Abstract

When the thermal and mass transfer boundary layers on the outside of a catalytic pellet provide significant resistances to heat and mass transfer between the pellet and the bulk gas stream moving through a packed bed, reactor design strategies must account for these rate processes to develop accurate predictions of reactor performance. It is necessary to relate temperature and reactant conversion in the bulk gas phase to their counterparts on the external surface of the catalytic pellets. Volumetrically averaged rates of reactant consumption throughout the catalyst can be predicted in terms of the effectiveness factor and the kinetic rate law, which employs temperature and molar densities on the external surface of the pellet. The methodology discussed in this article allows one to calculate temperature and species concentrations in the bulk gas stream and on the external surface of the catalytic pellet. As convective transport is negligible within the catalytic pores, stoichiometric relations among diffusional mass fluxes are employed extensively in the early sections of this article to develop the appropriate reactor design strategy. Generalized isothermal predictions that only consider the external resistance to mass transfer reveal that higher conversion of reactants to products is achieved at shorter residence times, over a restricted range of mass transfer Peclet numbers in the creeping flow regime, in nonideal reactors that include interpellet axial dispersion. Ideal design equations for packed catalytic tubular reactors overestimate reactant conversion in these simulations because the mass transfer Peclet number is below its critical value, and reactant molar densities on the external catalytic surface are significantly less than their counterparts in the bulk fluid phase moving through the packed bed. Nonisothermal design strategies are illustrated specifically for the gas-phase synthesis of methanol from carbon monoxide and hydrogen in fixed-bed catalytic reactors. The classic Prater equation is modified to estimate the intrapellet temperature rise for exothermic reactions in macroporous catalysts by considering (1) thermal diffusion, (2) the effect of collision integrals on ordinary molecular diffusion coefficients, and (3) temperature effects on the enthalpy change for chemical reaction.