A model for solid-gas reactions

Studies on the oxidation of zinc sulphide spheres suggested kinetic control in the temperature range 600-670°C and diffusion control in the range 740-820°C. In the intermediate range probably both chemical reaction and diffusion are simultaneously operative. In the kinetic regime, experimental data could be fitted to the established Levenspiel model, while in the diffusional regime the model represented by the following equation depicted the data very well: 3θ/AM=x+Bi[1.5-x-1.5(1-x)2/3]. This equation has been derived on the assumption that diffusion of oxygen through the "ash" layer (zinc oxide shell) controls the overall reaction. The effective diffusivity of the reacting and product gases through the ash layer was measured experimentally in a newly developed diffusion cell. The value of the tortuosity parameter (α) thus estimated form an independent set of diffusion experiments and that obtained from kinetic data by using the model represented by the above equation agreed very closely. The external mass transfer coefficient (kg) calculated from the model also need with the values calculated by standard methods. It may therefore be concluded that the above equation (based on the retreating core model) is an adequate representation of the diffusional regime. Similar equtaions can be readily written for other systems starting from Eq. (2) of the text