(m) (ym) (ym)
0.10 * 10
0.05
0.20
0.10
5
20
10
0.01
0.02
0.04
0.074
0.10
0.20
0.30
0.40
0.52
0.80
1.00
1.50
2.0
6.5
rate; V = superficial
particle diameter; dp = fiber diameter
overall mass removal efficiency; AP =
Raaa_ r*aaa r\ a K 4- i r* 1
a c i <7 A /^TGVipiHii4"'i/"\r» y
Eff.
e (%)
0.99 92.2
73.4
91.1
84.5
80.0
99.7
63.1
0.98 93.5
0.995 69.1
0.99 100.0
99.9
97.7
82.2
69.7
43.8
37.2
38.9
46.7
74.5
90.7
99.9
100.0
100.0
velcoity; L =
; e = porosity;
pressure drop.
Al'M'AOAn^A/^ K«» C.
AP
(Pa)
464
232
927
309
695
1,850
116
1,310
164
464
filter
Eff. =
quartile diameters.
46
-------
Increasing the fiber diameter to 20 ym reduces overall ef-
ficiency to 63%, but pressure drop is very low at approximately
120 Pa. The larger fiber diameter, particularly if it is stain-
less steel, will generally cost less per unit filter volume,
and the low pressure drop may be desirable. Therefore, use of
a fiber diameter greater than 10 ym may be advantageous, parti-
cularly if efficiency is found to increase with dust load.
Porosity—
A porosity of 0.99 for the prototype filter was selected
as a typical value. However, porosity of fibrous filter material
generally ranges between 0.98 and 0.995. Cases 8 and 9 show
the effect on filter performance for this range of porosity.
Efficiency and pressure drop are significantly affected by changes
in porosity, and porosity is, therefore, a key design parameter.
Particle Size—
Cases 10 through 23 and Figure 9 show the variation in ef-
ficiency of the prototype filter as a function of particle size
entering the filter. Figure 9 is the calculated fractional ef-
ficiency curve for the prototype fibrous filter operating at
base-case conditions. The variation reflects the effect of par-
ticle size on the fundamental collection mechanisms. Diffusion
appears to dominate below 0.05 ym, and impaction appears to
dominate above 1.0 ym. A minimum efficiency occurs near 0.3 ym
where all mechanisms are inadequate.
From the fractional efficiency curve, the overall mass ef-
ficiency of the prototype filter can be calculated for any inlet
particle size distribution. The use of only four quartile par-
ticle sizes from the base-case particle size distribution can
be seen to be a crude estimate of overall mass efficiency. If
the actual diesel exhaust particle size distribution is less
polydisperse than the base case but the mmd is still about 0.2 ym,
overall mass efficiency of the prototype filter will be signifi-
cantly less than predicted.
47
-------
100
oo
SUPERFICIAL VELOCITY = 0.315 m/s
THICKNESS - 0.10 m
FIBER DIAMETER - 10 )Um
POROSITY = 0.99
0.02 0.04 0.06 0.10 0.20 0.40 0.60 1.0
PARTICLE DIAMETER,
2.0
4.0 8.0
Figure 9. Fibrous filter efficiency versus particle diameter.
-------
SECTION 4
ELECTROSTATIC DEVICES
FUNDAMENTAL STEPS IN THE ELECTROSTATIC PRECIPITATION PROCESS
Creation of an Electric Field and Corona Current
The first step in the precipitation process is the creation
of an electric field and corona current. This is accomplished
by applying a large potential difference between a small-radius
electrode and a much larger radius electrode, where the two
electrodes are separated by a region of space containing an
insulating gas. For industrial applications, a large negative
potential is applied at the small-radius electrode and the large-
radius electrode is grounded.
At any applied voltage, an electric field exists in the
interelectrode space. For applied voltages less than a value
referred to as the "corona starting voltage", a purely electro-
static field is present. At applied voltages above the corona
starting voltage, the electric field in the vicinity of the small-
radius electrode is large enough to produce ionization by elec-
tron impact. Between collisions with neutral molecules, free
electrons are accelerated to high velocities and, upon collision
with a neutral molecule, their energies are sufficiencly high
to cause an electron to be separated from a neutral molecule.
Then, as the increased number of electrons moves out from the
vicinity of the small-radius electrode, further collisions be-
tween electrons and neutral molecules occur. In a limited high
electric field region near the small-radius electrode, each
collision between an electron and a neutral molecule has a cer-
tain probability of forming a positive molecular ion and another
electron, and an electron avalanche is established. The positive
ions migrate to the small-radius electrode and the electrons
migrate into the lower electric field regions toward the large-
radius electrode. These electrons quickly lose much of their
energy and, when one of them collides with a neutral electro-
negative molecule, there is a probability that attachment will
occur and a negative ion will be formed. Thus, negative ions,
along with any electrons which do not attach to a neutral mole-
cule, migrate under the influence of the electric field to the
large-radius electrode and provide the current necessary for
the precipitation process.
49
-------
Figure 10 is a schematic diagram showing the region very
near the small-radius electrode where the current-carrying nega-
tive ions are formed. As these negative ions migrate to the
large-radius electrode, they constitute a steady-state charge
distribution in the interelectrode space which is referred to
as an "ionic space charge." This "ionic space charge" estab-
lishes an electric field which adds to the electrostatic field
to give the total electric field. As the applied voltage is
increased, more ionizing sequences result and the "ionic space
charge" increases. This leads to a higher average electric field
and current density in the interelectrode space.
Figure 11 gives a qualitative representation of the elec-
tric field distribution and equipotential surfaces in a wire-
plate geometry, which is most commonly used. Although the elec- •
trie field is very nonuniform near the wire, it becomes essenti-
ally uniform near the collection plates. The current density
is very nonuniform throughout the interelectrode space and is
maximum along a line from the wire to the plate.
In order to maximize the collection efficiency obtainable
from the electrostatic precipitation process, the highest pos-
sible values of applied voltage and current density should be
employed. In practice, the highest useful values of applied
voltage and current density are limited by either electrical
breakdown of the gas throughout the interelectrode space or of
the gas in the collected particulate layer. High values of ap-
plied voltage and current density are desirable because of their
beneficial effect on particle charging and particle transport
to the collection electrode. In general, the voltage-current
characteristics of a precipitator depend on the geometry of the
electrodes, the gas composition, temperature and pressure, the
particulate mass loading and size distribution, and the resis-
tivity of the collected particulate layer. Thus, maximum values
of voltage and current can vary widely from one precipitator
to another and from one application to another.
Particle Charging
Once an electric field and current density are established,
particle charging can take place. Particle charging is essential
to the precipitation process because the electrical force which
causes a particle to migrate toward the collection electrode
is directly proportional to the charge on the particle. The
most significant factors influencing particle charging are par-
ticle diameter, applied electric field, current density, and
exposure time.
The particle charging process can be attributed mainly to
two physical mechanisms, field charging and thermal charging.35•36,a 7
These two mechanisms are discussed below.
50
-------
SMALL-RADIUS ELECTRODE AT
HIGH NEGATIVE POTENTIAL
REGION OF ELECTRON AVALANCHE
WHERE POSITIVE IONS AND ELECTRONS
ARE PRODUCED
REGION OF IONIZATION WHERE ELECTRONS
ATTACH TO NEUTRAL MOLECULES TO
FORM NEGATIVE IONS
Figure 10. Region near small-radius electrode.
SMALL-RADIUS ELECTRODE AT
HIGH NEGATIVE POTENTIAL
ELECTRIC FIELD
LINES
EQUIPOTENTIAL
SURFACES
IONS WHICH CONSTITUTE A CURRENT
AND A SPACE CHARGE FIELD
GROUNDED LARGE-
RADIUS ELECTRODE
Figure 11. Electric field configuration for wire-plate geometry.
51
-------
(1) At any instant in time and location in space near a
particle, the total electric field is the sum of the electric
field due to the charge on the particle and the applied electric
field. In the field charging mechanism, molecular ions are
visualized as drifting along electric field lines. Those ions
moving toward the particle along electric field lines which
intersect the particle surface impinge upon the particle surface
and place charge on the particle.
Figure 12 depicts the field charging mechanism during the
time it is effective in charging a particle. In this mechanism,
only a limited portion of the particle surface. (0 to 6
-------
X, Z. 6 - SPHERICAL COORDINATE SYSTEM
Z -*
NEGATIVELY CHARGED PARTICLE
ELECTRIC FIELD LINES
Figure 12. Electric field configuration during field charging.
53
-------
X, Z • COORDINATE AXES
PARTICLE HAS SATURATION CHARGE
€>-
Figure 13. Electric field configuration and ion distribution for particle charging
in an applied field after saturation charge is reached.
NEGATIVE IONS
X, Z - COORDINATE AXES
NEGATIVELY CHARGED
PARTICLE
ELECTRIC FIELD LINES
Figure 14. Electric field configuration and ion distribution for
particle charging with no applied field.
54
-------
Figure 13 depicts the thermal charging process in the presence
of an applied electric field after the particle has attained
the saturation charge determined from field charging theory.
The effect of the applied electric field is to cause a large
increase in ion concentration on one side of the particle while
causing only a relatively small decrease on the other side.
Although the ion concentration near the surface of the particle
becomes very nonuniform, the net effect is to increase the average
ion concentration, the probability of collisions between ions
and the particle, and the particle charging rate.
In thermal charging theories, the amount of charge accumu-
lated by a particle depends on the particle diameter, ion den-
sity, mean thermal velocity of the ions, absolute temperature
of the gas, particle dielectric constant, residence time, and
the applied electric field. The effect of the applied electric
field on the thermal charging process must be taken into account
for fine particles having diameters between 0.1 and 2.0 ym.
Depending most importantly on the applied electric field and
to a lesser extent on certain other variables, particles in this
size range can acquire values of charge which are two to three
times larger than that predicted from either the field or the
thermal charging theories. For these particles, neither field
nor thermal charging predominates and both mechanisms must be
taken into account simultaneously.
In most cases, particle charging has a noticeable effect
on the electrical conditions in a precipitator. The introduction
of a significant number of fine particles or a heavy concentra-
tion of large particles into an electrostatic precipitator signi-
ficantly influences the voltage-current characteristics. Qualita-
tively, the effect is seen by an increased voltage for a given
current compared to the particle-free situation. As the particles
acquire charge, they must carry part of the current, but they
are much less mobile than the ions. This results in a lower
"effective mobility" for the charge carriers and, in order to
obtain a given particle-free current, higher voltages must be
applied to increase the drift velocities of the charge carriers
and the ion densities.
The charged particles, which move very slowly, establish
a "particulate space charge" in the interelectrode space. The
distribution of the "particulate space charge" results in an
electric field distribution which adds to those due to the elec-
trostatic field and the ionic field to give the total electric
field distribution. It is desirable to determine the space
charge resulting from particles because of its influence on the
electric field distribution, especially near the collection
plate, where, for the same current, the electric field is raised
above the particle-free situation. In addition, the "particulate
space charge" is a function of position along the length of the
precipitator since particle charging and collection are a function
of length.
55
-------
Particle Collection
As the particle-laden gas moves through a precipitator,
each charged particle has a component of velocity directed to-
wards the collection electrode. This component of velocity is
called the electrical drift velocity, or migration velocity,
and results from the electrical and viscous drag forces acting
upon a suspended charged particle. For the particle sizes of
practical interest, the time required for a particle to achieve
a steady-state value of migration velocity is negligible and,
near the collection electrode, the magnitude of this quantity
is given by
w =
p
iray
(4.1)
WP =
q =
EP =
a =
y =
where w_ = migration velocity near the collection electrode
of a particle of radius a (m/s),
charge on particle (coul),
electric field near the collection electrode (volt/m)
particle radius (m),
gas viscosity (Pa-s),
C = Cunningham correction factor, or slip correction
f actor *9 = (1 + AX/a) ,
where A = 1.257 + 0.400 exp (-1.10 a/X), and
X = mean free path of gas molecules (m).
In industrial precipitators, laminar flow never occurs and
the effect of turbulent gas flow must be considered. The tur-
bulence is due to the complex motion of the gas itself, electric
wind effects of the corona, and transfer of momentum to the gas
by the movement of the particles. Average gas flow velocities
in most cases of practical interest are between 0.6 and 2.0 m/s.
Due to eddy formation, electric wind, and other possible effects
the instantaneous velocity of a small volume of gas surrounding '
a particle may reach peak values which are much higher than the
average gas velocity. In contrast, migration velocities for
particles smaller than 0.6 urn in diameter are usually less than
0.3 m/s. Therefore, the motion of these smaller particles tends
to be dominated by the turbulent motion of the gas stream. Under
these conditions, the paths taken by the particles are random
and the determination of the collection efficiency of a given
56
-------
particle becomes, in effect, the problem of determining the
probability that a particle will enter a laminar boundary zone
adjacent to the collection electrode in which capture is assured.
Using probability concepts and the statistical nature of
the large number of particles in a precipitator, White derived
an expression for the collection efficiency in the form
n = 1 - exp (-A w /Q)
(4.2)
where n = collection fraction for a monodisperse aerosol,
A = collection area (m )
w = migration velocity near the collection electrode of
p the particles in the monodisperse aerosol (m/s),
and
Q = gas volume flow rate (m3/s).
The simplifying assumptions on which the derivation of equa-
tion 4.2 is based are:
(1) The gas is flowing in a turbulent pattern at a constant,
mean forward-velocity.
(2) Turbulence is small scale (eddies are small compared
to the dimensions of the duct), fully developed, and
completely random.
(3) The particle migration velocity near the collecting
surface is constant for all particles and is small
compared with the average gas velocity.
(4) There is an absence of disturbing effects, such as
particle reentrainment, back corona, particle agglomera-
tion, or uneven corona.
Experimental data1*1 under conditions which are consistent with
the above assumptions demonstrate that equation 4.2 adequately
describes the collection of monodisperse aerosols in an electro-
static precipitator under certain idealized conditions.
In industrial precipitators, the above assumptions are never
completely satisfied but they can be approached closely. With
proper design, the ratio of the standard deviation of the gas
velocity distribution to the average gas velocity can be made
to be 0.25 or less so that an essentially uniform, mean forward-
velocity would exist. Although turbulence is not generally a
completely random process, a theoretical determination of the
57
-------
degree of correlation between successive states of flow and be-
tween adjacent regions of the flow pattern is a difficult problem
and simple descriptive equations do not presently exist for typ-
ical precipitator geometries. At the present, for purposes of
modeling, it appears practical and plausible to assume that the
turbulence is highly random. For particles larger than 10 jam
in diameter, the turbulence does not dominate the motion of these
particles due to their relatively high migration velocities.
Under these conditions, equation 4.2 would be expected to under-
predict collection efficiencies. The practical effect in model-
ing precipitator performance will be slight, however, since even
equation 4.2 predicts collection efficiencies greater than 99.6%
for 10-ym diameter particles at relatively low values of current
density and collection area [i.e., a current density of 1 x 10"1*
and a collection area to volume flow ratio of 39.4 m2/(m3/s)].
Removal of Collected Material
In dry collection, the removal of the precipitated material
from the collection plates and subsequent conveyance of the mate-
rial away from the precipitator represent fundamental steps in
t"he collection process. These steps are fundamental because
collected material must be removed from the precipitator and
because the buildup of excessively thick layers on the plates
must be prevented in order to ensure optimum electrical operating
conditions. Material which has been precipitated on the collec-
tion plates is usually dislodged by mechanical jarring or vibra-
tion of the plates, a process called rapping. The dislodged
material falls under the influence of gravity into hoppers located
below the plates and is subsequently removed from the precipitator
The effect of rapping on the collection process is deter-
mined primarily by the intensity and frequency of the force ap-
plied to the plates. Ideally, the rapping intensity must be
large enough to remove a significant fraction of the collected
material but not so large as to propel material back into the
main gas stream. The rapping frequency must be adjusted so that
a larger thickness which is easy to remove and does not signific-
antly degrade the electrical conditions is reached between raps.
In practice, the optimum rapping intensity and frequency must
be determined by experimentation. With perfect rapping, the
sheet of collected material would not reentrain, but would migrate
down the collection plate in a stick-slip mode, sticking by the
electrical holding forces and slipping when released by the
rapping forces.
LIMITING FACTORS AFFECTING PRECIPITATOR PERFORMANCE
Allowable Voltage and Current Density
The performance of a precipitator which has good mechanical
and structural features will be determined primarily by the
electrical operating conditions. Any limitations on applied
58
-------
voltage and current density will be reflected in the optimum
collection efficiency which can be obtained. A precipitator
should be operated at the highest useful values of applied vol-
tage and current density for the following reasons:
(1) high applied voltages produce high electric fields;
(2) high electric fields produce high values of the satura-
tion and limiting charge that a particle may obtain;
(3) high current densities produce high rates at which
particles charge to the saturation or limiting values
of charge;
(4) high current densities produce an increased electric
field near the collection electrode due to the "ionic
space charge" contribution to the field; and
(5) high values of electric field and particle charge pro-
duce high migration velocities and increased transport
of particles to the collection electrode.
Electrical conditions in a precipitator are limited by either
electrical breakdown of the gas in the interelectrode space or
by electrical breakdown of the gas in the collected particulate
layer. In a clean-gas, clean-plate environment, gas breakdown
can originate at the collection electrode due to surface irregu-
larities and edge effects which result in localized regions of
high electric field. If the electric field in the interelectrode
space is high enough, the gas breakdown will be evidenced by
a spark that propagates across the interelectrode space. The
operating applied voltage and current density will be limited
by these sparking conditions.
If a particulate layer is deposited on the collection elec-
trode, then the corona current must pass through the particulate
layer to the grounded, collection electrode. The voltage drop
across the particulate layer is
Vp = jpt, (4.3)
where V = voltage drop (volt),
j = current density (A/m2),
p = resistivity of particulate layer (ohm-m), and
t = thickness of the layer (m) .
59
-------
The average electric field in the particulate layer, Ep(volt/m),
is given by •
Ep = JP- (4.4)
The average electric field in the particulate layer can
be increased to the point that the gas in the interstitial space
breaks down electrically. This breakdown results from the ac-
celeration of free electrons to ionization velocity to produce
an avalanche condition similar to that at the corona electrode.
When this breakdown occurs, one of two possible situations will
ensue. If the electrical resistivity of the particulate layer
is moderate (sQ.1-1.0 x 109 ohm-m), then the applied voltage
may be sufficiently high so that a spark will propagate across
the interelectrode space. The rate of sparking for a given pre-
cipitator geometry will determine the operating electrical con-
ditions in such a circumstance. If the electrical resistivity
of the particulate layer is high (>1 x 109 ohm-m), then the ap-
plied voltage may not be high enough to cause a spark to pro-
pagate across the interelectrode space. In this case, the par-
ticulate layer will be continuously broken down electrically
and will discharge positive ions into the interelectrode space.
This condition is called back corona. The effect of these posi-
tive ions is to reduce the amount of negative charge on a par-
ticle due to bipolar charging and reduce the electric field
associated with the "ionic space charge". Both the magnitude
of particle charge and rate of particle charging are affected
by back corona. Useful precipitator current is therefore limited
to values which occur prior to electrical breakdown whether the
breakdown occurs as sparkover or back corona.
Field experience shows that current densities for precipi-
tators operating at about 150°C are limited to approximately
5 to 7 x 10""* A/m2 due to electrical breakdown of the gases in
the interelectrode space. Consequently, this constitutes a
current limit under conditions where breakdown of the particulate
layer does not occur.
Electrical breakdown of the particulate layer has been
studied extensively by Penney and Craig"2 and Pottinger1*3 and
can be influenced by many factors. Experimental measurements
show that particulate layers experience electrical breakdown
at average electric field strengths across the layers of approxi-
mately 5 x 10s V/m. Since it takes an electric field strength
of approximately 3 x 10s V/m to cause electrical breakdown of
air, this suggests that high localized fields exist in the par-
ticulate layer and produce the breakdown of the gas in the layer
The presence of dielectric or conducting particles can cause
localized regions of high electric field which constitute a
negligible contribution to the average electric field across
60
-------
the layer. The size distribution of the collected particles
also influences the electrical breakdown strength by changing
the volume of interstices. It has also been found that breakdown
strength varies with particulate resistivity with the higher
breakdown strength being associated with the higher resistivity.
Particle Reentrainment
Particle reentrainment occurs when collected material re-
enters the main gas stream. This can be caused by several dif-
ferent effects and, in certain cases, can severely reduce the
collection efficiency of a precipitator. Causes of particle
reentrainment include (1) rapping which propels collected mate-
rial into the interelectrode space, (2) action of the flowing
gas stream on the collected particulate layer, (3) sweepage of
material from hoppers due to poor gas flow conditions, air in-
leakage into the hoppers, or the boiling effect of rapped mate-
rial falling into the hoppers, and (4) excessive sparking which
dislodges collected material by electrical impulses and disrup-
tions in the current which is necessary to provide the electrical
force which holds the material to the collection plates.
Recent studies1*1*'1*5 have been made to determine the effect
of particle reentrainment on precipitator performance. In studies
where the rappers were not employed, real-time measurements of
outlet emissions at some installations showed that significant
reentrainment of mass was occurring due to factors other than
rapping. These studies also showed that for high-efficiency,
full-scale precipitators approximately 30 to 85% of the outlet
particulate emissions could be attributed to rapping reentrain-
ment. The results of these studies show that particle reentrain-
ment is a significant factor in limiting precipitator performance.
PREVIOUS USES OF ELECTROSTATIC DEVICES FOR DIESEL EXHAUST
Only two reports of the use of electrostatic devices on
diesel particulate have been located. One of these was a study
performed by Battelle Columbus Laboratories for General Motors,
in which an electric field was used to concentrate the particulate
into a portion of the exhaust.9 This work resulted in the device
shown in Figure 15. The device concentrated 50% of the parti-
culate into 16% of the exhaust gas. According to General Motors,
the insulators quickly became coated with particulate, resulting
in erratic operation and several failures.
In the other instance, Southwest Research Institute reported
testing an electrostatic precipitator built by Gordine Systems,
Inc.1*6 Although this device was designed to operate in the 1.2
to 1.6 x 10 -V region, continuous discharge was observed at
1.0 x 101* V, which was taken as the maximum operating voltage.
After 11 min of exposure to diesel exhaust, the device showed
61
-------
a\
to
0.008 INCH
CHARGING WIRE
DIRTY GAS
OUTLET
5 cm DIAMETER
STAINLESS STEEL TUBE
EXHAUST
GAS INLET
BORON NITRIDE
INSULATOR
ALIGNMENT
SLEEVES
CLEAN
GAS OUTLET
WIRE TIGHTENER
AND VOLTAGE POST
Figure 15. Electrostatic paniculate remover.^
-------
a dramatic increase in ionizer current indicating current leakage
through conduction paths of particulate. Measurements of the
collection efficiency were limited to smokemeter measurements,
which indicated that the device had no effect at all on the
exhaust.
APPLICATION OF ELECTROSTATIC FUNDAMENTALS TO DIESEL EXHAUST
introduction
The chief advantage in the use of an electrostatic device
to control diesel particulate lies in the high efficiency which
can be achieved at low backpressure. However, there are several
problem areas associated with this type of device. One of these
problems is reentrainment. Because the particles are primarily
carbon, they are expected to have a relatively low resistivity.
This implies that they would loose their charge readily when
attached to a conductive surface such as the collection plates
of a precipitator. Without electrostatic attraction between
the collected particles and the collection plates, the particles
have to depend on their stickiness due to adsorbed hydrocarbons
for adhesion to the collection surface. This may prove inade-
quate when subjected to a high velocity gas flow which pulsates
with the opening and closing of the exhaust valves and to the
mechanical vibration that occurs on a moving vehicle.
The water content of the gas presents another problem area
due to the fact that, at starting, the collection device will
pass through a temperature range which includes the dew point
temperature. The presence of condensed water films in conjunc-
tion with a conductive particulate may cause conductive paths
to accumulate on the insulator surfaces, which will in time short
out the device. After the device has reached operating tempera-
ture, all of the water will be in the vapor form and no further
accumulation should take place. Several possibilities exist
for dealing with the problem:
(1) Only electrically activate the device after it has
reached operating temperature. This would leave only
the mechanical collection forces active during the
starting cycle. This solution does not appear very
attractive because the emissions are particularly bad
during start up.
(2) Bypass the device altogether during starting by divert-
ing the exhaust through a regular muffler until the
control device has exceeded the dew point temperature.
This has the same drawback as solution 1.
(3) Use internal heating elements within the critical in-
sulators to keep the insulators above the dew point.
This would present problems due to the fact that the
63
-------
temperature would have to be above the dew point prior
to starting.
(4) Use insulators that are hydrophobia so that there would
be minimum chance of the water film forming on the
surface. This seems somewhat impractical due to the
fact that there is bound to be surface contamination
from the conductive particles alone.
(5) Design the insulators with sufficient surface path
length and dead air zones to minimize the effect of
tracking. This solution will not eliminate the prob-
lem, but it may lengthen the time period sufficiently
so that the insulators may be cleaned during regular
device maintenance.
Efficiency of an Electrostatic Precipitator
For a calculation of the size and efficiency of an electro-
static precipitator for collecting diesel exhaust particulate,
it is necessary to make several assumptions. First, assume a
two stage geometry in which the particle charging process pre-
ceeds and is separate from the particle collection process.
After particles are charged to saturation, we can find the num-
bers of charges on each particle by making some approximations.
If we assume a 0.20-m cylinder with a 0.25-cm wire, then from
an example in White1*7 we can achieve a charging field Ec of
1.0 x 106 V/m with a current density j = 2.0 x 10~2 A/m .
Assuming a 200°C operating temperature, we have a value for the
ion mobility b of 3.1 x lO'1* nr/V-s. Then using the expression
for the ion density
(4/5)
we obtain N = 4.0 x 10 ions/m .
o
If we assume a 0.14-m/s flow rate through the 0.20-m pipe,
we get a gas velocity of 4.32 m/s. If the charging is done by'
a 0.15-m long rod, we find that the particles are in the charging
region for a time t = 0.035 s. Smith and McDonald1*8 have plotted
the number of charges on a particle as a function of N0t, Ec,
and particle size. From this, we can estimate a value of 40
electron charges for a 0.30-um particle.
We can now calculate the ion drift velocity as given by
equation 4.1:
qE C
w = P c
p 6Ttay (4.1)
64
-------
For this example assume the following values:
q = 40 electron charges
E = 1 x 106 V/m
C = 1.7
c
a = 0.15 x 10~6 m (1/2 of particle mmd)
U = 2.5 x 10~5 Pa-s
to obtain a drift velocity of wp = 0.15 m/s. We can now solve
equation 4.2 for the collection efficiency in terms of the
collection area. Table 14 shows the plate area required for
a given collection efficiency.
TABLE 14. REQUIRED COLLECTION AREA FOR A DESIRED COLLECTION
EFFICIENCY FOR AN ELECTROSTATIC PRECIPITATOR
Efficiency, n Area, Ap (m2)
0.5
0.6
0.7
0.8
0.9
0.95
0.66
0.87
1.14
1.52
2.18
2.84
DESCRIPTION OF PROTOTYPE ELECTROSTATIC DEVICES
An electrostatic precipitator has been proposed to control
diesel particulate. This device features a periodic wet flushing
scheme to thoroughly clean the collected particulate from all
internal surfaces of the system.
A vertically oriented, cylindrical geometry appears best
suited to such a cleaning method and optimum from the standpoint
of structural strength. A conceptual sketch indicating the basic
features of such a system is shown in Figure 16. A two-stage
device was selected to provide a maximum collecting surface
within the space limitations of a vehicular installation. The
exhaust gas from the engine enters the precipitator tangentially
65
-------
TO SPRAY
NOZZLES
INSULATOR
CABLE TO HIGH
VOLTAGE SUPPLY
SPRAY NOZZLES
INLET fr(?
CHARGING
SECTION
COLLECTING
SECTION
0.30 m LONG
(12 in.)
CABLE TO HIGH
VOLTAGE SUPPLY
STAR-SHAPED
ELECTRODES
(DETAIL)
SUPPORT SPIDER FOR
GROUNDED CYLINDERS
SUPPORT SPIDER FOR
HIGH VOLTAGE
CYLINDERS
INSULATOR
SUPPORT SPIDER
OUTLET
REMOVABLE
FIBER FILTER
STORAGE TANK
Figure 16. First prototype electrostatic precipitator for collecting
diesel paniculate.
66
-------
so as to avoid immediate impaction on the high voltage insulators.
The cyclonic motion of the gas in the first stage (upper half)
of the precipitator has little effect on particulate collection
by impaction because most of the particles are very fine; how-
ever, the circular path allows for adequate charging time for
the particles before they enter the collecting stage.
Particle charging is achieved by means of an electrical
corona discharge from flat, star-shaped electrodes mounted on
the axial rod extending downward from the insulator at the top
of the device. A corona ball on the end of the rod suppresses
discharges to the grounded plates in the collecting stage. This
structure is preferred over a more conventional fine-wire corona
discharge electrode because of its ruggedness.
The collecting stage consists of a set of concentric cyl-
inders. In sequence of decreasing diameter, the odd-numbered
cylinders are connected to electrical ground and the even-num-
bered cylinders are connected to the output of a high voltage
power supply. The high voltage cylinders are connected together
by a metal bus bar and nested between the grounded insulators.
Insulating spacers between cylinders are avoided in order to
minimize leakage current due to fouling by low resistivity ma-
terial. Three stand-off insulators are used to support the
entire array of high voltage cylinders.
The precipitator is to be cleaned periodically by spraying
a nonvolatile liquid through the nozzles at the top of the de-
vice. The liquid is pulled through a fiber filter at the bottom
of the precipitator and then pumped into a storage tank for the
next cleaning cycle. The period between flushing operations
would probably be governed by the length of time required for
the particulate buildup on the insulators to develop a signifi-
cant currant leakage path between high voltage components and
ground. Provisions would be required to bypass the precipitator
during the cleaning operation, which might take 30 to 60 s.
The 0.20-m diameter and 0.15-m long charging region of this
device allow an estimation of collection efficiency using the
numbers obtained earlier in this section. From the detailed
drawings, the plate area can be calculated to be 1.46 m2. Accord-
ing to Table 14, 1.52 m2 is required for the 80% efficiency,
which leads to an estimated 79% efficiency for this device.
In practice, the efficiency should be somewhat higher due to
the star-shaped electrodes on the charging rod.
A second electrostatic device which has been proposed for
diesel exhaust is a radial flow device which uses dielectric
filter material in the collection region. This device, shown
in Figure 17, has the following features:
67
-------
o\
CO
0.15 m (6 in.)
0.13m (5.25 in.)
0.08m (3.25 jn.)
0.03 m (1.25 in.)
0.64m (25 in.)
0.61 m (24 in.)
INSULATOR
HIGH
VOLTAGE
COLLECTOR
PERFORATED
METAL
SCREENS
FILTER
MATERIAL
CONCENTRIC PRECIPITATOR
WITH DIELECTRIC COLLECTING
MEDIA
Figure 17. Second prototype electrostatic precipitator for collecting
diesel paniculate.
-------
(a) two stage operation, thus minimizing ozone generation
and power consumption,
(b) adaptation of high velocity gas throughput in small
diameter ducts to low radial velocities for collection,
thereby reducing reentrainment,
(c) high efficiency for the submicron particle size range,
(d) utilization of a combination of mechanical collection
forces as well as coulombic, dielectrophoretic, and
image electrical forces,
(e) convenient geometry for using electrified media in
the collection stage and ready adaptation to removable
cartridge form.
It is believed that the most effective form of collector
would involve a gradation using three collection zones. The
first would be a mechanical impaction collector utilizing the
high velocity jets of gas produced by the inner perforated screen
that changes the gas flow from the axial to the radial direction.
The second zone could be a relatively coarse fibrous bed of
collection media with superimposed electrostatic field. The
final zone would be a finer graded bed of fibrous media with
superimposed electrostatic field.
The efficiency of this device is harder to predict than
that shown in Figure 16. If the effects of the dielectric ma-
terial are neglected and the particles are assumed to have acquired
the same charge as in the wire-cylinder example earlier in this
section (admittedly a bad assumption), then for the metal col-
lection plate area of 1.2 m2 an efficiency of 0.7 (70%) can be
obtained from Table 14. The effects of the dielectric material
will be to increase the collection efficiency. Figure 18 shows
the results of using various geometries of dielectric material
in a similar device. Note that the collection efficiency for
the device was improved for any choice of dielectric geometry
and that the most efficient case was for a fibrous bed similar
to that proposed here.
This device does have two possible design drawbacks. The
first of these is the susceptibility to conductive contamination
on the insulators. This will probably be worse on the ionizer
and may necessitate the use of a different type of ionizer.
The second is the increase in backpressure that will result when
the dielectric fiber bed becomes loaded with particles. The
magnitude of this problem can only be determined experimentally.
69
-------
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ID'2
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FIRST STAGE ONLY
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GEOMETRY B
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-------
SECTION 5
WET SCRUBBERS
DESCRIPTION OF SCRUBBER PROCESSES FOR COLLECTION OF PARTICULATE
MATTER
Mechanisms
General—
There are a number of particle collection mechanisms utilized
in wet scrubbers. Inertial impaction is the primary collection
mechanism for particles of aerodynamic diameter larger than about
0.5 ym. The particle aerodynamic diameter is the diameter of
a sphere of unit density which has the same gravity settling
velocity as the real particle.
Inertial Impaction—
A gas stream approaching an object such as a scrubbing liquid
droplet has fluid streamlines curved around the droplet. The
inertial force causes migration of the particle across the curved
fluid streamlines to the droplet surface resulting in particle
collection. The collection of the particles on the droplet by
inertial impaction is dependent upon three factors: (1) the
velocity distribution of the gas flowing around the droplet,
(2) the particle trajectory (which is dependent upon factors
such as the air resistance on the particle, the particle mass,
the droplet diameter, and the gas velocity with respect to the
droplet), and (3) the adhesion of particles to the droplet once
impacted on the droplet surface.
The velocity of the particle with respect to the droplet
is characterized by the collecting body Reynolds number, Re :
Rec = UGpGdd^G (5-1}
where: UG = undisturbed gas stream velocity with respect to
the obstacle (m/s)
PG = gas stream density (kg/m3)
d = diameter of the droplet (m)
MG = gas stream viscosity (Pa-s)
71
-------
At low gas velocities, viscosity factors govern the streamlines,
so they smoothly spread around the object at a relatively large
distance upstream of the object. At high Reynolds numbers, the
streamlines abruptly diverge immediately upstream of the obstacle.
The effect of sudden spreading of the streamlines at a high Reynolds
number (high velocity) is to reduce the ability of the particles
to make the sudden change of direction required to stay with
the streamlines, thereby increasing inertial impaction and par-
ticle collection.
The dimensionless Stokes parameter, Kp, characterizes the
motion of the particle around a collecting droplet:
*p • -
where: P = particle density (kg/m3)
d = particle diameter (m)
UG = gas stream velocity (m/s)
VIG = gas stream viscosity (Pa-s)
d = droplet diameter (m)
c
CG = Cunningham correction factor (dimensionless)
The physical meaning of the Stokes parameter is the ratio of
the particle's "stopping distance" to the radius of the collec-
tor. The stopping distance is the distance it would take the
particle to stop in a still fluid. For spherical obstacles the
critical value of Kp, the value at which smaller values result
in no inertial deposition, is 0.083. Similarly, for cylindrical
obstacles, the critical Kp is 0.125. In reality, even below
the critical Kp values, some collection is made at the back of
the obstable because of turbulent eddies.
Gravitational Collection —
With gravitational collection, the collection obstacle
settles under free-fall through gas stationary with respect to
the velocity of the obstacle. The collection made by this method
is similar to that of the gas moving around a stationary obstacle.
Brownian Diffusion —
Very small particles (diameter <0.1 ym) can easily follow
the streamlines when they diverge around an obstacle, and are
thus rarely collected by inertial impaction. In gas streams
72
-------
with low velocities, small particles can move across the stream-
lines due to Brownian motion. If the particles diffuse into
the collection obstacles, they are collected and removed from
the gas stream.
The values for particle diffusivities are several orders
of magnitude smaller than that of gases and are close to those
values of solutes in liquid. Using the Einstein equation, the
particle diffusion coefficient can be calculated for particles
larger than 0.062 ym:
C kT
where:
D.
C = Cunningham correction factor (dimensionless)
c
(5.3)
k = Boltzmann constant (joule/K)
T = absolute temperature
yG = gas stream viscosity (Pa-s)
d = particle diameter (m)
One major assumption in using this equation is that the particle
concentration remains constant at a large distance from the
collecting obstacle, and that near the collection surface the
particle concentration is zero.
Electrostatic Collection—
There are five aspects of electrical forces that affect
the particle near the collecting obstacle; these are dependent
on particle and collector charging.
a. When both particle and collector are oppositely charged,
the coulombic force Kei dominates.
b. When only the collector is charged and this induces
an image charge of opposite polarity to the particle,
the force K62 dominates.
c. When only the particle is charged and it induces an
image charge of opposite polarity to the collector,
the force K63 has an influence on particle collection.
d. The repulsive force on the particle due to similarly
charged particles on the collector's surface, Keit,
has an influence on particle collection.
73
-------
e. The attractive force, Kes/ between a charged particle
and a grounded collector that has an induced charge
caused by the surrounding charged particles has an
influence on particle collection.
Both the repulsive force (K6lf) and the space charge attractive
force (Kes) are dependent on particle concentration and in most
cases can be considered negligible unless the scrubber uses bubbles
as the collector. The image force K63 is small and in most cases
will be negligible.
Thermophoresis and Diffusiophoresis—
Temperature or water vapor pressure gradients between the
surface of the collection droplets and the surrounding gases
can create a particle velocity toward the droplets, thereby in-
creasing particle collection efficiency. The magnitude of the
velocity, and its effect on collection efficiency, depends on
characteristics of the gas and the gradient involved.
Thermophoresis—When the gas temperatures on opposite sides
of a particle are different, gas molecules on the warmer side
will have a higher kinetic energy, and will randomly strike the
particle with more force than will molecules on the cooler side
of the particle. This unbalanced striking force causes the
particle to move away from the warmer temperature zone. The
velocity at which the particle moves depends on the temperature
gradient across the particle, the relative thermal conductivities
of the gas and particle (which govern the rate at which thermal
energy is conducted through the particle), and the density and
viscosity of the gas (which govern the drag force on the particle
at a given velocity). In general, thermophoresis does not have
a significant effect on particle collection because a large
temperature gradient is required to cause particle migration
velocities of a reasonable magnitude.
Piffusiophoresis—When the partial vapor pressure of water
in the bulk gas is greater than the water vapor pressure at the
surface of the collection droplets, water vapor will condense
from the gaseous state onto the droplets. A bulk motion of gas
toward the droplets is thus created to replace the condensed
vapor. This bulk motion of the gas toward the droplet and motion
of the water vapor towards the droplet sweep the particle toward
the droplet. In general, diffusiophoresis can have a significant
effect for the collection of fine particles.
Effect of Thermo- and Diffusiophoresis on Collection Effi-
ciency—The two phoretic forces can either increase or decrease
collection efficiency, depending on the direction of the appro-
priate gradient. Thermophoresis will increase collection effi-
74
-------
ciency when the collection droplets are cooler than the surround-
ing gases. Diffusiophoresis will increase collection efficiency
when collection droplets are condensing, and will reduce collec-
tion efficiency when the droplets are evaporating.
Types of Scrubbers
General—
Wet scrubbers are generally classified by the geometry or
physical configuration of the scrubber. Because there are many
gas-liquid contacting geometries possible, there is a consider-
able number of scrubber types available. However, the scrubber
types have design and operating parameters which can be compared,
as shown in Table 15.
Spray Towers—
A spray towers is the simplest type of scrubber. A spray
of liquid droplets is produced by nozzles and allowed to fall
through a rising stream of dirty gases. The simplest of spray
scrubbers produce droplets that are sufficiently large so that
the settling velocity is greater than the upward velocity of
the gas stream. To increase the residence time of the droplets,
the spray droplets are formed at such a size that the settling
velocity is less than the vertical gas velocity, entraining the
spray droplets in the gas stream. This will decrease the rela-
tive velocity between the particles and the droplet, resulting
in a smaller inertial impaction parameter, but it will increase
those collection mechanisms that require a longer residence time.
The predominant collection mechanism most spray scrubbers are
designed for is inertial impaction; thus the collection effi-
ciency for small particles is low.
Venturi Scrubbers—
In the venturi scrubber, the relative velocity between the
gas and the injected liquid causes the formation of the droplets.
The gas is accelerated through a mechanical constriction, causing
any water that is injected in the venturi throat to be sheared
off and atomized. The droplet diameters formed can range from
10 to 300 ym, depending upon the gas velocity. Within the venturi
throat the turbulence due to the gas acceleration enhances impinge-
ment collection. The particle removal efficiency is mainly a
function of the pressure drop, liquid-to-gas flow rate ratio,
and velocity of the gas through the venturi throat. The velocity
and liquid-to-gas ratio have this effect by controlling the
relative throat velocity and the degree of collection by the
droplets. The gas pressure drop and the gas velocity in the
venturi throat are related. It is necessary to have some means
75
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TABLE 15. OPERATING AND DESIGN PARAMETERS OF SCRUBBERS
Type
Spray
Venturi
Impingement
plate
Sieve plate
Charged
droplet
Floating
bed
Induced
draft
Liquid-to-gas
flow ratio
(m3/1000 m3)
0.27-4.0
0.13-2.7
0.13-0.27
per plate
0.13-0.27
per plate
0.13-2.7
1.3-6.7
0.53-1.3
Gas
velocity
(m/s)
0.60-3.0
30-120
15-30
through
jets
15-30
through
jets
0.60-3.0
3.0-6.0
1.2-6.0
Gas
pressure
drop
(Pa)
<250
2500-37300
250-1000
per plate
250-1000
per plate
250
1000-1250
per stage
500-2000
Gas
residence
time
(s)
5-20
0.1
2-4
2-4
5-20
1-5
1-2
Capability of
collecting
0.2-ym particles
low
possible with
high gas
velocity
low
low-medium
high
low-medium
low
-------
of collection of the droplets entrained in the gas stream by
the venturi. Mist droplet collectors include cyclone separators,
wire mesh filters, sieve plate scrubbers, and impingement baffle
vanes.
Impingement Plate Scrubbers—
Impingement plate scrubbers are generally towers with hori-
zontal plates placed at regular intervals. These plates have
a pattern of holes in them (0.1 to 1.0 cm and 6,400 to 32,000
holes/m2) to allow the gas to pass through. Impingement baffles
are located directly above the holes in the tray. As the gas
accelerates through the orifices in the tray, the entrained par-
ticles are also accelerated; many of these particles will impact
onto the baffle and be washed away by the liquid flowing over
the top of the trays. Due to the gas flow through the orifices
at a high velocity, some of the water flowing over the plates
and baffles is torn away creating water droplets. These droplets
have a low velocity compared to the gas stream, which allows
for the impaction of the particles onto the droplets. This
allows for two means of impaction for collecting the particles,
impaction on the droplets and impaction on the baffles. The
liquid flow is controlled by weirs. In most cases a mist elimi-
nator is needed; either vane axial cyclones or impingement baffle
vanes are effective for the size of droplets produced.
Sieve Plate Bubble Scrubbers—
Sieve plate scrubbers are also known as foam scrubbers.
The particles are collected on the inside surfaces of the foam
bubbles. In appearance these scrubbers look like an impingement
plate scrubber, without the impingement baffles. Generally,
the foam scrubbers have a larger quantity of orifices and also
a larger orifice diameter.
Collection by inertial impaction takes place during the
formation of the bubble. A secondary impaction mechanism works
as the bubble rises through the foam. This is due to gas and
particulate circulation inside the bubble, causing centrifugal
deposition of the particles on the bubble's wall. Because of
the long residence time due to bubble rise through the foam,
Brownian diffusion plays an important role in the collection
of small particles. Particle collection by Brownian diffusion
is also enhanced by gas circulation in the bubble during bubble
rise through the liquid.
The kinetic energy from the gas up-flow prevents the liquid
from weeping through the perforations. If the gas velocity de-
creases below the critical velocity for weeping, the liquid will
seep through the orifices, causing a large decrease in particle
77
-------
collection efficiency. On the other hand, if the gas velocity
becomes too great, the pressure drop will become so large that
the liquid will discontinue to flow, and flooding (build-up of
liquid in the tower as it cannot flow downwards) occurs.
Charged Droplet Scrubbers—
Electrostatics can enhance the performance of wet scrubbers,
making it a logical extension to any of the previously discussed
scrubbers. Charging of the particles can be accomplished by
passing the dust-laden gases through a high-voltage ionizing
section where corona discharge ionizes the air; through intercep-
tion with the air ions, the charge is transferred to the par-
ticles. These charged particles then enter the scrubber; if
the particles are not collected by the typical mechanisms of
the scrubber, collection may occur by the electrostatic attraction.
This particle-droplet attraction can be caused by the image
charge induced onto the droplet by the charged particle. The
droplet can be electrostatically charged to the polarity opposite
to the particle charge, and the particle-droplets undergo coulombic
attraction forces (which can be the largest electrostatic particle-
droplet attractive forces available).
DISCUSSION OF THE LITERATURE ON WET SCRUBBERS
Very little literature appears to exist on the subject of
wet scrubbers for diesel engines. Such literature as was located
in the literature survey for this project provides little informa-
tion on specific scrubber designs that would be applicable to
particulate control in light-duty vehicles.
Lawson and Vergeer50 reported on the results obtained with
a diesel engine intended for operation in an underground environ-
ment. They described two wet scrubbers as "a single path water
bath conditioner" and "a more sophisticated packed bed water
scrubber." For the single pass device, the authors reported
that particulate capture averaged only about 30%, which was con-
sidered unsatisfactory performance. For the packed bed device,
the removal efficiency of particulates was, at best, only margin-
ally better than that in the single pass device and under some
operating conditions was virtually identical to that in the single
pass device. Much of the authors' interest in the functioning
of wet scrubbers was for the control of hydrocarbons and gases.
Wood and Colburn51 also addressed scrubbers for use with
diesel equipment in an underground environment. They, too, were
concerned not only with the removal of particulates but with
the removal of gases such as nitrogen oxides. The authors pro-
vided little specific information on scrubber design or scrubber
78
-------
performance. Their general position seemed to be that the per-
formance of wet scrubbers has not been systematically investi-
gated but that such performance should be systematically studied
in view of the many design options that are available and the
large body of background information that has been accumulated
for scrubbers in other applications. The authors made one specific
recommendation: that water injection into the exhaust turbine
of a turbocharger be investigated as a means of particulate
control.
Sudar and Grantham52 described the use of a molten alkaline
carbonate scrubber on a diesel urban bus. This device was of
primary interest for controlling nitrogen oxide emissions. The
effect of the device on particulate emissions does not appear
to have been quantified. It is not apparent that a fused salt
scrubber would offer any advantages over a water scrubber for
particulate removal.
CALCULATION OF SCRUBBER EFFICIENCIES FOR PARTICULATE CONTROL
General Approach
The characteristics of diesel particulate that were used
in calculations of scrubber efficiencies were as follows:
Particle density - 2,000 kg/m3 (2.0 g/cm3)
Particle mass median diameter - 2 x 10~7 m (0.2 ym)
The assumption was made that the diesel exhaust gas would
enter the scrubber at either of two temperatures, 52 or 200°C.
The lower of these two temperatures is that prescribed by EPA
for measuring condensible gases as particulate. The upper tem-
perature is an estimated temperature of the exhaust gas as it
enters the muffler. The volume flow rate of gas at the upper
temperature was estimated as 0.14 m3/s (300 ft3/min) for an
automobile at highway cruising speed. The corresponding volume
flow rate at the lower temperature was estimated as 0.097 m3/min
(206 ft3/min) for the same conditions.
Estimates of the operating temperature and water vapor con-
centration in a scrubber were made by assuming that adiabatic
equilibration of the gas and water would be established with
respect to both temperature and water vapor concentration. The
operation of the water loop supplying the scrubber was assumed
to be virtually isothermal, and the concentration of water vapor
entering the scrubber was estimated as 3% by volume. The result-
ing estimates for the different operating conditions were then
as follows:
79
-------
Gas, water temperature, 52°C
Gas, water outlet temperatures, 30°C
Gas outlet water vapor concentration, 4.19%
Gas inlet temperature, 300°C
Gas, water outlet temperatures, 50°C
Gas outlet water vapor concentration, 12.17%
The calculations of scrubber efficiencies and related param-
eters followed the format presented in the Scrubber Handbook.53
These calculations were related to the pressure drops in the
scrubbers, which were assumed to be limited to a maximum of
2,500 Pa (25.4 cm or 10 in. of water head). In one type of ap-
proach, it was convenient to set the pressure drop at the maximum
desired and then to calculate the related parameters and finally
the efficiency. This approach was followed with the venturi
scrubber. In another approach, various parameters were assigned
reasonable values, and then the resulting efficiency and pressure
drop were calculated. This approach was followed with the sieve
plate bubble scrubber. In the calculations to follow, the SI
system of units used heretofore in this report was abandoned
because of the expression of equations in the Scrubber Handbook
in cgs or English units.
Calculations for a Venturi Scrubber
Design Considerations—
Calculations were made for a venturi operating at the maxi-
mum allowable pressure drop of 25.4 cm (10 in.) of water. This
was done even though a common pressure drop for a venturi is
frequently much higher. A series of selected liquid-to-gas
volume ratios was assumed, and then values of various parameters—
ultimately the efficiency—were calculated.
Calculations*—
1. Assume the maximum pressure drop and calculate the velocity
of the gas in the throat of the venturi for a series
of liquid-to-gas ratios.
M . r AP i . _
G !.Q3 , in-.
x 10~3(L/G)
* Definition of symbols and numerical values of certain parameters
are given in the following section, Notation.
80
-------
2. Calculate the diameter of the droplets produced in the
ventur i .
dd = jp. + 91.8(L/G)1-5 (5.5)
G
3. Calculate the Stokes impaction parameter.
P <3 2U_C
4 . Calculate the function given below involving the Stokes
impaction parameter and the wetting factor f, where
f is assumed to be 0.25 for a hydrophobic particle.
K f + 0.7
0.49 1 1_
0.7 + KpfJ Kp
F(Kp,f) = 1-0.7 - Kpf -f 1.4 in [^-^j ) + n ,V; f \ ^ (5.6)
5. Calculate the penetration of diesel particulate through
the venturi.
P = exp [ % L d(F(K .f))(L/G)] (5.7)
°°^G p
6. Calculate the efficiency of particulate removal.
n = 100(1 - P ) (5.8)
Notation—
CG = Cunningham correction factor
d = diameter of particle (2 x 10~5 cm)
d^ = diameter of droplet (cm)
f = wetting factor (0.25)
F(K ,f) = function of Stokes impaction parameter
p and wetting factor
K = Stokes impaction parameter
L/G = liquid-to-gas ratio (dimensionless)
AP = pressure drop (25.4 cm of water)
Pfc = penetration
UQ = gas velocity in throat (cm/s)
81
-------
n = efficiency, %
p = particle density (2.0 g/cm )
p. = water density (g/cm3)
LI
VG = gas viscosity (g/(cm-s))
Results—
The results of the calculations are given in the following
two tables. In Table 16, the assumed scrubber temperature is
30°C. In Table 17, the assumed temperature is 50°C. The results
indicate that a maximum efficiency of 4 to 5% would occur at
either temperature. The conclusion, therefore, is that a venturi
would be too inefficient for particulate removal from diesel
exhaust under the assumed constraint of pressure drop.
Calculations for a Sieve Plate Bubble Scrubber
Design Considerations—
The inefficiency of collecting 0.2-pm particles by impaction
in a venturi scrubber led to consideration of a sieve plate bubble
scrubber, in which collection by the combined mechanisms of im-
paction and diffusion was expected to give more promising results.
Reasonable values were assigned to the appropriate operating
parameters, and calculations were made of efficiency and pressure
drop.
Calculations*—
1. Assume an orifice diameter of 0.5 cm and (a) calculate
the minimum orifice velocity Uh to prevent weeping and
(b) calculate the maximum superficial gas velocity us
for a column to prevent flooding. The equations are:
jO.zs.GjO.izsjo.s (5<9)
-G " ML
Pr - P,,
Us = k(-=-r—S)°'5 where k = 6 (5.10)
For assumed values of the foam height h, calculate the
efficiency of particle collection n (a percentage) by
the combined mechanisms of impaction and diffusion.
The equation is:
* Definition of symbols and numerical values of certain parameters
are given in the following section, Notation.
82
-------
TABLE 16. EFFICIENCY OF PARTICULATE REMOVAL IN A VENTURI AT 30°C*
oo
u>
L/Gf U.
3.
6.
342
684
x 10"5
x ID"5
2
1
_ , cm/s
^
.72
.92
x 10"
x 10"
1.
2.
d , , cm
a
86 x
65 x
io-3
io-3
13
6
K_
P
.26
.572
F
-1
-6
(K.f)
P
.092
.734
x IO-1
x IO-2
n, %
3.53
4.36
1.337 x 10-" 1.36 x 10" 3.82 x 10~3 3.229 -3.351 x 10"2 4.43
2.674 x 10~" 9.60 x IO3 5.61 x 10~3 1.552 -1.304 x lO"2 3.59
4.010 x 10~" 7.84 x IO3 7.12 x 10~3 9.998 x 10"1 -6.657 x 1Q-3 2.86
1.337 x ID"3 4.29 x IO3 1.61 x 10~2 2.417 x 10"1 -6.144 x 10"" 1.10
2.674 x 1Q-3 3.04 x IO3 2.91 x lO"2 9.476 x 10"2 -9.080 x 10~5 0.42
* Constants: PL = 0.99567 g/cm3; MG = 1.86 x 10"" g/(cm/s); CG = 1.898.
These are dimensionless quantities corresponding to L/G ratios ranging
from 0.25 to 20 gal./lOOO ft3.
-------
TABLE 17. EFFICIENCY OF PARTICULATE REMOVAL IN A VENTURI AT 50°C*
oo
L/Gf
6.684
1.337
2.674
4.010
5.347
6.684
x
x
X
X
X
X
io-5
10"*
10~*
10"*
10"*
10"*
U , cm/s
1.92
1.36
9.60
7.84
6.79
6.07
x
x
X
X
X
X
10*
10*
IO3
IO3
IO3
10 3
dd<
2.65
3.82
5.61
7.12
8.50
9.82
x
x
x
x
x
x
cm
io-3
10- 3
10- 3
10- 3
io-3
io-3
KP
6.486
3.187
1.532
9.858 x IO"1
7.152 x 10"1
5.534 x IO"1
-6
-3
-1
-6
-3
-2
F(K ,f)
.667 x
.308 x
.278 x
.533 x
.805 x
.473 x
io-2
io-2
10"2
10'2
10~3
ID'3
n, %
4.09
4.14
3.33
2.66
2.14
1.80
* Constants: PL = 0.98807 g/cm3; VIQ = 1.95 x 10~* g/(cm-s); CG = 1.964.
These are dimensionless quantities corresponding to L/G ratios ranging
from 0.50 to 5 gal./lOOO ft3.
-------
D
n/100 = 1 - exp[-40F2K - 1.87h(- - -f— ) ° ' 5 ] (5. 11)
P (rb) b
The inertial impaction parameter K is given by the
equation: P
p d 2UC
K = P P 'h c (5.12)
P 9Vd
The foam density can be estimated by:
F = exp[-(0.184U P ° ' 5 + 0.45)] (5.13)
S ^j
The radius of bubbles formed, r. , is computed from:
-)"°-°5 (5.14)
The rising velocity of the bubble Vb is calculated from:
g(PT - P_)0'5 <5'17>
In this calculation, Ug is reduced to 80% of the value
previously calculated to ensure against flooding.
4. Calculate the pressure drop for a sieve tray plate (ex-
pressed in inches of water) :
ht = hw + how + hdp + hr
85
-------
= h'F (the pressure head up to the Level (5.19)
if the weir)
where: h
o
h = 0.48 F (nr)0*67 (the pressure head
ow w w
above the level of the weir)
(5.20)
"dp " 2C2pLg(2.54)
the dry plate)
(the pressure head from (5.21)
ij- = 1.14[0.4(1.25 - fh) + (1 - fh)M
h = 'ij i (the residual pressure head)
(5.22)
Notation—
Value at
Quantity
C - Cunningham correction factor
c
d = particle diameter (cm)
d = column diameter (cm)
c
d. = perforation diameter (cm)
30°C
1.898
2 x 10"5
28.50
0.5
50°C
1.964
2 x 10"5
28.85
0.5
d, ' = perforation diameter (in.)
D = diffusion coefficient (cm2/s)
f. = fraction of tray area per-
n forated
F = correction for column wall
w curvature
F = foam density
g = acceleration of gravity
(cm/s2)
h = foam height (cm)
h1 = foam height (in.)
hd = dry plate pressure drop
F (in. of water)
hr = residual pressure drop
(in. of water)
0.1968
2.252 x 10
0.07
1.2
0.263
981
0.1968
2.394 x 10~«
0.07
1.2
0.265
981
5, 10 5, 10
1.968, 3.937 1.968, 3.937
86
-------
Value at
Quantity 3Q°C 50lC
h = pressure head due to water
ow over weir height (in.
of water)
h = pressure head due to water up -
w to weir height (in. of
water)
h = total pressure head (in. of -
water)
k = Boltzmann constant 1.381 x 10~16 1.381 x 10~16
(ergs/K)
K = Stokes impaction parameter
L/G = liquid-to-gas ratio
(gal./lOOO ft3)
L/G1 = liquid-to-gas ratio (L/min3)
QG = gas flow rate (gal./min)
QG' = gas flow rate (cm3/s)
Q, = liquid flow rate (gal./min)
j-i
r. = bubble radius (cm)
T = temperature (K) 303 323
U = superficial gas velocity
s (cm/a)
U, = orifice gas velocity (cm/s)
Vfc = bubble velocity (cm/s) 24.891 24.648
W1 = weir length (in.)* 9.25 9.25
MG = gas viscosity (g/(cm-s)) 1.86 x 10~" 1.95 x 10-*
v = gas kinematic viscosity 0.163 0.186
^ (cm2/a)
PG = gas density (g/cm3) 1.1468 x 10 ~3 1.0428 x 10~3
* See Figure 19.
87
-------
Value at
Quantity 30°C 50°C
PT = liquid density (g/cm3) 0.99567 0.98807
i_»
PL' = liquid density (lb/ft3) 62.4 61.9
p = particle density (g/cm3) 2.0 2.0
6 = liquid surface tension 71.18 67.91
(dyne/cm)
n = single stage efficiency (%)
nfc = total efficiency (%)
Results—
The results of the calculations are given in the series
of tables that follows. Table 18 presents the results of calcula-
tions of Uh and Us for various assumed L/G values. Tables 19
and 20 give the results of efficiency calculations for a single-
stage sieve tray operating at 30 and 50°C, respectively, with
two assumed foam heights, 5 and 10 cm. Table 21 gives calculated
column diameters, and Table 22 gives calculated pressure drops
for a single-stage sieve tray, as visualized in Figure 19.
TABLE 18. MINIMUM ORIFICE VELOCITY AND MAXIMUM
SUPERFICIAL VELOCITY IN A SIEVE PLATE BUBBLE SCRUBBER
Temp,
°C
30
50
L/G,
gal./lOOO ft3
1.5
3.0
4.5
6.0
7.5
1.5
3.0
4.5
6.0
7.5
L/G;,
L/m3
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
V
cm/s
1162
1043
969
916
875
1222
1097
1021
966
922
Us'
cm/s
177
177
177
177
177
185
185
185
185
185
88
-------
TABLE 19. SCRUBBER PARAMETERS AND SINGLE-STAGE
EFFICIENCY IN A SIEVE PLATE BUBBLE SCRUBBER AT 30°C
h, cm
5
10
5
10
5
10
5
10
5
10
5
10
5
10
V
cm/s
1100
1100
1500
1500
2000
2000
2500
2500
3000
3000
3500
3500
4000
4000
V
cm
0.236
0.236
0.233
0.233
0.229
0.229
0.227
0.227
0.225
0.225
0.223
0.223
0.222
0.222
KP
0.001985
0.001985
0.002707
0.002707
0.003609
0.003609
0.004511
0.004511
0.005413
0.005413
0.006315
0.006315
0.007218
0.007218
n, %
2.96
5.31'
3.20
5.59
3.50
5.95
3.78
6.25
4.05
6.54
4.32
6.84
4.58
7.11
TABLE 20. SCRUBBER
EFFICIENCY IN A SIEVE
PARAMETERS AND SINGLE-STAGE
PLATE BUBBLE SCRUBBER AT 50°C
h, cm
5
10
5
10
5
10
5
10
5
10
5
10
5
10
V
cm/s
1100
1100
1500
1500
2000
2000
2500
2500
3000
3000
3500
3500
4000
4000
V
cm
0.238
0.238
0.234
0.234
0.231
0.231
0.228
0.228
0.226
0.226
0.224
0.224
0.223
0.223
KP
0.001980
0.001980
0.002700
0.002700
0.003600
0.003600
0.004499
0.004499
0.005399
0.005399
0.006299
0.006299
0.007199
0.007199
n, %
3.19
5.42
3.28
5.74
3.57
6.07
3.86
6.40
4.14
6.71
4.42
7.01
4.68
7.28
89
-------
TABLE 21. COLUMN DIAMETERS OF A SIEVE PLATE
BUBBLE SCRUBBER
Temp,
°C
30
50
QG'
ft3/min
192
192
205
205
QG''
cm/s
90,614
90,614
96,749
96,749
va
cm/s
142
142
148
148
h,
cm
5
10
5
10
dc'
cm
28.50
28.50
28.85
28.85
ft
0.935
0.935
0.947
0.947
a. Taken as 80% of the values listed in Table 17.
TABLE 22. PRESSURE DROP VALUES IN A SINGLE STAGE OF A
SIEVE PLATE BUBBLE SCRUBBER
Temp,
°C
30
50
F
0.263
0.263
0.265
0.265
h,
cm
5
10
5
10
h
w
0.518
1.035
0.522
1.043
h
ow
0.064
0.064
0.066
0.066
dp
1.409
1.409
1.291
1.291
h
r
0.349
0.349
0.332
0.332
h
nt
2.340
2.857
2.211
2.732
a. All values in inches of water. Uh was assumed to be 2000 cm/s.
L/G was assumed to be 1.5 gal./lOOO ft3 to assign a value to QT
in the calculation of h L
ow
90
-------
GAS OUT
2.18 IN.
PUMP
Figure 19. Diagram of a sieve plate bubble scrubber.
91
-------
The results of the calculations are summarized in Table 23.
This table is based on an assumed value of Uh of 2000 cm/s (which
is safely above the value required to prevent weeping) and as-
sumed values of Us of 142 and 148 cm/s at temperatures of 30
and 50°C, respectively, which are 80% of the maximum values
calculated to prevent flooding. The results are given for two
conditions: (1) the number of stages allowable to give a pres-
sure drop not exceeding 2,500 Pa and (2) the number of stages
required to give a total efficiency greater than 50%. It is
shown that with a pressure drop below 2,500 Pa, the maximum
efficiency obtainable is about 17%. It is shown, on the other
hand, that the pressure drop must reach at least 8,000 Pa to
give an efficiency exceeding 50%. In the first instance, the
estimated height of the scrubber is about 75 cm (2.5 ft); in
the second instance, the estimated height is about 300 cm (10 ft).
The conclusion, therefore, is that the sieve plate bubble scrub-
ber is not a practical solution to the problem of removing diesel
particulate.
Calculations for Other Scrubber Types
Approximate calculations were made for three scrubber types
in addition to the venturi and the sieve plate bubble scrubber.
The other three types were the impaction plate, the ordinary
spray scrubber, and the electrostatic spray scrubber. The essen-
tial conclusions reached were as follows:
• The impaction plate with orifices of 0.1 cm would pro-
duce a maximum efficiency of about 20% if constrained
to operate at a maximum pressure difference of 2,500 Pa.
• A spray tower would have to be roughly 275 cm (9 ft) tall
to accomplish a particulate removal efficiency of 50%
and would require an excessive volume flow of liquid.
• An electrostatic spray scrubber with a height not exceed-
ing 90 cm (3 ft) might produce a removal efficiency of
around 50%. However, this device would not offer design
advantages over other electrostatic devices discussed
in this report.
FURTHER CONSIDERATIONS
The general conclusion to be reached from the foregoing
material is that wet scrubbers employing water are not, in general
suitable for removing particulate from diesel exhaust. Under '
the constraint of a 2,500-Pa pressure drop, they are not effi-
cient enough in removing the fine particles present in diesel
smoke. A possible exception is the charged droplet scrubber,
but it is not clear that this device would be preferable to some
other type of electrostatic device.
92
-------
\o
CO
TABLE 23. SUMMARY OF CALCULATIONS FOR A SIEVE PLATE
BUBBLE SCRUBBER3
Design limitation
Ap < 10 in. of H20
n > 50%
Temp,
°C
30
50
30
50
h,
cm
5
10
5
10
5
10
5
10
dc'
cm
0.935
0.935
0.947
0.947
0.935
0.935
0.947
0.947
V
cm/s
142
142
148
148
142
142
148
148
No. of
plates
4
3
4
3
20
12
20
12
b
nt'
%
13.3
16.8
13.5
17.1
51.0
52.1
51.7
52.8
AP,b
Pa
2,330
2,130
2.200
2,040
11,700
8,540
11,000
8,160
a. Constant conditions: Uh = 2000 cm/s, L/G = 1.5 gal./lOOO ft3,
number of holes per tray = 150.
b. Total efficiency and total pressure drop.
-------
The above conclusion is reinforced by consideration of the
rate of water loss by evaporation in a wet scrubber. The water
loss by evaporation would obviously be more severe for diesel
exhaust entering a wet scrubber at 200°C rather than at 52°C.
Therefore, the first consideration will be made of the scrubber
with inlet gas at 200°C and outlet gas and water at 50°C.
Diesel exhaust entering a scrubber at 200°C at a rate of
0.14 m3/s (300 ft3/roin) will contain about 3% by volume water
vapor as the result of fuel combustion in the engine. This rate
corresponds to the flow of 1.97 g/s of water vapor. On leaving
the scrubber at 50°C with water vapor at the saturation level
(concentration, 12.17% by volume), the gas stream will carry
water vapor away at the rate of 8.83 g/s. Thus, water will be
lost at the rate of 6.86 g/s. This is equivalent to about
6.86 x 10~6 m3/s. For comparison, the rate of fuel consumption
may be calculated by assuming that the exhaust flow rate cor-
responds to a highway speed of 1.61 km/s (60 mi/h) and that the
fuel consumption rate is 2.52 x 10~6 m3/s (25 mi/gal, or
0.04 gal./min). The conclusion, therefore, is that water would
be consumed at 2.7 times the rate of consumption of diesel fuel.
For diesel exhaust entering a scrubber at 52°C and leaving
at 30°C with 4.19% water, the rate of water consumption will
be 2.05 g/s or 2.05 x 10~6 m3/s (roughly the same as the rate
of diesel fuel consumption). However, even if this rate of water
consumption were acceptable, it would not be clear that the
exhaust gas could be economically cooled to 52°C before entering
a scrubber.
The conclusion that a wet scrubber would not be a practical
device is further reinforced by considerations of the freezing
point of water. To avoid the freezing of water under normal
climatic conditions, some anti-freeze substance would have to
be added. The addition of such a substance would probably not
have a beneficial effect on the already low efficiencies of
various scrubber devices, and it might have a deleterious effect.
94
-------
SECTION 6
CONCLUSION
From the preceeding discussion, it is possible to draw some
conclusions regarding the applicability of scrubbers, electro-
static devices, and filters to the problem of controlling diesel
particulate emissions. The first of these, scrubbers, can be
eliminated from further consideration due to the large size
required to obtain adequate efficiencies at reasonable backpres-
sures, and to the high rate of water consumption.
Second, electrostatic devices merit further consideration
for the important reasons that they present very little back-
pressure to the system and that they maintain a relatively high
collection efficiency over the 0.1- to 0.8-um particle size range
over which, according to theory, a dip in filter efficiency occurs,
To offset this, they have the severe disadvantage of dysfunction
due to conductive particle contamination. Further research will
have to be performed to design a system which can eliminate this
difficulty. A lesser problem is that of disposal of the col-
lected particulate. A method must be developed whereby the device
may be cleaned and the collected particulate properly and safely
disposed of as a matter of routine maintenance.
The third mechanism, filtration, demonstrates the same
problem of disposal of the collected particulate. Another prob-
lem area is the possibility of high backpressure. Although it
is possible to design a device of appropriately low backpressure
when new, only experimentation will determine the amount of
particulate the filter can collect without elevating the back-
pressure beyond allowable limits and adversely effecting engine
performance. The problem of surface seal over can be avoided
by utilizing a filter with a large face area such as the proposed
prototype. The advantage of filtration is that it is a mechani-
cally simpler concept. Filters, while not necessarily simple
to design, are simpler to build and maintain in the field than
electrostatic devices and as a consequence should be cheaper
and more convenient in use.
Both electrostatic devices and filtration devices show
promise of becoming workable solutions to the diesel exhaust
problem. However, more research is needed on both types of
devices. Prototypes need to be fabricated and tested on actual
95
-------
diesel exhaust streams. Determinations of collection efficiency
as a function of particle size would be useful in the study of
the collection mechanisms involved, which should in turn lead
to the development of improved collection devices. Testing of
prototype devices will also yield additional insights into the
particulate problems that affect each device.
Additional study is also needed on the effect of gas stream
temperature on the hydrocarbon portion of the particulate. A
study by Black and High51* indicates that most of the condensation
and subsequent adsorption of vaporous hydrocarbons occurs in
the last few feet of the vehicle's tail pipe. This implies that
a temperature reduction in the exhaust system would increase
the amount of hydrocarbons which condense and can therefore be
collected as particulate. Supportive evidence has been given
by Masuda,55 who reports that Eikosha has seen a 20% by volume
increase in collected particulate occur when the cooling air
flow on the Aut-Ainer filter was increased.
96
-------
SECTION 7
REFERENCES
1. Springer, K.J., and T.M. Baines. Emissions from Diesel
Versions of Production Passenger Cars. SAE paper # 770818,
presented at the SAE Passenger Car Meeting, Detroit, Michi-
gan, September 26-30, 1977.
2. Broome, D., and I. Khan. The Mechanism of Soot Release
from Combustion of Hydrocarbon Fuels from Air Pollution
Control in Transport Engines. Institution of Mechanical
Engineering, London, England, 1971.
3. Lipkea, W.H., J.H. Johnson, and C.T. Vuk. The Physcial
and Chemical Character of Diesel Particulate Emissions -
Measurement Technique and Fundamental Considerations. SAE
Paper # 780108, presented at the SAE Congress and Exposi-
tion, Detroit, Michigan, February 27-March 3, 1978.
4. McCain, J.D., and D. Drehmel. Particle Size Measurements
of Automotive Diesel Emissions. Paper presented at the
Second Symposium on the Transfer and Utilization of Parti-
culate Control Technology, Denver, Colorado, July 23-27,
1979. (A reprint of this paper appears in this report as
Appendix I.)
5. Stern, A.C., Editor. Air Pollution Vol. Ill: Sources of
Air Pollution and Their Control. Academic Press, New York,
New York, 1968.
6. Briggs, T., J. Throgmorton, and M. Karaffa. Air Quality
Assessment of Particulate Emissions from Diesel-Powered
Vehicles. Final Report prepared for the Environmental Pro-
tection Agency by PEDCo Environmental, Inc., under Contract
No. 68-02-2515. EPA-450/3-78-038, U.S. Environmental Pro-
tection Agency, Research Triangle Park, North Carolina,
March 1978.
7. Pitts, J.N., Jr., A.M. Winer, and K.A. Van Cauwenberghe.
An Overview on Diesel Exhaust and Air Quality: Chemical
Implications. In: Proceedings of the Workshop on Unregu-
lated Diesel Emissions and Their Potential Health Effects,
National Highway Traffic Safety Administration, Environ-
mental Protection Agency and Dept. of Energy, April 27-28,
1978.
97
-------
8. Federal Register, 44(33) February 1, 1979.
9. General Motors Response to EPA Notice of Proposed Rulemaking
on Particulate Regulation for Light-Duty Diesel Vehicles,
April 19, 1979.
10. Springer, K., and R. Stahman. Removal of Exhaust Parti-
culate from a Mercedes 300D Diesel Car. SAE Paper # 770716,
presented at the SAE Off-Highway Vehicle Meeting and Exhibi-
tion, Mecca, Milwaukee, Wisconsin, September 12-15, 1977.
11. Frey, J.W., and M. Corn. Physcial and Chemical Character-
istics of Particulates in a Diesel Exhaust. American
Industrial Hygiene Association Journal, September-October,
1967.
12. Funkenbusch, E.F., D.G. Leddy, and J.H. Johnson. The
Characterization of the Soluble Organic Fraction of Diesel
Particulate Matter. SAE Paper # 790418, presented at the
Congress and Exposition Cobo Hall, Detroit, Michigan, February
26-March 2, 1979.
13. Mentser, M. and A.G. Sharky. Chemical Characterization
of Diesel Exhaust Particulates. Publication No. PERC/RI-
77/5, Pittsburgh Energy Research Center, Pittsburg, Pennsyl-
vania, March 1977.
14. Davies, C.N. Aerosol Science. Academic Press, Chapters
8 and 9, 1966.
15. Chen, C.Y. Filtration of Aerosols by Fibrous Media. Chemi-
cal Review, 55:595, 1955.
16. Fuchs, N.A., and I.E. Strechkina. A Note on the Theory
of Fibrous Aerosol Filters. Annals of Occupational Hygiene
6(1):27-30, 1963.
17. Davies, C.N. Air Filtration. Academic Press, 1973.
18. Saxena, S.C., and W.M. Swift. Dust Removal from Hot and
Compressed Gas Streams by Fibrous and Granular Bed Filters:
A State of the Art Review. Argonne National Laboratory,
May 1978.
19. Loffler, F. Collection of Particles by Fiber Filters.
Air Pollution Control, Part 1, W. Strauss, Ed. Wiley-Inter-
science, New York, New York, 1971.
20. Stenhouse, J.I.T., G.P. Bloom, and N.T.J. Chard. Dust Load-
ing Characteristics of High Inertial Fibrous Filters. Ameri-
can Industrial Hygiene Association Journal, 39(3):219, 1978.
98
-------
21 Loffler, F. Adhesion Probability in Fiber Filters. Clean
Air, 8(4), 1974.
22. Stenhouse, J.I.T., G.P. Bloom, and N.T.J. Chard. High
Inertia Fibrous Filtration—Optimum Conditions. Filtration
Separation, 15(2):128, 1978.
23. Paretsky, L., et al. Panel Bed Filter for Simultaneous
Removal of Fly Ash and SO2: II Filtration of Dilute Aerosol
by Sand Beds. Journal of the Air Pollution Control Associa-
tion, 21(4):204-209, April 1971.
24. Tardos, G.I., N. Abauf, and C. Gutfinger. Dust Deposition
in Granular Bed Filters: Theories and Experiments. Journal
of the Air Pollution Control Association, 28(4):354-363,
April 1978.
25. Eastwood, J., et al. Random Loose Porosity of Packed Beds.
British Chemical Engineering, 14(11):1542-1545, November
1969.
26. Lee, K.W., and J.A. Gieseke. Collection of Aerosol Par-
ticles by Packed Beds. Environmental Science and Technology,
13(4):466-470, April 1979.
27. Schmidt, E.W., et al. Filtration Theory for Granular Beds.
Journal of the Air Pollution Control Association, 28(2):143-
146, February 1978.
28. Jackson, S., and S. Calvert. Entrained Particle Collection
in Packed Beds. A.I.Ch.E. Journal, 12:1075, November 1966.
29. Ergun, S. Fluid Flow Through Packed Columns. Chemical
Engineering Progress, 48:89-94, 1952.
30. Lee, K.C., et al. The Panel Bed Filter. Electric Power
Research Institute (EPRI AF-560), May 1977.
31. Taub, S.I. Filtration Phenomena in a Packed Bed Filter,
Doctoral Thesis, Carnegie-Mellon University, 1971.
32. Goren, S.L. Aerosol Filtration by Granular Beds. Paper
presented at the EPA Particulate Control Technology Sym-
posium, Denver, Colorado, July 1978.
33. Sullivan, H., L. Tessier, C. Hermance, and G. Bragg. Reduc-
tion of Diesel Exhaust Emissions (Underground Mine Service).
Prepared for the Department of Energy, Mines and Resources,
Ottawa, by the Mechanical Engineering Department of the
University of Waterloo, Waterloo, Ontario, May 1977.
99
-------
34. Communication between Eikosha Company and Grady B. Nichols
of Southern Research Institute.
35. Pauthenier, M., and M. Moreau-Hanot. Charging of Spherical
Particles in an Ionizing Field. J. Phys. Radium, 3(7):590-
613, 1932.
36. White, H.J. Particle Charging in Electrostatic Precipita-
tion. Trans. Amer. Inst. Elec. Eng. Part 1, 70:1186-1191,
1951.
37. Murphy, A.T., F.T. Adler, and G.W. Penney. A Theoretical
Analysis of the Effects of an Electric Field on the Charging
of Fine Particles. Trans. Amer. Inst. Elec. Eng., 78:318-
326, 1959.
38. White, H.J. Industrial Electrostatic Precipitation. Addison-
Wesley, Reading, Massachusetts, 1963. p. 157.
39. Fuchs, N.A. The Mechanics of Aerosols, Chapter 2. Macmillan,
New York, New York, 1964.
40. White, H.J. Reference 8, pp. 166-170.
41. White, H.J. Reference 8, pp. 185-190.
42. Penney, G.W., and S. Craig. Pulsed Discharges Preceding
Sparkover at Low Voltage Gradients. AIEE Winter General
Meeting, New York, New York, 1961.
43. Pottinger, J.F. The Collection of Difficult Materials by
Electrostatic Precipitation. Australian Chem. Process Eng.,
20(2):17-23, 1967.
44. Gooch, J.P., and G.H. Marchant, Jr. Electrostatic Precipi-
tator Rapping Reentrainment and Computer Model Studies.
Final Draft Report prepared for the Electric Power Research
Institute by Southern Research Institute,-1977.
45. Spencer, H.w. A Study of Rapping Reentrainment in a Nearly
Full Scale Pilot Electrostatic Precipitator. EPA-600/2-
76-140, U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina, 1976.
46. Springer, K.I. An Investigation of Diesel-Powered Vehicle
Odor and Smoke. Final Report to the National Air Pollution
Control Administration, Department of Health, Education,
and Welfare, October 1969. Publication No. AR-695.
47. White, H.J. Industrial Electrostatic Precipitation. Ad-
dison Wesley, Reading, Massachusetts, 1963.
100
-------
48. Smith, W.B., and J.R. McDonald. Development of a Theory
for the Charging of Particles by Unipolar Ions. J. Aerosol
Sci., 7:151-166, 1976.
49. Inculet, I.I., and G.S.P. Castle. A Two-Stage Concentric
Geometry Electrostatic Precipitator with Electrified Media.
ASHRAE Journal, March 1971.
50. Lawson, A., and H. Vergeer. Analysis of Diesel Exhaust
Emitted from Water Scrubbers and Catalytic Purifiers.
Ontario Research Foundation, 1977.
51. Wood, C.D., and J.W. Colburn, Jr. Control Technology for
Diesel Equipment in the Underground Mining Environment-
A Review of Selected Topics. Final Report prepared by
Southwest Research Institute for U.S. Department of the
Interior, Bureau of Mines, Twin Cities, Minnesota, Purchase
Order P3381399, January 1979.
52. Sudar, S., and L. Grantham. Diesel Exhaust Emission Control
Program. Final Report prepared by Atomics International
for Southern California Rapid Transit District, Los Angeles,
California, January 1974.
53. Calvert, S., J. Goldschmid, D. Leith, and D.P. Mahta.
Scrubber Handbook, Volume I. Report prepared by Ambient
Purification Technology, Inc., for Environmental Protection
Agency. Contract CPA-70-95. August 1972.
54. Black, F. , and L. High. Diesel Hydrocarbon Emissions, Par-
ticulate and Gas Phase Symposium on Diesel Particulate Emis-
sion Measurement Characteristic. U.S. Environmental Pro-
tection Agency, Research Triangle Park, North Carolina.
55. Private Communication, S. Masuda with M.G. Faulkner of
Southern Research Institute.
101
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APPENDIX I
PARTICLE SIZE MEASUREMENTS OF
AUTOMOTIVE DIESEL EMISSIONS
by
Joseph D. McCain
Southern Research Institute
2000 Ninth Avenue, South
Birmingham, Alabama 35205
and
Dennis C. Drehmel
Industrial Environmental Research Laboratory
Environmental Protection Agency
Research Triangle Park, North Carolina 27711
Presented at
The Second Symposium on the Transfer
and Utilization of Particulate Control
Technology, Denver, Colorado, July 23-27, 1979
102
-------
Introduction
The federally mandated fuel economy standards for passenger
automobiles have resulted in considerable impetus being given
to the introduction of substantial numbers of diesel powered
automobiles into the passenger car fleet. The diesel engine
has long been acknowledged as being "dirtier" than the spark
ignition gasoline engine (by factors of 30 to 50 in particulate
emissions). The diesel particulate emissions are primarily car-
bonaceous, but 10 percent to 50 percent by weight of the material
is adsorbed higher molecular weight organics, a significant
portion of which may be polycyclic aromatics.1 Preliminary
results of Ames microbial mutagenicity bioassay tests have indi-
cated the possibility that these particulates may be carcino-
genic.
Possible solutions to the diesel particulate problem are
combustion modification or the use of aftertreatment devices
in the exhaust gas stream to collect and/or render the material
innocuous. Such treatment may be mandatory if the emissions
do prove to represent a significant carcinogenic risk. Selection
of candidate aftertreatment devices requires knowledge of the
chemical and physical properties of the particles. These include
particle morphology, particle size distribution, bulk densities
of the collected material, and particulate mass concentration
and emission rates in the exhaust gas stream. Because the organic
fraction of the particles appears to be adsorbed on the surfaces
of graphitic carbon base particles, the temperature history of
the gas stream may be important. If the sorption process takes
place at elevated temperatures, then collection of the particu-
late at the normal, relatively hot, exhaust gas temperatures
may be sufficient. However, if the sorption takes place only
during and after cooling of the exhaust stream to near ambient
conditions, the hot particle collection will not result in the
removal of the organic fraction.
The study reported here represents the first of a planned
series of experiments to characterize the exhaust emissions from
the point of view of aftertreatment exhaust gas cleanup and to
collect samples for bioassays to determine whether the biological
effects of particles collected at exhaust line temperatures are
the same as those collected after dilution and cooling by ambient
air.
Test Program
The tests described here were performed November 27 through
December 1, 1978, at a U. S. EPA facility located at Research
Triangle Park, North Carolina. A 1979 Oldsmobile 88 with a 350
Cubic Inch Displacement (CID) diesel engine was operated on a
103
-------
Burke E. Porter No. 1059 Chassis Dynamometer. The dynamometer
was programmed to emulate the Clayton roadload curve for water-
brake dynamometers used for vehicle certification. Test con-
ditions included the 13 minute Fuel Economy Test (FET) combined
city-highway test cycle, 97 kmph highway cruise, 56 kmph highway
cruise, and 56 kmph no load conditions. However, conditions were
not equivalent to those required by EPA for vehicle certification
and the test results should not be compared to those acquired by
official certification methods and conditions.
Sampling and measurement methods included Andersen cascade
impactors, conventional filtration techniques followed by con-
densers and organic sorbent traps, optical single particle counters,
electrical mobility methods, and diffusional methods. All samples
were taken directly from a modified exhaust pipe which was run
out from under the chassis and alongside the passenger side of
the automobile to permit reasonable access to the exhaust stream.
Andersen Model III cascade impactors with glass fiber impac-
tion substrates and backup filters were used to obtain total
particulate loadings and particle size distributions on a mass
basis over the size range from about 0.4 ymA to 5 ymA. The im-
pactors were operated in an oven close-coupled to the exhaust
pipe. During runs at a steady engine load (56 kmph and 97 kmph),
the oven was maintained at the same temperature as the exhaust
gas temperature at the sampling point. During the 13 minute
FET cycle testing, the impactors and ovens were maintained at
about the average exhaust gas temperature for the cycle, 175°C.
In addition to the cascade impactors, a Thermosystems Model
3030 Electrical Aerosol Analyzer (EAA) was used to determine
concentrations and size distributions of particles in the size
range of 0.01 urn to 0.5 pm. Particle concentrations ranging
from 0.3 urn to 2.5 urn were monitored using a Royco Model 225
optical particle counter. The Southern Research Institute
SEDS III sample extraction, conditioning, and dilution system
was used as an interface between the exhaust system and the EAA
and particle counter. This system provides a mechanism for the
removal of condensible vapors from the sample gas stream at ele-
vated temperatures followed by quantitative dilution to particle
concentrations within the operating ranges of the measurement
instruments.
Figure 1 is a diagrammatic sketch of the layout of the ex-
haust system and measurement instrumentation during the tests.
The vehicle operating conditions were selected to provide
samples collected at elevated exhaust gas temperatures prior
to cooling for the same engine cycle as the very large (10 kg)
sample collected for bioassay work. This 10 kg bioassay sample
was being collected using the FET test cycle using a standard
"constant volume" automatic dilution tunnel. The hot samples
collected were to be used for Ames tests to provide some indication
104
-------
BLOWER
EXHAUST
FOR ENGINE
COOLING
STANDARD
EXHAUST
SYSTEM TO
JUNCTION
DYNOMOMETER
ROLLERS
HEAT TRACED
SAMPLE LINE
OVEN FOR
IMPACTORS
AND FILTERS
MODIFIED EXHAUST
PIPE
FLEXIBLE PIPE
OUTSIDE OF
BUILDING
Figure 1. Modified exhaust pipe and test equipment layout for
diesel emission testing.
105
-------
of the relative mutagenicity of material collected at the exhaust
line temperatures and material collected after cooling and dilu-
tion. This information is intended to provide some insight into
whether hot collection of the particles will remove the carcino-
genic component of the exhaust. In the program as originally
conceived, samples were to be taken at a number of points between
the exhaust manifold and the tailpipe to provide samples col-
lected over a range of temperatures and engine loads. These
would have provided information on changes in particle size
distribution and composition as the exhasut gases were cooled.
Time limitations in preparing for the tests rendered it impos-
sible to carry out the proposed plan. However, it was found
that a considerable swing in gas temperature did occur with
changes in engine load. However it is not possible to differentiate
between engine load/speed induced and temperature induced con-
centration and composition changes in the data obtained during
this test.
Overall particulate loadings, engine gas flows, and sampling
temperatures for the cascade impactor samples are given in Table
I. Particle size distributions for the various conditions are
given in Figures 2, 3, 4 and 5. Each figure in this series
contains a plot of cumulative percentage smaller than the indi-
cated diameter versus diameter from the impactor data alone and
that obtained by integrating the distributions from the electrical
aerosol analyzer up to 0.5 urn and continuing the integration
from 0.5 \im to 10 urn with the impactor data. A particle density
of 1.0 g/cm3 was assumed for the integrations of the EAA data.
The overall size distributions from 0.01 urn to 10 um obtained
in this fashion agree very well with those obtained from the
impactors alone.
The variability in particulate concentrations through the
FET cycle is illustrated in Figure 6. This shows particle concen-
trations versus time in three particle size intervals through
several test cycles. These data were obtained using the optical
particle counter.
The particulates collected at exhaust gas temperatures were
found to be approximately 15 percent by mass organics. The re-
sults of the biotesting of the samples from the impactors, filters
and organic vapor traps will be reported elsewhere. '
Conclusions
Typical particulate concentrations at exhaust line conditions
for the Oldsmobile 350 CID diesel engine were found to be about
50 mg/NCM. Aerodynamic mass median diameters were about 0.3
to 0.5 um with larger medium diameters being obtained from higher
106
-------
TABLE I. RESULTS OF CASCADE IMPACTOR SAMPLING
Average exhaust
Average exhaust
temperature at
Aerodynamic
mass median
Operating volume flowrate sampling location
mode (m3/s) (°C)
FET cycle
97 kmph
56 kmph
(with load)
56 kmph
(no load)
0.
0.
0.
(Not
051
057
033
Available)
177
218
149
129
Particulate
loading (mg/NCM)
68
55
45
39
particle diameter
(pmA)
0
0
0
0
.26
.54
.46
.33
-------
nmanmp ffl fflpiEl FUEL 1ESHHP FET DUE
M) = 1.00 GUI
99.99
5f
IMPACTOR
ONLY
INTEGRATION OF
•EAA (0.01-0.5) AND IMPACTOR
(0.5-10)
•JM—\ i i mil 1—i i i niH 1 i i i HIM
10"5 10"1 10° 1O1
PARTICLE DIAhCTER (MICROKETERS)
Figure 2. Particle size distribution for FET-cycle.
108
-------
OUQDBUIBB DIESEL FUEL TCT-RTP 35 IM I/O UN)
RHD = 1.00 a/a
9.99-
§9?^
^j^j • • •
99.8-
99.5-;
99^
98 -;
95:
901
80 J
70 i
60 i
50 i
40 i
30 -i
20 1
±o\
5:
±\
O.5i
O.E:.
n_ rn -
1 1
- I
=" rtf
f ijjr
\ ll^1
[ IMPACTOR— ^^^«»*
r ONLY •-*
: •*
r • INTEGRATION OF EAA (0.01-0.5 Mm)
[ ^* ' AND IMPACTOR (0.5-10 ^m)
L •
I •
\
~ •
r
: •
:
r •
L •
— !Li — i — i i 1 1 1 u i_j — i i i iiu 1 i i i i ml
10~5 ID'1 10° 1O1
PARTICLE DIAMETER (MICROMETERS)
Figure 3. Particle size distribution for 35 mph no load condition.
109
-------
DUBBBILE m OE9EL FIB. TEST-RIP £ VN I/ UWI
no = tan aucc
99.99
99.B
99.5
99
98
95
90
SO
70
60
50
40
30
EO
10
5
O.E:
0.01
IMPACTORONLY
y
w
• INTEGRATION OF EAA (0.01-0.5 pm)
*' AND IMPACTOR (0.5-10 pm)
H—I I I Mill 1—I I im-H 1—I I I IIMI
10"5 10"1 10° 1O1
PARTICLE DIAMETER (MICROMETERS)
Figure 4. Particle size distribution for 35 mph with load condition.
110
-------
ffl EESEl FUEL TESHTO1 GO IM
MI = 1.00 a/a
H-
y
S
LJ
M
H
H
g
99.99-
93.95i
99.9-
99.8-
99.5^
99^
98^
95:
90^
80^
70 i
60 i
501
401
30^
20-
10^
5-
E-
1-
0.5-
O.E-
0.1-
0-05-
n.ni -
i
: r
, I
[ fJ.
^>
f «T^*
r IMPACTOR ONLY — -»»?•*
f •:*•
i- J%*
L •*
• »^ INTEGATION OF EAA (0.01-0.5 urn)
f • AND IMPACTOR (0.5-10 urn)
f •
1 •
r •
r •
r •
1 •
F
r •
* i i i 4 i 1 1 1 J i i i i 1 1 1 N i i i i i i nj
10"5 10'1 10P 101
PARTICLE DIAMETER (MICROMETERS)
Figure 5. Particle size distribution for 60 mph condition.
Ill
-------
ROYCO DIESEL FET CYCLE W/LOAD 11/30 9-10:40
^
^^
t"
X
in
2
§
9
g
7
6
4
3
—
™~
^
•••
r
0
-I 1 1 \ 1 1 1 1 1
CHANNELS CHANNEL , 0.36*.63pn, CYCLE
CHANNEL 3 0.9-1.1 JJm NO.
CHANNEL 5 1.2-2.0 fJm i
2
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4
5
0 6
* 7
• 8
/V •
C* '
0 0 A
o
o v
o °
• • • o
sis! i i
i i i i i i ii i
1 I
START
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8:58
9:11
9:24
9:37
9:50
10:02
10:15
10:28
9
0
O °
* i
A
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6789
CYCLE TIME, min
10 11 12 13
Figure 6. Relative concentration versus time for three particle
ranges during several FET cycles.
size
112
-------
engine speed/load conditions under steady state operating condi-
tions. The results reported here are qualitatively similar in
size distribution to those found by other investigators in mea-
surements of emissions from heavy duty diesel engines insofar
as the impactor data are concerned.2'
References
1. Blacker, S.M. EPA Program to Assess the Public Health
Significance of Diesel Emissions. Journal of the Air Pol-
lution Control Assoc. Vol 28, page 769, August 1978.
2. Lipkea, W.H., J.H. Johnson, and C.T. Vuk. The Physical
and Chemical Character of Diesel Particulate Emissions -
Measurement Technique and Fundamental Considerations. SAE
Paper # 780108, presented at the SAE Congress and Exposi-
tion, Detroit, Michigan, February 27-March 3, 1978.
3. Springer, K., and R. Stahman. Removal of Exhaust Particulate
from a Mercedes 300D Diesel Car. SAE Paper # 770716, pre-
sented at the SAE Off-Highway Vehicle Meeting and Exhibition,
Mecca, Milwaukee, Wisconsin, September 12-15, 1977.
113
-------
APPENDIX II
VAPOR PRESSURE VERSUS TEMPERATURE DATA
ON ORGANIC COMPOUNDS LISTED
BY MENTSER AND SHARKYl3
FOR DIESEL EXHAUST
114
-------
APPENDIX II. VAPOR PRESSURE VS. TEMPERATURE DATA ON ORGANIC COMPOUNDS LISTED FOR DIESEL EXHAUST
Possible compound
1 Crotonaldehyde
2 8-Propiolactone
3 2-Butanone
Tetrahydrofuran
4 Pentane
5 Ethyl Formate
G lye idol
Methyl Acetate
6 Carbon Disulfide
7 Benzene
8 Pyridine
9 Cyclohexene
10 Cyclohexane
11 Methyl Accylate
12 2-Pentanone
13 Hexane
14 Dioxane
Ethyl Acetate
15 Toluene
16 Aniline
17 Phenol
18 furfural
19 Furfuryl Alcohol
20 Mesityl Oxide
Cyclone xanone
21 Methyl Cyclohexane
22 Cyclohexanol
2-He xanone
23 Ethyl Acrylate
Methyl Methacrylate
Molecular
weight Formula
70.0417
72.0210
72.0573
72.0573
72.0936
74.0366
74.0366
74.0366
75.9442
78.0468
79.0421
82.0780
84.0936
86.0366
86.0729
86.1092
88.0522
88.0522
92.0624
93.0577
94.0417
96.0210
98.0366
98.0729
98.0729
98.1092
100.0085
100.0085
100.0522
100.0522
C,H,0
CjH»02
C.HeO
C»H,0
CsHi:
C,B,02
CjHiOi
CjH6Oj
CS2
C
-------
>IX II (continued)
Poulble compound
24 Beptane
25 Styrene
26 ethyl leniene
2-lyleae
3-xylem
4-xylene
27 MomxwtbyUnillM
o-Toluldlne
2* p-Qu inane
29 2-Cre*ol
3-Cr*eol
4-cceeol
30 lydroqulnon*
31 2-Mttbyleyeloheunon*
(dl)
32 Allyl«lyci*«
CRC
Antoine
CRC
SUB/CBC
CRC Value* reported arc those (or
3-ethylcuMne
VER. Reported value* baaed on only two P*T
data point*
SUB
CBC (continued)
-------
APPENDIX ii (continued)
43
44
45
46
47
48
49
49
50
Possible compound
Phenylether
Anthracene
Phenanthr«ne
Dinitro-o-cresol
Pyrene
Benz [a] anthracene
Chrysene
Benzo [c] pnenanthrene
7,12-Di»ethvl-
benz [a] anthracene
Benzo[k] £ luoranthene
Benzo tb]f luoranthene
Benzol j ] f luoranthenc
Cbolanthrene
Molecular
weight
170.0729
178.0780
178.0780
198.0275
202.0780
228.28
228.0936
256.1248
252.0936
252.0936
254.1092
Melting
Point,
Formula °C
Ci2H100 +27
€,„«,<, +217.5
CitHn +99.5
C7H,N205 +87.5
ClsH,j +156
C,,H12 =160
Ci.Bjj +225
+68
C2,H15 +122-3
+217
C20H,2 +168
C2,H12 +166
C2(Hi, +175 d
Boiling
point, °C
at 760 •»
258.5
342.0
340.2
-
404
Sublimes
448
-
-
480
-
-
Sublimes
25°C
3.3 x 10"2
6.2 x 10"'
1.7 x 10"*
1.1 x 10"*
7.D x 10"6
1.1 x 10"'
1.1 x 10"'
1.1 x 10"'
1.1 x 10"'
9.7 x 10""
9.7 x 10"'°
9.7 x 10""
9.7 x 10""
50°C
0.2
1.5 x 10"'
2.5 x 10"!
2.7 x 10"3
1.3 x 10"*
3.9 x 10"6
3.9 x 10"'
3.9 x 10"'
3.9 x 10"'
5.6 x 10"«
5.6 x 10"'
5.6 x 10""
5.6 x 10"'
75°C
1.0
2.3 x 10"3
2.6 x 10"2
4.3 x 10"2
1.5 x 10"3
8.1 x 10"s
8.1 x 10"5
8.1 x 10"5
8.1 x 10"5
1.8 x 10"'
1.8 x 10"'
1.8 x 10"'
1.8 x 10"'
ioo°c
3.9
2.4 x 10"2
0.2
0.5
1.3 x 10"2
1.1 x 10"3
1.1 x 10"3
1.1 x 10"3
1.1 x 10"3
3.7 x 10"s
3.7 x 10"s
3.7 x 10"s
3.7 x 10"s
125°C
12.3
0.2
1.4
3.8
8.3 x 1C"2
1.1 x 10"2
1.1 x 1C"2
1.1 x 10"2
1.1 x 10"2
5.1 x 10"»
5.1 x 10"*
5.1 x 10"*
5.1 x 10"*
150°C Ref
33.2 Antoine
1.4 SUB/CRC
4.1 SUB/CRC
23.9 SUB
4.3 x 10"' Pupp
8.4 » 10~* Murray
8.4 x 10"2
8.4 x 10"2
8.4 x 10"2
5.2 x 10"3 Pupp
5.2 x 10"5
5.2 x 10"3
5.2 x 10"'
Comments
Vapor pressure data was found only for
benz (a] anthracene. However, the com-
pounds in this group are structurally
similar and should all have very similar
vapor pressures.
Vapor pressure data was found only for
benzofk] f luoranthene. However, the corn-
similar and should all have very similar
fluoranthene based on measurements from
samples with si9nificant impurities
(-10%).
49 Benzo[«]pyr«n«
252.0936 C2IHi2
-177
5.6 x 10"' 2.3 x 10~7 5.4 x 10"' 8.3 x 10"5 9.1 x 10~* 7.5 x 10"3 Murray
-------
APPBID1X II (concluded)
Additional
possible compounds
Acetaldehyde
Acrolein
Acetone
Propr ionaldehyde
Isobutyraldehyde
n-Butyraldehyde
n-Valeraldehyde
Hexaldehyd.
Bstniji Idehyde
Molecular
weight Porsula
44.05
56.06
51.01
58.08
72.10
72.10
86.13
100. U
106.12
Helting Boiling
Point. point, *C
"C at 760 mm
VAPOR PRESSURE VS.
C2B,0
C,B,0
C,B,0
C,B,0
C.BiO
C,H,O
C,H,oO
C.BnO
C,B,O
-121
-87
-95
-81
-66
-99
-92
-56
-26
TEMPERATURE
»20.8
53
56.2
48.8
64
76
103
128
178.1
Vapor pressure (mmBq) at various temperatures
25°C
50°C
DATA ON ADDITIONAL ORGANIC
910.2
272.0
230.0
318.7
208.2
112.3
50.0
13.1
1.0
2096 . 0
700.7
612.7
816.5
521.6
319.4
138.0
44.5
4.8
75°C
COMPOUNDS
4029.5
-
1385.2
1793.1
-
765.8
329.2
126.9
18.1
100 °C
7606.6
-
2761.4
3495.2
-
1608.1
699.0
314.6
57.5
125'C
12646.2
-
4986.8
6201.6
-
3041.8
-
695.6
157.7
150°C
19664.3
-
8321.3
10204.1
-
5290.9
-
-
384.2
Ref
Smith
CRC .
Antoine 1
Smith j
VER. I
Smith
VER.
VER.
CRC
Conc'n in
diesel
exhaust, pp.T Co««ents
18.3 ppm
3.2 ppm
2-9 ppm
Reported values based on only two
P-T data points.
0.3 pp. Reported values based on only two
* P-T data points.
0.4 ppm
Reported values based on only
points.
0.2 pp.
K«y lp References
___ Vapor pressure values were obtained fiom a linear least squares analysis of log)VI> (Mdlg} vs 1/T(*K) froei data given in the Handbook of Chen is try
~ and Physics, 57th ed, Robert c. Heast, Bd. CRC Press. Cleveland. Ohio, 1976.
Vapor pressure value! were calculated fro* the Antoine Equation: logi oP(.»») - A - B/[Ot{*C)] . Values of A, B, and C were obtained fto» Lanqe's
- Undbook of chemistry, llth ed., J.A. Dean, Ed., NcGraw Hill Book Company, New York, H.Y., 1967.
Vapor preesure values were calculated froai the estpirical equation) log,0P(a.i) -A - B/T(*K). Values of A and B were obtained fro* the Handbook
800 " of CheaUstry and Physics, 57th ed, "Sublimation Data For Organic Compounds."
Vapor pressure values were obtained fcon a linear least squares analysis of loq^Pdam) vs. 1/T(*K) free, data given in Karel Verschueren's
" "Handbook of Environmental Data on Organic Chesiicals," Van Nottrand Reinhold Co.mpany, New York, M.Y., 1977, «5» pp.
Vapor pressure values were calculated frost the Antoine equation. Values of A, B, and C were obtained from:
DreUbach. R.R. and R.A. Martin, Ind.
g. chem. 41_, 2875 (1949).
Vapor pressure values were calculated from the empirical equation logjap(nm) » _A _ •» B. Values of A and B were obtained from: Pupp, C.,
».C. LK>, 3.3. Hurray, and R.p. Pottle, Atmos. Envir., «, 915 (1974). T(*K)
Vspor pressure values were calculated from the empirical equation log10P(mm) - A 4 B. Values of A and B were obtained from: Hurray, 3.3.
R.F. Pottie, and C. Pupp, Can. J. Chem., 52, 557 (1974).
T(*K)
Vapor pressure values were calculated from the Antoine equation. Values of A, B, and C were obtained from:
S*ith " Smith, I.E. and ».T. Bonner, Ind. Eng. Chem., 43, 1169 (1951).
Vogh, J.K. Mature of Odor Components in Diesel Exhaust. J. Air. Pollut. Control Assoc., 19, 773 (1969).
Combined concentration of crotonaldehyde and n-valeraldehyde
-------
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CONTROL OF DIESEL EMISSIONS
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Johnston, A. A., C. D. Wood, and K. Springer. A Proposal for
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Knight, B. E. and C. H. T. Wang. Some Experiments on the Mode
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Hare, C. T. , K. J. Springer, and R. L. Bradow. Fuel and Additive
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MEASUREMENT AND SAMPLING OF DIESEL EXHAUST
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Control of Air Pollution from New Motor Vehicles and New Motor
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Carey, A. W., Jr. Steady State Correlation of Diesel Smoke Meters-
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Laresgoiti, A., A. C. Loos, and G. S. Springer. Particulate and
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Dolan, D. F. and D. B. Kittelson. Diesel Exhaust Aerosol Particle
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Verrant, J. A. and D. B. Kittelson. Sampling and Physical Char-
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Vuk, C. T. , M. A. Jones, and J. H. Johnson. The Measurement and
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Black, F. and L. High. Methodology for Determining Particulate
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Stinton, H. C. Development of a Methodology for the Determination
of Diesel Total Hydrocarbon Emission Rates for Cyclic Driving
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Zweidinger, R. B., S. B. Tejada, D. Dropkin, J. Huisingh, and
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of Complex Environmental Mixtures, Williamsburg, Virginia,
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bustion Chamber Sampling - Hardware, Procedures, and Data
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Vehicle Meeting, Milwaukee, Wisconsin, September 8-11, 1975.
SAE Paper No. 750849.
Springer, K. J. and H. E. Dietzmann. Diesel Exhaust Hydrocarbon
Measurement - A Flame lonization Method. Presented at SAE
Meeting, Detroit, January 12-16, 1970. SAE Paper No. 700106.
Williams, R. L. and S. J. Swarin. Benzo(a)pyrene Emissions from
Gasoline and Diesel Automobiles. Paper presented at the
Congress and Exposition, Cobo Hall, Detroit, February 26-
March 2, 1979. SAE Paper No. 790419.
Lyons, M. J. Comparison of Aromatic Polycyclic Hydrocarbons
from Gasoline-engine and Diesel-engine Exhaust, General
Atmospheric Dust and Cigarette Smoke Condensate. NCI Mono-
graph No. 9, pp. 193-199, Washington, D.C., 1962.
Recommend Practice for Measurement of Gaseous and Particulate
Emissions from Light-Duty Diesel Vehicles. Draft Report,
U.S. Environmental Protection Agency, March 1978.
148
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Greeves, G. and J. 0. Meehan. Measurement of Instantaneous Soot
Concentration in a Diesel Combustion Chamber. I. Mech. E.
(London) Conference Paper C8.
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Engine Emissions - Pollutant Formation and Measurement.
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1973.
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Aromatic Hydrocarbons in Diesel Exhaust Gases. Presented at
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and Source Effluents. Health Lab. Sci. 7(Suppl. 1),:56-59,
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1),: 31-44, January 1970.
Sawicki, E., R. C. Corey, A. E. Dooley, et al. Tentative Method
of Routine Analysis for Polynuclear Aromatic Hydrocarbon
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HEALTH EFFECTS
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Turner, D. W. Workshop on Unregulated Diesel Emissions and Their
Potential Heal Effects, April 27-28, 1978. DOT-NHTSA, DOE
and EPA, Washington, D.C.
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Respirability of Exhaust Particulates Generated by a V-6,
Air-Cooled, Indirect-Injection Diesel Engine. Divisional
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Unregulated Diesel Emissions and Their Potential Health Effects.
Edited Transcript of Proceedings, Department of Transportation,
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152
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/7-79-232a
3. RECIPIENT'S ACCESSION NO.
. TITLE AND SUBTITLE
Assessment of Diesel Particulate Control:
Scrubbers, and Precipitators
Filters,
6. REPORT DATE
October 1979
6. PERFORMING ORGANIZATION CODE
. AUTHOR(S)
M.G. Faulkner, E.B. Dismukes, J.R. McDonald,
Pontius, and A.H. Dean
. PERFORMING ORGANIZATION NAME AND ADDRESS
D.H.
8. PERFORMING ORGANIZATION REPORT NO.
SORI-EAS-79-564
Project 3858-14-FR
. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
10. PROGRAM ELEMENT NO.
F.HF.
11. CONTRACT/GRANT NO.
68-02-2610, Task 14
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final; 3/79 - 7/79
14. SPONSORING AGENCY CODE
EPA/600/13
15. SUPPLEMENTARY NOTES IERL-RTP
541-2925.
project officer is Dennis C. Drehmel, Mail Drop 61, 919/
O. ABSTRACT.
The report discusses an investigation of three types of devices that might
be used for the aftertreatment of diesel exhaust to lower particulate emissions from
light-duty vehicles. The devices are filters, electrostatic precipitators (ESPs),
and wet scrubbers. The conclusions reached are that filters and ESPs merit further
consideration, but wet scrubbers do not. Wet scrubbers were eliminated from further
consideration on the basis of excessive size, low efficiency, and excessive fluid loss
by evaporation (assuming that water is the fluid of choice). Filters and ESPs,
although appearing to offer significant potential, have possible disadvantages that
can only be assessed experimentally. Prototype filters and electrostatic devices
that appear worthy of experimental study are described.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATi Field/Group
Pollution
Diesel Fuels
Diesel Engines
Dust
Aerosols
Dust Filters
Scrubbers
Electrostatic
Precipitators
Ground Vehicles
Pollution Control
Stationary Sources
Particulate
13B
21D
21G
11G
07D
13J
07A, 131
13F
13. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport/
Unclassified
21. NO. OF PAGES
174
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (»-73)
153
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