Freeboard region

If this is the case, the design and configuration of the gas outlet in the freeboard region can have an important effect on the fines residence time distribution and holdup above the bed. The importance of the freeboard region for further reaction of fines merits greater emphasis in both experimental and theoretical studies. 

In the following sections, we discuss reactor models for fine, intermediate, and large particles, based upon the Kunii-Levenspiel (KL) bubbling-bed model, restricting ourselves primarily to first-order kinetics. Performance for both simple and complex reactions is considered. Although the primary focus is on reactions within the bed, we conclude with a brief discussion of the consequences of reaction in the freeboard region and near the distributor. 

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 shows several frames of the video obtained for particles and clusters in the freeboard region. As expected, the FCC catalyst tended to cluster in the freeboard region. A statistical analysis of this video suggested that 30% of the FCC catalyst in the freeboard existed as particle clusters with an average size of 11 5.0 particles. 

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.

Reactions in the FCB are then discussed. The mechanism of successive contact (M25) is presented. Contact efficiency is greater in the dilute phase, i.e., freeboard region, than in the underlying dense phase. In the FCB, the dilute phase plays an important role in advancing the catalytic reaction when reaction rates are high. This factor provides a basis for identifying the appropriate reaction model and clarifying the effect of the dilute phase on selectivity and stability. 

Mixing and dispersion of gas and particles in the transition zone and in the freeboard region are important to know, in relation to the mechanisms of the reactions taking place (see Sections VII-IX). However, little work has thus far been done in these areas. 

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. 

Recently, Yates and Rowe (YIO) have observed, on the basis of their model for catalyst distribution in the freeboard region, that this region can usually exert a considerable influence on the course of the reaction. Their observation is essentially parallel with the concept of the successive contact mechanism. However, they use the bubbling bed model in calculating the reaction in the dense phase, so that the effect of directly contacting catalyst seems to be corrected two times, first partially in the dense phase and then in the freeboard region (see Section VII,A,3). 

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. 

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