Classification of Solid Particles

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
Geldart, D. (1986), Gas Fluidization Technology, Ed. John Wiley & Sons, Nova Iorque