BACKGROUND AND SUMMARY OF THE INVENTION Permeable membranes capable of selectively permeating components of a fluid mixture are considered in the art as a convenient, potentially highly advantageous means for achieving desirable fluid separations. Various types of permeable membranes have been proposed in the art. For example, U. S. 4,230,463 (Henis et al.), the complete disclosure of which is hereby incorporated by reference herein. discloses certain multicomponent membranes for separating at least one gas from gaseous mixtures by permeation in which the multicomponent membranes are comprised of a coating in occluding contact with a porous separation membrane. Also, U. S. (Nemser et al.) the complete disclosure of which is hereby incorporated by reference herein, discloses a selectively gas permeable membrane for the separation or enrichment of gaseous mixtures which membrane is formed from amorphous polymers of perfluoro-2,2-dimethyl-1, 3-dioxole. These membranes can be used for separation or enrichment of diverse gas mixtures including the oxygen enrichment of air and the separation or enrichment of gaseous organic compounds in admixture with air.

Selectively gas permeable membranes can be used for carrying out a variety of fluid separation operations having great practical and commercial importance. For example, U. S. 5,051,113 (Nemser) discloses the use of a selectively gas permeable membrane to provide oxygen enriched air for a mobile combustion engine and U. S. 5,053,059 (Nemser) discloses the use of similar membranes to provide oxygen enriched air to the air intake of residential furnaces.

In basic terms, conventional membrane separation or enrichment of gas mixtures usually are carried out in a vessel the interior of which is divided into two segregated

chambers by a selectively gas permeable membrane. The gas mixture is forced by a fan. blower or compressor through a feed port into the first chamber where it contacts one side of the membrane. Generally, the more preferentially permeable a component of the mixture, the faster that component permeates the membrane to enter the second chamber.

This produces in the second chamber a permeate composition enriched in preferentially permeable components and a retentate composition having reduced concentration of the preferentially permeable components in the first chamber. In continuous operation fresh gas mixture forced into the first chamber displaces the retentate composition which is allowed to leave the chamber through a retentate discharge port. It is customary to withdraw the permeate composition product from the permeate chamber. This withdrawal is typically accomplished by pulling the permeate from the permeate chamber with a vacuum pump.

Any conventional selectively gas permeable membrane system continuously operating at any given base set of conditions can produce a permeate composition enriched to a corresponding base concentration of the preferentially permeable components. It is often desirable to increase the concentration of the preferentially permeable components in the permeate composition. Heretofore, it was known to increase the enrichment by forcing more feed mixture into the system at a higher pressure. This technique normally calls for increasing the size or speed of the feed blower or compressor. The extent to which the gas mixture pressure can be increased may be limited by the maximum pressure that the membrane separation vessel or the membrane is designed to withstand. Use of more robust separation equipment may be needed to allow high pressure separation. Hence. power consumption and cost of operation increase.

U. S. 4,553,988 (Shimizu et al.) discloses a high temperature furnace which utilizes selectively permeable membranes to enrich the oxygen content of the air combusted with a gas source to increase the temperature of the furnace flame. Although the apparatus employs a suction fan to draw air through the oygen enriching membranes, the patent does not teach or suggest that placing the fan adjacent the outlet openings of the oxygen enriching apparatus increases the oxygen enrichment over that which would be attained by blowing air through the apparatus.

It has now been discovered that the enrichment of the permeate composition can be dramatically increased without forcing more feed through the system at higher than base

condition pressure. Surprisingly, it has been learned that the concentration of the preferentially permeable components will increase in the permeate composition when (a) the gas moving means, (e. g., a fan, blower or compressor) is removed to allow unrestricted entry of the feed gas mixture, and (b) a vacuum is drawn on the first chamber to withdraw the retentate gas mixture. Indeed enhanced enrichment can be accomplished by simply changing the connections between the gas moving means and the membrane separation unit of a conventional apparatus. The changes amounts to repositioning the fan, blower or compressor from feeding gas into the first chamber to drawing suction on the first chamber.

Accordingly, there is now provided a gas separation apparatus comprising a membrane separation unit comprising a gas containment vessel, a selectively gas permeable membrane defining within the vessel, a retentate chamber in fluid communication with one side of the membrane, the retentate chamber having a retentate discharge port, and a permeate chamber segregated from the retentate chamber and in fluid communication with the other side of the membrane, permeate gas moving means for continuously withdrawing gas from the permeate chamber, the permeate gas moving means having a suction port in fluid communication with the permeate chamber, and retentate gas moving means for continuously withdrawing gas from the retentate chamber, the retentate gas moving means having a suction port in fluid communication with the retentate discharge port.

The present invention also provides a method of producing an enriched gas composition from a gas mixture comprising the steps of providing a membrane separation unit comprising a gas containment vessel and a selectively gas permeable membrane within the vessel defining a permeate chamber in fluid communication with one side of the membrane and a retentate chamber in fluid communication with the opposite side of the membrane ; drawing a reduced pressure on the permeate chamber while continuously admitting unpressurized gas mixture into the retentate chamber, thereby

causing a preferentially permeable component of the gas mixture to permeate through the membrane at rates faster than other components, drawing a reduced pressure on the retentate cavity while continuously drawing reduced pressure on the permeate cavity, thereby increasing the concentration of the preferentially permeable component of the enriched gas composition in the permeate chamber.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a conventional membrane separation system operated in the"push pull"configuration.

Fig. 2 is a schematic diagram of a membrane separation system operated in the"pull pull"configuration according to the present invention.

Fig. 3 is a schematic diagram of a pull pull membrane separation system utilizing a hollow fiber membrane module.

DETAILED DESCRIPTION In simplest terms, the present invention involves the discovery that a significant increase in the concentration of preferentially permeable components of a gas mixture can be obtained using a selectively gas permeable membrane by drawing the retentate gas mixture by suction from the membrane separation unit instead of forcing feed gas mixture into the unit under pressure. Fig. 1 schematically shows the basic elements of a conventional membrane separation system. A membrane separation unit 10 has a gas containment vessel 2 within which a selectively gas permeable membrane 4 segregates two chambers, namely retentate chamber 6 and permeate chamber 8. A gas mixture to be separated or enriched 1 is forced into the retentate chamber 6 with a blower 3. The gas mixture contacts one side of the selectively gas permeable membrane 4 which is constructed of a material selected for capability to selectively permeate components of the gas mixture. Preferentially permeable components of the mixture permeate faster than less preferentially permeable components. Consequently, the concentration of preferentially permeable components increases in the permeate chamber 8 where the permeate gas 9 is withdrawn by vacuum pump 12 to be consumed by a process which

utilizes the composition of concentrated preferentially permeable components. Retentate gas 7 is displaced from the retentate chamber 6 by freshly incoming gas mixture stream 5.

Fig. I illustrates a so-called"push pull"membrane separation process. This name is derived from the characteristic feature that the feed gas mixture 5 is pushed into retentate chamber 6 under pressure while the permeate gas 9 is pulled from the permeate chamber 8 under vacuum. In comparison, Fig. 2 schematically shows so-called"pull pull"a membrane separation process according to this invention. Blower 3 (Fig. 1) has been removed to provide admission of unpressurized gas mixture 15 into retentate chamber 16.

Also, a gas moving device 13 has been installed to withdraw retentate gas 17 from the retentate chamber under vacuum.

It has been discovered that the configuration of a pull pull membrane separation process as shown in Fig. 2 can achieve a significant increase in the concentration of preferentially permeable components in the permeate gas relative to a push pull process (Fig. 1) while utilizing the same membrane separation unit and retentate vacuum pump 12.

An increased concentration also can be obtained when blower 3 is employed as the gas moving device 13. Consequently, a conventional push pull apparatus can be converted to the pull pull configuration by disconnecting the discharge of gas moving device 3 from retentate chamber feed port 11 and connecting the inlet of device 3 to retentate discharge port 14. Transfer line 5 should be connected directly to unpressurized gas mixture source 1.

The membrane separation unit utilized in the pull pull process according to this invention should have a selectively gas permeable membrane positioned in a gas containment vessel so as to define within the vessel a retentate chamber in fluid communication with one side of the membrane and a permeate chamber segregated from the retentate chamber and in fluid communication with the other side of the membrane.

The retentate chamber preferably should have an inlet port for introduction of the gas mixture to be enriched and a separate retentate discharge port through which the retentate gas is withdrawn by suction of a gas moving device. To increase efficiency of separation, the inlet port and retentate discharge port should be positioned to draw the gas mixture over the membrane surface. This should maximize the intimacy of contact between the gas mixture and the membrane. The gas mixture supplied to the separation unit should be unpressurized, that is, at only slightly higher pressure than the pressure in the retentate

chamber, for example, less than about 10% higher than the retentate chamber pressure.

The pressure drop between the unpressurized gas and the retentate chamber is controlled largely by the size of the orifice of the inlet port. The size of this orifice should be small enough that the mixture of gas through the inlet port is at a substantial velocity. The permeate chamber can have multiple discharge ports although one is preferred. The ports of the membrane separation unit should be adapted to mate in gas tight connection with conduits, such as tubes and pipes, which lead to suction ports of gas moving devices.

Selectively gas permeable membranes well known in the art can be used in this invention. The membrane can be an unsupported monolithic, nonporous, selectively gas permeable membrane composition. Preferably, a multilayer composite of a nonporous selectively permeable layer supported on a porous or microporous substrate layer is utilized. The shape of the membrane can be in sheet form. The sheet can be deployed as a flat sheet. or the sheet can be pleated or rolled into a spiral to increase the surface to volume ratio of the separation unit. The membrane can also be in tube or tube ribbon form. Tube ribbons are disclosed in U. S. Patent No. 5,565,166 which is incorporated herein by reference.

In a preferred embodiment, the structure of the membrane composite includes a microporous hollow fiber substrate which is coated on at. least one of its inner surface or outer surface with a nonporous, gas selectively permeable membrane composition. The fiber outer and inner diameter generally are about 0.1-1 mm and about 0.05-0.8 mm. respectively. A membrane unit of this type preferably will have a plurality of such coated hollow fiber membrane elements assembled in a bundle within a case. occasionally referred to herein as a module. A hollow fiber membrane module can be constructed so that the ends of the bundle of hollow fibers are potted using well known technique such as embedding the fibers in a mass of cured polymeric material. The potted ends can be cut in a direction perpendicular to the fiber axes to form tube sheets. The fibers can be coated with a nonporous, gas selectively permeable composition before or after potting and cutting. Modules containing multiple uncoated hollow fibers are commercially available from such manufacturers as Spectrum, Inc. and Celgard, LLC. A method of making coated hollow fiber membrane modules is disclosed in U. S. Patent Application Ser. No.

08/862,944 filed May 30,1997, the disclosure of which is incorporated herein by reference.

Figure 3 shows a schematic diagram of a hollow fiber membrane module 30 deployed in the pull pull configuration of this invention. Unpressurized inlet gas 31 is drawn through inlet port 35 into first plenum 32 upstream of a tube sheet 40 formed by the upstream potted ends of hollow fiber module 36. The downstream potted fiber ends form tube sheet 41 which together with the end of the module defines a second plenum 42. The volume within the first plenum, the hollow fibers and the second plenum thus collectively forms a tube side cavity. In the illustrated embodiment, the tube side cavity is utilized as the retentate chamber in view that tube side gas is withdrawn through via transfer line 38 through retentate discharge port 33 into the suction port of fan 43. The volume of the module outside the hollow fibers and inside the shell of the module, i. e., the space surrounding the fibers, defines a shell side cavity. As illustrated, the shell side cavity, operated as the permeate chamber, has two outlet ports, 34 and 39 proximate to the retentate discharge and inlet ends, respectively. In general, discharge ports can be placed at any location in the shell side cavity. Preferably, permeate gas is withdrawn from module 30 via transfer line 37 through port 39 while port 34 is kept closed. This establishes a countercurrent flow pattern between permeate and retentate streams inside the module that is often useful for increasing separation/enrichment efficiency.

Optionally. transfer line 44 (as shown by dashed lines) can be used in tandem with line 37 or alone to withdraw permeate through fan 45.

The type of gas moving devices utilized is not critical. For example, a fan, blower, compressor or vacuum pump can be used. The gas moving devices should have suction ports which should be connected to the discharge ports of the permeate and retentate chambers so as to withdraw the gases from these chambers, that is, by pulling the gases through the membrane separation unit. Representative types of gas moving devices suitable for use include axial flow fans, centrifugal fans, such as straight blade, forward curved blade and backward curved blade fans, and vacuum pumps, such as rotary vane pumps, piston compressors, diaphragm pumps, liquid seal vacuum pumps, linear pumps and rocking arm piston pumps. Centrifugal fans and rocking arm piston vacuum pumps are preferred due to high efficiency and low cost. The retentate gas moving device should be capable of producing flow at a rate at least about twice the permeate flow rate, and more preferably, between about 5 and 10 times the permeate flow rate.

The beneficial results of this invention can be obtained with membrane compositions of many types, provided that the component to be enriched from the gas mixture is preferentially permeable through the selectively permeable membrane composition.

Typical polymers suitable for the selectively gas permeable membrane according to this invention include synthetic rubber, natural rubber, poly (siloxane), polysilazane, polyurethane, poly (epichlorohydrin), polyamine, polyimine, polyamide, acrylonitrile-containing copolymers such as poly (alpha-chloroacrylonitrile) copolymers, polyester (including polylactam and polyacrylate), cellulosic polymer, polysulfone, polypyrrolidone, polyolefin, such as polyethylene, polypropylene, polybutadiene, poly (2. 3-dichlorobutadiene), polystyrene including polystyrene copolymers such as styrene-butadiene copolymer, polyvinyl, such as polyvinyl alcohol, polyvinyl aldehyde, polyvinyl butyral and polyvinyl halides, such as polyvinyl chloride, and fluorine-containing polymers. Preferred polymers include polyperfluorosulfonic acid. polysulfone, ethyl cellulose, silicone rubber, polycarbonate, poly (4-methylpentene-1), poly (l-trimethylsilyl-l propyne), poly (phenylene oxide), polyimide, poly (dimethylsiloxane), poly and an amorphous copolymer of the perfluorinated dioxole monomer perfluoro-2,2-dimethyl-1,3-dioxole ("PDD") and a complementary amount of at least one fluorine containing monomer.

In some particularly preferred embodiments, the copolymer is copolymerized PDD and at least one monomer selected from the group consisting of tetrafluoroethylene ("TFE"), perfluoromethyl vinyl ether, vinylidene fluoride and chlorotrifluoroethylene. In other preferred embodiments, the copolymer is a dipolymer of PDD and a complementary amount of TFE, especially such a polymer containing 50-95 mole % of PDD. Examples of dipolymers are described in further detail in U. S. Patents Nos. 4,754,009 of E. N.

Squire, which issued on June 28,1988 ; and 4,530,569 of E. N. Squire, which issued on July 23,1985. Perfluorinated dioxole monomers are disclosed in U. S. Patent No.

4,565,855 of B. C. Anderson, D. C. England and P. R. Resnick, which issued January 21, 1986. The disclosures of all of these U. S. patents are hereby incorporated herein by reference.

The amorphous copolymer can be characterized by its glass transition temperature ("Tg"). The polymer property of glass transition temperature is well understood in the art.

It is the temperature at which the copolymer changes from a brittle, vitreous or glassy state

to a rubbery or plastic state. The glass transition temperature of the amorphous copolymer will depend on the composition of the specific copolymer of the membrane, especially the amount of TFE or other comonomer that may be present. Examples of Tg are shown in FIG. 1 of the aforementioned U. S. Patent No. 4,754,009 of E. N. Squire as ranging from about 260°C for dipolymers with 15% tetrafluoroethylene comonomer down to less than 100°C for the dipolymers containing at least 60 mole % tetrafluoroethylene. It can be readily appreciated that perfluoro-2,2-dimethyl- 1, 3-dioxole copolymers according to this invention can be tailored to provide sufficiently high Tg that a membrane of such composition can withstand exposure to steam temperatures. Hence, membranes of this invention can be made steam sterilizable and thereby suitable for various uses requiring sterile materials, especially those involving biological materials. Preferably, the glass transition temperature of the amorphous copolymer should be at least 115°C.

The material useful for the substrate can be any solid natural or synthetic substance well known for this purpose. Often a polymeric substrate is desirable. Examples of polymers which can be used include polysulfones, polystyrenes, polycarbonates, cellulosic polymers, polyamide polyimides, polyarylene oxides, polyurethanes, polyesters, polysulfides, polyolefins, polyvinyls, and the like. Further examples include, styrene-butadiene copolymer, cellulose acetate-butryate polymer, polyphenylene oxide, polyethylene terephthalate, polyalkyl methacrylate, polyalkylacrylate, polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene and polyvinylidene fluoride.

Expanded polytetrafluoroethylene, sometimes referred to as ePTFE and polysulfone are particularly preferred.

Highly selective gas permeation can be employed to enhance the enrichment of the preferentially permeable gas components of many gas mixture systems. The gases amenable to membrane separation according to this invention typically include elemental gases, such as argon, neon, oxygen, ozone and nitrogen; hydrocarbons, such as methane, ethane and propane; halocarbons; halohydrocarbons; and others such as, oxides of nitrogen; carbon dioxide; hydrogen sulfide; ammonia; sulfur dioxide; carbon monoxide; phosgene and any mixture of any of them. This invention is ideally suited to and is particularly useful for producing oxygen enriched air from ambient air. Compared to the oxygen enrichment relative to ambient air produced by traditional push pull methods of

membrane separation, the pull pull method preferably can increase the oxygen enrichment by at least about 5%, more preferably by about 10% and most preferably about 20%.

The ability of the novel pull pull method to achieve superior enrichment of preferentially permeable gas components is particularly useful for increasing the effectiveness of membrane compositions that have moderate separation factors, for example, oxygen: nitrogen selectivity between about 1.5 and 3.5. PDD copolymers have oxygen: nitrogen selectivity of about 2.0-2.6 and generally high oxygen permeability (i. e., greater than about 300 barrers). One barrer equals 1x10-'° cm'-cm/cm'-cmHg-sec. Thus pull pull enables high permeability but moderately selective membrane materials to produce higher purity products such as more oxygen enriched air than would be possible to obtain by conventional push pull methods.

This invention is now illustrated by examples of certain representative embodiments thereof. wherein all parts, proportions and percentages are by weight unless otherwise indicated. All units of weight and measure not originally obtained in SI units have been converted to SI units.

EXAMPLES Example I Comparative Example 1 A gas separation system configured as shown in Fig. 1 was set up using a Model 3032-101-G609X Gast Rotary Vane Blower (Gast Corp., Benton Harbor, Michigan) to blow ambient air into one end of the tube side of a model CMS-3-2440-750-I hollow fiber membrane module (Compact Membrane Systems, Inc., Wilmington, Delaware). The module had fibers of 750 nm diameter provided a total 2,440 cm2 of membrane surface area. The fibers were coated with a nonporous layer of a dipolymer of 65 mole % and 35 mole % tetrafluoroethylene that gave an oxygen/nitrogen selectivity of 2.47 and an oxygen flux rating of 1075 GPU. Retentate air was discharged to atmosphere from the opposite end of the membrane module tube side port. The suction port of a model 2641-CE-564C Thomas Twin Head Vacuum Pump (Thomas Industries, Inc., Sheboygan, Wisconsin) was connected to the single shell side (i. e.. permeate chamber) port of the module. Oxygen enriched permeate was discharged to atmosphere. The system was operated in continuous mode and the data shown for Comp.

Ex. 1 in Table I was collected.

Table I Retentate Permeate Permeate Permeate Config-Feed Flow Flow Pressure Flow Oxygen uration (L/min.) (L/min.) (inches Hg) (L. min.) (vol. %) Comp. Ex. 1 push pull 75.3 69. 3 27. 0 6.0 33.8 Ex. I pull pull 80.3 75.3 27.0 5.0 36.8 Comp. Ex. 2 push pull 75.3 66.3 26.0 9.0 35.1 Ex. 2 pull pull 83.7 75. 3 25. 9 8. 4 35.9 The identical apparatus components were reconfigured according to the flow scheme of Fig. 2. Gas separation in the pull pull configuration was operated continuously and the data shown for Ex. 1 in Table I was collected. The pressure in the retentate chamber at the discharge end of the module was about 2 inches of water below atmospheric pressure.

Conversion to pull pull operation increased the oxygen concentration in the enriched permeate stream from 33.8 to 36.8 vol. %. These oxygen concentrations represent enrichment increases relative to 21 vol. % of oxygen in ambient air of 12.8 and 15.8 vol.

%, respectively. Thus pull pull provides a 23.4% improvement in oxygen enrichment over conventional push pull technology.

Example 2 Comparative Example 2 The procedure of Example 1 and Comparative Example 2 was repeated with the same blower and vacuum pump and a model CMS-3-2800-800-B hollow fiber membrane module (2800 cm-surface area, 800 um diameter fibers, same nonporous membrane layer composition as above, 1205 GPU oxygen flux rating). Data are presented in Table I. The data show that pull pull increased the oxygen concentration in the permeate from 35. 1 to 35.9 vol. %. As calculated in the previous examples, the enrichment increment above 21 vol. % was raised from 14.1 to 14.9 vol. % by pull pull which corresponds to 5.7% improvement.

Although specific forms of the invention have been selected for illustration in the drawings and examples, and the preceding description is drawn in specific terms for the purpose of describing these forms of the invention, this description is not intended to limit the scope of the invention which is defined in the claims.