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216 3. Heterogeneous Processes and Reactor Analysis

The Chung–Wen correlation can be used for fed and fluidized beds for 10 ix —3 < Re p <
10 3 W (Chung and en, 1968).
Pe
f p 0.2 0.011 Re 0.48 (3.513)
p

where
Re p,f m
(3.514)
Re p

In this equation, Re p,fm is the particle Reynolds number based on the minimum superf icial
velocity for fluidization. Moreo, for f we can set ed-beds, ix = 1 and = . The corre-
v
er
f
lation is applicable for void fractions between 0.4 and 0.8 with particle density up to 480
lb/ft 3 . Note that by changing the Re p number, the fluidized bed v oidage is changed.
f
According to Gunn (1968), the radial Peclet number in particulate fluidization (liq-
uid–solid systems) ranges between 1 and 10 for values of Re p in the range 4–1000.
Furthermore, the maximum mixing coeficient is found for f = 0.7. Finally the lateral
,
f
f (radial) mixing coeficients in gas–solid fluidized beds decrease constantly (for Re p > 10)
from about 10 – 0.05 by increasing the expansion ratio from 0.01 – 0.2.
Gas distribution in fluidized beds
The gas distributor has a considerable effect on the proper operation of the fluidized bed.
Basically, there are two types (Perry and Green, 1999): (a) for use when the inlet gas con-
tains solids and (b) for use when the inlet gas is free of solids. In the latter case, the dis-
tributor is designed to prevent the backflow of solids during normal operation, and in many
ution,
cases, it is designed to preent backflow during shutdown. To provide distrib it is
v
necessary to restrict the gas or the gas and solids flow so that the pressure drops across the
restriction amount of 0.5 to 20 kP a.
To fulfill the pressure requirements, a high velocity through the grid openings may be
er , needed. Ho values of velocity hae to be below 70 m/s to aoid attrition of the v
v
we
v
solids. In industrial applications, a shroud of pipe is often installed oer the opening, pro- v
viding velocities as high as necessary for the pressure requirements, and at the same time
enabling reduced velocities for entry to the bed, thus aoiding attrition. The technique is v
applied to both plate and pipe spargers. Pressure drop through a pipe or a drilled plate is
given by
u 2
P fo (3.515)
1.28 g

where:
u the velocity in the hole at inlet conditions
o
the fluid density in the hole at inlet conditions
f
P the pressure drop in consistent units.