Complex Geometries - Packed Beds and Foams

So far, we have considered catalytic materials that conform to the side walls of a microreactor. A downside to a functionalized coating at a channel wall is the limited catalytic surface area that can be provided. As an alternative, thin-film technology can be used for depositing catalytic materials on more complex three-dimensional surfaces inside microchannels [49]. Impregnation methods can also be used on porous silicon surfaces [50]. The mass transfer rates described for such structures are sufficient for all but the fastest heterogeneous reactions. 

Wada et al. [54] used a micropillar system with 50 pm pillars for a gas-liquid reaction that was mass-transfer limited, and estimated ka using ozonolysis reactions at 2.5 s, which was five times higher than the transfer rate in a similar reactor without posts. In such a configuration, the posts break up the gas-Uquid flow in the channel and increase the area for interphase mass transfer. 

Mass transfer coeflficient, gas-liquid-solid overall (m s Rate of first-order chemical reaction (s ) 

Dimensionless constant Coordinate in wall-normal direction (m) 

8o Constant mass transfer boundary layer thickness obtained from film theory (m) 

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