GB1597479A

GB1597479A - Gas separation processes

Info

Publication number: GB1597479A
Application number: GB49920/77A
Authority: GB
Original assignee: Cng Research Co
Current assignee: Cng Research Co
Priority date: 1976-12-01
Filing date: 1977-11-30
Publication date: 1981-09-09
Also published as: NL7713256A; FR2372650A1; CA1072000A; IT1104645B; JPS53102277A; ZA777157B; BE861384A; FR2372650B1; DE2753597A1

Description

(54) GAS SEPARATION PROCESSES (71) We, CNG Research Company, a corporation organized and exisiting under the laws of the State of Delaware, United States of America, of 4 Gateway Center, Pittsburgh, Pennsylvania 15222, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to gas separation processes.

As particularly disclosed hereinbelow, the invention relates to the selective removal of acid gases such as carbon dioxide, hydrogen sulfide, and sulfur dioxide, other sulfurcontaining compounds such as carbonyl sulfice, and other relatively high boiling 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 application 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 enhanced 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 hydrogen and nitrogen in the manufacture of ammonia.

A simplified form of the invention has particular application to the removal of sulfur-containing compounds and, also, suspended particulate matter from stack or flue gases. Various other uses for the invention will be recognized by those skilled in the art.

Many methods have been developed for effecting the selective separation of acid gases from other gases of primary value. Usually, a chemical or physical absorbent for the acid gases to be separated is contacted by the gas mixture being treated, the absorbent and absorbed acid gases are separated, and the absorbent is regenerated and recycled. The Benfield hot carbonate process is a typical example of processes using chemical absorption.

See Pipeline and Gas Journal, October 19, 1972, p. 58. The Rectisol refrigerated methanol process is a typical example of processes using physical absorption. See Industrial and Engineering Chemistry, July, 1970, pp. 39-43.

Particularly in the case of chemical absorption processes, but also to a substantial degree in the case of most physical absorption processes, there are substantial, inherent irreversibilities in both the absorption and regeneration steps. These irreversibilities necessitate substantial energy inputs ta the processes. For example, in the Benfield hot carbonate process, substantial amounts of steam are needed to regenerate the alkaline carbonate solution employed as the absorbent. And in the Rectisol refrigerated methanol process, substantial amounts of steam and refrigeration are needed to regenerate the methanol absorbent. Thus, an undesirable characteristic of prior acid gas removal processes is their inherent, substantial energy consumption in regenerating the absorbent.

In many of the prior gas separation processes, the absorbent streams gradually accumulate impurities that have no value and would cause objectionable pollution if discharged into the environment. In those cases, additional capital and operating costs must be incurred for processing contaminated absorbent bleed or slip streams. Many of the prior gas separation processes also inherently involve substantial losses of absorbents due to minor poisoning reactions, leaks, thermal degradation, evaporation into the purified gases, and the slow accumulation therein of tars and other heavy materials. Make-up for these absorbent losses represents a continuous operating cost.

Another undesirable characteristic 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 sufficiently high concentration for economical processing in a Claus plant to reduce them to elemental sulfur and non-polluting wastes. See Hydrocarbon Processing, April, 1971, p. 112.

Another undesirable characteristic inherent in some 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 equipment of the use of expensive corrosionresistant materials or expensive corrosion inhibiting chemicals.

Another disadvantage of many prior gas separation processes is that much of the available pressure and thermal energy of the purified gas stream and of the separated gases and reagents is not recovered. More reversible processes could recover and utilize such potentially available energy.

Another disadvantage of many prior gas separation processes is that they use relatively viscous absorbents or reagents, which decrease absorber stage efficiency and consume significant amounts of energy for pumping.

Another disadvantage of most prior gas absorption processes is 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. Typical trace materials, depending on the sources of the gases to be purified, may include metal carbonyls and sulfur-containing molecules (other than hydrogen sulfide), such as carbonyl sulfide, carbon disulfide, mercaptans, and the like, relatively high boiling point nitrogen-containing compounds, including ammonia, and hydrogen cyanide, and relatively high boiling hydrocarbons (as hereinafter defined). The inability to remove such trace impurities results in end product gases of lesser value or reduced utility. Of the various trace impurities encountered in acid gas removal processes, carbonyl sulfide is particularly objectionable and generally must be removed if present in a gas stream being treated. Some prior acid gas removal process are incapable of doing so.without substantially increasing absorbent flows, which requires large additional capital and operating costs.

By way of summary, all of the prior acid gas removal processes have had a number of serious disadvantages involving troublesome problems and/or excessive capital costs and/or operating costs.

In accordance with a process embodying the present invention and described in detail hereinbelow, all of the aforementioned relatively high boiling point contaminants may be selectively and substantially completely removed from gas mixtures containing lower boiling components of primary value, and a highly purified end product may be produced at greatly reduced captial and operating costs. For convenience in describing and defining the present invention, normal boiling or sublimination temperature (at one atmosphere absolute pressure) colder than -86 C. are considered to be relatively low, and normal boiling point or sublimination temperatures warmer than -86"C. 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 relatively low boiling gas, comprising 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 low boiling point gas; and separating material from the absorbed gas for processing into refrigerant-absorbent for use in the process.

Preferably, said at least one refrigerant-absorbent is selected from (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. It is also preferred that the step of contacting the gas mixture with said 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, said particulate solids being caused to undergo a change of phase, in whole or in part, during contact with said gas mixture; 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.

In another aspect, the invention provides a process for separating relatively high boiling point gases, for example hydrogen sulfide and other sulfur-containing gases, from a pressurized gas mixture also containing relatively low boiling point gas, comprising the steps of (a) contacting the gas mixture with liquid carbon dioxide to absorb and entrain primarily any hydrogen sulfide and other relatively high boiling point present, a part of said liquid carbon dioxide being caused to undergo a change of phase during the contacting of the gas mixture; (b) separating liquid carbon dioxide and gases entrained therewith from a first residual portion of said gas mixture including most of the relatively low boiling point gas; (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.

The above step (b) preferably includes the steps of recovering carbon dioxide from-the mixture of liquid carbon dioxide and gases entrained therewith and regenerating liquid carbon dioxide absorbent therefrom for reuse in step (a). The carbon dioxide separated in step (d) is preferably returned to the process as a liquid for use as absorbent in step (a). The above step (a) may include contacting the gas mixture at a pressure in excess of 80 psia with liquid carbon dioxide and step (c) may include 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 still another aspect of the present invention, a process is provided for separating relatively high boiling point gases, for example hydrogen sulfide and higher boiling point gases, from a pressurized gas mixture also containing relatively low boiling components, comprising the steps of contacting the gas mixture at a pressure above 80 psia with liquid carbon dioxide, a part of said liquid carbon dioxide being caused to undergo a change of phase during the contacting of the gas mixture, and separating the liquid carbon dioxide and gases entrained therewith from a residual portion of said gas mixture.

Preferably, the pressurized gas mixture enters the process in a substantially anhydrous condition and at a pressure of at least 250 psia. According to a further preferred embodiment, the separated liquid carbon dioxide and gases entrained therewith 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.

In yet another aspect, the present invention provides a process for separating carbon dioxide from other, relatively low boiling point components of a gas mixture, comprising the steps of contacting the gas mixture with a refrigerant-absorbent which is a sorbent comprising a particulate solid having a temperature below the triple point temperature of carbon dioxide and being caused to undergo a change of phase, in whole or in part, during contact with the gas mixture 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 low boiling point components.

In a preferred embodiment of the last 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 also preferred that the particulate solid of said sorbent be suspended in a liquid vehicle and flows countercurrent to a stream of said gas mixture for contact therewith. Preferably, said sorbent comprises 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 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 by the melting of said paticulate solid refrigerant during contact with said gas mixture. The step of contacting the gas mixture with said sorbent preferably includes transferring heat into the particulate solid, causing it to undergo said change of phase while effecting a progressive net reduction in the solid phase portion of the In a preferred form of the first-mentioned aspect of the present invention, the said one or more relatively high boiling 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, the process further comprising 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 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.

According to another preferred form of the first-mentioned aspect of the present invention, 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-aborbent 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 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.

In accordance with a still further preferred form of the first-recited aspect of the present invention, 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 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 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.

According to a yet another preferred form of the first-mentioned aspect of the present invention, said one or more relatively high boiling point gases include carbonyl sulfide, and said at least one refrigerant-absorbent comprises 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 which would be needed to remove hydrogen sulfide, if hydrogen sulfide were present in an amount equal to the amount of carbonyl sulphide actually present.

In accordance with a yet another preferred form of the first-mentioned aspect of the present invention, 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.

The process embodying the present invention and described hereinbelow is more nearly reversible and requires a lesser net enery input than prior processes; absorbent losses are inherently replaced in the process or 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 phase changes in the absorbents, which minimises heat exchange costs; no objectionable environmental pollution is caused; separated carbon dioxide can be recovered in a pure condition if desired; separated hydrogen 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 and is essentially free of sulfur-containing molecules, including carbonyl sulfide.

Although the different procedures utilized for removing sulfur-containing gases and for removing carbon dioxide may be practiced separately in accordance with the present invention, their integration into a single process as herein disclosed provides practical and economic advantages.

For removing hydrogen sulfide, carbonyl sulfice, and additional impurities other than carbon dioxide from a gas containing carbon dioxide in the process embodying the invention and described hereinbelow, a pressurized stream of the gases to be treated is first completely dehydrated. Tars and other very high boiling 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 dew point temperature (the temperature at which a liquid phase rich in carbon dioxide begins to condense) at the process 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 refrigerant-absorbent in one or a succession of absorption columns.

Hydrogen sulfide, carbonyl sulfide, other sulfur-containing molecules, and other relatively high boiling point molecules are essentially completely absorbed by and removed with the residual liquid carbon dioxide absorbent, the heat of absorption being dissipated as heat of vaporization of a portion of the absorbent. The partially purified residual gas stream emerges from 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.

The residual liquid carbon dioxide absorbent and the absorbed relatively high boiling point gases withdrawn from the absorption column or columns are stripped of any small amounts of valuable low boiling point gases unavoidably entrained therewith, the latter gases being recycled within the process. The stripped mixture of still liquid carbon dioxide absorbent and absorbed impurities (generally sulfur-containing gases) is then Processed to separate at least a part of the carbon dioxide and (as hereinafter described) to recover pressure energy and refrigeration potential as economics may dictate. This separation may be accomplished by fractional distillation, crystallization by freezing, extractive distillation, or other means while producing substantially pure carbon dioxide, which may be reused as absorbent or may be removed as by-product. When the thus concentrated mixture of impurities consists largely of sulfur-containing gases and carbon dioxide, such mixture may be further processed for the recovery of sulfur, for example, in a Claus plant, or it may be otherwise disposed of.

After the removal of sulfur-containing gases and other impurities as described above, the still pressurized, residual, main gas stream, is further cooled to near the triple point temperature of carbon dioxide to prepare it for a final carbon dioxide absorption operation.

Depending upon the main gas stream operating pressure, a considerable amount of carbon dioxide may be condensed and separated in the course of such cooling (more at high operating pressures). This cooling of the main gas stream is suitably performed by passing it through a series of indirect heat exchangers. The heat exchange medium used to effect such cooling may be any or a combination of serveral low temperature fluid streams produced in the process. The carbon dioxide condensed in this manner is withdrawn as a liquid stream at substantially the initial gas stream pressure and may provide some or all of the absorbent required in the first absorption step described above, any required make-up being supplied as also mentioned above.

The residual, further purified gas stream is discharged from that series of indirect heat exchangers at approximately its initial pressure but at about -55"C. and flows to the above mentioned, final, carbon dioxide absorption operation. In that final operation, the main gas stream is first brought into intimate contact with an absorbent that comprises a particulate solid having a high heat absorption capability and that is below the triple point temperature of carbon dioxide (i.e., colder than -56.6"C). Under these conditions, most of the remaining carbon dioxide content of the gas stream is condensed or frozen, removed with the absorbent, and vaporized or sublimed therefrom.

According to the presently preferred manner of practicing this invention, the particulate solid of the absorbent for this final carbon dioxide removal operation is suspended in a liquid vehicle as a pumpable slurry. In this case, the gas stream and absorbent slurry are brought into intimate contact by countercurrent flow through a conventional absorption column or series of columns.

The presently preferred absorbent for use at this point in the process is a partially frozen mixture of carbon dioxide and a liquid or liquids having low viscosity, low vapor pressure, high solubility for carbon dioxide, relatively low solubility for lower boiling point gases, low reactivity, good stability, and, in the mixture, a range of freezing points below -56.6"C.

Examples of such liquids are: ethers, such as di-n-propyl ether, di-n-butyl ether, and t-butyl methyl ether; ketones, such as 2-pentanone (methyl propyl ketone),-t-butyl methyl ketone, and methyl isobutyl ketone; methanol; hydrocarbons, such as heptane and hexane; aldehydes, such as butanal, pentanal, and 2-methyl butanal; and inorganic liquids, such as fluorosulfonic acid. The solid phase of the mixture is suspended in the liquid phase as a pumpable slurry, preferably provided at a temperature of about -70" to -750C. as it begins its flow through the absorption column or columns. When such an absorbent slurry contacts the gas mixture stream still containing 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 of the gas stream condenses and, by solution or entrainment, becomes part of the liquid phase of the absorbent, and the solid phase of the absorbent concurrently melts in absorbing the heat of condensation and solution of the carbon dioxide. Thus, carbon dioxide of the gas mixture stream is transferred from the gas to the liquid phase while the solid phase of the absorbent slurry liquefies, and both augment the liquid phase of the absorbent.

The term "refrigerant-absorbent" is used hereinafter to characterize both the first described liquid carbon dioxide absorbent for relatively high boiling point gases and the last described slurry absorbent for gaseous carbon dioxide. As so used, the term "refrigerantabsorbent" refers to an absorbent that, in whole or in part, undergoes a change of phase during the absorption process, the change of phase enabling it to utilize at least a portion of the heat of absorption to effect its own partial or complete phase change. Such an absorbent is to be distinguished from one that depends solely on its specific heat for its heat absorbing capability, without undergoing a phase change.

As part of the presently preferred form of the final carbon dioxide absorption operation, the gas mixture stream, still at substantially the same operating pressure, is finally contacted with only the liquid phase of the preferred refrigerant-absorbent slurry that has been processed to have a low content of dissolved carbon dioxide, as hereinbefore described in more detail. This final step can dissolve 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 final absorption column or columns at near the initial operating pressure of the system and at a temperature of about -70"C to -750C. The final, liquid absorbent and carbon dioxide dissolved therein continue to move downwardly so as to merge with and augment the liquid phase of the refrigerant-absorbent slurry.

The thus combined final liquid absorbent and completely melted refrigerant-absorbent, together with the carbon dioxide absorbed thereby, are withdrawn as a liquid stream from the opposite end of the column or columns at a temperature of around -560C. Any relatively low molecular weight components of the gas mixture being treated that were unavoidably also dissolved or absorbed thereby may be recovered therefrom by stripping them from the combined absorbents and absorbed carbon dioxide and may be added to the final, purified, gas stream product after recovering refrigeration potential from both.

Regeneration of the refrigerant-absorbent slurry for recycling is readily effected by reducing the pressure of the liquid stream ot combmed absorbents and condensed carbon dioxide entrained therewith so that the absorbed carbon dioxide is evolved therefrom and the absorbent liquid mixture is thereby cooled to about 700 to -75"C., thus regenerating the solid phase thereof by freezing. The evolved carbon dioxide gas is substantially pure by-product.

After regeneration of the refrigerant-absorbent slurry, a portion of the liquid phase thereof may be separated therefrom by decantation, warmed, and fed into a gas-liquid separator to facilitate depleting it of most of the remaining carbon dioxide therein. This liquid is then recooled to about -73"C. and pressurized to the main gas stream operating pressure for use as a final, liquid absorbent for carbon dioxide to complete the gas purification process.

In order for a liquid carbon dioxide refrigerant-absorbent to exist in contact with the gas stream being treated by the first absorption procedure described above for removing relatively high boiling point impurities, the absorbent preferably should be warm enough to ensure that no solid phase can form. Since the absorbent is predominantly carbon dioxide, which has a triple point temperature of -56.6"C., temperatures warmer than -56.6"C. are operational. Gases dissolved in the liquid carbon dioxide can depress the freezing point by several degrees, so that temperatures somewhat colder than -56.6"C. may also be operational.

An upper temperature limit for the requir

Theoretically, the limiting, 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 high pressure would be well above any economically practical operation pressure for most gas purification purposes, but might be used for special purposes. Thus, it is not possible to designate any meaningful, maximum, main gas stream operating pressure.

As will become apparent from the ensuing more detailed description of the process embodying the invention, the temperature to which the crude gases must be cooled for removing sulfur-containing molecules varies directly with the main gas stream pressure at which the raw shift gas is to be processed for that purpose. Also, the proportion of the carbon dioxide that can be removed by simple condensation in heat exchangers at temperatures above the triple point temperature of carbon dioxide increases markedly as the main gas stream operating pressure is increased. As a result, both captial and energy costs for carrying out the overall process embodying the invention are reduced as the main gas stream pressure is increased up to pressures at least as high as 1000 psia. Obviously, however, pressures can be reached at which such a trend in capital costs may no longer hold true. On the other hand, particular applications of the invention may warrant the use of much higher or lower pressures. Although it is not essential that the main gas stream pressure be maintained at the same level in the successive impurity removing operation of the complete process, practical and economic considerations will generally dictate maintaining a uniform main gas stream pressure throughout those operations.

The ability of a refrigerant-absorbent to utilize the heat of absorption to effect its own partial or complete phase change eliminates or reduces the need for supplying additional refrigeration to the absorption zone. When employing liquid carbon dioxide as an absorbent for relatively high boiling point gases in the manner first described 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 maintaining a substantially constant temperature throughout that zone. When using an absorbent slurry of a particulate solid that melts during the absorption process in the manner described above, a large part of the heat of absorption is utilized in melting the particulate solid, thus greatly reducing the amount of additional refrigeration required in the absorption zone to maintain a desirable temperature gradient through that zone.

Alternatively, as hereinafer 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 condensing significant amounts of carbon dioxide without requiring excessively frequent regeneration by recooling. Well known techniques for the handling of such particulate solids in fluidized bed operations may be employed for continuously removing particulate solids as they become coated with frozen carbon dioxide, subliming the frozen carbon dioxide therefrom, and recooling and recycling the particulate solid absorbent.

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

The successive operations of the process embodying the invention for separating carbon dioxide may also be used individually or in combination for that purpose when treating gas mixtures that are essentially free of still higher boiling point components such as sulfur-containing compounds. For example, if the gas to be treated is 50% carbon dioxide and its operating pressure is 1000 psia, then the preliminary removal of carbon dioxide by condensation at temperatures above its triple point temperature will reduce the carbon dioxide content of the gas from the initial 50% to about 15%. The final carbon dioxide absorption operation can then be used to remove as much of the remaining carbon dioxide as is desired. On the other hand, if the gas to be treated is only 30% carbon dioxide, and the pressure is only 100 psia, the preliminary removal of carbon dioxide by condensation at temperatures above its triple point temperature is not applicable, but the final carbon dioxide absorption operation can still be used to remove as much of the carbon dioxide as desired.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure I is a general flow diagram for a system in which four successive gas treatment operations of a process embodying the invention are integrated for primarily removing respectively, water, hydrogen sulfide and other sulfur-containing gases, a part of the carbon dioxide, and, finally, most of the residual carbon dioxide (down to a fraction of 1 mol percent if desired) from a gas mixture being treated; and Figures 2A and 2B are general flow diagrams showing in more detail a carbon dioxide crystallizer, indicated only generally in Figure 1, in two alternating conditions of operation thereof.

The invention will now be described in more detail and by way of illustrative and non-limiting example with particular reference to the removal of acid gases from the raw "shift gases" produced by coal gasification (as distinguished from coal liquefaction). As is well understood 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 of gasification sought, and the contemplated end uses for the gas product, as well as economic and environmental considerations. The raw shift gases are commonly discharged from such processes at elevated temperatures and pressures which vary from process to process.

For illustrative purposes, the composition of the raw shift gases to be treated by the process embodying this invention will be assumed to comprise the following typical components and proportions: Component Mol Fraction H2 0.4068 CO 0.1356 N2 0.0052 CH4 0.1218 CO2 0.3189 H2S 0.0111 H20 0.0005 COS 0.0001 HCN Trace NH3 Trace The exemplary processing of such a gas mixture will be described on the further assumption that the mixture is received from a coal gasification plant at 250C. and 1000 psia and is processed at that pressure. How the process and apparatus are desirably altered for processing such a gas mixture at a substantially lower pressure of about 300 psia will be explained in the course of that description.

Referring now to the accompanying drawings, the raw shift gas mixture of the foregoing composition is introduced into the illustrated purification system through a line 10 as a continuously flowing stream at a temperature of about 25"C., and a pressure of 1000 psia.

From the line 10, the gas mixture stream flows, first, through a heat exchange system 20 for dehydrating the stream and precooling it to its dew point temperature, that temperature in this example being about -27"C. The dehydrated stream when then flows via a line 25 through a sulfur absorption system 30 for removal of its content of relatively high boiling point components, particularly hydrogen sulfide and other sulfur containing molecules. The residual, partially purified 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 of carbon dioxide and to discharge the residual stream at a temperature of about -55"C., only slightly warmer than the triple point temperature of carbon dioxide. The residual partially purified stream then flows via a line 65 through a final carbon dioxide absorption system 70 for the removal of additional carbon dioxide while further lowering the main gas stream temperature to well below the triple point temperature of carbon dioxide. The final, residual, purified gas stream emerges from the final absorption system 70 via a line 100 at near the initial 1000 psia pressure of the raw shift gases and at a temperature of about -73"C. Before being discharged as the finally purified product, this gas stream in the line 100 is routed through another heat exchanger 102, then through a refrigeration unit 103 for recooling and, thence, back through additional heat exchangers for utilizing its refrigeration potential in the process.

The main gas flow to and through the several successive purification and heat exchange steps just described is emphasized in the drawing by heavy solid and dotted lines. Those several gas purification and heat exchange steps of the process and their interdependency in the presently preferred overall system will now be described in detail.

Precooling and Dehydration The precooling and dehydration system 20 may comprise a series of indirect heat exchangers of conventional design (only one being shown) for progressively cooling the raw shift gases entering this system via the line 10 to substantially the dew point temperature of the particular gas mixture, in this case -27 C. Such cooling condenses substantially all of the water content of the main gas stream entering the process through the line 10. The condensed water is removed via a condensate line 21 leading to a waste water clean-up system (not shown), which should be selected according to the nature and quantity of the impurities necessarily condensed and removed with the water. As indicated in the drawing and explained below, the finally purified gas stream and three other gas product streams of different temperatures may be used as the cooling media in these heat exchangers.

The precooling and dehydration system 20 will also include a final dehydration step (not shown) before cooling below 0 C. for removing the last traces of water from the main gas stream. Conventional water scavenging steps, such as those employing molecular sieves, activated alumina absorbents, etc. may be used for this purpose.

The precooled and dehydrated main gas stream flows from the system 20 via the line 25 to the absorption system 30 at near the initial gas stream pressure of 1000 psia.

Removal of Sulfur-Containing Gases The absorption system 30 for removing relatively high boiling gases, particularly hydrogen sulfide and other sulfur-containing gases, from the dehydrated main gas stream, will suitably comprise a multistage series of sieve tray absorption columns of conventional design. In each absorption column of such a system, the main gas stream moves upwardly, counter-current to and in intimate contact with a downward flow of liquid carbon dioxide absorbent supplied by a line 31 at 1000 psia and at the main gas stream temperature of about -270C.

During travel of the main gas stream through the absorption system 30, substantially all of the hydrogen sulfide and other sulfur-containing molecules, along with such other relatively high boiling point molecules as may be present, are absorbed and removed from the main gas stream by the liquid carbon dioxide absorbent. At the temperature and pressure prevailing in this system, the heat of absorption of the absorbed gases is utilized in vaporizing some of the liquid carbon dioxide absorbent so that the liquid carbon dioxide functions as a refrigerant-absorbent. This- causes a small net increase in the amount of gaseous carbon dioxide in the main gas stream at the expense of liquid carbon dioxide absorbent and permits the absorption to occur with negligible increase in temperature of the gas stream or of the absorbent.

The liquid carbon dioxide absorbent and absorbed hydrogen sulfide and other heavier molecules are withdrawn from the absorption system 30 at the pressure and temperature maintained therein (about 1000 psia and -27"C) via a line 32, are passed through an expander 33, by which their pressure is lowered to around 125 psia, and are discharged into a stripper-absorber column 35 equipped with a reboiler 36.

In the stripper-absorber 35, the lighter molecules (hydrogen, carbon monoxide, nitrogen, and methane that are also absorbed in small amount by the liquid carbon dioxide absorbent in the absorption system 30 are stripped from the still liquid absorbent and other absorbed gases. A relatively small amount of the fresh liquid carbon dioxide absorbent flowing in the line 31 is introduced into the upper end of the column 35 through a branch line 34 for countercurrent contact with the upwardly moving light fractions in the upper end of the column 35 for absorbing any traces of sulfur-containing molecules entrained with the light fractions. By using this supplemental absorption procedure in the column 35, the stripped light fractions and some gaseous carbon dioxide may be withdrawn from the upper end of the column 35 essentially free of sulfur-containing molecules and be moved through a line 37 for further processing as hereinafter described.

The reboiler 36 requires only a small amount of heat that may be supplied in any convenient manner, as by moving any available heating fluid therethrough via a line 38. The reboiler discharge via a line 39 consists essentially of liquid carbon dioxide absorbent and absorbed sulfur-containing molecules in a total concentration therein of about 4 mol percent. This stream is processed further to increase its concentration of sulfur-containing molecules to about 25 mol percent or more, as desired to produce an economical feed stock for a Claus plant for recovering the sulfur in elemental form. The desired Claus plant feed stock concentration may be economically produced by separating carbon dioxide by a combination of distillation and crystallization as hereinafter described. Distillatibn presently appears to be a more practical process for producing a carbon dioxide distillate having the requisite purity for reuse as absorbent in the sulfur absorber 30 and in the stripper-absorber 35.

At high main gas stream pressures of around 2000 psia, sulfur-free carbon dioxide may be condensed in the condenser system 60 in ample amount to satisfy the above described liquid carbon dioxide absorbent needs of the process in both the absorption system 30 and the stripper-absorber column 35. Therefore, when processing a main gas stream at such a high pressure, all of the stream flowing in the line 39 may be passed through a branch line 39a to a carbon dioxide crystallizing system 40.

At lower main gas stream pressures, less carbon dioxide can be condensed in the condenser system 60 for transmittal through the line 31 to the sulfur absorber 30 and through the line 34 to the stripper-absorber 35, in which case makeup carbon dioxide absorbent is required from another source. For this purpose, a part of the stream flowing in the line 39 is passed through a branch line 39b to a distillation system 55, described further below, and a liquified, substantially pure, carbon dioxide distillate produced in the distillation system supplies the makeup liquid carbon dioxide absorbent. When the main gas stream pressure is below about 250 psia, all of the flow through the line 39 may be to the distillation system 55 for maximizing the amount of substantially pure carbon dioxide produced therein for use as absorbent, and the crystallizer 40 may be idle.

Thus, in a plant designed solely for operation at a main gas stream pressure of about 2000 psia, the distillation system 55 may be omitted, and in a plant designed solely for operation at a main gas stream pressure below about 250 psia, the crystallizer 40 may be omitted. For a plant operating at intermediate main gas stream pressures, both the crystallizer and distillation system may be employed, each being tailored, of course, to process the desired flow therethrough.

Referring now to the carbon dioxide crystallizer 40 as shown in Figures 2A and 2B, it may suitably comprise four tanks 41, 42, 43, and 44. During one period of a 2-period operating cycle, the tanks 41 and 43 are connected in series with one side of an intervening heat exchanger 45 for 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 melting and discharging as a liquid the solid carbon dioxide formed and deposited in these two tanks during the preceding period of the operating cycle. Figure 2A shows that relationship. At the conclusion of the first described period, the tank interconnections are switched by appropriate valving so that, during the other period of the cycle, carbon dioxide is formed and deposited in tanks 42 and 44 by the same two-stage process as before while carbon dioxide is melted and discharged from the tanks 43 and 41 by the same two-stage process as before. Figure 2B shows the latter relationship.

Considering Figure 2A in more detail, the liquid carbon dioxide absorbent and absorbed sulfur-containing molecules flowing from the line 39 (see Figure 1) into the branch line 39a pass through a valve 46 for feeding them as a spray into the tank 41, which is maintained at a pressure of about 60 psia (well below the 75.1 psia triple point pressure of carbon dioxide).

To enhance atomization, the pressure of the liquid in the line 39a is desirably increased to around 200 psia by a pump (not shown). Under these conditions, part of the liquid carbon dioxide flashes into solid carbon dioxide that accumulates in the tank 41, and a gas mixture of carbon dioxide and sulfur-containing molecules is formed therein and withdrawn overhead via a line 47 at the required rate for maintaining the tank pressure near the desired 60 psia. A pressure relief valve (not shown) in the line 47 may be used to effect the required pressure control. This gas mixture is then compressed to about 85 psia by a suitable compressor 48 or the like in the line 47, is liquefied by indirect heat exchange in the heat exchanger 45 in the line 47, is pressurized to around 200 psia by a pump (not shown), and, as regulated by a valve 49, is sprayed into the tank 43 maintained at a pressure of about 18 psia. Additional carbon dioxide crystals are thus formed and accumulated in the tank 43, and a residual gas mixture of carbon dioxide and sulfur-containing molecules is again withdrawn overhead, this time via a line 50 (see Figure 1) for flow to a Claus plant. By the two stage removal of carbon dioxide, as described, the final, residual gas mixture taken off through the line 50 may be concentrated in sulfur-containing molecules to the 25 mol percent or so desired for an economical Claus plant feed.

During the above described formation and accumulation of solid carbon dioxide in the tanks 41 and 43, solid carbon dioxide similarly accumulated in the tanks 42 and 44 during the preceding half cycle is melted and removed. For this purpose, gaseous carbon dioxide introduced to the crystallizer 40 via a line 51 (see Figure 1) is compressed by a compressor 52 in the line 51 to about 85 psia 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 heat exchanger 45, where it is vaporized before flowing on through an extension of the line 53 into the tank 42. The melting of solid carbon dioxide and the condensing of gaseous carbon dioxide again occur, and the total of the thus formed liquid carbon dioxide is discharged from the crystallizer 40 through a line 54 (see Figure 1).

At the conclusion of the two operations in the tanks 41-44 described above, the system is converted by valve changes to the arrangement shown in Figure 2B. In the same manner, during the next half cycle of operation, solid carbon dioxide is formed and deposited in the tanks 42 and 44 while solid carbon dioxide previously formed in the tanks 41 and 43 is melted therein and is discharged via the line 54.

Liquid carbon dioxide discharged from the crystallizer 40 will have a concentration of sulfur-containing molecules below 2500 ppm. Any desired lower concentration of sulfur-containing molecules may be achieved by repeating the crystallization operation. The resulting liquid carbon dioxide is discharged, preferably after its pressure energy and refrigeration potential are recovered as hereinafter described with reference to Figure 1.

Referring now to the nature of the distillation system 55, shown only diagrammatically in Figure 1, it should be a multi-stage system including a feed vaporizer and a distillate vapor compressor. The system may be of conventional design for processing the liquid carbon dioxide absorbent and absorbed sulfur-containing gases introduced through the line 39b for separating a portion only of the carbon dioxide in a substantially pure form (about 1 ppm of sulfur-containing molecules) which, with recompression and heat exchange against the incoming liquid feed, may be discharged as a liquid through a line 56. A liquid residue, in which the concentration of sulfur-containing molecules is at or near 25 mol percent, rnay be withdrawn through a line 57. Depending upon the requirements of the process as determined primarily by the main gas stream pressure, part or all of the liquid carbon dioxide from the line 56 may flow through the line 56a, aided by an interposed pump 64, and into the liquid carbon dioxide absorbent supply line 31 as makeup absorbent, and part or all may flow through the line 56b into the line 54 that provides coolant for the carbon dioxide condenser system 60. The concentrate of sulfur-containing molecules in the residual liquid carbon dioxide vehicle flows in the line 57 through an interposed heat exchanger 58 and expander 59 for recovery of refrigeration potential and pressure energy before merging with the similar material discharged via line 50 from the carbon dioxide crystallizer and flowing therewith to a Claus plant.

The residual main gas stream discharged from the absorption system 30 via the line 45 will have had its original content of hydrogen sulfide and higher boiling point molecules substantially completely removed. By proper absorption column design and with appropriate flow rates, any trace of sulfur-containing compounds in the discharged gas mixture can readily be kept as low as 1 ppm by weight. This leaves only carbon dioxide as an acid gas contaminant still to be removed. As previously indicated herein, carbonyl sulfide is commonly encountered as a contaminant in very small amounts in gases also contaminated by other sulfur-containing molecules. By reason of its toxicity and its tendency to interfere with various chemical reactions, carbonyl sulfide must generally be removed from gas mixtures in which it is found. As also previously indicated herein, some prior acid gas removal processes are not capable of removing carbonyl sulfide or are capable of doing so only by substantially increasing absorbent flows relative to the flows of the crude gases being purified. The absorbent flow increases required for that purpose may be as great as 400%. By contrast, an advantageous characteristic of the above described operations for separating sulfurcontaining and other relatively high boiling point gases from relatively low boiling point gases is its inherent ability to remove carbonyl sulfide even more effectively than hydrogen sulfide. Stated differently the foregoing operations inherently separate all of the carbonyl sulfide when performed with the minimum liquid carbon dioxide absorbent flow that is capable of removing all of the hydrogen sulfide, so that no additional cost is entailed by carbonyl sulfide removal.

In addition to being effective for removing hydrogen sulfide, carbonyl sulfide, and other sulfur-containing gases, liquid carbon dioxide has other properties that contribute to the economy of this part of the overall process. It has an exceptionally low viscosity of about 0.3 to 0.5 centipoise over its range of temperatures in the sytem, a relatively high specific gravity of about 1.18, and a relatively low molecular weight of only 44. All of these properties contribute to keeping the size and cost of equipment and pumping costs to a minimum. Moreover, liquid carbon dioxide is produced 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 of potential economic value.

Initial Removal of Carbon Dioxide The partially purified gas mixture stream enters the first carbon dioxide removal system 60 via the line 45 at near the operating pressure of 1000 psia and a temperature of about -27"C. In this system, the gas mixture stream flows through one or a series of indirect heat exchanges of conventional design for lowering the gas mixture stream temperature to about -55"C., which is sufficiently low to condense a major portion of the carbon dioxide content of the stream while also further cooling the stream to near the triple point temperature of carbon dioxide for the purposes of the succeeding, final carbon dioxide removal steps of the process. The resulting liquid carbon dioxide condensate is substantially free of sulfurcontaining molecules and supplies a portion of the absorbent requirements of the absorption system 30 and the stripper absorber 35, to which it flows through the lines 31 and 34. The required additional liquid carbon dioxide absorbent is supplied from the distillation system 55 as described above.

Cooling in the heat exchange system 60 is most suitably performed in a series of indirect heat exchangers (not individually shown). The primary coolant for this purpose may be the liquid carbon 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 -35 C. via a line 61. A portion of the recovered coolant may be recycled back through the crystallizer 40 via the line 51 for melting the solid carbon dioxide formed therein, as described above with reference to Figures 2A and 2B. The balance of the coolant recovered from the line 61 may be discharged to the atmosphere via a line 62 after recovering additional refrigeration and pressure energy therefrom, as hereinafter described.

To the extent required, additional refrigeration for the condenser system 60 may be provided by using the final purified gas stream and other product streams of the process as supplemental coolants, as hereinafter described.

When the main gas stream of the resent example flows through the condenser system 60 at about 1000 psia, as much as 70o of the carbon dioxide content of the main gas stream may be removed by reducing its temperature to about -55 C., without the use of an absorption agent. As explained above, this is accomplished with relatively simple and inexpensive equipment, using refrigeration potential otherwise generated in the system and conveniently available for that purpose. Thus, the net energy input for this purpose is very small. At lower 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 gas stream flows from the condenser system 60 through the line 65 at near its original pressure of 1000 psia and only slightly above the triple point temperature of carbon dioxide. These main gas stream conditions are appropriate for the further and final carbon dioxide removal in the succeeding steps of the process.

Final Carbon Dioxide Removal The partially purified main gas stream, flowing to the final absorption system 70 via the line 65, is first moved into intimate contact with a refrigerant-a refrigerant-absorbent slurry. The optimum proportioning of solid to liquid in the slurry and the quantity required for treating a given quantity of gas will be determined by the carbon dioxide content of the gas stream being treated, by the specific heats of the several constituents of both the slurry and the gas stream being treated, by the specific temperatures at which the slurry absorbent and the gas stream are introduced into an absorption column or columns for countercurrent flow therethrough, by the absorption column design, and by the ability of pumping equipment employed to move the liquid vehicle and its entrained particulate solids in the system.

The residual, partially purified, main gas stream entering the final absorption system 70 from the line 65 at near 1000 psia and about -55 C. may still contain about 13% to 14% carbon dioxide; For treating such a gas mixture with the preferred refrigerant slurry described above, the slurry will suitably contain about 15% by weight of the particulate solid and be at the main gas stream pressure but at a temperature of about -73"C. This slurry is introduced from a line 71a into a sieve tray absorption column of conventional design at a level somewhat below the top of the column (or ahead of the last of a series of such columns) for downward movement countercurrent to an upward flow of the main gas stream. Thus, the uppermost part of the column (or the last one or more of a series of columns) is left available for a final scrubbing of the main gas stream with a carbon dioxide-depleted portion of the liquid phase only of the refrigerant slurry. This final, liquid absorbent is introduced through a line 71b for downward movement countercurrent to the upwardly flowing main gas stream before the latter is discharged from the absorption system 70 via the line 100. For this final scrubbing operation, the carbon dioxide-depleted, liquid phase absorbent will be at the main gas stream pressure and at a temperature of -730C. It dissolves the remaining carbon dioxide from the main gas stream down to a final carbon dioxide content of 1 mol percent, or less if desired, depending upon the degree to which the final, liquid absorbent was depleted of dissolved carbon dioxide and the severity of the final scrubbing operation. Continuing on down through the column or columns of the absorption system 70, the liquid phase absorbent and dissolved carbon dioxide merge with and augment 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 by the phase change mechanism described above.

During contact between the slurry absorbent and the main gas stream, melting of the solid phase of the slurry absorbent preferably proceeds to completion to provide as much cooling as possible for condensing a major portion of the residual carbon dioxide from the gas stream.

Additional refrigeration is required in the absorption system 70 to supplement the in situ refrigeration provided by the refrigerant-absorbent. Such additional refrigeration is provided by indirect heat exchange with the finally purified gas product or with other available cooling fluids as hereinafter explained. It is supplied near the main gas stream inlet end of system 70 where the solid phase of the refrigerant-absorbent is exhausted or is approaching exhaustion. Additional refrigeration is also required adjacent the opposite end of the absorption system 70 in the zone thereof where the main gas stream is finally contacted by the liquid absorbent introduced through the line 71b.

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 therewith through a line 72 at about -56 C. The finally purified gas stream is withdrawn from this absorption system 70 through the gas product line 100 at about -73 C.

and at near the initial main gas stream operating pressure of 1000 psia. The refrigeration potential of the finally purified gas stream in the line 100 is recovered in heat exchangers at various points in the overall system, as mentioned above and further detailed below.

The fully melted, combined, absorbent liquids and absorbed carbon dioxide withdrawn from the absorption system 70 through the line 72 at about - 560C. and near 1000 psia are first moved together through a pressure reducer 73 to lower their pressure to about 125 psia for movement via a line 74 into a stripper-absorber 75, along with the light fractions discharged via the line 37 from the prior stripper-absorber 35. This is indicated diagrammatically by the merging of the lines 37 and 72 into the line 74.

In the stripper-absorber 75, the light fractions (hydrogen, carbon monoxide, nitrogen, and methane) that have been unavoidably picked up from the main gas stream during its passage through the absorption system 70, together with similar light fractions received from the line 37 are stripped from the absorbent liquids and absorbed carbon dioxide. From the top of the stripping column 75, the stripped light fractions and only a minor amount of carbon dioxide entrained therewith are withdrawn via a line 76.

The absorbent liquids and most of the absorbed carbon dioxide, at about 125 psia, are withdrawn from the bottom of the stripper-absorber 75 through a valve 77 in a line 78 leading into a flask tank 79, which is maintained at a lower pressure of about 65 psia. This pressure drop results in the flashing off from the liquid of some of the carbon dioxide while the remaining liquid, including all of the absorbent vehicle and most of the carbon dioxide, is withdrawn via a line 80 to a refrigerant-absorbent slurry regenerating system 85. The flashed carbon dioxide gas is withdrawn from the flash tank as it is formed, through a line 81, and, with recompression in a compressor 82, is returned to the bottom of the stripper-absorber 75. There it bubbles upwardly through the liquid therein and assists in stripping the light fractions therefrom. Most of this carbon dioxide gas is then reabsorbed as it rises in the upper part of the stripper-absorber 75 by successively contacting downward flows of the same kind of refrigerant-absorbent slurry used in the absorption system 70 and the same kind of liquid absorbent used therein. These absorbents are respectively introduced into the stripper-absorber 75 via lines 83 and 71b from sources described hereinafter. Thus, the upper part of the stripper-absorber 75 functions similarly to the absorber system 70 in separating carbon dioxide from the stripped light fractions. As a result, the light fractions leaving the stripper-absorber 75 via the line 76 have very little carbon dioxide gas entrained therewith 4 mol percent or less).

The light fractions leaving the stripper-absorber 75 through the line 76 are at a temperature of about -73"C. Accordingly, they are recycled back through the process for recovery of their refrigeration potential before being combined with the purified main gas stream as described hereinafter.

The liquid effluent from the flash tank 79, flowing to the slurry regenerating system 85 at about 6 psia and about -55"C., has its carbon dioxide content and its temperature progressively reduced by a succession of further pressure reductions in a series of additional flash tanks (not individually shown). The separated carbon dioxide is flashed off as a gas in a high state of purity (less than 1 ppm of sulfur-containing molecules) and is withdrawn via a line 86 at only slightly above ambient pressure and at a temperature of about -75"C. It is then routed back through the process as hereinafter described for recovery of its refrigeration potential before it is withdrawn as a useful by-product or is released to the atmosphere, as economic considerations may dictate. Part of the remainder of the carbon dioxide in the liquid entering the slurry regenerating system 85 remains dissolved in the refrigerant-absorbent vehicle as the latter is progressively cooled by the succession of pressure reductions, and the balance is frozen and physically entrained in the refrigerantabsorbent vehicle as it moves through this system and out through a line 87 as a liquid-solid slurry.

The thus regenerated refrigerant-absorbent slurry flows through the line 87 to a decanting station 88 where a portion of the liquid vehicle of the slurry is separated therefrom, as by flowing over a weir 89 or the like. The remaining regenerated, refrigerant-absorbent slurry held back by the weir 89 is withdrawn through a line 90 at slightly above ambient pressure and at a temperature of about -75"C. for recycling to the absorption system 70 and to the stripper-absorber 75. For this purpose, the flow of regenerated refrigerant-absorbent slurry in the line 90 is divided, a major portion flowing from the line 90 into and through the line 71a and through a pump 91 therein for repressurizing 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 repressurizing the slurry to about 125 psia as required for it to flow 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 is fed via a line 93 through a heat exchanger 94 in which it is warmed as required for evolving most of its dissolved carbon dioxide in a gas-liquid separator 95, to which it flows through a line 96.

The substantially pure carbon dioxide evolved in the separator 95 is discharged therefrom slightly above ambient pressure and at a temperature of about -36"C. and is routed back through the process as hereinafter described for recovery of its refrigeration potential before it is withdrawn as a useful by-product or is released to the atmosphere as economic considerations may dictate.

The vehicle portion of the refrigerant-absorbent slurry that has been separated from the slurry at the decanting station 88 and depleted of dissolved carbon dioxide in the gas-liquid separator 95 is withdrawn from the latter via a line 97 that flows through the heat exchanger 94 where it is recooled by indirect heat exchange with the just separated, cold, vehicle portion of the regenerated refrigerant-absorbent slurry flowing from the line 93. Emerging from the heat exchanger 94 through a continuation of the line 97 at about -73"C. and only slightly above ambient pressure, the recooled, carbon dioxide-depleted vehicle portion of the regenerated refrigerant-absorbent slurry passes through a first pump 98 that repressurizes it to about 125 psia before it is divided by diverting a minor portion thereof through the line 83 and into the upper end of the stripper-absorber 75 for use therein as previously described. The major portion of the recooled, carbon dioxide-depleted, vehicle portion of the regenerated refrigerant-absorbent slurry continues on through a further extension of the line 97 and through a second pump 99 that further repressurizes it to about 1000 psia before it flows through the line 71b and into the gas discharge end of the absorption system 70 for use therein as previously described.

Although the desired low carbon dioxide content of the finally purified main gas stream emerging from the absorption system 70 could be achieved in the process of the present example by using only the refrigerant-absorbent slurry to absorb carbon dioxide, a slurry temperature of about -96"C. as it enters the absorption system 70 would be required to do so. That, in turn, either would require a greater total pressure drop in the succession of slurry flashers by which the refrigerant-absorbent slurry is regenerated, down to a final pressure below ambient, with obvious disadvantages in terms of capital costs and contamination of the system in the event of leaks, or would require substantially increased capital and operating costs for additional refrigeration. By using the carbon dioxidedepleted liquid vehicle portion of the refrigerant-absorbent slurry as the final absorbent for carbon dioxide, taking advantage of its relatively high capacity for dissolving carbon dioxide even at extremely cold temperatures, the coldest temperature required in the absorption system 70 in the present example is about -73"C., and the above mentioned disadvantages are avoided.

Recovery of Pressure and Refrigeration Energy As mentioned above, various product streams from the process are routed back through the system for the recovery of refrigeration potential therefrom. These product streams include the finally purified main gas stream flowing in the line 100, the light fractions withdrawn from the stripper-absorber 75 through the line 76, the high purity carbon dioxide gas withdrawn from the refrigerant slurry regenerating system 85 through the line 86 and from the gas-liquid separator 95 through a line 101, and a portion 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 product streams, in the lines 100 and 76', are at temperatures of about -73"C. 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 drawing for recovering refrigeration energy from these two product streams for use in the absorption system 70.

As previously indicated, supplemental cooling for the carbon dioxide condenser system 60 may also be required. The gases flowing out of the heat exchanger 102 through extensions of the lines 76 and 100 may be used for that purpose, along with the gases in the lines 86 and 101, which merge and flow together through an extension of the line 101, as shown. However, all of these last mentioned streams may require slight recooling or further cooling to provide a sufficient temperature driving force. Therefore, a suitable refrigeration unit 103 may be provided for recooling the gases flowing in the extended lines 76, 100, and 101 before they flow through a supplemental, indirect heat exchanger 104 that may be a part of the heat exchange system 60 but is shown separately in Figure 1 of the drawing for simplicity of illustration.

The three streams emerging from the supplemental heat exchanger 104 via further extensions of the lines 76, 100, and 101 and the stream emerging from the heat exchanger 60 via the lines 61 and 62 are all at temperature below -27"C. and may be used as the primary coolants for the incoming crude gas in the heat exchange system 20 mentioned above and shown in the drawing.

As previously explained, the carbon dioxide stream in the line 62 may be depleted of sulfur compounds to any extent desired. It may be discharged to the atmosphere or recovered as a by-product. Since it is still at a pressure of around 75 psia as it emerges from the heat exchange system 20 in the extended line 62, its pressure energy is recovered in an expansion turbine 106 or the like, as shown, before it is discharged.

The relatively low pressure stream of light fractions flowing through the line 76 from the stripper-absorber 75 should contain not more than 4 mol percent of carbon dioxide and may suitably be combined with the roughly ten times greater quantity of purified gas flowing at high pressure through the line 100. Accordingly, after passing through the heat exchange system 20, the further extended line 76 runs through a compressor 107 and then into the further extended line 100 to provide the maximum, purified, final gas product at close to the initial main gas stream pressure.

The high purity carbon dioxide (less than 1 ppm sulfur compounds) flowing through the further extended line 101 after it emerges from the heat exchange system 20 is at a pressure only slightly above ambient and may be discharged either to a by-product collection system (not shown) or to the atmosphere as economic considerations may dictate.

In the drawing and descriptions of the heat exchanger 58 for recovering heat from the Claus plant feed discharged from the distillation system 55 via the line 57, no particular source of a heat supplying medium is disclosed. Similarly, in the drawing and description of the heat exchanger 94 for warming the liquid absorbent flowing from the decanting station 88 via the line 93, no particular source of the second of the two heat supplying media indicated in the drawing is disclosed. As in the case of the reboiler 36 associated with the stripper-absorber 35, any suitable, available, heat supplying fluid may be used in the heat exchangers 58 and 94, thereby supplying additional cooling wherever needed in the overall system. Obviously, where the temperatures of available heat-supplying fluids are not suitable for their direct use as heat exchange fluids, heat pumps may be employed to effect the needed energy transfer. For example, this expedient may be employed for supplying refrigeration to the final carbon dioxide absorption system 70 near the product gas discharge end thereof, the need for such additional refrigeration being pointed out above.

Throughout the process, wherever significant pressure reductions of sizable fluid streams are required, as described above, expansion turbines driving electric generators may be used to recover the energy released by such pressure reductions and convert it to a form that is conveniently usable in operations that consume energy, as will be apparent to those skilled in the art.

Alternative Carbon Dioxide Absorbents As previously stated, many different particulate solid materials may be used in the final absorption system 70 as the solid phase of a refrigerant-absorbent slurry employed therein.

In the preferred example described above in detail, the solid phase melts as carbon dioxide in the main gas stream is condensed, and both are removed from the absorption zone as a liquid mixture with the refrigerant absorbent vehicle. Instead, the carbon dioxide of the gas stream may be condensed to the solid phase, i.e., frozen, either as a pure compound or in a mixture with material of the absorbent liquid vehicle. In both of these cases, the frozen carbon dioxide is removed from the absorption zone as a solid suspended in the liquid vehicle. In either case, the particulate solid component of such a slurry may be a solid phase of a liquid vehicle consisting of a single compound, or the particulate solid may be suspended in a liquid vehicle of different composition.

Another type of refrigerant-absorbent is a liquid-solid system which has a negligible carbon dioxide content and negligible capacity to dissolve carbon dioxide and other gases.

An example is a liquid/solid metal mixture with a freezing range below -56.6"C. Various mixtures of mercury, thallium, and potassium, for example, freeze at temperatures well below -56.6"C. Using such a refrigerant-absorbent system, carbon dioxide of the main gas stream will condense therefrom as a solid for entrainment with the liquid and progressively melting solid phases of such system. Separation of the frozen carbon dioxide from the depleted refrigerant-absorbent slurry and regeneration of the latter are readily accomplished by dropping the pressure of the mixture to below the triple point pressure of carbon dioxide so that the absorbed carbon dioxide sublimes and is separated as a gas. The cooling produced by that sublimation supplies most of the refrigeration required to refreeze the particulate solid material of the refrigerant-absorbent slurry for recycling to the absorber.

Composite materials may be employed as the particulate solid of a refrigerant-absorbent slurry for sorbing carbon dioxide by freezing it out of the main gas stream and adsorbing it onto the surfaces of the composite particles. Thus, one may employ a frozen fluid encased in durable solid walls in the form of small spheres or pellets. The uses of such a composite solid refrigerant combines the high heat absorption characteristics of a solid-liquid phase change with the handling characteristics of permanently solid spheres or pellets. Such spheres or pellets can be made by known technology in many shapes and sizes ranging from microscopic (microencapsulation) to macroscopic (on the order of inches in characteristic dimension). The pellet wall materials may be metal or plastic. The small size composite spheres or pellets of that character can be slurried in a suitable liquid vehicle and used in an absorption column system with only obvious differences in handling procedures and in the character of the process from what has been described above. Such a refrigerant-absorbent slurry may be regenerated while separating the adsorbed carbon dioxide by sublimation by the same regeneration procedure last described above.

However, it is not necessary that such a composite, particulate solid refrigerantabsorbent for carbon dioxide be suspended in a liquid vehicle as a liquid-solid slurry.

Instead, it may be suspended in a gaseous vehicle as a so-called fluidized bed, utilizing well known fluidized bed techniques for continuously removing the composite bodies as they become coated with frozen carbon dioxide, subliming the frozen carbon dioxide therefrom, refreezing the encapsulated refrigerant, and recycling the recooled composite bodies into the fluidized bed. Such composite bodies, including those of larger sizes, may be similarly used in fixed beds, moving beds, ebullating beds, and the like through which the gas stream being treated can be moved to absorbe carbon dioxide therefrom.

Other variants of the particulate solid, absorbent or adsorbent materials disclosed herein and of the methods for handling them to condense carbon dioxide below its triple point temperature may be employed as will be recognized by those skilled in the relevant arts.

In view of the foregoing examples of what have been referred to herein as 'refrigerant-absorbents', it will be apparent that the terms 'absorbent' and 'absorption' have been used in their broadest sense to include not only absorption by dissolving the gas component being separated in a liquid and absorption (or, more specifically; adsorption) by condensing or occluding the gas being separated onto the surface of a solid absorbent but, also, any mechanical entrainment of the gas being separated after condensing it to a more entrainable form. The term 'refrigerant-absorbent' as defined earlier herein is construed in accordance with the foregoing and, in all cases, refers to an absorbent which, in whole or in part, undergoes a change of phase during the gas separation process (i.e., a change of phase from a lower to a higher energy level such as from solid to liquid). This change of phase of the refrigerant-absorbent enables it to utilize at least a portion of the heat that must be taken up in dissolving, adsorbing and/or condensing the gas component to be separated as the latter also undergoes a change of phase (i.e., a change of phase from a higher to a lower energy level such as from gas to liquid). Both of these two phase changes occur in the process, and the change of phase of the refrigerant-absorbent is to be distinguished from the change of phase of the gas component to be separated.

Final Summary From the foregoing description of the process embodying this invention, it will be appreciated that it enables the complete separation of sulfur-containing gases from relatively low boiling point gases, and any desired degree of separation of other relatively high boiling point gases, including carbon dioxide, from relatively low boiling point gases, while operating over a wide range of main gas steam pressures. As capital equipment and operating cost analyses will further show, these results may be achieved with substantial savings in both categories compared to the capital equipment and operating costs of other processes heretofore available for obtaining the same or comparable results, and these savings may be realized over broad ranges of main gas stream operating pressures and compositions. In addition, the process embodying the invention has the many other practical and economic advantages set forth in the introduction to this specification.

The final purified gas product of the exemplary embodiment of the invention described in detail above is suitable for direct use as a relatively low B.T.U. fuel or for use as a feed to a relatively high B.T.U. fuel. Depending upon the particular crude gas mixture to be purified by the process embodying the invention many other uses for the purified product exist as will be appreciated by those skilled in the pertinent arts.

Although the invention has been described with detailed reference to a specific embodiment thereof and to certain optional modifications of that embodiment, it will also be appreciated that the invention is susceptible to many other modifications while utilizing the principles thereof and operating within the scope of the appended claims.

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

2. A process according to claim 1, wherein said at least one refrigerant-absorbent is selected from: (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 said 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, said particulate solids being caused to undergo a change of phase, in whole or in part, during contact with said gas mixture; 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.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Other variants of the particulate solid, absorbent or adsorbent materials disclosed herein and of the methods for handling them to condense carbon dioxide below its triple point temperature may be employed as will be recognized by those skilled in the relevant arts. In view of the foregoing examples of what have been referred to herein as 'refrigerant-absorbents', it will be apparent that the terms 'absorbent' and 'absorption' have been used in their broadest sense to include not only absorption by dissolving the gas component being separated in a liquid and absorption (or, more specifically; adsorption) by condensing or occluding the gas being separated onto the surface of a solid absorbent but, also, any mechanical entrainment of the gas being separated after condensing it to a more entrainable form. The term 'refrigerant-absorbent' as defined earlier herein is construed in accordance with the foregoing and, in all cases, refers to an absorbent which, in whole or in part, undergoes a change of phase during the gas separation process (i.e., a change of phase from a lower to a higher energy level such as from solid to liquid). This change of phase of the refrigerant-absorbent enables it to utilize at least a portion of the heat that must be taken up in dissolving, adsorbing and/or condensing the gas component to be separated as the latter also undergoes a change of phase (i.e., a change of phase from a higher to a lower energy level such as from gas to liquid). Both of these two phase changes occur in the process, and the change of phase of the refrigerant-absorbent is to be distinguished from the change of phase of the gas component to be separated. Final Summary From the foregoing description of the process embodying this invention, it will be appreciated that it enables the complete separation of sulfur-containing gases from relatively low boiling point gases, and any desired degree of separation of other relatively high boiling point gases, including carbon dioxide, from relatively low boiling point gases, while operating over a wide range of main gas steam pressures. As capital equipment and operating cost analyses will further show, these results may be achieved with substantial savings in both categories compared to the capital equipment and operating costs of other processes heretofore available for obtaining the same or comparable results, and these savings may be realized over broad ranges of main gas stream operating pressures and compositions. In addition, the process embodying the invention has the many other practical and economic advantages set forth in the introduction to this specification. The final purified gas product of the exemplary embodiment of the invention described in detail above is suitable for direct use as a relatively low B.T.U. fuel or for use as a feed to a relatively high B.T.U. fuel. Depending upon the particular crude gas mixture to be purified by the process embodying the invention many other uses for the purified product exist as will be appreciated by those skilled in the pertinent arts. Although the invention has been described with detailed reference to a specific embodiment thereof and to certain optional modifications of that embodiment, it will also be appreciated that the invention is susceptible to many other modifications while utilizing the principles thereof and operating within the scope of the appended claims. WHAT WE CLAIM IS:

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

4. A process for separating relatively high boiling point gases, for example hydrogen

sulfide and other sulfur-containing gases, from a pressurized gas mixture also containing relatively low boiling point gas, comprising the steps of (a) contacting the gas mixture with liquid carbon dioxide to absorb and entrain primarily any hydrogen sulfide and other relatively high boiling point gases present, a part of said liquid carbon dioxide being caused to undergo a change of phase during the contacting of the gas mixture; (b) separating liquid carbon dioxide and gases entrained therewith from a first residual portion of said gas mixture, including most of the relatively low boiling point gas; (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, wherein 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 therefrom for reuse in step (a).

6. A process according to claim 4, wherein 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, wherein 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, wherein step (c) includes contacting the gas mixture with a refrigerant-absorbent which is a sorbent comprising a particulate solid in a liquid vehicle and having a temperature below the triple point temperature of carbon dioxide, said particulate solid being caused to undergo a change of phase, in whole or in part, to condense and/or freeze carbon dioxide from the gas mixture, and separating the sorbent and condensed or frozen carbon dioxide from a residual portion of said gas mixture.

9. A process according to claim 7, wherein 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, for example hydrogen sulfide and higher boiling point gases, from a pressurized gas mixture also containing relatively low boiling point components, comprising the steps of contacting the gas mixture at a pressure above 80 psia with liquid carbon dioxide, a part of said liquid carbon dioxide being caused to undergo a change of phase during the contacting of the gas mixture, and separating the liquid carbon dioxide and gases entrained therewith from a residual portion of said gas mixture.

11. A process according to claim 10, wherein the pressurized gas mixture enters the process in a substantially anhydrous condition and at a pressure of at least 250 psia.

12. A process according to claim 10, wherein the separated liquid carbon dioxide and gases entrained therewith 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, relatively low boiling point components of a gas mixture, comprising the steps of contacting the gas mixture with a refrigerant-absorbent which is a sorbent comprising a particulate solid having a temperature below the triple point temperature of carbon dioxide and being caused to undergo a change of phase, in whole or in part, during contact with the gase mixture 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 low boiling point components.

14. A process according to claim 13, wherein 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, wherein 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, wherein said sorbent comprises a slurry of a articulate 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 progressively 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, wherein the particulate solid of said sorbent is predominantly 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, wherein the liquid vehicle of the sorbent is an organic liquid.

19. A process according to claim 17, wherein 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, wherein substantially all the cooling required to condense carbon dioxide from said gas mixture is provided by the melting of said particulate solid during contact with said gas mixture.

21. A process according to claim 13, wherein the step of contacting the gas mixture with said sorbent includes transferring heat into the particulate solid, causing it to undergo said change of phase while effecting a progressive net reduction in the solid phase portion of the sorbent.

22. A process according to claim 1, wherein 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 the process further comprising 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 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-regenerated sorbent, and returning the regenerated sorbent to said contact zone for reuse therein.

23. A process according to claim 22, wherein 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, wherein the particulate solid of the sorbent is predominantly carbon dioxide and the liquid vehicle of the sorbent is a solution of carbon dioxide in a liquid vehicle.

25. A process according to claim 24, wherein the liquid vehicle of the sorbent is an organic liquid.

26. A process according to claim 22, wherein a portion of the vehicle of the regenerated sorbent is separated therefrom, the remaining sorbent is returned to said contact tone for reuse therein, and the separated gas mixture stream is intimately connected by a countercurrent flow of the separated portion of the vehicle of the regenerated sorbent to remove additional carbon dioxide therefrom.

27. A process according to claim 1, wherein 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 having a temperature 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 said residual portion of said gas mixture.

28. A process according to claim 1, wherein 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 substantially all of the carbonyl sulfide present in said gas mixture; and the step of separating material from the absorbed gases for processing into refrigerant-absorbent for use in the process includes (b) separating liquid carbon dioxide and gases entrained therewith from said 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, wherein said one or more relatively high boiling point gases include carbonyl sulfide, and said at least one refrigerant-absorbent comprises liquid carbon dioxide, and the step of contacting said gas mixture includes contacting said gas mixture in counter-current 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 which would be adequate to remove hydrogen sulfide, if hydrogen sulfide were present, in an amount equal to the amount of carbonyl sulphide actually present.

30. A process according to claim 1, wherein 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.

31. A gas separation process substantially as herein described with reference to the accompanying drawings.