Role of distributed oxygen addition and product removal in the oxidative coupling of methane

Modeling and optimization of C2 hydrocarbon production via the oxidative coupling of methane (OCM) were studied. The model includes both homogeneous and heterogeneous reactions. Focusing on the use of detailed reaction networks, previously validated experimentally, and the critical role of oxygen in both methane activation and product degradation, this work systematically explores the use of controlled oxygen addition and product removal schemes that improve OCM performance. Based on a plug flow reactor that is divided into Np stages, within which oxygen is added and/or products are removed, a rigorous optimization algorithm is developed that simultaneously maximizes C2 yields and minimizes O2 consumption. In the absence of catalyst and product removal, the C2 yield is maximized at a fixed O2 /CH4 ratio, but this maximum yield is independent of the form in which the oxygen is added (cofeed or staged). When a catalyst is added, the optimal C2 yields show only gradual improvements with oxygen distribution because the benefits of the lower oxygen reaction order on the catalyst are adversely affected by concomitant surface degradation reactions. The largest yield improvements are obtained when the C2 hydrocarbons are removed at each stage before they undergo oxidation reactions. Thus, when staged oxygen addition is combined with product removal in the presence of a catalyst, C2 yields as high as 87% are achieved in about 20 stages. Such yield values are consistent with experiments in which continuous product separation schemes have been used.