(522a) Scale-out of Microreactor Stacks for Syngas Production From Methane

Microreactor technology has become an attractive means to intensify methane-to-syngas production processes for portable power applications as well as off-shore methane steam reforming. Conventional methane steam reforming reactors are characterized by residence times on the order of seconds while syngas generation processes using microreactor technology have demonstrated reaction time scales on the order of milliseconds [1]. This reduction in residence time is a product of shorter characteristic heat and mass transfer distances and it ultimately results in reduced equipment size. In this work, a scale-out strategy for microreactor stacks with alternating combustion and reforming channels with sub-millimeter gap size is proposed. The effect of scaling-out the stacks is then studied as a function of heat loss to the ambient.

Microreactor geometries were simulated using the computational fluid dynamics (CFD) software package Fluent®. Catalytic surface reaction rates were implemented using recently published reduced rate expressions derived from previous microkinetic models for methane combustion and steam reforming[2], [3]. Using this model, microreactor stacks of several sizes with alternating combustion and reforming channels were studied under varying degrees of heat loss. The maximum heat loss coefficient of each stack size where combustion processes are sustained and the stack is autothermal was determined and used to compare the stability of the different stack sizes.

It was found that for a given set of flow rates and material properties, there is a minimum number of combustion and reforming channels below which heat losses to the ambient are too high relative to heat generation via methane combustion. Below this stack size, combustion processes are not sustained and syngas is not produced. Significant transverse thermal gradients develop as the external heat loss coefficient is increased from adiabatic to the critical value. This causes the outer and inner channels to exhibit dissimilar methane conversions in both combustion and steam reforming channels. Temperature as well as methane conversion profiles for the individual channels were studied in order to understand the mechanism of extinction. It was found that near the critical heat loss coefficient of the stack, methane conversion within the combustion channel closest to the stack edge decreases significantly while inner combustion channels remain relatively unaffected by the heat losses. It is clear that the outermost combustion channel fails first, causing the entire stack to extinguish. The final part of the work examines methods for stabilizing microstacks for various power generation applications.

[1]Tonkovich, A.Y., et al. Chemical Engineering Science, 59, 4819 (2004).

[2]Deshmukh, S.; Vlachos, D.G., Combustion and Flame, 149, 366 (2007).

[3]Maestri, M., et al. Journal of Catalysis, (submitted)