Modeling and Simulation of a Steam-Selective Membrane Reactor for Enhanced CO2 Methanation

A modeling and simulation study of a hydroxy sodalite (H-SOD) membrane reactor for in situ steam removal during CO2 methanation reaction is presented. A nonisothermal, steady-state, pseudohomogeneous 1D mathematical model with axial dispersion was proposed to describe and compare the performance obtained with traditional fixed-bed and membrane reactors. The model was validated against experimental data taken from the literature, first concerning CO2 conversion in a fixed-bed reactor and then regarding mixture separation factors through the membrane. The reactor’s performance was assessed considering the CO2 conversion and CH4 outlet purity under different operating conditions. The membrane selectivity toward H2 is more significant than that for the other species (i.e., CH4 and CO2). Hence, if hydrogen is used as sweep gas it can be co-delivered from the permeate side, which was found to significantly enhance the membrane reactor performance. The permeate-to-retentate pressure ratio (P ratio) increases the CO2 conversion but only up to an optimum value (P ratio*), which depends on the reaction conditions and feed composition. This holds true particularly when processing raw biogas: increasing the P ratio leads to a trade-off situation, where CO2 conversion increases but CH4 purity decreases. High water selectivity of the membrane enables a significant improvement compared to the traditional reactor performance, particularly at high temperature, pressure, and contact time, even if some reactants are lost to the permeate. Notably, the membrane reactor excels over the traditional fixed-bed reactor even at milder temperatures and pressures, which is particularly relevant for improving the overall energy efficiency of power-to-methane processes.