Abstract:
The present invention is directed to a contactor for degrassing a liquid. The contractor includes a perforated core and a microporous membrane fabric wrapped around the core. The fabric includes a polymethyl pentene hollow fiber as a weft fiber and a warp yarn. A tube sheet secures the ends of the wound fiber and a shell encases the tube sheet and fabric. The shell has at least one opening to permit fluid flow through the shell and an end cap. In a further embodiment the invention is directed to a contactor for degrassing a liquid wherein the contactor is adapted to withstand pressures greater than 0.4 MPa and temperatures greater than 50° C.

Description:
FIELD OF THE INVENTION 
     This invention is directed to a hollow fiber membrane contactor. 
     BACKGROUND OF THE INVENTION 
     Hollow fiber membrane contactors are known. For example, see U.S. Pat. Nos. 3,288,877; 3,755,034; 4,220,535; 4,664,681; 4,940,617; 5,186,832; 5,264,171; 5,284,584; and 5,449,457, each is incorporated herein by reference. In general, such contactors utilize a thin walled membrane to separate, via diffusion, gaseous, solid or liquid components from a solution or colloidal mixture. Hollow fiber membrane diffusion contactors are commercially available under the name of LIQUI-CEL® from Celgard, Inc. of Charlotte, N.C. and under the name of SEPAREL® from Dianippon Ink and Chemicals of Tokyo, Japan (DIC). Such contactors have numerous uses, one being the degassing of fluids. 
     The SEPAREL® contactor comprises a shell surrounding a hollow fiber fabric that is wound around a perforated core. The SEPAREL® contactor uses a fabric made of polymethyl pentene (PMP) hollow fibers and polyester yarn. Hollow fibers made from PMP exhibits unique diffusion properties. See Japanese Kokai 2-102714 (published Apr. 16, 1990). Additionally, the SEPAREL® contactor operating parameters are limited to a maximum temperature of 50° C. and a maximum feed water pressure of 0.4 Mpa. See, Hollow Fiber Membrane Degassing Module—SEPAREL®, www.dic.co.jp. 
     Commercial PMP fabrics used in the manufacture of contactors utilize the PMP hollow fibers as the fill or weft and polyester yarns as the warp yarn. This fabric has a tendency to break if the fabric is wound under tension. One possible explanation for this weakness is the use of polyester warp yarn in the production of the fabric. Polyester is a relatively stiff material that does not bend and flex well. When a PMP fabric is wound around a mandrel the warp yarns absorb most of the applied load, thus fabrics using polyester warp yarns break and tear. Fabrics similar to those described in Japanese Kokai 2-102714 have been shown to break at essentially zero tension during winding. Some degree of winding tension is desirable to create a well-formed fiber bundle that fits properly within a contactor shell. 
     Another possible explanation for the tearing exhibited by such PMP fabrics is a failure to utilize properly spaced or sized warp yarn. For example, fabrics similar to those described in Japanese Kokai 2-102714 (which tear during winding) exhibit a maximum warp yarn count of approximately 5 lines (yarns) per inch. See JP 2-102714, Embodiment 3. 
     U.S. Pat. No. 4,911,846 discloses an artificial lung made with a hollow fiber cord fabric. Note, U.S. Pat. No. 4,911,846, FIGS. 11 and 12. The cord fabric comprises polyolefin hollow fibers (including PMP hollow fibers), as weft fibers and warp fibers (including polyesters, polyamides, polyimides, polyacrylonitriles, polypropylenes, polyarylates, polyvinyl alcohols, etc.). The warp yarns are preferably multifilament yarns of polyesters or polyamides having a yarn fineness of 10 to 150 deniers, more preferably 25 to 75 deniers. See U.S. Pat. No. 4,911,846 col. 6, lines 3-14. No information is provided regarding the spacing of the warp yarn or the makeup of non-polyester, non-polyamide warp yarns. 
     Accordingly, a need exists for an improved contactor preferably employing a fabric that is not susceptible to breakage and operable at higher temperatures and pressures than known PMP hollow fiber contactors. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a contactor for degassing a liquid comprising a perforated core and a microporous membrane fabric wrapped around the core. The fabric comprises a polymethyl pentene hollow fiber, as a weft fiber, and a polyolefin warp yarn. In preferred embodiments the fabric has a weft fiber count between 50 and 70 fibers per inch and a warp yarn count between 3 and 12 yarns per inch. A tube sheet secures the ends of the wound fabric and a shell encases the tube sheet and wound fabric. The shell has at least one opening to permit liquid flow through the shell and an end cap. 
     In a further embodiment, the invention is directed to a contactor for degassing a liquid wherein the contactor is adapted to withstand pressures greater than 0.4 MPa and temperatures greater than 50° C. The contactor according to this embodiment further comprises a shell and a microporous membrane fabric comprising a polymethyl pentene hollow fiber, as a weft fiber, and a warp yarn with fiber and yarn counts similar to those mentioned above. The fabric is preferably wrapped around a perforated core and situated inside the shell. The shell has at least one opening to permit the liquid flow through the shell. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 is a schematic illustration of a hollow fiber membrane contactor. 
     FIG. 2 is an illustration of the fabric according to the invention. 
     FIG. 3 is a schematic illustration of a second embodiment of the membrane contactor. 
     FIG. 4 is a schematic illustration of a third embodiment of the membrane contactor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein like numerals indicate like elements, there is shown in FIG. 1 an embodiment of a hollow fiber membrane contactor  10  according to the invention. The contactor  10  includes a core tube  12 . The core tube  12  has a plurality of perforations  14 . Hollow fibers  16  surround the core tube  12 . A shell  13  surrounds the fibers and the core tube  12 . Tube sheets  26  secure the lateral ends of fibers  16  to tube  12 . The lateral ends of shell  13  are closed with end caps  15 . 
     Referring to FIG. 1, a liquid  18  preferably enters the contactor  10  via a liquid inlet  20  of the core tube  12 . The liquid  18  travels through the inlet  20  of the core tube  12  and exits the core tube  12  via perforations  14  when a block  22  diverts the liquid. The liquid  18  then travels over the exterior surfaces of the hollow fibers  16 . The liquid  18  re-enters the core tube  12  via perforations  14  on the other side of the block  22  and exits the core tube  12  via a liquid outlet  24 . The hollow fibers  16  surround the core tube  12  and are maintained generally parallel to core tube&#39;s  12  axis via tube sheets  26 . The hollow fibers  16  extend through the tube sheet  26  and are in communication with headspaces  28  on either end of contactor  10 , so that a vacuum  30  drawn at ports  32  and  34  is in communication with the lumen side of hollow fibers  16  via headspaces  28 . Port  34 , for example, may also be used to introduce a sweep gas, which facilitates entrained gas removal. 
     The membrane contactor  10  is preferably an external flow, hollow fiber membrane module. The membrane contactor  10  has a lumen side and a shell side. The lumen side, also known as the internal side, is defined, in large part, by the lumen of the hollow fiber. The shell side, also known as the external side, is defined, in part, by the external surface of the hollow fiber. The liquid travels through the shell (or external) side, while the vacuum (or vacuum and sweep gas) is applied to the lumen (or internal) side. Thereby, entrained gases from the liquid pass, via diffusion, from the shell side through the membrane to the lumen side. 
     Preferably, the hollow fibers  16  are semi-permeable, gas selective, heterogeneous, integrally asymmetric, and liquid impermeable membranes. The membrane is, preferably, a single layer membrane (e.g., not a composite or multi-layered membrane) and is made from a homopolymer of PMP. The membrane is, preferably, a skinned membrane and the skin is on the shell side. The membrane has a permeability of less than 100 Barrers (10 −8  standard cm 3 .cm/sec.cm 2 .cm (Hg)). For example, see U.S. Pat. No. 4,664,681, incorporated herein by reference. The total membrane in the contactor preferably has an active surface area greater than 0.05 m 2  and most preferably between 0.1 m 2  to 350 m 2 . 
     Referring now to FIG. 2, the hollow fibers  16  are preferably made into a fabric  36  having a fill or weft yarn  38  and a warp yarn  40 . Preferably the fabric is a weft insertion knitted fabric where the warp yarn is the knitting yarn. The weft yarn  38  is the hollow fiber  16 . The fabric  36  preferably has a weft fiber count between 50 and 70 fibers per inch and most preferably between 60 and 65 fibers per inch. 
     The warp yarn  40  should be flexible, yet strong, and inert to the liquid flowing through the contactor. The warp yarns  40  are preferably multifilament polyolefin yarns. Most preferably the yarns are selected from the group consisting of polypropylene and polyethylene. Those skilled in the art recognize that the term filament is sometimes used synonymously with cut filament which is also called staple fiber. Accordingly, as used herein the term yarn should be interpreted to include yarns made from filament and staple fiber. Preferably, the yarn possesses a fineness sufficient to resist tearing but not too large as to cause noticeable gaps between fabric layers. Preferably the warp yarn should be between 80 denier/40 filament (i.e., a 80/40 yarn) and 20 denier/10 filament (a 20/10 yarn), most preferably around 40 denier/20 filament (a 40/20 yarn). Optionally, the warp yarn may include a surface finish, e.g. a silicon oil surface finish. 
     The count of the warp yarn is also an important factor in the design of the fabric. Too few warp yarns and the fabric will be susceptible to tearing. Too many will diminish the efficiency of the contactor by blocking surface area of the hollow fibers. In preferred embodiments the warp yarn count is between about 3 and 12 yarns per inch of fabric; most preferably around 6 to 7 yarns per inch. 
     When wound, the fabric  36  and the core tube  12  form a hollow membrane unit  42 . Unit  42  is preferably cylindrical. In use, it is expected that the unit  42  will have a diameter ranging between about 2 in. and 16 in. and a length ranging between about 8 in. and 72 in. Larger sizes are possible. The aspect ratio of the unit  42  is defined as L/D 2  where L is the nominal length of the unit and D is the nominal diameter of the unit. Preferably, the aspect ratio will range between 0.1 to 6.0 in. −1 . 
     Furthermore, the fabric  36  is preferably wound under tension to create a unit  42  having a packing fraction of between about 35% to 45%. Packing fraction (PF) is defined as the number of fibers (n) multiplied by the cross-sectional area of each fiber (A f ) divided by the cross-sectional area of the fiber bundle (A b ) where the cross-sectional area of the fiber bundle excludes the area occupied by the core tube  12 . Stated symbolically,        PF   =       n   *     A   f         A   b                              
     Additionally, PMP hollow fibers have a natural tendency to shrink which increases with temperature. Accordingly, in preferred embodiments of the invention, the PMP fabric  36  is preshrunk prior to winding. A preferred method of preshrinking and stabilizing the fabric is to heat the fabric to about 15° C. above the expected operating temperature for approximately 2 to 8 hours, preferably 4 hours. Heating the fabric between about 55° C. and about 65° C. for about 2 to 8 hours, preferably 4 hours, should provide adequate fiber stabilization for most anticipated applications. Preshrinking the fabric and winding the fabric under tension aids in achieving a well-formed bundle that contributes to the higher operating parameters (e.g., temperature and pressure) achieved by the invention. 
     Hollow fiber membrane units  42  formed according to the invention may be combined with other structural elements to create a contactor. Such structural elements are well known in the art and generally consist of an outer shell with at least one opening in the shell to permit fluid flow through the shell. Commonly assigned U.S. patent application Ser. No. 09/816,730, filed Mar. 22, 2001, incorporated herein by reference, discloses several possible structures for contactors, all of which are applicable to the present invention. 
     Referring to FIG. 3, contactor  10 ′ is the same as shown in FIG. 2 but for a flow diverting baffle  50  located within the shell side, and port  34  has been moved. The baffle  50  is added to promote distribution of liquid over all exterior surfaces of the hollow fibers  16 . Port  34  is moved to illustrate the non-criticality of port location. 
     Referring to FIG. 4, contactor  10 ″ differs from contactors  10  and  10 ′ by moving liquid outlet  24  from the terminal end of core tube  12  to the contactor shell, as illustrated. Vacuum  30  is in communication with headspace  28  which, in turn, is in communication with the lumens of hollow fibers  16 . The second headspace illustrated in the previous embodiments has been eliminated. Liquid  18  enters the liquid inlet  20  of the core tube  12 . Liquid  18  exits the tube  12  via perforations  14 , travels over the exterior surfaces of the hollow fibers  16 , and exits the shell side via an outlet  24 . The outlet designated  24  may be placed at other locations on the exterior of the contactor so that it maintains communication with the shell side. 
     The contactor according to the invention may be formed using any of the methods known by those skilled in the art. One such method is set forth in commonly assigned U.S. patent application Ser. No. 09/851,242, filed May 8, 2001. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.