On the reaction of solids with mixed gases

A mathematical formulation is presented, describing the rate of reaction of porous solids with mixed gases in regimes when the overall rate is controlled by pore diffusion or by external mass transfer. The reduction of metal oxides with a hydro gen-carbon monoxide mixture is a typical example of such systems.In the formulation, proper account was taken of the coupled nature of the fluxes, by writing the Stefan-Maxwell equations to represent the diffusion process. The overall rates obtained from the numerical solution of these simultaneous differential equations were compared with those calculated using a much simpler procedure, involving the concept of the pseudobinary diffusion coefficient. The maximum discrepancy between the total reducing gas flux obtained by these two methods was only some 15 pct at intermediate gas compositions. However, much more substantial discrepancies were obtained regarding the transfer of the individual components.