MODELING OF SOME DIFFUSION AND CHEMICAL REACTION PROCESSES IN FULLY-LOADED AND PARTIALLY-LOADED CATALYST PELLETS

Fully and partially impregnated porous catalysts are used in modeling simultaneous pore diffusion and chemical reaction.; In Part I of this work, we deal with a system that consists of a main reactant A, having some impurities B, which upon reaction over a given catalyst, forms the desired product R and highly undesired product S, respectively. In order to avoid or minimize the formation of product S, two pieces of equipment are conventionally used: namely, a separator and a catalytic reactor. It is shown that under certain conditions, partially filled, end-loaded catalysts can achieve several orders of magnitude reduction of the S-impurity level, without the need of a separator. This will happen in case that the desired reaction is under kinetic control and the undesired reaction is under diffusion rate control. Detailed derivations and results are given, showing the impurity reduction as a function of the catalyst loading, the Thiele moduli and the diffusivity ratio.; In Part II of this work, we deal with concentration dependent diffusion coefficients, which may be important especially when the reaction occurs over a molecular sieve catalyst. In this study, two models are examined which relate the binary diffusion coefficient with the self-diffusivities of reactant and product, and also with the reactant concentration inside the pore. These models account for any concentration dependence of the diffusion coefficient, with a self-diffusivity ratio of reactant and product ranging from zero to infinity. The effectiveness factor is significantly affected by such concentration dependence, attaining its highest value when the self-diffusivity of the product is infinitely higher than that of the reactant, and becoming zero in the case of non-diffusing product. The amount of catalyst loading has also a quantitative effect on the effectiveness factor. Exact analytical and numerical solutions have been developed for fully-loaded and end-loaded catalyst pellets of different geometries, and for various reaction orders, under isothermal conditions. In addition, asymptotic analytical solutions have been derived, which, after normalization, provide a bounded region for the effectiveness factor for reaction orders between 0 and 2, for any pellet geometry, and for any diffusivity ratio.