Page 261 - Materials Chemistry, Second Edition

244 Practical Design Calculations for Groundwater and Soil Remediation

(c) Use Equation (6.20) to determine the air-to-water-ratio:

H *  Q a   = S = =3(0.098)  Q a  


 Q w   Q w 
So, (Q /Q ) = 30.75 (dimensionless)
a
w
(d) Determine the required air flow rate by multiplying the liquid
flow rate with the air-to-water ratio:
Q = Q × (Q /Q ) = (120 gpm)(30.75) = 3,690 gal/min = 493 ft /min
3
a
a
w
w
Discussion:
A stripping factor of three means that the ratio of the design Q /Q and
w
a
the minimum Q /Q is also three. Consequently, the design Q /Q
a
w
a
w
can be obtained by multiplying the minimum Q /Q with the strip-
a
w
ping factor.
6.4.2.1 Column Diameter
One of the key components in sizing an air stripper is to determine the diam-
eter of the column. The diameter depends mainly on the liquid flow rate.
The higher the liquid flow rate is, the larger the column diameter would be.
Typical liquid hydraulic loading rate to an air-stripping column is kept to
20 gpm/ft or less. This parameter is often used to determine the required
2
cross-sectional area of the stripping column (A stripping ):
Q
=
A stripping (6.21)
surfaceloading rate
6.4.2.2 Packing Height
The required depth of the packing column (Z) for a specific removal effi-
ciency is another important design component. A taller column would be
required to achieve a larger removal efficiency. The packing height can be
determined using the transfer unit concept:

Z = (HTU) × (NTU) (6.22)

where HTU is the height of transfer unit and NTU is the number of trans-
fer units.
The HTU value depends on the hydraulic loading rate and the overall liq-
uid-phase mass-transfer coefficient, K a. (Note: K is the rate constant [m/s]
L
L
and a is the specific surface area [m /m ]. K a has a unit of 1/time.) The K a
3
2
L
L
value for a specific application can be best determined from pilot testing,