Dense-phase fluidized beds components

Bed components in a dense-phase fluidized bed include the gas distributor, cyclone, dipleg, heat exchanger, expanded section, and baffles, as shown in Fig. 9.4. The basic function of each component in a fluidized bed is described in the following. 

The effect of pressure on the heat transfer coefficient is influenced primarily by hgc (Botterill and Desai, 1972 Xavier etal., 1980). This component of h transfers heat from the interstitial gas flow in the dense phase of the fluidized bed to the heat transfer surface. For Group A and small Group B particles, the interstitial gas flow in the dense phase can be assumed to be approximately equal to Um ed. 6/i s extremely small for... 

A size distribution of particles is always desired rather than a single size in a fluidized bed. The two-phase theory of fluidized-bed operation is suspect when a bed contains appreciable lines, and models based on uniform particles should be used with caution. The dense phase in such cases should really be regarded as consisting of two phases emulsion and clusters of lines (d < 40 pm). Indeed, the results of Yadav et al. (1994) on commercial propylene ammoxidation catalyst clearly show that the lines agglomerate. A critical level of lines (30%) was found in terms of bed expansion, aeratability, and cluster size at which fluid-bed behavior is optimum. They proposed a model that takes the two dense phase components (emulsion and cluster) into account. Adding lines widens the limits of operable gas velocities and minimizes the segregation of particles. 

The best known model for slugging fluidized beds is that of Hovmand and Davidson (45,46). This is a variant on the Orcutt model (17,18) (see also Table 2) which assumes plug flow of gas in the dense phase. The interface mass transfer is again composed of two parts, a throughflow component and a diffusional component, with the dimensionless mass transfer coefficient given approximately by... 

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