Vortical fluidizing

It is shown in Fig. 3.24, that the vortical fluidized bed exists for definite ratios between the pressure differential and the gas flow velocity [48]. At small differential and velocity values (AB) powder particles are motionless. The gas flow velocity v corresponds to the onset of transfer to the fluidized bed state (B) and is called the first critical velocity. On reaching the second critical velocity, V2, powder particles are carried away from the vessel by the gas flow (C). 

Fig. 3.24. Pressure differential in the powder layer experiencing vortical fluidizing as a function of the gas flow velocity...

Large-diameter solid particles in a three-phase fluidized-bed system cause bubbles to be small, whereas, in a fine particle slurry, the bubbles can become large. Henriksen and Ostergaard40 showed that the large bubbles in the latter case can break as a result of Taylor instability at the root of the bubble. The wake properties of bubbles in a three-phase fluidized-bed system have been studied by Rigby and Capes.115 They showed that bubble wakes in a three-phase system consist not only of a stable portion carried with the bubbles but also of vortices shed by the bubbles. 

The vortical method of coating deposition consists of immersion of the heated article in the fluidized bed of the polymer powder formed by blowing a gas flow through the porous bottom of a vessel containing the powder (Fig. 3.23). Under certain parameters of the gas flow, the immovable layer of powder particles poured into the vessel to the level Ho expands up to the level H. The particles acquire mobility and transfer into the airborne state. The criterion for this change is a constant value of the gas pressure differential in the powder bed volume [52]. 

Fig. 3.23. Vortical state of the fluidized bed (a) normal, (b) stratification in fractions. (1) vessel (2) porous bottom, (3) inflation chamber, (4) gas socket, (5) polymer powder, (6) Cl particles, (7, 8) powder layers enriched with and withont Cl...

Electrostatic deposition of coatings in the fluidized bed is realized by the vibration, vortical or vibrovortex methods together with the electrical charging of suspended particles. 

The bubble-to-emulsion phase mass transfer can be described with a convection term and a diffusion term. The convection term describes the flow pattern of the fluidization gas from the emulsion phase inside the bubble which is dominant for the first part of our experiments. The molecular dif fusion of CO2 from the bubble phase (especially from the vortices) to the dense phase also influences the mass transfer process as discussed by Kunii and Levenspiel. 

The zero vorticity cell model has been extended numerically to intermediate Reynolds numbers for spheres (LeClair and Hamielec, 1968), and both the zero vorticity and the free-surface cell models (unmodified) to spheroids at low Reynolds numbers (Epstein and Masliyah, 1972). However, these extensions are more applicable to immobilized packed beds than to fluidized beds, and only in the absence of turbulence, which for unexpanded, fixed packed beds of spheres develops at Re > 110—150 (foils and Hanratty, 1966). 

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