US3763037A

US3763037A - Aromatic hydrocarbon

Info

Publication number: US3763037A
Application number: US00140776A
Authority: US
Inventors: H Thompson
Original assignee: Universal Oil Products Co
Current assignee: Honeywell UOP LLC; Universal Oil Products Co
Priority date: 1971-05-06
Filing date: 1971-05-06
Publication date: 1973-10-02
Anticipated expiration: 1990-10-02

Abstract

Loss of aromatics in the raffinate removed from an extractive distillation zone is diminished by reboiling a dilute watersolvent mixture with the extractive distillation lean solvent feed.

Description

United States Patent 11 1 Thompson 1 Oct. 2, 1973 1 AROMATIC HYDROCARBON 3,537,984 11/1970 Thompson 208/325 1 4 P l 2 1 1 1mm MM L- Thomson, Park Ridge, 3333333 3/1321 6151????fffififf? 233/313 Ill. 3,560,374 2/1971 Assignee: Universal P c C np ys Cl. al Des Plaines, [IL 1 I Primary ExaminerDelbert E. Gantz l [22] ed May 1971 Assistant Examiner-C. E. Spresser [2]] App]. No.: 140,776 Attorney.lames R. Hoatson, Jr. and Edward W.

, 1 Remus [52] US. Cl. 208/313, 208/321, 208/325,

208/326, 260/674 sE [51] Int. Cl. C07c 7/08 [57] ABSTRACT [58] Field of Search 260/674 SE; 208/313, Loss of aromatics in the raffinate removed from an ex- 208/321, 325, 326 tractive distillation zone is diminished by reboiling a dilute water-solvent mixture with the extractive distilla- [56] Reierences Cited tion lean solvent feed.

UNITED STATES PATENTS 3,642,614 2 1972 Van Tassell 260/674 11 1 Draw F'gure PATENTEDUBT 21 1s Y 3,763,037

N VE/V 7'0 5.- Herbert L. Thompson .4 TTORNEYS AROMATIC IIYDROCARBON BACKGROUND OF THE INVENTION The present invention relates to the extractive distillation and recovery of aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons. More specifically, the present invention relates to a method for controlling the amount of aromatics lost in the non-aromatic raffinate product removed from an aromatic selective, extractive distillation, particularly when sulfolane is utilized as the aromatic selective solvent.

Conventional processes for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feedstocks including catalytic reformates, hydrogenated pyrolysis gasolines and various selective cuts of these aromatic sources, utilize an aromatic selective solvent to facilitate the removal of the aromatics from nonaromatic hydrocarbons. In certain of these prior art processes, a hydrocarbon feedstock is first contacted in a solvent extraction zone with a solvent composition, preferably containing water to enhance the solvents selectivity, which selectively dissolves the aromatic components of the feedstock, thereby forming a raffinate phase comprising one or more non-aromatic hydrocarbons and'a minor amount of aromatic hydrocarbons and an extract phase comprising solvent having an aromatic hydrocarbon dissolved therein. The aromatic hydrocarbons contained in the extract may be recovered by distillation to yield an overhead distillate containing a portion of the extracted aromatics, an aromatic side-cut fraction and a solvent bottoms fraction which is recycled for reuse in the solvent extraction zone. Frequently, the solvent extraction zone extract phase is subjected to an extractive distillation (i.e., extractive stripping) to remove contaminating amounts of non-aromatic hydrocarbons which may be contained in the extract in order to make possible the recovery of nitration grade aromatic hydrocarbons.

Also not infrequently, particularly when processing a narrow boiling range or carbon number range mixture of aromatic and non-aromatic hydrocarbons, the aromatic hydrocarbons are recovered by passing the aromatic feedstock directly to an extractive distillation zone rather than going through the preliminary liquid phase solvent extraction. In this mode of operation, a hydrocarbon feedstock and a solvent composition which selectively dissolves the aromatic components, are contacted, thereby forming an aromatic-rich extract stream and a non-aromatic containing raffinate overhead. To maximize the amount of aromatic hydro- I carbons recovered from a given feedstock, it is necessary to substantially eliminate the passage of any aromatic hydrocarbons overhead in admixture with the non-aromatic raffinate. Further, when utilizing an aromatic selective solvent containing water, aromatic selective solvent is often removed overhead in admixture with the non-aromatic raffinate. These losses of aromatic selective solvent are controlled by water-washing the non-aromatic raffinate thereby removing. the sol" 2, ered and recycled back to the extractive distillation zone.

Accordingly, it is seen that the art has methods developed for recovering solvent removed overhead in the non-aromatic raffinate stream produced in the extrac tive distillation zone. However, since aromatic and non-aromatic hydrocarbons are miscible and cannot be removed by water-wash, those aromatic hydrocarbons which are inadvertently removed overhead in admixture with the raffinate streams are often lost in the process and ultimately lower the overall efficiency of a given extractive distillation process.

SUMMARY OF THE INVENTION It is an object of this invention to provide a process for the recovery of aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons by an improved, more facile and economical utilization of extractive distillation.

It is a more limited object of the present invention to provide a method for controlling the amounts of aromatic hydrocarbon which are undesirably removed in the non-aromatic raffinate product recovered from the extractive distillation zone, particularly when sulfolane solvent is utilized therein as the aromatic selective sol vent.

In a broad embodiment, therefore, the present invention relates to an improvement in a process for the separation of aromatic hydrocarbons: from a mixture of aromatic and non-aromatic,hydrocarbons wherein a lean, aromatic selective solvent, recovered from a solvent recovery fractionation column, is contacted with the hydrocarbon mixture in an extractive distillation zone to produce an aromatic-rich extract stream and a nonaromatic raffinate overhead and water is separated from a relatively dilute solvent containing water stream in a water stripping zone. The particular improvement involves controlling the amount of aromatic hydrocarbons removed in the non-aromatic raffinate by reboiling the water stripping zone, maintained at a pressure sufficient to vaporize at least a portion of the water passed thereto, with at least a portion of the lean solvent recovered from the solvent recovery zone, thereby cooling the lean solvent. At least a portion of the thus cooled solvent is passed to an upper portion of the extractive distillation zone. In a more limited embodiment, aromatic losses in the raffilnate overhead may be further controlled by lowering the pressure maintained on the water stripping zone when the amount of aromatic hydrocarbons removed overhead in the nonaromatic raffinate exceeds a predetermined level, thereby further cooling the lean solvent which lowers the amount of aromatic hydrocarbons removed in the overhead raffinate. This control method finds particular applicability when utilized in a sulfolane type solvent process or a process utilizing N-methyl-Z- pyrrolidone as solvent. r

In a further, more limited embodiment, the nonaromatic raffinate comprising hydrocarbons, solvent and water is removed in a vapor phase and condensed,

' with at least a portion of the condensed water being passed as reflux into an upper portion of the extractive distillation zone. Preferably, this water reflux is utilized only when the amount of aromatic hydrocarbons re- 1 moved in the raffinate exceeds a pre-determined level.

a process for separating non-aromatic hydrocarbons from aromatic hydrocarbons by extractive distillation. This novel method involves introducing a hydrocarbon feedstock containing aromatic and non-aromatic hydrocarbons into a sulfolane type extractive distillation zone maintained under extractive distillation conditions including the passing of a hereinafter specified lean solvent stream containing water into an upper portion of the extractive distillation zone to provide an aromatic, rich liquid extract stream relatively free of nonaromatic hydrocarbons and comprising sulfolane, aromatic hydrocarbons and water, and a vaporous rafflnate stream comprising non-aromatic hydrocarbons, water, a minor amount of sulfolane and a predetermined amount of aromatic hydrocarbons. The overhead raffinate stream is condensed to provide a first aqueous stream containing a minor amount of sulfolane and a hydrocarbon raffinate containing a minor amount of sulfolane. The hydrocarbon raffinate stream is washed with water to provide a relatively sulfolanefree, non-aromatic raffinate product and a second aqueous stream containing a minor amount of sulfolane. A portion of the first aqueous stream is passed as reflux into the upper portion of the extractive distillation zone, preferably only when the aromatics contained in the raffmate overhead exceeds a predetermined value. The liquid extract is passed into a solvent recovery zone maintained under solvent stripping conditions including the introduction of steam into a lower section of the recovery zone to provide a relatively non-aromatic and sulfolane free extract product and a lean solvent stream containing water at an elevated temperature, typically of about 250F to about 400F. The remaining portion of the first aqueous stream and the second aqueous stream are passed to a water stripping zone maintained at a temperature and pressure sufficient to provide a water vapor stream comprising water. This vapor stream is then condensed in admixture with the previously mentioned vaporous raffinate stream. The water stripping zone is reboiled with at least a portion of the lean solvent stream withdrawn from the solvent recovery zone at an elevated temperature to provide a cooled lean solvent stream which is then passed to the extractive distillation zone as the previously specified sulfolane stream. in a more limited embodiment, aromatic losses in the raftinate overhead are further controlled by lowering the pressure maintained in the water stripping zone when the amount of aromatic hydrocarbons contained in the raffinate exceeds the predetermined level. Lowering the pressure causes further cooling of the lean solvent passed to the extractive distillation zone thereby inducing greater amounts of internal reflux within the extractive distillation zone thereby lowering the amountof aromatic hydrocarbons removed overhead.

in a more limited embodiment the extract removed overhead from the solvent recovery zone is a vapor stream comprising aromatic hydrocarbons and water relatively free of sulfolane. This vapor overhead is condcnsed and the water recovered is utilized as the water with which the sulfolane containing raffinate is washed.

Other objects, embodiments and a more detailed description of the foregoing embodiments will be found in the following, more detailed description of the present invention.

DETAILED DESCRlPTlON OF THE INVENTION Feedstocks suitable for use in the practice of this invention include hydrocarbon fluid mixtures having a sufficiently high concentration of aromatic hydrocarbons to economically justify recovery of these aromatic hydrocarbons as a product stream. The present invention is particularly applicable to hydrocarbon feed mixtures which contain at least 25 percent by weight aromatic hydrocarbons, although hydrocarbon feed mixtures containing as little as l5 percent by weight aromatics and as high as 95 percent by weight aromatic hydrocarbons are suitable. A suitable carbon number range for the feedstock is from six carbon atoms per molecule to about 20 carbon atoms per molecule, and preferably from about six to 10 carbon atoms per molecule. A preferred feedstock source is the debutanized or depentanized effluent from a conventional catalytic reforming unit. Another satisfactory feedstock is the liquid by-product from a pyrolysis gasoline unit which has been hydrotreated to saturate olefins and diolefins, thereby producing an aromatic hydrocarbon concentrate suitable for processing by extractive distillation techniques. A typical feedstock from a catalytic reforming process unit contains single ring aromatic hydrocarbons of a wide boiling mixture including benzene, toluene and xylenes in admixture with the corresponding boiling range paraffms and naphthenes. Thus,- it is these single ring aromatic hydrocarbons whichare to be recovered from the preferred catalytic reforming reactor effluent. It is to'be pointed out, however, while the present invention is directed specifically to the extractive distillation of a hydrocarbon mixture, included within the broad scope of this invention are hydrocarbon mixtures in admixture with aromatic selective solvents such as a solvent-aromatic mixture containing non-aromatic hydrocarbons which is produced, as extract, in a liquid phase solvent extraction zone.

Solvents included within the scope of the present invention are those solvents which exhibit a tendency to selectively dissolve aromatic hydrocarbons when the aromatic hydrocarbons are contained in a nonaromatic mixture. The preferred solvent is a solvent of the sulfolane type. This solvent is a known article of commerce, particularly well known to those skilled in the art of aromatic recovery by selective solvent extraction and typically possesses a five-member ring containing one atom of sulfur and four atoms of carbon with two oxygen atoms bonded to the sulfur atom of the ring.

Other solvents which have a high selectivity for separating aromatic hydrocarbons from non-aromatic hydrocarbons included in the practice of this invention are the sulfolenes such as 2-sulfolene and 3-sulfolene, 2-methylsulfolane, 2-4-dimethyl-sulfolane, methyl-2- sulfonylether, n-aryl-3-sulfonylamine, 2-sulfonylacetate, diethyleneglycol, various polyethyleneglycols, dipropyleneglycol, various polypropyleneglycols, dimethylsulfoxide, N-methylpyrrolidone, glycol-amines, N-methyl-2-pyrrolidone, a N-methyl-2-pyrrolidoneethylene glycol mixture, and various mixtures of the above. As indicated, the specifically preferred solvent for use in the practice of the present invention is sulfolane. Also preferred is a N-methyl-Z-pyrrolidone mixture.

Since the aromatic selectivity of a solvent can be enhanced by the addition of water to the solvent, the pret'crrcd solvent utilized in the practice of this invention contains small quantities of water in order to increase the selectivity of the overall solvent phase for aromatic hydrocarbons over non-aromatic hydrocarbons without substantially reducing the solubility of the solvent phase for aromatics. The presence of water in the solvent composition also provides a relatively volatile material which is distilled from the solvent in the extractive distillation zone which is more fully discussed hereinafter, to help vaporize the last traces of non-aromatic hydrocarbons from the hydrocarbon stream by steam distillation. The composition of the present invention preferably contains from about 0.5 percent to about 20 percent by weight water, in particular from about 2 percent to about percent by weight depending upon the particular solvent utilized and the process conditions at which the extractive distillation zone is operated.

Extractive distillation conditions and techniques are generally well known to those trained in the art and vary depending upon the particular aromatic selective solvent utilized. Therefore, for brevity, the discussion will be generally limited to a sulfolane type solvent, particularly sulfolane, and no intent is made thereby to unduly limit the scope of this invention to this solvent species.

Broadly speaking, extractive distillation conditions for extracting aromatic hydrocarbons from nonaromatic hydrocarbons include low pressures and sufficiently high temperatures in order to vaporize all the non-aromatic hydrocarbons, thereby providing a predominantly non-aromatic fraction containing nonaromatics, a minor amount of aromatic hydrocarbons solvent, and water. It is the specific intent of this invention to limit the amount of aromatic hydrocarbons removed in the non-aromatic fraction recovered in the extractive distillation step. Also, provided is a relatively non-aromatic-free bottoms fraction containing aromatic hydrocarbons and solvent. Typically, this bottoms fraction contains less than 1,000 ppm and prefer ably less than 500 ppm by weight non-aromatic hydrocarbons.

Typical operating conditions in the extractive distillation zone when utilizing a sulfolane solvent, includes a pressure from about 90 mm Hg absolute to 40 psig, an overhead temperature of about l30F to about 330F, and a bottoms temperature of about I70F to about 355F. Sulfolane solvent to feed ratios may vary from about 1:1 to about 20:l depending on the pressure and temperature and feed composition. The preferred solvent to feed ratio when processing a C -C range naphtha is in the range of about 2:1 to about 6:1. A more detailed definitive description of the conditions, apparatus and process flows for an extractive distillation process utilizing a sulfolane type solvent may be found in Petroleum Refiner, September 1959, vol. 38, No. 9, pages 185 through 192, the teachings of which are specifically incorporated herein by reference. As indicated, the extractive distillation zone provides a rich, liquid extract stream relatively free of non-aromatic hydrocarbons comprising sulfolane solvent having dissolved therein the desired aromatic hydrocarbons. Also provided is a vaporous raffinate stream comprising non-aromatic hydrocarbons, water (steam) and a minor amount of sulfolane. The water is present because the lean sulfolane solvent utilized in the preferred embodiments contains water to enhance its selectivity. The vaporous overhead also contains a minor amount of aromatic hydrocarbons which are removed overhead and which represent a loss of aromatic hydrocarbons from the process. The essence of this invention lies in controlling the amount of aromatics, which are lost in this overhead stream, at a predetermined level. This level is dictated by the economics associated with a given unit and feedstock and can vary accordingly. In general, it is. desired to recover more than percent of the aromatics contained in a given feedstock. Simul' taneously associated with reducing aromatic losses will be the reduction in the amount of sulfolane which is present in the raffinate overhead. In any event, this vaporous raffinate stream is cooled and condensed. Upon condensation, two liquid phases are formed, a hydrocarbon phase containing minor amounts of sulfolane and a first aqueous phase, also containing a minor amount of sulfolane. The sulfolane contained in the hydrocarbon phase is removed by scrubbing the hydrocarbon raffinate with water by means well known to those trained in the art, including the utilization of a packed column or a rotating disc contactor. Preferably the water utilized is provided by a sulfolane free water stream recovered from a hereinafter described solvent recovery column. This water washing provides a sulfolane-free (i.e., less than 10 ppmsolvent) raffinate product and a second aqueous stream comprising water and sulfolane. The sulfolane contained in the described first and second aqueous stream is recovered in a water stripping zone to be described later.

The aromatics contained in the extract from the extractive distillation column are recovered in a solvent recovery column of the variety well known to those trained in the solvent extraction art. This column uti' lizes steam as a bottoms input stripping medium to aid in the separation of the aromatics and solvent. Low pressures and sufficiently high temperatures sufficient to distill the aromatic hydrocarbons overhead are utilized with the exact conditions being a function of feedstock composition and solvent. However, overhead pressures of about to about 400 mm Hg absolute and bottoms temperatures of about 250F to about 500F are utilized when sulfolane is the solvent. Produced is an overhead fraction of water and aromatics, free of solvent, which upon condensation yield a final extract product. The water is recovered and is typically utilized as the water input to the raffinate wash column.

The described loss of aromatics from an extractive distillation zone is a problem long recognized by the art. The exact amount of this loss is determined by a compromise dictated by economics; that is, the size of extractive distillation zone and processing conditions utilized which are required to produce a given purity aromatics product balanced against the amount of aromatics to be recovered. In other words, yield is balanced against purity. See, for example, US. Pats. Nos. 3,146,190 and 3,338,824 for a detailed discussion of these particular problems.

Overhead aromatic losses in sulfolane type units is,

particularly vexatious to solve, particularly when processing hydrocarbon feedstocks containing large amounts of aromatics, i.e., 80 percent by weight or more. Aromatic losses stem from trying to attain complete solvent miscibility on all trays of the extractive distillation zone. In conventional distillation processes, losses of higher boiling components as a vapor overhead are readily controlled by refluxing a portion of the overhead after cooling and condensation. However, in extractive distillation such as the sulfolane type, refluxing of the hydrocarbon rafflnate has a tendency to cause phase formation (two liquid phases) within the extractive distillation column, thereby upsetting the heat balance and operation of the column. This is a particular problem when processing highly aromatic feedstocks. One method which can be utilized is controlling the temperature of the lean solvent passed to the upper section of the extractive distillation zone. By choosing a proper temperature, the lean solvent entering acts also as a heat sink thereby causing an internal reflux to form. However, this temperature must be controlled within a relatively narrow range in order to prevent either an aromatic carryover, contamination of the extract with non-aromatics, or the formation of different liquid phases within the extractive distillation zone.

In actual practice, the lean solvent utilized in the extractive distillation zone has been recovered from the bottoms portion of a solvent recovery column at an elevated temperature, typically of about 250F to about 400F. The solvent must then be cooled before it can be used in the extractive distillation zone. The utilization of air cooling or cooling water streams, however, because of variations in atmospheric conditions, cause a wide fluctuation in the degree of cooling attained on the solvent and as a consequence, does not effectively control the aromatic losses from the column.

In the present invention, the temperature of the lean solvent passed to the extractive distillation column is more readily controlled by reboiling the column utilized within the process to recover sulfolane solvent from a dilute aqueous stream. In particular, at least a portion of the hot lean solvent emanating from the solvent recovery column is utilized to reboil a water stripping column wherein at least a portion of the water contained in the spent water stream from the rafflnate wash column and at least a portion of the water recovered from the condensed raffinate overhead stream from the extractive distillation column is vaporized to recover the sulfolane solvent contained therein. More particularly, the water is converted to steam and is used as the steam stripping medium within the solvent recovery column.

Broad conditions to be utilized within the water stripping column are well known to those trained in the art and include a temperature and pressure sufficient to vaporize at least a portion of the water. Preferably, subatmospheric pressures such as a pressure of about 300 mm Hg absolute up to about 760 mm Hg absolute, and a temperature of about 180F to about 250F are maintained in the bottom section of the stripping zone.

To also assist in controlling aromatic losses in the rafflnate overhead, a portion of the water recovered when the vaporous raffinate was condensed may be refluxed into the upper portion of the extractive distillation column. While the exact amount of water can vary according to feedstock and recovery sought, it is preferred that about 25 percent to about 75 percent of the condensed water be refluxed. In particular, it is preferred that this reflux be initiated only when the aromatic losses exceed a predetermined level.

A dilute aqueous solution of sulfolane provides a relatively constant boiling mixture which, when heat exchanged against. hot, lean solvent, provides smooth temperature control on the lean solvent passed from the solvent recovery column to the extractive distillation column. Afforded is a temperature control technique independent of ambient temperature fluctuations since the temperature within the water stripping column is readily maintained within a very narrow range by simply controlling the pressure on the column. Further, dilute solvent solutions are not very sensitive to composition changes. Accordingly, the temperature on the lean solvent stream is controlled by cooling at least a portion of the solvent recovery bottoms in the water stripper reboiler. The exact degree of cooling is readily maintained by either over cooling a portion of the lean solvent with exact control provided by admixing hot lean solvent with the cooled solvent, by varying the pressure of the water stripping zone, or a combination thereof whenever the aromatic losses in the raffinate overhead exceed a predetermined level.

DESCRIPTION OF THE DRAWING The present invention can be most clearly illustrated and described by reference to the attached schematic diagram illustrating the recovery of aromatic hydrocar bons from a catalytic reformate. Of necessity, certain limitations must be present in a schematic diagram of the type presented and no intention is made thereby to limit the scope of this invention as to feedstocks, rates, operating conditions, solvents, etc. Certain miscellaneous appurtenances including some valves, controls, pumps, compressors, separators, reboilers and the like have been eliminated. Only those vessels and lines necessary for a complete and clear understanding of the various embodiments of this invention are illustrated.

Referring now to the attached drawing, a C hydrocarbon feed stream derived from a catalytic reformate containing about 10 percent non-aromatics (paraffins and naphthenes) and about percent aromatic hydrocarbons, of which greater than 90 percent are C enters via line 1, is commingled with make-up or wet solvent entering via line 2 and passed via line 3 to extractive distillation column 4. Extractive distillation column 4 is of a conventional design for the extraction of aromatic hydrocarbons from an aromatic nonaromatic hydrocarbon mixture wherein sulfolane is utilized as the selective solvent. More particularly, sulfolane derived at a specified temperature in a manner to be described later, enters the upper portion of extractive distillation column 4 via line 5 and contacts therein the hydrocarbon feedstock entering through line 3. Removed overhead from extractive distillation column 4 via line 6 is a vaporous raflinate stream containing essentially. all of the non-aromatic hydrocarbons which are passed to the extractive distillation column, and a minor amount of aromatic hydrocarbons and sulfolane. The aromatics removed overhead from extractive distillation column 4 lower the expected aromatic hydrocarbon recovery desired and this amount is to be minimized. More specifically, the present invention controls the amount of aromatic losses by carefully controlling the temperature of the lean solvent feed, containing 3 mole percent water, entering line 5 in response to the amount of aromatics removed overhead.

Referring back to the rafflnate removed overhead via line 6, this vaporous fraction is condensed in heat exchange means 7 and passed to separation receiver 8 wherein a hydrocarbon stream containing sulfolane is separated and removed via line 9 and a water stream, also containing sulfolane, is removed via line 11. Illustrated is a reflux stream 10 which the art has utilized to control aromatic losses within extractive distillation column 4. According to the process of this invention, non-aromatic hydrocarbons removed from separator receiver 8 are seldom, if ever, refluxed to the upper portion of extractive distillation column 4 since their passage thereto may lead to separate liquid phases forming within the column. However, a portion of aqueous stream 11 may be withdrawn via line 12 and passed via line to the upper portion of extractive distillation column 4 to assist in controlling aromatic over head losses. Preferably, this reflux is passed to column 4 only when the aromatics contained in the vaporous raftinate exceeds a predetermined level which, as indicated, is a function of the aromatic recovery desired in the entire process.

Non-aromatic hydrocarbon raffinate is passed via line 9 to the lower portion of water wash column 24 wherein the hydrocarbon is scrubbed with water entering via line 23, the source of which is to be described later. Within wash column 24 the incoming water and hydrocarbon countercurrently contact each other. and the sulfolane solvent contained within the aromatic hydrocarbon is removed as a dilute aqueous medium (i.e., about 1 percent sulfolane mole basis) via line 26. An essentially sulfolane free raffinate is removed via line 25. The sulfolane contained within lines 26 and 11, as a dilute aqueous solution, are commingled and passed via line 27 to water stripping zone 28 to recover the sulfolane therein, in a manner to be described later.

Removed from the bottom portion of extractive distillation column 4 is a rich sulfolane stream relatively free of non-aromatic hydrocarbons. This rich aromatic extract is passed to recovery column 14 for separation therein of the aromatic hydrocarbons from the sulfolane solvent. More specifically, recovery column 14 is reboiled by a hot oil reboiling means 15 contained within liquid trap out tray 16 with stripping steam derived from a source to be described later, entering via line 37. Removed overhead from recovery column 14 is an aromatic extract relatively free of sulfolane solvent which is passed via line 17, commingled with condensate make-up entering via line 18, and condensed in heat exchange means 19. The resultant condensate is passed to separator receiver 20 wherein a sulfolane free aqueous stream is recovered and removed via line 23 and passed to raffinate wash column 24 as the described wash water medium. A hydrocarbon stream is removed via line 21 with a portion thereof passed via line 22 as reflux to the upper portion of recovery column 14. More specifically, the hydrocarbons recovered and removed from the process via line 21 represent at least 98.5 percent of the C aromatic hydrocarbons originally passed to extract distillation column 4 via line 1.

In a typical recovery column operation, recovery column 14 is maintained under sub-atmospheric pressures such as an overhead pressure of 100 mm Hg absolute and a bottoms pressure of about 440 mm Hg absolute. Further, hot oil reboiler is preferably maintained at a sufficient temperature so that the film temperature of the sulfolane never exceeds 450F to avoid sulfolane decomposition within the recovery column. Lean sulfolane solvent is removed from recovery column 14 via line 29 with a small portion thereof removed via line 30, passed through valve 31, and commingled with cooled sulfolane solvent 5 for reasons to be described later. The majority of the sulfolane solvent, typically 90 percent of that solvent removed from recovery column 14, is passed via line 29 to reboiler means 32. More specifically, the dilute aqueous stream in line 27 is passed to water stripper 28 for recovery therein of the sulfolane solvent removed overhead in the vaporous raffinate. Water stripper 28 is typically maintained at an overhead temperature of about 205F, an overhead pressure of 670 mm Hg absolute, a bottoms temperature of about 212F and a bottoms pressure of atmospheric pressure. Dilute, sulfolane: containing water, is removed from a lower portion of column 28 via line 34 and passed to reboiler means 32 for indirect heat exchange with the hot lean sulfolane solvent. Another portion of the dilute water stream is passed via line 35 to vaporizer 33. Water stripping column 28 and vaporizer 33 are maintained at a relatively constant temperature because of the dilute nature of the sulfolane solution passed thereto and provides a constant temperature source for cooling the lean sulfolane solvent removed from recovery column 14. In a specific situation, 90 percent of the lean solvent is removed via line 29 at a temperature of about 350F and is cooled in reboiling means 32 to a temperature of about 3l8F before passage to vaporizer 33. Within vaporizer 33 the lean solvent is further cooled to a temperature of about 250F thereby vaporizing the great majority of the water and sulfolane passed to the water stripper 28.

The lean sulfolane solvent is then passed via line 29 and commingled with the remaining 10 percent of the lean solvent to provide a final sulfolane feed stream which is maintained at a temperature of 260F. When processing an aromatic feedstock of the composition described, bymaintaining the lean solvent at a temperature of 260F, aromatic recoveries of 98.5 percent are economically feasible. Water and sulfolane heated and- /or vaporized in water stripper 28, are removed via line 36 and commingled with vaporizer 33 effluent, removed via line 35 and commingled in line 37 to provide the steam stripping stream necessary for efficient operation of recovery column 14. Removed overhead from water stripper 28 is an aqueous vapor fraction which is passed via line 38 and commingled with the vaporous raffinate in line 6.

The amount of aromatic hydrocarbon removed overhead from extractive distillation column 4 can be measured by installing analyzing means, not shown, in

line

6, 9 or 25. When the aromatic losses exceed a predetermined level necessary for 98.5 percent aromatic recovery, a portion of the aqueous stream recovered from the condensed raffinate overhead can be refluxed t0 the upper portion of the extractive distillation column 4. In addition, the lean solvent stream can be further cooled to a temperature below the 260F indicated. This additional cooling when aromatic losses exceed the predetermined value, can be accomplished by either cooling a greater portion of the lean solvent removed from recovery column 14 or by lowering the pressure on water receiver 28 and/or the pressure in vaporizer 33 thereby inducing a greater portion of the water to be vaporized which induces further cooling of the lean sulfolane solvent. Since the dilute sulfolane mixture processed in water stripper 28 and vaporizer 33 is a relatively constant boiling'mixture, this method of controlling the temperature of the lean solvent stream passed to the extractive distillation column is much more reliable than by cooling the hot lean solvent with conventional air cooling or cooling water means,

which are subject to fluctuations in ambient temperature conditions.

I claim as my invention:

1. In a process for the separation of aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons wherein a lean, aromatic selective solvent recovered from a solvent recovery fractionation column is contacted with said mixture in an extractive distillation zone to produce an aromatic rich extract stream and a non-aromatic raffinate overhead, and water is separated from a relatively dilute water stream containing solvent in a water stripping zone, the improved method of controlling the amount of aromatic hydrocarbons removed in the non-aromatic rafflnate which comprises the steps of:

i. reboiling the water stripping zone maintained at a pressure sufficient to vaporize at least a portion of the water passed thereto with at least a portion of the lean solvent recovered from the solvent recovery zone thereby cooling said lean solvent;

ii. passing at least a portion of the cooled solvent to an upper portion of the extractive distillation zone; and

iii. lowering the pressure in the water stripping zone when the amount of aromatic hydrocarbons removed overhead in the non-aromatic raffinate exceeds a predetermined level thereby further cooling the lean solvent and lowering the amount of aromatic hydrocarbons removed overhead in the raffinate.

2. The method of claim 1 wherein said selective solvent comprises a sulfolane type solvent.

3. The method of claim 1 wherein said selective solvent comprises sulfolane.

4. The method of claim 1 wherein said selective solvent comprises N-methyl-2-pyrrolidone.

5. The method of claim 1 wherein said non-aromatic raffinate comprises hydrocarbons, solvent and water in a vapor phase and is condensed, a condensed water phase recovered and at least a portion of the water phase is passed, as reflux, into an upper portion of the extractive distillation zone.

6. The method of claim 5 wherein said reflux is utilized only when the amount of aromatic hydrocarbons removed in the raffinate exceeds a predetermined level.

7. A method for controlling aromatic losses from a process for separating non-aromatic and aromatic hydrocarbons by extractive distillation which comprises the steps of:

i. introducing a hydrocarbon feedstock containing aromatic and non-aromatic hydrocarbons into a sulfolane type extractive distillation zone maintained under extractive distillation conditions including the passing of a hereinafter specified lean sulfolane stream containing water into an upper portion of the extractive distillation zone to provide a rich liquid extract stream relatively free of non-aromatic hydrocarbons comprising sulfolane and aromatic hydrocarbons and a vaporous raftinate stream comprising non-aromatic hydrocarbons, water, a minor amount of sulfolane and a predetermined amount of aromatic hydrocarbons;

ii. condensing the raffinate stream to provide a first aqueous stream containg a minor amount of sulfolane and a hydrocarbon raffinate stream containing a minor amount of sulfolane;

iii. washing said hydrocarbon rafflnate stream with water to provide a relatively sulfolane free raffimate product and a second aqueous stream contain a minor amount of sulfolane;

iv. passing a portion of the first aqueous stream as reflux into the upper portion of the extractive distillation zone;

v. passing the liquid extract into a solvent recovery zone maintained under solvent stripping conditions including the introduction of steam into a lower section of the recovery zone to provide a relatively non-aromatic and sulfolane free extract product and a lean sulfolane stream containing water at an elevated temperature;

vi. passing the second aqueous stream and the remaining portion of the first aqueous stream into a water stripping zone maintained under a temperature and pressure sufficient to provide a water vapor stream comprising water;

vii. condensing the water vapor stream in admixture with the raffinate stream in step (ii).

viii. reboiling the water stripping zone with at least a portion of the lean solvent stream to provide a cooled lean solvent stream; and

ix. passing at least a portion of the cooled solvent stream to the extractive distillation zone as said specified sulfolane stream.

8. The method of claim 7 wherein the pressure maintained on the water stripping zone is lowered when the amount of aromatic hydrocarbons contained in the rat'- finate exceeds the predetermined level.

9. The method of claim 7 wherein the reflux of step (iv) is effected only when the aromatic content in the raffinate exceeds a predetermined level.

10. The method of Claim 7 wherein the lean sulfolane stream of step (v) is at a temperature of about 250F to about 400F.

11. The method of claim 7 wherein the sulfolane free extract of step (v) is a vapor overhead relatively free of non-aromatic hydrocarbons and sulfolane comprising aromatics and water is condensed and separated to provide the water utilized inv washing the raffinate stream in step (iii).

Claims

2. The method of claim 1 wherein said selective solvent comprises a sulfolane type solvent.
3. The method of claim 1 wherein said selective solvent comprises sulfolane.
4. The method of claim 1 wherein said selective solvent comprises N-methyl-2-pyrrolidone.
5. The method of claim 1 wherein said non-aromatic raffinate comprises hydrocarbons, solvent and water in a vapor phase and is condensed, a condensed water phase recovered and at least a portion of the water phase is passed, as reflux, into an upper portion of the extractive distillation zone.
6. The method of claim 5 wherein said reflux is utilized only when the amount of aromatic hydrocarbons removed in the raffinate exceeds a predetermined level.
7. A method for controlling aromatic losses from a process for separating non-aromatic and aromatic hydrocarbons by extractive distillation which comprises the steps of: i. introducing a hydrocarbon feedstock containing aromatic and non-aromatic hydrocarbons into a sulfolane type extractive distillation zone maintained under extractive distillation conditions including the passing of a hereinafter specified lean sulfolane stream containing water into an upper portion of the extractive distillation zone to provide a rich liquid extract stream relatively free of non-aromatic hydrocarbons comprising sulfolane and aromatic hydrocarbons and a vaporous raffinate stream comprising non-aromatic hydrocarbons, water, a minor amount of sulfolane and a predetermined amount of aromatic hydrocarbons; ii. condensing the raffinate stream to provide a first aqueous stream containg a minor amount of sulfolane and a hydrocarbon raffinate stream containing a minor amount of sulfolane; iii. washing said hydrocarbon raffinate stream with water to provide a relatively sulfolane free raffinate product and a second aqueous stream contain a minor amount of sulfolane; iv. passing a portion of the first aqueous stream as reflux into the upper portion of the extractive distillation zone; v. passing the liquid extract into a solvent recovery zone maintained under solvent stripping conditions including the introduction of steam into a lower section of the recovery zone to provide a relatively non-aromatic and sulfolane free extract product and a lean sulfolane stream containing water at an elevated temperature; vi. passing the second aqueous stream and the remaining portion of the first aqueous stream into a water strIpping zone maintained under a temperature and pressure sufficient to provide a water vapor stream comprising water; vii. condensing the water vapor stream in admixture with the raffinate stream in step (ii). viii. reboiling the water stripping zone with at least a portion of the lean solvent stream to provide a cooled lean solvent stream; and ix. passing at least a portion of the cooled solvent stream to the extractive distillation zone as said specified sulfolane stream.
8. The method of claim 7 wherein the pressure maintained on the water stripping zone is lowered when the amount of aromatic hydrocarbons contained in the raffinate exceeds the predetermined level.
9. The method of claim 7 wherein the reflux of step (iv) is effected only when the aromatic content in the raffinate exceeds a predetermined level.
10. The method of Claim 7 wherein the lean sulfolane stream of step (v) is at a temperature of about 250*F to about 400*F.
11. The method of claim 7 wherein the sulfolane free extract of step (v) is a vapor overhead relatively free of non-aromatic hydrocarbons and sulfolane comprising aromatics and water is condensed and separated to provide the water utilized in washing the raffinate stream in step (iii).