US2615057A - Extraction of aromatic hydrocarbons with esters of thiolsulfonic acid - Google Patents

Description

Patented Oct. 21, 1952 XUNYITED ISTATES PATENT .7 OFFICE 7 EXTRACTION F AROMATIC" 'HYDROCAR- BONSWITH ESTERS 'OF THIOLSULFONIC ACID William F; Wolff; Park Forest;Il1:,"an'd Carl E. Johnson, Griifith, Ind.,assign0rs to Standard Oil Company, -Chicago,- -Ill., a 'corporation of 1 Indiana Application'Aug-us't 30, 1951; Serial No. 244,373

1 9 Claims. This invention relates to aprocess for the selective extraction of aromatic hydrocarbons and carbon stocks containing the same may be desired in" order to'prepare partially dearomatized I "hydrocarbon stocks" for :thermal 'or .catalytic "crackinga for hydrogenation operations, or for use as heater oils.

A-considerable number of" selective solvents have' been proposed for the removal of polycyclic aromatichydrocarbonsfrom hydrocarbon stocks "qcontaining the same, but heretofore: no solvent hasubeenfound tobe sufiiciently desirable' for .:all purposes to retard the further search forimproved' selectivetsolvents. We have discovered that certain esters of thiolsulfonic acids are'sur- I prisinglyieffective selective solvents for" aromatic hydrocarbons and particularly 1' polycyclic aromatics in admixture with other hydrocarbons :msuchvi as aliphatic or. saturated.hydrocarbons, notablyxnaturally occurring mixtures such as petroleum oil fractions, 'coal' tar-fractions or shalezoirfractions. For" example, the-esters of I thiolsulfonic acids are excellent solvents for v'mono-. and dimethylnaphthalenes, monoand polymethylanthracenes and the like.

further found'that said thiolsulfonic esters can 'beprepared'relatively cheaply, have relatively mild corrosive properties permittin their employment in conventional equipment, are characterized by surprisingly low' or negligible solu- We have bility' in the raffinate phase produced by the extraction" process, are readily recoverable, and 2 have physical'propertieswhich permit their employment as selective extractionsolvents'for organic sulfurcompounds and aromatic hydrocarbons at" readily attainable temperatures and pressures. Moreover, the molecular weight and chemical constitution of'the thiolsulfonic'esters can be: varied to obtain tailor-made extraction solvents best suited for the particular extraction l operations as they arise.

It is an object of'this invention to provid a process for the selective extraction of aromatic hydrocarbons, particularly polycyclic aromatics,

v -from their mixtures with other hydrocarbons. -Anotherobject of this invention is to provide the art of selective extraction of polycyclic aromatic I hydrocarbons with novel solvents, viz., certain thi'olsulionic esters. a An additional object of our r1 invention is to provide solventsfor selectiveextraction "processes. characterized by high selec- '-.tivity for polycyclicaromatic:hydrocarbons but I exhibiting little or no solvent capacity for allphatijrhydrocarbons, particularly. saturated hydrocarbons Yet another object of our invention is to provide thefartwith novel selective extraction processes and. solvent recovery procedures.

' These and other objects of our inventionwill be readily discerned from the ensuing description :"thereof'and from the appended drawing which is a schematic flow sheet illustrating our process.

The novel-solventsemployed in theselective wextraction process of'this invention are th-iolsul- .z'fonic esters containing between 2 andcarbon I a'toms, inclusive, permolecule/and havingthe general formulua:

' wherein R1 and Rz may be thesame or difierent andare selected from the classconsisting" ofhydrocarbon radicals and-substituted hydrocarbon radicals. Ordinarily, it ispreferred toemploy thiolsulfonic esters wherein R1 and Rzare'unsubstituted hydrocarbon radicals containing no aliphatic unsaturation, i. e., wherein Rr-and R2 'are selected from the class consisting of 'sat- 'uratedhydrocarbon radicals and aromatichyand R2 radicals in the aboveformula may-:con-

tainneg-ative substituents such as'halogen-atoms,

nitro groups, amino groups or the like.

" Examples, given by'way of illustration and not "necessarily for liini-tative purposes, of suitable R1 or R2 alkyl radicals are:- methyl, ethyl, n-propyl,

isopropyl, 'n-butyl, mixtures containingtwo' or more of these, amyl, octyl, nonyl, hexadecyland 1 'octadecyl. Illustrative. examples of...suitabl'e' R1 R2 cycl'oalkyl radicals include cyclopentyl,

- methylcyclopentyl," cyclohexyl, methylcyclohexyl,

idimethylcyclohexyl, ethylcyclohexyLrethylcyclopentyl, and endomethylen'ecyclohexyl' :(bicyclo [2,2,1] heptyl) and Z-methyl-bicyclo [2,2,1] heptyl. Examples of suitableR1 or Rs aralkyl radicals in the above general formula-include naphthomethyl, and the'like.

benzyl, 2-phenylethyl, 2-phenylpropy1=,-w-xylyl,

Examples of suitable R1 or R2 cycloalkyl-alkyl radicals=include cyclohexylmethyl, cyclohexylethyl, or cyclopentylpropyl, and the like. Examples of suitable R1 or R2 aryl radicals inc1ude -=pheny1-, naphthyl 3 and derivatives containnig nuclearly substituted chlorine atoms, nitro groups, etc. Examples of suitable R1 or R2 alkaryl radicals for substitution in the above general formula are tolyl, xylyl, dimethylphenyl, ethylphenyl, isopropylphenyl, butylphenyl, methylnaphthyl, dimethylnaphthyl, and the like. Examples of suitable R1 or R2 cycloalkyl-aryl radicals include cyclohexylphenyl, methylcyclopentylphenyl, cyclohexyltolyl, and the like. Specific examples of selective solvents for employment in the extraction process of the present invention include the following esters of methanethiolsulfonic acid: methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, n-amyl, octyl, dodecyl, hexadecyl, octadecyl; the corresponding esters of ethanethiolsulfonic acid; the corresponding esters, not however containing more than 20 carbon atoms per molecule, of l-propanethiolsulfonic acid, 2-propanethiolsulfonic acid, butanethiolsulfonic acid, pentanethiolsulfonic acid, hexanethiolsulfonic caid, octane'thiolsulfonic acid, dode'canethiolsulfonic acid, etc. Other specific examples include methyl benzenethiolsulfonate, methyl-ptoluenethiolsulfonate, ethyl benzenethiolsulfonate, phenyl benzenethiolsulfonate, chlorophenyl chlorobenzenethiolsulfonate, benzyl benzenethiolsulfonate, phenyl 3,5-dinitrobenzenethiolsulfonate, ethyl bisbenzenedithiolsulfonate, etc. It should be understood that we may employ mixtures of esters rather than individual esters. Be-

cause of their high selectivity and cheapness, we

prefer to employ esters of the above general formula wherein R1 and R2 are alkyl radicals containing from one to four carbon atoms.

Although there has been some dispute concerning the chemical structure of thiolsulfonic esters, sometimes called disulfoxides, the concensus of evidence favors the general formula which we have given above. The structures CH3SO2-SCH3 and C2H5SO2SC2H5 have, for example, been assigned by us on the basis of infrared spectra showing strong absorption bands at 7.6 and 8.8 microns, due to the SO2- group and the absence of absorption in the region of 9.l-10.0 microns, indicating the absence of the SO-- group. Structure assignment has also been based upon the discovery by us of a new acid-catalyzed addition reaction of thiolsulfonates to olefins to yield 1,4-sulfonethioethers, rather than the lA-disulfoxides which would be the reaction products expected from the symmetrical disulfoxide structure R1-SOSOR2. The selective solvents which we prefer to designate as sters of thiolsulfonic acid are prepared preferably by the oxidation of the corresponding disulfide. They can be prepared non-catalytically with dilute nitric acid, that is, about 40% nitric acid or with hydrogen peroxide as oxidizing agents at ambient temperatures up to about 80 C. Other oxidizing agents may be used. The disulfides also may be oxidized catalytically using air or oxygen as the oxidizing agent and the higher oxides of nitrogen as catalyst as taught in U. S. 2,433,395. The esters of thiolsulionic acid may also be prepared by other methods well known in the art.

It is not intended to imply that all the thiolsulfonates contemplated for use in this invention are precisely equivalent. However, they are all generally useful and suitable for the purposes of eral formula are either normally liquid or relatively low melting solids and may usually be used for the purposes of the present invention without auxiliary solvents or diluents. However, because of their solubility in hot water, methanol, ethanol, benzol, etc., it may be desirable to employ the solvents of the present invention together with more or less of such diluents or co-solvents in order to modify the selectivity of the thiolsulfonates, to lower their melting points, or for other reasons. The amount of auxiliary solvents employed can be selected with reference to specific cases; ordinarily, between about 1 and about 20 weight per cent based on the thiolsulfonate will be employed. Anti-solvents or diluents may also be employed in the practice of the present invention. Thus, diluents such as saturated hydrocarbons may be added to the feed stock to be dearomatized or introduced directly into the extraction zone. In the process of the present invention the selective solvent is employed as a liquid, melt or solution and the feed stock may be charged to the process as a liquid or solution.

In general, the extraction operations of the present invention are conducted at temperatures between about 20 C. and about 150 C., the particular extraction temperature depending upon the specific thiolsulfonate solvent, the melting point of the solvent, whether or not it is used alone or with an auxiliary solvent or diluent, the degree and selectivity of extraction sought to be effected, etc.

The volume of selective solvent employed depends, among other things, upon the polycyclic aromatic hydrocarbon and organic sulfur content of the feed stock, the temperature of operation and desired efficiency, but will generally fall within a range of about 0.2 to about 0 volumes of thiolsulfonate per volume of charging stock. Ordinarily, we prefer to employ between about 0.5 and about 2 volumes of solvent per volume of feed stock. Usually, pressures within the range of about 0 to p. s. i. g. are suflicient for this purpose, it being appreciated that the particular pressure necessary in a given case can readily be determined. The extractive process of the present invention is conducted under essentially neutral or slightly alkaline operating conditions.

Numerous hydrocarbon oil fractions derived from petroleum, coal, shale, etc., are known to contain polycyclic aromatic hydrocarbons and organic sulfur compounds whose removal is desired in order to produce refined hydrocarbon oils. Such oils may boil within the boiling range of (and can be generally characterized as) kerosene, Virgin gas oil, cracked gas oil, hydroformer bottoms, heater oil, furnace oil, diesel fuel, transformer oil, crud oil, reduced crude oil, visbroken crude oil, lubricating oil, etc. The present refining process can be applied for the purpose of desulfurizing and removing aromatic hydrocarbons and particularly polycyclic aromatic hydrocarbons from various petroleum stocks which are to be subsequently treated in refining or conversion operations in which polycyclic aromatic hydrocarbons, sulfur or sulfur compounds are undesirable, for example, catalytic cracking operations, catalytic reforming operations, catalytic hydroforming operations, catalytic hydrogenation or dehydrogenation in the presence of sulfur-sensitive, readily-coked catalysts, and the like.

The present process may also be applied to the refining of various coal tar fractions and coal tar distillates. In the refining of shale oil fractions the present refining agents serve not only gen compounds.

. presence of liquid acid catalysts.

to remove polycyclic aromatic hydrocarbons and organic sulfur compounds from the feed stock, but also to remove oxygen compounds and nitro- It should be understood that the above specific examples of charging stocks which may be refined in accordance with the present invention are illustrative only and are not intended to delimit the field of applicability .of the process of the present invention.

The present invention can be carried out in batch, continuous or semi-continuous operating cycles, and in one Or more actual or theoretical stages, employing contacting and separation equipment such as has heretofore been employed in the selective solvent refining of petroleum lubricating oil stocks or in effecting the alkyl-ation of isoparamnic hydrocarbons with olefins in the It should be understood that the specific equipment employed forms no part of the present invention and that any equipment adaptable for the purposes of contacting the solvent with the hydrocarbon charging stock and thereafter separating an extract phase from the refined charging stock can be employed for the purposes of the invention.

The data in the table afford a comparison of the performance of methyl methanethiolsulfonate in cracked cycle stock extraction with that of silica gel, nitromethane, furfural, dimethylformamide, S02, and diethylsulfoxide, which are among the most highly regarded solvents for the extraction of polycyclic aromatic hydrocarbons from the type of feed stock here under consideration. As a measure of the selectivity of the solvents for the extraction of sulfur compounds, the so-called aromatic selectivity factor (ASF) is determined in accordance with the following equation:

Percent dearomatization Volume percent extracted wherein Percent dearomatization:

n feed-c raffinate X 100 Wfeed-nfi silica gel percolate on the basis that silica gel percolation of the feed stock effects complete removal of aromatic hydrocarbons from said stock.

Thus, methyl methanethiolsulfonate was superior in aromatic selectivity to all of the excellent liquid solvents and fell only slightly below the highly selective solid adsorbent silica gel.

The light catalytic cycle stock employed in the examples was derived from the catalytic cracking of West'Texas gas oil in the presence of natural clay catalyst and had the following properties:

,AS TM distillation Vol. percent distilled:

I. B. P B. '1 10 B. P., F 455 20 B. P., 36.... 465 30 B. P., F 473 40 B. P., F 482 50 B. P., F 492 60 B. P., F 502 70 B. P., F 511 B. P., F 523 B. P., F 540 E. P B. P., F 578 71. 1.4967 Weight per cent s 1.10 Specific dispersion 156 Bromine No 11 A. P. I 29.3 Vol. per cent polycyclic aromatic hydrocarbons per cent 26.0

The following examples are intended to illustrate but not necessarily to limit the invention:

1 Example 1 Light catalytically cracked cycle oil having the properties set forth in the above table was agitated at room temperature and atmospheric pressure with an equal volume of methyl methanethiolsulfonate and the mixture was allowed to settle. The treatment of 50 volumes of cycle stock yielded 58 volumes of a dark-colored bottom or extract layer and 42 volumes of a top or raffinate layer of much lighter color than the feed stock. The raffinate layer was washed with boiling water in order to remove occluded solvent and was then found to have a refractive index (12 of 1.4798

. and a sulfur content of 0.72 weight per cent. The refractive index of the ramnate was further reduced to 1.4797 after caustic and water washing. The refractive index of the completely dearomatized feed, prepared by silica gel percolation, was 1.4500. Thus, the extraction process effected 36% dearomatization of the feed and 35% desulfurization, with the production of 16% of extract. The aromatic selectivity factor (ASF) was 225.

Example 2 The raffinate obtained by the process of Example 1 was agitated with 1.15 volumes per volume of methyl'methanethiolsulfonate to obtain a second raffinate having the refractive index of 1.4691. The second rafiinate was agitated with 1.17 volumes per volume of the same solvent to obtain a third raffinate with the refractive index of 1.4626. The third rafiinate was agitated with an equal volume of the same solvent to obtain a fourth raffinate with the refractive index of 1.4583. Thus, four batch extraction of the light catalytic cycle oil with methyl methanethiolsulfonate, employed in a total ratio of 4.32 volumes per volume of feed stock, resulted in 83% dearomatization of the feed stock and the production of 32 volume per cent of extract phase.

Example 3 The light catalytically cracked cycle oil employed as the feed stock in Example 1 was agitated for 5 minutes with an equal volume of methyl methanethiolsulfonate which had previously been saturated with ammonia gas and fil- 'tered. Within less than 1 minute after agitation had been discontinued, a sharp separation of a clearorange bottomor extract phase and a pale yellow topor rafiinate layer was observedtoocour. The amount of extract was 16 volume per cent of the feed stock. The water-washed rafiinate phase had a refractive index (11 of 1.4800. While the aromatic selectivity factor in this operation was about the same as that in Example 1, the rate of separation of phases and the colors of both the ralfinate and extract phases were all superior to those obtained in Example 1.

Example 4 We have observed that the selectivity of methyl methanethiolsulfonate for monocyclic aromatics is lower than that for polycyclic aromatic hydrocarbons. Thus, the extraction of a synthetic mixture containing equal volumes of benzene and n-heptane with 1 volume per volume of methyl methanethiolsulfonate at room temperature eifected 49% dearomatization and. the production of 45%, based on feed, of extract phase, corresponding to an aromatic selectivity factor of 111. In another test a 20% solution of amyl benzene in dodecane was extracted with an equal volume of methyl methanethiolsulfonate at room temperature. Dearomatization amounted to 10.6% with an observed aromatic selectivity of 181. An extraction of 20% methyl naphthalene in dodecane gave 56% dearomatization and an observed aromatic selectivity factor of 400. The extractions described in this example, that is Example 4, were carried out in the same manner as the procedure used in Examples 1, 2 and 3 above.

Example 5 As a further example of our process, 5 volumes of a light catalytically cracked cycle oil used as the feed stock in Example 1 were shaken for 5 minutes with 5 volumes of a mixture of alkyl alkanethiolsulfonates (C1-C4 range, prepared by the oxidation of a mixture of C1-C4 alkyl disulfides with dilute nitric acid) and the resultant mixture was allowed to stand, whereupon it settled to form 6.8 volumes of a bottom or extract layer and 3.2 volumes of a top or raffinate layer which had a substantially lighter color than the feed stock. The refractive index of the .rafhnate was 1.4708, as against 1.4967 of the feed stock and 1.4500 of completely dearomatized feed stock prepared by percolation of the feed stock through silica gel. Thus, 55.5% dearomatization of the feed stock was achieved with the production of 36 volume per cent, based on feed stock, of extract materials, corresponding to an aromatic selectivity factor of 154.

Example 6 In another test, 5 cc. light catalytic cycle stock with a refractive index of n :1.4967 was shaken for five minutes with 5 cc. of phenyl benzenethiolsulf-onate. This latter material was prepared by the oxidation of phenyl disulfide with hydrogen peroxide. It was used in the form of a supercooled liquid. The mixture was then allowed to stand, upon which it settled to 7.0 cc. dark colored bottom (extract) phase, and 3.0 cc. top (raffinate) phase which had better color than did the feed stock. The rafiinate phase contained about 0.1 cc. solvent which was removed by crystallization at low temperatures in the presence of pentane. After removal of the pentane the raihnate had a refractive index of 11 14684. The refractive index of the completely dearomatized feed (prepared by silica gel percolation) has been found to be n :1.4500. The refractive index of the raifinate corresponds to 61% dearomatization of the feed with 42% loss of feed into the extract phase.

ExampZe 7 In still another experiment, the above described light catalytic cycle stock was mixed with an equal volume of molten, peroxide-oxidized phenyl methyl disulfide which contained two gram atoms of oxygen per gram mol of product. Intimate contact of the components of the mixture was obtained by agitation at a temperature of 100 C. for a period of live minutes after which, upon standing, phase separation was obtained to give a dark colored extract layer equal to a of the total volume and a rafflnate layer of light colored oil equal to 30% of the volume of oil and extractant, mixture. Extractant was separated from the rafiinate by crystallization. The rafiinate, substantially free of extractant, mounted to 60% of the catalytic cycle stock used in the experiment, which was 49% dearomatized contact with phenyl methylthiolsulfonate.

An illustrative process flow and equipment are indicated in the drawing. The hydrocarbon mixture to be extracted, containing polycyclic aromatic hydrocarbons, and optionally organic sulfur compounds, for example, a light cycle stock from silica-alumina cracking of a West Texas gas oil, is passed through valved line I 0 into the contacting zone H. The feed stock is preferably dried and, if it contains hydrogen sulfide, is treated to eifect its removal, e. g., by washing with caustic, ethanolamine, N-dimethyl ethanolamine, etc.

We may employ conventional contacting and separation equipment such as has heretofore been employed in effecting selective solvent extraction of lubricating oils, illuminating oils, etc. The contacting equipment may comprise a vertical tower, which is preferably provided with packing or spacing materials to insure thorough contacting of the hydrocarbon feed stock and the thiolsulfonate solvent. Suitable materials of construction for the contacting zone are aluminum and stainless steel, although it should be understood that we may employ other construction materials, for example, glass-, ceramicor carbon-lined iron towers. Suitable spacing materials comprise shaped fragments, for example Berl saddles 0r Raschig rings made of carbon, porcelain, glass, aluminum, stainless steel, etc.; stainless steel jack chain; stainless steel or aluminum screens which may be shaped, for example, in the form of Scofield, McMahon or Stedman packing, etc. If desired, the contacting tower may be jacketed or provided with heat exchange coils to permit maintenance of the desired temperature.

In a desirable method of operation, the feed stock is passed into the lower portion of zone H against a counter-flow of a thlolsulfonate, for example, methyl methanethiolsulfonate, which is introduced into the upper portion of tower H through valved line l2. Diluents, e. g., pentane or hexane, or modifying solvents, e. g. ethanol, may be introduced with the feed stock, the thiolsulfonate solvent or separately through a line not shown, into zone H. Alkaline conditions during extraction can be maintained by introducing ammonia (gas) into zone II (by a line not shown).

The contacting or" the thiolsulfonate and hydrocarbon feed stock results in the production of raffinate and extract phases whose common boundary or interface is indicated at 3. The countercurrent extraction zone may be operated 9 with either the feed stockor'solvent as thecontinuous phase. In the mode of operation, illustrated in the drawing, the extract phase is shown as the continuous phase through which the hydrocarbon feed stock is introduced as the dispersed phase.

The raffinate phase forms a supernatant layer above interface 13 in zone ll, whence it is discharged, tcgether with the diluent employed, from said zone through valved line it. The raffinate phase is characterized by a substantially reduced content of polycyclic aromatic hydrocarbons and organic sulfur compounds as compared with the feed stock. It should be understood, however, that the rafiinate phase may be further treated to reduce its content of polycyclic aromatic hydrocarbons and organic sulfur compounds, if it is so desired. For this purpose, a portion at least of the raflinate phase may berecycled from line it by a line (not shown) to reenter zone i! with fresh feed stock. Alternatively, or in addition, the raffinate in line :4 may be sent to another contacting zone, identical in all substantial respects with zone Ii, for treatment in a second stage with fresh solvent, which may be the same thiolsulfonate or a different thiolsulfonate from that passing into line 12, or may be an entirely different type of solvent, e. g., liquid hydrogen fluoride, HF-BF3, liquid S02, phenol, furfural, dimethylformam'ide; bis-(betachloroethyl) ether, etc. It should be noted that further extraction of the rafiinate passing through line Hi with solvents different from the thiolsulfonate passing through line i2 is greatly facilitated by the fact that the thiolsulfonate is substantially insoluble in said r-aifinate, averting the necessity of special procedures for the removal of th'iolsulfonate from said raffinate. Small amounts of thiolsulfonate entrained in the raffinate can be removed by washing with hot water or aqueous hydrochloric acid.

The extract phase, a solution of polycyclic aromatic hydrocarbons and organic sulfur compounds in the thi'olsulf-onate solvent, is Withdrawn from the lower end of tower ll through valved line l5, whence it is passed into separation zone iii. If desired, a part of the extract phase may be recycled from line It to feed line It by means of a line not shown in the figure. Zone to may be a separating vessel into which hot water or aqueous solvents are introduced by valved line H, in suitable amounts, e. g., between about 0.5 and about 3.0 volumes per volume of extract, at a temperature between about 20 and about 100 C., to effect the resolution of the extract into an aqueous thiolsulf'onate layer and a second layer containing polycyclic aromatic hydrocarbons and organic sulfur compounds (and co-solvent, if any) employed in the extraction operation; said second layer flows overhead through line It! into settler It, wherein occluded water is discharged through line 20 and extracted materials through line 2!. Methyl methanethiolsulfonate is soluble to the extent of about 15 g. per 100 g. of water at 65 C. and about 6 g. per 100 g. of water at C; its solubility in 37.7% hydrochloric acid is about 10 times its solubility in water at the same temperature and the H01 solution of the thiolsulfonate can be resolved into two layers by blowing off I-ICl gas.

Alternatively, the separation zone It may take the form of a distillation vessel, preferably a steam distillation vessel in which the polycyclic aromatic hydrocarbons and sulfur com-pounds are vaporized and pass overhead, usually as azeotropes with water, through line l3 into a settling aqueous solution of thiolsulfonate in settler 23.

10 vessel [9, whence the water layer is withdrawn through valved line 20 and extracted; materials are withdrawn through valved line 2|. The extracted materials may be employed as plasticizers for vinyl resins, natural and synthetic rubber, asphalt, etc.

A bottoms fraction containing the thiolsulfonate is withdrawn from zone [5 through insulated line 22 into a settler 23 provided with weir 24 and cooling coil 25. The temperature of the is reduced to about room temperature or even to a lower temperature, e. g., down to about 10 C. in order to reduce the solubility of the thiolsulfonate solvent in said aqueous phase. If desired, CaClz, NaCl, MgSO4 or other salts may be added to the aqueous phase in settler 23 in order to, reduce the solubility of the'thiolsulfonate solvent therein. Two immiscible liquid layers are produced in settler 23, one of which is water or an aqueous salt solution and the other of which comprises essentially the thiolsulfonate solvent. The thiolsulfonates are in general of higher specific gravity than water but may be of lower specific gravity than the aqueous salt solutions employed in settler 23. The two layers are separately withdrawn from settler 23' at the appropriate points. For illustrative purposes, in the figure, anaqueous salt solution is shown being withdrawn from settler 23 through valved line 26- and the thiolsulfonate solvent through valved line 21, whence it may be recycled wholly or in part through valved line 28 and manifold 29 to line 12 and thence to extraction column ll. Preferably, at least a portion of the thiolsulfonate solvent is directed from line 21 into a distillation column as provided with a heating or reboiler coil 3!. Column 30 can suitably be a vacuum still from which steam can be separated from the wet solvent and removed overhead through line 32 into a barometric condenser 33. The dried solvent is discharged as bottoms through line 34 into manifold 28 for recycle to the extraction zone I I.

Alternatively, the recycle stream of solvent may be dehydrated in column 3!) by'azeotropic distillation of water therefrom with various azeotroping agents, e. g. benzene, toluene, n-heptane, ethyl acetate, etc.

Although the drawing represents a continuous countercurrent extraction operation, it will be apparent that the process of the present invention is amenable to batch proceessing, multi-unit operation, concurrent flow of solvent and feed stock, the use of knot-hole or other mechanical mixers for feed and solvent in series with one or more settling zones and other variations that will no doubt occur to those skilled in the art without departing from the spirit of this invention.

In our copending United States application for Letters Patent, Serial Number 244,374, filed of even date herewith, we have described and claimed the selective extraction of organic sulfur compounds from their mixtures with hydrocarbons by the employment of the aforesaid esters of thiolsulfonic acids.

Having thus described the process of our invention, what we claim is:

1. A process for the extraction of an aromatic hydrocarbon from a hydrocarbon mixture containing the same, which process comprises contacting said mixture with an ester of a thiolsulfonic acid containing 2 to 20 carbon atoms per. molecule and separating a raffinate phase from an extract phase comprising said ester and said aromatic hydrocarbon.

2. The process of claim 1 wherein the aromatic hydrocarbon is polycyclic.

3. The process of claim 1 wherein said mixture comprises a cracked gas oil containing polycyclic aromatic hydrocarbons.

4. A process for the extraction of a polycyclic aromatic hydrocarbon from a hydrocarbon mixture containing the same, which process comprises contacting said mixture with an ester of a thiolsulfonic acid containing 2 to 20 carbon atoms per molecule in liquid condition at a temperature between about 20 C. and about 150 0., and separating a railinate phase from an extract phase comprising said ester and said polycyclic aromatic hydrocarbon.

5. The process of claim 4 wherein said ester has the formula R1SO2SR2 wherein R1 and R2 are alkyl radicals containing 1 to 4 carbon atoms.

6. The process of claim 4 wherein said mixture comprises a cracked gas oil and said ester has the formula R1SO2-SR2 wherein R1 and R2 are alkyl radicals containing 1 to 4 carbon atoms.

'7. A process for the extraction of a polycyclic aromatic hydrocarbon from a hydrocarbon mixture containing the same, which process comprises contacting said mixture with an ester of a thiolsulfonic acid containing 2 to 20 carbon atoms per molecule in liquid condition, separating an extract layer containing said ester and said polycyclic aromatic hydrocarbon, selectively extracting said ester from said extract layer with an aqueous extractant at a temperature between about 20 C. and about 100 C. to produce an aqueous ester, drying said aqueous ester and recycling the resultant dried ester to further contact with said mixture.

8. The process of claim '7 wherein said ester has the formula RISO2SR2 wherein R1 and R2 are alkyl radicals containing 1 to 4 carbon atoms 9. The process of claim 7 wherein said mixture comprises a cracked gas oil and said ester has the formula R1SO2SR2 wherein R1 and R2 are alkyl radicals containing 1 to 4 carbon atoms.

l/VlLLIAM F. WOLFF'. CARL E. JOHNSON.

No references cited.

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