Simulataneous heat and mass transfer with and without reactant exhaustion in catalyst particles for reaction with volume change

Abstract

Asymptotic analytic expressions for the concentration and temperature profiles as well as effectiveness factor inside porous, thin-disk shaped catalyst particles have been developed for zeroth-order exothermic reaction with reactant-to-product volume change. The individual and combinative effects of volume-change, Thiele and thermal Thiele moduli, and of the dimensionless activation energy are discussed. As expected, the expressions revert to those for the corresponding isothermal and constant-volume cases. Errors arising from the approximation were calculated and found to be quite acceptable. MARSHALL SCOPEThe general objective of the authors' recent works has been to study the influence of volume or mole change on the mass transfer efficiency inside catalyst particles. In this paper, the heat transfer effect is and the system studied is a zeroth-order exothermic reaction which frequently occurs in industrial practice such as the hydrogenation and dehydrogenation of cyc!ic hydrocarbons. characteristic of zeroth-order reactions in exhibiting the reactant-exhaustion phenomenon which makes the interaction of heat and mass transfer highly interesting. HOWever, because of mathematical complications, the simplifying approximation of moderate temperature rise has been made. However, analysis shows that the general applicability of the result is not seriously affected. ~h~ present study is also confined to thin-disk geometry.This reaction was chosen also because of the unique The heat and transfer the film surrounding the catalyst particle is treated as a separate problem and thus not included here. CONCLUSIONS AND SIGNIFICANCEWe have developed analytic solutions for the concentration and temperature profiles as well as for the effectiveness factor for zeroth-order reaction occurring in porous, thin-disk type catalyst particles with reactant-toproduct volume change. Although an approximation was made, the error introduced is small enough to render the results useful in a large number of reaction systems, including such catalytic reactions as hydrogenation and dehydrogenation of cyclic hydrocarbons wherein the strong hydrocarbon adsorption allows the rate to be approximated by zeroth-order behavior.The results show that the effectiveness factor decreases it increases with increasing combination of the thermal Thiele modulus and dimensionless activation energy. However, the temperature profile inside the pellet does depend on the latter two factors separately. The study also confirms that when a reactant is exhausted before reaching the center of the pellet a flat temperature profile is maintained in this no-reaction central layer and the level of this temperature is a function of the thermal Thiele modulus (dimensionless heat generation-to-mass diffusion ratio) but not of the dimensionless activation energy. These dependencies and behaviors can be qualitatively applied to cylindrical and spherical particles as r re1. --with increasing volume-change modulus (expansion) while well.In a recent series of papers (Lih and Lin...