CATALYTIC OXIDATIVE COUPLING OF METHANE - REACTION-ENGINEERING ASPECTS AND PROCESS SCHEMES

Catalytic oxidative coupling of methane to higher hydrocarbons (OCM) has attracted widespread attention as a potential route to ethylene or liquid fuels. Besides tremendous research work in catalyst development, a lot of investigations dealt with reaction engineering aspects in order to make this process technically feasible, The objective of the paper is to evaluate the current status in this field. Various aspects as reaction kinetics, reactor selection and different modes of reactor operation are analysed with the aim to identify reaction engineering means which can be used to maximize C-2+ selectivity and yield. Particularly, available kinetic models for reaction engineering applications are revised and the importance of various reaction steps, also pre- and postcatalytic reactions is analysed. Since OCM is a complex reaction network of parallel and consecutive, heterogeneous as well as homogeneous reaction steps the selectivity to C-2+ hydrocarbons strongly depends on the reaction conditions; in the paper the effect of temperature, partial pressures of methane and oxygen, and total pressure is discussed. Central issue in development of the OCM process is the choice of the type of reactor. Possible designs of fixed-bed and fluidized-bed reactors, the status in their modelling for OCM, best C-2+ yields achieved in lab-scale units as well as novel reactor designs are reviewed. Furthermore, the possibility to improve catalytic performance by applying distributed feed of oxygen or secondary feed of ethane is discussed. Finally a general scheme of the OCM process which includes various options proposed in the literature is presented.