Three phase fluidization with countercurrent

Three-phase Fluidization with Countercurrent Flow... 

Countercurrent bubble flow with liquid-supported solids, which can be affected by downward liquid fluidization of particles having a density lower than that of the liquid, has been referred to as inverse three-phase fluidization. The mass transfer potential of such a countercurrent operation is worthy of study, especially for cases in which dispersion of the gas rather than the liquid is called for and the required gas-liquid ratio and throughput can be effected without flooding. In contrast, the corresponding cocurrent mode has received more attention than all other cases and constitutes the majority of the literature on three-phase fluidization. 

The three-phase fluidized-bed reactor with countercurrent mode of operation was used by Pruden and Weber 10 to study the hydrogenation of a-methyl styrene to cumene in the presence of palladium black catalysts. They used low gas velocities so that the gas was dispersed as bubbles in the slurry. They showed that the countercurrent mode of operation was better than the slurry operation (with no liquid flow), due to improved catalyst usage and improved gas holdup characteristics. 

The liquid holdup characteristics of a countercurrent, three-phase spouted-bed column are studied by Vukovic et al.141 They found that the total liquid holdup increased with the gas flow rate. Both the total and dynamic liquid holdups were comparable to those obtained in cocurrent-upflow three-phase fluidized-bed columns operating with similar gas and liquid flow conditions. 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

The particular characteristics of three-phase fluidized bed reactors have been covered in several recent reviews by Ostergaard [1], Wild [2], Epstein [3], Baker [4] and Muroyama and Fan [5]. Epstein [3] distinguished in particular the difference between three-phase fluidized beds and slurry reactors. In slurry reactors the size of the solid particles is normally smaller than 0.1 mm while in three-phase fluidization the particle diameter is bigger than 0.2 mm. The volumetric solid fraction is another significant difference, being 10% or below for slurry reactors and between 20-40% in three-phase fluidized bed units. In three-phase fluidized beds the particles are supported by the liquid and/or the gas while in slurry reactors the solid particles are suspended by the momentum transferred from the gas bubbles to the liquid and from the liquid to the solids. In slurry reactors the solid particles are normally carried into and out of the unit by the liquid stream. In three-phase fluidized beds the solids are not transported out of the unit by the liquid stream, they are fed and withdrawn independently of the liquid stream [3]. Epstein [3] introduced an interesting classification for three-phase reactors and particularly proposed four modes of operation for three-phase fluidized beds Mode I Cocurrent upflow circulation of gas and liquid with the liquid as a continuous phase. Mode II cocurrent upflow circulation of gas and liquid with the gas as a continuous phase. Mode III Countercurrent circulation of gas (upflow) and liquid (downflow) with the liquid as the continuous phase. Mode IV Countercurrent circulation of gas (upflow) and liquid (downflow) with the gas as the continuous phase. 

Three-phase reactions comprise gas-liquid-solid and gas-liquid-liquid reactions. Gas-liquid reactions using solid catalysts represent a very important class of reactions. Conventionally, they are carried out in slurry reactors, (bubble columns, stirred tanks), fluidized beds, fixed bed reactors (trickle beds with cocurrent downflow or cocurrent upflow, segmented bed, and countercurrent gas-liquid arrangements) and structured (catalytic wall) reactors. 

An alternative model for slugging fluidized bed reactors was proposed by Raghuraman and Potter (48). This Is an extension of their three-phase (bubbles, cloud-wake phase, emulsion phase) countercurrent backmlxlng model (21) which accounts for gas downflow In the dense-phase, said to occur for slugging beds when U > 2.5 U f Agreement with experimental results Is said (48,49) to be somewhat better for this model than for the Hovmand-... 

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