Recovery of polycarboxylic acid anhydrides

Claim 1

THE PROCESS FOR THE RECOVERY OF ANHYDRIDES OF POLYCARBOXYLIC ACIDS SELECTED FROM THE GROUP CONSISTING OF MALEIC ANHYDRIDE AND PHTHALIC ANHYDRIDE FROM A GASEOUS REACTION MIXTURE CONTAINING AN ANHYDRIDE IN THE VAPOR STATE, COMPRISING PRECOOLING THE GASEOUS REACITON MIXTURE TO A TEMPERATURE BELOW THE CONDENSATION POINT OF THE ANHYDRIDE TO LIQUIFY A PORTION OF THE ANHYDRIDE VAPOR PRESENT IN THE GASEOUS REACTION MIXTURE, SEPARATING THE LIQUIFIED ANHYDRIDE FROM THE GASEOUS REACTION MIXTURE, PASSING THE GASEOUS REACTION MIXTURE CONTAINING RESIDUAL ANHYDRIDE IN THE VAPOR STATE INTO CONTACT WITH A SOLVENT THEREFOR COMPRISING DIBUTYL PHTHALATE TO REMOVE RESIDUAL ANHYDRIDE, AND STRIPPING ANHYDRIDE FROM THE SOLVENT AT A TEMPERATURE BETWEEN 175* C. AND 250* C. AND A PRESSURE BETWEEN 5 MM. AND 50 MM. MERCURY, BELOW THE THERMAL DECOMPOSITION POINT OF THE SOLVENT AT THE EXISTING SUBATMOSPHERIC PRESSURE, WHEREBY A SUBSTANTIALLY COMPLETE RECOVERY OF ANHYDRIDE IS OBTAINED.