Fluidization freeboard region

Generally, it is advantageous to avoid reaction in the freeboard, as much as possible, since the temperature control and near-isothermal conditions observed in the fluidized bed are nearly impossible to achieve in the freeboard region. This is particularly problematic for a complex reaction, since the selectivity is often temperature-dependent. Experiments have shown that the following design features influence the extent of particle entrainment, and, by extension, the likelihood of reaction in the freeboard region ... 

FIGURE 11.7 Selected frames of FCC catalyst in the freeboard region of a 6 inch (15 cm) diameter fluidized bed at a superficial gas velocity of 2 ft/sec (0.61 m/sec). Images were collected at 4000 frames per second with a 20 J,s exposure time. 

These results suggest that particle clusters form in the fluidized bed and get ejected into the freeboard instead of clusters forming only in the freeboard region. Particle clustering most likely does happen in the freeboard, but it may not be the dominating contributor to particle cluster concentration in the freeboard. Kaye and... 

Figure 9.18. Solids holdup distribution in the freeboard region of a fluidized bed.

In the transition zone and the freeboard region, heat transfer between bed and wall is a function of bed density. Shirai et al. (S9, Sll) studied heat transfer from a sphere immersed in the fluidized bed and showed the trend of decreasing heat-transfer coefiicient with decreasing bed density. 

The freeboard region in a fluidized bed accommodates particles that are being entrained from the dense bed. Entrainment refers to the ejection of particles from the dense bed into the freeboard by the fluidizing gas. Elutriation refers to the separation of fine particles from a mixture of particles, which occurs at all heights of the freeboard, and their ultimate removal from the freeboard. The terms entrainment and elutriation are sometimes used interchangeably. The carryover rate relates to the quantities of the particles leaving the... 

Entrainment Most fluidized bed reactors employ one or more cyclone, either inside the freeboard region at the top of the vessel or located externally, to capture entrained solids that are then returned continuously to the base of the fluidized bed via a standpipe and a mechanical (e.g., slide) valve or aerated nonmechanical valve (see Ref. ° for details of solid return systems). A flapper gate, acting as a check valve, is commonly employed to prevent backflow of gas up the standpipe. While cyclones are by far the most popular, other gas-solid separators like impingement separators, electrostatic precipitators, filters, and scrubbers are sometimes provided, especially as second- or third stage separators. 

The bubbling fluidized bed (Figure 8.3) is divided vertically into two zones, namely, a dense phase and a freeboard region (also known as lean phase or dispersed phase). The... 

Recently, Mahecha-Botero and co-workers presented a generalized comprehensive model which characterizes multiple phases and regions (low-density phase, high-density phase, staged membranes, freeboard region) with the possibility to include new features or simplifications in order to simulate different fluidized bed (membrane) reactors. For a more detailed description of the model and assumptions an interested reader is referred to. ... 

Although, for example, Fig. 4.48A and D shows very similar characteristics, the difference in fluidization velocity causes the shape of the stagnant zones to the left and right to be different, and with 140% background fluidization particles are ejected into the freeboard region to a larger extent. 

D = Diameter of the fluidized bed column, m DpB = Diameter of the column in the freeboard region, m Dg = Hydraulic diameter of the fluidized bed column (4A/circumference), m... 

Peters MH, Fran L-S, Sweeney TL. Study of particle ejections in the freeboard region of a fluidized bed with an image carrying probe. Chem. Eng. Sci 38 485-487, 1983. 

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