Fast fluidization boundaries

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). 

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

Yang, W. Incipient Fast Fluidization Boundary and Operational Maps for Circulating Fluidized Bed Systems, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 61-71. Somerset, Pennsylvania (1993). 

The hydrodynamics of fast fluidization depends greatly on the boundaries of the retaining vessel, e.g., the wall, the inlet and the outlet. Inasmuch as different geometric structures were employed by different investogators in their experimental apparatus, inconsistencies in the resulting data are unavoidable. This will be the main topic for the present section. 

If the effect of boundaries on gas-solid flow in fast fluidized beds were overlooked, gas/solids interaction might well be misunderstood. 

Boundaries in fast fluidization refer mainly to the column wall as well as the inlet and outlet. Effect of the wall on pressure drop due to friction between the fluidized solids and the wall surface is minimal (Li et al, 1978), although it is the very cause of radial distribution of parameters. The configuration of the inlet and the outlet often strongly affect gas-solids flow, especially with regard to axial voidage profile. 

To describe the complicated variation of the flow structure in fast fluidization with operating parameters, material properties and boundary conditions, the following system of designation is proposed (MK-90-10-30). 

To account for the intrinsic characteristics of particle-fluid two-phase flow in fast fluidization, the particles and the fluid are considered to interact with each other on both a micro-scale and meso-scale level to produce local or meso-scale heterogeneity (phases), and the overall fluid-particle system interacts with the equipment boundaries on a much larger scale to produce macro-scale heterogeneity (regions). 

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. 

This locus is within the fast regime, near its upper boundary, on the map of Fig. 1. Therefore the proposed operation will be a fast fluidized bed. 

The core-annulus models treated in the previous section improve on the one-dimensional models covered in the preceding section by making some allowance for the difference in behavior between the relatively dense wall region and the dilute core of fast fluidized beds. However, the hydrodynamics are represented in a relatively crude manner. As illustrated in Fig. 33, experimental results show that reactant concentration varies continuously across the entire cross section of the riser, rather than there being a sharp discontinuity at a coreannulus boundary. Hence models are needed that provide for continuous variation across the riser, or at least a greater number of intervals in the lateral direction. Such models include the following ... 

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