Process for the desublimation of phthalic anhydride

In a process of the separation of phthalic anhydride from reaction gas obtained from the catalytic air oxidation of o-xylene which comprises cooling the gas in a separator in indirect heat exchange relationship with tubular cooling surfaces inside the separator maintained at 45.degree.-60.degree. C., collecting and withdrawing residual reaction gas through a gas outlet and melting off resultant desublimated phthalic anhydride from the cooling surfaces by heating the latter to a temperature in the range of 150.degree.-250.degree. C. said separator having a separator case section extending above the uppermost tubular cooling surfaces,

• the improvement wherein the internal surface of said separator case section extending above the uppermost tubular cooling surfaces is maintained during the cooling of the gas inside the separator at a temperature of 150.degree.-250.degree. C. to prevent a buildup of non-meltable impurities on said uppermost tubular cooling surfaces.