Incipient fast fluidization velocity

Local voidages for FCC catalyst at various radial positions were measured with an optical fiber probe in a Type A apparatus, from which radial volidage profiles and their probability density functions were computed by Li et al. (1980b), as shown in Figs 20 and 21. When gas velocity is less than the incipient fast fluidization velocity of 1.25 m/s, the radial voidage profile is relatively flat when gas velocity increases further, this profile becomes steeper high in the center. As flow is transformed into pneumatic transport, the... 

Fig. 32. Influence of solids circulation rate on gas backmixing at a velocity higher than the incipient fast fluidization velocity (after Li and Wu, 1991).

Based on these findings, it can well be appreciated that conditions for the onset of fast fluidization would change with gas and solids properties. Table I gives experimentally measured parameters at incipient fast fluidization, indicating that the U,f is roughly 3.5 to 4 times that of the fall-free velocity... 

Group B powders are of intermediate particle size, such as sand. They do not fluidize so smoothly as Group A, for bubbles form as soon as the incipient fluidization velocity is reached. It is thus evident that the ratio umJum is equal to unity. When the fluidizing gas is turned off suddenly, Group B powders would collapse immediately. In the fluidized state, the rising bubbles travel upward faster than the interstitial gas flow rate, and are therefore designated as fast bubbles. ... 

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