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194 3. Heterogeneous Processes and Reactor Analysis

Pressure drop in fluidized beds
For a bed of particles of hydraulic density (see Sections 3.9.6 and 3.9.8) fluidized by a
h
fluid of density to form a bed of depth Z and v oidage ε in a v , essel of crosssectional area
A , the pressure drop is (Richardson and Zaki, 1954)
p
) g (3.448)
)(
(1
Z h
The fluid pressure drop across the bed versus superficial fluid velocity through the bed
would appear as shown in Figure 3.53.
Figure 3.53 corresponds to an upflow operation, where the fluidized-bed pressure drop
ix remains constant after the minimum fluidization velocity. On the contrary, if a fed bed is
operated in downflow mode, the pressure drop continues to increase by increasing the fluid
velocity (dense line). This is the reason that fluidized beds may exhibit a lower pressure
, drop and thus the power cost is lo for high fluid v wer elocities.
The region BC is the fluidized-bed region where eq. (3.448) applies. The straight-line
ix
gion,
region OA is the fed-bed re where the particles do not moe relatie to one other v v .
The pressure drop in this region is described by the Ergun equation (Perry and Green,
1999):
150
u (1 ) 2 1.75 u 2 (1 ) p
s s (3.449)
s 2 d p 2 3 sp d 3 Z

or
 150 (1
G ) 1.75 2 (1 ) 1 G 2  p
  (3.450)
  s 2 d p 2 3 sp d 3   Z

Dense packed
ed First time fluidiz
Fixed bed
(downflow)
A

B C

Loose packed
0 u fm u s

Figure 3.53 The fluid pressure drop versus its velocity for packed and fluidized beds.