The invention relates broadly to the manufacture of pure t-butylstyrene and in particular to the refining of t-butylstyrene for removal of dialkenylbenzene impurities.
Tertiary-butylstyrene (tBS) is a compound which is advantageously prepared by catalytic oxidative dehydrogenation (OXD) of t-butylethylbenzene (tBEB). Tertiary-butylstyrene has many uses; e.g., as a chemical intermediate, as a monomer or comonomer in the production of polymeric materials, and the like. Teriary-butylstyrene has often replaced styrene in some applications because desirable physical and chemical product properties result from such a substitution. In addition, there are processes where styrene is not suitable but where tBS functions well.
Because tBS belongs to the same family as styrene, there are similarities in the chemistry of its preparation. 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.degree. C. higher than that of styrene, the tendency for tBS to polymerize is much greater than that of styrene in any of the commercial processes for purifying styrene.
Some of the differences between styrene and tBS derive 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.
In addition to some higher boiling hydrocarbon contaminants, there are particularly two dialkenylbenzene compounds which are present in the crude tBS stream obtained from the OXD reactor; i.e., isopropenylstyrene and butenylstyrene.
Careful conventional distillation of the recovered tBS fraction will remove the higher boiling impurities and also most of the butenylstyrene. However, isopropenylstyrene is very difficult to remove by such conventional distillation and will remain with the tBS at a concentration which is above the maximum limit for many applications. For instance, in some polymerization processes the isopropenylstyrene content that can be tolerated is less than about 100 ppm.
In copending application Ser. No. 646,267, filed Aug. 31, 1984, an extractive distillation process, has been disclosed for the removal of dialkenylbenzene contaminants from t-butylstyrene using anhydrous sulfolane as solvent. This process works well; however, the use of reduced pressure and elevated temperature contribute considerably to the equipment, inhibitor, and utilities costs of the overall process.
Trials with anhydrous sulfolane as solvent in liquid/liquid extraction at mild conditions indicated that pure tBS cannot be produced by this technique because tBS and anhydrous sulfolane are completely miscible.
It is, therefore, a principal object of the present invention to provide a refining process utilizing mild operating conditions to obtain a tBS product containing only trace quantities of dialkenylbenzene contaminants.