Bifurcation analysis of methane oxidative coupling without catalyst

Abstract

We present a detailed bifurcation analysis of methane oxidative coupling in the gas phase using a global kinetic model for the various oxidation, reforming and dehydrogenation reactions. The kinetic model satisfies the thermodynamic constraints and is validated with literature data as well as new data obtained under near isothermal conditions. It is used to determine the methane conversion and C2-products selectivity under various feed and operating conditions in large scale ideal adiabatic reactors. It is found that at higher CH4/O2 ratios (e.g. >4), ignition and extinction points exist only at either high feed temperatures and/or space times, which may not be of practical interest. Autothermal operation on the ignited branch with feed at near ambient conditions (∼300K and 1 bar) is feasible for practical range of space times (1 ms–1 s) only for low CH4/O2 ratios (e.g. 1.7–2.5), which includes the flammability range. Further, the best ethylene yields are obtained on the ignited branch close to the extinction point while best C2 yields may be obtained at higher space times or feed temperatures. Feeds with a high CH4/O2 ratio lead to higher selectivity of C2H4 but lower methane conversion and require higher inlet temperatures. The ratio C2H4/C2H2 decreases as the methane conversion increases or CH4/O2 ratio decreases. While oxidation (exothermic) chemistry dominates on the ignited branch near the extinction point, the dehydrogenation and reforming (endothermic) chemistries dominate as the space time or feed temperature is increased. It is shown that the highest yield of intermediate products and largest region of autothermal operation is obtained for an ideal reactor with perfect thermal back-mixing and zero species back-mixing (lumped thermal reactor model).