CA1128316A - Process for synthesis gas mixtures - Google Patents

Abstract

PROCESS FOR SYNTHESIS GAS MIXTURES
ABSTRACT OF THE DISCLOSURE
Synthesis gas mixtures of closely controlled compo-sition can be prepared by a process wherein a selectively permeable gas is separated by means of a plurality of semi-permeable hollow fiber membranes and concentrated in the bores of such hollow fiber membranes, a gas mixture dimin-ished in such selectively permeable gas is withdrawn as permeant from the outer surfaces of such hollow fiber membranes, and the composition of such permeant gas stream is controlled by adjusting the differential in pressure between the feed gas mixture and the permeated stream. A
synthesis gas mixture suitable for use as an OXO alcohol synthesis gas is produced by the process wherein a feed gas mixture comprising at least hydrogen and carbon monoxide is provided. There is also provided a method of controlling gas membranes of at least two differentially permeable gases treated by axial flow through a separation vessel containing a bundle of semipermeable hollow fiber membranes selectively permeable to at least one of such gases by adjusting the differential in pressure between the gas mixture reed and the permeated gas stream in response to changes in the composition of the permeant gas stream effluent.

Description

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PROCESS FOR SYNTHESIS GAS MIXTURES
~ nis invention relates to a process for preparing synthesis gas mixtures which employs semi-permeable hollow fiber membranes, and to methods of control of such process employing said hollow fiber membranes.

By this invention there is provided a process for the preparation of synthesis gas mixtures which is particul-arly advantageous for the production of such gas mixtures of closely controlled composition designed for specific synthe-sis reactions. There is also provided ~ novel means ofvery fle~ible control of such synthesis gas composition.

Many processes have been proposed to concentrate a less concentrated gas in a stream of mixed gases including cryogenic purification, absorption of some gases in solvents, adsorption of some gases on solid zeolites and the like, but generally these have involved concentrating andior purifying hydrogen, helium or other relatively light gas. Also membrane separation processes have been taught for separation of rela-tively pure streams of hydrogen or other light gases. One process utilizing membrane separation is that for the prepar-ation of synthesis gas mixtures for subsequent processing, i.e. synthesis gas mixtures to be subjected to the oxo reaction or other catalyzed synthesis reactions.

Such synthesis reactions frequently require a mixture of gases such as hydrogen and carbon monoxide very different from the amounts present in the effluen-t gases produced by primary reforming of hydrocarbons, i.e.
natural gas which also will generally contain impurities which may either harm the synthesis catalyst or lower the efficiency of the synthesis reaction. Consequently, those impurities which most deleteriously affect the catalyst or the reaction are generally removed. However, there remains

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a need to change the relative amounts of .he primary reactant gases, e.g. hydrogen and carbon monoxide, ~o a mixture more nearly stoichiometric for t'ne most efficien+
reaction.

l~his can be achieved in several ways, including adding pure or highly concentrated gases from anolher source, e.g. nighly concentrated carbon monoxide, Gr removing a portion of one of the constituent gases, e.g.
hydrogen. One method which has been suggested is to remove a portion of the hydrogen content by permeation of that light gas through a separation membrane in order to adjust ' the content of hydrogen to a desired level in respec. to the other constituent or constituents. Various types of membrane separation apparatus have been suggested for accomplishing such a removal of hydrogen, including flat membrane and hollow fiber apparatus adapted for contacting gas streams.

In most of these latter devices the desire lor very large membrane active surface area for contact with gas streams have led the developers to prescribe very small hollow fibers as membranes with small out~ide and inside diameters. For example i.n U.S.P. 3,422,008 McLain teaches a preference îor fibers witn diameters of from 0 to 50 microns and no more than 300 microns, see Col. 15, lines 25 63-68. T~ikewise in U.S.P. 3,339,341, Maxwell et al teach that holiow fibers having outside diameters of between 20 , and 250 microns are especially preferred, see Col. 4, lines 65-70, and at Col. 14, line 63 they record that a semi-comm-' ercial installation utilized hollow fibers of 29.2 microns.

Others have similarly found that hollow fibers having relatively small outside diameters are advantageous for utilization in separation apparatus in order to ?rovide sufficiently large ~.embrane surface areas and ot.-her advan.-ages such as capacities to withstand relatively large - r~-: .. . ~ .
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_3 pressure differentials. For example, Mahon in U.S,P. .

3,228,877 teaches that hollow fibers should have'relaLively small outside dizmeters and. that th'e more advantageous.
range of such outside diameters is between l~ and SO mi-crons, see Col. ll, lines 4-20.. Mahon also teaches at Col. lO, lines 57 et.seq. that the 2mount of pressure differential which can be withstood by a hollow fiber is directly related to the ratio.of the thickness of the wall of .he fiber to the inside diameter of that fiber, and, since grea~er pressur.e dif'ferentials usually result in' ~ greater flux through the fiber wall, the provision o~ very small hollow .ibers which can withstand high pressure difIerentials is desirable.. Thus Mahon concludes tha. the smaller the diameter of the fiber the smaller the corres-l~ ~onding wall thickness that is necess~ry to withstand a given pressure drop and that walls of lesser thickness çxhibit less resistance to permeate ~low and thus provide greater flux than that exhibited by fibers of walls of greater thickness.
However, it has now been foun'd that for purposes of improved axial or radial flow through a bundle of hollow fibers those of somewhat larger outside and inside diameters are desired. For example, it has been taught in Belgian Patent Number 192,024 filed November 29, 1978, that hollo~J
fibers having outside diameters of from about 150 to 800 mi-crons are preferred for most fluid separations, particularly gas separations. It is also taught in the above patent that the preferred wall thickness of the hollow fibers falls within the range of about 50 to 300 microns. and that such 30 hollow fibers' are preferably fabricated from a matërial hav-' .-ing a tensile modulus of at least about 15 kg~/mm~2l see page 15, lines 4-17. Also known ïs the fact that hollow fibers having low amplitude crimps result in fibers which exhibit rela-tively high packing factors, e.g.~ 40 percent while .
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maintaining acceptable flow 2xially and radially of a bundle of such .~ibers, see page 9, line 8 .o page 10, line 28 and p2ge 12, line 30 to page 14, line 26.
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It has been proposed to employ semi-permecble membr2nes contained in sep2ration appara.us to selec.ively separate one or more gases from a gas mixture con.aining at le2s. one additionzl gas, wherein the ~er~eant (non-permeated~ g2S mixture is subjec,ed to-processing subseauent t~ .he sep2ration operz,ion. If the sepzration app2ratus provides signirican- resistance to the low of the gas mix~ures, ~ubstan.ial energy expenditures ~ay be required .o recompress the permean. gas-~ixture to desired pressures for su~sequent processi.~g. ~ressure drops in a g2s mixrure caused by flow resis.ance or .he sepzr2.ion 1~ ~3par2tus a~e ,~requently substantial when the feed gas mixture is ~ed to the bores of the f~bers. ~or exzmp1e, G2rdner et al disclose in Chemical Engineering Progress, October 1977, p2ges 76-78, the use of separation ap?aratus cont2ining hollow fiber membr~nes L or removing hydrogen from a hydrogen znd c2rbon monoxide ~eed stream in an oxo alcohol synthesis process. The eed s~re~m ~t a pressure-of 350 lbs. per sa. inch gauge (psig)(24.6 kg./cm.2) is com-pressed to a pressure of 600 psig, (42.2 kg./cm.2) passed through the bores of the hollow fibers in the separation ap-2S paratus, and recovered as a permeant stream from-the bores of the hollow fibers of the separation apparatus at 330 psig.
(23.2 kg./cm.2). Thus the compression and, in some cases, re-compression of the feed gas stream is e~pen~ive in terms of ~
i capital expenditure for ccmpressors ~nd of operating costs and processes in which such excess costs could be avoided areclearly desirable. In the Gardner reference above, although a substantial pressure drop is incurred due to the boreside feed of the feed gas stream to the separation apparatus, the bore-side feed is apparently necessary due to the lack of distribution and loss of efficiency if the feed ~as stream were fed to the exterior or shell side of the hollow fibers. Sheil side feed to hollow fiber separation apparatus can provide other .
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advantages~ For instance, a greater surface area for effecting the se2aration is provided at the exterior surfaces of hollow fibers than at the in~erior surface of those fibers. ~oreover, hollow- fibers are generally able to withstand higher pressure differentials when the higher pressure is at the exterior as opposed to the interior of the fibers since generally most materials exhibit greater compressive than tensile strengths.

Advantages have been found in the use of hollow fiber membranes when such membranes have low amplitude crimp and relatively large outside diameters and wall thicknesses as described above. These advantages are best observed when the feed gas mixture is fed radially, i.e. the gas mixtures introduced in the mid portion of a bundle of hollow fibers and flow substantially perpendicular-ly to the orientation of the hollow fibers, or predomin-antly axially, i.e. the gas mixture is introduced at an outside portion of the fiber bundle, flows generally in ; the same direction as the orientation of the fibers, and exits at another portion of the fiber bundle. While often radial feed is considered to provide better gas separation efficiencies, axial feed is frequently more desirable since the separation apparatus may be less complex in ; design than radially fed separation apparatus, and since no radial feed conduit need be positioned within the bundle, the bundles employed for axial flow may comprise the greater ratio of available membrane surface area per given volum~
of separation apparatus than that of a radially fed appara-tus.
In accordance with this invention, it has been found that processes for providing synthesis gas mixtures derived from feed gas mixtures having different compositions than those desired in the synthesis process are most advantageously produced by means of hollow fiber membranes positioned in bundles within separation apparatus vessels when axial flow on the outside or shell side of the hollow .

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~8~.i c fiber is employed in producing desired permeant s~nthesis gas mixture which is withdrawn from the outside of such hollow fiber membr~nes at a point remote from the initial feed gas entry, and in which a permeate gas stream is S withdrawn from the bores of said hollow fiber membranes.
It has also been found in accordance with the present invention that the most advantageous and f].exible means of control of the ratio of the desired gases in the permeant gas streams produced is accomplished by adjusting the differential pressure between the feed gas mixture admitted to the shell side of said fiber membranes and the pressure of the permeate gas stream withdrawn from the bores thereof. Furthermore, this control in the pressure differ-ential is frequently most advantageously and conveniently controlled by establishing a fixed or set pressure for the feed gas mixture and varying the pressure at which the permeate gas stream is withdrawn in order to vary the total differential pressure. In accordance with the present invention, the composition of the desired permeant gas mixture product is determined by an analyzing means and adjustments to the differential pressure between the gas streams previously referred to are made on the basis of changes in the composition of the permeant gas stream product.

For convenience the present invention will be described in detail in relation to the process for preparing a synthesis gas mixture for an oxo alcohol synthesis in which the desired ratio of hydrogen to carbon monoxide is approximately 1:1. However~ it is to be understood that the same process of preparing synthesis gas mixtures and for controlling gas mixtures can be applied to the production of many and various mixtures of gases other than the two specific gases mentioned and in varying ratios of one gas to another. Thus the present invention is not limited to the specifically described process for the production of a specific oxo alcohol reaction synthesis gas detailed herein-after.

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The preparation of an oxo reaction synthesis gas generallv begins with a source of the major gases required, hydrogen and carbon monoxide, usually in ~d-mixture with other gases in minor amount. Such gas streams may be found as by-product of refining and/or petrochemical operations. ~here no such source is convenient such gas streams are generally generated by various catalytic or thermal primary reforming reactions for hydrocarbons such as natural gas, naptha, gas oil or the like. The content of various product gases will vary with the nature of the reacting hydrocarbon and the type of reforming and nature of the catalyst employed. For example when natural gas is employed and reformed over the usual catalysts various other gases than hydrogen and carbon monoxide are present in generally smaller quantities, such as carbon dioxide, methane, nitrogen and water. In order not to poison the oxo catalyst or reduce the effectiveness of that reaction most of such contaminating gases are removed from the desired oxo synthesis gas stream.

2~ In order that the efficiency of the oxo reaction process not be reduced because of competing reactions it is generally considered desirable to remove as much as possible of any carbon dioxide which may have been produced in the primary reforming reaction or may be present in the gas mixture from any other desired source. Generally such carbon dioxide is removed by an absorp-tion, or scrubbing, operation or by an adsorption, or solid treatment, opera-tion. Absorption can be by any good solvent for the carbon dioxide such as water, methanol or monoethanolamine.
One convenient method involves a scrubbing of the gas stream by monoethanolamine to remove carbon dioxide. Another method is to subject the gas stream to a methanol scrubbing wherein the carbon dioxide removed can be stripped from the methanol with another gas and the methanol absorbent thus regenerated for further reuse. Any of such carbon dioxide removal operations may be employed in the present process as desired, and generally such a method will be employed to reduce .he carbon dioxid- content of any feed gas mixture to below 100 ?arts per million and pre~erably selow about 30 p~TS per million or CarbOIl dioxide.

Further gaseous contaminan's in the typical primary reformer product gas stream can also be removed if desired by any sui.able treatment of the gas stream.
One convenient method is to subject the r-eed gas stream to an adsorption treatment over activated carbon, molecular sieves or zeolites, or the liXe for the removal by adsorption of the other gaseous contaminants as well as any methanol entrained from the solvent adsorption .reatments described above. The adsorbent can then be regenerated by stripping with a dry stripping gas as required .

In general feed gas mixtures produced by primary reforming operations will be relatively high tempera-ture and will require cooling to nearer ambient temperatures, for example about -50 to 50C, and preferably from about -20 to 20C. prior to subjecting the gas stream to removal of the contaminating gases as described above. Approximate-ly ambient temperatures are preferred for carrying out the subsequent steps of the process including contact with the hollow fiber separation membranes. Thus, if desired, the treated reed gas stream can be adjusted to a temperature of ~rom 10 to 50C, and preferably to a temperature of ~rom 20 to 40C. prior to contact with such separa.ion membranes.
Such feed gas streams will also be at supera-tmospheric pressure, varying depending upon the source of the mixed gas stream employed. If generated in a primary reforming operation the product mixed gas stream will be delivered at some superatmospheric pressure ranging from 10 to 100 atmospheres and preferably at a pressure of from about 20 to 50 atmospheres. When other sources of the mixed gas eed strea~l are employed they will either be supplied at such `
superatmospheric pressures or, if desired, can if compressed '~) . - ~,.

to the' desired pressure. Advantageously, thé mixed g2s feed stream is brought into contact with the hollow fiber membrane at substantially the desired superatm pheric pressure Ior the utilization of the product synthesis gas. Fven more preferably, such mixed gas feed stream is brought into contact,with,the hollow fiber ' membranes at from l/3 to 3 atmospheres (0.34 to 3.1 kg /cm~) higher pressure t~an that desired in the product synthesis gas stream.
The ~eed gas mixture comprisi~g hydrogen and carbon monoxide, whether or not subjected to the preferred pretreabment for removal of carbon dioxide and other contaminant gases, is contacted with a hollow fiber separ-ation membrane, which exhibits selectivity to the permea-tion of hydrogen as compared to the permeation of carbon monoxide, and generally any other gases present in trace amounts. In view of the generally substantially higher volume concentrations of hydrogen in the feed gas streams, 2s comp red to that of carbon ~onoxide, the separztion membrane need not exhibit high selectivity of separation of hydrogen from carbon monoxide in-order to pro~ide an enhanced process for the preparation of synthesis gas mixtures. Generally the selectivity of separation of a membrane is described in terms of the ratio o~ the permea-bility of the fast permezting gas, i.e. ~ydrogen, to the permeability of the slow permeating gas, i.e. carbon monoxide, wherein the permeability of the gas through the membrane can be defined as a volume of gas, a. standard ,.
temperature and pressure, which passes through the membrane per square centimeter of surface area per second for a partial pressure drop of one centimeter of mercury across the membrane per unit thickness. This ratio is referred to as the separation factor for a membrane. Often the sep2r- .
ation factor of the membrane for the permeation of hydroge~
over carbon monoxide is at least about lO. Separation factors for hydrogen over carbon monoxide o~ 80 or greater may be provided by certain membranes, but such highly ........... ~
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selective membranes offer little advantage. Most frequentl;, membranes exhibiting a separation factor for nydrogen over carbon monoxide of about 10 to 50 are adequate.
Likewise, the higher the permeability of hydrogen through 2 membrane, the less effective membrane surface area is required to pass a desired amount of hydrogen througn that membrane. Particularly desirable membranes exhibit hydrogen permeabilities of at least about lxlO 6, preferably at least about lxlO S cubic centimeters of hydrogen per square centimeter of membrane surface area per second at a partial pressure drop of one centimeter of mercury across the membrane.

The effective membrane surface area should be sufficient to allow an amount of hydrogen permeate such as lS to produce the desired ratio of hydrogen to carbon monoxide in the permeant gas st-ream. Factors influencing the deter-mination of the amount of effective membrane surface area ~include the permeation rate of hydrogen through the membrane under the separation conditions including temperature, absolute pressure, pressure differential across the membrane and partial pressure differentials of hydrogen across the membrane.

The partial pressure differential of hydrogen across a membrane provides a driving force for the permeation of hydrogen and depends not only on the pressure but on the concentration of hydrogen on each side of the membrane.
Advan-tageous pressure differentials across the membrane are at least about 3 atmospheres. However, the pressure differential should not be so great as to unduly stress the membrane such that it ruptures or is prone to easy rupturing. In many instances pressure differentials across the membrane can be considerably greater, from about S to about S0 a-tmospheres~ Desirably, sufficient effective g membrane area and pressure differential is provided that at least about 30%, and preferably from about 40 to 80%
of the hydrogen in the mixed gas feed stream permeates the separation membrane.

In the present invention the separator vessel contains membranes in hollow fiber membrane form with a plurality of the hollow fiber membranes arranged substan-tially parallel in bundle form, ~he feed gas mixture is contacted on the outside surfaces (shell side) of the hollow fiber membranes and axial flow along and about the hollow fiber membrane is established. Such shell side effluent or permeant synthesis gas product from the separator can be within less than 1 to 3, in fact within less than 0.1 atmospheres of the pressure of the feed gas stream fed to the separator. Since the concentration of hydrogen on the feed gas side of the membrane is continually diminishing as the hydrogen permeates .hrough to the bore of the hollow fiber membranes which in turn has increasing concentrations of hydrogen, the hydrogen partial pressure differential across the hollow fiber membrane is continually changing. Thus, flow patterns across the hollow fiber membranes can be utilized to provide desirably improved separation of hydrogen from the feed gas mixtures. Although the present process is found to be advantageous either in concurrent or countercurrent mode, it is preferred to operate in a countercurrent manner when -the shell side feed stream is established in axial flow such that the feed gas stream disperses within the bundle and generally flows in the direction in which the hollow fiber membranes are oriented. Thus, by establishing countercurrent flow by admitting the feed gas stream at the end of a hollow fiber membrane separator from which the bore effluent is removed an increased hydrogen partial pressure differential across the hollow fiber membranes is maintained, since the concentration of hydrogen increases in the bore as it flows in the direction in which the higher concentration of hydrogen is present in the feed gas mixture.

The separator containing the hollow fiber se?ar-ation membranes may be oî any suitable design for gas se?arations, ?roviding for shell side axial flow about the hollow fiber membrane. Thus the se?arator vessel may be of double-ended design wherein the feed gas stream is admitted in the mid portion of the separator vessel and the permeant gas removed from both ends thereof ~hile .he permeate gas stream may be removed from the bore at either one or both ends of the separator vessel. More preferably the separator vessel is of single-ended design in which the permeate gas from the bore is removed from one end only and the permeant gas can be removed from either end of the separator vessel, while the feed gas can be admitted to the separator vessel at any point from one end to the opposi-te end thereof. In order to establish the most desirable countercurrent flow it is preferable to admit the feed gas at the same end of the separation vessel at which the ?ermeate gas from the bore is removed and to remove the permeant gas from the opposite end of the separator vessel.

Any suitable material selectively permeable to hydrogen in favor of carbon monoxide or other heavier ~ases may be employed for the hollow fiber sepa~ation membrane.
Typical membrane materials include organic polymer or organic polymers mixed with inorganics such as fillers, reinforcements, and the like. Metallic and metal-containing membranes may also be employed. Polymers which may be suitable for the separation membranes can be substituted or unsubstituted polymers and may be selected from polysulfones;
polystyrenes, including styrene-containing polymers such as acrylonitrile-styrene copolymers, styrene-butadiene copoly-mers and styrenevinylbenzylhalide copolymers; polycarbonates;
cellulosic polymers, such as cellulose acetate, cellulose acetate-butyrate, cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose, etc.; polyamides and polyimides, including aryl polyamides and aryl polyimides;
polyethers; polyarylene oxides, such as polyphenylene oxide and polyxylylene oxide; polyesteramide-diisocyanate;

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polyurethanes; polyesters including pol~acryla'es, su^h as polye.hy'-^n2 ter~?hth21ate, polyalkyl mG-.`racry1-'es, ,l ?oly21kyl acrylat~s, polyphenylene .ere?h.r~la.2, e.c.; '' ?olysul~id-s; polymers lrom monomers naving alph2-olefinic unsa.ura.ion other .han men.ion2d ~ove such ~s poly~.`nyl2re, ?olypro?ylene, polybutene-l, poly-4-methyl?er.._ne-1, ?oly-vinyls, e.g. polyvirlylchloride, ?olyvinylfluoride~ ?lY-vinylidene chloride, polyvinylidene fluoride, polyvinyl alcohol, poly-~inyl esters such as polyvinyl acetate and polyvinyl propionale, ?oiyvinyl ?yridir~s, ?olyvinyl pyrrolidones, polyvinyl e.hers, ?olyvinyl ketones, ?oly- ¦
v-nyl aldehydes such as ?olyv-nyl formal and ?olyvinyl ~u.yral, polyvinyl ~mides, olyvinyl ~m~nes, ?olyvinvl urethanes, polyvinyl ureas, polyvinyl pnosphatss and ?oly-'-5 v~nyl sulrat2s; ?olyallylsj polytriazoles; ?ol;~en-zimidazol~s; ?olycarbodiimides; polyphosphazines; e.c., j' and int2rpolymers including block interpolymers containing repeating uni-s from the above such as terpolymers or acrylonitrile-vinyl bromide-sodium salt of ?ara-sul o-~henyim2thallyl etherj and grarts and blends con.2ini?.g any or the roregoing. Typic21 substituents providing ';
subs.itut~d poly~ers include halogen, such as fluorin2, chlorine and bromine; hydroxyl groups; iower alkyl grou3s; '' lower 21koxy groups; monocyclic aryl; 10~2r 2cyl grou2s and the like.

The hol'ow fiber membrane material is ~re,~~-ably 25 thin zs possible in ord2r ~o improve th- rate of pe-rmea-ticn through the m2mbrane, yet of sufficient thickness to i~sure ad2quate strength to the hollcw fiber membrans to withstand the sep2ration conditions including .he ~irfe-ren-tial 3ressures and diflerential partial préssu-es em.?'oysG.
Ho7low iber ~._mbranes may be 'sotro?ic, i.e.h2~2 subs'~..-~i~lly th2 s2m2 ~-nsit-y i:hroughout, or thsy may aniso ropic, i.~. h-~;ng aL 'e_st one zon2 of grsater densi.y ~ n a 3j le~st Gn2 o-her zone of the fiber me.nbran2. ~ne hol:v~ -'~sr membrane may be chemically homoO~nous, i.e. cors.ructed o~
.he sa~2 ma.sri2l, or m2y be a composit2 membrare. Suitable _ ..

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com?osite membranes may comprise a .hin layer which e~fec.s the separa-ion on a porous physical SUpport which ~rovides .he necessary s~eng.h to .he hollow fiber ~embrar,e ~o withs.and the se?æY2.ion conditions O.her suitzble composite hollow _iber me.~brcnes are he mul.icom~onent hollow fiber membr2nes àisclosed in 3el~ian Patent Number 860,811 filed November 14 i977 These me~branes com~rise a porous se-paration membrane which substantially effects tne separation and a coating material in occluding contact with the porous separation membrane These m~lticomponent mem-branes are particularly attractive for gas separations in-cluding those for seDarating hydrogen from carbon monoxide and other heavier gases, in that good selectivity of separa-tion and high flux through the membrane can be obtained The ma.erials for .he coatin~ ol these mul.ico~?on-ent membr2n2s may be na.~al or syn.heric su~siances, znd æ~e o~~ en po~ers, which adv2ntageously exhi~it .he a3?ro-pri2.e properties ,o p~ovide occluding contac, wi.h .he porous sepcration membr2ne. Syn~hetic su~stcnces include bo,h addition 2nd condensation polyme~s Typical OI .he use~ul ~a,erials which ccn comprise ,he coa.ing 2re pol~-,ers which c2n be subs.ituted or unsubs,i,u.ed, 2n~ which æ~e solid or liquid under g2S sep2r2.ion condi,ions, 2nd inclu~e syn.he-ic rubbers; natural rubbers; rel2.ively`high molecul2r weig:n. 2nd/or high boiling liauids; orgenic pre-poly~ers; polysiloxanes; silicone poly~ersj polysil2,anes;
?olyurethanes; polyepichlorhydrin; polyamines; polyimines;
polyamides including polylact~ms; acrylonitrile-con-aining copolymers such zs poly (CC-chloroacrylonitrile) copoly~ers;
polyesters including polyacrylates, e.g. polyalkylac~yla.es and polyalkyl methacrylates wherein the ~lkyl g~oups have rom 1 to about 8 czrbon 2 .oms ~ ?olysebac~tes ~ pol ysuccin2 .es, ~nd 21kyd resins; terpenoid -esins; linseed oil; cellulosic ~5 ?olymers; polysulfones, especially ali?h2tic-c0ntcining .

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polysulfones; polyalkylene glycols such as polyethylene glycol, polypropylene glycol, etc.; polyalkylene poly-sulfates; polypyrrolidones; polymers from monomers having GC-olefinic unsaturation such as polyolefins, e.g. poly~
S ethylene, polypropylene, polybutadiene, poly(2,3-dichloro-butadiene), polyisoprene, polychloroprene, polystyrene including polystyrene copolymers, e.g., styrene-butadiene copalymers, polyvinyls such as polyvinylalcohols, polyvinyi aldehydes, e.g. polyvinyl formal and polyvinyl butyral, polyvinyl ketones, e.g. polymethylvinylketone, polyvinyl esters, e.g. polyvinyl benzoate, polyvinyl halides, e.g.
polyvinyl bromide, polyvinylidene halides, polyvinylidene carbonate, poly(N-vinylmaleimide), etc., poly(l,5-cyclo-octadiene), poly(methylisopropenylketone), fluorinated ethylene copolymers, polyarylene oxides, e.g. polyxylylene oxide; polycarbonates; polyphosphates, e.g.,polyethyl-enemethyl phosphate; and the like, and any interpolymers including block interpolymers containing repeating units from the above, and grafts and blends containing any of the foregoing. The polymers may or may not be polymerized after application to the porous separation membrane.

In the present process the desired product synthes- --is gas which is withdrawn from the separator vessel, and hence from the outside surface of the hollow fiber membranes, has a diminished hydrogen content in comparison of that of the feed gas mixture charged to the separator vessel. It is also withdrawn at a pressure which is very little lower, frequently as little as 0.1 to 0.3 atmospheres lower, than the pressure at which the feed gas mixture was charged to the separator vessel by virtue of the very low pressure drop imposed by the axial flow of the gas stream through the separator vessel and about the outside surfaces of the hollow fiber membranes therein. A wide range of composition of the permeant synthesis gas product can be realized through use of the present invention depending upon the co~position of the feed gas mixture, the flow rate of the feed gas :

mixture and the pressure differential maintained between the feed gas mixture and the hollow fiber membrare bores, and thus the hydrogen partial pressure differential maintained between the outside and bore side of the hollow fiber membranes.

Any feed gas composition in which the h~drogen content is desirably to be diminished in respect to the other gaseous components can be treated by the present process. In the specific instance of a synthesis gas desired for an oxo synthesis process reaction a feed gas composition containing greater than the stoichiometric equivalent molar volumes of hydrogen to carbon monoxide can be employed as a feed gas. In general such feed gases range in hvdrogen to carbon monoxide ratios of from about lS 1.3:1 up to 4 or 5:1. Any desired flow rate can be main-tained in the separator vessels employed in the present process by varying the size and number of such vessels employed. It is apparent that increasing flow rates within a single vessel will afford less opportunit-y for permeation of the hydrogen content of the feed gas mixture.

The differentïal in pressures maintained between the feed gas stream and the permeate stream within the bores of the hollow fiber membranes is the chief variable involved in the present process and affords the most advantageous means of control of the composltion of the permeant synthesis gas stream product. Thus it has been found tha-t the pressure differential may range from as low as about 5 to as high as about 70 or more atmospheres depending upon the inherent strength and resistance to rupture of the hollow fiber membranes employed. The pressure of the feed gas mixture, i.e. the shell side gas charged to the separator vessel, can range from as low as about 5 to about 70 atmospheres, but preferably ranges from about 8 to about 50 ; atmospheres and more preferably from about 12 to about 30 atmospheres. In contrast the pressure on the permeate gas , stream in the bores of the hollow fiber membranes can range from about 1 to 50 atmospheres, preferably from about 1 ~o 20 atmospheres and more ?referably from about 1 to 10 atmospheres. Thus the pressure differential is preferably mainatined from about ~to 30 atmospheres and more prefer-ably from about 10 to 20 atmospheres.

By suitable adjustment of the pressure differen-tial in the present process a permeant synthesis gas stream can be co~trolled with wide flexibility to produce a composition in which the ratio of hydrogen to carbon monox-ide can be varied from 0.8 to 3.1 or higher, depending upon the composition of the feed gas stream. For most synthesis gas mixtures designed for application in an oxo reaction process and depending upon other gas mixtures which may be blended therewith, the composition can be controlled to a ratio of hydrogen to carbon monoxide of from about 0.9 to 2.1. Most preferably when blending of other gas streams is not ~o be employed the composition of the permeant synthesis gas stream is controlled to a ratio of hydrogen to carbon monoxide of from about 1.0 to 1.2.

Such permeant synthesis gas streams have been found to be very suitable and advantageous for direct use in a synthesis reaction for oxo alcohols employing a cobalt oxo catalyst. When ancillary gas streams are available for blending with the permeant synthesis gas stream to comprise the final synthesis gas subjected to the oxo catalyst the streams of wider variation in hydrogen to carbon monoxide ratio indicated above are entirely suitable and useful.

There is simultaneously produced a permeate gas stream from the bores of the hollow fiber membrane of relatively higher hydrogen content than the feed gas mixture charged to the separator vessel. Generally this hydrogen-rich permeate stream will comprise at least 80 volume per-cent hydrogen. More frequently, and depending upon the .. . . .
.

t relative permeabili-ty to hydrogen as opposed to carbon monoxide of the specific hollow fiber membrane employed, such a permeate gas stream will comprise greater than 90 volume percent hydrogen and most frequently will be comprised of from about 92 to 9~ volume percent hydrogen.
This hydrogen-rich permeate gas stream is suitable for any desired use in a particular installation. For example, such dry hydrogen gas streams can be employed to regener-ate molecular sieve adsorption units by sweeping out the contaminant gases previously adsorbed thereon. Likewise, this hydrogen stream can be employed to regenerate the carbon dioxide solvents such as methanol or monoethanolamine by stripping the absorbed carbon dioxide therefrom. Such permeate gas stream is also suitable for any other uses to which a hydrogen stream of relatively high purity may be directed as desired.

As indicated hereinabove, control of the differ-ential pressures between the feed gas mixture and the permeate gas stream in the bores of the hollow fiber membranes may be achieved by any convenient method, including adjustment of the compression impressed on the feed gas stream or, more conveniently,adjusting the pressures maintained by the control valves in the respective streams. The most conven-ient method ror control of this pressure differential is frequently found to be that of establishing a stabilized pressure for the feed gas mixture and thereafter varyingthe pressure maintained within the bores of the hollow fiber membranes by adjusting suitable valve means in the permeate gas line immediately downstream from the separator vessel. Such control of the permeate gas stream pressure, and hence of the pressure differential across the total separator vessel, is adjusted in response to changes in the composition of the permeant synthesis gas stream derived from the shell side of the se?arator vessel. Thus, once a desired composition, i.e. a hydrogen/carbon monoxide ratio, is determined, any change from this desired ratio will resul in an adjustment in the pressure maintained on the permeate gas stream in th~ bores. A desired composi~ion may 3~i comprise, for example, the highest carbon monoxide content in the permeant synthesis gas stream, and the pressure of the permeate gas stream would then be adlusted to the lowest value required by subsequent desi~ed processing 5 of such permeate g~as stream.

Any suitable method of analyzing the permeant synthesis gas stream can be employed. Manual analysis by means of liquid reactants is generally too slow in order to respond to adjustments in the required pressures.
10 However, vari~us analytical means are available to rapidly record any changes in the carbon monoxide and/or hydrogen content of the permeant synthesis gas stream. One of the most convenient of these is a recording gas chromatograph.
In the preferred form of the present process such a gas 15 chromatograph is provided with an associated signal generating device, which signal is used to control the setting of the valve means which controls the pressure of the bore permeate gas stream. Such a signal from a process control analyzer, such as a gas chromatograph 20 can also be routed to a process control compu-ter, which in turn generates a signal for control of the valve means on the permeate gas stream line from the separator vessel.
Such control circuitry for process control by means of gas chromatograph analyzers is well-known and will not be 25 further described herein.

Although the present invention has been described in terms of a process for providing a synthesis gas mixture it will be apparent that the process control feature of adjusting the differential pressure between the feed gas 30 mixture and the permeate gas stream in order to control the composition of a permeant gas mixture is widely appli-cable to many various mixtures of feed gases and product gases. Thus, in any gas separation system in which a per-meant gas mixture is desired for further processing or 35 treatment after permeation oi a lighter more selectively permeable gas from the original feed gas mixture, such a process for controlling gases as outlined would prove to be particularly advantageous. Sucn permeant gas mixtures could be composed of any other series of gases less selectively permeable than a lighter gas whose concentra-tion in the permeant gas stream is desired to be reduced.
Likewise, the same control system for gases can be applied with equal advantage to those systems wherein the permeate gas is the principal stream desired for further reaction or treatment. In that instance the control would still depend upon the analysis of the permeant gas stream for the content of either the desired gas or one of the other principal gases present in the permeant gas stream for control in the same way of the pressure imposed upon the permeate gas stream.

For example, whenever it is desired to recover excess or waste hydrogen from various refinery or petro-chemical plant processes such as hydrogenations of hydro-carbons, such a method of control of the gas streams result-ing from separation of gases by means of hollow fiber mem-branes can be employed. Thus, the applicability of the present invention to control of mixed gas streams is broad and the advantages thereof can be realized in a number of industrial operations.

The following examples are provided to further illustrate the invention. All parts and percentages of gases are by volume, unless otherwise indicated.

EXPERIMENT I
. . . _ .
A binary mixture of 75 mol per cent hydrogen and 25 mol per cent carbon monoxide was treated in a separator vessel containing anisotropic cellulose acetate hollow fiber membranes having an inside diameter (bore) of 100 microns, an outside diameter of 300 microns and a wall thickness of 100 microns of whish less than 10 microns constituted the dense, anisotropic layer. The hollow fiber membranes had an effective surface area of approximately 9600 square centimeters. The separator vessel was immersed in a constant temperature bath at 23C. The pressure of the feed gas to the outside surface of the hollow fiber membranes was varied by a pressure controller and that on the bore was maintained at a fixed value. There was thus generated two gas streams (permeant and permeate) of the compositions indicated below at the indicated pressure differentials between the shell and bore of the membranes at flow rates of fromll8.5 to 19.2 liters per minute of feed gas mixture. The separation factor determined from the indicated flux and determined permeabilities of each ~--constituent gas is recorded in Table 1 below.
, .
Table 1 Diff. Pressure Permeant Permeate Sep. Factor Atm.(Shell) (Bore) H /CO
Mol % Mol % 2 H2 C0 H2 C~
3.4 73.9 26.0 94.5 5.5 45.8 6.8 68.0 31.0 95.9 4.1 40.8 10.2 60.9 38.9 97.6 2.4 86.5 ': ;, ~22-FXAMPLE II

A feed gas de~ived from the primary reforming of natural gas which had been scrubbed wi~h methanol and passed through a bed of zeolite having the following composition W2S treated .for separation of a portion of its hydrogen content.
.
C~m~onent Mole %
. . .
Hydrogen 71.5 Carbon Monoxide 25.6 . Methane 2.8 Nitrogen 0.1 The. separati.on vessel contal.ne~ poly(siloxane) coated anisotropic polysulfone hollow fiber membranes sub-stantially prepared in accordance with the method disclosed in ~xample 64 of the above Belgian patent hav-ing an inside diameter of 250 microns, and outside diameter lS of 500 microns and a wall thi-ckness of 125 microns. The hol-low fiber membranes had an effective surface area of a~proxi- :
mately 550.0 square centimeters. The hollow fiber membranes had a determined separation factor of hydrogen over carbon monoxide of 46.4. The separation vessel contained approxi-mately 1200 individual fiber membranes.

.

, . ., . _ . .

~8 ~
.

The separation unit was operated in co-current axial flow with the permeant gzs stream ~ithdr~wn from the shell side of the membrane at the same end of the separation vessel as the permeate gas.stream.from the bores.of the membranes while the feed was to the shell side at the opposite-.end of the vessel. ~e separation unit was operated at.ambient temperature of approximately 25~ for, all runs. Pressures of the feed g~s mixture of the above composition and of the permeant and permeate gas streams were constantly controlled.- Ihe composition of both the.
permeant and permeate. gas streams were also constantly determined by gas ~hromatographs. All data was monitored by an IBM Model 1800 process computer and flow rates, pressures.and permeant and permeate gas compositions were . ..
recorded every three minutes during a run. The separation unit w25 operated continuously for a period in excess of 1200 hours.

The pressure differential between the shell inlet pressure which was held constant at about 19.7 atmospheres, and the bore outlet pressure was varied from about lO.S
. atmospheres to 6.54 atmospheres in .increments of approximately one atmosphere to establish differentials in pressure between the shell inlet and bore outlet pressures of from 9.2 to 13.2 atmospheres. The flow rates of the gas having the 2bove composition were varied within the appr.oximate range of 4000 to 9000 standard cubic centimeters per minute at the various differential pressures ~oted. The pressure drop from the shell inlet to the shell (~ermean.) outlet of .he separat~on vessel was found to vary mi~imally over 30 the range of pressure differentials and_flow_rates tested~ - :
averaging about 0.07 to 0.O9.atmos. (0.072 to 0.093 kg.!cm2) within a range of 0.06 to 0.12 atmos. (0.062 to 0.124 kg./cm.2) The results of some 28 runs conducted for sufficient periods to establish substantially steady-state op-eration are set out in Table 2 below in terms of feed gas flow rate, pressure differentials, H2/C0 ratios in permeant gas, hydrogen content of permeate gas, percent of hydrogen directed to permeate and . ___.. ,...... . .. _.... _ ......... .

percent of carbon monoxide recovered in permeant gas stream.

~3L2 :`

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.
d~
............................
~ ~ ~ C~ O ~ U~ O ~ O ~ o c~ ~D ~O ~ ~D ~I CO ~ (D
C~ o ~
~ ~ C!) . ~
.,. o ,~
~ a) o ~ a- a~ t--~ ~ Lt~ c~ (D CO 0~ 0 0 0 CO C`l ~7 ~ ~ ~ ~ ~ O u~ ~ ~ C`l ~ ............................
! ~ ~ U) U') t~ * O ~ a') Ll') N (D ~ O ~I O 1-') 0 0 0 C~l O tD
0\ ~

~ Q) t' ~ o\ o ~ ~ o o ~ .n ~ ~ C~ o co ~ ~ C~ C~ ~ ~ ~ ~ C~l ~ ci~ o ~ ~ u7 L
"~ ............................
o ~ V c~ O O o ~ o ~ ~ ~ ~ c~ c~
:~
r p~ ~
O 11) O ~ co ~` (.D r-l O a- Q ~ l O ~ ) O ~
~0 ~ O O O O ~ ~ O O O ~1 ~ ~ O O ~ ~ ~ O ~ ~1 ~ O O ~ ~ O O
L
X `r~ C~
.~

h o +.
:) h n~ ~` O ~ C~ ~ r` O C~ ~ r` O C`~ ` O ~ ~ ~ O C~ CC ~
~1 ................. --~ ~ ~ cn o ~ c~l ~ a) o ,~ O ~ ~ ~ ~ O ~ ~ ~7 o ~ C~

"~
~1 U~ O CO C`~ r~ O O O C~ ~ o cn ~ co ~ ;~ ~ o ~ ~
'~1 ' Ei ~ C~l ~D O O r--I (D al ~ ~ 0 3 ~ t-- O CD ~ J r~~ ~D C~ O CJ~ ~t N~ o o a- c~ O O O o a~ ~) o o o o o a- o o c)~ O o o co O O O ~ ~) t) , , `:

.. , ;

EXAMPLE IIB
...
A series of four runs were conducted with the same separation unit described above operated in both `~ co-current and countercurrent manner with other conditions 5 varied between flow rates of from 5000 to 9000 standard cubic centimeters per minute and pressure differentials of 9.14 and 12.31 atmospheres. The results of such directly comparative runs in terms of percent of hydrogen directed ~ to permeate gas and percent of carbon monoxide recovered ;~ ` 10 in permeant gas are set out in Table 3 below. It will be noted that the percent of hydrogen directed to permeate gas is consistently higher and the percent of carbon monox-. ide recovered in permeant gas are generally somewhat im-proved when operating in the countercurrent manner.

Table 3 ~, , Feed Pressure Co.-Current Countercurrent Rate Difr- %H2 %C0 to %H to %C0 to . Std. erential, Permeate Permeant Pe~meate Permeant Run cc/min` atm. Gas Gas Gas Gas ;, . ~:
20 ~ 50009.14 64.0 81.0 73.6 84.8 B 70009.14 58.3 85.1 68.9 88.4 ` C 750012.31 73.0 85~0 87.6 86.0 `
D ~00012.31 70.1 88.2 79.2 87.3 , ' ' :

``; :~:

''`` ,:
" :

, . ~

: -27-., ~.
-; EXAMP~ III
~ .
. A synthesis gas prepara-tion installation was `
provided comprising 15 separator vessels containing multicomponent anisotropic polysulfone hollow fiber membranes, the polysulfone having a molecular weight in t` excess of 10,000, coated with poly(siloxane), which had a molecular weight in excess of 1000 prior to cross-linklng to provide a silicone-rubber, the membranes produced as described in Example II. The hollow fiber membranes had inside diameters of approximately 265 to 295 microns, outside diameters of approximately 535 to ~;
575 microns, wall thicknesses of from about 125 to 150 microns and each bundle had an effective sur~ace area of approximately 93 square meters.
'~
15The installation was operated for a period of three months, during which period relatively stable condi- ~ -tions were maintained by an automatic control system. The installation comprised a manifold carrying the feed gas to three headers each supplying a bank of 5 separators 20 operated in axial countercurrent flow. Three headers and ~ ;~
a manifold collected the permeant gas for supplying - - `
synthesis gas, after blending with other gas streams, to ;;
an OX0 alcohol synthesis process. The permeate gas was collected from the bores of the hollow fiber membranes by three headers and a manifold and supplied to further processing. The dry feed gas line to each separator and each permeate gas header were provided with sampling means ;~
supplying signals to an automatic analyzer which was adapted ; to continuously record composition and pressure data at 30 selected intervals. he manifold supplied by the three -~
permeant gas headers was provided with sampling means supplying signals to a separate automatic continuous analyzer which was operated for process control by coupling to a process control computer. The process control computer `; ;
i 35 supplied a signal to a pressure control valve in the ~` permeate gas manifold line in response to any departure from , .
:.
.. ^- , E3 . . ; :
~'`:` '' , the sel point for carbon monoxide content of the permeant gas stream as detected by the separate automatic analyzer.
Thus, the process control was effected in response to the composition of the permeant gas stream by varying the differential pressure imposed by the pressure of the permeate gas stream.

The separation unît was operated at am~ient temperatures of from 30 to 40C. in axial countercurrent flow under relatively stable conditions. The manner of control of the process is illustrated by two sets of data recorded at an interval of twenty days of continu-ous operation. The conditions and data recorded are set forth in Table 4 below, wherein the control was to a set point of 36.0 mol. percent carbon monoxide in the permeant gas stream.
- Table 4 Day 1 - Separation Factor H2/CO=23.5 Flow Rate Feed Gas Composition, Mol %
kg/hr Pressure, ~P Feed Permeant Permeate Atm. Atm. GasGas Gas 1402 22.65 14.76 H2 73 9 59 5 94 0 CO 23.0 36.0 5.1 `
CH4 2.9 4~2 0.5 N2 0 3 0,2 0.5 Day 20 - Separation Factor H2/CO=23.5 138522.52 14.70 H2 72.7 59.8 94.1 CO 24.0 36.0 5.4 CH4 3.2 4.1 0.4 N2 0.1 0.2 0.1 At a different period the same process was oper-ated so as to accomodate a large reduction in feed gas flow rate. In this instance the control set point was changed to a lower value to accomodate the new condition. A period of approximately 4-1/2 hours was required to effect a lower , . . ~:,- ~ .
: :
.

`
~12~

;~s : -29-steady operating state. The data recorded at 30 minute ,- intervals during this period is set forth in Table S
below and comprises the record of feed gas flow rates, ,-v permeate stream pressures and permeant stream carbon S monoxide content. The initial figure of 33 mole percent CO represented the initial set poînt and the figure of - 29 mole percent C0 represented the final set point for process control at steady state operation.
Table 5 Feed Gas Permeate Gas Permeant Gas Time Flow Rate Pressure C0 Content ' ' Min. ka/hr atm. mol %
c~

0 1610 6.87 33 1134 " 24 1140-1220 " 28 " " 27 ` 120-210 " " 27 240 " 6.12 27 270 1180 6.12 29 300 1180 6.20 29

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows;

1. A process for preparing a synthesis gas mixture characterized by:
providing a feed gas mixture comprising at least hydrogen and carbon monoxide, maintaining said feed gas mixture at superatmos-pheric pressure, contacting said feed gas mixture in axial flow with the outside surfaces of a plurality of semiper-meable hollow fiber membranes selectively permeable to hydrogen at said superatmospheric pressure and having an outside diameter of from about 150 to 800 microns and a wall thickness of from about 50 to 300 microns, selectively permeating a portion of the hydrogen present in said feed gas mixture through said hollow fiber membranes and into the bores thereof, withdrawing from contact with said outside surfaces a permeant synthesis gas stream diminished in hydrogen content suitable as at least a portion of a synthesis gas mixture, withdrawing from the bores of said hollow fiber membranes a permeate stream of relatively high hydrogen content at a lower pressure, and substantially controlling the composition of said permeant synthesis gas stream by adjusting the differential in pressure between said feed gas mixture contacting said outside surfaces and said permeate stream, said differential in pressure being adjusted by varying permeate gas stream valve means to vary the pressure of said permeate stream in response to a signal generated by a permeant gas stream composition analysis means.

2. The process of claim 1 characterized in that said semipermeable hollow fiber membranes comprise a hollow fiber porous separation memebrane having a coating of a different material in occluding contact with the outside surface thereof.

3. The process of claim 1 characterized in that said porous separation membrane comprises polysulfone.

4. The process of claim 1 characterized in that said coating comprises poly(siloxane).