Taylor flow, monolithic catalysts

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

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). 

G. Taylor, Dispersion of a viscous fluid on the wall of a tube, J. Fluid Mechanics 70 161 (1961). S. Irandoust, B. Andersson, E. Bengtsson, and M. Siverstrdm, Scaling up of a monolithic catalyst reactor with two-phase flow, Ind. Eng. Chem. Res. 28 1489 (1989). 

However, when considering monoliths having comparable fractional catalyst volumes and SA/V ratios as typical catalyst particles in fixed beds, countercurrent flow of gas and liquid is still very problematic. At the small channel diameter of about 1 mm (see Table 2) and at realistic velocities of gas and liquid, the liquid, which should flow downward as a film along the wall, will easily bridge the channel and form a slug, which will be transported upward by the gas. Thus, instead of the desired annular countercurrent flow, a segmented flow, or Taylor flow, in the upward direction will be obtained. This phenomenon is akin to the flooding in packed beds. 

Nijhuis et al. also carried out the hydrogenation of a-methylstyrene in both a monolith and trickle bed reactor [49]. The monolith was 10 mm in diameter with a cell density of 400 cpsi, whereas the trickle bed was 47 mm in diameter. Both reactions were carried out in the Taylor flow regime. The catalyst productivity, defined as the rate of product formation per unit volume of catalyst, was found to be 6.2 mol m s , compared with 4.6 mol m s . To test the importance of Taylor flow in the reduction of mass transfer limitation and enhancement of the observed reaction rate, the researchers also carried out a liquid-full experiment, where only liquid presaturated with hydrogen was fed to the monolith. The catalyst productivity in this case was 1.5molm s . This experiment clearly indicates that the mass transfer rate of hydrogen through the phase interface in Taylor flow is much faster than in the bulk liquid. 

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]. 

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