Fluidization aggregative

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

Foscolo, P. U. Gibilaro, L. G. A Fully Predictive Criterion for the Transition between Particulate and Aggregate Fluidization, Chem. Eng. Sci., 39 1667... 

Figure 57. Computation results showing gradual transition from particulate fluidization to aggregative fluidization. (Li, 1987.)...

Essentially aggregative fluidization is a two-phase system there is a dense phase (sometimes reterred to as the emulsion phase), which is continuous, and a discontinuous phase called the lean or bubble phase. The simplitied assumption that all the gas over and above that required tor minimum fluidization flows up through the bed in the form ot bubbles is known as the two-phase theory. It the total volumetric flow ot gas is Q then... 

The phenomena of rapid particle movement and the intimate contact between solids and at least a portion of the gas give rise to a series of characteristics of aggregative fluidization such as good mixing, near isothermal conditions and high rates of heat and mass transfer which are exploited in a wide range of unit operations. 

Minimum bubbling velocity timb is defined as the gas velocity at which bubbles first appear in aggregative fluidization. For coarse uniformly-sized particles, for example those in Geldart group B, it is usually the case that M i, = u /- However, very fine non-uniformly sized particles such as those in group A exhibit smooth bed expansion and no bubbling until a gas velocity considerably in excess of the minimum... 

See aggregative fluidization. Entrained particles which leave the fluidized bed column entirely. 

A system of classifying particles into four groups (A, B, C and D) according to their fluidization behaviour, first proposed by Geldart. See aggregative fluidization. 

The first two conditions seem to be fulfilled only for (a) liquid-liquid systems (b) liquid-gas systems spray towers and bubble columns and (c) gas fluidized systems in an aggregative fluidization state. To what extent the third condition is fulfilled depends, in most cases, on mass transfer limitation of reactions between two or more components. 

Zabrodsky, S.S. (1963). Heat Transfer Between Solid Particles and a Gas in a Nonuniformly Aggregated Fluidized Bed. Int. J. Heat Mass Transfer, 6, 23 and 991. 

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