Spouted fluidised bed reactor for kinetic measurements of the reduction of Fe2O3 in a CO/ CO2 atmosphere part II: An extended random pore model for solid-state diffusion

Abstract

A generalised random pore model, including both gaseous diffusion and solid-state diffusion in the product layer was developed, and was successfully applied to the reduction of hematite to magnetite by CO. The initial surface rate constant, ks,0′ and effective diffusivity, DS in the original model were formulated to describe the surface reaction and solid-state diffusion in the product layer. This approach is also particularly useful to determine the solid-state diffusivity during the reduction of a metal oxide, which is difficult to obtain via direct measurement. As an extension of previous work, the generalised random pore model was satisfactorily applied to the kinetic data obtained from a spouted fluidised bed reactor for the reduction of pure iron oxide with CO. Kinetic and diffusion constants, ks,0′ and Ds, were derived by fitting the model to the kinetics data for temperatures up to 650 °C. The activation energy for ks,0′ and Ds were calculated to be 35 kJ mol-1 and 100 kJ mol-1. The overall reaction kinetics were likely to be control by both surface reaction and the product layer diffusion of iron ions, Fen+. The fitted random pore model was fed into a two-phase fluidised bed model to test against experimental measurements from the spouted fluidised bed reactor. The model outlet concentration was shown to agree closely with that observed in the experiment. This validated the key assumption required for intrinsic kinetic measurements using the fluidised bed reactor, and also give credence to the applicability of the random pore model to reactions limited by solid state diffusion.