A simulation of an inert membrane reactor in the partial oxidation of butane to maleic anhydride in high butane concentrations (2 - 13%) is presented A porous membrane was used to distribute oxygen to a fixed bed of VPO catalyst, to which butane was fed. Two different reactor configurations were simulated: a standard inert membrane reactor, in which oxygen permeates inward from the shell side and the catalyst is packed on the tube side; an outward flow inert membrane reactor; in which the catalyst bed is packed on the shell side and oxygen flows outward from the tube side. The model explains the different performances of both types of reactor in terms of the interaction between the oxygen and hydrocarbon concentration profiles and the temperature profiles that are obtained with both reactor configurations.