Fast fluidization fluid velocity

Figure 23.2(b) shows a fast-fluidized-bed reactor, together with external equipment, such as cyclones, for separation of fluid and solid particles carried out of the reactor, and subsequent recirculation to the reactor. In a fast-fluidized bed, the fluidization velocity... 

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. 

According to the preceding definition, the relationship between the saturation carrying capacity K and fluid velocity Ut can be calculated, as shown in Fig. 5, which defines the transition from the PFC regime to the FD regime, that is, from fast fluidization to dilute transport, as corroborated by the experimental points (the data at high velocities was transported from Fig. 7). 

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. 

As the velocity is further increased, a point is reached when the bed density becomes a strong function of the rate of solids feed. This corresponds to the fast fluidization regime and is characterized by the significant fact that fluid-bed densities as high as those in a bubbling bed can be maintained by adjnsting the solids flow rate. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

Nucleation in fluidized-bed granulation by necessity occurs within a drop-controlled regime, which requires fast drop penetration and low spray flux [Eq. (21-107), Figure. 21-107]. Spray flux / should be no more than 0.2, and quite possibly much lower. Increasing wettability has been shown to increase nuclei size, presumably due to more stable operation (Fig. 21-99). Figure 21-168 illustrates the impact of increasing spray flux and fluid-bed gas velocity on size distribution. Decreasing dimensional spray flux (which is inverse to... 

From Table 11.25, four different (but somewhat overlapping) categories of fluidized-bed reactors can be recognized bubbling-bed, turbulent- (or fluid-) bed, fast-bed, and pneumatic- (or transport-) bed. Occasionally, reactor operation at velocities close to has been attempted. 

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