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4.2 Design of Adsorption and Ion-Exchange Processes 343

we hae v
15 D
s
Q rel 2 Q rel 6.66 BV/h
1.2311 r
o
As will be analyzed in the following section, a minimum relative flow rate arises for down-
flow operation.
Hydraulic analysis and controlling mechanism : F or the determination of the mass trans-
aste
w
icient, fer coef and the case of water treatment, the correlation proposed by Chern
f
and Chien (eq. (3.345)) is used. As a criterion, the Biot number as defined by Hand et al .
(eq. (4.105)) is used.
For the ealuation of the particle Peclet number and the liquid holdup, the correlations
v
proposed by Inglezakis et al. are used, i.e. eqs. (3.313) and (3.332), respecti . The Biot ely v
number, liquid holdup, and bed Peclet number for do v e v ol- ersus relati wnflo w operation v
umetric flow rate are presented in Figure 4.35.
It is obvious that for the whole flow-rate range, the rate-controlling mechanism is
expected to be the solid diffusion control ( Bi > 41). Furthermore, the flow can be charac-
terized as ideal plug flow for flow rates aboe 2.15 BV/h, where v Pe L is higher than about
100. Hover, the liquid holdup is very low (56.83%) and this could be proed a serious v
we
problem, een with the use of a liquid distributor at the top of the bed. In order to hae a v
v
satisfying liquid holdup, i.e about 80%, the relatiw rate should be about 5.62 BV/h. e flo v
Then, by means of a liquid distributor at the top of the bed, it is possible to achieve a holdup
near 100%. Thus, for do w rate are (BV/h) v w operation the limits of the relati e flo wnflo

5.62 Q 6.67
rel

On the other hand, in upflo w operation, Pe L is higher than 140 for the whole relative flow-
rate region up to the value of 6.66 BV/h. At the same time, the liquid holdup is al w ays

140
120
100
80

60 Bi
h%
40
Pe L
20
0 1 2 3 4 5 6 7
Q(BV/h)
Figure 4.35 Biot number, liquid holdup, and bed Peclet number for do ersus rel- w operation v wnflo
ative volumetric flow rate.