REACTIVITY, EXPERIMENTAL STRATEGY AND REACTION-RATE DATA EVALUATION IN NONCATALYTIC GAS-SOLID SYSTEMS

Important aspects of noncatalytic gas-solid reactions including mass transfer and pore structure, heat transfer, chemical reaction kinetics and equilibrium, mechanical stresses, phase change and sintering are briefly reviewed. An improved strategy for the determination of reaction kinetics is formulated, akin to the systematic procedure which exists for catalytic systems. The synthesis of titanium nitride is utilized as a case study. The approach includes the use of nonisothermal data in addition to the traditional isothermal data as a crucial step to obtain data with sufficient orthogonality to allow rigorous evaluation of competing rate laws. Two particle morphologies are tested and experiments are conducted over a wide range of conversion, temperature and heating rate. Four kinetic laws are evaluated by nonlinear optimization using the BET surface area as a measure of reactivity. The logarithmic law provides a surprisingly good fit of the experimental data. The apparent observed activation energy is a function of temperature, conversion and particle size even though the intrinsic activation energy remains constant. This offers an explanation for discrepancies that exist between kinetic parameters obtained in previous investigations.