(508b) Gas-Liquid Reactor Model for the Liquid-Phase Oxidation of Hydrocarbons

Gas-liquid reactors are very common in industrial practice (i. e. oxidations, hydroformylations, chlorinations). The description of such processes still presents a difficulty. The reaction kinetics of the absorbed gas with liquid phase is often fast compared to diffusion and a local analysis of diffusion effects near the gas-liquid interface is needed for proper reactor modeling. In addition, the reactions are often highly exothermic and the associated temperature increase at the interface affects not only the kinetic rate constants but also the solubility of the gaseous reactants. Also, the liquid-phase reactants and/or products are in some cases volatile which adds another degree of complexity to the problem. Hence, the modeling of the gas-liquid reactors is by necessity a multi-scale approach with the phenomena at the gas-liquid interface (length scale of order of the order of 100 mm) being couples with the macro-scale reactor model (length scale of the order of 1 m).

In this paper, a comprehensive model for gas-liquid reactors, using combination of mixing cell approach and film model for mass and heat transfer, is developed. The film model appears as a sub-model and the fluxes obtained using the boundary element method (Ramachandran, 1991) at the interface are then directly used as the link to the reactor model. Through the mixing cell approach, the reactor model is applicable to full spectrum of degrees of mixing-from a well mixed system (one cell) to a plug flow behavior (cell in series). Key features of the model include also vaporizations of the reactant and the effect of evaporative cooling (change of vapor flow rate and gas concentration due to vaporization), and application to wide class of oxidation reactions (e.g. cyclohexane, toluene, p-xylene) in stirred tank, stirred tank in series, and staged bubble column reactors.

Application to two important industrial processes, cyclohexane and p-xylene oxidation, is illustrated. The oxidation kinetics proposed by Alagy (1974) for cyclohexane and Wang (2005) for p-xylene oxidation are used. The results of these two oxidation reactions are disscused as a function of oxygen partial pressure, gas and liquid flow rate, mixing speed, and temperature.

References:

1) Ramachandran, P.A. (1991) “Boundary element method for diffusion reaction with boundary condition” Chem. Eng. J., 47, 69-85.

2) Alagy, J., Trambouze, P., and Van Landeghem, H. (1974). “Designing a cyclohexane oxidation reactor.” Ind. & Eng. Chem. Process Design and Development, 13(4), 317-23.

3) Kongto, A., Limtrakul, S., Ngaowsuwan, K., Ramachandran, P. A., and Vatanatham, T. (2005). “Mathematical modeling and simulation for gas-liquid reactors.” Com. Chem. Eng., 29(11-12), 2461-2473.

4) Wang, Q., Li, X., Wang, L., Cheng, Y., and Xie, G. (2005). “Kinetics of p-Xylene Liquid-Phase Catalytic Oxidation to Terephthalic Acid.” Ind. Eng. Chem. Res, 44(2), 261-266.