Fast fluidization and dense suspension

However, it is not always easy to distinguish between the flow behavior encountered in the fast fluidization and the transport bed reactors [56]. The transport reactors are sometimes called dilute riser (transport) reactors because they are operated at very low solids mass fluxes. The dense riser transport reactors are operated in the fast fluidization regime with higher solids mass fluxes. The transition between these two flow regimes appears to be gradual rather than abrupt. However, fast fluidization generally applies to a higher overall suspension density (typically 2 to 15% by volume solids) and to a situation wherein the flow continues to develop over virtually the entire... 

Figure 4 Transitions between dense suspension upflow, fast fluidization, and pneumatic transport with increasing solids circulation flux at a constant gas velocity.

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. 

Fast fluidization. To operate in the fast fluidization regime requires both that U exceed the transport velocity, and that solid particles be fed to the base of the unit with a sufficient flow rate, typically 20-200 kg/m -s, that a relatively dense suspension can be maintained in the reactor. Typical reported values (7) of are 1.5 m/s for 49 pm silica alumina cracking catalyst (pp= 10/0 kg/m ) and 3.8 m/s for hydrated alumina particles (Pp " 2460 kg/m ). There are insufficient data to allow to be correlated in a general way. 

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