Solids entrainment in a large gas fluidized bed

Fournol, A. B.; Bergougnou, M.A.; Baker, C. G. J.

doi:10.1002/cjce.5450510402

Cited by 53 publications

(23 citation statements)

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“…Lewis et al (1962) performed entrainment experiments using column diameter ranging from 0.019 to 0.146 m and concluded that for column diameters greater than 0.08 to 0 1 m, the entrainment or elutriation becomes constant and independent of vessel diameter; however, a sharp rise in entrainment rates was found at small bed diameters (0.009 to 0.02 m). Recently, more data from large-scale fluidized bed have been reported (Fournol et al, 1973;Nazemi et al, 1974;Merrick and Highley, 1974, Large et al, 1976;Colakyan et al, 1979;Lin et al, 1980) It is found that under similar operating conditions, the elutriation rate constant from the large-scale bed tends to be much higher than that from the small-scale bed as shown in Figure 9. At a low gas velocity (particle Reynolds Number less than the critical Reynolds Number), the friction between the particles and the column wall is important.…”

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Fluidized bed freeboard phenomena: Entrainment and elutriation

1982

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A model describing the entrainment of solid particles in the freeboard region of a fluidized bed is proposed. Published experimental data have been used to correlate the entrainment rate near the bed surface and the elutriation rate above the bed. It has been shown that the column diameter can have an important influence on the amount of fines elutriated. SCOPEWhen the fluidized bed is used as a chemical reactor the freeboard region not only provides the space for the disengagement of solids, but also for additional reaction between the projected particles and the gas. For fast reactions, this region could become very important providing considerable conversion of gas (Yates and Rowe, 1977).In order to model physical and chemical phenomena in this region, it is necessary to understand the entrainment of particles in the freeboard. Extensive studies on entrainment and elutriation have been made, and many correlations have been proposed (Leva, 1951 Lin et al., 1980). However, most of them are based on the experimental data of smallscale fluidized beds which are found to be much lower than that of large-scale fluidized beds. Extrapolation of these empirical correlations usually leads to strange results. It is the objective of this study to correlate the entrainment and elutriation data of different column diameters and to propose a more realistic model for estimation of the entrainment rate of solid particles in the freeboard. In addition, where the projected particles originate and how the bed internals affect the entrainment and elutriation rates are discussed. CONCLUSIONS AND SIGNIFICANCEThe solid particles inside the bed are splashed into the freeboard when bubbles burst at the bed surface. The particles that are thrown above the bed either rise or fall in this region, depending on the size, density and the gas velocity. The entrainment rate of particles decreases exponentially along the freeboard height. The rate of particles ejected at the bed surface can be correlated in terms of hydrodynamic parameters of the bed, such as bubble diameter and excess gas velocity above minimum fluidization. The elutriation rate of fines, on the other hand, is practically independent of the bed hydrodynamics, namely, bed particle size, presence or absence of baffles or tubes in the bed, etc. However, the installation of internals above the bed will reduce the entrainment and elutriation due to the direct impingment on the obstructions. The elutriation rate is related to the saturation carrying capacity of the gas stream, which can be calculated from the equations based on material and force balances. Both the entrainment rate and the elutriation rate are affected by the size of the column. The elutriation rate is especially affected by the wall of the column when the solid velocity is low. This is because the particles tend to descend along the wall of the column. INTRODUCTIONIn order to examine gas and solid flow patterns, a fluidized bed may be divided into three distinct zones as shown in Figure 1. Close to the bottom of the...

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