Flow structure fast fluidization

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)... 

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. 

Bai, D., Jin, Y and Yang, C. Influence of the inlet structures on flow in fast fluidized bed, Fifth National Conference on Fluidization (in Chinese), Beijing, China, April 9-14, pp. 94-97 (1990b). 

Figure 1 graphically summarizes the flow structure of fast fluidization introduced in the last chapter, as characterized by... 

To describe the complicated variation of the flow structure in fast fluidization with operating parameters, material properties and boundary conditions, the following system of designation is proposed (MK-90-10-30). 

Guo Q, Werther J (2004) Flow Behaviours in a Circulating Fluidized Bed with Various Bubble Cap Distributors. Ind Eng Chem Res 43 1756-1764 Hartge E-U, Li Y, Werther J (1986) Analysis of the local structure of the two phase flow in a fast fluidized bed. In CFB Technology, Pergamon press, pp 153-160... 

E.-U. Hartge, Y. Li, and J. Werther, Analysis of the Local Structure of the Two-Phase Flow in a Fast Fluidized Bed, in Circulating Fluidized Bed Technology, P. Basu ed., Pergamon Press, Toronto, 1986. 

Thus for a fast fluidization, internals are used to redistribute the axial and radial gas-solid flow structure, that is, to improve the uniformity of gas-solid flow structure in space and to change the core-annulus flow structure, so as to promote radial gas-solid exchange. 

In general, nonuniform structures, in both time and space, is widespread in bubbling, turbulent, and fast fluidization regimes. On the one hand, such nonuniformity can enhance the mass and heat transfer of a bed. On the other hand, it decreases the contact efficiency of gas and solids and makes the scale-up rather difficult. Internals are usually introduced not to eliminate the nonuniform flow structure completely but to control its effect on chemical reactions. The function of internals varies in different fluidization regimes, as do the types and parameters of internals. Taking these purposes into consideration, internals may be successfully applied to catalytic reactors with high conversion and selectivity, and some other physical processes. 

Hartige EU, Li Y, Werther J. Flow structures in fast fluidized beds. In Fluidization V. New York, Engineering Foundation, 1986, pp 345-352.. 

Bai D, Shibuya E, Masuda Y, Nakawaga N, Kato K. Flow structure in a fast fluidized bed. Chem Eng Sci 51 957-966, 1996. 

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