Oxidation of o ‐Xylene and Naphthalene to Phthalic Anhydride

Phthalic anhydride (PA) is produced for many years on an industrial scale by the vapor-phase oxidation of o -xylene and/or naphthalene with air over a vanadium oxidation catalyst in a multitubular fixed-bed reactor. The reaction typically occurs under explosive conditions at slightly above ambient pressure and high temperature (390–450 °C) with an excess of air. The exothermic heat of reaction is used for the production of high-pressure steam. The reaction gas is cooled for additional heat recovery, followed by an optional post-reactor, and further cooling before entering the recovery section in which the phthalic product is condensed as a liquid and/or a solid. The latter is performed by so-called switch condensers in which the PA is removed from the gas phase to the solid phase followed by melting. The waste gas is purified by thermal or catalytic incineration. The crude PA receives thermal treatment followed by a two-stage distillation. The paper discusses how the increase of feed concentration over the past 50 years has been handled by improvement of the vaporizers, the reactor, the recovery, and purification sections. An important aspect has been the operation of the reactor and associated equipment under increasingly sensitive explosive conditions requiring mitigating measures to prevent damage and destruction of the reaction section.