(150d) Design of Membrane Reactor for Oxidative Coupling of Methane: Effects of Interphase Mass and Heat Transport Limitations

The oxidative coupling of methane to ethane and ethylene has been studied using different reactor concepts, among which the membrane reactor is tipped to be a potential candidate for its commercialization. Utilizing membrane reactors for OCM still suffer from the lack of membrane materials with high mechanical strength, thermal stability, and large surface area which at the same time is compatible with high performing OCM catalyst. Also, due to the high exothermic nature of OCM reactions, the comprehension of the interaction between the mass and thermal effects couple with the additional radial gradients introduced in the membrane reactor are indispensable for its optimal design. In this work, the latter problem is addressed via modelling that accounts for the OCM reaction path dependencies. A packed bed membrane reactor is design for oxidative coupling of methane (OCM); 1D and 2D heterogeneous models that distinguishes the bulk and solid phase are applied. A reaction network comprising of 15 reversible catalytic reactions and 39 gas phase reactions is implemented in the model. The interphase (concentration on the external surface of the catalyst) concentration profiles of molecules are shown to highly depend on the catalyst diameter and the gas flow velocity, of which concentration gradients are present for gas and surface radicals for all the conditions considered. These concentration gradient only marginally affected the selectivity towards C₂ (ethane and ethylene) but the radial concentration gradient are more detrimental for C₂ selectivity, even at conditions (large particle sizes & low flow velocities) where the OCM reactions is highly limited by mass transfer. Similarly, the radial heat gradient are more important than the interphase heat transfer gradient for the range of operating conditions in the membrane reactor. A further comparison of the performance of a membrane reactor with that of the conventional packed bed reactor, shows that the interphase heat transfer is more important in the latter than the former.