CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority based on U.S. Provisional Application No. 60/556,891, filed on Mar. 29, 2004, and U.S. Provisional Application No. 60/648,394, filed on Feb. 1, 2005, both of which are incorporated by reference. 
     
    
     DISCLOSURE OF THE INVENTION  
       [0002]    The present invention relates to fluid treatment elements which may be used to treat fluids, including gases, liquids, or mixtures of gases, liquids and/or solids, in a wide variety of ways. For example, some of the fluid treatment elements may be used to remove one or more substances from the fluid and may then function as concentrators or filters or separators. Others of the fluid treatment elements may be used to transfer substances between two fluid streams and may then function as mass transfer devices. 
         [0003]    In particular, the present invention relates to pleated fluid treatment elements which are structured to treat a fluid in a crossflow mode of operation. The pleated fluid treatment elements may include a single sheet or a multilayer composite having a fluid treatment medium. The single sheet or the multilayer composite may be folded or corrugated in a zigzag fashion to create several pleats. Each pleat has a folded end, an open end and two legs that extend between the folded end and the open end. The opposite end edges of the pleated sheet or composite are sealed to one another to form a generally cylindrical fluid treatment pack with each pleat extending generally axially along the fluid treatment pack. 
         [0004]    The pleated, crossflow fluid treatment elements may include a first fluid flow path that passes tangentially along the pleats of the fluid treatment pack and a second fluid flow path that passes through the pleated fluid treatment medium from or to the first fluid flow path. For example, feed fluid may enter the fluid treatment element along the first fluid flow path. The feed fluid then passes via the first fluid flow path axially along the fluid treatment pack and tangentially within the pleats of the pack, where the feed fluid is treated. For example, one or more substances, including one or more constituents of the feed fluid, may be removed from the feed fluid by passing out of the feed fluid along the second fluid flow path through the fluid treatment medium. Alternatively, one or more substances may be added to the feed fluid by passing into the feed fluid along the second fluid flow path through the fluid treatment medium. The treated feed fluid then continues along the first fluid flow path out of the fluid treatment element. 
         [0005]    In accordance with one aspect of the invention, fluid treatment elements embodying the invention may comprise a fluid treatment pack, a spacer arrangement, and first and second fluid flow paths. The fluid treatment pack includes a fluid treatment medium, an axis, first and second opposite ends, and a plurality of pleats which extend axially between the first and second ends of the pack. Each pleat has a folded end, an open end, and first and second legs which extend between the folded end and the open end. The spacer arrangement is associated with the pleats to define a first region within each pleat which is occupied by the spacer arrangement and a second region within each pleat which is substantially free of structure. The first fluid flow path extends axially along the pleated fluid treatment pack within the pleats and includes the second region of each pleat. The second fluid flow path extends through the pleated fluid treatment medium from or to the first fluid flow path. 
         [0006]    In accordance with another aspect of the invention, fluid treatment elements embodying the invention may comprise a hollow, generally cylindrical fluid treatment pack, a spacer arrangement, and first and second end caps. The fluid treatment pack includes an axis, an interior, first and second opposite ends, and a pleated composite. The pleated composite defines a plurality of pleats extending axially between the first and second ends of the fluid treatment pack. Each pleat has a folded outer end, an open inner end, and first and second legs first extending between the folded outer end and the open inner end. The pleated composite includes a fluid treatment medium having an inner surface and an outer surface and a drainage medium positioned along the outer surface of the fluid treatment medium. The spacer arrangement is associated with the pleats of the fluid treatment pack inwardly from the inner surface of the fluid treatment medium. The spacer arrangement includes a first spacer positioned proximate the first end of the fluid treatment pack, a second spacer positioned proximate the second end of the fluid treatment pack, and a region which extends axially along each pleat between the first and second spacers and which is substantially free of structure. The core arrangement is positioned in the hollow interior of the fluid treatment pack and includes a blind portion axially spaced from the first end and the second end of the fluid treatment pack. The blind portion resists fluid flow radially inwardly from the axially extending regions of the fluid treatment pack. The first and second end caps are respectively sealed to the first and second ends of the fluid treatment pack, and each end cap has a central opening which fluidly communicates with the axially extending regions of the pleats. 
         [0007]    In accordance with another aspect of the invention, fluid treatment elements embodying the invention may comprise a generally cylindrical fluid treatment pack, a spacer arrangement, an outer surround, and first and second end caps. The fluid treatment pack includes an axis, an interior and an exterior, first and second opposite ends, and a pleated composite. The pleated composite defines a plurality of pleats extending axially between the first and second ends of the fluid treatment pack. Each pleat has a folded inner end, an open outer end, and first and second legs extending between the folded inner end and the open outer end. The pleated composite includes a fluid treatment medium having an inner surface and an outer surface and a drainage medium positioned along the inner surface of the fluid treatment medium. The spacer arrangement is associated with the pleats of the fluid treatment pack outwardly from the outer surface of the fluid treatment medium. The spacer arrangement includes a first spacer positioned proximate the first end of the fluid treatment pack, a second spacer positioned proximate the second end of the fluid treatment pack, and a region which extends axially along each pleat between the first and second spacers and which is substantially free of structure. The outer surround is positioned around the exterior of the fluid treatment pack and includes a blind portion axially spaced from the first and second ends of the fluid treatment pack. The blind portion of the outer surround resists fluid flow radially outwardly from the axially extending regions of the fluid treatment pack. The first and second end caps are respectively sealed to the first and second ends of the fluid treatment pack, and each end cap has a central opening which fluidly communicates with the interior of the fluid treatment pack. 
         [0008]    In accordance with another aspect of the invention, methods of making a fluid treatment element embodying the invention may comprise corrugating a fluid treatment medium to form a plurality of axially extending pleats. Each pleat has a folded end, an open end, and first and second legs extending between the folded end and the open end. The method further comprises forming the plurality of pleats into a generally cylindrical fluid treatment pack, the pleats extending axially along the fluid treatment pack. The method further comprises positioning a spacer arrangement between the legs of the pleats. The spacer arrangement spaces the legs apart and defines a region within each pleat that extends axially and is substantially free of structure. The method further comprises sealing the first and second ends of the fluid treatment pack to form a first fluid flow path and a second fluid flow path. The first fluid flow path extends axially along the fluid treatment pack via the regions which are substantially free of structure. The second fluid flow path extends through the fluid treatment pack from or to the first fluid flow path. 
         [0009]    In accordance with another aspect of the invention, a method of making a fluid treatment element embodying the invention may comprise corrugating a composite to form a plurality of pleats. The composite includes a fluid treatment medium having first and second opposite side edges and first and second opposite surfaces, a drainage medium positioned along the first surface of the fluid treatment medium, a spacer positioned along the second surface of the fluid treatment medium proximate the first side edge and a material positioned along the second surface of the fluid treatment medium. The method further comprises forming the corrugated composite into a generally cylindrical fluid treatment pack having first and second ends, where the pleats extend axially along the fluid treatment pack. The method further comprises stripping the material from the corrugated composite to form a region within each pleat that is substantially free of structure. The method further comprises sealing the first and second ends of the fluid treatment packs to form a first fluid flow path and a second fluid flow path. The first fluid flow path extends axially along the fluid treatment pack via the regions which are substantially free of structure and the second fluid flow path extends through the fluid treatment pack from or to the first fluid flow path. 
         [0010]    Embodiments of the invention may be configured in a variety of ways. For example, for some embodiments, the spacer arrangement may be associated with the pleats in a manner that positions the spacer arrangement between the legs of the pleats. For other embodiments, the spacer arrangement may be associated with the pleats in a manner that positions the spacer arrangement within the legs of the pleats, for example, between the fluid treatment medium and an additional porous medium, such as a drainage medium. For most embodiments, the spacer arrangement defines a region substantially free of structure that is located directly next to, i.e., adjoins, a surface of the fluid treatment medium. 
         [0011]    Embodiments of the invention provide many advantages over conventional fluid treatment elements. For example, by providing regions within the pleats that are substantially free of structure, feed fluid can flow along these regions with less resistance to fluid flow. Consequently, feed fluid may flow tangentially through the fluid treatment pack with less pressure drop. Further, by locating the regions substantially free of structure directly next to the surface of the fluid treatment medium, fluid flowing along the tangential flow path through these regions can more thoroughly clear foulants from the surface of the fluid treatment medium. Consequently, the service life of the fluid treatment elements can be extended. In addition, many feed fluids are subject to damage when they flow through or around structures in the fluid flow path. For example, when a feed solution containing cells and/or cellular components flows through a porous netting, many cells may be disrupted, destroying the cells and contaminating the solution with cellular debris. By providing regions within the pleats that are substantially free of structure, embodiments of the invention allow feed fluid to flow tangentially along the fluid treatment pack with little or no damage. 
         [0012]    Thus, in accordance with another aspect of the invention, a method for treating a cellular solution comprises passing the cellular solution axially along a pleated fluid treatment element within the pleats. Passing the cellular solution axially within the pleats includes directing the cellular solution axially along regions between the legs of the pleats that are substantially free of structure. The method further comprises passing one or more substances through a fluid treatment medium of the fluid treatment element from or to the cellular solution in the regions substantially free of structure. 
         [0013]    Embodiments in which the spacer arrangement is positioned between the fluid treatment medium and an additional porous medium, e.g., a drainage medium, offer additional advantages. In these embodiments, fluid flowing axially along the pleats via the first fluid flow path flows mostly along the regions substantially free of structure but partly along the additional porous medium. During normal modes of operation, the fluid in the first fluid flow path on one side of the fluid treatment medium is generally at a higher pressure than the fluid on the opposite side of the fluid treatment medium. The higher pressure fluid keeps adjacent legs of each pleat spaced from each other and the regions substantially free of structure open to fluid flow. There may be instances when the pressure of the fluid in the first fluid flow path falls below the pressure of the fluid on the opposite side of the fluid treatment medium. Adjacent pleat legs may then expand toward each other, collapsing and pinching off all or some portion of the regions substantially free of structure. However, when the normal mode of operation resumes and the fluid pressure in the first fluid flow path is again greater than the fluid pressure on the opposite side of the fluid treatment medium, the regions free of structure can be immediately reestablished. The higher pressure fluid easily flows along the first fluid flow path within each pleat via the additional porous medium, where it forces the adjacent legs of each pleat back to their normal spaced position and reopens the regions substantially free of structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]      FIG. 1  is a view of a fluid treatment element which is partially sectioned as shown in  FIG. 2 . 
           [0015]      FIG. 2  is a partial section view of the foreground of a fluid treatment element showing regions that are substantially free of structure. 
           [0016]      FIG. 3  is a partial section view of the fluid treatment element showing a spacer arrangement. 
           [0017]      FIG. 4  is a representational view of a portion of a system for making the fluid treatment element. 
           [0018]      FIG. 5  is an oblique view of a fluid treatment pack and a side seal. 
           [0019]      FIG. 6  is a partial section view of another fluid treatment element showing regions substantially free of structure. 
           [0020]      FIG. 7  is a partial section view of the fluid treatment element of  FIG. 6  showing a spacer arrangement. 
           [0021]      FIG. 8  is a representational view of a system for making the fluid treatment element of  FIG. 6 . 
           [0022]      FIG. 9  is a partially sectioned view of a fluid treatment assembly. 
           [0023]      FIG. 10  is a plan view of a fluid treatment element showing a surround. 
           [0024]      FIG. 11  is an end view of a core arrangement and a spacer arrangement. 
           [0025]      FIG. 12  is a view of a fluid treatment assembly which is partially sectioned as shown in  FIG. 13 . 
           [0026]      FIG. 13  is a partial sectional view of the foreground of a fluid treatment element showing regions that are substantially free of structure. 
           [0027]      FIG. 14  is a partial sectional view of the fluid treatment element of  FIG. 13  showing a spacer arrangement. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
       [0028]    Many different fluid treatment elements may embody the invention. An example of one fluid treatment element  10  embodying the invention is shown in  FIGS. 1-3 . The fluid treatment element  10  generally includes a fluid treatment pack  11  having an axis  12 , opposite ends  13 ,  14 , a fluid treatment medium, and a plurality of pleats  15 . The pleats  15  extend generally axially between the ends  13 ,  14  of the pack  11 , and each pleat  15  generally includes a folded end, e.g., a folded outer end  20 , an open end, e.g., an open inner end  21 , and two legs  22 ,  23  which extend between the folded end  20  and the open end  21 . A spacer arrangement may be associated with the pleats of the fluid treatment pack in any of a variety of ways. For example, a spacer arrangement  24  may be positioned between the legs  22 ,  23  of each pleat  15  to space the legs  22 , 23  apart. The spacer arrangement  24  defines a region  30  within the pleat  15 , e.g., between the legs  22 ,  23  of the pleat  15 , that is occupied by the spacer arrangement  24  and another region  31  within the pleat  15 , e.g., between the legs  22 ,  23  of the pleat  15 , that is substantially free of structure. The spacer arrangement may be associated with a plurality of pleats, including some, most, or all of the pleats, to define regions substantially free of structure. For example, the spacer arrangement may define regions substantially free of structure in up to about 40% or more of the pleats, including at least about 60%, at least about 80%, at least about 90%, or at least about 95% of the pleats, or all of the pleats. Greater percentages are preferred because they provide more regions substantially free of structure within the pleats of the fluid treatment element. The fluid treatment element  10  further includes a tangential fluid flow path  32  and a lateral fluid flow path  33 . The tangential fluid flow path  32  extends generally axially along the fluid treatment pack  11  within the pleats  15 , including the regions  31  that are substantially free of structure. The lateral fluid flow path  33  fluidly communicates with the tangential fluid flow path  32  and extends laterally through the fluid treatment medium to or from the tangential fluid flow path  32 . 
         [0029]    In operation, feed fluid may pass tangentially along the fluid treatment pack  11  via the tangential fluid flow path  32 . The feed fluid within the regions  31  that are substantially free of structure may be treated by removing one or more substances from the feed fluid via the lateral fluid flow path  33  through the fluid treatment medium or by adding one or more substances to the feed fluid via the lateral fluid flow path  33  through the fluid treatment medium. The fluid treatment element  10  may thus be considered a pleated, crossflow fluid treatment element. International Publication No. WO 00/13767 also discloses pleated, crossflow fluid treatment elements and is incorporated by reference in its entirety. 
         [0030]    The fluid treatment pack  11  may be structured in a wide variety of ways. For example, the fluid treatment pack may be a pleated, single-layer sheet (not shown) or may include a pleated, multilayered composite  40 . Some or all of the layers of the composite  40  may be integrally joined to or formed with one another. However, in many embodiments, the layers of the composite  40  comprise separate layers positioned adjacent to one another. 
         [0031]    The fluid treatment pack  11  includes a fluid treatment medium  41 , for example, as a single sheet or a region of a single sheet or as one or more of the layers of a composite  40 . The fluid treatment medium  41  may have opposite surfaces, e.g., an inner surface  42  and an outer surface  43 . Suitable fluid treatment media may vary widely depending on such factors as the nature of the feed fluid and how the feed fluid is to be treated. For example, the fluid treatment medium may have, or may be modified to have, any of a myriad of properties. The fluid treatment medium may be porous, permeable or semipermeable and may have removal efficiencies from the Angstrom or Dalton range or less, through the submicron range, to the micron or tens of microns range or more. For example, the fluid treatment medium may comprise a nanofiltration, an ultrafiltration, or a microfiltration medium. The fluid treatment may allow gas and liquid to pass through it or just gas and not liquid. The fluid treatment medium may be liquiphobic or liquiphilic, may have an electrically neutral or charged surface, and/or may include one or more functional groups which may, for example, be arranged to bind to one or more substances in the feed fluid. The fluid treatment medium may be configured in a variety of ways, including, for example, as a membrane or a fibrous sheet, and may be formed from any suitable material, including metal, natural or synthetic polymers, or a ceramic or glass. For many embodiments, a polymeric filtration membrane having a submicron removal rating may be used as the fluid treatment medium. 
         [0032]    The fluid treatment pack  11  may include one or more porous media in addition to the fluid treatment medium  41 . For example, the fluid treatment pack  11  may include a porous drainage medium, e.g., as a region of a single sheet or as another layer of the multilayer composite  40 . The drainage medium  44  may be positioned along at least one surface, e.g., the outer surface  43 , of the fluid treatment medium  41  either adjoining the surface, or spaced from the surface, of the fluid treatment medium. The drainage medium  44  may comprise any of a variety of materials having a sufficiently low edgewise flow resistance to enable fluid to adequately flow to or from the surface of the pleated fluid treatment medium. Many suitable drainage axially media are disclosed, for example, in U.S. Pat. No. 5,543,047 and U.S. Pat. No. 5,252,207, both of which are incorporated by reference in their entirety. For many embodiments, a woven or nonwoven polymeric material or a polymeric mesh may be used as the drainage medium. For example, the drainage material may comprise a mesh having first and second sets of strands, e.g., machine direction strands and cross direction strands. Either set of strands, or neither set of strands, may be oriented within the composite parallel to the pleats. 
         [0033]    A multilayer composite  40  may include other porous media layers. For example, the drainage medium  44  may be positioned along the outer surface  43  of the fluid treatment medium  41  with a cushioning layer (not shown) between them. The cushioning layer protects the fluid treatment medium  41  from abrasion by the drainage medium  44 , as disclosed in U.S. Pat. No. 5,252,207. As another example, if the fluid treatment medium  41  is flimsy, a support medium (not shown) may be positioned along a surface of the fluid treatment medium  41 , for example, along the surface opposite the drainage medium  44 , e.g., along the inner surface  42 . Many of the stiffer drainage media may be used as a support medium, which more rigidly supports the fluid treatment medium  41 . 
         [0034]    The spacer arrangement may also be structured in a wide variety of ways and may be incorporated in the composite or may be distinct from the composite. For example, the spacer arrangement may comprise one or more spacers associated with the pleats of the fluid treatment pack. In the illustrated embodiment, the spacers  50  may be positioned within the pleats, e.g., between the legs  22 , 23  of each pleat  15  to space the legs  22 , 23  apart. The spacer may have a variety of configurations. For example, the spacer may comprise a plurality of pads, each pad being inserted within a pleat. Alternatively, the spacer  50  may comprise one or more strips  51 , 52  of material which extend along and are pleated with the single sheet or the multiple layers of the composite  40 , the strips being incorporated in the composite. 
         [0035]    The thickness of the spacer arrangement  24 , e.g., the spacers including the strips or the pads, generally corresponds to the thickness of the regions  31  that are substantially free of structure and may be selected in accordance with desired number of pleats and the desired size of each region substantially free of structure. For some embodiments, the thickness of the spacer arrangement  24  may be in the range from about 0.05 mm or less to about 5 mm or more or in the range from about 0.1 mm to about 2.0 mm or in the range from about 0.1 mm to about 1 mm, e.g., about 0.5 mm. Various widths are suitable for the spacer arrangement  24 . For example, the width may be in the range from about 3 mm or less to about 20 mm or more, e.g., from about 5 mm to about 10 mm. The height of the spacer arrangement  24  may be at least about 25%, or at least about 50%, or at least about 80%, or at least about 95%, or about 100% of the height of the pleat. 
         [0036]    The spacer arrangement may be arranged along one or both surfaces of the fluid treatment medium. For example, the spacer arrangement may be positioned only along one surface. In the embodiment illustrated in  FIGS. 1-3 , the spacer arrangement  24  may be positioned along the surface of the fluid treatment medium  41  opposite the drainage medium  44 , e.g., along the inner surface  42 . The spacer arrangement may also be positioned at various axial locations along the fluid treatment pack. For example, the spacer arrangement may be positioned along only one end of the fluid treatment pack. Thus, the spacer arrangement may be only one strip of material, and the strip may be pleated along one end of the fluid treatment pack between the legs of the pleats. Alternatively, the spacer arrangement  24  may be positioned only along both ends of the fluid treatment pack  11 , as shown in  FIGS. 1-3 . Thus, the spacer arrangement  24  may be only two strips  51 , 52  of material, and the strips  51 , 52  may be pleated along each end  13 , 14  of the fluid treatment pack  11 , for example, between the legs  22 , 23  of the pleats  15 . Alternatively, the spacer arrangement may include three or more strips of material. Spacers may be positioned at axial locations spaced from the ends of the fluid treatment pack, e.g., in the middle of the pack or at multiple intervals along the axial length of the fluid treatment pack. 
         [0037]    The spacer arrangement may be formed from a material which resists compression, and the material may be channeled or perforated or porous or may be impermeable, depending on factors such as the location of the spacer arrangement. For example, a spacer arrangement may comprise a spacer located at an end of the fluid treatment pack. If that end of the fluid treatment pack at the spacer is sealed against fluid flow, for example, by an end cap or a sealant, the spacer material may be channeled, perforated, porous, or impervious. If the end of the fluid treatment pack at the spacer is not sealed against fluid flow, the spacer material may be channeled, perforated or porous to allow fluid to flow into or out of the regions free of structure within the pleats. Spacers which are positioned at locations spaced from the ends of the fluid treatment pack may also be channeled, perforated or porous to allow fluid flow axially along the regions free of structure within the pleats. For many embodiments, spacer arrangements formed from material having a low edgewise flow resistance, as disclosed for various drainage media in U.S. Pat. No. 5,543,047, are suitable. 
         [0038]    The spacer arrangement  24  defines a region  30  within each pleat which is occupied by the spacer material and another region  31  which is substantially free of structure. To increase the size of the region  31  substantially free of structure, the size of the spacer region  30  may be small. For example, the total width of the region  30  occupied by the spacer arrangement  24  may be less than about 20%, or less than about 10%, or less than about 5%, or less than about 3%, or less than about 1%, of the axial length of the fluid treatment pack  11 . Further, the region  31  within each pleat  15  that is substantially free of structure may have some structure in the region, but at least about 50%, or at least about 70%, or at least about 90%, or at least about 95%, or 100% of the region  31  is completely free of structure. For many preferred embodiments, the fluid treatment elements  10  have smaller spacer regions  30 , e.g., spacer regions  30  which are less than about 10% of the axial length, and less structure in the regions  31  which are substantially structure free, e.g., regions  31  which are at least about 95% completely free of structure. For some embodiments, each region  31  extends from one spacer at one end of the fluid treatment pack to the other spacer at the other end of the fluid treatment pack and is completely free of structure. 
         [0039]    In addition to the fluid treatment pack  11  and the spacer arrangement  24 , the fluid treatment element  10  may include other features. For example, the fluid treatment element  10  may include a core arrangement  53  positioned in the interior  54  of the hollow fluid treatment pack  11 . The core arrangement  53  may be variously configured. For example, the core arrangement  53  may comprise a blind portion  55  which serves to block off the open inner ends  21  of the pleats  15  and/or resist, e.g., prevent, radially inward fluid flow from the regions  31  that are substantially free of structure. The blind portion  55  may terminate at locations spaced axially inwardly from one or both ends  13 , 14  of the fluid treatment pack  11 . The blind portion  55  may be hollow or solid, may have a generally cylindrical outer surface that abuts the pleats  15 , may be neither porous nor perforated, and may be without surface textures that would allow fluid to drain axially between the core arrangement  53  and the fluid treatment pack  11 . Feed fluid flowing along the tangential fluid flow path  32  may thus be prevented from flowing radially inwardly from the regions  31  that are substantially free of structure. 
         [0040]    The core arrangement  53  may also comprise an end portion  60  in the interior  54  of the fluid treatment pack  11 . The end portion  60  may extend from each end of the blind portion  55  to an end  13 , 14  of the fluid treatment pack  11 , or beyond, and a barrier  61  between the blind portion  55  and each end portion  60 . Each end portion  60  may include a generally cylindrical side wall  62  which is porous or perforated and an opening  63  opposite the barrier  61 . The barriers  61 , which may be impervious, may serve to direct fluid flow between the interior of the end portions  60  and the regions  31  substantially free of structure via the open inner ends  21  of the pleats  15 . 
         [0041]    The core arrangement may be configured in many other ways. For example, the core arrangement may include the barriers and the blind portion but the end portions may be eliminated. The barriers may then direct fluid between the interior of the fluid treatment pack and the regions substantially free of structure via the open inner ends of the pleats. Alternatively or additionally, the core arrangement may include the barriers without the blind portion and the end portions. A sealant may be disposed along the open inner ends of the pleats in place of the blind portion, blocking off the open inner ends. As another alternative, the core arrangement may comprise a blind portion which extends through the interior of the fluid treatment pack to one or both ends of the fluid treatment pack. The spacers between the legs of the pleats at one or both ends of the fluid treatment pack may then be channeled or porous or perforated to allow fluid to pass through the spacer into or out of the regions  31  that are substantially free of structure. 
         [0042]    In some embodiments, the core arrangement may be omitted. For example, a sealant may be applied along the interior of the fluid treatment pack, blocking off the open inner ends of the pleats. Alternatively or additionally, the pleats of the fluid treatment pack may be packed tightly enough that the spacers are compressed at the open inner ends of the pleats but are not compressed radially outwardly from the open inner ends. The open inner ends of the pleats may thus be pinched off without collapsing the regions that are substantially free of structure. With the core arrangement omitted, the spacers between the legs of the pleats at both ends of the fluid treatment pack may be porous or perforated to allow fluid to pass through the spacer into or out of the regions that are substantially free of structure. 
         [0043]    The fluid treatment element  10  may also include a surround  64 , such as a cage, a sleeve, and/or a wrap, positioned around the exterior  65  of the fluid treatment pack  11 . The surround  64  may be porous or perforated, or may have other openings, along a portion or all of the axial length of the fluid treatment pack  11 , allowing fluid to flow along the lateral fluid flow path  33 . The surround may adjoin or abut the outer folded ends of the pleats and may hold the pleats in position. 
         [0044]    The fluid treatment element  10  may also include a sealing mechanism at the ends  13 , 14  of the fluid treatment pack  11 . The sealing mechanism seals the end, e.g., all or a portion of the end, of the fluid treatment pack to establish the tangential fluid flow path along the fluid treatment element and the lateral fluid flow path through the fluid treatment medium. Various mechanisms may be used to seal the ends  13 , 14 . For example, the fluid treatment element  10  may include one or two end caps  70 , 71  respectively joined to one or both ends  13 , 14  of the fluid treatment pack  11 . The end caps  70 , 71  may be formed from any suitably impervious material and may be joined to the ends  13 , 14  in any suitable manner which seals the ends  13 , 14 . For example, the end caps may be melt bonded, welded, solvent bonded, or adhesively bonded, e.g., with a potting compound or a sealant, to the ends  13 , 14 . For many embodiments, a spacer at the end of the fluid treatment pack may be joined to the end cap along with the end of the pack. Alternatively, the spacer may be positioned inwardly from the end of the pack and may not be joined to the end cap along with the end of the pack. The end caps may or may not be bonded to the core arrangement  53  and/or the surround  64 . 
         [0045]    The end caps may be configured in any suitable manner. In the embodiment illustrated in  FIGS. 1-3 , each end cap  70 , 71  comprises an open end cap which has an opening  72  that fluidly communicates with the interior  54  of the fluid treatment pack  11  and the respective opening  63  in the end portion  60  of the core arrangement  53 . An end cap may be generally flat or may have a protrusion  73  that extends, for example, away from the fluid treatment pack. The protrusion may be unitarily formed with the end cap or may be a separate piece attached to the end cap. The protrusion may be configured as any of a wide variety of adapters or fittings that enable the fluid treatment element to be coupled to another component, e.g., a housing or another fluid treatment element. For example, the fitting on one end of the fluid treatment element  10  may comprise a feed or process fluid inlet fitting  68 , while the fitting on the other end may comprise a concentrate a retentate outlet fitting  69 . Alternatively, the adapters, fittings, inlet opening, and outlet opening may be configured as part of an end cap without a protrusion. One or both end caps may also include one or more seals, such as a gasket or an O-ring, for sealingly coupling the fluid treatment element to another component. 
         [0046]    Alternatively or additionally, the end sealing mechanism may comprise a sealant, including, for example, an epoxy, a silicone or a polyurethane, applied to all or a portion of one or both ends of the fluid treatment pack. For example, a sealant may be applied to the drainage medium along the outer surface of the fluid treatment medium at one or both ends of the fluid treatment pack. The sealant may also be applied around the exterior of the fluid treatment pack at the one or both ends of the pack to form a gasket for sealing the fluid treatment element to another component. However, the sealant may not be applied along the inner surface of the fluid treatment pack at the one or both ends. The spacers between the legs of the pleats along the inner surface of the fluid treatment medium at the one or both ends of the fluid treatment pack may be channeled, porous or perforated to allow fluid to enter or exit the regions. Further, the interior of the fluid treatment may be completely blinded off, for example, by a core arrangement that has a blind portion extending through the interior of the fluid treatment pack from one end to the other. Fluid, e.g., feed fluid, may then flow into the fluid treatment pack along the tangential fluid flow path, e.g., through the spacers at one end, along the regions substantially free of structure and out of fluid treatment pack through the opposite spacers. The tangential fluid flow path and the lateral fluid flow path may then be separated by the sealant except via the fluid treatment medium. 
         [0047]    The fluid treatment element may be made in any of several different ways. According to one general example, the fluid treatment medium, either as a single sheet or as part of a composite, may be corrugated to form a plurality of axially extending pleats. Any suitable type of corrugator may be used to form the pleats, including, for example, a pusher bar type corrugator, a blade type corrugator, a rotary type corrugator, or an in-line type corrugator. Various corrugation techniques are disclosed in U.S. Pat. No. 5,543,047. The plurality of pleats may then be arranged into a generally cylindrical fluid treatment pack. A spacer arrangement may be associated with some, most or all of the pleats in a variety of ways. For example, the spacer arrangement may be positioned within the pleats, e.g., between the legs of the pleats to space the legs apart, and define regions which are substantially free of structure. The ends of the fluid treatment pack may be sealed, e.g., completely or partially, to establish the tangential fluid flow path via the regions that are substantially free of structure and the lateral flow path through the fluid treatment medium. The spacer arrangement may be positioned within the pleats during or after corrugation; before, as, or after the pleats are arranged into the fluid treatment pack; and/or before, as, or after the ends of the fluid treatment pack are sealed. 
         [0048]    Another of many examples of a method of making a fluid treatment element  10  is represented in  FIGS. 4 and 5 . For example, a composite  40  may be formed from separate layers of a fluid treatment medium  41 , a drainage medium,  44 , a spacer arrangement  24 , and a strip-out material  74 , as shown in  FIG. 4 . The fluid treatment medium  41  may include opposite surfaces  42 ,  43  and opposite side edges  75 ,  76 . The drainage medium  44  may be positioned along one surface of the fluid treatment medium  41 , e.g., along the surface  43  which becomes the outer surface  43  of the fluid treatment medium  41  in a cylindrical fluid treatment pack. 
         [0049]    The spacer arrangement  24  may also be positioned along one surface of the fluid treatment medium  41 , e.g., along the surface  42  which becomes the inner surface  42  of the fluid treatment medium  41  in a cylindrical fluid treatment pack. The spacer arrangement may comprise only one spacer, e.g., one strip which may be positioned along one of the side edges of the fluid treatment medium. In the illustrated embodiment, the spacer arrangement  24  comprises two spacers, e.g., two strips  51 , 52  positioned along both side edges  75 , 76  of the fluid treatment medium  41 . Positioning the spacer strips  51 , 52  along the side edges  75 , 76  allows the spacer arrangement  24  to be securely held in place when the sealing mechanism is joined to the ends of the fluid treatment pack  11 . However, the sealing arrangement may comprise fewer or more than two spacer strips and may be positioned at different locations within the composite. 
         [0050]    The strip-out material  74  may be positioned along the same surface  42  of the fluid treatment medium  41  as the spacer arrangement  24  and may at least partially occupy the regions not occupied by the spacer arrangement  24 . For example, in the illustrated embodiment the strip-out material  74  comprises only one sheet of material that extends from one spacer strip  51  to the other spacer strip  52 . Alternatively, the strip-out material may be narrower than the distance between the strips  51 , 52  and/or may comprise two or more sheets or strips. For many embodiments, the strip-out material may be similar to the materials described as drainage media in U.S. Pat. No. 5,543,047 and U.S. Pat. No. 5,252,207. The thickness of the strip-out material may be approximately equal to the thickness of the spacer arrangement  24 . For some embodiments, the strip-out material may be omitted. 
         [0051]    Additional layers, e.g., a cushioning medium, may be fed to the composite, and each of the layers may be fed form a source, such as a roll. All of the layers may come together to form the composite at the same time, e.g., at the corrugator. Alternatively, subcomposites of fewer than all of the layers may be formed at different times, and the subcomposites may subsequently be formed into the composite. The layers, subcomposites, and composites may undergo various processes, e.g., trimming, calendaring and/or heat-setting, prior to corrugation. 
         [0052]    After the composite  40  is formed, it may be corrugated by any suitable corrugator. For example, the composite  40  may be fed into a pusher-bar type corrugator  80  as disclosed in U.S. Pat. No. 5,543,047. The corrugator  80  folds the composite  40  into a plurality of pleats  15  which extend in an axial direction. The heights of the pleats and/or the lengths of the legs  22 , 23  of the pleats  15  may vary, depending, for example, on the desired size and configuration of the fluid treatment pack  11 . For each pleat  15  the legs  22 , 23  may have about equal lengths or one leg may be longer than the other leg. Further, the legs may be straight or curved. In addition to forming the pleats  15 , the corrugator  80  may perform various other processes, including heat-setting and trimming. For example, after the composite  40  is pleated, a cutter (not shown) may cut the pleated composite axially, providing a leading edge  81 , a trailing edge  82  and a predetermined number of pleats  15  for the fluid treatment pack  11 . 
         [0053]    Incorporating the spacer arrangement  24  in the composite  40  and folding the spacer strips  51 , 52  with the other layers of the composite  40  has many advantages. For example, it reduces the complexity of the manufacturing process because no additional steps are required to insert spacers between the pleats after the pleats are formed. Further, it ensures that every pleat has a spacer because the spacer strips are folded into each pleat, along with the other layers of the composite, as the pleat is being formed. 
         [0054]    Incorporating the strip-out material  74  in the composite  40  along with the spacer arrangement  24  and folding strip-out material  74  with the other layers of the composite  40  also has many advantages. For example, it prevents undue compression of the spacer arrangement  24  as the composite  40  is pressed between the pleating surfaces of the corrugator  80 . Further, because the strip-out material  74  is folded with the remainder of the composite  40 , the radius at the inner surface of the folded end  20  of the pleat  15  may be larger than it would be if the strip-out material  74  were omitted during corrugation. This larger radius provides a more open and, therefore, a less restricted region  31  substantially free of structure within each pleat  15  after the strip-out material has been removed. 
         [0055]    The strip-out material  74  may be removed from the pleated composite  40  at any suitable time. For example, after the pleats  15  have been formed by the corrugator  80  but before the predetermined number of pleats  15  have been cut from the pleated composite  40 , the strip-out material  74  may be stripped from the pleated composite  40 . Alternatively, the strip-out material  74  may be stripped from the pleated composite  40  after the predetermined number of pleats  15  have been cut. Once the strip-out material  74  has been removed, the regions  31  between the spacer strips  51 , 52  and between the legs  22 , 23  of the pleats  15  are substantially free of structure. As yet another alternative, the strip-out material  74  may remain with the pleated composite  40  as the fluid treatment pack  11  is being formed. 
         [0056]    The pleated composite  40  may be arranged into a generally cylindrical fluid treatment pack  11  in a variety of ways. For example, the leading edge  81  and the trailing edge  82  of the pleated composite  40  may be brought around and positioned next to one another, as shown in  FIG. 5 , forming a hollow, generally cylindrical fluid treatment pack  11  having an interior  54 , an exterior  65 , and opposite ends  13 , 14 . A side seal  83  may then be formed along the edges  81 , 82  in any number of ways, including, for example, melt bonding, adhesive bonding and/or mechanically connecting, e.g., crimping. If the strip-out material  74  is still part of the pleated composite  40 , the strip-out material may be incorporated in the side seal  83 . Alternatively, the strip-out material  74  may be trimmed back from the leading and trailing edges  81 , 82  prior to forming the side seal  83 . After the side seal  83  is formed, the strip-out material  74  may be removed, for example, by stripping the strip-out material  74  from the fluid treatment pack  11 , e.g., from the interior  54  of the fluid treatment pack  11 . If the strip-out material  74  is incorporated in the side seal  83 , it may be stripped from the composite  40  and then cut inwardly from the side seal  83  prior to removal. With the strip-out material  74  removed, the regions  31  between the spacer strips  51 , 52  and between the legs  22 , 23  of the pleats  15  are substantially free of structure, and the open ends  21  of the pleats  15  communicate between these regions  31  and the interior  54  of the fluid treatment pack  11 . 
         [0057]    Prior to sealing the ends  13 , 14  of the fluid treatment pack  11 , the core arrangement  53  and the surround  64  may be fitted to the fluid treatment pack  11 . The core arrangement  53  may be inserted within the interior  54  of the fluid treatment pack  11 . The open inner ends  21  of the pleats  15  may be blinded by the blind portion  55  of the core arrangement  53  but may communicate with the end portions  60  of the core arrangement  53  beyond the barriers  61  via the perforated or porous side wall  62 . The surround  64  may be fitted around the exterior  65  of the fluid treatment pack  11 . The surround  64  may, or may not, be bonded to the folded ends  20  of the pleats  15 . The pleats  15  may be arranged to extend in a generally radial direction, in a straight non-radial direction, or in an angled or curved non-radial direction from the open ends  21  at the core arrangement  24  to the folded ends  20  at the surround  64 . For pleats that extend generally radially, the pleats may have a height approximately equal to (D-d)/2 and for pleats that extend non radially, the pleats may have a height greater than (D-d)/2, where D and d are the outer and inner diameters, respectively, of the fluid treatment pack  11 . For some embodiments, the pleats  15  may be arranged between the core arrangement  53  and the surround  64  with all of the pleat heights being the same. For other embodiments, the pleat heights within the fluid treatment pack may differ from one to another. 
         [0058]    The ends  13 , 14  of the fluid treatment pack  11  may then be sealed, for example, by applying a sealant or by attaching end caps  70 , 71  to the ends  13 , 14 . The end caps  70 , 71  may be attached to the ends  13 , 14  of the fluid treatment pack  11 , as well as the ends of the core arrangement  53  and/or the surround  64 , in a variety of ways. For example, the end caps  70 , 71  may be melt-bonded or adhesively bonded to the ends of the fluid treatment pack  11 , the core arrangement  53 , and the surround  64 . Bonding the spacer arrangement  24 , e.g., the spacer strips  51 , 52 , to the end caps  70 , 71  along with the other layers of the composite  40  may fixedly secure the spacer arrangement  24  as well as the other layers of the composite  40 , in place. Once the ends  13 , 14  of the fluid treatment pack  11  are sealed, e.g., by attaching the end caps  70 , 71 , the tangential fluid flow path  32  and the lateral fluid flow path  34  through the fluid treatment element  10  are established. 
         [0059]    Another example of a fluid treatment element  100  embodying the invention is shown in  FIGS. 6 and 7 . The fluid treatment element  100  shown in  FIGS. 6 and 7  has many features which are similar to those described and suggested for the fluid treatment element  10  shown in  FIGS. 1-3 , and corresponding components are identified with the same reference numbers. For example, both the fluid treatment element  100  shown in  FIGS. 6 and 7  and the fluid treatment element  10  shown in  FIGS. 1-3  may include a fluid treatment pack  11  having an axis  12 , opposite ends  13 , 14 , a fluid treatment medium  41 , and a plurality of axially extending pleats  15 . Each pleat  15  includes a folded end, e.g., a folded outer end  20 , an open end, e.g., an open inner end  21 , and two legs  22 , 23  which extend between the folded end  20  and the open end  21 . The ends of the fluid treatment pack  11  are sealed by a sealing mechanism, such as pair of open end caps  70 , 71 . A core arrangement  53  having a blind portion  55  and opposite end portions  60  and/or a surround  64  may be associated with the interior and the exterior of the fluid treatment pack  11 . Both fluid treatment elements  10 , 100  include a tangential fluid flow path  32  that extends generally axially along the pleats  15  and a lateral fluid flow path  33  that fluidly communicates with the tangential fluid flow path  32  and extends laterally through the fluid treatment medium  41 . 
         [0060]    Further, in both the fluid treatment element  100  shown in  FIGS. 6 and 7  and the fluid treatment element  10  shown in  FIGS. 1-3 , the fluid treatment pack  11  may comprise a multilayer composite  40  which includes a fluid treatment medium  41  having an inner surface  42  and an outer surface  43 . A drainage medium  44  may be positioned along the outer surface  43  of the fluid treatment medium  41 . Other porous media, such as a cushioning layer (not shown), may also be positioned along the outer surface  43  of the fluid treatment medium  41 . 
         [0061]    The fluid treatment medium  100  shown in  FIGS. 6 and 7  may include one or more additional porous media. For example, one or more porous media  101  may be positioned along the inner surface  42  of the fluid treatment medium  41 . A spacer arrangement  24  may be associated with the pleats  15  to define a region  31  substantially free of structure between the porous medium  101  and the surface  42  of the fluid treatment medium  41 . While the porous medium may comprise any of a wide variety of woven, nonwoven, or mesh sheets, the porous medium  101  for many embodiments may comprise a drainage medium. For example, a drainage medium may comprise a mesh layer, including an extruded mesh layer having one set of parallel strands fixed atop another set of parallel strands, e.g., a machine direction set of strands fixed atop a cross direction set of strands. Either set of strands, or neither set of strands, may be situated in the multilayer composite  40  with the strands running generally axially, parallel to the pleats  15 . 
         [0062]    The spacer arrangement may be configured in a variety of ways, as previously described with respect to the fluid treatment element  10  shown in  FIGS. 1-3 . In the fluid treatment element  100  shown in  FIGS. 6 and 7 , the spacer arrangement  24  may comprise one or more spacers  50  as one or more layers of the multilayer composite  40 . For example, the spacers  50  may comprise two spacer strips  51 , 52  which are incorporated in, and corrugated with, the composite  40  at the first and second ends of the fluid treatment pack  11 . 
         [0063]    The spacer arrangement may be positioned at a variety of locations to define a region substantially free of structure within the legs of the pleats and between the porous medium and the surface of the fluid treatment medium. For example, the spacer arrangement may be positioned in the multilayer composite to locate the region substantially free of structure directly next to, i.e., adjoining, the surface of the fluid treatment medium. In the fluid treatment element  100  shown in  FIGS. 6 and 7 , the spacers  50 , e.g., the spacer strips  51 , 52 , adjoin the inner surface  42  of the fluid treatment medium  41  on one side and the outer surface  102  of the porous medium  101 , e.g., the drainage medium on the other side. The spacer arrangement  24  thus defines a region  30  within the legs  22 , 23  of each pleat  15  which is occupied by the spacer arrangement  24  and a region  31  within the legs  22 , 23  of each pleat  15  which is substantially free of structure and which adjoins the inner surface  42  of the fluid treatment medium  41  and the outer surface  102  of the porous drainage medium  101 . 
         [0064]    The thickness of the spacer arrangement  24 , which generally corresponds to the thickness of the region  31  substantially free of structure, and the thickness of the porous medium  101  may be selected in accordance with several factors, including the desired number of pleats and the desired size of the region substantially free of structure. For many embodiments the combined thicknesses may be in the range from about 0.05 mm or less to about 5 mm or more, or in the range from about 0.1 mm to about 2.0 mm, or in the range from about 0.1 mm to about 1.0 mm. The thickness of the spacer arrangement  24  may be greater than the thickness of the porous medium  101 , thereby increasing the size of the region  31  substantially free of structure. For example, the thickness of the spacer arrangement  24  may be at least about 1.1 or about 1.2 or about 1.5 or about 2 or about 2.5 or about 3 or about 4 or about 5 or more times the thickness of the porous medium  101 . For some embodiments, the thickness of the porous medium  101  may be about 0.2 mm and the thickness of the spacer arrangement maybe about 0.4 mm. For many embodiments, the thickness of the porous medium  101  along the inner surface  42  of the fluid treatment medium  41  may be less than the thickness of the drainage medium  44  along the outer surface  43  of the fluid treatment medium  41 . 
         [0065]    In the fluid treatment element  100  shown in  FIGS. 6 and 7 , the fluid treatment pack  11  may have only one porous medium  101  extending along the inner surface  42  of the fluid treatment medium  41 , and the spacer arrangement  24  may be positioned between the fluid treatment medium  41  and the porous medium  101 , with the region  31  substantially free of structure adjoining the inner surface  42  of the fluid treatment medium  41 . However other embodiments may include more than one porous medium extending along the surface of the fluid treatment medium and/or a spacer arrangement which may be a differently positioned. For example, in addition to the drainage medium, a support medium and/or a cushioning layer may extend along the inner surface of the fluid treatment medium between the fluid treatment medium and the drainage medium. The spacer arrangement, as well as the region substantially free of structure, may then be positioned between the inner surface of the fluid treatment medium and the cushioning layer, or between the cushioning layer, or the support medium, and the drainage medium. 
         [0066]    Fluid treatment elements similar to the fluid treatment element  100  shown in  FIGS. 6 and 7  may be made in any of several different ways, including many of the ways described with respect to the fluid treatment element  10  shown in  FIGS. 1-3 . For example, a multilayer composite  40  may be formed from separate layers of fluid treatment medium  41 , two drainage media  44 ,  101 , and a spacer arrangement  24 , as shown in  FIG. 8 . The outer drainage medium  44  may be positioned along the surface  43  of the fluid treatment medium  41  that becomes the outer surface  43 , and the inner drainage medium  101  may be positioned along the surface  42  of the fluid treatment medium  41  that becomes the inner surface  42 . The spacer arrangement  24 , for example, two spacer strips  51 , 52 , may be positioned between the inner surface  42  of the fluid treatment medium  41  and the inner drainage medium  101 , for example, along the side edges  75 ,  76  of the fluid treatment medium  41 . For example, the spacer strips  51 ,  52  may be positioned in contact with the inner surface  42  of the fluid treatment medium  41  and/or the outer surface  102  of the inner drainage medium  101 . 
         [0067]    After the multilayer composite  40  is formed, it may be corrugated and arranged into a generally cylindrical fluid treatment pack  11  in a manner similar to that described and suggested with respect to the embodiments shown in  FIGS. 4 and 5 , except the embodiment shown in  FIG. 8  may have no strip-out material. As shown in  FIG. 6 , the fluid treatment pack  11  may have a region  31  substantially free of structure within each pleat  15 , e.g., within each leg  22 ,  23  of each pleat  15 . The spacer arrangement  24  and the region  31  substantially free of structure may be positioned between the inner surface  42  of the fluid treatment medium  41  and the outer surface  102  of the inner drainage medium  101 , for example, adjoining the inner surface  42  of the fluid treatment medium  41  and the outer surface  102  of the inner drainage medium  101 . The inner surface  103  of the inner drainage medium  101  of one leg may face or even contact the inner surface  103  of the inner drainage medium  101  of an adjacent leg. 
         [0068]    The fluid treatment pack  11  may be fashioned into a fluid treatment element  100  by the addition of sealing mechanisms, e.g., open end caps  70 , 71 , a core arrangement  53 , and/or a surround  64  as previously described with respect to the fluid treatment element  10  shown in  FIGS. 1-3 . 
         [0069]    Fluid treatment elements embodying the invention may be contained within a wide variety of housings to form a fluid treatment assembly. For example, a housing may be formed from any impermeable material, e.g., a metallic material or a polymeric material, which is chemically compatible with the fluids and mechanically capable of withstanding the process parameters, e.g., pressure and temperature. The fluid treatment element may be permanently contained within the housing, forming a disposable fluid treatment assembly, or it may be removably contained within the housing, allowing a used fluid treatment element to be replaced by a new fluid treatment element within a reusable housing. 
         [0070]    Fluid treatment assemblies may be configured in many different ways. For example, a fluid treatment assembly may comprise a housing containing only a single fluid treatment element or a housing containing multiple fluid treatment elements arranged serially or in parallel within the housing. The fluid treatment assembly may have three or more principal ports. For example, where the fluid treatment assembly serves a separator such as a filter, it may include a feed or process fluid port, a concentrate or retentate port, and a filtrate or permeate port. Where the fluid treatment assembly serves as a mass transfer device for transferring a substance from one fluid to another, it may include an inlet port and an outlet port for each fluid. The fluid treatment assembly may further include various supplemental ports, e.g., ports associated with venting or backwashing. 
         [0071]    One of many different examples of a fluid treatment assembly  200  is shown in  FIG. 9 . The fluid treatment assembly  200  includes a housing  201  which may permanently contain a fluid treatment element, such as a fluid treatment element  10 , 100  similar to those previously described and suggested. The fluid treatment assembly  200  may serve as a filtration module and may include a feed or process fluid port  202  coupled to the feed fluid inlet fitting  68  of the fluid treatment element  10 , 100 , a concentrate or retentate port  203  coupled to the concentrate outlet fitting  69  of the fluid treatment element  10 , 100 , and a filtrate or permeate port  204  which fluidly communicates with the exterior  65  of the fluid treatment pack  11 . The fluid treatment element  10 , 100  is sealed within the housing  201  to isolate the feed port  202  and the concentrate port  203  from the permeate  204 . 
         [0072]    Fluid treatment elements and assemblies embodying the invention may be used to treat any of a myriad of fluids in any of numerous crossflow processes. In one of many applications, a fluid treatment element and assembly may be used to treat cellular solutions, e.g., liquid solutions containing cells and/or cellular components. Cellular solutions may contain any of a wide variety of cells, including bacterial cells, fungal cells, yeast cells, and mammalian cells, especially Chinese Hamster Ovary (CHO) cells. Cellular solutions may contain a wide variety of cellular components, including cellular products such as proteins and enzymes, cellular structures, and substances to be incorporated into cells. Cellular solutions may be treated in numerous ways. For example, cellular solutions may be processed to harvest, separate, concentrate, and/or purify the cells and/or cellular components. Cellular solutions can be somewhat viscous and very sensitive, the cells and/or cellular components of the solution being easily damaged. Fluid treatment elements embodying the invention are particularly well suited for processing liquid cellular solutions. 
         [0073]    A feed fluid, such as a cellular solution including a suspending liquid and one or more types of cells and/or cellular components, may be directed axially along the fluid treatment element  10  within the pleats via the tangential fluid flow path  32 , as shown in  FIG. 9 . For example, the cellular solution may be directed through the feed port  202  of the fluid treatment assembly  200  and generally axially into the feed inlet  68  and the end portion  60  of the core arrangement  53  of the fluid treatment element  10 , 100 . The barrier  61  then directs the cellular solution generally radially outwardly through the openings  63  in the perforated side wall  62  of the end portion  60  and the open inner ends of the pleats. 
         [0074]    The feed fluid, e.g., the cellular solution, then passes axially along the tangential fluid flow path  32  between or within the legs of the pleats from one end of the element  10 , 100  to the opposite end. In the fluid treatment element  10  shown in  FIGS. 1-3 , the fluid may flow along this portion of tangential fluid flow path  32  solely through the regions  31  that are substantially free of structure. In the fluid treatment element  100  shown in  FIGS. 6 and 7 , the fluid may flow along this portion of the tangential fluid flow path  32  through both the regions  31  that are substantially free of structure and the inner porous medium, e.g., the drainage medium  101 . Most of the fluid may flow through the regions  31  substantially free of structure because these regions  31  may be larger and have less flow resistance than the inner porous medium  101 . 
         [0075]    For many embodiments, the feed fluid, e.g., the cellular solution, may be initially directed into and along the regions  31  substantially free of structure at a lower pressure and/or flow rate, and the pressure and/or flow rate of the feed fluid may then be increased to the normal operating pressure and/or flow rate over a period of time. For example, the cellular solution may be initially directed into and along the regions  31  substantially free of structure at a pressure which provides a flow rate of about 0.5 to about 1 liter/minute. The pressure of the cellular solution may then be increased over a period of about 3 to about 5 minutes to provide a normal operating flow rate of about 7 liters/minute. For many embodiments, the transmembrane pressure along most or all of the fluid treatment medium during normal operation may be arranged to be positive, and the transmembrane pressure at the retentate end of the fluid treatment element is preferably no less than about negative 0.5 psi. 
         [0076]    Within the tangential fluid flow path  32  along the fluid treatment medium  41 , the feed fluid, e.g., the cellular solution, may be treated in any of a variety of ways. For example, the cellular solution may be treated, for example, by removing some of the suspending liquid and/or one or more other substances in the cellular solution via the lateral fluid flow path  33  through the fluid treatment medium  41  of the fluid treatment pack  11 . The fluid treatment element  10 , 100  may thus serve as a concentrator, for example, principally allowing the suspending liquid to pass as permeate generally radially outwardly through the fluid treatment medium  41  via the lateral fluid flow path  33  to the exterior  65  of the fluid treatment pack  11  and, hence, to the permeate port  204  of the fluid treatment assembly  200 . Alternatively or additionally, the fluid treatment element  10 , 100  may serve as a separator, for example, allowing one or more substances, such as proteins or enzymes, to bind to the fluid treatment medium  41  or to pass as permeate generally radially outwardly through the fluid treatment medium  41  via the lateral fluid flow path  33  to the permeate port  204 . Alternatively, the cellular solution may be treated by adding one or more substances to the cellular solution radially inwardly through the fluid treatment medium  41  via the lateral fluid flow path  33 . 
         [0077]    Fluids and substances in the fluids pass to or from the tangential fluid flow path  32  through the fluid treatment medium  41  via the lateral fluid flow path  33 . Many embodiments of the fluid treatment element  10 , 100  may include a surround  64  which may have openings such as open pores or perforations along most or all of its length. The fluids and substances in the fluids may then pass via a lateral fluid flow path  33  which extends through the fluid treatment medium  41  and any other downstream layers of the fluid treatment pack  11  and generally radially through the openings in the surround  64 . 
         [0078]    Other embodiments of the fluid treatment  10 , 100  may include a surround  64  which may have openings only along a specific region of the surround  64 , e.g., only at one or both ends of the fluid treatment element  10 , 100 . For example, as shown in  FIG. 10 , the surround  64  may have openings  66  only at one end of the fluid treatment element  10 , 100 . In the illustrated embodiment, the openings, e.g., perforations  66 , may be only at the end near the retentate outlet opening  69 . The surround  64  may be variously configured. For example, the surround  64  may comprise a flexible, impermeable sleeve or wrap having a blind portion  67  along most of its length and openings  66  only at one end, e.g., the retentate end. Alternatively, the surround  64  may comprise a more rigid, impermeable cage or tube having a blind portion  67  along most of its length and openings  66  only at one end, e.g., the retentate end. As yet another alternative, the surround may comprise a cylindrical side wall of a housing which may be fitted against the exterior of the fluid treatment pack. The housing may then have a port communicating with a channel, e.g., an annular channel, at one end, e.g., the retentate end, of the fluid treatment pack. 
         [0079]    The fluids and the substances in the fluids, e.g., the permeate, may pass via a lateral fluid flow path  33  which extends through the fluid treatment medium along the entire height of the fluid treatment element  10 , 100 . Away from the openings  66 , the lateral fluid flow path  33  further extends axially or tangentially between the outer or downstream side of the fluid treatment medium and the inner surface of the surround  64  to the openings  66  and then generally radially through the openings  66 . The fluids and the substances in the fluids may flow toward the openings  66  axially within the pleats, e.g., along the downstream drainage layer, or axially between the folded outer ends of the pleats and the inner surface of the surround  64 . The surround  64  may have structures such as ribs or lands along the inner surface of the surround which define passageways leading to the openings  66 . By providing the openings  66  in the surround  64  at the retentate end of the fluid treatment element  10 , 100 , the fluid and the substances in the fluid flowing outwardly through the fluid treatment medium  41  along the lateral fluid flow path  33  may flow in the same direction, or co-currently, as the direction that the feed fluid flows along the tangential fluid flow path  32 . Alternatively, the openings may be located at the inlet end of the fluid treatment element, and the fluid and the substances in the fluid flowing outwardly through the fluid treatment medium along the lateral fluid flow path may flow in the direction opposite, or counter-currently, to the direction that the feed fluid flows along the tangential fluid flow path. 
         [0080]    As shown in  FIG. 9 , at the end of the fluid treatment element  10 , 100  opposite the feed inlet  68 , the remaining feed fluid, e.g., the remaining cellular solution, then passes generally radially inwardly along the tangential fluid flow path  32  from the open inner ends of the pleats and the perforated side wall  62  into the end portion  60  of the core arrangement  53 . The barrier  61  then directs the cellular solution generally axially along the tangential fluid flow path  32  from the end portion  60  of the core arrangement  53  through the concentrate outlet  69  of the fluid treatment element  10  and out through the concentrate port  203  of the fluid treatment assembly  200 . In a single pass mode of operation, the cellular solution may be directed from the concentrate port  203  of the fluid treatment assembly  200  to other components of the fluid system (not shown). In a multipass mode or a recirculating mode of operation, the cellular solution may be directed back to the feed port  202  of the fluid treatment assembly  200  for continued treatment within the fluid treatment element  10 , 100 . 
         [0081]    Many advantages are associated with fluid treatment elements embodying one or more aspects of the invention. For example, by providing a tangential fluid flow path  32  through regions  31  which are substantially free of structure, fluid treatment elements embodying the invention offer less resistance to the flow of fluids, such as the cellular solution. The fluids may thus flow through the fluid treatment element with a smaller pressure drop. Further, because at least 50%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the tangential fluid flow path  32  through the fluid treatment element may be through regions  31  which are substantially free of structure, fluids, such as the cellular solution, may flow through the fluid treatment element with little or no damage, for example, to the cells and/or cellular components in the solution. Fluid treatment elements embodying the invention may thus reduce the need for additional downstream processing to remove cellular debris and prevent the release of dangerous endotoxins into the retentate or the permeate. In addition, by locating the regions substantially free of structure directly next to the surface of the fluid treatment medium, fluid flowing along the tangential flow path can more thoroughly clear foulants form the surface of the fluid treatment medium. This can extend the service life of the fluid treatment element. 
         [0082]    Fluid treatment elements having a region substantially free of structure and an adjacent porous medium, including elements similar to those disclosed in  FIGS. 6 and 7 , have additional advantages. For example, by providing both a region substantially free of structure and a porous medium, e.g., a drainage medium, in the tangential fluid flow path along the fluid treatment medium, the fluid treatment element can operate more effectively. During many normal modes of operation, the fluid in the tangential flow path in the regions substantially free of structure is generally at a higher pressure than the fluid on the opposite side of the fluid treatment medium, and the regions substantially free of structure remain open to fluid flow. However, there may be instances when the fluid pressure along all or some portion of the regions substantially free of structure is less than the fluid pressure on the opposite side of the fluid treatment medium. Adjacent pleat legs may then expand toward one another, collapsing some portion or all of the region substantially free of structure within each pleat. However, when the normal mode of operation is resumed, the regions substantially free of structure can be immediately reestablished. The higher pressure fluid easily flows along the tangential flow path within each pleat via the porous medium, e.g., the drainage medium, where it forces the adjacent legs of each pleat back to their normal position, quickly reestablishing the regions substantially free of structure within each pleat. 
         [0083]    Fluid treatment elements having a surround with openings only in a specific region, including elements similar to those disclosed in  FIG. 10 , also have additional advantages. By directing fluid and substances in the fluid along a substantial portion of the lateral fluid flow path co-currently or counter-currently with the feed fluid along the tangential fluid flow path, various operating parameters, such as transmembrane pressure or flux, may be enhanced. This can lead to improved efficiency or longer service life. 
         [0084]    Some of the benefits of fluid treatment elements embodying various aspects of the invention are illustrated in the following Examples. 
       EXAMPLE 1 
       [0085]    This example demonstrates an effect of directing a cellular solution through a pleated, crossflow fluid treatment element without regions substantially free of structure. 
         [0086]    The crossflow fluid treatment element is similar to the element  10  shown in  FIGS. 1-3 , except the crossflow fluid treatment the element has no spacer arrangement  24  and no regions  31  which are substantially free of structure. In place of the spacer arrangement and the regions substantially free of structure is an inner net drainage medium comprising a symmetrical polypropylene extruded net having a thickness of 0.33 mm and a strand count of 31 strands per inch in both directions. The pleated fluid treatment pack consists of the inner net drainage medium, a fluid treatment medium comprising a PES membrane having a removal rating of 0.65 microns, and an outer drainage medium comprising a spunbonded polypropylene nonwoven material having a thickness of 0.3 mm. The pleats extend from the core arrangement to the surround in a curved direction. The outer diameter of the core arrangement is 46 mm, the inner diameter of the surround is 66.5 mm and the crossflow fluid treatment element is 24.5 cm long. 
         [0087]    The cellular solution includes an aqueous-based liquid culture medium containing mammalian CHO cells. 
         [0088]    The cellular solution is recirculated through the crossflow fluid treatment element from the feed inlet through the inner net drainage medium to the concentrate outlet at a crossflow rate of 10 L/minute for 155 minutes. The permeate port is closed for the first 145 minutes and open for the last 10 minutes. 
         [0089]    The initial concentration is 5.6×10 5  cells/ml of the cellular solution and the initial viability is 96%. 
         [0090]    The final concentration is 1.5×10 4  cells/ml of cellular solution and the final viability is unmeasurable due to low cell count. 
         [0091]    Cell debris is observed in the permeate fluid. 
       EXAMPLE 2 
       [0092]    This example demonstrates an effect of directing a feed fluid such as a cellular solution through a pleated, crossflow fluid treatment element embodying an aspect of the invention. 
         [0093]    The crossflow fluid treatment element is similar to the element  10  shown in  FIGS. 1-3  and includes spacer strips at each end of the pleated fluid treatment pack and regions between the strips that are substantially free of structure. The pleated fluid treatment pack consists of two layers of a fluid treatment medium, each comprising a PES membrane having a removal rating of 0.65 microns, and an outer drainage medium comprising a spunbonded polypropylene nonwoven material having a thickness of 0.30 mm. The spacer strips consist of two strips of a spunbonded polypropylene nonwoven material having a width of 10 mm and a thickness of 0.30 mm. The spacer strips are pleated with the fluid treatment pack, one at each end of the pack. The pleats extend from the core arrangement to the surround in a curved direction. The number of pleats, the outer diameter of the core arrangement, the inner diameter of the surround, and the length of the element are substantially the same as in Example 1. 
         [0094]    The cellular solution is an aqueous-based liquid culture medium containing mammalian CHO cells. 
         [0095]    The cellular solution is recirculated through the crossflow fluid treatment element from the feed inlet through the regions substantially free of structure to the concentrate outlet at a crossflow rate of 7 L/minute for 177 minutes. The permeate port is open for the entire 177 minutes. 
         [0096]    The initial concentration is 7.7×10 4  cells/ml of cellular solution and the initial viability is 63%. 
         [0097]    The final concentration is 2.13×10 5  cells/ml of cellular solution and the final viability is 72%. 
         [0098]    No cellular debris is observed in the permeate fluid. 
       EXAMPLE 3 
       [0099]    This example demonstrates an effect of directing a feed fluid such as a cellular solution through a pleated, crossflow fluid treatment element embodying an aspect of the invention. 
         [0100]    The crossflow fluid treatment element is similar to the element  100  shown in  FIGS. 6 and 7  and includes an inner porous medium, spacer strips at each end of the fluid treatment pack between the inner porous medium and the fluid treatment medium, and regions between the strips that are substantially free of structure. The pleated fluid treatment pack  11  consists of (1) an inner porous drainage medium comprising a 0.125 mm thick extruded polypropylene mesh having 41 strands per inch (25.4 mm) in the machine direction and 23 strands per inch (25.4 mm) in the cross direction; (2) a fluid treatment medium comprising two layers of a PES membrane having a removal rating of 0.65 micron; (3) two spacer strips comprising a spunbonded polypropylene nonwoven material having a thickness of 0.3 mm and being respectively positioned at each end of the pack between the inner porous medium and the fluid treatment medium; and (4) an outer drainage material comprising a spunbonded polypropylene nonwoven material having a thickness of 0.3 mm. There are 95 pleats, each pleat having a height of about 13 mm, and the pleats extend from the core arrangement to the surround in a curved direction. The outer diameter of the core arrangement is about 46 mm, the inner diameter of the surround is about 66.5 mm, and the length of the crossflow fluid treatment element is about 24.5 cm. 
         [0101]    The cellular solution includes an aqueous-based liquid culture medium containing mammalian CHO cells. 
         [0102]    The cellular solution is recirculated through the crossflow fluid treatment element from the feed inlet through the regions substantially free of structure and the inner porous medium to the concentrate outlet at a crossflow rate of 7 L/minute for 15 minutes. A peristaltic pump controls the permeate flow to a set rate of 1.7 L/minute. The flux rate is approximately 200 LMH. 
         [0103]    The initial concentration is 9.1×10 5  cells/ml of cellular solution and the initial viability is 98%. 
         [0104]    The final concentration is 1.1×10 7  cells/ml of cellular solution and the final viability is 97%. 
         [0105]    No cell debris is observed in the permeate fluid. 
         [0106]    While various aspects of the invention have previously been described and/or illustrated in the Figures, the invention is not limited to these embodiments. For instance, one or more of the features of these embodiments may be eliminated without departing from the scope of the invention. For example, as previously described, the core arrangement  53  or the strip-out material  74  may be omitted from some embodiments. As another example, the surround  64  may be omitted from some embodiments. 
         [0107]    Further, one or more features of one or more embodiments may be combined with one or more features of other embodiments without departing from the scope of the invention. For example, in some embodiments an end cap  70  as shown in  FIG. 1  at one end of the fluid treatment pack may be combined with a sealant as previously described at the other end of the fluid treatment pack. The core arrangement may then include an end portion having openings and a barrier only at the end of the fluid treatment element near the end cap. 
         [0108]    Further, one or more features of the embodiments may be modified without departing from the scope of the invention. For example, the spacer arrangement may be associated with the core arrangement or one or both end caps rather than being incorporated in the composite. As shown in  FIG. 11 , the spacer arrangement  24  may comprise a plurality of spacer fins  110  which extend outwardly in a straight or curved direction from the core arrangement  53 . Spacer fins  110  extending from the ends of the core arrangement  53  may be solid, porous, perforated, or channeled. Each spacer fin may have a thickness, width and height similar to spacers previously described. These spacer fins  110  may be arranged between the legs  22 ,  23  of the pleats  15  as the core arrangement  53  is inserted in the fluid treatment pack, replacing the spacer strips  51 , 52  of  FIGS. 3 and 4 . As the end caps are attached to the ends of the fluid treatment pack, the spacer fins  110  may also be attached to the end caps. While the illustrated spacer fins  110  project outwardly from the ends of the core arrangement  53 , spacer fins may also, or instead, project from the core arrangement at locations intermediate the ends. The intermediate spacer fins may be porous, perforated or channeled to allow feed fluid to flow past the spacer fins and axially along the regions which are substantially free of structure. As yet another modification, the spacer fins may project inwardly from an end cap, for example, between layers of a multilayer composite, rather than outwardly from the core arrangement. 
         [0109]    Further, embodiments having different features can still be within the scope of the invention. For example, fluid treatment assemblies and fluid treatment elements may have a tangential fluid flow path which extends within the pleats along the outer surface of the fluid treatment medium rather than the inner surface. One example of such a fluid treatment assembly  300  and a fluid treatment element  301  is shown in  FIGS. 12-14 . The features of the fluid treatment element  301  shown in  FIGS. 12-14 , including the components and the methods of making and using the element, may be analogous to those of the fluid treatment element  10  shown in  FIGS. 1-3 , and corresponding components may be identified with the same reference numerals. However, the geometry of the fluid treatment pack  11  of the fluid treatment element  301  may generally be reversed with respect to the geometry of the fluid treatment pack  11  of the fluid treatment element  10  shown in  FIGS. 1-3 . For example, the drainage medium  44  may be positioned along the inner surface  42  of the fluid treatment medium  41 , while the spacer arrangement  24 , e.g., the spacers  50  such as the spacer strips  51 , 52 , and the regions  31  substantially free of structure may be positioned along the outer surface  43  of the fluid treatment medium  41 . 
         [0110]    For example, as shown in  FIGS. 12-14 , a fluid treatment element  301  may include a fluid treatment pack  11  having an axis  12 , opposite ends  13 ,  14 , a fluid treatment medium  41 , and a plurality of axially extending pleats  15 . Each pleat  15  includes a folded end, e.g., a folded inner end  20 , an open end, e.g., an open outer end  21 , and two legs which extend between the folded end  20  and the open end  21 . The fluid treatment medium  41  may have an inner surface  42  and an outer surface  43 , and the fluid treatment pack  11  may comprise a multilayer composite  40  having two or more layers. For example, an inner drainage layer  44  may extend along the inner surface  43  of the fluid treatment medium  41 , either adjoining or spaced from the inner surface  43 . Other porous media, such as a cushioning layer (not shown), may also be positioned along the inner surface  43  of the fluid treatment medium  41 . 
         [0111]    The spacer arrangement may be configured and positioned in the fluid treatment pack  11  in a variety of ways, as previously described with respect to the fluid treatment element  10  shown in  FIGS. 1-3 . In the fluid treatment element  301  shown in  FIGS. 12-14 , the spacer arrangement  24  may comprise one or more spacers  50 ,.e.g., two spacer strips  50 , 51 , which are incorporated in, and corrugated with, the composite  40  along the outer surface  42  of the fluid treatment medium  41 . The spacer arrangement defines one or more regions  30  that are occupied by the spacer arrangement  24  and one or more regions  31  that are substantially free of structure along the outer surface  43  of the fluid treatment medium  41 . The spacer arrangement, the regions occupied by the spacer arrangement, and the regions substantially free of structure may adjoin the outer surface of the fluid treatment medium or may be spaced from the outer surface with one or more layers between them. The fluid treatment pack  11  may be positioned between a core arrangement  53  along the interior of the fluid treatment pack  11  and a surround  64  along the exterior  65  of the fluid treatment pack  11 . 
         [0112]    The surround  64 , which may be analogous to the surround  64  shown in  FIG. 10 , may comprise a blind portion, e.g., a central blind portion  67 , which serves to block off the open outer ends  21  of the pleats  15  and/or resist radially outward flow from the regions  31  that are substantially free of structure. The blind portion  67  may terminate at locations spaced from the ends  13 , 14  of the fluid treatment pack  11 . The blind portion  67  may comprise an impermeable wrap or sleeve or may comprise a portion of the side wall  306  of the housing  302  of the fluid treatment assembly  300 , all of which may fit tightly around the exterior  65  of the fluid treatment pack  11 , adjoining the outer folded ends of the pleats  15 . A portion of the surround  64  with perforations  66  or other openings may extend from the blind portion  67  to each end  13 , 14 . 
         [0113]    The end sealing mechanism, e.g., the end caps  70 , 71 , and the housing  302  may be arranged to direct feed fluid along a tangential fluid flow path  32  which extends along the outer surfaces of the legs  22 , 23  of the pleats  15 . For example, feed fluid may pass from the feed port  303  generally radially inwardly through the perforation  66  in the surround  64  and the open outer ends  21  of the pleats  15  near one end  13  of the fluid treatment pack  11  to the regions  31  that are substantially free of structure. The feed fluid then passes generally axially along the tangential fluid flow path  32  via the regions  31  that are substantially free of structure to the other end  14  of the fluid treatment pack  11 . The feed fluid then passes generally radially outwardly from the regions  31  that are substantially free of structure through the open outer ends  21  of the pleats  15  and the perforations  66  in the surround  64  to the concentrate port  304  of the fluid treatment assembly  300 . 
         [0114]    The core arrangement  53  may be without any barrier and may be perforated throughout its length or may have an outer surface texture which allows fluid to drain along the core arrangement. One or both end caps  70 , 71  may be an open end cap that communicates between the core arrangement  53  at the interior  54  of the fluid treatment pack  11  and the permeate port  305  of the fluid treatment assembly  300 . One or more substances may pass from the tangential fluid flow path  32  and the regions  31  which are substantially free of structure through the fluid treatment medium  41  to the interior  54  of the fluid treatment pack  11  and the core arrangement  53 , or vice versa, via the lateral fluid flow path  33 . 
         [0115]    The fluid treatment element  100  shown in  FIGS. 6 and 7  may similarly be modified to provide a tangential fluid flow path within the pleats along the outer surface of the fluid treatment medium. For example, the geometry of the fluid treatment pack  11  shown in  FIGS. 6 and 7  may be reversed. The fluid treatment pack may comprise a multilayer composite which includes a porous medium e.g., a drainage medium, positioned along the outer surface of the fluid treatment medium. The spacer arrangement, the regions occupied by the spacer arrangement, and the regions substantially free of structure may be positioned between the outer surface of the fluid treatment medium and the porous medium. For example, the spacer arrangement, the regions occupied by the spacer arrangement, and/or the regions substantially free of structure may adjoin the outer surface of the fluid treatment medium. The multilayer composite may further include a drainage medium positioned along the inner surface of the fluid treatment medium. The fluid treatment pack may be positioned between a surround and a core arrangement. For example, the surround  64 , the end caps  70 , 71 , and the core arrangement  53  of the fluid treatment element  301 , as well as the housing  302  of the fluid treatment assembly  300 , shown in  FIGS. 12-14 , may be associated with the fluid treatment pack. 
         [0116]    The present invention is thus not restricted to the particular embodiments which have been described and/or illustrated but includes all embodiments and modifications that may fall within the scope of the claims.
Fluid treatment elements and methods for treating fluids may be used to treat fluids, including gases, liquids, or mixtures of gases, and/or solids, in a crossflow mode of operation. For example, some fluid treatment elements and methods may be used to remove one or more substances from the fluid and may then function as concentrators or filters or separators. Other fluid treatment elements or methods may be used to transfer substances between two fluid streams and may then function as mass transfer devices.