Scale-out of microreactor stacks for syngas production from methane

Microreactor technology is an attractive means to intensify methane-to-syngas production processes as microreactors have shorter characteristic heat and mass transfer distances relative to conventional methane-to-syngas reactors. Current industrial syngas production processes are characterized by contact times on the order of seconds while syngas generation processes using microreactor technology have demonstrated reaction time scales on the order of milliseconds. In this work a scale-out strategy for microreactor stacks with alternating combustion and reforming channels with sub-millimeter gap size is proposed. Microreactor geometries were simulated using the computational fluid dynamics (CFD) software package Fluent® and catalytic surface reaction rates were implemented using recently published reduced rate expressions derived from microkinetic modeling for methane combustion over Pt and methane steam reforming and watergas-shift reactions over Rh. Using this model, microreactor stacks of several sizes were studied under varying degrees of heat loss by changing the external heat loss coefficient. The maximum heat loss of a stack occurs at the critical heat loss coefficient and this quantity was used to compare the stability of the different stacks. It was found that for a given set of flow rates and material properties, there is a minimum stack size below which methane combustion cannot be sustained and thus the stack will not function. For a wall thermal conductivity of 100 W/m-K, the minimum size was seven channels while at 23 W/m-K the minimum size was eleven channels. The behavior of individual channels within one stack was investigated in order to understand the extinction mechanism as well as heat generation and consumption within the stacks. It was found that near the critical heat loss coefficient of the stack, methane conversion within the combustion channel closest to the stack edge drops significantly and upon further heat loss combustion in this channel dies and causes extinction of the entire stack.status: publishe