Transport Phenomena in Microscale Reacting Flows

Niket S. Kaisare, Ceorgios D. Stefanidis, and Dionisios C. Vlachos 

Transport phenomena are crudal in the scale-up of conventional, large-scale chemical reactors because many processes are heat and/or mass transfer controlled. Since transport coeflEcients are typically inversely proportional to the characteristic dimension of the system, miniaturization of chemical systems leads to a substantial increase in transport rates. This increase in turn enhances the overall rate of processes that are transport limited, leading to considerable process intensification, i.e. the same throughput can be achieved with a much smaller device and thus with much lower capital. Alternatively, much higher throughput can be achieved using a system of the same size as a conventional one, but made up of many small components (scaling out). 

Aside from process intensification, the enhanced transport rates may render a microscale system gradientless. This fact has important ramifications for operation and fundamental studies. Examples include elimination of detrimental hot spots caused by very exothermic reactions and the possibility of a microchemical system being isothermal and kinetically controlled. The latter makes microchemica] systems ideal for extraction of intrinsic chemical kinetics. 

Micro Process Engineering, Vol.l Fundamentals, Operations and Catalysts Edited by V. Hessel, A. Renken, J.C. Schouten, and J.-I. Yoshida Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31550-5 

Due to the small characteristic dimension, the flow in microchemical systems is laminar. As a result, mixing relies only on molecular diffusion instead of the more efficient turbulence that large-scale systems typically exhibit. At the same time, the diffusion time scale is much shorter due to the small size of a microscale device. However, structural elements that play the role of static micromixers may be necessary to spread fast flows, enhance fluid-solid contact, increase mixing of incoming gases, etc. One such example is the post-micromixer discussed in Ref. [5]. 

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