A novel reverse flow reactor coupling endothermic and exothermic reactions: an experimental study

Abstract

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation, i.e. with incorporation of regenerative heat exchange inside the reactor via periodic gas flow reversals. In a small laboratory scale reactor the concept of this ‘reaction coupling reverse flow reactor’ (RCRFR) has been investigated experimentally for the propane dehydrogenation coupled with methane combustion over a monolithic catalyst, aiming for a proof of principle. Despite the inherently and inevitably large influences of radial heat losses on the axial temperature profiles in a laboratory scale reactor, as shown with some experiments with propane and methane combustion in reverse flow without propane dehydrogenation reaction steps, the experimental results show that indeed endothermic and exothermic reactions can be integrated inside the reactor together with recuperative heat exchange. The periodic steady state was easily obtained without any problems associated with process control. Furthermore, intermediate flushing with nitrogen between the propane dehydrogenation and methane combustion steps could be safely omitted. However, it was necessary to reduce the oxygen concentration during the methane combustion steps in order to avoid too high temperatures due to local combustion of carbonaceous products in the washcoat deposited during the preceding propane dehydrogenation reaction step. Propane dehydrogenation experiments in a reactor filled entirely with active catalyst demonstrated the seriousness of ‘back-conversion’, a term used to indicate the loss of propane conversion due to propylene hydrogenation because of the low exit temperatures. Experiments performed in a reactor with inactive sections flanking the active catalyst section at both ends showed that the back-conversion could be effectively counteracted.