GB2163741A - Recovery of sulfolane - Google Patents

Abstract

A process for the purification of sulfolane which has been used as a solvent in the preparation of t- butylstyrene comprises contacting an aqueous sulfolane solution with a liquid hydrocarbon separating the hydrocarbon and the sulfonate solution, and distillating the sulfolane solution to recover sulfolane containing only small quantities of water and substantially free of organic contaminants. <IMAGE>

Description

SPECIFICATION Recovery of sulfolane The invention relates broadly to a solvent recovery process and in particular to the recovery of sulfolane used in the purification of t-butylstyrene.

Tertiary-butylstyrene (tBS) is a compound which is advantageously prepared by the catalytic oxydehydrogenation of t-butylethylbenzene (tBEB). It has many uses, e.g. as a chemical intermediate or as a monomer or comonomer in the production of polymeric materials. Tertiary-butylstyrene has replaced styrene in some applications because desirable physical and chemical product properties result from such a substitution. In addition, there are proceses where styrene is not suitable but where tertiary-butylstyrene functions well.

Because tBS belongs to the same family as styrene, there are similarities in the chemistry.

One of the common properties is the tendency for the styrenics to polymerize whenever they are activated by chemicals or by heat. Some of the techniques used in purifying styrene can be used to purify tBS. However, because the boiling point of tBS is about 70"C higher than that of sytrene, the tendency for tBS to polymerize is m4ch greater than that of styrene in any of the commercial processes for purifying styrene.

Some of the differences between styrene and tBS are derived from the compounds of the dialkenylbenzene family, that are present in tBS but not in styrene. These crosslinking compounds can polymerize to give a type of polymer that interferes with the operation of refining equipment. The crosslinked polymer has a tendency to collect in the equipment and to resist attempts to dissolve it.

As described and claimed in Application No.

(N.39246), unreacted tBEB feed can be separated from tBS while minimizing operational problems due to equipment fouling by extractive distillation under reduced pressure using sulfolane as solvent and removing solfolane by water dilution and separation. The sulfolane, containing small amounts of unsaturated and saturated hydrocarbon, is then recovered by distillation to remove water and can be recycled to the process.

Also, in Application No. (N.39235) filed concurrently herewith, there is disclosed and claimed a process for the refining of tBS wherein the concentrations of the aforementioned dialkenylbenzenes notably isopropenylstyrene and butenylstyrene, are reduced to very low levels. The process also involves the extractive distillation of tBS feed with sulfolane under reduced pressure, recovering refined tBS as overhead product and sulfolane bottoms product containing the divinyl contaminants, small amounts of tBS as well as other higher boiling hydrocarbons. The sulfolane bottoms is recycled to the process and preferably at least a portion thereof is treated by periodic or continuous distillation for removal of impurities to prevent excessive accumulation thereof within this process loop.

It has been found, however, that even very careful distillation of used sulfolane, either aqueous or non-aqueous, is not sufficient for maintaining extended trouble free operations in the recovery and purification of tBS. In fact, after a relatively few passes through the system the sulfolane becomes darker and more viscous and the efficiency of the extractive distillation process in which it is used is not as high as with fresh sulfolane.

The present invention seeks to provide a process for the purification of sulfolane which has been employed in the recovery of tBS.

In accordance with the present invention there is provided a process for the purification of used sulfolane from a tertiary butylstyrene recovery process wherein the used sulfolane contains polymeric and other hydrocarbon contaminants, which process comprises: (a) introducing the used sulfolane as an aqueous solution to an extraction zone; (b) intimately contacting said solution in the extraction zone with a liquid hydrocarbon solvent; (c) separating an aqueous solution of sulfolane depleted of polymeric and other contaminants from the solvent; (d) distilling in a distillation zone the separated sulfolane to remove water overhead and (e) recovering purified sulfolane as distillation zone bottoms.

The invention will be described in more detail by reference to the accompanying figure.

which is a schematic drawing of an apparatus in which the process of the invention may be performed.

In a typical operation, aqueous sulfolane in line 1 can be one obtained from a treatment zone (not shown) for recovery of tBS from an oxydehydrogenation effluent also containing tBEB as well as hydrocarbon by-products including divinyl aromatic contaminants. The used sulfolane will contain these compounds in small concentrations as impurities. However, the main impurity is a polymer accumulated from the previous treatment zone. Also the used sulfolane will contain water, typically between 25 and 75 wt.%, used to separate the tBS from the tBS-sulfolane stream exiting the treatment zone. Alternatively, the stream in line 1 can be obtained before the water dilution from a treatment zone where a tBS stream is purified to reduce the content of divinyl aromatic contaminants. Again, such a stream will contain the same type of polymeric and divinyl aromatic impurities as described before.Obviously, the stream in line 1 can also be a mixture of effluents from the two aforementioned treatment zones.

The used sulfolane is contacted as an aque ous solution in extraction zone 2 with a hydrocarbon solvent introduced in line 3. Suitable solvents are liquid aliphatic hydrocarbons such as pentanes, hexanes, heptanes and octanes or aromatic hydrocarbons such as benzene, ethylbenzene, xylene, toluene and tBEB, and others which are liquid under the extraction conditions. Preferably the solvent is an aromatic. The extraction is advantageously carried out by countercurrent operation employing a liquid hydrocarbon solvent to aqueous sulfolane solution volumetric ratio of from 1:10 to 10:1. The extraction can be carried out over a wide range of conditions, however, for practical reasons the temperature is usually from 20"C to 900C. If so desired the raffinate can be subjected to a further extraction (not shown) similar to that described above.An extract containing at least a portion of the impurities present in the used sulfolane feed in line 1 is withdrawn in conduit 4. The aqueous soluflane depleted in impurities is introduced via line 5 into conventional distillation zone 6 equipped with means for providing reflux 7 and with a reboiler arrangement as shown at 8. Water is withdrawn from the overhead condenser via line 9 and it is advantageously returned to the point in the overall tBS process where water is required for separating sulfolane from hydrncar,bons e.g. from tBS.

The distillation operating conditions should be chosen such that the temperature of the sulfolane is not allowed to exceed 220"C and preferably not more than 200"C. The pressure is controlled by the temperature of the available cooling water for condensation of the water recovered overhead and returned as reflux to the top of the distillation zone. Suitable pressures for the distillation step are between 4 and 20 kPa (30 and 150 mm Hg).

The sulfolane stream recovered as bottoms in line 10 contains negligible quantities of undesired impurities, however, further reduction of these can be achieved if so desired by a redistillation of the bottoms. Also, if desired, the dry sulfolane may be filtered to remove any insoluble material that may be present. In addition, any residual quantities of water can be removed by treatment with desiccants.

The following Example illustrates the invention.

EXAMPLE One volume of a used sulfolane solution containing about 50 wt. /0 water, about 0.8 wt. % hydrocarbon impurities, mostly in the form of tBS, and 0.3 wt. % polymerization inhibitors is intimately contacted two times in a 380 dm3 (100 U.S. gallon) size glass-lined agitated extraction kettle with 1/2 volume of ethylbenzene in each step. The extraction temperature is about 25"C. After the final separation of ethylbenzene layer from the aqueous sulfolane layer, the latter is distilled in a vacuum still equipped with 3.05 m (10 ft.) packed distillation column having a diameter of 7.6 cm (3 inches). The operating conditions include a reflux of 1:10, and a maximum reboiler temperature of 170"C and a pressure of about 12 kPa (90 mm Hg). The bottom product is pure sulfolane containing about 0.1 wt % water.

Integration of the aforementioned purification as a step in an overall tBS recovery process resulted in sustained, contiruous, trouble-free operation of the extractive distillation column used for separation of tBS from unreacted tBEB in a program lasting about 2 months.

However, after a few passes through the process cycle when the extraction step was not included in the sulfolane purification, the sulfolane became dark and viscous, and the efficiency of the extractive distillation column became less efficient. If operations had been allowed to continue, there were strong indications that the reboiler would completly plug up.

Claims (12)

1. A process for the purification of used sulfolane from a t-butylstyrene recovery process, wherein the sulfolane contains polymeric and other hydrocarbon contaminants, which process comprises: (a) introducing the used sulfolane as an aqueous solution to an extraction zone; (b) intimately contacting said solution in the extraction zone with a liquid hydrocarbon solvent; (c) separating an aqueous solution of sulfolane depleted of polymeric and other contaminants from the solvent; (d) distilling in a distillation zone the separated sulfolane to remove water overhead and (e) recovering purified sulfolane as distillation zone bottoms.

2. A process according to claim 1 wherein the separated sulfoiane of step (c) is subjected to a second extraction before distilling step (d).

3. A process according to claim 1 or 2 wherein the liquid hydrocarbon solvent is an aiiphatic hydrocarbon selected from pentanes, hexanes, heptanes and octanes.

4. A process according to claim 1 or 2 wherein the liquid hydrocarbon solvent is an aromatic hydrocarbon selected from benzene, ethylbenzene, toluene, xylene and t-butylethylbenzene.

5. A process according to any one of the preceding claims wherein the liquid hydrocarbon solvent to aqueous solution of sulfolane volumetric ratio in step (b) is between 1:10 and 10:1.

6. A process according to any one of the preceding claims wherein the temperature of step (b) is from 20"C to 90"C.

7. A process according to any one of the preceding claims wherein the distillation zone bottoms are maintained at a temperature not exceeding 220 C.

8 A process according to claim 7 wherein said temperature does not exceed 200 C.

9. A process according to any one of the preceding claims wherein the pressure of the distillation step (d) is between 4 and 20 kPa (30mm Hg and 150 mm Hg).

10. A process according to any one of the preceding claims wherein the water content of the aqueous sulfolane solution is between 25 and 75 wt. %.

11. A process according to claim 1 sub stantially as described with reference to the Examples.

12. Use of sulfolane purified by a process as claimed in any one of the preceding claims in the purification of t-butylstyrene by extrac tive distillation.