STRUCTURE FOR A FILTER OR A HEAT EXCHANGER AND A METHOD FOR MAKING THE STRUCTURE

Abstract: A structure (10 or 100) useful for separating contaminants from fluids or exchanging heat between two fluids includes a fused single crystal acicular ceramic support (12 or 112) having a discriminating layer (38 or 128) disposed on at leat part of the support. The support has at least one channel that extends entirely through the body. If the structure is used as a filter, the discriminating layer is suitably a sintered, porous alpha-alumina membrane or other porous membrane. If the structure is used as a heat exchanger, the discriminating layer is impermeable. The support is preferably composed of a non-stoichiometric acicular mullite. The support may be in either a dead-end or cross-flow configuration. The structure may include plugs (22 and 23) to close off one end or the other of the channels in the support. The structure preferably includes end caps (118) over the ends of the support. The end caps should be of the same ceramic as used in the support. The ceramic material used in the support, plugs and end caps is interlocked, thereby connecting the plugs and end caps to the support. The filter is preferably covered by a fired, enamel glaze on the outside of the support and the end caps. A method for making the filter includes the step of converting a green ceramic support in situ into acicular ceramic particles, preferably into whiskers. The method preferably entails affixing the support, plugs and end caps together by interlocking them by the conversion step.

Description: STRUCTURE FOR A FILTER OR A HEAT EXCHANGER AND A METHOD FOR MAKING THE STRUCTU RE Technical Field -. This invention relates generally to a ceramic structure for filtering of, or heat exchange between, fluids, either liquids or gases. Background of the Invention Ceramic filter media are commonly used in a wide range of fluid handling procedures, including filtration, diffusion, recovery, transfer, mixing and foaming. Ceramic ,. 0 filter media are also employed as catalysts themsel es, or as carriers for catalysts. Ceramics possess several advantages as media for fluid handling over alternatives such as organic or metallic filter media. One advantage is a superior resistance to deterioration from heat or chemical exposure. Ceramic filter media are most often used in the form of an aggregation of .. ceramic particles, either loose or bound to one another. The ceramic particles can be formed as spheres, platelets or needles. The particles are routinely obtained by crushing and classifying (that is, sorting by size) a previously manufactured mass of the desired material. This method of manufacture is subject to some drawbacks, however. Crushing a formed material can degrade some desirable structural characteristics of the material, such as its impact strength, - n mechanical strength, rigidity, porosity or aspect ratio. Even if ceramic particles can be successfully classified, subsequent aggregation of the particles to attain rigidity or improve a physical or mechanical property may require sintering or the use of a bonding agent. Alternatively, the particles may be aggregated by placing them in a metal container. The containers can, however, be expensive and may not be ... completely resistant to the gas or liquid being treated. It can also be difficult to achieve a good seal between the metal container and the particles If a good seal is not obtained, fluid may leak around and bypass the particles. Ceramic filter media have also been used as supports for discriminating layers such asfluorocarbon polymers or sintered ceramic membranes. These supports have typically been made from previously fired spherical particles of alpha-alumina (α-A^O.) orcordierite. The particles have then been lightly sintered to bond them together and give them mechanical strength. Unfortunately, many of the resulting supports did not have possess sufficient strength, particularly against impact or against the pressure of fluid flowing through them. The resulting supports typically had a porosity of no more than about 30% (or 70% of theoretical density, defined as 100% minus the volume% porosity). Although many solutions to these problems have been suggested, each has its own drawbacks- For example, JP 63-103877A discloses a process for preparing a porous ceramic compact that may be useful for industrial filtering. The compact is described as having fine porous structure with a relatively high deflection strength. The compact consists of acicular mullite crystals formed by compression molding and sintering of stoichiometric mullite (3 A! 2 θ3-2Siθ2)- The starting material includes additives so as to allowthe transfer of any unreacted or excess silica into a glass phase, which is then eluted with an acid such as hydrofluoric acid (HF). Acids such as HF require special handling procedures and degrade or attack any metal that may be used in a device. U.S.-A 3,993,449 discloses a process for preparing single crystal mullite fibrils useful as fillers, catalysts, or catalyst supports. The fibrils are made from aluminum suifate, a silica (Si0 2 ) source and an alkali metal salt (fluxing agent). While both of these disclosures suggest that a fibrous mullite body or support can be obtained that is relatively strong, the degree to which the whiskers forming the bodies bind to each other is not clear. Moreover, control of pore size in such bodies is not as great as could be desired and the average pore sizes are typically quite small. The bodies are often not useful for applications requiring a porosity as high as 50 up to 85% . The use of HF to elute the glass phase is inconvenient because it requires handling precautions. Additionally, devices constructed from metal generally cannot be used in processes employing HF as an elution agent. U.S.-A 4,894J60 discloses a honeycomb structure for fluid filtration. The structure comprises a porous ceramic support having a multiplicity of parallel passageways formed through it by uniformly spaced porous partition walls. The passageways permit pressurized fluid to flow through the support. A selective membrane is coated onto the surface of the passageways to separate one or more components from the fluid. The filtrate is carried through the partition walls to the exterior surface of the partition walls for collection. The honeycomb structure is preferably contained within a cylindrical casing and held in place by a pair of support plates near the ends of the ceramic support element. The external surface of the partition walls between the support plates is coated with a glaze, to direct the flow of filtrate into the discharge port. While useful for its intended purpose, the device of U.S.-A 4,894,160 appears to have several drawbacks. Joints such as those between the honeycomb structure and the support plates are subject to leakage, especially during repeated thermal cycling. The honeycomb support, being constructed of α-AI 2 0 3 and kaolin, necessarily shares the drawbacks of the prior ceramic supports. Also, the ceramic support has a porosity of only 7 to 25 percent. It would be desirable to have a ceramic filter structure wherein a discriminating layer is deposited upon a support structure or material that has been grown in situ to form a network of interlocked needles or platelets that has high mechanical strength, high impact strength, sufficient heat resistance and good resistance to thermal cycling. The filter structure should be prepared by a process that avoids hazards associated with handling acids such as HF. The process should also include a method for interlocking two or more pieces into a ceramic o body that has a uniform composition and structure. The uniform composition and structure should continue including throughoutthe locations pieces of the structure are joined. Disclosure of the Invention One aspect of the present invention is a structure for filtering a fluid or for exchanging heat or a constituent or contaminant between two fluids passing through the 5 structure, comprising: a body composed of a fused, interlocked, single crystal acicular ceramic material, the body having porous partition walls defining at least one open channel extending entirely through the body; and means for collecting fluid exiting the body through the partition walls. The body is preferably a honeycomb extrusion of porous partition walls that define a plurality of open parallel channels in fluid communication with one another. Any fluid not passing through the porous partition walls exits the body unhindered, through the open channel(s). The means for collecting fluid exiting through the partition walls can include an impermeable coating, such as a glaze, substantially covering the exterior of the body so as to define an exit for the exiting fluid. The structure can further comprise a membrane disposed on the partition walls, thereby lining the open channel(s). The membrane can be a porous discriminating layer of sintered ceramic, a polymeric organic compound, a molecular sieve, a gel filtration, gel, or a microporous gaseous diffusion barrier. This makes the structure useful for cross-flow filtration. The sintered ceramic material is desirably sintered C1-AI2O3. Alternatively, the membrane can be impermeable, preventing a first fluid passing through the channels from mixing with a second fluid passing through the partition walls. This makes the structure useful for heat exchange between the fluids. Other elements may be added to make the structure useful for a variety of purposes. For example, the structure can include a pair of porous end caps composed of the same acicular ceramic material as the body. The end caps preferably abut and interlock with the partition walls. Each end cap preferably includes a hole therethrough in fluid communication with the open channels. Each end cap also preferably includes at least one recess that extends from the end cap external surface remote from, rather than proximate to, the partition walls only partway through the end cap. Thus, the holes provide an entrance and exit for a fluid to pass through the open channels, while the recesses provide for eitherthe exit, orthe entrance and exit, of fluid passing through the porous partition walls. The holes and recesses preferably have axes that parallel those of the open channels. Conveniently, the collecting means can then include a pair of connecting tubes received in the end cap recesses to directthe flow of filtrate exiting the porous walls and end caps. Another pair of connecting tubes can be received in the end cap holes to directthe flow of fluid into and out of the open channels in the ceramic body. The body is desirably composed of non-stoichiometric acicular mullite, preferably of 76 percent by weight Al 2 0 3 and 24 percent by weight Siθ2- The body is preferably formed as a honeycomb extrusion of plural parallel channels. For filter applications, the body desirably further comprises plugs of porous, non-stoichiometric, acicular mullite that are disposed in, and interlocked with, single, alternating ends of the channels. In yet another related aspect, the body is preferably from 50 to 85 percent porous. A second aspect of the present invention is a method of forming a structure useful for filtering a fluid or for exchanging heat or a constituent between two fluids passing through the structure, comprising the steps of: forming a green support composed of a ceramic material capable of being converted in situ into fused, interlocked, single crystal, acicular ceramic particles; converting said ceramic material in situ into such particles, thereby forming a rigid and porous ceramic support; and applying a membrane layer on at least part of the support. The membrane layer is impermeable for heat exchanger applications and permeable forfiltering applications. The green support is preferably a honeycomb extrusion having a plurality of open, parallel channels defined therein. The support is desirably formed by abutting together at least two green support pieces, such as a honeycomb extrusion and either porous plugs or end caps or both. Each piece is composed of a non-stoichiometric mixture of AI2O3 and Siθ2- The pieces become interlocked with each other when they are converted to acicular mullite. A permeable membrane layer is desirably formed in two steps. First, at least one surface of the porous ceramic support, preferably surfaces of the open channels in a honeycomb extrusion, is coated with a slurry of C1-AI2O3. In the second step, the support and the -1-AI2O3 are sintered after removal of excess slurry. This provides a thin, continuous membrane of porous α-AI 2 03 on the surface of the support. An impermeable layer may be formed in a similarmanner using a suitable material. The nature and extent of the present invention will be clear from the following detailed description of the particular embodiments thereof, taken in conjunction with the appendant drawings, in which: FIGURE 1 is a partial cut away side view of a structure prepared in accordance with the present invention; FIGURE 2 is a cross-sectional view taken along line 2-2 of Figure 1 showing how the end of the structure is cut to place outlet channels and inlet channels in fluid communication, respectively, with other outlet channels and inlet channels; FIGURE 3 is a partial cross-sectional view similarto Figure 1 , more clearly showing the flow of fluid therethrough and the α-Al 2 03 or other membrane coating part of the interior of the structure; FIGURE 4 is a cross-sectional view of a cross- flow filter incorporating the structure of the present invention; FIGURE 5 is a cross-sectional view taken along line 5-5 in Figure 4; FIGURE 6 is a cross-sectional view taken along line 6-6 in Figure 4; FIGURE 7 is an end view of a ceramic support used in making the cross-flow filter shown in Figure 4 disclosing how the end of the support is cut so as to yield the structure shown in Figure 6; and FIGURE 8 is an enlarged, partial cross- sectional view of the cross-flow filter of Figure 4, showing the flow of fluid through open filter channels and through porous support walls that define the open channels. Referring first to Figure 1 , a filter 10 is shown. Filter 10 is useful for removing an entrained contaminant from a fluid, for example, for removing insoluble organic material from an aqueous slurry. Filter 10 comprises a support 1 1 composed of a fused single crystal acicular ceramic of interlocked needles, whiskers or platelets (collectively, "whiskers" or "acicular particles"). Support 1 1 is distinguished from prior supports because it is composed of individual acicular ceramic particles that are rigidly interlocked at the locations where they cross rather than merely being interwoven as in prior supports. As described in more detail below, the conversion of the ceramic into fused and interlocked acicular particles occurs in situ. The fusion and interlocking of the whiskers yields a support that, in comparison to prior supports formed from preformed, lightly bonded whiskers, possesses significantly improved structural strength and resistance to high temperatures. The acicular ceramic material is preferably a non-stoichiometric acicular mullite composed of about 76 percent by weight (wt-%) Al 2 0 3 and 2 wt-% Si0 2 . Other ceramic materials that are capable of forming fused, interlocking, single crystal, acicular structures upon in situ formation are also useful. Such other ceramic materials are believed to include aluminum borate whiskers, alumina whiskers and α-AI 2 03 platelets, or other materials sharing these characteristics. Support 1 1 can be formed in any of a variety of conventional shapes, including a disk, a barrel, a tube, a dead-end body, or a cross-flow body. Support 1 1 can also be formed into more complex shapes than prior filter supports. Support 1 1 can be a single piece or be formed from a plurality of pieces having a uniform composition and structure. Uniformity of composition is especially desirable through locations where pieces are joined together. The shape of support 1 1 should be appropriate for the function desired of the structure incorporating the support. For example, support 11 can be configured as a honeycomb extrusion for either a dead-end or cross-flow filter. In the dead-end configuration, half of the holes at each end of the honeycomb are plugged in a checkerboard arrangement. 5 In this arrangement, holes open at one end are plugged at the other end. in the cross-flow configuration, all holes or channels in the honeycomb are connected by a series of shallow slits in the support, and no end plugs are used. A honeycomb support shape is shown in U.S.-A 5,098,455. In filter 10 (Figure 1), support 11 comprises a honeycomb extrusion 12 having a 10 fluid inletend 14 and a fluid outlet end 16. A pluralityof longitudinally extending inlet channels 18and parallel longitudinally extending outlet channels 20 extend from fluid inlet end 14to fluid outlet end 16. Inlet channels 18 are formed by plugging their downstream ends adjacent outlet end 16 with downstream plugs 22. Outlet channels 20 are formed by plugging their upstream ends adjacent inlet end 14 with upstream plugs 23. 15 Once extruded, the honeycomb extrusion 12 is allowed to dry to a self-sustaining green body. As more clearly shown in Figure 2, alternating single holes at opposing ends of inlet channels 18and outlet channels 20 are then plugged in checkerboard fashion. Plugging occurs with green plugs 22 and 23 made of a paste of the same composition as honeycomb extrusion 12. When green, plugs 22 and 23 may possess the same amount of filler as the 20 composition making up extrusion 12. However, it is preferred for handling purposes that green plugs 22 and 23 have a higher porosity than extrusion 12, for example, typically on the order of 70 % (30 % theoretical density). This higher porosity is achieved by mixing more filler or water with the mixture of alumina and either clay or silica. After plugs 22 and 23 have dried to a green stage, all inlet channels 18 at inlet end 25 14 are placed in fluid communication with each other. Similarly, at outlet end 16, all outlet channels 20 are placed in fluid communication with each other. Fluid communication is desirably established by saw cuts 24 extending across honeycomb extrusion 12 in the direction of arrows A on both fluid inlet end 14 and fluid outlet end 16. End caps 26 and 30 are then fastened to ends 14 and 16 of honeycomb extrusion 12 either by moistening end caps 26 and 30 0 with water, or by applying to end caps 26 and 30 a diluted slurry of the paste used for plugs 22 and 23. End caps 26 and 30 are also fashioned from the same composition as honeycomb extrusion 12, that is, having the same ratio of aluminum to silicon as in extrusion 12. Support 11 also comprises an inlet end cap 26 that is attached to inlet end 14 of honeycomb extrusion 12. Inlet end cap 26 has a hole 28 formed through it, in fluid 5 communication with inlet channels 18 through spaces provided by saw cuts 24. The surfaces of downstream plugs 22 and inlet channels 18, together with the interior surface of inlet end cap 26, define an inlet chamber 36 within filter 10. Support 11 further comprises an outlet end cap 30 that is attached to outlet end 16 of honeycomb extrusion 12. Outlet end cap 30 has a hole 32 (shown in Figure 3) formed through it, in fluid communication with outlet channels 20 through spaces provided by saw cuts 24. The exterior of honeycomb extrusion 12, inlet end cap 26 and outlet end cap 30 are substantially covered with a fired glaze layer 34 (except for the inlet and outlet holes 28 and 32). Layer 34 is impermeable to, and nonreactive with, the fluid to be treated and its entrained contaminants. In addition to these elements, filter 10 comprises a porous discriminating layer 38 on at least part of support 11. Discriminating layer 38 is preferably disposed on the surfaces of downstream plugs 22, inlet channels 18, and the interior surface of the inlet end cap 26 that define inlet chamber 36. Discriminating layer 38 is preferably a sintered α-AI 2 0 3 membrane. It can, however, be any conventional layer suitable for filtration, microfiltration, ultrafiltration (for example, sterilization, or purification of crystals), reverse osmosis (for example, desalination of sea water), or gas separation. Classes of materials useful for discriminating layer 38 include sintered ceramics (of which C1-AI2O3 is an example), polymeric organic compounds, molecular sieves, gels, and microporous or ultraporous gaseous diffusion barriers. "Moiecular sieves" include both zeolites and crystalline aluminophosphates derived from mixtures containing an organic amine or a quaternary ammonium salt. "Gels" are simply those gels (such as dextran gels) that are useful in gel filtration. "Polymeric organic compounds" include hydrocarbon, halogenated hydrocarbon, fluorocarbon, and chlorofluorocarbon resins and polymers, such as polytetra- fluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/perchloro-alkylvinylether copolymer (PFA), polychlorotrifluoroethylene (PCTFE), polyvinyl- difluoride (PVDF), polypropylene resin, and polyvinylchloride resin. The foregoing materials suitably provide, either singly or in combination, pore sizes within a useful range of 0.5 nanometers (nm) to 50 micrometers (μm). The material(s) chosen for discriminating layer 38 should also be compatible with the material of support 1 1. Discriminating layer 38 can be formed by any fabrication process and any constituents that yield a layer useful for its intended purpose. For example, U.S. -A 4,874,516 discloses a method for applying a PTFE membrane filter to a ceramic substrate so as to permeate the surface layer of the substrate. The substrate and PTFE are heat-treated to yield a supported membrane of a high polymer resinous material having a useful pore size as small as about OJ μm for ultra- or semiultra-filtration. Appropriate fabrication processes for membranes of other materials are well known, and can be adapted for use in the present invention without undue experimentation. In use, fluid enters filter 10, as shown by unlabeled arrows in Figure 3, through inlet hole 28 and passes, via the inlet chamber 36, through discriminating layer 38 and the walls of honeycomb extrusion 12, into outlet channels 20. The surfaces of outlet channels 20 and upstream plugs 23, together with the interior surface of outlet end cap 30, define an outlet chamber 40 for collection of the permeate fluid. The permeate fluid exits filter 10 through outlet hole 32. Referring to Figure 4, cross-flow filter 100 comprises a body 1 11. Body 111 is preferably composed of the same fused single crystal whiskers as support 11 of structure 10. Like support 11, body 111 can be a single piece or be formed from a plurality of pieces of uniform composition. Body 1 11 is suitably configured as a honeycomb extrusion 1 13 having a number of continuously formed partition walls 1 12. Walls 1 12 define a plurality of parallel open channels 114 that extend entirely through body 111. Open channels 1 14 define a path for the flow of a first fluid through body 11 1. As shown in Figure 5, open channels 1 14 are preferably circular in cross section, although they may have any convenient or desired cross-sectional shape. All open channels 1 14 are preferably in fluid communication with one another. Fluid communication can be established in a variety of ways. One way includes positioning a hollow dome over each end of extrusion 1 13 so that all channels are open to the hollow dome. A preferred way is to form a plurality oftransverse clear openings 116 through partition walls 112. Openings 116 are desirably positioned at opposing ends of the open channels 114. Openings 116 are preferably recesses or channels formed by cuts that extend fully across the ends of extrusion 113 and intersect open channels 114. More particularly, as shown in Figure 7, when open channels 114 are arranged in a closest-packed (hexagonal) fashion, the cuts are made along every other row of open channels 1 14 in each of the three directions in which the rows run. The cuts, also known as saw cuts, can be made in any convenient fashion. They are usually made before extrusion 1 13 is converted into the preferred single crystal acicular ceramic. One way to make the cuts is to draw a thread, wetted with water, across the ends of the extrusion 113 longitudinally in the directions of arrows A, B, and C. Figure 6 shows howan end of extrusion 113 looks after the cuts are made. Referring again to Figure 4, body 11 1 further comprises a pair of porous end caps 118 that abut opposing ends of extrusion 113. End caps 118 are preferably in physical contact with the ends of porous partition walls 1 12. Each end cap 118 includes a hole 120 that extends fullythrough end cap 118 and is in fluid communication with open channels 1 14. Each end cap 118 also includes a recess 122 that extends only partway through end cap 1 18 and is spaced apart from, and opposed to, partition walls 1 12Jn otherwords, recesses 122 do not communicate directly with open channels 1 14. End caps 118 are desirably composed of the same single crystal acicular material as partition walls 112. End caps 1 18 are also desirably interlocked with walls 122. The material is preferably non-stoichiometric acicular mullite. A first connecting tube 124 is received in, and operatively connected to, each end cap hole 120. The first connecting tubes 124 fluidly communicate with all of the open channels 114, preferably by wayof openings 116. As such, they define a means for directing the flow of a first fluid from one first tube 124, into, through and out of the open channels 114, and through the other first tube 124. First connecting tubes 124 can be made of a ceramic and may be co-fired with the conversion of body 1 1 1 to the preferred single crystal acicular ceramic. First connecting tubes 124 are preferably made of, or coated with, a material that is impermeable to the fluids being handled. This precludes mixing of the fluids. Second connecting tubes 130 and 131 are received in, and operatively connected to, end cap recesses 122 of opposing end caps 1 18. Second connecting tubes 130 and 131 are, I ike first connecting tubes 124 preferably made of, or coated with, a material that is impermeable to the fluids being handled. An impermeable coati ng 134 substantially covers the exterior of the body 1 1 1 , 0 except for holes 120, recesses 122, and the open ends of connecting tubes 124, 130 and 131. Coating 134 may extend partway along connecting tubes 124, 130 and 131 in order to assure a good seal to body 1 1 1. Coating 134, preferably together with either of second connecting tubes 130 or 131 , defines a means for collecting fluid exiting body 1 1 1 through partition walls 1 12. The exiting fluid can be a second fluid supplied to filter 100, a filtrate derived from the 5 first fluid, or a combination of both. Similarly, coating 134, second connecting tubes 130 and 131 and end caps 1 18 define a means for directing the flow of the second fluid into, through, and out of partition walls 1 12. The means for collecting fluid exiting body 1 1 1 is preferably formed integrally with body 1 1 1. In other words, the collecting means and body 1 1 1 constitute a single piece. o This does not mean that the collecting means and body 1 1 1 are composed of the same material. To contrary, the body 1 1 1 is porous, while the collecting means is impermeable. The integral formation of the collecting means and body 11 1 is most conveniently carried out by making coating 134 an impermeable glaze fired on exterior surfaces of body 1 1 1. Coating 134 is desirably formed from the same material as fired glaze layer 34 of structure 10. 5 The surfaces of porous partition walls 1 12 that define open channels 1 14, as well as any surfaces of end caps 1 18 exposed to the first fluid flowing through open channels 1 14, are preferably coated with a membrane 126 so as to line channels 1 14. In cross-flow filter 100, membrane 126 is a porous discriminating layer 128. Discriminating layer 128 is preferably formed from the same classes of materials as discriminating layer 38 of structure 10. The use of cross-flow filter 100 is straightforward, as shown in Figure 8. A flow of a first fluid is established through open channels 1 14 by supplying the first fluid through either of first connecting tubes 124 and withdrawing it through the other. The flow of the first fluid is generally designated by long arrows 136. A flow of a second fluid is established through porous walls 1 12 and porous end caps 118, preferably in a direction opposite to the flow of the first fluid. The second fluid may be supplied by and withdrawn, respectively, through second connecting tubes 130 and 131. The flow of the second fluid is generally designated by short, tailless arrows 138. Impermeable coating 134 collects and directs the flow of the second fluid. The transfer of a contaminant or constituent across membrane 126 is generally designated by short, tailed arrows 140. The flow ofthe contaminantor constituent is thereafter concurrent with the flow of the second fluid. That is, the contaminant or constituent passes through membrane 126, into porous walls 1 12 where is conveyed along with the second 5 ffui to the end caps 118 into which second connecting tube 131 is inserted. The second fluid and the contaminantor constituent are then collected and directed by the impermeable coating 134 to second connecting tube 131 through which they then exit or pass out of body 111. The designation of second connecting tubes 130 and 131 is merely a matter of convenience. Indeed, to save costs during assembly, end caps 118, first connecting tubes 124 10 and the second connecting tubes 130 and 131 are preferably composed as identical units. The use of body 1 1 1 in devices other than cross-flow filter 100 is also straightforward. For example, body 111 is useful in a heat exchanger for exchanging heat between two preferably opposed fluid flows. The construction of such a heat exchanger is identical to that of cross-flow filter 100, except forthe specific nature of membrane 126. In a 15 heat exchangerJt is critical that no exchange of fluids takes place. Therefore, membrane 126 of a heat exchanger must be an impermeable layer. Many such impermeable layers are known and would be useful in the practice of the present invention. However, because ofthe superiority of support offered by single crystal acicular ceramic body 111 in both mechanical and thermal resistivity, an impermeable membrane 126 can be made thinner than past dividing 0 walls or septa for heat exchange. Indeed, it is expected that a broader range of impermeable materials will be useful in heat exchangers incorporating body 111 for this reason. Moreover, the high porosity of body 111 permits a greater rate of fluid flow through it, specifically, through or along the length of porous walls 112. These factors allow a more rapid exchange of heat than could be obtained in prior heat exchangers. The flow paths through a heat exchanger incorporating body 1 1 1 would be essentially the same as those for cross-flow filter 100 shown in Figure 8, except, of course, that there would be no flow of material across membrane 126. The small, tailed arrows 140 would, instead, represent the exchange of heat, in one direction orthe other, depending upon which fluid was hotter. Q Body 111 is also useful in a simple filter, presenting generally unimpeded flow of a fluid through open channels 1 14, but having a cross-flow ofthe filtrate. The construction of such a filter can be identical to that of filter 100, except that membrane 126 is only optionally applied to porous walls 1 12. The flow path through the simple filter is essentially the same as that of the cross-flow filter 100, except, of course, that there is no second fluid. Filtrate from the flow ofthe first fluid passes through walls 112 and end caps 118JS collected and directed by impermeable coating 134, and exits the body 111 through either of the second connecting tubes 130 and 131. Filter 10 and cross-flow filter 100 are preferably constructed as follows: a mixture containing AI2O3 and Si0 2 in a molar ratio of about 2 to 1 (an atomic ratio of aluminum to silicon of about 4 to 1) is first prepared. The mixture can be prepared by combining clay and AI2O3, having a net composition of about 76 wt-% AI2O3 and 24 wt-% Si0 2 , the clay and Al 2 0 3 being mixed according to the amount of AI2O3 and Siθ2 in the clay. All of the foil owing percentages, unless indicated otherwise, are also by weight. Suitable clays typically contain about 35 % Siθ2 and about 50 % AI 2 θ3 # so that a typical starting composition prepared from AI2O3 and clay can include about 60 percent clay and about 40 percent Al 2 0 3 . However, the mixture is advantageously and preferably prepared directly from Al 2 0 3 and fused (amorphous) Siθ2 powders of high purity. The mixture of AI2O3 and clay or Si0 2 may be blended with a conventional filler for ceramics, such as wood flour or saw dust, and a convenient amount of water for handling. The filler provides porosity to the mixture upon conversion to acicular mullite, for example, by combustion of an organic filler, or by evolution of water from a hydrated form of AI2O3 or Siθ2- The non-stoichiometric mullite will retain its overall dimensions and its theoretical density upon conversion to its acicular form. The mixture of AI 2 θ3, filler and either S1O2 or clay is formed into any convenient or desired shape for support 1 1 or body 1 1 1. For example, a first portion of the mixture can be extruded in a honeycomb shape to yield honeycomb extrusions 12 and 1 13. The remainder of the mixture can then be used to form other pieces of support 1 1 or body 1 1 1 in a similar fashion. The theoretical density of support 1 1 and body 1 1 1 will vary upon the volume percentage of filler employed in the mixture and the amount of pressure applied during the shape-forming process. Typically, a honeycomb extrusion can be made having sufficient green strength at 40 to 50 % theoretical density. The inclusion of a greater volume percentage of filler allows other shapes to be obtained having theoretical densities as low as 15 %, that is, having 85 % porosity. The shape and theoretical density of support 1 1 and body 1 1 1 are selected to match, respectively, the needs to which filter 10 and cross-flow filter 100 will be put. The shape and theoretical density of support 1 1 and body 1 1 1 are also selected to match, respectively, discriminating layer 38 and membrane 126. Honeycomb extrusion 113 is suitably allowed to dry to a self-sustaining body in the same manner as honeycomb extrusion 12. The open ends of all open channels 1 14 are then placed in fluid communication with each other, preferably by the cuts 16 across said open channel ends, in the diagonal directions shown by arrows A, B and C in Figure 7. End caps 1 18 can be fastened to honeycomb extrusion 1 13 in the same manner as end caps 26 and 30 are fastened to honeycomb extrusion 12. Honeycomb extrusions 12 and 1 13, their respective end caps and, where present, plugs are then oriented vertically and all converted to acicular, non- stoichiometric mullite. With regard to extrusion 113, connecting tubes 124, 130 and 131 can be inserted into end cap holes 120 and recesses 122 and co-fired with extrusion 113 and end caps 118. Alternatively, connecting tubes 124, 130 and 131 can be previouslyformed orfired. In either case, conversion of end caps 118 desirably affixes connecting tubes 124, 130 and 131 to end caps 118. Although not shown, connecting tubes could also be inserted into holes 28 and 32 of structure 10 and processed or converted in the same manner. The conversion is preferably carried out in such a fashion that extrusions 12 and 113, plugs 22 and 23 in extrusion 12, and their respective end caps retain the same theoretical density or porosity as they possessed in their green state. The conversion preferably occurs in a two step process in which AI2O3 and Siθ2 in a molar ratio of about 2 to 1 are heated in a closed system at 500° to 950°C in the presence of SΪF4. The AI2O3, Si0 2 and SiF 4 are heated for a time sufficientto react and form fluorotopaz. The resulting fluorotopaz is then heated to a temperature within a range of 800°C to 1500°C, preferably 975° to 1150°C, more preferably 1 100°to 1 150°C. The actual temperature depends upon SiF 4 vapor pressure. A high vapor pressure favors lowertemperatures and a low vapor pressure requires higher temperatures within said range. The fluorotopaz transforms to non- stoichiometric mullite whiskers, with the evolution of all of the SiF 4 previously absorbed. The reaction is preferably conducted in a closed system so the SiF 4 is recaptured and used for subsequent reactions. If necessary, the temperature can be cycled between these two ranges until SiF^ is no longer absorbed by the material upon cooling down to the lower temperature range. The lack of absorption indicates that no unreacted AI2O3 and Siθ2 remain inthe mixture. Unexpectedly, not only does this process convert the honeycomb extrusions 12 and 113 and their respective plugs and end caps to non-stoichiometric acicular mullite, the whiskers forming them fuse and become interlocked in a three dimensional fashion, so as to fixthe plugs end caps steadfastly to their respective honeycomb extrusion. The resulting support 1 1 or body 11 1 is thus composed of a plurality of pieces that are extremely resistant to separation, since they are structurally uniform with and interlocked with each other, even through the locations at which the pieces abut. If desired, the source of fluorine can be different from SiF 4 . Materials such as AIF3, HF, a2SiF6, NaF, and NH4F are also expected to be useful for this purpose. Once converted to acicular mullite, the exterior of honeycomb extrusions 12 and 113 and their respective end caps are preferably coated with any conventional, compatible glaze, and fired. The glaze forms a nonreactϊve, fluid-impermeable surface, designated as 34 in Figures 1 and 3 and as 134 in Figures 4-6 and 8, over the extrusions 12 and 113 and their respective end caps. The preferred glaze is a mixture of glass frit and clay. One particularly preferred high temperature, commercial glaze for this purpose is sold under the trademark "PEMCO" glaze by Mobay Chemical Company. If desired, conventional techniques that render the exterior of honeycombs 12 and 113 and their respective end caps impermeable without a glaze may be used. An α-Al2θ3 membrane or other discriminating layer 38 or 128 is then applied, respectively, to the surfaces defining the plurality of inlet chambers 36 of the filter 10 or to the surfaces of porous walls 1 12 that define the plurality of open channels 1 14 of filter 100. Layers 38 and 128 can be applied in any fashion conventional for the application of discriminating layers. Layers 38 and 128 may be formed from any conventional coating so long as the resultant layer possesses a pore size less than the pore size, respectively, of honeycomb extrusions 12 and 113. The layers should also meet the respective service requirements of filters 10 and 100. Although many hydrocarbon polymers, fluorocarbon polymers and ceramics may form satisfactory layers 38 and 128, α-Al 2 03 is preferred. One advantage of an α-AI 2 0 3 membrane is that it can be regenerated in filtering operations such as separating insoluble organics from an aqueous slurry. Regeneration occurs by igniting and burning organics trapped on the membrane. Discriminating layers 38 and 128 may include a sintered ceramic layer, such as an C1-AI2O3 membrane having a pore size between 0.15 and 50 μm, useful for a microfilter. If an ultrafilter is desired, smaller pore sizes may be made as described hereinbelow. An α-AI 2 03 discriminating membrane may be applied from an aqueous slurry of -1-AI2O3. The slurry typically contains 50 % or more water. The slurry can also include conventional dispersants in order to prevent agglomeration of the α-Al 2 0 3 powder in the slurry. The slurry can further include an acid or a base in order to adjust the pH of the slurry and improve dispersion of the (1-AI2O3 in the slurry. In addition, the slurry can include an agent for controlling the viscosity of the slurry, such as "METHOCEL" (a registered trademark of The Dow Chemical Company, Midland, Michigan, for its brand of methylcellulose). The powdered C1-AI2O3 used to prepare of the membrane is preferably of a high grade of purity. Impurities in the powder affect the range and uniformity of membrane pore sizes. Alcoa Aluminum Corporation and Vista Corporation each sell C1-AI2O3 powders of near 100 % purity that are useful in the present invention. Typical useful grades of Alcoa α-AI 2 0 3 are grades A-13, A- 16, and A-99. Alcoa grade A-99 powders range in size from 0.15 to 1.0 μm in diameter, with an average diameter of 0.2 μm. Grade A-16 ranges from 0.2 to 10 μm in size, with an average diameter of about 0.4 μm. Grade A-13 ranges in size from 1 to 50 μm, with a mean diameter between 5 and 10 μm. Mixtures of various particle sizes can also be employed in the present invention, to yield any desired pore size in the ceramic membrane. Once the slurry of powdered α-Al 2 03 is prepared, it is introduced into the inlet chamber 36 of filter through the inlet hole 28 in the inlet end cap 26, and allowed to coat the surfaces of inlet channels 18, downstream plugs 22, and the interior surface of inlet end cap 26. Similarly, the slurry is introduced into the cross-flow filter 100 through a first connecting tube 124 and allowed to coat the surfaces of open channels 1 14 and any exposed surfaces of end caps 1 18 within filter 100. Excess slurry is then removed from filter 10 or cross-flow filter 100. The filter, either 10 or 100, is then sintered in order to form a thin, continuous layer of microporous 1-AI2O3. In the event that an ultrafilter is desired, a similar procedure is followed, although a slurry of finer particles is used. This will result in a pore size of somewhat greater than about 0.5 nm. The discriminating layer may also be graded from the highly porous support through an intermediate discriminating layer of microporous material and topped with a layer of ultra- fine porous material, in order to effect more efficient ultrafiltration. If a heat exchanger is desired, membrane 126 is impermeable to the fluids being treated rather than porous. Any of a variety of materials can be used to form impermeable membrane 126. One material is the glaze used for external impermeable coating 136 on body 111. The impermeable membrane is preferably as thin as possible, subject to the strength required of itduring use. Some of the criteria to be considered in assessing the necessary thickness and character of an impermeable membrane include, but are not limited to: high thermal conductivity; low difference in thermal expansivity from that of porous body 1 1 1 ; resistance to deformation under the pressure ofthe fluids to which it is exposed; and resistance to corrosion atthe intended temperature of use. Selection of a particular material and a material thickness for impermeable membrane 126 may be made without undue experimentation. The firing schedules and optimal temperatures for conversion ofthe non- stoichrometric mixture of AI2O3 and Siθ2 or clay to acicular mullite are subjectto ready determination without undue experimentation. However, for the preferred mullite, C1-AI2O3 and glaze compositions disclosed, the following heating schedule is convenient: 1. Filler burn-out and bisque firing. A bisque composed of either assembled green honeycomb extrusion 12, plugs 22 and 23, and end caps 26 and 30 or assembled green honeycomb extrusion 113 and end caps 118 is heated at3"C per minute from room temperature up to 350°C, then at 5°C per minute up to 650°C, and held at that temperature for 60 minutes. The assembly is then heated to 1100 c Cat 5°C per minute, and finally allowed to cool to room temperature at 20°C per minute. 2. Conversion to acicular mullite. The assembled pieces are reheated in nitrogen at a rate of 10°C per minute from room temperature up to 950°C, preferably up to about 750°C. The pieces are then subjected to vacuum, and exposed to a SiF 4 atmosphere over a period of 60 minutes. The assembly is then heated at 20°C per minute up to 1100°C, and held at 1100°C for 60 minutes while SiF4 is removed, for example, by sublimation into another furnace at 750°C. As indicated earlier, the assembly may be allowed to cycle between these two temperatures until conversion is completed, as indicated by a lack of uptake of SiF 4 atthe lower temperature. Nitrogen is then admitted, and the assembly is allowed to cool to room temperature at 25°C per minute. 3. Creation of external glaze. Glazing is carried out by heating the assembly and the applied glaze to 1000°C at a rate of 10°C per minute. The assembly is held at 1000°C for 15 minutes and then cooled to room temperature at a rate of 25°C per minute. 4. Sintering of alpha-alumina membrane. After applying the Al-O slurry to the appropriate surfaces, the assembly is heated at a rate of about 5°C per minute from room temperature to 1550°C. The filter 100 is then held at 1550°C for about 2 hours, and allowed to cool to room temperature at 10°C per minute. The present invention possesses numerous advantages. The interlocked whiskers are bound to each other where they cross, so that a very strong three-dimensional network of o very high porosity is achieved over a wide range of support pore sizes. Relatively large whiskers that are formed in accordance with the present invention yield larger pore sizes than small whiskers. The larger pore sizes allow devices incorporating the present invention to achieve very high flow rates through filters 10 and 100. This advantage is especially important in heat exchangers. The strong network tends to resist flattening under pressure during use, a 5 problem sometimes experienced with conventional Al 2 0 3 or cordierite supports. Perhaps the most significant advantage enjoyed by the present invention, however, is the very economical manner in which ceramic filter supports of a variety of complex shapes can be assembled. Many modifications to the disclosed filters 10 and 100 can be made while retaining the advantages enjoyed by the use of a non-stoichiometric acicular mullite or other fused single crystal acicular ceramic as a membrane or discriminating layer support. The support 1 1 need not be a honeycomb extrusion, but can be any conventional or useful shape, depending upon the environment of filtration or material treatment. Body 1 1 1 disclosed herein is only one example of the variety of shapes a device within the scope of the present invention can take. The present invention therefore provides a filter, a single or dual fluid cross-flow filter or a heat exchanger and a method for maki ng the same that offer numerous advantages. The advantages stem from the high permeability, high mechanical and impact strength, and high thermal resistivity enjoyed by non- stoichiometric acicular mullite and other fused single crystal acicular ceramics that are used as substrates. These characteristics make the present invention useful in a wide variety of environments over an improved range of service temperatures. While our invention has been described in terms of several specific embodiments, other embodiments could readily be adapted by one skilled in the art. Accordingly, the scope of our invention is limited only by the following claims.

Claims: 1. A structure for filtering a fluid or for exchanging heat or a constituent between two fluids, comprising: a body composed of a fused, interlocked, single crystal acicular ceramic material, the body having porous partition walls defining at feast one open channel extending entirely through said body; and means for collecting fluid exiting said bodythrough said partition walls. 2. Astructure as claimed in Claim 1 wherein the means for collecting exiting fluid includes an impermeable coating substantially covering the exterior of said body. 3. Astructure as claimed in of Claim 2 wherein the coating is a glaze. 4. Astructure as claimed in anyone of Claims 1-3 further comprising a membrane disposed on said porous partition walls, thereby lining said at least one open channel. 5. Astructure as claimed in Claim 4, wherein the membrane is impermeable and the structure is used for exchanging heat between two fluids. 6. Astructure as claimed in Claim 4, wherein the structure is used for filtering a fluid or for exchanging a constituent between two fluids and the membrane is a porous discriminating layer selected from a sintered ceramic layer, a layer of a polymeric organic compound, a molecular sieve, a gel filtration layer, or a microporous gaseous diffusion barrier. 7. A structure as claimed in Claim 6, wherein the discriminating layer is a sintered alpha-alumina membrane. 8. A structure as claimed in any one of Claims 1-7 wherein the body includes a honeycomb extrusion having a plurality of parallel open channels defined by said partition walls. 9. A structure as claimed in Claim 8 wherein all ofthe parallel channels are in fluid communication with one another. 10. A structure as claimed in Claim 8, wherein the structure is used for filtering a fluid and further comprises porous plugs of said ceramic material that are disposed in single, alternating ends of said channels. 1 1. A structure as claimed in any of Claims 1 , 9 and 10 wherein the body includes a pair of porous end caps that are composed of the acicular ceramic material, the end caps being abutted to, and interlocked with, the partition walls, each of the end caps having a hole therethrough in fluid communication with the open channel(s), each hole having a connecting tube disposed in and connected thereto 12. A structure as claimed in Claim 1 1 wherein the end caps each include a recess opposite said partition walls extending only partway through said end caps, each recess having a connecting tube disposed in and connected thereto 13 A structure as claimed m any one of Claims 1-12 wherein the ceramic material is non-stoichiometric acicular mullite 14. A method of forming a structure useful for filtering a fluid or for exchanging heat or a constituent between two fluids passing through said structure, comprising: forming a porous green ceramic support from a ceramic material that is capable of being converted in situ into fused, interlocked, single crystal, acicular ceramic particles; converting the ceramic material in situ into fused, interlocked, single crystal, acicular ceramic particles, thereby forming a rigid and porous ceramic support; and applying a porous discriminating layer on at least part of said support 15. A method as claimed in Claim 14, wherein said ceramic material is a non- stoichiometric mullite yielding acicular mullite upon said converting step and said applying step is accomplished by coating at least one surface of said support with a slurry of alpha- alumina, and sintering said support and said alpha-alumina so as to form a thin, continuous membrane of porous alpha- alumina on said at least one surface of said support 16. A method as claimed in Claim 14 or Claim 15 wherein the porous green ceramic support is formed by abutting at least two support pieces together

Inventors: HUGHES NEAL N, MOYER JOHN R, HUGHES, NEAL, N., MOYER, JOHN, R.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: B01D29
IPC codes: B01D29, B01D39, B01D46, B01D63, B01D69, B01D71, B01J35, C04B35, C04B38, F28F21