Solid—gas reactions: effect of solid shape on proposed diffusion model

Abstract

The diffusion model for gas-solid reactions, proposed by Phadtare and Doraiswamy [9] and applied for the oxidation of zinc sulphide by Gokarn and Doraiswamy [6] for spherical pellets, has been extended to include different geometrical shapes. Model equations have been derived for the long cylinder, right circular cylinder ( L = D), infinite cylinder and flat plate. Cylindrical ZnS pellets have been prepared at three different compression pressures, and oxidation carried out at various temperatures for each compression pressure. It has been confirmed that there is a definite shift in the controlling regime and that the “critical temperatures” [i.e. the temperature at which the shift occurs] is dependent on the porosity of the ZnS pellet, shifting to a lower temperature as the porosity is decreased. It has also been observed that the modified kinetic and diffusion models satisfactorily represent the experimental data in the respective zones of control for all the shapes studied. The value of the effective diffusivity obtained by the application of the model to the experimental data for various shapes at a particular temperature has been found to be the same irrespective of the pellet geometry, thus providing further confirmation of the proposed models. In the kinetic regime the activation energy of the reaction has been estimated to be 7·55 kcal/g mole and in the diffusion regime 1·92 kcal/g mole. The Aris approximation for the diffusion length has been found to be applicable to the various geometrical configurations examined, thus proving that this useful approximation, which was so far limited to catalytic reactions, can also be employed for gas—solid reactions.