CA1305074C

CA1305074C - Process for separating co -from other gases

Info

Publication number: CA1305074C
Application number: CA000517436A
Authority: CA
Inventors: Ralph M. Kelly
Original assignee: CYNARA COMPANY (THE)
Current assignee: CYNARA Co (THE)
Priority date: 1985-09-09
Filing date: 1986-09-04
Publication date: 1992-07-14
Anticipated expiration: 2009-07-14
Also published as: NO168872C; EP0214641A2; NO863576L; NO168872B; DE3688724D1; NZ217495A; EP0214641B1; AU6217686A; US4659343A; AU599331B2; EP0214641A3; DE3688724T2; NO863576D0

Abstract

ABSTRACT

An improved process is described for separating carbon dioxide from light hydrocarbons by means of a membrane. The membrane separation is operated so that hydrocarbon liquids are condensed in contact with the membrane. This process is particularly valuable for the separation of gas mixtures into separate acid gas, gaseous hydrocarbon and liquid hydrocarbon streams.

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Description

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IMPROVED PROCESS FOR SEPARATING

This is an improved process for separating carbon dioxide from hydrocarbons in a 1uid mixture using a semi-permeable membrane. In particular, this process involves condensation of hydrocarbon liquids from the fluid in contact with the membrane.

Numerous references exist which describe various processes for separating gases by means of semi-permeable membranes~ U.S. Patent No. 3,415,038 described a method for separating a first gas from a gaseous mi~ture using a thin, dry, asymm~tric cellulose acetate membrane. U.S. Patent Nos. 3,842,515, 4,080,744, 4,080,743 and 4,127,625 describe other techniques for drying a water-wet membrane to be used in gas separation. U.S. Patent 4,130,403 teaches the separation of CO2 from a natural gas stream by means of a dry cellulose~ester membrane. U.S Patent. 4,374,657 discloses the separation of acid gases from gaseous hydrocarbon mixtures substantially free of methane by use of membranes. U.S. Patent 3,616,928 describes : : :

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vertical orientation of hollow fiber membrane devices to promote drainage of condensed liquid hydrocarbons during separation of hydrogen from gas mixtures.

The spiral membranes employed in the prio~
art for separation of carbon dioxide from light hydro-- carbons are-generally not suited to handling liquids.
Many membrane compositions, such as those in U.S.
` Patent 4,23~,463, are deleteriously affected by liquid hydrocarbons. Heretofore, it has generally been thought that condensible hydrocarbons should be first separated from the carbon dioxide-containing gas mixture prior to bringing it in contact with the membrane.

The subject invention is a process for sep-arating carbon dioxide from a fluid mixture comprising carbon dioxide, at least one Cl or C2 hydrocarbon and at least one C3 to C8 hydrocarbon which process is characterized by (a) contacting a liquid-hydrocarbon tolerant membrane selectively permeable to carbon . dioxide with said fluid mixture; (b) permeating carbon dioxide through the membrane at conditions which result in the presence of liquid hydrocarbons in the fluid in con-tact with the membrane; and (c) separating the liguid hydrocarbons from the gases which do not per-meate the membrane; and, optionally (d) contacting the non-permeate gases ~rom Step (c) with a membrane selec-tively permeable to carbon dioxide, (e) permeating carbon dioxide through the membrane to produce permeate and non-permeate gas streams, and (f) recovering gaseous hydrocarbons from the non-permeate, Surprisingly, it has been found that the recovery and gas purity of the carbon dioxide stream from the membrane is not adversely 34,~68-F -2-.,, ~

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affected by the presence of liquid hydrocarbons in the feed.

In another embodiment, this inven-tion repre-sents a process for separation of carbon dioxide from a - 5 mixture of at least one Cl or C2 hydrocarbon and at least one C3 to C8 hydrocarbon. In this process, the fluid mixture is contacted with a first membrane and carbon dioxide is selectively permeated through the membrane. The non-permeate is separated intc a gaseous stxeam containing essentially all the carbon dioxide and methane present in the non-permeate and a liquid~
stream containing essentially all the C4 and heavier hydrocarbons. The resulting gaseous stream is sep-arated with a second membrane through which carbon dioxide is selectively permeated. The gaseous non--permeate consists predominantly of ethane or methane.
In this alternative process, liquids are not neces-sarily present in contact with either the first or second membrane, but optionally can be present.
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Figures 1 and 2 are schematic illustrations of embodiments of the process of this invention.

Membranes selectively permeable to carbon dioxide as used in this inYention are known in the prior.art. To be operable in the process wherein liquid hydrocarbons contact the membrane, a membrane should be selected which is not deleteriously affected by exposure to Cl-C8 liquid hydrocarbons. Illustrative membranes are described in U.S. Patents 3,415,038;
3,842,515; 4,080,743; 4,127,625; 4,130,403; and 4,421,529, as well as British Patent Mo. 1,478,085.

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~L3~J5~4 ~4--The skilled artisan can readily determine experi-mentally which of these membranes are adversely affected by exposure to li~uid hydrocarbons.

Operable membranes include organic polymers and copolymers, optionally containing adjuvants such as - fillers, plasticizers, stabilizers and permeability modifiers. Illustrative polymer compositions suitable for use in membranes can be selected from polysul~one, polyethersulfone, styrenic polymers and copolymers, polycarbonateæ, cellulosic polymers, polyamides, poly-imides, polyethers, polyarylene oxides, polyurethanes, polyesters, polyacrylates, polysulfides, polyolefins, polyvinyls and polyvinyl esters. Interpolymers, including block repeating units corresponding to the foregoing polymers, as well as graft polymers and blends of the foregoing, are suitable for use in mem- -branes. The aforementioned polymers can operably bear substituents, such as, fluoro, chloro, bromo, hydroxyl, alkyl, alkoxy, acyl or monocyclic aryl groups, so long as the substituents do not deleteriously affect the membrane properties.

Preferred as membranes are cellulose esters, e.g., cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose propionate, cellulose butyra~e, cellulose cyanoethylate, cellulose methacrylate and mixtures thereof. Mixed esters of cellulose, such as cellulose acetate butyrate, mixed cellulose acetates and cellulose acetate methacrylate, are also operable.
Commercial cellulose triacetate, containing from 42.7 to 44 w~ight percent acetate, is the material of choice for the mem~ranes used in the subject method.

3~,268-F -4-': ' o~

Inasmuch as the flux of materials permeatingis generally inversely related to the membrane thick-ness, it is desirable that the d~scriminating layer of the membrane be as thin as possible while main-taining adequate membrane strength and good rejection. Homo-geneous membranes are operable, but asymmetric mem-branes are preferred. The preferred asymmetric cel-lulose ester membrane.will typically have a dense discriminating layer less than one micron (0.000001 m) thick and a much thicker, relatively porous supporting sublayer.

Composite membranes, which have a porous supporting layer of dissimilar composition providing additional strength and integrity to the discriminating layer, are also preferred. For example, microporous polysulfone materials can be used as a supporting layer. Of course, this dissimilar support can include a second discriminating layer, but generally a second discriminating layer is neither necessary nor desir-able.

The term "membrane" as used herein isintended to encompass a wide variety of possible con-figurations known in the prior art. For example, the . membrane may be used as a flat film, tubular film o-r hollow iber. The membrane can also be a spiral device, provided it is designed to accommodate liquids present in contact with the membrane.
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A hollow fiber membrane is generally pre-ferred and can be readily prepared by techniques known in the art. The internal and outside diameter of the 34,268-F -5-,~

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fiber can operably be varied to modify membrane charac-teristics. The inside diameter is preferably 30 to 400 microns (0.00003 to o.oooa m) with a wall thickness of 5 to 150 microns (0.000005 to 0.00015 m). Especially preferred are cellulose triacetate fibers having an inside diameter of 70 to 130 microns (0.00007 to 0.00013 m) and a wall thickness of 7-5 to 110 microns (0.000075 to 0.00011 m). Preferably, the wall thick-` ness is at least 15 percent of the internal diameter Membranes suitable for use in the subject invention are commercially available. For example, water-wet cellulose ester membranes are available in both hollow fiber and spiral devices. To render these films suitable for the separation of non-aqueous fluids the film must be carefully dried so as to avoid sig-nificant disruption of the membrane structure. A
preferred method for drying cellulose ester membranes is described in U.S. Patent 4,~30,807. One preferred technique for drying the water-wet membrane is to first anneal the fiber in 80C. water for about 1.5 minutes. The water is then extracted from the fiber with isopropanol and the isopropanol displaced with hexane, heptane or isooctane in the manner taught in U.S. Patent 3,842,515. A particularly preferred technique for drying water-wet hollow fiber membxane bundles is to introduce a 50:50 volume percent mixture of isopropanol and isooctane down the bore of each fiber while an inert gas stream is passed over the hollow fiber's outer surface. When the fiber is essentially free of water, the introduction of the isopropanol/isooctane mixture is terminated and the guid remaining in the bore pervaporated through the . 34,268-F -6-, membrane. Generally, such membranes will exhibit a C02 flux of at least about lX10 6 cm3/(sec cm2 cm of Hg) at 10C. and 50 psig (345 kPa) with a pure CO2 feed.

As some snrinkage occurs in drying the hollow fiber, if the fiber is assembled in a bundle prior to drying, the construc~ion of the bundle should allow tolerance for some shrinkage. For example, if a per-forated core is employed, it should be designed so that some reduction in length will take place as the fibers shrink. Also, the epoxy resin tubesheet should be cured with an agent which pxomotes good adhesion with the hollow fibers, e.g., a commercial aliphatic amine curing agent.

The 1uid in the feed stream contains carbon dioxide along with hydrocarbon liquids, gases or mix-tures of hydrocarbon liquids and gases. Preferably, the predominant components present in addition to carbon dioxide are lower (C1-C5) saturated aliphatic hydrocarbons, such as methane, ethane, butane and propane, nitrogen and other components produced in association with crude oil. Preferably, such feed streams will contain from 20 to 90 volume percent CO2 with a remaining amount of lower aliphatic hydrocarbons and optionally a small amount (preferably less than 10 percent) of H2S. For the instant separation process to ; work most efficiently, the feed gas preferably contains at leàst 10 volume percent CO2.

The feed stream is advantageously substan-tially free of water. Water can lead to undesirable hydrate formation and may necessitate drying of the ' ~ 34,268-F -7-.
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~l3~ 74 carbon dioxide product later. With some membranes, minor amounts of water can be tolerated although the long-term performance of the membrane may sufer. Of course, the feed stream can be dried by use of desic-cants or other techniques well known in the art.

- - Depending on the pressure and temperature of the feed stream, it can be present as eithe~ a gas or liquid. As the carbon dioxide selectively permeates through the membrane, the hydrocarbon dew point of the fluid not permeated gradually increases with increasing concentration of C3 and higher hydrocarbons. Moreover, in certain preferred embodiments of the invention the expansion of the permeate reduces the temperature of the permeate, which efficiently cools the non-permeate through thermal cross exchange in hollow fiber membrane devices. Unlike prior art processes, li~uid conden-sation of C3 and higher hydrocarbon components in contact with the membrane is encouraged.

Advantageously, the condensed liquid present in the feed stream is not vaporized while in contact with the membrane. The fluid in contact with the membrane can exist as a two-phase mixture of gas and liquid. Optionally, the membrane device can be oriented vertically and adjacQnt membrane hollow fibers or membrane sheets spaced apart to promote movement of condensed liquid to a collection point in the housing of the membrane device where liquid can be separated and removed. Alternatively, the two-phase mixture can be conveyed from the membrane device to a conventional gas/liquid separator.

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In the prior art membrane processes for separation of carbon dioxide from metha~e, the feed is pretreated by condensation at low temperatures to remove components which might otherwise condense in the membrane and the feed temperature is adjusted as neces-sary to prevent condensation. Alternatively, the feed gas is heated substantially a-bove the hydrocarbon dew point in prior art techni~ues. It has now been found that these precaut;ons are not necessary. Accordingly, improved separation factors are possible by operation at lower temperatures and the operating and capital cost of a system can be reduced due to elimination of pretreatment equipment to remove liquifiable components.

The pressure of the fluid feed stream can vary over a wide range dependent upon physical charac-teristics of the membrane. A pressure of from 25 to 1,000 pounds per square inch gauge (psig) (172 kPa to 6895 kPa) is typically operable. Higher pressures are operable if the membrane is not deleteriously affected.
All other operating parameters being e~ual, the carbon dioxide flux of the membrane generally increases with increased pressure in the feed stream.

The differential pressure across the membrane ca~ also operably vary over a considerable range.
Preferably, the pressure on the feed side of the mem-brane should be at least ~0 psi (276 kPa) greater than that on the permeate side of the membrane. A pressure differential of at least 100 psi (689 kPa) is more preferred.

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The temperature at which the carbon dioxideseparation is conducted should be such that liquids will be present in contact with cir formed in contact with the membrane on the feed side in at least one stage. Temperatures in the range from 19C to 40C
are typically operable, dependent on a variety of factors. The optimum operating temperature depends- -upon the feed composition, the identity of the membrane and the pressure. The optimum temperature can be determined empirically.

Cooling o the feed stream and membrane from ambient temperatures to the temperature desired for separation can be accomplished by conventional refr:i-geration techniques or any other convenient means. In some embodiments of the invention, it is desirable to promote hydrocarbon condensation by cooling the feed before introduction to the membrane. The feed temper-ature is then adjusted as desired by heating. In one preferred embodiment, the feed stream is cooled in part by heat exchange with the permeate or non-permeate from the membranes.

The subject process can be conducted in a single membrane stage, but multiple membrane stages optimized for specific temperatures and feed composi-tions are generally preferred.

Several of the instant membrane separationunits can be operated in parallel to increase the overall capacity of the separation device. Alter-natively, several membranes can be employed in series to improve separation performance. The optimum number ::

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of membrane stages depends upon the feed composition, nature of the membrane, the process pressure, the permeate pressure, feed ternperature and other ~actors.
Not all the membrane stages need operate with liquids present in the fluid feed. The most advantageous number of stages can be determined empirically.

The liquids removed at different stages in a multiple stage membrane separation process will vary in composition and vapor pressure. Generally, these liquids are advantageously removed from the fluid in the membrane or in a separator and passed to a dis-tillation column to enhance separation of hydrocarbon liquids from lighter fractions and compounds. It is desirable that the liquids separated via the membranes be introduced to the distillation column at the appro-priate level to minimize heat required for efficient separation.

The carbon dioxide-enriched permeate has a variety of uses. The carbon dioxide can be injected into oil-bearing formations to enhance oil recovery in accordance with known methods. The carbon dioxide can also be used as an inerting gas~ If H2S is present in the gas feed, it will generally permeate along with the CO2. Advantageously, from 1 to 20 mole percent H2S can be present. It is desirable to selectively eliminate the H2S from the permeate by known gas conditioning methods, if the gas is to be used for inerting or H2S
removal is otherwise desirable. The removal of carbon dioxide enhances the commercial value of the hydro-carbons present in the feed.

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The liquid hydrocarbons separated in the subject process generally are predominantly C3 and higher hydrocarbons. Essentially all C4 and higher hydrocarbons (at least 90 percent) will be in the liquid phase, but a significant percentage of the C3 hydrocarbons can be present in the gas. These hydro-carbons can be saturated or unsaturated and branched or straight chain. The methane and ethane fractions present in the feed are also selectively separated b~t are predominantly present in the hydrocarbon gas frac-tion. Essentially all methane will be present in the gas phase, but a significant percentage of ethane in the fluid feed may be present in the liquid phase. In preferred embodiments of the invention, the hydrocarbon gas product is suitable for sale to natural gas pipe-line systems or for use in equipment and machinery fueled with natural gas.

In prior membrane separation systems, a part of the fluid which does not permeate is recycled to the feed to the membrane to further reduce the concentra-tion of carbon dioxide and hydrogen sulfide. However, this recycle in the subject process increases the mole percent of light hydrocarbons in the ~luid contacting the membrane thereby increasing the cooling required to separate the liquid hydrocarbons. It has now been found desirable to conduct further separation of carbon dioxide from the light hydrocarbon gas in one or more membranes connected in parallel with the membranes used to separate liquid hydrocarbons. The hydrocarbon gas from this parallel train of membranes can then be combined with the hydrocarbon gas from the membranes contacting the liquid hydrocarbons.

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The concentration of C3 and higher hydro-carbons in the feed to parallel membrane train is relatively lcw. Consequently, the pressure and tem-peratures of the hydrocarbon stream remains above the hydrocarbon dew point during contact with the second membrane train. The heat removed as a result of expan-sion of the permeated-gas results in a significant temperature reduction, since no change of state occurs.
The permeate gas also absorbs heat from the non-perm~ate, thereby reducing the temperature of both streams. The reduction in temperature of the non-permeate generally enhances the separation achieved in subse~uent membrane stages. Alternatively, the cooled permeate and non-permeate streams can be used to cool feed to other membrane stages through heat exchange in conventional apparat-~s.

Figure 1 depicts in cross-section, for pur-poses of illustration, a hollow fiber membrane device disposed vertically that could be used in the process of this invention. The fluid to be separated is intro-duced via inlet 1 to the fluid feed pipe 2~ The fluid feed pipe 2 passes through a first tubesheet 3 and terminates in a second tubesheet 4. The section of the fluid~feed pipe 2, between the first and secon~ tube-sheets 3 and 4, contains a plurality of perforations 5through which the feed fluid will pass and thereafter contact a plurality of hollow fibers 6 oriented gener-ally parallel with the longitudinal axis of the fluid feed~pipe 2. Wrapped about the bundle of hollow fibers 6 is a porous polymer outer wrap 9, which helps to prevent shifting of hollow fibers 6. The feed fluid passes axially and radially through the bundle of :: .
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hollow fibers 6 with most of the condensed hydrocarbon liquids present exiting through a first outlet 19 in the pressure case 8 surrounding the hollow fibers 6.
The hydrocarbon gas which does not permeate through the hollow fibers 6 typically exits through a second outlet 7. The remainder of the feed fluid will permeate through the hollow fibers 6. The bores of the-holl-ow fibers 6 communicate at each end through the tubesheets 3 and~4 with a first and second header space 11 and 12.
The fluid which permeates through the hollow fibers ~
ultimately exits through a first and second outlet 13 and 14 in the pressure case 8.

The membrane device depicted in Figure 1 can also be operated by passage of fluid feed to an inlet in the housing 7. The outlet 19 may be sealed in this embodiment. The non-permeate fluid exits through the core 2 after passing axially and radially through the bundle of hollow fibers 6. This non-permeate fluid can optionally contain both gas and liquid~ Operation of the membrane device with outside in flow through the hollow fiber bundle generally is advantageous where a significant percentage of the fluid feed permeates through the hollow fiber membranes.
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A simplified schematic diagram of a system for separating acid gases, i.e., carbon dioxide, natural or fuel gas, i.e., Cl and C2 hydrocarbons and liquid hydrocarbons from mixtures is presented in Figure 2. The fluid feed mixture passes through a line 30 to a first gas/liquid separator 32. The separator 32 diverts the gas present to line 34 and ~; most of the free liquid present to line 36. The fluid in line 34 is conveyed to a dehydrator 38 and the dry fluid exits through line 56.
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The fluid in line 56 is cooled in a heat exchan~er 58 and then passed through line 59 to a second separator 62. The liquid stream separated in separator 62 is conveyed through line 64. The pre-dominantly gaseous fluid is passed from the separator62 through line 66 to a first membrane 68. The per-meate-from the mem~rane 68 exits via line 72. The liquid non-permeate condensed in the membrane 68 exits through line 73. The gaseous non-permeate exits the membrane 68 through line 70 and is conveyed to a heat exchanger 74 where it is cooled.

The cooled non-permeate exits the heat exchanger 74 and is conveyed via line 76 to a third separator 78. The gaseous fraction removed by the separator 78 is passed through line 80 to a second memb~ane 82. The liquid fraction from the separator 78 passes through line 98.

The second membrane 82 separates an acid gas permeate which exits through line 84, a gaseous non--permeate stream conveyed through line 86 and a liquid permeate passed through line 81. The non-permeate from line 86 is conveyed first to a heat exchanger 88 and the cooled fluid is ~assed via line 90 to a separator 92. The separator 92 conveys a gas consisting mostly -of ethane and methane through line 94. The liquidfraction from the separator 92 passes through line 100.
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The liquids from the second, third and fourth separators 62, 78 and 92 and from the first and second membranes 68 and 82 are conveyed from lines 64, 98, 100, 73 and 81 to line 102 and then to a distillation 34,268-F . -15-~ 3~ 4 . ~

column 104. The higher boiling fraction exits the column 104 through line 116 and a portion is conveyed in line 117 to be heated in a reboiler 118 and recycled through line 120 -to the column 104. The remainder of S heavies in line 116 axe passed to line 121.
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The distill~te from the column 104 is con-veyed through line 105 to a heat exchanger 106. The cooled fluid exits the heat exchanger 106 through line 108 and passes to a separator llO. The liquids 10 from the se~arator 110 are returned to the column 104 via line 112.

The gas fractions essentially free of C3 and higher hydrocarbons from separator 110 are conveyed via line 114 to a third membrane 122. The permeate from 15 the membrane 122 exits through 124. The non-permeate from membrane 122 is conveyed through line 126 to a second-stage membrane 130. The permeate from the membrane 130 is conveyed through line 128 to line 134, where it is combined with permeate from line 124. The non-permeate from the membrane 130 is con~eyed via line 132 to line 140.

The permeate from the membranes passed through lines 72, 84 and 134 is combined in line 95.
: This permeate contains high concentrations of carbon : 25 dioxide and optionally hydrogen sulfide relative to that in the fluid feed in line 30.
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: The gas rich in methane and/or ethane con-: veyed in lines 94 and 132 is combined in line 1~0. The : fuel gas emergin~ from line 140 can be further treated b~ wet gas conditioning techniques to remove residual hydrogen sulfide and carbon dioxide, if desired.
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The liquid hydrocarbons are conveyed in line 121. These li~uid hydrocarbons have value as feed-stoFks for refined petroleum products.

The schematic diagram presen-ted in Figure 2 can be readily adapted for handling fluid feeds of variou-s compositions. Selection of suitable columns, separators, heat exchangers, dehydrators, membranes and comprèssors is within the ordinary skill in the art.' In a preferred embodiment of the invention, the indi-vidual or combined permeate fluids, non-permeate gas and/or condensed liquid streams can cross-exchange heat with fluids in lines 56, 70 and/or 86 to lessen or eliminate refrigeration equipment associated with heat exchangers 58, 74 or 88. In another preferred embodi-ment of the invention, the fluids from lines 64, 73,81, ~8 and 100 are introduced separately to different levels of the separation column 104 dependent upon their composition to enhance efficiency in separation.
Additional membrane stages, compressors and other auxiliary equipment can be added as necessary.

Projected recoveries of components in gaseous hydrocarbon streams and liquid hydrocarbon s-treams, separated by a typical system similar to that in Figure 2, is presented in Table I for fluid feeds of various compositions. The gas hydrocarbon stream corresponds to that in line 140 in Figure 2 and the liquid composition is that in line 121. The feed gas pressure is,in the range of 350 to 500 pounds per square inch gauge (psig) (2413 kPa to 3448 kPa). The feed gas composition in mole percent exclusive of carbon dioxide is 0.56 percent H2S, 2.50 percent N2, .

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50.52 percent CH4, 16.92 percent C2H6, 15.37 percent C3~, 2.14 percent i-C4H1o, 5.12 percent n-C4H10 and 6.87 percent C5 and higher alkanes.

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Cryogenic separation processes are used in the prior art to separate C3 and higher hydrocarbons from methane, ethane and ethylene. The process of this invention offers lower capital cost, lower energy cost, equivalent liquid hydrocarbGn recovery, recovery of hydrocarbon gas recovery in a compact processing unit.

34,268-F -20-:

Claims

1. A process for separating carbon dioxide from a fluid mixture comprising carbon dioxide, at least one C1 or C2 hydrocarbon and at least one C3 to C8 hydrocarbon which process is characterized by:
(a) contacting a first liquid-hydrocarbon tolerant membrane selectively permeable to carbon dioxide with said fluid mixture;
(b) permeating carbon dioxide through the first membrane at conditions which result in the presence of liquid hydrocarbons in the fluid in contact with the membrane; and (c) separating the liquid hydrocarbons from the gases which do not permeate the first membrane.

2. The process of Claim 1, further comprising:
(d) contacting the non-permeate gases from Step (c) with a second membrane, separate from Step (a), that is selectively permeable to carbon dioxide;

34,268-F -21-(e) permeating carbon dioxide through the second membrane to produce permeate and non-permeate gas streams; and (f) recovering gaseous hydrocarbons from the non-permeate.

3. The process according to Claim 1 wherein the first membrane is a water-dry cellulose ester membrane.

4. The process according to claim 1 wherein the first membrane is a hollow fiber membrane.

5. The process according to Claim 1 wherein the permeate or non-permeate gas from Step (e) is employed to cool feed gas introduced to the first membrane in Step (a).

6. The process according to Claim 1 wherein the first membrane exhibits a CO2 flux with pure CO2 of at least about 1X10-6 cm3/(sec cm2 cm of Hg) at 10°C and 50 psig (345 kPa).

7. The process according to Claim 4 wherein the first membrane has an inside diameter of 30 to 400 microns and a wall thickness of 5 to 150 microns.

8. The process according to Claim 4 wherein the first membrane has an inside diameter of 30 to 400 microns and a wall thickness of 5 to 150 microns.

9. The process according to Claim 1 wherein in step (b) carbon dioxide is permeated through the first membrane at the feed stream pressure of 25 to 1000 psig (172 to 6894 kPa) and the pressure on the feedside of 34,268-F -22-the first membrane is at least 40 psi (276 kPa) greater than the pressure on the permeate side of the membrane.

10. The process according to any one of Claims 1 to 9 wherein said process is conducted in a single membrane stage.

11. The process according to any one of Claims 1 to 9 wherein said process is conducted in multiple membrane stages.

34,268-F -23-