The Maleic Anhydride Manufacturing Process

The traditional maleic anhydride manufacturing process involves reacting benzene with excess air. A low benzene concentration is used in order not to exceed the flammability limit of the mixture. The reaction gas mixture is passed over a multitubular, fixed bed catalyst reactor at a pressure of 0.15-0.25 MPa. In addition to maleic anhydride, the reaction produces two moles of CO2 and water. The reaction is highly exothermic, causing hotspots of 340-500°C. For each ton of benzene that is reacted, 27MJ of heat are generated. Molten eutectic salts circulate outside the reactor tubes to dissipate the heat. Steam is generated when the molten salts are cooled, which is used to drive air compressors (see Fig. 9.31). 

The vapor mixture leaving the reactor is cooled down to 55°C by heat exchangers, and 40-60% of the maleic anhydride is recovered as liquid. Contact of maleic anhydride with water results in the undesired formation of maleic acid. The maleic anhydride 

FIGURE 9.31. Maleic anhydride manufacturing process based on benzene. 

The modified process is based on the oxidation of butane (see Fig. 9.32). A major advantage of C4 over benzene is that no carbon is lost in the reaction. The yield from butane process is 30% greater than that from the benzene process. Also, C4 is much cheaper than benzene. Benzene is a known carcinogen. So, the fixed bed process with n-butane as the raw material has been the only MA route used commercially since 1985 in the United States. In the fixed bed process, a low concentration of butane is passed over the catalyst at 400 80°C and 0.3-0.4MPa. The process generates more water than the benzene process. Unsupported vanadium phosphorus oxide (VPO) catalysts with promoters such as lithium, zinc, and molybdenum are commonly used. 

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