Mass flux, fast fluidization

Figure 14. Radial profiles of solid mass flux in fast fluidized bed. (From Herb, Dou, Tuzla and Chen, 1992.)...

The interaction of parametric effects of solid mass flux and axial location is illustrated by the data of Dou et al. (1991), shown in Fig. 19. These authors measured the heat transfer coefficient on the surface of a vertical tube suspended within the fast fluidized bed at different elevations. The data of Fig. 19 show that for a given size particle, at a given superficial gas velocity, the heat transfer coefficient consistently decreases with elevation along the bed for any given solid mass flux Gs. At a given elevation position, the heat transfer coefficient consistently increases with increasing solid mass flux at the highest elevation of 6.5 m, where hydrodynamic conditions are most likely to be fully developed, it is seen that the heat transfer coefficient increases by approximately 50% as Gv increased from 30 to 50 kg/rrfs. 

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. 

Monceaux, L., Azzi, M., Molodtsof, Y., and Large, J. F. Particle mass flux profiles and flow regime characterization in a pilot-scale fast fluidized bed unit, in Fluidization V (K. Ostergaard and A. Sorensen, eds.), pp. 337-344. Eng. Found., New York, 1986b. 

However, it is not always easy to distinguish between the flow behavior encountered in the fast fluidization and the transport bed reactors [56]. The transport reactors are sometimes called dilute riser (transport) reactors because they are operated at very low solids mass fluxes. The dense riser transport reactors are operated in the fast fluidization regime with higher solids mass fluxes. The transition between these two flow regimes appears to be gradual rather than abrupt. However, fast fluidization generally applies to a higher overall suspension density (typically 2 to 15% by volume solids) and to a situation wherein the flow continues to develop over virtually the entire... 

Therefore, a CFB may be operated under turbulent fluidization, fast fluidization, or pneumatic transport regimes. This broad definition is consistent with experimental observations reported in the literature dealing with the general appearance of the bed. In this chapter, the discussion focuses mainly on systems where the gas velocity and solids mass flux are independent variables (VIS), and concentrates on the fast fluidization regime. 

Solid mass flow flux and velocity also vary across the radius of fast fluidized beds. Experimental measurements obtained by Herb et al. (1992) show that while local solid fluxes are positive upward in the core of the bed, they can become negative downward in the region near the bed wall. The difference between core and wall regions becomes increasingly greater as total solid mass flux increases. The downward net flow of solid in the region near the bed wall has significance for heat transfer at the wall. 

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