A variety of gas-liquid microchannel reactors have been developed so far, using different contacting principles. Some devices utilize continuous-phase contacting (i.e., nondispersed separate phases with large specific interfaces). Among these are microstructured falling film, overlapping channel, and mesh reactors. Dispersed-phase contacting is obtained when one of the phases is interdispersed into the other phase. Regular flow patterns are provided by the segmented (Taylor) flow in a single microchannel or numbered-up versions such as the microbubble column; other flow patterns such as annular flow may be achieved as well. Foam microreactors utilize a moving rigid 3-D bubble network at high gas content. Miniaturized packedbed microreactors follow the paths of classical engineering by enabling trickle-bed operation. Because of the often highly regular flow pattern, not obtained in conventional gas-liquid contactors, an understanding of the underlying hydrodynamics and heat and mass transfer is crucial for optimal performance of all types of gas-liquid microstructural reactors. Several examples are given, including film-thickness measurements, flow-pattern maps, determination of mass-transfer coefficients, residence-time distributions, scale-out issues, etc. Numerous applications demonstrate the improved performance of gas-liquid microreactors. Among these are fluorinations, chlorinations, hydrogenations, sulfonations, photo-oxidations, etc. Recently, the scope of reactions has been widened, since there is now the possibility to carry out gasliquid-solid processes in the same microreactors as used for noncatalytic reactions because of the development of catalyst washcoats and other materials deposited onto microchannels. Some relevant examples are given for illustration.

Computational fluid dynamics simulations are used to study the mixing characteristics of a microscale mixer for gaseous flow as a function of various operating and design parameters. The device is based on a T configuration and gases of different viscosities are employed. Simulations show that the mixing length increases with the fluid speed and is also influenced by the mixer aspect ratio. Altering the angle between the inlet channels does not significantly affect the mixing performance, whilst throttling the fluid considerably decreases the mixing length. The results are compared with Fourier number predictions and it is demonstrated that these can provide useful limits for a preliminary design.

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The improved mass transfer characteristics of Taylor flow, make it an attractive flow pattern for carrying out gas—liquid operations in microchannels. Mass transfer characteristics are affected by the hydrodynamic properties of the flow such as thickness of the liquid film that surrounds the bubbles, bubble velocity, bubble and slug lengths, mixing, and flow circulation in the liquid slugs, and pressure drop. Experimental, theoretical, and modelling attempts to predict these properties are reviewed and relevant correlations are given. Most of these refer to capillaries but there are number of studies on square channels. In general, flow properties are well understood and predicted for fully formed Taylor bubbles in a developed flow and in clean systems, particularly in circular channels. However, the presence of impurities and their effect on interfacial tension cannot be fully accounted for. In addition, there is still uncertainty on the size of bubbles and slugs that form under certain operating and inlet conditions, while there is little information for channels with non-circular cross-sections.

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