Page 259 - Materials Chemistry, Second Edition

242 Practical Design Calculations for Groundwater and Soil Remediation

where
C = COC concentration in the liquid phase (mg/L)
G = COC concentration in the air phase (mg/L)
Q = air flow rate (L/min)
a
Q = liquid flow rate (L/min)
w
For an ideal case where the influent air contains no COCs (G = 0) and the
in
groundwater is completely decontaminated (C = 0), Equation (6.16) can be
out
simplified as:
QC( ) = QG( ) (6.17)
w in a out
Assume that Henry’s law applies and the effluent air is in equilibrium with
the influent water; then:

* (6.18)
G out = H C in
where H* is the Henry’s constant of the COC in a dimensionless form.
Combining Equations (6.17) and (6.18), the following relationship can
be developed:

H *  Q a   = 1 (6.19)

 Q w  min
The (Q /Q ) is the minimum air-to-water ratio (in vol/vol), and this is
a
w min
the air-to-water ratio for the previously mentioned ideal case. The actual
air-to-water ratio is often chosen to be a few times larger than the minimum
air-to-water ratio.
The stripping factor (S), which is the product of the dimensionless Henry’s
constant and the air-to-water ratio, is commonly used in design:

S = H *  Q a   (6.20)

 Q w 
The stripping factor is equal to unity for the previously mentioned ideal case.
It would require a packing height of infinity to achieve the perfect removal.
For field applications, the values of S should be greater than 1. Practical values
of S range from 2 to 10. Operating the system with a value of S larger than 10
may not be economical. In addition, a high air-to-water ratio may cause an
unfavorable phenomenon, called flooding, in air-stripping operations.
The following procedure can be used to determine the air flow rate for a
given liquid flow rate:

Step 1: Convert the Henry’s constant to its dimensionless value using
the formula given in Table 2.4.