Automatic kinetic model generation: A novel modeling approach for liquid-phase processes

The liquid-phase oxidation of cyclohexane is an important industrial process for the production of cyclohexanol and cyclohexanone, key intermediates in the production of nylon. Despite its industrial significance, its complex free radical mechanism is not yet fully understood, posing challenges for process optimization and reactor design towards a more sustainable future. Notable advances have been made in the past decade regarding the automatic construction of kinetic models for various gas-phase processes. ALKIMO, which stands for "Automatic Liquid-phase Kinetic Modeler", aims to fill the present gap for liquid-phase processes. This enables the automatic transformation of a kinetic model developed for the gas phase to the liquid phase, by incorporating the impact of a liquid. ALKIMO was applied to the liquid-phase oxidation of cyclohexane as a proof of concept, and the developed model will be validated against experiments performed in a tubular reactor operating in the slug flow regime. Such a regime consists of alternating cyclohexane liquid slugs and oxygen gas bubbles, resulting in diffusional limitations between the phases, and necessitating the development of an accurate slug flow reactor model. The developed slug flow reactor model uses an overall volumetric mass transfer coefficient to describe these diffusional limitations. Simulated conversions form a more gradual profile compared to a plug flow reactor model, where perfect mixing between the phases is assumed at the inlet. Subsequently, the kinetic model was extended to include formation of 6 hydroxyhexanoic acid, a significant by-product observed experimentally. The extended kinetic model, in combination with the developed slug flow reactor model, will be validated through newly acquired experimental data in the future. The model correctly predicts the expected products to be the major products in the reactor outlet, namely cyclohexane, cyclohexanol, cyclohexyl hydroperoxide, and 6-hydroxyhexanoic acid.