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3.6 T ed Beds o-Phase Fix w 139

v
coefficient. This correlation has been deried for gas–liquid systems. Ho it can be v er , we
used for agitated slurry reactors as well (Smith, 1981). CGS units are used with this
correlation.
Another equation is the Yaki–Yoshida correlation, which is proposed for agitated slurry
reactors (Ramachandran and Chaudhari, 1984):

2 1.5 0.19
ka D  2  D N  2 
fg Gtot , L a DN a L a
0.06    
D 
  g 
T L
 
L  0.5 
u  0.6  ND a  0.32 (3.269)
LsG
 D     
    
T L L  u sG
where N is the impeller rotational speed (r/s). CGS units should be used with this equation.
The CGS unit of surface tension is mN/m (dyn/cm).
Finally, for double-impeller combinations and gas–liquid systems, the Bouaif i et al .
(2001) correlation can be used:

P  g  0.5
ka 0.0218 u 0.6 (3.270)
fg GL  V  L   sG

where a GL is based on the volume of liquid and u sG 0.018 m/s. SI units are used with
this correlation.
The effect of surface aeration on k fg
where, As has been noted else the aeration of liquid is generally not desirable. This is why
the gas phase that is employed for the reaction is mixed with the air dissolved due to sur-
face aeration, and thus the mass-transfer coeficient decreases due to the reduction of the f
partial pressure of the reacting gas in this gas–air mixture. Calderbank proposed an equa-
tion for the minimum stirring rate for surface aeration for gas–liquid systems (Panja and
Phaneswara Rao, 1993):
ND a a 1.98 1.34 
L g  0.15 (3.271)
D T 1.1    
L

Thus, aeration tak es place for N N a (in s 1 ). SI units should be used with this correlation.

3.6 TWO-PHASE FIXED BEDS

3.6.1 Introduction

xc Adsorption and ion e hang e
Fixed bed is the most frequently used operation for ion-exchange and adsorption
processes. The adsorption or ion-exchange medium is contained inside a steel v essel,