Abstract:
Membranes are arranged in a stack with flat sheets of feed channel spacer and permeate carrier. Flat feed channels and permeate channels alternate through the thickness of the stack. Edges of the feed channels are sealed along the length of the stack. Edges of the permeate channels are sealed across the width of the stack. The stack may be more than 1.5 m long. Optionally, membranes may be sealed to each other without being folded. A filtration element comprises a stack and a shell. The shell has at least an inlet to the feed channels and a permeate outlet. Optionally, the element may be operated in a permeate side cross flow configuration. Parts of the stack may be pre-assembled, in some cases by an automated process. The filtration element may be used for reverse osmosis, forward osmosis, pressure retarded osmosis or nanofiltration.

Description:
FIELD 
       [0001]    This specification relates to membrane filtration modules, for example reverse osmosis or nanofiltration modules, in which flat membrane sheets are arranged in a stack in a module, and to methods of making them. 
       BACKGROUND 
       [0002]    Flat sheet membranes have been used in immersed ultrafiltration or microfiltration modules. In modules produced by Kubota, membrane sheets are provided on both sides of a plastic frame to form a hollow pocket. The pockets are placed in a spaced apart arrangement in a module and immersed in an open tank. Permeate is withdrawn by suction applied through a port in the frame to the inside of the pocket. In a module described in U.S. Pat. No. 7,892,430, filter elements are made up of two membrane sheets provided on both sides of a drainage element. The elements are arranged in a spaced apart relationship and immersed in an open tank. Permeate is withdrawn by suction through a pipe that passes through bores in the elements. Operating immersed in a tank of feed water and at low transmembrane pressure differential avoids the need for these modules to be rigid or strong. 
         [0003]    Flat sheet membranes have also been used in reverse osmosis. However, reverse osmosis membranes are typically formed into spiral wound modules. The spiral wound configuration is inherently suited to high pressure applications but only when there is no cross flow on the permeate side. Attempts to make flat sheet pressure driven modules, some with cross flow, are described in U.S. Pat. No. 5,104,532, U.S. Pat. No. 5,681,464, U.S. Pat. No. 6,524,478, European Patent 1355730 and Japanese publication 7068137. 
       SUMMARY OF INVENTION 
       [0004]    The following section is intended to introduce the reader to the detailed description to follow and not to limit or define the claims. 
         [0005]    This specification describes a stack comprising flat sheet membranes. The membranes are arranged in a stack with flat sheets of feed channel spacer and permeate carrier. The stack has planar feed channels and permeate channels alternating through the thickness of the stack. Edges of the feed channels are sealed along the length of the stack. Edges of the permeate channels are sealed along the width of the stack. In an embodiment, the length of the stack is greater than its width. The stack may also be more than 1.5 m long. Optionally, membrane sheets may be sealed to each other where required without being folded. 
         [0006]    The specification also describes a filtration element. The element comprises a stack as described above and a shell. The shell has an inlet at one end in communication with the feed channels. The shell has at least one permeate outlet in communication with the permeate channels. The permeate outlet may further communicate with a permeate conduit along the length of the stack or perpendicular to the sheets of the stack. Optionally, the element may have a permeate inlet and an outlet such that the element may be operated in a cross flow configuration. The element may be used, for example, for reverse osmosis, forward osmosis, pressure retarded osmosis or nanofiltration. 
         [0007]    This specification also describes methods of making a stack. Parts of the stack may be pre-assembled, optionally by way of a substantially continuous or automated process. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]      FIG. 1  is an exploded isometric view of an assembly of sheets forming a stack. 
           [0009]      FIG. 2  is an isometric view of an element including the stack of  FIG. 1 . 
           [0010]      FIG. 3  is a cross section of the element of  FIG. 2  along line  3 - 3  of  FIG. 2 . 
           [0011]      FIG. 4  is a cross section of the element of  FIG. 2  along line  4 - 4  of  FIG. 2 . 
           [0012]      FIG. 5  is an isometric view of a second stack. 
           [0013]      FIG. 6  is an exploded cross section of a second element including the second stack of  FIG. 5 . 
           [0014]      FIG. 7  is an assemble cross section of the second element. 
           [0015]      FIG. 8  is an exploded isometric view of a third stack during a step in an assembly process. 
           [0016]      FIG. 9  is a cross section of a third element having the third stack of  FIG. 8  during a step in an assembly process. 
           [0017]      FIG. 10  is a plan view of the third element of  FIG. 9 . 
           [0018]      FIG. 11  is a schematic drawing of a machine for making a stack or a portion of a stack. 
           [0019]      FIG. 12  is a schematic drawing of a machine for making permeate holes in a stack or a portion of a stack. 
           [0020]      FIG. 13  is a plan view of a permeate carrier having a reinforced permeate holes. 
           [0021]      FIG. 14  is a plan view of a feed spacer having a permeate hole reinforced with a ring and pre-applied edge seals. 
           [0022]      FIG. 15  is a schematic drawing of a device for setting the thickness of the ring of  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Most reverse osmosis modules are made in a spiral wound configuration. In a typical module, the feed flows along the length of the module. Membranes are used in the form of a folded sheet with the fold abutting a permeate collection tube. The length of the feed path is limited by the width of the membrane material which is typically less than 1.5 meters. The length of the sheet is limited by resistance to permeate flow, which would limit the efficiency of the modules. Glue lines are applied to the membranes or permeate carriers with large tolerances for movement of the layers as they are wound up. In combination, these procedures result in the active membrane surface area in a spiral wound module being significantly less than the surface area of the membrane material consumed in manufacturing the module. 
         [0024]    In place of a leaf in a spiral wound module, a flat filtration module as described in this specification may be made from a stack of materials that remain flat in the finished material. Optionally, the stack may be assembled from pre-made clips comprising a permeate carrier, a feed spacer, and two membranes. The two membranes may be formed by folding a single piece of membrane material or from two separate pieces of membrane material. Two membranes can be bonded to each other in the stack for example by ultrasonic or thermal welding, adhesives such as hot melt glues or urethane resin, or by tape. The bonded membranes create barriers between a feed sides and a permeate side of the module. The flat filtration module can be used, for example, for reverse osmosis or nanofiltration or as an alternative to a spiral wound membrane. 
         [0025]      FIG. 1  shows a stack  10 . The stack  10  is composed of flat layers of materials. Optionally, a sheet of material may be folded, across its length or its width, to form multiple layers. The individual layers are: membrane  12 , feed spacer  14 , permeate carrier  16  and, optionally, barrier sheet  18 . The barrier sheet  18  is an impervious layer, for example a polyethylene sheet. Optionally, the barrier sheet  18  may be replaced by a shell of a module. 
         [0026]    A sub-assembly comprising two sheets of membrane  12 , a feed spacer  14  and a permeate carrier  16  will be referred to as a clip  20 . In the clip  20 , the two membranes  12  are spaced apart by one of the feed spacer  14  and the permeate carrier  16 , and the other of the feed spacer  14  and the permeate carrier  16  is on the outside of the clip  20 . The separation layers of the membranes  12  face the feed spacer  14 . The clip  20  shown is ordered, starting from the bottom of the clip  20  as first membrane  12 , feed spacer  14 , second membrane  12 , and permeate carrier  16 . However, the clip  20  could alternatively start with the feed spacer  14 , the second membrane  12  or the permeate carrier  16 . When multiple clips  20  are stacked on top of each other, successive membranes  12  are spaced apart by alternating layers of feed spacer  14  and permeate carrier  16 . Optionally, there may be additional layers that do not form a complete clip  20  at the top or bottom, or both, of the stack  10 . A stack  10  may have one clip  20  or a plurality of clips  20 . Multiple clips  20  may be pre-assembled and the stacked as clips  20  to form the stack  10 . 
         [0027]    The layers of material in the stack  10  may be the same materials used in making spiral wound membranes. For example, the membrane  12  may be a thin film composite reverse osmosis or nanofiltration membrane cast on a supporting structure. The feed spacer  14  may be an expanded plastic mesh. The permeate carrier  16  may be a tricot knit fabric. 
         [0028]    For the purposes of this specification, the stack  10  will be described with reference dimensions as shown in  FIG. 1 . The longer dimension of the sheets of material will be referred to as a length L. The shorter dimension of the sheets of material will be referred to as the width W. The dimension perpendicular to the plane of the material will be referred to as thickness T. The stack  10  is not limited in length L to the width of typical membrane materials and may be longer than 1.5 meters. A long stack  10  has fewer seals perpendicular to the length L of the stack  10  per unit area and so may achieve a higher effective filtration area per unit area of membrane  12 . A long stack  10  can also avoid the anti-telescoping devices, O-rings, module interconnectors and other parts that are required to create between modules in a chain of spiral wound modules of similar length. 
         [0029]    Still referring to  FIG. 1 , a line of X marks (i.e. XXXXXXXXX) on a layer indicates a seal between the two membranes  12  on either side of the seal. The seal may be made directly between the membranes  12 , through an intermediate layer, or by seals from both membranes  12  to the intermediate layer. The membranes  12  above and below a feed spacer  14  are sealed along the length of the feed spacer  14 . The membranes  12  above and below a permeate carrier  16  are sealed along the width of the permeate carrier  16 . A membrane  12  separated from a barrier sheet  18  by a feed spacer  14  or permeate carrier  16  adjacent to a barrier sheet  18  is sealed to the barrier sheet  18 , directly, through the intermediate layer, or by a seal to the intermediate layer which is sealed to the barrier sheet  18 . The feed spacers  14  and associated seals form generally flat feed channels open at the ends of the stack  10  and running through the length of the stack  10 . The permeate carriers  16  and associated seals form generally flat permeate channels closed at the ends of the stack  10 . The permeate channels may be open on both sides of the stack  10 , be open on one side of the stack  10 , or be closed on all four sides of the stack  10 . 
         [0030]    Seals may be made by any method known for making a spiral wound membrane. For example, a seal may be a fold in a sheet of material or made by a sealant. Suitable sealants include urethanes, epoxies, silicones, acrylates and hot melt adhesives. For example, seals may be made with ethylene vinyl acetate (EVA) based hot melt adhesive. Seals made with sealants are cured after or while the stack  10  is compressed according to an embodiment. However, unlike spiral wound membranes modules, the stack  10  may be assembled without requiring sheets of material to slide against each other while a sealant is curing. A seal may therefore be made by methods that would bond too quickly for use in making spiral wound modules. For example, seals may be made by thermal, laser welding or ultrasonic welding, or by a fast setting sealant. Alternatively, a seal may be made by a line of tape joining two membranes  12  together around a feed spacer  14  or permeate carrier  16 . 
         [0031]    In an embodiment, the seals are sized and placed as close to the edge of the stack as possible while still being strong enough to resist a design pressure. Movement of the layers during rolling does not need to be accommodated and so, relative to a spiral wound module, a higher percentage of the membrane source material may become active membrane area in the stack  10 . Longer element lengths allow for a higher active membrane area as a percentage of the membrane material used. Seals that cure faster can be placed more precisely, thereby reducing the need for wide seals and further increasing active membrane area. Movement of the layers can be controlled precisely in a web based process, which also helps reduce the need for wide seals. 
         [0032]    In  FIG. 1 , a feed spacer  14  is sealed along its edges in length to the membranes  12  above and below it. Optionally, two membranes  12  may be sealed to each directly beside the edges of the feed spacer  14 . In this case, the width of the feed spacer  14  is less than the width of the membranes  12 . The edges of the membranes  12  are drawn together and attached, for example by a sealant or sonic or thermal welding. The feed spacer  14  does not need to be included in the seal. However, the feed spacer  14  may optionally protrude at least partially into the seal to inhibit movement of the feed spacer  14 . This may allow for a higher cross flow velocity or feed pressure to be applied to the stack  10 . In  FIG. 1 , a permeate carrier  16  is also sealed along its edges in width to the membranes  12  above and below it. As described for the feed spacer  14 , two membranes  12  may be sealed to each other beside the edge of the permeate carrier  16 . 
         [0033]    A sub-assembly of two membranes  12  with a feed spacer  14  sealed between them may be made essentially continuously by unrolling these three layers through an edge sealing device. The sub-assembly can be cut into segments after passing through the edge sealing device to create segments for use in building clips  20  and stacks  10 . Optionally, a permeate carrier  16  may be rolled out over the upper membrane  12  to create a clip  20  as shown in  FIG. 1  before the layers are cut into segments. Dots of sealant or spot welds can be used to prevent the permeate carrier  16  from sliding relative to the membrane  12 . 
         [0034]    With or without the clips  20  pre-made in this way, the stack  10  is assembled by applying sealant to the permeate carrier  16  of a clip  20  and placing another clip  20  on top, repeating these steps until a desired stack  10  height is reached. Barrier layers  18  and a lower permeate carrier  16  with associated lines of sealant may be added as shown in  FIG. 1  but are not necessarily required. Sealant may be applied to the permeate carrier  16  in two lines as shown in  FIG. 1  to create a permeate-side cross flow stack. Alternatively, sealant may be applied to the permeate carrier  16  in a three sided pattern (similar to glue lines  60  in  FIG. 8 ) to provide a stack in which permeate is withdrawn from one edge only. In an embodiment, the stack  10  is compressed between two flat plates while the glue cures. 
         [0035]      FIG. 2  shows a filtering element  22 , alternatively called a module. The element  22  has a shell  24  surrounding a stack  10 . In an embodiment, the shell  24  is rigid and able to withstand feed water pressure applied to the stack  10  with minimal deflection. The shell  24  is non-porous and may be made, for example, from a plastic such as ABS, or stainless steel. The shell  24  shown is made from two top panels  26 , two side panels  28  and two end panels  30 . The top panels  26  may alternatively be called a top panel  26  and a bottom panel  26  when referring to a specific one of them. The panels  26 ,  28 ,  30  may be glued or welded together. However, other shapes and methods of construction may be used. The shell  24 , particularly the top panels  26 , may be shaped so as to increase its effective thickness and reduce deformation. 
         [0036]    Referring particularly to  FIG. 4 , the stack  10  is shown with only a few layers spaced apart for ease of illustration only. However, when assembled the layers of a stack  10  are placed directly on top of each other. The top and bottom layers of the stack bear against the top panels  26  of the shell  24  at least in use. In this way, the top panels  26  prevent the stack  10  from expanding to an extent that would damage the seals when feed water is applied under pressure to the stack  10 . Optionally, the top panels  26  may be sealed to the stack  10 . 
         [0037]    The panels  26 ,  28 ,  30  are attached and sealed to each other to make the shell  24 , for example by adhesive or ultrasonic welding. In one assembly procedure, the shell  24  is made but for one of the top panels  26 . A stack  10  is placed in the shell  24  and optionally sealed to the bottom panel  26 . The corner seals  32  are cast in place. The remaining top panel  26  is attached, and optionally sealed to the stack  10 , while the corner seals  32  are curing. In another assembly procedure, the shell  24  is assembled but for one of the side panels  28 . The stack  10  is inserted into the shell  24 . The corners seals  32  are injected into the shell  24 . Optionally, the exterior of the corners of the shell  24  may be formed by the corners seals  32 . In any of these options, the side panels  28  and end panels  30  may be sized such that the top panels  26  compress the stack  20 . 
         [0038]    Referring particularly to  FIG. 3 , corner seals  32  separate the interior of the shell around the stack  10  into one or more end compartments  34  and, optionally, into one or more side compartments  36 . The corners seals  32  seal to the shell  24  and to the stack  10 . The corner seals  32  may be made, for example, of a sealant such as hot melt glue, epoxy or urethane. Each end compartment  34  is in fluid communication with the feed spacers  14 . Each side compartment  36 , if any, is in fluid communication with the permeate carriers  16 . 
         [0039]    A feed port  38  is provided in one end panel  30  to connect a source of feed water to the element  22 . Optionally, a retentate port  40  may be provided in the other end panel  30  to remove retentate, alternatively called concentrate or brine. In another option, feed can be provided from ports  38 ,  40  on both ends of the element  22 . A permeate port  42  is provided for each side compartment  36  to remove permeate from the element  22 . 
         [0040]    Although side compartments  36  are optional, having side compartments  36  on both sides of the permeate carrier  16  allows for a reduced permeate path length per unit width W of the stack  10 . This can result in an increased net filtration pressure relative to a module with one side compartment  36 . Alternatively, an element  22  with two side compartments  36  may be used with cross flow on the permeate side of the element  22 . 
         [0041]    An element  22  can be built around a stack  10  of essentially any thickness T. Side panels  28  and end panels  30  need to be stronger with increasing thickness T, but the number of top panels  22  per unit membrane area is reduced with increasing thickness T. The thickness T can be chosen to optimize material consumed by the shell  24 . Alternatively, a lesser thickness T may be chosen to allow for more easily scalable systems and to provide smaller individual elements  22  for replacement or repair. 
         [0042]      FIGS. 5 to 7  show a second element  50 . The second element  50  is similar to the element  22  but the second element  50  is essentially without side compartments  36 . Instead, permeate is removed through a spigot  52  that passes through holes in the stack  10  and the top panels  26 . The spigot  54  has one or more openings  54  to collect permeate from within a second stack  48 . Feed water is kept from entering the spigot  54  by rings  56  around holes in the feed spacer  14 . The thickness of the rings  56  is exaggerated in  FIG. 6 . The rings  56  pass through the feeds spacer  14  and are at least thick enough to be compressed against the adjacent membranes  12  when the second stack  48  is in the shell  24 . 
         [0043]    The rings  56  may be made, for example, of a pre-made elastomeric material placed within a hole in the feed spacer  14 . Alternatively, the rings  56  may be made of a curable sealant, for example hot melt adhesive, cast in place with part of the feed spacer embedded in the ring  56 . The stack  10  may be assembled before the sealant cures such that it binds to the adjacent membranes  12 . Alternatively, the sealant may be pre-cured. With a ring  56  made of an elastomeric material or pre-cured sealant, when the stack  10  is assembled additional sealant can be applied between the ring  56  and the adjacent membranes  12  or the ring  56  may be re-heated after the membranes  12  have been added to seal the ring  56  to the membranes  12 . Seals along the edges of the feed spacer  14  may be made in similar ways, for example with strips sealed to the membranes as described for the rings  56 . The spigot  52  is sealed to the shell  24 , for example with glue  58 . 
         [0044]    Membranes  12  on either side of a permeate carrier  16  are typically sealed together on all four edges since permeate is withdrawn from the spigots  52 . In this case, the permeate side of the second element  50  is separated from the feed side but corner seals  32  may still be used on at least one end of the second element  50  to prevent the feed water from by-passing the feed spacer  14 . The second stack  48  does not need to be sealed to the shell  24  or barrier layers  18 . The second stack  48  may have feed spacer  14  as its first and last layer. Alternatively, one or more additional corner seals  32  may be use and one or more edges of the permeate carrier  16  may be left open to also collect permeate from one or two side compartments  36  as in  FIGS. 2 to 4 . 
         [0045]      FIG. 8  shows part of a process for making a third stack  62 . In the third stack  62 , two layers of membrane  12  are provided, and one seal is formed, by folding a sheet or membrane material around a feed spacer  14 . Glue lines  60  are made using a sealant, such as hot melt adhesive, of a type used in making spiral wound membranes. The glue lines  60  are laid out in the pattern that is visible on the upper membrane  12  in  FIG. 8 . However, the glue lines  60  may be laid out on either the permeate spacer  16  or the membrane sheet  12  of each set of permeate spacer  16  and adjacent membranes sheet  12 . After all of the layers in the third stack  62  have been assembled, the stack is compressed which forces the glue lines to penetrate through the permeate spacers  16 . The glue is allowed to cure while the third stack  62  is under compression. In this way, membranes  12 , or pairs of a membrane  12  and a barrier layer  18 , that are separated by a permeate spacer  16  are sealed to each other. 
         [0046]    To help align the layers of the third stack  62  during assembly, the length L of one edge of the third stack  62  may be clamped while the glue lines  60  are applied. Alternatively, that edge of the third stack  62  may be ultrasonically welded, which may also avoid the need to apply the portion of the glue lines  60  that would be parallel to the weld. Upper layers of the third stack  62  are initially folded back over the clamp or weld until any required glue lines  60  have been applied to lower layers. 
         [0047]    The third stack  62  may be installed in a shell  24  as shown in  FIGS. 2 to 4  except that only one side compartment  36  and permeate port  42  are used. The third stack  62  releases permeate only along its length at the right side of the third stack  62  as it is oriented in  FIG. 8 . The feed spacer  14  also needs to be sealed to the adjacent membranes  12  along the left side of the third stack  62  (as it is oriented in  FIG. 8 ). This seal may have been accomplished by welding through the entire left side of the third stack. Alternatively, the feed spacer  14  may have a sealant along its left side that forms a seal in any of the ways described for the rings  56  or edge pre-seal  114  in  FIGS. 5-7  and  14 . 
         [0048]    As another alternative, the left side of the feed spacer  14  may be sealed by potting after the third stack  62  is assembled as shown in  FIG. 9 . In  FIG. 9 , the third stack  62  is inserted into the edge of a second shell  64 . The second shell  64  has a sheet forming the equivalent of two top panels  26  and a side panel  28 . A side compartment  36  is defined by a generally semi-circular curved portion of the second shell  64 . Additional curved sheets, shown in  FIG. 10 , provide the equivalent of end panels  30 . The curved parts of the second shell  64  allow it to be expanded to insert the third stack  62 . Two corner seals  32 , also visible in  FIG. 10 , are cast in placed after the third stack  62  is inserted and join the corners of the curved sections. Any excess material in the third stack  62  protruding from the second shell  64  may be trimmed to be flush with, or at a desired distance from, the edge of the second shell  64 . 
         [0049]    Potting the feed spacer  14  seals the membranes  12  to the feed spacer  14  and forms the equivalent of a second side panel  28  and two more corner seals  32 . To pot the feed spacer  14 , the assembly described above is inserted into a pan  70  containing liquid potting resin  74 . Optionally, potting resin  74  may be prevented from flowing far into the feed spacer  14  by a blocking strip  72 . The blocking strip  72  shown in  FIG. 9  protrudes from the stack  10 , but the blocking strip  72  may alternatively be recessed within the stack  10  to allow some potting resin  74  to penetrate into the stack  10 . The blocking strip  72  may be made by pre-curing a sealant such as a hot melt adhesive in the feed spacer  14 . Alternatively, a viscous potting resin  74  may be drawn into the feed spacer by omitting the blocking strip  72  and applying a vacuum to the feed port  38  or retentate port  40 . After the potting resin  74  cures into a solid, the assembly, now forming a fourth element  78 , is withdrawn from the pan  70 . Optionally, the resin block may be cut along trim lines  76 .  FIG. 10  shows a completed fourth element  78 . 
         [0050]      FIG. 11  shows a system  80  for assembling a clip  20  in a generally continuous manner. The clip  20  can be made in any length and later cut into segments for making a stack  10 ,  48 ,  62 . The clip  20  in this example has, starting from the bottom, a feed spacer  14 , a membrane  12 , a permeate carrier  16  and another membrane  12 . Optionally, part of the clip comprising a membrane  12 , a permeate carrier  16  and another membrane  12  may be made in the system  80  with the feed spacer  14  added later when assembling a stack  10 ,  48 ,  62 . In another option, part of the clip  20  comprising a membrane  12 , a feed spacer  14 , and another membrane  12  may be made in the system  80  (the order of layers is changed relative to  FIG. 11 ) with the permeate carrier  16  added later when assembling a stack  10 ,  48 ,  62 . 
         [0051]    Each of the layers is fed from a roll. In the example of  FIG. 11 , a feed spacer roll  82  is located below a first membrane roll  84  which is below a permeate carrier roll  86  which is below a second membrane roll  88 . The layers may pass over various idler rolls  81 . The idler rolls  81  may position a layer as required for a tool (to be described below) to operate on the layer. The idler rolls  81  also align the layers for feeding into a pair of nip rollers  87 . The nip rollers  87  compress sealant applied to the layers to cause the sealant to penetrate through one or more of a feed spacer  14 , permeate carrier  16  or the support layer of a membrane  12 . One or both of the nip rollers  87  may be made of, or covered with, an elastomeric material such as rubber or silicone. The elastomeric material helps the nip rollers  87  take in the layers with beads of sealant and yet produce a clip  20  compressed to about the sum of the thickness of the layers. 
         [0052]    The nip rollers  87  also flatten the resulting clip  20  in the direction of the length of the nip rollers  87 . One or both of the nip rollers  87  may be heated to help the sealant flow during a short residence time in the nip. Sealants of the type used in making spiral wound membranes may be used, but, in an embodiment, with formulations that are less viscous and faster setting. 
         [0053]    Sealant is applied to the permeate carrier  16  from one or more nozzles  85 . A nozzle  85  may be suspended on a servo controlled table such that the nozzle  85  can be moved across and along the permeate carrier  16 . For example, to produce a line of sealant across the width of the permeate carrier  16 , the nozzle  85  moves across the permeate carrier  16  while also moving towards the nip rollers  87  at the same speed as the permeate carrier  16 . To produce a line of sealant along the edge of the permeate carrier  16 , the nozzle  85  stays in position relative to the width of the permeate carrier  16  but retracts away from the nip rollers  87  to be ready to make another line across the width of the permeate carrier  16 . By combining these movements with turning a metering pump supplying sealant on and off, the nozzle  85  can produce various patterns on the permeate carrier. For example, the nozzle  85  can produce parallel lines of sealant as shown in  FIG. 1 , a three sided pattern as shown in  FIG. 8  or a four sided pattern as shown in  FIG. 6 . 
         [0054]    Optional nozzle  83  applies sealant to the feed spacer  14 . For example, dots of sealant may be applied to keep the feed spacer  14  relative to the other layers. In this case, full lines of sealant are applied to the feed spacer  14  when clips  20  are assembled into a stack. Alternatively, nozzle  83  may apply full lines of sealant along the edges of the feed spacer as shown in  FIG. 1 . In this case, the sealant may be a hot thermoplastic sealant reactivated by applying heat to a stack  10  of clips  20  to seal adjacent clips  20  together, or additional sealant may be applied while assembling the stack  10 . A temporary barrier sheet  18  may be unrolled under the feed spacer  14  if required to prevent sealant from being deposited on the lower nip roller  87 . Alternatively, feed spacer  14  maybe rolled out between two membranes  12 . The nozzle  83 , or another specialized nozzle, may also be used to apply rings  56  as shown in  FIGS. 5 to 7  to the feed spacer  14 . 
         [0055]    In a case where the system  80  is used to create a second element  50  as in  FIGS. 5 to 7 , the four sided sealant pattern on the permeate carrier  16  can capture a pocket of excess air between membranes  12 . This may prevent the layers from being compressed together. The nip rollers  87  inhibit this problem by squeezing out excess air as the layers advance. However, since holes will be punched for the spigots  52  in any event, a hole can be punched through the membranes  12  before the membranes  12  are compressed around the permeate carrier  16  to provide another path for air to escape. In the system  80 , holes are punched by a block  93  and die  91  upstream of the nip rollers  87 . The die  91  is actuated at a frequency that, given the line speed of the system  80 , produces holes at the spacing of the spigots. 
         [0056]    In other methods of constructing a second element  50  without using nip rollers  87 , it is also helpful to perforate the membranes  12  in the area where the spigots  52  will be located before constructing a pocket of two membranes  12  sealed to a permeate carrier  16 . One or more holes are only required in one membrane  12  of a packet comprising two membranes  12  sealed around a permeate carrier  16 . 
         [0057]    A hole made in the membranes  12  before sealing to the permeate carrier  16  may be the final size required to accommodate the spigot  52 . However, in the system  80  the layers may be moving at a line speed that makes it difficult to punch a large hole with precision. In that case, a small hole sufficient to release air may be punched by the system  80  and a larger hole for the spigot  52  can be made later. 
         [0058]      FIG. 12  shows a machine  90  for making larger holes for a spigot  52 . A clip  20  or other assembly of layers is fed by geared rollers  92  controlled by a controller  98 . The controller  98  is also connected to a sensor  94  and a punch  96 . The controller  98  advances the clip  20  until the sensor  94  detects an air release hole. The controller  98  then causes the rollers  92  to advance the air release hole to a position within the area of the die  96 . Optionally, the controller may stop the rollers  92  at this point while the punch  96  operates. The controller  98  instructs the punch  96  to punch a hole against block  100 . However, because the air release holes may not be accurately located, the controller  98  advances the clip as required to provide a desired spacing between the spigots  52 . Sensing the position of the air release hole is done to check whether the air release hole will be located within the spigot hole. If so, then the spigot hole is punched and the machine  90  goes to make the next spigot hole. If not, the controller  98  sends an alarm to indicate that part of the clip  20  is defective and resets by putting the next air release hole in the center of the punch  96 . The process of putting the holes in the membranes  12  or a clip  20  could alternatively be done with a rotary die cutter. This applies to air relief holes or to spigot holes in the machine  90  or the system  80 . 
         [0059]    Machine  90  may also be used to make spigot holes when no air release holes are made by system  80 . In this case, sensor  94  is omitted and machine  90  advances the clip  20  as required to produce spigot holes in desired locations. Optionally, machine  90  may also be fitted with a cutter and produce clip segments of a required length with spigot holes in specified locations. 
         [0060]    In an optional assembly method, air release holes, spigot holes or other registration holes are used to align multiple clips  20  as they are placed on top of each other to from a stack. For example, the clips  20  can be placed over a jig having vertical pins on the centers of where the spigots  52  will be located. Once all layers are in place, the pins are withdrawn. If required, and a punch, or other hole making device, is pushed through the entire stack  10  to enlarge the holes to the size of spigot holes. 
         [0061]    Still considering a second element  50  as in  FIGS. 5 to 7 , when a stack  10  is assembled there is a tendency for the rings  56  to compress the permeate carrier  16  in areas between, or above or below, the rings  56 . Compressing the permeate carrier  16  increases its resistance to the flow of permeate to the spigot  52 . Referring to  FIG. 13 , a spigot hole  110  in the permeate carrier  16  is optionally reinforced to resist compression by the rings  56 . In this example, radial lines  112  of sealant, such as EVA or other hot melt adhesive, are embedded in the permeate carrier  16  around the spigot hole  110 . 
         [0062]      FIG. 14  shows an example of a feed spacer  14  pre-conditioned for assembly into a second element  50  as in  FIGS. 5 to 7 . The feed spacer  14  has a ring made by applying and curing a hot melt adhesive such as EVA around an air release hole, registration hole, or full sized spigot hole. Edge pre-seals  114  are applied along the length of the feed space by applying and curing a hot melt adhesive. Optionally, one or more corners of the feed spacer  14  may have a recess  116  to help a corner seal  32  attach to membranes  12  around the feed spacer  14 . Similar recesses  116  may be used with other elements having corner seals  32 . The feed spacer  14  is assembled into a stack  10 , for example by being alternated with packets of membranes  12  pre-sealed around a permeate carrier  16 . The rings  56  and edge pre-seals  114  can be sealed to adjacent membranes  12  by applying an additional sealant to the rings  56  and edge pre-seals  114  before they are pressed against membranes  12 . In this case, the additional sealant may have a low viscosity and fast setting time. Alternatively, the stack  10  may be assembled without additional sealant. In this case, the stack  10  is re-heated after assembly such that the hot melt adhesive of the rings  56  and edge pre-seals  114  melt at least partially and adhere to the membranes  12 . 
         [0063]    In an embodiment, the height of the rings  56  and edge pre-seals  114  is close to the thickness of the feed spacer  14 .  FIG. 15  shows a press  120  used in a process of applying rings  56 , edge pre-seals  114  or both. A portion of the press  120  around ring  56  is shown, but a larger press maybe used to also apply the edge pre-seals  114 . The press  120  has an upper plate  122  and a lower plate  128 . In an embodiment, at least one of the plates  122 ,  128  has a heating element  130 . A feed spacer  114  with a molten hot melt adhesive is inserted between the plates  122 ,  128  and the plates  122 ,  128  are brought together to the thickness of the feed spacer  14 . However, the feed spacer  14  is very thin (for example about 0.029 inches thick) and simply pressing the hot melt adhesive tends to produce a ring  56  of uneven thickness with parts that may be 30% or more thicker than the feed spacer  14 . Heating at least one of the plates  122 ,  128  and leaving the feed spacer  14  in the press  120  for a period of time, for example 10 minutes or more, reduces the excess thickness to within a few percent of the desired thickness. Optionally, release layers  126  may be used above and below the feed spacer  14 . Insulating layers  124  prevent the release layers  126  from melting to the presses  120 . In the example shown, the insulating layers  124  are sheets of permeate carrier  16 . The permeate carrier  16  additionally provides a path for air to escape from the press  120 . 
         [0064]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.