US3619419A

US3619419A - Solvent recovery process

Info

Publication number: US3619419A
Application number: US835544A
Authority: US
Inventors: Herbert Lytle Thompson
Original assignee: Universal Oil Products Co
Current assignee: Honeywell UOP LLC; Universal Oil Products Co
Priority date: 1969-06-23
Filing date: 1969-06-23
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09

Abstract

An improved process for the extraction of water soluble primary solvent contained in a raffinate hydrocarbon stream produced by a primary extraction process, wherein the primary solvent is contacted with an aqueous secondary solvent under extraction conditions in an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks. The improvement comprises passing the solvent-containing raffinate through the perforations of the extraction decks at a hole velocity greater than 2 feet per second, and preferably in the range of from about 5 to about 8 feet per second. The process has particular application where the raffinate stream comprises nonaromatic hydrocarbons, the primary extraction process comprises an aromatic process, the secondary solvent comprises water, and the primary solvent comprises a sulfolane-type chemical solvent or a polyalkylene glycol solvent. One preferred application is in the recovery of sulfolane from nonaromatic raffinates in the water wash tower of an aromatic extraction processing plant.

Description

United States Patent I 72] Inventor Herbert Lytle Thompson Park Ridge, 111. [21] Appl. No. 835,544 [22] Filed June 23, 1969 [45] Patented Nov. 9, 1971 [73] Assignee Universal Oil Products Company Des P1aines,lll.

[54] SOLVENT RECOVERY PROCESS 15 Claims, 3 Drawing Figs.

[52] US. Cl 208/321, 196/ 14.52 [51] 1nt.C1 Cl0g 21/28 [50] Field of Search 208/321; 196/ 14.52

[56] References Cited UNITED STATES PATENTS 2,614,031 10/1952 Tickler 196/1452 2,667,407 1/1954 Fenske et a1 196/1452 2,767,068 10/1956 Maycock et a1. 196/1452 2,921,015 1/1960 Shiras 208/321 RAFFINATE 3,209,047 9/1965 Young Primary ExaminerHerbert Levine Att0rneys.lames R. Hoatson, Jr. and Philip T. Liggett ABSTRACT: An improved process for the extraction of water soluble primary solvent contained in a raffmate hydrocarbon stream produced by a primary extraction process, wherein the primary solvent is contacted with an aqueous secondary solvent under extraction conditions in an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks. The improvement comprises passing the solvent-containing raffinate through the perforations of the extraction decks at a hole velocity greater than 2 feet per second, and preferably in the range of from about 5 to about 8 feet per second. The process has particular application where the raffinate stream comprises nonaromatic hydrocarbons, the primary extraction process comprises an aromatic process, the secondary solvent comprises water, and the primary solvent comprises a sulfolane-type chemical solvent or a polyalkylene glycol solvent. One preferred application is in the recovery of sulfolane from nonaromatic raffinates in the water wash tower of an aromatic extraction processing plant.

NET r fi RAFFINATE 7 CLEAN WASH L. Y WATER l 1 l J l 3 I T l i.

| l l I l RICH WASH WATER PATENTEUNDV SHEET 1 OF 2 NET - RAFFINATE Z LEAN WASH T WATER (FEW i 1 J H- I L I 1 I I I '1 i 9 T 1 I3 '0 |4\ H 1 3 T L C I RAFF|NAT'\ w l l l FIGURE l RICH WASH WATER I INVENTOR. HERBERT LYTLE THOMPSON A 7' TOR/V575 SOLVENT RECOVERY PROCESS FIELD OF THE INVENTION The present invention relates to the solvent extraction of aromatic hydrocarbons from a hydrocarbon charge stream. More particularly, the present invention relates to the recovery of solvent from the nonaromatic raffmate produced by the solvent extraction of aromatics form a hydrocarbon charge stream. Most specifically, the present invention relates to an improved process for the recovery of solvent from the nonaromatic raffmate by means of a secondary extraction process.

It is well known in the art that the nonaromatic raftinate which leaves the extraction zone of an aromatic hydrocarbon extraction process contains solvent. The solvent which is withdrawn in the raffimate stream must be recovered not only because it may interfere with subsequent raffmate processing or ultimate raffinate use, but primarily because continual loss of solvent in the raffinate stream is a prohibitive economic expense in the aromatic extraction process. The recovery of the solvent from the raffmate stream may be accomplished by distillation, or adsorption, or by a secondary solvent extraction process.

A typical solvent which is utilized in commercial aromatics extraction and which may be recovered in accordance with the practice of this invention is a solvent of the sulfolane type. The solvent possesses a five-membered ring containing one atom of sulfur and four atoms of carbon, with two oxygen atoms bonded to the sulfur atom of the ring. Generically, the sulfolane type solvents may be indicated as having the following structural formula:

wherein R,, R R and R are independently selected from the group comprising a hydrogen atom, and alkyl group having from one to ten carbon atoms, and alkoxy radical having from one to eight carbon atoms, and an arylalkyl radical having from one to 12 carbon atoms.

Other solvents which may be included within this process are the sulfolenes such as 2-sulfolene or 3-sulfolene which have the following structures:

2-Sulfolene Other typical solvents which have a high selectivity for separating aromatics from non-aromatic hydrocarbons and which may be processed within the scope of the present invention are 2-methylsulfolane, 2,4-dimethylsulfolane, methyl 2- sulfonyl ether, n-aryl-3-sulfonyl amine, 2-sulfonyl acetate, diethylene glycol, various polyethylene glycols, dipropylene glycol, various polypropylene glycols, dimethyl sulfoxide, N- methyl pyrollidone, etc.

The specifically preferred solvent chemical which is processed within the scope of the present invention is sulfolane, having the following structural formula:

DESCRIPTION OF THE PRIOR ART Because the typical solvents which are utilized in aromatics extraction are water soluble, it is the practice to extract the solvent from the raftinate stream by contact with an aqueous stream in a subsequent extraction means. The extraction of the solvent from the raffinate with water may be undertaken in any suitable liquid-liquid contacting means, as in a tower containing suitable packing such as Berl Saddles or Raschig Rings, or in a tower containing suitable trays, or in a rotating disc contactor (RDC). The solvent may then be readily recovered from the aqueous solution by distillation.

One typical extraction tower containing extraction trays, wherein the solvent is recovered by extraction from the nonaromatic raftinate with water, is the so-called "rain deck" extraction tower. In this type of operation, the extraction tower contains a plurality of perforated extraction trays or decks containing upcomer conduit means. The water phase collects above the deck and passes down through the perforations contained therein, while the hydrocarbon phase is contained under the deck and passes up to the next contacting stage via the upcomer conduit means contained in the deck. The water falling through the perforations of the extraction deck rains down through the hydrocarbon phase which is immediately under the deck. Thus the name rain deck" is typically utilized for this type of water wash tower.

Another version wherein a suitable tray device is used in water washing the nonaromatic rafi'mate in order to recover the solvent therefrom, comprises a plurality of perforated decks wherein downcomer conduit means are provided. In this type of deck, the water is retained above the trays and passes down to the contacting stage below via the downcomer conduits. The hydrocarbon which is contained below the deck passes up through the perforations of the deck and up through the water phase thereby providing for extraction of the solvent contained in the raffinate by the water phase which is on the deck.

IN utilizing this type of water wash extraction tray or deck, it is common in the art to provide a 2 foot spacing between adjacent trays of the plurality. It is further the practice to provide that the space between the decks contain about I foot of liquid level of aqueous phase above the deck, and about 1 foot of hydrocarbon phase above the aqueous phase. In addition, it is the practice to pass the hydrocarbon up through the perforations in the decks at flow rates below a hole velocity of 2 ft./sec., and preferably to operate with a hole velocity of about 1 ft./sec. These limitations of tray spacing and hole velocity are imposed in such water wash operations in order to minimize entrainment of the phases, and maximize the extraction efficiency for the decks.

It has been discovered, however, that in commercial aromatic extraction units the recovery of solvent from the raffinate by extraction with the water does not correspond to the recovery which is to be anticipated based upon solubility data and the assumption of the reasonable efficiency of the extractor.

It is obvious in the art to provide additional physical stages in the aqueous extractor in order to enhance the recovery of the solvent. Such a solution to the problem of poor extraction efficiency is technically feasible but it is not a preferred solution since it requires that the number of physical stages in the aqueous extractor must be increased. Not only in this a prohibitively uneconomical expedient but once a commercial unit has been placed on stream it is often a physical impossibility to modify the existing facility to provide the required additional contacting stages. The preferred solution to the problem, therefore, is to subject the solvent rich raffmate stream to extraction conditions which will render the raffinate means.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a process for the recovery of water soluble solvent from a nonaromatic rafiinate stream by aqueous extraction.

It is a particular object of the present invention to provide a means for the recovery of water soluble solvent from a nonaromatic raffinate stream in an aqueous extraction means containing a minimum number of physical stages.

it is a more specific object of the present invention to minimize the number of physical stages in the aqueous extraction means and simultaneously achieve recovery of water soluble solvent in a more facile and economical manner.

It has been determined that these objectives may be achieved by bringing the nonaromatic raffinate stream into contact with an aqueous phase in an extraction zone containing not a greater number of physical stages, but containing a fewer number of physical stages than have heretofore been utilized in the art.

it has now been determined that increased efficiency of extraction may be obtained and a fewer number of physical stages may be utilized within the extraction zone, if the nonaromatic raffinate is passed upward through the perforations of the extraction trays at hole velocities which are substantially greater than what has been heretofore utilized in the prior art. As noted hereinabove, in extraction operations wherein the raffinate is water washed by being passed up through the perforations of the extraction trays, while the water phase passes to the tray below via downcomer means, the raffinate has been passed through the perforations of the extraction trays at hole velocities consistently below 2 feet per second, and more normally at hole velocities of about 1 foot per second. It has now been determined that by the practice of the present invention, increased efficiency of extraction is obtained at hole velocities substantially greater than 2 ft./sec., and in particular that increased economic benefits can be achieved with hole velocities in the range of from about 5 ft./sec. to about 8 ft./sec.

Therefore, in accordance with the practice of the present invention, one embodiment comprises an improved process for the extraction of water soluble primary solvent contained in a raffinate stream produced by a primary extraction process, wherein the primary solvent is contacted with an aqueous secondary solvent under extraction conditions in an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks, wherein the improvement comprises passing the solvent containing raffinate stream through the perforations of the extraction decks at a hole velocity greater than 2 ft./sec.

As shall be set forth more clearly hereinafter, the present invention may be further characterized as a process for separating a water soluble primary solvent from a solvent containing raffinate hydrocarbon stream produced in a primary extraction process which comprises, contacting the raffinate hydrocarbon stream in a contacting zone with a hereinafter specified hydrocarbon stream under conditions sufficient to provide a mixed hydrocarbon stream containing the water.

soluble solvent; passing the mixed hydrocarbon stream into an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks, wherein the mixed hydrocarbon stream is contacted with a first stream of aqueous secondary solvent under extraction conditions, said conditions comprising the flow of the mixed hydrocarbon through the perforations of the extraction decks at a hole velocity greater than 2 ft./sec.; withdrawing from the extraction zone a second stream of aqueous secondary solvent containing the primary solvent; and, withdrawing from the extraction zone a mixed hydrocarbon stream having substantial freedom from the primary solvent, and passing a portion thereof to contacting zone as the specified hydrocarbon stream.

The present invention may be further characterized by the two embodiments set forth immediately hereinabove wherein the raffinate stream comprises nonaromatic hydrocarbons, the

primary extraction process comprises an aromatic extraction process, the secondary solvent comprises water, the primary solvent comprises either a sulfolane type chemical or at least one polyalkylene glycol.

The process of the present invention and its operational basis, is clearly set forth in the accompanying figures. FIG. 1 consists of a simplified schematic flow diagram illustrating the broad embodiment of the invention and one preferred process wherein the invention is practiced. FIG. 2 comprises a set of curves obtained form laboratory data wherein there is shown the enhanced extraction of solvent from the nonaromatic raffinate within increased hole velocities as the raffinate passes through the perforations of the extraction tray. FIG. 3 illustrates design data for a specific example of raffinate composition and shows the influence of hole velocity upon different design considerations for the extraction tower.

The inventive process may now be more readily understood by discussing the figures in greater detail.

FIGURE I As noted hereinabove, FIG. 1 sets forth a simplified schematic flow diagram of the process wherein the present invention may be practiced.

Referring now to FIG. 1, a typical nonaromatic raffinate stream containing a primary solvent in solution, leaves an aromatic extraction zone, not shown, at a temperature normally in excess of 150 F. This raffinate stream is cooled in heat exchanger means, not shown, and enters the process of the present invention via line 1 at a temperature of F. or less. The raffinate stream passes via line 1 to an aqueous extraction or water wash tower 2 which contains a plurality of perforated extraction trays 3. Each extraction tray comprises a perforated deck and suitable downcomer means. Since those skilled in the extraction art are familiar with extraction tray construction, the perforated trays 3 are not shown in great detail and, for illustrative purposes only, are shown as single pass extraction trays.

Water Wash Tower 2 is maintained under operating conditions sufficient to remove the primary solvent from the nonaromatic raffinate by means of a lean wash water which passes into the water wash tower via line 4. Among these operating conditions are a temperature in the range of from 60 to F., and normally a temperature of about 100 F. The pressure must be sufficient to keep all constituents in a liquid state, and the pressure will, therefore, be normally in excess of 30 p.s.i.g. The lean wash water is injected into the water wash tower 2 at a mol ratio of water to raffinate hydrocarbon, in the range of from 0.5 mols per mol to 2.0 mols per mol. Normally, when sulfolane is the primary solvent, the wash water passes into the extraction tower 2 via line 4 at a rate sufficient to maintain from 1 mol to 1.4 mols of water for every mol of nonaromatic raffinate entering the tower. When a polyalkylene glycol solvent is the primary solvent to be recovered from the raffinate, the normal range of operation comprises 0.5 mols to 1.0 mols of water for every mol of raffinate containing the glycol solvent.

The extraction trays within the water wash tower 2 may be any type of suitable extraction tray, provided that they are operated with the raffinate passing up through the perforations in the decks. This means that each deck will support a layer of aqueous phase which is maintained thereon and overflows via downcomer means to the extraction tray below. The hydrocarbon phase is confined below each extraction tray and above the aqueous phase which is held on the extraction tray below. As the rafiinate passes up through the tower, it passes through the perforations of each extraction tray 3 at a hole velocity greater than 2 fi./sec., and preferably at a hole velocity in the range of from about 5 to about 8 ft./sec.

By operating the water wash tower 2 under the conditions set forth hereinabove, an improved extraction efficiency is obtained and the primary solvent is removed from the raffinate with increased recovery. A rich wash water leaves the bottom of the raffinate water wash tower 2 via line 5 and passes to subsequent processing for the recovery of the primary solvent therefrom. The net raffinate leaves the water wash tower 2 via line 6, and passes either to subsequent processing or to storage facilities, not shown. The net raffinate will contain a lower quantity of primary solvent than has been heretofore experienced in the prior art manner of operation of hole velocities below 2 ft./sec.

It is common in the art of aromatic extraction to design the aromatic extraction plant with the capability of running at 50 percent of design flows. Consequently, provision must be made to maintain the operation of the water wash tower 2 at the desired hole velocities in the extraction tray in order to continue to obtain increased efficiencies of extraction despite fluctuations in raffinate production. In order to achieve this, a portion of the net raffinate may be recycled so that the hole velocities in the extraction trays will be maintained at the desired rates.

REferring again to FIG. 1, when the raffinate entering the process via line 1 decreases in rate of flow, either through a decrease in the rate of charge of hydrocarbon to the aromatic extraction tower, not shown, or due to change in the composition of the feed to the aromatic extraction tower, the raffinate rate to water wash tower 2 will be reduced. Under such circumstances, a portion of the net raffinate is withdrawn from line 6 via line 7 at a controlled rate. The rate of flow via line 7 is controlled by flow control valve 8 in a manner sufficient to maintain the total flow of hydrocarbon through the water wash tower 2 at rates sufficient to produce the desired hole velocity through the perforations in the trays 3. The recycled portion of the net raffinate passes through the control valve 8 and into line 9. It hereafter passes through a block valve 10 and into the water wash tower 2 via line 11. This is the preferred method of return for the net raffinate which is recycled to the water wash tower 2. The fresh raffinate containing the primary solvent is contacted with the recycle raffinate under the lowest of the extraction trays 3, and the mixed hydrocarbon stream then passes up through the perforations of the extraction tray at the desired hole velocity.

As an alternative method, the portion of the raffinate which is recycled in order to maintain the hole velocity within the water wash tower 2, may be withdrawn via line 12 and passed through block valve 13 before passing into line 1 via line 14. When such an operation is utilized, the block valve 10 in line 9 remains closed and the-contacting of the fresh raffinate containing the primary solvent, with the recycle raffinate which is substantially free of solvent, occurs in line 1 before the mixed raffinate stream passes into the water wash tower 2.

Those skilled in the art will thus perceive that the contacting zone wherein the solvent-containing feed raffinate is contacted with the recycled portion of solvent-free rafi'inate, may be either internal or external to the water wash tower which contains the aqueous extraction zone.

FIGURE II As indicated hereinabove, FIG. 2 sets forth an example of experimental laboratory data which shows that increased hole velocity produces an increased extraction efiiciency in accordance with the present invention.

The data shown in FIG. 2 were obtained from laboratory experiments which were made for a single extraction deck utilizing a nonaromatic raffinate from a commercial extraction plant. The raffinate was blended with various concentrations of primary solvent and synthetic wash water was made by blending pure water with various concentrations of primary solvent. Various concentrations of primary solvent in the wash water were made in order to simulate operation at various locations (upper and lower) of a typical water wash tower.

The raffinate which was utilized in the series of experiments had a gravity of 69.8 API at 60 F., and it had a boiling range of 155 F. to 357 F. This raffinate comprised 98.2 liquid volume percent of nonaromatics and L8 liquid volume percent of aromatics. The raffinate was blended with various amounts of primary solvent and utilized in all the experimental work.

Referring now to FIG. 2, there is shown a plot of the percent extracted vs. hole velocity. Three curves are containing in FIG. 2 and all curves are for an extraction deck containing circular perforations 0.2083 centimeters in diameter. Curve A shows the increased efficiency which is obtained by increasing hole velocity when contacting the raffinate containing 50 ppm. of chemical sulfolane with a wash water containing ppm. of the sulfolane solvent. Curve B is a similar curve for contacting the raffinate containing 550 ppm. sulfolane with water containing 150 ppm. sulfolane. Curve C illustrates the contacting of raffinate containing 550 ppm. of sulfolane with water containing 10,000 ppm. of sulfolane.

The curves of FIG. 2 clearly show that increased hole velocity increases extraction efficiency regardless of the amount of solvent contained in the hydrocarbon phase and regardless of the amount of solvent contained in the aqueous phase. Similar curves were obtained on other extraction tests utilizing other hole sizes.

FIGURE In FIG. 3 sets forth curves for illustrating maximum effectiveness of the present invention as applied to a commercial water wash tower design. In FIG. 3, design considerations are set forth for water washing a primary solvent comprising chemical sulfolane from a raffinate having he same characteristics as the raffinate disclosed in the discussion relative to FIG. 2. That is to say, the design basis comprises a raffinate having a gravity of 69.8 API, containing 1.8 liquid volume percent of aromatics, and having a boiling range of from F. to 357 F.

The design apparatus basis for the curve contained in FIG. 3 is a 4-foot diameter extraction vessel containing extraction decks having %-inch diameter holes. Those skilled in the art realize, of course, the hole size is not of specific significance since the hole velocity is the critical factor in the present invention. The design basis for this water wash tower is a hole velocity at 75 percent of flooding velocity. The design basis, further, is for an extractor tray containing downcomers sufficient to hold I foot of aqueous phase on each deck.

The design operational basis is for an aqueous extraction on the raffinate to reduce the raffinate content from 550 ppm. of sulfolane solvent in the input raffinate, to produce a net raffinate hydrocarbon containing only 1.0 ppm. of sulfolane.

Utilizing the data obtained in the laboratory for sulfolane solvent systems, a curve is obtained which relates the number of extraction trays required within the water wash tower vs. the hole velocity through the extraction trays in feet per second. This curve illustrates that as the hole velocity increases the total number of trays required is decreased for a specific recovery of sulfolane solvent.

The next curve which is illustrated in FIG. 3 is a plot of the tray spacing within the water wash tower vs. the hole velocity through the extraction deck in feet per second. Those skilled in the art realize that tray spacing is a calculated figure obtained by the hydraulic balance for the specific extractor deck being utilized. More simply put, the tray spacing is in effect the amount of liquid head of water contained within the downcomer which is needed to drive the hydrocarbon phase up through the holes of the extraction tray, and through 1 foot of water phase contained on the deck above, at a given hole velocity. Referring to FIG. 3 it is seen that as hole velocity increases the tray spacing required between adjacent extraction trays increases accordingly.

The next curve contained within FIG. 3 is a plot of column height vs. hole velocity. This curve is based upon the knowledge of the number of trays utilized at a given hole velocity and the tray spacing required for that hole velocity. This curve of column height, however, is not proportional to these two factors since in addition to the calculated column height due to number of trays and tray spacing, there is provided a 14 foot additional tower height for the bottom and the top of the water wash tower. This additional distance of 14 feet is provided in order to provide a settling zone at the top of the water wash tower and a similar zone at the bottom of the water wash tower. These zones provide a quiescent zone at the top and the bottom wherein the hydrocarbon and aqueous phases may be settled out before being withdrawn from the water wash tower. It is seen from this curve in FIG. 3, that as the hole velocity increases and tray spacing thereby increases, the column height additionally increases to an apparent maximum at a hole velocity of about 9 ft./sec. Above this hole velocity, so few extraction trays are'required in the water wash tower that it is readily apparent why the column height decreases.

The three curves of FIG. 3 define operation at a maximum extraction efficiency. That is to say, the design basis provided for aqueous extraction of a nonaromatic raffinate containing 550 p.p.m. of chemical sulfolane in a water wash tower to produce a raft'inate containing only 1.0 p.p.m. of sulfolane. Thus the extraction efficiency was in excess of 99 percent.

The next question, therefore, is where this maximum efficiency can be obtained at lowest expense. The cost of a water wash tower is essentially equal to the cost of the column and the cost of the extraction trays contained therein. Cost estimates of the water wash tower were made based upon the curves of FIG. 3. It was found that at hole velocities in excess of 5 ft./sec. the cost of the water wash tower was substantially equivalent. However, it was also found that at velocities below 5 ft./sec. the cost of the water wash tower increased with decreasing hole velocity. At low hole velocities, the column was shorter but a greater number of extraction trays were needed to obtain the constant degree of extraction. Those skilled in the art realize that extraction trays are an expensive item of equipment. Thus, having a shorter water wash tower containing more extraction trays, is more expensive than having a taller tower containing a fewer number of extraction trays positioned at a larger tray spacing.

It may, therefore, be concluded from the from the data that reduced capital expense is obtained by maintaining the extraction hole velocity in excess of 2 ft./sec. and preferably in excess of 5 ft./sec.

From the curves contained within FIG. 3 it might be inferred that maximum economy and maximum efficiency would be obtained at a higher hole velocity. However, other considerations limit the preferred operation to a maximum hole velocity of about 8 ft./sec.

As noted hereinabove, tray spacing is fixed by hydraulic balance for a given hole velocity. Higher hole velocities require a higher tray spacing up to about 8 ft./sec. It was found in the laboratory, that the required tray spacing was adequate for settling of phases so that entrainment was not so great that the extraction efficiency benefits derived by high bole velocity was lost. Thus, it was found that a hole velocity of up to 8 ft./sec. and a tray spacing of up to 12 feet could be utilized to obtain a water wash tower at minimum cost while still obtaining the benefit of improved extraction efficiency. The laboratory data indicated that at hole velocities exceeding 8 ft./sec., entrainment became a problem such that there is the danger that under some operating conditions, the tray spacing demanded for proper hydraulic balance may not be sufficient to allow proper settling out of the phases between the trays. Therefore, the efficiency of extraction derived by high velocity could be lost due to entrainment problems. While additional tray spacing could be added to obtain proper settling of phases, the added spacing results in a tower height such that the capital cost may become prohibitive.

By referring again to FIG. 3, another reason may be found for limiting the operation to a maximum hole velocity of 8 ft./sec. In FIG. 3, it will be seen that the number of trays required for the extraction at 8 ftJsec. is only four, and that at hole velocities above 8 ft./sec. a reduced number of extraction trays are required. By limiting the water wash tower to such a small number of extraction trays, there is introduced into the water wash extraction step, the danger of an ineffective water washing of the primary solvent from the nonaromatic raffinate. This danger is that under some circumstances of operation, the aromatic extraction unit wherein the water wash tower comprises one operating step, may be subjected to flow fluctuation and operational upsets. If such an upset should occur and the water wash tower contains less than four extraction trays, a substantial slut" of the primary solvent may enter the water wash tower with the nonaromatic raftinate and pass out of the tower without being extracted out of the raffinate. It is, therefore, desirable to provide a sufficient number of extraction trays within the water wash tower to minimize the danger that any upset or operational swings at the.aromatic extraction column will produce upsets or flow fluctuations at the water wash tower sufficient to allow the loss of a substantial amount of the primary solvent. Thus, in the preferred embodiment the water wash tower should contain at least four extraction trays and preferably more than four, so that hole velocities in excess of 8 ft./sec. must be avoided.

In conclusion, therefore, it may be summarized that the effectiveness of the present invention is enhanced by operating the solvent recovery step with the raffinate passing through the perforations of the extraction trays contained within the water wash tower at hole velocities in excess of 2 ft./sec. and preferably with hole velocities within the range of from 5 to 8 ft./sec. The effectiveness of this preferred range of operation may be illustrated by the following example which is based upon the operations of a commercial aromatic extraction unit.

EXAMPLE A commercial aromatics extraction unit was operated on an aromatics concentrate feedstock to extract benzene, toluene, xylene and C +ar0matics form the hydrocarbon feed. A net raffinate was withdrawn from the aromatic extraction process which had a boiling range in the C,,C range and a gravity of 62.9 API. This raftinate stock contained 2.7 mol percent of aromatic hydrocarbons and 1.2 mol percent sulfolane solvent.

Referring now to FIG. 1, the raftinate entered the water wash tower of the commercial aromatic extraction unit via line 1 at a rate of 2263 b.p.s.d. (barrels per stream day) or a rate of 212.3 mols per hour. At these flow rates, the rafiinate feed to the water wash tower 2 contained 2.5 mols per hour of sulfolane solvent which was required to be recovered in the commercial process.

The water wash tower 2 contained seven extraction trays 3 having 54-inch holes. The tray spacing within the extraction tower was 92 inches between adjacent extraction trays (7 ft. 8 inches A wash water stream entered the water wash tower 2 via line 4 at a rate of 371 b.p.s.d. or a rate of 300 mols per hour. This lean wash water was derived from the extract stripping column of the commercial aromatic extraction unit, and the wash water therefore contained about I00 p.p.m. of sulfolane solvent and a trace of low boiling aromatic hydrocarbon.

The raffinate stream entered the extractor column 2 via line 1 and passed upward through the perforations of the seven extraction trays at a hole velocity of about 6.1 ft./sec. The water wash operation was maintained at a temperature of about I00 F. and a pressure of 65 p.s.i.g.

A net wash water was withdrawn from the bottom of the water wash tower 2 via line 5 at a rate of 387 b.p.s.d. or 302.5 mols per hour. This net wash water contained 2.5 mols per hour of sulfolane solvent which was extracted from the hydrocarbon phase in the extraction zone comprising the seven extraction trays.

The net raffinate product was withdrawn from the top of the water wash tower 2 via line 6 at a rate of 2247 b.p.s.d. or 209.8 mols per hour. The net raffinate product from the water wash tower consistently analyzed a sulfolane solvent content in the range of from about 10 p.p.m. of sulfolane to about 12 p.p.m. of sulfolane.

amount of sulfolane solvent. Prior art experience indicates that typical operation would require water washing of the raffinate at hole velocities less than 2 ft./sec., and normally 1 ft./sec., through a water wash tower containing a substantially greater number of extraction trays than was utilized in the commercial operation described herein, Under such prior art operating conditions, it is anticipated that the net raffinate product recovered via line 6 would contain from about 50 ppm. of sulfolane solvent to about 100 ppm. sulfolane solvent.

During the commercial operation described hereinabove, no recycle ,of raffinate was required since sufficient raffinate product was produced at the aromatics extraction column to maintain the hole velocities required within the water wash tower. However, during other .periods of the commercial operation, charge rates to the aromatic extraction tower were cut back and recycle of the net raffnate wasrequired'at the water wash tower. The recycle of the raffinate was conducted via line 7, control valve 8, line 9, line 12, block valve 13, and line 14, thereby effecting contact of the recycle-raffinate and the fresh raffinate feed externally to the water wash tower. Under suchcircumstances of operation it was found that since the hole velocity was maintained in range of from 6.0 to 6.4 ft./sec. at all times, the extraction of the primary sulfolane solvent from the rafi'rnate phase by the wash water continued to be maximized sothat the net raffinate product contained substantially the same low quantity of sulfolane solvent as was experienced when no recycle of raffinate was required.

PREFERRED EMBODIMENT The effectivenessv of the present invention and the process of the present invention have been clearly set forth by the disclosure hereinabove.

It is particularly noteworthy that the above disclosure sets forth a method whereby increased extraction efficiencyis effected while simultaneously reducing the number of physical stages required within, the extraction zone. Therefore, the process of the present invention sets forth a method whereby an existing aqueous extraction or water wash tower may be modified to improve extraction of the primary solvent from the raffinate, without requiring the insertion of additional mass transfer stages. Contrary to what the prior art. would lead one to expect, improved extraction efficiency may be obtained by removing extraction trays from the existing aqueous extraction tower instead of adding trays. Of course, the operation must be further modified to provide for the passage of raffinate through the perforations of the extraction trays at greater hole velocities, but this is simply provided by installing a process line for-the recycleof a portion of the net product raffinate in the manner disclosed hereinabove.

Thus, the present invention provides increased recovery of primary solvent at minimum expense for future water wash systems to be constructed, and for existing water wash systems which must be modified to correctproblems of poor solvent recovery.

Those skilled in the art realize that the effectiveness of the present invention is influenced by a great many factors; For example, the degree of extraction depends upon the specific primary solvent which is being removed from the raffinate hydrocarbon stream by the water wash step. .In addition, the effectiveness and the operating conditions required will be influenced by the temperature of the raffinate stream entering the water wash tower, the solvent content of the raffinate stream entering the,inventive process from the aromatic extraction unit, the solvent content of the lean wash water, and the specific operating conditionswithin the water wash tower. It must further be noted that .the solvent content of the nonaromatic raffinate will vary, since it is dependent upon the temperature level of the preceding aromatic extraction processing unit and upon the mo] percent'of aromatics remaining in the nonaromatic raffinate stream. lt is, therefore, not possible to define specifically the operating conditions which are required within the water wash tower. However,

those skilled in the art can readily ascertain the operating con- I ditions which may be required for any specific raffinate composition by utilizing the teachings which have been presented hereinabove.

Therefore, it may now be summarized that a preferred embodiment of the present invention comprises an improved process for the extraction of water soluble primary solvent contained in a raffinate stream produced by a primary extraction process, wherein the primary solvent is'contacted with an aqueous secondary solvent under extraction conditions in an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks, wherein the improvement comprises passing the solvent containing rafi'rnate stream through the perforations of the extraction decks at a hole velocity in the range of from about 5' ft./sec. to about 8 it may further be summarized, that another preferred embodiment of the 'present:invention comprises a process for separating a water soluble primary solvent from a solvent containing raffinate hydrocarbon stream produced in a primary vsoluble solvent; passing the mixed hydrocarbon stream into an extraction zone comprising an extraction tower containing a plurality of perforated extraction decks wherein the mixed hydrocarbon stream is contacted with a first stream of aqueous secondarysolvent under extraction conditions, the conditions comprising the flow of mixed hydrocarbon through the perforation 'ofthe extraction deck at a hold velocity in the range of from about 5 ft./sec. to'about 8 ft./sec.; withdrawing from the extraction zone a second stream of aqueous secondary solvent containing the primary solvent; and withdrawing from the extraction zone a-mixed hydrocarbon stream having substantial freedom from the primary solvent, and passing a portion thereof to the contacting zone as the specified hydrocarbon stream.

The invention claimed:

1. In a process for the extraction of water soluble primary solvent contained in a nonaromatic hydrocarbon raffinate stream produced by a primary aromatic extraction process,

. wherein said primary solvent is contacted with an aqueous wherein R R,,-R and R are independently selected from the group comprising a hydrogen atom,an alkyl group having from one to ten carbon atoms, an arylalkyl radical having from one to 12 carbon atoms, and an alkoxy radical having from one to eight carbon atoms.

4. Process of claim 3 wherein said primary solvent comprises sulfolane.

5. Process of claim 2 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolene selected from the group consisting of Z-sulfolene and 3-sulfolene.

6. Process of claim 2 wherein said secondary solvent comprises water, and said primary solvent comprises at least one polyalkylene glycol.

7. Process of claim 6 wherein said primary solvent comprises at least one glycol selected from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.

8. Process for separating a water soluble primary solvent from a solvent-containing nonaromatic raffinate hydrocarbon stream produced in a primary aromatic extraction process which comprises:

a. contacting said raffinate hydrocarbon stream in a contacting zone with a hereinafter specified hydrocarbon stream under conditions sufficient to provide a mixed hydrocarbon stream containing said water soluble solvent;

b. passing said mixed hydrocarbon stream into an extraction zone comprising an extraction tower containing a plurality of stationary perforated extraction decks, wherein said mixed hydrocarbon stream is contacted with a first stream of aqueous secondary solvent under extraction conditions,said conditions comprising the flow of mixed hydrocarbon through the perforations of said extraction decks at a hole velocity greater than 2 feet per second;

c. withdrawing from said extraction zone a second stream of aqueous secondary solvent containing said primary solvent; and,

d. withdrawing from said extraction zone a mixed hydrocarbon stream having substantial freedom from said primary solvent, and passing a portion thereof to said contacting zone as said specified hydrocarbon stream.

9. Process of claim 8 wherein said hole velocity is maintained in the range of from about 5 ft./sec. to about 8 ft./sec.

10. Process of claim 9 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolanetype chemical of the general formula:

wherein R R R and R are independently selected from the group comprising a hydrogen atom, an alkyl group having from one to ten carbon atoms, an arylalkyl radical having from one to 12 carbon atoms, and an alkoxy radical having from one to eight carbon atoms.

11. Process of claim 10 wherein said primary solvent comprises sulfolane.

12. Process of claim 9 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.

13. Process of claim 9 wherein said secondary solvent comprises water, and said primary solvent comprises at least one polyalkylene glycol.

14. Process of claim 13 wherein said primary solvent comprises at least one glycol selected from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.

15. Process of claim 8 wherein said contacting zone comprises a lower section of said extraction tower and said extraction zone comprises an upper section of said extraction tower containing at least a portion of said plurality of extraction decks.

Claims

2. Process of claim 1 wherein said improvement comprises a hole velocity in the range of from about 5 ft./sec. to about 8 ft./sec.
3. Process of claim 2 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolane-type chemical of the general formula:
4. Process of claim 3 wherein said primary solvent comprises sulfolane.
5. Process of claim 2 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.
6. Process of claim 2 wherein said secondary solvent comprises water, and said primary solvent comprises at least one polyalkylene glycol.
7. Process of claim 6 wherein said primary solvent comprises at least one glycol selected from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.
8. Process for separating a water soluble primary solvent from a solvent-containing nonaromatic raffinate hydrocarbon stream produced in a primary aromatic extraction process which comprises: a. contacting said raffinate hydrocarbon stream in a contacting zone with a hereinafter specified hydrocarbon stream under conditions sufficient to provide a mixed hydrocarbon stream containing said water soluble solvent; b. passing said mixed hydrocarbon stream into an extraction zone comprising an extraction tower containing a plurality of stationary perforated extraction decks, wherein said mixed hydrocarbon stream is contacted with a first stream of aqueous secondary solvent under extraction conditions, said conditions comprising the flow of mixed hydrocarbon through the perforations of said extraction decks at a hole velocity greater than 2 feet per second; c. withdrawing from said extraction zone a second stream of aqueous secondary solvent containing said primary solvent; and, d. withdrawing from said extraction zone a mixed hydrocarbon stream having substantial freedom from said primary solvent, and passing a portion thereof to said contacting zone as said specified hydrocarbon stream.
9. Process of claim 8 wherein said hole velocity is maintained in the range of from about 5 ft./sec. to about 8 ft./sec.
10. Process of claim 9 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolane-type chemical of the general formula:
11. Process of claim 10 wherein said primary solvent comprises sulfolane.
12. Process of claim 9 wherein said secondary solvent comprises water, and said primary solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.
13. Process of claim 9 wherein said secondary solvent comprises water, and said primary solvent comprises at least one polyalkylene glycol.
14. Process of claim 13 wherein said primary solvent comprises at least one glycol selected from the group consisting of diethylene glycol, dipropylene glycol, and triethylene glycol.
15. Process of claim 8 wherein said contacting zone comprises a lower section of said extraction tower and said extraction zone comprises an upper section of said extraction tower containing at least a portion of said plurality of extraction decks.