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138 3. Heterogeneous Processes and Reactor Analysis

3.5.3 External mass transfer

Mass transfer coeficients: liquid to particle (k f )
f
Acording to Fishwick et al . (2003), the injection of gas in a baffled vessel leads to a
f
decrease in the mass transfer coeficients and this effect becomes more intense at higher
er
,
gas rates. The significance of gas dispersion is, ho less pronounced at higher
we
v
agitation speeds. It is also observed that under high agitation speeds in baffled v essels,
v
a considerable amount of air is dispersed inside the vessel een in the absence of an
injected gas.
The proposed correlation for agitated slurry reactors is that of Sano et al . (1974;
Ramachandran and Chaudhari, 1984; K 1996): oide,
3
kd  d 4  0.25 
 13
fp p L L (3.266)
   
2 0.4
D F 
3   D 
fs L L f
where F is a shape factor and can be taken as unity for spherical particles. CGS units
s
should be used with this equation.
The Hiraoka correlation proposed for liquid–liquid dispersion in an impeller mixing
vessel (Kato et al., 2001) is
kd  dP 4  0.193
fp p s 13 (3.267)
0.45  3  Sc
D  
f LL
The observed values of the mass transfer coefficient- in three-phase systems between solid
and liquid for the conventional impellers and a typical baffled vessel (e.g. Rushton turbine,
propeller) are between the values predicted by Hiraoka (liquid–liquid dispersion,
eq. (3.267)) and Levins and Glastonbuty (solid–liquid dispersion, eq. (3.118)) correlations.
However, as an approximation, the Levins and Glastonbuty correlation could be used for
three-phase systems (Smith, 1981).

f Mass transfer coeficients: gas bubble to liquid (k )
fg
Normally, ethe major mass transfer resistance ubble phase is a gas mixture, en when the b v
for slightly soluble gases is in the liquid. Thus, the mass transfer coeficient in the liquid f
film around the bubble is the important one in gas bubble-to-liquid mass transfer (Smith,
1981; Treybal, 1980).
The mass transfer coeficient in the liquid film around the gas bubble can be estimated f
W ills, as follows (Prasher and 1973; Smith, 1981):
  0.25 (3.268)
k 0.592 D 0.5
fg fg    
L
fusi
where D is the molecular difvity of solute gas in the liquid and the kinematic
fg L
viscosity of the liquid. A higher gas rate results in a lower liquid side mass transfer