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

The oxidation of cyclohexanol/cyclohexanone with nitric acid as a fast and highly exothermic process was hard to control at low molar ratios of oxidant to organic substrate even in microreactors. Process of runaway gas generation was observed due to the local hot spots, which decreased the selectivity of adipic acid. Additional gas was introduced into microreactors before the reaction, changing the previous wave annular flow to the uniform slug flow. Through the microscopic high-speed photography and CFD simulation, it was found that the internal convection made the reactant molecules in the aqueous slug distribute evenly instead of gathering near the interface before, thus eliminating the local hot spots and stabilizing the hydrodynamic characteristics. The reaction performance was evaluated under different conditions, and a higher adipic acid selectivity of 93.6% was achieved at a molar ratio of 6.7 with the combination of microreactors and conventional reactors.