CA1312428C

CA1312428C - Acid gas scrubbing by composite solvent-swollen membranes

Info

Publication number: CA1312428C
Application number: CA000554872A
Authority: CA
Inventors: Eric K.L. Lee; Stephen L. Matson; Dwayne T. Friesen; Donald J. Kelly
Original assignee: Bend Research Inc
Current assignee: Bend Research Inc
Priority date: 1986-12-30
Filing date: 1987-12-18
Publication date: 1993-01-12
Anticipated expiration: 2010-01-12
Also published as: NO875397D0; EP0273724A3; EP0273724B1; NO875397L; ATE77257T1; US4737166A; DE3779893T2; NO171248C; NO171248B; JPS63175617A; AU599185B2; DE3779893D1; EP0273724A2; AU8314187A

Abstract

Abstract of the Disclosure A composite immobilized liquid membrane suitable for acid gas scrubbing is disclosed, the membrane comprising a solvent-swollen polymer and a microporous polymeric support, the solvent comprising at least one solvent selected from a class of solvents comprising those solvents with a highly polar group in the molecular structure of the solvent, said highly polar group containing at least one atom selected from nitrogen, oxygen, phosphorous and sulfur, said solvents having a boiling point of at least 100°C and a solubility parameter of from about 7.5 to about 13.5 (cal/cm3-atm)1/2. Said solvents are homogeneously distributed through the solvent-swollen polymer from 20%
to 95% by weight. Also disclosed are methods of acid gas scrubbing of high- and low-Btu gas effluents with such solvent-swollen membranes.

Description

I 1312~28 ¦ !
- I ACID GAS SCRUBBING BY COMPOSITE
SOLVENT-SWOLLEN MEMBRANES

Background of_the Invention Removal of the acid gase~ carbon dioxide and/or hydrogen sulfide from natural ga3, petroleum~
hydrogen, and coal gaq is important from an envlron-mental standpoint since such gases are highly toxic and corrosive, often contributing to the phenomenon known as ~acid rain. n Such ac~d gases are also quite destructive to meth2nation catalysts. Generally, removal is accom-plished by a number of scrubbing processes utilizingab~orbents or solvents which rever~ibly absorb the acid gas. For example, the Benfie~d and Catacarb processes utilize activated carbonate absorbents, the monoethan-olamine and diglycolamine processes use aqueous am~ne solutions, while the Puriso~ and Sulfl!nol processes use simple phy~ical solvents. In the case of the Purisol process, the solvent is N-methyl-Z-pyrrolidone, and raw gas i9 contacted with a countercurrent flow of the absorbing solvent, the solvent thereafter being regen-erated by flashing and ~tripping.
However, all such conventional scrubbing processes are quite costly in terms of capital and operating expen~e since they require absorption in large-volume, high pressure towers, desorption in low pressure generators or stripping columns~ exten~ive ` 1312428 ! pumping for ~olvent recirculation, and the generation of ; ~ubstantial amount~ of ~team for ~tripping. It i~ e~ti-mated that nearly a third of the cost of prodùcing gaseous fuels such as hydrogen and methane from coal S is attributable to coal gaa cleanup by such proce~e3.
Removal of hydrogen sulfide from coal gas with an immobilized liquid membrane comprising carbonate solution ~upported in discrete pore~ of an un~pecified microporous hydrophilic polymer membrane wa~ made by Matson et al. and reported in 16 Ind. En~. Chem._Proc.
Des. Dev~, 370 (1977), the extent of the removal being limited to 15 to 30~. Hydrogen ~ulfide was also removed from a mixture of hydrogen sulfide and nitrogen by Heyd et al. and reported in 2 J. Memb._Sci. 375 (1977), the , removal being effected by un~upported vinylidene flouride polymeric membrane~ modified by the addition of 10~ by weight of various amines and other agent~.
Although the use of 1-methyl-2-pyrrolidone ia di~cloaed as one of the modifiers, the results o~tained were lesa sati~factory than with an unmodified membrane.
In ~he production of ~ynthetic natural gas from coal ga~ (compriaing steam, hydrogen, carbon monoxide, carbon dioxide, methane and ~mall amount~ of hydrogen sulfide)~ the concentration of methane is increased in a ~eries of stepa which involve the removal of carbon dioxide and hydrogen sulfide since carbon dioxidej interferes with the shift conver~ion reaction step and hydrogen aulfide tends to poi~on methanation cataly~t~. In the ~eries of methane-enrichment steps~

.

i312~28 hydrogen and carbon monoxide are deqirably left in the proce~s ga~ ~tream ~ince they partake in the ~hift con-ver~ion reaction prior to the methanation reaction. It would therefore be de~irable to have a method of effi-S ciently and selectively removing carbon,dioxide andhydrogen ~ulfide from coal gas proce~ ~tream~ while leaving carbon monoxideJhydrogen and methane in the ~tream.
Low-stu coal gas i5 produced at lower pre~ure (about 300 p~i) as an alternative fuel for combined-cycle power generation. Removal of hydrogen sulfidefrom such coal gas ~tream3 i~ es~ential to minimize atmospheric pollution by ~ulfur dioxide formed during combu3tion. Bulk removal of carbon dioxide i~ neither nece~ary nor desirable, becau~e expanslon of th$~ inert ga~ in the turbine contributes to its power-generating efficiency. Thus, a proces~ capable of selectively removing hydrogen sulfide from ~uch gas steams while leaving carbon dioxide in the stream i~ requlred.
It i8 therefore an object of this invent~on to provide a novel, inexpen~ive, and effic$ent means for the removal of acid ga~e~ such a~ carbon dioxide and hydrogen sulfide from other ga eY.
It i~ a further object of thi~ invention to provide a novel and efficient means of ~electively removing hydrogen sulfide from a mixture of hydrogen sulfide and carbon dioxide.
It is a further object of thi~ invention to provide a novel and efficient means of selectively separating carbon dioxide from a mixture of carbon dioxide and hydrogen.

J ~

1312~8 , I
It is a further object of thls ~nvention to 'provide a novel and efficient'means of selectively separating hydrogen ~ulfide and/or carbon dioxide from a mixture containing ~uch gases and methane.
It is a still further object o,f this invention to provide a novel means of cleaning both coal gas and natural gas.
These and other object~ are accomplished by the present invention, which i~ summari2ed and particularly de~cribed below.

Brief Description of the Drawinq~
' FIG. 1 is a schematic diagram illustrating an exemplary embodiment of the present invention for acid gaq removal from hiqh-Btu coal gas.
FIG. 2 is a schematic diagram illustrating another exemplary embodiment of the present invention for hydrogen Aulfide removal from low-Btu coal gas.
FIG. 3 is a graph showing a relationship between the composition of a membrane of the pre~ent invention and flux of one acid gas therethrough.

Summary of the Invention According to the pre~ent invention, novel hybrid'membranes are provided that are capable of ~elective removal of the acid gase~ carbon dioxide and hydrogen sulfide from other gases and gas mixtures and that are further capable of selective removal of hydro-gen ~ulfide in preference to carbon dioxide and carbon , -4-` J

.

dioxide in preference to hydrogen. The novel hybrid membrane~ comprise compo~ite immobilized liquid mem-brane~ made of polymer~ that are compatible with and swellable by a class of high boiling point, highly polar S ~olvent~ containing nitrogen, oxygen, phosphorous or ~ulfur atoms, the swollen liquid membranes being sup-ported either on or in the pores of other microporous polymeric support~. The swellable polymer ~ay be crosslinked before, after, or ~imultaneously with infusion of the solvent so as to further improve lts performance characteristics.

Detaile~ Descrietion of the Invention There are broadly two aspects to the present invention. One aspect compri~es novel composite immo-bilized liquid membraneq and the other a~pect compri3es methodq for the selective removal of the acid gases hydrogen sulfide and carbon dioxide from process streams conta~ning such gases.
The novel composite immobilized liquid membrane~ of the pre~ent invention comprise essentially two components: (1) a solvent-swollen polymer supported upon the surface of or in the pores of (2) a microporous polymeric ~upport.
The solvent-swollen polymer is compatible with and swellable by at least one solvent selected from a clas~ of solvents compriqing those solvent~ with a highly polar group in the molecular structure of th~
solvent, ~aid highly polar group containing at least one J
1312~28 ` , .
atom selected from nitrogen, oxygen, pho~phorous and sulfur, said solvents having a boiling point of at least 100C and a ~olubility parameter of from about 7.5 to about 13.5 (cal/cm3-atm)l/2. Such solvents may include S alcohols, amine~, amide~, carbamates, carbonates~ e~tQrs~
ethers, lactams, lactones, morpholines, nitriles, phosphate~, phosphines, phosphites, pyridines, sulfones, 3ul foxides, thiols, thioamides, thioester~, thioether~, thioureas, ureas, and urethanes. Mixtures of such classes of solvents work quite well in the present ~nvention and, in many cases, yield a membrane having performance characteristics superior to those using a ~ingle solvent. An especially preferred mixture compriYes a mix of alkyl and aryl-~ubstituted phosphates with alkyl- and aryl-substituted pyrrolidones, e.g., trialkylphosphates and alkylpyrrolidones: a specific example i8 tri-2-ethylhexylpho~phate with N-cyclohexyl-pyrrolidone. Another preferred mixture of ~olvents comprises dialkylphthalates and alkyl-substituted pyrrolidone~, e.g., dioctylphthalate and N-cyclohex~1-2-pyrrolidone.
; A preferred class of such amine solvents includes ter~iary amine solvents of the general formula ~R3 wherein R is selected from any of alkyl~ substituted alkyl~ cycloakyl~ substituted cycloalkyl, aryl or substituted aryl, the alkyl groups containing from 1 to 20 carbon atoms.
A preferred cla~s of such lactam solvent~ are the cyclic lactams comprising pyrrolidone-type solvents of the general formula ,.
R ' R ~ = o R ' wherein R' i3 alkyl and ~ub~tituted alkyl containing from 1 to 20 carbon atoms.
Substituentq on the alkyl chains~ the cycloalkyl ringq and the aryl group~ in both the ter-tiary amine and pyrrolidone formulas generally includenonreactive groups ~uch as hydroxy, amino, halide, and ether groups. Specific examples of preferred tertiary amine ~lvents with such characteri!~tic~ include octa-decyldimethylamine, tri-N-octylamine, dodecyldimethyl-amine, tri-n-dodecylaminé, tetradecyldimethylamine, hexadecyldimethylamine~ and dimethylhydrogenated tallow amine.
Specific example~ of preferred pyrrolidone solvent~ with such characteristics include N-methyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, 5-methyl-N-cyclohexyl-2-pyrrolidone, N-(2-hydroxyethyl)-2-pyrrolidone, cocoalkyl-2-pyrrolidone, N-dodecyl-2-pyrrolidone, l-dimethyl-2-imidazolidone~ 1~3-dimethyl-3~4,5,6-tetrahydro2(1H)pyrimidone and N-tallowalkyl-2-pyrrolidone.
Specific examples of lactone ~olvent~ having the characteri~tics mentioned above include dodecanolac-tone, qamma-butyrolactone, delta-valerolactone~ alpha-methyl~ -butyrolactone~ valerolactone, .
epsilon-caprolactone, delta-decalactone, 3-methyl-valerolactone, 5,5-dimethyl-ene-butyrolactone, and qamma-decalactone.
Specific examples of ester ~olvents having uch characteristic~ include dimethylmalonate, diethyl-malonate, dibutylmalonate, diethylphthalate, dipentyl-phthalate, dioctylphthalate, diheptylphthalate, dihexylphthalate, ethyllactate, diisodecylphthalate, heptylnonylphthalate, diisodecylfumarate, dinonylglu-tarate and di-n-butylisosebacate.
Specific examplea of carbonate ~olvent~ having ~uch characteri3tics include propylene carbonate~
bis(nonylphenyl)carbonate, bi~(2-ethoxyethyl)carbonate~
dlphenylcarbonate, dibutylcarbonate, and 2,3-butylene carbonate.
I Specific examples of phosphate ~olvent~ having such characteristics include tri(2-ethylhexyl)pho3phate, tributylpho~phate, dibutylphenylphosphate, isode-cyldiphenylphosphate, i30propylphenyldiphenylpho~phate, and trinonylphosphate.
Specific examples of phosphite solvents having such characteristics include trimethylphosphite, triethylphosphite, tripropylphosphite, trii30amylphos-phite, triphenylphosphite, and trinonylphenylphosphite.
Specific example~ of pyridine solvents having ~uch characteristics include 4-(3-pentyl)pyridine, 5-(4-pyridyl)-5-(2-butenyl)-2,7-nonadiene, 4-(5-nonyl)-pyridine~ and 4-(4'-methylpiperidino)pyridine.

. , Specific example~ of amide ~olvents having ~uch characteristics include N,N-dimethylformamide, N,N-dimethylacetamide, tetramethyloxamide, N,N-dibutyl-stearamide, N-ethylacetamide, and N,N-diethylacetamide.
Specific examples of nitrile 301vent~ having ~uch characteri~tics include valeronitrile, octyl-nitrile, glutaronitrile, undecylnitrile, dodecylnitrile, malononitrile, adiponitrile, oleylnitrile, benzonitrile, and phenylacetonitrile.
Specific examples of alcohol solvent~ having such characteri~tics include sec-butylalcohol, l-pentanol, heptanol, l-octanol, l-dodecanol, cyclohexanol, allyl-alcohol, benzylalcohol, 2-ehtylhexanol, triethyleneglycol, polyethyleneglycol-200, ~-cresol, and nonylphenol.
Specific examples of thiol solvents having such characteri~tic~ include dodecylthiol, hexadecyl-thiol, benzylthiol, and butylthiol.
SpQcific exampl`es of thioether ~olvents hav$ng such characteristics include dihexylsulfide, didecyl-~ulfide, diphenylsulfide, thiophene, and tetrahydrothiophene.
Specific examples o sulfoxide ~olvent~ having such characteristic~ include dimethylsulfoxide and tetramethylenesulfoxide.
Specific examples of ~ulfone solvents having such characteristics include 2,~-dimethyl~ulfolane, 3-methylsulfolane, tetrahydrothiophene-l,l-dioxide, methylpropyl~ulfone~ dipropylsulfone~ and tolylxylylsulfone.

131242~

Speci~ic examples of solvents having such characteristics and containing mixed functional groups include 2-acetylbutyrolactone, 4-t2-(dimethylamino)ethyl]-morpholine, N,N'-dimethylaminopropyl-pyrrolidone, anethole, 2-ethoxyethylacetate, tributoxyethylphosphate, tetrahydrofurfuryl alcohol, triethanolamine, 2-amino-ethanol, l,1,3,3-tetramethylurea, N-cyclohexyl-p-toluenesulfonamide, and thiomorpholine.
Specific examples of ether solvents having such characteristics include tetraethylene glycol dimethyl ether, polyethylene glycol, polyphenylether and ethylene glycol dibutyl ether.
Specific examples of morpholines having ~uch characteristics include morpholine, l-morpholino-l-cyclohexene, 4-morpholinecarbonitrile and 3-morpholine-1,2-propanediol.
Specific examples of phosphines having such characteristics include trioctylphosphine oxide, triphe-nylphosphine, and triphenylphosphine dibromide.
Specific examples of thioamideq having such characteristic~ include thioacetamide~ thiobenzamide, thioacetanilide and l,l'-thiocarbonyldiimidazole.
Specific examples of thioe~ters having such characteristics include gamma-thiobutyrolactone, thiocaprolactam and thioethylacetate.
Specific examples of thioureas having such characteri~tic3 include tetramethyl-2-thiourea, 1,1~3~3-tetramethyl-2-thiourea and 2~3-diphenyl-2-thiourea.

131242~

, Specific examples of ureas having such characteristics include tetramethylurea, tetraethylurea and trimethylurea.
By a polymer "compatible" with the clas~ of solvents noted is meant, generally, a polymer that retains such solvents in a single phase, i.e., the polymer forms a homogeneous mixture with the ~olvent.
~bjective tests for determinin~ polymer-solvent com-patibility are known in the art. One such test, popu-larly known as the "blooming" test, comprise~ mixin~ the~ubject polymer and solvent in a volatile solvent such as methylene chloride, tetrahydrofuran or toluene, fla~hing off the solvent, and observing the mixture to determine whether there is either phase ~eparation or "bloomingn--the phenomenon of the solvent in question oo~ing out of the polymer. If either type of separation does occur, the test is considered to show that the polymer and solvent are not compatible.
~y a polymer "swellable" by the class of solvents of the present invention is meant one that can imbibe the solvent in question to the extent that the polymer comprises from about 20 to about 90 wt% of the swollen polymer while at the same time allowing homo-gen00us distribution thereof throughout the polymer.
Under such circumstances the polymer typically ~wells or increases in volume from about 20 to about 1000~.
Suitable solvent-swellable polymers may be broadly described a~ ~lightly polar polymers. Examples of cla~ses of polymers found to be compatible with and '~ ' 1~, 13~2~28 swellable by the solvents of the present invention include polymers and compatible copolymers of any of polyvinylpyrrolidones, polymethacrylate~, polyamide~
polysulfonamides, polysulfones, cellulose acetate, regenerated cellulo~e, polyurethanes, ethylene-vinylacetate copolymers, ethylene-propylene-butadiene terpolymers, polyvinylhalide~, and nitrile rubbers.
Preferred solvent~~wellable polymers are polyvinylpyrro-lidones, polysulfonamides, polyurethanes, and poly-10 methacrylates.
Such solvent-swellable polymer~ may be cross-linked before, after, or simultaneously with infusion by solvent so a to further enhance performance character-istic~ of the novel composite membranes of the present invention. Exemplary suitable methods of crosslinking include crosslinking by free radical generators such as peroxides (e~g., ammonium peroxydisulfate and dicumyl peroxide) and diazo compounds (e.g., 2,2'-azobis(iso-butyronitrile)) and by other crosslinking agents such as ethylenedimethacrylate, tetraethyleneglycoldimethacry-late, trimethylolmethacrylate, ethoxylated bisphenol-A
diacrylate, divinylbenzene, N,N-diallyltartardiamide, triallyl-1,3,5-benzenetricarboxylate, N-N'-methylene bisacrylamide, methyl diisocyanate, toluyl diisocyanate, trimesoyl chloride, and other di- and tri-functional isocyanates, acid halides and vinyl compounds. When crosAllnking is accomplished simultaneously with solvent addition, the polymer may even be in its monomeric form~

the cro3slinking taking place simultaneously with polymerization.

1312~2~

The microporous polymeric support of the composite membrane of the present invention may be generally de3cribed a~ being re~istant to attack by the solvents of the present invention, as having surface pore~ in the range of from about 0.001 to about 1.0 micron in diameter, and as having ~ufficient tenslle strength to withstand transmembrane pressure differen-tial~ of at least 100 psi, but preferably 200 to 1500 p9i. Suitable candidates include polyamide~, especially the polycondensation product of hexamethyl-enediamine with adipic acid, or nylon 66, cellulose acetate, crosslinked polysulfone, regenerated cellulose, polyethersulfone, polypropylene, and polytetrafluoro-ethylene, with thicknesses varying between 20 and 300 microns. The support may be in the form of flat 31leet~
or hollow fibers, with the solvent-swollen polymer coating on either the outside or in~ide (lumens) of the hollow fibers.
The solvent-swollen polymer portion of the composite membrane~ of the present invention may be sup-ported either directly upon the surface of the micro-porous polymeric support or within the pores thereof.
When the solvent-swollen membrane is on the ~urface of the upport, a particularly preferred form of the solvent-swollen polymer is that of a thin (from about 0.1 to about 20 microns) asymmetric film. By "asymmetric film" is meant a microporous film with a generally non-porous "skin" over the top thereof.

1312~28 Referring now to the drawlngs, the removal of acid ga~es from high-Btu-containing coal gas in a variation of the Synthetic Natural Gas (SNG) process is illustrated in FIG. l, while FIG. 2 illustrates the removal of hydrogen sulfide from low-Btu-containing coal gaq in combination with a Combined-Cycle Power Plant and a Claus Plant.
In the SNG process, the concentra~ion of methane gas from the off gas of coal gasification is enriched in a series of steps to produce a clean, high-Btu-containing gas. The crude gaseous byproduct of coal gasification is a high pressure (on the order of 1000 psi) mlxture of steam, hydrogen, carbon monoxide~
carbon dioxlde, methane and trace quantities of hydrogen sulfide and nltrogen. Thls mlxture iq passed through a shift converter where the reaction noted below occurs, CO + H20~ C2 + H2 thereby increaslng the H2/C0 ratio to about 3:1, a~
required ln the qubsequent methanatlon reaction 3H2 ~ C0 - ~ CH4 + H20 As illuqtrated in FIG. l~ a hot high-Btu-containing coal gas qtream from a shift converter at about lO00 psla i5 passed through a heat exchanger to cool it to an intermediate temperature, for example, about 30C above ambient, and thence through a ga~/liquid contactor where the stream is saturated with solvent of the same type with which the membrane of the present invention is swollen. Solvent i~ provided both from a storage source and from recycled condensed J
1312~28 solvent, a~ explained further below. The so-saturated gas feed stream is directed against the membrane of the present invention at substantially the same pressure it leaves the ~hift converter (about 800 to 1200 psia~.
The membrane separation unit may comprise cylindrical modules of spirally-wound flat sheet membranes or longitudinally-oriented hollow fibers. The feed stream comprising C02, H2, CH4, C0, N2 and H2S as major com-ponents (excluding water vapor) is split by the membrane unit into two streams on either 3ide of the membrane--a permeate stream and a residue stream. In additlon to containing solvent vapor from the membrane, the permeate stream is rich in concentrations of those gaseous com-ponents to which the membrane is more permeable, that is, hydrogen sulfide and carbon dioxide, while the resi-due stream is rich in concentrations of the remainder of the gaseou~ components of the feed stream, which includes the hydrocarbon gas CH4 and its mutually reac-tive component~ H2 and C0. A sweep stream composed of an inert ga~ i~ in con~tant contact with the permeate side of the membrane to dilute and entrain the permeate gaqes and separate them from the process system. Pres-sure of approximately l atm on the permeate side of the membrane is preferably maintained, and preferably at least lO0 psi less than on the feed stream side. In connection with the SNG process, the inert gas is pre-ferably nitrogen ina~much as that gas is generally available as an off gas from an air separation unit (not shown) that provide~ oxygen to the coal gaq gasifier.

-15- ~

1312~28 Each of the permeate and residue streams may be cooled in heat exchangers whereby the solvent vapor in the Qtreams i~ condensed and thereafter returned to the gas/liquid contactor for recycling.
As illustrated in FIG. 2, a lo~-Btu-containing coal gas stream at about 300 psia, useful in a Combined-Cycle Power Plant iq similarly cooled to an intermediate temperature in a heat exchanger, the ~tream being split by a membrane ~eparator into a permeate stream relati-vely rich in H2S and a residue stream containing the remainder of the gaseou3 components and being relatively rich in C02. In this case, the permeate sweep stream may comprise air, which entrains the H2S and advan-tageously helps oxidize H2S to elemental ~ulfur in a Claus plant. Pressure on the permeate side of the membrane is maintained at least 100 psi lower than that on ~he feed side, and preferably on the order of 14 to 20 psia. Relatively clean (~0.1% H2q) coal gas remains ln the residue ~tream, which is useful as a fuel in Combined-Cycle Power plant.
Example 1 A solution was prepared comprising 5.46 g of N-vinyl-2-pyrrolidone (the monomeric precursor of polyvinylpyrrolidone), 1.24 g of isodecylmethacrylate (a co-monomer to N-vinyl-2-pyrrolidone), 0.15 g polyvinylpyrrolidone (PVP)~ 1.15 g of ethoxylated Bisphenol-A-diacrylate (a crosslinking agent)~ 0.1 g of ; dicumyl peroxide (a polymerization initiator)~ 0.1 g of 2,2-dimethoxy-2-phenylacetophenone (an activator for t ~ .

the polymerization initiator), and 2.0 g N-methyl-2-pyrrolidone (NMP). A th~n film (approximately 50 micron~) of this ~olution wa~ cast onto a gla~s plate and irradiated with ultraviolet light for one minute, cau~ing partial polymerization of the NMP-swollen polymer, whereupon it was contacted with a microporous polymeric support of nylon 66 approximately 125 micron~
in thickness ~old under the trade name Ultipor*NDG and made by Pall Corporation of Glen Cove, New York. After contact with the support, polymerization was completed by ultraviolet radiation for another ~ix minutes. The support membrane wa~ removed from the glass plate, carrying the NMP-swollen PVP crosslinked polymer with it and containing 20% by weight NMP.
Example 2 An anisotropic microporou~ polymeric support membrane of cellulo~e acetate (CA) manufactured by Gracesep Mfg~ Ltd. of ~end, Oregon was partially hydro-lyzed by cutting it into 4 x 10-inch strips and placing them in a vessel containing a solution of 2.0 wt%
triethylamine in water, The solution was constantly agitated during the hydrolysis reaction for six hoursO
The partially hydrolyzed strips were removed from the solution and rinsed in running water for two hours.
Water was then removed from them by solvent exchange with first isopropanol and then hexane for 20 minutes each, followed by air drying.
The compatibility of a polyurethane polymer made by Hexcel Corp, of Chatsworth, California and sold *trade-mark under the name Uralite*6115 with the solvent N-cyclohexyl-2-pyrrolidone was verified by allowing the polymer to imbibe up to 400 wt~ of the solvent with no phase separation or 103s of solvent, the polymer swelling in volume approximately 750~.
The CA support prepared as outlined above was then coated with a solvent-swollen crosslinked poly-urethane gel prepared as follows: 20 wt% of the mono-meric precursor components of Uralite*6115 containing an initiator, a catalyst, and a methyl-diisocyanate cross-linking agent was mixed with 80 wt% N-cyclohexyl-2-pyrrolidone and placed in an oven at 100C for six hours. The resulting partially-gelled crosslinked poly-meric mixture was diluted to about 35 wt% with toluene and sprayed onto the cellulose acetate support with an air brush to a thickne~s of about 15 microns. The so-coated support was covered with aluminum foil and left at room temperature for 24 hours before use.
Examples 3-6 The gas permeation properties of the composite gel-coated membrane of Example 2 were studied with respect to carbon dioxide, hydrogen, hydrogen sulfide and methane. Discs of the membrane 47 mm in diameter were placed in Millipore~ high-pressure filter holders and the coated side exposed to 100 psig of each of the gases except h~drogen sulfide, which was at 10 psig~ and the flow of gas mea~ured after five minutes of s~ch expo~ure. The flux (reported in units of *trade-mark t -18-SCFH/ft2-10,000 psi) and selectivity (ratio of fluxes) for each membrane is shown in Table I below.
Table I

Ex. Flux Selectlvit No. C02H2 -H2S CH4Co2/H2H2S/C~2 H2S/CH4 C02/C~4 _ _ . , 3 11613.8 695 4.5 8.4 6.0 153 25.8 4 91 7.9 821 5.6 11.69.0 146 16.3 5 87 5.9 721 5.1 14.78.3 140 17.1 6 12523.3 9~9 9.8 5.4 7.3 92 12.8 As is apparent from the above data, the com-posite membranes of the present invention showed high selectivity toward carbon doxide over hydrogen, hydro-gen sulfide over carbon dioxide, hydrogen sulfide over methane and carbon dioxide over methane, thus making them excellent candidates for acid gas scrubbing appli-cations.
Exa~le 7 A simulated study was conducted for 98%
removal of H2S and 93% removal of C02 from a high-Btu-containing coal gas feed 3tream from a shift converter at 1000 psia and 20DC and having the volumetric compo-sition noted in Table II (omitting water vapor). The feed ~tream in the 3tudy was split into a permeate and a residue Qtream by a serîes of spiral-wound modules containing the solvent-swollen membrane of Example 2 and having a combined surface area of 1.89 x 106 ft2.

--19-- ., J , 1~

, ..
Pressure on the permeate side of the membrane was main-tained at 20 psia. The results are shown in Table II.
Table II
_, , _ Compo~i tion (vol% 1 Purified Permeating 5ComponentFeed _ Coal Gasl Gases* _ 2S 0.5 <0.01 >0.49 C2 33.3 2.~ 7~.0 H2 32.2 48.5 9.0 ~0 10.1 15.5 2,5 10 CH4 23.1 32.0 10~4 N2 0.8 1.5 0.8 Total 100 100 100 Flow Rate*670.5395.5 275 (106 SCFD) _ _ _ *The N2 ~weep steam flow rate of 308.6 x 106 SCFD add~
to the total flow rate of the permeate.
Example 8 A ~imulated study was conducted for 92%
removal of H2s from a low~Btu-containing coal gas feed ~tream at 300 psia and 20C and having the volumetric composition noted in Table III (omitting water vapor)O
The feed ~t~eam in the study was split into a permeate and a residue stream by the same type of membrane modu-les as in Example 7 having a combined ~urface area of 2.57 x 104 ft2. Pressure on the permeate side of the membrane was maintained at 20 psia. The re~ults are shown in Table III.

, -20- ~

`,:

13~2~28 Table III

~ ~ Com osition (vol~) P Purified Permeating Component Feed Coal Gas Gases H25 1 0.086 14.4 S C2 15 11 73.7 H2 22 23 4.41 CO 19 20 3.4 CH4 5 5 1.6 N2 38 41 2.5 Total 100 100 100 .

Flow Rate _ 250 233 17 Exampleq 9-13 Other highly permeable and acid-gas selective compoqite gel-coated membranes were prepared ln the ~ame manner as in Example 2 u~ing the solventq noted in Table IV, except that the CA support membrane wa~ not partially hydrolized (an unneces~ary ~tep in these examples that is otherwi~e performed to improve the reqistance of the CA support membrane toward swellin~ by certain solvents, such as N-cyclohexyl-2-pyrrolidone ~NCHP)).
The gas permeation properties of the composite gel-coated membranes were studied using the methods described in Examples 3-6. The results are shown in ~able IV below.

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J
1312~28 Table IV

. _ _ _ Ex. Flux _ Selectivitv No. Solvent C02 H2 H2S cH4 C027H2 H2S/C2 H2S/CH4 C02/CH4 9 dipentyl- 480 - 1180 10.7 _ 2.5 110 44.9 phthalate 4-(4'methyl- 340 350 3090 8.8 0.97 9.1 350 39 piperidino)-pyrldine . 11 anethole 270 - - 7.5 _ _ _ 36 12 diethyl~ 360 - - 10.4 _ _ _ 34.6 phthalate 13 N-dodecyl- 270 250 990 7.0 1.1 3.7 140 39 pyrrolidone __ . _ . ._ Example 14 For specifi~ separations, membrane properties may be optimized in terms of permeate flux and selec-tivity by using suitable swelling solvent mixtures in the gel-composite membrane. Membranes were prepared and tested as described in Examples 9-13 u~ing swelling-solvent mixtures of from 20 wt~ to 75 wt% dioctyl-phthalate ~DOP) in NCHP. The result~ are shown inFIG. 3 with flux reported in the same units as in Example~ 3'6. As seen in FIG. 3, the optimum DOP~NCHP
solvent ratio for a high C02-flux gel-membrane is 1:1.
The high C02-flux gel-membrane also gave a high C02/CH4 separation factor of 27~ making it a very useful membrane for separating carbon dioxide from methane gas streams.

-22- .

1312~28 Example 15 A membrane was prepared and tested as described in Examples 9-13, using a swelling solvent mixture of 75 wt% tri-2-ethylhexylphosphate (TEHP) in NCHP. This membrane exhibited high fluxes for CO2 and H2S (850 and 3,250 SCFH/ft2-10,000 psi, respectively), while maintaining high C02/CH4 and H2S/CH4 separation factor~ of 30 and 125, respectively.
Example 16 Solvent-swollen gel membranes were prepared by first mixing 2 g of an ethylene-vinyl acetate copolymer resin that had 25 wt% vinyl acetate content and 6 ml NCHP in 20 ml of toluene until a homogeneous polymer solution was obtained. A 4-mil layer of solution was then ca~t on a glas~ plate using a doctor blade. The plate was allowed to stand at room temperature for 90 minutes to allow evaporation of the toluene and coagula-tion of the polymer/NCHP into a rubbery gel film about 1.1 mil thick. The gel film was removed from the glass plate and placed on the surface of a microporous cellu-lose acetate support membrane of the type described in Example 2.
The gas-permeation properties of the so-prepared compo~ite gel-cellulo~e acetate membrane was measured ~or carbon dioxide~ nitrogen~ and hydrogen.
Discs of the membrane 47 mm in diameter were placed ln a MilliporeO high-pressure filter holder, the gel side was exposed to 114 psia carbon dioxide, 914 psia nitrogen~
and 64 psia hydrogen~ and the flow of gas was measured after 20 minutes of such exposure.

f~

1312~28 The pressure-normalized gas fluxe~ for the membrane reported in the same units as ln Examples 3-6 were 3900 for carbon dioxide, 430 for hydrogen~ and 480 for nitrogen; selectivities were 9.0 for C02/H2 and 81 for C02/N2. Estimated selectivit~es fo~ H2S/CO2 H25/CH4 were >5 and >25, respectively.
The terms and expre~sions which have been employed in the foregoing specification are u3ed therein as term~ of description and not of limitation, and there 0 i9 no intention, in the use of such terms and expres-sions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims

1. A composite immobilized liquid membrane comprising:
(a) a microporous polymeric support; and (b) a solvent-swollen polymer compatible with and swellable by at least one solvent selected from a class of solvents comprising those solvents with a highly polar group in the molecular structure of the solvent, said highly polar group con-taining at least one atom selected from nitrogen, oxygen, phosphorous and sulfur, said solvents having a boiling point of at least 100°C and a solubility parameter of from about 7.5 to about 13.5 (cal/cm3-atm)1/2.

2. The membrane of claim 1 wherein the solvent is selected from alcohols, amines, amides, carbamates, carbonates, esters, ethers, lactams, lactones, morpholines, nitriles, phosphates, phosphines, phosphites, pyridines, sulfones, sulfoxides, thiols, thioamides, thioesters, thioethers, thioureas, ureas, urethanes and mixtures thereof.

3. The membrane of claim 1 wherein the solvent is a solvent of the formula NR3 or wherein R is alkyl and substituted alkyl containing from 1 to 20 carbon atoms, cycloalkyl, substituted cycloalkyl, aryl and substituted aryl, R' is alkyl and substituted alkyl containing from 1 to 20 carbon atoms, the substituents in both R and R' groups being selected from the group consisting essentially of hydroxy, amino, halides and ethers.

4. The membrane of claim 1 wherein said solvent-swollen polymer has 20% to 95% by weight of said solvent homogeneously distributed therethrough.

5. The membrane of claim 1 wherein said solvent-swollen polymer is contained within the pores of said microporous polymeric support.

6. The membrane of claim 1 wherein said solvent-swollen polymer is in the form of a thin film on the surface of said microporous polymeric support.

7. The membrane of claim 1 wherein said solvent-swollen polymer is selected from polyvinyl-pyrrolidones, polysulfonamides, polyureas, polyurethanes, polyacrylates, polymethacrylates, polyesters, poly-amides, polysulfones, cellulose acetates, regenerated celluloses, ethylene-vinylacetate copolymers, ethylene-propylene-butadiene terpolymers, polyvinylhalides, nitrile rubbers, copolymers, and mixtures thereof.

8. The membrane of claim 1 wherein said solvent is selected from mixtures of (a) alkyl- and aryl-substituted phosphates and (b) alkyl- and aryl-substituted pyrrolidones.

9. The membrane of claim 8 wherein said alkyl- and aryl-substituted phosphate is selected from trialkyl- and triaryl-substituted phosphates.

10. The membrane of claim 9 wherein said solvent comprises a mixture of tri-2-ethylhexylphosphate and N-cyclohexylpyrrolidone.

11. The membrane of claim 1 wherein said solvent is selected from N-methyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, N-dodecyl-2-pyrrolidone, N-(2-hydroxyethyl)-2-pyrrolidone, cocoalkyl-2-pyrrolidone and N-tallowalkyl-2-pyrrolidone.

12. The membrane of claim 1 wherein said microporous polymeric support is selected from nylon 66, asymmetric cellulose acetate, regenerated cellulose, crosslinked polysulfone, polyethersulfone, polyethylene, polypropylene, and polytetrafluoroethylene.

13. The membrane of claim 1 wherein said solvent-swollen polymer is selected from polyvinyl-pyrrolidone and polyurethane and said solvent is selected from N-methyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone diethylphthalate, dipentylphthalate, 4-(4'-methyl-piperidino)pyridine, anethole, N-dodecyl-pyrrolidone, mixture of dioctylphthalate and N-cyclohexyl-2-pyrrolidone, and mixtures of tri-2-ethylhexylphosphate and N-cyclohexyl-2 pyrrolidone.

14. The membrane of claim 1 wherein said solvent-swollen polymer is an ethylene-vinyl acetate copolymer and said solvent is N-cyclohexyl-2-pyrrolidone.

15. The membrane of claim 1 wherein said solvent-swollen polymer is crosslinked by a crosslinking agent selected from peroxides, diazos, vinyls, acid halides, and isocyanates.

16. The membrane of claim 15 wherein cross-linking of said crosslinked solvent-swollen polymer is accomplished by the use of a crosslinking agent selected from ethylenedimethacrylate, tetraethylenaglycoldimetha-crylate, trimethylolmethacrylate, ethoxylated Bisphenol-A diacrylate, divinylbenzene, N,N-diallyltartardiamide, triallyl-1,3,5-benzenetricarboxylate, N-N'-methylene bisacrylamide, methyl-diisocyanate, toluyl diisocyanate and trimesoylchloride.

17. The membrane of claim 15 wherein said solvent-swollen polymer is polyvinylpyrrolidone and crosslinking is accomplished simultaneously with polymerization of a monomeric precursor of polyvinylpyrrolidone.

18. A method for the separation of hydrogen sulfide and carbon dioxide gases from hydrogen, carbon monoxide and hydrocarbon gases comprising splitting a feed stream comprising all of said gases with the membrane of claim 1, 7, 8, 11, 13, 14 or 15 into a permeate stream on one side of said membrane rich in hydrogen sulfide and carbon dioxide and a residue stream on the other stream side of said membrane rich in the remainder of said gases.

19. The method of claim 18 wherein the partial pressures of hydrogen sulfide and carbon dioxide on the permeate stream side of said membrane are less than the partial pressures of such gases on the feed stream side of said membrane.

20. The method of claim 18, additionally comprising a sweep gas stream on the permeate side of said membrane.

21. The method of claim 20 wherein said sweep gas comprises air.

22. The method of claim 20 wherein said sweep gas stream comprises an inert gas.

23. The method of claim 22 wherein said inert gas comprises nitrogen.

24. The method of claim 18, additionally comprising saturating either or both of said feed stream and said sweep stream with the same solvent with which said membrane has been swollen.

25. The method of claim 24 wherein said saturating solvent is recovered from either or both of said permeate stream and said residue stream and is recycled to either or both of said feed stream and said sweep stream.

26. The method of claim 25 wherein said recovery is by condensation of solvent vapor in said permeate and residue streams.

27. The method of claim 26 wherein said con-densation is accomplished by means of a heat exchanger.

28. A method for the separation of hydrogen sulfide gas from carbon dioxide gas comprising splitting a feed stream comprising both of said gases with the membrane of claim 1, 7, 8, 11, 13, 14 or 15 into a hydrogen sulfide-rich permeate stream and a carbon dioxide rich residue stream.

29. The method of claim 28 wherein the partial pressures of hydrogen sulfide and carbon dioxide on the permeate stream side of said membrane are less than the partial pressures of such gases on the feed stream side of said membrane.

30. The method of claim 28, additionally comprising a sweep gas stream on the permeate side of said membrane.

31. The method of claim 30 wherein said sweep gas comprises air.

32. The method of claim 30 wherein said sweep gas comprises an inert gas.

33. The method of claim 32 wherein said inert gas comprises nitrogen.

34. The method of claim 30, additionally comprising saturating either or both of said feed stream and said sweep stream with the same solvent with which said membrane has been swollen.

35. The method of claim 34 wherein aid saturating solvent is recovered from either or both of said permeate stream and said residue stream and is recycled to either or both of said feed stream and said sweep stream.

36. The method of claim 35 wherein said recovery is by condensation of solvent vapor in said permeate and residue streams.

37. The method of claim 36 wherein said con-densation is accomplished by means of a heat exchanger.