Fast fluidized beds

Eor turbulent and fast-fluidized beds, bubbles are not present as distinct entities. The following expression for bed voidage, bed occupied by gas, where U is in m/s, has been suggested (17) ... 

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

One of the advanced concepts for capturing CO2 is an absorption process that utilizes dry regenerable sorbents. Pure sodium bicarbonate from Dongyang Chemical Company and spray-dried sorbents were used to examine the characteristics of CO2 reaction in a flue gas environment. The chemical characteristics were investigated in a fast fluidized reactor of 0.025 m i.d., and the effects of several variables on sorbent activity, including gas velocity (1.5 to 3.5 m/s), temperature (40 to 70 °C), and solid concentration (15 to 25 kg/m /s)], were examined in a fast fluidized-bed. Spray-dried Sorb NX30 showed fast kinetics in the fluidized reactor. 

The fluidized reactor can be a bubbling fluidized type or a fast fluidized type, depending on the gas velocity and the reactivity of the sorbents. We adopted a fast fluidized bed type reactor for carbonation and regeneration reactions in order to identify the chemical characteristics of sorbents in a fast fluidized reactor of 0.025 m i.d. 

Reactivity of spray dried Sorb sorbents in a fast fluidized bed... 

The reactivities of spray-dried sorbents were examined in a fast fluidized bed. The reactor was operated at a carbonation temperature of 50 °C, and a gas velocity of 2 m/s with an initial sorbent inventory of 7 kg to compare CO2 concentration profiles in effluent gas for spray-dried Sorb NH series and NX30 sorbent. Figure 5 shows the comparison of CO2 concentration profiles in effluent gas of Sorb NHR, NHR5, and NX30 in a fast fluidized-bed reactor. The CO2 removals of Sorb NHR and NHR5 were initially maintained at a level of 100 % for a short period of time and quickly dropped to a 10 to 20 % removal level. 

Fig. 3, Effect of carbonation temperature on CO2 removal in a fast fluidized-bed reactor.

Hartge, E.-U., Li, Y., and Werther, J., Flow Structures in Fast Fluidized Beds, Fluidization V, Proc. 5th Eng. Foundation Conf. on Fluidization, Elsinore Denmark, p. 345 (1985)... 

Rowe, P. N., and Stapleton, W. M., The Behavior of 12-inch Diameter Fast Fluidized Beds, Trans. Instn. Chem. Engrs., 39 181 (1961)... 

Zhang, W., Tung, Y., and Johnsson, F., Radial Voidage Profiles in Fast Fluidized Beds of Different Diameters, Chem. Eng. Sci., 46(12) 3045 (1991)... 

Figure 11. Axial variation of solid concentration for fast fluidized bed of sand particles, at U— 5 m/s and Gs = 30 kg/m2 s. (From Herb, Dou, Tuzla and Chen,...

Figure 13. Radial profiles of solid volume fraction in fast fluidized bed. Fr = Us/Jgd P,M = Gs/ pgUg (Fro m Beaude and Louge, 1995.)...

Figure 15. Concentration of solid in clusters in fast fluidized bed. (From Soong, Tuzla and Chen, 1993.)...

General Characteristics. Energy addition or extraction from fast fluidized beds are commonly accomplished through vertical heat transfer surfaces in the form of membrane walls or submerged vertical tubes. Horizontal tubes or tube bundles are almost never used due to concern with... 

Figure 16, General heat transfer characteristics of fast fluidized bed. (Front Kiartg el at., 1976.)...

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. 

Figure 18. Parametric effects of solid flux and particle diameter on heat transfer in fast fluidized beds. (From Furchi et al, 1988).

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. 

The parametric effect of system pressure on the heat transfer coefficient was studied by Wirth (1995). They obtained experimental measurements of the heat transfer Nusselt number for fast fluidized beds... 

Figure 21. Significance of local suspension density for heat transfer in fast fluidized beds. (Data of Don, Herb, Tuzla and Chen, 1991.)...

Figure 22. Heat transfer coefficients for fast fluidized beds at various pressures. (DataofWirth, 1995.)... 

Models and Correlations. A multitude of different models and correlations have been proposed for prediction of the heat transfer coefficient at vertical surfaces in fast fluidized beds. To organize the various models in some context, it is helpful to consider the total heat transfer coefficient as comprised of convective contributions from the lean-gas... 

The simplest correlations are of the form shown by Eq. (15), in attempts to recognize the strong influence of solid concentration (i.e., suspension density) on the convective heat transfer coefficient. Some examples of this type of correlation, for heat transfer at vertical wall of fast fluidized beds are ... 

The lean/gas phase convection contribution has received the least attention in the literature. Many models in fact assume it to be negligible in comparison to dense phase convection and set hl to be zero. Compared to experimental data, such an approach appears to be approximately valid for fast fluidized beds where average solid concentration is above 8% by volume. Measurements obtained by Ebert, Glicksman and Lints (1993) indicate that the lean phase convection can contribute up to 20% of total... 

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