Niket S Kaisare

The reduction in size of catalytic microreactors results in high heat and mass transfer rates and a significant increase in the surface area to volume ratio. A further increase in the catalytic surface area can be achieved in a scaled-down version of fixed bed reactors. Since micro-fixed bed reactors are often deemed impractical due to their large pressure drops, one could use precisely structured inserts to increase the surface area, enhance mixing and manipulate the flow distribution. Catalytic propane combustion in microreactors with multi-channel and posted inserts, which consist of multiple static structures (walls separating various channels and pillar-like structures, respectively) in the flow channel of a microreactor, is considered in this series of two papers. In this first paper, we present numerical comparison of multi-channel and posted catalytic inserts for non-adiabatic self-sustained propane combustion. The inserts are oriented axially along the flow direction. We show that channel and post microreactors have similar performance for low thermal conductivity of the inserts. The in-line arrangement of the posted structures is preferred over a staggered arrangement because the former provides higher propane conversion and more stable combustion. The role of thermal conductivity of the microreactor wall structure and the catalytic inserts is investigated. The thermal conductivity of the microreactor structure affects the performance of the posts but not the channels; this is contrary to the effect of catalyst insert thermal conductivity where it is vice-versa. The channel microreactor is more stable towards high flow-rate blowout limit, whereas the post microreactor is significantly more stable at the lower flow-rate extinction limit. This results in stable operation of the post microreactor under more fuel-lean mixtures than the channel microreactor. © 2010 Elsevier Ltd.

The potential of methane steam, reforming at microscale is theoretically explored. To this end, a multifunctional catalytic plate microreactor, comprising of a propane combustion channel and a methane steam reforming channel, separated by a solid wall, is simulated with a pseudo 2-D (two-dimensional) reactor model. Newly developed lumped kinetic rate expressions for both processes, obtained front a posteriori reduction of detailed microkinetic models, are used. It is shown that the steam, reforming at millisecond contact times is feasible at microscale, and in agreement with a recent experimental report. Furthermore, the attainable operating regions delimited from the materials stability limit, the breakthrough limit, and the maximum, power output limit are mapped out. A simple operation strategy is presented for obtaining variable power output along the breakthrough line (a nearly iso-flow rate ratio linej, while ensuring good overlap of reaction zones, and provide guidelines for reactor sizing. Finally, it is shown that the choice of the wall material depeneis on the targeted operating regime. Low-conductivity materials increase the methane conversion and power output at the expense of higher wall temperatures and steeper temperature gradients along the wall. For operation close to the breakthrough limit, intermediate conductivity materials, such as stainless steel, offer a good compromise between methane conversion and wall temperature. Even without recuperative heat exchange, the thermal efficiency of the multifunctional device and the reformer approaches -65% and -85%, respectively. © 2008 American Institute of Chemical Engineers.

A pseudo-2D reactor model is used to perform a comprehensive study on the catalytic ignition of a lean propane/air mixture in a microreactor with Pt-coated walls. The results of the in-house code are verified against the commercial package FLUENT. The roles of inlet velocity, wall conductivity, heat losses or heat transfer to an adjacent endothermic channel, channel gap size, and wall thickness in steady-state ignition via inlet preheating and resistive heating, are studied. The results show that the heat loss/transfer has the largest effect on ignition for both ignition modes. For an adiabatic reactor, the ignition inlet temperature decreases with increasing wall conductivity. The exact opposite trend is observed at high heat losses. On the other hand, in the resistive heating mode, the external power input at ignition decreases with increasing wall conductivity, irrespective of the heat losses. Lower values of the inlet velocity result in a lower required power input for ignition. The two ignition modes are contrasted, and microreactor and multifunctional devices start-up strategies are suggested. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.