Process for separating acid gases from gaseous mixtures utilizing composite membranes formed from salt-polymer blends

The present invention is a process for separating acid gas from gaseous mixtures containing acid gas and at least one non-acid gas. The process comprises bringing the gas stream into contact with a multilayer composite membrane comprising a non-selective polymeric support layer and a separating layer comprising a blend of a water soluble polymer and one-half equivalent or more of an acid gas reactive salt based upon the repeating unit of the water soluble polymer, the acid gas reactive salt which is formed from a monovalent cation and an anion for which the pK.sub.a of the conjugate acid is greater than 3, wherein the multilayer composite membrane separates the acid gas from the gaseous mixture by selectively permeating the acid gas.

FIELD OF THE INVENTION
 The present invention is a membrane-based process for removing acid gases
 from gaseous mixtures containing one or more acid gases and at least one
 non-acid gas. A general embodiment of the composite membrane comprises a
 microporous polymer support layer and a contiguous separating layer formed
 from a blend of a water-soluble polymer and one-half equivalent or more of
 an acid gas reactive salt based upon the repeating unit of the water
 soluble polymer.
 BACKGROUND OF THE INVENTION
 Numerous industrial processes require separating one or more acid gases
 such as CO.sub.2 and H.sub.2 S from gaseous mixtures containing such acid
 gases and additional non-acid gas components such as CH.sub.4 and H.sub.2.
 Such processes include separating carbon dioxide from hydrogen gas in
 hydrogen synthesis plants and removing H.sub.2 S and CO.sub.2 during the
 cleanup of natural gas. Numerous types of membranes have been evaluated
 for use in such processes taking into account economic and energy
 consumption considerations.
 Preferred membranes for separating acid gases from gaseous mixtures are
 those which permeate the desired acid gases at a preferential rate over
 non-acid gas components such as hydrogen gas and methane in the process
 stream to be treated and which are highly permeable with respect to such
 acid gases. However, such membranes are relatively rare and sometimes have
 limited utility because the membranes exhibit insufficient acid gas
 permeability.
 U.S. Pat. No. 4,500,667 discloses gas separation membranes comprising an
 organic polymer-inorganic compound blend which is prepared by admixing an
 organic polymer such as poly(vinyl alcohol) with a heteropoly acid or salt
 thereof such as dodecamolybdophosphoric acid in a mutually miscible
 solvent. After allowing the mixture to react for a period of time
 sufficient to form a blend, the solution is cast on an appropriate casting
 surface, the solvent is evaporated and the desired membrane is recovered.
 The membranes are suitable for separating water from a water-containing
 hydrogen stream.
 U.S. Pat. No. 4,780,114 discloses membranes which selectively permeate acid
 gases such as CO.sub.2 or H.sub.2 S over non-acid gas components. The
 membranes comprise a thin film of a molten salt hydrate which may be
 immobilized within the pores of a thin, porous inert support or
 alternatively, may be encapsulated in a non-porous, gas permeable polymer
 such as poly(trimethylsilylpropyne), polymer blends or silicone rubber.
 The term, molten salt hydrate, refers to a salt, which upon heating, melts
 to yield a liquid system which contains bound water. Molten salt hydrates
 are represented by the formula A.sub.x B.sub.y.nH.sub.2 O wherein A and B
 are ionic species of opposite charge and n represents the number of moles
 of bound water per mole of salt. Representative molten salt hydrates
 include tetramethylammonium fluoride tetrahydrate and tetramethylammonium
 acetate tetrahydrate.
 U.S. Pat. No. 4,913,818 discloses a process for removing water vapor from a
 gas/vapor mixture such as a water-alcohol mixture, which utilizes a
 regenerated cellulose membrane which is impregnated with a hygroscopic
 electrolyte salt. The electrolyte is desirably a salt of an alkali metal,
 an alkaline earth metal or a transition metal wherein the anion is a
 chloride, bromide, fluoride, sulphate or nitrate. Representative salts
 include LiBr, KCl, MgCl.sub.2, CaCl.sub.2, SrSO.sub.4 and NaNO.sub.3. A
 cellulose membrane impregnated with LiBr provided a 2.5-fold increase in
 water vapor flux over the same cellulose membrane containing no salt.
 Jansen and coworkers, (Proc. Int. Conf. Pervaporation Processes Chem. Ind.,
 3rd, 338-341, 1988) disclose cellulose and poly(vinyl alcohol) (PVOH)
 membranes which are impregnated with CsF. The CsF-impregnated PVOH
 membrane provided an approximate doubling of water vapor flux although
 water/alcohol selectivity decreased by an unspecified amount. Higher water
 vapor fluxes were obtained by repeatedly impregnating the membrane with
 CsF. The reference does not teach or suggest that the disclosed membranes
 permeate carbon dioxide.
 U.S. Pat. No. 4,973,456 discloses a process for reversibly absorbing acid
 gases such as CO.sub.2, H.sub.2 S, SO.sub.2 and HCN present in a gaseous
 mixture wherein the gaseous mixture is contacted with a hydrate salt
 represented by the formula A.sub.x.sup.m+ B.sub.y.sup.n-.rH.sub.2 O
 wherein A.sup.m+ is a cation, B.sup.n- is the conjugate base of a weak
 acid having a pK.sub.a corresponding to an ionization constant of the acid
 greater than 3 as measured in dilute aqueous solution, m and n are
 independently selected integers from 1-4, x and y are integers such that
 the ratio of x to y provides a neutral salt and r is any number greater
 than zero up to the maximum number of moles of water which can be bound to
 the salt. Representative salt hydrates include tetramethylammonium
 fluoride tetrahydrate, tetramethylammonium acetate tetrahydrate and cesium
 fluoride.
 U.S. Pat. No. 5,062,866 discloses a process for separating an unsaturated
 hydrocarbon from a feed stream containing such unsaturated hydrocarbon
 wherein the feed stream is contacted with a membrane comprising a blend of
 hydrophilic polymer and a hydrophilic alkali metal salt and metals which
 are reactive with respect to the desired unsaturated hydrocarbon. Suitable
 polymers include polyvinyl alcohol, polyvinyl acetate, sulfonyl-containing
 polymers polyvinylpyrrolidone and the like. Suitable metal salts include
 silver nitrate.
 U.S. Pat. No. 5,336,298, assigned to Air Products and Chemicals, Inc.,
 Allentown, Pa., teaches a process for separating acid gases from a gaseous
 mixture containing acid gas and at least one non-acid gas component
 wherein the gaseous mixture is brought into contact with a multilayer
 composite membrane comprising an essentially non-selective polymeric
 support layer and a separating layer comprising a polyelectrolyte polymer
 which contains cationic groups which are electrostatically associated with
 anions for which the pK.sub.a of the conjugate acid is greater than 3. The
 multilayer composite membrane selectively permeates acid gas thereby
 removing the same from the gaseous mixture. Suitable polyelectrolyte
 polymers include poly(diallyldimethylammonium fluoride),
 poly(diallyldimethylammonium acetate), poly(vinylbenzyltrimethylammonium
 fluoride) and poly(vinylbenzylammonium acetate).
 Industry is searching for improved membranes for separating acid gases from
 acid-gas containing gaseous mixtures wherein the membrane would desirably
 exhibit substantially improved permeability to the acid gas to be
 separated from the feedstream without sacrificing selectivity for the
 component to be passed through the membrane.
 SUMMARY OF THE INVENTION
 The present invention relates to a process for selectively removing acid
 gases from a gaseous mixture containing such acid gases. The process
 utilizes a composite membrane comprising a non-selective polymeric support
 layer and a separating layer which comprises a blend of a water soluble
 polymer and one-half equivalent or more of at least one acid gas reactive
 salt based upon the repeating unit of the water soluble polymer, each acid
 gas reactive salt comprising a monovalent cation and an anion for which
 the pK.sub.a of the conjugate acid is greater than 3. Suitable acid gas
 reactive salts can be represented by the formula A.sub.x B.sub.y.nH.sub.2
 O wherein A and B are ionic species of opposite charge and n is the number
 of moles of bound water per mole of salt and x and y are integers such
 that the salt remains charge neutral. While "n" presented in the formula
 must be a number greater than 1 and is bounded by the maximum number of
 moles of water which can be bound to the salt, the membranes of the
 present invention may be installed for operation when "n" equals zero
 provided the gaseous mixture to be treated contains water vapor in an
 amount sufficient to hydrate the acid gas reactive salt. Optionally, a
 third layer referred to as a protective layer, may be deposited onto the
 active separating layer to provide added strength and resistance to
 abrasion.
 The membranes of the present process are unique compared to conventional
 polymeric membranes because they selectively permeate CO.sub.2, H.sub.2 S
 and other acid gases while retaining non-acid gases such as H.sub.2 and
 CH.sub.4 at feed pressure. Thus, contaminant acid gases can be removed
 from a feedstream while maintaining the bulk components of the feedstream
 at pressure. The present process accomplishes this result because acidic
 gases can react with the acid gas reactive salt and/or the water soluble
 polymer of the separating layer of the membrane in a manner which enhances
 acid gas permeation. In contrast, non-acid gases such as H.sub.2 and
 CH.sub.4 can only permeate through the membrane by physically dissolving
 and diffusing across the membrane. Since the membrane is highly ionic,
 solubilities of H.sub.2 and CH.sub.4 are very low, permeance is minimized
 and the separating layer serves essentially as a barrier to the non-acid
 gas components of the feedstream.
 Representative acid gas reactive salts suitable for use in the separating
 layer of the membrane include cesium fluoride, tetramethylammonium
 fluoride, cesium acetate, cesium pipecolinate and choline fluoride. Water
 soluble polymers can be selected from a variety of polyelectrolytes or
 polymers containing polar functional groups. Particularly useful water
 soluble polymers include poly(vinylbenzyltrimethylammonium fluoride),
 poly(vinyl alcohol) and poly(vinyl amine).
 Compared to prior art facilitated transport membranes wherein the salts and
 salt hydrates are immobilized within the pores of the supporting layer of
 the membrane, the acid gas reactine salts of the present invention are
 substantially soluble in the water soluble polymer substrate thereby
 reducing problems associated with hole formation which might occur in
 prior art membranes when feed pressure exceeds the bubble point of the
 substrate.
 DETAILED DESCRIPTION OF THE INVENTION
 The present invention is a process which uses a multilayer composite
 membrane for selectively removing acid gases such as CO.sub.2, H.sub.2 S,
 COS, SO.sub.2 and NO.sub.x from gas mixtures while retaining non-acid
 gases such as H.sub.2, CH.sub.4, N.sub.2, O.sub.2 and CO, at pressure in
 the feed stream contacting the membrane. The membranes used in this
 process comprise a minimum of two layers, and more typically three or more
 layers, at least one of which consists of a separating layer comprising a
 blend of a water soluble polymer, typically a polyelectrolyte or a polymer
 containing polar function groups, and one-half equivalent or more of at
 least one acid gas reactive salt, the number of equivalents being based
 upon the repeating unit of the water soluble polymer. Suitable acid gas
 reactive salts can be represented by the formula A.sub.x B.sub.y.nH.sub.2
 O wherein A and B are ionic species of opposite charge and n is the number
 of moles of bound water per mole of salt and x and y are integers such
 that the salt remains charge neutral. While "n" presented in the formula
 must be a number greater than 1 and is bounded by the maximum number of
 moles of water which can be bound to the salt, the membranes of the
 present invention may be installed for operation when "n" equals zero
 provided the gaseous mixture to be treated contains water vapor in an
 amount sufficient to hydrate the acid gas reactive salt.
 In the most general embodiment of the invention, the process for separating
 acid gases from a gaseous mixture containing acid gas and at least one
 non-acid gas comprises contacting the gaseous mixture with a multilayer
 composite membrane comprising a first polymeric support layer and a
 separating layer comprising a blend of a water soluble polymer and
 one-half equivalent or more of at least one acid gas reactive salt based
 upon the repeating unit of the water soluble polymer, the acid gas
 reactive salt which is formed from a monovalent cation and an anion for
 which the pK.sub.a of the conjugate acid is greater than 3, wherein the
 multilayer composite membrane separates the acid gases from the gaseous
 mixture by selectively permeating the acid gases. Suitable acid gas
 reactive salts can be represented by the formula A.sub.x B.sub.y.nH.sub.2
 O wherein A and B are ionic species of opposite charge and n is the number
 of moles of bound water per mole of salt and x and y are integers such
 that the salt remains charge neutral. The composite membranes may be used
 in any configuration known in the art, such as flat sheets, spiral wound,
 hollow fiber, plate and frame as conventionally known in the art, and the
 like.
 The membranes of the present process are particularly useful for separating
 CO.sub.2 from CH.sub.4 and H.sub.2, and H.sub.2 S from CH.sub.4 and
 H.sub.2. The process comprises contacting a gaseous feed stream containing
 one or more acid gases and at least one non-acid component with the feed
 side of the membrane and recovering a gas stream which is enriched with
 the desired acid gas at the permeate side of the membrane. The permeate
 gases may be collected directly or alternately, their recovery may be
 facilitated by sweeping the permeate side of the membrane with an inert
 gas or the acid gas being collected. Suitable sweep gases include inert
 gases such as nitrogen, helium and argon. However, the process can be
 operated without using a sweep gas.
 In most cases, the composite membranes are effectively operated by
 maintaining a minimal partial vapor pressure difference of water between
 the feed stream and permeate sweep) streams to the membrane in order to
 retain the activity of the acid gas reactive salt toward transporting acid
 gases across the composite membrane.
 The composite membranes of the present process are unique relative to
 typical polymeric membranes in that they are substantially impermeable to
 non-acid gases such as H.sub.2 and CH.sub.4 while they are highly
 permeable toward acid gases. Typically, H.sub.2 can permeate conventional
 polymeric membranes at a rate comparable to or greater than CO.sub.2 and
 H.sub.2 S. Thus, CO.sub.2 to H.sub.2 and H.sub.2 S to H.sub.2
 selectivities for conventional polymeric membranes are relatively small
 and usually less than one. In contrast, the multilayer composite membranes
 of the present invention exhibit relatively high CO.sub.2 to H.sub.2 and
 H.sub.2 S to H.sub.2 selectivities. Consequently, the membranes of the
 present process permit the application of membrane technology to
 separations which were unachieveable using conventional polymeric
 membranes.
 In an alternate embodiment of the process, the multilayer composite
 membrane comprises a third layer, referred to as a protective layer, which
 is deposited onto the separating layer to provide the membrane with added
 strength and resistance to abrasion and the like. Suitable protective
 films are selected from dense permeable polymer films such as
 poly(dimethylsiloxane) (PDMS) and poly(trimethylsilylpropyne) (PTMSP).
 The membranes of the present invention exhibit gas permeance rates which
 are dependent upon the acid gas partial pressure of the gaseous mixture to
 be separated. For example, the data shall demonstrate that CO.sub.2
 permeance of the subject composite membranes increases with decreasing
 CO.sub.2 feed partial pressure which is consistent with facilitated
 transport of CO.sub.2 and implies chemical reactivity of CO.sub.2, an acid
 gas, with the separating layer. The chemical reactivity of the acid gas
 components of the gaseous mixture to be separated with the acid gas
 reactive salt blended in the water soluble polymer of the active
 separating layer results in relatively high acid gas permeances while
 desirably limiting permeance of the non-acid gas components. Moreover, the
 acid gas components may exhibit chemical reactivity with the separating
 layer when formed from a polyelectrolyte. For purposes of this invention,
 the term, facilitated transport, refers to an acid gas transfer mechanism
 wherein the acid gases to be separated from the gaseous mixture are
 capable of reacting with one or both components of the separating layer of
 the membrane.
 The microporous polymer support layer of the composite membrane is highly
 permeable but typically non-selective and serves principally to support
 the active separating layer. The support layer can be fabricated from a
 wide variety of materials including dense materials such as
 poly(dimethylsiloxane) and poly(trimethylsilyl propyne) or microporous
 materials such as polysulfones or conventional ceramics. The microporous
 layer can be fabricated in any conventional manner including flat sheets
 or hollow fiber configurations. Such polymeric materials optionally may
 have asymmetrically distributed pore sizes.
 The separating layer of the multilayer composite membrane comprises a blend
 of two components. The first component is a water soluble polymer which
 may be a polyelectrolyte or a polymer containing polar functional groups.
 The second component is an acid gas reactive salt comprising a monovalent
 cation and an anion for which the pK.sub.a of the conjugate acid is
 greater than 3. Suitable acid gas reactive salts can be represented by the
 formula A.sub.x B.sub.y.nH.sub.2 O wherein A and B are ionic species of
 opposite charge and n is the number of moles of bound water per mole of
 salt and x and y are integers such that the salt remains charge neutral.
 The term, blend, means that the recited components are thoroughly mixed
 whereby the acid gas reactive salt is dispersed throughout the water
 soluble polymer or polyelectrolyte. This blend can be a true solution in
 which the salt is solubilized in the polymer or the blend can be a mixture
 where two distinct phases are present and the salt is dispersed in the
 polymer. Applicants have discovered that the membranes of the present
 process are not adversely affected if a minor amount of salt crystallizes
 out the blend. Therefore, the practitioner need not be concerned with
 adding too much salt to the water soluble polymer such that the blend
 becomes saturated with salt.
 The term, water soluble polymers, means polymers which are soluble in water
 or which are dispersible in water. Such materials can be summarized in two
 categories, polyclectrolytes and polymers containing polar functional
 groups. The water soluble polymer should preferably exhibit low permeance
 of the non-acid gas components of the gaseous mixture to be separated.
 Examples of suitable water soluble polymers include polyelectrolytes such
 as poly(diallyldimethylammonium fluoride) (PDADMAF),
 poly(vinylbenzyltrimethylammonium fluoride) (PVBTAF),
 poly(vinylbenzyltrimethylammonium chloride) (PVBTCl),
 poly(diallyldimethylammonium acetate) (PDADMAOAc),
 poly(vinylbenzyltrimethylammonium acetate) (PVBTAOAc), poly(vinyl
 N-methylpyridinium fluoride), poly(vinyl N-methylpyridinium acetate),
 poly(vinyl N,N-dimethylimidazolium fluoride), poly(vinyl
 N,N-dimethylimidazolium acetate), poly(N,N-dimethylethyleneimine
 fluoride), poly(N,N-dimethylethyleneimine acetate), poly(2-hydroxypropyl
 dimethylammonium fluoride), poly(2-hydroxypropyl dimethylammonium
 acetate), and the like. The molecular weight of the water soluble polymer
 is not believed to be critical and the polymer need only have a molecular
 weight sufficient to cast suitable films.
 Polyelectrolytes as used herein are distinguishable from substantially
 water-insoluble ionic polymers and ion-exchange polymers in that
 polyelectrolytes have a relatively high ionic content, i.e., up to one
 ionic unit per polymer repeat unit. Ionic polymers, on the other hand, are
 compounds possessing organic or inorganic salt groups which are attached
 to a polymer chain and which have relatively low ionic content, usually
 less than 10 mole % with respect to the polymer chain repeat unit. Ion
 exchange polymers generally consist of an insoluble polymer matrix or
 resin to which are attached ionizable functional groups. Cation exchange
 resins have a fixed negative charge on the polymer matrix with
 exchangeable cations (and vice-versa for anion-exchange resins).
 Preferably, the water soluble polymer reversibly reacts with the acid gas
 in the presence of water vapor. However, such reactivity is not required
 in order for the membranes to provide the benefits discussed in this
 Specification. For example, polyelectrolytes which contain non-reactive
 anions, such as Cl.sup.- in PVBTACl, exhibits low gas permeance and
 non-facilitated transport of acid gases as evidenced by the independence
 of acid gas permeance on feed partial pressure. However, significantly
 higher permeances and selectivities are obtained when an acid gas reactive
 salt is blended with such non-reactive water soluble polymers wherein the
 resulting salt/polymer blends exhibit facilitated transport of acid gases
 as demonstrated by the relationship between permeance and feed partial
 pressure. Moreover, amino acid salts such as cesiumn pipecolinate which
 are more reactive with respect to H.sub.2 S over CO.sub.2 can be used to
 fabricate membranes which selectively permeate H.sub.2 S over CO.sub.2.
 Other polymers which are not polyelectrolytes but contain polar functional
 groups can be used in conjunction with appropriate acid gas reactive salts
 to fabricate the active separating layer of the composite membranes.
 Suitable water soluble polymers which contain polar functional groups
 include poly(vinyl alcohol) (PVOH), poly(vinylamine) (PVAm) and
 carbonyl-containing polymers such as polyvinylpyrrolidone. While membranes
 formed from these materials are essentially unreactive with respect to
 acid gases and exhibit relatively low gas permeance, unexpectedly superior
 acid gas permeability and selectivity is achieved by adding the enumerated
 acid gas reactive salts to these polymers.
 The separating layer of these composite membranes further comprises an acid
 gas reactive salt which is blended with the water soluble polymer. The
 acid gas reactive salt contains cationic groups which are associated
 electrostatically with anions for which the pK.sub.a of the conjugate acid
 is greater than 3. The value of the pK.sub.a is that obtained for the
 conjugate acid as determined in a dilute aqueous solution. Examples of
 suitable anions include F.sup.- (pK.sub.a HF=3.45) and the acetate ion
 (pK.sub.a acetic acid=4.75). Preferred salts are those which contain
 fluoride, acetate or carboxylate anions.
 Suitable acid gas reactive salts can be represented by the formula A.sub.x
 B.sub.y.nH.sub.2 O wherein A and B are ionic species of opposite charge
 and n is the number of moles of bound water per mole of salt and x and y
 are integers such that the salt remain charge neutral. Suitable salts
 include those which react reversibly with acid gases, in particular
 CO.sub.2 and H.sub.2 S. Such reactive salts consist of monovalent cations
 and anions for which the pK.sub.a of the conjugate acid of the anion is
 greater than 3. Representative salts include cesium fluoride,
 tetramethylammonium fluoride, cesium acetate and choline fluoride.
 Additionally, amino acid salts such as cesium pipecolinate which are more
 reactive with respect to H.sub.2 S than CO.sub.2 can be used to fabricate
 membranes which selectively permeate H.sub.2 S over CO.sub.2.
 The concentration of acid gas reactive salt to be added to the water
 soluble polymer can be varied to alter the permselective properties of the
 composite membrane. As the quantity of added gas reactive salt to be
 blended with the water soluble polymer is increased, acid gas permeance
 for the membrane increases without substantial loss of selectivity.
 The following examples are presented to better illustrate the present
 invention and are not meant to be limiting.
 Experimental
 Membranes were prepared using aqueous or methanolic casting solutions
 containing the desired water soluble polymer and acid gas reactive salt.
 In general, casting solutions were prepared by dropwise addition of an
 aqueous acid gas reactive salt solution with rapid stirring to a polymer
 solution. Casting solutions were applied to either flat sleet or hollow
 fiber microporous supports and dried at room temperature in a shroud which
 was constantly purged with N.sub.2. In some cases, a protective layer of a
 poly(dimethylsiloxane)-like material such as Petrarch MB PS254 obtained
 from Huls America, Bristol, Pa. was applied using a 1-5 wt. % polymer
 solution in CH.sub.2 Cl.sub.2.
 The membrane apparatus used was similar to that described by Bateman, et
 al., (Sep. Sci. Tech. 19, 21-32 (1984)). The membrane was sealed in a
 stainless steel cell similar to that described by Otto, et al., (J. Appl.
 Polym. Sci. 38, 2131-2147 (1989)). The feed gas to the membrane cell was
 obtained from premixed gas cylinders. Either nitrogen or helium was used
 as a sweep gas. Generally, thermal mass flow controllers were used to
 maintain feed pressure and sweep gas flow rates were maintained between 10
 and 20 cm.sup.3 (STP)/min. Feed gas pressures greater than ambient were
 maintained by employing a back pressure regulator.
 The sweep gas pressure was atmospheric. Both the feed and sweep gases,
 except where indicated, were humidified by passage through constant
 temperature water bubblers. The feed gas was passed over one surface of
 the membrane and the sweep gas over the other. The sweep gas stream was
 then passed through the sample loop of a gas chromatograph and a sample
 for analysis was injected periodically. Data was collected over a minimum
 of 24 h. The determination of the concentration of permeating gases
 permitted calculation of gas flux. Permeance, P.sub.o /l, in units of
 cm.sup.3 (NTP)/cm.sup.2.multidot.s.multidot.cmHg was calculated using the
 equation below:
EQU P.sub.o /l=J/A.DELTA.P
 where J represents gas flow in cm.sup.3 (NTP)/s, A is the membrane area in
 cm.sup.2 and .DELTA.P is the difference in the feed and permeate partial
 pressures of gas in cmHg. For flat sheet membranes, A was 3.77 cm.sup.2
 and 3-15 cm.sup.2 for hollow fiber modules. Selectivity is the ratio of
 permeances or permeabilities of two gases. The term equivalent (equiv) is
 taken as the number of moles of added salt per mole of polymer repeat
 unit. Unless otherwise stated, aqueous casting solutions were employed to
 make films of the present invention.
 Because many of the membranes described in the Examples exhibited low
 CH.sub.4 permeances, CH.sub.4 was often not detected in the sweep gas.
 When CH.sub.4 was not detected, selectivity was estimated in one of two
 ways as indicated in each example.

EXAMPLE 1 (COMATIVE)
 SEATION OF A CO.sub.2 /CH.sub.4 /H.sub.2 GASEOUS MIXTURE
 USING A MEMBRANE FORMED FROM POLY(VINYL BENZYLTRIMETHYLAMMONIUM FLUORIDE)
 (PVBTAF)
 A poly(vinylbenzyltrimethylammonium fluoride) (PVBTAF) membrane containing
 no added acid gas reactive salt was fabricated on a flat sheet microporous
 polysulfone support using a methanolic casting solution containing 5.0 wt.
 % of PVBTAF. PVBTAF was prepared by the following procedure wherein 42 g
 of 30% PVBTACl (obtained from Scientific Polymer Products, Inc., Ontario,
 N.Y.) was mixed with 155 g water. The resulting mixture was added slowly
 with rapid stirring to a solution of 53.4 g KF in 150 ml of water. The
 solution was loaded into dialysis tubing and dialyzed against three
 changes of deionized water. To the resulting solution was added slowly and
 with rapid stirring 57.1 g KF in 150 ml water. The volume of solution had
 become so large that about half of the water was removed Linder vacuum at
 60.degree. C. The solution was dialyzed against four changes of deionized
 water. The resulting solution was filtered and the solvent volume was
 reduced as above to about one-third of the original volume. A polymer film
 was prepared by placing the solution under flowing nitrogen for an
 extended period of time. Permselective data was obtained as a function of
 feed pressure under the conditions listed below.
 PVBTAF flat sheet membrane at 23.degree. C.; feed: 30.6% CO.sub.2, 34.4%
 CH.sub.4 in H.sub.2 ; N.sub.2 sweep; 5.degree. C. feed and sweep water
 bubblers.

CO.sub.2 absorption capacities
 Pressure CO.sub.2 (mole CO.sub.2 /mole polymer
 (kPa) repeat unit)
 Absorption data 77.5 0.440
 134.1 0.477
 204.5 0.594
 Desorption data 60.2 0.482
 19.6 0.379
 4.2 0.316
 3.6 0.259
 The examples demonstrate that acid gas permeance of the composite membranes
 increases when an acid gas reactive salt of the enumerated compositions is
 blended with the water soluble polymer component of the active separating
 layer of the composite membrane. Applicant's process which utilizes a
 membrane having a separating layer formed from a water soluble polymer and
 an acid gas reactive salt provides substantially improved selectivity over
 the prior art facilitated transport membranes having a separating layer
 which does not contain the subject salts. Moreover, the improved
 selectivity is obtained without substantial losses in permeability and the
 feed stream is maintained at pressure.
 Having thus described the present invention, what is now deemed appropriate
 for Letters Patent is set forth in the following Claims.