Adipic Acid Production from Cyclohexane

The process under analysis comprises two major sections: (1) First oxadation step; and (2) Second oxidation step.

Fist oxidation step. Initially, cyclohexane is fed to the first of several staged air oxidation vessels, along with meta-boric acid. The cyclohexane is oxidized in the liquid-phase to cyclohexyl hydroperoxide (CHHP). In the presence of boric acid, the cyclohexyl hydroperoxide is trapped as cyclohexyl perborate ester, which in turn reacts with cyclohexane yielding borate ester and cyclohexanol. The borate ester formed is relatively stable, in such a way that the generation of undesired by-products is minimized. Next, the ester is hydrolyzed, yielding cyclohexanol and boric acid, which is dehydrated and recycled to the oxidation. Subsequently, CHHP still present in the oxidation product is decomposed by another oxidation reaction over a fixed bed of cobalt catalysts, yielding a cyclohexanol-cyclohexanone mixture (KA oil).

Second oxidation step. The KA oil is then reacted with nitric acid in the presence of copper/vanadium catalyst to form Adipic Acid. While this second oxidation is very exothermic, a high ratio of nitric acid to KA oil is employed, so that the nitric acid also works as heat sink, avoiding uncontrollable reactions. 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 gas are released from the reaction. While this can be viewed as nitric acid losses, NO2 and NO are recovered, further oxidized, and recycled as nitric acid. 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.