Reduçao termica zno

Ind. Eng. Chem. Res. 2004, 43, 7981-7988

7981

Solar Carbothermal Reduction of ZnO: Shrinking Packed-Bed Reactor Modeling and Experimental Validation
Thomas Osinga,† Gabriel Olalde,‡ and Aldo Steinfeld*,†,§
Department of Mechanical and Process Engineering, ETH - Swiss Federal Institute of Technology Zurich, ¨ CH-8092 Zurich, Switzerland, Institut de Science et de Genie des Materiaux et Procedes, CNRS-IMP, ¨ ´ ´ ´ ´ 66125 Font-Romeu Cedex, France, and Solar Technology Laboratory, Paul Scherrer Institute, CH-5232 Villigen, Switzerland

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 firstorder 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 thermogravimetric data is EA ) 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 1D 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 × 10-5 m/s as the