Studies in gas-solid reactions

Abstract

Non-catalytic gas-solid reactions are of great industrial importance and have therefore attracted considerable research effort. As a result, a number of models have been proposed in literature. Among the early models, the shrinking unreacted core model and the homogeneous model are applicable to non-porous and highly porous solids respectively. Apart from other limitations, these models do not take into consideration the structural parameters of the solid such as porosity, pore size, grain size, etc., which are of considerable importance for many actual systems. Among the few models developed based on assumed idealized structures of the solid, the pore model and the parallel plate model are rather unrealistic. Though the grain model is fairly satisfactory, it assumes constant porosity and effective diffusivity in the solid during reaction, apart from pseudo steady state. In the present investigation, a structural model has been proposed for a gas-solid reaction, which attempts to take into consideration the effects due to the variations in surface area, porosity, and effective diffusivity with the progress of the reaction. Unsteady state has also been incorporated. Differential equations have been formulated for the situation of unidirectional diffusion and reaction of a gas within the solid. The dimensionless form of the governing differential equations with the pertinent initial and boundary conditions have been solved by numerical techniques. Experiments have been carried out with zinc sulphide and air for verifying the above model. Weight loss of the solid pellet with time and conversions in the pellet with position at specific intervals have been measured. Some additional experiments have also been carried out to determine the properties such as porosity, pore size, grain size, surface area, and intrinsic reaction rate constant which characterize the solid pellet. The experimental results, namely the weight loss with time and conversions with position in the pellet, have been compared with the values evaluated from the model equations. Agreement is found to be quite satisfactory. The sensitivity of the model to various simplifying assumptions has also been tested. It is found that pseudo-steady state assumption is quite reasonable, but assumptions regarding constant porosity, surface area, effective diffusivity, etc., can lead to significant errors.