Dense-phase fluidized beds freeboard

Consider a riser where the voidage in the bottom dense region is uniform and the top dilute region behaves as the freeboard of a dense-phase fluidized bed. The axial profile of the voidage in the top dilute region can be expressed by [Kunii and Levenspiel, 1990] (see 10.4.1)... 

Pemberton, S. T. and Davidson, J. F. Chem. Eng. Sci. 41 (1986) 243, 253. Elutriation from fluidized beds. I Particle ejection from the dense phase into the freeboard. II Disengagement of particles from gas in the freeboard. 

The bubbling fluidized bed (Figure 8.3) is divided vertically into two zones, namely, a dense phase and a freeboard region (also known as lean phase or dispersed phase). The... 

The linear interpolation approach cannot accurately treat the abrupt changes of diffusivity that may occur in some locations in multiphase reactors where the phase fractions change rapidly (e.g., across the transition zone from the dense bed to the freeboard section in a bubbling fluidized bed). To improve the interpolation procedure, one seeks an approximation of the diffusivity that gives an accurate approximation of the diffusive flux for cases with large changes in the material properties. 

When bubbles burst and release their gases at the surface of a fluidized bed, it is generally observed that particles are ejected into the freeboard space above the surface. These particles can originate either from the dense phase just ahead of the bubble at the moment of eruption (Do et al., 1972) or from those solids that travel in the wake of the rising bubbles (Basov et al., 1969 Leva and Wen, 1971 George and Grace, 1978). 

---