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3.8 T Fluid–Solid Fluidized Bed Reactors w o-Phase, 191

McCabe et al ., 1983). Hov er , v we en though v e ery small particles frequently act as if damp,
dry, forming agglomerates or fissures in the bed, or spouting (Perry and Green, 1999).
According to Gunn (1968), the grinding of solids to very small particle sizes is e e, v xpensi
and as a result in many cases particles of sizes greater than 0.5mm are used, as in the
v
case of combustion of coal, or een as large as 6cm in other applications (Perry
v
er
and Green, 1999). Ho large particles cause instability and result in slugging or
we
,
massive surges.
,
For velocities higher than the minimum fluidization v the appearance of flu-
elocity
idized beds is often quite different (McCabe, 1983). In most liquid systems, as the v eloc-
ity is increased, the motion of the particles becomes more vigorous, whereas the bed
density at a given velocity is the same in all sections of the bed. This is called “particulate
fluidization” (or smooth fluidization/nonbubbling fluidization/homogeneous fluidization,
Figure 3.51) and its characteristic is the large but uniform expansion of the bed at high
velocities. This type of fluidization appears when the fluid and the solids have similar den-
sities. In contrast, the density difery high when the fluid is a gas, which results ference is v
ati
in the so-called bubbling fluidization (or aggree fluidization or heterogeneous flu-
v
g
v idization)—the gas moes through the reactor either forming “b that contain rela- ubbles”
tively few solid particles, or as a continuous “dense” phase where the particle
concentration is high (particulate or emulsion phase).
Although, in general, liquids are associated with particulate fluidization and gases with
w
bubbling fluidization, it is not alays the case. The density difference is the decisi e v
parameter and thus bubbling fluidization appears in water systems of heavy solids, and
particulate fluidization in high-pressure gas systems of fine particles (McCabe, 1983).
However, a gas is usually the fluid in fluidized beds and the bubbling regime pre ails v
(Smith, 1981). Industrial reactors, particularly for solid-catalyzed gas-phase reactions,
often operate in that re with typical values of gas velocities in the range 5–30 u fm or
gime,
even 250 u fm , where u fm is the minimum fluidization velocity (Le 1972) enspiel, v .
Another type of fluidization is the slugging fluidization. It represents the case where the
bubbles form slugs of gas, usually when the size of the bubbles is about one-third the diam-
eter of the bed. In general, slugging is undesirable because it is accompanied by high pres-
sure, which may cause dangerous vibrations to the reactor .
Finally, it should be noted that in the case of multisized solids, the operating v elocity
should be higher than the minimum fluidization velocity of the largest particle and smaller
that the elutriation velocity of the smallest particles.
Geldart (1973) classifwders into four groups according to their fluidization prop- ied po
erties by air at ambient conditions. This classification is now used widely in all fields of
powder technology.

v
• Group A powder s: They gie a region of nonbubbling fluidization beginning at the
minimum fluidization velocity ( u fm ), followed by bubbling fluidization as fluidizing
velocity increases. This velocity limit is called minimum bubbling velocity ( u bm > u fm ).
These materials have small mean particle size ( d p < 30 µm) and/or low particle density
(<~1.4 g/cm 3 ). Fluid-cracking catalysts are typically in this cate . gory
• Group B powders: They give only bubbling fluidization. Bubbles are formed as soon as
elocity e the gas vxceeds the minimum fluidization v elocity ( u bm = u fm ). Most particles