Two-Stage Temperature Control for the Synthesis of Adipic Acid through K/A Oil Oxidation in a Microreactor System

Abstract

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 6hydroxyimino-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 °C in LTS and 85 °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 .

Continuous Synthesis of N-Cyclohexyl-2-benzothiazole Sulfenamide with Microfluidics and Its Kinetic Study

“…Microreaction technology as a potential strategy has gained much attention in both academia and industry for process intensification. − The particular structure of a microreactor ensures high surface-to-volume ratios, small characteristic dimensions, and readily tunable conditions, which could significantly enhance the mass and heat transfer. − Meanwhile, the reduced amount of fluid allows the oxidation to be safely performed, which is important for production under both high temperature and pressure upon using pure oxygen . In the aspect of practical industrial production, the advantages of microreactors are highly conductive to continuous operation, , as well as the scale-up ability. , Special attention has been paid to segmented flow (i.e., slug or Taylor flow) as this regime enables more efficient contact for gas–liquid systems. − These features of microreactors are particularly attractive for the synthesis of CBS or other substituted sulfenamides in the presence of pure oxygen, as well as the highly exothermic phenomenon and high-pressure conditions.…”

Introduced gas regulating fast and highly exothermic reactions with in situ gas generation in microreactors

“…In addition, the obvious temperature gradient and local hot spots in the reactor are still unavoidable. Conversely, microreactors, benefiting from their good mass and heat transfer capacity, have been utilized to efficiently intensify this oxidation process 13,14 . In our previous work, the application of microreactors improved the yield of adipic acid from 93% to 97% and greatly reduced the reaction time from 1 h to several minutes when the molar ratio of nitric acid to the organic substrate ( M ‐ratio) was around 22, while a value of 7 was applied in chemical industry.…”

“…Conversely, microreactors, benefiting from their good mass and heat transfer capacity, have been utilized to efficiently intensify this oxidation process. 13,14 In our previous work, the application of microreactors improved the yield of adipic acid from 93% to 97% and greatly reduced the reaction time from 1 h to several minutes when the molar ratio of nitric acid to the organic substrate (M-ratio) was around 22, while a value of 7 was applied in chemical industry. We hoped that a smaller M-ratio could be applied without the cumbersome multi-step feed operation for better efficiency and lower costs.…”