Gas-solid reactions with solid product are encountered in a number of chemical process industries with applications ranging from ceramic material manufacture to gas scrubbing for pollution control. Because of the formation of the solid product, a number of other processes must be considered in the study of this type of reactions in addition to mass and heat transport and chemical reaction. Most of these processes are related to the evolution of the morphology of the solid because of the formation of the solid product and the diffusion of the reactive species through the formed layer of solid product. If the solid product occupies stoichiometrically more volume than the solid reactant from which it results, phenomena of pore plugging and, as a result, incomplete conversion may arise. Since pores of different size are expected to be plugged at different times - the smaller the pores, the earlier they will be plugged - the transient behavior of the reacting particles should in general depend not only on the average structural properties of the porous calcined solid (i.e, internal surface area and porosity) but on the form of the pore size distribution as well, even under reaction conditions characterized by negligible intraparticle concentration gradients. In the presence of significant intraparticle diffusion limitations, the overall picture of a gas-solid reaction with pore closure behavior becomes considerably more complex. If the reaction rate at the external surface of the particles is much higher than that in the interior, complete pore closure may take place at the external surface while there is still open space left in the interior. Such an event can lead to ultimate conversions much lower than those predicted by the stoichiometry of the reaction for complete plugging of the internal space. Based on these observations, the PIs are experimentally studying gas-solid reactions with solid product. This equipment grant is for an automated laboratory system capable of determining the physical microstructure of any solid material. It is to be used to elucidate the effects of processing conditions on the microstructural properties of solid products and reactants and their effects on the behavior of gas-solid reaction systems. Specifically the equipment is an automated mercury porosimeter with appropriate hardware and software for data acquisition and analysis. The system will be used to analyze solid samples for a number of experimental projects involving powders and porous media including: (1) reactions of metal oxides with hydrogen sulfide and sulfur dioxide employed to control pollutant emissions from coal utilization systems, (2) densification of porous preforms in manufacturing ceramic matrix composites by chemical vapor infiltration, and (3) characterization of powders and powder compacts (fresh or partially sintered) involved in the fabrication of metal and ceramic composites using powdered material techniques.
