Fast fluidization regions

Fig. 60. Regime diagram for A1203. 1. bubbling region 2. turbulent region 3. fast fluidization region 4. pneumatic transport region. [After Chen and Kwauk, 1985.]...

Then, overall hydrodynamics of fast fluidization—region—will be discussed by extending the EMMS model to both axial and radial directions. Other two aspects of local hydrodynamics—regime and pattern—will not be involved as this book is limited to the fast fluidization regime. 

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. 

As a result, gas velocity and solids circulation rate can be adjusted separately and independently. For any given gas velocity and solids circulation rate, there is only a unique equilibrium solids inventory in the fast fluidized column, which determines the height of the dense-phase region at the bottom. 

To account for the intrinsic characteristics of particle-fluid two-phase flow in fast fluidization, the particles and the fluid are considered to interact with each other on both a micro-scale and meso-scale level to produce local or meso-scale heterogeneity (phases), and the overall fluid-particle system interacts with the equipment boundaries on a much larger scale to produce macro-scale heterogeneity (regions). 

The FD region at the top is characterized by the dominance of the fluid over the movement of particles, as already shown in Fig. 11. When the fluid-dominated FD regime is first formed, the clusters of fast fluidization are disintegrated to form an essentially one-phase structure in which the particles are, however, not completely discretely suspended, that is, at a much higher concentration as compared to the ef computed for the broth before Upt. This is shown by the fluctuating voidage considerably above zero, as can be seen in the upper right-hand side of Fig. 11. 

Inasmuch as heat transfer depends on the hydrodynamic features of fast fluidization, if the fast fluidized bed is equipped with an abrupt exit, the axial distribution of solids concentration will have a C-shaped curve (Jin et al., 1988 Bai et al., 1992 Glicksman et al., 1991. See Chapter 3, Section III.F.l). The heat transfer coefficient will consequently increase in the region near the exit, as reported by Wu et al. (1987). 

---