Taylor-flow microreactors

Taylor-flow microreactors contain a dispersing mixing element for gas and liquid streams, typically of T- and Y-shape, followed by a reaction channel for the segmented gas-Uquid flow, often of quite extended length, as the Taylor flow is dominant in typical flow-pattern maps [4,26-27]. In general, all Taylor-flow microreactors can induce other flow patterns as well those which were mentioned above. 

A universal transition prediction model of the transition capillary number as the functions of the Reynolds numbers of gas and liquid phases and the contact angle of the channel wall is formulated as 

The most striking advantages of this concept are simplicity in approach and operation, as well as fast installation, since micromixers are commercially available at comparatively low cost, and ability to reach high productivity already with one device. 

However, coalescence of the foam may occur, as surface forces are less dominant here due to the larger scale. In aqueous systems, this can be prevented by adding surfactants to lower surface tension. For organic solvents, there is no straightforward solution and only short-term contacting may be realized. In addition, the interface may be not as defined as for two-phase continuous and some of the disperse microreactors, with the exception of the foams that can be quite regular [80]. 

A large number of micromixers have been used, most often their design was not oriented on a special use for gas-liquid reactions, but rather on mixing of miscible liquids [78]. Interdigital micromixers comprise respective feed charmel arrays that 

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In one version, Taylor-flow microreactors comprised two types of mixer designs followed by a single microchannel (see Figure 4.36) [279]. 

Figure 4.37 Design of two mixing units in Taylor-flow microreactors used for achieving dispersed flow in a downstream channel (100pm width x 50pm depth, 2 cm length). L liquid C gas (by courtesy of Wiley-VCH Verlag GmbH) [279].

Figure 4.40 Schematic of the microbubble column , a numbered-up Taylor-flow microreactor with a mixing for each reaction channel (left) scanning electron micrograph of the mixing element, top view (right). L liquid C gas. The small channels with the semicircular openings are the gas feed and the larger rectangular ones are for liquid feed (by courtesy of VDI Verlag) [275],...

The Taylor-flow microreactor comprised a micromixer for mixing of the precursors for the particle synthesis followed by a gas inlet for separating this continuous mixed liquid stream into segments separated by gas bubbles [328,329]. 

Figure 9.12 Design of a Taylor flow microreactor with feeds and mixing zone (left) and reaction channel and outlet (right). Please note that in the image an improved T-type mixer design is given, which differs somewhat from the one shown in Figure 9.10. Dimensions of...

Two classes of gas-liquid microchannel reactors were developed in the past years -continuous-phase contacting falling film, overlapping charmel, mesh, and annular flow approaches, and dispersed-phase contacting by Taylor flow reactors, micromixers for bubble and foam formation, and miniaturized packed bed microreactors, which follow classical trickle-bed operation at smaller scale. Recently integration of operations inside a microdevice has been studied and led to the development of membrane microreactors. 

The gas-liquid flow in microreactors is characterized by uniform flow patterns, which are usually realized in Taylor flow or film flow [50]. In this type of reactions, it is very important to regulate the contact between gas and liquid, that is, to control the film thickness. 

Despite the small dimensions, conventional models for dispersion have been applied to microreactors. Since flow is laminar, a Taylor-Aris or shear-induced dispersion model generally describes dispersion. Beard has applied the Taylor-Aris dispersion model to... 

For residence times up to several minutes (and same reaction time of the synthesis in microreactors), the insertion of large Taylor gas bubbles into a Uquid flow reduces RTD broadening, since the liquid volumes are then segmented and have less axial dispersion (see Figure 14.2). The RTD of the segmented gas-Uquid microflows is significantly narrower than for the single-phase liquid flow. 

Both reactor types R3 and R4 use the segmented flow (Taylor) principle. They are divided into two categories R3 has very small channels (<1 mm) and R4 are monolith reactors (honeycomb), well developed on the laboratory scale with at least one example of industrial application. Category R3 includes single-channel and multi-ple-channel reactors [10], etched in silicon [10] or glass [10,11], with wall-coated or immobilized catalysts in the case of gas-liquid-solid additions [12], and capillary microreactors for gas-liquid-liquid systems [13]. 

S. McGovern, G. Harish, C. S. Pai, W. Mansfield, J. A. Taylor, S. Pau, R. S. Bessei Multiphase flow regimes for hydrogenation in a catalyst-trap microreactor, Chem. Eng. J. 2008, 135, S229-S236. 

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