CA1072000A - Gas separation process - Google Patents

Abstract

ABSTRACT

A continuous gas purification process for selec-tively removing acid gases such as CO2, H2S, and SO2, other sulfur-containing molecules such as COS, and other relatively high boiling point impurities from a gas mixture stream con-taining lower boiling point components, such as H2, N2, CO, CH4, C2H4, C2H2, and the like, comprises: Providing the gas mixture to be treated as a dehydrated stream at an initial pressure above atmospheric and at a temperature which is sub-stantially the dew point temperature of carbon dioxide in the gas stream; contacting the gas stream at such pressure and temperature with a liquid carbon dioxide refrigerant-absorbent to absorb such impurities other than CO2, and separating these impurities together with still liquid carbon dioxide absorbent from a first residual gas stream, separating a portion of the carbon dioxide component of said first residual gas stream at such pressure, preferably by condensation while further cooling the pressurized stream by indirect heat exchange, leaving a second residual gas stream; contacting said second residual gas stream at such pressure with a second refrigerant-absorbent maintained below the triple point temperature of carbon dioxide for absorbing additional carbon dioxide and leaving a third residual gas stream; and preferably, performing a final separa-tion of additional carbon dioxide from said third residual gas stream by absorption into a liquid absorbent maintained below said triple point temperature, which liquid absorbent may be a liquid phase of the second refrigerant-absorbent.
The second refrigerant-absorbent preferably comprises particu-late solid carbon dioxide suspended in an organic liquid vehicle as a slurry in which the solid phase is a mixture of carbon dioxide and organic liquid and melts to provide in situ refrigeration for condensing carbon dioxide from the second residual gas stream. The finally purified gas stream is heat exchanged to recover its refrigeration potential and is dis-charged at substantially ambient temperature and at substan-tially said initial pressure. Some of the liquid carbon dioxide refrigerant-absorbent is separated from sulfur-containing impurities absorbed thereby to concentrate the latter for pro-cessing in a Claus plant to recover elemental sulfur. The absorbents used in the process are regenerated therein.
Additional carbon dioxide is recovered with a purity acceptable for discharge to the atmosphere or recovery as a by-product.
In various parts of the process, pressure energy and refriger-ation potential are recovered and used to minimize the net energy input to the process.

Description

I~ACKGROU~D OF TIIE INVI~NTION

This invention relates to the selective rcmoval of acid gases such as carbon dioxide, hy~ro~ell sulfide, and sulfur dioxide, other sulfur-containing`compounds such as carbonyl sulficle, and other rela~ively high ~)oiling point gases, generally regarded as contaminants, from gas mixtures also containing lower boiling point components such as hydrogen, carbon monoxide, methane, and other light molecules such as nitrogen, some or all of which may be of primary value.
The invention has particular applica~ion to the selective removal of acid gases and other sulfur-containing gases from, for example, the gaseous products of coal gasification, so as to produce a fuel gas end product of enhaneed value and utility. The invention is particularly useful, also, in the selective removal of similar contaminants from the products of combustion of methane or other carbon-containing fuels to produce l~ydrogen and nitrogen in the manufacture of ~mmonia. i simplified form o the invention has particular applicatlon ~o ~he removal of sulfur-containing compouncls ancl, also, suspend-ed particulate matter from stack or~flue gases. Various other uses for the invention will be recognized by those s~illed in the art.

~ lany methods have been developed for effecting the selective separation of acid ~ases from other gases of primary - -3- ¢~

` ioq2~00 value. ~sually, a cl~cmical or physical absorbent ~or the acid gases to be separated is contacted by the gas mixture being treated, the absorbent and absorbed acid gases are separa~ed, and the absorbent is regenerate~ and recycled. The Benfield hot carbonate process is a typical example of processes usin~ chemical absorption. See Pipeline and Gas Journal, October 19, 1972, p. 58. The Rectisol refri~era~ed methanol process is a typical example of processes using physical absorption.
See Industrial and Engineering Chemistry, July, 1970, pp. 39-43.
O~ !
Particularly in the case of chemical absorption processes, but also to a substantial degrce in the case of most physical absorption processes, there are substantial, inherent irreversibilities in both the absorption and regenera-tion steps. These irrevcrsibilities necessitate substantial energy inputs to the processes. ~or example, in the Benfield hot carbonate process, substantial amounts of steam are needed to regenerate the allcaline carbonate solution employed as the absorbent. ~nd in the Rectisol refrigerated methanol process, substantial amounts o~ steam and refrigeration are necdcd to regenerate the methanol absorbent. ~hus, an undesirable characteristic of prior acid gas removal processes is their inherent, substantial energy conswnption in regenerating the absorbent.

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~z~oo In many of the prior gas separation processes, the absorbent streams gradually accumulate impurities that have no value and would cause objectionable pollution i~
discharged in~o the environment. In those cases, additional capital and operating costs must be incurred ~or processing contalllinated absorbent bleed or slip streams. Many of the prior gas separation processes also inher~ntly inv~lve sub-stantial losses of absorbents due to minor poisonin~ réactions, leaks, thermal degradation, evaporation into t~e purified gases, and the slow accumulation therein of ~ars and other heavy materials Make-up for these absorben~ losses represents a continuous operating cost.

~ nother undesirable characteristlc of many prior acid gas~removal processes is that they require high capital and~operating~ costs to~recover the separated hydrogen sulfide and other sulfur-containing gases in a suf~iciently high con-centration for economical processin~ in a Claus plan~ to re- ~i3t~
duce them to elemental sulfur and non-polluting wastes. See ll~drocarbon Processing, April, 1971, p. 112.

:
Another undesirable characteristic inherent in sorne of the prior gas separation processes is that they require the use of absorbent solutions that are corrosive or become corrosive in use. This requires periodic replacement of ~5--:

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equipment or the use of ex~ensive corrosion-resistant materials or expensive corrosion inhibiting chemicals.

Another disadvantage of many prior gas separation processes is that much of the available pressure and thermal energy o the purified gas stream and of the separated gases and xea~,ents is not~recovered. More reversible processes could recover and utilize such potentia.lly available energy.

~ notller disadvan~a~e o~ many prior gas separation processes is that tbey use relatively vlscous absorbents or rea~ents, which decrease absorber stage eEficiency and .consume significant amounts of.energ~ for pumping.

Another disadvan~age of most prior~gas absorp~lon :processes lS that expensive heat exchangers or excessive absorbent flows are necessary to remove heat of absorption when large amounts of gas are absorbed.

Still another undesirable characteristic inherent in some of the prior;~acid gas removal~processes is their inability to remove trace impurities that:are~undesirable in the purified product gases. l`ypical~trace materials, dependin~
on the sources of the gases to be purified, may include metal ~, ~ bon~ls and sul~ur-contIu~ng molecules (other thi~ hydrogen sulfide), such as carbonyl sulfide, carbon disul~ide, mRrcaptans,i~d the like, relatively high boiling nitxogen-corlta~ng co~unds, Lncludingi~m~nia and hydrogen cyanide, and relatively hi~h ~oiling hydrocarbons (as hereinafter defined). The inability to remove such trace -impurities results in end product gases of lesser value or reduced utility. Of tlle various tr~ce im~uri~ies encountered -~
in acid gas removal processes, carbonyl sulide is particularly -~
objectionable and generally must be removed if present in a ~`
g~s stream bein~ treated. Some prior acid gas removal processes are incapable of doin~ so without substantially increasin~
absorbent flows, which requires large additional capi~al and operatin~ costs.

By way of summary, all of the prior acid gas removal ~rocesses have llad a number of serious disadvantages involvin~ troublesome problems and/or excessive ca~ital costs and/or operating costs. ~

SU~r.lARY 0~ TI~E I~r`~'rION ~`
', ''.~
In accordance ~7ith the present invention, all o~
the aforementioned relatively high boilin~, point con~aminants may be selectively and substantially com~letcly remo~2d ~rom gas mixtures containing lower boiling components o~ primary value, and a highly purified end product ~ay be ~roduced at ~,rcatl~J re~uced ca~ital and opcrating costs. For convcnience -7~

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in describing and defining the present invention, normal ;
boiliny or sublimination temperatuxes (at one atmosphere absolute pressure) colder than -86C. are considered to be relatively low, and normal boiling point or sublimination temperatures warmer than -86C. are considered to be relatively high.
In accordance with one aspect of the present invention, a process is provided for separating one or more relatively high boiling point gases from a gas mixture also containing lower boiling point gases, characterized in the steps of contacting the gas mixture with at least one refrigerant~
absorbent; separating the refrigerant-absorbent or absorbents ~ ~-together with relatively high boiling point gas entrained therewith from a residual portion of said gas mixture including most of the relatively lower boiling point gases; and `~;
separating material from the absorbed gases for processing into ;~
refrigerant-absorbent for use in the process. ~-Pre~erably, said at least one refrigerant-absorbent is selected from the class consisting of ~a) li~uid carbon dioxide; (b) a solid phase frozen from a liquid mixture of carbon dioxide and a liquid vehicle, and (c) slurries of said solid phase in said liquid mixture. It is also preferred "~
that the step of contacting the gas mixture with at least one refrigerant-absorbent comprise the steps of contacting the gas mixture first with one and then with the o~her of the -~
following two sorbents: (a) liquid carbon dioxide, and (b) particulate solids having a ~emperature below the triple point temperature of carbon dioxide; and, after each such contact, separating sorbent together with relatively high boiling point gas sorbed thereby from a residual portion of said gas mixture; and, in each instance, separating sorbent from the sorbed gas for reuse of the sorbent in the pro~ess.
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Z~O~ ,, In another aspect, the invention provides ~. process for separating relatively high boiling point gases, including .:
hydrogen sulfide and other sulfur-containing gases, from a pressurized gas mixture also containing lower boiling point gases, characterized in the steps of (a) contacting the gas ;~ ~
mix~ure with liquid carbon dioxide to absor~ and entrain : ~:
primarily any hydrogen sulfide and higher boiling point gases present, (b) separating liquid carbon dioxide and gases :~.
entrained therewith from a first xesidual poxtion of said gas mixture:including most o the relatively lower boiling point .
gases; (c) reducing the temperature of said first residual :
gas mixture portion to con.dense and/or freeze carbon dioxide therefrom; and (d) separating the condensed or frozen carbon .- :
dioxide from a second residual portion of said gas mixture.
The above step (b) includes preferably the steps of recovering carb~n dioxide from the mixture of li~uid carbon dioxide and gases entrained ~herewith and regenerating liquid carbon dioxide absorbent therefrom for reuse in step (a).
The carbon dioxi~e separated in step (d) is returned to the -~
process as a liquid for use as absorbent in step (a). The - .
-~:
abo~e step (a) may include contacting the gas miX~ure at a pressure in excess of 80 psia with liquid carbon dioxide and step (c) includes reducing the temperature of said first ~ :~
residual gas mixture portion below the triple point temperature ~, of carbon dioxide to condense or freeze carbon dioxide therefrom. ~ ;
In a still another aspect of the present invention, a process is provided for separating relatively high boiling point gases, such as hydrogen sulfide and higher boiling poin~
gases~ from a pressurized gas mixture also containing lower boiling components, characteriæed in the steps of contacting the gas mixture at a pressure above 80 psia with liquid carbon dioxide, and separating the liquid carbon dioxide and gases -'., :; .

'' '' :, - 8a -1~2~0 entrained therewith rom a residual portion of said gas mixture.
Preferably, the pressurized gas mixture enters the ;
process in a substantially anhydrous condition and at a pressure o~ at least 250 psia. According to a further preferred embodiment, the separated liquid carbon dioxide and entrain~d sulfur-containing gases are processed to recover at least a part of the carbon dioxide component thexeof in liquid form ;~
for return to the process at said pressure as a part of the absorbent used therein.
In a yet another aspect, the Present invention provides a process for separating carbon dioxide from other, lower boiling components of a gas mixture, characterized in the steps of contacting the gas mixture with a sorbent comprising a particulate solid refrigerant having a temperature below the triple point temperature of carbon dioxide to condense carbon dioxide from the gas mixture~ and separating the sorbent and condensed carbon dioxide from a residuaI portion of said gas mixture which includes most of the relatively lower boiling point components.
In a pre~erred embodiment of the l~st mentioned method, :`
the gas mixture, at a pressure of at least 250 psia, is first cooled to substantially below its dew point temperature at that pressure for condensing a part of its carbon dioxide ~ -content therefrom prior to contact with said sorbent. It is ., i also preferred that the particulate solid of said sorbent be suspended in a liquid vehicle and fIows countercurrent to a stream of said gas mixture for contact therewith. ~referably, said sorbent ccaprises a slurry of a particulate solid material in a solution of the same material in a liquid vehicle, said slurry flows countercurrent to a stream of said gas mixture ~;~
for contact therewith, and the particulate solid of the slurry - 8b - -lO~ZQO~
;, `
progressively melts during such contact and absorbs heat evolved from the condensing carbon dioxide from the gas mixture stream. Preferably, substantially all the cooling required to condense carbon dioxide from said gas mixture is provided b-~ the melting of said particulate solid refrigerant during contact with said gas mixture. The step of contacting the-gas mixture with said sorbent prefera~ly includes transferring heat into the solid sorbent, causing it to undergo a change of phase while effecting a progressive net reduction in the solid phase portion of the sorbent.
A still further preferred form of the first-mentioned embodiment of the present invention, the said one or more relatively high boiling point gases include carbon dioxide-and the step of contacting the gas mixture with said at ;- -least one refrigeran~-absorbent includes moving a stream of said gas mixture through a contact zone in countercurrent contact with a fIow of said at least one refrigerant~
absorbent which is a sorbent initially comprising a particulate solid suspended in a liquid vehicle and having a temperature substantially below the triple point te~perature of carbon dioxide~ whereby carbon dioxide is condensed from said gas mixture and is entrained with said sorbent, separating a ;
gas mixture stream largely depleted of carbon dioxide from the flow of sorbent entering said conkact zone, separating the sorbent and entrained, condensed carbon dioxide from the gas mixture stream entering said contact zone, reducing the pressure on the sorbent and entrained, condensed carbon dioxide to evolve carbon dioxide gas while cooling the sorbent below the triple point temperature of carbon dioxide and regenerating the sorbent to its initial condition, separating the so-formed gaseous carbon dioxide from the thus-generated sorbent, and returning the regenerated sorbent to said contact zone for reuse therein.
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According to another preferred folrm of the first ~ntioned general definition of the present invention, the said one or more relatively high boiIing point gases include ~:~
carbon dioxide and the step of contacting the gas mixture ~:~
with said at least one refri~erant-absorbent includes contacting the gas mixture with said at least one refrigerant- ~.
absorbent which is a sorbent comprising a particulate solid `~
refrigerant having a temperature below the.triple point ~
.:
tempera~ure of carbon dioxide to condense carbon dioxide from :
the gas mixture by transfer of heat into the solid sorbent, . .-.
causing it to undergo a change o phase while effe~ting a ~ :
pr~gr~ssive net reduction .in the solid phase portion of the ~.
sorbent and separating the sor~ent and condensed carbon dioxide from a residuaI portion of said gas mixture.
In accordance with a still further preferred ~ !
embodiment of the first recited general definition of the present invention, the said one or more relatively high boiling point gas0s include hydrogen sulfide, carbonyl sulfLde, . `~ . -and carbon dioxide and the step of contacting the gas mixture -.
with at least one refrigeran~-absorbent includes: (a) contacting the gas mixture in:countercurrent flow with liquid carbon dioxide in an amount suf~icient, but not exceeding that required, to absorb and entrain substantially all of the hydrogen sulfide present in the gas mixture and thereby also ;
to absorb and entrain substantially all of the carbonyl sulfide ..
present in said gas mixture; and the step of separating ;:
material from absorbed gases for processing into refrigerant~
absorbent includes (b) separating liquid carbon dioxide and ;
gases entrained therewith from a first residual portion of said gas mixture; (c) reducing the ~em~erature of said first residual gas mixture portion to condense carbon dioxide ~ therefrom; and (d) separating the condensed carbon dioxide .~:
from a second residual portion of said gas mixture. -:

20~0 ` ~`~ ,`,, ' According to a yet ano~her preferred embodiment of the first me~tioned general definition of the present invention, the said one or more relatively high boiling point gases include carbonyl sulfide, and said at least one ~ :
refrigerant-absorbent comprises liquid ~arbon dioxide, and ~;.
the step of contacting said gas mixture includes contacting said gas mixture in countercurrent flow with said liquid carbon dioxide to absorb substantially all of the carbonyl sulfide by using a li~uid carbon dioxide flow about equal to or less than that needed to absorb hydrogen sulfide.
In accor~ance with a yet another preferred embodiment :~
of the first-mentioned general definition of the present .~. -invention, the said one or more relatively high boiling point gases comprise sulfur-containing molecules, said at least .-one refrigerant-absorbent includes carbon dioxidet and the step of separating material ~rom~the absorbed gase~s includes separating the carbon dioxide .from the sulfur-c~ntaining molecules by crystallization.
The process of ~he present inven~ion is more nearly~
reversible and-requires~a lesser ne~ energy input than prior .~ . .
. , processes; absorbent losses are inherently replaced in the ~ -~
process o* are minimal; relatively low viscosity absorbents are used, with improved stage efficiency and savings in pumping costs; the removal of heat of absorption is facilitated by 1;. -phase changes in the absorbents, which minimizes heat exchange -.
costs; no objectionable environmental pollution is caused; the . ` -~
separated carbon dioxide can be recovered in a pure condition if desired; the separated h~drogen sulfide is recoverable in a desirably high concentration for processing in a Claus plant;
corrosion problems are minimal; and a residual primary gas :,~
product is recovered that is low in carbon dioxide content ¦-a~d is essentially free of sulfur-containing molecules, including ~1 ~
~ 8e -qz~oo carbonyl sul~ide. .
Although the di~erent procedures utilized for ~
removing sulfur-containing gases and for removing carbon ~:
.. .. -dioxiae may be practiced separately in accordance with the ,~
present invention, their integration into a single process ;- `
as herein ~ .
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1.7-71 ~ 2000 disclosed provides practical and economic advanta~es.

~ or removing hyclrogen sulfide, carbonyl sulfide, and additional impurities other than carbon dioxide from a ~as containin~ carbon dioxide, a pressuri~ed stream o the gases to be treated is first completely dehydrated. Tars and other very high boilin~ impurities are condensed and removed with the water in this step. The dehydrated gas stream is then cooled to a temperature as close as possible to its de~ point temperature(the tcmperatllre at which a liquid phase rich in carbon dioxide ber`ins to condense~ at the proce~s pressure.

.
The dehydrated and still pressurized gas stream at substantially the dew point temperature mentioned above, is then contacted by a countercurrent stream of a liquid carbon dioxide refrl~erant-absorbent in one or a succession of absorption columns. ~iydrogen sulfide, carbonyl sulfide; o~L~er sulfur-containing molecules, and other relatively hig~h boiling point molecules are essentially completely absorbed by and removed with the residual liquid carbon dioxicle absorbent, the heat of absorption being dissipated as heat of vaporization o~ a portion of the absorbent. The partially purifiecl residual gas stream emerges f~om this section of the~system at substantially the pressure and temperature at which it entered and with a somewhat increased content of carbon dioxide constituting .
:
the only significant impurity still to be removed therefrom.

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The residual liguid carbon dioxide absorbent and the absorbed relatively higll boiling point gasès withdrawn from the absorption column or columns are stripred of any small amounts.of valuable low boiling point gases unavoidably entrained tllerewith, the latter gases being recycled within the process. The stripped mixture of still liquid carbon dio~ide absorbent and absorbed impurities (~enerally sulfur-containin~ ~ases) i.s then processed to separate a~ léast a par~ of ~he carbon dioxide and (as here-inafter described) to recover pressure energ~y and rerigeration potential as economics may di¢tate. This separation may be accomplished by fractional distillation, crys~allization by freezin~, extractive distillation, or other means while pro-ducin~ substantially pu~e carbon dioxide, which may be reused as absorbent or may be removed as by-product. When the thus concentrated mix~ure of impurities consists largely of sulfur-containing gases~and carbon dioxide, such mixture may be ~-further processed for the recovery of sulfur, or example, in ;~ ~
a Claus plant, or it may be otherwise disposed of. , After the removal of sulfur-containin~ ~ases and other impurlties as described above, the still pr¢ssurized, residual, maln ~as stream, is further cooled to near the triple point temperature of carbon dioxide to prepar¢ it for a final carbon dioxide absorption opeEation. DependiD~ upon the main gas stream operatiDg pressurej a consid¢rable amount of carbon dioxide may be condensed and separated in the course of such co`oling (more at hi~h operatin~ pressures). This cooling of ,, -la-07~000 the main ~as s~ream is sui-~al~ly performed by passing it through a series of indirect heat exchangers. The heat exchange medium used to ef~ec~ such cooling may be any or a coml~ination of several lo~ t~mpera~ure ~luid streams ~roduced in the process.
The carbon dioxide conden~ed in this manner is withdrawn as a liquid s~ream at su~stantially the initial ~,as stream pressure and may provide some or aLl of the absorbent required in the ~irst absorption step described above, any required make-up being supplie~ as`also mentioned above.

The residual, further puri~ied gas stream is dis~
charged from that series of indirect heat exchangers at approximately its initial pressure hut at about -55C. and flows to the above mentioned, final, carbon dioxide absorption opera~ion. In tha-~ final operation, the main -gas stream is first brought into intimate contact with a~ absorbent that comprises a particulate solid llaving a high heat absoxption capability and that is below the triple point temperature of carbon dioxide (i.e., colder than -56.6C). Under these conditions, most of the remaining carbon dioxi~e content of the gas stream is condensed or ~rozen, removed with the absorbent, and vaporized or sublimed there~rom, According to the presently pre~erred manner of practicing this invention, ~he particulate solid o~ the absorbent for this final carbon dioxide removal operation is suspended in a`liquid vehicle as a pumpable slurry. In this case, the 107Z()~)0 gas stream ancl absorbent s]~ ry are brou~,ht into intimate contact by countercurrcnt flow through a conventional absor~tion column o series of columns.

The presently preferred absorbent for use at this point in the process is a ~artially frozen mixture o~ carbon dioxide and a liauid or liquids havin~ low viscosity, low vapor pressure, high solubility ~or carbon dioxi~e, relatively low solubility ~or lower boilin~. ~oint gases, 10~7 reactivitv, good stability, and, in the mixture, a ran~e of ~reezing ~oints belo~7 -56.6C. Examples of such liquids are: ethers, such as di-n-propyl ether, di n-butyl ether, and t-butyl methyl ether; I;etones, such as 2-pentanone (metllyl propyl ketone), t-l)utyl methvl ketone, and methyl isobutyl ketone; methanol; hydrocarbons, such as heptane and hexane; aldehydes, such as butanal, pentanal, and 2-methyl !, butanal; ancl inorganic liquids, such as 1uorosulfonic acid. The solid phase of the mixture is suspended in the liquicl ~hase as a pumpable slurry, preferably providecl at a tem~erature of about -70 to -75~. as it be~ins its flow throu~,h the absorption column or columns. When such an absorbent slurry contacts the ~as mixture stream still containing an appreciable amount o~ ~aseous carbon dioxide, simultaneous direct heat transfer and mass transfer occur between the gas, liquid, and solid phases. ~aseous carbon dioxicle o~ the gas stream conclenses and, by solution or entrainment, be-comes part of the liguid phase of the absorbent, and the solid phase of the al)sorbent concurrently melts in ab~sorl-inF, the heat of condensation and solution of the carbon dioxicle. Thus, carbon dioxide of the gas mixture stream is transferred rom the~gas to the liquid ~hase while the solid phase oE the absorbent slurry 1/ - / i.~

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liquefies, and both au~men~ the liquid phase cf the absorhent.

The term "refri~erant-absorbent" is used herein-after to characterize both the first described liquid carbon dioxide absorbent for relatlvely high boiling point ~ases and the last descri~ed slurry absorbent for ~aseous carbon dioxide.
As so used, the term "refr~erant-ahsorben~" refers to an absorbent that, in whole or in part, undergoes a chan~,e of .
phase durin~ the absorption process, the cllange of phase enablin~7 it to utilize at least a portion of the heat of absorption to ~ effect its own partial or complete phase change. Such an i ; absorbent is to be distinguished from one that depends solely on its s~eclfic heat for its heat absorbln~ c'apability, without undergoing a phase change. ~
, :

As part~of the presently preferred orm of the final carbon dloxlde~a~sorpti~on operation, the gas mixture stream, still at substantially the same o~eratin~ pr:essure, is ~, finally contacted with~only the liquid phase of the preferred refrigerant-absorbent slurry that has been processed to have a .i ~ ~ low content of dlssolved carbon dioxide, as hereinafter described : ~ , in more de~tail. This final step can ~issolve and remove carbon dioxide to almost any desired degree without further temperature reduction. As a result, a finally purified gas mixture may be withdrawn from one end of the flnal absorption column or columns at near the initial operatlng press~re of the system and at a temperature of about -70C to -75C. The final, liquid absorbent and carbon dioxide dissolved therein continue to move downwardly so as to merge with and au~ment the liquid ~hase of the refri~erant-~7ZO~O

absor~ent s lurry .

The thus combined final liquid absorbent and com-pletely melted refrigerant-absorbent, together ~ith the carbon dioxide absorbed thereby, are withdra~m as a liquid stream from the opposite end o~ the column or columns at a temperature of around -56C. Any relatively low molecular weight components of the ~as mixture being treated that were unavoidably also dissolved or absorbed thercby may be recovered therefrom by stripping them ~rom the combined absorbents and absorbed carbon dioxide and may ~)e added ~o the final, purified, ~as stream product after recovcring refrigeration potential from both.

Rcg,eneration o~ the rerigerant-absorbent slurry for recycling is readily effected by reducing the pressure of the liquid stream of combined absorbents ar.d condensed carbon dioxlde entrained therewith so that the absorbed carbon dioxide is evolved therefrom and the absorbent liquid mixture is thereby cooled to about -70 to -75C., thus regeneratir.g the solid phase thereof by freezing. The evolved carbon dioxide ~as is a substantially pure by-product.

After regenera~ion of ~he refrigerant-absorbent slurry, a portion of the liquid phase thereof may be separated therefrom by decantation, warmed, and fed into a ~as-liguid separator to facilitate deple~ing it of most of the remaining carbon dioxide therein. This liquid is then recooled to about -73C.
and pressurized to the main gas stream operatin~ pressure for use as a final, li~uid absorbent for carbon dioxidc to complete the r ., '.' : ` '' . , - / J. ~J
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' i~720~0 gas purification process.

In order for a liquid carbon dioxide refrip~erant-absorbent to exist in contact with ~he gas stream being treated by the first absorption procedure described above for removing relatively high boiling point impurities, tlle absorbent,prefer-ably should be warm enou~,h to insure that no solid phase can orm. Since the absorbent is predominantly carbon dioxide, which has a triple point temperature of -56.6C., temperatures warmer than -56.6C. are operational. ~ases dissolved in the ~' liquid carbon dioxide can depress the freezing point by several degrees, so that temperatures somewhat colder than -56.6C. may ! :
also be operationaL.
~ .

An upper.temperature llmit;or the~re~uired co- ~
; exlstence of:liquld and gas phases ln the flrst ab~sorption pro- ~ ;
cedure described above is the critical temperature o~ the system.
; ~ Since the critical temperature varies with the compositionl an upper limit is the highest value amon~ the critical temperatures of the pure components o the system. 0 the major components presen~ in the main gas stream leaving this absorption system, carbon dioxide usually has the highest crltical temperature, 31C.
, ~lence, the theoretical upper temperature limit in the absorption zon~
~, for any such system is 31C.
:, ' The lowest pressure at which liquld carbon dioxide`can be used as an absorbent depends upon the composition o the ~as . .
:~ being treated. The pressure must be sufficient or carbon dioxide ~ .
in the ~as and liquid phases to coexist in equilibrium.

,, -15-1~2~

If thc ~as is pure car~on dioxide, the 10~7es t operational pressure is about 80 psia, which is the lowest pressure at which pure liquid carbon dioxide can exist. As the ~raction of the carbon dioxide in the ~as is reduced, the minimum operational pressure rises. At 50% carbon dioxide and wher~ ~he remaining gas components are low boiling, the minimum operational pressure is about l6Q psia. Since ~as streams requirin~, ~reatment by this procedure necessarily also contain other high boiling point impurities along with the gaseous components to be purified, with the lattcr generally constitu~ing more than half of the to~al, -the carbon dioxide concentration in the main p,as stream at this point in the process will generally be less than 50/~. Therefore, for the purpose of removin~ relatively hi~h boilin~ point im-purlties, the required gas stream pressure will usually exceed 160 psia and be progressively higher with lower and lower carbon .
dioxide concentrations in the gas stream.

Theoretically, the limlting, maximum gas :stream pressure is the highest pressure at which the various liquid and gas phases can exist in equilibrium and is many thousand psia, although varying widely with the compositions involved.
Such extremely hi~h pressure would be well above any cconomically practical operating pressure for most ~as purification purposes, but might be used for special purposes. Thus, it is not possible to designate any meaningful, maximum, main gas stream o~eratin~
pressure.

As will become apparent from the ensuin~ description of the invention, the temperattlre to which the crude ~ases.

~0~20~

must be cooled for removing sulfur~contalnin~ molecules varies directly ~ith the main gas stream prcssure at which the raw f shift gas is to be processecl for tllat purpose. Also, ~he proportion of the carbon dioxide that can be removed by simple condensation in heat exchangers a~ temperatures above the triple point temperature of carbon dioxide increases markedly as the main gas sLream operatin~7 ~ressllre is increased. ~s a result, both capital and ener~y costs for carryin~ out the over-all process of the invention are reduced as the main ~as stream pressure is increased up to pressures at least as high as 10~ psia.
Obviously, however, pressures can be reached at which such a trend in capital costs may no lon~er hold true. On the other hand, particular applications of the invention may warrant the use of much highcr or lo~er pressures. Although it is not essential that the main ~as stream pressure be maintai.ned at the same level in the succe:ssive impurity removin~ operations of the complete process, practi.cal and economic considerations will ~,enerally dictate maintaining a uniform main gas stream pressure throu~hout those operations.

The ability o~ a refrigerant-absorbent to utilize the heàt of absorption to effect its own partial or complete phase change eliminates or reduces the need for sup~lying additional refrigeration to the absorption zone. ~nlen e~ploying7 liquid carbon dioxide as an absorbent Eor relatively hi~h boilin~, point gases in the manner ~irst describecl above, substantially all`of the heat of absorption is utilized in vaporizing a portion of the liquid absorbent so that no additional refrigeration is required in the absorption zone for maintainin~ a substantially constant L I / L~

~(3'72~0~

temperature throughout that zone. When using an absorbent slurry of a particulate solid that melts during thc absorption process in the manner described above, a large part of the heat of ab-sorption is utilized in melting the par~iculate solid, thus great ly reducing the amount of additional refri~eration required in the absorption zone to main~ain a desirable temperature gradient through that zone.

Alternatively, as hereinafter described in more detail, the particulate solid of the absorbent used for a final carbon dioxide removal operation may be any of a variety of particulate solid materials that are sufficiently resistant to abrasion to retain their particulate solid form and that have a high specific heat for condensin~ significant amounts of caxbon dioxide without requirin~ excessively requent regeneration by recooling.
Well known ~`echni~ues for the handling of such particulate solids in fluidized bed operations may be employed for continuously re-moving particulate solids as they become coated wit~ frozen carbon dioxide, subliming the frozen carbon dioxide therefrom, and recooling and recycling the particulate solid absorbent.

As will he apparent from the foregoing and from the ensuing detailed descrlption of the invention hereinaf~er, the portion of the process of the invention by which hydrogen sulfide, other sulfur-containing gases, and other gases having higher boiling points than carbon dioxide are removed rom the gas mixture being treated may be used, alone, for separating such high boiling components l^rom relatively low boiling point ~ases and carbon dioxide.

J

1~20~
'l`lle successive op.iations of the invention ~or separating carbon ~ioxide may also be used individually or in combination ~or that purpose when treatin~, ~as mixtures that are essentially free o still higher boiling point components such as sulfur-containin~, com~ounds. ~or example, if the ~as to be treated is 50% carbon dioxide and i~s operatinp~ pressure is 1000 psia, then the preliminclry removal of carboll dioxide by condensation at temperatures above its trlple point temperature will reduce the carbon dioxide content of the ~as from the ; initial 50% to about 15%. The final carbon dioxide absorption operation can then be used to re~.ove as much of the remaining carbon dioxide as is desired. On the other hand, i the gas to be treated is only 30% carbon dioxide, and the pressure is only lOO psia, the preliminary removal o carbon dioxide by condensation at temperatures above its triple point temperature is not applicable, but ~he final carbon dioxide absorption operation can still be used to remove as much of the carbon , , dioxide as desired.

~: ' -,, . -., , l~YZ~

BRIEI~ D~'SCRIPTl:ON OI;' TI~F: DRA~ GS
_ In the accompanyin~, drawings:

Figure 1 is a general flow dlagram for a system in ~hich the four successive gas treatment operatiolls of the in-vention are integrated for ~rimarily removing, respectively, water, hydrogen sulfide and ;other sulfur-containing gases, a part of the carbon dioxide, and, finally, most o~ the residual carbon ~ioxide (down ~o a fraction of 1 mol percent i~ desired) from a gas mixture being treated.

:;, . -. Figures 2A and 2B are general flow dia~rams showing in more detail the carbon dioxide crystallizer, indicated only ; generally ln Figure 1, in its two alternating conditions of .. . operation.
., !.~ `

.`,', ' " .
.. - 1 ~ ' `' `
l~qZ~O

D~T~ILED D~:SCRIPTION OF Tllr I~lVI,NTION

The invention will now be describe-l in more detail with particular re~erence to the removal o acid ~ases from the raw "shi~t ~,ases" produced by coal gasification (as distin~uished from coal liquefaction). ~s is well under~stood in that art, such processes and the composition of their gaseous end products may be varied according to the particular coal being processed, ;~
the degree o gasification sou~ht, and the contemplated end uses for tlle gas product, as well as economic and envirorlmental considerations. The raw shit gases are commonly ~ischarged from such ~rocesses at elevated temperatures and pressures which vary from process to process.

For illustrative purposes, the composition of the .
- raw shift gases to be trea~ed by the process of this invention , . -' ` ''" "'' ' 1 / '' ~ . ~

iO~2000 will ~e assumed to comprise the following typical components and proportioIIs:

Ilol Irac~ion O.t~06~
CO ~.1356 ~2 0.0052 CII~+ 0.1~
C2 0.31~9 112S. O. 0111 I~O 0.0005 COS 0. 0001 , IICN Trace i~il3 Trace i . ~ .
The exemplary processing of such a gas mixture will bc des-.i cri~cd on tlle further assumption that tlle mi~ture is received , .
froIn a coal gasification plant at 25~C. and 1000 psia and is ?
: processed at tlIat pressure. lIow the process and apparatus are ~, '~ desirably altered Eor processing such a gas mixture at a su~-~, stantially lower pressure of about 300 psia will bc explained in.tlle course of that description.
~ .

ReEerring now to tlle accompan~ing ~rawing, the raw s!lift gas mix~ure o~ the Eoregoing composition is introduced .
into the illustrated purifica~ion system through a line lO as a continuously Elowing stream at a temperature of about 25C., and a pressure o~ 1000 psia. ~rom the line 10, the gas mixture stream flows, first, through a heat exchange system 20 ~or dehydrating the stream and precooling it to i~s dew point .

, -22- ~

temperature, that temperature in this exam~le bein~ about -27C. The dellydrated stream then ~lows via a line 25 through a sulfur absorption system 30 for removal of its content o~ .
-relatively high boiling point compc)nen~s, ~articularly hydro-gen sul~ide and other sulur containing molecules, l'he residual, partially purifiecl stream then flows via a line 45 through another heat exchange system 60 for further cooling it to condense and remove the bulk of its content oE carbon dioxide and to dischar~e the residual stream at a temperature o:E about . ;
-55C., only slightly warmer than the triple point temperacure :
of carbon dioxide. The residual partially puriied stream then -~ flows via a line 65 through a inal carbon dioxide absor~tion ;~ system 70 for the removal of additional carbon dioxide wllile further lowering the main gas stream temperature to well below ` : the triple point temperature o~ carbon dioxide. The final, ., resi.dual, purified gas stream emerges from the final absorption ., , `` system 70 via a line 100 at near the initial 1000 psia pressure ., . ~,.
of the ra~ shi~t gases and at a temperature of abou~ -73C.
Before b~ing discharged as the finally purified product, this gas stream in the line lO0 is routed through another heat exchanger 102, then through a refrigeration unit 103 for recooling, and, thence, back through additional heat exchangers or utilizing its rerigeration potential in the proccss.

--','' , .

/ i Z~

Tlle main ~.as flo~ to and throu~,h the several successive puri.flcation and heat exchan~e ste~s just descri~e~
is em~hasizecl in the drawing hy heclvy solitl an~ dotted lines.
Those several ~as puriicati.on and heat exchan~e Stt?.~s of the ~rocess and their interde~entlency in tht?. ~re~ently preerred overall system ~ill no~.~7 he described in detail.

Prccoolin~. ancl Dehyclration The ~recoolin~, and dchydration system 20 may com-prise a series of indirect heat exchan~ers of conventional ' desi~,n (only one being shown) ~or rro~ressivelv coolin~. the raw shi~t gases entcring this system via the line 10 to sub-stantially the dew point temperature o the narticular gas mixture, in this case about -2,7C. ~uch cooling condenses substantial-ly all of the ~7a.ter content of the main ~as stream enterin~ the process through the line lO. The condensed water is removetl via a condensa~e line 21 l.eadil-g to a waste ~7att-~r clean-ur system (not shown), which should be selectcd accord- ?~''' ' ing to tlle nature and quantity o the impurities nccessarily condensed and removed with the ~7ater. As indicated in the drawin~ and ex~lained helow, the finally purified ~as stream and three otiler c~as product streams o different temperatures may be used as the coolincs:~. media in ~hese heat exchan~,ers, The precoolin~ and dehydration sys~em 20 will also include a Einal dehydration step (not sho~n) ~eore coolln~
below O~C. for removin~ the last traces o~ water rom the main c~,as s~ream. Conventional water scavenging steps, such as those em~loyin~. molecular sieves, activa~ed aluMina ~ 07ZQ~
ahsorbelits, etc. may he uscl .~or this ~-urpose.

The ~recooled and dehydrated main gas stream flows -' from the system 20 via the line 25 to t~le absorPtion system 30 at near the initial ~as stream pressure oE 1~00 psia.

Removal of ~ulEur-Contalnin~ ~ases .;
Thc absorption system 30 ~or removill~ relatively high boiling gases, particularly hyclrogen sul~ide and other sulur-containing ~ases, from the dehydrated main ~as stream, will suit-ably com~rise a multista~,e series o~ sieve tray absorption columns o~ conven~ional design. In each absorption column of such a system, the main gas stream moves upwardly, counter-current to and in intimate contact with a down~ard flow of liquid carbon clioxide ab~sorbent suprlie~ by a line 31 at 1000 psia and at the main ~,as stream tempera~ure oE about -27C, During travel of the main ~,as stream throu~,h the absorption system 30, substantially all o~ the hydro~en sulfide and other sulfur-containing molecules, alon~, with such other relatively hi~h boiling point molecules as may be present, are absorbed and removed from the main ~as stream by the liquid carhon dioxlde absorbent. At the temperature and pressure prevailing in this system, the heat o~ absorption of the absorbed gases is utilized in vaporizin~ some of the liquid carbon dioxide absorbent so that the liquid carbon dioxide functions as a refri~,erant-absorbent, This causes a small net increase in the amount of gaseous carbon dioxide in the main ~,as stream at the expense of liquid carbon ~ioxide , ~z~o :`
absor~en~ ancl ~ermits tae absorption to occur ~ith nep,li~ib1e ; increase in temperature o the ~as stream or of the absorbent.
;., . ' ' l`he liquid carbon dloxide absorbent and absorbed hydro~en su]fide and other heavier molecules are ~ithdra~n from the absorption system 30 at the ~ressure and ~emneratllre ., .
maintainecl tllerein (about 1000 ~sia and -27C) via a line 32, are passecl throu~,h an expander 33, by whicll their l~ressure is lowered to around 125 psia, and are ciischar~ed into a stripper-absorber co1umn 35 equi~ed with a reboi]er 36. `

In the stripper absorber 35, the 1i~,hter mo1ccules(hydro~en, carbon monoxide, nitrogen, and metllane) that are also absorbed in sr.lal1 amount by tlle li~uicl carbon di.oxide absorbent ln the absor~tion system 30 are strlpped from the still li~uid absorbent and other absorbed ~ases. ~ relatively small amount of the ~resh li~uid carbon dioxide absorbent flo~in~ in the line 31 is introducecl into the upper end of the ;~
column 35 through a branch line 34 for countercurrent contact ' , ~ -26-lO~Y2BOO

with ~he up~ardly movi-llg light fractions in the upper en~ of the column 35 for absor~ing any traccs of sulfur-containing moleculcs cntrained Wit~l the ligllt fractions. By using this supplemental absorption procedure in tlle column 35, ~he strippcd li~llt fractions and some gaseous carbon clioxide may ; be wi~l~drawn from the upper end of the colullm 35 essentially ~ree of sulfur-containing molecules and be moved througll a line 37 or fur~her processing as llereillafter described.
' .

The reboiler 36 requires only a small amount o~ hea~
tllat may 've supplied in any convenient manner, as by moving any available lleatin~ fluid therethrougll Vicl a line 38. The reboiler discharge via a line 39 consists essentially of liquicl carbon dioxide absorbent and absorbed sul~ur-containing molecules in a total concentration therein o~ about 4 mol percent. Tl~is stream is processed further to increase its concentration o~ sul~ur-containing molecules to abou~ 25 mol pcrcen~ or more, as desircd ~o procluce an economical feed stock for a Claus plant for recovering the sul~ur in elemental form. The desired Claus plant fced stoclc con-centration may be economically procluced ~y separating carbon dioxide by a combination of distillation and crystallization as hereinafter described. Distillation presen~ly appears to be a more prac~ical process ~or producing a carbon dioxide .~ .

' ` ~ lO~ZOOO

distillate having the requisite purity for reuse as a~sorbent ~, in the sulfur absorber 30 and in the stripper-absorber 35. ,-:. ;
At hi~h main gas stream pressures oE around 2000 psia, i . sulfur-free carbon dioxide may be condense~ in the conclenser .,: systcm 60 in ample amount to satisfy the above described liquid carbon dioxide absorbent needs o the process in both the absorption system 30 and the stripper-ahsorber column 35.
Therefore, wl~en processing a main gas strearn at such a high pressure, all o the stream flowing in thc line 39 may be passcd througll a branch line 39a to a carbon dio~ide crystall-.; izing system 40.
' At lower main gas stream pressures, less carbon .. ; dioxicle can be condense~ in the condenser systcm 60 for trans-;: ~ : : :
~ mittal througll the line 31 to ~he sulfur absorber 30 and ~
. ~ :
through the line 34 to tlle strippcr-absorbcr 35, in wllicll case makeup carbon dioxicle absorbent is require~ from anotller source. 'i~;7 ~ For this ~urpose, a part of the stream ~lowing in the line 39 ~ is passed through a brancll line 39b to a distillatioIl system 55, ' described further below, and a liquiied; substantially pure, carbon dioxide distillate produced in tlle ~istillation~sys~em .supplies the:makeup liqui~ carbon ~ioxi~c absorbent. I~hen the main gas stream pressure is below about 250 psia, aIl of :
~ ~ the flow through the lLne 39 may be to thc distillation system ., :;

" //
' ' 10~000 55 for maximizing the aMoun~ of substantially purc carbon ~io~idc produced thcrein Eor use as absorbe~t, and tile crys~all-izer 40 may bc idle.

o Thus, in a plant designed solely or oyeration at a main gas stream yressure o~ about 2000 psia, ~ . distilla~ion system 55 may ~e omit~ed, and in a plant designed solely for operation at a main gas stream pressure below about 250 psia, the crystallizer 40 m~y be omitted. ~or a plant operating at intermediatc main gas stream pressures, both the crystallizer and dis~illation system may be employed. each being tailored, of course, to process the desired flow ~hcrethrough.

Referring now to the carbon dioxide crystallizer 40 f as shown in Figures 2~ and 2B, it may sultably comprise four tanks 41, 42, 43, and 44. During one period of a ~-period operating cycle, the tanks 41 and 43 are colmected in series with one sidc of an intervening heat exchanger 45 ~pr performing a two-stage process of forming and depositing solid carbon dioxide in each tank 41 and 43. The other two tanks 42 and 44 are connected in series with the other side of the intervening heat exchanger 45 for performing a two-stage process of n~elting and discharging as a liquid the solid carbon dioxide formed and deposited in these two ~anks during tlle preceding period of the operating cycle. Figure 2~ shows tllat relationship.

g_ At tht~ eonelusion o~ tlle ~irst'(leseribe~l ~eri.o(l, tlle tan~
interconneetions are s~7i.tclled hy c~nnrt~riate vc~lvin~r ~o t~l~t, clurin~r tlle otller periocl o~ tlle eyele, earllo~ clioxi(le is ~ormed ~ , ancl c'e~oscited in tan~s 42 ancl 4~! h~r tl~e same ~ o-sta~re rroeess c~s before while earl..on dioxicle is meltecl ancl (lisel~ar~r~e~l Erom the tanl;s ~l3 ar~ 1 by tlle s,amt? t~lo-st~p& ~roeeC;s a.s l!e-~ore.
~i~r,ure 2L s}io~ts t~le latte.r relationsl~ir.

Consiclerin~ ~ip~ure 2~ in rnore detail, the litluid earbon clioxide al)sorhent and al~sorl)ed sul~ur-contai.nin~ i molecules flo~inp frol~ ~lle line 39 ~see 7Fi~ure ].) i.nto the' ~raneh line-3C~,a !)ass tllrou~h a v~lve 4G or ~ee~lin~ tl~em as a s~ra.y into tlle tnnl; ~ 7hieh is mai.ntairlecl a~ ressure o.~
a~eout G0 psia (wc~ e.1O~.7 t~e 75.1 rs.ia tri.rle~oint rressure of earhon dioxide). To enhanee atomiæation, the rressure o~ the linui.d in tlle I.ine 3'3a is desirahly inereasecl to around 20n r;sia a pul;~r (no~ sllo~.7n~. ~nder these eonclitions" part of ~lle licluld car~on cl~.oxi.de flasl~es into soli(l carl~on di.ox'i.(]e that '`' aeeumulates in~the tanl~ 41, ancl a gas miY-ture o.f ear~on clioxide and sulfur-eoTltainin~ moleeules i.s forn~e(l therein an~ Titll.-dra~n overheac] vla.a line 47~at,tlle re~uired rate; or Tnaintainin~
the tank 7,rcessllre near the desired 6~ rsia. ~ ~ressure relief valve (not sllown) in the line 47 may l~e usecl to eE~ee~ the recluired pressure eontrol. This ~as mixture is then eom7,~resse(l to ahout 85 psia by a suitahle eompressor 4~ or the lile in the line ~7, is li~ue~led ~y inclireet lleat e~;chall~e in the heat exchanter 45 in the line ~!7 ~ iS ~ress~lri.zccl to arounc~
200 rsi~ hy 1~ ~uTn~ (not sho~7n), and, as re~llate(] I~,r a valve -~3n-0~2000 ~9, is spraycd into the ~ank 43 maintaine~ a~ a pressure of about 18 psia A~di~ional carbon dioxide crys~als are thus formc~ an~ accumulated in the tank,43, and a residual gas mixture of carbon dioxide and sulfur-containing molecules is again wi~hdrawn overhead, this time ~ia a line 50 (sec ligure 1) for flow to a Claus plant By the two's~age re~oval of carbon dioxide, as dcscribed, the final, residual gas mix~ure taken off ~hrougll the linc 50 may be concentra~cd in sulfur-containing molccules to the 25 mol percent or so desired for an economical Claus plant feed During the~ above describe~ forma~ion and accumulation of solid carbon dioxide in tlle tanks 41 and 43, solid carbon dioxide similarly accumulated in~the tanks 42'and 44 ~uring ~ I ' ~he preceding half cycle is melted and removed For this ~ ' purpose, gaseous carbon dioxide lntroduccd to the crystallizer, 40 via a line 51 (see ~igure l) is compressed by a compressor 52 in the line 51 to about 85 psla and is discharged into the tank 44 where it condenses while melting the solid carbon dioxide therein The resulting liquid carbon dioxide flows via a line 53 through the hcat exchanger 45, where it is , vaporized before flowing on througll an extension of the line 53 into the tank 42 The melting of solid carbon dioxi~e and the condensing of~gaseous carbon ~ioxide again occur, and the total of the thus formed liquid'carbon dioxide is dLscharged from the crys~al.izer 40 chrough a line 54 (see `' , -~V720 Fi~ure 1).

,~t the conclusion o~ the two o~erations in the tanks ~ 4~1 clescribed al?ove, the sy~stem is convert~d ~y v~lve chan~,es ~o the arran~ement shown in ~i~ure 2I,. In the same nlanner, durin~ tlle next hal.~ cycle o~ o~erati.on, snlid carl?on dio~ide i8 fornlecJ. and cle~ositecl in. the tanl;s l~2 ancl l,4 ~hil.e ~ ;~
solicl carl.)on dioxide previously ~ormetl in the tanks ~1 ancl 43 is meltcd tllerein and is dischar~ecl via ~lle line 5~.

Liclulcl carbon clloxide dischclr~ed ~rom tl~e cr)Tstallizer 40 will have a concentration o~ sul~ur-contai.ni.n~ molecules below 2500 ppm~ Any desired lowe~ conce,n~ration o.~ sul~ur-containinp, mol.ecules may be achieved ~,y re~eatinp, the crystalliza~ion operation. The resultinp li~ui.d car~-on dio~ide is discl~arp,ed, ~re~erably after its ~ressure enerp,y and refrip,eration ~otential are recovered as hereina~ter clescribecl .- .
with re,~erence to ~i~,ure 1. ;' P~e~erri.n~ no~ to the na~ure of tl~e dis~illation system 55, sl~own only dia~rammatically in li~ure 1, it should he a ~ ' multi-sLa~.e system includin~, a ~eed va~ori7.er ~.tncl a distillate va~or compressor. The syste,m mav be of convelltional desi~,n ~or ~rocessin~ the liaui~ carhon dioY~icle ahsorhcnt ancl . - .

~ 32-107~0~0 absorbed sulfur-containing ~ases introduced ~llrough the line 39b for separating a portion only of thc carbon dioxide in a substantially ~ure form (about 1 ~pm of sulur-containin~, molecules) which, with recom~ression and heat exchan~e against the incomi.n~! liquid feed, may be dischar~ed as a liquid through a line 56. ~ liquid residue, in whicll the conccntration of sulfur-containing molecules is at or near 25 mol ~crcen~, may be withdrawn through a line 57. De~endin~ upon the re-quirements of the process as determined primarily by the main ~as stream ~ressure, part or all of the liauid carbon dioxide . t from the line 56 may flow through the line 56a, aided by an interposed ~un-p 64, and into the liquid carbon dioxi~e absorbent supply line 31 as makeup absorbent, and ~art or all may flow throu~h the line 56b into the line 54 that provldes coolant for the carbon dioxide condenser system 60. The concentrate `,;' of sulfur-containing molecules in the residua]. liquid carbon~ ~
dioxide vehicle flows i.n the line 57 through an interposed : -hea~t exchan~er 58 and expander 59 for recovery of refrigeration ~otential and pressure energy before mergin~ with the similar material discharged via line 50 from the ca.rbon dioxide crystall-izer and flowin~ therewith to a Claus plan~, -The residual main gas stream discharged rom the absorption system 30 via the line 45 will have had its original content of hydrop,en sulfide and hi~,her boilin~ ~Oillt molecules :,. 10~000 ~ >stantially co~letely removed. ~y ~ro~er absorption column desito~n ~nd ~-ith ~ppro~riate ~lo~ rates, any trace of sulur-containin~, compounds in the dischar~)ed ~,as mixture can readily be kept as low as 1 p~m by wei~ht. This leaves only carbon dioxide as an acid ~s c~ntaminant ~ti]l to he removed.

~s previously indicated h~rcin, carbonyl sulfide is commonly encountered as a contaminant in very small amou~ts ~ :
in gases also contaminated by other sulfur-containin~ molecules. '' By reason of its toxicity and its ~endenc~ to interfere ~li.th ~' various chen~ical reactions, carbonyl sul~i(le mu~st ~enerally be removed from ~as mixtures in which it is foun~. ~s also previously indicated herein, som~ prior acid ~as removal processes :~ :
are not capa~le of removin~ carbonyl sulfide or are ca~ahle of doin~, so only by su~stantially increasin~, absorbent ~lows relative to tlle flows of the crude ~ases beinF~ purified. The ,~
absor~ent flow increases re~uired or that ~ur~ose may be as great at 400~. By contrast, an advantageous characteristic of the above described operations for separating sulfur-containing -~
and o~her relatively high boilin~ ~oint gases from relatively low boiling point ~ases is its inherent a~ility to remove carbonyl sulfi~e even more effectively than hydrogen sul~ide. ,`:
Stated differently the fore~oin~, operations inherently s~parate all of the carbonyl sulfi~e when performed with the minimum liqui~ carbon ~ioxide absorbent flow that is capal~le o~ re~oving all of the hydro~o.,en sulfide, so that no additional cost is entailed for c~rbonyl sulfide rem~val, and also the flcw of li ~ d carbon dioxide absorbent for rem~l of carbonyl sulfide can be equal to or less , .
~ that required for the removal of hydrc~en sulfide. As will be appreciated by one skilled in the art, for purposes of the r~al of carbonyl sulfide, the flow of carbon dioxide can r~nge from about 70% of ~
that required for removal of hydrogen sulfide up to an equal or econcmacally :.
acceptable flow in excess thereof.
B _ ZO~

In addi~on to being effe~ive for removing hydrogen s~lfide, carbonyl sulfide ~d o~er sulfur-~ont~ng gases, liquid carbon '~
~oxide has other prop~i~ that contr~ute to the ec ~ ny of ~s p~ ; .~, of the overall p~x~ss. It has an exceptionally low visoosity of ~ ut 0.3 .'' to 0.5 centipoise over its range of temperatures in the syst~m, : , a relatively high specific gravity of about 1.18, and a relatively low molecular weight of only 44. Ali of these ' ., properties contribute to keeping the si~e and cost of equi~ment ;." , and pumping costs to a minimum. Moreover, liquid carbon `, dioxide ls ~roduced in the process in greater amount than .. - .
needed as an absorbent so that it imposes no replacement cost . -'-but~ instead, is produced as a useful by-product o~ potential economic value.

.
Initial Re~oval of Carbon Dioxide , ' ,.

The partially purified g,as mixture stream enters the first carbon dio~ide removal system 60 via the line 45 at-near the operating pressure of 1000 psia and a tem~erature of '~
about -27C. In,this system, the gas mixture stream flo~s through ,one or a series of indirect heat exchangers of con-ventional design for lowering the.gas mixture stream temperature ~ .
to about -55C., which is sufficiently low to condense a major ,:

B `" ~
. . _ . .

i~Y20~0 portion of t~e carbon ~ioxide content oE the stre~rn while also further coolin~, the stream to near the triple point temperature of carbon dioxide for the pur~oses of the succeed- -ing, final carbon dioxide removal steps of the process. Tlle resultin~ liquid carbon dioxide condensate is substantially free of sulur-containin~ molecules and su~plies a portion of the absorbent requirements of the absorption system 30 and the stripper abs`orber 35, to which it flows through the lines 31 and 34. The required additional liquid carbon dioxide absorben~ is supplied rom the distillation system 55 as ^~
described above.

Coolin~ in the heat exchan~e system 60 is most suitably perormed in a series of indirect heat exchangers (not individ-ually shown). Tlle primary coolant for this purpose may be the liquid carbQn dioxide of moderate purity that is discharged from the crystallizer 40 via the line 54 as described above.
This liquid carbon dioxide coolant may suitably be recovered from the heat exchange system 60 as a gas at about 75 psia and about -35C. via a line 61. A portion of the recovered coolant may be recycled back through the crystallizer 40 via the line 51 for meltin~ the solid carbon dioxide formed therein, as described above with reference to ~igures 2A and 2B. The balance o~ the coolant recovered from the line 61 may be ~36-.

... .... , . _ .. . . . . . . ... ..

10~2~0 dischar~ed to the atmosphere via a line 62 after recoverin~
additional refrigeration and pressure ener~y thcrcfrom, as hereinater described.

To the extent re~uired, additional refri.~eration for the condenser system 60 may be provide~ hv usin~ the final purificd gas s~ream and other product streams o~ the process as supplemental coolants, as hereina~ter described.

~,~
When the main gas s~ream of the present example flows through tile condenser system 60 at about 1000 psia, as much as 70% of the carbon dioxide content of the main ~as stream may be removed by reducin~ its temperature to about -55C., wi~hout the use of an absorption agent. As explained above, this is -accomplished with relatively simple and iDexpensive equi~ment, usin~ refrig.eration potential otherwise generated in ~he system ~ -and conveniently available for that purpose. Thus, the net ener~y input for this purpose is very small. At lo~er main gas stream pressures, less carbon dioxide is removed in this manner, as pointed out above, and more must be removed in the succeeding steps of the process.

The main ~as stream flows from the condenser system 60 throu~h the line 65 at near its original pressure of 1000 ~sia !

- ~7-:``
~qZOQO

an~ only sli~htly above the triple point temT~erature of carbon dioxide. These main ~as stream conditions are appropriate for the further and final carbon clioxide removal in the succeedin~ steps of the process.

~inal Carbon Dioxicle ~emoval .. . . . .. . .. .

l`he partially puriied main ~as stream, ~lowin~
to the final absorption system 70 via the line 65, i5 first moved into intimate contact with a refri~,erant-absorbent d~ i introduced into that system at a temperature ~7ell belo~J
the triple point tem~erature of carbon dioxide. In this case, the refri~.erant-absorbent advanta~eously comprises a ~articulate solid that melts over a tempera~ure ran~e below the triple point temperature of carbon dioxide. Most con-veniently, the particulate solid is suspended.as a slurry in an appropriate liquid vehicle.

Many different particulate solid ma.terials may be used in the absorption sYStem 70 as the refri~erant-absorbent, or as a ~art thereof. However, amon~ all of the useful, particulate solid materials, a soli~ frozen from a liquid mixture of carbon dioxide ancl liquicl vehicle is unique, both in its behavior as a refrigerant-absorbent and in its re-lationship wi.th thc liquid vehicle in which it is preferably suspended as a slurry. The composition of that solid ma~ vary from 0% to 100% carbon dioxide depending upon the choice of liquid vehicle. With 2-pentanone as the liquid vehicle, the solicl is pure carbon dioxide. ~Tith di-n-propyl ether and -38- :

~ 200~
: `
di-n-butyl ethcr, ~he soli~l is a mix~ure rich in carbon dioxide.
"

Usin~ a solid of ~lle character menti.oned above, the liquid vehicle of the refri~erant-absorl~ent s]urry shoul(l be a ~,ood solvent for car~)on dioxide ancl be miscibl~ with liquid carbon dioxide so as to funct:ion as a sink Eor the liquid pro-duced by progressive meltin~, o~ the soli~ ~hase and absorption from the ~,as phase as the tem~erature o the re~riperant-absorbent slurry increases cluring contact with thc main gas stream. ~everal exam~les of suitable liquid vehicles are enumerated above herein.
Of those mentioned, ~-pentanone is ~resently ~referred, and the follo~7inp~ detailed process descri~ion a~plies wllen usin~

2-pentanone as tl~e liquid vehicle.

The ~ro~ressive melting oE the ~articulate solid in the liquid vehicle of the refrigerant-ahsorbent slurry dis-trlbutes the cooling effect so that only smali tem~erature and carbon dioxide partial pressure driving forces are required throu~hout the direct heat transfer-absor~tion contact bet~een the slurry a~d the main gas stream. Thus, as pointed out in the fore~oin~, summary of the invention, when such a refri~,erant-absorbent slurry contacts a warmer ~as mixture stream still containin~ an appreciable amount of gaseous carbon dioxide simultaneous direct heat transfer and mass transfer occur between the gas, liquid, and solid phases. Gaseous carbon dioxide o~
the gas stream condenses, and solid of the refri~!erant slurry simultaneously melts, whereby carbon dioxide of the ~as mixture stream is transferred from the gas to the li~uid pllase, solid ~39-`` ' ~ ZOOO

of the refrigerant-absorbent slurr~ liyue~ies, and both au~,ment the liquid phase of the slurry, ~7ith the heat of condensation o.~ the gaseous carbon dioxide bein~ absorbed mainly by the meltin~ of solid. Because of these unique relationships and the resulting ahsorptioll mechanism, a solid ph~se frozen from a liquid mixture of carbon dioxide and a liquid vehlcle is the presently pre~erred particulate solid material for the refri~erant-absorbent slurry.

~ !

.. . , . .. . . . _ _ . . . . _ _ _ . ..

`` -``'` l~Z

l`he oT)timum ~roportionin~ of solid to liquid in the slurry and the quantity reauire~ ~or treating a ~!iven quantity of gas ~ill be determine~ by the car~on dioxide content o the-~as stream being treated, by the specific heats of the several constituents of both the slurry and the ~as stre~m bein~
treated, by the specific temperatures at ~hich the sl~trry absorhent ancl tlle gas stream are introduccd lnco atl absor~tion column or columns Eor countercurrent flow therethrou~h, by ~he absorption column desi~n, and by the ability oE pumping equip-ment employed to move the liquid vehicle ancl its entraine~
particulate soli~s in the system.

The residual, partially purified, main ~as stream entering the final absorption system 70 from the line 65 at near 1000 psia and about 55C. may still contain about 13%
, to 14% carbon dioxide. For treating such a gas mixture with the preferred refrigerant slurry described above, the slurry ill suitably contain about 15% by wei~,ht of the particulate `.'~
solid and be at the main ~as stream pressure but at a temperature of about -73~C. This slurry is introduce~ rom a line 71a into a sieve tray absorption column o~ conven~ional design at a level some~1hat below the top of the column (or ahead of the ~z~o~

last of a series of s~1ch columns) or downward movement counter-current ~o an upward flow of the main ~as stream. Thus, the uppermost part of the column (or the last one or more of a series of columns) is le~t available for a final scrubbing of the main ~as stream with a carbon dioxide-deple~ed portion o the liguid phase only of the re~ri~erant slurry. This inal, liqui~ ahsorben~ is introduced through a line 71b for ~o~nward movement coun~ercurrent ~o the upwardly Elowin~ main gas stream before the latter is dischar~ed from the absor~tion system 70 via t1le line lOO. For this final scrubbin~ operati.on, the carbon dioxide-depleted, liquid phase absorbent will be at the main gas stream pressure an~ at a temperature of -73C. It dissolves the remainin~ carbon dioxide from the main ~,as stream down to a final carbon dioxide content of l mol ~er-cent, or less if desired, depending upon the degree to which the final, li~uid absorbent was depleted of dissolved carbon dioxide and tl~e severity of the final scrubbin~ operation.
Continuing on down throu~,h the column or columns of ~he absorption system 70, the liquid phase absorbent and dissolved carbon dioxide merge wlth and au~ment the slurry absorbent introduced through the line 71a. Both then move together to the bottom of the column (or bottom of the first of a series of columns), absorbing additional carbon dioxide b~ the phase chan~e mechanism described above.

o During contact between the slurry absorbent and ' the main ~as stream, meltin~, of the solid phase o the slurry absorbent preferably ~roceeds to completion . , to provide as much cooling as possi~le or condensing a major portion of the rcsidual carbon dioxide rom the ~,as stream.

Additional refri~eration is required in the absorp-tion system 70 to supplement ~he ln situ refri~!e~ation provided by the refrigerant-absorbent. Such ~ddi~ional refrigeration ,.~
is providecl by indirect heat exchange with the ~inally purified ~as product or with other available coolin~ fluids as herein-after explained. It is supplied near the main gas s~ream inlet end o system 70 where the solid ~hase of the refri~erant-absorbent is exhausted or is approachin~ exhaustion. Additlonal refrigeration is also required adjacent the opposite end of the absorption system 70 in the zone thereof where tile main ~as stream is finally contacted by the liquid ab~sorbent introduced ~ i through the lLne 7lb.

Liquid carbon dioxide formed in the absorption system 70, both by melting and by condensation, is entrained with the liquid vehicle portion of the refrigerant-absorbent and is removed therewlth~through a line 72 at about -56C. The finally purified gas stream is withdrawn from this absorption sy,stem 70 tllrou~h the gas product line 100 at about -73C. and :. , ~0~2~0~

at near tlle ini~ial main gas stream opera~ing pressure of J
1000 psia. Tile refrigeration potential of the finally puriied gas strearn in tlle line 100 is recovered in heat cxchangcrs at various point~ in the overall system, as mentioned above and furtller detailed ~elow.

l`lle fully mcited, combined,absorbcr~ uids and absorbe~ carl)on dioxide withdra~m :Erom ~lle absorp~ion system 70 througll the line 7~ at about 56C. and near 1000 psia are firs~ moved ~ogether through a pressure reducer 73 to lowcr their ~ressure to about 125 psia for movement via a line 74 into a stripper-absorber 75, along wi~h the light ractions discllarged via the line 37 from tlle prior stripper-absorber 35.
This is indicated diagrammatically by the mer~ing of the lines E
37 and 72 into the line 74.

.
In the stripper-absorber 75, the light fractions (hydrogen, carbon mono~ide, nitrogen, and methane) that h~ve ;~
been unavoidably picked up from the main ~as s~ream during its passage througll tlle absorption systeln 70,toge~her with similar light fractions received fro~ the line 37iare stripped from the absorbent liquids and absorbed carbon dioxide. lrom the top of the stripping colun~ 75, ~he stripped light fractions and only a minor amount of carbon dioxi.de cntrained therewith are withdra~m via a line 76.

- ~44-1~ 00 The absorbent liquids and most o~ tlle absorbed' carbon dioxide, at about 125 psia, are ~ hdrawn from the bottom o the stripper-absorber 75 througll a valvc 77 in a line 7S lea~ing into a flash tanl; 79, ~hich is maintained at a lo~er pressure of about 65 psia. This pressurc drop results in the flaslling off from the li-luid ol: some of the carbon dio~i~c whilc ~he remaining li(luicl, inclu~ling all of thc ~sorbent vehiclc and most of the carbon ~ioxide, is withdrawn via a line ~0 to a refrigerant-absorbent slurry regenera~ing sys~em 85. The flaslled carbon dioxide gas is ~ithdrawn from the flash tank as it is ~ormcd, through a linc Sl, and, witll recomprcssion in a compressor ~2, is returned to the bottom of tlle stripper-absorber 75. 'l`here it bubbles upwardly through the liquid therein and assists in stripping tlle light fractions therefrom. ~lost of this carbon ~ioxide gas is ~hen reabsorbed as i~ rises in the upper part o the s~ripper-absorber 75 by successively contacting do~nward : flows o~ the same kind of refrigerant-absorbent slu~-ry use~ -in the absorption system 70 and tl~e same kind of liqui~
absorben~ uscd therein. These absorben~s are respccti-~ely introduced into the stripper-absorber 75 via lines 83 and 71b ~rom sources described llereinafter. I'hus, tile upper part of tlle stripyer-absorber 75 functions similarly ~o the absorber system 70 in separating carbon dio~ide ~rom the stripped light fractions, ~s a result, tlle light ~ractions leavlng thc , ' lO~J200~

.
s~ripper-a~sorber 75 via tlle line 76 have very li~le carbon ~ioxi~c ~,as en~rain~d ~l~erewi~h (4 mol percen~ or less).

Tl~e ligh~ fractions leavin~ the s~ripper-a~sorber 75 througll the line 76 are at a tempera~ure o~ abou~ -73C.
~ccor~ingly, ~hey are recycled ~ack tllrougll the ~rocess Eor recovcry of tLleir reri~era~ion potcntial l)e~ore ~cin~ com~ined with the pu~iEied maill gas s~ream as described h~rcina~ter.

The liquid effluen~ from tlle flasll tank 79, flowing .~ !
to tlle slurry re~,enera~in~ system 35 at abou~ 65 psia and about -55C., has its carbon dioxide content and i~s tem~eracure pro-gressively reduced by a succession of fur~ller prcssure re-ductions in a series of additional flash tanl;s (not individually shown). The separatcd carbon dioxide is 1ashed off as a gas ln a high s~ate of purity (less ~han 1 ppm o~ sul~ur-containing moleculcs) and is withdra~n via a line ~6 at only sligh~ly above ambient pressure and at a tempera~ure of about -75C.
It is thcn routed back through the proccss as hereinaftcr described ~or recovery of i~s refrigera~ion po~ential before it is withdra~n as a useful by-product or is released to the atmosphere, as economic considerations may dictate. Part o~
the remainder o~ the carbon ~ioY~idc in the liquid entering the slurry regenerating ~ystem 85 remains dissolved in thc -L~6-: lOqZ~

refri~erant-a~sorben~ vehicle as the la~ter is progressively cooled by the succession of pressure reductions, and the r balance is frozen and physically entrainc~ in the refrigerant-absorbcnt vehicle as it moves througl~ is systcm and out throu~h a line 87 as a liquid-solid slurry.

'l`hc thus re~encrated r~frigerant-a~sorbenL slurry flows throu~ll the line 87 to a clecan~ing station ~8 where a portion of the liquid vehicle oE tlle slurry is separaLe~
therefrom, as by flowin~ over a weir ~9 or the like. The remaining re~enerate~, refrigerant-absorbent slurry held back by the wcir 89 is withdrawn through a line 90 at slightly above ambicnt pressure and at a temperature o~ about -75C. for recyclin~ to the absorption system 70 and to the stripper-absorber 75. l~or this purpose, the flow of Iegenerated re-frig~erant-a~sorbent slurry in ~he line 90 is divided, a major portion flowing from ~he line 90 into an~ througll ~he line 71a and throu~h a pump 9l therein for rcpressurizi~g the slurry to about 1000 psia as required for it to flow into the absorption system 70 for use therein as previously described.
The remainder of that slurry flows~.into and through the line 71b and through a pump 92 therein for repressurizin~ the slurry to about 125 psia as required for it to ~low into the stripper-absorber 75 for use therein as previously described.

The liquid vehicle of the slurry that is separated at the decanting station 88 lS fed via a line 93 through a heat -~7--- --10~2~

exchan~er '~ in ~hicll it is warmed as required for evolving most o~ i~s dissolved carbon dioxide in a gas-].i~uid separator 95, to which it flows through a line 96. The substantially pure carbon dioxide evolved in the scparator 95 is discharged there~rom slightly above ambient pressure and at a temperature of about -36C. and is routed back througLI the process as hereinafter described ~or recovcry o~ i~s re~ri~eration potential be~ore it is withdrawn as a useful by-pro~uct or is release~
to the atmosphcre as economic consi~crations may dictate.

The vehicle portion of the refri~erant-absorbent slurry that has been separated from thc slurry at the dccanting station 88 and depleted of dissolved carbon dioxide in the gas-liquid separator 95 is withdrawn from the la~ter via a line 97 that flows througll the heat exchanger 94~wllere it is recooled by Lndirect heat exchange ~ith the just separated, cold, vehicle portion o the regeneratcd refrigerant-absorben~ slurry ~lo~-, ~ .
lng from the line 93. Emerging from the heat exchanger 94through a continuation o~ the line 97 at about -73C. and only sli~l~tly above ambient pressure, the recooled, carbon dioxide depleted vehicle portion of the regenerated refrigerant-absorbent slurry passes ~hrough a first pump 98 that re-pressurizes it to about 125 psia before it is divided by diverting a minor portion thereof througll the line 83 and into the upper end of the stripper absorber 75 for use thcrein ~Q~2~0~

as prcviousl~J ~escribe~. Thc major portion oE the rccooled, carbon dioxide-depletecl, vehicle portion of tl~e regenera~ed reirigerant-absorbellt slurry continues on through a further ex-tension of the line 97 and through a second ~ump 99 tha-t furtller repressurizes it to about 1000 psia beorc it flows thro~gh the line 71b and into the gas discharge end of the absorption systeln 70 or use thercin as previously describcd.

~lthough the desired low carbon dioxide content of ~ j .;~.
the finally ~urified main gas streaTn emergin~ from the absorption system 70 could be achieved in the process of the prescnt example by using only the refrigerant-absorbent slurry to absorb carbon dio~.ide, a slurry temperature of about -96C. as it cnters the absorption system 70 would be required to do so. That, in turn, either would require a grea~er total pressure drop in the success:ion o~ slurry flashers by wllich the refrigerant-absorbent slurry is regenerated, do~ to a finaI pressure , ~ t bclo~ ambient, with obvious disadvanta~es in terms of capital costs an~ contamination of the system in the event of leaks, or ~ould require substantially increased capital and operating costs for additional refrigeration. By using the carbon dioxi~e-depleted,liquid vehicle portion of the refrigerant-absorbent slurry as the final absorben~ for carbon dioxide, talcing advantage of its relatively high capacity for dissolving carbon dioxi~e. even at extremely cold temperatures, the coldest temperature required in the absorption system 70 ~72~O~

in tl~c yrcsent exam~le is about -73C., and the above men~ioned disadvantages arc avoided.

Recovcry o Pressure and Refri~eration l~ncry~y , ~s mentioned above, various procluct streams from the process are rou~ed back througll the sys~em for thc re-covery of re~rigera~ion potential thercfrom. These product streams include the finally purified main gas stream flowin~
in the line 100, the light fractions witlldrawn from the s stripper-absorber 75 through the line 76, the l~igh purity carbon dioxide gas withdrawn from the rcfrigerant slurry regenerating system 85 through the line 86 and from tlle gas-liquid separator 95 through a line 101, and a por~ion of the carbon dioxide coolant withdrawn as a gas from the carbon dioxide condenser system 60 through the lines 61 and 62. The first two of those four yroduc~ streams, in the lines 100 and 76 are at temperaturcs of about -73C. and are directly usable in the absorption system 70 as supplemental, indirect heat exchange refrigerants for maintaining the desired temperature gradient therein. For simplicity of illustration, a separate heat exchanger 102 is shown in Figure 1 of the ~rawing for recovering refrigeration energy from thoæ two product streams for use in the absorption system 70.

As previousl~ indica~ed, supplemental cooling for the carbon dioxide condenser system 60 may also be required.

, .

t)O~

-The ~ascs flowin~ ou~ of the heat exchan~er 102 throu~h extensions of the lines 76 and 100 may he used for that purpose, along ~ith the gases in the lines 8G and 101, which mer~e and flow together through an extension of the line 101, as shown.
~lowever, all of these last ~entione~ streams may require sli~ht recoolin~1 or Eurther coolin~ to provide a suEicient te~era-ture ~riving force. Therefore, a suitablc re~ri~,eration unit 103 may be provided for recooiing the gases flowing in the extend-ed lines 76, 100, and 101 be~ore they flow ~hrough a supple-men~al, inclirect heat exchanger 10~ that may be a part of `
the heat exchange system 60 but is shown aeparately in ~igure 1 of the drawin~ for simplicity of illustration.
'lhe three streams emerging from the sup~lemental heat exchanger 104 vla further extensions of the line5 76, 100, and lOl and the stream emcrging fram the heat exchanger 6~via the llnes 61 and 62 are all at temperatures below -27C. and may be used as the primary coolants for the incomi.ng cru~e ~as in the~heat exchange system 20 mentloned above and shown in the drawln~

As previously explained, tl~e carbon dioxide stream ~ -in tlle line 62 may be depleted of sulfur compounds to any extend desired. It may be dischar~,ed to tlle atmosphere or re covered as a by-product. Since it is still at a pressure of around 75 psi~ as it emerges from the heat exchan~e system 20 in the extended line 62, its ~ressure energy is recovered in an expansion turbine 106 or the like, as shown, before it is dischar~ed.

_, .. . . .. . . . .. ... .... ....

iV'Y~O~O

The relatively low pressure stream of li~ht fractions -flowing ~hrough the line 76 from the stri~er-absorber 75 should contain not more than ~ mol percent of car~on dioxide and may suita~ly he co~bined with the roughly ten time~s greater quantity of purifiecl gas flowing at high pressure through the line lO0. Accordingly, after ~assing through the heat ex chang,e system 20, the further extencled line 76 runs throu~h a compressor 107 and then into the further exten~ed line lO0 to provide the maximum, p~rified~ final gas ~roduct at close to the initial main gas stream pressure.

The high purity carbon dioxide (less than l rpm sulfur com~ounds) flowin~ through the further extended line 101 after i~ emerges from the heat exchan~e system 20 is at a pressure only slip,htly above ambient and may be discharged either to a by-~roduct collection system (not shown) or to the atmosphere as econornic considerations may dictate.

In the drawin~ and descri~tions of the heat exchan~er 5~ for recovering, hea~ from the Claus plant feed discharg,ed from the distillation system 55 via the line 57, no particular ~0~20~0 source of a heat supplyin~, medium is disclosed, ~imilarly, in the drawing and description of the heat exchan~er 9~i for warming the liquid absorbent 10win~from the decanting station 88 via the line 93, no particular source of the second of the t~o heat su~pl~ing media indicated in tlle drawin~ is disclosed. As in the case of the reboiler 36 associated ~ith the stripper absorber 35, any suitable, availab].e, heat ~supplyi.n~. 1uid ma~ be used in the heat exchan~,ers 58 and 9l~, thereby supplyln~ additional coolin~, wherever needed in the overall system, Obviously, ~here the ten~peratures of available heat-su~plyin~ fluids are not suitable for their dircct use as heat exchan~e fluids, heat pumps may be employed to ef~ect the nee~ed ener~y transfer, For example, this expedient may be employed ~or supplyinp~
refri~eration to the final carbon dioxide absorption system 70 ,. .
near the product ~,as dischar~,e end thereo~, the need for such additional refrigeration bein~ pointed out above.

Throughout the process, wherever signific~nt ~ressure ~ :;
reductions of sizable 1uid streams are required, as described above, expansion turbines drivin~ electric ~enerators may be used to recover the ener~y released,by such ~ressure reductions and convert it to a form that.is conveniently usable in operations that consume energy, as will ~e apparent to those skilled in tlle art. :

:;

, 53-17-71$

:10~2~ 0 .
~lternativc Carl~on Di.o~icle Al,sorl~e7l'ts ~s rreviou~sly sta~ed, rmany differen~ ~articulate solicl n.laterials ma~ ~.e u~secl in tlle final allc.orr~tion system 70 as the solid 7,hasc of a refri~erant-al~sorl~en~ .s].urrv eln~loyecl therein. ln the rreferrecl exam~1e ~escri.l~e~l al~ove in detail, r the so1icl 7,hase me1ts a.s carhon di.oxicle in t~le mai.n 5~as strean. i.~s con~len.qe(ll an~ oth are removed Erom tlle aksor7,ltion zone as a li(lui.d mixture with thc~ refrigeratl-t al)sor~en~ vehicle.
., "
Instead, the carllon clioxide of tlle ~,as stream may l.e conclen~sed "
to the sol.icl ~)hase, i.e., fro7,en, either as a n~lre com~oun~
or in a mixture witl~ material of the al~sorl~ent li.~ui~l vehicle.
In both of these cases, the Erozen carl~on clioxi(l~. i.s re~ovecl from the absor~tion zone as a solid ~sus~ended i.n the l.i~ui.d vehicle. In either case, the particu1ate solid component of such a slurr~ may ~e a solicl ~l~ase of a liquid vehicle .
consistin~ o~ a .si.n~1c com~ound, or tlle ~articulate solicl may .
l~e sus~ended in a liquid vellicle o:E di:Eferent colnl~os~tion.

Another tyre o reri~erant-aLsorl~cnt is a li~lui.d-solicl system which has a ne~liF,ible;carl-on cli.oxide content and ne~ 1e ca~acity to dissolve carl)on dioxi~le ancl otller p~ases.
An exam~1e is a licluid metal mixture witll a free7,ill~ ran~e below -5G.6C. Various mixtures of mercury, thalliun~, ancl potassi.um, for exam~1e, free7,e at tem~eratures well helo~ -S6.~C.
llsin~ such a refrigeran~-absor~ent svstem, car~nn clioxi~l~ of'tl~e main ~as stream wi11 condense tllerefrom as a ~solicl for entraillment ~ith tl~e l.i~ui..cl and ~rop~ressively meltin~ so1id ~ ases of such systcm. ~.c~aration of the fro7~en carl?on di.oY~ide From _ rj ~1 _ tlle clcl~letecl re~ri.~crant-al~c;orL~ent slurr.y .~n(.l rer~cner(~tic)n of the latter are reacli.ly accompllslle(l hv dro~infT the pressure o~ the mixture to.below t]le trirle roint rres~surc o~ carborl dioxiclc so tha~ the absor~ecl ca~ on clioxi.de suh-limes and is se~arated as a sTas- l`he coolillF rro~lucecl l-y tllat sublimation supT"lies nlo~st of the refri~eration re~uirecl to refreeze the partlculate solicl ma~eri.~l o-~ ~he r.e~rigerant-absorbent slurry for recyclin5T. to t~le ~ sorl~cl-.
~ .

Com~ositc materials may be eml-loYed as tlle ~articulate solicl of a re:Eri~erant-al~sorhent slurry ~or sorhin~ carbon di.oY~icle by fl~ee~in~ i.t out o~ tl-c main ~as stream and adsorbin~
it onto the sur~aces of the com~osite particles. lhus one mav employ a frozen fluid encased ln ~lurahle soli(~ ~Tall~s in the form o~ small s~heres or ~ellets. The use o~ such a composite solid refrlgerant combines the hi~h heat absorrtion characteristics of a solid~ uicl phase chan~Te ~Jith tlle handlin~ characteristics of permanently solid s~lleres or pellets. Such s~heres or pellets can ~e made by kno~m technoloFly ln many shapes ~nd sizes r~n~inF~
from microscopic (microencapsulation) to macroscopic (on the ;
order of inches in ch~racteristic dimension). The ~ellet wall materials may be metal or plas LiC . The small size composite spheres or ~ellets of that character can he slurried in a suitable li~ul.(l vellicle ancl used in an absorption col~lmn system ~ith only obvious differences in l~andlin~ procedures and in the character of the ~rocess ~rom ~7hat has ~een ~le.scrihe~l abo~Te. ~n.lch a refrigerant-absorbent slurry may be re~ene~ated ~hile sel~ratinfT~
the adsorbed carbon dioxide by sublimation by the same re~eneration procedure last ~cscribe~ above.

_5s lOq2~00 :: t l!o~;tver it is not necessary th~t slc11 a composite, ~articula~e solicl refrif.rerant-absor~ent ~or carbon dioxide he su~s~ended in a litluicl vehicle as a linuid-sollcl slurry.
Insteacl it may be suspended ln a gaseous vellicle as a so-called ~luidiztd bed utilizi.n~ well l~no~n luidized be1 tec1nilues for contlnuously removi.nf~ the ct~m~o~site bodies ~s tlley becomt- coated with frozen carhon dioxile, sul)limin~ the frozen carbon (1ioxide titere~ro11l reEreezinf~ Lhe encapsulatecl refrifrerant and recycling the recoole~ com~osite bodies into the ~luidi~ecl1)t-(l. Such composlt:e Lodits includin7. tl~os o larf~er ~si.r~e~s may be similarly used :iD ixed bcds movin beds ebullatin~ beds and the lil:e througl ~hic1 tl-t- f'~aS
stream beirif ~tr.eatec1 can be n-ovéd to absorb carhon cllo.~lde there~rom. .
,, nally :althoufrh Less e~ficient particulate solid materials which do not underFo ct plase cllan~e but hi l rely upon their s~eciflc heat for~their he~t al~sorbil1fr capabllity may also be- em~loyed in simiIar ~ays as carbor dloxide sorben~s.
Ohviously suitable solld materials for use in that manner should have as:hifrh a s~ecific heat~as ~ossible. . :

Other variants o~ tl~e ~artlcul.ltt solitl a1>sorbent or adsorbent materials disclosed herein an1 o~ the ~ethocls for handling them to conclens~e carbon clioxide belo~7 its tri~le ~Ol11t tem~erature may l~e.employecl as will be recof~n.ir~ed l~y those s~illel in tle reltvant arts.

: -56- - .

~inal Summarv ' `

~ rom the foregoin~ descriptiQn of thi.s invention, it ~ill be ap~rcciated that it enablcs the coml~lete se~aration of sul~ur-containin~, gases from relatively lo~ boiling point gases, and an~ desired degree of separation o~ other relatively high bailing point gases, including carl~on dioxidc, from lower boiling point gases, whilc operating ovcr a wi.clc ran~e of main f~aS stream pressures. As capital equiprnellt and operating cost analyses will ~urther sl~ow~ these results ma~ be achieve~ ~ith subs~antial savin~,s in both categories compare~ ~o the cal~ital equil~ment ancl operating costs of o~ller processcs 1l2retoore avai.lable Eor obtaining the same or comparable results, and these savin~,s may be realized over broad range.s o~ main gas stream operatin~, ~ressures and composi.tions. ~n addition, the process of the invention has the many other practical and economic advanta~es set forth i.n the .~oreg,oin~,~Summary o thè Invcntion.

'l'he ~inal purified gas product o~ the e~:emplary embodiment of the invention described in dctail above is suitable .~or ~irect use as a relatively low ~,T.~. ~ucl or for use as a feed to a relatively high B.T.U. ~uel. I)cpending upon the particu-lar crude gas miYture to be puri~ied by ~he process of the invcntio iO~2~

many otl~er uses for the purified procluct exist as will be appreciated by thosc sl;illed in ~lle per~inent arts.

AlLhou~h the inven~ion has bccn described witl dctailecl reference ~o a specific cmbocliment tllereof and to certain optional r.locli~ica~ions of ~hat embocliri~ent, it will also be appreciatecl tha~ the invcntion is susccptible to many other modi~ications while utilizin~ Llle principles tllereo~
and operating within the scope of the appended claims.

. .

Claims (19)

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

1. A process for separating one or more relatively high boiling point gases from a gas mixture also containing lower boil-ing point gases, characterized in the steps of contacting the gas mixture with at least one refrigerant-absorbent; separating the refrigerant-absorbent or absorbents together with relatively high boiling point gas entrained therewith from a residual portion of said gas mixture including most of the relatively lower boiling point gases; and separating material from the absorbed gases for processing into refrigerant-absorbent for use in the process.

2. A process according to claim 1, characterized in that said at least one refrigerant-absorbent is selected from the class consisting of (a) liquid carbon dioxide (b) a solid phase-frozen from a liquid mixture of carbon dioxide and a liquid vehicle, and (c) slurries of said solid phase in said liquid mixture.

3. A process according to claim 1, wherein the step of contacting the gas mixture with at least one refrigerant-absorbent comprises the steps of contacting the gas mixture first with one and then with the other of the following two sorbents:
(a) liquid carbon dioxide, and (b) particulate solids having a temperature below the triple point temperature of carbon dioxide;
and, after each such contact, separating sorbent together with relatively high boiling point gas sorbed thereby from a residual portion of said gas mixture; and, in each instance, separating sorbent from the sorbed gas for reuse of the sorbent in the process.

4. A process for separating relatively high boiling point gases, including hydrogen sulfide and other sulfur-containing gases, from a pressurized gas mixture also containing lower boiling point gases, characterized in the steps of (a) contacting the gas mixture with liquid carbon dioxide to absorb and entrain primarily any hydrogen sulfide and higher boiling point gases present;
(b) separating liquid carbon dioxide and gases entrained therewith from a first residual portion of said gas mixture;
including most of the relatively lower boiling point gases;
(c) reducing the temperature of said first residual gas mixture portion to condense and/or freeze carbon dioxide therefrom, and (d) separating the condensed or frozen carbon dioxide from a second residual portion of said gas mixture.

5. A process according to claim 4, characterized in that step (b) includes the steps of recovering carbon dioxide from the mixture of liquid carbon dioxide and gases entrained therewith and regenerating liquid carbon dioxide absorbent there-from for reuse in step (a).

6. A process according to claim 4, characterized in that the carbon dioxide separated in step (d) is returned to the process as a liquid for use as absorbent in step (a).

7. A process according to claim 4, characterized in that step (a) includes contacting the gas mixture at a pressure in excess of 80 psia with liquid carbon dioxide and step (c) includes reducing the temperature of said first residual gas mixture portion below the triple point temperature of carbon dioxide to condense or freeze carbon dioxide therefrom.

8. A process according to claim 7, characterized in that step (c) includes contacting the gas mixture with a sorbent comprising a particulate solid having a temperature below the triple point temperature of carbon dioxide to condense and/or freeze carbon dioxide from the gas mixture, and separating the sorbent and condensed carbon dioxide from a residual portion of said gas mixture.

9. A process according to claim 7, characterized in that said pressure is at least 250 psia and, prior to step (c), the temperature of said first residual gas mixture portion, at said pressure, is reduced to below its dew point temperature at that pressure but above the triple point temperature of carbon dioxide to condense carbon dioxide for recovery and use as absorbent in step (a).

10, A process for separating relatively high boiling point gases, such as hydrogen sulfide and higher boiling point gases, from a pressurized gas mixture also containing lower boiling components, characterized in the steps of contacting the gas mixture at a pressure above 80 psia with liquid carbon dioxide, and separating the liquid carbon dioxide and gases en-trained therewith from a residual portion of said gas mixture.

11. A process according to claim 10, characterized in that the pressurized gas mixture enters the process in a sub-stantially anhydrous condition and at a pressure of at least 250 psia.

12. A process according to claim 10, characterized in that the separated liquid carbon dioxide and entrained sulfur-containing gases are processed to recover at least a part of the carbon dioxide component thereof in liquid form for return to the process at said pressure as a part of the absorbent used therein.

13. A process for separating carbon dioxide from other, lower boiling components of a gas mixture, characterized in the steps of contacting the gas mixture with a sorbent comprising a particulate solid refrigerant having a temperature below the triple point temperature of carbon dioxide to condense carbon dioxide from the gas mixture, and separating the sorbent and condensed carbon dioxide from a residual portion of said gas mixture which includes most of the relatively lower boiling point components.

14. A process according to claim 13, characterized in that said gas mixture, at a pressure of at least 250 psia, is first cooled to substantially below its dew point temperature at that pressure for condensing a part of its carbon dioxide content therefrom prior to contact with said sorbent.

15. A process according to claim 13, characterized in that the particulate solid of said sorbent is suspended in a liquid vehicle and flows countercurrent to a stream of said gas mixture for contact therewith.

16. A process according to claim 15, characterized in that said sorbent comprises a slurry of a particulate solid mater-ial in a solution of the same material in a liquid vehicle, said slurry flows countercurrent to a stream of said gas mixture for contact therewith, and the particulate solid of the slurry pro-gressively melts during such contact and absorbs heat evolved from the condensing carbon dioxide from the gas mixture stream.

17. A process according to claim 15, characterized in that the particulate solid of said sorbent is largely carbon dioxide and the liquid vehicle thereof is a solution of carbon dioxide in a liquid vehicle whereby the solid of said sorbent progressively melts into and augments the liquid phase thereof, and carbon dioxide of the gas mixture stream is progressively transferred from the gas to the liquid phase and is entrained with and augments the liquid phase of the sorbent as the solid phase thereof is depleted.

18. A process according to claim 15, characterized in that the liquid vehicle of the sorbent is an organic liquid.

19. A process according to claim 17, characterized in that the liquid vehicle of the sorbent is a member of the class consisting of di-n-propyl ether, di-n-butyl ether, t-butyl methyl ether, 2-pentanone, t-butyl methyl ketone, methyl isobutyl ketone, methanol, heptane, hexane, butanal, pentanal, 2-methyl butanal, and fluorosulfonic acid.

--20. A process according to claim 13, characterized in that substantially all the cooling required to condense carbon dioxide from said gas mixture is provided by the melting of said particulate solid refrigerant during contact with said gas mixture.

--21. A process according to claim 13, characterized in that the step of contacting the gas mixture with said sorbent in-cludes transferring heat into the solid sorbent, causing it to undergo a change of phase while effecting a progressive net reduc-tion in the solid phase portion of the sorbent.

--22. A process according to claim 1, characterized in that said one or more relatively high boiling point gases include carbon dioxide and the step of contacting the gas mixture with said at least one refrigerant-absorbent includes moving a stream of said gas mixture through a contact zone in countercurrent contact with a flow of said at least one refrigerant-absorbent which is a sorbent initially comprising a particulate solid suspended in a liquid vehicle and having a temperature substantially below the triple point temperature of carbon dioxide, whereby carbon dioxide is condensed from said gas mixture and is entrained with said sorbent, separating a gas mixture stream largely depleted of carbon dioxide from the flow of sorbent entering said contact zone, separating the sorbent and entrained, condensed carbon dioxide from the gas mixture stream entering said contact zone, reducing the pressure on the sor-bent and entrained, condensed carbon dioxide to evolve carbon diox-ide gas while cooling the sorbent below the triple point temperature of carbon dioxide and regenerating the sorbent to its initial condi-tion, separating the so-formed gaseous carbon dioxide from the thus-regenerated sorbent, and returning the regenerated sorbent to said contact zone for reuse therein.

--23. A process according to claim 22, characterized in that the particulate solid of the sorbent is soluble in the liquid vehicle thereof and progressively melts therein while passing through said contact zone, and the particulate solid content of the sorbent is regenerated by freezing as the sorbent is cooled by pressure reduction and the evolution of carbon dioxide therefrom.

--24. A process according to claim 23, characterized in that the particulate solid of the sorbent is largely carbon dioxide and the liquid vehicle of the sorbent is a solution of carbon dioxide in liquid vehicle.

--25. A process according to claim 24, characterized in that the liquid vehicle of the sorbent is an organic liquid.

--26. A process according to claim 22, characterized in that a portion of the vehicle of the regenerated sorbent is separ-ated therefrom, the remaining sorbent is returned to said contact zone for reuse therein, and the separated gas mixture stream is intimately contacted by a countercurrent flow of the separated por-tion of the vehicle of the regenerated sorbent to remove additional carbon dioxide therefrom.

--27. A process according to claim 1, characterized in that said one or more relatively high boiling point gases include carbon dioxide and the step of contacting the gas mixture with said at least one refrigerant-absorbent includes contacting the gas mixture with said at least one refrigerant-absorbent which is a sorbent comprising a particulate solid refrigerant having a tempera-ture below the triple point temperature of carbon dioxide to condense carbon dioxide from the gas mixture by transfer of heat into the solid sorbent, causing it to undergo a change of phase while effecting a progressive net reduction in the solid phase portion of the sorbent and separating the sorbent and condensed carbon dioxide from a residual portion of said gas mixture.

--28. A process according to claim 1, characterized in that said one or more relatively high boiling point gases include hydrogen sulfide, carbonyl sulfide, and carbon dioxide and the step of contacting the gas mixture with at least one refrigerant-absorbent includes:
(a) contacting the gas mixture in countercurrent flow with liquid carbon dioxide in an amount sufficient, but not exceeding that required, to absorb and entrain substantially all of the hydrogen sulfide present in the gas mixture and thereby also to absorb and entrain substanti-ally all of the carbonyl sulfide present in said gas mixture;
and the step of separating material from absorbed gases for process-ing into refrigerant-absorbent includes (b) separating liquid carbon dioxide and gases entrained therewith from a first residual portion of said gas mixture;

(c) reducing the temperature of said first residual gas mixture portion to condense carbon dioxide therefrom and (d) separating the condensed carbon dioxide from a second residual portion of said gas mixture.

--29. A process according to claim 1, characterized in that said one or more relatively high boiling point gases include carbonyl sulfide, and said at least one refrigerant-absorbent com-prises liquid carbon dioxide, and the step of contacting said gas mixture includes contacting said gas mixture in countercurrent flow with said liquid carbon dioxide to absorb substantially all of the carbonyl sulfide by using a liquid carbon dioxide flow about equal to or less than that needed to absorb hydrogen sulfide.

--30. A process according to claim 1, characterized in that said one or more relatively high boiling point gases comprise sulfur-containing molecules, said at least one refrigerant-absorbent includes carbon dioxide, and the step of separating material from the absorbed gases includes separating the carbon dioxide from the sulfur-containing molecules by crystallization.--