Fluidization classification

The main interest here has been their effect on solids carryover and heat transfer (see below). Some work on transition velocities between flow regimes has been reported, however. Thus Cai et al. (1989) studied the effect of operating temperature (50 to 500°C) and pressure (0.1 to 0.8 MPa) on the transition from bubbling to turbulent fluidization of eight powders in Groups A and B of the Geldart classification fluidized in a column 150 mm in diameter and 3.8 m in height. 

Whereas Geldart s classification relates fluidized-bed behavior to the average particle size in a bed, particle feed sizes maybe quite different. For example, in fluidized-bed coal (qv) combustion, large coal particles are fed to a bed made up mostly of smaller limestone particles (see Coal conversion processes). 

FIG, 17-1 Powder-classification diagram for fluidization by air (ambient conditions). [From Geldart, Powder TecbnoL, 7, 285-292 (1973).]... 

Classification The separation of fine particles from coarse can be effected by use of a fluidized bed (see Drying ). However, for economic reasons (i.e., initial cost, power requirements for compression of fluidizing gas, etc.), it is doubtful except in special cases if a fluidized-bed classifier would be built for this purpose alone. 

For group B and D particles, nearly all the excess gas velocity (U — U,nj) flows as bubbles tnrough the bed. The flow of bubbles controls particle mixing, attrition, and elutriation. Therefore, ehitriation and attrition rates are proportional to excess gas velocity. Readers should refer to Sec. 17 for important information and correlations on Gel-dart s powder classification, minimum fluidization velocity, bubble growth and bed expansion, and elutriation. 

The Subpart O standards apply to units that treat or destroy hazardous waste and which meet the definition of an incinerator. An incinerator is any enclosed device that uses controlled flame combustion and does not meet the criteria for classification as a boiler, sludge dryer, carbon regeneration unit, or industrial furnace. Typical incinerators1 2 3 include rotary kilns, liquid injectors, fixed hearth units, and fluidized bed incinerators (Table 23.1). The definition of an incinerator also includes units that meet the definition of an infrared incinerator or plasma arc incinerator. An infrared incinerator is any enclosed device that uses electric-powered resistance as a source of heat and which is not listed as an industrial furnace. A plasma arc incinerator is any enclosed device that uses a high-intensity electrical discharge as a source of heat and which is not listed as an industrial furnace. 

Figure 10. Fluidization regimes, adapted from Grace (1986) by Kunii and Levenspiel (1991) particles labeled by Geldart Classifications A, B, D.

Powder Classification Techniques. The Geldart (1973) fluidization, and Dixon (1981) slugging classifications have been found useful in explaining ... 

Perhaps the greatest difficulty in predicting fluidization performance via the Geldart (1973) classification is deciding on a single diameter to represent the complete material, especially if the product possesses a wide particle size distribution. This is supported to some extent by the more recent bulk density approach proposed by Geldart et al. (1984). 

Test the bulk solid in a fluidization chamber to confirm both the Geldart (1973) classification and the material s air retention properties (i.e., by undertaking deaeration experiments). 

This survey focuses on the so-called fixed beds, according to the Kuuni and Levenspiel classification, but these authors will refer to packed beds. Fluidized beds are outside the scope of this review. Furthermore, there exist no fixed beds in a physical sense if by fixed we mean the relative movement of both the interstitial gas... 

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