Modelling of a Reverse Flow Catalytic Membrane Reactor for the Partial Oxidation of Methane

Abstract

Gas-To-Liquid (GTL) processes have great potential as alternative to conventional oil and coal processing for the production of liquid fuels. In GTL-processes the partial oxidation of methane (POM) is combined with the Fischer-Tropsch reaction. An important part of the investment costs of a conventional GTL-plant is related to cryogenic air separation. These costs could be substantially reduced by separating air with recently developed oxygen perm-selective perovskite membranes, which operate at similar temperatures as a POM reactor. Integration of these membranes in the POM reactor seems very attractive because oxygen reacts at the membrane surface resulting in a high driving force over the membrane increasing the oxygen permeation.Because the POM-reaction is only slightly exothermic, the natural gas and air feed have to be preheated to high operating temperatures to obtain high syngas yields and because the Fischer-Tropsch reactor operates at much lower temperatures, recuperative heat exchange is essential for an air-based POM process. External heat transfer at elevated temperatures is expensive and therefore recuperative heat exchange is preferably carried out inside the reactor, which can be achieved with the reverse flow concept. To combine the POM reaction, air separation and recuperative heat exchange in a single apparatus a novel, multi-functional reactor is proposed, called the Reverse Flow Catalytic Membrane Reactor (RFCMR). In this reactor a relatively uniform temperature profile should be established at the membrane section and the temperature fronts should be located in the inert in- and outlet sections.To study the RFCMR concept, reactor models have been developed assuming a shell-and-tube geometry, based on models that are commonly used to describe conventional reverse flow reactors. Simulations of the novel reactor concept revealed that a small amount of methane has to combusted on the air side to create the reverse flow behaviour. Also a small amount of steam has to be injected distributively along the perovskite membrane section to maintain the centre of the reactor at nearly isothermal conditions. With these modifications it was found that the desired temperature profile could indeed be created in the RFCMR and that high overall syngas yields can be achieved.