Fluidized beds are being used in industry as a dryer and reactors. The operation of the bed mainly depends on the particle in the bed. The following classification is based on the size of the particles.
Geldart Group A
In the group A class, particles tend to be aeratable and fluidize well. The particle size ranges from 30 micrometers to 125 micrometers in this group. Indeed, most particles used in fluidization are group A powders because they can easily be made by spray drying. Typically, powders from this group don’t promote maximum bubble sizes larger than 20 cm. They exhibit considerable bed expansion without the formulation of bubbles. The bed expansion can exceed greater than 100% at high pressure. Therefore, the bed size should the large enough to avoid loss of mass in the downstream equipment. The catalytic oxidation, oxychlorination, and acrylonitrile processes are the example of Group A fluidized bed powders.
Geldart Group B
The particles of this group tend not to undergo smooth fluidization and are used in a wide range of fluidized unit operations. The size of these particles ranges from 150 micrometers to 1000 micrometers. These particles form vary large bubbles therefore, slugging can occur in even larger units. Fluidized combustors and pyrolysis units are generally classified in Geldart Group B.
Geldart Group C
The group of powder is most difficult to fluidize due to its small size (< 30 micrometers). These are considered cohesive and experienced channeling during operation. These small particles behave like a cluster instead of independent particles. These types of powders are fluidized with the aid of baffles, microjets, and mechanical vibration. Larger particle size powder may also be added to promote smoother fluidization.
Geldart Group D
The particle size of this group is larger than other three groups hence require larger gas flow rate for fluidization. The bubble size of this group is enormous and slugging can be observed even in larger beds. Therefore, these powders are processed in spouting beds where it requires lower gas than the standard fluidized bed.
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