Formation, purification and recovery of phthalic anhydride

摘要

Phthalic anhydride of commercially acceptable quality is recovered in high (94-96%) yields from a mixture containing on a weight basis from 70 to 90% o-phthalic acid, 1.5 to 21% water, 0.3 up to 13% benzoic acid, 0.2 up to 2% o-toluic acid, 0.2 to 1% 2-carboxybenzaldehyde, 0.1 up to 2% phthalide and from 1.3 up to 10% higher boiling materials by rapid dehydration of o-phthalic acid to its anhydride and rapid evaporation thereof followed by contact of the resulting vapor mixture with an inert reflux liquid in a fractionation zone to remove water and to provide a partial purification of the anhydride, removal of phthalide therefrom by only heating said partially purified anhydride in the presence of a catalytic amount of an alkali metal hydroxide having a molecular weight of at least 56 followed by fractionation of the phthalide-free mixture. Such process is more commercially attractive than melting said mixture containing 70 to 90% o-phthalic acid to dehydrate it to its anhydride and recovering the anhydride by fractionation because such apparently simple process cannot produce phthalic anhydride of a color or purity to be commercially acceptable nor can it provide an anhydride product substantially free of phthalide.