Effect of Pressure on Reduction Rate of Hematite Sphere with Hydrogen at High Pressure

Abstract

A hematite sphere of 3.3cm in diameter was reduced by H₂ in pressure up to 27 atm at 900°C. The temperature profiles within the solid sphere and the weight change were continuously measured. The experimental results were analyzed by the three interface unreacted-core shrinking model with dynamic effective diffusivity. Influence of gas flow through product layers on the reduction rate elucidated reasonably by using the known structural constants obtained from the permeability and the isobaric diffusion experiments.
The effect of pressure on the reduction rate was found to be governed essentially by three rate parameters : the effective molecular diffusivity at 1 atm (D°_AB) _eff', Knudsen diffusivity K_A, and chemical reaction rate constant k_s. The larger (D°_AB) _eff' was, the faster reduction rate became and the larger pressure effect became. In the case that k_s and K_A became larger under the condition of constant (D°_AB) _eff' the reduction rate increased but the pressure effect decreased. The reduction rate increased with the increase of the pressure, while it became maxmum when the molecular diffusion through the products layers governed the overall reduction rate.