Characteristics of Oxygen Permeation and Partial Oxidation of Methane in a Catalytic Membrane Reactor for Syngas Production

In this study, the use of membranes for partial oxidation of methane (POM) for syngas production is investigated numerically in a catalytic membrane reactor (CMR). The reactor considers using a multilayer oxygen-permeable membrane made of macroporous Ni-Al foam substrate in the presence of LaNi0.9Pt0.1O3 catalyst. The CMR was fed with air in one side and a mixture of fuel (CH4) and helium in the sweep side. Because of its symmetry, half of the domain was meshed in the full three-dimensional (3-D) domain and ANSYS-2019-R2 was used to perform the numerical simulations. For modeling oxygen permeation across the membrane, a source/sink term was created in mass and momentum equations using a series of user-defined functions (in C++) to be compiled and attached to the software. The indirect approach for methane conversion into syngas was adapted in the numerical model considering steam and dry reforming reactions. The model was validated using the available literature experimental data on the same CMR. The simulations were performed over ranges of operating parameters in order to improve the performance of the CMR for syngas production. The effects of sweep fuel concentration, feed air flow rate, and sweep gas flow rate on CMR performance were investigated. The results showed that increasing sweep fuel concentration reduces its conversion. All reactions were quenched in the outlet tube, because of the increased flow velocity and lack of O-2. The counter-current flow configuration resulted in much better selectivities of CO and H-2, compared to the cases of cocurrent flow configuration (89% and 73%, compared to 38% and 23%, respectively). The increase of oxygen concentration at the permeate side of the membrane while increasing the feed air flow rate leads to a reduction of syngas selectivity, along with an increase in fuel conversion. The fuel conversion showed a sharp reduction from similar to 53% to similar to 5% for an increase in sweep gas flow rate from 5 L/h to 30 L/h at a fixed sweep fuel concentration of 10% (by volume). The selectivities of CO and H-2 decrease to <5% for sweep flow rates of >30 L/min. This indicates insufficient availability of oxygen for fuel conversion at higher sweep gas flow rates, because of reduced temperature within the CMR. For sweep flow rates of >30 L/h, the gas temperature even cools in the axial direction toward the membrane, resulting in deteriorated performance of the CMR.