Flooded bed reactor

The bubble flow reactor (or flooded bed reactor) does not seem to have gained wide acceptance. In this reactor liquid and gas flow rates are limited by the necessity of avoiding to carry the catalyst away. Moreover it is claimed to be unstable reactor runaway can be initiated by hot spots which form near the gas distributor, in the case of exothermic reactions. Nevertheless flooded bed reactors have been used in a few processes where heat transfer between the cold liquid feed and the hot liquid of the reactor is realized by direct contacting and so ensures easily the initiation of the catalytic reaction (Figure 3). 

Caprolactone is continuously converted into hexanediol in a fixed bed reactor with the catalyst submerged by the liquid (flooded bed reactor). Hydrogen is charged into the bottom of the reactor in the ratio 10/1 to the caprolactone. It bubbles up through the liquid phase catalyst be mixture. The reaction conditions are 250 C and 280 bar. Backmixing, rather important in this type of reactor, and thermodynamic equilibrium impede a complete conversion. After the recovery of hydrogen in flash drums, the hydrogenated product is distilled in a series of columns where pure 1,6-hexanediol is extracted and unreacted caprolactone returned for recycle (21,22). 

Some contrasting characteristics of the main lands of three-phase reactors are summarized in Table 23-15. In trickle bed reactors both phases usually flow down, the liquid as a film over the packing. In flooded reactors, the gas and hquid flow upward through a fixed oed. Slurry reactors keep the solids in suspension mechanically the overflow may be a clear liquid or a slurry, and the gas disengages from the... 

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)...

Under practical conditions, countercurrent operation in a packed bed reactor is not feasible, because flooding occurs (55,56). The reason is that in the small interstitial space, extended momentum transfer takes place between the liquid flowing down and the gas flowing upward. At velocities used in industry this would imply... 

A heterogeneous tubular reactor that incorporates three phases where gas and liquid reactants are contacted with the solid catalyst particles, is classified as a trickle-bed reactor. The liquid is usually allowed to flow down over the bed of catalyst, while the gas flows either up or down through the void spaces between the wetted pellets. Co-current downflow of the gas is generally preferred because it allows for better distribution of liquid over the catalyst bed and higher liquid flow rates are possible without flooding. 

The GSLI are often operated under very high gas and liquid flow rates (near flooding) compared to the ones used in GSLC (in particular trickle-bed) reactors. An exception is the SYNTHOIL reactor for coal liquefaction. 

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. 

The hollow-fiber trickle-bed reactor, according to Yang and Cussler [59], is another variant of the hollow-tube theme. In this case the porous tube is not coated with a catalytic material. The outer shell surrounding the fibers is instead filled with catalyst pellets. The liquid is added to this outer shell, and the gas reactant is added to the inside of the fibers. Since no catalyst is present in the gas-liquid contact, this type of reactor functions merely as an effective gas-absorber. In comparison with the trickle bed, no flooding occurs with the hollow-fiber trickle-bed reactor at high liquid loads, which means a much higher reaction rate at high liquid flow rates than obtained with the traditional trickle bed. 

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... 

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