C2 yield enhancement during oxidative coupling of methane in a nonpermselective porous membrane reactor

Abstract

A systematic comparison between a packed bed membrane reactor (PBMR) and packed bed reactor (PBR) for oxidative coupling of methane (OCM) on Na2WO4-Mn/SiO2 catalyst reveals the utility of distributing the oxygen feed along the catalyst bed length. A sufficiently high transmembrane pressure difference is needed to realize a moderate C2+ yield enhancement (∼30%). The enhancement is attributed to a shift in selectivity towards the desired coupling reaction by maintaining a high methane to oxygen ratio along the bed length. The elevated transmembrane pressure is achieved by adjusting the shell and tube side parameters including catalyst particle size, inert dilution ratio, residence time, and temperature. Under identical reaction conditions, a C2+ yield of 23.6% is obtained for the PBMR compared to 18% for the PBR. The potential detrimental impact of an axial pressure drop in the catalyst bed is compensated by increasing the shell side pressure through an increase in the shell-side nitrogen dilution ratio. Experiments were conducted at lower nitrogen dilution by increasing the catalyst particle size and reducing the tube side pressure. At a dilution ratio of ∼38%, a C2+ yield of 20% compared to 17% in PBR was obtained, which is on the higher end of the C2+ yield reported in literature for a distributed feed membrane reactor at a low dilution. Ways to overcome bypassing of the catalyst bed by shell side O2 as well as back diffusion of methane were also investigated. The results emphasize the crucial role of transmembrane pressure in enhancing the distribution of oxygen feed along the reactor length for improved OCM catalyst performance.