Surfactant treatment of aromatic polyimide gas separation membranes

Abstract: A process for improving the permselectivity of an aromatic polyimide gas separation membrane comprising contacting said membrane with a solution of a surfactant in a solvent which swells the membrane but in which the membrane material is soluble to an extent of no more than 3% under the conditions used to contact the membrane.

Description: FIELD OF THE INVENTION The present invention relates to a treatment for aromatic polyimide gas separation membranes to improve their permselectivity with respect to at least one pair of gases in a multicomponent gas mixture. The treatment involves applying a solution of a surfactant in a liquid which swells but is a non-solvent for the aromatic polyimide forming the permselective gas separation membrane. Said solution of a surfactant in the non-solvent liquid is capable of enhancing permselectivity to a greater degree than the liquid non-solvent alone. PRIOR ART US-A-4,472,175 discloses asymmetric gas separation membranes which have been treated on one or both surfaces with a Bronsted-Lowry acid to improve the permselectivity of the membrane for at least one gas of a gaseous mixture. US-A-4,554,076 discloses a method for improving the separatory properties of membranes by depositing on the surface of such membrane a fluorinated amphiphilic compound in an oriented Langmuir-Blodgett layer to increase membrane selectivity. The patent is primarily directed to liquid-liquid separations, but gases are mentioned. US-A-4,654,055 discloses asymmetric gas separation membranes which have been treated on one or both surfaces with a Bronsted-Lowry base to improve the permeation selectivity of the membrane for at least one gas of a gaseous mixture. EP-A- 0,179,474 discloses treating various polymeric gas separation membranes with a dilute solution of a cationic surfactant in a volatile non-polar organic solvent to improve the selectivity of the membrane with respect to separating two gases selected from CO 2 , CH 4 and He. The membrane may be formed of a polyamide. JP-A-89/123617 discloses a porous carrier in the form of a sheet or a hollow yarn coated with a thin membrane of polyorganosiloxane, etc., having excellent gas separability. A surfactant [RfCOONH 4 , RfCONH(CH 2 ) 3 (CH 2 CH 2 O) 2 CH 3 , etc.] having ≦30dyn/cm surface tension at the critical micelle concentration is preferably applied thereon to eliminate minute defects generated in the formation of the thin membrane, etc. By this method, the defects can be eliminated without substantially deteriorating the gas permeability. JP-A-89/123618 discloses a composite membrane having excellent gas permeability and gas separability by applying a solution of a non-ionic surfactant having a fluorocarbon chain-containing long-chain alkyl group and an amine oxide hydrophilic group on a porous supporting membrane, and drying the solution. The non-ionic surfactant having an amine oxide hydrophilic group and a fluorocarbon chain-containing a specified long-chain alkyl group is applied on the porous supporting membrane, etc., and dried. JP-A-85/64602 discloses a dried translucent membrane composed of a surfactant contained in a dried translucent membrane of a polymer selected from among the polyamide, polyimide and polysulfon groups. The surfactant used is at least of such a type as representing either of the anionic, cationic, amphoteric and non-ionic surfactant. In addition, the surfactant should preferably be of such a type as having a solubility of at least 2 weight parts against 100pts.wt. of solvent at room temperature. A preferable anionic surfactant is represented by the formula of R 1 A - M + (R is an aliphatic, and alicyclic or an aromatic hydrocarbon group, A - an anionic group selected from among -COO - , -OSO 3 - , - SO 3 - and PO 3 - , and M + a cation). EP-A-0241937 discloses a highly permeable aromatic polyimide gas separation membrane and the process of using it. The aromatic polyimide has the formula wherein -Ar-is or mixtures thereof, -X, -X 1 , -X 2 and -X 3 independently are alkyl groups having 1 to 6 carbon atoms or aromatic groups having 6 to 13 carbon atoms, and -Z is -H, -X, -X 1 , -X 2 , or -X 3 ; and R or mixtures thereof. After formation the membrane is dried and the dried membrane treated with Dow Corning SYLGARD-184 solution in FREON-113. EP-A-0244635 discloses a process for the increase in selectivity of assymetric membranes. The membrane which may be formed from a polyetherimide is contacted with a solvent capable of swelling but not dissolving the membrane and optionally comprising dissolved polyetherimide and the membrane is dried. EP-A-141793 discloses asymmetric gas separation membranes of polymeric materials, e.g. a polyimide, having selective permeation of at least one gas of a gaseous mixture over that of one or more remaining gases of the gaseous mixture. These exhibit improved permeation selectivities for the at least one gas when the asymmetric membranes are contacted on one or both surfaces with a solvent, the solvent having a solubility parameter similar to the solubility parameter of the polymeric materials comprising the membranes. The process for producing the improved membranes, and processes for utilizing such membranes for selectively separating at least one gas from a gaseous mixture by permeation are disclosed. EP-A-0286090 discloses post-treatment of composite membranes with a volatile solvent system, with or without minute amounts of additives therein, to repair defects in the thin separation layer thereof. The membrane is then dried by the evaporation of said solvent. Minute amounts of non-volatile additives remain in and plug imperfections in the separation layer thereby enhancing the separation, and the overall separation/permeability, characteristics of the composite membrane. SUMMARY OF THE INVENTION Gas separation membranes desirably have a high permeability to gases. This means that the effective portion of the membrane should be as thin as possible. Making the membranes as thin as possible necessarily leads to the membrane containing imperfections or defects. These defects pass gases indiscriminately, which reduces the selectivity of the membrane with respect to one gas from a mixture of gases preferentially permeating the membrane. The present invention relates to a process for apparently healing these defects to provide an aromatic polyimide membrane having improved permselectivity with respect to at least one gas in a gaseous mixture by the treatment as defined in Claim 1. DETAILED DESCRIPTION Most commercial aromatic polyimide gas separation membranes are asymmetric in nature. They are made by casting a film or extruding a hollow fiber from a solution of an aromatic polyimide or polyamic acid precursor in a solvent mixture, evaporating a portion of the solvent from one side of the film or the outside of the hollow fiber and quenching in a nonsolvent. In the case of a polyamic acid precursor it is then cyclized by a suitable treatment such as the application of heat. The resulting asymmetric membrane is characterized by a thin skin of polymer supported by a generally cellular structure. This provides a membrane having a thin effective separation member, which results in a high flux or permeation rate to gases, which is highly desirable. However, this effort to form a highly permeable membrane also leads to the formation of submicroscopic defects which pass gases indiscriminately causing the membrane to have an effective separation value for most pairs of gases which is less than the intrinsic separation value of the polymer from which the membrane is made. The selectivity of one gas over another in a multicomponent mixture by permeation through a gas separations membrane is controlled, in part, by the molecular free volume in the dense separating layer of the membrane. To obtain the intrinsic gas selectivity of a membrane, a perfect, defect-free, dense separating layer must be formed during the membrane formation process. The integrity of this separating layer must be maintained throughout the gas separations module to retain a high gas membrane selectivity. This idealized defect-free membrane separating layer could be prepared by increasing the thickness of the layer. In so doing, defects would not be able to protrude through the separating layer. However, this results in a reduction of the gas permeation rate through the membrane, which is not desirable. In the past, these membrane defects have been sealed to improve gas separations membrane performance. One approach has been to treat the membrane with a swelling agent. The swelling agent may comprise a single component or a multicomponent mixture, such as solvent-nonsolvent mixtures. It is believed that the outer layers of the swollen membrane compact upon drying to yield the observed improvement to the membrane selectivity. Recent examples of such processes include Makino in US-A-4,486,376; Makino in US-A-4,512,893; Wang in US-A-4,512,894; Lee in US-A-4,527,999; and Thompson in US-A-4,715,960. Membrane materials may vary greatly in their intrinsic parameters. What may constitute a swelling agent for one material may be a solvent or an inert substance to another material. Examples of this are shown in the before mentioned prior art. Further, certain swelling agents may be effective only for specific polymers or for certain types of defects. Another approach to healing such defects in gas separation membranes has been to superimpose two layers on top of each other. For example, Robb, in US-A-3,325,330, teaches that a multicomponent gas membrane prepared from two dense separating layers laminated together is unlikely to have defects because defects in one dense layer are probably not going to align with defects in the other. Browall et al, in US-A-3,874,986 and US-A-3,980,456, extend these teachings to include multicomponent gas membranes comprising a laminate between an asymmetric membrane and a dense film. These teachings have been further extended to include multicomponent gas membranes prepared by coating an asymmetric membrane with a solution which upon drying and curing forms a dense polymeric layer (US-A-4,230,463). The teachings of the prior art suffer from not taking into account long-term performance of gas membranes. Commercial gas separation membranes preferably retain high performance levels throughout long operational lifetimes of five years or greater. Gas membranes, however, undergo severe environmental stresses such as short membrane contacts with liquids and/or physical shocks during their operational lifetime. EP-A-0,107,636 teaches that the sealed multicomponent gas membrane compositions exemplified in US-A-4,230,463 suffer from catastrophic membrane performance losses when contacted with low molecular weight fluids, an expected environmental concern during typical gas separation membrane operation. It is believed that these performance declines are due to the adhesion losses between the asymmetric membrane and the sealing coating. The resulting delamination of the sealing coating and the asymmetric membrane reopens the defects in the dense separating layer of the asymmetric membrane. The prior art has not heretofore considered potential performance losses from environmental physical shocks to the gas membrane. Such environmental physical shocks can occur, for example, during the transport of gas membrane modules, installation of gas membrane modules, or as a result of uncontrolled transmembranegas-pressure cycling typical of gas membrane operation. Environmental physical shocks would also severely test the adhesion between the sealing treatment and the asymmetric membranes of the multicomponent gas membranes disclosed in the prior art. US-A-3,980,456 discloses the use of a preformed organopolysilaxane-polycarbonate copolymer sealing material. The polymeric sealing materials used previously cannot effectively intrude into pores to plug them and are, therefore, only effective by applying a thin coating on top of the membrane material. This causes a substantial loss in membrane productivity. Henis and Tripodi in US-A-4,230,463, teach that membrane sealing materials are only effective if their molecular size is large enough not to be drawn through the pores of the porous separation membrane during coating and/or separation operations (US-A-4,230,463, col. 19, line 25-32, line 51-56). Further, they teach that when poly(siloxanes) are used as the membrane sealing material, the polysiloxane molecular weight must be higher than about 1,000 (US-A-4,230,463; col. 20; line 23-27). EP-A-0,107,636 attempts to correct environmental degradation concerns, but suffers from applying a coating of polysiloxane diols with molecular weights greater than 10,000. The present invention circumvents the above shortcomings and provides high performance aromatic polyimide gas separation membranes which are resistant to severe environmental stresses. The intimate mechanistic details of this procedure are not well understood. They may vary for different material compositions. It is clear that the procedure reduces the effects that membrane defects and imperfections have on the gas-pair selectively. The synergistic effects of the surfactant and the swelling liquid serve effectively to seal a broad variety of aromatic polyimide membrane types which incorporate a wider distribution of pore and defect sizes than is found in the prior art. The membrane treated by the process of this invention does not suffer from the end-use environmentally-induced delamination of the prior art. The process of the present invention effects greater selectivity for gas separations membranes used among other things in recovery of hydrogen in refinery and ammonia plants; separation of carbon monoxide from hydrogen in syngas systems; separation of carbon dioxide or hydrogen sulfide from hydrocarbons; enrichment of oxygen and nitrogen from air for increased combustion or inerting streams, respectively. The present invention finds its greatest utility in effecting greater selectivity for gas membranes used for enrichments of oxygen and nitrogen from air for increased combustion or inserting streams, respectively. The process of the present invention entails contacting an aromatic polyimide gas separation membrane with a solution of an effective amount of a surfactant in a liquid which swells but is a non-solvent for the aromatic polyimide forming the membrane, followed by drying. The solution is incapable of dissolving more than 3 weight per cent of the membrane material under conditions of contacting the membrane. The gas separation membrane healing technology of the present invention is useful for membranes comprising essentially all aromatic polyimide membrane material composition. Further, this procedure will find utility for many aromatic polyimide membrane morphologies, such as asymmetric or composite membranes, particularly asymmetric. The preferred material composition of these asymmetric membranes are soluble aromatic polyimide materials. These polyimides may be prepared by typical procedures as taught in the prior art, such as US-E-30,351. More preferably, the aromatic polyimide compositions incorporate pendant aliphatic side chains. Examples of such compositions are disclosed in, for example, US-A-4,705,540 and US-A-4,717,394. The preferred aromatic polyimides for use in the present invention consist essentially of repeating units of the formula wherein R is selected from the group consisting of where -R I - is alkylene groups of 1 to 5 carbon atoms, where -R II - is or alkylene groups of 1 to 5 carbon atoms; -Ar- is or mixtures thereof where -R III - is alkylene groups of 1 to 5 carbon atoms alkoxy groups of 1 to 5 carbon atoms, phenyl or phenoxy groups, where -R IV -is or alkylene groups of 1 to 5 carbon atoms, -X-, -X 1 -, -X 2 -, and -X 3 - independently are alkyl groups of 1 to 5 carbon atoms, alkoxy groups of 1 to 5 carbon atoms, phenyl or phenoxy groups, -Y-, -Y 1 -, -Y 2 - and -Y 3 -independently are -H, -X-, -X 1 -, -X 2 -, -X 3 - or halogen, -Ar'- is or mixtures thereof where -R II - has the above-defined meaning, m is 0 to 100% and preferably 20 to 100% of m plus n, n is 0 to 100% and preferably 20 to 80% of m plus n and m = 100% minus n, and p is 0 to 4. The aromatic polyimide gas separations membrane is no limited to any specific type or design. The membrane is preferably in the form of a hollow fiber. The polymeric membrane may be formed as described in the prior art. For example, they may be of the type described in U.S-A-4,230,463 or by Cabasso et al in Research and Development of NS-1 and Related Polysulfone Hollow Fibers for Reverse Osmosis Desalination of Seawater (NTIS PB-248666, 7/1975). In the present invention, an aromatic polyimide gas separation membrane is contacted with a solution of a surfactant in a liquid which swells but is a non-solvent for the aromatic polyimide membrane. The membrane is then dried. The solution is defined as being incapable of dissolving greater than 3% by weight of the membrane material at 20°C. The nonsolvent liquid will comprise a swelling agent for the membrane material. The swelling liquid may be composed of a single component or be a multicomponent mixture incorporating non-interacting substances and/or solvents for the membrane material. Specific non-solvent liquids will be dependent on the intrinsic parameters of the membrane material and the membrane morphology. In practical terms, a swelling agent for a certain membrane may be defined as a substance which modifies the membrane's gas permeation properties through contact. The membrane treatment with said solution comprising the surfactant and the swelling agent as described herein gives at least a 5% improvement in the selectivity of oxygen over nitrogen from the gas pair oxygen/nitrogen (21/79, mole). More preferably, said selectivity enhancement is 10% or greater. Surfactants effective in the herein described procedure may include anionic, cationic, or neutral (nonionic) types. They may be aliphatic, aromatic, or fluorinated. Said surfactants are to be incorporated in an effective amount. This effective amount will vary broadly based on the chemical nature of the surfactant, on the nature of the nonsolvent liquid, on the chemical nature of the aromatic polyimide membrane material, and on the morphology of the membrane to be treated. Membranes treated with an effective amount of surfactant, as described herein, will demonstrate at least a 5% enhancement in the selectivity of oxygen over nitrogen from the gas pair oxygen/nitrogen (21/79, mole) when compared to an identical membrane treated with the same nonsolvent liquid which does not incorporate said surfactant. EXAMPLES Referential Example This referential example describes the material and processes by which asymmetric hollow fiber aromatic polyimide gas separation membranes were produced. The hollow fiber membranes are spun from a hollow fiber, spinnerette into a water coagulant as has been taught in the prior art. A polymer solution was prepared with approximately 21% (weight) solids content of the aromatic polyimide prepared by the polycondensation of 2,4,6-trimethyl-1,3-phenylene diamine with a 1:1 (molar) mixture of 4,4′-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis(1,2-benzenedicarboxylic acid anhydride) and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride, 20% (weight, based on polymer) lithium nitrate, 20% (weight, based on polymer) tetramethylene sulfone, 3% (weight, based on polymer) 4,4′-[2,2,2-trifluoro-1-(trifluoromethyl)-ethylidene bis(1,2-benzenedicarboxylic acid anhydride] in a 1:1 (weight) mixture of N,N-dimethylacetamide and dimethylsulfoxide. Similar polymers are described in US-A-4,705,540 and US-A-4,717,394. The above polymer solution was extruded through a hollow fiber spinnerette with fiber channel dimensions of outer diameter (OD) equal to 22 mils (5.6 x 10 -4 m) and inner diameter (ID) equal to 10 mils (2.54 x 10 -4 m) at the rate of 213 cc per hour at 115°C. A solution of 1% (weight) lithium nitrate in a 47.5:47.5:5.0% (weight solution of N,N-dimethyl-acetamide:dimethylsulfoxide:water was injected into the fiber bore at a rate of 85.7 cc per hour. The spun fiber passed through an air gap length of 1 cm at 20°C into a water coagulant bath maintained at 20°C. The fiber was wound up on a drum at the rate of 75 meters per minute. The water-wet fiber was dehydrated as taught in US-A-4,080,743; US-A-4,080,744; US-A-4,120,098; and EP-A-219,878. This specifically involved the sequential replacement of water with methanol, the replacement of methanol with hexane, and drying. The fibers were cut into lengths approximately 20 inches (0.51 m) long and the open ends of the fibers were potted in an epoxy resin within a 1/4 inch (0.00635 m) diameter stainless steel tube to provide loops approximately 9 inches (0.23 m) long. From 10 to 24 fibers were so potted for the individual tests. Examples 1-7 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 1. The outer surfaces of the polyimide membranes were contacted with an aqueous solution of a nonionic fluorosurfactant which is a 50% solution of a fluoroaliphatic polymeric ester in ethyl acetate (surfactant A) at the concentrations (weight, based on active agent) reported in Table 1 at room temperature for 0.25 hours. The solutions were drained and the membranes dried in a vacuum oven at 20 inches (0.51 m) mercury (68 kPa) and 60°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 1. (1cm Hg = 1,33 kPa). GPU = 10 -6 X cm 3 (STP) cm 2 x sec x (cmHg) Example Preliminary Treatment Concentration (wt %) Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 1 140 2.9 0.05 35 4.4 2 145 2.9 0.025 40 4.5 3 140 3.1 0.005 40 4.5 4 155 2.7 0.005 30 4.4 5 155 3.0 0.0025 45 4.3 6 125 3.4 0.0005 60 4.2 7 120 3.1 0.00025 65 4.1 Control Examples 1-2 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (989 kPa), 24°C ± 2°C. Results are reported in Table 2. The outer surfaces of the polyimide membranes were contacted with deionized water at room temperature for 0.25 hours. The water was drained and the membranes dried in a vacuum oven at 20 inches (0.51m) mercury (68 kPa) and 60°C ± 10°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 2. Control Example Preliminary Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 1 120 3.2 95 3.6 2 115 3.1 55 3.9 Examples 8-25 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example 1, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 3. The outer surfaces of the polyimide membranes were contacted with aqueous solutions of the aliphatic surfactants listed in Table 3 at room temperature for 0.25 hour. The solutions were drained and the membranes dried in a vacuum oven at 20 inches (0.51m) mercury (68 kPa), and 60°C ± 10°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 3. Example Preliminary Treatment Solution (wt %) Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 8 130 3.1 0.1 SURFACTANT B 60 4.4 9 115 3.4 0.1% SURFACTANT C 50 4.2 10 130 3.2 0.1% SURFACTANT D 20 4.9 11 140 3.0 0.01% SURFACTANT D 30 4.9 12 130 3.0 0.005% SURFACTANT D 35 4.6 13 115 3.2 0.055% SURFACTANT E 25 4.7 14 150 2.9 0.0055% SURFACTANT E 35 4.7 15 120 3.2 0.00175% SURFACTANT E 35 4.6 16 140 3.1 0.00055% SURFACTANT E 60 4.0 17 145 3.0 0.055% SURFACTANT F 35 4.2 18 170 2.7 0.1% SURFACTANT G 25 4.0 19 165 2.8 0.01% SURFACTANT G 40 4.2 20 155 2.9 0.1% SURFACTANT H 10 4.8 21 170 2.8 0.005% SURFACTANT H 55 4.0 22 160 2.8 0.005% SURFACTANT H 55 4.0 23 150 2.8 0.0475% SURFACTANT I 20 5.0 24 150 2.9 0.00475% SURFACTANT I 35 4.8 25 170 3.0 0.00238% SURFACTANT I 65 4.2 In Table 3, surfactant B is an anionic surfactant which is 90% of sodium alkyl naphthalene sulfonate. Surfactant C is 90-96% active ingredient anionic surfactant sodium lauryl sulfate. Surfactant D is 100% active ingredient nonionic surfactant which is ethoxylated phosphate. Surfactant E is a 60% active ingredient nonionic surfactant which is an amine ethoxylate. Surfactant F is a 60% active ingredient nonionic surfactant which is alcohol ethoxylate. Surfactant G is 100% active ingredient nonionic surfactant which is alcohol ethoxylate. Surfactant H is 95% active ingredient nonionic surfactant which is alcohol ethoxylate. Surfactant I is a 50% active ingredient nonionic surfactant which is alcohol ethoxylate. Examples 26-34 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa) 24°C ± 2°C. Results are reported in Table 4. The outer surfaces of the polyimide membranes were contacted with aqueous solutions of the fluorosurfactants listed in Table 4 at the concentrations (weight, based on active agent) listed in Table 4 at room temperature for 0.25 hours. The solutions were drained and the membranes dried in a vacuum oven at 20 inches (0.51m) mercury (68 kPa) and 60°C ± 10°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa) 24°C ± 2°C. Results are reported in Table 4. Example Preliminary Treatment Solution (wt %) Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 26 145 2.9 0.04% SURFACTANT J 35 4.6 26 150 2.9 0.004% SURFACTANT J 50 4.2 28 130 2.9 0.047% SURFACTANT K 50 4.1 29 140 3.1 0.04% SURFACTANT L 30 4.6 30 135 3.0 0.004% SURFACTANT L 50 4.2 31 165 3.0 0.05% SURFACTANT M 35 4.5 32 140 3.0 0.005% SURFACTANT M 50 4.1 33 150 2.9 0.1% SURFACTANT N 30 4.4 34 125 2.9 0.01% SURFACTANT N 45 4.1 In Table 4 surfactant J is an anionic fluorosurfactant supplied as a 40 wt % solution in a 45/15 water/isopropanol mixture, and is a mixture of (R f CH 2 CH 2 0)P(0)(0NH4) and a nonfluorinated surfactant. Surfactant K is an amphoteric fluorosurfactant present as a 47 wt % solution in acetic acid, which surfactant has the formula R f CH 2 CH 2 (0C0CH 3 )CH 2 N + (CH 3 ) 2 CH 2 C0 2 - . Surfactant L is a nonionic fluorosurfactant supplied as a 40 wt % solution in 30/30 water/isopropyl alcohol which surfactant has the formula R f CH 2 CH 2 0(CH 2 CH 2 0) x H. Surfactant M is a nonionic fluorosurfactant supplied as a 50 wt % solution in a 25/25 water/ethylene glycol mixture which has the formula R f CH 2 CH 2 0(CH 2 CH 2 0) y H. Surfactant N is an nonionic fluorosurfactant having the formula (R f CH 2 CH 2 0)P(0)(0NH 4 ) 2 . In each of the above formulae R f is F(CF 2 CF 2 ) 3-8 . Examples 35-43 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example 1, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 5. The outer surfaces of the polyimide membranes were contacted with aqueous solutions of the fluorosurfactants listed in Table 5 at the concentrations (weight, based on active agent) listed in Table 5 at room temperature for 0.25 hours. The solutions were drained and the membranes dried in a vacuum oven at 20 inches (0.51m) mercury (68 kPa) and 60°C ± 10°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 5. Example Preliminary Treatment Solution (wt %) Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 35 135 2.9 0.05% SURFACTANT O 35 4.4 36 130 2.9 0.005% SURFACTANT O 50 4.1 37 145 2.8 0.05% SURFACTANT P 35 4.0 38 145 2.8 0.081% SURFACTANT P 40 4.5 39 165 2.9 0.081% SURFACTANT Q 45 4.3 40 160 2.9 0.05% SURFACTANT Q 30 4.7 41 180 2.9 0.025% SURFACTANT R 25 4.9 42 165 2.8 0.1% SURFACTANT R 20 5.2 43 150 3.0 0.001% SURFACTANT S 30 4.8 In Table 5, Surfactant O is a cationic fluorosurfactant comprising 50% active solids, 33% isopropyl alcohol and 17% water. Surfactant P is an anionic fluorosurfactant which is a 25% active ingredient solution of amine perfluoroalkyl sulfonates in water. Surfactant Q is a nonionic fluorosurfactant which is 95% of the active ingredient fluorinated alkyl polyoxyethylene ethanols. Surfactant R is an amphoteric fluorosurfactant comprising 25% sodium salt of fluoroalkyl sulfonate in 25% diethylene glycol monobutyl ether and 50% water. Surfactant S is 100% of a nonionic fluorosurfactant which is fluoroaliphatic polymeric esters. Control Examples 3-5 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 6. The outer surfaces of the polyimide membranes were contacted with FREON® 113 (1,1,2-trichloro-1,2,2-trifluoroethane) at room temperature for 0.24 hours. The FREON® 113 was drained and the membranes dried in a vacuum oven at 20 inches (0.51m) mercury (68 kPa) and 60°C ± 10°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 KPa), 24°C ± 2°C. Results are reported in Table 6. Control Example Preliminary Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 3 165 2.9 30 4.4 4 150 3.0 20 4.7 5 150 3.2 25 4.8 Examples 44-48 Asymmetric polyimide hollow fiber membranes, prepared as described in Referential Example, were tested for mixed gas oxygen/nitrogen (21/79 mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 7. The outer surfaces of the polyimide membranes were contacted with the mixtures of surfactants listed in Table 7 (descriptions of surfactants in earlier examples) FREON® 113 at the concentrations (weight, based on active agent) listed in Table 7 at room temperature for 0.25 hours. The mixtures were drained and the membranes dried in a vacuum oven at 20 inches (0.51m) mercury (68 kPa) and 60°C ± 10°C overnight. The membranes were tested for mixed gas oxygen/nitrogen (21/79, mole ratio) permeabilities at 100 psig (689 kPa), 24°C ± 2°C. Results are reported in Table 7. Example Preliminary Treatment Mixture (wt %) Treated PO 2 (GPU) PO 2 /PN 2 PO 2 (GPU) PO 2 /PN 2 44 135 2.9 0.1% SURFACTANT D 10 5.1 45 150 2.9 0.04% SURFACTANT J 15 4.9 46 135 2.9 0.1% SURFACTANT N 15 5.0 47 135 2.9 0.081% SURFACTANT Q 10 5.0 48 150 2.9 0.1% SURFACTANT S 20 4.9

Claims: A process for improving the permselectivity of a gas separation membrane comprising aromatic polyimides with respect to at least one gas of a gaseous mixture comprising (a) contacting said membrane with a solution of an effective amount of a surfactant in a liquid, (b) the liquid being capable of swelling the membrane but the solution being incapable of dissolving more than 3 weight percent of the membrane material under conditions of contacting the membrane, (c) the amount of surfactant being sufficient to provide at least a 5% improvement in the selectivity of the membrane with respect to oxygen over nitrogen from a 21/79 mole ratio oxygen/nitrogen mixture over the selectivity achieved with the liquid alone, and (d) drying the membrane. The process of Claim 1 wherein the aromatic polyimide consists essentially of repeating units of the formula wherein R is selected from where -R I - is or alkylene groups of 1 to 5 carbon atoms where -R II -is or alkylene groups of 1 to 5 carbon atoms; -Ar- is or mixtures thereof where -R III - or alkylene groups of 1 to 5 carbon atoms, alkoxy groups of 1 to 5 carbon atoms, phenyl or phenoxy groups, where -R IV - is or alkylene groups of 1 to 5 carbon atoms, -X-, -X 1 -, -X 2 -, and -X 3 - independently are alkyl groups of 1 to 5 carbon atoms, alkoxy groups of 1 to 5 carbon atoms, phenyl or phenoxy groups, -Y, -Y 1 , -Y 2 and -Y 3 independently are -H, -X, -X 1 , -X 2 , -X 3 or halogen, -Ar'- is or mixtures thereof where -R II - has the above-defined meaning, m is 0 to 100% of m plus n, n is 0 to 100% of m plus n, and n = 100% minus m, and p is 0 to 4. The process of Claim 2 wherein -m- is 20-100% of m plus n. The process of Claim 1, 2 or 3 wherein the surfactant is a nonionic surfactant. The process of Claim 4 wherein the surfactant is a fluorinated surfactant. The process of Claim 1, 2 or 3 wherein the surfactant is an anionic surfactant. The process of Claim 6 wherein the surfactant is a fluorinated surfactant. The process of Claim 6 wherein the hydrophobic portion of the surfactant is a hydrocarbon. The process of Claim 2 or 3 wherein -Ar- is The process of Claim 9 wherein R is The process of Claim 10 wherein -R I - is

Inventors: HAYES RICHARD ALLEN, HAYES, RICHARD ALLEN
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1990
Main classification: B01D67
IPC codes: B01D69, B01D71, B01D67