Chapter 3 - Gas–Solid Reactions and Reactors

Abstract

Gas–solid reactions involving porous catalysts are ubiquitous in chemical and petroleum industries, and it is vital to understand the kinetics of heterogeneous catalytic reactions to develop robust reactor models. This chapter begins by providing an insight on effective diffusivity, which accounts for various mechanisms by which the gas-phase species get transported to the active sites inside a porous catalyst. The influence of pore diffusion on reaction rate is described by considering an isothermal first-order reaction occurring inside the pores of catalyst particles of different geometries. The relative importance of reaction and pore diffusion resistance is described using dimensionless Thiele modulus, and the actual rate of reaction is described using effectiveness factor. After elucidating this treatment for reactions occurring under nonisothermal conditions, various examples involving series and parallel reactions are discussed in terms of selectivity of the catalyst toward a specific product. The external mass transfer of the reactant from the fluid stream to the catalyst pore is then integrated with the rate expression to determine the overall rate. The elementary steps, viz., adsorption, surface reaction, and desorption, involved in a catalytic reaction are discussed, and the approach to derive the overall rate expression at steady state in presence of various rate-determining steps is explained. The effect of catalyst deactivation on the reaction rate is also described. Modeling of ideal and nonideal catalytic packed bed reactors operating under both isothermal and nonisothermal conditions is elucidated using examples. Finally, an approach to model the conversion of solid particles in presence of a reactive gas stream involved in noncatalytic reactions is elucidated using the shrinking unreacted core model. Expressions for solid conversion under various controlling resistances are derived, and its extension to shrinking particles and film growth is outlined.