(37a) Modeling Of A Micro-Scale Heat Exchanger Reactor For Portable Hydrogen Production

Integrated microreactors for hydrogen generation via coupled endothermic and exothermic reforming have been widely investigated^1,2. Understanding the characteristics, behavior and stability of these systems as a function of geometry, material properties and reaction kinetics is essential to designing an efficient system.

In the present study, a one-dimensional steady-state model describing heat convection and conduction is employed to investigate the effects of key kinetic, fluid and solid phase properties upon thermal efficiency and reactor conversion. This model accounts for convection and conduction of heat and mass in two separate reacting fluids, coupled via heat conduction across a solid wall. Selection of appropriate boundary conditions allows investigation of packaging (adiabatic or isothermal) and flow configuration (co-current vs. counter-current) upon system performance.

First we analyze the case of heat transfer between two homogeneous fluids in the absence of chemical reaction or phase change in counter current configuration. The same analysis is extended for the case of heat transfer between one reacting and one non-reacting fluid, and two reacting fluids (coupling of endothermic-exothermic reactions). A parametric study is carried out in order to identify the key groups that most influence the heat exchanger reactor performance and the existence of steady state multiplicity.

Results demonstrate that the use of high thermal conductivity materials (e.g. silicon, stainless steel) limits thermal efficiency due to significant axial conduction losses, ultimately leading to isothermal-slab behavior. Low thermal conductivity materials (e.g. cordierite ceramics) yield superior thermal efficiencies, resulting from development of temperatures gradients along the solid phase axial length. Also, we show that the use of adiabatic packaging presents higher thermal efficiencies. Modeling results are supported by an experimental system capable of creating large networks of parallel ceramic microchannels for heat integration.

1 Arana, L.R., S.B. Schaevitz, A.J. Franz, M.A. Schmidt and K.F. Jensen, ?A Microfabricated Suspended-Tube Chemical Reactor for Thermally Efficient Fuel Processing,? J. Microelectromech. Syst., 12(5), 600-612 (2003). 2 Kolios, G., Gloeckler, B., Gritsch, A., Morillo, A., Eigenberger, G., ?Heat-integrated Reactor Concepts for Hydrogen Production by Methane Steam Reforming?. Fuel Cells. 5(1), 52 (2005)