Fast fluidization regime

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. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

The boundary between the turbulent and the fast fluidization regimes has been of some dispute in the fluidization field. However, the choking velocity (Uch) appears to be a practical lower-velocity boundary for this regime (Karri and Knowlton, 1991 Takeuchi et al., 1986). 

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. 

The flow behavior in the riser varies with gas velocity, solids circulation rate, and system geometry. On the basis of the flow behavior, the fast fluidization regime can be distinguished from neighboring regimes. 

Figure E10.1. Fast fluidization regime boundaries for Example 10.1.

A0 Opening area for the mechanical valve or L-valve turbulent regime or choking to the fast fluidization regime... 

In the first zone (1a), the polymer is kept in a fast fluidization regime when leaving this zone, the gas is separated and the polymer crosses the second zone (1 b) in a packed bed mode and is then reintroduced in the first zone. A complete and massive solid re-circulation is obtained between the two zones. 

Reddy Karri, S. B., and Knowlton, T. M. A practical definition of the fast fluidization regime, in Circulating Fluidized Bed Technology III (P. Basu, M. Horio and M. Hasatani, eds.), pp. 67-72. Pergamon Press, 1991. 

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. 

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