Flooded fixed bed

In this type of reactor, gas superficial velocity is of the order of 0.1-0.3 m/s which is low enough to avoid mechanical interactions between gas and liquid. The velocity of the liquid, in the range 1 - 8.10 m/s is still low, but sufficient to guarantee satisfactory external wetting of the catalyst particles. Table 1 shows advantages and disadvantages of Fixed Bed Multiphase Reactors. Table 2 shows the characteristic parameters of TBRs compared to the two other important multiphase reactors Stirred Slurry and Flooded Fixed Bed Reactor. 

Heterogeneous catalysts can be divided into two types those for use in fixed-bed processing wherein the catalyst is stationary and the reactants pass upward (flooded-bed) or downward (trickle-bed) over it, and those for use it slurry or fluidized-bed processing. Fixed-bed catalysts are relatively large particles, I/32 to 1 /4 inch, in the form of cylinders, spheres, or granules. Slurry or fluidized-bed catalysts are fine powders, which can be suspended readily in a liquid or gas, respectively. Fixed-bed processing is especially suited to large-scale production, and many important bulk chemicals are made in this mode. 

Some contrasting characteristics of the main kinds of three- phase reactors are summarized in Table 8.3. In trickle bed reactors both phases usually flow down, the liquid as a film over the packing. In flooded reactors the gas and liquid flow upward through a fixed bed. Slurry reactors keep the solids in suspension mechanically the overflow may be a clear liquid or a slurry, and the gas disengages from the vessel. The fluidized three-phase... 

Fig. 4.17. Flow regimes in three-phase fixed-bed reactors, (a) Gas and liquid in co-current downwards flow (trickle-bed operation). (b) Gas and liquid in co-current upwards flow (liquid floods bed), (c) Gas and liquid in countercurrent flow (not often used for catalytic reactors)...

Figure 8-1 Correlation of Lobo for the prediction of flooding rates in fixed-bed columns.24...

An interesting monolithic configuration has recently been disclosed that can be suitable for three-phase processes carried out in countercurrent mode [10]. This can be particularly important for processes in which both thermodynamic and kinetic factors favor countercurrent operation, such as catalytic hydrodesulfurization. The flooding of a reactor is a considerable limitation for the countercurrent process run in conventional fixed-bed reactors. Flooding will not occur to that extent in the new monolith. A configuration of channels of the new monolith is such that subchannels open to the eentcrline are formed at the walls. The liquid flows downward, being confined in these subchannels and kept there by surface tension forces. The gas flows upward in the center of the channel. The results of studies on the new monolith concept are presented in Chapter 11 of this book. 

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. 

At termination of the loading cycle under fixed bed operation the column remains flooded with partially treated feed. In processes where the feed liquor commands a high monetary value with respect to its constituents, as in for example, such processes as carbohydrate refining, metal recovery, and pharmaceutical production it is common practice to sweeten off the column by displacing up to about one bed volume of column residual which is saved for reprocessing. 

In general, when a fixed bed is selected, the issue whether to employ a concurrent upflow or downflow operation must be considered. Operating a randomly packed bed reactor in the countercurrent mode is usually not feasible because flooding occurs at gas velocities far below industrial relevance. In a concurrent upflow, complete catalyst wetting is obtained at the expense of much larger liquid holdup compared to a concurrent downflow. High liquid holdup... 

In continuous and fixed-bed extractors there is a critical through pot rate beyond which flooding will occur. The viscosity of extract usually iucreases as it moves toward the fresh solids end of extraction systems. Houce, in crossflow extractors nod slalioanry-basket extractors, progressive changes in recirculation rates may have to he used to compensate for such changes in viscosity. 

Figure 8.10 Fixed-bed reactors for gas-liquid-solid systems (a) Trickle bed (b) upflow flooded (c) counterflow.

A trickle bed reactor utilizes a fixed bed over which liquid flows without entirely filling the void spaces between particles. The liquid usually flows downward under the influence of gravity, while the gas flows upward or downward through the void spaces between the catalyst pellets and the liquid holdup. Generally, co-current downward flow of liquid and gas is preferred because this mode of operation facilitates a uniform distribution of the liqnid across the catalyst bed and permits the employment of higher liquid flow rates before encountering flooding constraints. 

---