Mass transfer, fast fluidized beds

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. 

In this chapter, emphasis will be given to heat transfer in fast fluidized beds between suspension and immersed surfaces to demonstrate how heat transfer depends on gas velocity, solids circulation rate, gas/solid properties, and temperature, as well as on the geometry and size of the heat transfer surfaces. Both radial and axial profiles of heat transfer coefficients are presented to reveal the relations between hydrodynamic features and heat transfer behavior. For the design of commercial equipment, the influence of the length of heat transfer surface and the variation of heat transfer coefficient along the surface will be discussed. These will be followed by a description of current mechanistic models and methods for enhancing heat transfer on large heat transfer surfaces in fast fluidized beds. Heat and mass transfer between gas and solids in fast fluidized beds will then be briefly discussed. 

The residual carbon contents at different axial locations of the combustor were measured in the pilot plant tests (Li et al., 1991), as shown in Fig. 18. These data show that axial variations in carbon content with temperature (from 810 °C-923 °C) are as a whole rather slight, but mean carbon content increases with decreasing excess air ratio. Besides, for excess air ratios greater than 1.2, the carbon content at the top of the combustor is somewhat less than that at the bottom, while for excess air ratio less than 1.2, the opposite tendency is evident. In conclusion, for this improved combustor, an excess air ratio of 1.2 is considered enough for carbon burn-out, leading to reduced flue gas and increased heat efficiency as compared to bubbling fluidized bed combustion. That is probably attributable to bubbleless gas-solid contacting for increased mass transfer between gas and solids in the fast fluidized bed, as explained by combustion kinetics. 

The fast fluidized bed operates with high gas velocity and high solid mass flow rate as compared to the ordinary turbulent bed, thus intensifying gas/solid contacting and improving the mass transfer of oxygen from the gas phase to the solid phase, thereby resulting in a severalfold increase in CBI. 

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. 

Since gas density is usually negligible compared to solid density, pb is essentially equal to the mass concentration of solid per unit mixture volume. Some examples of this type of correlation, for convective heat transfer at vertical walls of fast fluidized beds, are given below. Due to accumulating evidence that the vertical size of the heat transfer surface affects the average the correlations are divided with regard to short or long surfaces. 

As in the Orcutt model, for a slow reaction, i.e. a small kvs, the overall conversion of the fluidized bed is less sensitive to mass transfer and thus to bed hydrodynamics than for a fast reaction, i.e. large kws. 

Two basic types of flow methods can be distinguished those with mixing and those without. The chief limitation of unmixed flow reactors is that mass transfer processes are frequently limiting and in the case of fast chemical reactions are probably always limiting. Stirred-flow reactors and fluidized bed reactors may often overcome the mass transfer limitation, and indeed these hybrid techniques may represent the best attributes of batch and flow methods. Each of these approaches i.s considered in turn. 

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