Patent Publication Number: US-2005126982-A1

Title: Membrane module having fiber breakage protection

Description:
This is an application claiming the benefit under 35 USC 119(e) of U.S. Provisional Patent Application Ser. No. 60/528,493, filed Dec. 11, 2003. U.S. Ser. No. 60/528,493 is incorporated herein, in its entirety, by this reference to it. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to membrane modules for water treatment.  
     BACKGROUND OF THE INVENTION  
      Hollow fiber membranes can advantageously be used in water treatment units to extract permeate from a supply of water. The hollow fiber membranes may be potted in headers. A source of suction is provided to the headers to withdraw permeate through the membrane walls and into the lumens of the fibers. The permeate is then drawn into a permeate collection cavity in or adjacent to the headers.  
      Immersed membranes may be used for extracting clean water (permeate) from a tank of contaminated water containing solids or mixed liquor. The membranes are often provided in the form of assembled modules, each module having many hollow fiber membranes extending from a header for collecting permeate that passes through pores of the membrane walls into the lumens of the membranes. Streams of air bubbles may be provided in the tank to rise past the membranes for cleaning purposes. The air bubbles also help to create circulation patterns in the tank.  
      However, the air bubbles or circulating water in the tank apply forces to the membranes. These forces may cause the fibers to break. A broken fiber allows unfiltered water from the tank to enter the permeate collection cavity. Since this is undesirable, the broken fiber must be located and repaired resulting in loss of productivity and additional expense.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to improve on, or at least provide a useful alternative to, 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 or a method of potting hollow fiber membranes. It is another object of the present invention to provide a membrane module having hollow fiber membranes that are protected, or have been potted, to inhibit their breakage. The following summary provides an introduction to the invention but is not intended to define or limit the invention which may reside in a combination or sub-combination of features provided in this summary or in other parts of this document for example the claims.  
      When using immersed membrane modules, the fiber membranes can sway or vibrate back and forth. The inventors have observed that this motion can induce some stress in the fibers, particularly near the headers where the fibers exit the potting material which seals and secures the outsides of the membranes, and particularly in fibers positioned at the outer periphery of any particular bundle of potted fibers. As a result, the fibers at the outer periphery of a bundle of fibers, and any stray fibers sticking out from a bundle of fibers into a circulation channel for the water in the tank, are particularly susceptible to breakage. By protecting even only these fibers, the rate of breakage in the bundle may be drastically reduced. Alternately, a thinner or weaker, but less expensive, fiber may be used and result in the same breakage rate as a strong fiber that is not so protected.  
      According to one aspect of the present invention, an assembly of hollow fiber membranes has at least one bundle of permeating hollow fiber membranes that are potted in a block of solid potting material, and a cushioning wall extending along at least a portion of the axial length of the fibers adjacent the potting material around at least a portion of the perimeter of the bundle. The portion of the perimeter may be an area where forces on the fibers from bubbles or circulating water are highest. Optionally, the entire perimeter may be protected.  
      The cushioning wall can have a fixed edge adjacent the potting material, and the wall can extend away from the potting material in a direction generally parallel to the length of the hollow fiber membranes. The wall may extend away from the potting material for only a short distance, for example, less than 15 cm, since the need for protection is primarily where the fibers join the potting material. If the potting material has wicking asperities with upper edges extending away from the potting material, and the cushioning wall can extend past the upper edges of the wicking asperities. The cushioning wall can be attached to the potting material of the header. Alternately, the header can be provided with a shell in which the potting material is held, and the cushioning wall can be attached to the shell.  
      The cushioning wall can have a free edge opposite the fixed edge. The cushioning wall can comprise a layer of liquid material solidified after application to the potting material or shell. Alternatively, the cushioning wall can comprise a strip of flexible material applied as a solid and the fixed edge can be attached to or potted in the potting material.  
      In other aspects, protective or non-permeating fibers can be added to a bundle to form a cushioning wall protecting the other fibers. For example, a layer of protective fibers of generally the same length as the other bundled fibers, may be provided on the outside of a bundle, either in one or more selected high stress locations, or around the entire perimeter of the bundle. The layer of protective fibers can include non-permeating hollow fibers, with ends that terminate and are embedded within the potting material. Alternatively, the non-permeating hollow fibers can have fluid isolating obstructions within the fibers, between the ends of the non-permeating fibers and where the non-permeating fibers exit the potting material. The fluid isolating obstructions can be plugs or pinch seals. Because the fibers are non-permeating they may break without allowing water in the tank to enter the permeate collection cavity. But even when broken, the protective fibers continue to protect the other fibers in the bundle from breakage. As a further alternative, the layer of protective fibers can include non-permeating solid filaments. The solid filaments can have at least an outer layer of fiber material that is generally the same as the material from which the permeating hollow fiber membranes are constructed or be of such a material throughout. As yet a further alternative, the layer of protective fibers can comprise reinforced permeating fibers having ends potted in the cured resin. The reinforced hollow fibers can be hollow fibers supported on a braided tube.  
      The layer of protective fibers can additionally or alternatively include a porous fabric sheet. The fabric sheet can be constructed of, for example, but without limitation, cheesecloth or gauze or other macroporous fabric constructions, and can extend along substantially the entire axial length of the bundled permeating hollow fibers.  
      In another aspect of the present invention, a method of constructing a header having potted filtering hollow fiber membranes and breakage protection for the membranes is provided. The method includes providing a bundle of permeating hollow fiber membranes inside a header shell or other potting container in a first layer of potting material and providing a cushioning wall along at least a portion of the axial length of the fibers and along at least a portion of the perimeter of the bundle to reduce stress on the fibers where the fibers exit the resin and to prevent stray fibers from sticking out beyond the periphery of the bundle.  
      The cushioning wall can be provided around substantially the entire perimeter of the bundle. Providing the wall can include applying a liquid along a portion of the perimeter of the bundle which is to have the wall and allowing the liquid to solidify in contact with the potting material or shell. Alternatively, the cushioning wall can be in the form of a strip of solid flexible material having a first edge positioned to generally abut the first layer of potting material, and the method can include supplying a second layer of potting material to pot the first edge of the cushioning wall, fixing the first edge of the cushioning wall to the header.  
      In another aspect of the present invention, a method of potting hollow fiber membranes in a header that controls against potting stray fibers is provided. The method includes arranging a plurality of hollow fiber membranes in a bundle, providing gathering elements around the perimeter of the bundle of fibers and/or adjacent one end of the bundle and providing a solidifying potting material such that the potting material flows between adjacent fibers in the bundle.  
      The gathering elements can be substantially inelastic, and can be provided between groups of fibers in each bundle to form sub-bundles. The gathering elements can comprise, for example, but not limited to, string-like elements such as fiber membranes, filaments, or yarn. The method for providing the gather elements can include threading, stitching, or weaving the gathering elements in a matrix pattern around and/or through the bundle, in a plane generally perpendicular to the axis of the fibers. 
    
    
     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 a filtration module according to the present invention;  
       FIG. 2  is a top view of a header portion of the module of  FIG. 1 ;  
       FIG. 3  is an enlarged cross-sectional view of a portion of the header of  FIG. 2  cut in a vertical plane;  
       FIG. 4  is another view of  FIG. 3  showing a fugitive material for a process for potting the header of  FIG. 2 ;  
       FIG. 5  is an enlarged view of a header portion of the module of  FIG. 1 ;  
       FIG. 6  is a cross-sectional view of a portion of the header of  FIG. 5 ;  
       FIG. 7  is a similar view as  FIG. 6  for an alternate header embodiment;  
       FIG. 8  is an end view of an alternate module embodiment with a further alternate header embodiment;  
       FIGS. 9   a  and  9   b  are partial cross-sectional views of the header of  FIG. 8  showing alternate embodiments of a protective layer of non-permeating fibers;  
       FIG. 10  is a cross-sectional view of an alternate non-permeating fiber for use with the header of  FIG. 8 ;  
       FIG. 11  is a cross-sectional view of an alternate protective layer for use with the header of  FIG. 8 ;  
       FIG. 12  is a perspective view of a further alternate protective layer for use with the header of  FIG. 8 ; and  
       FIG. 13  is a plan view of a lower part of a bundle of fibers with gathering elements, the bundle cut in a horizontal plane. 
    
    
     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 at least one bundle  102  of permeating hollow fiber membranes  104  extending between the headers  100 . In the embodiment illustrated, the module  90  has two bundles  102 . Each bundle  102  is configured in an elongate rectangular shape when viewed from above ( FIG. 2 ), so that each bundle has a generally rectangular perimeter  103  (shown in phantom line) in a plane perpendicular to the axis of the hollow fiber membranes. The two bundles  102  are arranged in parallel in the header  100 . Other configurations, such as, for example and without limitation, modules with a single header at one end of the bundles, modules with only one bundle of fibers, and headers/bundles with circular perimeters can also be provided within the scope of the present invention.  
      As best seen in  FIG. 3 , showing a portion of the header  100  holding one bundle  102 , each header  100  is provided with a block of potting material, such as a cured resin  106 , in which the hollow fibers  104  are potted such that they are attached to the potting material and sealed such that water to be filtered does not contaminate the permeate. The resin  106  can hold the hollow fiber membranes  104  in a generally closely packed, but spaced-apart, relationship, to provide a desired fiber potting density.  
      The header  100  can have a shell  108 , and the resin  106  can fill an upper portion of the shell. A permeate cavity  110  is provided between the underside of the resin and the inner surface of the shell. The permeating fibers  104  have lumens  112  that are in fluid communication with the permeate cavity  110 . The resin  106  serves to hold and seal the fibers of the bundles in the shell, and to seal off the permeate cavity  110 . In a module  90  with two headers  100 , one header  100  can be a non-permeating header with the resin  106  filling the shell  108  entirely, that is the area occupied by the permeate cavity  101  in  FIG. 3  is filled with resin  106 .  
      Referring to  FIG. 4 , the cavity  110  can be formed, for example, by providing a fugitive material  114  inside a lower portion of the shell  108 , then pouring the resin  106  on top, allow resin  106  to cure, and then removing the fugitive material  114 . This and other processes for potting that may be used with the present invention are described further in U.S. Pat. No. 6,592,759, which is hereby incorporated in its entirety by this reference to it. Alternatively, other methods of potting the fibers may be used. For example, the ends of the fibers may be placed in liquid resin in a container. The resin is allowed to cure around the fibers, optionally while the fibers and container are centrifuged. The resin is then cut to open the ends of the fibers, and the block of resin, optionally with part of the walls of the container still attached, is glued into a header shell.  
      Referring now to  FIGS. 3 and 5 , the header  100  is further provided with a cushioning sidewall  123  that extends along at least a portion of the perimeter  103  ( FIG. 2 ) of each bundle  102 , and along at least a portion of the axial length of the membranes  104  adjacent the cured resin  106 . The cushioning sidewall  123  can be provided by a flexible protective strip  120  having a lower edge  122  adjacent the resin  106 , and an upper edge  124  opposite the lower edge  122 . The lower edge  122  can be attached to the resin  106 . As the fibers  104  sway back and forth, the outermost fibers  104  can press against the cushioning sidewall  123 , thereby reducing and/or distributing the stresses on the fibers  104 . The sidewall  123  may also keep fibers that would otherwise stray, close to the rest of the bundle. This can reduce breakage of the fibers  104 , particularly at the point where the fibers exit the resin  106 . The sidewall  123  can be provided along portions of the perimeter  103  of each bundle  102  where potentially damaging forces are experienced. As an example, in bundles as shown in  FIGS. 1 and 2  having a rectangular perimeter with a major (longer) axis and a minor (shorter) axis, and the membranes  104  being oriented horizontally when in use, the sidewalls  123  can be provided along the sides of the perimeter  103  parallel to the major axis. Alternatively, the sidewalls  123  can be provided around the entire perimeter  103 . The sidewalls  123  need not be of a continuous, solid extent, but can have, for example, but not limited to, apertures, openings, or gaps.  
      Referring to  FIGS. 3 and 6 , in certain potting processes, the exposed surface of the resin  106  can present wicking asperities  128  having upper edges  130  that extend away from the main volume of the resin  106 . The wicking asperities are generally formed as a result of capillary action or other surface effects of the liquid resin along the outer surface of the hollow fiber membranes  104 . The resin typically hardens to form a relatively hard, non-compliant material compared to the membrane material. The inventors have observed that the likely point of breakage of the outermost fibers  104  in the bundle  102  is near the point along the length of the fibers  104  where the upper edges  130  of the asperities  128  extend. To improve the cushioning effect of the wall  123 , the cushioning sidewall  123  can be provided along an axial extent of the membranes  104  that extends beyond the upper edges  130  of the asperities  128 . In the illustrated embodiment, the strip  120  has a lower edge  122  that is attached to the resin  106 , and an upper edge  124  that extends past the upper edges  130  of the wicking asperities  128 .  
      The material of the strip  120  can be any flexible material that is able to yield to a force exerted on it by a fiber  104  that may be attached to, or pressed against, the wall  123 , while providing some resistance to such force so that any stress induced on the fiber  104  at its point of exit from the resin  106  is reduced. Furthermore, the strip  120  should be sufficiently soft and/or smooth so that any rubbing action that may occur between the strip  120  and the fibers  104  does not damage the fibers  104 .  
      In the embodiment illustrated, the strip  120  is formed of a silicone material that is applied in a generally liquid form around the perimeter  103  of each bundle  102 , near the resin  106 . The silicone solidifies to form the flexible strip  120 . The lower edge  122  of the strip  120  so formed bonds to an exposed surface of the header  100 , which can include at least one of the surfaces consisting of the resin  106  and the shell  108 . The silicone may also bond to the membranes  104 , although that bond may be broken as the membranes move in service.  
      Another embodiment of a header  200  according to the present invention is best seen in  FIG. 7 . The header  200  has bundles  102  of hollow fibers  104  potted in resin  106  in a rectangular configuration, and a cushioning wall  223 , provided by a protective strip  220 , that extends around the perimeter  103  of the bundles  102 . The cushioning wall  223  extends axially along at least a portion of the axial length of membranes  104  adjacent the cured resin. The strip  220  comprises a solid strip of flexible material having a lower edge  222  that is potted in the resin  106  of the header  200 . The material can be, for example, but not limited to, a strip of flexible plastic or rubber.  
      To facilitate construction of the header  200 , a two-stage potting process can be employed. The bundle is prepared with the protection strip  220  wrapped around one end. The protective strip  220  may have a circumference the same as or slightly longer than the circumference of the bundle to inhibit fibers from straying from the bundle. The lower ends of the fibers  104  in the bundle  102  can be potted in a first layer of resin  106   a.  The lower edge  222  of the strip  220  can then be positioned along the perimeter  103  of the bundle  102  and against the upper surface of the resin layer  106   a,  and a second, final layer of resin  106   b  can then be provided on top of the first layer  106   a  and around the lower edge  222  of the strip  220 . The layers  106   a  and  106   b  together form the cured resin  106  of the header  200 , and hold the fibers  104  and the strip  220  securely in place. Alternatively, one or more strips  220  may be provided only at selected locations, for example along the two long sides of a bundle, and inserted into the resin before it cures, or provided on top of a first layer  106   a  as described above.  
      In the embodiment illustrated, the strip  220  of the header  200  has an upper edge  224  opposite the first edge  222 , and the cushioning wall  223  extends between the edges  222  and  224 . The resin  106  has wicking asperities  128  with upper edges  130  extending away from the exposed surface of the resin  106  and adjacent the fibers  104 . The upper edge  224  of the strip  220  extends past the upper edges  130  of the wicking asperities  128 . Accordingly, the cushioning wall  223  straddles the axial position of the upper edges  130  of the wicking asperities  128  along the bundle  102 . The wall  123 ,  223  may be, for example, between 1 cm and 15 cm high.  
      A third embodiment of a header according to the present invention is illustrated at  300  in  FIG. 8 . In the embodiment illustrated, two opposed headers  300  are shown as part of a filtration module  92 . Each header  300  has two bundles  102  of permeating hollow fiber membranes  104  potted in resin  106 , and a cushioning wall  323  around a portion of the perimeter of each bundle  102 . The cushioning wall  323  extends axially along substantially the entire length of the fibers  104  in the bundle  102  and is potted in both headers  300 .  
      The cushioning wall  323  can be provided in the form of a protective layer  325  of non-permeating fibers  327 . The non-permeating fibers  327  are fibers that do not provide fluid communication between an external surface of the fibers and the permeate cavity  110  in the header  300 . The non-permeating fibers  327  may be potted in the resin  106 , around at least a portion of the perimeter  103  of each bundle  102  of fibers  104 , and may be potted at a spacing or potting density generally equal to that of the permeating fibers  104  in the bundle  102 . The protective layer  325  may comprise about one to five or two to three rows of fibers  327  in thickness. In the embodiment illustrated, the protective layer  325  comprises two rows of non-permeating fibers  327 .  
      Referring to  FIG. 9   a,  the non-permeating fibers  327  of the protective layer  325  can comprise additional hollow fibers of generally the same construction as the permeating hollow fibers  104 , but potted so that the lumens of the fibers  327  are fluidly isolated from the permeate cavity  110 . This fluid isolation can be provided by embedding the ends of the non-permeating fibers  327  within the resin  106 . In other words, the non-permeating fibers  327  can be shorter than the permeating fibers  104  so that the non-permeating fibers  327  terminate within and do not pass through the resin  106 .  
      Referring to  FIG. 9   b,  in an alternative configuration, the non-permeating fibers  327  can be fluidly isolated from the permeate cavity  110  by providing a fluid isolating obstruction  329  in the lumens. Such an obstruction  329  can be positioned in the lumens  112  anywhere at a point between the permeate cavity  110  and the expected breakpoint along the length of the fibers. Typically the position of the obstruction  329  would be adjacent the ends of the fibers near the potting resin  106  of the header  300 . The obstruction  329  can be in the form of, for example, but not limited to, a bonded pinch seal or a plug. Such a plug can be provided by dipping the ends of the fibers  327  in a liquid resin and allowing the resin to cure or by ultrasonically welding the ends closed.  
      Alternatively, as best seen in  FIG. 10 , the non-permeating fibers  327  of the protective layer  325  can be solid fiber strands (filaments)  337 . The solid filaments  337  can be constructed of, for example, a polyester yarn filament core  339  that is coated with a sleeve layer  341  of material similar to the material forming the permeating hollow fibers  104 .  
      In use, the non-permeating fibers  327  of the protective layer  325  protect against breakage of the permeating fibers  104  by acting as sacrificial elements that can fracture or break without causing undesirable consequences, such as contamination of the permeate. As well, the protective layer can inhibit the occurrence of “stray” permeating fibers  104  potted in the header  300 , since the permeating fibers  104  in the bundle  102  are gathered within the non-permeating fibers  327  of the outer protective layer  325 . In other words, if any stray fibers were potted in the header  300 , such a stray fiber would more likely be a non-permeating fiber  327 , rather than a permeating fiber  104 .  
      In use, it is expected that a certain proportion of the outermost fibers  327  in the header  300  will experience sufficient stress forces to cause those fibers  327  to fracture, rupture, or otherwise break. Since the fibers  327  in the outermost layer  325  are non-permeating, no contamination of the permeate results from such breakage. Furthermore, even if some of the fibers  327  in the layer  325  break, enough of the fibers  327  of the protective layer  325  can remain intact to provide sufficient breakage protection to the underlying permeating fibers  104 . As well, any broken fibers may leave stubs emerging from the resin  106  that may continue to act as a cushioning wall  323 . Stubs of non-permeating fibers may also be used as the protective layer  325 . Ultimately, some of the permeating fibers  104  may also eventually break, but the protective layer  325  will have delayed the breakage, so that the mean operating time between servicing the module  92  has been extended.  
      Referring now to  FIG. 11 , an alternate protective layer  325   a  providing the cushioning wall  323  comprises reinforced hollow fibers  347 . The reinforced hollow fibers  347  may be, for example, hollow fibers supported on a braided tube, having sufficient strength to resist breakage during normal use. The reinforced hollow fibers  347  can be permeating or non-permeating. In general, the reinforced hollow fibers  347  would be non-permeating, since the fibers  347  have a different construction compared to the permeating hollow fibers  104  in the bundles  102 . These differences in construction could result in differences in filtration characteristics, and hence, contamination of the permeate collected through the permeating hollow fibers  104 . However, if the reinforced fibers  347  are constructed to provide similar filtration characteristics as those of the permeating fibers  104 , then the reinforced fibers  347  could also perform a permeating function.  
      Referring now to  FIG. 12 , in another embodiment, the cushioning wall  323  is provided in the form of a protective layer  325   b  that comprises a fabric sheet  357 . The fabric sheet  357  can be a porous textile sheet that permits transverse flow of non-permeated liquid to the interior permeating fibers  104  of the bundles  102 . The sheet  357  can comprise, for example, but not limited to, cheesecloth, gauze, or a loosely woven fabric. In the embodiment illustrated, the wall  323  comprising the sheet  357  has a lower end  322  and an upper end  324  (not shown), each of which are potted in a header  300 . The cushioning wall  323  extends between the lower and upper ends  322 ,  324 , adjacent the outer fibers  104  of the bundle  102 .  
      Referring now to  FIG. 13 , the modules  90 ,  92  according to the present invention can be provided with gathering elements  401  that extend around the perimeter  103  of the bundles  102  of permeating hollow fiber membranes  104 . The gathering elements  401  can be provided to corral the ends of the hollow fiber membranes  104  before potting the membranes  104  in the header  100 . The gathering elements  401  can be, for example, but not limited to, string elements such as hollow fibers, filaments, or yarn. The gathering elements  401  can be substantially inelastic or unstretched so as to avoid applying compressive forces on the bundles  102 , and thereby maintain spaces between adjacent fibers  104  in the bundle  102 . Maintaining space between the fibers  104  in the bundles  102  can facilitate the provision of resin  106  between adjacent fibers  104  in the bundle  102 . By corralling the ends of the fibers  104 , the gathering elements  401  can aid in preventing the occurrence of stray fibers potted in the resin  106 . A stray fiber is a fiber  104  that emerges from the cured resin  106  in a position spaced apart from the main bundle  102  of fibers  104 , such as, for example, between the perimeter  103  of the bundle  102  and the shell  108  of the header  100 .  
      The gathering elements  401  can comprise yarn coarsely weaved in a plane perpendicular to the axis of the fibers  104 . The grid pattern formed by the coarsely weaved gathering elements  401  can thereby provide sub-bundles  403  of fibers  104  within each bundle  102 . Grouping the fibers  104  into sub-bundles can facilitate the corralling function of the gathering elements  401  in the bundle  102 .  
      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.