Optimum effectiveness factors for porous catalysts



In the manufacture of catalyst pellets from particles of powder, for example alumina, one of the parameters is pellet density. The opposing effects of density on surface available for reaction and on pore diffusion result in a specific pellet density for maximum reaction rate per unit volume of reactor. The method for predicting the optimum density and rate is presented for a first-order reaction. The data required are pore volumes and pore-size distributions as a function of density, as well as kinetic measurements. Such data are available for the ortho-para hydrogen reaction using a nickel oxide-on-alumina catalyst. Using this system as an illustration, it can be seen that the optimum effectiveness factors are less than unity and range from about 0.68 for 1/8-in. diam. spherical catalyst pellets to 0.29 for 1-in. pellets. Corresponding optimum densities are from about 1.3 to 0.6 g./cc

The results demonstrate the possibility of a tailor-made pelleted catalyst for optimum reaction rate.