Production of ortho-phthalic acid and its conversion and recovery of

Abstract

Phthalic anhydride of commercially acceptable quality can be produced continuously in yields of from about 80 up to about 83 mole percent (% of theoretical) through a combination of steps starting with the neat oxidation of liquid o-xylene with air in the presence of catalysis provided by cobalt, manganese and bromine. Said oxidation is conducted in two steps wherein the first step is conducted under mild conditions and short residence time to provide a liquid effluent containing 8 to 40 weight percent o-phthalic acid and 6 to 30 weight percent o-xylene and the second step oxidizes such liquid effluent composition under more vigorous conditions including higher reaction temperature and longer residence time until substantially all the o-xylene and precursors of o-phthalic acid have been converted to o-phthalic acid. Thereafter, the second liquid oxidation effluent is subjected to simultaneous rapid dehydration and phthalic anhydride evaporation followed by contact of the resulting mixture of vapors with a reflux liquid in a fractionation system to remove water as part of the top fraction and provide a liquid partially purified anhydride as a separate bottom fraction, heating said liquid anhydride with a catalytic amount of KOH to remove phthalide and then fractionating the phthalide-free liquid partially purified anhydride to obtain, in known manner, high purity phthalic anhydride product.