Fast fluidization terminal velocity

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

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. 

As noted earlier, increasing gas velocity for any given fluidized bed beyond the terminal velocity of bed particles leads to upward entrainment of particles out of the bed. To maintain solid concentration in the fluidized bed, an equal flux of solid particles must be injected at the bottom of the bed as makeup. Operation in this regime, with balanced injection of particles into the bed and entrainment of particles out of the bed, may be termed fast fluidization, FFB. Figure 10 presents an approximate map of this fast fluidization regime, in terms of a dimensionless gas velocity and dimensionless particle diameter. 

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. 

With increasing fluid velocity, a particle-fluid system starts with the particle-dominated fixed bed terminating at UmC, spans the particle-fluidcompromising regimes of particulate, bubbling, turbulent and fast fluidization,... 

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. 

A closer look at the bed permits a more detailed description, schematically represented in Fig. 13.3-4, in which the void fraction is plotted versus the relative superficial gas velocity. As soon as the minimum fluidization velocity is exceeded, bubbles appear in the bed. Beyond the bubbling regime, a turbulent regime is obtained in which the bubble life time is short, so that, overall, the bed looks more uniform. The terminal and blow-out velocities coincide. Beyond that velocity, the regime of fast fluidization is reached, with a net entrainment of solids. If the velocity is further increased, the transport regime is reached, with a very steep decline in the solid volume fraction. 

Fig. 10.2 Flow regime map of gas-soUd contacting, a Characteristics of turbulent flow regime, b Characteristics of spouted beds, bubbling fluidized beds, fast fluidized beds and pneumatic transport regimes. In the figure notation the ordinate u = U p / n(pp — pg)g) is a dimensionless gas velocity, the abscissa d = dp[pg pp — Pg)glp ] a dimensionless particle size, the terminal velocity of a particle falling through the gas (m/s), and Umf the gas velocity at minimum fluidization (m/s). Letters A, B, C and D refer to the Geldart classification of solid particles. Reprinted from [49] with permission from Elsevier...

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