The fundamental transport phenomena in microchannel steam reforming reactors for hydrogen production are studied using dimensionless numbers. Numerical simulations are performed using chemical kinetics coupled with fluid mechanics to study different phenomena that lead to transport. The dimensionless numbers in transport phenomena are principally analyzed to provide insight into the reaction behaviors and characteristics. The effects of reaction pressure, catalyst porosity, and inlet Reynolds number are evaluated to understand the physical and chemical processes involved in the systems. The results indicate that hydrogen productivity is favored kinetically at higher pressures, but high process efficiency is difficult to achieve. Momentum diffusivity dominates the diffusion behavior, and heat diffuses slowly. Catalyst porosity is vital to ensuring effective operation. Higher levels of porosity allow more efficient hydrogen production and may resolve the problem of mass-transfer limitation. Convective rates dominate mass transfer, and diffusion contributes insignificantly. The tortuous Reynolds number depends heavily on the level of porosity. The Nusselt and Sherwood numbers depend heavily on the pressure but do not vary significantly with the Reynolds number.

Optimization of catalyst porosity arrangements is carried out for hydrogen production through computational modeling of a thermally integrated microchannel reactor. The reactor has parallel flow channels for conducting simultaneous oxidation and reforming reactions. Numerical simulations are performed under a variety of velocity conditions to evaluate the effect of reforming catalyst porosity arrangement on the transport phenomena in the reactor system. The oxidation catalyst has a uniform porosity, and the porosity range of the reforming catalyst is from 30 to 70 percent. The porosity is uniform in each segmented region and the overall porosity is maintained 50 percent. The heat and mass transfer issues for the reactor system are highly complex. Performance comparisons are made in terms of methanol conversion, hydrogen yield, and heat of reaction between these porosity cases under different inlet velocity conditions. Dimensionless Nusselt and Sherwood number analyses are performed to understand the underlying cause for the performance difference. The dimensionless numbers in transport phenomena are principally analyzed to understand how important the transverse transport components are. The results indicate that optimization of catalyst porosity arrangements is required for thermal matching purposes. The optimum porosity arrangement depends upon the flow rates. The catalyst porosities must be configured to improve the kinetics in the upstream or downstream sections of the reactor so that the endothermic and exothermic processes are thermally matched. While advantages can be realized by using the two-segment design, the three-segment design yields no advantage. The processes of transverse transport are of great importance to the chemical reactions.