(21b) Design Principles of Multifunctional Microdevices for Hydrogen Production Via Short Contact Time Steam Reforming

Autothermal hydrogen generation in a catalytic reactor requires coupling of endothermic reforming reactions with exothermic combustion or partial oxidation reactions. The oxidation and reforming reactions can be carried out in separate reactors, as in the current industrial practice, or combined in a multifunctional device (1). Steam methane reforming is a major industrial process for hydrogen production in refineries and petrochemical plants and may become an important player in biorefineries. Therefore, investigation of its potential to provide hydrogen generation at the microscale is of great interest for portable and distributed power generation.

We use a pseudo two-dimensional (2D) model developed in (2) to model a microdevice, which consists of two parallel catalytically coated, closely spaced plates. Propane/air combustion on Pt takes place in one of them and steam methane reforming on Rh in the other. Fast heat exchange is achieved through the thin wall that separates them. This relatively simple configuration provides compactness, intimate coupling between the two processes, and the flexibility of different catalysts, fuels and flow configurations. We investigate whether we can make steam methane reforming workable at small scales at short contact times by capitalizing on process intensification while avoid microburner extinction. This requires not only fast reforming chemistry but also fast heat transfer. Aside from that, we aim at developing general design guidelines of integrated multifunctional microreactors. To this end, the effect of operating conditions, of the choice of the wall material and the reactor size are studied.

Our results show that the intrinsic steam methane reforming chemistry on Rh is very fast and only slightly slower than methane partial oxidation. Catalytic plate reactor operation in co-current mode enables compact steam methane reforming, fast transverse heat transfer and millisecond operation. Design maps for efficient operation have been generated; they are demarcated from the maximum power output point, the breakthrough point and the material stability limit (an upper temperature above which materials' stability is an issue). Low conductivity wall materials result in increased methane conversion at the expense of hot spot formation and steeper axial wall temperature gradients. Finally, it was found that decrease in the reforming channel gap size at constant inlet velocity drastically increases fuel conversion and system efficiency.

References

1. Deshmukh, S. R.; Vlachos, D. G. Chemical Engineering Science 2005, 60, 5718-5728.

2. Kaisare, N. S., Vlachos, D. G. Proceedings of the Combustion Institute 2007, 31, 3293-3300.