Abstract:
A module has a housing and a tube-sheet. Hollow fiber membranes are potted in the tube-sheet with their ends open. Mechanical interference prevents excessive movement of the tube-sheet along the length of the housing. A gasket is placed in a groove between the tube-sheet and the housing, opening towards the outer face of the tube-sheet. A cap is secured and sealed to the end of the housing. To construct the module, membranes are placed inside of the housing with their ends protruding. A potting fixture is attached to the end of the housing over the ends of the membranes and protruding into the end of the housing so as to form the groove. Liquid potting material is injected into the potting fixture and solidified into a tube-sheet. The potting fixture is removed and the ends of the fibers are cut open. The module may be used, for example, to dehydrate a fermentation product vapour mixture fed through the lumens of the fibres by extracting water vapour permeated into the housing under partial vacuum.

Description:
FIELD 
       [0001]    This specification relates to hollow fibre membrane modules and methods of making them. 
       BACKGROUND 
       [0002]    The following background description is not an admission that anything discussed herein is prior art or part of the common general knowledge of persons skilled in the art. 
         [0003]    A polyimide hollow fibre membrane is described in US Publication No. 2006/0117955 which is incorporated herein by this reference to it. These membranes may be used, for example, for to remove water vapour from vapour mixtures containing fermentation products such as ethanol. 
         [0004]    Multiple segments of hollow fibre membrane may be grouped together into modules by potting their ends in one or two tube-sheets. The tube-sheets and membranes are assembled with a shell so as to create a shell side plenum between the outsides of the membranes and the inside of the shell. Caps placed over the open ends of the fibers at the tube-sheets complete a lumen side plenum. The lumen side plenum is separated from the shell side plenum except for by way of permeation through the membrane walls. Subject to numerous considerations, permeation can occur through the membranes in either direction such that either the feed side or the shell side may be used as the feed side or the permeate side. Transmembrane pressure (TMP) may be applied to withdraw permeate by way of feed side pressure, permeate side suction, or both. With high TMP, significant forces are created on the tube-sheets. Using shell side feed reduces these forces because the area of the tube-sheet occupied by the fibers reduces the area of the tube-sheet exposed to the TMP. Using shell side feed also allows the outside face of the tube-sheet to be supported against a module end plate but a large, complex module may result and the feed side hydraulics may be poor. 
         [0005]    In addition to the forces on the tube-sheets, module design is complicated by the need to cast the tube-sheet around the membrane ends, produce open ends of the membranes exposed to the outside of one or both tube-sheets, locate the membranes inside the shell, use materials that are chemically resistant to the intended feed substances, and create robust seals between the solidified tube-sheets, the shell and the caps. Making robust seals is complicated by the need to account for different materials used in the shell and the tube-sheet. In particular, the shell and tube-sheets tend to expand at different rates in response to temperature changes or absorption of components in the feed. To deal with these issues, arrangements of numerous gaskets or other components are often required. 
       SUMMARY 
       [0006]    The following summary is intended to introduce the reader to the disclosure and not to limit or define any claimed invention. 
         [0007]    A module is described having a housing and a tube-sheet at one end. Hollow fiber membranes are potted in the tube-sheet with their ends open at the outer face of the tube-sheet. The tube-sheet and housing have corresponding features of shape providing mechanical interference against movement of the tube-sheet along the length of the housing. A gasket is placed in a groove between the tube-sheet and the housing, the groove opening towards the outer face of the tube-sheet. A cap is secured and sealed to the end of the housing. Optionally, the cap or a seal between the shell and cap may cover all or part of the groove to mechanically capture the gasket. If both ends of the membranes are to be open, a similar construction may be used at the other end of the module. 
         [0008]    A method of constructing a module is described. A bundle of membranes are placed inside of a housing with their ends protruding from the housing. A potting fixture is attached to the end of the housing over the ends of the membranes. The potting fixture protrudes into the end of the housing around an inside edge of the housing. Liquid potting resin is injected into the potting fixture and cured into a tube-sheet around the ends of the membranes. The potting fixture is removed and the ends of the fibers are opened by cutting through the tube-sheet close to the end of the housing. A gasket is placed inside a groove left when the protruding portion of the potting fixture was removed. A cap is attached over the end of the housing. 
         [0009]    The module has utility for separating a mixture of gases, which may be a mixture of vapours or include vapours. One particular example is dehydrating fermentation broths that have been converted to vapour mixtures for example by boiling, distillation or pervaporation. The fermentation broths may include ethanol, ABE or other substances useful as fuel. The vapour mixture may be fed under pressure into one end of the lumens of the fibres, with a mixture reduced in water vapour concentration produced at the other end of the fibers, and water vapour enriched permeate withdrawn from the shell side under a partial vacuum. While this example is given to demonstrate that the module has utility, the module may also be used for other purposes. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0010]      FIGS. 1 and 2  are side and isometric views of a module housing. 
           [0011]      FIG. 3  is an exploded cross-sectional view of an end of a module having a housing as in  FIG. 1 . 
           [0012]      FIG. 4  is an assembled cross-sectional view of the end of the module shown in  FIG. 3 . 
           [0013]      FIG. 5  is a cross-sectional view of the end of the module shown in  FIG. 3  during an intermediate step in a process of potting membranes. 
           [0014]      FIG. 6  shows a set of completed modules assembled into a rack. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIGS. 1 and 2  show a housing  10 . The housing is made of a section of tubing  12  with two end flanges  14  attached one at each end. A shell side port  16  is provided at one end of the housing  10 . Optionally, a second shell side port can be added at the other end of the housing if a shell side sweep is desired. The housing  10  shown is made of stainless steel with the flanges  14  and shell side port  16  welded to the tube  12 , although other materials or methods of assembly might be used. 
         [0016]    Referring to  FIGS. 3 and 4 , a tube sheet  18  is located inside the flange  14 . The tube sheet  18  is made of a solidified potting material cast around the ends of numerous membranes  20 . In the Figures, only a few membranes  20  are shown to simplify the drawing although in practice there may be thousands. The ends of the membranes  20  are open and optionally flush with the outside face  22  of the tube-sheet  18 . The outside face  22  of the tube sheet  18  may protrude by a small distance from the end  28  of the flange  14 , but preferably is close to the end of the flange  14 . Ridges  34  in the tube-sheet  18  fit inside of recesses  36  in the flange  14  to hold the tube-sheet  18  in the flange  14 . A groove  20  between the circumference of the tube-sheet  18  and the inside of the flange  14  opens to an outside face  22  of the tube-sheet  18 . An O-ring gasket  24 , for example of elastomeric material, is inserted into the groove  20 . A flat gasket  26  covers the end of the flange  14 . A cap  30 , which may be a section of feed or retentate piping, is attached to the flange  14 . In the example shown, the cap  30  buts against the flat gasket  26  and is bolted by way of a lap flange  32  to the flange  14 . The flat gasket  26  provides a seal between the cap  30  and the flange  14 . Preferably, the flat gasket  26  or the cap  30  cover some or all of the opening of the groove  20  to trap the O-ring gasket  24  in the groove  20 . Optionally, the O-ring gasket  24  may be replaced by a flexible sealing material formed in place to the shape of at least the inner part of groove  20 . For example, silicone rubber can be poured or squeezed into the groove  20 . The sealing material may undergo a physical or chemical change to a more viscous or solid form after being placed in the groove  20 . The sealing material is preferably chosen to be one that will adhere to the tube-sheet  18  and flange  14  and be flexible enough to accommodate changes in groove  20  width, for example due to thermal expansion. 
         [0017]    The O-ring gasket  24  provides a seal between the tube-sheet  18  and the flange  14  to separate the lumen side and shell side of the module. It is not necessary to seal the cap  30  to the tube-sheet  18 . The O-ring gasket  24  is sized to account for shrinkage of the tube-sheet  18  during curing and thermal expansion of the flange  14 . Pressure applied in use by gases fed into the lumens of the membranes  20  though the cap  30 , or by suction on the shell side port  16 , pushes and compresses the O-ring gasket  24  into the groove  20 . In the example applications described in the introduction, with feed gases flowing though the lumens of the membranes  20  and shell side suction, the feed pressure relative to suction on the port  16  is such that O-ring gaskets on both the feed and product/retentate sides of the housing  12  are drawn into the grooves  20 . The membranes  20  in the example applications are not backwashed but pressure may be applied in the reverse direction during integrity tests. During integrity tests, the test pressure may be high enough to move the O-ring gaskets  24  out of the grooves  20  but the O-ring gaskets  24  are retained in the groove  20  by the flat gasket  28  or cap  30 . 
         [0018]    The tube-sheet  18  may be made of, for example, epoxy resin. The resin does not stick to various other materials, for example stainless steel, that could be used for the flange  14 . The tube-sheet  18  may also shrink during curing and expand less than the flange  14  when heated. The tube-sheet  18  is therefore likely to have an outer diameter slightly less than the inside diameter of flange  14  when first put in use. The tube-sheet  18  may expand as it absorbs some of the feed components and the initial shrinkage of the tube-sheet  18  provides a useful allowance for this expansion. Over time, the tube-sheet may expand to the point that it bears against the inside of the flange  18 . However, even then any resulting pressure is unlikely to cause sufficient friction to overcome forces against the tube-sheet  18  along the length of the housing  10  created by the TMP. 
         [0019]    The ridges  34  and recesses  36  are shaped to provide mechanical interference preventing the tube-sheet  14  from being pushed along the length of the housing  10  in use. For example, the recesses  36  may be generally square sectioned annular grooves. In this case, the housing  10  can be re-used by breaking the tube-sheet  18  and removing it in pieces. The membranes  20  can be cut through port  16  to allow the pieces of tube-sheet  18  to be removed. Alternatively, the recesses  36  may be cut in a spiral thread form. This allows the tube-sheet  18  to be screwed out of the housing  10  to re-use the housing  10  after the membranes  20  have exceeded their service life. 
         [0020]    The membranes  20  are potted as part of a process of casting the tube-sheet  18  in the flange  14 . Referring to  FIG. 5 , one or more bundles of membranes  20  are placed in the housing  10  with their ends protruding from the end of the housing  10 . A potting fixture  40  is attached to the end of the housing  10  and fills a notch in the end of the flange  14 . The membranes  20  are typically inserted into the housing  10  before the potting fixture  40  is added, although at one end of the housing  10  the order of these steps can be reversed. The potting density of the membranes  20  is typically large enough, for example 45-55% to prevent the membranes  20  from moving excessively during potting. 
         [0021]    The potting fixture  40  has one or more ports  44  that admit one or more nozzles  46 . Nozzle  46  is used to inject liquid potting material  48 , for example polyester resin, into the flange  14  and potting fixture  40 . After the potting material  48  has been injected, nozzle  46  may be removed from port  44  and replaced by a plug (not shown). Alternately, nozzle  46  may be left in potting material  48 . The potting material  48  flows around the ends of the membrane  20  and into the recesses  36  and then is allowed to solidify in situ. The potting fixture  40  may be wrapped with a coil of tubing or covered by a jacket to allow a heated or cooled liquid to be circulated around it to control the temperature profile of the potting material  48  as it solidifies. If the potting material  48  would also flow into the ends of the membranes  20  up to the height of the bottom of the flange  14 , the ends of the membranes  20  may be sealed before the membranes  20  are placed in the housing  10 . After the potting material  48  has at least partially solidified, the potting fixture  40  is removed. The block of potting material  48  is later cut along cut line  50  to open the ends of the membranes  20 . The assembly may be shipped at this stage, with the membranes  20  protected inside the housing  10 , or after the gaskets  24 ,  26  and cap  30  are added. 
         [0022]    If a module is being made for dead end flow only, the other ends of the membranes  20  may be sealed and the other end of the housing  10  capped or otherwise closed. Alternately, as shown in  FIG. 6 , a module  60  can be made with the membranes  20  potted with their ends open at both ends of the housing  10 . In that case, the steps described above are also performed to pot the membranes  20  and form a tube-sheet  18  at the other end of the module  60 . If the steps above were performed statically, with the housing  10  vertically and the potting material  48  flowing by gravity, then the housing is inverted after a first tube-sheet  18  is formed to allow the second tube-sheet  18  to be formed. In the module  60  shown, the membranes  20  are integrally skinned poyimide membranes as described in US Publication No. 2006/0117955 and available from Vaperma Gas Separation Solutions under the trademark SIFTEK. These membranes have a sufficiently high Tg to allow the potting material  48 , epoxy resin, to be used at 70-100 C. At this temperature, viscosity of the potting material  48  is low enough to allow the potting material to penetrate into the bundle of membranes  20  under the force of gravity and surface attractions alone. Alternately, if a more viscous potting material  48  will be used, or if it is desirable to form both tube-sheets  18  simultaneously, the tube-sheets  18  may be formed in a centrifuge. 
         [0023]    Still referring to  FIG. 6 , a second cap  62  at the other end of the module  60  may optionally have a different configuration from the cap  30 . Several modules  60  may be joined together to form a rack  64 . The caps  30  may be attached to or be part of a feed manifold  60 . Second caps  62  may be attached to or be part of a retentate header  68 . Ports  16  may be connected to a permeate header  70 . All of these components may be mounted to a frame  72 . A bellows or other means may be used in one or more locations, for example adjacent the ports  16 , to allow for manufacturing tolerances. For other applications, different arrangements of modules  60  and piping may be made. 
         [0024]    In one example, a module  60  has a length of about 180 cm and a diameter of about 25 cm. The module  60  contains about 28,000 hollow fibre membranes  20 . The membranes are integrally skinned polyimide membranes as described above, formed for inside out permeation. The membranes  20  have an outside diameter in the range of about 0.5 to 1.5 mm, for example about 1.0 mm, an inside diameter of about 0.2 to 1.0 mm, for example about 0.7 mm, and an OD/ID ration of between about 1.3 to 2.0. The membranes  20  are potted at both ends and about 20 modules  60  are provided in a rack  64  as shown in  FIG. 6 . The modules  60  are used by feeding a vapour mixture, for example vapours including water extracted from a fermentation broth, at high temperatures and under pressure to the feed manifold  60 . The vapour mixture flows through the lumens of the membranes  20  and out to the retentate header  68 . Water vapour is preferentially permeated through the walls of the membranes  20  via the driving force of a vacuum applied to the permeate header  70 . The retentate/product is a dehydrated vapour mixture. Under appropriate process conditions, and optionally with multiple stages of modules  60 , the retentate may be sufficiently dehydrated to be condensed and used as a fuel or fuel supplement.