Steam reforming of methanol is a reaction of considerable interest for generating hydrogen. Hydrogen has, by far, the highest gas diffusivity compared to the chemical species involved in the reforming reaction. Thus, intuitively, it is not difficult to fathom that significant back diffusion of hydrogen may occur. The extent of hydrogen back diffusion has neither been investigated nor discussed in the literature.
A computational fluid dynamics (CFD) model for steam reforming of methanol in packed bed reactor accounting of multi-step reaction mechanism proposed by Peppley et al. and multi-component diffusive-convective transport was developed. The model, not originally intended to investigate hydrogen back diffusion, yielded startling results. The model, allowing for axial diffusion, predicted that for the experimental reactor employed by Peppley et al, that the influence of hydrogen back diffusion extends as far upstream as 10 cm from the bed inlet.

This back diffusion has significantly affects the concentration field and hence the kinetics of methanol steam reforming. It is concluded that this mechanism must be accounted for both in commercial reactor sizing and in the design of experiments to extract kinetc paramters