METHOD AND APPARATUS FOR GAS SEPARATION

Abstract: Feed gas is directed along the non-skin surface of gas separation membrane modules (21) comprising a cylindrical bundle of parallel contiguous hollow fibers (11) supported to allow feed gas to flow from an inlet (22) at one end of a cylindrical housing (21) through the bores (12) of the bundled fibers (11) to an outlet (24) at the other end while a component of the feed gas permeates through the fibers (11), each having the skin side (13) on the outside, through a permeate outlet (26) in the cylindrical casing (21).

Description: The present invention relates in general to gas separating and more particularly concerns novel apparatus and techniques for advantageously using membrane systems for separation through an especially convenient approach that facilitates using a less costly membrane cartridge structure to attain a given degree of separation. A typical membrane system comprises a hollow tubular membrane having a porous side inside and a membrane separating skin on the outside. A problem with prior art membrane systems for liquid/solute separations, such as reverse osomosis (RO) or ultrafiltration (UF), results from a phenomenon known as "concentration polarization" which results in an increased concentration of the species to be separated at the separating layer (membrane "skin"). This increase in concentration tends to reduce the permeation rate of permeable species across the membrane and to reduce the degree of separation possible. To reduce "concentration polarization" in liquid/solute systems, it has been essential to sweep the feed stream tangentially past the separating membrane layer at sufficient velocities to induce mass transfer of retained component away from the membrane skin and into the bulk feed stream. Introducing feed streams from the side opposite the skin side aggravates concentration polarization by forming an additional, essentially stagnant, layer presented by the fluid inside the porous backing through which solute to be separate is transported predominantly by diffusion. This same mode of operation has been adopted in gas separation with membranes; that is, maintaining the feed gas stream on the "skin" side of the membrane to reduce polarisation. An example of this type of system is disclosed in US-A-4329157 and illustrated in FIG. 7 thereof. For hollow fibre membrane geometry, the skin side of the membrane may be either on the inside or the outside of the fibre. Technically, it is far easier and generally less expensive (and more common commercially) to cast a so-called asymmetric membrane with an integral skin on the outside of the fibre. Most, if not all, commercially available membrane gas separation systems with asymmetric or composite membranes flow the feed gas on the skin side of the membranes in an effort to prevent or reduce concentration polarization. For hollow fibres with outside skins, this approach requires flowing the feed gas on the outside of the fibre bundle as disclosed in the aforesaid patent. Uniform flow of the feed gas on the outside of the membrane fibres is more difficult to achieve compared with inside flow and results in sometimes elaborate schemes for prevention of dead spots and maintenance of good, even flow distribution. GB-A-2122103 discloses membranes consisting of a porous support membrane having a high polymer thin film formed on the surface by plasma polymerization. In a specific example the porous support membranes are hollow fibres or porous glass, and the high polymer thin film plasma polymerized on the porous support membranes is an organosilicon compound. It is an important object of this invention to provide improved apparatus and techniques for gas separation with membrane modules. According to the invention, a method of gas separation using asymmetric membranes with an integral skin, said asymmetric membranes having a skin side and a non-skin side, comprises the steps of directing a feed gas having a plurality of components along the non-skin side of each asymmetric membrane to cause a component of the feed gas to permeate through the asymmetric membrane; and withdrawing the permeated gas component. Apparatus according to the invention is as set out in claim 4. In a specific cocurrent flow arrangement of the apparatus, a hollow tubular membrane having its skin side on the outside circumference is open at both ends and is seated in a cylindrical housing having an opening at one end for receiving feed gas, an opening at the other end for delivering a gas component, and near the other end in the circumferential wall, a third opening for passing the permeated component that diffuses through the membrane. In a countercurrent flow arrangement, membranes may be confined to develop a flow of diffusate in a direction opposite the flow of the feed gas for withdrawal though an opening in the casing. The casing may be flexible to reduce weight or conform to an available shape and need not be a pressure vessel. Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which: FIG. 1 is a fragmentary view of a single hollow fibre with gas flow through the bore and the separating skin layer on the outside of the lumen; FIG. 2 is a view, partially in section, of a system according to the invention comprising a number of cocurrent parallel hollow fibres in a gas separating system, and FIG. 3 is a view, partially in section, of a countercurrent system according to the invention. With reference to the drawings and more particularly FIG. 1, there is shown a fragmentary view of a single hollow fibre with gas flow through the bore and a separating skin layer on the outside of the lumen. Fibre 11 has a wall surrounding the lumen 12 and a skin 13 on the circumferential surface of fibre 11. Feed gas having components to be separated enters lumen 12 in fibre 11 at end 14 as indicated by arrow 15. Permeate diffuses through the wall of tube 11 and its outside skin 13 to produce a permeate flow of a component separated from the feed gas transverse to the tube axis as indicated by arrows 16. Reject exists at end 17 of hollow fibre 11 along the direction indicated by arrow 18. It has been discovered surprisingly and in conflict with general expectations based on observations of prior art liquid/solute membrane separation systems that concentration polarization is not aggravated, and feed gas flow on the membrane side opposite the skin separating layer has no detrimental effect on system performance. This property is believed to result from the exceptionally high interdiffusion of gases at the separation barrier back into the bulk stream in gas separation via membranes and because the prevailing economical mass flux levels are generally significantly lower than in liquid separation systems. Referring to FIG. 2, there is shown a view partially in section of a gas separation membrane system according to the invention. A cylindrical housing 21 has an axial inlet 22 at one end for receiving feed gas flowing as indicated by arrow 23, an axial outlet 24 at the opposite end for discharging reject along the flow direction indicated by arrow 25 and a transverse outlet 26 for ejecting permeate along the transverse flow direction indicated by arrow 27. Housing 21 includes a cartridge of hollow fibers 11. Hollow fibers 11 are supported in exit and entrance headers 31 and 32 annularly sealed against the inside wall of housing 21 so that essentially the only fluid communication through the headers are through the bores of the hollow fibers 11. The cartridge of hollow fiber 11 may be encased in an impervious cover, such as a mylar film, anchored in header 32 and open opposite outlet 27 to promote cocurrent flow. This approach is structurally different from the prior art, such as exemplified in FIG. 7 of U. S. Patent No. 4,329,157, which expressly teaches that fibers are imbedded in the header at the feed end so as to block the feed end openings of the fibers. This blocking is essential to the prior art approach of having the feed gas flow along the outside skin of the fibers. With the prior art system, if the feed pressure is greater than atmospheric pressure, the fibers must be housed in a pressure vessel. With the present invention, a pressure vessel is not required, and the casing may be flexible to reduce weight or conform to available shape. Referring to FIG. 3, there is shown an alternative embodiment of the invention that is a countercurrent flow system. Corresponding elements are identified by the same reference symbols in FIGS. 2 and 3. The structure of the embodiment of FIG. 3 is similar to that of FIG. 2 except that hollow fibers 11 are encased in a impervious cover, such as mylar sleeve 33, that is sealed at the end against header 31 and open at the end facing header 32 so that permeate exits through outlet 26' along the direction of arrow 27'. The invention has a number of advantages. Cartridge design fabrication is relatively simple and a part of a less expensive system. When operating in a pressure mode, the casing need not be designed for as high a pressure as when directing the feed gas along the outside skin. The outside skin remains intact although in tension in response to the feed gas stream being directed along the inside of the tubes. Countercurrent flow is more readily achieved. Furthermore, the membrane need not be straight so that the reject could exit on the same side of the casing as the feed enters. To verify that it is practical to feed on the non-skin side; that is, on the inside of the fibers, experiments were conducted using air feed to cartridges at 10 psig and 10% conversion. The effective permeability data listed in the following table setting forth these results were temperature corrected to 77 o F. Thus, operation with the feed on the non-skin side through the bores of the fibers produces essentially identical results to operation with the feed on the skin side; that is, outside the fibers. And countercurrent flow provides better cartridge overall apparent selectivity than cocurrent flow. While the invention has been described in connection with tubes, it is within the principles of the invention to use membranes of other configurations. For example, plate membranes might be used with feed gas flow through opposed pairs of membranes between the non-skin side of each membrane in a pair. The feed-flow may be effected by having a pressure above atmospheric at the feed end or below atmospheric at the exit end, or combinations thereof. There has been described novel apparatus and techniques for gas separation using membrane modules characterized by numerous advantages, including ease of obtaining a given degree of separation with relatively inexpensive modules capable of operating satisfactorily over relatively long periods of time. It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiments described herein without departing from the inventive concepts.

Claims: A method of gas separation using asymmetric membranes with an integral skin, said asymmetric membranes having a skin side (13) and a non-skin side, comprising the steps of directing a feed gas having a plurality of components along the non-skin side of each asymmetric membrane (12) to cause a component of the feed gas to permeate through the asymmetric membrane (12); and withdrawing the permeated gas component. A method according to claim 1, wherein each asymmetric membrane (12) comprises a hollow fibre (11) having a central bore with the skin side (13) being on the outside of the fibre and the non-skin side being the bore side thereof, wherein the step of directing the feed gas includes directing the gas through the bore in the fibre. A method according to claim 2, wherein the asymmetric membranes include a plurality of contiguous hollow fibres (11), and the step of directing feed gas along the non-skin side includes directing the feed gas through the bores in the contiguous fibres (11). Apparatus for carrying out the method of claim 1, the apparatus comprising, casing means (21) for supporting a plurality of asymmetric membranes with an integral skin, said asymmetric membranes having a skin side and a non-skin side, and having an inlet (22) for receiving the feed gas, and at least first (26) and second (24) outlets for expelling the permeated component and a rejected component respectively, and means (31,32) for supporting the asymmetric membranes (11) arranged to present a single passage from the inlet (22) along the non-skin side to the second outlet (24) for the rejected component. Apparatus according to claim 4, wherein the asymmetric membranes comprise a plurality of contiguous hollow fibres (11) each having a bore (23), and wherein the supporting means comprises means (31,32) at each end of the plurality of fibres for supporting the fibres (11) in the casing (21) to allow feed gas to flow through the fibre bores (120) from the inlet (22) to the second outlet (24). Apparatus according to claim 4 or claim 5, further comprising means for establishing countercurrent flow of the permeated component relative to the flow of feed gas.

Inventors: GOLLAN, AYRE Z.
Applicants: A/G TECHNOLOGY CORPORATION
Application year: 1986
Main classification: B01D53
IPC codes: B01D69, B01D53