(296a) Autothermal Reactor Design for Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) is considered one of the more promising direct processes for valorizing methane in the form of ethylene. In the last decades, OCM research has focused primarily on developing catalysts with the potential to improve the low C2 yields. But is the primary issue of OCM truly a catalyst problem? Because of the high exothermicity of the OCM process, thermal effects and path dependence are dominating in all OCM reactors of practical importance. Furthermore, irreducible diffusion limitations exist on the pellet scale. Understanding how to exploit these mass and heat transfer effects by reactor engineering is a prerequisite for the breakthrough of OCM. From bifurcation analyses performed by Balakotaiah and co-workers, it followed that the key features of an OCM reactor are high effective thermal conductivity and a narrow residence time distribution. The latter is necessary to control and maximize the selectivity towards the C2 products. High effective thermal conductivity provides positive thermal feedback facilitating possible ignition while operating at close to ambient inlet temperatures and harnessing the exothermicity of the reaction to perform OCM adiabatically and autothermally. Some feasible autothermal reactor designs include the shallow packed bed reactor and monoliths with a high conductivity substrate, both described in detail by Balakotaiah. Alternatively a gas-solid vortex reactor (GSVR) can be considered. CFD-based bifurcation diagrams show that the GSVR indeed exhibits sufficient thermal backmixing to create steady-state multiplicity, which will eventually allow autothermal operation. Next to the hydrodynamic aspects detailed kinetics are accounted for during the CFD simulations. In doing so minimal catalyst performance criteria can be formulated highlighting the importance of considering the combined effects of the intrinsic kinetics of OCM on one hand and of the reactor design and operation on the other hand.