US 4,215,051 AGranted

Formation, purification and recovery of phthalic anhydride

AssigneeStandard Oil Company (Indiana)

InventorsHobe Schroeder, David A. Palmer

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Abstract

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 20% 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.

A process for the formation, purification and recovery of phthalic anhydride from a mixture comprising on a weight basis from 70 up to 90 percent o-phthalic acid, from 1.5 up to 21 percent water, from 0.3 up to 13 percent benzoic acid, from 0.2 up to 2 percent o-toluic acid, from 0.1 up to 2 percent phthalide, from 0.2 up to one percent 2-carboxybenzaldehyde, and from 1.3 up to 10 percent higher boiling compounds;

which process comprises subjecting said mixture as a liquid to the sequential steps of (a) rapid dehydration of o-phthalic acid to its anhydride under conditions for rapid evaporation of said anhydride wherein a mixture of vapors of said anhydride, water, and the oxygen-containing aromatic compounds comprising benzoic acid, o-toluic acid, phthalide and 2-carboxybenzaldehyde is formed;
(b) separating water vapor from said mixture of vapors by contacting in a fractionation zone such mixture of vapors in countercurrent flow with a reflux liquid at a temperature below the boiling temperature of said anhydride which liquid is inert to the components of said mixture of vapors, a solvent for said anhydride at a temperature between its freezing point and its melting point, remains liquid at and below the freezing point temperature of said anhydride, and boils at a temperature below the boiling point temperature of said anhydride;
(c) removal of phthalide from the resulting water free mixture of said anhydride and said oxygen-containing aromatic compounds by heating to a temperature above 200.degree. C. such mixture of said anhydride and oxygen-containing aromatic compounds in the presence of an alkali metal hydroxide having a molecular weight of at least 56 until the presence of phthalide is no longer detectable;
(d) fractionation of the mixture resulting from step (c) to remove:
as a first fraction substantially all of the benzoic acid, o-toluic acid and 2-carboxybenzaldehyde;
as a second fraction product phthalic anhydride and leave as a third fraction the compounds boiling higher than phthalic anhydride;
and (e) contacting said product fraction and 0.25 to 0.5 weight percent water vapor with a solid noble metal catalyst to remove bromine therefrom.

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Abstract

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