Geldart (1973) classified powders into four groups according
to their fluidization properties at ambient conditions.
The Geldart Classification of Powders is now used widely
in all fields of powder technology.
Group A powders, when fluidized by air at ambient conditions,
give a region of non-bubbling fluidization beginning at Umf ,
followed by bubbling fluidization as fluidizing velocity increases .
Group B powders give only bubbling fluidization under these conditions.
Geldart identified two further groups:
Group C powders - very fine, cohesive powders which are incapable
of fluidization in the strict sense.
Group D powders are large particles that are distinguished
by their ability to produce deep spouting beds (see Figure 5 and Video 4).
Figure 5: An image from Video 4
[11.3 min to download at 28.8kbaud]:
A spouted fluidized bed of rice (Group D solids)
Figure 6 shows how the group classifications are related
to the particle and gas properties.
Figure 6: Geldart's classification of powders
The fluidization properties of a powder in air may be predicted
by establishing in which group it lies. It is important to note that
at operating temperature and pressures above ambient a powder may appear
in a different group from that which it occupies at ambient conditions.
This is due to the effect of gas properties on the grouping and may
have serious implications as far as the operation of the fluidized bed
is concerned. Table 1 presents a summary of the typical properties of
the different powder classes.
Since the range of gas velocities over which non-bubbling fluidization
occurs in Group A powders is small, bubbling fluidization is the type
most commonly encountered in gas-fluidized systems in commercial use.
The superficial gas velocity at which bubbles first appear is known as
the minimum bubbling velocity Umb. Premature bubbling can be caused by
poor distributor design or protuberances inside the bed. Abrahamsen and
Geldart (1980) correlated the maximum values of Umb with gas
and particle properties using the following correlation:
where F is the fraction of powder less than 45 
m.
In Group A powders when Umb > Umf
bubbles are constantly splitting and coalescing, and a maximum stable
bubble size is achieved. This makes for good quality, smooth fluidization.
Figure 7 and Video 5 show bubbles in a Group A powder in a two-dimensional
fluidized bed.
Figure 7: An image from Video 5
[2.4 min to download at 28.8kbaud]:
Bubbles in a two-dimensional fluidized bed
of Group A powder. Through splitting and coalescence,
bubbles achieve a maximum stable size, effectively independent
of gas velocity or vessel size.
Video 6 [3.8 min to download at 28.8kbaud]:
shows the non-bubbling expansion
of a fluidized bed of Group A powder as the gas velocity is gradually
increased.
Video 7 [3.3 min to download at 28.8kbaud]:
shows the collapse of a fluidized bed of Group A powder. When the air supply
is interrupted there is a rapid drop in bed height as bubbles escape,
then a slow linear decrease as the aerated powder structure collapses.
In Groups B and D powders Umb= Umf ,
bubbles continue to grow in size, never achieving a maximum size
(see Figure 3 and Video 8). This makes for rather poor quality fluidization
since there are large pressure fluctuations.
Video 8 [5.6 min to download at 28.8kbaud]:
shows the continual increase
in size of bubbles in a two-dimensional fluidized bed of a Group B powder.
Bubbles increase in size with distance from the distributor
and with increasing gas velocity. Bubble size in limited only
by the vessel size.
In Group C powders the interparticle forces are large compared
with the inertial forces on the particles. As a result, the particles
are unable to achieve the separation they require to be totally
supported by drag and buoyancy forces and true fluidization does
not occur. Bubbles, as such, do not appear; instead the gas flow
forms channels through the powder (see Figure 8 and Video 9).
