Patent Publication Number: US-2005126978-A1

Title: Potting method for membrane module

Description:
This is an application claiming the benefit under 35 USC 119(e) of U.S. Provisional Application Ser. No. 60/531,995, filed Dec. 24, 2003. All of U.S. Ser. No. 60/531,995 is incorporated herein by this reference to it. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to membrane modules for water treatment and, more particularly, to potting membranes into a header.  
     BACKGROUND OF THE INVENTION  
      Membranes can be used in water treatment units to extract permeate from a supply of water. Immersed membranes may be used for extracting clean water (permeate) from a tank of contaminated water containing solids or mixed liquor. The membranes may be potted in headers, and can be assembled in modules, each module having many membranes extending from a header. A source of suction can be provided to the headers to withdraw permeate through the membrane walls and into the lumens of the fibers. The permeate can then be drawn into a permeate collection cavity in or adjacent to the headers.  
      In one potting method, a layer of a fugitive material is provided in a potting container that can be a header shell. Ends of membranes to be potted are then inserted partway into the gel. Potting resin in a substantially liquid form can then be provided in a layer on top of the fugitive material. Once the resin has at least partially cured, the fugitive material can be evacuated, leaving a permeate cavity with which the insides of the membranes are in fluid communication.  
     SUMMARY OF THE INVENTION  
      The inventors have observed that when potting membranes, for example hollow fiber membranes, using a known method such as, for example, but not limited to, the fugitive method, described above, a gap is typically provided between the outermost fibers in a bundle and the inner surface of the sidewall of the potting container. This gap provides room for a nozzle to be passed along the gap to lay down an amount of potting resin. The resin can migrate through the bundle of fibers (for example, under the force of gravity) and eventually make its way to the center of the bundle so that all of the fibers are satisfactorily potted.  
      This migration and leveling off of the resin generally takes a considerable amount of time. Also, uneven initial application of the resin (along the edges of the bundles) can disturb the underlying fugitive material and result in uneven resin thickness. Furthermore, the gap reduces the number of fibers that could otherwise be potted in the shell, since the potted header will be left with a “dead space” in the form of a border of cured resin where the gap formerly existed and in which generally no fibers are potted. Occasionally a stray fiber may become potted in the dead space border rather than with the bundle of fibers. Such stray fibers lack support of neighboring fibers, and for that reason, among others, are particularly prone to breakage. Breakage of the permeating fiber membranes can result in undesired contamination of the permeate.  
      It is an object of the invention to improve on the prior art. It is another object of the present invention to provide a membrane module of hollow fiber membranes and a method of making such a membrane module. It is another object of the present invention to provide a header for a membrane module having internal injection ducts that can be used for potting the hollow fiber membranes. It is another object of the present invention to provide a header for a membrane module that can, for a particular size of header, accommodate a greater amount of filtering membranes (i.e. improved ratio of active header area to “dead space”). It is another object of the invention to provide a module with structural elements spanning a permeate cavity. It is another object of the invention to provide a fugitive potting process that preserves a space for fluid flow between the end of the membranes and the walls of a header. These and other objects are provided by the features described in the claims. The following summary provides an introduction to the invention which may reside in a combination or sub-combination of features provided in this summary or in other parts of this document.  
      According to one aspect of the present invention, a header pan is provided with a plurality of tubes or ducts extending into the header. For example, the ducts may pass through what will be a permeate cavity in the header. The ducts have a first opening for introducing potting resin into them and one or more second openings for ejecting the resins. The second openings are located in an area to be filled with potting resin. The first openings may be located inside or outside of the header pan and are connected to a runner or other means for supplying liquid resin to the ducts. For example, the runner may be a channel glued or clamped to a side or bottom of the header pan with one or more openings in communication with the first openings of the ducts. To make a header, a fugitive material is placed in a part of the header pan that will be a permeate cavity. Ends of the membranes are inserted into the fugitive material. Liquid resin is injected into the first openings of the ducts, for example through the runner, flows through the ducts and out the second openings. The resin may be applied in steps separated by waiting periods which give time for the resin to flow across the header. The liquid resin is allowed to cure and the fugitive material is removed, for example through a permeate port. The ducts, filled with solid resin, may remain in the header where they provide a structural link between the resin and the header pan. The runner, if used, may also be left with the header or it may be removed and re-used. The ducts may be sized and shaped to provide minimal physical interference with the membranes such that membranes can be provided nearly uniformly across the header pan. The method may be adapted to other potting methods, for example methods not using fugitive materials, methods involving centrifugation and methods in which the fibers are potted first in a cavity that is not the header pan itself.  
      According to another aspect of the invention, a fugitive potting method uses two layers of fugitive materials. A first fugitive layer is provided adjacent the surface, for example the top or bottom surface, of a header shell. A second fugitive layer is provided adjacent the first layer. The first layer resists penetration of the fibers more than the second layer. The membranes are inserted into the second layer and may pass partially or completely through it. However, when the ends of the membranes reach the first layer, resistance to further penetration increases and the membranes are not inserted all the way through the first layer. A potting material is then provided adjacent the second layer and hardened. Both fugitive layers are then removed leaving a gap between the ends of the membranes and the inside surface or surfaces of the header shell. This gap provides a clear channel of about the thickness of the first layer for permeate flow through the header. The increased resistance to penetration of the first layer assists in providing this gap by providing a physical barrier to insertion of the fibers or by signaling to a person or machine inserting the fibers that the interface between the two fugitive layers has been reached. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made by way of example, to the accompanying drawings that show embodiments of the present invention, and in which:  
       FIG. 1  is a perspective view of an embodiment of a module according to the present invention;  
       FIG. 2  is a top view of the module of  FIG. 1 ;  
       FIG. 3  is a top view of a header shell of the module of  FIG. 1 ;  
       FIG. 4  is a cross-sectional view of the header shell of  FIG. 3  taken along the lines  4 - 4 ;  
       FIG. 5  is detailed cross-sectional view of a needle shown in  FIG. 4 ;  
       FIGS. 6   a - 6   c  are cross sectional views showing the header of  FIG. 1  at various stages in a potting process;  
       FIG. 7  is a side view of a runner attached to the header of  FIG. 4 ;  
       FIG. 8  is a top view of an alternate module according to the present invention showing only the header shell;  
       FIG. 9  is cross-sectional view of the module of  FIG. 7  taken along the lines  9 - 9 ; and  
       FIGS. 10   a - 10   c  are cross-sectional views showing the header of  FIG. 1  at various stages in another potting process according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A filtration module  90  having a header potted according to the present invention is shown generally in  FIG. 1 . The module  90  has opposed headers  100  and a bundle  102  of permeating hollow fiber membranes  104  extending between the headers  100 . The bundle  102  is configured in an elongate rectangular shape when viewed from above ( FIG. 2 ), having a generally rectangular perimeter  103  (shown in phantom line) in a plane perpendicular to the axis of the hollow fiber membranes. Other configurations, such as, for example but without limitation, modules with a single header at one end of a bundle, modules with tow or more bundles of fibers, and headers/bundles with circular perimeters, or perimeters of other shapes, can also be provided within the scope of the present invention.  
      Referring now to  FIGS. 3 and 4 , each header  100  has a shell  106  that can be generally channel shaped and rectangular in cross-section. Each shell  106  has a base  108 , and sidewalls  110  and end walls  112  that extend generally perpendicularly from the base  108 . The sidewalls  110  and end walls  112  are spaced apart to define a recess  116  for receiving ends of the hollow fiber membranes  104 . The surfaces of the base  108 , sidewalls  110 , and end walls  112  facing towards the recess  116  define the inner surface  118  of the shell  106 . The surfaces of the base  108 , sidewalls  110 , and end walls  112  facing away from the recess  116  define the outer surface  120  of the shell  106 .  
      In the embodiment illustrated, the shell  106  of the header  100  is further provided with an optional rib  122  that extends from the base  108 , generally parallel to, and spaced between, the sidewalls  110 . The rib  122  can divide the recess  116  into two smaller recesses  116   a  and  116   b.    
      The header  100  is further provided with protrusions  123  that can be in the form of ducts or needles  124  extending from the shell  106  into the recess  116 . As further described hereinafter, the needles  124  have internal injection ducts  126  for injecting a generally liquid material into the recess  116 . In the embodiment illustrated, the needles  124  are generally cylindrical in shape, having lower ends  134  fixed to the base  108  of the shell  106 , and upper ends  136  positioned between the sidewalls  110  of the shell  106 . The needles  124  are arranged in two rows  138  of needles  124 , each row  138  extending parallel to, and generally centrally between, the rib  122  and one of the sidewalls  110  ( FIG. 3 ). In this way, a first row  138   a  of needles  124  is provided in the recess  116   a , and a second row  138   b  is provided in the recess  116   b.    
      Referring now to  FIGS. 4 and 5 , the ducts  126  of the needles  124  have inlet ports  130  that open to the outer surface  120  of the shell  106 , and discharge outlets  132  positioned in the interior of the recess  116 . The ducts  126  comprise an axial passage  140  and a radial passage  142  ( FIG. 5 ). The axial passage  140  has a lower portion  144  extending from the lower end of the needle  124  to a point about one third of the way along the height of the needle  124 . The axial passage  140  has an upper portion  146  that extends from the lower portion  144  to a point about two thirds of the way along the height of the needle  124 . The diameter of the lower portion  144  can be greater than the diameter of the upper portion  146 .  
      The radial passage  142  comprises a cross-bore  148  that passes through the width of the needle  124  and intersects the upper portion  146  of the axial passage  140 . The cross-bore  148  provides two discharge outlets  132  on opposite sides of the needle  124 . In the header  100 , the cross-bore  148  can be oriented generally parallel to the sidewalls  110  of the shell  106 , so that the discharge outlets  132  are directed towards opposed end walls  112 .  
      The needles  124  can be further provided with a deflector cap  150  at their upper ends  136 . In the embodiment illustrated, the deflector cap  150  is an upwardly pointing conically shaped feature provided at the upper end  136  of each needle  124 . The deflector cap  150  can facilitate potting of the fibers  104  in the header  100 , as described in greater detail hereafter.  
      Each needle  124  can be a distinct element, separately attached to the shell  106  of the header  100 . To facilitate attachment of the needles  124  to the shell  106 , each needle  124  can be provided with a mounting surface  152  adjacent the lower end of the needle  124 . The mounting surface  152  can include a cylindrical undercut portion that has a smaller outer diameter than an upper portion of the needle  124 . The base  108  of the shell  106  can be provided with bores  154  each sized to receive the mounting surface  152  in a snug fit. An adhesive can be applied to the mounting surface  152  for securing the needle  124  to the shell  106 .  
      Alternatively, at least a portion of the needles  124  may be provided integrally with the shell  106 , by a process such as, for example, but not limited to, injection molding. In one embodiment, a lower portion of the needles including the axial passage could be integrally molded with the shell, and an upper portion having the deflector cap and radial passage could be separately attached to the lower portion.  
      Each needle  124  can further be provided with at least one annular groove  158  positioned to become embedded in the cured resin after potting. In the embodiment illustrated, each needle  124  has two spaced-apart annular grooves  158  positioned between the discharge outlets  132  and the deflector cap  150 . The annular grooves  158  improve the physical bonds between the needle  124  and the cured resin which enhances the ability of the needle  124  to strengthen the header structure.  
      For potting the header  100 , a fugitive material, such as a gel  160 , can first be provided in a layer along the base of the shell ( FIG. 6   a ). The ends of bundled hollow fiber membranes  104  can then be inserted into the recess  116  from above, and lowered into the gel  160  ( FIG. 6   b ). The membranes  104  can be lowered partway down into the gel  160  to leave a space  159  between the ends of the membranes  104  and the inner surface of the base  108  of the shell  106 . As the membranes  104  are lowered into the recess  116 , the deflector cap can deflect the ends of the membranes  104  around the needles  124 . As well, the sidewalls  110  and end walls  112  can guide the membranes  104  into the recess  116 , thereby corralling the ends of the membranes  104  into the closely packed bundle  102  of spaced-apart membranes.  
      Potting resin  162  (see cutaway portion of  FIG. 6   c ) can then be injected into the recess  116  through the needles  124 , surrounding a portion of the length of the fibers  104  above the gel  160 . To inject the resin  162 , a potting runner  164  can be provided. Referring to  FIGS. 6   b  and  7 , the runner  164  can be in the form of a conduit  166  that extends along the outer surface  120  of the base  108  of the shell  106 , with spaced-apart nozzles  168  extending from the conduit to engage the inlet ports of the ducts  126 . The conduit  166  can be, for example, a tubular member constructed of plastic and may be of various cross sections such as round, square of C-channel. The runner  164  can be secured to the outer surface  120  of the shell  106  by, for example, a suitable adhesive. Alternatively, or additionally, the nozzles  168  can be sealingly secured to the inlet ports  130  of the ducts  126  and so fix the runner  164  to the shell  106 .  
      The runner  164  can have an inlet  170  connected to a supply of resin and outlet nozzles  168  to dispense resin into the injection ducts  126 . The runner  164  can have as many nozzles  168  as there are inlet ports  130  of the ducts  126 , with each nozzle  168  being in fluid communication with one of the ports  130 .  
      To pot the membranes  104 , the potting resin  162  can be pumped through the runner  164 , so that the resin  162  flows through the nozzles  168 , into the ducts  126 , and then into the recess  116  of the shell  106 . The resin  162  can thus be supplied directly to an interior portion of the recess  116 , and simultaneously at more than one location in the interior of the recess  116 . In one method of potting the membranes  104  in the header  100 , the resin  162  is pumped through the nozzles  168  in alternating cycles of higher pressure and lower pressure. The lower pressure cycle can allow some migration or leveling of the resin, between the higher pressure cycles. The lower pressure cycle can be an “off” condition in which no or virtually no resin pressure is provided at all.  
      Referring again to  FIG. 6   c , after injecting a desired amount of resin, the resin can cure, and once at least partially cured, the fugitive gel  160  can be removed, leaving a permeate collection cavity  161  with which the lumens of the membranes  104  are in fluid communication. At least some resin  162  will generally remain in the injection ducts  126  to seal the ducts  126  off from the permeate cavity  161 . The runner  164  will generally also retain resin  162  that can cure, to further plug and seal off the permeate cavity  161  from untreated liquid outside the shell  106  when immersed. Alternately, the runner  164  can be stopped before the resin is cured and re-used or discarded, the resin in the ducts  126  sealing off the openings created as the runner is removed.  
      The inventors have observed that significant pressure differentials can be experienced between the permeate cavity  161  and the surrounding untreated water. This can place stress on the header shell  106 , and cause arching of the header shell  106  and/or the resin layer  162 . Without sufficient strength, the header shell  106  and/or the resin layer  162  can rupture, causing failure of the header. To strengthen the headers, one or more of the following techniques can be employed: using thicker wall sections to construct the header shell, providing reinforcing ribs in the shell, reducing the cross-sectional size of the header shell, or providing a thicker layer of resin.  
      Alternatively, or additionally, according to the present invention, the runner  164  can remain attached to the shell  106  to aid in reinforcing the header  100 . The needles  124  also remain attached to the header, being generally embedded within the cured resin  162 . The mechanical bond between the resin  162  and the needles  124  can reinforce the header by tying the resin layer  162  to the shell  106  along positions of the resin layer  162  disposed between the sidewalls  110  of the shell  106 . This can permit an increase in the width of the header shell, while maintaining sufficient strength to withstand pressure differentials experience by the header during use or a reduction in sizes of other components for the same width of header. Resin filling the annular grooves  158  provided along the outer surface of the needles  124  can enhance the mechanical bond between the cured resin layer  162  and the needles  124 .  
      As best seen in  FIGS. 8 and 9 , an alternate embodiment of a header  200  according to the present invention has a shell  206  that is similar to the shell  106 , but with an increased width. As well, the header  206  does not have a longitudinal rib (like the rib  122  of the header  100 ), but is instead provided with a transverse rib  222  at about the midpoint along the length of the shell  206 .  
      The shell  206  can have a base  208 , sidewalls  210 , and end walls  212  to define a recess  216 . The shell  206  has protrusions  223  in the form of needles  224  extending from the base  208  of the shell  206 . The needles  224  extend into the interior of the recess  216 , and injection ducts  226  are provided through the interior of the needles  224 . The ducts are grouped into two sets of ducts. A first set of ducts  226   a  has discharge outlets  232   a  at a first, shorter distance measured generally normal from the base  208  of the shell  206 , and a second set of ducts  226   b  has outlets  230   b  at a second, greater distance from the base  208  of the shell  206 , relative to the first set of ducts  226   a.    
      The distinct sets of ducts  226   a ,  226   b  can be used to inject two separate layers of material for potting the fibers  104 . In the embodiment illustrated, the first set of ducts  226   a , having lower outlets  232   a , is used to supply a layer of potting resin  262  in the shell  206 . The layer of resin  262  has a thickness that extends from above the permeate cavity  261  to a point below the outlets  232   b  of the second set of ducts  226   b . The second set of ducts  226   b , having higher outlets  232   b , is used to inject a layer of cushioning material  263  on top of the potting resin  262 . The potting layer  262  can be of a material such as, for example, but not limited to, an epoxy that cures to form a relatively hard, chemical resistant block of material. The cushioning layer  263  can be of a material such as, for example, but not limited to, silicone that cures to form a relatively softer material. The cushioning layer  263  can reduce the occurrence of fiber breakage, which, the inventors have observed, occurs most frequently at the point where the fibers  104  exit the potting.  
      The distinct ducts  262   a  and  262   b  can be provided in separate needles  224   a ,  224   b , respectively. Alternatively, each needle  224  can have one duct  226   a  and one duct  226   b , extending as separate passages between distinct inlet ports  230   a ,  230   b  and outlets  232   a ,  232   b , respectively, which are connected to respective runners.  
      In other embodiments, the runner can be provided inside the header shell. Injection ducts can also be provided in other locations such as through the sidewalls of the header shell. Alternatively or additionally, ribs in the shell can be provided with injection ducts or an injection slot along part or all of the length of the rib. The runner can be constructed to be re-usable. Such a re-usable runner could have a conduit of steel, and nozzles that can engage the inlet ports of the injection ducts in a releasable sealed manner, for example through o-rings. The runner can be temporarily attached to the shell, for example with clamps, screws or other fasteners.  
      Referring again to  FIGS. 6   a - 6   c , the inventors have found that providing the space  159  between the ends of the potted fibers  104  and the base  108  of the shell  106  can facilitate evacuating permeate from the collection cavity  161  and/or the lumens of the fibers  104  at reduced head loss by providing a channel clear of membranes. Improved permeate evacuation can be particularly noticeable in headers where the potted bundle of fibers  104  has fibers  104  across substantially the entire width of the shell  106 , such that little or no gap is provided between the outermost fibers  104  in the bundle and the inner surface of the sidewalls  110  of the shell  106 .  
      Referring now to  FIGS. 10   a - 10   c , to facilitate providing the space  159  between the ends of the fibers  104  and the shell  106 , a fugitive multilayer  360  having two or more layers can be used in place of the single fugitive gel layer  160 . More particularly, the fugitive multilayer  360  can have, with respect to insertion of the fibers  104  during potting, a generally less penetrable base layer  360   a  and a more penetrable upper layer  360   b  positioned on top of the base layer  360   a  ( FIG. 10   a ).  
      The base layer  360   a  can be, for example, but not limited to, a a solid, a deformable solid, solid particles, a viscous liquid or gel, water that has at least partially frozen to form a layer of ice along its upper surface, or other material resistant to insertion of the fibers  104 . For further example, a solidified gelatin may be used. The base layer  360   a  can have a depth of, for example, about 5 mm to 20 mm or more, the depth being chosen to create a free channel for permeate flow of a desired volume. The upper layer  160   b  can be, for example, a viscous gel in non-solidified form, through which the fibers  104  can be inserted but on top of which the potting resin  162  can be supported until cured or partially cured. Other materials, for example but without limitation waxes or powders, may also be used as may be appropriate to work with the base layer. The depth of the upper layer  160   b  can be, for example, about 5 mm to 20 mm or more.  
      During a potting process using the fugitive multilayer  360 , the generally less penetrable base layer  360   a  can be provided by, in one particular embodiment, pouring liquid gelatin material into the shell  106 . The gelatin can then set or solidify, to form the base layer  360   a . The process by which the gelatin sets or solidifies can simply require waiting an appropriate length of time, or the process can include reducing the temperature of the gelatin. Once the base layer  360   a  has been formed, the more penetrable upper layer  360   b  of the fugitive multilayer  360  can be provided by applying gel on top of the base layer  360   a  ( FIG. 10   a ).  
      The fibers  104  can then be lowered into the fugitive multilayer  360 . The fibers  104  can penetrate partially or fully through the depth of the upper layer  360   b , but will encounter increased resistance to further penetration when lowered to a depth where the ends of the fibers  104  contact the base layer  360   a . Insertion of the fibers  104 , if they have gone that far, can then be stopped. Even if some further lowering of the fibers  104  does occur, the fibers  104  will generally not penetrate deeply into the base layer  160   a  ( FIG. 10   b ) and will not penetrate all of the way through it and so will still provide a space  159 .  
      Potting resin  162  can then be injected into the shell  106  above the upper layer  360   b  of the multilayer fugitive  360  ( FIG. 10   c ). A potting method using injection ducts as described previously herein can be used to provide the resin  162 . Alternately, other methods of providing the potting resin may be used. Once the resin has cured or at least partially cured, the fugitive multilayer  360  can be removed from the shell  106 . Removal of the multilayer fugitive  360  can include, for example, but not limited to, heating the multilayer fugitive  360 , dissolving with water, pouring and/or flushing with chemicals such that the fugitive multilayer  360  is removed, and the membranes  104  and resin  162  are undamaged.  
      Once the fugitive multilayer  360  is removed, a two-part permeate cavity  361  remains between the base  108  of the shell  106  and the resin  162 . The two-part cavity  361  has a lower part  361   a  formerly occupied by the base layer  360   a  of the multilayer fugitive  360 , and an upper part  361   b  formerly occupied by the upper layer  360   b  of the fugitive multilayer  360 . The lower part  361   a  of the permeate cavity  360  is substantially empty, devoid of any fibers  104 , resin  162 , or fugitive material. The upper part  361   b  of the permeate cavity  361  is generally occupied by the lower ends of the fibers  104  to the extend that they protrude from the resin  162 .  
      Materials other than a solidified gelatin can be used for the generally impenetrable base layer  360   a  of the fugitive multilayer  360 . Any layer of material that can provide a detectable resistance to the insertion of the fibers  104  can be used. Complete impenetrability is not required. For example, if the fibers  104  are inserted into the fugitive multilayer  360  by hand, a base layer  360   a  that is more viscous than the upper layer  360   b  can be sufficient. A person potting the fiber  104  can sense the difference in resistance to insertion of the fibers  104  as the ends of the fibers  104  pass through the upper layer  360   b  and engage the base layer  360   a . Insertion of the fibers can then be halted, so that the space  159  remains between the shell  106  and the fibers  104 . Similarly, a machine inserting the fibers  104  may be fitted with a sensor to detect that difference in resistance provided by the base layer  360   a . The sensor is linked to the machine controls and instructs the machine to not insert the fibers  104  further when the base layer  360   a  has been reached. The base layer  360   a  and the upper layer  360   b  need not be of different material, but can be of the same material at different degrees of solidification or viscosity. For example, the base layer  360   a  can be of solidified gelatin, and the upper layer can be of gelatin that is only partially solidified or still generally liquid. Having the same material throughout the fugitive multilayer  360  can simplify removal of the fugitive multilayer, since a particular process and/or chemical effective for removal of one layer  360   a  or  360   b  can be used for the other layer as well.  
      While preferred embodiments of the invention have been described herein in detail, it is to be understood that this description is by way of example only, and is not intended to be limiting. The full scope of the invention is to be determined by reference to the appended claims.