US3470088A

US3470088A - Method for aromatic hydrocarbon recovery

Info

Publication number: US3470088A
Application number: US680857A
Authority: US
Inventors: Anthony G Vickers
Original assignee: Universal Oil Products Co
Current assignee: Universal Oil Products Co
Priority date: 1967-11-06
Filing date: 1967-11-06
Publication date: 1969-09-30
Anticipated expiration: 1986-09-30

Description

Sept. 30, 1969 A. G. vlcKERs METHOD FOR AROMATIC HYDROCARBON RECOVERY Filed Nov. 6, 1967 /N VEN TOR Anl/)any G. Vic/fers United States Patent 3,470,088 METHOD FOR AROMATIC HYDROCARBON RECOVERY Anthony G. Vickers, Raynes Park, England, assignor to Universal Oil Products Company, Des Plaines, Ill., a

corporation of Delaware Filed Nov. 6, 1967, Ser. No. 680,857 Int. Cl. C07c 7/02; C10g 21/06' U.S. Cl. 208-321 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to the extraction and recovery of aromatic hydrocarbons from feedstocks utilizing solvent extraction. It particularly relates to an improved method for the recovery of aromatic hydrocarbons from the extract phase from such solvent extraction operation.

It is known in the art that a conventional process for the recovery of high purity aromatic hydrocarbons from various feedstocks, such as catalytic reformate, PYrolysis gasoline, etc., is liquid-liquid extraction utilizing a solvent, such as diethylene glycol or sulfolane, each of which has high selectivity for the aromatic hydrocarbon components contained in the feedstock. Typically, in the practice of such prior art process, a hydrocarbon feed mixture is contacted in an extraction zone with an aqueous solvent composition which selectively dissolves the aromatic components from the hydrocarbon feedstock thereby forming a rainate phase comprising one or more non-aromatic hydrocarbons and an extract phase comprising solvent having aromatic components dissolved therein. The extract phase is then usually distilled, yielding an overhead distillate containing a portion of the extracted aromatic component, a side-cut fraction comprising aromatic hydrocarbons, and a bottoms fraction comprising lean solvent generally suitable for reuse in the extraction zone. Frequently, to prevent losses of the solvent, the railinate phase is washed with water in a washing zone in order to remove solvent from the rainate phase.

Also, not infrequently, the extract phase is subjected to extractive distillation in order to remove a contaminating quantity of nonaromatic hydrocarbons from the extract phase. The extractive distillation operation is normally performed in order to make possible the recovery of nitrogen grade aromatic hydrocarbons, such as benzene and toluene. In many cases it is common practice in the prior art to introduce added solvent into the extractive distillation column in order to enhance the separation therein of the non-aromatic hydrocarbon contaminants from the desired aromatic hydrocarbons. The source of the added solvent is either fresh solvent introduced from the outside or a returning portion of lean solvent from the solvent regeneration facilities or the solvent recovery column associated with the process. Therefore, a typical prior art process for the recovery of aromatic hydrocarbons en- Compasses a solvent extraction step, an extractive distillation step, and a iinal distillation step for the recovery of the aromatic hydrocarbons from the solvent phase.

As previously mentioned, in the extractive distillation operation, it is commen practice to add to the extract Mice phase considerable quantities of additional solvent so that the relative volatilities between a nonaromatic hydrocarbon component and an aromatic hydrocarbon component are substantially increased in order to electuate an almost complete separation between the two via distillation. This, of course, requires a distillation column of some complexity utilizing large quantities of utilities, such as steam for heat input in order to properly perform the distillation. However, the extractive distillation column is severely limited in the amount of heat input which is possible because care must be taken to minimize the quantity of aromatics in the overhead ash which would represent a loss in yield of desirable aromatic hydrocarbons by virtue of adding inefficiencies to the extraction operation to which the overhead stream is normally returned. Accordingly, the extractive distillation operation achieves a balance between the desire to remove non-aromatic hydrocarbons from the aromatic hydrocarbons and the desire to maximize the recovery of aromatic hydrocarbons.

In similar fashion, the operation of the aromatic recovery column is one of achieving proper balance. It would generally be desirable to have the feed to the aromatic recovery column as high as possible in temperature so that a minimum amount of reboiler heat may be added to the column. It is also desirable to control the temperature of the feed to the recovery column since relatively small changes in temperature have a large effect on the heat balance of the column. In short, the achievement of thermal balance around the entire process for the recovery of aromatic hydrocarbons is of utmost importance if the economics of the process are to become commercially desirable.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a method for the recovery of aromatic hydrocarbons from the extract phase of a solvent extraction operation in a more facile and economical manner.

It is another object of this invention to provide an im proved method for the separation and recovery of aromatic hydrocarbons from a suitable feedstock in a facile and economical manner.

Thus, according to one embodiment of this invention there is provided a method for recovery of aromatic hydrocarbons from the extract phase from a solvent extraction zone which comprises the steps of: (a) introducing an extract phase containing solvent having aromatic hydrocarbons dissolved therein, said extract also contaminated with nonaromatic hydrocarbons, into a iirst distillation zone maintained under distillation conditions including the presence of hereinafter specified added solvent stream; (b) withdrawing from said iirst zone a distillate fraction containing said contaminants and a bottoms fraction comprising solvent and aromatic hydrocarbons; (c) passing said bottoms fraction into a `separation zone maintained under conditions sufficient to produce a. vapor stream comprising solvent and aromatic hydrocarbons and a liquid stream comprising solvent contaminated with aromatic hydrocarbons; (d) introducing said vapor stream of step (c) together with a controlled portion of said liquid stream of step (c) into a second distillation zone maintained under distillation conditions; (e) cooling the remaining portion of said liquid stream; (f) withdrawing from said second distillation zone a distillate product stream comprising aromatic hydrocarbons and a lean solvent stream substantially free of aromatic hydrocarbons; and, (g) returning said cooled portion of said liquid stream of step (c) to step (a) as said added solvent.

Another embodiment of this invention includes the method hereinabove wherein said solvent comprises sulfolane.

It is noted from the hereinabove brief description of the present invention relative to the prior art that significant economies of operation are achieved by the expedient of introducing the bottom stream from the extractive distillation column into a separation zone, more fully discussed hereinbelow, for the purpose of producing a vapor stream comprising aromatic hydrocarbons and solvent, and a liquid stream comprising solvent. A controlled portion of the liquid is introduced into the aromatic recovery column lin an amount suflicient to provide adequate lean solvent for extraction purposes, as more fully discussed hereinafter. 'I'he remaining liquid stream is cooled and returned to the extractive distillation column as added solvent thereby, in effect, by-passing the solvent recovery column, and by doing so, effects considerable economies in the operation of the solvent recovery column. It was discovered that by operating in this manner the capital expense and operating expense, particularly, for the aromatic recovery column, were signilicantly decreased and that the amount of aromatic hydrocarbons in the solvent being returned to the extractive distillation operation did not, in any way, decrease the ultimate yield of aromatic hydrocarbons, nor did it effect in any adverse way the elciency of removal of any nonaromatic hydrocarbons in the extractive distillation operation.

The hydrocarbon feedstock which may be separated by the improved method of the present invention comprises many different aromatic-nonaromatic mixtures. Typically, feedstocks applicable to the solvent extraction step include hydrocarbon distillate fractions (usually boiling within or near the gasoline boiling range) of natural gasoline or straight-run petroleum distillates and especially comprise reformed naphthas which are rich in aromatic compounds and which are particularly valuable as a source of mono-nuclear aromatic hydrocarbons, such as benzene, toluene, and xylene. Still further, the recovery of high purity xylene, for example, from pyrolysis gasoline fractions presents unique problems to this processing scheme due to the relatively high boiling nature of the non-aromatic contaminants in the xylene fraction.

Accordingly, the improved method of the present invention is uniquely applicable to feedstocks comprising a pyrolysis gasoline fraction from which it is desired to recover xylenes in high yield and in Ihigh purity. In each case, for suitable feedstocks of this invention, it is understood that such feedstocks contain non-aromatic hydrocarbons as well as aromatic hydrocarbons; that is, the aromatic extract from the solvent extraction step comprises solvent having aromatic hydrocarbonsdissolved therein, but this extract is also contaminated with nonaromatic hydrocarbons. Typically, the feedstock of the present invention as charged to the aromatic extraction step will contain from about 30% to about 60% by weight aromatic hydrocarbons, although, aromatic concentrations as high as 95% by weight may be used in some cases.

Solvent compositions which may be utilized in the practice of the present invention are those selected from the classes which have high selectivity for aromatic hydrocarbons. These aromatic selective solvents generally contain one or more organic compounds containing in their molecule at least one polar group such as hydroxyl, amino, cyano, carboxyl, or nitro radical. In order to be eective, the organic compounds of the solvent composition having the polar radical must have a boiling point substantially greater than the boiling point of water which preferably is included in the solvent composition for enhancing its selectivity, and in general, -must also have a boiling point substantially greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon feed mixture.

Organic compounds suitable for use as part of the solvent composition preferably are selected from the group of those organic-containing compounds which include the aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as the glycol esters and glycol ether esters. The monoand polyalkylene glycols in which the alkylene group contains from 2 to 3 carbon atoms, such as ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol, as well as the methyl, ethyl, propyl, and butyl ethers of the glycol hydroxyl groups and the acetic acid esters thereof, constitute a satisfactory class of organic solvents useful in admixture with water as the solvent composition for use in the present invention. An illustrative glycol comprises triethylene glycol.

Additionally, excellent results may be obtained utilizing the cyclic monomeric sulfone, such as tetrahydrotriophene-l,ldioxide. Still further, an organic compound particularly useful in the practice of this invention is a sulfolane which may fbe made by condensing a conjugated diolefn with sulfur dioxide and then subjecting the resulting product to hydrogenation, alkylation, hydration and/ or other substitution or addition reactions. Typically, organic compounds belonging to the sulfolane class are 2-sulfolene, 2methylsulfolane, 2,4-dimethylsulfolane, 2,4- dirnethyl 4 sulfolane, methyl-3-sulfonyl ether, ethyl-3- sulfonyl sulfide, and others.

The apparatus embodied in the practice of the present invention may be any conventional or convenient type known to those skilled in the art. Also, the operating conditions suitable for the practice of this invention are conventional and well known to those skilled in the art, with exception of the precise temperatures and pressures for operating the extractive distillation column, bottoms flash zone, and the aromatic recovery column, according to the teachings of this invention. In any event, from the teachings presented and from a general knowledge of the art, those skilled in the art will be able to choose the proper operating conditions to achieve the benefits ascribed to the practice of the present invention.

The amount of solvent composition utilized should be at least suflcient to dissolve the constituent to be extracted. It may be desirable to use a considerable excess over the theoretical amount of solvent composition necessary, especially when maximum purty and maximum recovery of the aromatic hydrocarbons are desired. Usually, in the extraction step the solvent composition to feed ratios will range from about 1:1 to about 20:1 by volume, preferably, from about 5:1 to about 15:1 by volume. A summary of the conditions necessary for the practice of the sulfolane-type of operation may be found in Petroleum Rener, volume 38, No. 9, September 1959, pages 18S- 192, the entire disclosure of which is incorporated herein by reference.

The solvent extraction step, as previously mentioned, is well known and may utilize apparatus of any type suitable for effecting counter-current contact between two liquid phases, at least partially, but not wholly miscible with each other and wherein the relatively more dense solvent may be brought into intimate contact with the relatively less dense hydrocarbon phase. Thus, the extraction zone which produces the solvent extract which is used as feedstock to the practice of the present invention may comprise a packed column or may contain a series of horizontal plates through which the liquid solvent flows in dispersed form and in countercurrent flow relationship to the ascending hydrocarbon stream.

The invention may be more fully understood with reference to the appended drawing.

DESCRIPTION OF THE DRAWING Referring now to the accompanying drawing, a suitable feedstock, such as pyrolysis gasoline, is introduced into the process via line 1. If desired, additional nonaromatic hydrocarbons from a source hereinafter described also enters the process via line 37 in admixture with the feed in line 1. Lean solvent is introduced into the upper part of extractor 2 through line 20 and countercurrently contacts the feed entering extractor 2 via line 1. Extractor 2 is maintained at an elevated temperature and pressure, for example, 100 F. and 75 p.s.i.g. suflicient to maintain the solvent and feedstock in liquid phase. Since the solvent has selectivity for the aromatics in the feed, the aromatic hydrocarbons are preferentially dissolved into the solvent phase. The rejected non-aromatic hydrocarbons are withdrawn from extractor 2 via line 23, and if desired, a portion thereof is recycled back to the extractor by flowing through pump 36 and line 37 as previously mentioned hereinabove.

The remaining portion of the raffinate phase continues through line 23 where, in a preferred embodiment of the present invention, it is fractionated and subsequently water washed, thy means not shown, to remove small quantities of dissolved solvent. The details of the splitter operation on the rainate phase will be more fully developed hereinbelow.

The operating conditions chosen for extractor 2 are sufficient to provide substantial dissolving of the aromatic hydrocarbons into the selective solvent. However, in all commercial practices, the eiciency of aromatic removal is influenced to a considerable extent by the amount of non-aromatic hydrocarbons which are also left behind in the solvent phase. Therefore, in a preferred embodiment of this invention a light non-aromatic hydrocarbon stream commonly called a back-wash stream is introduced into the lower portion of extractor 2 via line 11 wherein it counter-currently contacts the extract phase being withdrawn from extractor 2 via line 3. This back-washing step apparently results in the displacement from the extract phase of the heavier non-aromatic contaminants which would have been carried into the extract phase. Basically, the object of the back-washing is to displace the heavy feed non-aromatic hydrocarbons with lighter non-aromatic hydrocarbons, and this is conveniently done by introducing a light, primarily non-aromatic, hydrocarbon fraction into the lower point of extractor 2.

The extract phase comprising solventhaving aromatic hydrocarbons dissolved therein, but also still being contaminated with non-aromatic hydrocarbons, is withdrawn from extractor 2 via line 3 and passed into extractive stripper column 4. The extract phase material in line 3 is mixed with an added solvent stream, at a temperature substantially the same as the temperature of the extract phase, from line 13, the source of which is more fully described hereinafter.

The operating conditions for distillation column 4 are conventional in that sufficient heat must be added to the column in order for separation to take place between the non-aromatic hydrocarbons and the aromatic hydrocarbons contained in the solvent. Typical operating conditions for extractive stripper column 4 for use with, say, sulfolane type solvent, include a pressure from 90 mm. Hg to 15 p.s.i.g., an overhead temperature from 140 F. to 330 F. and a bottoms temperature from 170 F. to 370 F., typically, about 350 F.

Operating under these hereinabove described conditions a relatively light non-aromatic hydrocarbon upper stream is withdrawn from column 4 via line 5. It was found iu the practice of the present invention that when processing feedstocks from which high purity xylenes are desired, the extractive stripper 4 will have difficulty in removing the heavier non-aromatics overhead without also vaporizing the lighter boiling aromatic hydrocarbons. For example, if the C9 naphthenes are not removed in this stripper, they remain in the aromatic hydrocarbon product when the solvent is subsequently removed from the extract phase. This, of course, results in the production of undesirably low purity C8 aromatic hydrocarbons. Therefore, the heavier nonaromatic hydrocarbons are displaced by the lighter nonaromatic hydrocarbons in the back-wash step to permit the lighter non-aromatics to be more readily vaporized in the operation of extractive stripper 4.

The relatively light non-aromatic hydrocarbon overhead stream is removed from stripper 4 through line 5, condenser 6, line 7, and into overhead receiver 8. This overhead material is condensed and separated into two phases: one a water-solvent phase and the other a light hydrocarbon phase. The relatively light hydrocarbon phase is withdrawn from receiver 8 via line 11 and recycled to the lower end of extractor 2, as previously mentioned. The water-solvent phase settles in boot 9 from which it is withdrawn via line 10 and usually sent to solvent recovery means, not shown. A bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein is withdrawn from extractive stripper 4 via line 12 land a portion thereof passed via line 38 through reboiler heater 39 for return to stripper 4 as a means of providing distillation heat therein. The remaining bottoms fraction passes via line 12 into ash column 44.

In the practice of this invention, ash column 44 is simply an equilibrium flash pot wherein a vapor stream comprising solvent and aromatic hydrocarbons is produced and withdrawn via line 45, The unvaporized liquid stream comprising primarily solvent Ahaving aromatic hydrocarbons remaining therein is withdrawn from ash column 44 via line 13.

The operating conditions applicable to ash column 44 include those which are suicient to ash into the vapor stream a large portion of the aromatic hydrocarbons. The precise operating conditions will depend, of course, on the kind and quantity of aromatic hydrocarbons contained in the solvent and on the particular volatility characteristics of the solvent and of the solvent-hydrocarbon mixture. For the case Where the solvent is sulfolane and the aromatic hydrocarbons comprise a mixture of benzenes, toluenes, and xylenes, typical operating conditions include a temperature of about 310 F. and a pressure of about 450 mm. Hg absolute. If necessary for proper temperature control in the ash zone of column 44, the material in line 12 may be passed through au auxiliary heater, not shown, prior to column 44.

A controlled portion of the liquid stream in line 13 is passed via line 46, mixed with the vapor material in line 45, and the admixture passed into solvent recovery column 14. As used herein, the term controlled portion is intended to embody the concept that the amount of material passed via line 46 is only sutiicient to provide the necessary amount of lean solvent to extractor 2. Alternatively it is with the scope of this invention for the material in line 45 to be sent directly and separately into column 14 by means not shown. In such case, the vapor material would enter column 14 at a locus above the locus for entry of the liquid from line 46.

The remaining liquid stream in line 13 is now passed to cooling means which according to a preferred embodiment of this invention is reboiler 34 which is supplying heat to the previously mentioned rainate splitter column 24. After leaving reboiler 34, the cooled liquid stream in line 13 is passed into admixture with the extract phase in line 3 and ultimately into extractive stripper 4 discussed in detail hereinabove. It was found that the material in line 13 contained sufiicient heat to supply heat for operating :splitter column 24.

The cooling of the material in line 13 is necessary for proper thermal balance of the extractive stripper 4. For example, if the material is sent from line 13 directly to extractive stripper 4, the feed temperature to the column would go up, the amount of heat duty from reboiler 39 would go down and, thus, stripping would be seriously impaired in this column. However, it is distinctly preferred to cool this stream by reboiling the splitter column in order to maximize heat recovery in the system. On the other hand, the invention only requires cooling of this stream and, therefore, embodies any cooling means desired by those skilled in the art.

Returning now to splitter column 24, since in some instances a portion of the relatively light hydrocarbon back-wash material introduced into extractor 2 may pass through the extractor and leave in the railinate phase being withdrawn via line 23, it would be necessary to supply sulicient additional back-wash material makeup for this loss. In a preferred embodiment of this invention this is conveniently accomplished by passing the rainate phase via line 23 into splitter column 24. A light rafnate portion is removed overhead via line 25 where it is passed through condenser 26 into overhead receiver 27. Conventionally, a portion of the condensed light raflnate is returned to column 24 via line 28 .as reux thereon. The remaining portion of the light raiinate is withdrawn from receiver 27 via line 29 and, as necessary for the hereinabove described makeup, passed via line 31 in line 7 into receiver 8 for return to extractor 2 via line 11, as previously mentioned. The remaining portion, if any, of light vraflnate may be withdrawn from the system via line 30 as a net product stream. However, it is preferable that no light rainate net product be taken; rather, all raflinate product is withdrawn from the bottom of column 24.

Thus, a rafnate stream is removed from the bottom of column 24 via line 32 and withdrawn from the system via line 35 as another net product. In order to supply distillation heat to column 24, a portion of the material in line 32 is passed via line 33 through reboiler 34 in indirect heat exchange with the liquid stream in line 13, as previously mentioned. The heated material is passed then from exchanger 34 back into splitter column 24.

Returning now to the operation of flash column 44, as previously mentioned, a vapor stream comprising solvent and relatively pure aromatic hydrocarbons is withdrawn via line 45 and passed into solvent recovery column 14. Additionally, a controlled portion of the liquid in line 13 is also passed via line 46 into column 14. Distillation column 14 is operated in conventional manner for the separation of aromatic hydrocarbons from the selective solvent. Depending upon the volatility characteristics of the solvent, the operating conditions are chosen so that extremely high purity aromatic hydrocarbons may be withdrawn from distillation column 14 via line 15, passed through condenser 16 into overhead receiver 17. In many cases, the operation of the solvent recovery column will include the introduction of steam in the column and the subsequent water produced thereby may be withdrawn from boot 42 via line 43. The relatively pure aromatic product stream containing desirably benzene, toluene. and xylenes is withdrawn from receiver 17 via line 18 wherefrom a portion is returned to the column as reflux and the remaining fraction withdrawn from the system via line 19 as a high purity aromatic hydrocarbon product. The material in line 19 may be subsequently sent to conventional separation means for the recovery therefrom as separate product streams high purity benzene, toluene, and/ or xylenes.

Lean solvent generally suitable for reuse in the extraction process is withdrawn from distillation column 14 via line 20 and a portion therefrom is passed via line 40 through reboiler 41 for the supply of distillation heat to the column. The remaining net product from the bottom of solvent recovery column 14 is passed via line 20 into the upper portion of extractor 2 as the lean solvent therein, previously discussed. If desirable or necessary, a portion of the lean solvent in line 20 may be sent to solvent regeneration means via line 21 and returned from the solvent regeneration means via line 22.

Typical operating conditions for distillation column 14, when utilizing sulfolane as the solvent and desiring to recover a mixture of benzene and toluene as the aromatic hydrocarbons, includa an overhead temperature from 120 F. and 306 F., a bottoms temperature from 300 F. to 350 F. and a column pressure from 50 mm. Hg to 700 mm. Hg absolute. Those skilled in the art, from general knowledge and from the teachings presented herein will know how to choose the proper operating conditions to effectuate recovery of high purity aromatic hydrocarbons from the solvent.

PREFERRED EMBODIMENT From the teachings presented hereinabove, the preferred embodiment of this invention includes the method previously mentioned wherein the separation zone referred to comprises a ash zone maintained under equilibrium conditions.

Another preferred embodiment of this invention provides an improvement in a process for the separation and recovery of aromatic hydrocarbons from a feedstock containing aromatic and non-aromatic hydrocarbons utilizing a solvent extraction step wherein an extract stream comprising aromatic hydrocarbons dissolved in said solvent is withdrawn from one end of a solvent extraction zone, said extract being contaminated with non-aromatic hydrocarbons, and a raflnate stream comprising non-aromatic hydrocarbons is withdrwan from the other end of said zone which comprises the steps of: (a) introducing said extract phase into a trst distillation zone maintained under conditions sufficient to produce an overhead fraction comprising non-aromatic hydrocarbons and a bottoms fraction comprising said solvent having aromatic hydrocarbons dissolved therein (b) passing said bottoms fraction of step (a) into a tlash zone under conditions suflcient to produce a vapor stream comprising aromatic hydrocarbons and solvent and a liquid stream comprising solvent containing aromatic hydrocarbons; (c) passing said raffinate stream into a reboiled splitter column under conditions sulcient to produce a distillate stream comprising relatively light non-aromatic hydrocarbons and a bottoms stream comprising relatively heavy non-aromatic hydrocarbons; (d) passing said distillate stream of step (c) and said overhead fraction of step (a) into said extraction zone under conditions sufficient to remove at least a portion of said contaminants from said extract; (e) introducing said vapor stream of step (b) together with a controlled portion of said liquid stream of step (b) into a second distillation zone under conditions sufficient to produce a distillate product stream comprising aromatic hydrocarbons and a bottoms stream comprising lean solvent suitable for reuse in the extraction zone; (f) cooling the remaining portion of said liquid stream to substantially the temperature of said extract phase of step (a); and, (g) returning said cooled liquid stream of step (b) to step (a) as added solvent therein.

A still further preferred embodiment of this invention includes the improvement hereinabove wherein said liquid stream of step (b) is passed into indirect heat exchange with the bottoms of said splitter column of step (c) to provide reboiler heat therein and to cool said remaining liquid portion.

The invention claimed:

1. Method for recovering aromatic hydrocarbons from the extract phase from a solvent extraction zone which comprises the steps of:

(a) introducing an extract phase containing solvent having aromatic hydrocarbons dissolved therein, said extract also contaminated with non-aromatic hydrocarbons, into a first distillation zone maintained under distillation conditions including the presence of hereinafter specified added solvent stream;

(b) withdrawing from said first zone a distillate fraction containing said contaminants, and a bottoms `fraction comprising solvent and aromatic hydrocarbons;

(c) passing said bottoms fraction into a separation zone maintained under conditions sufficient to produce a vapor stream comprising solvent and aromatic hydrocarbons and a liquid stream comprising solvent contaminated with aromatic hydrocarbons;

(d) introducing said vapor stream of step (c) together with a controlled portion of said liquid stream of step (c) into a second distillation zone m-aintained under distillation conditions;

(e) cooling the remaining portion of said liquid stream;

(f) withdrawing from said second distillation zone a distillate product stream comprising aromatic hydrocarbons and a lean solvent stream substantially free of aromatic hydrocarbons; and

(g) returning said remaining cooled portion of said liquid stream of step (c) to step I(a) as said added solvent.

2. Method according to claim 1 wherein s-aid solvent comprises sulfolane.

3. Method according to claim 1 wherein said separation zone comprises a flash zone maintained under equilibrium flash conditions.

4. In a process for the separation and recovery of aromatic hydrocarbons from a feedstock containing aromatic and non-aromatic hydrocarbons utilizing a solvent extraction step wherein an extract stream comprising aromatic hydrocarbons dissolved in said solvent is withdrawn from one end of a solvent extraction zone, said extract being contaminated with non-aromatic hydrocarbons, and a raffinate stream comprising non-aromatic hydrocarbons is withdrawn from the other end of Said zone, the improvement which comprises the steps of (a) introducing said extract phase into a irst distillation zone mainta-ined under conditions sufcient to produce an overhead fraction comprising non-aro- -matic hydrocarbons and a bottoms fraction cornprising said solvent having aromatic hydrocarbons dissolved therein;

(b) passing said bottoms fraction of step (a) into a ash zone under conditions suicient to produce a vapor stream comprising aromatic hydrocarbons and solvent and a liquid stream comprising solvent containing aromatic hydrocarbons;

(c) passing sa-id raffinate stream int-o a reboiled splitter column under conditions suficient to produce a distillate stream comprising relatively light non-aromatic hydrocarbons and a bottoms stream comprising relatively heavy non-aromatic hydrocarbons;

(d) passing said distillate stream of step (c) and said overhead fraction of step (a) into said extraction zone under conditions suicient to remove at least a portion of said contaminants from said extract; (e) introducing said vapor stream of step (b) together with a controlled portion of said liquid stream of step -(b) into a second distillation zone under conditions sufficient to produce a distillate product stream comprising aromatic hydrocarbons and a bottoms stream comprising lean solvent suitable for reuse in the extraction zone;

(f) cooling the remaining portion of said liquid stream t-o substantially the temperature of said extract phase of step (a); and,

(g) returning said cooled portion of said liquid stream of step (b) to step (a) as added solvent therein.

5. Improvement according to claim 4 wherein said remaining portion of the liquid stream of step (b) is passed into indirect heat exchange with the bottoms of said splitter column of step (c) to provide reboil heat therein and to cool said remaining portion.

6. Improvement according to claim 4 wherein said solvent comprises sulfolane.

I7. Improvement according to claim 4 wherein said feedstock comprises a pyrolysis gasoline fraction and said distillate product stream vof step (e) contains xylenes in high purity.

References Cited UNITED STATES PATENTS 2,730,558 1/1956 Gerhold 20s-321 3,167,501 1/1965 WOOdle 20832l 3,247,101 4/1966 Woodle 208-321 3,262,875 7/1966 Girotti et al. 208-321 3,361,664 l/196'8 Broughton et al. 208--321 HERBERT LEVINE, Primary Examiner U.S. Cl. X.R.