In 1973, Geldart classified powders into 4 categories (Figure 1) according to their physical properties in environmental conditions. Such classification is broadly used in every solid technology field.
Figure 1: Geldart's classification
Group A
Corresponds to fine particles (20< dp <100 mm) and relatively light (rp<1400 kg/m3). These particles present a particulate fluidization before the appearance of bubbles; the minimum bubbling velocity (Umb) is higher than the minimum fluidization velocity (Umf).
Group B
The inter-particle forces are disregarded and the bubbling phenomenon starts at the beginning of fluidization, it means., Umb/Umf=1. The bubble rising speed is greater than the interstitial gas velocity and the bubble diameter increases with bed height and excess gas velocity (U-Umf). Bed expansion is small and uniform.
Group C
Cohesive and very fine particles. Fluidization is extremely difficult because inter-particle forces are greater than gas force (drag force). In small diameter beds, such particles go up as a block of solids, whereas in large diameter beds preferential channels are formed from the distributor to bed surface. The particles can be fluidizable through mechanic agitation, vibration or addition of fluidizable particles (cat.B.).
Group D
Dense materials with important diameters (dp > 600mm). Large bubbles move up more slowly than the interstitial gas so that the gas enters through the base of the bubble and goes out through the top (slow bubbles). In order to fluidize such particles, special distributors are generally used so that fluidization of the bed-can be favored.
|
Group C
|
Group A
|
Group B
|
Group D
|
|
|
Outstanding
features |
Cohesive;
fluidization with the help of accessories |
Range fluidization
non-bubbling |
Start of bubbling
at the minimum fluidization velocity |
Start of bubbling
at the minimum fluidization velocity |
|
Typical examples
|
Wheat flour
Cement |
Catalyst
|
Sand
|
Shale, coal,
grains in general |
|
Bed Expansion
|
Low in the presence
of preferential channels; high when fluidized |
High
|
Moderate
|
Low
|
|
Aeration
rate |
Initially rapid then exponential
|
Slow, Linear
|
Fast
|
Fast
|
|
Bubbles
Properties |
Without Bubbles
(Preferential channels and colapses) |
The bubbles break
and coalesce continuously; maximum bubble size |
No Limits
of size |
No limits
of size |
|
Mixture of solids
|
Very Low
|
High
|
Moderate
|
Low
|
|
Gas mixture
(bubble) |
Very low
|
High
|
Moderate
|
Low
|
|
Spouted bed
|
No
|
No, except for
very shallow beds |
Only shallow beds
|
Yes, even in
deep beds |