Studies in gas-solid catalytic reactions : redox kinetics in pore-diffusion regime

Abstract

Knowledge about the effect of diffusional intrusion on the kinetics of a gas–solid catalytic reaction-very common in process industries-is essential for a better understanding of these reactions in industrial reactors. The oxidation of hydrocarbons and alcohols is often explained by the Redox mechanism. This work presents a study of the effect of intrusion of transport processes on the kinetics of such reactions. The intraparticle effectiveness factor under isothermal and non isothermal conditions has been obtained for a spherical pellet for various order combinations using the Orthogonal Collocation technique and the concept of the burnt out zone, covering the entire range of the Thiele parameter from the kinetic regime to the regime of severe pore diffusion. The results of this analysis have been extended to geometries other than the sphere through a normalisation of the Thiele parameter. An interesting feature of the non isothermal effectiveness factor has been observed. The expression for the asymptotic effectiveness factor for large Thiele parameter values has been used to extract the kinetic parameters-namely, reaction orders and rate constants of the Redox model-from isothermal experimental data obtained with different feed compositions but only one particle size in the pore diffusion regime. Experimental data on toluene oxidation on a vanadium pentoxide catalyst in a spinning basket reactor have been obtained, and the kinetic parameters derived from this data in this way are found to be consistent and in agreement with reported literature values. An algorithmic approach for optimal ordering of particle sizes along the length of a catalytic packed bed reactor has been developed by operating the reactor at effectiveness factors close to the non isothermal maximum. Computations of the formulated steady state stiff differential equations show a substantial increase in conversion for different types of non isothermal fixed bed reactors for a simple irreversible first order reaction, and their performance has been compared.