Fluidized particles, classification

The transport behavior of fluidized bed systems can be described in light of the properties of the fluidized particles and the flow regimes. In the following, particle and regime classifications along with general components in a fluidized bed are given. 

Figure 9.1. Geldart s classification of fluidized particles (from Geldart, 1973).

Intensive studies have been carried out on the ascending bubble diameters in free fluidized beds (C5, K27, R14, R16, W9). Various correlations for estimating bubble diameters have appeared (M36, R13, W9). However, the particles utilized in these experiments belong to group B of Geldart s classification. For this type of particle, bubble diameters are expressed as a function of bed diameter, of distance of the bubble above the distributor, of initial bubble diameter, and of physical properties of the fluidized particles. Mori and Wen (M36) emphasized the former three factors and proposed the equation ... 

Heat transfer in gas-fluidized bed can occur by conduction, convection, and radiation depending on the operating conditions. The contribution of the respective modes of heat transfer to the coefficient of heat transfer depends on particle classification, flow condition, fluidization regimes, type of distributor, operating temperature, and pressure. Heat transfer between a single particle and gas phase can be defined by the conventional equation of heat transfer ... 

Goossens WRA. Classification of fluidized particles by Archimedes number. Powder Technol 98 48-53, 1998. 

Some applications of liquid-fluidized beds, such as particle classification, are over a century old, while many. 

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). 

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. 

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

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). 

It is noteworthy that the group classification depends not only on the particle but also on the gas properties. Moreover, the above classification is related to the fluidization in the presence of air at ambient conditions. For a different fluid and operating conditions, a powder may appear in a different group. Thus, Figure 3.52 can be helpful in classifying a powder only for ambient conditions and with air as the fluid. 

Thus, a more general classification should be based on the fluidization regime rather than die particle and fluid characteristics. The following classification is introduced in the present book. 

In these devices classification occurs in the space over a fluidized bed from which fine particles are carried away with the air stream. The air velocity must essentially exceed the maximal fluidization velocity of fine fraction. These classifiers operate on the gravitational counterflow separation principle with fluidized bed used to increase particles residence time in the separator. Coarse particles can move horizontally across the bed to the exit, which is another advantage of this device. 

In principle, the experimental protocol of fluidized bed adsorption does not deviate from packed bed operations, the main difference being the direction of liquid flow. The standard sequence of frontal chromatography, equilibration, sample application, wash, elution, and cleaning (CIP) is performed with an upward direction of flow as shown in Fig. 3. During equilibration of the matrix the stabilization of the fluidized bed occurs, in case of size and/or density distribution of the adsorbent particles the classification within the bed may be detected by visual observation of the bed. As discussed below, bed stability may... 

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