US3267028A - Separation of wet pyrolysis gases by sorbent treating and fractionation - Google Patents

Description

Aug. 16, 1966 R. E. M HARG 3,

SEPARATION OF WET PYROLYSIS GASES BY SORBENT TREATING AND FRACTIONATION Filed OCt. 18, 1963 Glycol & g Con/actor;

a Discharge Drum- 1 N M a I g g Compressor Q) "8 3 INVENTOR- g Roberf E. McHorg A ml 1/ W 4 g J W ATTORNEYS.

United States Patent 3,267,028 SEPARATION OF WET PYROLYSIS GASES BY SOR- BENT TREATING AND FRACTIONATION Robert E. McHarg, Arlington Heights, 111., assignor to Universal Oil Products Company, Des Plaines, 11]., a

corporation of Delaware Filed Oct. 18, 1963, Ser. No. 317,258 6 Claims. (Cl. 208341) This invention relates broadly to the separation of complex hydrocarbon mixtures by fractional distillation and more particularly to a process for drying wet hydrocarbon gases prior to separating same by low temperature fractionation. Still more particularly, the present invention is directed to the pretreatment of pyrolysis gases, comprising substantial amounts of polymerizable light olefins, gasoline boiling range material and water vapor, with a liquid sorbent having selective solubility both for the water vapor and gasoline boiling range material, to remove these components from the pyrolysis gases whereby the latter may be more efficiently separated by subsequent fractional distillation.

One of the commercially attractive routes to the production of valuable light olefins is the thermal cracking of hydrocarbons, typically light paraffins such as ethane, propane or butane, and of light or heavy naphtha fractions. As is well-known in the art, the cracking is effected at high temperatures, often in the presence of superheated steam, preferably in a tubular reactor or plurality of cracking furnace coils. Depending on the charge stock and specific conditions employed, the cracking zone eflluent may comprise ethylene, propylene, butylenes, etc. or mixtures thereof, any or all of which may constitute the principal product or products, A very flexible operation involves the simultaneous cracking, in separate parallel cracking zones, of a normally gaseous paraffinic charge stock and of a naphtha charge stock; the effluents from the two cracking zones are combined and charged to a prefractionation zone to effect a rough split between light and heavy ends, and the prefractionator overhead material, containing the desired olefinc products, is passed through a fractionator train from which the several product streams are recovered. The naphtha cracking eflluent contains a minor proportion of light paraflins which can be advantageously separated and recycled to the paraflin cracking zone. fractionator net overhead material comprises a major proportion of gas and a minor proportion of liquid. The prefractionator overhead gas is typically a multicomponent mixture containing light non-hydrocarbons such as hydrogen, hydrogen sulfide, carbon monoxide and carbon dioxide; hydrocarbons including methane, ethane, ethylene, acetylene, propane, propylene, butane, C -olefins, pentanes, and C materials; and wator vapor in approximately saturation quantity.

Preliminary to product recovery, the prefractionator overhead gas is compressed to superatomspheric pressure and is charged as feed to a first distillation zone, the overhead product from which is rich in C and lighter hydrocarbons and the bottoms product is rich in C and heavier hydrocarbons. The feed composition is often such that this first distillation zone, which may be designated herein as a C /C splitter, must be operated with refrigerated reflux to provide a commercially satisfactory separation. The C fraction is then separated in a series of low temperature fractionating columns, while the C fraction is separated in a separate series of fractionating columns. As used in this application, the term refrigerated reflux means a fractionating column reflux stream which is cooled to a temperature substantially below that obtainable from normal refinery cooling water.

The pre- Patented August 16, 1966 Whereas the latter provides reflux temperatures typically in the range of -l20 F., temperatures achievable by refrigeration are substantially lower, for example, in the range of 30 F. to +40 F. Since, however, the splitter feed is saturated with water vapor, means must be provided to adequately dry this wet hydrocarbon feed in order to inhibit ice and hydrate formation within the splitter column, particularly in the rectification section thereof. It is well-known that below a critical temperature water vapor will combine with normally gaseous paraflinic and olefinic hydrocarbons, particularly CH C H 0 1-1 C H and C H to form an ice-like solid or hydrate. The temperature of hydrate formation is a function of water partial pressure, hydrocarbon composition, and total pressure; for example, a pyrolysis gas of typical composition such as herein dealt with, saturated with water vapor at a total pressure of 230 p.s.i.g., will .have a hydrate formation temperature of approximately 40 F. Such hydrates tend to plug valves, heat exchange tubes, fractionator trays and other process equipment. Hence the necessity for reducing the water content of the wet hydrocarbon mixture so that hydrates will not be formed at the lowest temperature to which the mixture will be subjected.

Removal of water from an ethylene plant gas stream is conventionally accomplished by passing such gas stream through a fixed bed of solid desiccant such as activated alumina, activated charcoal or silica gel. The use of solid desiccants in this service has not been eminently satisfactory by reason of the fact that the desiccant tends to become fouled by polymerizable light ends, coke and other impurities and further because the drying operation is only semi-continuous, requiring multiple swinging vessels, extensive capital investment in standby equipment, and frequent manual replacement of the desiccant.

In accordance with the present invention, the dehydration of wet hydrocarbon pyrolysis gases is carried out by contacting the gases with a liquid sorbent comprising a polyhydric alcohol containing from 2 to 6 carbon atoms. Although liquid desiccants such as alkylene and polyalkylene glycols have been used to dry natural gas streams in gasoline plants, so far as I am aware, such liquid desiccants have never been employed to treat cracking plant effluent gas preliminary to their separation. Use of a liquid desiccant in this service is not only advantageous in providing continuous drying and eliminating the fouling problem associated with solid desiccants, but also affords an unexpected improvement in the operation of the aforesaid C /C splitter, as set forth below.

The cracking plant eflluent gas, particularly when the charge stock to the cracking zone comprises normally liquid hydrocarbons such as naphtha, includes a small proportion of relatively high :boiling material, that is, a mixture of compounds boiling above the C fraction. Such compounds typically include benzene, toluene, xylene, other aromatics having an end point of about 400 F., and also paraffinic, naphthenic, and olefinic hydrocarbons having an end point of about 400 F. These materials are generically classified herein, and are so to be interpreted in the appended claims, as pseudo-gasoline compounds inasmuch as they boil at least partly in the gasoline boiling range but exist in the vapor phase in the eflluent gas. Even after the efiluent gas is compressed to superatmospheric pressure, for example, in the range of 50-550 p.s,i.g. and the heat of compression is removed by cooling to ambient temperature or below, the uncondensed eflluent gas still comprises an appreciable proportion of pseudo-gasoline compounds. In the prior art processes the charge to the C /C splitter therefore includes the pseudo-gasoline compounds which necessarily appear in the splitter bottoms product. To presence of these heavy materials in the splitter bottoms is often deleterious because, for an economically feasible distillation pressure as determined by reflux temperature, the reboiler temperature must be set high enough to cause thermal polymerization of the C olefins which are, of course, also present in the splitter bottoms and which constitute a valuable byproduct of the cracking process. Incipient thermal polymerization of the C olefins may be expected to occur at about 220 F., and the rate increases rapidly with increasing temperature. Such undesirable side reactions in the splitter reboiler lead to fouling of the reboiler heat exchange tubes and loss of product. While theoretically this condition could be minimized by reducing distillation pressure and thereby lowering reboiler temperature, the very low top tower temperatures required make this approach commercially impractical I have found that the aforesaid vapor phase pseudogasoline compounds can be nearly completely removed from the splitter charge gas by scrubbing or otherwise suitably contacting said gas with a liquid polyhydric alcohol containing from 2 to 6 carbon atoms. The absorption of pseudo-gasoline compounds is highly selective in that the C material is nearly completely removed, while the C C fractions are not absorbed to any appreciable extent. Removal of these small quantities of heavy ends from the splitter feed depresses the bubble point of the splitter bottoms product by as much as 20- 40 F., thereby lowering the reboiler temperature to below that at which thermal polymerization of the C olefins tends to occur. Fouling of the reboiler heat exchange tubes and loss of product are therefore largely prevented. It will be seen that the polyhydric alcohol simultaneously functions in a dual capacity: (1) as a desiccant for drying the splitter feed gas to minimize ice and hydrate formation in the rectification section of the splitter column and (2) as a sorbent for absorbing pseudogasoline compounds from the splitter feed to depress the bubble point of the splitter bottoms product. The absorbed pseudo-gasoline compounds are recovered from the spent sorbent by any suitable means, preferably separately from and in advance of the step of regenerating or dehydrating the sorbent for reuse.

A broad embodiment of this invention provides an improvement in the separation of wet hydrocarbon pyrolysis gases by fractional distillation into light and heavy fractions, said gases comprising a minor amount of high-boiling pseudo-gasoline compounds, including the steps of compressing said gases to superatmospheric pressure and passing the compressed gases as feed to a distillation zone operating with refrigerated reflux, which improvement comprises first contacting the compressed gases, at a temperature above the temperature of hydrate formation between the water vapor and light hydrocarbons contained therein, with a liquid sorbent comprising a polyhydric alcohol containing from 2 to 6 carbon atoms; absorbing in the sorbent a substantial portion of said water vapor and pseudo gasoline compounds; passing the thusly treated pyrolysis gases to said distillation as aforesaid, whereby to minimize ice and hydrate formation in the upper portion of the distillation zone and to lower the bubble point of the bottoms product therefrom; and recovering from the thusly contacted sorbent said pseudogasoline compounds.

In a more specific embodiment, the present invention is directed to an improvement in the separation of hydrocarbon pyrolysis gases by fractional distillation into light and heavy fractions, said gases comprising minor amounts of water vapor and high-boiling pseudo-gasoline compounds, including the steps of compressing the gases to superatmospheric pressure and passing the compressed gases as feed to a distillation zone operating with refrigerated reflux, which improvement comprises first contacting the compressed gases, under a pressure of 150- 300 p.s.i.g. and at a temperature of 40l40 F., with a liquid sorbent comprising a glycol containing not more 4.- than 6 carbon atoms; absorbing in the sorbent a substantial portion of said water vapor and pseudo-gasoline compounds; passing the thusly treated pyrolysis gases to said distillation zone as aforesaid; and recovering from the thusly contacted sorbent said pseudo-gasoline compounds.

The pyrolysis gas as received from the cracking plant prefractionating column is generally under low pressure ranging from slightly above atmospheric to about 20 p.s.i.g. This gas is first compressed to distillation pressure or somewhat thereabove in one, or preferably several, compressor stages. The final pressure will of course depend on the gas composition and desired cut point, among other factors. For the gas compositions herewith concerned, such pressure will lie in the range of 50-550 p.s.i.g. and more particularly in the range of 150-300 p.s.i.g. The gas leaving the final compression stage may be cooled to about -100 F. to condense out water and hydrocarbon vapors. In a preferred embodiment of the invention the compressed gas is chilled by indirect heat exchange with a suitable refrigerant to a temperature of about 40-80 F. in order to allow advantageously lower temperatures to obtain in the sorbent treating zone. The compressed pre-cooled pyrolysis gas may be passed through a knockout drum in which condensed water and hydrocarbons are separated, prior to charging the gas to the sorbent treating zone.

In the sorbent treating zone the pyrolysis gas is intimately contacted with a liquid polyhydric alcohol containing from 2 to 6 carbon atoms. This may be carried out in any suitable continuous gas-liquid contacting apparatus such as a cocurrent or countercurrent column provided with vertically spaced sieve decks or bubble cap trays, or filled with a solid packing material such as Rashig rings, Berle saddles and the like, or the contacting may be effected in a mechanically or sonically agitated vessel or pulse column. Polyhydric alcohols containing from 2 to 6 carbon atoms possess the best combination of selective absorptivity for Water and also for the pseudogasoline compounds. It is essential that the polyhydric alcohol contain not more than 6 carbon atoms because higher alcohols have too great a solubility for C -C hydrocarbons which would result in their loss from the C /C splitter overhead stream. Another factor operating against the use of such higher alcohols is their higher melting points which would necessitite undesirably elevated temperatures in the sorbent treating zone to insure liquid phase flow. Suitable polyhydric alcohols for use in this invention include ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, 1,2- propanediol, 1,3-butanediol, 2,3-butanediol, diethylene glycol, triethylene glycol and glycerol. Preferred polyhydric alcohols are ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, triethylene glycol and glycerol The polyhydric alcohol may be used in a substantially pure state, or as a water-alcohol solution or in admixture with liquid desiccants such as monoethanolamine, dicthanolamine and triethanolamine.

The temperature maintained in the sorbent treating zone, for the most efficient absorption of Water vapor and pseudo-gasoline compounds, should be as low as practicable, consistent with process economics, but should be above the temperature of hydrate formation and, of course, above the melting point of the particular polyhydric alcohol employed. The preferred temperature of the sorbent-pyrolysis gas contacting step is from about 40 to about F. Such temperatures may be readily attained by Water cooling or refrigerant chilling the pyrolysis gas, the inflowing fresh sorbent stream, or both streams. When a conventional multi-stage column is employed, a moderate temperature gradient will exist along the length thereof by reason of the heat of absorption but, where desired, internal or external interstage cooling means may be used to more closely approximate isothermal conditions.

columns may be of conventional construction.

The dehydrated pyrolysis gas, now also substantially free of pseudo-gasoline compounds, is charged without appreciable pressure reduction as feed to a first distillation column, reflux to which is cooled by refrigeration to a temperature well below that obtainable by usual refinery cooling water, for example, in the range of 20 F. to +20 F. This column may be operated as a C /C splitter in which case the net overhead product therefrom will be rich in C -C olefins and paraflins, together with fairly substantial amounts of methane, hydrogen and small quantities of light nonhydrocarbon gases, and the bottoms product will be rich in C C olefins and paraffins. However, since the pseudo-gasoline compounds have been removed from the splitter feed by the sorbent treating step, they-are not present in the bottoms product so that the bubble point thereof is depressed 2040 F. below that which would obtain if the pseudo-gasoline compounds were present. Thermal polymerization of the C olefins in the bottoms product is greatly inhibited, with out having to employ a substantially lower reflux temperature and reduced distillation pressurethe only practical alternative for avoiding polymerization of the C olefins. The splitter net overhead stream may be subjected to one or more low temperature distillation steps from which relatively pure ethane, ethylene, propane and propylene product may be recovered. The splitter bottoms stream may be subjected to one or more distillation steps, using either refrigerated reflux or normal temperature reflux, from which relatively pure butane, butylenes, pentane and a C fraction may be recovered. The splitter bottoms stream may be combined with pyrolysis liquid (cracking plant prefractionator net overhead liquid) before being charged to the next distillation column.

After contact with the pyrolysis gas the liquid polyhydric alcohol sorbent contains appreciable amounts of absorbed water and pseudo-gasoline compounds. Such spent sorbent is preferably utilized in closed cycle operation and is therefore regenerated to recover the pseudo-gasoline compounds and to remove the absorbed water. Even if the sorbent were not recycled, the pseudo-gasoline compounds are desirably recovered since they also constitute a valuable product. In a preferred embodiment of the invention the spent sorbent from the treating zone is warmed and flashed at an intermediate reduced pressure under conditions favorable towards selective vaporization of the pseudo-gasoline compounds without appreciable vaporization of the sorbent or of the water dissolved therein. The flash vapors, rich in pseudo-gasoline compounds, may be condensed and sent to storage or they may be combined, with or without condensation, with the C /C splitter bottoms product and/or the pyrolysis liquid from the cracking plant. The unflashed liquid is then passed to a vacuum distillation column or water stripper to effect the requisite dehydration of the sorbent, and the resulting dried sorbent is then returned to the treating zone for reuse.

In the drawing there is illustrated a simplified flow chart of the process of the present invention. All distillation Omitted are such details as pumps, process instrumentation and controls, certain heat recovery circuits, valving, start-up lines and similar hardware, the design and application of which will be well within the purview of one skilled in the art. For the purpose of demonstrating a specific embodiment of the invention, the drawing will be described in connection with the treatment of pyrolysis gas from a commercially scaled hydrocarbon cracking plant comprising two parallel thermal cracking zones, the charge to which is respectively ethane and naphtha (55 API, boiling range of 244-385 F., and average MW of 125). It will be understood that charge stocks, stream compositions, operating conditions, order of fractionation and the like are exemplary only and may be varied widely without departing from the spirit of the invention, the scope of which is to be limited only in conformance with the claims of this application.

Pyrolysis gas at 6 p.s.i.g. and 100 F. is Withdrawn from a prefractionator overhead receiver (not shown) through line 1 at the rate of 138,794 pounds per hour. Compressor 2 raises the pressure of the pyrolysis gas to 235 p.s.i.g. Although shown here as a single stage machine, compressor 2 preferably comprises 2, 3 or more stages in a series, with interstage cooling, liquid knockout and spillback to suction for pressure control. The compressed pyrolysis gas is passed through indirect cooling means 3, which includes a water aftercooler followed by a propylene subcooler, whereby the temperature of the high pressure gas is reduced to F., condensing substantial quantities of water and hydrocarbon vapor. This mixed phase stream is passed to discharge drum 4 for gravity separation of the liquid phases. Water is removed through line 5 at the rate of 7,434 pounds per hour and hydrocarbon condensate is withdrawn through line 6 at the rate of 25,528 pounds per hour. The pyrolysis gas at 231 p.s.i.g. and 75 F. is charged via line 7 to glycol contactor 8 at the rate of 105,832 pounds per hour.

Contactor 8 is a vertical column containing 12 sieve decks. Substantially dry triethylene glycol at 70 F. is introduced to the top deck of contactor 8 through line 9 at the rate of 14,308 pounds per hour. The glycol flows downwardly through contactor 8 countercurrently to the ascending gas stream. The spent glycol taken off through line 11 has absorbed from the pyrolysis gas 126 pounds per hour of water and 594 pounds per hour of pseudo-gasoline compounds. The treated pyrolysis gas, now substantially free of water and pseudo-gasoline compounds, is taken overhead through line 10 and charged to C /C, splitter 24. The spent glycol in line 11, which is under a pressure of 230 p.s.i.g., is warmed by heat exchanger 12 to a temperature of 180 F., flashed across valve 13 to a pressure of about 5 p.s.i.g., and the resulting mixed phase material flows to flash drum 14. Flash vapors, consisting essentially of pseudo-gasoline compounds, are condensed in condenser 15 and passed via line 16 to flash receiver 17. Unflashed wet glycol flows through line 19 to glycol regeneration still 20 which is operated at an absolute pressure of millimeters Hg and a reboiler temperature of 360 F. The absorbed water is distilled off and taken overhead through line 21 at the rate of 126 pounds per hour. Hot regenerated glycol is withdrawn through line 22, heat exchanged against spent glycol in exchanger 12, and cooled by cooler 23 to 70 F. Cooler 23 includes a water-cooled exchanger followed by a propylene-refrigerated chiller. Cooled regenerated triethylene glycol is thence returned via line 9 to contactor 8.

The C /C splitter column is operated at a pressure of about 230 p.s.i.g., a top tower temperature of l0 F. and a reboiler temperature of 200 F. Overhead vapors in line 25 are partially condensed by propylene refrigerant in condenser 26. Net overhead vapor at a flow of 80,482 pounds per hour is taken from receiver 27 through line 29 to further separation facilities; its composition is approximately 17% H and inerts, 30% CH 45% C -C olefins and 8% C -C paraflins. Reflux at a temperature of 10 F. is returned to column 24 through line 28 at a rate of 37,000 pounds per hour. Bottoms product is withdrawn from column 24 through line 30 at a rate of 24,630 pounds per hour; its composition is approximately 17% C s, 60% C olefins, 2% butanes and 20% pentanes.

The C /C splitter bottoms is passed to a heavy ends fractionator train, only the first column of which, depentanizer 33, is shown. The depentanizer feed may comprise several commingled streams, for examples as illustrated here, the depentanizer feed consists of the splitter bottoms from line 30, 10,350 pounds per hour of pyrolysis liquid (net overhead liquid from prefractionator receiver, not shown) in line 31, hydrocarbon condensate from line 6 and flash condensate (pseudo-gasoline com- 7 pounds) from line 18. The combined feed to depentanizer 33 is at a rate of 61,102 pounds per hour and its composition is approximately 10% C s and lighter, 33% C olefins, 1% butanes, 13% pentanes and 42% pseudoprises: contacting the compressed feed in a gaseous state, at temperatures above those at which water vapor and light hydrocarbons contained therein form a hydrate, and at a pressure of about 50-550 .p.s. i.g., with a liquid sorbent gasolines. The depentanizer column is operated at a 5 consisting essentially of a polyhydric alcohol containing substantially lower pressure than splitter 24, for example, from 2 to 6 carbon atoms, thereby to absorb a substanat about 40 p.s.i.g. Depentanizer reflux temperature is tial portion of said water vapor and pseudo-gasoline comabout 80 F. and the bottoms temperature about 300 F. pounds without appreciable absorption of the C -C (the overhead receiver and reflux system are not illusgaseous hydrocarbons; thereafter passing the C -C hytrated). Net overhead liquid is taken off through line 10 drocaribon gases to said distillation zone as aforesaid, 34 at a rate of 27,196 pounds per hour; its composition whereby to minimize ice and hydrate formation in the is approximately 2% C C hydrocarbons, 72% C C upper portion of the distillation zone and to lower the olefins, 4% C C paraflins and 22% pentanes. The bubble point of the bottoms product therefrom; and treatdepentanizer overhead liquid may be sent to additional ing the used sorbent to recover the pseudo-gasoline comdistillation facilities to separately recover the various pounds therefrom. fractions at such purities as may be desired. Depentan- 2. The process of claim 1 further characterized in that izer bottoms liquid is sent to storage via line 35 at a rate the sorbent material is selected from the group consisting of 33,906 pounds per hour; its composition is approxiessentially of ethylene glycol, trimethylene glycol, diethmately 100% pseudo-gasolines. It will be appreciated ylene glycol, triethylene glycol and glycerol. that since the C olefins are taken overhead in this column, 3. In the process of separating wet hydrocarbon pythey are not subjected to thermal polymerization temperarolysis gases containing a major proportion of C -C hytures. drocarbons including C -C olefins and a minor propor- A more complete composition analysis of the several tion of water vapor and high boiling pseudo-gasoline cornstreams in the above-described process is set forth below. pounds by the fractional distillation thereof into light and These, of course, are illustrative only and may vary wideheavy fractions, including the steps of compressing said 1y depending on the feed stock, flow rates and other gases to superatmospheric pressure and passing them as a processing conditions in this table, light non-hydrocarfeed to a distillation zone operating with refrigerated rebons consist of H 8, CO and C0 The term C olefins flux, the improvement which comprises: contacting the embraces isobutylene, butene-l and 'butene-Z. All percompressed feed in a gaseous state, in a treating zone centages are given in mol percent: under a pressure of about 150-300 p.s.i.g. and at a tem- Table I Gas to Con- Gas to Splitter Depen- Depen- Depen- Comdensate Con- Over- Splitter Pyrolysis tanizer tanizer tanizer pressor +H O tactor head Bottoms Liquid Feed Over- Bottoms (Line 1) (Lines 5 (Line 7) (Line 29) (Line 30) (Line 31) (Line 32) head (Line 35) and 6) (Line Light nonhydrocarbons, percent 0. 4 0. 5 Hz do- 12.3 14.4

It will be seen from Table I that pseudo-gasolines are absent from the splitter bottoms. If they were present, even in a fair low concentration of 13 mol percent, the splitter reboiler temperature would have to be raised from 200 F. to at least 220 F. At this latter tempera ture thermal polymerization of the C olefins, constituting 60 mol percent of the bottoms product, would take place.

The pseudo-gasoline compounds may be recovered from the spent sorbent by a method other than a single stage flash vaporization. For example, one may employ for this purpose multiple stage flash vaporization, distillation, or liquid-liquid extraction. If desired, the absorbed water and pseudo-gasoline compounds may be removed simultaneously in a single distillation step in which both water and pseudo-gasoline compounds are driven overhead and the hydrocarbon phase is then recovered by gravity separation.

I claim as my invention:

'1. In the process of separating wet hydrocarbon pyrolysis gases containing a major proportion of C -C hydrocarbons including C -C olefins and a minor proportion of water vapor and high boiling pseudo-gasoline compounds therein by fractional distillation thereof into light and heavy fractions, including the steps of compressing the gases to superatmospheric pressure and passing the compressed gases as a feed to a distillation zone operating with refrigerated reflux, the improvement which comperat-ure of about 40 F.140 F., with a liquid sorbent consisting essentially of a polyhydric alcohol containing not more than 6 carbon atoms; thereby to absorb a substantial portion of said Water vapor and pseudo-gasoline compounds without appreciable absorption of the C1-C5 gaseous hydrocarbons; thereafter passing the C -C hydrocarbon gases to said distillation zone as aforesaid, whereby to minimize ice and hydrate formation in the upper portion of the distillation zone and to lower the 'bubble point of the bottoms product therefrom; withdrawing spent sorbent from said treating zone and regenerating the same by separating said pseudo-gasoline compounds and water therefrom, then recycling the regenerated sorbent back to the treating zone.

4. The process of claim 3 wherein said compressed gases, prior to contact with said sorbent, are first precooled to a temperature in the range of 4080 F. to condense out a portion of the Water vapor therein and the resulting aqueous phase is separated from the precooled gases.

5. The process of claim 3 further characterized in that said regenerated sorbent is cooled to a temperature below about F. before returning it to the treating zone.

6. A process for the separation of compressed wet hydrocarbon pyrolysis gases comprising a major proportion of C -C hydrocarbons including C -C olefins and a minor proportion of high boiling pseudo-gasoline compounds which comprises:

(a) passing said pyrolysis gases under a pressure of 150-300 p.s.i.g. to a treating zone and herein contacting the gases at a temperature in the range of 40- 140 F. with a liquid sorbent consisting essentially of a glycol containing not more than 6 carbon atoms;

(b) absorbing in the sorbent a substantial portion of the Water vapor and pseudo-gasoline compounds contained in said pyrolysis gases;

(c) withdrawing spent sorbent from said treating zone and heating it to an elevated temperature;

((1) flashing the heated spent sorbent to yield an unflashed wet sorbent fraction and a flashed fraction rich in pseudo-gasoline compounds;

(e) distilling said unflashed fraction under reduced pressure to remove Water therefrom;

(f) returning the resulting dried sonbent to said treating zone;

(g) withdrawing the thus treated pyrolysis gases from said treating zone and passing them as feed to a first distillation zone operating with refrigerated reflux and maintained under a pressure of 150-300 p.s.i. g.

(h) recovering from said first distillation zone an overhead product rich in C hydrocarbons and lighter materials and a bottoms product;

(i) passing said bottoms product and said flashed fraction as feed to a second distillation zone maintained at a substantially lower pressure than the [first distillation zone; and

(j) recovering from said second distillation zone an overhead product rich in C -C hydrocarbons and a bottoms product rich in said pseudo-gasoline compounds.

References Cited by the Examiner UNITED STATES PATENTS NORMAN YUDKOF'F, Primary Examiner.

V. W. PRETKA, J. C. JOHNSON, Assistant Examiners.

Claims (1)

1. IN THE PROCESS OF SEPARATING WET HYDROCARBON PYROLYSIS GASES CONTAINING A MAJOR PROPORTION OF C1-C5 HYDROCARBONS INCLUDING C2-C4 OLEFINS AND A MINOR PROPORTION OF WATER VAPOR AND HIGH BOILING PSEUDO-GASOLINE COMPOUNDS THEREIN BY FRACTIONAL DISTILLATION THEREOF INTO LIGHT AND HEAVY FRACTIONS, INCLUDING THE STEPS OF COMPRESSING THE GASES TO SUPERATMOSPHERIC PRESSURE AND PASSING THE COMPRESSED GASES AS A FEED TO A DISTILLATION ZONE OPERATING WITH REFRIGERATED REFLUX, THE IMPROVEMENT WHICH COMPRESES: CONTACTING THE COMPRESSED FREED IN A GASEOUS STATE, AT TEMPERATURES ABOVE THOSE AT WHICH WATER VAPOR AND LIGHT HYDROCARBONS CONTAINED THEREIN FORM A HYDRATE, AND AT A PRESSURE OF ABOUT 50-550 P.S.I.G., WITH A LIQUID SORBENT CONSISTING ESSENTIALLY OF A POLYHYDRIC ALCOHOL CONTANING FROM 2 TO 6 CARBON ATOMS, THEREBY TO ABSORB A SUBSTANTIAL PORTION OF SAID WATER VAPOR AND PSEUDO-GASOLINE COM-

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