Gas—solid fluidized beds

Kunii, D., Levenspiel, 0., "Bubbling Bed Model for Kinetic Processes in Fluidized Bed-Gas-Solid Mass and Heat Transfer and Catalytic Reactions", lEC Proc. Des. Dev., 1968, J7,... 

A unified approach has been developed for the prediction of transition in multiphase reactors such as gas-hquid bubble columns, liquid-liquid spray columns, solid-liquid fluidized beds, gas-solid fluidized beds, and three-phase fluidized beds. 

Investigator Type of correlation Phases involved Molerus et al. [55, 56] Wall-to-bed heat transfer coefficient in bubbling fluidized beds Gas-solid... 

A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

Bed Density—p. The average density of a fluidized bed of solid particles and gas. Bed density is mainly a function of gas velocity and, to a lesser extent, the temperature. 

Martin, H., Heat Transfer Between Gas Fluidized Beds of Solid Particles and the Surface of Immersed Heat Exchanger Element, Parts I II, Chem. Eng. Process, 18 157-169,199-223 (1984)... 

Typical fluid-bed processor elements can be seen in Figure 1. The fluidizing gas enters the bed at the bottom through an air distributor. The gas passes up through the bed of solids, causing it to fluidize. This gas/solid mixture behaves much like liquid of similar bulk density. Above the fluidized bed, a freeboard section (expansion chamber) is provided to slow down the... 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

The mixing and residence-time distribution of the gas arc particularly important for catalytic reactions but arc also significant for gas-solid reactions when gaseous reactants arc to be converted to the greatest possible extent in fluidized beds. Gas mixing is closely linked to the motion and mixing of the solids in the bed. 

Figure 14. Vertical gas dispersion in a fluidized bed of solids of Geldart group A (measurements by various workers [2]).

Thus, both of these assumptions are restrictive, and in the absence of additional information, one of these assumptions needs to be made. For the case of gas-solid fluidized beds and solid-liquid fluidized beds, if we assume Da = Dl = 0, we get a criterion [Eq. (61)] that is almost same as the criterion of Gibilaro and Foscolo (1984), given in Eq. (50). Therefore, this assumption was made. 

The direct contact model has some difiiculties, however. In fluidized beds, gas bubbles of very low solid content are usually considered to exist in the dense phase (H14, K13, T19). Also, the cloud layer is negligibly thin, due to small (/ r for the usual fluid catalyst beds, according to equa-ticMis of Davidson and Harrison (D3) and Murray (M47). The streamlines of gas phase through a bubble have been observed to pass through the cloud, but not through the bubble wake (R17). Thus there seems little possibility of believing that the bubble gas is in direct contact with a substantial amount of catalyst in the bubble phase (see also Secticxi VI,A). Furthermore, the direct contact model is applied to the data by Gilliland and Knudsen, and v in Eq. (7-9) is calculated to fit the data. Calculation (M26) shows that the volume of catalyst, with an apparent density the same as for the emulsion, which contacts the bubble gas freely exceeds the volume of bubble gas itself (v/ib = 3.3, 2.0, and 1.5, respectively, for Uc. = 10, 20, and 30 cm/sec). This seems to be unsound physically. 

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... 

One of the more interesting methods for continuous countercurrent ion exchange is the use of fluidized bed techniques for continuous circulation of the resin. Figure 1.6 shows the Dorrco Hydro-softener. In the fluidized bed, a solid phase is suspended in a liquid or gas. Consequently, the solid behaves like a fluid and can be pumped, gravity fed, and handled very much like a liquid. The fluidized resin moves down through the softener on the right and is then picked up by a brine-carrier fluid and transferred to the regenerator on the left. 

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