CA1188233A

CA1188233A - Honeycomb filter

Info

Publication number: CA1188233A
Application number: CA000421869A
Authority: CA
Inventors: Wayne H. Pitcher, Jr.
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1982-02-22
Filing date: 1983-02-17
Publication date: 1985-06-04
Also published as: US4417908A; EP0089751A1; EP0089751B1; DE3371290D1; JPH0349608B2; JPS58196820A

Abstract

ABSTRACT
The capacity and operating time of a solid particulate filter of honeycomb construction may be increased by forming the filter with a matrix of thin, interconnected walls which define a multiplicity of cells and which have interconnected open porosity of a volume and size sufficient to enable the fluid to flow completely across the thin walls in their narrow dimensions between adjoining cells and through the thin walls in their longer dimensions between adjoining or neighboring cells and to restrict the particulates in the fluid from flowing either completely across or through any of the thin walls and by dividing the cells into an inlet group and an outlet group and providing the inlet group of cells with substantially greater collective thin wall surface area than is provided to the out-let group of cells.

Description

Pitcher 7 IMPROVED HONEYCOMB FILTER

s~e~
The invention relates to filters for trapping solid particulates present in fluid streams and, in particular, to filter bodies formed from thin porous walled, honeycomb st-uc-tures~
~he removal of solid particulates from fluids, gases or liquids, in which the particulates are mixed is typically accomplished by means of filters made from solid materials which are formed into articles or masses having a plurality of pores o small cross-sPctional size extending therethrough~ which may be interconnected, such that the solid materials are both per-meable to the fluid~ which flow through the article or mass and capable of restraining most or all of the particulates mixed in the ~luid from passing through the article or mass, as desired.
Such pores constitute what is termed "open porosity" or "acces-sible porosity". The restrained par~iculates are collected on the inlet surface(s) andJor within the pores of the material.
~he minimum cross-sectional size of some or all of the pores can be larger than the size of ~ome or all of the particulates to be removed from the fluid, but only to the extent that sig-nifîcant or desired amounts of sufficiently large particulates become trapped on or within the filters during the transit o~
contaminated Eluid. As the mass of collected particulates in-creases, the flow rate of the fluid through tne ilter generall~
decrease3 to an undesirable level. The filter is then either ,.~

~ 3 3 discarded as a disposable, replaceable element or regenerated by suitably removing the collected particulates so that it may be reused~
Certain considerations are helpful in evaluating and comparing filters. One is filter efficiency, that iso the amount of the suspended particulates of concern that are removed from the fluid as it passes through the filter (usually express-ed as a percentage of the weight of all particulates in the 1uid prior to its passing through the filter). Another con-sideration is flow rate, that is the volume of fluid per unit of time that passes through the filter and collected particu-lates~ In a closed, continuous feed system, a comparable con-sideration is back pressure, i~e. the fluid pressure upstream from the filter which depend~ upon the downstream fluid pressure and the filter presure drop, i~e. the difference between the fluid pressure upstream and downstream from the filter caused by the presence of the filter and particulates thereon, and which is itself dependent upon the flow rate. ~et another consideration is operating tim~, the cumulative time of service of a filter before its flow rate or the back pressure becomes unacceptable so as to necessitate replacement or regeneration of ~he filter. Yet another consideration is capacity, the amount of particulate that the filter can hold while still providing a minimum acceptable flow rate or maximum acceptable back pressure. Other desirable features of the filter includ compact structure, mechanical integrity, inertness or non-dele-terious reaction with the fluid and/or particulate material.

~ 3 3 It has been known for some time that honeycomb struc-tures having a plurality o hollow passages or cells extending through them, which are formed in whole or in part by thin porous interconnected walls, may be successfully employed in filtering applications. U.SO Patents 4,060,488, and 4/069,157 describe fluid filtering apparatus constructed by applying to a porous support body such as a thin porous walled honeycomb structure, a suitable filtering membrane. The membrane allows a ~eparable component of a fluid solution or mixture trans-versing the honeycomb structure passages supporting the membrane to pass into the porous support structure. The filtrate fluid migrates through the open porosity of the support structure to an internal reservoir or to an outer surface of the structure where it is removed.
~.S. Patents 4,041,591 and 4,041,5g2, assigned to the assigne~ hereof, describe multiple ~low path bodies and methods for fabricating the sa~e from honeycomb st,-uctures having col-umns or layers of hollow cells which extend in a substantially mutually parallel fashion through the structure and which are formed by a plurality of thin intersecting walls. The structure is modified so that two separate fluids may be transported through alternate columns or layers of cells. It is suggested that the described multiflow path body might optionally be used in iltration and osmotic separation applications by using porous materials to produce the original honeycomb structure.
As described, the device would function like those of the afore-said U.S. Patents 4,060,488, and 4,069,1i7 which allow only a -- -- . . ~ ~

~ ~8 ~3 ~

fraction of a fluid which is passed through the alterna~e columns or layers of cells ~o migrate across the thin porous walls into the adjoining interleaved columns or layers of cells while the remainder of t~e fluid, having a higher concentration of contam-inant or separable constituent, can continue to pass through and exit the structure.
Porous walled honeycomb structures may also be used directly ~i.e~ without a filter membrane or other covering~ to filter all fluid passed through the structure. Canadian applica-tion Serial No. 380,875, filed June 30, 1981, and assigned to the assignee hereof, and U S. Pakent 4,~76l071 both describe fil~ers formed from honeycomb s~ructures, themselves formed a multipli-city of thin, porous, intersectins walls which define a pair of open, opposing end faces and a multiplicity of hollow, substan-tially mutually parallel passages or cells extending longitudi-nally through the structure betw~en the end faces. At each end face where the open transverse areas of the cells ~ould normally be exposed, the ends or alternate cells are closed in a chec~er-ed or checkerboard pattern.
The pattern is reversed at either end face so that each cell of the structure is closed at only one Pnd face and shares common thin walls with adjoining cells which are closed only at the opposing end face of the structure. A
contaminated fluid is introduced under pressure to a ~inlet"
end face of the filter body and passes into those cells open at the inlet end face ( n inlet" cells). Because the inlet cells are closed at the opposing, "outlet~ end face of the structure~

~ 3 3 the contaminated fluid is forced to pass acros~ the narrow dimension of the thin, porous walls into the adjoining "outlet"
cells lwhich are all closed at the inlet end face and open at the outlet end face of the structure) and tbrough them from the outlet end face of the filter. All or substantially all of the solid particulate matter in the fluid is deposited on the thin wall surfaces defining the interior of the inlet cells or is trapped in the pore~ Eorming the open porosity of the thin walls. All of the contaminated fluid passing through the struc-ture is filtered and all of the internal, thin walls of the structure, each Oc which is shared in co~mon between an adjoin-ing inlet and outlet cell, are utilized in filtering.
U.S~ Patent 4,276,071 also describes 2 second filter embodiment formed from a crossflow honeycomb structure in which layers of longitudinally extending and laterally extending pas-sages are alternately stacked. Fluid containing solid particu~
late contaminant is passed into either end of one of the two commonly extending sets of passages and is recovered after fil-tering from the remaining set of interleaved, commonly extending passages. As only the thin walls separating the laterally and longitudinally extending layers of cells are permeable to the fluid, filtration reportedly occurs only through those thln walls which are shared in common between adjoining inlet and outlet passages. Accordingly, this device is only about one~
half as efficient as a comparably sized and dimensioned honey-comb structure incorporating the previously described checkered pattern of al.ernate cells.

Hy providing a filter of honeycomb structure with thin walls having at least a minimum requisite amoun~ of inter connected open porosity, filtration can be achieved through all thin walls defining each inlet cell irrespective of the type of adjoining cell (i.e. inlet or outlet) with which the inlet cell shares its defining thin walls. Such filters, however, do not generally have as much particulate capacity or useful operating time as comparable (i.e. same geometric configuration, dimensions and material composition) checkered plugged filters.

10 SUMMARY OF THE :I:NVENTION
It is an object of the invention to provide a filter of honeycomb structur~ having greater capacity than a comparable checkered plugged filter.
It is yet another object of the invention to improve the useful operating time of the filter of honeycomb structure by reducing the rate at which solid particulate contaminant is built up on the inlet cell surfaces of the filter.
Thus, the present invention provides a filter for removing all or substantially all of the solid particulates in fluids passed through the filter comprising a multiplicity of interconnected thin porous walls defining inlet and outlet end faces of the filter and a multiplicity of cells, each cell extending through the filter from at least one of the inlet and outlet end faces and having a surface area defined by surfaces of the thin walls exposed within the cell. The thin walls contain interconnected open porosity of a volume and size sufficient to enable the fluid to flow across the '~,;~r~

narrow dimension of the thin walls between adjoining cells and through the longer dimensions of the thin walls bPtween adjoining or neighboring cells and to restrain at least a significant portion o~ the solid particulates from passing either across or through any of the thin walls. An outlet group of cells is provided, each cell open at an outlet face and closed where adjoining any inlet end face. An inlet group of cells is provided each cell open at an inlet end face and closed where adjoining any outlet end face. The collective thin wall surface area of the inlet group of cells is signifi-cantly greater than the collective thin wall surface area of the outlet group of cells.
In another aspect, the invention provides a method of fabricating a filter for removing solid particulates from fluid streams comprising the steps of providing a honeycomb structure formed from a matrix of thin, intersecting porous walls which define a pair of open end faces and a multiplicity of hollow cells extending in a substantially mutually parallel fashion, closing a first group of cells near their end portions at one end face and the remaining cells near their end portion at the remaining end face, the interconnected open porosity of the thin walls being of volume and size sufficient to enable fluid flowing into the first group of cells to flow completely across the narrow dimensions of the thin walls into adjoining cells of the second group and through the thin walls in their longer dimension to adjoining or neighboring cells of the second group and preventing at least a significant portion of the solid particulates from passing completely across or through the walls in any of their dimensions. An improvement comprises the step of further - 6a -8~33 closing said cells to p.ro~ide substantially greater collective thin wall surface area to the inlet group of cells than the col lective thin wall surface area provided the outlet ~roup of cells.
In another aspect, the present invention provides a compact through flow exhaust particulate filter element for diesel engines said element comprising a ceramic monolith having a plurality of interlaced, thin gas filtering porous internal walls defining a plurality of parallel passages extending to opposite ends of the element, said passages including a first group comprising inlet passages open at one end of the element and closed at the other and a second group comprising outlet passages closed at said one ena of the element and open at the other, the lnlet passages having collective cross-sectional areas substantially greater than the respective collective cross-sectional areas of the out-let passages. ~n improvement is provided comprising inletpassages adjoining one another and sharing in common defining interlaced, porous internal walls, and the plurality of interlaced thin porous internal walls having internal interconnected open poxosity of a volume and size sufficient to enable the fluid to flow across said walls in their narrow dimensions and through said walls in their longer dimensions and to restrain at least a significant portion of the particulates from passing either completely across or through the walls in any of their dimensions.
In accordance with certain aspects of the claimed invention, a filter of honeycomb structure comprises a multiplicity of interconnected thin porous walls defining at least one inlet face and at least one outlet end face of the filter and a multiplicity of cells each extending 1~8~

through the filter ~rom at least one of the inle~ or outlet end facesr The thin walls are formed with interconnected open poro~ity of a size a~ volume sufficient to enable the fluid to flow completely across the narrow dimension of the thin walls and completely through the longer dimensions of the thin walls and to also restrain at least a significant portion of the solid particulates in the fluid from passing either completely across or through the thin walls. Open porosity of at least about 25~ and, preferably, o at least about 35~ by volume is provided to achieve the nacessary fluid flow through the longer dimensio~s of the thin walls. The open porosity may be provided by pores having a mean pore diameter as small as about 0.1 micron although larser pores are typically used. Both volumetric open porosity and mean pore size are determined by conventional 1~ mercury intrusion porosimetry.
All or subs.antially all of the cells are divided into ~n inlet group of cells, each of which is open at at least one inlet end face of the structure and closed where it may adjoin outlet end faces of the structure, and an outlet group, each of which is open at at least one outlet end face of the struc-ture and clo~ed where i. m~y adjoin inlet end f~ces of the structure. Most importantly, the colle_tive ~urface area of the thin walls defining the inlet group of cells is significant-ly greater than the collective surface area of the thin walls defining the outlet group of cells. Desirably, the collective surface area of the inlet group of cells should be at least about 25% greater than the collective surface area of the outlet group of cells and, preferably, about 2 to ~ times ~ 3 3 greater than the collective thin wall surface area of the out-let group of cells.
A significantly greater portion of the total collec-tive cell wall area is provided to the inlet group of cells by positioning at least a portion of the inlet cells adjoining one another whereby the adjoining inlet cells share the same defin-ing thin wall in common. Exemplary configura~ions are described wherein all or substantially all cells of the inlet group share at least one and, typically, two or more of their defining thin walls in common with other adjoining cells of the inlet group or, from a different perspective, share at least 33~ and, tvpi ca~ly, at least 50~ or more of their defining thin walls in common with other adjoining inle. cells. Significantly greater collectivs inlet than outlet cell wall thin wall sur~ace area lS may be provided by providing a greater number or inlet cells than the number of outlet cells provided or by differentially sizing the transverse cross-sectional areas of the inlet and outlet cells, or both.
Filter embodiments according to the present invention ~0 having open pore volumes preferably between approximately 40 and 70% ~ormed by pores having mean diam2ters of between about 1 and 60 microns and preferably, betw*en about 10 and 50 microns, and with thin walls less than about 0.060 in. (1.; mm.) and preferably between about 0.010 and 0.030 in. (.~5 and .76 mm.) thick are used to remove particulate matter from diesel engine exhaust gases.

RIEF DESCRI?TION OF T~E DRAWINGS
Various embodiments of the invention will now be de-scribed with reference to the accompanying figures in which:
Figs. la, lb, and lc depict various views of an exem-plary embodiment filter of honeycomb structure according to the present invention;
Fig. 2 depicts dia~rammatically an end face of an exemplary honeycomb filter employing the checkered plug~ins pattern depicted in U.S. Patent 4,276,071 and pending Canadian application Serial No. 380,875 filed June 30, 1981.
~igs. 3 through 17 depict diagrammatically end ace views of other alternate embodiments of the presen' invention;
Fig. 18 depicts an exemplary diesel particulate fil-tering apparatus incorporating a filter of honeycomb structure according to the present invention; and Fig. 19 depicts a crossflow type honeycomb fllter utilizing the present invention.

DETAILED D~SCRIPTION OF THE INVE~TIO~
According to the invention, ~n improved filter of thin, pcrous walled honeycomb construction is provided by prc-viding a total collective inlet cell thin wall surface area which is relatively greater than the total collec~ive outlet cell surface area provided; Figs. la, lb, and lc provide three views of an exemplary filter 2~ of honeycomb structure accordir.~
to the present invention. The filter 20 is cylindrically shaped and comprises a multiplicity of thin, porous interconnec'in~

_ g _ walls 22 which define a pair of substantially identical circular end faces 24 and 26 on opposite sides of the filter 2Q and a multiplicity of hollow, open-ended passages or cells 28 and 29 which extend in a substantially mutually parallel fashion sub-~tantially longitudinally through the filter 20 between the end faces ~4 and 26. One end of each of the cells 28 and 29 is plugged, covered or otherwise closed by suitable means near one or the other of the end faces 24 or 26 to provide groups of inlet and outlet cells 28 and 29, respectively, as is better seen in F g. lb, which is a longitudinally sectioned view of tAe filter 20 of Fig. la exposing a vertical column of inlet cells 28, which are open at the inlet end face ~4 and closed by plugs 32 at the outlet end face 2~, and outlet cPlls 29, which are open at the outlet end face 26 and closed by additional l; plugs 32 at the inlet end face 24, of the filter 20. As each of ~he cells 28 and 29 has been plugged at only one end thereof, the pattern of plugged cells visible on the inlet end face 24 o~ the filter 20 in Fig. la is reversed on the opposing outlet end .ace 2~ (hidden in Fig. la).
The thin walls 22 can be made of any suitable material provided they have internal interconnected open porosity of at least 25~ and preferably at least 35~ of the bul~ volume of the thin walls. This provides sufficient interstitial pore channels to allow the fluid to flow completely across their narrow dimen-sion (e.g. between adjoining inlet and outlet cells) and most impor~antly, through their lo~ger dimensions (e.g. between an inlet cell and an adjoining or neighboring outlet cell or the outlet end face 26~ and to prevent at least a significant por i tion of the particulates from flowing either completely across or through the thin walls 22 in any direction. Thin walls 22 are typically formed with the same uniform thickness (i.e. their narrow dimension), although the thickness of the thin walls may be varied to achieve desired flow rates therethrough. Each cell 28 and 29 is defined by a subset of intersecting thin walls 22 or by a portion of the skin 30 and a subset of the thin walls 22~ Each of the thin walls 22 is shared in common by a pair of 10 adjoining cells 28 and/or 29 with each of the opposing outer surfaces of each thin wall 22 forming an inner surface 33 or 34 of an inlet or outlet cell 28 or 29, respectively.
By forming each subset with uniform thicknesses which differ between the subsets, the flow rates through each subset of walls may be controllably varied.
Fig. 2 depicts an inlet end face 124 of an exemplary honeycomb fil-er 120 employing the checkered inlet/outlet cell pattern referred to above in the BACKGROUND OF THE INVENTION
and described in the aforesaid Canadian appln. Serial No. 380,875 20 and U.S. Patent 4,276,071. Inlet and outlet cells 128 and 129, respectively, are alternated with one another sc that each inlet cell 12B shares thin walls 122 in common only with adjoining outlet cells 129 and vice versa~ This requirement that each type of cell share thin walls with only the other type of cell results in the filter 120 having substantially equal collective inlet and outlet cell thin wall surface areas. Those familiar , ~ 3 with honeycomb filter art will appreciate that the back pressure of a filter of honeycomh structuré is determined by sev~ral contributing factors includin~ thin W2l1 chara~teristics (s~-dth, open porosity, mean pore size, etc.), i~let and outlet cell characteristics (cell density and sizes, inlet/outlet cell ratios, relative sizes and arrangement, etc.) and particulate characte~istics (rate of deposit, effective porosity, etc.).
ASYmmQtriCa11Y apportioning the thin wall area of a filter of given geometry in favor of greater collective inlet cell surface area typically reduces its effective capacity and increas~s its initial back pressure. However, I have appreciated that by providing thin walls havins the described requisite open poros~
ity this increase in initial back pressure can be offset by a decrease in the contribution to back pressure due to particulate ~uildup~ as the particulates may be filtered through all inlet cell wall surfaces and thus, spread over a relativ~ly greater area. As a result, the capa~ity of a honeycomb filter of given conf~guration and having the requisite porosity in its thin walls may be increased by asymmetrically appor~ioning more of the thin wall surface area to the inlet than to the outlet cells. This, in turn, effectively increases the useful operat-ing time of the filter.
Thin wall surface area may be asymmetrically appor-tioned in several ways. For e~ample, as substantially all of the inlet and outlet cells 28 and 29 of the exemplary filter 20 of Figs. la - lc have the same surface area ( i .e~ the same length, transverse cross-sectional geometry and size) subs~an-tially greater collective inlet cell thin wall surface area as compared to collective outlet cell surface area has been provid-ed ~y creating more inlet cells 28 than outlet cells 29. Approxi-mately 67% of the cells and collective cell surface area of the filter 20 of Figs. la - lc are inlet cells 28 and inlet cell surface areas 33, the remaining 33~ being outlet cells 29 and outlet cell surface areas 34. Thus, the collective thin wall surface area of the inlet cells 28 is about twice that of the outlet cells 29.
Figs. 3 through 17 depict diagrammatically various in-let end face patterns of inlet cells 228 and outlet cells 229 extending longitudinally through a filter 220 of honeycomb st.ucture between an inlet end face 224 (depicted in part) and an opposing outlet end face ~not depicted), in a fashion similar to the cells 28 and 29 of the filter 20 of Figs. la - lc. The depicted inlet/outlet cell patterns provide collective inlet cell surace areas substantially greater than the collective outle~ cell surface areas provided. The inlet end face 224 and an opposing outlet end face (hidden) and the plurality of cells 228 and 229 are again formed by thin interconnected walls 2~2 having the previously described re~uisite open porosity. Again each filter 220 ~ay be provided with a skin if desired around the cells 228 and 22~ between the inlet 224 and outlet end faces. The outlet cells 229 have been shaded to indicate their closure by plugs or other suitable means near the inlet end ~aceO The outlet cells 228 are again closed near the opposing outlet end face. The inlet end face patterns depicted in Figs.

~ 33 3 through 17 would be repeated across the entire inlet face 228 of the filter 220 and reversed across the entire outlet end face (not depicted) of each filter 220~
As can be ~een from the Figs. 3 through 17, various cellular transverse cross-sectional geometries may be employed în practicing the invention. In add ition to the s~uares, rec-tangles, triangles and hexagons depicted, other equilateral shapes such as, for example, pentagons and octagons, other polylateral shapes such as, for example, rhomboids, and even continuous curved shapes such as, for example, circles and elipses, or combinations of linear and curved wall cell shapes may be utilized. In accordance with the teachings of the afore-said Canadian application Serial No. 380,875, the included angles formed by and ~etween the intersecting thin walls (or adjoining thin wall sections in the case of continu-ously curved cell geometries) are preferably greater than about 60 to provide sufficient access of the fluid to all interior cell areas.
In Figs. 3 through 5 and 7 through 15 the cel's 228 and 229 have been formed with transverse cross-sectional geo-metries of the same uniform size and shape. Substantially greater collective inlet cell to collective outlet cell thin wall surface areas are provided, as in the case of the embodi-ment of Figs~ la - lc by providiny a greater number of inlet 228 than outlet cells 229. Alternatively, greater collective inlet cell to outlet cell thin wall surface areas can be provld-ed by varying the sizes and hence the individual thin wall ,i~i 8f~33 surface areas of the inlet 228 and outlet cells ~29, (i.e. as is depicted in Figs~ 6 ar.td 17) or by varying bQth the numbers and si2es of the inlet 228 and outlet ~79 cells ~i~e. as l.s depicted in ~ig. 16). Figs. 7 through 11 and 15 depict uniform cellular geometry in inlet and outlet cell patterns that provide approximately 67% collective inlet and 33~ collective outlet cell thin wall surface areas. In Fi~. 6, if the width of the larger, square cells is twice that of the narrower, rectangular cells, approximately 57~ collective inlet and 43~ collective outlet cell thin wall surface areas are provided. If the same ra~ios are maintair.ted in Fig. 16, approximately 73% collective inlet and 27~ collective outlet cell thin wall surface areas are providedO Figs. 3 through 5 and 12 through 15 also depict dif~erent ratios ~f uniformly sized and shaped inlet and outlet lS cells, 228 and 229, the cells in Figs. 3, 4, 12, and 13 provid-ing approximately 75% collective inlet and ~5~ collective out-let cell thin wall surface areas, in Figs. 5 and 15 providing approximately 80% collective inlet and ~0~ collective outlet cell thin wall surface areas, and in Fig. 14 providing approxi~
mately 89% collecti~e inlet and 11~ collective outlet cell thin wall surface areas.
The thin walls 22 and 222 can be made of any suitable material that pro~ides the aforesaid requisite interconnected open porosity including powdered metals, glasses~ ceramics (generally crystalline), resins or organic polymer~, papers or textile fabrics ~with or without fillers), etc and combincttions thereof includlng, for example, glass-ceramic mix.tures and ~L8~3~33 cermets. It ls preferred to fabricate the thin walls ~Z and 222 from plastically formable and sinterable finely divided particulates and/or short leng~h fibers of substances that yield a porous ~intered material after being fired to effect the sinterin~ thereof, especially powdered metals, glasses, ceramics, glass-~eramics, cermets or other ceramic based mix-tures. In addition to volatizable plasticizers and/or binders, which may be used to prepare a workable batch mix~ure, any suitable or conventional fugitive or combustible (burn-out) additive can be dispersed within the formable and sinterable mixture so as to provide appropriate and ade~uate interconnected open porosity in the sintered intersec~ing thin walls 22 and 222. The requisite open porosity can be designed into the thin walls 22 and 222 by raw material selection as described in U.S. Patent 3,950,175. Although the matrix of thin walls 2~ and 222 may be fabricated by any suitabie technique for the material selected, it is preferabl y formed monolîthically with a skin 30, by extrusion from a sinterable mixture in a manner as disclosed in U.S. Patents Z0 3,790,654, 3,919,384, and 4,008,033 and a U.S. Patent No.
4,364,888, filed May 4, 1981, and assigned to the assignee hereof.

The cell ends may be closed with any material and in any marner compatible with the material of the thin walls 22 and 2Z2 under the envisioned service conditions of the filter 20 or 220. This includes non-deleterious reaction to the thin ~ 3 3 wall material and the contaminated fluid, either singularly or in common, good adhesion to the thin walls, suitable durability to the fluid flsws at the desired flow rates, similar coeffi-ci~nts of thermal expansion ( if the filter is to be used at elevated temperatures)~ etc. Typically, plugs 32 are formed by chargin~ a flowable or formable plugging material into selected cell ends and then performing steps such as curing, drying, firing, etc. which transform the formableJflowable material into a solid closure which adheres mechanically and/or chemically to the thin walls 22 or 222 and completely covers or f ills the cell endO The plugs 32 or oth~r closure means may be either porous or nonporous, al though in the former case, the open porosity (i.e. pore volume and mean pore size) should be su~fi-ciently small so as to prevent the passage of higher than desir-1~ ed levels of the solid particulates through or around the plugs 32 and hence through the filter 20 or 220.
Where z sinterable honeycomb matrix is used, com-patible sinterable ce~ent mixtures are typically used to for~
the plugs 32 in the selected cell ends. Methods and apparatus for charging ~lowable and/or ormable materials such as sin-terable cement mixtures into selected cell ends of a rigid honeycomb structure are described in the a~oresald Canadian application Serial No. 380,875 and in the following cases which are assigned to the assignee hereof: Canadian Serial No. 405,931, filed June 24, 1982; published European applica-tion 82/303724; and U.S. Patent No. 4,411,856, and ~i~l8~
Canadian Serial No. 405,930, filed June 24, 1982, and U.S.
Patents 4,427,728 and 4,432,918, both filed August 24, 1981.
Generally, these methods and/or apparatus may be used with either green (i.e. dried but not sintered) or sintered honeycomb matrices or with other rigid, non-sinterable honeycomb structures. Sinter-able cement mixtures may also be used with green (i.e. sinterable) honeycomb matrices, if their sintering temperatures are suffi-ciently close to that of the matrices, as is described and claimed in another U.S. Patent No. 4,455,180 filed August 24, 1981, or a cement having a lower sintering temperature may be charged into a previously fired structure having a higher sinter-ing temperature, as is described in the aforementioned Canadian application Serial No. 380,875. Dimensional changes (typically shrinkage or possibly expansion) of a sinterable substrate and/or plugging material(s) upon drying and/or sintering may be com-pensated for by the use of a plugging material which foams when fired during sintering, such as the foam-type ceramic cements described in U.S. Patents 3,189,512 and 3,634,111 and in U.S~
Patent No. 4,297,140~ filed July 3, 1980 and assigned to the assignee hereof, or a plugging material which undergoes an appropriate net relative expansion with respect to the honevcomb structure, such as is described and claimed in the aforesaid : U.S. Patent No. 4,455,180.

~ 3~

Figs. lb and lc illustrate fluid flow through and across the thin walls 22. Similar flow will occur in the other described em~odiments of- the invention~ Again, ~ig. lb depicts a vertical column o the cells 28 and 29 of the filter 20 of Fig. la. Inlat cells 28 (open at the inlet end face 24 and closed at the outlet end face 26) and outlet cells 29 (closed at the inlet end face 24 and open at the outlet end face 2~) are in~erspersed ~ith one another along the column. Fig. lc is an expanded interior transverse sectioned view of the filter 20 of Figs. la and lb, depicting the cross sections of several inlet and outlet cells 28 and 29, respectively, and the inter-connected thin walls 22 definins those cells . Contaminated fluid, represented by the arrows 36, is introduced to the filter 20 at its inlet end face 24 and enters through the inlet cells 28. Where an inlet cell 28 shares a thin wall 22 in co~mon with an adjoining outlet cell 29 (such as, for example, the thin wall 22a shared by adjoining inlet and outlet cells 28a and 29a, respectively, in each of Figs. lb and lc) primary fluid flow is across the thickness ( i.e. narrow dimension) of the thin walls 2?a, as is indicated by the lined arrows 38.
Where a thin wall 22 is shared in common between a pair of inlet cells 28 ( such as, for example, the thin wall 2~b between the adjoining inlet cells 23a and 28b), primary fluid flow ls through the longer dimensions or those thin walls ~i.e. the oute~ surfaces 33 of the thin ~alls 22 forming the inner sur-faces of the lnlet cells 28a and 28b and 7 nto other intercon-nected thin walls ~ forming adjoining or neighboring outlet ~8~33 cells 29, as is indicated by the arrows 40 in Fig. lc. Due to the interconnected open porosity, some fluid flow also occurs through the thin walls 22 in their remaining longer dimension (i.e. in the horizontal direction through the thin walls 22 in Fig. lb and in directions normal to the plan o~ Fig.
lc) between the inlet cells 28 and the outlet cells 29 or the end face 26 or both. If the walls 22 are of uniform width (i.e. thickness), the incoming fluid will flow at a greater rate between those thin walls 22 shared in common between adjoining inlet and outlet cells 28 and 29, respectively (i.e. the flow indicated by the arrows 38 in Figs. lb and lc) as that pathway offers the least initial resistance. As a layer of solid particulate begins building up on those thin wall surfaces of the inlet cells 28, back pressure across those walls increases and fluid flow rates into those walls tends to equalize to the flow rates into the thin walls 22 between adjoining inlet cells 28, as represented by the lined arrows 40. Flow through the various thin walls 22 forming the inlet cells 28 will constantly adjust as solid particulate is built up on any particular inner wall surface 33 or portion of an inner wall surface 33 of an inlet cell 28.

The inlet end face of an exemplary preferred embodi-ment of the subject invention is depicted in Fig. 17.
As was previously stated, the filter 220 is of honeycomb ~ 3 3 structure and is provided with a first subset of inlet cells 228 (plug~ed at the opposing, hidden outlet end face of the filter 220) having ~u~stantially larger individual transverse cross-sectional areas than the areas of a subset of outlet cells 229 closed (indicated by shading) with appropriate means at the depicted inlet end face of the filter 220. The outlet cells 229 are located at ends of the thin walls 222b lying between adjoining inlet cells 228 and provide increased thermal shock r~sistance to the structure as compared to simple polylat-eral cell shapes such as squares, rectangles, other parallelo-grams and triangles formed ~y intersecting subsets of parallel thin walls extending in continuous planes across the end ~aces.
In addition, the thin walls 22~a forming tne outlet cells 229 are different in thickness from the walls 222b separating and defining adjoining inlet cells 2~8 so as to penmit differential fluid flow across and through the thin walls 222a and 222b, respectively, to equalize flltration on all inl~t cell surfaces 33 or to maximize flow rates through the filter 220.
The comoosition and physical parameters of the thin walls 22 and 222, including their dimensions, percent, open porosity and mean pore sizet will vary to satisfy the userts o~erating requirements such as filter strenath, durability and efficiency; contaminant size and concentration; fluid flow rate, density and ~iscosity; etc. Envisioned uses of filters accordiny to the present invention include exhaust gas and molten metal filters and heat recovery wheels, as are described in the aforesaid Canadian application Serial No. 380,875.

- 21 =

~ 8~ 3 ~

A particularly desirable use of the present invention is for the removal of carbonaceous solid particul~tes from diesel engine exhaust gas so as to avoid air poilution by such particulates, which can range in diameter from about 5 microme-ters (i.e. microns~ down to 0.05 microns or less . Fig . 18 shows an exemplary form of such apparatus, which comprises the filter 20' held within a container or can 50. The filter body 20' may be any of those depicted in Figs. la through 17, or any other according to the teachings of this invention which also have inlet cells 2B' and outlet cells 29' formed by thin porous intersecting walls 22' and extending between opposing inlet and outlet end faces 24' and 26'l respectively. A skin 30' has also been provided as the outer surface of the body 20l between the end faces 24' and 26'. The can 50 is similar to a conventional type of can employed for mounting catalytic con-verter honeycomb substrates in exhaust systems of internal combustion engines as is described, for example, in U.S. Patent 3,441,381. The can 50 comprises two parts 51 and 52 respectively formed of filter-holding portions 53 and 54, conduit-connectors 55 and 56, conical portions S7 and 58 respectively joining connectors 55 and 56 to portions 53 and 54, and mating flanges 59 and 60 which are mechanically fastened together (e.g. by bolts and r.uts not shown) to keep the can 50 properly assembled for use and so as to be capable of being unfastened in order to open the can 50 for replacement of ilter 20'. Internal annular mounting members 61 and 62 of L-shaped cross-section are respectively fastened to portions - - 22 ~

38~33 53 and 54 so as to respectively abut against end faces 24' and 26' and hold filter 20' in its proper fi~ed axial position within can 50. To cushion filter 20' against mechanical shock and vibration, it is ordinarily desirable to surround filter 20' with a wrapping or mat 63 of metal mesh, refractory fiber or the like, which may fill the annular space between filter 20' and portions 53 and 54. To minimize heat loss from filter 20' and excessive heating oE portions 53 and 54, a layer of insulating material 64, such as glass or mineral wool mat, may also bs wrapped around skin 30'.
Connectors 55 and 56 are suitably fastened (eOg. by welding or mechanical coupling) to upstream and downstream por tions, respectively, of the exhaust gas pipe o a diesel engine.
~hile the can 50 can be located in and form part of the exhaust gas conduit some distance downstream from the engine exhaust manifold, it desirably can be located near or at the exit ~rom the exhaust manifold. The latter arrangement facilitates regen-eration of the filter 20' by utilizing the higher temperature of the exhaust gas upon exiting the exhaust manifold to cause, with excess air in the exhaust gas, the combustion of carbona-ceous particulates restrained in the filter 20'. The gaseous combustion products formed during regeneration can tnen pass on through and out o the filter 20' for emission through the con-nector 56 to the tailpipe (not shown) fastened to the connector 56. I~ desirable (especially when the can 50 is located down-stream along the exhaust conduit some distance from the exhaust manifold), a combustion ignition device may be positioned in the can 50, such as a glow plug in the conical portion 57 or an electric heater within the cen~ral axis of filter 20' (similar to the device of U.S~ Patent 3~768,982, and secondary air may be injected into the can 50 up-S stream from the filter 20' to assist in regeneration of the ~ilter 20' without removing it from the can 50. Additionally, catalyst substance( 5 ) can be placed on and in thin walls 22' of filter 20' (similar to catalytic converter honeycomb sub r strates) to ~acilitate regeneration combustion in the filter 20'. In ordinary usage, frequent higher rotational (iOe. rpm) speed of the diesel engine can contribute sufficient heat (eO
g. 400-500C or higher) requiring less frequent replacement o~ filter 20'. Nevertheless, removed fil~ers 20' can be reverse flushed with air to blow much of the trapped particulates out : 15 of it into a collector bag and then fully regenerated by high temperature air passed through it before reinstalling in can 50.
For diesel particulate filtering applications, honey-comb stru~tures having cellular densities between about 10 and 300 cells/in.2 tabout 1.5 to 46.5 cells/cm.2) are conceivably useful, with densities between about 100 and 200 cells/in.2 (about 1505 to 31 cells/cm~2) preferred for light automotive applications~ Wall thicknesses can vary upwards from the mini-mum dimension required ~or structural integrity, bout .002 in. (about .05 mm.) for the extruded ceramic matarials to be subsequently discussed, but a range of between about .010 and .030 inches ~about .25 and .76 mm.) is preferaole with these ~ i~

3~

materials ~t the desirable cellular densities. Volumetric open poro~ity of th~ thin walls may vary~ but should not be less than about 25% and desirably not less than about 35% of thin wall volume with a range of between about 40 to 70% preferred to minimize hydraulic back pressure to fluid flow throu~h the longer dimensions of the thin walls. Mean pore diameters within the walls may also vary from about l to 60 microns with a range between about 10 and sa microns preferred., Desired efficiencies may range from less than about 5G% to about 90~
or ~ore depending upon the operating charateristics and operat-ing schedule of the engine.
Ceramic materials such as cordierite material disclos-ed in U.S. Patents 3,885,977 and 4,00l,028, arP generally ~referred for diesel particulate trap ilters because, 2S was earlier found in their use as catalyst substrates in internal combustion engine exhaust systems, these materials have properties that enable them to withstand and be durable under the thermal, chemical, and physical conditions to which they are subjected in such systems including those of Z0 diesel engines.
~hin wall matrices for diesel exhaust particulate fil-ters may be ~rmed by extrusion from the p~rticular batch mix-tures of TABLE I of the aforesaid Canadian application No. 380,875 and plugged with the described manganese-magnesium, foam-type cordierite cement, preferred for diesel exhaust and other fil-~ ~ 33 tering applications having high sodium ionic con-tent, in the manner described in that application. In particular, it was found that filters utilizing the plugging pa~terns depicted in Figs. la, 8, and 4 of this application were found to have great-er average particulate capacity at back pressures above about 100 cm. of water than did checkered plugged filters (i.e. that illustrated in Fig. 2) of similar yeometry when both types o filters were formed from the preferred batch mixture listed for SAMPLES D-E-F-G-~ in TA~LE I of the aforesaid application Serial No. 165,646 and were comparison tested in a manner simi-lar to the testing described therein. This ~apacity comparison was based upon cylindrical filters approximately 3.6 inches (about 9.3 cm.) in diameter and about 12 inches (30.5 cm.1 overall length having square cells formed at a de~sity of about 100 cells/in.2 (about 15.5 cells/cm.2) and fo~med ~y thin walls about .017 inches (.6 mm.) in uniform thickness.
The reduc~ion in back pressure build-up from lower particulate build-up rates in filters of the same composition and geometry bu~ plugged according to the pattern of Fig. 5 was not suffi-cient to compensate for their higher initial back pressures and at the ~40 cm. water back pressure cut-off of the test~
these filters had less particulate capacity than did the compa-rable c~eckered plugged filters. Other filters or ,dentical composition and cellular geometry but having plugging pa~terns allocating more than 80% of the collective cell thin wall surface area to the inlet cells faired even more poorly with respect to the checkered filters at the 140 cm. water back pressure limit as would be expected from the results of the filters plugged in the Fig. 5 pattern.
In addition to honeycomb filters having cells extend-ing in a single uniform direction through the filter, it is S envisioned that the invention may be used with other types of honeycomb filters such as the exemplary cross-flow filter 100 depicted in Fig. 19. The filter 320 is ayain formed by a mul-tiplicity of thin, interconnected porous walls 322 which define a pair of opposing inlet end faces 324 and 325 and an outlet end face 326 on three OL- the outer surfaces of the filter 320, A group of inlet cells 328 extend laterally through the filter 320 and between the inlet end faces 324 and 325 in layers.
The inlet cells 328 are open at the inlet end faces 324 and 325 and those located at the outlet end face 326 are closed along that surface. Outlet cells 329 extend longitudinally through the filter 320 in layers interspersed among the layers of inlet cells 328. The outlet cells 329 are open at the outlet end face 326 and closed where they adjoin the inlet end faces 324 and 325. Flui.d with solid particulate contaminant enters the filter 320 through the open ends of the inlet cells 328 at both inlet end faces 324 and 325. Again, the thin walls 322 have interconnected open porosity of a size and volume ~ufficient to allow the fluid to pass completely across and through the thin walls 322 in their narrow and longer dimensions while preventing contaminant from passing either completely through or across any dimension of the thin walls 322. Again, collective inlet cell area, which may be substan-8~Z33 tially greater than the collective outlet cell area, can typi-cally be provided by providing more inlet than outlet cells, larger inlet than outlet cells or, as has been provided in the filter 320, both. ~lternately, the filter 320 may be provided with a single inlet end face or with opposing outlet end faces or a pair of inlet and pair of outlet end faces~
While various embodiments of the invention and sug-gested modifications thereto have been desc.ibed, it should be understood that other modifications could be made in the struc-ture, arrangement, and composition of the described embo2iments without departing from the scope of the invention which is more fully defined in the following claims.

Claims

I CLAIM:

1. A filter for removing all or substantially all of the solid particulates in fluids passed through the filter comprising:
a multiplicity of interconnected thin porous walls defining inlet and outlet end faces of the filter and a multiplicity of cells, each cell extending through the filter from at least one of the inlet and outlet end faces and having a surface area defined by surfaces of the thin walls exposed within the cell, the thin walls containing interconnected open porosity of a volume and size sufficient to enable the fluid to flow across the narrow dimension of the thin walls between adjoining cells and through the longer dimensions of the thin walls between adjoining or neighboring cells and to restrain at least a significant portion of the solid particulates from passing either across or through any of the thin walls, an outlet group of cells, each cell open at an outlet face and closed where adjoining any inlet end face, an inlet group of cells, each cell open at an inlet end face and closed where adjoining any outlet end face, and the collective thin wall surface area of the in-let group of cells being significantly greater than the collec-tive thin wall surface area of the outlet group of cells.

2. The filter of claim 1 having a single inlet end face and a single outlet end face on opposing sides of the fil-ter and said plurality of cells extends longitudinally through the filter between the inlet and outlet end faces.

3. The filter of claim 1 wherein said collective thin wall surface area of the inlet group of cells is at least about 25% greater than said collective thin wall surface area of the outlet group of cells.

4. The filter of claim 3 wherein said collective thin wall surface area of the inlet group of cells is between about 2 and 4 times greater than said collective thin wall sur-face area of the outlet group of cells.

5. The filter of claim 1 wherein all or substantially all cells of the inlet group each shares two or more of its defining thin walls in common with other adjoining cells of the inlet group.

6. The filter of claim 5 wherein all or substantially all cells of the inlet group each shares at least 75% of its defining thin walls in common with other adjoining cells of the inlet group.

7. The filter of claim 1 wherein the number of inlet cells is also substantially greater than the number of outlet cells.

8. The filter of claim 1 wherein the number of cells in the inlet group and the outlet group are substantially equal and the average individual, transverse cross-sectional areas of the inlet cells is greater than the average individual, trans-verse cross-sectional areas of the outlet cells.

9. The filter of claim 1 or 5 wherein substantially all of said plurality of cells have substantially square, trans-verse cross-sectional geometries.

10. The filter of claim 1 wherein the open porosity of the thin walls is at least about 25% or more by volume.

11. The filter of claim 10 wherein said open porosity is formed by pores having mean diameters of about 1 micron or more.

12. The filter of claim 11 wherein the open porosity of the thin walls is at least about 35% or more by volume.

13. The filter of claim 12 wherein the open porosity of the thin walls is also less than about 70% by volume.

14. The filter of claim 13 when used to remove car-bonaceous solid particulate from diesel engine exhaust gas and wherein said pores have mean diameters of between about 10 and 50 microns.

15. The filter of claim 1 wherein said matrix of thin walls is formed from a ceramic-based material.

16. In the method of fabricating a filter for remov-ing solid particulates from fluid streams comprising the steps of providing a honeycomb structure formed from a matrix of thin, intersecting porous walls which define a pair of open end faces and a multiplicity of hollow cells extending in a substantially mutually parallel fashion, closing a first group of cells near their end portions at one end face and the remaining cells near their end portion at the remaining end fare, the interconnected open porosity of the thin walls being of volume and size suffi-cient to enable fluid flowing into the first group of cells to flow completely across the narrow dimensions of the thin walls into adjoining cells of the second group and through the thin walls in their longer dimension to adjoining or neighboring cells of the second group and preventing at least a significant portion of the solid particulates from passing completely across or through the walls in any of their dimensions, the improvement comprising the step of:
further closing said cells to provide substan-tially greater collective thin wall surface area to the inlet group of cells than the collective thin wall surface area provided the outlet group of cells.

17. A compact through flow exhaust particulate filter element for diesel engines, said element comprising a ceramic monolith having a plurality of interlaced, thin gas filtering porous internal walls defining a plurality of parallel passages extending to opposite ends of the element, said passages includ-ing a first group comprising inlet passages open at one end of the element and closed at the other and a second group compris-ing outlet passages closed at said one end of the element and open at the other, the inlet passages having collective cross-sectional areas substantially greater than the respective col-lective cross-sectional areas of the outlet passages, the im-provement comprising:
inlet passages adjoining one another and sharing in common defining interlaced, porous internal walls, and the plurality of interlaced thin porous internal walls having internal interconnected open porosity of a volume and size sufficient to enable the fluid to flow across said walls in their narrow dimensions and through said walls in their longer dimensions and to restrain at least a significant portion of the particulates from passing either completely across or through the walls in any of their dimensions.

18. The filter element of claim 17 wherein the indi-vidual cross-sectional areas of the inlet passages are substan-tially greater than the respective individual cross-sectional areas of the outlet passages.

19. The filter element of claim 17, wherein the open porosity is provided by pores having a mean pore diameter of 0.1 micron or larger.