Fast fluidization voidage

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) ... 

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 22. Vertical voidage distribution in fast fluidization. (Li and Kwauk, 1980.)...

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. 

The three-dimensional voidage distribution provides the basic correlation for building a reactor model for fast fluidization, given data on particle-fluid transfer coefficients and intrinsic particle reaction kinetics. 

Tung, Y., Li, J., Zhang, J., and Kwauk, M., Preliminary Experiments on Radial Voidage Distribution in Fast Fluidization, Fourth Nat. Conf. Fluidization, Lanzhou, China (1987)... 

Wang, N., Li, Y., Zheng, X., and Kwauk, M., Voidage Profiling for Fast Fluidization, First Intern. Conf. Circulating Fluidized Beds, Halifax, Canada (1985)... 

On the basis of the observations in the macroscale, the flow of a fast fluidized bed can be represented by the core-annulus flow structure in the radial direction, and coexistence of a bottom dense region and a top dilute region in the axial direction. Particle clusters are an indication of the heterogeneity in the mesoscale. A complete characterization of the hydrodynamics of a CFB requires the determination of the voidage and velocity profiles. There are a number of mathematical models accounting for the macro- or mesoaspects of the flow pattern in a CFB that are available. In the following, basic features of several types of models are discussed. 

Zheng, C. G., Ding, Y., Xia, Y. S., Bin, H. and Kwauk, M. (1991). Voidage Redistribution by Ring Internals in Fast Fluidization. In Fluidization 91 Science and Technology. Ed. Kwauk and Hasatani. Beijing Science Press. 

The influence of gas velocity and solids circulation rate on flow regime transition was measured in a fast fluidization, 90-mm i.d. and 8-m high (Li and Kwauk, 1980a). Figure 4 shows the change of the average voidage, e,... 

Fig. 5. Types of apparatus and their respective effects on axial voidage profile. 1, fast fluidized bed 2, intermediate hopper 3, cyclone 4, slow fluidized bed 5, downcomer 6, solids rate controlling device 7, solids rate measurement device 8, suspension section.

Voidage profiles represent one of the most important aspects of the flow structure of fast fluidization, which play an important role in gas and solids mixing, mass and heat transfer, and conversion in a chemical reactor. Considerable efforts have been given to studying the axial and radial variation of solids concentration axially, dilute at the top and dense at the bottom, and radially, dilute in the center and dense in the vicinity of the wall. As already mentioned in Section II, these variations depend mainly on gas velocity and solids circulation rate and are also influenced by the configuration of the apparatus. 

Fig. 15. Axial voidage profile Tor four powders in a Type A fast fluidized bed (after Li and...

Fig. 16. Axial voidage profile for different inventories of pulverized coal in a Type B fast fluidized bed (after Li et al., 1984).

Local voidages for FCC catalyst at various radial positions were measured with an optical fiber probe in a Type A apparatus, from which radial volidage profiles and their probability density functions were computed by Li et al. (1980b), as shown in Figs 20 and 21. When gas velocity is less than the incipient fast fluidization velocity of 1.25 m/s, the radial voidage profile is relatively flat when gas velocity increases further, this profile becomes steeper high in the center. As flow is transformed into pneumatic transport, the... 

Fig. 20. Radial voidage profile for fast fluidized beds (after Li et aU 1980, 1985).

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. 

Li, J., Tung, Y., and Kwauk, M. Axial voidage profiles of fast fluidized beds, in Circulating Fluidized Bed Technology II (P. Basu and J. F. Large, eds.), p. 193. Pergamon Press, 1988. 

Tung, Y., Zhang, W., Wang, Z., Tiu, X., and Ching, X. Further study of radial voidage profiles in fast fluidized beds, Eng. Chemistry and Metallurgy 10(2), 18-23 (1989). 

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