EPA-600/2-77-056
February 1977
Environmental Protection Technology Series
EVALUATION OF CERAMIC FILTERS FOR HIGH-
TEMPERATURE/HIGH-PRESSURE FINE
PARTICIPATE CONTROL
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to develop and
demonstrate instrumentation, equipment, and methodology to repair or prevent
environmental degradation from point and non-point sources of.pollution. This
work provides the new or improved technology required for the control and
treatment of pollution sources to meet environmental quality standards.
EPA REVIEW NOTICE
This report has been reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the
views and policy of the Agency, nor does mention of trade
names or commercial products constitute endorsement or
recommendation for use.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-056
February 1977
EVALUATION OF CERAMIC FILTERS
FOR HIGH-TEMPERATURE/HIGH-PRESSURE
FINE PARTICULATE CONTROL
by
G. G. Poe, R. M. Evans,
W.S. Bonnett, andL.R. Waterland
Aerotherm Corporation
485 Clyde Avenue
Mt. View, California 94042
Contract No. 68-02-1319, Task 25
ROAPNo. 21ADL-029
Program Element No. 1AB012
EPA Task Officer: B.C. Drehmel
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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FOREWORD
This document contains a review and evaluation of ceramic membrane filtration as a novel con-
cept for the control of fine particulate. Several materials are identified as candidates for testing
as high temperature, fine particulate filters and a test matrix is proposed.
The study was performed for the Environmental Protection Agency, Research Triangle Park, North
Carolina. Dr. D. C. Drehmel was the EPA Task Officer. The Aerotherm Program Manager was Mr. Fred
Moreno. Acting as Technical Advisor for the task was Dr. C. B. Moyer. The study was performed
during the period December 1975 through June 1976.
n
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TABLE OF CONTENTS
Page
SUMMARY 1
INTRODUCTION 3
THEORETICAL DISCUSSION 4
3.1 Filter Efficiency 5
3.2 Pressure Drop 7
3.3 Actual Filtration 10
4 CURRENT RESEARCH 13
5 LITERATURE AND MANUFACTURERS SURVEYS 14
5.1 Thick Walled Elements 17
5.2 Thin Walled Elements -Catalyst Monolith Supports 20
5.3 Potential Cleaning Methods 20
6 PROPOSED TEST PROGRAM 31
7 PROCESS ECONOMICS 36
8 CONCLUSIONS AND RECOMMENDATIONS 38
REFERENCES 39
APPENDIX A - FILTER TEST PLAN 40
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LIST OF ILLUSTRATIONS
Figure Page
1 Collection efficiency versus particle size 8
2 Collection efficiency versus particle size 9
3 Pressure response in constant rate gas filtration 11
4 Multiple element filter assembly 18
5 Flow rate of air at standard conditions for a typical ceramic cylindrical .
element, wall thickness - 6.0 mm 19
6 Examples of 3M Company's ThermaComb corrugated ceramic 21
7 Structural shapes for 3M ThermaComb 22
8 Examples of Coming's Celcor cordierite monoliths 23
' , 9 Examples of Dupont's Turvex honeycomb 24
10 Example of General Refractory Company's Versagrid ceramic honeycomb 25
11 Examples of Norton Company's silicon carbide Spectramic honeycomb . . 26
12 Pore size distribution for 3M Company's ThermaComb AlSiMag 795 27
13 Pressure drop across an 0.2 mm (0.008 in) AlSiMag 795 flat ceramic piece .... 28
14 Air flow through 0.2 mm (0.008 in) AlSiMag 795 flat ceramic piece 29
15 3M Crossflow Ceramic Monolith 32
16 3M Element Low Temperature Holder 33
17 3M Element and Holder Mounted Inside Pipe 34
LIST OF TABLES
Table Page
1 Filter Operating Conditions 15
2 Survey Results 16
3 Comparison of Filter Element Costs for the Conditions in Table 1 37
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SECTION 1
SUMMARY
The purpose of this study was to analyze and evaluate ceramic membrane filters as a new, fine
(<3um) particulate control concept for high temperature (~900°C), high pressure (HTHP) processes.
The results of this effort are summarized in the following paragraphs.
The concept of membrane filters is not a new one. A review of the theory of operation, from
the standpoint of both particle collection efficiency and fluid flow behavior, is presented here.
The theory is the same as that for fiber filter operation. The complexity of the problem necessi-
tates that many simplifying assumptions be made; consequently, theoretical calculations are not
very reliable for predicting actual filter operation. However, these calculations can be very use-
ful in explaining the quantitative behavior of a filter and in investigating the sensitivity of
filter operation to changes in fluid parameters and filter material characteristics. Actual filter
operation can be predicted only with the aid of sufficient experimental data at similar operating
conditions.
Current research to develop ceramic membrane filters for use in HTHP processes is limited to
an EPA sponsored project with Westinghouse and Horizon Research Inc. The purpose of this project
is to develop a thin-walled (-0.6 mm) ceramic material with a pore size of approximately 0.5 urn.
Success in producing a sample suitable for laboratory testing as a filter material has been limited.
Several ceramic filters were identified in this study as potential candidates for fine parti-
culate removal. There does not seem to be any inherent material limitation to high temperature
operation; however, no evidence of high temperature filter application was found. These filters are
typically 2 to 6 mnTthick, cylindrical in shape, and available with various pore size increasing up-
wards from 0.5 \m. It appears that these elements may be suitable for fine particulate control in
hot gas streams.
The most promising, although undeveloped, idea for a ceramic filter is to use ceramic honey-
comb monoliths similar to those available for catalyst supports and heat exchangers. The walls of
the monoliths are approximately 0.2 to 0.4 mm thick and of varying pore size and porosity. Geomet-
ric configurations are available which would force the gas to flow through the membrane walls.
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Pressure losses would be very small relative to those of standard ceramic filter elements. The
application of ceramic monoliths to high temperature fine particulate control appears very promising,
and it is strongly recommended that this concept be further investigated.
Three ceramic materials were selected for testing and were delivered to Westinghouse. The
materials were a standard ceramic filter material (Selas), a ceramic monolith (3M) and a new ma-
terial called ceramic FiberForm (FMI). A test matrix was prepared and is included as Appendix A.
A combined-cycle power plant using ceramic membrane filters appears to be cost competitive
with other combined-cycle fine particulate removal systems. The ceramic monolith concept offers
a substantial cost savings (perhaps a factor of 10) over other ceramic filter elements.
Ceramic filters appear to be both technically and economically feasible as fine particulate
control devices in high temperature gas flows; however, suitable experimental data must be obtained
in order to adequately evaluate them. Each type of filter should be tested under conditions sub-
stantially similar to the expected operating conditions. Data concerning the collection efficiency,
pressure loss, duty cycle, filter life, and methods of cleaning should be obtained.
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SECTION 2
INTRODUCTION
High temperature gas turbines used to generate electric power require gas streams virtually
free of particulate matter. Gas streams from high temperature, high pressure coal processes, such
as low Btu gasification and pressurized fluidized bed combustion, require considerable particulate
removal. In order to maintain high thermal efficiency the particulate clean-up must be done at the
high temperatures of the process. Coarse particles (>10ym) can be successfully removed by high
temperature cyclones. Many new concepts for fine particulate control at elevated temperatures are
presently being proposed. One such concept utilizes ceramic membrane filters.
Ceramic materials have been used extensively in high temperature environments. High melting
temperature and mechanical strength, low thermal expansion, and resistance to chemical attack are
some of their attractive features. These materials have found application as filters in medicine,
biology, aerospace, and electronics where extremely clean gas and liquid streams are required.
There are no obvious reasons why ceramic filters would not also be suitable for the high tempera-
ture, fine particulate control applications described here.
This report presents a review and evaluation of current, proposed and potential ceramic mem-
brane particulate control technology. The theory of membrane filters is reviewed in Section 3.
Section 4 is a brief summary of the current EPA-Westinghouse ceramic membrane filter program, which
is currently the only developmental research program concerning ceramic filters for high temperature
applications. Section 5 presents the results of the literature and manufacturers surveys. Three
materials, proposed for testing at Westinghouse, and a test plan are discussed in Section 6. The
economics of ceramic filters are briefly discussed in Section 7, and the conclusion and recommenda-
tions presented in Section 8.
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SECTION 3
THEORETICAL DISCUSSION
Filter materials are usually classified as one of two types, membrane filters or fiber fil-
ters. In a two dimensional representation, membrane filters may be thought of as a uniform solid
phase interrupted by a fluid phase (holes), and are generally characterized by low porosity (void
fraction). A fiber filter may be thought of as a uniform fluid phase (holes) interrupted by a
solid phase (fibers) and is characterized by high porosity. In an oversimplified view, membrane
filters work because the membrane pore diameter is smaller than that of the particles to be col-
lected; the particles are thus prevented from passina through the membrane. On the other hand,
fiber filters operate most effectively after an initial "cake" (layer) of particles has been de-
posited; thus, the cake serves as the filter. In either case the theoretical analysis of the fil-
ter operation is the same and follows directly from the theory of flow through porous media. The
analysis results in the development of two measures of filter performance - particle collection ef-
ficiency and fluid pressure drop across the filter.
Usually, the theoretical development of filter collection efficiency has been approached
microscopically from a very detailed analysis of small scale events inside the filter. Important
parameters are the particle size, filter pore size, fluid Reynolds number, and diffusivity. Each
collection mechanism is studied separately and an expression for the collection efficiency derived.
The individual expressions are then combined to yield an estimate of the total collection efficiency
of the filter. These expressions are usually derived for a single size particle and well-defined
flow conditions. Unfortunately, the procedure is quite complicated, necessitating the use of many
simplifying assumptions which can lead to considerable inaccuracy.
The analysis of fluid pressure drop, on the other hand, is approached macroscopically. The
system is treated as a continuum and the suitably modified equations of motion are applied. A fil-
ter porosity (e) is introduced in the continuity equation and a filter permeability (K) modifies
the momentum conservation equation. Porosity is a straightforward measure of filter void fraction
and may be easily calculated from density measurements. The permeability, however, is a measure of
the fluid drag introduced by the filter and cannot be accurately predicted at present. It thus
becomes an empirical constant.
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Expressions for the efficiency and the pressure drop (from Darcy's law) of a filter are
developed for highly idealized systems in the following sections. This is followed by a discussion
of the possible effects of some real system complications.
3.1 FILTER EFFICIENCY
Particle collection by filtration results from the action of three basic mechanisms:
• Diffusion of particles to solid surfaces
• Impaction of particles on solid surfaces due to deviation of the particle motion from the
streamlines of fluid motion
• Interception of particles by solid surfaces due to particle motion along streamlines.
Clearly, the last two mechanisms depend on the details of the fluid flow adjacent to the pore sur-
faces. Some authors describe a fourth mechanism - sieving, in which a particle with a radius greater
than the largest pore radius is "strained" out of the fluid. However, this is just a special case
of impaction.
A detailed analysis of each collection mechanism is quite tedious and complicated, and will
not be presented here. The resulting equations, as given by Davies (Reference 1), are presented in-
stead. The analysis assumes that particles are chemically inert, uncharged, solid spheres of iden-
tical size (mono-disperse). In addition, particles are assumed to be at infinite dilution so that
particle — particle interactions and pore clogging may be neglected. Thus the porosity and permea-
bility can be considered constants. The resulting expressions for filter efficiency include the
pore radius (R), the mean fluid velocity in each pore (v) and the average length of the pore (h) as
parameters. These quantities are determined from measurements of the porosity, the average number
of pores per unit area, and the fluid flowrate.
The efficiency due to diffusion (ED) has been given by Spurny & Pich (References 2,3,4) as:
ED = 1 - 0.81904 exp (-3.6568 NQ) - 0.09752 exp (-22.3045 ND)
(1)
- 0.03248 exp (-56.95 ND) - 0.0157 exp (-107.6 ND),
-------
where ND = h V/R2v
V = kTF/6iray = Diffusion coefficient
F = Cunningham slip factor
k = Boltzman's constant
a = particle radius
y = fluid viscosity.
The expression is valid for pore radii smaller than 10 percent of the filter thickness.
The efficiency due to inertia! deposition (Ej) is given by Pich (References 5,6) as
Ej = 1 - (a + B)2/(a + I)2, (2)
where a = l/(\Ar/2,/3~e - l)
e = porosity
B = 1 - 2S Sa + 2S2 a {1 - exp (-1/S^a)}
S = mFv/6irayR = Stokes number of particle
m = mass of particle.
This is an unproven expression in that many unsupported approximations were used in its derivation;
however, it is possibly the most accurate expression presently available.
The efficiency due to interception (ER) is determined geometrically as the ratio of the inter-
ception area of the pore (irR2 - ir(R - a)2) to the total area of the pore (irR2). Thus,
c _ i /? a\ , ,
fcR " R r " R) • (i}
The overall collection efficiency (E) is then given by:
E ' Ei + 0 - EI)(ED + **)> <«>
where y = 0.6 ^ " a/R\
When this number exceeds unity the efficiency is assumed to be 100 percent. The overall collection
efficiency is seen to be a function of particle size, pore size, porosity, and particle density.
Fluid properties of importance are the viscosity, the Cunningham slip factor, the flowrate and the
fluid mean velocity in the pore. The effects of pore size distribution on collection efficiency
have been found to be quite significant (Reference 7); however, these effects are not included here.
6
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Figure 1 shows the- predicted overall collection efficiency as a function of particle size
with superficial fluid velocity as a parameter. These results are for a membrane filter 0.25 mm
thick with an e of 0.3 and uniform pore radius of 5 microns. As expected, for larger particles
(>0.5vim radius) collection efficiency increases with increasing particle size. However, note that
for very small particles the predicted collection efficiency increases with decreasing particle
radius. This results from enhanced diffusion of these particles to the collector surface. Figure
2 shows overall collection efficiency as a function of particle size with filter pore radius as a
parameter.
3.2 PRESSURE DROP
The flow of a continuum fluid through a porous medium is described by the continuity and con-
servation of momentum equations. For a chemically inert, electrically neutral, homogeneous Newtonian
fluid these equations are:
->• ->
V - p V0 (5)
(6)
where V is the superficial velocity of the fluid at the surface of the porous medium, e is the
porosity, K is the permeability and y is the gas viscosity. Equation (6) is commonly called Darcy's
law. For steady flow of a clean, ideal gas of constant viscosity these equations may be solved for
a variety of geometries. The most common form of Darcy's law for flow through a planar filter of
thickness L is:
Vo = A = Hi? ^
where Q is the volumetric flowrate and A is the surface area and AP has been assumed to be small
compared to P. This equation is often used to calculate the permeability of a material, since
permeability is not derivable from other basic considerations. For a system of two planar filters
of thickness L-, and I?
AP = AP, + AP2 = (8)
where L = 1^ + L2 and
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1.0
oo
Superficial velocity
V = Q/A = 10 cm/secQ
7 cm/secA
4 cni/secQ
2 cm/sec<>
1 cm/sec>
h = 0.25 mm
y = 0.035 cp
E = 0.3
R = Sum
0.05
0.1
0.5 1.0
Particle radius (a) in microns
2.0
5.0
Figure 1. Collection efficiency versus particle size.
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h = 0.25 mm
V = Q/A = 10 cm/sec
u = 0.035 cp
e = 0.3
0.05
Particle radius (a) in microns
Figure 2. Collection efficiency versus particle size.
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K = lK^K2/(l2!^ + L^). (9)
This form of Darcy's law is useful for determining the permeability of a cake-filter system with
clean air. Similar results can be obtained for a cylindrical system.
If particles are contained in the fluid the pressure drop is affected in two ways. First
the permeability of the filter changes with time as the smaller particles plug the pores. Second,
a dust cake builds up on the surface and forms a new porous medium. These effects can be quantified
by appropriate use of Equation (0) or (9). The dust cake is considered to be one of the filters
and its thickness (l_2) and permeability (ic2) are taken as functions of time. After a short initial
transient period the permeability and rate of growth (k) of the dust cake are approximately constant;
thus the pressure drop of the combination of filter and dust cake is given by:
AP = AP + , do)
where
QuL,
This linear time dependence of the pressure drop is illustrated in Figure 3 which shows data for a
fiber filter.
3.3 ACTUAL FILTRATION
The development described above is based on a number of simplifying assumptions. Few, if
any, of these assumptions are true in actual practice. Real effects not considered in the above
theoretical development include:
• Filter pores are not uniformly sized, right circular cylinders
• Particles are not spheres of uniform radii
• Chemical reactions may become important at high temperatures and pressures
• Corrosion of filter material may be important at high temperatures or in adverse environ-
ments
• Pressure fluctuations may cause changes in dust cake properties
• The effects of reentrainment may be significant.
10
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A - Establishment of cake
B — Homogeneous dust deposit
Total filtering cycle
16 20 24 28
Time, minutes
32 36 40 44
Figure 3. Pressure response in constant rate gas filtration.
11
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Consequently, the expressions for efficiency and pressure drop should be taken as approximations,
useful only to illustrate functional relationships of important system parameters.
The effects of temperature and pressure on filter operation are introduced largely through
changes in the fluid viscosity. The viscosity of air, for example, is relatively unaffected by
pressure, but is (to a good approximation) directly proportional to temperature. Consequently, as
temperature increases, both viscosity and filter pressure drop increase (cf. Equation (10)). In
addition, thermal expansion can decrease the porosity and permeability. These effects also give
rise to increased filter pressure drop.
The effects of temperature on filter collection efficiency are similarly adverse. Strauss
and Lancaster (Reference 8) have shown that collection efficiency (through the mechanisms of impac-
tion and interception) decreases as the gas viscosity increases. Conversely, collection efficiency
due to particle diffusion may increase slightly with increasing temperature. Therefore lower col-
lection efficiencies are expected at increased temperatures for particles larger than 0.2 microns
in diameter. The converse is true for particles smaller than 0.2 microns.
12
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SECTION 4
CURRENT RESEARCH
Westinghouse Research Laboratories is currently performing the only large scale effort
directed toward applying ceramic membrane filters to particulate removal from hot gas streams.
This study has potential application to cleaning high temperature fuel gas from coal gasifiers,
and is being funded by the EPA (Reference 9). The Westinghouse effort seeks to develop thin
(0.25 mm) cylindrical membrane filter elements with small, uniform pore size (0.5ym). These fil-
ter elements are then to be tested and evaluated as a technique for collecting sub-micron particu-
late from a high temperature (>1500°F) fuel gas stream.
To date progress in obtaining appropriate planar samples of the filter material for perfor-
mance testing has been limited. Fabrication problems have included:
• Difficulty in laying down a planar membrane free of oxide irregularities, which can
become large holes when the membrane is etched
• Difficulty in converting the initially deposited yalumina to a-alumina without warping
the filter disk (warped disks break when installed in filter holders for performance
testing)
• Difficulty in uniformly enlarging the pores throughout the membrane to produce a filter
with acceptable pore size.
As a result of these problems, few data for either clean gas flow characteristics of particulate
removal performance are available. Recently Horizon Research has fabricated a suitable ceramic mem-
brane and support structure, and has performed several successful preliminary performance tests.
However, it appears that development of this material will be curtailed due to continued difficulties
in preparation of suitable samples.
13
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SECTION 5
LITERATURE AND MANUFACTURERS SURVEYS
There is a paucity of data available on the use of ceramic filters to collect particulate from
hot gas streams. A limited survey of filtration literature and of ceramic manufacturers/suppliers was
therefore undertaken to determine:
• The available types of ceramic filter materials
• The use of ceramic filters in current filtering applications
• The types of ceramic filter materials which may be suitable for collecting sub-micron
particulate from a high temperature gas stream.
The survey was accomplished in four steps:
• Review of existing filtration literature
• Telephone contact with ceramic filter suppliers
• Telephone contact with ceramic manufacturers
0 Telephone contact with monolithic catalyst support manufacturers.
The literature survey revealed that sub-micron particle ceramic filters are in use in medical
aerospace, electronic and biological applications for both gases and liquids. However, no discussion
of the operation of these filters at high temperatures was found. This appeared to be more a case of
lack of present applications rather than ceramic material limitations.
These findings prompted the telephone survey discussed below. The purpose of the survey was
to contact a representative sample of ceramic manufacturers and suppliers, and to determine if capabil-
ities exist to develop a suitable rigid ceramic filter for application to hot gas clean-up. As a guide,
a representative set of operating conditions was established (see Table 1). The conditions in Table 1
approximate those from a low BTU coal gasifier downstream of the cyclones. The key parameters are the
temperature and the particle size limits. A summary of the relevant telephone contacts is given in
Table 2.
14
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TABLE 1. FILTER OPERATING CONDITIONS
Temperature
Pressure
Gas Flow Rate
Gas Composition (% Vol)
Molecular Weight
Density
Heat Capacity
Viscosity
Particulate Loading
(upstream of filter)
Distribution (by weight)
Particulate Loading
(downstream of filter)
Distribution (by weight)
1100 to 1400°K
2000 kPa
200 mVmin
H2
CO
H20
co2
N2
CH4
H2S
COS
NH3
tars, heavy HC,
23
4.8 kg/m3
2510 J/kg°C
3.5 x 10-5 N-s/m
1.144 x 10'3 £f
100% < 30ym
80% < lOym
50% < 4ym
15% < lum
1.144 x 10-5 |jjf
100% < 6ym
98% < 4ym
95% < 2ym
80% < lym
(1500 to 2000°F)
(300 psia)
(7000 SCFM)
21
14
13
n
38
2.5
0.5
30 ppm
600 ppm
others trace
(0.3 Ibm/ACF)
(0.6 Btu/lbm°F)
2 (0.035 centipoise)
(0.5 grain/SCF)
(0.005 grain/SCF)
15
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TABLE 2. SURVEY RESULTS
Company
1. Babcock & Mil cox Co. ,
Refractories Div.
Augusta, Georgia
2. Coors Porcelain Co.
Golden, Colorado
3. Corning Glass Co.
4. Do! linger Corp.
Rochester, New York
5. General Refractories
Bala Cynwyd, Pa.
6. Horizons, Inc.
Cleveland, Ohio
7. 3-H Company
Saint Paul , Minn.
8. Norton Company
Worcester, Mass.
9. Selas Flotronics
Springhouse, Pa.
10. Wisconsin Porcelain Co.
Sun Prairie, Wisconsin
Code*
c
c,f
f,s
f
c.s
c
c,s
c,s
f
c,f
Comments
Did not think they had any materials
that would work for the application.
Several small scale filters for this
particle size. No high temperature
filter experience. Samples sent to
Aerotherm. Nominal wall thickness
~6 mm (1/4").
Nothing for this application. Celcor
cordierite monoliths a possibility.
Some filter elements that would work
for this application. Relatively
thick wall, large pressure drop
(-100 kPa (1 atm)). Elements
available for testing.
Nothing for this application. Ver-
sagrid cordierite honeycomb is a
possibility; however geometrical
constraints a problem.
This membrane (-0.25 mm thick) being
developed for testing in this applica-
tion under Westinghouse — EPA program.
AlSiMag 795 (cordierite) honeycomb
structure should work for this appli-
cation. Elements available for test-
ing. Low pressure drop. High sur-
face area to volume ratio. Good
process control over pore size,
porosity and membrane thickness.
Samples sent to Aerotherm.
Two materials suitable for thick
walled filters. Also Spectramic
honeycomb is a possibility.
Micro-porous porcelain element
would be suitable. Pressure drop
~50 kPa. Sample sent to Aerotherm.
Thick walled filter elements
similar to 4 and 9. No membranes.
c - ceramic manufacturer
f - filter supplier
s - catalyst support manufacturer
16
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All contacts were asked to supply information pertaining to:
• High temperature operation
• High flowrate operation
• Cleaning procedures
• Cost
t Novel applications and design ideas.
In addition, all contacts were asked if they were supplying anyone with ceramic elements suitable
for hot gas cleaning, if they were developing any materials suitable for this application, and what
if any, material limitations should be considered for this application.
In general, two types of ceramic materials suitable for this application were identified.
These are discussed separately in the following sections, and a brief discussion of possible filter
element cleaning methods is also given.
5.1 THICK WALLED ELEMENTS
The type of filter element generally available from ceramic manufacturers and filter suppliers
is referred to as a "thick walled element". It is usually cylindrical and has a wall thickness of
4 to 6 mm which is "thick" in comparison to the Westinghouse/Horizons membrane and ceramic monoliths
(wall thickness ~0.25 mm). For large flowrate applications, the elements (typically 50-100 mm in
diameter and 250-300 mm in length) would be grouped in an assembly similar to the one shown in Figure
4. The wall thickness of the element is somewhat limited by the ceramic forming process; however,
a more significant limitation is mechanical strength. The structural strength of the wall decreases
with decreasing wall thickness; hence, thin walls in the cyclindrical geometry would probably be too
fragile for heavy duty use.
Thick walled elements have several inherent disadvantages. First, the pressure drop across
an element increases directly with the wall thickness (see Equation (11)). Typical pressure losses
for a 6 mm thick filter of this type are shown in Figure 5 with pore size as a parameter. Values on
the order of 35 to 70 kPa (5 to 10 psi) may be expected for filters of this type. A second problem
with thick walled elements is the possible occurrence of thermal stress and spelling when the element
is subjected to temperature cycling. This requires considerable care in start-up and shut-down pro-
cedures. Finally, material costs for thick walled elements are greater than for thin walled elements.
This is discussed in more detail in Section 6.
17
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Figure 4. -Multiple element filter assembly.
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200
150-
3.
Q.
TOO.
3
CO
to
S-
o.
50-,
10 100
Flow rate — 'cm3/sec-cm2
psi
t-25
-20
-15
-10
-5
1000
Figure 5. Flow rate of air at standard conditions for a typical ceramic
cylindrical element, wall thickness ~ 6.0 mm.
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The impact of the disadvantages described above will have to be assessed in ngnt or outer
ceramic devices and participate removal methods. An adequate assessment can be made on-ly if opera-
tional performance data at the conditions of interest are available.
5.2 THIN WALLED ELEMENTS - CATALYST MONOLITH SUPPORTS
The ceramic monoliths used as catalyst supports for combustion processes were considered as
potential filters, since the desired characteristics of a ceramic membrane filter are very similar
to those of a monolith, i.e.,
• High temperature capability
t Structural integrity
t Large surface area to volume ratio
• Moderate porosity (25-30 percent)
• Small pore diameter (less than 10pm).
These ceramic monoliths are manufactured by several companies (see Table 2) in a variety of
configurations. Typically they have a honeycomb appearance with a wall thickness of approximately
0.25 mm. The honeycomb construction provides excellent structural strength. Figures 6 through 11
illustrate some of the various structural configurations. Those monoliths manufactured by an extru-
sion process have rather limited geometric flexibility. However, some processes (e.g., 3M Company
as shown in Figure 7) allow very flexible geometries. The 3M process is similar to that used in
making corregated paper products. 3M reports that the process allows independent control over wall
thickness, pore size and porosity. A typical pore size distribution for one of 3M's products is shown
in Figure 12. Preliminary flowrate/pressure drop relationships for the 3M material are shown in
Figures 13 and 14. It should be noted that pressure drop is in the 1 to 10 kPa range which is a fac-
tor of 10 to 100 less than that for thick walled ceramic elements. The decrease in flowrate with in-
creasing temperature shown in Figure 14 is primarily due to increases in gas viscosity. The advantages
discussed above strongly suggest that this type of material be tested for use as a fine particulate
filter.
5.3 POTENTIAL"CLEANING METHODS
During the course of the survey several methods were proposed for cleaning the filter elements.
These fell into three general categories:
• Ultrasonic treatment
• Washing
• Back flow
20
-------
a. 8x enlargment
b. Rolled structures
c. Rolled and stacked structures
Figure 6. Examples of 3M Company's ThermaComb
corrugated ceramic.
21
-------
SC SPIH CELL (Note Separator)
XFSC
CROSS-FLOW,
SPLIT CELL
Note separators
and corrugations
at 90°
XXSC
CRISS-CROSS,
SPLIT CELL
Note separators
and corrugations
at 45°
XXHC
CRISS-CROSS,
HONEYCOMB
with corrugations
at 45°
Note there is no
separator.
Figure 7. Structural shapes for 3M ThermaComb.
22
-------
ej
r
oo
5
Figure 8. Examples of Coming's Celcor cordierite monoliths,
23
-------
a. Straight honeycomb
b. Slant cell honeycomb
c. Cross-flow honeycomb
Figure 9. Examples of Dupont's Turvex honeycomb.
24
-------
Qj
eJ
to
1 1 ••*•••*•**••
i I i > ) • !««** It* »»«)••••••••• ) >•••••»•••*>
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Figure 10. Example of General Refractory Company's
Versagrid ceramic honeycomb.
25
-------
Figure 11. Examples of Norton Company's silicon
carbide Spectramic honeycomb.
26
-------
c
(0
-C
re
O)
en
OJ
S-
o
CL
IOC
80-
50.
40-
20-
(0
I
^_
100 10 1.0 0.1 0.01
Pore diameter (microns)
Figure 12. Pore size distribution for 3M Company's ThermaComb AISiMag 795.
-------
100-i
AP (in of H20)
0.5
1.0
I
1.5
2.0
i
80-
60-
o
01
40 -
o
20-
r
0.1
r
0.2
0.3
AP (kPa)
r
0.4
I
0.5
0.6
Figure 13. Pressure drop across an 0.2 mm (0.008 in)
AlSiMag 795 flat ceramic piece.
28
-------
560-
480
400
320
E
(j
c
•i—
E
-------
No specific information was obtained concerning either possible implementation methods or possible
treatment efficiencies of sonic cleaning although it was generally thought that the application of
ultrasonic energy would aid and speed a washing cycle. Washing of a filter would necessarily'be done
off-line due to time and temperature requirements. Thick walled elements would require slow cooling
to prevent thermal stress, cracking and spalling. Thin walled elements probably would not require as
lengthy a cool-down period.
Back flow appears to be the most promising cleaning method at this time. There are numerous
ways this can be accomplished. One is to briefly take a filter element off-line and use the pressure
in the filter assembly to blow down the element in a reverse flow pulse. Alternatively, part of the
clean gas could be used to reverse flow one element of a multi-element assembly. Gravity would carry
the particles to the bottom of the assembly and possibly out of the vessel. Another possibility
is the use of a pressure pulse similar to baghouse cleaning methods. Cleaning methods and duty cycles
still require considerable experimental investigation.
30
-------
SECTION 6.
PROPOSED TEST PROGRAM
At the conclusion of the work described in Sections 4 and 5 it was evident that the only way
to realistically evaluate high temperature, high pressure ceramic filters would be to test the
available materials at the conditions of interest. Three materials were identified for testing,
and a test matrix was prepared. The materials and the test matrix are discussed below.
A standard ceramic filter manufactured by Selas Flotronics was selected as representative
of the "thick walled" elements described in Section 5.1. This material has been tested and used in
low temperature environments; however, there are no data available on the high temperature (T > 200°C)
performance of this material. There is a relatively large pressure drop (50 - 100 kPa) across the
element. It is expected that the pressure drop will increase at higher temperatures.
A cross-flow ceramic monolith manufactured by 3M was selected to represent the ceramic mono-
lith elements described in Section 5.2. The primary reason for this choice was that this was the
only monolith available in a cross-flow geometry. Preliminary data for the pressure drop-flowrate-
temperature relationships were available from 3M (Figures 13 and 14). However, there are no data on
the filtration characteristics of this material. Several samples of this material were provided by
3M. These samples and a low temperature element holder were delivered to Westinghouse for testing.
The split cell cross-flow construction is shown in Figure 3. Sketches of the material and holder
are shown in Figures 15, 16 and 17.
A third material, which is significantly different from the two described above, was also
identified. This material, called alumina FiberForm, is manufactured by Fiber Materials, Inc. It
is made of alumina-silica fibers bonded with a high purity alumina binder and produced using vacuum
slurry molding techniques. FiberForm has a useful temperature limit of about 1450°C (2600°F). It
is a very high porosity, low density material and has a texture somewhat like styrofoam. This ma-
terial has never been tested as a filter; however, its pore size, porosity and temperature limits
make it an attractive candidate. Samples of FiberForm have been delivered to Westinghouse.
31
-------
Figure 15. 3M crossflow ceramic monolith.
-------
CO
oo
-j-
Figure 16. 3M element low temperature holder.
-------
Figure 17. 3M element and holder mounted inside pipe.
-------
A test plan for the evaluation of potential filter materials was prepared and is included
as Appendix A. The major test objectives for both low and high temperature testing are:
• Establish pressure drop-flowrate relationships for clean air flow
• Determine fine particle collection efficiency
• Measure pressure drop as a function of time for dirty air
• Investigate various cleaning methods
The testing would be done in two phases. The first phase is for low temperature testing and
screening of materials. Only those materials that perform satisfactory will be tested at high
temperatures.
35
-------
SECTION 7
PROCESS ECONOMICS
Lack of sufficient data at present precludes accurate calculations of process costs for a
high temperature particulate cleaning device utilizing ceramic membrane filters. However, suffi-
cient preliminary data do exist to obtain an order of magnitude estimate of the capital require-
ments and operating costs for a ceramic filter, gas cleaning plant.
Westinghouse Research Laboratories has compiled extensive cost data on ceramic filtration
for cleaning fuel gas from a coal gasifier for use in a combined cycle power plant. These data
appear in Reference 9 and will only be briefly summarized here.
Westinghouse compared the costs of a ceramic membrane filtration system to those of a com-
bined granular bed/conventional baghouse filtration system. For purposes of illustration, the
initial capital investment and the annual operating expenses were estimated for each particulate
cleanup system which treats approximately 30 mVsec (63,600 ACFM) of fuel gas at 870°C and 1620 kPa
(235 psia). This is equivalent to the output of a coal gasifier which supplies fuel to a 250 MW
combined cycle plant. An installed capital cost of $8.25 million and total annual operating ex-
penses of $1.8 million were estimated for the granular bed/conventional baghouse system. Esti-
mated costs for the ceramic membrane filter system were made for various filter characteristics —
pore diameter, membrane thickness, gas velocity, free cross section, etc. For a filter with 1
micron diameter pores, the estimated capital investment ranged from about $0.7 to $7.0 million,
and total annual operating expenses ranged from $0.7 to $2.3 million. For example, for a case
requiring 800 m2* of 0.25 mm thick ceramic membrane filter, the capital investment is $2.1 million
and the annual costs are $0.9 million, which is easily competitive with the above alternative.
Several ceramic materials currently produced by ceramic and catalyst monolith suppliers
were described in a previous section. Many of these materials have the potential to meet the re-
quirements of a hot gas filtration system. A cost comparison of various filter systems operating
at the conditions shown in Table 1 is given in Table 3. These results are for a single
*
Pore size = lym, superficial gas velocity = 4.5 cm/sec, pressure loss = 60 kPa.
36
-------
filter system and do not include any duplication which may be required for on-line/off-line opera-
tion. The costs shown are for the filter materials only; no pressure vessel or tube sheet costs
are included. It is interesting to note that the pressure drop required for either the thick- or
thin-walled membranes is significantly lower than the 50 to 80 kPa estimated for reasonable flows
through the Westinghouse/Horizons material. This is primarily due to the different pore size and
permeability of these materials.
TABLE 3. COMPARISON OF' FILTER ELEMENT COSTS FOR THE CONDITIONS IN TABLE 1
Wall Thickness - mm
Flow Area - m2(ft2)
Superficial Velocity - cm/sec
(in/sec)
Approximate Pressure Drop, Clean -
kPa (psi)
Size of Elements - mm (in)
Number of Elements
Approximate Cost of Elements
Thick-Wall
4.76
8.17
(88)
9.1
(3.57)
34.5
(5)
66.7 dia x 254 x 4.8
(2-5/8 dia x 10 x 3/16)
153
$4600
($560/m2)
Thin-Wall
0.25
24.00
(258)
2.54
(1.0)
3.732
(0.54)
152 x 152 x 152
(6x6x6)
17
$850
($35.50/m2)
Westinghouse
0.25
17.0
(177)
4.5
(1.77)
60.0
(8.7)
-
-
$4080
($240/m2)
The most interesting aspect of the cost comparison is the projected cost for thin-wall ce-
ramic material in monolith form. Currently available material at $35.50/m2 for 4 cells/cm is
roughly 10 times less expensive than the projected cost of the Westinghouse ceramic material. If,
for either material, the costs associated with plant construction are approximately equivalent, and
if the element cost is a significant portion of total cost of the system, then the use of ceramic
monoliths offers a great potential cost savings.
It appears that the economics of ceramic membrane filters, in particular the ceramic monoliths,
compare quite favorably with other gas cleanup schemes for particulate removal from high temperature
gas streams.
37
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SECTION 8
CONCLUSIONS AND RECOMMENDATIONS
This investigation resulted in the following conclusions:
• The theory for the operation of membrane filters is not adequate to predict the opera-
tion of real filters.
• The Westinghouse project has had only limited success thus far in developing and testing
a ceramic membrane filter.
0 Current available ceramic filters
— appear promising for high temperature fine particulate control
— appear to be economically feasible
- need to be experimentally tested for high temperature operation.
• Ceramic honeycomb monoliths
offer excellent possibilities for use in high temperature particulate control
- are economically superior to present ceramic filter elements
- need to be experimentally tested in particulate control applications.
Before adequate assessment of these control methods can be made, several important questions
must be answered. These are:
• What is the relationship of collection efficiency to pore size and pore size distribu-
tion and to particulate size and size distribution?
t What is the relationship of pressure loss to pore size, porosity, and permeability?
• How susceptible to clogging are the ceramic elements?
• How can the elements be cleaned?
An experimental program that will provide answers to the above questions is recommended.
This program utilizes currently available ceramic filter elements and ceramic monoliths, and thus,
requires no material development.
38
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REFERENCES
1. Oavies, C. N., Air Filtration, Academic Press, London, 1973.
2. Spumy, K. and Pich, J., "Analytical Methods for Determination of Aerosols by*Means of Membrane
Filters: VI, On the Mechanisms of Membrane Ultrafilter Action. VII, Diffusion and Impaction
Precipitation of Aerosol Particles by Membrane Ultrafliters," Collect. Czech. Chem. Commun.,
Vol. 28, 1963, pp. 2886-2894; Vol. 30, 1965, pp. 2276-2287.
3. Spurny, K. and Pich, 0., "Zur frage der Filtrationmechanismen bei Membranfiltern," Staub, Vol.
24(7), 1964, pp. 250-256.
4. Spurny, K. and Pich, J., "Auffangen von Aerosolteilchen mittels Membranfilter unter Wirkung der
Diffusion und Impaction," Zent. Biolog. Aerosol Forsch., Vol. 11(6), 1964, pp. 508-511.
5. Pich, J., "Abscheidung von Aerosolteilchen durch Ausschleuderung in der Umgeburg einer kreis-
formigen Offnung," Staub. Vol. 24(2), 1964, pp. 60-62.
6. Pich, J., "Impaction of Aerosol Particles in the Neighborhood of a Circular Hole," Collect.
Czech. Chem. Commun., Vol. 29, 1964, pp. 2223-2227.
7. Caroff, M., Choudhary, K. R., and Gentry, J. W., "Effect of Pore and Particle Size Distribution
on Efficiencies of Membrane Filters," J. Aerosol Sci., Vol. 4(2), 1973, pp. 93-102.
8. Strauss, W. and Lancaster, 8. W., "Prediction of Effectiveness of Gas Cleaning Methods at High
Temperatures and Pressures," Atmospheric Environment, Vol. 2, 1968, pp. 135-144.
9. Westinghouse Research Laboratories, Monthly Progress Reports, "Fine Particle Collection," EPA
Contract 68-02-1887.
39
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APPENDIX A
FILTER TEST PLAN
1. NOMENCLATURE
A - Surface area available for filtration (cm2)
B - Inlet gas particle loading (g/cm3)
P - Pressure (kPa)
AP - Pressure drop across a filter element (kPa)
Q - Gas flowrate (cmVsec)
t - Time (sec)
T - Temperature (°K)
V - Superficial gas velocity, Q/A (cm/sec)
TI — Particle collection efficiency
2. AVAILABLE TEST FACILITIES
Several of Westinghouse Research Laboratories' filter test facilities are available for use.
The flow conditions for three of them, which are to be used in the initial phase of the testing, are
given below.
2.1 10 Liter/min Facility
• Q from 15 to 150 cmVsec (1 to 10 £/min)
• Maximum AP of 50 kPa (7 psi)
• T to 1150°K (1600°F)
• Atmospheric pressure
• Polydisperse particles, sub-10 ym diameter
• Particle loading limits unspecified (but nominally 0.1 to 0.5 grain/SCF)
40
-------
2-2 100 Liter/min Facility
• Q from 500 cmVsec to 5000 cmVsec (30 fc/min. to 300 fc/min)
• Maximum AP of 6 kPa (1 psi)
• Ambient temperature
• Atmospheric pressure
• Polydisperse particles, sub-10 pm diameter
• Particle loading limits unspecified (but nominally 0.1 to 0.5 grain/SCF)
2.3 High Flowrate Facility
• Q up to 2.36 x 105cm3/sec (14,158 tjm, 500 acfm at 1500°F)
• Maximum AP of 7.5 kPa (7.1 psi)
• Temperatures up to 1150°K (1500°F)
• Atmospheric pressure
• Polydisperse particles, sub-10 ym diameter
• Particle loading limits unspecified (but nominally 0.1 to 0.5 grain/scf)
3. FILTER TEST MATERIALS
Three types of ceramic filter materials will be tested. If other materials become available
they will be included in the Phase 1 testing.
3.1 Thick Walled (1/8" to 3/8") Ceramic Filter Disks
Several disks will be tested in the 1 to 10 £/min. facility. Filter properties to be varied
include pore size and disk thickness.
3.2 Thin Walled (-0.010") Ceramic Catalyst Monoliths
Several monoliths will be tested in the 30 to 300 ft/mln. facility. The cell size and wall
thickness may be varied.
3.3 Alumina FiberForm
Alumina FiberForm, manufactured by Fiber Materials, Inc., is a new material made of alumina-
silica fibers bonded with a high purity alumina binder and produced using vacuum slurry molding
techniques. This material is significantly different from the ceramic filter materials and the
41
-------
ceramic monoliths. It has a useful temperature unit of about 1450°C (2600°F) and a very high
porosity.
4. PRETEST ACTIVITY
• Westinghouse will procure suitable ceramic filter disks and disk holders
• Aerotherm will procure catalyst monolith materials and FiberForm, and deliver them to
Westinghouse
5. FILTER TEST MATRIX
Filter testing will proceed in two phases - Phase 1 for ambient temperature tests and Phase
2 for high temperature tests. The Phase 1 testing is intended to screen out materials which are
not suitable for use as fine particulate filters.
5.1 Phase 1 - Ambient Temperature Testing (~80°F)
This test phase has the following general objectives:
• Obtain AP versus V relationships for clean gas flow at ambient temperature
• Evaluate n as a function of t, V , and particle size at ambient temperature
• Evaluate the effectiveness of various backflow filter cleaning methods
• Evaluate various filter cycle possibilities
Testing to fulfill these objectives will be accomplished in four test series.
5.1.1 Test Series 1 - Clean Flow Characterization
• Objective:
- Establish the AP versus VQ relationships as functions of filter pore size and thickness
for clean gas flow at ambient temperature
• Test Variables:
-Q - vary over appropriate facility test range
- Pore size - vary by testing several elements of each type
- Thickness - vary by testing several elements of each type
• Measure:
- AP as a function of Q
42
-------
Results from this series Will consist of plots of AP vs VQ, with filter pore size and thickness as
possible parameters.
5.1.2 Test Series 2 -Participate Removal Efficiency Characterization
• Objectives:
- Determine AP versus t as a function of C, VQ, and filter pore size at ambient tempera-
ture.
- Determine filter collection efficiency (n) versus particle size as a function of C, V ,
and filter pore size.
— Investigate the effects of cake buildup and compression on n-
• Test Variables:
- Q - vary over appropriate facility test range
-C - vary in the range 0.2 to 1.0 x 10~5 g/cm3 (0.1 to 0.5 grain/SCF) for each flowrate
— Pore size - vary by testing several elements of each type
t Measure:
- AP vs t
— Inlet particle loading and size distribution
— Outlet particle loading and size distribution as a function of time
Inlet gas particulate load will consist of a polydisperse sub-lOpm particle mixture. Each
filter element is to be tested at several gas flowrates and several inlet particle loadings. Mea-
surements of AP versus t will give the rate of dust cake accumulation on the filter. The influences
of gas superficial velocity, inlet particle loading, and filter pore size on this rate of accumula-
tion will be investigated. Comparison of outlet particle load and size distribution with inlet load
and distribution will allow calculation of particle collection efficiency versus particle size. The
influence of V , C, and filter pore size on n will also be investigated. Measurement of n versus t
will allow assessment of the relative contributions to particle collection from filter collection
and dust cake collection mechanisms. Obviously, the filter element should be thoroughly cleaned be-
tween individual experiments. Results from this series will include plots of AP versus t, and n as
a function of particle size versus t, with C, VQ, and pore size as parameters. The possibility of
combining this series with test series 3 should be considered.
43
-------
5.1.3 Test Series 3 -Cleaning Method Study
• Objective:
- Investigate various backflow filter cleaning methods including steady backflow and
pulsed backflow
t Test Variables:
- Backflow Q and duration of backflow for steady backflow methods
- Pulse pressure and duration for pulsed backflow methods
• Measure:
- Initial AP after the cleaning operation
The primary purpose of this study is to observe dust cake removal and pore cleaning. Two
types of backflow cleaning experiments should be performed: steady backflow, in which the rate and
duration of backflow are varied, and pulsed backflow, where the backflow pressure pulse magnitude
and duration are varied. Measurement of the initial, clean gas AP (forward flow) will serve as a
reference point to define the degree of dust cake removal achieved. Assessment of the degree of
short term (one cycle) pore clogging and the effect of V on pore clogging will also be possible.
This series will result in the determination of the "best" backflow cleaning methods for each fil-
ter element type. The possibility of combining experiments in this series with those in Series 2
should be considered.
5.1.4 Test Series 4 -Cleaning Cycle Study
• Objectives:
-Determine feasible, long-term filter cycles, including filter times and cleaning times
for the "best" cleaning methods from Series 3
- Investigate the effects of filter pore clogging on long term filter operations, includ-
ing an estimate of filter lifetime at ambient temperature
— Investigate the effects of flow channel clogging in monolith elements on long-term
filter operation
• Test Variables:
- Filter time and cleaning time in a complete filter cycle
-Q
-C
44
-------
• Measure
— AP versus t during filtering operation
— TI as a function of particle size
This test series will extend the work of Series 3 by investigating feasible filtering cycles,
using the "best" backflow filter cleaning method(s) defined in Series 3. A "best" combination of
filtering time, followed by cleaning time, over many filter cycles will be described for long-term
filter operations. Since the rate of dust cake accumulation is dependent on filtering flowrate and
inlet particulate loading, these variables will be parameters in the cleaning cycle description. As
such, Q and C will be varied during this test series. Measurements of AP versus t and initial filter
cycle AP will be used to evaluate candidate filter cycles. Occasional measurements of particulate
collection efficiency should also be performed to monitor filter performance. It is expected that
initial cycle AP will increase with time as filter pores clog; therefore, the length of the filter-
ing portion of a cycle will have to decrease as number of cycles performed increases. The rate of
this increase in initial AP with time will serve to define the useful life of a filter element. An
additional concern relates to flow channel clogging in monolith elements. Backflow alone may not
completely clear these channels. The effects of this clogging on element usable lifetime may also
be assessed by noting the time rate of increase in initial cycle AP. Results from this series will
consist of time specifications for "best" feasible filter-clean cycles, together with estimates of
filter element usable life.
5.2 Phase II - High Temperature Testing
High temperature testing will be performed for those materials identified in the Phase 1 test-
ing as potential filters. General objectives for this phase of testing include:
• Obtain AP versus V relationships for clean gas as a function of temperature
• Assess potential filter porosity decreases, due to material phase change or material sin-
tering, on AP vs VQ relationships
• Evaluate n as a function of particle size and temperature at constant VQ and C
• Evaluate candidate filter-cleaning cycles at high temperatures
Testing to fulfill these objectives at temperatures up to 1100°K (1500°F) will be accomplished in
four additional test series.
45
-------
5.2.1 Test Series 5 — High Temperature Clean Flow Characterization
• Objectives:
— Determine AP vs V for clean gas flow as a function of temperature
— Determine if changes in the AP, V curves can be accounted for solely by gas viscosity
changes or are material pore structure changes also important
i Test Variables:
-T - vary temperature to 1100°K (1500°F)
— Q - vary flowrate over appropriate facility test range
— Pore size - vary by testing several elements of each type
• Measure:
— AP as a function of Q at each T
AP is expected to increase with increasing temperatures, at constant V , due to
t Increasing gas viscosity with increasing T
• Potential porosity decreases due to phase change, or material sintering
Results of this series will consist of plots of AP versus V with temperature as a parameter
for each pore size tested, and a determination of the relative effects of gas viscosity and decreases
in porosity on AP-V curves.
5.2.2 Test Series 6 —High Temperature Particulate Removal Efficiency Characterization
• Objectives:
— Investigate AP versus t as a function of filter pore size and temperature
— Investigate n as a function of filter pore size and temperature
• Test Variables:
- T - vary to 1100°K (1500°F)
— Pore size - vary by testing several elements of each type
• Measure:
— AP versus t
— Inlet particle size distribution
- Outlet particle loading and size distribution as a function of time
The test series is essentially a repeat of Series 2 with the introduction of temperature as
a primary test variable. The theory of filter particle collection mechanisms predicts that n should
decrease with increasing temperature. This would imply that the rate of dust cake accumulation
46
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(and the slope of a AP versus t plot) should be smaller at high temperatures. However, these effects
may be offset by potential porosity decreases as discussed in Section 5.2.1. The findings of this
test series, taken with those of Series 5, will serve to define the relative magnitude of the effects
of possible decreased porosity versus decreased collection efficiency on overall filter collection at
high temperature. Results from this series will consist of plots of AP versus t, with T as a param-
eter, at varying pore size, and n as a function of particle size versus T at varying filter pore size.
5.2.3 Test Series 7 - High Temperature Cleaning Cycle Study
• Objective:
— Determine feasible high temperature cleaning cycles
§ Test Variables: ,
— Filter time, cleaning time
• Measure:
— AP versus t during filtering operation
— n as a function of particle size
This series is essentially a high temperature repeat of Test Series 4. In this series, test-
ing at one flowrate, one particle loading, and one temperature should suffice. Test temperature
should be the highest temperature expected in eventual ceramic filtration applications (~1500°F).
The purpose of this test series is to refine and extend the conclusions derived in Test Series 4 to
high temperature. Results from this test series will consist of the time specifications for "best"
feasible high temperature filter-clean cycles, together with estimates of filter element usable life
at high temperature.
5.2.4 Test Series 8 - High temperature, High Flow Tests
This test series will be conducted only with those materials that perform satisfactorily in
Test Series 5 through 7. The objective of this test is to simulate as close as possible "real world
conditions. These tests will be done in the high flow facility described in Section 2.3. The test
will encompass the activities described in Test Series 6 and 7.
47
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before cornpleting)
. REPORT NO.
EPA- 600/2 -77 -056
3. RECIPIENT'S ACCESSION-NO.
4. T.TLE AND SUBTITLE Evaluation of Ceramic Filters for High-
Temperature/High-Pressure Fine Particulate Control
5. REPORT DATE
February 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
G.G. Poe, R. M. Evans, W. S. Bonnett, and
L.R. Water land
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Aer other m Corp.
485 Clyde Avenue
Mt. View, California 94042
10. PROGRAM ELEMENT NO.
1AB012; ROAP 21ADL-029
11. CONTRACT/GRANT NO.
68-02-1318, Task 25
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
Task Final; 12/75-6/76
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES j£RL-RTP Task Officer for this report is D.C. Drehmel, Mail
Drop 61, 919/549-8411 Ext 2925.
is. ABSTRACT
repOrt gives results of a study to analyze and evaluate ceramic mem-
brane filters as a new, fine particulate «3 micrometers) control concept for high-
temperature (approx. 900 C), high-pressure processes. Several ceramic filters
were identified as potential candidates for fine particulate removal. There does not
seem to be any inherent material limitation to high-temperature operation; however,
no evidence of high-temperature filter application was found. The filters typically are
2-6 mm thick, cylindrical, and available with various pore sizes, increasing upward
from 0. 5 micrometer. These elements may be suitable for fine particulate control in
hot gas streams. The most promising, although undeveloped, idea for a ceramic fil-
ter is to use ceramic honeycomb monoliths similar to those available for catalyst
supports and heat exchangers. The walls o.f the monoliths are about 0. 2-0. 4 mm
thick and of varying pore size and porosity. Geometric configurations are available
which would force the gas to flow through the membrane walls. Pressure losses
would be very small relative to those of standard ceramic filter elements.. The
application of ceramic monoliths to high-temperature fine particulate control appears
very promising. It is strongly recommended that this concept be investigated further.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Flue Gases
Particles
eramics
Fluid Filters
Membranes
Filtration
Air Pollution Control
Stationary Sources
Particulate Control
High Temperature
High Pressure
13B
21B
11B
13K
11G,06C
07D
3. DISTRIBUTION STATEMENT
Unlimited
9^ SECURITY CLASS (This Report)'
Unclassified
21. NO. OF PAGES
52
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
48
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