Synthesis of adipic acid through oxidation of K/A oil and its kinetic study in a microreactor system

Abstract

Synthesis of adipic acid (AA) through the oxidation of cyclohexanol and cyclohexanone (K/A oil) with nitric acid was conducted in a capillary microreactor system. Effects of the temperature, the nitric acid concentration, the volumetric flow rate ratio of nitric acid to K/A oil, and the capillary length on the selectivity and the product yield were investigated systematically to achieve optimal reaction conditions. Notably, a high yield of AA (i.e., 90%) was achieved just in 6 seconds at 85°C with the use of 55 wt% nitric acid. Gas components produced in this oxidation process and its total volumetric flow rate were determined under various operational conditions, which was beneficial for reaction mechanism characterization and process optimization. Finally, a kinetic model was established, which was of crucial theoretical significance and practical value for optimizing the reactor design and better understanding such fast and highly exothermic multiphase processes with abundant gas production.

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

“…According to our previous study, the selectivity for AA increased first and then decreased with increasing reaction temperature . Therefore, it is crucial to control the reaction temperature precisely with enough heat exchange capacity.…”

“…Two analysis units, including a liquid analysis unit and a gas analysis unit, were connected to the end of the capillary microreactor, as shown in Figure . More details about the setup and the analysis procedure have been provided in our previous study …”

“…As a powerful process intensification platform, microreactor technology has undergone rapid development and has widespread utilization in both academia and industry due to its advantages such as high mixing efficiency, fast heat and mass transfer rates, and ensured process safety. − Employing microreactors for fast and exothermic reaction processes (e.g., oxidation, nitration, and sulfonation) will enhance the process efficiency and safety. − In our previous study, a continuous-flow capillary microreactor system was developed to synthesize AA and a high yield of AA (90%) was obtained at a residence time of only 6 s and 85 °C using 55 wt % nitric acid . In addition, we studied the effects of operational parameters on the yield of each dicarboxylic acid (AA, GA, and SA) and gas products and proposed a power-law kinetic model based on the experimental results under the operating conditions similar to the industrial process.…”

“…As mentioned before, a series of research studies demonstrated the complexity of this oxidation process, but we can simply divide it into two main steps as illustrated in Figure . ,,, The conversion of K/A oil to intermediates (e.g., 2-nitrosocyclohexanone (2-NC), 6-hydroxyimino-6-nitrohexanoic acid (AMNA), and hemihydrate of cyclohexanedione (HCD)) proceeds rapidly and is a highly exothermic process (Step 1), while the conversion of intermediates to AA (Step 2) is a relatively slow reaction and gives a small contribution to the heat release in the whole process . Based on our previous results from a single-stage temperature control microreactor system, the reaction temperature is considered to be the most decisive operating parameter in this oxidation process, which will directly affect the reaction route and the selectivity of AA . The large quantity of heat released from the system, especially in Step 1, should be removed immediately, which requires excellent heat exchange performance of the reactor and precise temperature control.…”

“…Two simplified reaction steps for the synthesis of AA through the oxidation of cyclohexanol with nitric acid (elaborated in refs , , , ) and the whole reaction network is detailed in ref .…”