Slide 1

Jangam, Mujumdar - Classification and Selection

Table 3.4 shows the properties of various groups of particles/powders. Again, these
figures are applicable to dry powders and may need adjustments for wet particles and
poly-disperse particles where additional surface forces come into play. There are vari-
ous configurations of fluidized bed dryers and selecting proper fluidized bed dryer for
certain application is a big task itself. Figure 3. 2 shows the classification of fluidized bed
dryers based on various criteria such as the mode of operation, flow regime, tempera-
ture and pressure used, the drying media used, the way of fluidization and the type of
heat transfer. There are different designs of fluidized bed dryers and the commonly used
are the following; well mixed, plug flow, spouted bed, agitated, vibrated and pulsating
fluid bed. Each of these types is suitable for product with specific properties. Figure 3. 3
shows various types of fluidized bed dryers available for feed particles of different cha-
racteristics. Note that different design strategies are needed for different dryer types.
Also, often the same dryer type can behave differently when different materials are
dried in it. Presence of sugar in fruits makes drying especially "tricky" due to unquantifi-
able effects of stickiness. Note that Table 3.4 is located on very extensive experimental
observations using "dry" particulates. Hence, the classification scheme may not in gen-
eral work for surface-wet particles. Experimental testing is therefore required.

Table 3.4. Characteristics and properties of various types of dry powders and particu-

lates

Group Characteristics and Properties

A

• Good fluidization quality, aeratable, easily fluidized, smooth at low
  velocity and bubbling at higher velocity and slug at high velocity,
  bed expands, Good solids mixing.


• Small mean particle size, low density, typically 30< dp <100μm and
  ρ<1400 kgm-^3.

B

• Good fluidization quality, sand-like particles, vigorous bubbling,
  slug at high velocity, small bed expansion, good solids mixing in
  bubbling.


• Typically 40μm<dp<500μm, 1400kgm-^3 <ρ<4000kgm-^3.

C

• Bad fluidization quality, cohesive due to strong interparticle force,
  severe slugging and agglomeration, may generates electrostatic
  charges, poor solids mixing.


• Fine and ultra-fine particles.

D

• Poor fluidization quality, spoutable, difficult to fluidize in deep bed
  depth, large bubbles, severe channeling, relatively poor solids
  mixing.


• Large and/or dense particles, typically dp >500μm, ρ>1400kgm-^3.