A Review of Mass Transfer Controlling the Reaction Rate in Heterogeneous Catalytic Systems

Abstract

Mass transfer limitations play an important role on the rate of reaction; the rate of conversion and product formation, including in the catalytic systems. In a homogeneous catalytic reaction in which all substances (reactant(s), product(s), and catalyst) are in the same phase, the effect of mass transfer between phases is mostly negligible. In a heterogeneous catalytic reaction; however, the catalyst is usually in a different phase from the reactant(s). Commonly the catalyst is in the solid phase embedded in the reacting species which usually are in the liquid or gaseous phase. Consequently, the reaction rate is principally relied on the mass transfer or diffusion between these phases. A lot of efforts have been made, due to the important roles of mass transfer effects on the reaction rate. The main purpose for this chapter is to apply the basic restriction of mass transfer on the heterogeneous catalytic reaction. The illustrations based on the literature reviews in the heterogeneous catalytic processes are conducted for elucidation. Since the reaction catalyzed by solid catalysts occurs when the reactant molecules come in contact with the active sites, which are usually located inside the catalyst pores. In other words, the catalytic reaction is taken place after the reactant molecules diffuse through the fluid layer surrounding the catalyst particles (external diffusion or film diffusion), then through the pore with in the particle (internal diffusion). The internal diffusion of the molecules competes with the reaction; at the same time, the external mass transfer is dependent on the stagnant film thickness and the activity on the outer layer. Hence, the diffusion of molecules is not only hindered by the other molecules, but also by the physical hindrances. The classical seven steps for a catalytic reaction (Fig. 1), i.e. (1) diffusion of the reactants from the bulk phase (boundary layer) to the external surface of the catalyst pellet (film diffusion or interphase diffusion), (2) diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface; the point where the chemical transformation occurs, (pore diffusion or intraparticle diffusion), (3) adsorption of reactants on the inner catalytic surface, (4) reaction at specific active sites on the catalyst surface, (5) desorption of the products from the inner surface, (6) diffusion of the products from the interior of the pellet to the pore mouth at the external surface, and (7) diffusion of the products from the external pellet surface to the bulk fluid (interphase diffusion), are generally used as the key for explanation.