Radial voidage profile, fast fluidization

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

Tung, Y., Li, J., and Kwauk, M., Radial Voidage Profiles in a Fast Fluidized Bed, Proc. of Third China-Jpn. Symp. on Fluidization, p. 139, Beijing, China (1988)... 

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).

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). 

Zhang W, Tung Y, Johnsson F. Radial voidage profiles in fast fluidized beds of different diameters. Chem Eng Sci 46(12) 3045-3052, 1991. 

Tung Y, Li Y, Kwauk M. Radial voidage profiles in a fast fluidized bed. In Kwauk M, Kunii D, eds. Fluidization 88 Science and Technology. Beijing Science Press, 1988, pp 139-145. 

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. 

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. 

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. 

Owing to the rapid formation and dissolution of particle clusters which contribute to high slip velocities and solid backmixing but preserve a limited extent of gas backmixing, the fast fluidized bed regenerator exhibits unique axial and radial profiles for voidage, temperature and carbon concentration (see Figs. 9 and 11 and Table VIII). 

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