US3520946A - Recovery of aromatics with improved xylene purity - Google Patents

Description

July 21., 1970 D. B. BRoUGHToN RECOVERY 0F AROMATICS WITH IMPROVED XYLEN -PURITY Filed Sept. 11, 1967 /A/ VE/V TOR' Dona/d 5. Broughfon A T TOR/VEYS United States Patent Oce 3,520,946 Patented July 21, 1970 3,520,946 RECOVERY OF AROMATICS WITH IMPROVED A XYLENE PURITY Donald B. Broughton, Evanston, lll., assignor to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Sept. 11, 1967, Ser. No. 666,731 Int. Cl. C07c 7/10 U.S. Cl. 260--674 14 Claims ABSTRACT OF THE DISCLOSURE A technique to be integrated into a process designed to recover aromatic hydrocarbons from a mixture thereo-f with non-aromatic hydrocarbons via extraction. A light aromatic-rich intermediate stream is withdrawn from the extractive stripper and rectified. The tray temperatures in the extractive stripper are thereby more readily maintained at a desired elevated level, and the C8 and C9 nonaromatics are more easily stripped from the aromatic-containing solvent. The invention is adaptable to other solvent-extraction processes wherein polar hydrocarbons are separated from non-polar hydrocarbons.

APPLICABILITY OF INVENTION The invention described herein is adaptable for use in the separation, and ultimate recovery of polar hydrocarbons from non-polar hydrocarbons, which separation is effected by means of a solvent characteristically selective for adsorbing the polar hydrocarbons. More specifically,-

my invention is directed toward the separation and recovery of aromatic hydrocarbons from various mixtures there-` of with non-aromatic hydrocarbons.

The use of the terms polar and non-polar inthe present specification and appended claims, isintended to distinguish between classes of hydrocarbons wherein one particular, type is more polar while another is less polar. For example, in an extraction process for recovering naphthenes from a mixture `thereof with parains, the former are polarand the latter non-polar. In the extraction of aromatics from av mixture thereof with naphthenes, the naphthenes arepconsidered non-polar with respect to aromatics which are polarf ln onev of its specific applications, the process evolved from the present invention serves to` segregate particular. species of aromatic hydrocarbons, such as benzene, toluene and/or C9 aromatics from other hydrocarbons normally contained in'petroleum fractions and distillates. The proc-` ess utilizes a solvent which may be indefinitely recycled Within the system, yields the desired hydrocarbon product in a state of high purity, particularly the mixed xylenes (C9 aromatics), and separates the same substantially in its entirety from the feed stocks charged to the process.

The present invention is particularly concerned with an improvement in the type of separation process wherein a vmixture of various classes of hydrocarbons lis introduced into an extraction zone at an intermediate locus in the height thereof, and is countercurrently contacted therein with a solvent selective for aromatic hydrocarbons. A raffinate phase comprising substantially all of the non-aromatic hydrocarbons in the feed stock is removed from one end portion of the extractionzoneat ywhich the solvent is introduced, an extract phase comprising the,

aromatic components of the feed stock is removed from the other end portion of the extraction zone and the aromatic solute is subsequently recovered by stripping and fractionating the extract phase.

Although my invention is applicable for use with any hydrocarbon feed stock having a sufficiently high aromatic concentration to justify the recovery thereofi.e. at least about 25.0% by volume-unusual advantages are afforded when processing those charge stocks wherein the aromatic portion comprises at least about 65.0% by volume of benzene. The overall carbon number range of suitable charge stocks is from about six to about ten. These charge stocks will generally include, in addition to C6, C7 and C8 aromatics, non-aromatics which predominate in C3 and C9 paraflins and naphthenes. Exemplary of various sources of suitable charge stocks are the depentanized effluent from a catalytic reforming unit, coke oven by-products, wash oils, hydrotreated pyrolysis naphtha (the hydrotreating is generally required for olen saturation and contaminant removal), etc.

As the ratio of benzene to heavier aromatics increases, the accompanying effect is an increase in the degree to which the tray temperatures, especially in the lower section, in the extractive stripper become depressed. When charge stocks, such as those above set forth, contain a high ratio of benzene to other aromatics, upwards of 65.0% by volume of benzene in the total aromatic portion, the subsequent recovery of a high-purity xylene concentrate is virtually unattainable. The tray temperatures in the extractive stripper become depressed to the extent that the stripping of heavy non-aromatics (chiefly C9 and C9 paraffins and naphthenes) becomes inefficient. These non-aromatics ultimately contaminate the xylene product. The lighter non-aromatics-i.e. having six and seven carbon atoms per molecule-are, however, readily extracted notwithstanding high ratios of benzene to heavier aromatics. While the adverse effect on tray temperatures can be partially obviated by process changes such as, for example, an increase in flow rate of solvent through the stripper, accompanied by a simultaneous increase in the reflux rate to the extractor, the resulting increase in equipment sizes and utility requirements to satisfy heat loads produces a process which cannot be economically justified.

PRIOR ART Processes for the recovery of aromatic hydrocarbons from hydrocarbon mixtures have been known for many years. These are usually accomplished by extractive distillation, or alternately by countercurrent liquid phase contacting of the feed with a solvent selective for aromatics. The present inventive concept relates to both of these processes. The prior art also teaches the use of a light parafiin backwash as a recycle stream to the lower portion of the extractor to displace at least a portion of the heavier non-aromatic components in the extract phase. This is shown in U.S. Pat. NO. 3,037,062. In recovering aromatics from a reformate feed using a glycoltype solvent, the use of a light paraflin backflush has been satisfactory. It has been found, however, that when using a sulfolane-type solvent it becomes increasingly difficult to remove the heavy non-aromatic components from the extract phase with a light paratlin backwash. Although sulfolane-type solvents are highly satisfactory as selective solvents for recovering aromatics, their high solubility results in higher concentrations of non-aromatic component in the extract phase. To displace these non-aromaticV components from the extract phase with a light paraffin stream requires a large backush stream which must be eventually separated, and accordingly involves a large capital expenditure. If the feed stock non-aromatic components present in the extract phase are not displaced, an additional burden is placed on the extractive stripper, and some of these components will not be readily stripped out of the aromatic-solvent mixture in the extractive stripper. It has been observed that, when recovering aromatics from a C6-C9 feed using a sulfolane solvent, some of the C9 naphthenes remain in the extract phase and 3 eventually appear as contaminants in the ultimate xylene product stream. This problem is further compounded by the use of feed stocks containing high ratios of benzene to heavier aromatics.

OBJECTS AND EMBODIMENTS An object of my invention is the separation of polar hydrocarbons from a mixture thereof with non-polar hydrocarbons and a solvent characteristically selective for adsorbing polar hydrocarbons. A corollary objective is to separate aromatic from a mixture thereof with non-aromatics and a solvent selective for aromatic hydrocarbons.

A principal object of my invention is to increase the purity of an ultimate xylene product stream.

Another object is to inhibit depressed tray temperatures in the extractive stripper while utilizing a solvent extraction process with charge stocks having a high ratio of benzene to heavier aromatics.

Therefore., in a broad embodiment, my inventive concept encompasses a process for separating polar hydrocarbons from a mixture thereof with non-polar hydrocarbons and a solvent characteristically selective for adsorbing polar hydrocarbons, which process comprises the steps of: (a) introducing said mixture into a stripper column, removing a non-polar hydrocarbon-rich stream from an upper portion of said stripper column, and removing a solvent-containing, heavy polar hydrocarbon-rich stream from a lower portion of said stripper column; (b) removing from an intermediate portion of said stripper column, a light polar hydrocarbon-containing stream substantially free from non-polar hydrocarbons, introducing said light polar hydrocarbon stream into a rectifying column; and, (c) removing a solvent-containing, heavy polar hydrocarbon stream from the bottom portion of said rectifying column and a solvent-free, light polar hydrocarbon stream from the upper portion thereof.

A more limited embodiment affords an improved process for the separation and recovery of aromatic hydrocarbons from a hydrocarbon mixture, which process comprises the steps of: (a) introducing said mixture into an intermediate point of an extraction zone, contacting said mixture therein with a solvent characteristically selective for adsorbing aromatic hydrocarbons, and at elevated temperature and pressure sufficient to maintain said mixture and solvent in liquid phase, thereby forming a raffinate phase and an extract phase; (b) removing said raffinate phase from an upper portion of said zone; (c) re moving said extract phase from a lower portion of said zone, and introducing said extract phase into a stripper column; (d) removing a non-aromatic hydrocarbon containing stream from an upper portion of said stripper, a rich solvent, aromatic-containing stream from a bottom portion of said stripper, and removing a benzene-rich, solvent-containing stream from a middle portion of said stripper; (e) introducing the stripper bottoms stream into a fractionating tower, removing therefrom an overhead 'stream comprising aromatic hydrocarbons and a bottoms stream comprising lean solvent, recycling at least a portion of the latter to an upper portion of said extraction zone; (f) introducing said benzene-rich, solvent-containing stream into a rectifying column, removing as overhead therefrom a solvent-free benzene-rich stream and a heavy aromatic, solvent-containing stream from the bottom portion thereof; and (g), introducing said heavy aromatic, solvent-containing stream into said stripper column.

fThese and other objects and embodiments will become apparent from the following detailed description of my invention. With respect briefly, however, to other embodiments, these involve operating conditions, particular solvents, and especially internal recycle streams. Exemplary of the latter includes introducing the solvent-containing heavy polar hydrocarbon stream, withdrawn from the bottom of the rectifying column into the extractive stripper with the extract phase, or polar hydrocarbon-solvent mixture. The intermediate stream removed from the extractive stripper is substantially free from lighter non-polar or non-aromatic hydrocarbons, and is in an amount such that the ratio of light polar hydrocarbons, or benzene, therein to that contained in the solvent-containing stream removed from the bottom portion of said stripper column, is greater than about 1.7: l, and preferably at least about 2.0: 1.

The non-aromatic stream removed from the upper portion of the extractive stripper is, at least in part, preferably returned to the bottom portion of the extractor Zone as reflux, or backflush. Prior to such return, of course, the stream is condensed and passed into an overhead receiver equipped With a Water-leg for the separation therein of a water phase.

Preferred solvents for use in the process of extraction of aromatics incorporating the present concept are hereafter described in greater detail.

DESCRIPTION OF DRAWING The integration of my invention into a process for the separation and recovery of aromatic hydrocarbons may be more clearly understood upon reference to the accompanying drawing wherein one specific embodiment is illustrated. In the drawing, various heaters, coolers, control valves, start-up lines, instrumentation, and other miscellaneous appurtenances have been reduced in number, or eliminated entirely as not being necessary for the purposes of illustration. Such modifications will be well within the purview of those possessing ordinary skill in the art. One item is, however, worthy of brief mention; the aromatic product (Shown in the drawing as line 19) is virtually always sent to an after fractionation section generally consisting of a benzene column, a toluene column and a xylene column, for separation into individual product streams. It is understood that such a fractionation scheme forms no essential part of my invention, the scope and spirit of which is defined by the appended claims.

With reference to the drawing, the mixed hydrocarbon feed is introduced into extractor 2 via line 1. It is preferred to introduce the feed into the extractor at a locus of about 1/3 of its height from the bottom. This permits the feed to contact the solvent in the upper 27s of the extractor and permits the extract phase to Contact a light naphthene-containing backwash stream in the bottom 1/3 of the extractor. Lean solvent is introduced into the upper section of extractor 2 through line 20, the source of which is hereafter described, whereby thel lean solvent countercurrently contacts the feed. The extractor is maintained at an elevated temperature and pressure sufficient to maintain the solvent and the hydrocarbon feed mixture in the liquid phase. Since the solvent has a solubility selectivity for aromatics over non-aromatics, and is immiscible Awith the feed, aromatics rwill preferentially dissolve into the solvent phase. Thus, as the solvent passes downflow through the upper 2/3 of the extractor, the aromatics concentration therein gradually increases. Appropriately, the solvent that is first introduced into the extractor is called lean solvent and the solventA containing dissolved aromatics is called fat, or rich solvent. The feed hydrocarbon phase passes upflow through the extractor since it is less dense than thefsolvent phase, the extractor having disposed therein suitable means to contact the feed phase and the solvent phase efficiently. As the feed hydrocarbon phase continues upflow through the extractor, the aromatics and a small quantity of non-aromatics gradually dissolve into the solvent phase such that when the hydrocarbon phase reaches the top of the extractor, it contains essentially no aromatics and is called the raffinate phase. The raffinate phase is withdrawn from extractor 2 through line 23. This raffinate phase may lbe treated to remove small quantities of dissolved solvent. The extract phase continues through the lower 1/3 of the extractor where it countercurrently contacts an irnmiscible light naphthene-containing hydrocarbon stream employed as the backwash, or reflux. The backwashing step results in the displacement from the extract phase of at least a portion of the heavier non-aromatic feed components. It must be remembered that sulfolane-type solvents will dissolve some non-aromatics along -with the aromatics of the feed and the object of the backwashing, as hereinafter stated, is to displace the heavier feed nonaromatic hydrocarbons with the lighter naphthene-containing hydrocarbons. Preferably, the light naphthene-containing hydrocarbon is a fluid having from about to 7 carbon atoms per molecule when feeds in the Cfr-C10 carbon range are used. The backwash is accomplished by introducing the light naphthene-containing hydrocarbon into a lower portion of extractor 2 through line 11.

The extract phase is withdrawn from the lower portion of extractor 2 through line 3, and is introduced into extractive stripper 4. Since the presence of the solvent in the stripper has less effect on the boiling point of the non-aromatic compounds dissolved in the solvent than it does on the aromatic compounds also dissolved in the solvent, stripper 4 is capable of removing the nonaromatic components from the extract phase. This effect is further enhanced, where desired, by introducing additional lean solvent into the extract phase through bypass line 13. In general, the solvent tends to retain the more soluble aromatic component, even at temperatures of the extract considerably above the boiling point of the aromatics alone. However, when processing feed stocks of wider boiling'ranges such as in the C6-C10 range, the stripper will effectively remove the lighter non-aromatics overhead but will have difficulty in removing the heavier non-aromatics overhead without also vaporizing the lighter aromatics. Furthermore, when the heavier non-aromatics contain appreciable amounts of naphthenes this difficulty is increased since the solvent will possess the tendency to retain the naphthenes in solution more readily than the parafiins. In the presence of sulfolane, benzene and C9 napthenes have about the same volatility and therefore it becomes difficult to remove C9 naphthenes in the stripper without also removing some C6 aromatics. This, of course, ultimately results in low purity C8 aromatics: Therefore, at least a portion of the heavier non-aromatics are displaced by the lighter naphthene-containing hydrocarbons in the backwash to permit the lighter nonaromatics to be readily vaporized in the stripper.` Light parafiins are less effective in displacing heavy napthenes than are light naphthenes and accordingly, it is preferred that the backwash material contain an appreciable concentration of naphthenes-at least about` 15% and preferably at least about 25% by volume. When using saturated sulfolane as the solvent, the ratio of the solvents selectivity for naphthene compared to a paraffin of the same carbon number is about 1.4 to 1.0. Also, as the molecular weight of the naphthenes increase, the selectivity of saturated sulfolane for said naphthene decreases by a factor of about 1.2 per carbon atom. Thus, a light naphthene containing backwash material is efiicient and satisfactory to displace the heavier non-aromatic Cs and Cgs from the extract phase.

As hereinbefore set forth, when the aromatic portion of the feed stock is high in the ratio of benzene to heavier aromatics, at least about 65.0% by volume is considered high, the problems associated with efficient non-aromatic C8 and C9 (C10 when in the feed stock) stripping continues to be present notwithstanding the backwash, or reflux technique in the extractor zone. These heavier non-aromatic hydrocarbons continue to be carried `into the after fractionator train, and appear as contaminants in the xylene product stream. The higher benzene concentration in the extractive stripper results in depressed tray temperatures, thereby increasing the difficulty of stripping heavy non-aromatics.

In accordance with my invention, these difliculties are alleviated, and the ultimate Xylene purity significantly improved, by withdrawing an intermediate vapor stream from the extractive stripper at a locus where the light non-aromatics, which could contaminate the final benzene and toluene streams, have already been stripped from the solvent. This vaporous stream is passed into a nonreboiled rectifier, the reflux rate to which is such that only the lighter aromatics are removed as an overhead stream, and directed to the benzene column in the after fractionation facilities. The bottoms stream from the rectifier contains principally xylenes and heavier aromatics, in addition to contaminants not stripped from the solvent in the upper section of the stripper. rThis stream is returned to the upper section of the extractive stripper with the extract phase. The lower section of the extractive stripper will now be functioning with a relatively low ratio of benzene to higher aromatics and, consequently, tray temperatures will be higher and the heavier non-aromatics more readily stripped from the solvent.

With reference once again to the accompanying drawing, the extractive stripper is operated at elevated temperatures and intermediate pressures in order to remove substantially all of the non-aromatics, some of the water and aromatics and a small amount of the solvent overhead. This overhead stream is removed from stripper 4 through line 5, wherein it passes through condenser 6, line 7 and into stripper overhead receiver 8. The condensed overhead material is separated into two phases therein, one a water-solvent phase and the other a light naphthene-containing hydrocarbon stream. The light hydrocarbon phase is 'withdrawn from receiver 8 through line 11 to` be recycled to the lower section of extractor 2. The water-solvent phase settles in boot 9, withdrawn therefrom through line 10, and sent to solvent recovery means not shown. A portion of the bottoms fraction from the stripper flows through line 25 wherein it passes through reboiler heater 26 and returns to stripper 4.

The remaining bottoms portion comprising aromatics and solvent is withdrawn from stripper 4 and passes through line 12, and into solvent recovery column 14.

An intermediate stream, stripped of light non-aromatic hydrocarbons, is withdrawn from stripper 4 through line 31, and is introduced into the lower section of rectifier 32. The overhead stream in line 33 passes through condenser 34 into overhead receiver 35. A refiux stream is returned to the top of the rectifier in such an amount that only lighter aromatics are taken overhead in line 33. The remainder of the condensed overhead stream is withdrawn from condenser 35 via line 37, and is ultimately introduced into the after fractionation facilities. The bottoms stream in line 38, containing chiefly xylenes, higher aromatics and solvent, is returned therethrough 1to stripper 4, conveniently by way of the extract phase in ine 3.

Solvent recovery column 14 is operated at low pressures and elevated temperatures to separate the solvent from the aromatics. The aromatics and a small amount of water are removed as a vapor overhead from column 14 and pass through line 15, condenser 16 and into overhead receiver 17. The overhead material is separated into a hydrocarbon phase comprising aromatics, and a water phase. The aromatics are withdrawn from receiver 17 through line 18 Where a portion thereof returns to column 14 as reflux while the remaining portion of aromatics is withdrawn through line 19 as net product, in combination with the light aromatics in line 37. The water phase which settles in boot 29 is withdrawn through line 30. Generally, the net product is sent to a series of fractionators to recover the individual aromatics as substantially pure components, such as benzene, toluene, ortho-xylene, ethylbenzene, etc. A portion of the bottoms material from column 14 is withdrawn through line 27 where it passes through reboiler 28 before returning to column 14. The remaining bottoms portion of the lean solvent is withdrawn through flow conduit 20 wherein it is recycled back to the upper point in extractor 2.

A small amount of decomposition of solvent and other sludge forming reactions may occur in the process, and it is desirable to remove a small slipstream of lean solvent from line 20 through line 21 and regenerate the solvent in equipment not illustrated, in order to prevent a buildup of sludge. The regeneration preferably is done by rerunning the solvent and thereafter the regenerated solvent is returned to line 20 by means of line 22.

Since in some instances a portion of the light naphthene-containing backwash may pass through extractor 20 and leave in the rafnate phase in line 23, it would be necessary to supply sufficient backwash material as makeup. This is conveniently accomplished either by passing the raffinate fraction therein and recycling at least a portion of the light raffinate fraction through flow conduit 24 into receiver 8, or alternately passing the feed stock owing in line 1 into a splitter fractionator, producing alight feed stock and a heavy feed stock, introducing the heavy feed stock into the intermediate locus in extractor 2 and passing the light feed stock into line 11 and ultimately into the lower point in extractor 2.

SUMMARY OF INVENTION R2CB H-Rs wherein R1, R2, R3 and R4 are independently selected from the group consisting of a hydrogen atom, an alkyl group having up to carbon atoms, an alkoxy radical having up to 8 carbon atoms and an arylalkyl radical having up to 12 carbon atoms. Other solvents preferably included are the sulfolenes such as 2-sulfolene or 3sulfolene which have the following structure:

o o o o s Hl/ \CH2 HZC/ \CH2 HC (13H2 Hozon The sulfolane solvents may be made by condensing a conjugated diolen with sulfur dioxide and then subjecting the resulting product to hydrogenation, alkylation, hydration and/or other substitution or addition reactions. A 2-sulfolene may be made by isomerizing the product resulting from condensing the conjugated diolefn with sulfur dioxide. Other solvents which have high selectivity for separating aromatics from non-aromatic hydrocarbons are Z-methylsulfolane, 2,4-dimethylsulfolane, methyl 2- sulfonyl ether, n-aryl-3-sulfonyl amine, 2sulfonyl acetate, diethylene glycol, various polyethylene glycols, dipropylene glycol, various polypropylene glycols, dimethyl sulfoxide, N-methyl pyrrolidon and others.

The aromatic selectivity of the sulfolane solvents can be further enhanced by the addition of Water to the solvent. Preferably, the solvents contain a small amount of water dissolved therein to increase the selectivity of the overall solvent phase for aromatic hydrocarbons over non-aromatic hydrocarbons without reducing substantially the solubility of the solvent phase for aromatic hydrocarbons. The presence of water in the solvent composition provides a relatively volatile material therein which is distilled from the fat solvent in the extractive stripper to Vaporize the last traces of non-aromatic hydrocarbon from the fat solvent stream by steam distillation. The solvent composition contains from about `0.5% to about 20.0% by Weight of water and preferably from about 3.0% to about 15.0% depending on the particular sulfolane solvent utilized and the process conditions under which the extractor and eX- tractive stripper are operated.

The extractor is operated at elevated temperature and at a sufficiently elevated pressure to maintain the feed stock, solvent and light backwash streams in the liquid phase. Suitable temperatures are within the range of from F. to about 400 F. and preferably at an intermediate level from about 175 F. to about 300 F. Suitable pressures are within the range of from about atmospheric pressure up to aobut 400 p.s.i.g. and preferably from about 50 p.s.i.g. to about 150 p.s.i.g. Generally, the volume of reflux introduced into the lower point in the extractor is at least 10% by volume of the extract phase leaving the extractor.

The extractive stripper is operated at moderate pressures and sufciently high reboiler temperatures to drive all the light backwash non-aromatic components and some of the aromatics, water and solvent overhead. Typical stripper pressures are from atomspheric to about p.s.i.g., although the top of the stripper is generally maintained at from about 1.0 p.s.i.g. up to about 20 p.s.i.g. The reboiler temperature is dependent upon the composition of the feed stock and the solvent. Generally for a C6 feed stock using saturated sulfolane stripper bottom temperatures of from 275 to about 360 F. are satisfactory.

The solvent recovery column is operated at low pressures and suciently high temperatures to drive the aromatic hydrocarbons overhead and thus produce a lean solvent bottoms. Again the choice of operating conditions depends on the feed stock and the solvent composition. Preferably the top of the solvent recovery column is operated at a vacuum of from about 100 to about 400 mm. of mercury absolute. Low pressures must be employed in order to maintain a sufficiently low reboiler temperature to avoid thermal decomposition of the solvent. Preferably, the reboiler temperature should be maintained below Iabout 360 F. when using saturated sulfolane as the solvent.

The quantity of vapor withdrawn as an intermediate stream from the extractive stripper is dependent upon the ratio of benzene to heavier aromatics in the feed stock. Those feed stocks which give rise to the depressed tray temperature diiculties in the extractive stripper generally have an aromatic portion which is at least 65.0% by volume benzene. Proper tray temperatures, and thus most efficient stripping of heavier non-aromatics can be maintained if sucient vapor is withdrawn through line 31 lnto rectifier 32 such that the ratio of benzene in line 37 to benzene in line 12 is at least about 1.7:1, and preferably at least 2.0:1. The rectifying column is reuxed to the extent necessary to insure that only light aromatics, benzene and toluene are removed overhead.

The following example is presented to illustrate 'an extraction process incorporating my invention; it is not intended to limit the invention to the materials used, or to the operating conditions.

EXAMPLE .streams to the extractor zone are 616.6 mols/hr. of lean solvent, light backwash, or reux, into a lower section 1n an amount of about 205.5 mols/ hr., and a feed stream containing 169.0 mols/hr. of benzene, 38.5 mols/hr. of toluene, 17.9 mols/ hr. of C8 aromatics and 34.6 mols/hr.

of non-aromatics of which about 82.0% are naphthenes. The backwash employed as reflux contains about 85.0% of C6 hydrocarbons, the remainder being primarily C7 hydrocarbons. The extractor is a rotary disc contactor, and is maintained at a pressure of about 85 p.s.i.g. and a temperature of about 200 F.

A raffinate stream, in an amount of about 37.8 mols'/ hr., is removed from the upper section of the extractor, and 1044.3 mols/ hr. of an extract phase is removed from the bottom section of the extractor. In the following Table I, component analyses of the various extractor streams, in mols/hr., are conveniently presented by designating the streams according to the line numbers employed in the accompanying drawing. Thus, the backwash stream is line 11, the lean solvent stream is line 20 (as it enters the extractor), the raffinate phase is designated as line 23 and the extract phase as line 3. Line 1, of course, is the feed stock.

TABLE I.EXTRACTOR STREAM ANALYSES The extract phase is passed into an extractiva stripper in admixture with about 625.1 mols/hr. of additional lean solvent. In addition to the added lean solvent, the extract phase is admixed with 82.5 mols/hr. of a rectifier bottoms stream, the source of which is hereinafter set lforth. The mixture enters an extractiva stripper which is operating with a top pressure of about 2.0 p.s.i.g., a top temperature of 220 F. and a bottoms reboiler temperature of about 330 F. An overhead stream is withdrawn (line in an amount of 227.6 mols/hr., condensed and passed into an overhead receiver equipped with a water-boot. Two phase separation is effected: a hydrocarbon phase, returned as reux to the extractor in an amount of 205.5 mols/hr. (line 11), and a water phase in an amount of 22.1 mols/ hr., of which about 1.8 mols is solvent.

A bottoms stream is removed in an amount of 1316.4 mols/hr., (line 12) and is introduced into a solvent recovery column maintained at a top pressure of about 200 mm. Hg absolute, a top temperature of about 145 F. and a bottoms reboiler temperature of about 350 F. An overhead, aromatic concentrate is recovered in an amount of about 110.4 mols/hr. (line 19), and the solvent recycled to the precess via lines 13 and 20 as herebefore described. A benzene-rich intermediate vapor phase is withdrawn from the stripper in an amount of about 207.9 mols/hr. (line 31), and introduced into a non-reboiled rectifying column.

In the following Table II, the component analyses are presented for the total feed to the extractive stripper (line 3), the overhead therefrom (line 5), the bottoms stream sent to solvent recovery (line 21) and the intermediate vapor stream to the rectifying column (line 31).

TABLE II.STRIPPER STREAM ANALYSES The rectifying column is operated at a reflux ratio such that only the lighter aromatics are removed overhead in line 33. These arecondensed, and the portion not serving as reflux is removed via line 37 in an amount of 125.4 mols/ hr. The heavier aromatics in the vapor side-cut are returned to the extractive stripper in an amount of 82.5 mols/hr. (line 38). The following Table III indicates the separation effected in the rectifying column.

TABLE IIL-RECTIFIER STREAM ANALSYES The overhead from the rectifying column is conveniently combined with the aromatic overhead from the solvent recovery system for further separation into individual components in the after fractionation scheme. The combined aromatic stream (line 37 and the aromatics in line 12) consists of 168.6 mols/hr. of benzene, 38.0 mols/hr. of toluene, 17.0 mols/hr. of C8 aromatics ond less than 0.02 mol/hr. of heavier non-aromatics. The C8 aromatics, chiefly xylenes, can, therefore, be recovered in an extremely pure concentrate, contaminated by 0.1% non-aromatics, or less. In the absence of the intermediate vapor side-cut and rectification, design calculations indicate that the tray temperatures in the stripper are depressed to the extent that the bottoms stream (line 12) contains about 0.04 mol/hr. of heavy non-aromatics, or about 0.2% of the utlimately recoverable xylenes.

I claim as my invention:

1. A process for separating polar hydrocarbons from a mixture thereof with non-polar hydrocarbons and a solvent characteristically selective for adsorbing polar hydrocarbons, which process comprises the steps of (a) introducing said mixture into a stripper column, removing a non-polar hydrocarbon-rich stream from an upper portion of said stripper column, and removing a solvent-containing, heavy polar hydrocarbonrich stream from a lower portion of said stripper column;

(b) removing from a middle portion of said stripper column, a light polar hydrocarbon-containing stream substantially free from non-polar hydrocarbons, introducing said light polar hydrocarbon stream into a rectifying column; and,

(c) removing a solvent-containing, heavy polar hydrocarbon stream from the -bottom portion of said rectifying column and a solvent-free, light polar hydrocarbon stream from the upper portion thereof.

2. The process of claim 1 further characterized in that the polar hydrocarbon portion of said mixture comprises at least 65.0% by volume of light' polar hydrocarbons.

3. The process of claim 1 further characterized in that the solvent-containing heavy polar hydrocarbon stream from said rectifyin g column is introduced into said stripper column.

4. The process of claim 1 further characterized in that the ratio of light polar hydrocarbons in the solvent-free stream removed from the upper lportion of said rectifying column to the light polar hydrocarbons in the solventcontaining stream removed from the bottom portion of said stripper column is greater than about 1.7:1.

5. The process of claim 1 further characterized in that said polar hydrocarbons are aromatic, and said light polar hydrocarbon is benzene.

6. A process for the separation from and recovery of aromatic hydrocarbons from a hydrocarbon mixture which comprises the steps of:

(a) introducing said mixture into an intermediate point of an extraction zone, contacting said mixture therein with a solvent characteristically selective for adsorbing aromatic hydrocarbons, and at elevated temperature and pressure suflicient to maintain said mixture and solvent in liquid phase, forming a raffinate phase and an extract phase;

(b) removing said raffinate phase from an upper portion of said zone;

(c) removing said extract phase from a lower portion of said zone, and introducing said extract phase into a stripper column;

(d) removing a non-aromatic hydrocarbon containing stream from an upper portion of said stripper, a rich solvent, aromatic-containing stream from a bottom portion of said stripper, and removing a benzenerich, solvent-containing stream, substantially free from non-aromatic hydrocarbons, from a middle portion of said stripper;

(e) introducing the stripper bottoms stream into a fractionating tower, removing therefrom an overhead stream comprising aromatic hydrocarbons and a bottoms stream comprising lean solvent, recycling at least a portion of the latter to an upper portion of said extraction zone;`

(f) introducing said benzene-rich, solvent-containing stream into a rectifying column, removing as overhead therefrom a solvent-free, benzene-rich stream and a heavy aromatic, solvent-containing stream from the bottom portion thereof; and,

(g) introducing said heavy aromatic, solvent containing stream into said stripper column.

7. The process of claim 6 further characterized in that the nonaromatic hydrocarbon containing stream removed from the upper portion of said stripper column is at least in part returned to said extractor zone.

8. The process of claim 6 further characterized in that the solvent comprises a sulfolane of the general formula:

O O \S Ri-C \CH-R4 Rz-tlH-CH-Ra where R1, R2, R3 and R4 are independently selected from the group consisting of a hydrogen atom, an alkyl group having up to 10 carbon atoms, an alkoxy radical having up to 8 carbon atoms, and an arylalkyl radical having up to 12 carbon atoms.

9, The process of claim 8 further characterized in that R1, R2, R3 and R4 are hydrogen atoms.

10. The process of claim 8 further characterized in that the sulfolane solvent contains up to 20% by volume of water.

11. The process of claim 6 further characterized in that the solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.

12. The process of claim 6 further characterized in that the raiinate removed from the upper portion of the eX- traction zone is passed into a splitter fractionator and separated therein into a light raffinate and a heavy rafinate, a portion of the light raffinate being recycled back to the lower portion of the extraction zone.

13. The process of claim 6 further characterized in that said hydrocarbon mixture is introduced into a splitter fractionator and separated therein into a light overhead portion and a heavy bottom portion, the heavy bottom portion being introduced into the intermediate point of the extraction zone and the light overhead portion being introduced into the stripper overhead receiver.

14. The process of claim 6 further characterized in that the hydrocarbon mixture comprises a hydrotreated pyrolysis oil.

References Cited UNITED STATES PATENTS 3,065,168 11/1962 Zuiderweg et al 208-325 3,210,269 10/1965 Kosters et al. 208-325 3,396,101 8/1968 Broughton 208-325 DELBERT E. GANTZ, Primary Examiner C. R. DAVIS, Assistant Examiner U.S. Cl. X.R. 208-325, 332

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