(390b) Enhancing Catalyst Effectiveness and Loading in Microreactors

Microreactors allow for increased viability of multiple energy & fuels applications through the ability to achieve breakthroughs in thermal efficiencies through process integration.  By reducing the characteristic transport lengths from millimeters to microns, order-of-magnitude improvements in heat transfer rates per unit reactor volume are achieved. This enables breakthroughs in reactor capacity and performance for highly endothermic processes including methane steam reforming. The reduced hydraulic diameter in turn necessitates the use of catalyst washcoatings in lieu of conventional packed-beds. This catalyst configuration inherently alters the conventional design rubric for catalyst thickness selection, owing to the fact that catalyst films experience internal heat addition, in contrast to conventional catalyst particles which must rely upon external solid-fluid contacting for heat addition.

In this presentation, a 1-D model was used to determine the possibility of gaining higher effectiveness factors for conventionally-determined film thicknesses, or conversely achieving comparable effectiveness at higher than conventional film thicknesses, through exploiting internal heat addition. Mass and energy balances were used with new boundary conditions to allow for this internal heat addition.  The conditions for methane steam reforming were tested using this system, showing high thermal efficiencies and higher effectiveness factors at higher catalyst thickness than conventionally used. This hypothesis was then tested using an experimentally validated computational fluid dynamics model of an Annular Microreactor (AMR) device, currently under development by Power+Energy, Inc., to test the sensitivity of the system to catalyst thickness. The effectiveness factor for each catalyst thickness was compared to the isothermal system for the cases of a flow rate resulting in 95% of equilibrium conversion of methane, and a flow rate much greater than equilibrium conversion. The thermal efficiency was calculated for the non-isotheral system for each case. In all cases the non-isothermal case with external heating had a higher relative average effectiveness with a thermal efficiency greater than 100%.  This investigation proves that a thicker catalyst can be used to increase hydrogen production with little loss in effectiveness, thus improving the reforming capacity in existing microreactors.