Environmental Engineering Reference
In-Depth Information
Operating line
I
I
I
Initial
loading
Equilibrium
Clean purge
Actual
F
Equilibrium curve
Actual
Equilibrium
Fluid concentration
Y
Distance
z
Time
t
G
qY
b
Design Equation
:
V
= z
/
t
=
ρ ∆∆
/
b
ρ ∆∆
⋅
qY
G
/
Desorption Time:
t
dt
=
b
L
b
Minimum Purge Requirement:
m
=
G
∆∆kg purge/kg bed
/
qY
Figure 7.10
Type III isotherm: desorption [10].
7.10
Design of fixed-bed adsorption columns
Two macroscopic methods to design adsorption columns are the scale-up and kinetic
approaches. Both methods rely on breakthrough data obtained from pilot columns. The
scale-up method is very simple, but the kinetic method takes into account the rate of
adsorption (determined by the kinetics of surface diffusion to the inside of the adsorbent
pore). The scale-up approach is useful for determining the breakthrough time and volume
(time elapsed and volume treated before the maximum allowable effluent concentration
is achieved) of an existing column, while the kinetic approach will determine the size
requirements of a column based on a known breakthrough volume.
7.10.1 Scale-up approach
Initially, a pilot column with a bed volume (
V
p
) and volumetric flowrate of fluid (
Q
p
)
is used. As shown in Figure 7.11, the total volume (
V
pilo
T
) of fluid that passes through
the column is measured until the outlet solute concentration is observed to rise to the
maximum allowable value (
C
a
).
The (plant-scale) design column should operate such that:
1 Fluid residence time in the pilot and design column are the same.
2 The total volume of fluid processed until breakthrough per mass of sorbent in the column
is the same for both columns.
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