(82c) Microchannel Reactor Modeling and Design for Catalytic Hydrocarbon Steam Reforming

Microchannel reactors are known as the units characterized by parallel, identical channels with characteristic dimensions in the order of micrometers. These units are constructed using mostly a metallic substrate, have surface area-to-volume ratios ca. 50-100 times higher than those of their conventional counterparts, and involve catalysts in a form coated to the walls of the microflow paths. Due to the presence of high surface areas, sub-millimeter dimensions and use of metallic substrates, heat can be distributed quickly and uniformly along the channel walls and the coated catalyst layers allowing the efficient use of the catalyst. All of these features can turn into significant size and cost savings; when compared with a conventional reactor at an identical capacity, a microchannel reactor can lead to reductions of ~90% in size, ~30% in capex and ~25% in opex.

This study addresses quantitative investigation of steam reforming of iso-octane over Ni-based catalyst coated, microchannels to produce hydrogen needed to power a 1-kW PEM fuel cell. Heat needed to drive endothermic (endo) steam reforming is supplied by the exothermic (exo) methane combustion running in parallel channels coated with a Pt-based catalyst. Sets of parallel, straight exo and endo channels, all of which have identical-sized square cross-sections, are placed in the forms of layers and successive channels are equally separated by metallic walls. Heat flow between the intra-layer (identical) channels are neglected, allowing the multichannel system to be characterized by a ?representative unit' involving a single inter-layer pair of endo and exo channels. The system is assumed to operate at steady-state and reactive flows are assumed to be in a co-current mode.

The representative unit is modeled by incorporating two-dimensional continuity, momentum, energy conservation and species mass transport equations for the fluid and catalytic washcoat phases, and energy conservation equation for the metallic wall phase. Simultaneous solution of these equations are carried out using the method of finite elements under the Comsol Multiphysics CFD (Computational Fluid Dynamics) environment. The model is then used to figure out the effects of various configurational parameters such as wall thickness, type of wall material and the presence of micro-baffles on temperature distribution and on hydrogen yield, defined as number of moles of hydrogen produced per mole of iso-octane fed. The results show how these parameters can affect the degree of heat flow between the channels and can provide useful insights for the design of microchannel reactors that give increased hydrogen yields via improving the extent of endothermic steam reforming.