Fast fluidization clustering

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. 

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

Single particles will tend to be carried out of the bed if the fluid velocity exceeds the terminal falling speed u, of the particles given by equation 9.5. Thus the normal range of fluidization velocity is from umf to a,. However, it may be found that the fluid velocity required to bring about fast fluidization is significantly higher than u, because particles tend to form clusters. 

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. 

In early studies on fast fluidization (Yerushalmi et al., 1979 Li and Kwauk, 1980a), clustering of particles was recognized as a fundamental phenomenon, and much attention has since been given to the visualization of bed structure. Qin and Liu (1982) developed a fluorescent particle visualization technique, as shown in Fig. 10. Particles covered with a long-decay fluorescent powder... 

Fig. 12. Axial variation of appearance for clusters in a two-dimensional fast fluidized bed (after Bai et al., 1991). 1, riser 2, distributor 3, two-dimensional bed 4, cyclone 5, butterfly valve 6, downcomer.

It has been widely accepted that the phenomenon of fast fluidization is attributed to cluster formation. Photomicrography of the fast fluidization... 

The high frequency of cluster formation and dissolution in fast fluidization is reflected in high-frequency random voidage fluctuations as shown at the top subfigure of Fig. 11. Such a change in two-phase behavior promotes efficient gas/solids contacting. 

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. 

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). 

Fast fluidization (FF) has become one of the most widely used forms of bubbleless fluidization. It exploits the phenomenon that fine powders do not conform to the correlations for particulate fluidization but can admit far greater gas flow than would be permitted by the terminal velocity of the constituent particles. The particles aggregate into clusters or strands to make way for the rapid fluid flow. These clusters or strands frequently dissolve and reform with fresh particle members, thus leading to high rates of particle-fluid mass and heat transfer that are hardly realizable with bubbling fluidization. 

Yerushalmi and Cankurt considered the transport velocity as the boundary between the turbulent and the fast fluidization [4], The transition takes place gradually through a turbulent state where both voids and clusters coexist. During fast fluidization a dense phase exists at the bottom of the bed while a lean phase exists at the top. 

Another hydrodynamic complication found in fast fluidized beds is the tendency for particles to aggregate into strands or clusters, as reported by Horio et al. (1988) and Chen (1996). The concentration of solid particles in such clusters is significantly greater than in the bed itself, and it increases with increasing radial position and with increasing total solid flux (see Soong et al., 1993). This characteristic also directly affects heat transfer at walls. 

Heat transfer models are usually written in terms of either clusters or dense wall layers, based on the hydrodynamics of fast fluidization. For cluster models (Fig. 26), heat can be transferred between the suspension and wall by (1) transient conduction to particle clusters arriving at the wall from the bulk, supplemented by radiation (2) convection and radiation from the dispersed phase (gas containing a small fraction of solid material). The various components are usually assumed to be additive, ignoring interaction between the convective and radiation components. 

Above 3.5 Ft/S bubbles become unstable and break down to jagged tongues of gas that dart to and fro — the turbulent regime. Eventually by 6 Ft/S, clusters of solids are conveyed up the centre and out of the bed while other clusters fall down at the wall (fast fluidization). Beyond about 6 Ft/S the large clusters break down to small clusters that are pneumatically conveyed out of the pipe with only relatively minor slippage back downwards at the wall. 

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