Study on oxidative coupling of methane in oxygen permeable tubular membrane reactor

Mixed-conducting oxygen permeable membrane is a kind of ceramic membrane, which exhibits both electronic and oxygen ionic conductivity. At high temperature, oxygen can permeate through the membrane in the form of oxygen ions. So on the membrane surface, there are many oxygen species (O-, O-2(-), O-2(2-) and O-2(-)), which can improve C-2 selectivity of oxidative coupling of methane (OCM). A dense tubular membrane made of Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCFO) was prepared by the plastic extrusion method. OCM was investigated in the tubular membrane reactor. C-2 production rate in the BSCFO membrane reactor is much higher than that in La0.6Sr0.4Co0.8Fe0.2O3-delta ( LSCFO) and La-Ba-Co-Fe-O (LBCFO) membrane reactors. The reason is that the vacancy concentration in the BSCFO is higher than that in the LSCFO and LBCFO. The higher vacancy concentration not only accelerates the diffusion rate of oxygen ion but also increases the activation rate of methane. So the BSCFO is more active for OCM than the LSCFO and LBCFO. The C-2 production rate increases with the increase of temperature and methane concentration in the feed. However, the C-2 selectivity is determined by the competitive result of oxygen ion recombination and OCM reaction. High rate of oxygen ion recombination will decrease the C-2 selectivity. The methane conversion increases while C-2 selectivity decreases with the increase of oxygen partial pressure in the oxygen-rich side. This result indicates that the gas-phase reaction plays an important role in the OCM reaction. There is a large amount of O-2 in the products which results in the modest increase of the oxygen permeation flux, compared with the case of air/He.