The functioning of thermochemical transformers includes several phenomena: heat and mass transfers and solid/gas chemical reaction. Their physical meaning is studied, modelled by partial differential equations and solved by the finite element method. The model has been validated from experimental results. Main parameters for the reactive material transformation are the operating pressure and the permeability of the porous medium used, called Impex. In this article, optimum gas diffuser implantation is proposed in order to increase mass transfer and improve reactor performances.