Fluidized regimes bubbling fluidization

The bubbling fluidization regime, as shown in Fig. 9.3(b), is reached with an increase in the gas velocity beyond (7mb. Bubbles form and induce vigorous motion of the particles. In the bubbling fluidization regime, bubble coalescence and breakup take place. With increasing gas velocity, the tendency of bubble coalescence is enhanced. Two distinct phases, i.e., the bubble phase and the emulsion phase, are present in this regime. 

Flow regime Bubbling, slugging or turbulent, distinct upper interface Fast fluidization Dilute transport... 

Particle Regimes. In 1973, particles were classified with respect to how they fluidize in air at ambient conditions into Geldart groups (6) (Fig. 4). Particles that formed bubbles immediately after the gas superficial velocity exceeded were designated as Group B particles. For these particles, the... 

Classical bubbles do not exist in the vigorously bubbling, or turbulent fluidization regimes. Rather, bubbles coalesce constantly, and the bed can be treated as a pseudohomogenous reactor. Small bubble size improves heat transfer and conversion, as shown in Figure 5b. Increasing fines levels beyond 30—40% tends to lower heat transfer and conversion as the powder moves into Group C. 

Fig. 7. Axial density profiles in the (—) bubbling, (------) turbulent, and (----) fast and ( ) riser circulating fluidization regimes. Typical gas velocities for...

Fig. 8. (a) Schematic for an FCC unit showing where the various fluidization regimes are found and (b) a corresponding phase diagram for Group A powder (FCC catalyst) where the numbers on the curves represent the superficial soHd velocity in m/s. A represents the bubbling regime B, the turbulent ... 

Yerushalmi and Avidan (1985) suggest that the axial dispersion coefficient of solids in slugging and turbulent flow varies approximately linearly with the bed diameter, similar to Thiel and Potter (1978). The data are shown in Fig. 17 although May s results are probably in the bubbling fluidization regime rather than turbulent flow. 

As fluidized beds are scaled up from bench scale to commercial plant size the hydrodynamic behavior of the bed changes, resulting, in many cases, in a loss of performance. Although there have been some studies of the influence of bed diameter on overall performance as well as detailed behavior such as solids mixing and bubble characteristics, generalized rules to guide scale-up are not available. The influence of bed diameter on performance will differ for different flow regimes of fluidization. 

The chapter begins by describing how temperature and pressure affect parameters important for bubbling fluidized beds, and then discusses their effect on regime transitions, circulating fluidized beds, and cyclones. 

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

Gas jets in fluidized beds were reviewed by Massimilla (1985). A more recent review is by Roach (1993) who also developed models to differentiate three jet flow regimes jetting, bubbling and the transition. However, most of the data were from jets smaller than 25 mm. The discussion here will emphasize primarily large jets, up to 0.4 m in diameter, and operation at high temperatures and high pressures. The gas jets can also carry solids and are referred to as gas-solid two-phase jets in this discussion. 

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