Graded particle-size retention filter medium for cell-type filter unit

A cell-type filter unit having two or more filter media layers and/or zones, at least one of the layers/zones having a different particle retention capability disposed on each side of a non-filtering separator element. The filter media are preferably positioned such that each succeedingly distal filter layer or zone from the separator has a decreased particle retention capability than each proceeding filter layer or zone. The filter media layer most proximal to the separator element may be separated from the separator by a support material for supporting such filter media element and preventing collapsing of the media into any separator conduit.

DETAILED DESCRIPTION OF THE INVENTION There is disclosed a cell-type filter unit having one or more filter media installed on each side of a separator element, the separator element being of such composition as to have minimal, if any, filtering capability for the fluid to be filtered at its position in the unit, but being sufficient to effectively separate the filter media on each side thereof, and each filter media comprising two or more zones or layers of filter material which differ in their ability to retain different-sized particles and/or total contaminant mass given ambient filtration conditions. The two or more zones or layers may comprise one or more sheets of filter media, which may be composed of similar materials (in which the materials are formulated and processed to create a media with different retention capabilities), or may be composed of different materials having distinctly different particle retention characteristics. The two or more zones or layers may be contiguous or non-contiguous with one another as long as the fluid being filtered communicates between the zones or layers. Filter media having different PRC may be produced discretely by standard manufacturing methods. Such media may then be physically stacked onto each other to create finished multi-layer media structure within a cell. Alternatively, the multi-zone media structure may be produced by forming a first media zone of a certain PRC by standard manufacturing methods, and then overfelting this first media zone with other media zones of different PRC. Such alternative methodology yields a single contiguous sheet of media, which contains multiple filtration zones. This sheet can be assembled into a cell in the selected orientation. In a presently preferred embodiment, a first filter medium zone capable of retaining the smallest particle sizes, as compared to other filter medium zone, is located adjacent to the separator (downstream side) to act as the qualifying zone which determines particle removal efficiency. Each succeeding filter medium zone installed distal to the first filter medium (i.e., upstream) is less capable of removing smaller-sized particles than the filter medium more proximal to the separator. That is, preferably the PRC of the filter media zones increase in the direction of fluid flow so that contaminants that are desired to be removed are progressively retained throughout the filter medium thickness as a function of the filtered particle size and proximity from the separator. Preferably the zones or layers on one side of the separator are substantially the same in construct (fabrication, composition, dimension and charge) and positioned in the same manner with respect to the separator. In cell-type filter units having two or more filter media elements of graded PRC, preferably the gradation is such that the PRC increases from upstream (from the filtering surface of the filter medium) to downstream (adjacent to the separator). An advantage of such gradation, is that the CHC of the combined layers is greater than the CHC of either layer alone, even when such layers individually are taken to the same depth of the combined layers. The PRC of the filter media may be varied by altering the composition, which makes up the media, and/or fabrication of the media. For example, Zeta-Plus® filter media is made from a combination of fiber, filter aids and resin. Fibers, such as cellulose, glass or synthetic fibers, may be selected to alter the PRC. PRC may also be affected by the particular filter aid chosen, such as one of the variety of grades of Diatomaceous Earth (DE) or Perlite. Likewise, variation of the resin that is incorporated to act as a binder may also effect PRC due to the electrokinetic properties imparted by the resin to the media. PRC of materials of the same general composition may be altered by varying the ratio of the components, for example, the amount of cellulose used. PRC may also be modified by changes in the process used in making or fabricating the filter media, as, for example, in adding a calendering operation to densify the media. As stated above, the filter media may be comprised of one or more zones made from dissimilar material. One zone, for example, could be of a Zeta-Plus® construct, while the other zone may be a media typically used in a pleated filter, such as a melt-blown material, a membrane, etc. Typically, the thickness of such zones will need to be adjusted such that they can be made into a cell unit using conventional machinery. Each media filtration zone may be produced discretely by its own standard manufacturing methods and then physically stacked onto the other media filtration layers to create a finished multi-layer structure within the cell unit. It is preferred that the media layer having the highest PRC be located as the downstream zone. When Zeta-Plus® media is used as the upstream filtration zone, the downstream zone may advantageously be a calendared melt-blown polypropylene media of the type used in the Polypro XL® pleated filter, or a symmetric cast nylon membrane of the type used in Cuno's Zetapor® or BevAssure® pleated filter. An asymetric cast nylon membrane may also be used. When the Zeta-Plus® media is used as the downstream filtration zone, the upstream zone may advantageously include an un-calendared melt-blown polypropylene media of the type used in the more open retention ratings of the Polypro XL® pleated filter. The filter media may alternatively be comprised of one or more zones made from a material of substantially the same construct (formulation and fabrication) and charge (i.e., having substantially the same zeta-potential). In such case, the PRC of each zone is directly correlateable with the air flow resistance across the medium zone (i.e., the higher the air flow resistance, the greater the PRC). Preferably the zone oriented most-upstream (in a fluid flow) will have a smaller air flow resistance (and therefore the pressure) and therefore lower PRC, than the each succeeding downstream zone. Preferably the difference in air flow resistance between each succeeding zone differs by more than about 10%, preferably more than about 25%, and yet more preferably more than about 50%, but not more than 80%. The filter media zone most proximal to the separator element may be separated from the separator by an intervening support material for supporting such filter media zone and preventing intrusion of the any portion of the filter media zone under pressure differential into any conduit, groove or indentation in the separator. Support zones may also be interposed between filter media zones. Although standard media thickness may be utilized for each filter media zone in the multi-zone filter medium cell-type filter, it is preferred that the total filter medium thickness in the multi-zone cell-type filter be about 0.13 to 0.218 inches. Such total filter medium thickness is preferred as the increase in total filter media thickness per cell may cause a significant reduction in the number of cells and ultimately reduce the associated filter surface area in a defined cartridge housing. The thickness of each zone in a multi-zone filter medium may differ. In order to require minimum modifications to presently employed cell-type filter unit assembly equipment and molds, it may be preferred to limit zones additional to a filter medium zone of standard thickness (between about 0.1 to about 0.25 inches) to membrane-like thickness, and in particular to less than about 30 mils. Any thin membrane that increases particle removal efficiency performance versus the overlying filter medium layer may be used in conjunction with a filter medium of standard thickness. Presently preferred are zones comprising melt-blown media, particularly polypropylene material (e.g., Polypro®XL) and cast nylon microporous membrane (e.g., Zetapor®). The separator preferably should support the filter media under differential pressure while providing flow conduits for the clean fluid to exit the cell. Filter units of the present invention may be stacked in a conventional manner to form a cartridge. Cell-type filter units are preferably stacked along a central axis. Typically, the number of units making up such a cartridge are known to vary between 2-21 cells, commonly about 16 cells. While a membrane filter medium zone may contact directly onto each side of the separator, a support material zone may be interposed between any such zone and the separator to add protection against abrasion, collapse, etc. The support material zone should preferably be relatively stiff and strong, but have a relatively open pore size such that it does not contribute significantly to change in pressure, or act as a filter medium. Presently preferred materials include spun bound non-woven material (e.g., Typar®, Reemay®) or a plastic netting (e.g., AET Plastinet®, Conwed Vexar®). Preferably, the support material and the filter medium zones are sealed together in their outer perimeters, presently preferably, by an injection molded polymeric edge seal, or by other process and materials, that provide support to perform the sealing function. Preferably the filter medium, separator, and any support material are centered about a central void of the same size and dimension. In a lenticular filter, such void is generally circular. Presently it is preferred that the filter media are bounded along their perimeters by an insert molding process that encapsulates the perimeters in plastic. Sealing along the central void perimeter may be provided by axial compressive forces generated during cartridge-housing installation for double-open end (“DOE”) style cartridges, or by assembly force for single open end (“SOE”) cartridge, or by other methods presently known in the art. As would be readily apparent to one of ordinary skill in the art from the present disclosure, the multi-zone cell-type filter unit of the present invention provides for significant advantages over cell-type filter units of the prior art. By incorporating additional filter medium zones having larger PRCs and/or CHCs into a conventional cell-type filter unit in the manner described, particle removal efficiency and retention performance of the stacked filter assembly is significantly improved without affecting the life of the filter unit. Another major benefit for the filter customer is improved filtration economics. As previously noted, in many filtering process applications, stacked cell-type filter unit cartridges are used as a pre-filter to a downstream membrane filter. By incorporating the membrane media into the pre-filter assembly in the manner described, the downstream membrane filter and its housing may be eliminated or its useful life significantly lengthened (if it can't be removed from service due to integrity test requirements). Further, less down time would be anticipated to be spent in checking and replacing one filter rather than in checking and replacing two filters. One also gets, for a wide variety of filter media, the benefits of longer life with the same PRC versus that of single layer media. The examples which follow are representative of a few of the many scenarios in which such filter construct might find advantageous use. 
 EXAMPLE 1 A customer is currently using a ZetaPlus® grade 60S product. The customer asserts that the product provides acceptable in-line life, but only marginally meets the effluent quality standards that it demands. While a tighter 90S grade ZetaPlus® is found to provide the desired effluent quality, it is deemed by the customer to provide for an unacceptable life. By serially-combining the two filter media, the necessary effluent quality and longer in-line life may be obtained, however, the serial combination would require installation of a second housing which unacceptably adds to the client's capital and operational costs. Further, the client understands that there is greater down time involved in replacing filters that are housed in separate housings. A graded pore size ZetaPlus® cartridge with 60S and 30S grade layers is found to be the best option since it maintains the acceptable in-line life, while improving effluent quality, without the need to install and maintain a second housing. 
 EXAMPLE 2 A customer is currently using a ZetaPlus® grade 50S product as a pre-filter to a downstream membrane filter. The customer asserts that the combined filters meet the effluent quality standards that it demands, but fails to meet its requirement for in-line life. A more open 30S grade of ZetaPlus®, while not significantly affecting effluent quality, is found to reduce in-line life by permitting more rapid build-up on the membrane filter. A tighter 60S grade, while not significantly affecting effluent quality, is found to reduce in-line life by permitting more rapid build-up on the 60S media. A media of graded-pore size construction from 30S to 60S is found-to increase in-line life by minimizing build-up on both the membrane and graded-pore size pre-filter. 
 EXAMPLE 3 A customer is currently using a ZetaPlus® grade 90S product as a pre-filter to a downstream membrane filter. The customer asserts that the combined filters provide acceptable in-line life, but only marginal to unacceptable effluent quality, as it allows the membrane to plug and have a short service life. No tighter ZetaPlus® grade exits than the grade 90S product. One option is to install a non-ZetaPlus® media prior to the membrane that traps more particulates, such as the Polypro XL 0202P1 pleated filter medium. This option provides good effluent quality and in-line life but requires another type of housing to be inserted in-line adapted for housing the Polypro XL 0202P1 pleated filter medium, thus adding to capital and operational costs. Adding the Polypro XL 0202P1 medium between the 90S medium and the membrane also permits enhanced in-line life, however, requires yet a third housing to be place in line with the other housings, again adding to capital and operational costs. Another option is to provide a filter medium comprised of layered ZetaPlus® grade 90S and Polypro XL 0202P1 in place of the ZetaPlus® grade 90S pre-filter alone. Such system does not require a third filter housing, and if fabricated in the shape of the ZetaPlus® grade 90S filter, a new housing to fit the filter. Such system would provide good in-line filter life and good effluent quality. A third option is to provide a layered ZetaPlus® grade 90S and membrane medium in the shape of the ZetaPlus® grade 90S pre-filter, which would also provide good in-line filter life and effluent quality. Referring now to the drawings, wherein like reference numerals identify similar structural elements of the subject invention, and which set forth representative embodiments of the present invention, additional advantages of the present invention become readily apparent. Referring to FIG. 1 , there is shown a side perspective elevational view of a representative lenticular cell-type filter unit 20 , having a relatively large upper filter medium filtration area 21 , an outer edge seal 22 disposed along the circumference of the filter cell, to retain the various components of the filter cell, and an aperture void 23 . Now referring to FIG. 2 , there is shown a cross-section of a representative lenticular cell-type filter unit 20 cut along the 2 - 2 line of FIG. 1 , wherein the cell-type filter unit includes an upper 27 and lower 28 filter medium structure. As can be seen upper filter medium structure 27 is composed of a first upper filter medium layer 29 and a second upper filter medium layer 30 . In a similar manner, lower filter medium 28 is composed of a first lower filter medium layer 31 and a second lower filter medium layer 32 . As illustrated, first upper filter medium layer 29 and second upper filter medium layer 30 , as well as first lower filter medium layer 31 and second lower filter medium layer 32 , may be generally of the same thickness. The first, 29 , 31 , and second, 30 , 32 , filter medium layers of the present invention are manufactured to have different PRCs. Upper filter medium 27 and lower filter medium 28 may be circular in shape and joined by a circular edge seal 22 which grips the upper filter medium 27 and lower filter medium 28 filter media on either side to form a liquid tight seal at the circumference of the unit. Lenticular cell-type filter unit 20 also includes a separator element, generally indicated at 33 . Now referring to FIG. 3 , there is shown a cross-section of the a representative lenticular cell-type filter unit 20 cut along the 2 - 2 line of FIG. 1 , wherein the cell-type filter unit includes an upper 25 and lower 26 support layer inferior to upper membrane filter layer 24 , lower membrane filter layer 19 , which in turn is inferior to upper filter medium 27 and lower filter medium 28 . Upper filter medium 27 and lower filter medium 28 are manufactured to have different PRC than upper membrane filter layer 24 and lower membrane filter layer 19 . Upper 25 and lower 26 support layers provide, respectively, support to upper membrane filter layer 24 and lower membrane filter layer 19 . Now referring to FIG. 4 , there is shown a side elevational representation showing assembly process of the individual components of the cell-type filter unit of FIG. 3 using a representative cell unit assembly mandrel 34 . Separator 33 is initially placed on mandrel 34 . One either side of separator 33 is upper 25 and lower 26 support layers, followed by upper membrane filter layer 30 and lower membrane filter layer 31 , respectively, such membrane filter layers capable of retaining relatively smaller-sized particles than upper filter medium 27 and lower filter medium 28 which follow thereafter. In one embodiment (not shown), filter medium layers 27 and 28 , relatively large pore size filter media, are further covered by a filter netting to aid in holding the filter medium together. Turning now to FIG. 5 , there is shown a perspective view of a representative lenticular cell-type filter unit assembly 45 comprising a plurality of cell units of the present invention positioned in filter housing 48 . Filter assembly 45 is comprised of a series of stacked lenticular cell-type filter units 20 positioned about a central axis 46 communicating with out-take pipe 47 of filter housing 48 . In operation, the fluid to be filtered is passed through inlet pipe 49 into housing interior 50 . The fluid passes through the filter medium of filter cell 20 and is conducted by conduits in separator 33 (not shown) to central axis 46 and out of out-take pipe 47 . In order to demonstrate the efficacy of the presently described invention with respect to commercially available grades of lenticular filter material, a series of experiments (Examples 5-7) were undertaken using Zeta-Plus™ brand filter media having different degrees of pore openness designated by grade. Permeability of the filter media was measured as the pressure drop in inches of water when 20 SCFH of air was passed through a three-inch diameter, 7.1 square inch, cross section of the media. Life expectancy of the filter, as well as efficiency of filtration, was adjudged by challenging the filter media with a cell lysate prepared as follows: E. coli ATCC &num;49696 was grown in Luria-Bertani Broth (10 g/l tryptone, 5 g/l yeast extract, 10 g/l NaCl, distilled water 5 liters). Cells were cultured until they reached mid to late exponential stage, and then centrifuged down to a pellet at 17,000× g (10,000 RPM in a JA-10 rotor) for 30 minutes at 4° C. The cells were then re-suspended in 10 mM Tris HCl (ph 8.0), respun, and washed once more. After the second washing phase, the cells were lyzed by re-suspending the pellets (1 g/80 ml) in 30 mM Tris HCl (pH 8.0) containing 20% sucrose. After stirring from 60 to 90 minutes, potassium EDTA and lysozyme were added to 10 mM and 0.5 mg/ml respectively. The resulting solution was stirred for 30-45 minutes. The cell solutions were then aseptically returned to centrifuge tubes and a pellet was obtained. The pellets were re-suspended in sterile distilled water and the tubes were placed into a freezer at −70° C. overnight. The tubes were then allowed to thaw. Such freeze/thaw procedure was repeated a total of three times to ensure adequate lysis. After the final freeze/thaw, the tubes were pooled and stirring was performed for at least 30 minutes. In order to minimize enzymatic breakdown of the lysate components by various proteases, the lysate was placed in a refrigerator at 4° C. or freezer at −20° C. Filter life was adjudged by the initial volume of filtrate passed through the filter to reach 20 psid over initial pressure (measured in gallons/ft 2 ). Efficiency was adjudged from the clarity of the filtrate collected from the filtration system tested. Challenge with the cell lysate was carried out at a pH of about 6.8 to 7.3. 
 EXAMPLE 5 Full-thickness 60 grade medium was compared to half-thickness 60 grade medium combined with either half-thickness 30, or half thickness 10, grade medium. When combined the two half-thickness filter media were of the same dimension as the full thickness 60 grade medium. Likewise, each half-thickness filter medium layer was substantially dimensioned the same as the other. As demonstrated by the data in Table 1, life of the filter was dramatically improved by combining half-thickness 30 grade medium to half-thickness 60 grade medium as compared to full-thickness 30 or 60 grade alone. Addition of half-thickness 10 grade medium with the half-thickness 60 grade medium provided significantly improved life over full thickness 30 and 60 grade medium, and the combined half-thickness 30/60 grade media. No practically significant difference between filter efficiencies was discerned between the grades and grade combinations. 1 TABLE 1 UPPER LAYER LOWER LAYER Grade Life Weight Permeability Weight Permeability 30 23.8 — — 16.3 17 60 8.9 — — 17.6 93 30/60 28.8 8.3 12 9.6 63 10/60 36.5 8.7 8 9.6 63 
 EXAMPLE 6 Full-thickness 90 grade medium was compared to half-thickness 90 grade medium combined with either half-thickness 60, half thickness 30, or half-thickness 10, grade medium. When combined the two half-thickness filter media were of the same dimension as the full thickness 90 grade medium. Likewise, each half-thickness filter medium layer was substantially dimensioned the same as the other. As demonstrated by the data in Table 2, life of the filter was dramatically improved by combining half-thickness 10, 30 and 60 grade medium to half-thickness 90 grade medium as compared to full-thickness 90 grade alone. Improvement in life of the filter paralleled the openness of the particular grade. No practically significant difference between filter efficiencies was discerned between the grades and grade combinations. 2 TABLE 2 UPPER LAYER LOWER LAYER Grade Life Weight Permeability Weight Permeability 90 4.06 — — 16.9 196 60/90 5.59 9.6 63 7.7 92 30/90 17.74 8.3 12 7.7 92 10/90 43.3 8.7 8 7.7 92 
 EXAMPLE 6 Half-thickness 120 grade medium was combined with either half-thickness 60, half-thickness 30, or half-thickness 05, grade medium to form combination filters of approximately the same dimension. As demonstrated by the data in Table 3, life of the filter was dramatically improved by up to 60 grade, but remained relatively flat, or slightly diminished, thereafter. No practically significant difference between filter efficiencies was discerned between the grade combinations. 3 TABLE 3 UPPER LAYER LOWER LAYER Grade Life Weight Permeability Weight Permeability 90/120 1.38 7.7 92 12.7 276 60/120 4.24 9.6 63 12.7 276 30/120 4.05 8.3 12 12.7 276 05/120 3.97 8.9 2 12.7 276 While the invention has been described with respect to preferred embodiments, those skilled in the art will readily appreciate that various changes and/or modifications can be made to the invention without departing from the spirit or scope of the invention as defined by the appended claims.