Taylor Flows

Depending on flow rates and fluid properties, the bubbles often have hemispherical shaped tops and flattened tails [16]. 

The thickness of the wall film in Taylor flow in capillaries is mainly dependent on the ratio of viscous to interfacial forces, which is given by the capillary number, Ca. 

Ca is mostly referred to the continuous phase i = C) this corresponds to the liquid phase (i=L) in gas-liquid systems. 

Different correlations can be found in the literature for estimating the thickness of the wall film. The majority of the studies suggest that the film thickness (5gj ) is a function of capillary diameter (li ) and capillary number (Ca). Two correlations, Bretherton [28] and AussUlous and Quere [29], are widely used. 

Copyright (2005) American Chemical Society.) (c) Comparison between flow behavior in the liquids slugs of Taylor flow in straight and meandering channel. (Adapted from Ref. [27] with permission of The Royal Society of Chemistry.) 

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In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

Figure 1.2 Normal and focused multilamination flow patterns, slug flow composed of gas/liquid segments (Taylor flow ), and ordered foam flow ( hexagon flow ) (from top to bottom).

Figure 16 Relative increase of friction and mass transfer due to gas-liquid Taylor flow, compared to developed laminar flow in small tubes. represents the dimensionless length of a liquid slug. Re the Reynolds number based on the liquid.

In Table 3 the three common reactor types are compared. Obviously, the monolithic reactor in the Taylor-flow regime leads to a high degree of process intensification. When these numbers are recalculated into production rates, values of 40 mol/m3reactor-s were found. Figure 17 illustrates the high value in relation to the Weisz window of reality. This demonstrates the attractiveness of using monoliths in fast catalyzed gas-liquid-solid reactions. 

In fast reactions, mass transfer or intraparticle diffusion becomes controlling. Thinner catalyst coatings, Taylor flow, etc. can be applied to optimize these... 

Figure 24 Schematic representation of the operation of a monolith channel, washcoated with a zeolite catalyst, under Taylor-flow conditions.

Irandoust S, Andersson B. Simulation of flow and mass transfer in Taylor flow through a capillary. Computers Chem Eng 1989 13 519-526. 

As shown in Fig. 8.6, several typical flow patterns can be found in the monolith channels, depending on gas-liquid ratio, flow rates, viscosity, surface tension, and channel diameter. All of these flow patterns show a very low static hold-up, but only two are regular and allow stable operation the so-called Taylor-flow and the film-flow regime. 

Taylor-Flow Microreactors Taylor-flow microreactors contain a dispersing mixing element for gas and liquid streams, typically of Tand Y shapes, followed by a reaction channel for the segmented gas-liquid flow, often of quite extended length, as the Taylor flow is dominant in typical flow-pattern maps [230,248-250]. In general, all Taylor-flow microreactors can induce other flow patterns as well as the ones mentioned above. 

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],...

Segmented gas-liquid (Taylor) flow was used for particle synthesis within the liquid slugs. Tetraethylorthosilicate in ethanol was hydrolyzed by a solution of ammonia, water and ethanol (Stober synthesis) [329]. The resulting silicic acid monomer Si (OH)4 is then converted by polycondensation to colloidal monodisperse silica nanoparticles. These particles have industrial application, for example, in pigments, catalysts, sensors, health care, antireflective coatings and chromatography. 

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 12 Film flow versus Taylor flow. Film flow is suited to catalytic distillation and stripping. 

In cocurrenf gas-liquid flow, several flow regimes can occur. The preferred one usually is Taylor flow. This type of flow is characterized by gas bubbles and liquid slugs flowing consecutively through the small monolith charmels. The gas bubbles occupy (nearly) the whole cross section of fhe channel and are elongated. Only a thin liquid film separates the gas bubbles from fhe catalyst (Figure 13). 

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