(603b) Coupling of Endothermic and Exothermic Reactions in Cross-Flow Microrectors

The possibility of tailoring catalyst patterns and flow configuration, and fast heat and mass transfer rates make microreactors ideal for thermal coupling of exothermic combustion reaction with endothermic reactions. Examples of such coupling include hydrogen generation through endothermic reforming or ammonia decomposition, dehydrogenation, hydrocarbon cracking, etc. Effective utilization of heat released from combustion to drive the endothermic reaction is a prerequisite for improving device efficiency, ensuring auto-thermal operation and managing temperature surges in the multi-function microreactor. The past decade has witnessed growing interest in using thermally coupled microreactors. Kolios et al. [1] reviewed various options for thermal coupling in fixed-bed reactors for auto-thermal reactor operation. Deshmukh and Vlachos [2] compared co-current and counter-current flow configurations for coupling propane combustion with ammonia decomposition in microreactors. The advantage of co-current coupling is that the hot-spot formation is minimized due to overlap of reaction zones in the exo- and endothermic channels.

The objective of this work is to investigate cross-flow reactor design through Computational Fluid Dynamics (CFD) simulations for thermal coupling between endothermic and exothermic reactions. A planar device structure will be used and 3D CFD simulations will be performed. A cross-flow configuration is used for the combustion zone so that the flow direction is perpendicular to that of the reforming channels. The advantages of using cross-flow configuration include lower pressure drop in combustion channel [3] and good thermal management [4,5]. Specifically, low temperature gradients were obtained in cross-flow microreactor even for adiabatic rise of 1400 K in combustion chamber [4]. On the other hand, Arzamendi [5] showed that the temperature difference in the solid structure for cross-flow microreactor is between those observed in co- and counter-current flow arrangements.

Our initial simulations with combustion confirmed the observation of lower pressure drop (reported by [3]). These results also indicate that careful design of flow channels is required to ensure high conversion. Likewise, appropriate catalyst placement is necessary for better thermal management and improved device stability. The effect of flow rate, material choice, and the role of geometry will be discussed.

References [1] Kolios, G., Frauhammer, J. and Eigenberger, G. (2000). Autothermal fixed-bed reactor concepts. Chemical Engineering Science, 55, 5945?5967. [2] Deshmukh, S.R. and Vlachos, D.G. (2005). Effect of flow configuration on the operation of coupled combustor/reformer micro devices for hydrogen production. Chemical Engineering Science, 60, 5718-5728. [3] Ajmera, S.K, Delattre, C, Scmidt, M.A, Jensen, K.F(2002). ?Microfabricated cross-flow chemical reactor for catalyst?. Sensors and Actuators B 82, 297-306. [4] Rebrove, E.V, de Croon, M.H.J.M, Schouten, J.C (2001). ?Design of a microstructured reactor with integrated heat-exchanger for optimum performance of a highly exothermic reaction?. Catalysis Today, 69, 183-192. [5] Arzamendi, G, Deiguez, P.M, Montes, M, Centento, M.A, Odriozola, J.A. and Gandia, L.M (2009). ?Integration of methanol steam reforming and combustion in a microchannel reactor for H2 production? Catalysis Today, 143, 25-31.