CA1282712C

CA1282712C - Enhanced gas separation process

Info

Publication number: CA1282712C
Application number: CA000509375A
Authority: CA
Inventors: Kishore Jasraj Doshi
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1985-05-24
Filing date: 1986-05-16
Publication date: 1991-04-09
Anticipated expiration: 2008-04-09
Also published as: DE3682717D1; US4645516A; KR900006052B1; AU5784986A; CN86104191B; ATE69967T1; JPS6211527A; ZA863868B; EP0209970A1; CN86104191A; JPH031049B2; AU582442B2; EP0209970B1; IN167308B; KR860008792A

Abstract

ENHANCED GAS SEPARATION PROCESS
Abstract of the Disclosure In the purification of gas streams by contact with a permeable membrane selective for the separation of desired components from impurities, harmful gas stream components are removed by means of a pressure swing adsorption system prior to passage of said gas streams to the permeable membrane. Where the harmful components nevertheless comprise valuable components of said gas streams, the purified portion of said gas stream, either the permeate or the non-permeate from said membrane, is used to supply purge gas to the pressure swing adsorption system for desorption and recovery of said harmful components together with said desired components purified upon contact with said membrane. Where said harmful components are not valuable components of said gas streams, the impurity portion of said gas stream, either permeate or non-permeate from said membrane, is used to supply purge gas to said pressure swing adsorption system for desorption and discard of said impurities and said harmful components from the gas purification operation.

D-14,803

Description

~27~L~

ENHANCED GAS SEPARATION PROCESS
BACKGROUND OF THE INVENTION
Fielq of the Invention The invention relates to the separation of desired components of ~ gas stream from lmpurities contained therein. More particularly, it relates to the enhancing of the recovery of high purity product in such gas separation operstions.
DescriPtion of the Prior Art Permeable membrane systems provide a commercially feasible means for separating and purifying a desired component or components from a feed gas mixture thereof with other less desired components or impurities. For such purposes, the feed gas mixture ls passed to a gas permeable membrane capable of selectively permesting certain components of the mixture that are withdrawn from the discharge side of the membrane as a permeate gflS
at a reduced pressure. The non-permeate portion of the gas mixture is withdrawn from the feed side of the membrane essentially at the feed gas pressure.
Typically, the component desired to be purlfied wlll be recovered as the permeate portlon of the feed gas mixture, while the impurlties ~ssociated therewith ~re withdrawn as the non-permeate portlon of sald feed gas mlxture. In spplicatlons lnvolvlng the purlflcation of purge gases from the reaction loop of ammonla or methanol plants, such purge gases are commonly pa~sed to 8 permeable membrane system at substantislly the reaction pressure level. The D-14,803 g~2827~LZ

components desired to be recoYered, i.e., hydrogen and nitrogen in ammonia plants, and hydrogen and carbon dioxide in methanol plants, are recovered as permeate gas at a reduced pressure and, fffter recompression, ere recycled to the ammonia or methanol loop for further production of the desired product~ The purge gas from the reaction loop also typically contain~ ammonia or methanol together with various process impurlties. Before such purge gas is brought into contact with the permeable membrane, such ammonia or methanol is des1rebly removed to the extent that such components might be harmful to the permeable membrane so as to affect lts effectiveness over its intended operating life. For this purpose, a water wash or distillstion process is typically employed, with said ammonia or methanol being removed in aqueous form.
The use of permeable separation membranes in the treatment of ammonia synthesis purge streams to recover hydrogen is t~ught by Gardner et al, in "Hollow Fiber Permeator for Separating Gases", Chemicel Engineering Progress, October, 1977, pages 76 to 78. Thls approach was affirmed by Perry, U.S. 4~172,885, with reference to the known reaction of hydrogen end nltrogen at a superatmospheric pressure to produce ammonis. As stated therein, an ammonia-containing reaction effluent is withdrewn from the ammonla synthesis reaction zone. This reaction effluent, which contsins substantial amounts of unreacted hydrogen and nitrogen, is recycled ln a well-known ammonia synthesis loop to the ammonia synthesis reaction zone. Combined with D-14,803 7~

this recycling reaction effLuent in the ammonia synthesis loop is ~ synthesis feed gas stre~m of hydrcgen, nitrogen and impurities such as meth~ne and argon. In addition to the removal of product effluent from the reactlon effluent in the ammonia synthesis loop, ~ purge streAm is removed from the re~ction effluent in an amount such as to maintain the proportion of impurlties desirably low. In the process 8S described by Perry, the purge stream from the ammonia synthesis is chilled to condense and coalesce ammonia and is then scrubbed with water to provide a purge stream h~ving less than about 0.5 volume % smmonia and wster vapor therein.
Alternstively, the ammonia content of the purge stream is reduced to less than 0.1 volume % by either chilling to coalesce ammonia, or adsorption or absorption of ~mmonis. The thus-treated purge stream, substantially at the synthesis reaction pressure level, is thereafter passed to the permeable membrane for permeation of hydrogen therethrough and reintroduction to the ammonia synthesis reaction zone.
It will be appreciated that smmonia removal pretreatment ,~s indicated above necess~rlly results in the application of coalescers, scrubbers and the like, as well ss the need for regenerating smmonia that is removed in the form of squeous smmonia in order to svoid undue loss of product ammonia. While such sn approsch csn be utilized in commercial operations, the development of improved techniques and systems would be desirable in the art. Thus, it is desirable that, in svoiding contact of the D-14,803 32~

permeable membrsne with feed gas components h~rmful thereto ~t partlcular concentrations, the pretreQtment process be simplified ~nd the need for employing w~ter scrubbing and ammonia regeneration be avoided. At the ssme time, the enhanced recovery of the ammoni~ content of the purge g~s i5 desirable so that the overall product recovery of the flmmonia productlon oper~tlon can be enhanced. Those skilled in the art will apprecihte that slmllsr concerns and desires pertaln to the field oÇ methanol production.
Similarly, ~ desire for such improvements exists with respect to other gas puriflcstion operatlons, not necessarily assoclated with a re~ction synthesis loop, but whereln components of 8 feed stre~m harmful to a permeable membrane but otherwise vsluable sre desirably removed there~rom prlor to contsct with sald membrane. In such operAtlons, it is also gener~lly deslred to ~vold the loss of such valuable components so that an enhanced recovery of 2~ desired, hlgh purity product csn be achieYed.
The present invention is directed towards the provision of an improved process for the separation of desirab1e components of a gas stream from impurities associated t:herewith, wherein enhanced separation of impurities f`rom a gas stream is achieved upon contact with a gas permeable membrane.

~8~
~ 5 SUMMARY _ F THE~INV_NTION
Components of a feed g8S stresm deemed harmful to ~ permeable separatlon membrane are removed therefrom prior to cont&ct of said g8s stream with the membrane. For this purpose, a pretreatment pressure swing ~dsorption system is employed. The purified gas stresm from the membrsne can be used 8s a purge g8s for the pressure swing adsorptlon system to enhsnce recovery when said harmful components constitute valuable constituents of the feed gas stresm. When s~id hsrmful components ~re not desired constltuents of the feed g~s stre~m, the waste stream from the membrane is similarly employed as a purge gss so AS to facilltate the enhsnced recovery of desired high purity product.
DETAILED DESCRIPTION OF THE INVENTION
The present invention employs a novel combination of pressure swing adsorp-tion (PSA) processing and permeab1e membrsne separation operation. Such a combinstion, as descrlbed ~nd clalmed herein, serves to obviate the need to water scrub from 8 feed gas stream varlous components that msy be harmful to the permeable membrane. Regeneratlon of the PSA system ls facilitsted ln a manner enhancing the recovery of desired hlgh purity product ~rom the PSA-permesble membr~ne combln~tlonj.

D-14,803 ~ 82 ~ 1~

In the practice of the invention for the above-indicated, illustrative treatment of 8 purge gas from sn ammonia synthesls loop, a PSA system is used to reduce the concentration of said ammonia in the purge gas prlor to its contact with a permeable membrane. Thus, said ammonia is selectively adsorbed as a more readily adsorbable component of the purge gas, while unreacted hydrogen and nitrogen, together with impurities such as methane and argon, pass through the PSA system as a less readily adsorbable component thereof. Thus, ammonia is removed from the purge gas without the need for water scrubbing, and a treated gas stream in dry, ammonia-depleted form is passed from the PSA system to a separator zone containing a gas permeable membrane capable of selectively permeating the hydrogen and nitrogen components thereof. Methane and argon impurities are removed from the separator zone as non-permeable gas. While the ammonia removed in the pretreatment PSA system may be harmful to the particular permesble membrane being employed, it nevertheless comprises valuable product gas, the loss of which would adversely affect the high recovery desired from the overall ammonis production operations. The separation and desired recovery of ammonia from the synthesis loop purge gas by the prior art approaches are, however, necessarily costly and involve inconvenient processing operations. 8y contrast, the invention facllitates the recovery of such ammonia in a convenient manner conducive to the enhanced recovery of high purity ammonia product. Thus, the purlfied D-14,803 ~2~:

g~s stresm con~aining hydrogen and nitrogen removed from the separator zone as the permeate portion of the treated gas stream is recycled to the PSA system as a purge gas to facilitate desorption and recovery of the previously adsorbed ammonia from said system~ The essentially dry purge g~s effluent discharged from the PSA system, and containing hydrogen, nitrogen and said desorbed ammonia, is convenlently recycled to the ammonia loop as purified gas stream substantially free of the impurities present in said ammonia synthesis loop purge gas, but substantially without undesired loss of ammonl~ product.
It will be appreciated that the process ~nd system of the invention can be applied to a variety of other permeable mem~rane gas separstion operstions. For example, methanol production operations conventionally include the removal of purge gas from 8 methanol synthesis loop. This purge gas contains valuhble methanol product gas desired to be recovered, but comprising a component deemed harm~ul to various typical permeable membrane materials. The purge gas is thus advantageously passed to a PSA system for the selective adsorption of said methanol prlor to passing the resulting essentially dry, methanol-depleted, treated purge gas stream to said separator zone containing a gas permeable membrane capable of selectively separat~ng the useful components from impurities present therein. In the methanol application, the desired product components, nsmely hydrogen and c~rbon dioxide, are selectively permeated through the D-14,803 ~32~7~2 membrane and recovered as the permeate portion of the treated gas stream. The impuritles associated therewith, namely methane and nitrogen, are removed as the non-permeate portion of the treated gas stream. In the practice of the invention, said permeate portion is recycled to the PSA system, and an adsorbent bed thereof, as 8 purge g8S to facilitate desorption and removal of methsnol therefrom. The essentially dry purge gas effluent discharged from the PSA system thus contains hydrogen, carbon dioxide and said desorbed methanol, and is recycled to the methAnol loop as a purified gas stream subs'cantlslly free of the impurities present in said methanol synthesis loop purge gas, but substantlally without undesired loss of methanol product. Another practical, lllustrative embodiment of the invention resides in the treatment of a feed gas stream comprising natural gas to be separated from carbon dioxide, H2S and water, with SAid stream also containing various hydrocarbons that are both harmful to various permeable membrane msterials and impurities with respect to a desired natural gas processing operation. Said harmful hydrocarbon impurities are selectively adsorbed in a pressure swing adsorpl:ion system, together with sald H2S
and water, prior to pass~ge of the hydrocarbon, H2S and water-depleted natursl gas stresm to the separator zone containlng said permeable membrane.
The carbon dloxlde 8as is selectlvely permeated through said membrane, and the carbon dioxlde containing permeate portion of the treated gas stream is recycled to the PSA .system as a purge gas D-14,803 to desorb and remove the hydrocarbons and other ~dsorbed components constituting impurities with respect to the desired natursl g~5 processing operation from the PSA system. In this instance, S the purge gas effluent discharged from the PSA
system is passed to w~ste or other use, whlle the non-permeate portion of the treated gas stream, constituting a pur~fied naturhl gas stream, ~s removed ~s product gAS.
It will be apprec~ated that the invention enables e clean, dry feed g8S stream to be psssed to the permeable membrane, thereby enhancing mem~rsne life Qnd effectiveness, and the components of the ~nltlal feed stream deemed harmful to the membrane to be conveniently recovered in a hi8h purity gas form rather than in the aqueous stste. Thus, substantislly less energy is required to achieve the desired separation and recovery of components as compared with water wash or distillation process.
Furthermore, either the permeate or the non-permeate portion of the trested gas stream can be recycled to a reaction loop, or otherw~se processed in particular applications, in advantageously water-free form.
The PSA system employed in the practice oF
the invention will contain one or more beds, with at least two ~dsorbent beds commonly being employed.
It will be understood that the feed gas stream, such BS the natural gas stream or the flmmonia or methanol loop purge gsses or the natur~l gas stre&m referred to above, is passed, on Q cyclic bssis, to each bed at the appropriate intervals in the processing D-14,~03 sequence ~hat includes (a) adsorption ~t hn upper adsorptio~ pressure level, (b) cocurrent depressurizstion to an intermediate pressure level or levels, wlth release of void space gas rom the product end of the bed, (c) countercurrent depressurizatIon to a lower desorption pressure level, with release of g~s ~rom the feed end of the bed, (d) purge and (e) repressuriz~tion from said lower desorption pressure to said upper adsorption pressure. Those skilled in the art will appreciate that the PSA system will commonly contain at least three or four adsorbent beds, wlth systems containing up to ten or more beds being useful in particular applications depending on the production, purity and recovery requirements of ~ given appl1cation. Each bed of the system will generally comprise adsorbent particles known and commercially svailsble in the art that are capable of selectively adsorbing the component(s) of the feed gas stream deemed harmful to the permesble membrane being employed. Each adsorbent bed will also be understood t:o include means for passing the feed gas to be treated to the feed end of the bed, and means for discharging the less readily adsorbed, treated gas stream f`rom the discharge end thereof, and to be adapted for the carrying out of the processing steps indicated above on sn spproprlate cyclic basls.
In conventional pressure swing adsorption operations, the void space gas released from one bed ls commonly passed, directly or indirectly, to snother bed initially at a lower pressure for pressure equali~ation purposes. Additional vod D-14,803 ~28;~2 space gas can be rele~sed from the product end of the bed being cocurrently depressurized, with such void space gas be~ng used to provide purge gas to another bed in the PSA system before such other bed is repressurized from its lower desorptlon pressure. In the practice of the invention, however, such use of cocurrent depressuriz~tion gas to desorb ~nd recover components of the feed gas mixture hsrmful to the downstream perme~ble membrane, but constitutlng valusble product gas and/or a desirable reactant or reactants, would result in said components belng recovered together with the impurities content of the cocurrent depressurizstion gas as well as with the useful components of the feed gas stream. Contrary to said convention~l PSA practice, therefore, the tre~ted gas stream removed from the PSA system is passed to the separation zone for the indicated separation of useful components of said gas stream from impurities sssoclated therewith, with the purified g~s stream wlthdrawn from the sep~rator zone being recycled as purge gas to the adsorbent beds of the PSA system in place of cocurrent depressurization gas. Thus, lmpurities interlded to be separated out of the feed gas stre~m by the PSA - membrane combinatlon of the invention sre not lncluded ln the purge gas for the PSA system. It will be sppreciated that, ln embodlments in which the harmful components removed from the feed gas stream do not constitute a desirable reactsrlt or product gas, the practice of the invention en~bles such components to be desorbed from the adQorbent beds and removed from the PSA

D-14,803 ~2~3Z7~X

system wlthout undesired loss of valuable product gas and/or desirQble re~ctents. If, ln such case, the void space gas relessed from one bed upon cocurrent depressurization were to be used as purge gas, as in conventionel oper&tions such as those descrlbed ln the Wagner Patent, U.S. 3,430,418, the desorptlon and removal of harmful components would be accompsnied by the necessary loss of valuable re&ctants and/or product gas as well as by the dlscharge from the systems of undesired impurities.
By recycllng the gas stream contsining a substantial amount of impuritles from the separstor zone to the PSA system as a purge gRS to remove harmful, non-valuable components, prior to dlscharglng said gas stream, l.e. either the permeate or more commonly non-permeate portion of the treated gas stream, the loss of useful resctants snd/or product gss can be mlnimized. Thus, depending on the particulars of any given oper~tion, means wlll be provlded for recycling either the permeste portlon or the non-permeate portlon of the treated gas stream from the separator zone to the PSA system for use as purge gas therein. Conduit means are likewise provlded to pass the purge gas effluent removed from the PSA system elther to the applicable synthesis reaction zone or to waste or other purpose as the clrcumstances o~ a given applicatlon suggest.
It i8 within the scope of the invention to employ any suitable adsorbent in the adsorbent beds of the pretreatment PSA system. Thus, the ~dsorbent used will be such as to selectively adsorb the harmful components AS a more readily adsorbable D-14,803 component of the feed gas mixture, while enabling the treated g~s comporlents to be separsted in the separat10n zone to p~ss through the PSA system as a less readily sdsorbable component of sald feed g~s mixture. A vflriety of sdsorbent msteri&ls are well known in the art and suitable for use in the practice of the invention, depending upon the performance character~stics desired of ~dsorbent and the g~s separstion performance levels desired in any g~ven application. Zeolitic molecular sieves, activated carbon, silic~ gel, activsted ~lumina and the like sre ex~mples of suitable, commercially available adsorbents, with psrticular types of molecular sleve msterial being generally preferred for vsrious prActicsl commercial ~ppllcstions of the invention.
The sepsr~tor zone of the invention will contsin a permeable membr~ne of sny suitsble form for the desired gas separ~tion operations. Thus, the membrsne m~y be employed in plate and fr~me form, or may comprise spirsl wound film membranes, tubular membrenes, hollow fiber membranes, or the like. The use of hollow fiber membrsnes is generslly preferred due to the high surface area per unit of membrane that can be obtalned thereby. It will be apprecisted th~t, when membranes are used in tubulflr or hollow fiber form, fl plurality of such membrsnes c~n conventionally be arrsnged in parallel bundle form. In such embodiments, the feed gss stream can be brought lnto cont~ct with either the outer or shell side, or the lnner or tube side of the membrane p~rtlcles. It ls commonly preferred to D-14,803 ~82~

pass the feed gas stream to be tre~ted to the shell side o the sep~rstion zone, QS this arrengement generally results in ~ relatively lower pressure drop across the membrane than when the feed g~s i8 passed to the tube s~de of the membrsne. Tho~e skilled in the art will appreclate th~t the flow of the feed gas and of the permeat~ng gas within the separation zone can be either cocurrent or countercurrent. Using bundles of hollow fiber and tubular membranes, the psssage of feed gas can be either radlal or ~xial with respect to the direction in which the hollow fibers or tubular membranes are positioned within the separator zone.
It is within the scope of the invention to employ finy suitable permeable membrane materisl capable of selectively separating the useful components of the treated gas stream passed from the PSA system by the permeable membrane - cont~ining sepsrator zone from the impurities contained therein. The invention m~y be practiced with either the useful components or the impurlties being recovered ~s the permeate g8S, and thus with either the permeate or the non-permeate portion of the trested gas stream belng recycled to the PSA system &S purge gas depending on the pflrtlcular gas separation operation belng carrled out. In the ammonl~ ~nd methanol loop purge gas appllcatlons referred to flbove, the permeate portion of the treAted gas stre~m withdrawn from the separ~tlon zone is typically recycled to the PSA system fiS
purge gas to facllltate desorptlon of the harmful but valu~ble components, i.e. ~mmonls and methflnol ~-14,803 in such appllcations, and removal thereof for recycle back to the ammonis and methsnol synthesis loops in such respective production sperations. In the natural gas tre~tment operation, the C2 containing non-permeate portlon of the treated gas strea~ is reduced In pressure and recycled to the PSA system as purge gas to facilitate desorption of harmful, non valuable impuritles. lhose skilled ln the srt will appreciate th~t the membrane materials that may be employed include metallic snd inorganic membranes as well ~s v&rious organic polymeric msterials or such orgsn~c polymeric materials mixed with inorganic materials æuch as fillers reinforcements and the like. Organic polymers that may be considered for various practical commercial operations include such m~terials es polysulfones; polystyrenes; includlng such styrene - containing polymers as scrylonitrile - styrene copolymers, styrene -but~diene and styrene - vinylbenzyl halide copolymers; cellulosic polymers, such ~s cellulose acetate, cellulose acetate - butyrste~ methyl or ethyl cellulose; polyamides and polyimides;
polyc~rbon&tes; polyurethanes~ polyesters, including polyacrylates, polyethylene; polypropylene;
polyvinyl pyrldines, and the like. Such polymers may be either substituted or unsubstituted, with typic~l substituents of such substituted polymers ~ncluding halogens, such as chlorine, fluorine and bromine; hydroxyl groups; lower alkyl groups; lower elkoxy groups; monocycllc aryl; lower acyl groups, etc.

D-14,803 As confirmed in the sbove ind~cated Perry patent, U.S. 4,172,885, the ~rt generally desires to utilize a membrane material that is as thin as possible, while belng sufficiently thick to assure ~hat the membrane has sufftcient strength to withstand the membrane separation condit~ons to which it is to be exposed. The use of a permeable membrane in con~unction wlth a coating material is also known, wlth such combinations enabling good selectlvlty of separatlon to be achleved together with high flux through the membrane. Typicsl costings include substituted or unsubstltuted polymers that are either svlid or liquid under gas separation conditlons. Examples of such coating materlals include synthetic and natur~l rubbers, organlc prepolymers, polyurethanes, polyamines and the like. It will be understood that such co~tings may be polymerized elther before or after the appllcatlon thereof to the permeable membrane with which they sre to be employed.
It wlll be appreclsted that the components of the feed gas stream that are deemed harmful to the permeable membrane belng employed need not necessarily be essentlally completely removed in the pretrefltment PSA system. Rather, the treated gas stream removed from the PSA system wlll comprise an essentially dry gas stresm depleted of said harmful components such that the concentrat1On of said harmful components ln the treated gas stream is at a desired residual level such as to avoid undue damage to the permeable membrane belng used upon contact of the treated gas stream therewith. Such residual D-14,803 level will, of course, vary depending upon the gas separation being made, the opersting conditions employed, the particular PSA system employed, the composition and form of the permeable membrane, and the like. Those skilled in the art wlll appreciste that, as employed herein, the term "hsrmful" is used to denote (1) sny effect adverse to the contemplated life of the membrane and its effectlveness over the course of such membrane life and/or (2) any components that are undesirable or unacceptable in the membrane product ges.
As a typical example, the purge gas stream in the ammonla synthesis loop purge gas embodiment commonly contains on the order of about 0.5 to 5 vol.% ammonis. It is generally not economically feasible to employ the PSA-membrsne combination of the invention with the residual ammonia level being ln excess of about 0.5% after pretreatment in the PSA system. Practical commercifll application can be msde of the invention, in varlous embodiments thereof, with the PSA system being used to selectively adsorb ammonia such that the residual ammonla concentration of the ammonia - depleted, essentially dry treated gas stream is less th~n 0.5 ~5 volume %, as in the range of from about 0.1~ to about 0.5 volume ~. In other embodiments, it may be desirable to further reduce the ammonia level to less than 0.1 volume ~, such as from about 10 to about 1,000 ppm. A similar flexibillty will be understood to pertain to other gas separation operatlons carried out in accordance with the practice of the invention.

D-14,803 ~Z8Z~

The operating conditions employed in the prsct1ce of the invention will be understood to depend upon the details of the particular gas separ~tion operstion and of the part~cular PSA
system and perme~ble membrane unit employed.
Referring to the smmonia synthesis loop purge gas applic~tion, it is known in the art that the ammonia synthesis reactlon can be carried out over a relatively wide range of temperflture and pressure conditions, with temperatures generally in the r~nge of from about 150C up to about 600C. The reaction effluent is used to heat the feed gas to the reactor by indirect hest exchange, as indicated above, so that the reaction effluent is typicslly at a temperature of less than 100C at the point where the purge gas stream is withdrawn from the loop.
The ammonia synthesis pressure is generally in the range of from about 100 to about 200 ~tmospheres absolute, or more. The purge gas stream removed from the ammonia loop is gener~lly passed to the PSA
system at a pressure at least substantially ~n the ran8e of said synthesis pressure, although hlgher or lower pressures can be employed depending upon the circumst~nces of a given applicatlon. This pressure thus constitutes the upper adsorptlon pressure of the PSA system. The treated g~s stream removed from the PSA ~ystem, l.e. the less refldlly adsorbable component of the purge gss stream, wlll be at said upper ~dsorption pressure level. It is ~enerally ~dv~ntageous to pass the treated gas stream to the separator zone at 8S h~gh a pressure as possible so as to svoid unnecess~ry recompresslon costs upon D-14,803 ~2 ~

recycle oF the purified gas stream from the separation zone to the ammonla loop. Thus, the trested gas stream ls preferably psssed to the sepsr~tor zone at said upper adsorptlon pressure level, or essentially at said pressure level, although it will be understood that it ls withln the scope of the invention to pass the treated gas stream to the separator zone at ~ny pressure level desired for a given application. The permeate portion of the treeted gas stream will be understood to be obtained at a reduced pressure, with pressure drops of at least about 20 atmospheres and up to about 120 atmospheres, typically about 50-100 atmospheres, occurring across the permeable membrsne. The non-permeate portion of the treated gas stream is passed from the separ~tor zone at substantially the same pressure as that at which the treated feed gas is passed to the separator zone.
In the ammonia purge g8S application as discussed above, the permeate portion of the treated gas stream is recycled to the PSA system at a reduced pressure for use as purge gas to facilitate removal of the selectively adsorbed components from the adsorbent beds. Said reduced pressure comprises the lower desorption pressure level employed in the PSA
processing cycle. In other applications, the non-permeate portion of the treated gas stream may be recycled to the PSA system at a reduced pressure corresponding to the lower desorption pressure of the PSA cycle, as ln the natural gas treatment application referred to above. In a typical ammonia purge loop application, the purge gas is passed to 8 D-14,803 3-bed PSA system containing a suitable adsorbent st an upper adsorption pressure of 130 atmospheres absolute, with the treated gas passing to a separator zone containing a sultable commercisl membrane at essentially said upper ~dsorptiGn pressure. The permeate g~s, recovered at about 30 atmospheres and containing hydrogen and nitrogen free of undesired impurities, such as methane and srgon, is recycled to the PSA system flS A purge at said lower pressure, employed 8S the lower desorption pressure in the PSA processing cycle.
SAid purge gas facilitstes desorption and recovery of ammonla selectively absorbed so that the treated gas stream is passed to the membrane-containing separator zone with a depleted ammonia concentration of about 0.1 volume ~ of said treated gas stream.
The purge gas effluent removed from the PSA system is recycled to the ammonia synthesis compression train.
Those skilled in the srt will appreciate that vsrious changes and modifications can be made in the details of the process and system herein described without departing from the scope of the invention 8S~ set forth in the flppended claims. If, for example, the permeate hydrogen contAining gas stream i8 not available ln sufficlent quantity, the PSA bed regeneration step can be ~ided by the warming up of the permeate g8S indlrectly by means of steam or another heat containing source to facilit~te desorptlon an~ removal of h~rmful components fro~ the bed. Alternatively but less desirably, a portion of the non-permeate gas can be D-14,803 ~z~

utilized for supplemental purge purposes after appropriate pressure reduction. As indicated above, the port10n of the g8S sepsrated on the sepsration zone that is employed for purge purposes in the PSA
system csn be either the permeate or the non-permeate portion depending upon whether the selectively adsorbed component is to be removed or rejected. Thus, in a practical refinery ~pplicstion, sdsorbed ~rom~tics, H2S or H20 along with other hydrocsrbons can be desirably re~ected by the use of the undeslred residue, or non-permeate, gas as the regenerating purge gas. In ~ toluene deslkylation application/ adsorbed aromatlcs compri~ing useful components csn be recovered and recycled to the main loop by utilizing the purified hydrogen stream recovered as the permeate portion as the regener~ting purge gas.
The invention provides a highly advant~geous and convenient means for echieving a product of desired specification ln membrane gas separation processes ln which the gas stream to be sep~rsted cc)ntsins a component or components deemed harmful to the llfe and performance capabilities of the membrane~ The pretreatment PSA ~ystem as employed in the pr~ctice of the inventlon provides desiruble processing flexibllity depending upon the circumstances of ~ given gas separ~tion operation.
The PSA-membr~ne combination, ss employed in the practlce of the inventlon, prov~des a significant sdvance in the gas separstion art as applied to natural gss tre~tment, ammoni~ or methanol synthesis loop purge gas purificstion and other deslrable industrisl applications.

D-14,803

Claims

1. In the process for the production of a desired product wherein a feed gas stream is treated to reduce its concentration of a component(s) harmful to a permeable membrane prior to said feed gas stream being contacted with said permeable membrane for the separation of components useful in said production of said desired product from undesired impurities present therewith, the improvement comprising:
(a) passing said feed gas stream at an upper adsorption pressure to the feed end of an adsorbent bed in a pressure swing adsorption system capable of selectively adsorbing said harmful component(s) present therein, with unadsorbed useful components of said feed gas stream being withdrawn from the product end of the bed at said upper pressure level as a treated gas stream depleted of said harmful component(s);
(b) passing said treated gas stream essentially at said upper adsorption pressure to a separator zone containing a gas permeable membrane capable of selectively separating said useful components from said undesired impurities present therewith, the concentration of said harmful components(s) in said treated gas stream being at a desired residual level;
(c) separately withdrawing the non-permeate and the permeate portions of the treated gas stream from the separator zone, one such portion comprising a purified gas containing said useful components of the feed gas stream, the other D-14,803 such portion containing a substantial amount of the impurities content of said treated gas stream passed to the separator zone;
(d) passing said purified gas withdrawn from the separator zone for use in the production of said desired product, said purified gas, or a portion thereof, being first recycled to an adsorbent bed in said pressure swing adsorption system at a pressure lower than said upper adsorption pressure as a purge gas to remove said harmful components(s) from the bed where said harmful component(s) comprises product gas and/or a desirable reactant(s) in the production of desired product;
(e) discharging said gas stream containing a substantial amount of impurities to waste or other use, with said gas stream being recycled to an adsorbent bed in said pressure swing adsorption system at a pressure lower than said upper adsorption pressure as a purge gas to remove said harmful component(s) from the bed where said harmful component(s) does not comprise a desirable reactant(s) in the production of desired product, whereby undesired contact of the permeable membrane with harmful components of said feed gas stream is avoided.

2. The process of Claim 1 in which said purified gas stream comprises the permeate portion of the treated gas stream withdrawn from the separator zone.

3. The process of Claim 2 in which said permeate portion of the treated gas stream is D-14,803 recycled to said pressure swing adsorption system at reduced pressure as a purge gas to remove therefrom harmful component(s) comprising product gas and/or reactant(s) in the production of desired product gas.

4. The process of Claim 3 in which said non-permeate portion of the treated gas stream is discharged to waste or other use.

5. The process of Claim 4 in which said feed gas stream comprises an ammonia loop purge gas removed from an ammonia synthesis loop, said ammonia loop purge gas being passed to said pressure swing adsorption system for the selective adsorption of ammonia therefrom, said ammonia being harmful to the permeable membrane but comprising valuable product gas.

6. The process of Claim 5 in which said treated gas stream in ammonia depleted form is passed to said gas permeable membrane capable of selectively permeating the hydrogen and nitrogen components thereof at reduced pressure, with methane and argon impurities comprising non-permeate components thereof.

7. The process of Claim 6 in which said purified gas stream comprising essentially dry purge gas effluent discharged from the pressure swing adsorption system and containing hydrogen, nitrogen and desorbed ammonia is recycled to said ammonia loop.

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8. The process of Claim 7 in which said treated gas stream passed to the separator zone has an ammonia concentration of less than 0.5 volume %.

9. The process of Claim 8 in which said ammonia concentration is from about 0.1% to about 0.5 volume %.

10. The process of Claim 8 in which said ammonia concentration is less than 0.1 volume %.

11. The process of Claim 10 in which said ammonia concentration is from about 10 to about 1,000 ppm.

12. The process of Claim 8 in which said purge gas effluent is recycled to the ammonia synthesis feed gas being passed to the ammonia loop.

13. The process of Claim 8 in which said purge gas effluent is recycled directly to said ammonia synthesis loop.

14. The process of Claim 13 in which said ammonia concentration is from about 0.1 to about 0.5 volume %.

15. The process of Claim 13 in which said ammonia concentration is less than about 0.1 volume %.

16. The process of Claim 1 in which said feed gas stream comprises a methanol synthesis loop purge gas removed from a methanol loop, said methanol loop purge gas being passed to said D-14,803 pressure swing adsorption system for the selective adsorption of methanol therefrom, said methanol being harmful to the permeable membrane.

17. The process of Claim 2 in which said feed gas stream comprises a natural gas stream containing H2S and water as well as hydrocarbons that are harmful to the permeable membrane and impurities with respect to a desired natural gas processing operation, said harmful impurities being selectively adsorbed in the pressure swing adsorption system.

18. The process of Claim 17 in which said carbon dioxide-containing non-permeate portion of the treated gas stream is recycled to said pressure swing adsorption system at reduced pressure as a purge gas to remove therefrom said components harmful to the permeable membrane and impurities with respect to the desired processing operation, the purge gas effluent discharged from the pressure swing adsorption system and containing said components being discharged to waste or other use.

19. The process of claim 1 and including heating the purge gas prior to its recycle to an adsorbent bed to facilitate desorption and removal of harmful components from the bed.

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