Back-flushable filtration device and method of forming and using same

A back-flushable filtration device is provided. The device includes a monolith of porous material having an inlet end and an outlet end. The passageways of the monolith are plugged at the inlet and outlet ends of the monolith, thereby preventing direct passage of feed stock through the passageways from the inlet end to the outlet end. A microporous membrane of mean pore size smaller than the mean pore size of the monolith material covers the surface of the passageways. Methods for making the back-flushable filter as well as methods for using the back-flushable filter also are provided.

BACKGROUND OF THE INVENTION 
The invention relates to a particulate surface filter, regenerable by 
back-flushing, formed from a porous honeycomb monolith structure with 
selectively plugged passageways and microporous membrane coatings applied 
to the passageway surfaces. 
Diesel particulate filters for removal of soot from combustion sources have 
been commercially available for several years, since the early 1980's. 
These devices are fabricated from a porous honeycomb ceramic monolith 
which contains a multiplicity of longitudinal passageways extending 
through the monolith between a pair of opposing end faces where the open 
cross sections of the passageways are exposed. The passageways themselves 
are formed by thin, porous walls which extend continuously between the end 
faces. The passageway densities can range from below 25 passageways per 
square inch to over 1000 passageways per square inch of cross-sectional 
area of the monolith. This monolith structure represents a highly-compact, 
high surface area filter. 
The diesel filter is typically formed from such a monolith structure by 
plugging alternate ends of adjacent passageways. In this structure, 
exhaust gas flow is introduced into open passageways at the inlet face of 
the structure. These passageways are plugged at the downstream end face, 
and thereby gas flow is forced to flow through the porous walls of the 
monolith structure. Carbon soot is collected on and within the walls of 
the monolith structure. The soot so collected is removed by intermittent 
regeneration which is accomplished by thermal oxidation. 
Unless the trapped, particulate matter is removed by combustion, it would 
be extremely difficult to regenerate a diesel filter. For example, 
regeneration by back flushing, a method frequently used for filters which 
function as surface filters, will be highly ineffective because 
particulate matter will have entered and plugged the pore structure. 
Filter structures which function in this manner, so called depth filters, 
are generally single use disposable filters. 
The mean pore size of the ceramic materials used for honeycomb monoliths 
suitable for diesel particulate filters can vary, and is typically in the 
range of from about 10 microns to 50 microns. The pore size distribution 
of such materials is generally quite broad. A relatively large pore size 
is chosen so that the devices have a low pressure drop at the desired gas 
flow rate per unit filter area. The walls of such diesel filters have a 
pore structure which is substantially homogeneous across its thickness, 
and therefore, for the wall thickness typically employed, a finer pore 
size cannot be employed without creating a pressure drop undesirable for 
diesel applications. 
The pore size and pore size distribution in diesel filters are such that if 
used in typical surface filtration applications, particulate matter would 
enter and clog the pore structure. Backflushing to regenerate the filter 
would be ineffective due to such clogging of the pore structure. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a new filtration 
device which is regenerable by back flushing. 
It is a further object of this invention to provide such a filter which has 
a large amount of surface area relative to the volume of the device. 
A still further object of this invention is to provide a method for making 
such a filtration device 
This invention results from the realization that a back flushable filter 
can be fabricated from a large pore size monolith if the filtration 
surfaces of the monolith passageways are coated with a microporous 
membrane coating with a pore size sufficiently small such that particulate 
matter in the fluid to be filtered is removed on the surface of the 
membrane coating and is prevented from entering the pore structure of the 
monolith material. The device has wide utility for filtration of gases and 
liquids for removal of particulate matter of particle size from a few 
microns down to sub-micron size. 
This invention features a filtration device for receiving a feed stock at 
an inlet end face and for separating the feed stock into filtrate and a 
filter cake. The filter is comprised of a monolith of porous material 
containing a plurality of passageways extending longitudinally from the 
inlet end face to an outlet end face of the monolith through which 
filtrate is discharged. A plurality of plugs in the ends of passageways at 
the inlet end face and the outlet end face of the monolith prevents direct 
passage of the feed stock through the passageways from the inlet end face 
to the outlet end face, and a microporous membrane of mean pore size 
smaller than the mean pore size of the monolith material is applied to the 
surfaces of the passageways. 
In one embodiment, the monolith material is a porous ceramic, and may be 
selected from the group including cordierite, alumina, silica, mullite, 
zirconia, titania, spinel, silicon carbide, silicon nitride, and mixtures 
thereof. 
In another embodiment, the membrane is a polymeric membrane, and may be 
selected from the group including cellulose, cellulose acetates, cellulose 
nitrate, polyethylene, polypropylene, nylons and other polyamides, 
polyesters, polycarbonates, polyvinylidene difluoride, 
polytetrafluoroethylene, polysulfones, polyethersulfones, 
polyacrylonitriles, and mixtures thereof. 
In yet another embodiment the membrane is a ceramic membrane and may be 
selected from the group including alumina, zirconia, titania, silica, 
zircon, cordierite, mullite, spinel, silicon carbide, silicon nitride, and 
mixtures thereof, bonded by thermal sintering or with a reactive inorganic 
binder. 
The pore size of the membrane preferably is in the range of from 0.1 micron 
to 5 microns, and the ratio of the mean pore size of the monolith material 
relative to that of the membrane may be in the range of 2 to 500, or more 
preferably 10 to 250. 
In one embodiment the plugs are in alternate ends of adjacent passageways. 
The initial retention efficiency of the filter may be greater than 99% for 
5 micron particles, and preferably, greater than 99% for 0.5 micron 
particles. 
The invention features a method of forming a filtration device from a 
monolith of porous material having a plurality of passageways extending 
longitudinally from an inlet end face to an outlet end face of the 
monolith, including plugging passageways at the outlet end face while 
leaving them unplugged at the inlet end face, thereby becoming inlet 
passageways, plugging other passageways at the inlet end face while 
leaving them unplugged at the outlet end face, thereby becoming outlet 
passageways, and applying a microporous membrane of pore size smaller than 
the pore size of the monolith to at least the surfaces of the walls of the 
inlet passageways. 
The invention further features a method for filtering a feed stock. 
According to this method, feed stock is introduced into a monolith of 
porous material having a plurality of longitudinal passageways and having 
an inlet end and an outlet end. The monolith is constructed and arranged 
so that feed stock entering the inlet end must pass through a monolith 
wall separating longitudinal passageways in order to be discharged from 
the outlet end. The monolith wall, at least on the side in contact with 
the feed stock, is coated with a microporous membrane of mean pore size 
smaller than the mean pore size of the monolith material. A filter cake is 
formed on the microporous membrane during filtration, and flow then is 
reversed to remove the filter cake from the microporous membrane. Further, 
the filter cake removed may be collected.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, the invention includes a monolith 10 in a housing 12. 
The monolith has porous, longitudinal walls 14 forming a plurality of 
longitudinal passageways extending from an inlet end face 16 of the 
monolith to an outlet end face 18 of the monolith. A filter body is formed 
from such a monolith by plugging alternate ends of adjacent passageways, 
thereby creating inlet and outlet passageways. Thus, plugs 20 block the 
outlet end of inlet passages 22 and plugs 24 block the inlet end of outlet 
passages 26. This construction prevents direct passage of a feed stock 
through the passageways from the inlet end face to the outlet end face of 
the monolith through which filtrate is discharged. Instead, feed stock 
entering an inlet passageway from the inlet end must pass through the 
porous monolith walls 14 separating the inlet and outlet passageways in 
order to be discharged. 
A thin microporous membrane 28 is formed on at least the surfaces of the 
walls forming the inlet passageways. The pore size of the membrane is 
smaller than that of the monolith material, and preferably less than that 
of the size of the particulate matter to be removed by the filter. 
As shown in FIG. 2, during filtration, flow (arrow 30) of a feed stock to 
be filtered is introduced into inlet passageways 22 and is forced to flow 
through the microporous membrane 28 and supporting monolith walls 14 
separating inlet and outlet passageways. Particulate matter is retained on 
the surface of the membrane as a filter cake 32. Filtrate is removed at 
the outlet end face 18. 
As shown in FIG. 3, the filter is regenerated by reversing flow (arrow 34), 
that is, back flushing from the outlet end face into the outlet 
passageways, through the monolith walls, 14 and into the inlet 
passageways. The filter cake 32 is detached from the surface of the 
membrane and is swept away in the back-flush fluid exiting from the inlet 
end face. 
While a back flushable filter according to the invention may be plugged at 
alternate ends of adjacent passageways, as shown in FIG. 4, other plugging 
configurations are possible. For example, this includes asymmetric 
plugging configurations, such as shown in FIG. 5, in which passageways 
other than alternate passageways are plugged. 
In FIG. 4a, at the inlet end face 16, alternate passageways are plugged as 
at 24 and become outlet passageways 26; unplugged passageways are the 
inlet passageways 22. At the outlet end face 18, shown in Fig. 4b, those 
passageways not plugged at the inlet end face are plugged as at 20, and 
passageways plugged at the inlet end face are left unplugged. 
In FIG. 5a, a different plugging configuration is shown. At the inlet end 
face 16 in this configuration only 25% of the passageways are blocked by 
plugs 24. At the outlet end face 18, 75% of the passageways 23 are blocked 
by plugs 20. This configuration allows more filtration area for the 
incoming feed stock. 
For each possible plugging configuration there is no passageway which 
remains open from the inlet end face to the outlet end face. That is, once 
a plurality of passageways is plugged at one end face of the monolith, all 
other passageways are to be plugged at the opposite end face. This 
plugging requirement prevents direct passage of the feed stock through a 
passageway from the inlet end face to the outlet end face. 
A passageway could be plugged at both end faces, but it would be isolated 
from each end face and therefore be inactive for filtration. This 
isolation of specific passageways may be practiced for small passageways 
at the outer surface of the monolith, as is shown in FIGS. 4 and 5, to 
isolate passageways which are difficult to coat with the microporous 
membrane because of the reduced size of the passageways. The porous 
monolith can be formed from a variety of porous materials, including 
ceramics, glass bonded ceramics, glasses, sintered metals, cermets, resins 
or organic polymers, papers or textile fabrics, and various combinations 
thereof. Among ceramics are included cordierite, alumina, silica, mullite, 
zirconia, titania, spinel, silicon carbide, silicon nitride, and mixtures 
thereof. These ceramic materials may also be used in monoliths in which 
the ceramic materials are bonded with a glass. 
In order to have a suitably high hydraulic permeability, the mean pore size 
of the monolith material preferably is greater than about 5 microns, and 
the porosity of the material preferably is greater than about 40 volume 
percent. 
The plugs used to seal the alternate ends of the adjacent passageways can 
be polymeric or inorganic, and are normally selected to have good adhesion 
and chemical and thermal compatibility with the monolith material. 
The membrane coating can be formed from a variety of materials, including 
polymeric materials and inorganic materials. Polymeric materials which can 
be used include cellulose, cellulose acetates, cellulose nitrate, 
polyethylene, polypropylene, nylons and other polyamides, polyesters, 
polycarbonates, polyvinylidene difluoride, polytetrafluoroethylene, 
polysulfones, polyethersulfones, polyacrylonitriles, and mixtures thereof. 
Inorganic materials which can be used include sintered metals and ceramic 
membranes. Ceramic membranes can include alumina, zirconia, titania, 
silica, zircon, cordierite, mullite, spinel, silicon carbide, silicon 
nitride, and mixtures thereof, bonded by thermal sintering or with a 
reactive inorganic binder as described in co-pending application Ser. No. 
07/198,195, filed May 24, 1988, and entitled "Porous Inorganic Membrane 
with Reactive Inorganic Binder, and Method of Forming Same", the entire 
disclosure of which is incorporated herein by reference. 
Mean pore size of the membrane coating is preferably in the range of from 
0.1 micron to 5 microns. The thickness of the membrane coating should be 
as thin as possible so as to minimize the hydraulic resistance of the 
membrane coating, and preferably is less than 100 microns. 
The membrane coating may be applied only to the inlet passageways, or 
alternately, it may be applied to both inlet and outlet passageways. If 
applied to both sets of passageways, the clean filter resistance to flow 
is increased. However, a membrane coating on the outlet passageways 
prevents possible plugging of the monolith material by particulate matter 
which may be present in the back flushing fluid. It also permits the 
device to be used as a back flushable filter with flow moving in either 
direction. Also, the membrane coating is more readily applied to the 
passageways before plugging the ends of the passageways. 
The membrane coating may be applied by various techniques, including 
viscous coating, filtration, and slip casting. Viscous coating is useful 
for coating of polymeric membranes. Filtration and slip casting may be 
used to apply coatings of ceramic or metal powders, which are subsequently 
stabilized and made strongly coherent and adherent to the passageway walls 
by thermal sintering, chemical reaction bonding, or other bonding 
techniques. 
It is important that the membrane be a true membrane, and not a partial 
coating. Thus, by membrane it is meant that a continuous coating be formed 
over the monolith surfaces such that access to the pores in the monolith 
is only via the membrane. Most preferably, the membrane covers the surface 
of the monolith but does not enter the pores of the monolith to any 
substantial degree. This is believed to be accomplished using the coating 
materials and methods described in greater detail in the Examples below. 
The filter may be used to filter either a gaseous or liquid feed stock. In 
either instance, the clean filter flow resistance is to be minimized. This 
is achieved by proper selection of a monolith material with sufficiently 
large pore size and porosity so that the monolith material has a high 
hydraulic permeability. The resistance of the membrane coating is kept 
small by controlling membrane pore size, porosity, and thickness. The 
preferred membrane pore size is greater than about 0.1 micron and less 
than 5 microns; the preferred porosity is greater than 40 volume percent; 
and the preferred membrane coating thickness is less than 100 microns. The 
ratio of the mean pore size of the monolith material relative to that of 
the membrane coating is generally in the range of 2 to 500, and preferably 
in the range of from 10 to 250. The ratio of the thickness of the 
passageway walls of the monolith to the membrane coating is generally in 
the range of 2 to 100, and preferably in the range of 5 to 50. 
The filter is regenerated by back-flushing with a fluid normally free of 
particulate matter. In many instances, the fluid used for back-flushing 
can be filtrate produced from the feed stock. 
The following examples provide a comparison of permeability and separation 
efficiency for a monolith filter without a membrane coating and a monolith 
filter with a membrane coating according to this invention. 
EXAMPLE 1 
A six inch long cordierite monolith with a square cross section about 0.75 
inches on a side was cut from a larger monolith sample obtained from 
Corning Inc. (Corning, N.Y.). The monolith material was EX66 which has a 
50% porosity and a 35 micron mean pore size. The passageway configuration 
was 100 square passageways per square inch, uniformly spaced. The 
passageway side dimension was about 0.075 inch and the wall thickness was 
about 0.025 inch. The monolith, as cut, had 49 parallel passageways, 7 on 
a side. 
Polyvinylchloride end rings were glued onto each end of the monolith as 
sealing surfaces. A silicone adhesive, RTV41 (General Electric Company, 
Waterford, N.Y.) was used. After gluing on the end rings, only 25 
passageways were available to be used. These remaining passageways were 
plugged to make a dead ended filter. A total of twelve alternate 
passageways on the inlet face of the device were plugged with the silicone 
adhesive. Passageways which were open on the inlet face (a total of 
thirteen) were plugged at the outlet face. Fluid to be filtered was 
thereby constrained to flow through the porous passageway walls. There was 
about 0.16 square feet of wall passageway area for fluid filtration. 
The dead ended filter was tested for pressure drop at a fixed nitrogen gas 
flow . A pressure drop of 0.5 inches of water was measured at room 
temperature and 2.5 feet per minute face velocity through the filter. 
After this gas flow test, the initial retention efficiency of 5 micron 
alumina (Norton Company, Code 7921) suspended in water was measured by 
determining the turbidity of the feed stock and the initial filtrate 
(approximately first 50 cc of filtrate) in a filtration test. The test was 
conducted at room temperature and a feed stock flow of about 500 cc/min. 
The initial retention efficiency was 17% for a feed suspension with a 
turbidity of about 1000 NTU. 
EXAMPLE 2 
A cordierite monolith identical to that in EXAMPLE 1 was coated on all 
passageways with a ceramic membrane by slip casting generally in 
accordance with the methods taught in U.S. patent application Ser. No. 
07/198,195. The membrane composition in weight percent as fired was 75% 
TAM zircon milled fine (TAM Ceramics Inc., Niagra Falls, N.Y.) and 25% 
glass frit P941 (Pemco Products, Industrial Chemicals Division, Baltimore, 
Md.). The membrane thickness was measured by scanning electron microscopy 
to be about 50 microns and the membrane porosity was estimated to be about 
40-50 vol. %. The ratio of monolith wall thickness to membrane thickness 
was about 13. 
End rings were glued on the specimen and its passageways were plugged 
identically to the specimen of EXAMPLE 1. 
The dead-ended filter with the membrane coating was tested for pressure 
drop at a fixed nitrogen gas flow. A pressure drop of 4 inches of water 
was measured at room temperature and 2.5 feet per minute face velocity 
through the filter. 
After this test, the initial retention efficiency of 5 micron and finer 
alumina particles (Norton Company, Code 7920) suspended in water was 
measured as before. For feed stocks with approximately 1000 NTU, the 
initial retention efficiency was 99.8% for 5 micron alumina, 99.7% for 3 
micron alumina, and 99.4% for 0.5 micron alumina. The filter was 
regenerated between tests by thoroughly back flushing with water. 
Another test was performed with an aqueous dispersion of a monodisperse 
polystyrene latex of particle size in the range of 0.35 to 0.55 micron 
(Dow Chemical Company, Midland, MI, type DL247A). Latex initial retention 
efficiency for a feed stock with about 1300 NTU was 25.2%. 
Based on these retention data, the membrane mean pore size was estimated to 
be about 0.2 to 0.5 micron. The ratio of mean pore size of the monolith 
material to that of the membrane coating was estimated to be about 70 to 
175. 
Another test was conducted for the filtration of flour suspended in air. In 
this test flour was dispersed in air and aspirated by a vacuum through the 
filter. The filter was backflushed by reversing flow through the filter 
and the test was repeated for several cycles. During the filtration cycles 
the filtrate was visibly free of dust. No apparent blockage or plugging of 
the filter was observed over the filtration and regeneration cycles. 
Although specific features of the invention are shown in some drawings and 
not others, this for convenience only as each feature may be combined with 
any or all of the other features in accordance with the invention. 
Other embodiments will occur to those skilled in the art and are within the 
following claims: