Bubbling fluidized beds zones

Two-Phase theory of Davidson According to the two-phase theory, two phases exist in the bubbling fluidized bed (a) the bubbling phase consisting of gas bubbles, and (b) the particulate phase, namely the solids around the bubbles. The particulate phase is alternatively called the emulsion phase. Bubbles stay in the bubble phase and penetrate only a small distance into the emulsion phase. This zone of penetration is called cloud since it envelops the rising bubble. 

Figure 16.5 The different zones of a bubbling fluidized-bed pyrolyser where reactions take place, with the indication of the specific hydrodynamic and thermodynamic features...

A first estimate for Qb is given by the two-phase theory of fluidization, proposed by Toomey and Johnstone [130] and developed by Davidson and Harrison [29, 30]. In this theory a bubbling fluidized bed consists of two zones or phases, referred to as the bubble phase consisting of pure gas and the emulsion phase consisting of uniformly distributed particles in a supporting gas steam. The emulsion phase is assumed to be operating at minimum fluidization conditions (7 m/> while the bubble phase carries the remaining gas flow U ... 

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. 

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... 

Vaux, W. G., Attrition of Particles in the Bubbling Zone of a Fluidized Bed, ... 

The presence of vertical membrane elements or modules also helps to prevent bubble coalescence, thus favoring heat and mass transfer in the reactor. Their spacing should be sufficiently small so that the maximum number of the membrane tubes or modules may be provided in the reaction zone and large enough that no blockage or bridging of the fluidized bed occurs. 

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