Design and Optimization of a Novel Monolith Reactor for the Conversion of Natural Gas to Ethylene by Oxidative Coupling

We examine the oxidative coupling of methane (OCM) with ethane as a cofeed using a two-mode reduced-order model and present a comprehensive ignition-extinction analysis of the process in an adiabatic monolith reactor. We determine the optimal feed composition to convert methane and ethane simultaneously, expand the region of autothermal operation, and increase selectivity to ethylene compared to the case of pure methane feed. Our results show that reactant conversions and selectivity to ethylene are nonmonotonic on the ignited branch, and there exists an optimum point away from the extinction point maximizing the per-pass ethylene yield. It is demonstrated that ethylene selectivity can be enhanced in low cell density monoliths (high hydraulic radius) by operating under an external mass transfer limit. The computations reveal that shorter beds with strong thermal back-mixing close to the thin-bed and homogeneous limit lead to a high ethylene yield by maintaining a low oxygen concentration across the bed. The results also indicate the potential of direct utilization of natural gas (of typical composition 90% CH4 and 10% C2H6) in OCM with a 30% enhancement in ethylene yield as compared to pure methane feed. Finally, the feasibility of autothermal operation at a higher pressure of 5 bar is also presented.