Fast fluidization solids distribution

The data of Fig. 20 also point out an interesting phenomenon—while the heat transfer coefficients at bed wall and bed centerline both correlate with suspension density, their correlations are quantitatively different. This strongly suggests that the cross-sectional solid concentration is an important, but not primary parameter. Dou et al. speculated that the difference may be attributed to variations in the local solid concentration across the diameter of the fast fluidized bed. They show that when the cross-sectional averaged density is modified by an empirical radial distribution to obtain local suspension densities, the heat transfer coefficient indeed than correlates as a single function with local suspension density. This is shown in Fig. 21 where the two sets of data for different radial positions now correlate as a single function with local mixture density. The conclusion is That the convective heat transfer coefficient for surfaces in a fast fluidized bed is determined primarily by the local two-phase mixture density (solid concentration) at the location of that surface, for any given type of particle. The early observed parametric effects of elevation, gas velocity, solid mass flux, and radial position are all secondary to this primary functional dependence. 

Thus, the three-dimensional voidage distribution in a fast fluidized bed can be determined, semi-empirically as our understanding stands at the present, from the physical properties of the solids and the gas and the operating variables. 

Boundaries in fast fluidization refer mainly to the column wall as well as the inlet and outlet. Effect of the wall on pressure drop due to friction between the fluidized solids and the wall surface is minimal (Li et al, 1978), although it is the very cause of radial distribution of parameters. The configuration of the inlet and the outlet often strongly affect gas-solids flow, especially with regard to axial voidage profile. 

Inasmuch as heat transfer depends on the hydrodynamic features of fast fluidization, if the fast fluidized bed is equipped with an abrupt exit, the axial distribution of solids concentration will have a C-shaped curve (Jin et al., 1988 Bai et al., 1992 Glicksman et al., 1991. See Chapter 3, Section III.F.l). The heat transfer coefficient will consequently increase in the region near the exit, as reported by Wu et al. (1987). 

Figure 19 shows the size distribution of solids sampled at the top, middle, and bottom of the fast fluidized bed combustor and the downcomer (Li et al., 1991). It can be seen that all particle size distribution curves are rather... 

In vertical pneumatic transport the radial particle concentration distribution is almost uniform, but some particle strands may still be identified near the wall. Little or no axial variation of solids concentration except in the bottom acceleration section is observed [58]. The flow associated with transport bed reactors tends to be dilute (typically 1 to 5 % by volume solids) and uniform. By virtue of the smaller reflux and density of the suspension within the dilute pneumatic conveying regime, there might be larger temperature gradients than within the fast fluidization regime [56]. 

For pneumatic conveying all the particles are evenly dispersed in the gas. This makes contacting ideal or close to ideal. The plug flow model is thus well suited for the dilute transport reactors, but has also been used for the denser fast fluidization regime neglecting gradients in the solids distribution. For first order reactions the model can be written as ... 

The fluidized bed reactor can also handle fast, complex reactions, with mixing and temperature control being especially good when stirring is provided. Unfortunately, the extent of back mixing is difficult to assess so that the residence time distribution of the reactants in the reactor is uncertain. In addition, only small catalyst particles can be used, and attrition, with the consequent breakdown and loss of catalyst, is a problem. Finally, a catalyst bed is adequately fluidized over only a comparatively narrow range of flow rates. More information about kinetic reactors can be found in reviews [33,34,50], Applications of the basket-type mixed reactor to liquid-solid systems are discussed by Suzuki and Kawazo [62] and by Teshima and Ohashi [63], and the development of a laminar flow, liquid-solid reactor by Schmalzer et al. [64], In the latter reactor the wall is coated with a catalyst layer. 

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