(603a) Stability and Performance of Microreactor Stacks for Coupling of Exothermic and Endothermic Reactions

It is often assumed that production within microreactor stacks increases linearly with the number of channels and therefore scale-up (or scale-out) of microchannel reactors is easier relative to conventional methods [1-3]. This linear scale-out assumption may breakdown since it has not been established that physical phenomena caused by stack edges affect all channels equally. This work makes the first stride in determining the effects of scaling-out microreactor stacks for an example process, methane steam reforming. Computational fluid dynamics (CFD) simulations are used to study stacks of different sizes in order to understand nonlinear effects which arise in scale-out of microchemical systems. As an example process, syngas production from methane is studied using a multifunctional, parallel plate reactor with alternating combustion and steam reforming channels. A scale-out strategy is proposed which creates larger stacks from a small base-unit. Stability under external heat loss is evaluated for both high and moderate wall thermal conductivities and stack performance is evaluated using performance indices developed in this work. Our calculations indicate that microsystems provide one to three orders of magnitude larger volumetric and gravimetric throughputs than conventional technology irrespective of model uncertainty, and such intensification is central to portable and distributed processing. Microreactors exhibit energy efficiency that is a strong function of size and heat loss but can outperform conventional processing under many conditions. However, they result in higher cost per unit syngas volume unless system optimization is carried out. We find that smaller stacks are unstable (not autothermal) under laboratory heat loss conditions. Stacks with high wall thermal conductivities are more stable than those with moderate wall conductivities under our conditions. At high heat loss coefficients, significant transverse thermal gradients exist between interior and edge channels of the stacks that result in significant loss of efficiency. A transition from all channels ignited to some ignited and some extinguished and finally to all channels extinguished is discovered as criticality is approached in moderate size stacks. Methods are investigated to improve the stability of stacks. Lastly, scale-out is studied for methanol steam reforming stacks, which operate at lower temperatures relative to methane powered microsystems.

[1] Lerou, J.J. and K.M. Ng, Chemical reaction engineering: A multiscale approach to a multiobjective task. Chemical Engineering Science, 1996. 51(10): p. 1595-1614.

[2] Jensen, K.F., Microreaction engineering - is small better? Chem. Eng. Sci., 2001. 56(2): p. 293-303.

[3] Stefanidis, G.D. and D.G. Vlachos, Millisecond methane steam reforming via process and catalyst intensification. Chem. Eng. Tech., 2008. 31(8): p. 1201-1209.