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3.8 T Fluid–Solid Fluidized Bed Reactors o-Phase, w 201

The use of internals can improe the smoothness of fluidization, when this is desirable. v
The use of vertical cylindrical tubes immersed in the bed is a representatie e v xample.
f
These tubes are suficiently long to extend from the distributor to the top of the bed. F or
this type of beds and for perforated plate distributors, the following correlation can be used
for the estimation of fluidized-bed porosity in gas–solid fluidization (Hilal, 2000):

u fm  f  w (3.479)
u
s
fm
u u   1  
ter fm fm
where:
w 1.28 0.00033 P 3 (3.480)

where u ter is the terminal velocity of the particles and P is the hole pitch of the perforated
plate. A square pitch of 7–12 mm is typical.

e Criteria for distinguishing fluidization r gimes

,
The minimum bubbling velocity for Group A particles (or more generally for TA ype
fluidization) and gas–solid systems is (Abrahamsen and Geldart, 1980; e Y et al ., 2005)
2.07 0.06 G d exp(0.716 p ) X 4 5
u (3.481)
bm 0.347

G
where X 45 is the fraction of particles with size smaller than 45 µm and d p the siee diam- v
eter. In this equation, SI units should be used. In the region of Group B particles (for ambi-
ent conditions and air as gas phase) or in the Type B fluidization regime (for other
conditions), where u bm = u fm , the aboe equation results in v u bm < u fm , which is not accept-
xample,
able. For e for ambient conditions and air as gas, if d p = 0.24 mm and = 1500
h
kg/m 3 , we are in the re (Figure 3.52) and the result is gion of Group B particles u bm = 0.022
m/s, while u fm = 0.028 m/s.
Using the Bayens equation (3.460) and eq. (3.481), the ratio of minimum b ubbling
velocity to minimum fluidization velocity is
u 2300 0.126 exp(0.716
0.523 ) X
bm G G 45 (3.482)
u fm d 0.8 p ( p G ) 0.934 0. g 9 934
u b m u b m
For type B fluidization, we obtain ( = 1), while for type A fluidization ( 1). In
u u
fm fm
the second case, the fluidization regime is
• particulate, if u fm u s u bm
• type A bubbling, if u s u bm

In Figure 3.54, the ratio of the minimum b elocity to the minimum fluidization ubbling v
velocity is plotted for air at 20 °C, and is 1000–3000 kg/m 3 . From Figure 3.52 and the
p