Phthalic anhydride formation and separation

A method for obtaining a partially purified phthalic anhydride product from the liquid effluent containing from 70 up to 92% o-phthalic acid, from 1.5 up to 21% water, from 0.3 up to 13% benzoic acid and/or acetic acid by weight and impurities boiling above and below the boiling point of phthalic anhydride which are oxygen containing aromatic compounds including precursors of o-phthalic acid as well as compounds containing cobalt, manganese;

• or bromine derived from the catalysis used for the preparation of such liquid effluent by the neat air oxidation of liquid xylene at a temperature of from 150.degree. C. up to 230.degree. C. under a gauge pressure of from 17 up to 30 kg/cm.sup.2 ;
• which method comprises the steps of continuous in situ dehydration of o-phthalic acid to phthalic anhydride and flash evaporation of it from said liquid effluent at a pressure of from 760 mm Hg down to 40 mm Hg and a temperature of from 180.degree. C. up to 250.degree. C. leaving a fluid residue fraction containing materials boiling above said anhydride;
• continuous removal of the vapor fraction and the liquid residue fraction as separate streams from said dehydration-evaporation;
• continuous removal of heat from the vapor fraction to cool it to a temperature below the boiling temperature of phthalic anhydride but above the dew point of water whereat partially purified phthalic anhydride condenses to the liquid state, and the continuous separation of uncondensed vapors from the liquid partially purified phthalic anhydride condensate.