This invention relates to a process for the preparation of phthalic anhydride by catalytic gas-phase oxidation of o- xylene . According to this method, o- xylene and an oxygen-containing gaseous mixture of unreacted gas to the reaction in the main reactor o- xylene, low-service and phthalic anhydride reaction product gas phase containing the intermediate reaction product and the oxidation state of the anhydride , the reaction heat generated in the main reactor is passed, at least in part by indirect cooling by the heat exchange medium, is introduced into the intermediate reaction product to the reactor unit. The concentration of unreacted o- xylene in the intermediate reaction product is 1% by weight or more, of the intermediate reaction product that - the concentration of phthalic anhydride in the oxidation state of 0.5% or more. The process according to the invention to increase the overall yield of phthalic anhydride without being at all or significantly reduce product quality.
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