Adipic Acid Production from Benzene (Cyclohexane Intermediate)

The process under analysis comprises four major sections: (1) Benzene Hydrogenation; (2) Nitric Acid Synthesis; (3) Cyclohexane Oxidation; and (4) KA Oil Oxidation.

Benzene Hydrogenation. Fresh Benzene and hydrogen are fed to a bubble column reactor, then the reactor top product is fed to a finishing fixed-bed reactor to complete benzene hydrogenation. In both steps the reaction is catalyzed by nickel based catalysts. The output from the finishing reactor is separated in a knock-out drum in two phases, a hydrogen rich gaseous phase that is recycled to the primary reactor, and a liquid stream rich in cyclohexane that is purified in a distillation column before it is directed to the Cyclohexane Oxidation section.

Nitric Acid Synthesis. Ammonia is initially oxidized to nitric oxide at medium pressure (4 - 6 bar abs) and temperatures between 870 to 930°C in the presence of platinum-rhodium catalyst gauzes. Then, nitric oxide is oxidized to form nitrogen dioxide, which is then absorbed (at the same pressure) by water, producing 65 wt% Nitric Acid.

Cyclohexane Oxidation. Cyclohexane is fed to the first of several staged air oxidation vessels, along with boric acid. In the presence of cobalt naphthenate catalysts, cyclohexane undergoes several reactions until it is finally converted into a mixture of cyclohexanol-cyclohexanone (KA oil).

KA Oil Oxidation. The KA oil is then reacted with nitric acid in the presence of copper/vanadium catalyst to form Adipic Acid. The oxidation reaction is extremely quick; it is carried out in two converters, operating at different temperatures. Most of the KA oil is oxidized in the first, low-temperature converter, with the remainder being converted in the high-temperature converter downstream. Nitrogen oxides and nitrogen are released from the reaction. After separation of water and monobasic acids from the oxidation product, crystallization under vacuum and centrifugation, a crude Adipic Acid is obtained. Finally, the crude Adipic Acid is re-dissolved in water, recrystallized, centrifuged, and dried, yielding fiber-grade Adipic Acid.