Production of Maleic Anhydride from Oxidation of Benzene

The many processes for the oxidation of benzene to maleic anhydride all use a similar catalyst based on V2O5, which may be modified with, e.g., MoO3 or H3PO4. Due to the strongly exothermic reaction, tube bundle reactors with, e. g., 13 000 externally cooled tubes in a reactor diameter of about 5 m are used.

Using fused salts as the circulating heat-exchange liquid, the heat of reaction is removed and used to generate high pressure steam.

A benzene/air mixture is oxidized to maleic anhydride over the catalyst at 2-5 bar, 400-450°C, and with a residence time of about 0.1 s:

The benzene conversion reaches 85-95%. The selectivity to maleic anhydride is only about 60-65% (up to 75% using newer developments). About a quarter of the benzene is completely oxidized so that the total evolution of heat is about 6500-7000 kcal (27200- 29400 kJ) per kilogram of converted benzene.

As described in the oxidation of butene/butane to maleic anhydride, fixed-bed technology for the oxidation of benzene has also been replaced by more economical fluidized-bed or moving-bed processes.

The reaction gas is cooled in several hcat exchangers. Since the temperature in the last cooler is below the condensation temperature of anhydride, about 50-60% is obtained directly as an anhydride melt. The remainder is washed out with water in the form of maleic acid and converted into maleic anhydride in a dehydration column or a thin-film evaporator.

This last stage can be run continuously using o-xylene as a water entraining agent. Both crude products are then fractionated to a purity of 99.7%. The distillation residue contains fumaric acid and higher boiling products.