Solar carbothermal reduction of ZnO: Shrinking packed-bed reactor modeling and experimental validation

The thermodynamics and kinetics of the carbothermic reduction of ZnO are examined over the temperature range 400-1600 K. Above 1340 K, the equilibrium composition of the stoichiometric chemical system consists of an equimolar gas mixture of Zn (vapor) and CO. Assuming a first-order rate constant for the surface reaction kinetics between ZnO(s) and CO and further applying a shrinking spherical particle model with an unreacted core, the apparent activation energy obtained by linear regression of the thermogravinietric data is E-A = 201.5 kJ/mol. A numerical model is formulated for a solar chemical reactor that uses concentrated solar radiation as the energy source of process heat. The model involves solving, by the finite-volume technique, a ID unsteady-state energy equation that couples heat transfer to the chemical kinetics for a shrinking packed bed exposed to thermal radiation. Validation is accomplished by comparison with experimentally measured temperature profiles and Zn production rates as a function of time, obtained for a 5-kW solar reactor tested in a high-flux solar furnace. Application of the model for a scaled-up reactor predicts a large temperature gradient at the top layer, which is typical of ablation processes where heat transfer through the bed becomes the rate-controlling mechanism. Once the temperature of the top layer exceeds 1200 K, the bed shrinks at an approximately constant speed of 2.8 x 10(-5) m/s as the reaction proceeds under a constant radiative flux input.