Effects of product separation on the kinetics and selectivity of oxidative coupling

Abstract

It is generally accepted that secondary reactions of ethylene limit the yields that can be achieved in oxidative coupling of methane. It seems obvious that a means of removing ethylene before secondary reactions occur might overcome this limitation. We have used a variety of separation techniques to remove selected products during the catalytic conversion of methane and oxygen in a fully-backmixed, batch reactor. Time-resolved measurements of the evolution of the reactants and stable products allows us to determine the global reaction kinetics, and the influence of the removal of specific products on these kinetics. We first employed a condensation/ adsorption technique in which ethane, ethylene and CO2 are removed during the course of methane conversion. In this case, secondary reactions that convert ethane to ethylene are prevented, and the dominant product is ethane. Ethane selectivities in excess of 80% even at methane conversions above 80%, were obtained. We also employed a membrane separation technique in which product separation is specific for ethylene. Ethane and CO2 remain in the reactor. Under these conditions, high yields of ethylene, by far the predominant hydrocarbon product recovered, are achieved. We find that product removal influences the global kinetics. For example, removal of ethane reduces chain propagation, reducing the rate of methane conversion. This effect is small, however, compared to removal of CO2, which significantly reduces the energy barrier to methane conversion, increasing the rate of conversion. These effects, and results from detailed kinetic modelling of the consequences of specific product removal on the complex reaction network will be discussed.