A model for membrane reactors is proposed, taking into account the radial concentration profiles for each reactant, as well as the temperature profiles. This model is applicable for a membrane reactor with a fixed bed of catalyst inside the membrane and with distribution of one of the reactants along the reactor through the membrane. The model predictions are compared with the results obtained in an experimental reactor, and the effect of an increase in the reactor diameter and other changes in the operation conditions are studied by means of the model.