Kinetic aspects of deactivation of catalysts in prolonged use

Abstract

A theoretical approach to the dynamic analysis of deactivation and aging of heterogeneous catalysts is proposed -analysis of the rate derivative in a kinetic equation of a given form. Methodology was demonstrated for the simplest kinetic equations -zero-and first-order. Regardless of the form of the kinetic equation for the dynamic model of deactivation, the rate constant is a function of an independent reaction coordinate. The method can also be used for solving reactivation problems and for explaining the dynamics of cation exchange in zeolite. Problems related to aging and deactivation of solid catalysts arise in the course of heterogeneous catalytic reactions [1]. Empirical dynamic models based on intuition are used in practice for solving these problems. In particular, problems in cracking [2] and dehydrogenation [3-7] of hydrocarbons have been solved by this method.Gradual coking of the surface of the catalysts, up to total poisoning, take place in these processes in relatively severe conditions [8].We attempted to analyze how the second derivative of the concentration c changes in time t as a function of the form of the kinetic equation, i.e., the laws by which the reaction slows and in particular, how the kinetic order is reflected in the dynamics of the change in activity.The proposed methodology involves the analysis of the rate derivative in a kinetic equation of a given form. The simplest kinetic equations -zero-and first-order -are examined. It is shown that regardless of the form of the equation, rate constant k is a function of an independent coordinate of the reaction.During prolonged use of coked heterogeneous catalysts, the proposed approaches can be substantiated by previously used semiempirical models of deactivation. In a first approximation, this problem can be solved with the existing linear correlation in [9] between the activation energy and the preexponential factor.As the analysis of the literature shows, no attention was previously turned to the possibility of predicting the dynamics of deactivation with consideration of the form of the kinetic equation. Our proposed mathematical approach combined with additional methods of physicochemical investigation of the coked surface in situ can be a very useful tool for analyzing dynamic models of heterogeneous catalytic processes not only in block reactors but also in a continuous-flow system.