Minimum bubbling condition

The fluidized-bed voidage at minimum bubbling conditions (emb) also has been shown to increase with increasing temperature (Fig. 7) and increasing pressure (Fig. 8). 

Pavlov s equation (3.475) is used for voidage determination at minimum bubbling conditions (for it, = bm), and since dp < 0.5 mm, the Broadhurst-Becker equation (3.466) is used for lm. For a wide range of particle sizes, the values of efm and ebm are close to each other, within a ratio between 0.95 and 1.1 for all particle densities. Values of bm/efm ratio... 

For a bed with Group A particles, bubbles do not form when the gas velocity reaches Umf. The bed enters the particulate fluidization regime under this condition. The operation under the particulate fluidization regime is characterized by a smooth bed expansion with an apparent uniform bed structure for Umf < U < Umb, where Umb is the superficial gas velocity at the minimum bubbling condition. The height of the bed expansion in terms of a can be estimated by [Abrahamsen and Geldart, 1980a]... 

Abrahamsen and Geldart [29] also proposed the following correlation for the bed voidage at minimum bubbling conditions ... 

It is clear from previous sections of this chapter that fluid-dynamic phenomena can account for a wide spectrum of behaviour patterns in unstable heterogeneous fluidized beds, and there is no reason to suppose that the same should not be so for stable homogeneous systems. The problem becomes that of identifying a relevant criterion for characterizing such differences. As no minimum-bubbling condition is predicted for fully homogeneous beds, fluidization quality criteria based on Smb and Aa are inapplicable in these cases. 

Figure 14.4 contains solutions to eqn (14.28) for a typical group B air-fluidized system it shows bubble void fractions ej for all values of dense phase void fraction e. There is a lot of information in this diagram, but only one solution is of relevance from a strictly practical point of view the jump at the minimum fluidization condition, ei = e f = 0.4, to a virtually completely void bubble, 2 1 (In fact this value computes to over 0.999.) This result provides a truly theoretical justification for the long established two-phase theory of gas fluidization for moderately sized powders, which postulates a dense particle phase that remains at the minimum bubbling condition for all fluid fluxes in excess of 17 /, with the remaining gas forming completely void bubbles (Toomey and Johnstone, 1952)... 

Edoa dense phase void fraction at high fluid flux Emb void fraction at minimum bubbling condition Emf void fraction at minimum fluidization condition 6 particle layer spacing, m... 

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