GB787577A - Improvements in or relating to the manufacture of isopropenyl acetate - Google Patents

Abstract

Isopropenyl acetate is obtained by a continuous process which comprises passing the mixture of gases obtained from the pyrolysis of acetone to keten and a catalyst as defined below into a reaction vessel containing already condensed gases from the said pyrolysis and the catalyst, wherein the resulting reaction mixture is boiled and a distillate comprising essentially acetone and isopropenyl acetate is removed, the reaction conditions being such as to maintain the concentration of isopropenyl acetate (with respect to acetone) in the boiling reaction mixture at a higher value than in the vapours withdrawn from the reaction vessel. The catalyst which is added continuously to the reaction mixture is one known to promote the reaction of keten and acetone to give isopropenyl acetate, specified catalysts being sulphuric acid, p-toluenesulphonic acid, o-nitrobenzenesulphonic acid and diacetic naphthalene-1:5-disulphonic acid dianhydride. The concentration of catalyst in the reaction mixture is preferably maintained at about 0.25 per cent by weight. The distillate obtained from the reaction vessel consists of isopropenyl acetate, acetone, and by-product acetic anhydride, and in order to maintain the volume of liquor in the reaction vessel reasonably constant a proportion of the distillate may be fed back to the reaction vessel at the liquor level and the remainder of the distillate is passed continuously to other equipment for purification of the isopropenyl acetate and recovery of acetone for re-use in the process. The heat required to maintain the reaction mixture at the boil may conveniently be supplied by the sensible heat in the pyrolysis gases and the reaction may be controlled by partial cooling of the pyrolysis gases or by varying the proportion of the distillate recycled to the reaction vessel. In examples: (1) acetone is pyrolysed at 650 DEG C. to yield a gas comprising acetone and keten, with small amounts of methane, ethylene, and carbon monoxide, and the hot gas is partially cooled and fed continuously into the bottom of an agitated reaction vessel containing a reaction liquor comprising acetone, isopropenyl acetate and by-products (mainly acetic anhydride). The reaction liquor is kept boiling by the heat of the feed gas and the vapours are cooled to give a condensate and some noncondensable gas which is allowed to pass out of the system. The condensate has the composition acetone 77.8 per cent, isopropenyl acetate 21.1 per cent and by-products (mainly acetic anhydride) 1.1 per cent. Half of this condensate is returned to the reaction vessel at the liquor level and other half is fractionally distilled to remove high boiling products (mainly acetic anhydride) and to recover acetone for re-use in the process. The gas inlet temperature to the reaction vessel is suitably between 200 DEG and 400 DEG C. and the equilibrium composition of the liquid in the reaction vessel is controlled by the reaction conditions and is such that acetone and isopropenyl acetate are present in the ratio of about 54 to 46 parts by weight. Diacetic naphthalene 1,5-disulphonic acid dianhydride is added continuously to the reaction liquor as catalyst and small amounts of nonvolatile impurities such as spent catalyst which tend to accumulate in the reaction liquor are controlled by continuously purging liquid from the reaction vessel; (2) as in (1) except that the keten-containing gas is cooled to a constant temperature of 300 DEG C. and the proportions of the condensate recycled to the reaction vessel are controlled to give the same product take-off rate as in (1). Specification 723,417 is referred to.