Two-Stage Temperature Control For The Synthesis Of Adipic Acid Through K/A Oil Oxidation In A Microreactor System

Aiming to further increase the yield of adipic acid (AA) through the oxidation of K/A oil (i.e., the mixture of cyclohexanol and cyclohexanone) by nitric acid, a two-stage temperature control strategy was employed using a capillary microreactor system. The primary intermediate in this oxidation process was determined as 6-hydroxyimino-6-nitrohexanoic acid (AMNA), which was beneficial for both reaction mechanism characterization and process optimization. The oxidation process could be divided into two stages according to the reaction phenomena observed using a high-speed camera, that is, the K/A oil conversion and the gas production. Effects of the temperature combination for the low-temperature stage (LTS) and high-temperature stage (HTS) and the capillary length on the yield of AA, the relative concentration of AMNA, and the gas composition were investigated systematically to obtain optimal reaction conditions. Notably, a high yield of AA (i.e., 93%) was achieved just in a 10 m-long capillary microreactor by 50 wt % nitric acid using an optimal temperature strategy (i.e., 45 degrees C in LTS and 85 degrees C in HTS) with 40% less gas production than the single-stage temperature control process. Moreover, the AA yield of 97% with 10% lower specific nitric acid consumption could be achieved with the combined application of a microreactor and 5 min of aging operation, which was 4% higher compared with the yield obtained in CSTR.