Abstract:
Fluid treatment assemblies and elements may include a plurality of axially extending pleats and 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 assemblies and elements may be used to remove one or more substances from the fluid and may then function as separators, including filters and concentrators. Other fluid treatment assemblies and elements may be used to transfer substances between two fluid streams and may then function as mass transfer devices.

Description:
DISCLOSURE OF THE INVENTION 
       [0001]    The present invention relates to fluid treatment assemblies and 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 assemblies and elements may be used to remove one or more substances from the fluid and may then function as concentrators or filters or other types of separators. Others of the fluid treatment assemblies and elements may be used to transfer substances between two fluid streams and may then function as mass transfer devices. 
         [0002]    In particular, the present invention relates to pleated fluid treatment assemblies and elements which are structured to treat a fluid in a crossflow mode of operation. The pleated fluid treatment elements include a fluid treatment medium, e.g., either as a single sheet or as one or more layers of a multilayer composite. 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. Opposite end edges of the pleated sheet or composite are sealed to one another along an edge seal to form a generally cylindrical fluid treatment pack with each pleat extending generally axially along the fluid treatment pack. 
         [0003]    The fluid treatment assemblies and 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. Feed fluid may enter the fluid treatment assembly or 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 assembly or element. 
         [0004]    In accordance with one aspect of the invention, fluid treatment elements may comprise a fluid treatment pack, a sealant, and first and second fluid flow paths. The fluid treatment pack includes a fluid treatment medium which has first and second surfaces. The fluid treatment pack also includes 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 includes first and second axial ends at the first and second ends of the fluid treatment pack, respectively. Each pleat further includes a folded end, an open end, first and second legs which extend between the folded end and the open end of the pleat, and a region free of structure. The region free of structure extends axially within the pleat between the first and second axial ends along the first surface of the fluid treatment medium and opens onto the first and second axial ends of the pleat. The sealant seals each end of the fluid treatment pack from the second surface of the fluid treatment medium. The first fluid flow path extends axially along the pleated fluid treatment pack within the pleats and includes the regions free of structure. The second fluid flow path extends through the fluid treatment medium from or to the first fluid flow path. 
         [0005]    In accordance with another aspect of the invention, fluid treatment elements may comprise a hollow, generally cylindrical fluid treatment pack, a sealant, and a core. The fluid treatment pack includes an axis, an interior, first and second opposite ends, and a pleated composite. The pleated composite includes a fluid treatment medium having an inner surface and an outer surface and defines a plurality of pleats extending axially between the first and second ends of the fluid treatment pack. Each pleat includes first and second axial ends at the first and second ends of the fluid treatment pack, respectively. Each pleat further includes a folded outer end, an open inner end, first and second legs which extend between the folded outer end and the open inner end, and a region free of structure. The region free of structure extends axially within the pleat between the first and second axial ends along the inner surface of the fluid treatment medium and opens onto the first and second axial ends of the pleat. The sealant seals each end of the fluid treatment pack from the outer surface of the fluid treatment medium. The core is positioned in the hollow interior of the fluid treatment pack along the open inner ends of the pleats. 
         [0006]    In accordance with another aspect of the invention, fluid treatment elements may comprise a generally cylindrical fluid treatment pack, a sealant, and an outer surround. The fluid treatment pack includes an axis, an exterior, first and second opposite ends, and a pleated composite. The pleated composite includes a fluid treatment medium having an inner surface and an outer surface and defines a plurality of pleats extending axially between the first and second ends of the fluid treatment pack. Each pleat includes first and second axial ends at the first and second ends of the fluid treatment pack, respectively. Each pleat further includes a folded inner end, an open outer end, first and second legs which extend between the folded inner end and the open outer end, and a region free of structure. The region free of structure extends axially within the pleat between the first and second axial ends along the outer surface of the fluid treatment medium and opens onto the first and second axial ends of the pleat. The sealant seals each end of the fluid treatment pack from the inner surface of the fluid treatment medium. The outer surround is positioned around the exterior of the fluid treatment pack along the open outer ends of the pleats. 
         [0007]    In accordance with another aspect of the invention, methods of making a fluid treatment element may comprise corrugating a fluid treatment medium having first and second opposite surfaces to form a plurality of pleats and forming the corrugated fluid treatment medium into a generally cylindrical fluid treatment pack. The fluid treatment pack has first and second opposite ends and the pleats extend axially along the fluid treatment pack between the first and second ends. The methods also comprise positioning a stripout material along the first surface of the fluid treatment medium and applying a sealant to the fluid treatment pack near the first and second ends to seal the ends of the fluid treatment pack from the second surface of the fluid treatment medium. The methods further comprise removing the stripout material from the corrugated fluid treatment pack to form a region within each pleat that is free of structure. The stripout material is removed from the fluid treatment pack after the sealant is applied to the ends of the fluid treatment pack. 
         [0008]    In accordance with another aspect of the invention, methods of making a fluid treatment element may comprise forming a composite which includes a fluid treatment medium having first and second opposite surfaces and a stripout material positioned along the first surface of the fluid treatment medium. The methods also comprise corrugating the composite to form a plurality of pleats and 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 methods further comprise applying a sealant to the fluid treatment pack near the first and second ends to seal the ends of the fluid treatment pack from the second surface of the fluid treatment medium. The methods further comprise removing the stripout material from the corrugated composite to form a region within each pleat that is free of structure. The stripout material is removed from the composite after the sealant is applied to the ends of the fluid treatment pack. 
         [0009]    Embodiments of the invention have many advantages. For example, by providing regions within the pleats that are 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 free of structure directly next to a surface of the fluid treatment medium, fluid flowing along the tangential flow path through these regions may have higher and more uniform shear rates and maintain the surface of the fluid treatment medium more thoroughly free of foulants. In addition, many feed fluids deposit foulants when they flow through or around obstructions in the flow path in the fluid flow path. By providing regions within the pleats that are free of structure, embodiments of the invention allow feed fluid to flow along the tangential flow path through the fluid treatment pack without depositing significant amounts of foulants on the fluid treatment medium. Consequently, the performance of the fluid treatment element can be enhanced and the service life of the fluid treatment elements can be extended. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a sectioned view of a fluid treatment assembly. 
           [0011]      FIG. 2  is an end view of the fluid treatment assembly shown in  FIG. 1 . 
           [0012]      FIG. 3  is a sectioned view showing the foreground of the fluid treatment assembly, as shown in  FIG. 1 . 
           [0013]      FIG. 4  is a representational view of a multilayer composite including a fluid treatment medium. 
           [0014]      FIG. 5  is an oblique view of a cylindrical fluid treatment pack. 
           [0015]      FIG. 6  is a sectioned side view of a portion of a fluid treatment pack in a fixture. 
           [0016]      FIG. 7  is an oblique view of a stripout material in a fluid treatment pack. 
           [0017]      FIG. 8  is a sectioned view of another fluid treatment assembly. 
           [0018]      FIG. 9  is a sectioned view of the fluid treatment element shown in  FIG. 8 . 
           [0019]      FIG. 10  is a sectioned view showing the foreground of the fluid treatment assembly shown in  FIG. 8 . 
           [0020]      FIG. 11  is a sectioned view of a portion of a fluid treatment pack in a fixture. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Many different fluid treatment elements and assemblies may embody the invention. An example of a fluid treatment assembly  10  is shown in  FIGS. 1-3  and comprises a fluid treatment element  11  contained in a housing  12 . The fluid treatment element  11  generally includes a hollow, generally cylindrical fluid treatment pack  13  having a central axis  14 , opposite ends  15 ,  16 , a fluid treatment medium  17 , and a plurality of pleats  20 . The pleats  20  extend generally axially between the ends  15 ,  16  of the pack  13 . The pleats  20  may extend inwardly or outwardly in a radial direction or in a curved, arcuate, angled or straight nonradial direction. Each pleat  20  may include opposite axial ends  21 ,  22  at the respective opposite ends  15 ,  16  of the fluid treatment pack  13 . Each pleat  20  may further include a folded end, e.g., a folded outer end  23 , an open end, e.g., an open inner end  24 , and two legs  25 ,  26  which extend between the folded end  23  and the open end  24 . All or substantially all, i.e., at least about 85% or at least about 90% or at least about 95% or 100%, of the pleats  20  of the fluid treatment pack  13  include a region  27  free of structure. Greater percentages are preferred because they provide more regions free of structure within the fluid treatment pack. The region  27  free of structure extends axially within each pleat  20  along the full length of the pleat  20  between the axial ends  21 ,  22  and opens onto the axial ends  21 ,  22  of the pleat  20 . The fluid treatment medium  17  has an inner surface and an outer surface, and the region  27  free of structure extends axially within the pleat  20  along one of the surfaces, e.g., the inner surface, of the fluid treatment medium  17 . A sealant  18  at each end of the fluid treatment pack  13  seals the other of the surfaces, e.g., the outer surface, of the fluid treatment medium  17  from the ends  15 ,  16  of the fluid treatment pack  13 . The fluid treatment element  11  further includes a tangential fluid flow path  30  and a lateral fluid flow path  31 . The tangential fluid flow path  30  extends generally axially along the fluid treatment pack  13  within the pleats  20 , including the regions  27  free of structure. The lateral fluid flow path  31  fluidly communicates with the tangential fluid flow path  30  and extends laterally through the fluid treatment medium  17  to or from the tangential fluid flow path  30 . 
         [0022]    In operation, feed fluid may pass tangentially along the fluid treatment pack  13  via the tangential fluid flow path  30 . For example, feed fluid may pass along the tangential fluid flow path  30  though a feed or process fluid inlet  32  of the housing  12  into the regions  27  free of structure at the open axial ends  21  of the pleats  20 . The feed fluid then passes via the regions  27  free of structure along one surface of the fluid treatment medium  17 , exiting the fluid treatment pack  13  and the housing  12  at the opposite open axial ends  22  of the pleats  20  and a retentate or concentrate outlet  33 . Within the regions  27  free of structure the feed fluid may be treated, for example, by removing one or more substances from the feed fluid via the lateral fluid flow path  31  through the fluid treatment medium  17  or by adding one or more substances to the feed fluid via the lateral fluid flow path  31  through the fluid treatment medium  17 . For example, one or more components of the feed fluid may be removed by passing through the fluid treatment medium  17  and through one or more permeate or filtrate outlets  34  of the housing  12 . The fluid treatment element  11  may thus be considered a pleated, crossflow fluid treatment element. International Publication No. WO 00/13767 and International Publication No. WO 2005/094963 also disclose pleated, crossflow fluid treatment elements and are incorporated by reference in their entirety for any and all purposes. 
         [0023]    The fluid treatment pack  13  may be structured in a wide variety of ways. For example, the fluid treatment pack may include a pleated, multilayer composite  40 , as shown in  FIG. 3 . Some or all of the layers of the composite  40  may be integrally joined to or formed with one another. For example, they may be heat bonded, solvent bonded, or adhesively bonded to one another along all or a portion of the adjacent surfaces of the layers. However, in many embodiments, the layers of the composite  40  may comprise separate layers positioned adjacent to one another but not affixed to one another except in the vicinity of the sealant, where the sealant may affix the layers to one another. 
         [0024]    The fluid treatment pack  13  includes a fluid treatment medium  17  as one or more of the layers of the composite  40 . The fluid treatment medium  17  may have opposite surfaces, e.g., an inner surface  41  and an outer surface  42 . 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, including permeable, semipermeable, or permselective, and may have removal efficiencies, including molecular weight cutoffs, 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 reverse osmosis, 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 permeable, semipermeable, or permselective membrane or a porous sheet, such as 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 permeable polymeric membrane having a submicron removal rating or finer may be used as the fluid treatment medium. 
         [0025]    The fluid treatment pack  13  may include one or more layers in addition to the fluid treatment medium  17 . For example, the fluid treatment pack  13  may include a porous drainage medium as another layer of the multilayer composite  40 . The drainage medium may comprise a downstream or permeate drainage layer  43  which is positioned along a surface, e.g., the outer surface  42 , of the fluid treatment medium  17  either adjoining the outer surface  42  or spaced from the outer surface  42  of the fluid treatment medium  17 . The drainage layer  43  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 outer surface  42  of the pleated fluid treatment medium  17  parallel to the pleat legs. Many suitable drainage 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 any and all purposes. For many embodiments, a woven or nonwoven polymeric fibrous material or a polymeric mesh may be used as the drainage medium. For example, the permeate drainage layer  43  may comprise an extruded polymeric mesh having first and second biplanar sets of strands. For the extruded mesh layer, as well as any other mesh layer, either set of strands, or neither set of strands, may be oriented within the composite  40  parallel to the axis  14  of the fluid treatment pack  13 . 
         [0026]    The multilayer composite  40  may also include other layers. For example, the permeate drainage layer  43  may be positioned along the outer surface  42  of the fluid treatment medium  17  with a downstream or permeate porous support layer  44  and a downstream or permeate porous cushioning layer  45  between them. The permeate support layer  45  extends along the outer surface  42  of the fluid treatment medium  17  and an inner surface of the permeate drainage layer  43  and supports the fluid treatment medium  17 , as well as the permeate cushioning layer  45 . For example, the permeate support layer  44  may support the fluid treatment medium  17  against forces, such as the forces associated with the transmembrane pressure, which press the fluid treatment medium  17  into the permeate drainage layer  43 , thereby resisting extrusion of the fluid treatment medium  17 , as well as the permeate cushioning layer  45 , into the channels and openings of the permeate drainage layer  43 . The permeate support layer  44  may comprise a wide variety of woven or nonwoven polymeric fibrous materials or polymeric meshes which may be finer than the mesh of the permeate drainage layer  43 . 
         [0027]    The downstream cushioning layer  45  may be immediately adjacent to the fluid treatment medium  17  and extend along the outer surface of the fluid treatment medium  17  and inner surfaces of the permeate support layer  44  and the permeate drainage layer  43 . The permeate cushioning layer  45  protects the fluid treatment medium  17  from damage from the other downstream layers  43 ,  44 . For example, the permeate cushioning layer  45  protects the fluid treatment medium  17  from abrasion by the downstream support layer  44  or the downstream drainage layer  43 . For many embodiments the cushioning layer may comprise a nonwoven fibrous material which is thin, smooth, and tough. 
         [0028]    The regions  27  free of structure within the pleats  20  generally provide spaces or channels within the pleats  20  through which fluid, e.g., feed fluid, may flow from one end  15 ,  16  of the fluid treatment pack  13  to the other end  16 ,  15 . Each region  27  extends axially along a pleat  20  between the legs  25 ,  26  of the pleat  20 . The widths of the regions  27  free of structure, i.e., the distance from one pleat leg to the other, may vary or may be constant from region  27  to region  27 . Further, the width of a region  27  free of structure may vary or may be constant along the axial length of the region  27  and/or along the height of the pleat  20 . For many embodiments, the nominal width of the region  27  free of structure may be in the range from about 10 thousandths of an inch or less to about 200 thousandths of an inch or more, e.g., from about 20 to about 150 thousandths of an inch or from about 40 to about 130 thousandths of an inch. In addition, the height of each region  27  free of structure may be at least about 50% or at least about 75% or at least about 90% or about 100% of the height of the pleat  20  between the folded end  23  and the open end  24  of the pleat  20 . Again, greater percentages are preferred because they provide larger regions  27  free of structure. 
         [0029]    Each region  27  free of structure may extend between the axial ends  21 ,  22  of the pleat  20  without obstruction. For example, the regions  27  free of structure may not include a spacer arrangement, such as one or more spacers inserted within the filter pack and/or between the legs of the pleat, to define or maintain a region free of structure within the pleat  20 . Further, for most embodiments each region  27  free of structure may be positioned immediately adjacent to the fluid treatment medium  17 , adjoining a surface, e.g., the inner surface  41 , of the fluid treatment medium  17 . Consequently, fluid such as feed fluid flowing along the tangential flow path  30  passes through the regions  27  free of structure with less pressure drop and at a higher, more uniform shear rate along the surface of the fluid treatment element  17 . 
         [0030]    In addition to the fluid treatment pack  13  and the regions  27  free of structure, the fluid treatment element  11  may include other features. For example, the fluid treatment element  11  may include a core  50  positioned in the interior of the fluid treatment pack  13 , for example, along the open inner ends  24  of the pleats  20 . The core  50 , which may serve to inhibit fluid flow from the open inner ends  24  of the pleats  20 , may be variously configured. For example, the core  50  may comprise a solid rod, or a hollow tube capped at each end. The core may have a outer surface  51  which closes off the open inner ends  24  of the pleats  20 . The core  50  then directs fluid into or out of the open axial ends  21 ,  22  of the pleats  20  and inhibits fluid flow radially inwardly through the open inner ends  24  of the pleats  20 , confining the fluid along the tangential flow path  30  within the regions  27  free of structure. 
         [0031]    The fluid treatment element  11  may also include a surround  52 , such as a cage, a sleeve, or a wrap, positioned around the exterior of the fluid treatment pack  13 . The surround may be perforated or porous, or may have other openings, along all or only a portion of the axial length of the surround. For some embodiments, the surround may be impermeable. In the illustrated embodiment, the surround  52  has openings along the entire axial length and may comprise one or more layers of a polymeric mesh, e.g., an extruded polymeric mesh, circumferentially wrapped around the exterior of the fluid treatment pack  13 . The surround may abut the pleats  20 , e.g., the folded outer ends  23  of the pleats  20 , and assist in holding the pleats  20  in position and maintaining fluid communication between the fluid treatment medium and the permeate outlet. 
         [0032]    The housing  12  may be configured in many different ways to contain the fluid treatment element  11 . For example, the housing  12  may include a shell  53  which surrounds the exterior of the fluid treatment element  11  and has opposing open ends  54 ,  55 . The housing  12  may also include a plurality of ports which may be variously configured. For example, the housing  12  may have a feed or process fluid inlet port  32 , and the feed inlet port  32  may simply comprise one of the open ends  54 ,  55  of the shell  53 . The housing  12  may also have a retentate or concentrate outlet port  33 , and the retentate outlet port  33  may simply comprise the other of the open ends  55 ,  54  of the shell  53 . At least one permeate or filtrate outlet port  34 , e.g., two permeate outlets  34  located near the ends of the shell  53  and angularly displaced by 180°, may also be associated with the housing  12 . 
         [0033]    A sealant  18 , which may extend radially outwardly from the fluid treatment medium  17  to the shell  53  of the housing  12  at each end of the fluid treatment element  11 , seals each end  15 ,  16  of the fluid treatment pack  13  from one surface, e.g., the outer surface  42  or permeate side, of the fluid treatment medium  17 . The sealant  18  may also serve to affix the fluid treatment element  11  to the housing  12  and/or to affix the fluid treatment medium  17  in position, e.g., to affix the fluid treatment medium  17  to one or more other layers of the fluid treatment pack  13 . Various sealants, including, for example, epoxies, urethanes, or polyolefins, may be utilized. For many embodiments the sealant may be an epoxy. The sealant  18  may extend axially inwardly from each end of the fluid treatment element  11  a distance in the range from about ⅛ inch or less to about one inch or more. The sealant  18 , in conjunction with the housing  12 , directs fluid along the lateral flow path  31  from or to the tangential flow path  30  through the fluid treatment medium  17 . For example, permeate or filtrate may be directed along the lateral flow path  31  from the tangential flow path  30  and the regions  27  free of structure through the fluid treatment medium  17  to the surround  52  and along the surround  52  to a permeate outlet port  34  of the housing  12 . 
         [0034]    A sealant  64  may also be positioned between the core  50  and the pleats  20  at each end of the fluid treatment pack  13 , e.g., on the feed side of the fluid treatment medium  17 . The sealant  64  may affix the legs  25 ,  26  of adjacent pleats  20  to the core  52  without significantly obstructing the regions  27  free of structure. The sealant  64  may extend axially along the entire length of the core  52 . For some embodiments, the sealant  64  may extend axially from an end into interior of the fluid treatment pack a distance in the range from about ⅛ inch or less to about one inch or more. With the core  52  and the shell  53  of the housing  12  affixed to the fluid treatment element  11 , the housing  12  may not include, e.g., may be free of, any end pieces which extend between the core  50  and the shell  53 , as shown in  FIG. 1 . No portion of the housing  12  then has an inner diameter less than the outer diameter of the fluid treatment pack. This simplifies manufacture and reduces the amount of waste to be discarded when the fluid treatment assembly  10  is spent. The ends of the shell  53  may each be configured as a fitting, e.g., a portion of a tri-clamp fitting  65 , and connected to corresponding fittings on an inlet line and an outlet line (not shown). 
         [0035]    The fluid treatment element or assembly may be made in any of numerous ways. For example, one method may generally comprise corrugating at least a fluid treatment medium having first and second surfaces to form a plurality of pleats and forming the corrugated fluid treatment medium into a generally cylindrical fluid treatment pack. The fluid treatment pack has first and second ends and the pleats extend axially along the fluid treatment pack between the first and second ends of the pack. The method also comprises positioning a stripout material along the first surface of the fluid treatment medium and applying a sealant to the fluid treatment pack near the first and second ends to seal the ends from the second surface of the fluid treatment medium. The method further comprises removing the stripout material from the corrugated fluid treatment pack to form a region within each pleat that is free of structure. 
         [0036]    Another method may generally comprise corrugating a multilayer composite to form a plurality of pleats and forming the corrugated composite into a generally cylindrical fluid treatment pack. The multilayer composite includes at least a fluid treatment medium having first and second opposite surfaces and a stripout material positioned along the first surface of the fluid treatment medium. The cylindrical fluid treatment pack has first and second ends and the pleats extend axially along the fluid treatment pack. The method further comprises applying a sealant to the fluid treatment pack near the first and second ends to seal the ends from the second surface of the fluid treatment medium and, after applying the sealant, stripping the stripout material from the corrugated composite to form a region free of structure within each pleat. 
         [0037]    A more specific example of a method of making a fluid treatment assembly may initially involve forming the multilayer composite  40 , as shown in  FIG. 4 . Forming the composite  40  may include arranging the permeate drainage layer  43 , the permeate support layer  44  and the permeate cushioning layer  45  along the surface of the fluid treatment medium  17  that will become the outer surface  42 . The permeate drainage layer  43  may be the same width as the fluid treatment medium  17  and their side edges may be aligned. The permeate support layer  44  and the permeate cushioning layer  45  may be somewhat narrower than the fluid treatment medium  17 . For example, the side edges of the permeate support layer  44  and the permeate cushioning layer  45  may be parallel to but spaced inwardly from the side edges of the fluid treatment medium  17 , for example, by up to about ¾ inch. 
         [0038]    The multilayer composite  40  may also be formed with one or more layers along the surface of the fluid treatment medium  17  that will become the inner surface  41 . For example, a sealant barrier layer  70  and a stripout material  71  may be arranged along the inner surface  41  of the fluid treatment medium  17 . The sealant barrier layer may comprise a single sheet having a width similar to the width of the fluid treatment medium. However, for most embodiments the sealant barrier layer  70  may comprise two narrow strips positioned along the side edges of the fluid treatment medium  17  and, in many instances, spaced inwardly from the side edges of the fluid treatment medium. For example, the strips of the sealant barrier layer  70  may be up to about 1 inch wide or more, and a side edge of each strip may be arranged parallel to but spaced inwardly by up to about ¼ inch or more from the side edge of the fluid treatment medium  17 . The sealant barrier layer  70  may be formed from a variety of materials that will provide a barrier to the sealant. For many embodiments, the sealant barrier layer  70  may be formed from a thin, impermeable polymeric film that resists bonding to the sealant. 
         [0039]    The stripout material  71  may be arranged along the inner surface  41  of the fluid treatment medium  17 . The stripout material  71  may have the same width as the fluid treatment medium  17 , and their side edges may be aligned. Removal of the stripout material  71  from the fluid treatment pack  13  establishes the regions  27  free of structure, and the thickness of the stripout material generally corresponds to, e.g., may be about one half of, the width of each region  27  free of structure. Consequently, the thickness of the stripout material  71  may be selected in accordance with the desired number of pleats  20  and the desired size of the regions  27  free of structure. The stripout material  71  may be structured in a wide variety of ways and may be fashioned from any of numerous materials. For example, the stripout material may comprise a single layer or multiple layers that are flexible enough to be corrugated with the fluid treatment medium  17 . Further, the stripout material may be fashioned from a sheet material which resists compression and which prevents damage to the fluid treatment medium during corrugation. For many embodiments, the stripout material  71  may comprise a multilayer composite, e.g., a three-layer composite including a cushioning layer, a support layer and a drainage layer similar to the permeate cushioning layer  45 , the permeate support layer  44 , and the permeate drainage layer  43  previously described. 
         [0040]    The multilayer composite  40  may then be corrugated to form a plurality of pleats  20 . The composite may be corrugated by any of numerous corrugators and the pleats may be variously configured. For example, the legs of each pleat may have about equal lengths or one leg may be longer than the other. After the multilayer composite  40  has been corrugated, a cutter may cut the pleated composite in a direction parallel to the pleats  20 , providing a leading edge  72 , a trailing edge  73 , and a predetermined number of pleats  20 . The pleated composite is then formed into a generally cylindrical pack  13 . For example, the leading edge  72  and the trailing edge  73  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  13  having an interior, an exterior, and opposite ends  15 ,  16 . A side seal  74  may be formed along the leading and trailing edges  72 ,  73  in any number of ways including, for example, melt bonding, adhesive bonding, and/or mechanically connecting. For most embodiments, the sealant barrier layer  70  and the stripout material  71  may be trimmed back from the leading and trailing edges  72 ,  73  so they do not form part of the side seal  74 . 
         [0041]    After the fluid treatment pack  13  is formed, one or more sealants may be applied. The amount and viscosity of any of the sealants and the wetability of the various layers which contact the sealant may be selected to prevent insufficient or excessive wicking of the sealant axially along the layers. The sealant may be applied in any of several ways. For example, each end  15 ,  16  of the pack  13  may be dipped into a sealant  81  such that the sealant  81  fills the end  15 ,  16  of the pack  13  to an axial depth of up to about ⅛ inch or more from each end  15 ,  16  of the pack  13 . As shown in  FIG. 6 , each end  15 ,  16  of the fluid treatment pack  13  may be dipped in a cup-shaped fixture  80  containing the sealant  81  at the bottom. The fixture  80  may have an inner diameter which corresponds to the desired outer diameter of the fluid treatment pack  13  and a depth of about 1 inch or less to about 1¼ inch or more, with a slight taper near the top. A core  50  or a core substitute may be inserted in the interior of the fluid treatment pack  13  before the pack  13  is dipped in the sealant  81  in the fixture  80 . 
         [0042]    After the initially-applied sealant  81  solidifies, a sealant  18  may then be applied between the fluid treatment pack  13  and the inner-diameter of cup-shaped fixture  80 , filling all void spaces in the fluid treatment pack  13  radially between the outer surface  42  of the fluid treatment medium  17  and the inner diameter of the fixture  80  and axially between the initially-applied sealant  81  and, for example, the bottom of the taper near the top of the fixture  80 , as shown in  FIG. 6 . The sealant  18  penetrates into the voids of the permeate drainage layer  43  in an end region of the fluid treatment pack  13  extending from near the bottom to near the top of the fixture  80 . Because the side edges the permeate cushioning and support layers  44 ,  45  are spaced inwardly from the side edge of the fluid treatment medium  17 , as shown in  FIG. 4 , the sealant  18  may penetrate into the voids of the permeate cushioning and support layers  44 ,  45  only about 1/16 inch to about ½ inch axially inwardly from the edges of the permeate cushioning and support layers  44 ,  45 . The sealant  18  may also penetrate into the voids of the fluid treatment medium  17  in an end region extending from near the bottom to near the top of the fixture  80 , passing through the fluid treatment medium  17  to the inner surface  41  of the fluid treatment medium  17  where it is blocked by the sealant barrier strip  70 . Alternatively, the sealant  18  may simply bond to the outer surface  42  of the fluid treatment medium  17  without passing through to the inner surface  41 . 
         [0043]    After the sealant  18  solidifies, the outer surface  42  of the fluid treatment medium  17  is sealed from the ends  15 ,  16  of the fluid treatment pack  13 . The permeate drainage layer  43  and the fluid treatment medium  17 , as well as the permeate support layer  44  and the permeate cushioning layer  45 , are effectively bonded to one another in the end regions of the fluid treatment pack  13  by the sealant  18 . Further, the legs  25 ,  26  of adjacent pleats  20  are embedded in the sealant  18  and are effectively bonded to one another outwardly from the fluid treatment medium  17  in the end regions of the fluid treatment pack  13 . However, the stripout material  71  and the sealant barrier strips  70 , may be bonded to the fluid treatment pack  13  only by the initially-applied sealant  81 . 
         [0044]    Although the surround  52  may be installed around the fluid treatment pack  13  before any sealant  81 ,  18  is applied, for many embodiments, the surround  52  may be installed after the sealant  81 ,  18  is applied. The surround  52  may be in the form of a sheet or strip and may be circumferentially or helically wound around the fluid treatment pack  13  in one or more layers. Alternatively, the surround may be in the form of a cylindrical cage or sleeve and may be axially slid onto the fluid treatment pack. For many embodiments, the outer diameter of the surround  52  may closely correspond to the inner diameter of the housing  12 . 
         [0045]    The fluid treatment element  11 , including the fluid treatment pack  13 , the sealant  81 ,  18 , the core  50 , and the surround  52 , may be installed in the housing  12 . For example, the fluid treatment element  11  may be axially slid into the shell  53  of the housing  12 , with the surround  52  abutting the housing  12  and fluidly communicating between the outer surface  42  of the fluid treatment medium  17  and one or more permeate outlets  34 . A sealant may be applied to the fluid treatment element  11  before it is slid into the shell  53  to assist in bonding the fluid treatment pack  13  to the surround  52  and/or the fluid treatment element  11  to the shell  53 . To allow fluid communication between the outer surface  42  of the fluid treatment medium  17  and a permeate outlet  34 , the sealant may be applied in a manner that does not significantly inhibit permeate flow. For example, the sealant may be applied in a pattern of dots or dashes along the outer surface of the folded ends  23  of the pleats  20  between the opposite ends  15 ,  16  of the fluid treatment element  11 . The shell  53  may be somewhat shorter than the fluid treatment element  11  allowing at least the initially-applied sealant  81  at each end of the fluid treatment element  11  to extend beyond the ends of the shell  53 . Further, the length of the shell  53  may be greater than the axial extent of the surround  52 , e.g., greater by about ½ inch or less to about 1½ inch or more, leaving a small annulus which extends radially from the outer diameter of the sealant  18  to the inner diameter of the shell  53  and axially from the end of the shell  53  to the axial end of the surround  52 . 
         [0046]    Additional sealant  18  may be applied between fluid treatment element  11  and the housing  12  to further seal the second surface  42  of the fluid treatment medium  17  from the ends of the fluid treatment element  11 . The additional sealant  18  may also bond the fluid treatment element  11  to the housing  12 . For example, the shell  53  and the fluid treatment element  11  may be positioned with the axis  14  vertical, and sealant  18  may be injected into the annulus at the lower end of the shell  53 . The sealant  18  may fill the annulus and extend axially a short distance into the surround  52 . Once the sealant  18  in the annulus solidifies, the shell  53  and the fluid treatment element  11  may be inverted and the sealant  18  may be injected into the annulus at the other end of the shell  53 , completely sealing the second surface  42  of the fluid treatment medium  17  from the ends of the fluid treatment assembly  10  and firmly fixing the fluid treatment element  11  to the housing  12 . 
         [0047]    The stripout material  71  may be removed from the fluid treatment pack  13  at various times including, after the legs  25 ,  26  of adjacent pleats  20  are bonded to one another at the pack ends. For example, after the sealant  18  in the annulus solidifies, the end portion of the fluid treatment element  11  which extends beyond the housing  12  and contains the initially-applied sealant  81  may be cut, e.g., sliced, from the fluid treatment element  11 . Cutting the initially-applied sealant  81  from the fluid treatment element  11  exposes the sealant barrier layer  70  and the stripout material  71  at the ends  15 ,  16  of the fluid treatment pack  13 . The core  50  or core substitute may also be removed from the interior of the fluid treatment pack  13 , further exposing the stripout material  71  along the interior of the fluid treatment pack  13 . The stripout material  71  may then be pulled from the fluid treatment pack  13 . The sealant barrier layer  70  prevented contact between the sealant  18  applied along the outer surface  42  of the fluid treatment medium  17  and the stripout material  71 . Consequently, the stripout material  71  is not bonded to anything within the pack  13  and may simply be pulled from between the legs  25 ,  26  of the pleats  20  via one or both axial ends of the pack  13 , leaving the regions  27  free of structure in place of the stripout material  71 . Because the legs  25 ,  26  of the adjacent pleats  20  are bonded to one another in the end regions of the fluid treatment pack  13  and the fluid treatment medium  17  is bonded to the sealant  18  in the end regions of the fluid treatment pack  13 , the regions  27  free of structure are maintained without obstruction along the inner surface of the fluid treatment medium  17 . For most embodiments, the thin sealant barrier layer  70  may be fashioned from a material which does not bond to the sealant  18 . Consequently, the sealant barrier layer  70  may be removed from the inner surface  41  of the fluid treatment medium  17  as or after the stripout material  71  is removed. 
         [0048]    After the stripout material  71  has been removed and the regions  27  free of structure have been exposed, a core  50  may be installed in the interior of the fluid treatment pack  13 . The core  50  may fit closely against the legs  25 ,  26  of the pleats  20  at the open ends  24 , preventing axial flow from end to end of the fluid treatment pack that bypasses the regions  27  free of structure. For many embodiments, the core  50  may be bonded to the legs  25 ,  26  of the pleats  20  at the open ends  24  by a sealant. The sealant may be applied along the entire length of the core, e.g., as the core is inserted into the interior of the fluid treatment pack  13 , or along only a portion of the core. For example, the sealant  64  may be applied between the core  50  and the pleats  20  only in the end regions of the fluid treatment pack  13 , either as or after the core  50  is inserted in the interior of the fluid treatment pack  13 . Once the sealant solidifies, the core  50  is firmly held in place within the pack  13 . 
         [0049]    Fluid treatment assemblies embodying the invention may be used to treat any of a myriad of fluids in any of numerous crossflow processes. For example, the fluid treatment assembly  10  shown in  FIGS. 1-3  may be used in a separation process. The feed fluid inlet  32  and the retentate outlet  33  may be coupled to a feed line and a retentate outlet line (not shown), and a feed fluid may be introduced into the fluid treatment assembly at the open axial ends  21  of the pleats. The core  50  and the sealant  18  between the outer surface  42  of the fluid treatment medium  17  and the housing  12  direct the feed fluid from the feed line straight into the regions  27  free of structure, where it flows within the regions  27  free of structure axially along the tangential flow path  30  to the opposite open axial ends  22  of the pleats  20 . Each region  27  free of structure is surrounded by the inner surface  41  of the fluid treatment medium  17  along the legs  25 ,  26  and the folded end  23  of the pleat  20 . At the opposite open axial ends  22  of the pleats  20 , the feed fluid exits the fluid treatment pack  13  and the fluid treatment assembly  10  via the retentate outlet  33 , the retentate exiting the regions  27  free of structure straight into the retentate outlet line. 
         [0050]    Within the regions  27  free of structure, one or more substances may be removed from the feed fluid via the lateral fluid flow path  31 . The one or more substances may pass as permeate generally radially from the inner surface  41  to the outer surface  42  through the fluid treatment medium  17  and through the permeate drainage layer  43  to the surround  52 . The permeate may then pass along the lateral flow path  31  generally axially along the surround  52  to the one of permeate outlets  34 , which may be connected to permeate outline lines (not shown). The permeate then exits the fluid treatment assembly  10  via the permeate outlets  34  and the permeate outlet line. Either the permeate or the retentate or both may be the desired product of the separation process. 
         [0051]    Many advantages are associated with fluid treatment elements and assemblies embodying the invention. For example, by providing regions free of structure, fluid treatment elements and assemblies embodying the invention offer less resistance to the flow of fluids, e.g., feed fluids. Where the fluids flow straight from the feel inlet line into the open axial ends of the regions free of structure and/or from the open axial ends of the regions free of structure straight into the retentate outlet line, there is even less resistance to fluid flow because there are fewer turning losses at the feed fluid inlet and the retentate outlet. Fluids may thus flow through the fluid treatment element or assembly with a smaller pressure drop. Further, by locating the regions free of structure immediately against the inner surface of the fluid treatment medium, fluid flowing along the tangential flow path can more thoroughly keep foulants clear of the surface of the fluid treatment medium, for example, because a much higher, more uniform shear rate can be provided at the surface of the medium. In addition, where regions free of structure extend along the fluid treatment medium free of obstructions, fewer foulants are deposited on the fluid treatment medium, and sensitive feed fluids, such as cellular solutions, may flow along the regions free of structure with little or no damage to the sensitive fluid or its components. 
         [0052]    While various aspects of the invention have been previously described and/or illustrated, the invention is not limited to these embodiments. For instance, one or more of the features of any embodiment may be eliminated without departing from the scope of the invention. For example, the surround may be eliminated. Fluid may then flow between the permeate outlet and the fluid treatment medium via the permeate drainage layer. Alternatively or additionally, the inside surface of the housing may have passages which direct fluid between the permeate outlet and the fluid treatment medium. As another example, either or both of the permeate cushioning layer and the permeate support layer may be eliminated for fluid treatment media that are less susceptible to damage and better resist forces associated with fluid flow. 
         [0053]    Further, one or more features of any embodiment may be modified without departing form the scope of the invention. For example, the permeate outlet may be positioned on the shell of the housing near one end and in fluid communication with the outer surface of the fluid treatment medium. The surround may then include a blind portion that may be impermeable and imperforate and may extend axially from the opposite end to near the permeate outlet. The blind portion of the surround may then serve to force permeate to flow axially within the permeate drainage layer along the lateral flow path to the permeate outlet. 
         [0054]    As an example of another modification, the permeate cushioning layer and the permeate support layer may be merged into a single layer which serves the functions of both a cushioning layer and a support layer. 
         [0055]    As an example of another modification, the housing may include end pieces and the end pieces may be joined to the ends of the shell. The end pieces may be generally circular and have a hollow, central fitting that protrudes outwardly and has a smaller outer diameter than the shell. The inside surface of each end piece may have lands, ribs or other structures that define passageways which communicate between the open axial ends of the regions free of structure and the interior of the central fitting of the end piece. The end pieces may be connected to the ends of the shell with the lands or ribs contacting the sealant at the ends of the fluid treatment element and the ends of the core. The fluid treatment element and the core may then be supported axially within the housing by the end pieces. 
         [0056]    As an example of another modification, the stripout material may comprise a more rigid material and may be inserted in the pleats, e.g., in the axial ends of the pleats or the open ends of the pleats, after the corrugated fluid treatment pack is formed. For example, the stripout material may be configured as rigid fins which extend along a portion of, or the entire, axial length of the pleats. The fins may be separate, flat pieces having a width and height corresponding to the desired width and height of each region free of structure. Alternatively, the fins  82  may be attached to a core substitute  83 , as shown in  FIG. 7 . The core substitute  83  may be slid into the interior of the fluid treatment pack  13  with the fins  82  sliding through the open axial ends  24  and between the legs  25 ,  26  of the pleats  20 . The fluid treatment pack and the more rigid stripout material may be dipped into the cup-shaped fixture containing the initially-applied sealant. The surround and the other sealants may be applied and the fluid treatment element may be disposed in the housing, as previously described. After the initially-applied sealant is cut from each end of the fluid treatment element, the more rigid stripout material may be pulled from the fluid treatment pack, leaving the regions free of structure in place of the stripout material, e.g., in place of the fins. 
         [0057]    As another example of a modification, the stripout material may be removed from the fluid treatment pack after the sealant is applied to the surface, e.g., the outer surface, of the fluid treatment medium but before the fluid treatment element is inserted in the shell or before the surround is placed around the pack. After the sealant has been applied to the outer surface of the fluid treatment medium and has solidified, the end portion of the fluid treatment packing having the originally-applied sealant may be cut from the fluid treatment pack, exposing the stripout material at the ends of the pack. The core or the core substitute may be removed, and the stripout material may be removed, leaving the regions free of structure within the fluid treatment pack along the inner surface of the fluid treatment medium. The surround may then be placed around the fluid treatment pack, the fluid treatment element may be placed inside the shell, the remaining sealant may be applied in the annulus, and the core may be installed in the interior of the fluid treatment pack. 
         [0058]    Further, one or more features of any embodiment may be combined with one or more features of other embodiments. For example, the end pieces may be combined with an embodiment having fluid passages on the inner surface of the shell of the housing. 
         [0059]    Further, embodiments having very different features can still be within the scope of the invention. For example, the fluid treatment assembly may comprise a disposable fluid treatment element which is removably mounted in and sealed to a reusable housing, such as a reusable shell. After the disposable fluid treatment element is spent, it may be removed from the reusable housing and replaced by a new or cleaned fluid treatment element. 
         [0060]    As another example, fluid treatment elements and assemblies may have regions free of structure which extend 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  90  and fluid treatment element  91  is shown in  FIGS. 8-10 . The features of the fluid treatment assembly  90  shown in  FIGS. 8-10 , including the components and the methods of making and using the assembly  90 , are similar to those of the fluid treatment assembly  10  shown in  FIGS. 1-3 , and corresponding components are identified by the same reference numerals. However, the geometry of the fluid treatment assembly  90  may generally be reversed with respect to the fluid treatment assembly  90  shown in  FIGS. 1-3 . For example, the permeate cushioning layer  45 , the permeate support layer  44 , and the permeate drainage layer  43  may be positioned along the inner surface  41  of the fluid treatment medium  17 , while the regions  27  free of structure may extend axially along the outer surface  42  of the fluid treatment medium  17 . 
         [0061]    The fluid treatment assembly  90  may include a fluid treatment element  11  disposed in a housing  12 . The fluid treatment element  11  may include a fluid treatment pack  13  having a central axis, opposite ends  15 ,  16 , a fluid treatment medium  17 , and a plurality of axially extending pleats  20 . Each pleat  20  has a folded inner end  23 , an open outer end  24 , two legs  25 ,  26  which extend between the folded end and the open end, and opposite open axial ends  21 ,  22  at the opposite ends  15 ,  16  of the fluid treatment pack  13 . All or substantially all of the pleats  20  include a region  27  free of structure which extends axially within each pleat  20  along the full length of the pleat  20  between the axial ends  21 ,  22  and opens onto the axial ends  21 ,  22  of the pleat  20 . Each region  27  free of structure extends axially within the pleat  20  along the outer surface of the fluid treatment medium  17 . A sealant  18  at each end of the fluid treatment pack  13  seals the inner surface  41 , e.g., the permeate side, of the fluid treatment medium  17  from the ends  15 ,  16  of the fluid treatment pack  13 . The fluid treatment element  11  further includes a tangential fluid flow path  30  and a lateral fluid flow path  31 . The tangential fluid flow path  30  extends generally axially along the fluid treatment pack  13  within the pleats  20 , including the regions  27  free of structure. The lateral fluid flow path  31  fluidly communicates with the tangential fluid flow path  30  and extends laterally through the fluid treatment medium  17  to or from the tangential fluid flow path  30 . 
         [0062]    Fluid treatment elements and assemblies having regions free of structure which extend along the outer surface of the fluid treatment medium may be coreless. However, many embodiments may include a core  50 . The core  50  may be hollow and perforated or permeable, and the interior of the core  50  may fluidly communicate along all or most of the axial length of the core  50  with the inner surface  41  of the fluid treatment medium  17  via the permeate cushioning layer  45 , the permeate support layer  44 , and the permeate drainage layer  43 . The core  50  may have an open end  48  and an opposite blind end  49  or two open ends. The surround  52  may be blind, e.g., impermeable and imperforate, along its entire length, or it may be omitted. 
         [0063]    The housing  12  may include an outer shell  53 , and opposite end pieces  92 ,  93  attached to the shell  53 . The shell  53  may be closely fitted around the exterior of the fluid treatment element  11 . A sealant  64  may be positioned between the shell  53  and the pleats  20  at each end of the fluid treatment pack  13 . Additionally, a sealant may be positioned between the surround  52  and the pleats  20  along the length of the fluid treatment pack  13 . One end piece  92  may include a feed fluid inlet  32  and a manifold  94  which fluidly communicates between the feed fluid inlet  32  and the regions  27  free of structure at the open axial ends  21  of the pleats  20  at one end of the fluid treatment pack  13 . The opposite end piece  93  may include a retentate outlet  33  and a manifold  95  which fluidly communicates between the retentate outlet  33  and the regions  27  free of structure at the open axial ends  22  of the pleats  20  at the opposite end of the fluid treatment pack  13 . Either or both end pieces may include a permeate outlet. In the illustrated embodiment, the end piece  92  on the feed end may include a permeate outlet  34 , which may be sealed to the open end  48  of the core  50 , such that the retentate and the permeate are separated from one another. The other end of the core  50  may be a blind end  49 . The end pieces  92 ,  93  may be attached to the shell  53  to support the fluid treatment element  11  and the core  50  within the housing  12 . 
         [0064]    Although the fluid treatment assembly  90  shown in  FIGS. 8-10  may include end pieces  92 ,  93 , fluid treatment assemblies having regions free of structure which extend along the outer surface of the fluid treatment medium may be free of end pieces. For example, fluid treatment assemblies having regions free of structure along the outer surface of the fluid treatment medium may include a housing similar to the housing  12  of the fluid treatment assembly  10  shown in  FIG. 1 , including a shell having fittings at each end. A feed pipe and a retentate pipe having inner diameters comparable to the inner diameter of the shell may be attached to the fittings at the feed inlet end and the retentate outlet end of the shell, respectively. A permeate outlet pipe may be attached to the core at one or both ends of the fluid treatment element and extend a distance within the feed pipe and/or the retentate pipe. Either the permeate outlet pipe or the feed pipe and/or the retentate pipe may bend, allowing the permeate outlet pipe to extend through the wall of the feed pipe and/or retentate pipe, separating the permeate fluid from the feed fluid and/or the retentate fluid. 
         [0065]    Methods of making a fluid treatment assembly having regions free of structure along the outer surface of the fluid treatment medium may be similar to those for making a fluid treatment assembly having regions free of structure along the inner surface of the fluid treatment medium, as previously described. However, the sealant barrier layer and the stripout layer may be positioned in the multilayer composite  40  along the surface of the fluid treatment medium  17  which will become the outer surface  42  or feed side. The permeate drainage layer  43 , the permeate support layer  44 , and the permeate cushioning layer  45  may be positioned in the composite  40  along the surface of the fluid treatment medium  17  which will become the inner surface  41  or permeate side. 
         [0066]    Additionally, the initially-applied sealant  81  and the sealant  18  which seals the inner surface  41  of the fluid treatment medium  17  from the ends of the fluid treatment pack  13  may be applied in a cup-shaped fixture  100 , as shown in  FIG. 11 , which is similar to the cup-shaped fixture  80  shown in  FIG. 6 . However, the fixture  100  shown in  FIG. 11  further includes a central cylindrical protrusion  101  that has an outer diameter which corresponds to the inner diameter of the fluid treatment pack  13 . The central protrusion  101  extends upwardly about 1 inch or less to about 1¼ inch or more and may have a slight inward taper near the top. For each end, the fluid treatment pack  13  is dipped into an initially-applied sealant  81  in the bottom of the fixture  100 , filling the entire end of the fluid treatment pack  13  to a depth of up to about ⅛ inch or more. The sealant  81  then solidifies. The sealant  18  for the inner surface  41  of the fluid treatment medium  17  may then be applied between the fluid treatment pack  13  and the central cylindrical protrusion  101  of the fixture  100 , for example, by means of a long needle inserted into the hollow interior of the fluid treatment pack  13  from the opposite end of the pack  13 . The sealant  18  fills all of the void spaces in the fluid treatment pack  13  at each end  15 ,  16  radially between the inner surface  41  of the fluid treatment medium  17  and the outer diameter of the central cylindrical protrusion  101  and axially from the initially-applied sealant near the bottom of the fixture  100  to the bottom of the taper near the top of the cylindrical protrusion  101 . 
         [0067]    After the sealant  18  solidifies, the inner surface  41  of the fluid treatment medium  17  may be sealed from the ends  15 ,  16  of the fluid treatment pack  13 . The permeate drainage layer  43  and the fluid treatment medium  17 , as well as the permeate support layer  44  and the permeate cushioning layer  45 , may be effectively bonded to one another inwardly from the fluid treatment medium  17  in the end region of the fluid treatment pack  13 . Further, the legs  25 ,  26  of adjacent pleats  20  may be effectively bonded to one another inwardly from the fluid treatment medium  17  in the end region of the fluid treatment pack  13 . However, the stripout material  71  and the sealant barrier layer  70  may be bonded to the fluid treatment pack  13  only by the initially-applied sealant  81 . 
         [0068]    After the sealant  18  solidifies and the legs  25 ,  26  of adjacent pleats  20  are bonded to one another in the end regions of the fluid treatment pack  13 , the stripout material  71  may be removed from the pack  13 . For example, the portion of the fluid treatment pack  13  containing the initially-applied sealant  81  may be cut from the pack  13 , allowing the stripout material  71  and the sealant barrier layer  70  to be removed from the pack  13 . The stripout material  71  may then be pulled from the exterior of the fluid treatment pack  13 , leaving the regions  27  free of structure to extend axially along each pleat  20  and open at both axial ends  21 ,  22  of the pleats. The sealant barrier layer  70  may also be removed from exterior of the fluid treatment pack  13 . 
         [0069]    After the stripout material  71  is removed from the fluid treatment pack  13 , a surround  52  may, or may not, be applied to the exterior of the fluid treatment pack  13  at the open outer ends  24  of the pleats  20 . The fluid treatment element  11  may be inserted into the shell  53  of the housing  12  and may, or may not, be bonded to the shell  53  by a sealant positioned between the inner surface of the shell  53  and surround  52  or the legs  25 ,  26  of adjacent pleats  20  at the open outer ends  24  of the pleats  20 . 
         [0070]    The perforated core  50  may be installed at various times, e.g., after the sealant  18  is applied along the inner surface  41  of the fluid treatment medium  17  and the initially-applied sealant  81  is cut from the fluid treatment pack  13 . Additional sealant  18  may be applied between the core  50  and the folded inner ends  23  of the pleats  20  in the end regions of the pack  13  to more completely seal the inner surface  41  of the fluid treatment medium  17  from the manifold  94 ,  95  of each end piece  92 ,  93  and to fix the core  52  to the fluid treatment pack  13 . After the core  50  is installed in the fluid treatment pack  13  and the fluid treatment element  10  is installed in the shell  53 , the end pieces  92 ,  93  may be attached to the ends of the shell  53  and sealed to the core  50  to form the fluid treatment assembly  90 . 
         [0071]    In use, feed fluid may be directed along the tangential flow path  30  into the feed inlet  32  and the inlet manifold  94 . From the inlet manifold  94 , the feed fluid flows generally axially into the open axial ends  21  of the pleats  20  at one end  15  of the fluid treatment pack  13 , through the regions  27  free of structure along the outer surface  42  of the fluid treatment medium  17 , to the open axial ends  22  of the pleats  20  at the opposite end  16  of the pack  13 , and into the outlet manifold  104 . From the outlet manifold  95 , the retentate exits the fluid treatment assembly  90  via the retentate outlet  33 . For many embodiments, one or more components of the feed fluid are removed in the regions  27  free of structure via the lateral flow path  31 . The components flow generally radially from the outer surface  42  to the inner surface  41  through the fluid treatment medium  17  and through the permeate cushioning layer  45 , the permeate support layer  44  and the permeate drainage layer  44  to the interior of the perforated core  50 . The permeate then flows along the lateral flow path generally axially along the interior of the core  50  and the through the permeate outlet  34 . 
         [0072]    As another example of an embodiment having very different features, a fluid treatment assembly may be configured as a mass transfer device. For example, a fluid treatment assembly similar to the fluid treatment assembly  10  shown in  FIGS. 1-3 . An impermeable, imperforate surround may be positioned around the exterior of the fluid treatment pack most of the axial distance between the two ports in the shell. A first fluid, e.g., a gas or a liquid, may enter the fluid treatment assembly through one of the ports, pass axially along the outer surface of the fluid treatment medium through the outer drainage layer, the outer support layer, and the outer cushioning layer, and exit through the other port. Another fluid may flow co-current or counter current along tangential flow path from the fluid inlet axially along the regions free of structure to the fluid outlet. One or more components of one of the fluids, e.g., the first fluid flowing along the outer surface of the fluid treatment medium, may pass through the fluid treatment medium along the lateral flow path to the fluid flowing into the regions free of structure. 
         [0073]    The present invention is thus not restricted to the particular embodiments which have been described and/or illustrated but includes all modifications, combinations, and different embodiments that fall within the scope of the claims.