FILTRATION CASSETTE ARTICLE AND FILTER COMPRISING SAME

Abstract: A filtration cassette (12) of a type comprising filter sheets arranged in a multilaminate peripherally bonded array, wherein the filter sheets (104) alternate with permeate (106) and retentate (102) sheets. A stacked assembly (200) of such filtration cassettes may be employed to form stacked cassette filter. The filtration cassettes and filter of the invention may be advantageously employed for dewatering of aqueous biomass suspensions, desalting of proteins, removal of secreted metabolites from cellular suspensions, and the like.

Description: "FILTRATION CASSETTE ARTICLE AND FILTER COMPRISING SAME" BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to filtration cassettes, of a type comprising membrane filter sheets arranged in a multilaminate, peripherally bonded array wherein the filter sheets alternate with foraminous (e.g., screen or mesh) permeate sheet elements, and ribbed retentate channel elements. The invention further relates generally to cross-flow filters comprising a multiplicity of stacked filtration cassettes of such type, as well as to a method of making such filtration cassettes. Description of the Related Art Stacked plate cross-flow filters are utilized in a variety of solids-liquid separation operations, including the dewatering of solids-liquid suspensions such as aqueous biomass suspensions, the desalting of proteins, and the removal of secreted metabolites from cellular cultures. In such systems, the stacked plates making up the cross-flow filter are typically mounted in a unitary frame structure whereby the respective plates are retained in alignment with one another, in a so-called "plate and frame" construction. The plate and frame filter typically utilizes a unitary liquid feed conduit provided with openings at spaced intervals along its length and extending through the stacked plates as a means to introduce influent solids-containing liquid into the flow channels defined between adjacent plates in the stacked plate assembly. The flow channels in the plate and frame filter contain filter elements, such as disposable filter paper sheets or polymeric membranes, with which the solids-containing liquid is contacted and through which solids-depleted liquid passes. A unitary liquid withdrawal conduit featuring openings at spaced intervals along its length correspondingly extends through the stacked plates, in liquid flow communication with the respective flow channels of the stacked plate assembly, and conveys solids-depleted liquid out of the filter system. As filtration proceeds, the filtered solids build up in the flow channels of the filter, on the "feed liquid sides", i.e., active filtration surfaces, of the filter sheets. The filter then is periodically backwashed, or alternatively, it may be fully shut down after a predetermined time or after a predetermined level of solids has accumulated in the flow channels on the filtration surfaces of the filter sheet elements, following which the system is drained of liquid, and the filter sheets replaced as necessary. In one type of presently marketed stacked filter system, commercially available from Millipore Corporation (Bedford, Massachusetts) as the Prostak® cross-flow filter, the adjacent filter plates define a flow channel. Solids-containing influent liquid is fed at one side of the plate from a central location into a transversely extending feed distribution conduit, which is provided with openings at spaced-apart intervals along the length of the conduit for egress of the solids-containing liquid. At the opposite side of the adjacent plates, the flow channel is similarly constructed with a liquid collection conduit having openings along its length to collect the solids-depleted liquid and discharge same from a central outlet communicating with the collection conduit. A major problem which has been encountered in cross-flow filters of the above- described type is that the liquid flow distribution, as for example reflected by the volumetric liquid flow rate or the liquid superficial velocity, is highly non-uniform in the transverse direction of the flow channel. Such maldistribution of the solids-containing liquid is a result of the fact that (1) the influent liquid is introduced into the feed distribution conduit at a central location, and (2) the cross sectional area of the liquid inlet is smaller than the sum of the inlet cross sectional areas of the cross flow subchannels. Due to the pressure drop in the transverse direction, from the medial inlet port of filter plate assembly to the extremities of the feed distribution conduit, the local longitudinal flow (cross-flow) of liquid from the inlet side to the outlet side of the stacked plates, at progressively farther transverse distances from the central liquid inlet port, is progressively reduced in correspondence to the pressure drop experienced as the liquid is directed transversely to the outer extremities of the distribution conduit. As a result, there is preferential channeling of the liquid at the central part of the flow channel from the inlet side to the outlet side thereof, and concomitant under-utilization of the peripheral areas of the filter. The available filter surface therefore is partially utilized in cross flow and partially in dead end filtration. When the solids in the central portion have been built up to a point requiring backwashing or cleaning of the filter, only that portion of the filter utilized in cross flow can be cleaned. The peripheral areas of the filter sheet remain fouled and cause carry-over of fouled material from one batch process to the next. Such transverse maldistribution of the feed liquid in cross-flow filters of the aforementioned type could conceivably be overcome by the provision of header manifolds to introduce feed liquid into the filtration channels at multiple introduction points along the sides of the stacked filter plates, with a corresponding outlet header manifold arrangement at the opposite side of the stacked plates. Unfortunately, however, such provision would significantly increase the overall system pressure drop as well as the complexity of the filter system, since it would be necessary to positively seal the multiplicity of feed liquid branch lines passing from the manifold into the filter. Another type of stacked plate cross-flow filter which has been commercialized employs a transversely extending liquid distribution conduit with spaced-apart openings therein to introduce solids-containing liquid into the flow channel between adjacent stacked plates, but instead of a central inlet port to flow the solids-containing liquid to such conduit, the liquid is axially fed into the conduit from a feed line connected to a transverse extremity of the conduit. Filters of such type are available from Millipore Corporation (Bedford, Massachusetts) under the trademark Pellicon®. This feed arrangement results in a progressive diminution of the liquid pressure at increasing transverse distances in the feed end of the distribution conduit, which in turn results in progressively transversely decreased cross-flow rates of liquid in the flow channel. In an effort to overcome the aforementioned liquid flow maldistribution characteristics of stacked plate filters, filter plates have been constructed with baffle elements defining discrete flow channels, with the intent of achieving a more uniform distribution of the solids-containing influent liquid across the full areal extent of the filter elements in the flow channels of the filter. A filter plate commercially available from Toyo Soda Manufacturing Company, Ltd. (Tokyo, Japan) has a structure in which solids-containing influent liquid is introduced to the flow channel at a central inlet port along side of the plate. A wall is disposed in front of the liquid inlet, extending upwardly from the floor of the flow channel and transversely toward the extremities of the flow channel, to divide the influent system into two outwardly directed streams. Downstream from such stream-splitting wall is a longitudinally extending divider partition, the steam-splitting wall and the divider partition together forming a "T" construction when viewed in plan view. Longitudinally spaced from a parallel to the stream- splitting wall are a series of baffle partitions on either side of the divider partition. The baffles extend transversely part-way across the flow channel on either side of the divider partition, so that there is formed a serpentine flow path for each of the split streams, on the respective sides of the partition. A unitary liquid outlet port is provided at the opposite side of the stacked plates from the inlet port, whereby the respective serpentine flows are finally joined and discharged from the flow channels of the filter. Although the dual serpentine flow path arrangement described above provides a somewhat better distribution of liquid flow across the area extent of the filter paper element, the sharp turns in the flow path at the extremities of the baffles create edge and entrance effects in the flow streams which produce substantial dead space and bypassing therein. As a result of such anomalous flow phenomena, the filtration efficiency of the baffled serpentine flow arrangement is significantly reduced. A microporous filter article is commercially available from Millipore Corporation (Bedford, Massachusetts) under the trademark MF-Millipore, as a stacked array comprising a top gasket sheet member, a filtrate separator plate, a filter sheet of microporous filtration medium, a retentate separator plate, a second filter sheet of microporous filtration medium, a filtrate separator plate, and a bottom gasket sheet member. The filtrate separator plates and the retentate separator plate include a peripheral border including manifold openings therein, circumscribingly enclosing a central screen or mesh flow channel area. In operation, a recirculating fluid path across the filter sheets is directed by the retentate screen, in the flow channel area of the retentate separator plate, which contacts the two filter sheets. The filter sheets separate the solution or other feed material to be separated, according to the porosity rating of the filter sheets. The filtrate screen, in the flow channel area of the filtrate separator plate, carries away the filtrate to the manifold openings in the peripheral borders of the filtrate separator plates. The filtrate then flows through the manifold openings of the filtrate separator plates to a filtrate collection means such as a vessel which is provided exteriorly of the filter comprising the stacked array of such filter articles. in Tangential ultrafiltration and microfiltration membrane cassette systems are available from Filtron Technology Corporation (Northborough, Massachusetts) under the trademarks Minisette, Centrasette, and Maxisette, which comprise multiple layers of membrane assemblies composed of ultrafiltration or microfiltration membranes disposed between polymeric screen or linear open channel retentate separators and screen filtrate separators. Blocked borders on the filtrate and retentate screens direct the filtrate to collection ports on a bottom cell manifold. Cassettes can be provided in a stacked array, or cell, mounted between end plates, which may be provided with suitable ports, for introduction of feed material to be separated in the cassettes, and for discharge or withdrawal of filtrate and retentate. My prior U.S. Patent 4,882,050 issued September 19, 1989 describes a filter plate characterized by substantially uniform transverse distribution of liquid from a unitary liquid feed port, and highly uniform liquid cross-flow across the full transverse extent of the flow channel formed when plates of such type are stacked to form a cross-flow filter. The filter plate of this prior patent has a generally planar and rectangular shape with a substantially flat bottom surface. A top surface of the plate is provided with an upwardly extending wall circumscribingly bounding a flow channel of generally rectangular shape. A liquid inlet port is disposed at a medial part of a first side of the flow channel, with the liquid outlet port at a medial part of a second side of the flow channel opposite the first side thereof. The liquid inlet port is joined in liquid flow communication with a liquid feed trough extending transversely across the first side of the flow channel, and the liquid outlet port is 1 joined in liquid flow communication with a liquid collection trough extending transversely across the second side of the flow channel. In this construction, a plurality of spaced-apart partitions extend upwardly from the floor of the flow channel between the liquid feed trough and the liquid collection trough. These partitions are of lesser height than the walls circumscribing the flow channel and are substantially parallel to one another, to define a series of sub-channels extending longitudinally between the liquid feed trough and the liquid collection trough. Both the liquid feed trough and the liquid collection trough are of progressively increasing depth from their respective medial portions to their marginal extremities. Plates according to this prior patent may be utilized in stacked pairs to form enclosed flow channels within which filtration may take place in a highly efficient manner. Specifically, a first plate of the type broadly described above is paired with a structurally identical second plate positioned in inverted relationship to the first plate, such that the respective circumscribingly bounding walls of the first and second plates are in abutting sealing contact with one another. In this stacked arrangement, a filter element support of generally rectangular shape approximating the dimensions of the flow channel is interposed between the adjacent plates, with filter sheet elements between the support and each of the respective pair filter plates. My prior U.S. Patent 4,867,876 discloses a filter plate suitable for use with filter elements to form a stacked plate filter. In the stacked plate filter, pairs of such filter plates ? are mated with filter elements therebetween, to form flow channels wherein solids-containing liquid may be contacted with the filter sheet elements for filtration thereof to produce solids- reduced liquid, and permeate. The filter plate of U.S. Patent 4,867,876 has a generally planar shape with a substantially flat bottom surface. A top surface of the plate is provided with a first upwardly extending wall circumscribingly bounding a flow channel of generally rectangular shape. The flow channel in this prior design is circumscribingly bounded by a second upwardly extending wall interior to and of lesser height than the first circumscribingly bounding wall, the second wall being in spaced-relation to the outer wall along diagonally opposed L-shaped peripheral sections of the flow channel, each such L-shaped peripheral sections comprising a leg extending transversely across the flow channel for a major portion of the length thereof, and a leg dimension of the flow channel and communicating at its extremity with an opening extending through the plate, with the portions of the periphery of the flow channel not comprising such L-shaped sections comprising ridge elements extending between the first and second circumscribingly bounding walls. A liquid inlet port is disposed at a first side of the flow channel in this prior design, with a liquid outlet port at a second side of the flow channel opposite the first side thereof. The liquid inlet port in this prior filter plate is joined in liquid flow communication with a liquid feed trough interior to the second bounding wall and extending transversely across the first side of the flow channel. The liquid outlet port is joined in liquid flow 1 communication with a liquid collection trough interior to the second bounding wall and extending transversely across the second side of the flow channel. A plurality of spaced-apart partitions extend upwardly from the floor of the flow channel between the liquid feed trough and the liquid collection trough. Such partitions are substantially parallel to one another to define a series of sub-channels extending longitudinally between the liquid feed trough and the liquid collection trough. These partitions preferably are of lesser height than the first (outer) wall circumscribing the flow channel and of substantially the same height as the second (inner) wall circumscribing the flow channel. Plates of the foregoing type may be utilized in stacked pairs to form enclosed flow channels within which filtration may take place in a highly efficient manner. Specifically, a first plate of this type broadly described above may be paired with a plate of the type broadly described above may be paired with a corresponding second plate positioned in inverted relationship to the first plate, such that the respective first circumscribingly bounding walls of the first and second plates are abutting sealing contact with one another. In such stacked arrangement, a filter element of generally rectangular shape approximating the dimensions of the flow channel is interposed between the adjacent plates, suitably with its peripheral edges reposed on the second bounding wall. Such filter element is provided with an interior flow structure, whereby permeate entering the interior of the element is conveyed to the edge portions of the element for discharge into the aforementioned L-shaped peripheral sections of the flow channel between the respective first and second bounding walls. In an illustrative aspect, the filter element may comprise a foraminous support of generally rectangular shape approximating the dimensions of the flow channel, interposed between the adjacent plates, with filter sheet elements between the foraminous support and each of the respective filter plates. In the operation of a stacked filter plate assembly of the type disclosed in prior U.S. Patent 4,867,876, liquid introduced via the liquid inlet port enters the liquid feed trough and is laterally distributed from the associated portion of the feed trough to outer extremities thereof. The liquid flow is directed into the sub-channels to yield a longitudinal liquid cross- flow which is highly uniform over the full transverse extent of the flow channel, so that the full areal extent of the filter element is highly effectively utilized. As a result, the solids filtration capacity of the stacked plate assembly is substantially increased and the assembly is capable of significantly extended operation prior to regeneration of the filter, as compared to various prior art cross-flow plate and frame filters. My prior U.S. Patent 5,049,268 discloses a filter plate which may be used in invertedly facing pairs, with a sheet holder having filter sheets disposed on its respective faces, mounted between the facing filter plates. The filter plate is characterized by (a) a generally rectangular and generally planar shape with main top and bottom surfaces; (b) a first wall extending upwardly from said main top surface and circumscribingly bounding a flow channel area of generally rectangular shape thereon; (c) a second wall on said main top surface extending upwardly therefrom, said second wall being interior to and of lesser height than said first wall and in spaced relation to the first along diagonally opposed L-shaped peripheral sections of the flow channel area; (d) each L-shaped peripheral section comprising a leg extending transversely across the flow channel area for a major portion of the width thereof and a leg extending longitudinally for a portion of the length of the flow channel area and communicating at its extremity with an opening extending through the plate, with peripheral portions of the flow channel area not comprising such L-shaped sections comprising ridges extending between the first and second walls; (e) an inlet port opening at a corner portion of the filter plate at a first end thereof, extending through the plate, interiorly positioned in a polygonal-shaped distribution basin on the main bottom surface of the plate, said distribution basin being bounded by generally linear side edges defining corners of the basin at respective intersections of the side edges; (f) an inlet trough opening extending through said plate and transversely across a major portion of the width of said flow channel area and along a first side edge of said distribution basin, such that said liquid inlet trough opening communicates said distribution basin on the main bottom surface of the plate with said flow channel area on the main top surface of the plate; (g) an outlet port opening at a corner portion of said plate diagonally opposite said inlet port opening, said outlet port opening extending through the plate, interiorly positioned in a polygonal-shaped collection basin on the main bottom surface of the plate, said collection basin being bounded by generally linear side edges defining corners of the collection basin at respective intersections of the side edges thereof; (h) an outlet trough opening extending through the plate and transversely across a major portion of the width of said flow channel area and long a first side edge of said collection basin, such that said outlet trough opening communicates said collection basin on the main bottom surface of the plate with said flow channel area on the main top surface of the plate; and (i) a plurality of transversely spaced-apart partitions ( > extending upwardly from the floor of the flow channel area between the liquid feed trough and the liquid collection trough, said partitions being of substantially the same height as the second wall and substantially parallel to one another to define a series of channels between the partitions, extending longitudinally between the liquid feed trough and the liquid collection trough. In one specific embodiment disclosed in prior U.S. Patent 5,049,268, the liquid distribution basin and liquid collection basin each have quadrilateral shape, wherein each basin comprises: the port opening being disposed at a first said corner and the side edges intersecting at the first corner defining a first included angle w therebetween of from about 60° to about 110°; a second corner diagonally opposite the first corner, and the side edges intersecting at the second corner defining a second included angle x therebetween of from about 45° to about 90°; a third corner transversely adjacent the first corner and longitudinally adjacent the second corner, with the side edges intersecting at the third corner defining a third included angle y therebetween of from about 70° to about 135°; a fourth corner longitudinally adjacent the first corner and transversely adjacent the third corner, with the sides edges intersecting at the fourth corner defining a fourth angle z therebetween of from about 60° to about 90°; and the side edge extending transversely between the second and fourth corners also bounding the associated trough opening extending through the plate and communicating the basin with the flow channel area. The filter plate disclosed in prior U.S. Patent 5,049,268 is very highly efficient in effecting mass transfer operations, e.g., dewatering of aqueous biomass suspensions, desalting of proteins, and removal of secreted metabolites from cellular suspensions. Such filter plate may be readily formed as a unitary article by injection molding thereof from a variety of plastics materials, such as polypropylene, polysulfone, polyvinylchloride. etc., but due to the relatively complex geometric character of the filter plate, the injection molded plates are relatively expensive to produce. Accordingly, it would be a significant advance in the art, and is therefore an object of the present invention, to provide a filtration article of a type which provides the superior mass transfer efficiency and utility of the filter plate disclosed in prior U.S. Patent 5,049,268, but which is much more economical to produce than the filter plate and sheet holder of such prior U.S. Patent 5,049,268. It is another object of the invention to provide a filter comprising a stack of improved filtration articles of such type. It is another object of the invention to provide a method of making such improved filtration article, in a simple, efficient, and economical manner. Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims. SUMMARY OF THE INVENTION The present invention relates generally to filtration cassettes, of a type comprismg membrane filter sheets arranged in a multilaminate, peripherally bonded array, wherein the filter sheets alternate with permeate sheet elements, and retentate channel elements. In another aspect, the present invention relates to cross-flow filters comprising a multiplicity of stacked filtration cassettes of the general type described in the preceding paragraph, as well as to a method of making such filtration cassettes. In one particular aspect, the present invention relates to a filtration cassette comprising a multilaminate array of sheet members of generally rectangular and generally planar shape with main top and bottom surfaces, wherein the sheet members include in sequence in the array a first retentate sheet, a first filter sheet, a permeate sheet, and second filter sheet, and a second retentate sheet, wherein each of the sheet members in the array has at least one inlet basin opening at one end thereof, and at least one outlet base opening at an opposite end thereof, with at least one permeate passage opening at longitudinal side margin portions of the sheet members; each of the first and second retentate sheets having at least one channel opening therein, wherein each channel opening extends longitudinally between the inlet and outlet basin openings of the sheets in the array and is open through the entire thickness of the retentate sheet, and with each of the first and second retentate sheets being bonded to an adjacent filter sheet about peripheral and side portions thereof, with their basin openings and permeate passage openings and register with one another, and arranged to permit flow of I S ' filtrate through the channel openings of the retentate sheet between the mlet and outlet basin openings to permit permeate flow through the filter sheet to the permeate sheet to the permeate passage openings; the filtration cassette comprising a unitary article of inter-bonded sheet members. In another particular aspect, the present invention relates to a filtration cassette comprising a multilaminate array of sheet members of generally rectangular and generally planar shape with main top and bottom surfaces, wherein the sheet members include in sequence in said array a first retentate sheet, a first filter sheet, a permeate sheet, a second filter sheet, and a second retentate sheet, wherein each of the sheet members in said array has at least one inlet basin opening at one end thereof, and at least one outlet basin opening at an opposite end thereof, with at least one permeate passage opening at a longitudinal side margin portion of the sheet members; each of the first and second retentate sheets having at least one channel opening therein, extending longitudinally between the inlet and outlet basin openings of the sheets in the array, and being bonded (e.g.., compression bonded) to an adjacent filter sheet about peripheral end and side portions thereof, with their basin openings and permeate passage openings in register with one another and the filtrate passage openings of each of the retentate sheets being circumscribingly bonded to the adjacent filter sheet, and with a central portion of each of the retentate sheets and adjacent filter sheets being unbonded to permit l b permeate contacting the retentate sheet to flow through the filter sheet to the permeate sheet; and each of the filter sheets being secured at its peripheral portions on a face thereof opposite the retentate sheet, to the permeate sheet. In another particular aspect, the present invention relates to a filtration cassette comprising a multilaminate array of sheet members of generally rectangular and generally planar shape with main top and bottom surfaces, wherein the sheet members include: a first retentate sheet of suitable material, e.g. polysulfone, polyethersulfone, polycarbonate, urethane, silicone, or other material of construction, having (i) at least one longitudinally extending rib or partition element, such partition element(s) when provided in multiple configuration being transversely spaced-apart from one another and being of substantially the same height and substantially parallel to one another to define a single or a series of channels between the partitions, extending longitudinally between the respective inlet and outlet basin openings of associated filter elements and permeate sheet members, on both faces thereof, (ii) filtrate passage openings at side portions of the sheets, and (iii) the retentate sheet aligned to the first sheet of filter material at respective end and side portions thereof, with the basin openings and filtrate passage openings of the associated sheet members in register with one another and the filtrate passage opening of the retentate sheet member being circumscribingly compressed against the first sheet of filter material, and with a central portion of the first sheet of filter material and the retentate sheet member being unbonded to permit permeate contacting the retentate sheet member to flow through the first sheet member of filter material to the permeate sheet member; a first sheet member of filter material having (i) at least one basin opening, of a suitable shape, e.g., polygonal, semicircular, oval or sector shape, at each of opposite end portions of the sheet member defining respective inlet and outlet passages, and (ii) at least one filtrate passage opening at the side portions of the sheet member, wherein the first sheet member of filter material is bonded to the permeate sheet member at their respective end and side portions, with their basin openings and filtrate passage openings in register with one another and the basin openings being circumscribingly bonded at respective end portions of the first sheet member of filter material and the permeate sheet member, and with a central portion of the first sheet member of filter material and the permeate sheet member being unbonded so as to define a central portion permeate channel of the permeate sheet communicating with the filtrate passages in the first sheet member of filter material and in the permeate sheet member; a permeate sheet member, having (i) multiple basin openings of suitable shape at each of opposite end portions of the sheet member defining respective inlet and outlet passages, and (ii) filtrate passage openings at the side portions of the sheet member; a second sheet member of filter material having (i) at least one basin opening at each of opposite end portions of the sheet member defining respective inlet and outlet passages, and (ii) at least one filtrate passage opening at the side portions of the sheet member, wherein the second sheet member of filter material is compression sealed to the retentate sheet member at their respective end and side portions, with their basin openings and filtrate passage openings in register with one another and the filtrate passage opening of the retentate sheet member being compression sealed to the second sheet member of filter material, and with a central portion of the second sheet member of filter material and the retentate sheet member being unbonded to permit permeate contacting the retentate sheet member to flow through the second sheet member of filter material; and a second retentate sheet member of suitable material, e.g. polysulfone, polyethersulfone, polycarbonate, urethane, silicone, having (i) at least one longitudinally extending rib or partition element, provided that when multiple partition elements are employed, the partition elements are transversely spaced-apart from one another, such partition elements being of substantially the same height and substantially parallel to one another, to define a single channel or a series of channels between the partitions, extending longitudinally between the respective inlet and outlet basin openings of the filter elements and permeate sheet members, on both faces thereof, (ii) filtrate passage openings at the side portions of the sheet member, and (iii) the retentate sheet compression sealed to the second sheet of filter material at respective end and side portions thereof, with their basin openings and filtrate passage openings in register with one another and the filtrate passage opening of the retentate sheet member being compression sealed to the second sheet member of filter material, and with a central portion of the first sheet member of filter material and the retentate sheet member being unbonded to permit permeate contacting the retentate sheet n member to flow through the second sheet member of filter material to the permeate sheet member. As used hereinafter, the term "sheet member" employed in reference to the various laminae of the cassette - the cassette being the stack of generally planar sheet members forming an operative mass transfer unit when coupled to fluid supply, retentate and filtrate flow means - will be variously referred to as simply "sheet." It will therefore be noted that the term "sheet" will denote the generally planar members of the cassette, the cassette thus comprising an assembly of permeate sheets, filter sheets, and retentate sheets, coupled to one another in such manner as to permit flow of the fluid to be separated through the flow channel(s) of the device, for mass transfer involving passage of the filtrate through the filter sheets, and retention of the retentate on the side of the filter sheet opposite the side from which the filtrate emerges. The term "compressible" in reference to the retentate sheet or other structural feature or sheet member of the present invention means that such component or member is compressively deformable by application of load or pressure thereon. The above-described filtration cassette of the invention comprises a "base sequence" of elements, defined as a sequence of sheet elements constituting a retentate sheet (hereafter designated by the symbol "R"), a filter sheet (hereafter designated by the symbol "F"), a permeate sheet (hereafter designated by the symbol "P"), a second filter sheet ("F"), and a second retentate sheet ("R"), thereby providing a sequence of sheet elements, R/F/P/F/R. D The base sequence of sheet elements may be utilized in construction of filters comprising a plurality of filtration cassettes, wherein the retentate sheet is utilized to seal the top and bottom elements of a plurality of filtration cassettes of a sequence, comprising retentate sheet "R", filter sheet "F", permeate sheet P, filter sheet "F", retentate sheet "R", filter sheet "F", permeate sheet P, filter sheet "F", and retentate sheet "R". An illustrative stacked cassette filter according to the invention may for example feature the sheet sequence R/F/P/F/R/F/P/F/R/F/P/F/R, comprising a first retentate sheet, two base sequences of sheets FPFR in a repetitive sequence, and a second retentate sheet. In all repetitive sequences, other than a single cassette base sequence, the following relationship is observed: where X is the number of filter sheets F, the quantity 0.5 X -1 is the number of interior retentate sheets R, and the quantity 0.5 X is the number of permeate sheets P, with two outer retentate sheets being utilized to seal the top and bottom extremities of the intervening sequence. Thus, it is possible to utilize a large number of base sequence cassettes in a repetitive sequence, to provide a stacked cassette filter of the desired mass transfer area. Many configurations are possible. It is feasible in some instances, e.g., for mass transfer studies and system quantitation, to utilize a single cassette comprising the base sequence R/F/P/F/R wherein the outermost retentate sheets in the sequence are compression-sealed at their outer faces to an end plate accommodating removal of permeate from the filtrate passage openings of the constituent sheet members in the cassette. The sheets of filter material used in the cassette article of the present invention may be of any suitable porosity rating. As used herein, the porosity rating of a sheet of filter material is the smallest particle size which cannot pass through the pores of the filter material. Typical porosity ratings are expressed in molecular weight (MW) and micrometer units, e.g., a 2 micron filter media sheet being a material which will pass particles smaller than 2 microns in diameter through the pores of the material, while particles larger than 2 microns will not be passed through the filter material, and as a further example, a 10,000 MW filter media sheet being a material which will pass particles smaller than 10,000 MW in diameter through the pores of the material, while particles larger than 10,000 MW will not be passed through the filter material. In one preferred embodiment of the cassette article of the invention, a retentate sheet is provided with a plurality of transversely spaced-apart, longitudinally extending ribs or partitions, extending upwardly from (the central portion of) each of the main top and bottom faces of the retentate sheet, such ribs or partitions being of substantially the same height and substantially parallel to one another to define a series of channels between the partitions, extending longitudinally between the respective basin openings of the retentate sheet, on both faces thereof. The adjacent filter sheets may be further bonded to the outer extremities of the ribs or partitions, and the ribs or partitions may be formed of any suitable material, e.g., a flexible resilient adhesive bonding medium, such as a urethanes, epoxy or silicone adhesive sealant medium, e.g., applied in a "bead" in the longitudinal direction of the retentate sheet on both main top and bottom faces thereof. As used herein, the term "bonded" in reference to adjacent sheets in the multilaminate cassette means that the adjacent sheets are secured to one another in such manner as to prevent flow of the material being processed, e.g., the feed material to be separated, as well as component materials therefrom (filtrate or permeate, as well as retentate), from flowing through such secured areas or between the adjacent sheets at such secured areas. Preferably, the bonding is carried out with a suitable adhesive or sealant medium, e.g., a urethane, epoxy, cyanoacrylate, or silicone adhesive material, which fills the interstices in the bonded pair of sheets, and adhesively joins one of the adjacent sheets to the other in the bonded areas. As used herein, the term "compressive bonding" and "compress ively bonded" refer to bonding and bonding operations in which the structure being bonded is subjected to a compressive load or force, for sufficient time and under sufficient period to effect the bonding securement of the structure. The invention may for example be carried out with bonding of sheets in the multilaminate array to one another with cyanoacrylate or other "fast" adhesives, or alternatively the adhesive or sealant medium may require extended cure at ambient temperature or other appropriate cure conditions, and it may be advantageous to conduct such cure with the laminate structure in a fixture or other assembly in which the compressive bonding is effectively completed. Compressive bonding of laminae in the practice of the invention is highly desirable, in order to assure the leak-tightness and structural integrity of the resulting multilaminate assembly of the cassette. In one specific embodiment of the cassette article of the invention, the inlet and outlet basins each comprise two quadrilateral openings, wherein each basin opening comprises: V side edges intersecting at a first corner of the basin opening defining a first included angle w therebetween of 90°; a second corner diagonally opposite said first corner, and the side edges intersecting at such second corner defining a second included angle x therebetween of 90°; a third corner transversely adjacent the first corner and longitudinally adjacent the second corner, with the side edges intersecting at the third corner defining a third included angle y therebetween of 90°; a fourth corner longitudinally adjacent the first corner and transversely adjacent the third corner, with the sides edges intersecting at the fourth corner defining a fourth angle z therebetween of 90°. In another embodiment of the cassette article of the invention, the inlet and outlet basins each comprise four quadrilateral openings, wherein each basin opening comprises: side edges intersecting at a first corner of the basin opening defining a first included angle w therebetween of 90°; a second corner diagonally opposite said first corner, and the side edges intersecting at such second corner defining a second included angle x therebetween of 90°; a third corner transversely adjacent the first corner and longitudinally adjacent the second corner, with the side edges intersecting at the third corner defining a third included angle y therebetween of 90°; a fourth corner longitudinally adjacent the first corner and transversely adjacent the third corner, with the sides edges intersecting at the fourth corner defining a fourth angle z therebetween of 90°. Each sheet member in the cassette broadly described hereinabove may suitably comprise filtrate passage openings at the side portions (margins) of the sheet member. In a specific aspect of the invention, each of the permeate and retentate sheets may constitute a foraminous material of from about 80 to about 300 mesh size. Each of the foraminous permeate and retentate sheets may for example comprise a woven polymeric mesh, e.g., of a material selected from the group consisting of polyester, polypropylene, nylon, fluorocarbon polymers such as polytetrafluoroethylene, polyethylene, and polysulfone, and composites comprising one or more of such materials. The permeate and retentate sheets alternatively may comprise a non-woven material, if of suitable foraminous character, or alternatively may comprise a sintered metal matrix, or other structural element of the desired porosity and permeability characteristics. Alternatively, the permeate and retentate sheets may be of an imperforate or solid rigid character. - ^ The filter sheets used in the filtration cassette of the present invention may be of any suitable materials, such as a material selected from the group consisting of cellulose, polyphenylene oxide, polysulfone, cellulose nitrate, cellulose acetate, regenerated cellulose, polyether amide, polyphenylene oxide/polysulfone blends, mixed esters of cellulose, and polyether sulf one. In one specific embodiment of the cassette article of the invention, the inlet and outlet basins of the sheet members are structurally reinforced by radio frequency (RF) welding of urethane or other suitable material such as ethylvinylacetate, silicone, and EPDM. In a particularly preferred specific embodiment of the invention, a first assembly consisting of a first filter sheet, permeate sheet, and second filter sheet are urethane bonded and then die cut to form a first cutout of inlet and outlet basins. The longitudinal ends of the first assembly then are covered on both sides (main faces) thereof with a thin sheet of urethane which completely overlays the inlet and outlet basins. The first assembly with the respective laminae, in the sequence of: urethane sheet, filter sheet, permeate sheet, filter sheet, and urethane sheet, then is RF welded to form a second assembly. The RF welding fuses the two sheets of urethane inside the inlet and outlet basins resulting in an encapsulation of the inlet and outlet basins. The second assembly then is die cut a second time, to create an opening at the inlet and outlet basins which is dimensionally smaller then the first cut. The resulting newly formed inlet and outlet basins are now entirely encapsulated and reinforced by the fused urethane sheets. This specific technique is a substantial improvement to existing stacked cassette elements because it prevents the delamination of ultrafiltration membranes. Numerous membrane types, especially ultrafiltration membranes and reverse osmosis membranes, are cast (manufactured) as a skin layered on a support structure. In use, the turbulence associated with the fluid in the inlet and outlet basins results in delamination and membrane cassette failure. The aforementioned encapsulation procedure reinforces and seals the exposed skin of two part membrane structures wherein the membrane and cassette life is greatly extended. Additionally, the reinforced inlet and outlet basins can be operated at higher velocities for improved operation and cleaning without the occurrence of membrane delamination. Another aspect of the present invention relates to a stacked cassette cross-flow filter comprising cassette articles of the type described above. Still another aspect of the present invention relates to a filter assembly including a pair of end plates or manifold plates between which the cassettes are secured for operation. A still further aspect of the invention relates to a filtration cassette, e.g., of a type as variously described above, comprising bonded portions which are bonded by a bonding technique selected from the group consisting of heat bonding, ultrasonic bonding, radio frequency bonding, and microwave bonding. In yet another aspect of the invention, the filtration cassette comprises a permeate sheet of a rigid character. Another aspect of the invention relates to a filtration cassette of the aforementioned type, wherein the permeate channel is longitudinally continuous. A further aspect of the invention relates to a filtration cassette of theabove described type, in which the basins thereof are devoid of sharp corners. Another aspect of the invention relates to a filtration cassette of the previously described type, in which the inlet and outlet portions of the cassette include a transversely extending weld line interior to a basin thereof, and with adhesive bonding of such end portion interior to the weld line. A still further aspect of the invention relates to a stacked cassette filter comprising a filtration cassette of the type previously described, wherein an outermost retentate sheet is bonded to a rigid endplate, and a compressible gasket is disposed between the rigid endplate and a manifold plate of the filter. Other aspects and features of the present invention will be more fully apparent from the ensuing disclosure and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS M Figure 1 is a perspective view of a filter assembly comprising a filtration cassette according to one embodiment of the present invention with compressible end gaskets and end plates. Figure 2 is an exploded perspective view of a filtration cassette according to one embodiment of the present invention. Figure 3 is a top plan view of a portion of a filtration cassette, featuring two quadrilateral-shaped basins. Figure 4 is a top plan view of a portion of a filtration cassette according to another embodiment of the invention, featuring four quadrilateral-shaped basins. Figure 5 is a perspective elevation view of a filter comprising a multiplicity of cassette articles according to the present invention. Figure 6 is a plan view of a filter sheet of a type which may be usefully employed in the cassette of Figure 2. Figure 7 is a plan view of a retentate sheet of a type which may be usefully employed in the cassette of Figure 2. «, Figure 7A is a top plan view of a portion of a retentate sheet according to another embodiment of the invention, featuring four quadrilateral-shaped basins. Figure 8 is a plan view of a compressible end gasket of a type which may be usefully employed in the cassette of Figure 2. Figure 8A is a plan view of a compressible end gasket according to another embodiment of the invention, featuring four quadrilateral- shaped basins. Figure 9 is a plan view of a formaminous permeate sheet of a type which may be usefully employed in the cassette of Figure 2. Figure 10 is a perspective view of a filter comprising a multiplicity of cassettes and compressible end gaskets of the type shown in Figures 1 and 2. Figure 11 is a top plan view of a filtration cassette according to one embodiment of the invention. Figure 12 is a top plan view of a portion of a cassette, composed of sheet members P/F/P featuring two quadrilateral-shaped basins after the first die cut. 3 c Figure 12A is a top plan view of a portion of a filtration cassette, composed of sheet members P/F/P according to another embodiment of the invention, featuring four quadrilateral-shaped basins after the first die cut. Figure 13 is a top plan view of a portion of a cassette, composed of sheet members P/F/P after RF welding the encapsulating material, and featuring two quadrilateral-shaped basins. Figure 13A is a top plan view of a portion of a filtration cassette, composed of sheet members P/F/P after RF welding the encapsulating material according to another embodiment of the invention, and featuring four quadrilateral-shaped basins. Figure 14 is a top plan view of a portion of a cassette, composed of sheet members P/F/P after RF welding the encapsulating material, and featuring two quadrilateral-shaped basins after the second die cut. Figure 14A is a top plan view of a portion of a filtration cassette, composed of sheet members P/F/P after RF welding the encapsulating material according to another embodiment of the invention, featuring four quadrilateral-shaped basins after the second die cut. Figure 15 is a top plan view of a rigid retentate plate according to one embodiment of the present invention, such as may be employed in the filtration cassette of the invention. Figure 16 is a top plan view of a further embodiment of a rigid permeate plate such as may be employed in the filtration cassette of the invention. Figure 17 is a top plan view of a compressible resilient gasket, such as may be employed between a rigid endplate and a manifold plate of a filter assembly comprising a filtration cassette in accordance with the present invention. Figure 18 is an exploded elevation view of a filter assembly comprising a filtration cassette according to the invention, and utilizing a "hard shell" arrangement with a compressible gasket between the hard shell endplate and the manifold plate of the filter assembly. DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF The disclosures of my prior U.S. Patent Application No. 08/405,493, now issued as U.S. Patent 5,593,580, and my prior U. S. Design Patent No. 357,059 issued April 4, 1995 hereby are incorporated herein by reference, in their entirety. The filter cassette article of the present invention is adapted to be employed in stacked arrays to form a stacked cassette filter assembly in which the base sequence of retentate sheet (R), filter sheet (F), permeate sheet (P), filter sheet (F), and retentate sheet (R) may be repeated in the sequence of sheets in the filter assembly as desired, e.g., in a repetitive sequence of retentate sheet (R), filter sheet (F) , retentate sheet (R), filter sheet (F), permeate sheet (P), filter sheet (F) , retentate sheet (R), filter sheet (F), permeate sheet (P), filter sheet (F), retentate sheet (R), filter sheet (F), retentate sheet (R). Thus, the filter cassette of a desired total mass transfer area is readily formed from a stack of the repetitive sequences. In all repetitive sequences, except for a single unit sequence, the following relationship is observed: where X is the number of filter sheets, 0.5X-1 is the number of interior retentate sheets, and 0.5X is the number of permeate sheets, with two outer retentate sheets being provided at the outer extremities of the stacked sheet array. The filter sheets, and the retentate and permeate sheets employed therewith, may be formed of any suitable materials of construction, including, for example, polymers, such as polypropylene, polyethylene, polysulfone, polyethersulfone, polyetherimide, polyimide, polyvinylchloride, polyester, etc.; nylon, silicone, urethane, regenerated cellulose, polycarbonate, cellulose acetate, cellulose triacetate, cellulose nitrate, mixed esters of cellulose, etc.; ceramics, e.g., oxides of silicon, zirconium, and/or aluminum; metals such as stainless steel; polymeric fluorocarbons such as polytetrafluoroethylene; and compatible alloys, mixtures and composites of such materials. Preferably, the filter sheets and the retentate and permeate sheets are made of materials which are adapted to accommodate high temperatures and chemical sterilants, so that the interior surfaces of the filter may be steam sterilized and/or chemically sanitized for regeneration and reuse, as "steam-in-place" and/or "sterilizable in situ" structures, respectively. Steam sterilization typically may be carried out at temperatures on the order of from about 121°C to about 130°C, at steam pressures of 15-30 psi, and at a sterilization exposure time typically on the order of from about 15 minutes to about 2 hours, or even longer. Alternatively, the entire cassette structure may be formed of materials which render the cassette article disposable in character. The end plates used with the cassette articles of the invention to form a unitary filter assembly may be formed of any suitable materials of construction, including, for example, stainless steel or other suitable metal, or polymers such as polypropylene, polysulfone, and polyetherimide. Figure 1 is a perspective view of a cross-flow filter 10 comprising a cassette article 12 according to one embodiment of the present invention. The cassette is of a multilaminate construction, comprising a plurality of sheets as described hereinafter in greater detail, peripherally bonded to one another at their end and side margins. For ease of description in the ensuing discussion, the respective corners of the cassette as shown in Figure 1 are lettered consecutively, beginning at the lower right-hand corner A as shown and, proceeding counterclockwise, including upper right-hand corner B, upper left-hand corner C, and lower left-hand corner D. In the vicinity of these consecutive corners A, B, C, and D of the cassette are provided openings 16, 18, 20, and 22, respectively, which extend through the cassette and are employed for mounting of the cassette on rods of diameter closely approximate but slightly smaller than the respective openings. In this respect, it is to be noted that opening 22 is of larger size (diameter) than the remaining openings 16, 18, and 20. The purpose of such disparity in opening size is to provide a "keying" feature whereby the proper alignment of the plate is secured, since only opening 22 will fit over a large-sized rod of corresponding diameter, whereas openings 16, 18, and 20 will not accommodate passage over such a large- sized rod. Other features or structures may be employed for the same purpose, viz., of keying the cassette to a predetermined proper orientation, in place of the different sized opening 22 relative to the remaining same-sized corner openings 16, 18, and 20. For example, one of the respective corner openings may be of a different shape than the others, e.g., square or triangular in cross-section, rather than circular. Alternatively, the corner profile of one corner of the cassette may be notched or differently shaped in relation to the other corners, to provide a readily visibly discernible keying or orientational structure of the cassette article. The filtration cassette 12 is provided at its respective side margins with openings 64, 66, 68, and 70 which are smaller than openings 16, 18, and 20 and which are employed for keeping the compressible retentate sheet in place. The filtration cassette 12 also is provided at its respective side margins, at the mid- section of the longitudinally extending cassette, with openings 24 A, B, C, D, E and 26 A, B, C, D, E extending through the cassette. These openings may be employed for egress of permeate produced in the filtration operation when the cassette is deployed in the stacked cassette filter assembly illustrated, and/or otherwise for accommodating ingress/egress of a selected fluid, such as steam or other sterilant fluid for effecting cleaning and regeneration of the filter, or a secondary fluid for mass transfer contacting with a primary fluid passage through the filter. This filter 10 comprises a base including a mounting plate 30 having vertically upwardly extending rods 34, 36, and 38 at its respective corner portions as shown. Each of the rods 34, and 36 are of the same diameter, while the fourth rod 38 is of larger diameter, to provide the plate orientation keying structure, which will ensure that the constituent plates of the filter assembly are assembled in the proper orientation, since the corresponding rod mounting openings 18 and 20 in the cassettes, e.g., cassette 12, are of the same diameter, accommodating the smaller diameter rods, while the third rod mounting opening 22 is of larger diameter, to accommodate rod 38. Thus, by providing a rod of larger diameter, and forming the cassettes 12 with correspondingly shaped openings, the proper registration of the cassette openings with the proper rods is assured, resulting in correct orientation of the respective stacked filtration cassettes in the array. This filter 10 comprises a base including a mounting plate 30 having vertically upwardly extending rods 72, 74, 76, and 78 correspond to openings 64, 66, 68, and 70 for mounting and alignment of the compressible retentate sheets 14A and 14B. 3L It will be appreciated from the foregoing that any other cassette orientation registration device may be employed to ensure to correct positioning of the successive stacked filtration cassettes on the mounting plate 30. For example, the cassettes are oriented with their successive notches superposed with respect to one another. Alternatively, the cassette itself may be embossed, etched, or otherwise manufactured with an orientational device, e.g., a raised protrusion in the shape of an arrow, to indicate the correct orientation of the filtration cassettes when stacked on the mounting plate 30. Although only one filtration cassette is illustratively shown in the Figure 1 embodiment, it will be recognized that one or a plurality of cassettes may be employed to form a filter in accordance with the present invention. Between the cassette 12 and mounting plate 30, there is provided a compressible retentate sheet 14B which is equipped with openings to accommodate its positioning over the respective rods 34, 36, 38, 72, 74, 76, and 78 so that the compressible retentate sheet seals the bottom surface of the cassette 12. The retentate sheets in the multilaminate array may for example have a thickness between 0.125 mm and 6.0 mm, and the other sheets in the array may have correspondingly dimensioned thicknesses appropriate to the mass transfer operation being conducted with the cassette. Overlying the cassette 12 in the exploded array of Figure 1 there is provided a compressible retentate sheet 14A which is equipped with openings to accommodate its positioning over the respective rods 34, 36, 38, 72, 74, 76, and 78 so that the compressible retentate sheet seals the top surface of the cassette 12. 31 Overlying the cassette 12 in the exploded array of Figure 1 is an upper end plate 60, which is provided with suitable openings 18, 20, 22, 64, 66, 68, and 70 accommodating the insertion therethrough of the rods 34, 36, 38, 72, 74, 76, and 78. Overlying the cassette 12 in the exploded array of Figure 1 is an upper end plate 60, which as shown is suitably engaged by mechanical fastener assemblies 85, comprising washer 86 and lock-nut 88, which threadably engages the complementarily threaded upper ends of the respective rods 84A...84K, and 84L. At the upper left-hand corner portion C of the mounting plate 30 shown in Figure 1, adjacent to rod 36, there is provided a liquid outlet conduit 8 in flow communication with openings 50 A and 50B extending through the end plate 30 and communicating with the quadrilateral-shaped collection basins 54 A and 54B of the cassette. The mounting plate 30 also is provided at its right-hand side margins, at the distal section of the longitudinally extending plate, with permeate collection trough 58 which is in flow communication with permeate outlet conduits 62 A and 62B. This is employed for egress of permeate produced in the filtration operation (and issuing from filtrate or permeate opening 26 A, B, C, D, E in cassette 12) and/or otherwise for accommodating ingress/egress of a selected fluid, such as steam or other sterilant fluid for effecting cleaning and regeneration of the filter. At the lower right-hand corner portion A of the end plate 60 shown in Figure 1 there is provided a liquid inlet conduit 44 in flow communication with openings 46A and 46B (not shown) extending through the end plate 60 and communicating with the quadrilateral- shaped collection basins 48 A and 48B of the cassette. The end plate 60 also is provided at its left-hand side margin, at the near-section of the longitudinally extending plate, with permeate collection trough 56 (not shown) in flow communication with permeate outlet conduits 52A and 52B. This conduit is employed for egress of permeate produced in the filtration operation (and issuing from filtrate or permeate opening 24 A, B, C, D, E in cassette 12), and/or otherwise for accommodating ingress/egress of a selected fluid, such as steam or other sterilant fluid for effecting cleaning and regeneration of the filter. The cassette 12 shown in Figure 1 has a quadrilateral-shaped feed distribution trough, in which liquid entering in feed liquid conduit 44 issues from feed liquid openings 46A and 46B (not shown) therein and is distributed in the feed distribution basin 48A and 48B, from which it passes longitudinally through the cassette 12, in a flow channel provided between the retentate sheet and adjacent filter sheets, as hereinafter more fully described. In the flow channel, the permeate components of the feed material passes through the filter sheets adjacent to the retentate sheet, and flows into next-adjacent permeate channels, from which the permeate flows to openings 24 A, B, C, D, E and 26 A, B, C, D, E. The retentate then issues from an opposite end portion of the central flow channel of the cassette into the collection basins 54A and 54B, from which the retentate is discharged from the filter through openings 50 A and 5 OB into liquid conduit 8. It will be appreciated that the permeate openings 24 A, B, C, D, E and 26 A, B, C, D, E may, as previously described, be coupled to a flow circuit including a second mass transfer medium which is to be passed in mass transfer relationship to the primary feed material stream entering the filter in conduit 44. By such arrangement, mass transfer is able to be carried out in both directions across the filter sheets in the cassette. Alternatively, the filter may be used as shown, with the openings 24 A, B, C, D, E and 26 A, B, C, D, E being used for discharge of permeate. It will be recognized that instead of two permeate conduits 62A and 62B on the mounting plate 30, and two permeate conduits 52A and 52B on the end plate 60, both permeate outlet conduits may be provided on mounting plate 30, or on end plate 60, or alternatively, both plates may feature any number of permeate discharge conduits secured thereto and in communication with the permeate openings of the cassette(s). Figure 2 is an exploded perspective view of a cassette 12 employed in the Figure 1 filter, and comprising, a compressible retentate sheet 102, a filter sheet 104, a foraminous permeate sheet 106, a filter sheet 108, and a compressible retentate sheet 110, as constituent sheets in the multilayer array. As shown, each of the constituent filter sheets 104, 106, and 108 has a generally quadrilateral-shaped cut-out opening 114A and 114B at one end thereof, and a similar cut out Ho opening 112A and 112B at the opposite end thereof, and each of the sheets is provided with corner openings 16, 18, 20, 22, 64, 66, 68, and 70. As shown, each of the compressible retentate sheets 102 and 110 have quadrilateral retentate channels 116A and 116B. Each of sheets is generally co-extensive in areal extent with the others, and when consolidated into a unitary cassette article, the cutout openings 112A and 112B and 114A and 114B and retentate channels 116 A and 116B in each of the respective sheet elements are in registration, as are the rod openings 16, 18, 20, 22, 64, 66, 68, and 70. The sheets are each bonded at their side and end extremities to the next adjacent sheet in the cassette, and the retentate sheet 102 is bonded at its peripheral region 120 to the corresponding peripheral region of filter sheet 104, and in like manner the retentate sheet 110 is bonded at its peripheral portion 122 to filter sheet 108. Permeate sheet 106 is bonded at its outer periphery 124 on its top face to filter sheet 104, and the permeate sheet at its bottom face is bonded along its peripheral region 126 to filter sheet 108. On the retentate sheets, a longitudinally extending, transversely spaced-apart rib 128 is provided, extending from the inlet basin opening 112 and 112B to the collection basin opening 114A and 114B. The permeate sheet 106 is suitably bonded to the adjacent filter sheets (108 and 104, respectively) in such manner as to leave an unbonded central area 130 on permeate sheet 106 which communicates with the filtrate or permeate openings 24 A, B, C, D, E and 26 A, B, C, D, E whereby filtrate is readily removed from the cassette in use. Figure 3 is a top plan view of a portion of a cassette article 12, wherein the collection basin is composed of two quadrilateral shaped basins 54A and 54B. Figure 4 is a top plan view of a cassette 12 according to another embodiment of the invention featuring four quadrilateral shaped basins 54A, 54B, 54C, and 54D. Shown in dashed line representation in the basin is the liquid collection conduit 50A, 50B, 50C, and 50D, denoting the position of the liquid withdrawal conduit relative to the basin structure. It will be recognized that the inlet distribution basins corresponding to the outlet collection basin shown in Figures 3 and 4, respectively, are similarly configured, with respect to their shape and component angles, relative to the same cassette article, although the distribution and collection basins could in some applications advantageously be hemispherical or of different shape and size characteristics. Although the quadrilateral shape of the respective liquid distribution basins 48A and 48B and collection basins 54A and 54B may be widely varied in the broad practice of the present invention, as regards the specific values of the corner angles of such basins, the specific shape and angles shown in Figures 3 and 4 are most preferred to facilitate uniformity of flow path length for liquid across the entire areal extent of the respective basins and flow channel area, i.e., a generally uniform velocity profile of the fluid flowing longitudinally across the flow channel area of the cassette. In the filtration cassette shown in Figure 4, wherein the distribution basins each comprise four quadrilateral sub-basins 54A, 54B, 54C, and 54D, each sub-basin suitably comprises: side edges intersecting at a first corner defining a first included angle a therebetween of 90°; side edges intersecting at a second comer defining an included angle b therebetween of from about 90°; a third comer transversely opposite the first comer, with the side edges intersecting at the third comer defining an included angle c therebetween of 90°; and a fourth comer transversely opposite the third corner, with the side edges intersecting at the fourth comer defining an included angle d therebetween of 90°. In like manner the distribution and collection basins could in some applications advantageously be hemispherical or of different shape and size characteristics. The filter cassette of the present invention may be fabricated in any suitable manner, including casting, injection molding, dielectric bonding, solvent bonding, adhesive bonding, etc., the specific method of fabrication depending on the material of construction and the desired end use. For example, polysulfone is a preferred material of construction for the filter sheets, alternating with polysulfone retentate sheets, and foraminous polypropylene permeate sheets, and bonded by urethane or cyanoacrylate adhesive. In one particularly preferred embodiment of the invention, the cyanoacrylate adhesive Super Bonder #4981 from Loctite Corporation, Newington, CT is utilized the adhesive bonding medium for construction of the cassette. M In another particularly preferred embodiment of the invention, a two part urethane CONATHAMES #DPST-20256 (part A and part B) Conap, Inc., Olean, NY is employed as the cassette bonding material. When polysulfone, polypropylene, and urethane are utilized as materials of construction, dielectric bonding may suitably be employed as the method of fabrication. Alternatively, when polypropylene, polypropylene, and cyanoacrylates are utilized as the materials of construction, wet bonding may suitably be employed as the method of fabrication. Figure 5 shows a perspective elevation view of a filter according to one embodiment of the invention, utilizing a plurality of cassettes according to the invention, to form a stacked cassette array. This stacked cassette filter 200 comprises a base 202 including a mounting plate 30 having vertically upwardly extending rods 34, 36, 38, 72, 74, 76, and 78 at the respective comer and side portions as shown. Each of the rods 34 and 36 is of the same diameter, while the third rod 38 is of larger diameter, to provide a cassette orientation keying structure, which will ensure that the constituent cassettes and compressible retentate sheets of the filter assembly are assembled in the proper orientation, with the corresponding rod mounting openings in the cassette being appropriately sized for such rods. Rods 72, 74, 76, and 78 are of the same diameter to provide support and alignment of the compressible retentate sheet. The stacked cassettes 220 form a stack cassette array 222 which is retained between the plate 30 and end plate 60, as shown, with mechanical fastener means 85 being provided on the respective threaded rods to provide the necessary compressive action on the cassette array 222. The upper end plate features a permeate outlet conduit 52A and 52B, and a corresponding permeate egress conduit is provided on the opposite side at the bottom of plate 30, 62 A and 62B. At the upper left-hand corner portion of mounting plate 30 is provided a retentate discharge conduit 8, and at the lower right-hand of the filter is provided a feed material inlet conduit 44, as shown. Inlet conduit 44 may be joined to a suitable source of material to be separated, which then can be provided under suitable temperature, pressure, and flow rate conditions to the filter 200 for separation therein into retentate, discharged in conduit 8, and permeate, discharged in conduit 52A and 52B (as well as the corresponding filtrate or permeate conduits 62 A and 62B at the opposite lower portion of the filter). Figure 6 is a plan view of a filter sheet of a type which may be usefully employed in the cassette of Figure 2, and wherein all parts and elements are correspondingly numbered to Figure 2. Ir Figure 7 is a plan view of a retentate sheet of a type which may be usefully employed in the cassette of Figure 2 , and wherein all parts and elements are correspondingly numbered to Figure 2. Figure 7A is a top plan view of a portion of a retentate sheet according to another embodiment of the invention, featuring four quadrilateral-shaped basins 116 A, B, C, and D, and featuring additional permeate channels 24 F-L and 26 F-L, beyond the corresponding two basin structure of Figure 7, but wherein all other elements of Figure 7A are numbered correspondingly to Figure 7. Figure 8 is a plan view of a compressible end gasket of a type which may be usefully employed in the cassette of Figure 2, featuring longitudinal retentate channels 80 A and B separated by rib 90, and wherein the permeate openings and rod mounting openings are numbered correspondingly to Figure 2. Figure 8A is a plan view of a compressible end gasket according to another embodiment of the invention, featuring four quadrilateral-shaped basins 80 A-D, separated by longitudinally extending, transversely spaced-apart ribs 90 A-C, and wherein the permeate openings and rod mounting openings are numbered correspondingly to Figure 8 and Figure 2. Figure 9 is a plan view of a formaminous permeate sheet of a type which may be usefully employed in the cassette of Figure 2, and which is numbered correspondingly to Figure 2. Figure 10 is a perspective exploded view of a filter 222 comprising a multiplicity of cassettes and compressible end gaskets of the type shown in Figures 1 and 2, and wherein corresponding parts and elements are numbered correspondingly to Figures 1 and 2. Figure 11 is a perspective view of a filtration cassette according to one embodiment of the invention, wherein the parts and elements are numbered correspondingly to Figure 2. Figure 12 is a top plan view of a portion of a cassette 302, composed of sheet members P/F/P featuring two quadrilateral-shaped basins 314 A, B at one end portion of the multilaminate stack, and two quadrilateral-shaped basins 312 A, B at the other end portion of the multilaminate stack, after the first die cut. Figure 12A is a top plan view of a portion of a filtration cassette, composed of sheet members P/F/P according to another embodiment of the invention, featuring four quadrilateral-shaped basins 314 A-D, after the first die cut. Figure 13 is a top plan view of a portion of the cassette of Figure 12, numbered correspondingly thereto, and composed of sheet members P/F/P after RF welding the encapsulating material 324. l Figure 13A is a top plan view of a portion of the filtration cassette of Figure 12A, numbered correspondingly thereto, and composed of sheet members P/F/P after RF welding the encapsulating material 324. Figure 14 is a top plan view of the cassette of Figures 12 and 13, numbered correspondingly thereto, and composed of sheet members P/F/P after RF welding the encapsulating material, and featuring two quadrilateral-shaped basins after the second die cut. Figure 14A is a top plan view of the filtration cassette of Figures 12A and 13 A, numbered correspondingly thereto, and composed of sheet members P/F/P after RF welding the encapsulating material, and featuring four quadrilateral-shaped basins after the second die cut. Figure 15 is a top plan view of a rigid permeate sheet 400 which may be employed in the practice of the present invention, as a component sheet of the filtration cassette. As shown, the permeate sheet comprises a plate member 402 of generally rectangular form, being elongate in the vertical direction as illustrated. The permeate sheet has a main top surface 430 and an opposite main bottom surface (not shown in the view of Figure 15). At the corners of the permeate sheet 400 are provided mounting openings 406, 408, 410 and 412. Opening 406 is of larger size than the remaining openings 408, 410, and 412, as a keying feature of the sheet. The permeate sheet, which may be of significant thickness in relation to the filter sheets and permeate sheets, is provided at its respective ends with basin openings 422 and 424. The basin openings may be single openings at each end of the sheet, or as in the illustrated embodiment, may comprise multiple openings at each end. Preferably such basin openings are characterized by the absence of sharp corners and thus may be provided with the rounded edge corner conformation shown. The permeate sheet also is provided with registration openings 414, 416, 418 and 420, which likewise may provide a keying function, relative to correct orientation of the permeate sheet and associated sheets of the filtration cassette as assembled therewith. The permeate sheet shown in Figure 15 is provided with a central permeate collection region 450 which is characterized by a series of longitudinally spaced-apart ridge elements having valleys therebetween so as to provide transversely extending channels in the sheet. By means of this permeate collection region, permeate passing through the filtration sheet at such face of the permeate sheet contacts the ridged surface of the collection region and is channeled to the marginal, longitudinally extending permeate troughs 436 and 438. The channeled permeate flows through the left-hand passages 432 to the permeate discharge openings 426, with permeate concurrently and likewise flowing through right-hand passages 434 to the right-hand permeate discharge openings 428. In this manner, the permeate passing through the filter sheet at each of the main top and bottom faces of the permeate sheet is contacted with the collection region 450 of the sheet and is channeled into the longitudinal troughs and respective permeate discharge openings 426, 428. It will be understood that the central collection region 450 of the permeate sheet may be configured in any of a wide variety of structural and geometric arrangements to channel permeate to the respective discharge openings of the sheet. Figure 16 is a top plan view of a permeate sheet 500 according to another embodiment of the invention. The corresponding parts and elements of Figure 16 are numbered correspondingly to the parts and elements of Figure 15, by addition of 100 to the reference numbers of the corresponding parts and elements of Figure 15. In the Figure 16 embodiment, the permeate discharge openings 526 and 528 on the respective left-hand and right-hand margins of the permeate sheet are each longitudinally continuous. Otherwise the permeate sheet of Figure 16 corresponds to the structure and characteristics of the permeate sheet of Figure 15. The longitudinally continuous permeate openings 526 and 528 are employed in the plate of Figure 16 to facilitate lay-up and assembly of the corresponding filtration cassette. It has been found that when the filtration cassette is of substantial length (elongate or longitudinal extent), and the permeate plate includes a series of longitudinally spaced-apart permeate openings, registration of the respective filter sheets, permeate sheets, and retentate sheets is frequently difficult, and that the provision of a continuous permeate opening at each $0 of the respective sides of the elongate permeate sheet permits a more easily facilitated registration of the filter sheet and retentate sheets in the fabrication of the filtration cassette. Figure 17 is a top plan view of a flexible resilient gasket 600, such as may be employed between an exterior rigid endplate when employed in a filtration cassette assembly, and a manifold plate of the filter comprising such filtration cassette. The gasket 600 comprises an elongate body 602 having a top main surface 630 and a corresponding bottom main surface (not shown in the view of Figure 17), with mounting openings 606, 608, 610 and 612. Basin opening 622, 624 provided at opposite end portions of the gasket. The gasket also features alignment opening 614, 616, 618 and 620, and longitudinally spaced- apart left-hand permeate openings 626 and longitudinally spaced-apart permeate openings 628. The gasket 600 permits a "hard shell" filtration cassette, i.e., a filtration cassette comprising an outermost rigid endplate, to be efficiently sealingly mated to a manifold plate of a filter comprising the filtration cassette. The gasket may be compressively retained between the endplate of the filtration cassette and the manifold plate of the filter, as hereafter described. Figure 18 is an exploded elevation view of a filter assembly FA comprising a filtration cassette FC according to the invention, utilizing a "hard shell" arrangement, with 5- compressible gaskets G,, G 2 between the respective hard shell endplates EP,, EP 2 and the respective manifold plates MP,, MP 2 of the filter assembly. The filtration cassette FC includes a central permeate sheet P. to the respective faces of which are bonded the filter sheets F, and F 2 . Exterior to the the filter sheets in the cassette array are retentate sheets R, and R . The retentate sheets R, and R 2 may be of rigid and solid form, and are respectively bonded to the end plates EP,, EP 2 to complete the structure of the filtration cassette. It will be appreciated that the filter assembly FA may comprise a number of sequentially arranged filtration cassettes FC, only one being shown in Figure 18 for ease of simplicity and to facilitate description. In this manner, a "hard shell" cassette is formed. A gasket Gi is provided between the endplate EP, and the manifold plate MP, of the filter assembly, such manifold plate having an outlet O from which the filtered retentate may be discharged from the filter assembly, as well as a permeate port PT from which the collected permeate is discharged from the filter assembly. The permeate port PT may be provided in duplicate, with one port located behind the visible port PT in the schematic drawing of Figure 18, so that a separate port is provided at each of the margins of the assembly communicating with the permeate collection troughs of the side margins of the filtration cassette(s) in the filter assembly. In like manner, a gasket G 2 is provided between the endplate EP 2 and the manifold plate MP 2 of the filter assembly, such manifold plate having an inlet I to which the material to be filtered is introduced to the filter assembly. In such filter assembly, the gaskets effect sealing of the hard shell filtration cassette against the manifold plates of the filter assembly, with each of the sheets EP,, Rj, F,, P, F 2 , R 2 , and EP 2 being interbonded to form a unitary cassette article. It will be appreciated that the filtration cassette of the present invention may be variously fabricated, utilizing foraminous, e.g., screen or mesh, or rigid sheets in its construction, the choice of a specific type component sheet being readily determinable for a given end use application of the invention. As mentioned, the filtration cassette of the present invention comprises a unitary particle including inter-bonded sheet members, wherein bonding may be variously effected, e.g., by techniques including heat bonding, ultrasonic bonding, radio frequency bonding, microwave bonding, adhesive bonding, etc. In some instances it may be desirable to form one or more of the sheets with portions thereof including carbon filler, whereby mated sheets in the array of sheet members may be microwave bonded to one another. For example, the retentate sheet could be formed with a carbon impregnated peripheral portion which when sandwiched with the filter sheet and the permeate sheet in sequence may be subjected to microwave heating in a microwave oven to bond the constituent retentate sheets to the adjacent sheets. Alternatively, the constituent sheet members may be laid up with microwave heating tapes being applied to the areas to be bonded, whereby such tapes under microwave -T 3 conditions are heated to effect bonding of the sheets between which the tape is interposed. Microwave susceptor strips, ribbons, or particles may for example be employed on or within the corresponding sheets, so that fabrication can be carried out in a conventional microwave oven environment. Accordingly, the invention contemplates the provisions of sheet members including regions or portions thereof which are bondable to corresponding areas of adjacent sheets under exposure to microwave radiation. Alternatively, radiation frequency or ultrasonic bonding could be utilized with a radiation-susceptible material being employed to effect bonding of the appertaining sheet areas. As mentioned herein, it is preferred that the inlet and outlet basins of the filtration cassette be devoid of sharp corners, so that any intersecting linear surfaces converge at a rounded or curvate corner. Such configuration assists the hydrodynamic characteristics of the cassette and facilitates its cleaning, since the presence of sharp corners can produce stagnant areas in which solids, e.g., biomass, proteins, or other solid or depositable species can accumulate and adversely effect the clean (e.g., sterile) character of the filtration cassette. It will be apparent from the foregoing that the respective sections of stacked cassettes may be variously joined in fluid flow communication with one another, in parallel or in series, to form stacked filter "trains" whose constituent sections may be employed to carry out a number of unit operations on an influent or feed material, such as concentrating (dewatering), washing, dialyzing, desalting, etc. $\ For example, a stacked filter train of series-connected sections may be employed in a culturing system of the type disclosed and claimed in my prior U.S. Patent 4,885,087, and U.S. Serial No. 07/207,655 filed June 21, 1988 (and corresponding international patent application no. PCT/US89/02011 filed 10 May 1989 for "CULTURE DEVICE AND METHOD"), the disclosures of which hereby are incorporated by reference, in applications such as the production in vitro of human immunodeficiency virus (HIV) on cellular or synthetic substrates. In such HIV production application, a first stacked cassette section could be employed to concentrate HIV, a second section could be utilized to add media to or withdraw media from the system, all without withdrawing any virus, such as may otherwise present a risk of immunosuppressive infection. Thus, a closed system virus culturing arrangement is provided, which is highly advantageous not only for the production of HIV but also the culturing or other processing of pathogenic as well as non-pathogenic bacterial, viral, and yeast species. Industrial Applicability The filtration cassettes of the invention may be usefully employed to perform various mass transfer operations, such as liquid-liquid extraction, dewatering and concentration of solids suspensions, desalting of proteins, introduction of nutrient media for biomass culturing, harvesting of secreted metabolites and other products for cellular suspensions, etc. The cassette articles of the invention are particularly useful for forming stacked cassette filter assemblies. In such assemblies, respective sections of stacked cassettes may be variously joined in fluid flow communication with one another, in parrallel or in series, to form stacked filter "trains" whose constituent sections may be employed to carry out a number of unit operations on an influent or feed material, such as concentrating (dewatering), washing, dialyzing, desalting, etc. For example, a stacked filter train of series-connected sections may be employed in a culturing system of the type disclosed and claimed in my prior U.S. Patent 4,885,087, and U.S. Serial No. 07/207,655 filed June 21, 1988 (and corresponding international patent application no. PCT/US89/02011 filed 10 May 1989 for "CULTURE DEVICE AND METHOD"), the disclosures of which hereby are incorporated by reference, in applications such as the production in vitro of human immunodeficiency virus (HIV) on cellular or synthetic substrates. In such HIV production application, a first stacked cassette section could be employed to concentrate HIV, a second section could be utilized to add media to or withdraw media from the system, all without withdrawing any virus, such as may otherwise present a risk of immunosuppressive infection. Thus, a closed system virus culturing arrangement is provided, which is highly advantageous not only for the production of HIV but also the culturing or other processing of pathogenic as well as non-pathogenic bacterial, viral, and yeast species.

Claims: THE CLAIMS 1 „ A filtration cassette comprising a multilaminate array of sheet members of generally rectangular and generally planar shape with main top and bottom surfaces, wherein the sheet members include in sequence in the array a first retentate sheet, a first filter sheet, a permeate sheet, and second filter sheet, and a second retentate sheet, wherein each of the sheet members in the array has at least one inlet basin opening at one end thereof, and at least one outlet base opening at an opposite end thereof, with at least one permeate passage opening at longitudinal side margin portions of the sheet members; each of the first and second retentate sheets having at least one channel opening therein, wherein each channel opening extends longitudinally between the inlet and outlet basin openings of the sheets in the array and is open through the entire thickness of the retentate sheet, and with each of the first and second retentate sheets being bonded to an adjacent filter sheet about peripheral and side portions thereof, with their basin openings and permeate passage openings and register with one another, and arranged to permit flow of filtrate through the channel openings of the retentate sheet between the inlet and outlet basin openings to permit permeate flow through the filter sheet to the permeate sheet to the permeate passage openings; the filtration cassette comprising a unitary article of inter-bonded sheet members. π 2. A filtration cassette according to claim 1, wherein the permeate sheet is of rigid character. 3. A filtration cassette according to claim 1, wherein the permeate sheet is of screen or mesh construction. 4. A filtration cassette according to claim 1, wherein the sheet members are inter- bonded by a bonding technique selecting from the group consisting of heat bonding, ultrasonic bonding, radio frequency bonding, and microwave bonding. 5. A filtration cassette according to claim 1, wherein at least two sheet members of the cassette are at least in part microwave bonded to one another. 6. A filtration cassette according to claim 1, wherein inlet and outlet basin openings are devoid of sharp comers. 7. A filtration cassette according to claim 1 , wherein filter sheets are bonded to respective faces of the permeate sheet, with a weld line interior to the basin openings, and with adhesive bonding interior to the weld lines. 8. A filtration cassette according to claim 7, wherein said weld line is formed by radio frequency welding or ultrasonic welding. 9. A filtration cassette according to claim 1 , wherein permeate passage openings at longitudinal side margin portions of the sheet members are longitudinally continuous. 10. A stacked cassette filter including a stacked assembly of filtration cassettes, wherein each filtration cassette comprises a multilaminate array of sheet members of generally rectangular and generally planar shape with main top and bottom surfaces, wherein the sheet members include in sequence in the array a first retentate sheet, a first filter sheet, a permeate sheet, and second filter sheet, and a second retentate sheet, wherein each of the sheet members in the array has at least one inlet basin opening at one end thereof, and at least one outlet base opening at an opposite end thereof, with at least one permeate passage opening at longitudinal side margin portions of the sheet members; each of the first and second retentate sheets having at least one channel opening therein, wherein each channel opening extends longitudinally between the inlet and outlet basin openings of the sheets in the array and is open through the entire thickness of the retentate sheet, and with each of the first and second retentate sheets being bonded to an adjacent filter sheet about peripheral and side portions thereof, with their basin openings and permeate passage openings and register with one another, and arranged to permit flow of filtrate through the channel openings of the retentate sheet between the inlet and outlet basin openings to permit permeate flow through the filter sheet to the permeate sheet to the permeate passage openings; the filtration cassette comprising a unitary article of inter-bonded sheet members. 11. A stacked cassette filter according to claim 10, wherein an outermost filtration cassette comprises an outermost retentate sheet of rigid character. 12. A stacked cassette filter according to claim 10, wherein an outermost filtration cassette comprises a retentate sheet bonded to an endplate to provide a hard shell cassette structure, and the endplate is positioned adjacent a manifold plate of the filter with a compressible gasket between the endplate and the manifold plate.

Applicants: KOPF HENRY B, KOPF, HENRY, B.
Application year: 1998
Main classification: B01D25
IPC codes: B01D25, B01D63, C12M3