Page 202 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH003.qxd 7/13/2006 1:46 PM Page 198
198 3. Heterogeneous Processes and Reactor Analysis

be too small. If a value below about 0.4 is predicted, it should be considered suspicious.
Values around 0.5 are typical.
By using the experimental data of Wen and Yu (1996), it can be shown that the proposed
alues of correlation predicts the lo wer v well, while it fails to predict higher v alues.
fm
fm
Furthermore, in a number of published e xperimental data for gas–solid fluidized beds pre-
sented by Broadhurst and Becker (1975), fm is considerably influenced by particle size
and it is higher for lower particle sizes. The We been tested for 0.05 u equations ha v en–Y
d p 50 mm, resulting in an athe de , v er viation of 34%. Ho we v erage standard de viation
is 21% for d p 0.5 mm and 38% for d 0.5 mm. It seems that the ef fect of particle size
p
is the reason for the higher deviation of the u equations. Thus, it can be stated that en–Y W
these equations could be used safely for particle sizes greater than 0.5 mm. Broadhurst and
Becker (1975) proposed the following correlation for gas–solid fluidized beds, which takes
into account the effect of particle size on :
fm

2  0.029   0.02 1 1
0.586 0.72   G (3.466)
fm S 3  
  dg ( p )    
p
Gp
G
ed for v This equation has been deri
0.5 1
S

500 p 50000

G

2  1
1   10 5
  dg 3 ( p G )  
Gp
Eq. (3.466) has correlated several experimental data giving a maximum deviation of 0.04, for
low particle sizes ( 0.4 mm diameter). This correlation is v w alid only in the creeping flo
alue of poros-
regime, i.e. Re 10, for relati The minimum predicted v ely small particles. v
fm
ity is 0.37. Thus, the lo alues predicted by the Broadhurst–Becker equation should not wer v
be used. This is the case of large particles, where the Wen–Yu correlations should be used.
Using the aboe equation for air at 20 °C and hydraulic density within 1500–4000
v
kg/m 3 , the porosity at minimum fluidization is
• within the range 0.4–0.5 for particle size of 0.03–0.25 mm and 1
S
• within the range 0.56–0.71 for particle size of 0.03–0.25 mm and 0.6
S
W An equation similar to that of en and Yu that has been proposed for the liquid–solid flu-
idization is that of Limas–Ballesteros (Lee et al. , 2005):

0.42

fm 0.376 (3.467)

S