Effect of heat and mass transfer on the thermal decomposition of SrCO 3 compacts
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I. INTRODUCTION
MODELING of gas-solid reactions has been carried [1–4]
Many models are used to out over the last 30 years. analyze experimental results from kinetic experiments in order to provide information regarding the physical and chemical nature of the processes. Some of these models were developed under isothermal conditions, i.e., the temperature at the reaction interface was assumed to be the ambient temperature. Heat-transfer phenomena have been treated in earlier kinetic studies,[5–10] but the solutions of the heat and mass equations for the reaction of a compact reactant are seldom compared to experimental results. In general, a process caused by a chemical reaction may involve both heat and mass transfer. In the past, external mass transfer and diffusion of reactants and products have been considered together with chemical reactions in developing models and, in general, minor importance was attached to heat-transfer phenomena. However, many chemical reactions have relatively high exothermic or endothermic reaction enthalpies, which would affect the actual temperature at the reaction interface. In the present work, the chemical reaction is treated together with external mass transfer, external heat transfer, gaseous pore diffusion, and effective heat conduction through the porous solid. The analysis is based on the results of thermogravimetric measurements of the decomposition of spheres of pressed SrCO3 powder. Thermocouples were mounted either on the surface or at the center of the sphere. The results from previous work on the thermal decomposition of SrCO3 powder, where the chemical reaction was controlling the rate,[11] are incorporated in the present work.
I. ARVANITIDIS, Research Associate, and S. SEETHARAMAN, Professor, are with the Department of Metallurgy, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. X. XIAO, Professor, is with the Department of Ferrous Metallurgy, Northeast University, Shenyang, 110006, People’s Republic of China. Manuscript submitted June 16, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
II. EXPERIMENTAL A. Materials SrCO3 powder (99.95 pct pure, with an average particle size of 5 mm) was supplied by Johnson Matthey GmbH (Karlsruhe, Germany). The strontium carbonate powder was ground in an agate mortar, heated at 673 K for 24 hours in order to remove any physically and chemically bounded water from the atmsosphere, and then cooled in a dessicator. Spheres were made by isostatic pressing of SrCO3 powder in a mold made of transparent silicon rubber. The silicon rubber, with the product name Rhodorsil RTV 1556, was supplied by SIKEMA AB (Stockholm). The apparant porosities of the SrCO3 spheres are presented in Table I. Some spheres were pressed together with a thermocouple in the center. The argon gas (maximum of 2 ppm impurities) used during thermal analysis was supplied by AGA Gas (Stockholm). The gas was further purified from moisture and CO2 using silica gel, ascarite, and magnesium perchlorate before it entered the reaction chamber. Pt-10 pct Rh/Pt wir
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