Liquid distribution in trickle bed reactor

Liquid distribution in trickle bed reactors has been mainly discussed from the aspect of flow channels between particles [6, 7]. However, since most of the commercial catalysts are extrudates, an effect of the particle orientation on liquid distribution is much more important than flow channel, which relates to mass flow rate and a particle size. Shaped catalysts have a higher volume activity than cylindrical catalysts when an effect of diffusion on the reaction rate is large [8]. Therefore, the shaped catalysts have been commonly used for hydrodemetallation of residue. However, since an effect of liquid distribution on the catalyst performance is important in large-scale commercial reactors, catalyst shape should be carefully selected to maximize the effectiveness of the catalyst usage in a commercial application. 

Herskowitz, M. and J. M. Smith. Liquid Distribution in Trickle-Bed Reactors - 2. Tracer Studies. AIChE Journal 24 (1978) 450. 

Herskowitz, M., "Liquid Distribution in Trickle Bed Reactors", Ph.D. Thesis, Univ. California, (1978), Davis, California,... 

C. Boyer, B. Fanget, Measurement of liquid flow distribution in trickle bed reactor of large diameter with a new gamma-ray tomographic system, Chem. Eng. Sci. 57 (2002) 1079-1089. 

As can be observed, the main difference between conventional three-phase reactors and catalytic membrane reactors hes in the relative positions of the mass transfer resistances with respect to the catalytic phase. In a conventional porous catalyst the catalytic sites in the pores have only one way or path of access. The gaseous reactant will encounter the first two mass transfer resistances at the gas-liquid interface, where the solvation equilibrium of the species from one phase to the other wiU take place. The dissolved species will diffuse towards the surface of the catalytic pellet for quite a long path in the hquid phase and will meet an additional mass transfer resistance at the hquid-sohd catalyst interface. It then needs to diffuse and react in the porous structure of the catalyst as well as the other reactant already present in the liquid phase. In the case of a traditional three-phase reactor (Fig. 4.3a), the concentration of at least one of the reacting species is hmited by its solubility and diffusion in the other fluid phase with a long diffusion path and in some cases unknown interfadal area (e.g., bubbles with variable size depending on the type of the gas feeding and distribution device in slurry reactors, not uniform phase contact and distribution in trickle-bed reactors). 

In trickle bed reactors the gas and liquid both flow downward through a fixed bed of catalyst. The gas phase is continuous, and the liquid also is continuous as a film on the particles. Provided that the initial distribution is good, liquid distribution remains substantially uniform at rates of 10-30 m3/... 

Saderman, A.J. Gladden, L.F. Magnetic resonance imaging as a quantitative probe of gas-liquid distribution and wetting efficiency in trickle-bed reactors. Chem. Eng. Sci. 2001, 56, 2615. 

Kundu, A. Saroha, A.K. Nigam, K.D.P. Liquid 57. distribution studies in trickle-bed reactors. Chem. 

The changing hydrodynamics is one important influence factor under periodical liquid flow rate in trickle-bed reactors. Because until now an exact mathematical description of the hydrodynamics in periodically operated trickle-bed reactors especially the liquid flow field and the liquid distribution within the catalyst bed, is impossible and detailed further experimental and theoretical studies of the hydrodynamics under unsteady-state conditions are required. Despite a broad basis of experimental data published in the literature no correlation with general applicability is yet available, if an accuracy of the liquid holdup level better than 30% is desired [37]. 

Figure 1 Schematic examples of multiphase systems in which a discrete dispersed phase is moving through, or moved by, a continuous fluid phase. The discrete phase can be a solid (left), a gas (center), or a liquid (right). In many cases, inhomogeneous mesoscale structures appear in the spatial distribution of the discrete phase, caused by interplay of hydrodynamic flow and local energy dissipation. More complicated cases with three or more phases are also possible, such as encountered in slurry reactors (where solid particles are also present in the continuous liquid phase) or trickle bed reactors (where the droplets are sprayed on a packed bed of particles). To focus on the essentials, the topical sections will focus mostly on the two-phase examples depicted here.

Ross (R2) reported measurements of desulfurization efficiency of fixed-bed pilot and commercial units operated under trickle-flow conditions. The percentage of retained sulfur is given as a function of reciprocal space velocity, and the curve for a 2-in. diameter pilot reactor was found to lie below the curves for commercial units it is argued that this is proof of bad liquid distribution in the commercial units. The efficiency of the commercial units increased with increasing nominal liquid velocity. This may be an effect either of mass-transfer resistance or liquid distribution. 

Burghardt et al. (1995) studied, among others, the liquid distribution using needle-type distributors in trickle beds and found that the density of the liquid feed points does have an important effect on the value of the liquid holdup, and thus on the performance of the reactor. They concluded that for a density of more than 5000 feeding points per square meter, the liquid holdup was stabilized. 

The trickle bed reactors that operate in the downflow configuration and have a number of operational problems, including poor distribution of liquid and pulsing operation at high liquid and gas loading. Scaleup of these liquid-gas-solid reactors is much more difficult than a gas-solid or gas-liquid reactor. Nevertheless, the downflow system is convenient when the bed is filled with small catalyst particles. And, because the catalyst particles are small, these reactors are quite effective as filters of the incoming feed. Any suspended fine solids, such as fine clays from production operations, accumulate at the front end of the bed. Eventually, this will lead to high pressure differentials between the inlet and outlet end of the reactor. 

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