AIR STRIPPING

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AIR STRIPPING

• The removal of volatile contaminants from water
  and contaminated soils

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Air Stripping Tower Use
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Air Stripping Towers
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Some column internals
Clockwise from top left Packing, bubble caps,
mist eliminator, sieve tray
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Case Study TCE Contaminated Site Remediation
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Henrys Law
Henry's Law states that the amount of a gas that
dissolves into a liquid is proportional to the
partial pressure that gas exerts on the surface
of the liquid. In equation form, that is
where, CA concentration of A, mol/L or
mg/L KH equilibrium constant (Henry's
Law constant), mol/L-atm or mg/L-atm pA
partial pressure of A, atm
See also http//en.wikipedia.org/wiki/Henry's_law
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Example Solubility of O2 in Water
Although the atmosphere we breathe is comprised
of approximately 20.9 percent oxygen, oxygen is
only slightly soluble in water. In addition, the
solubility decreases as the temperature
increases. Thus, oxygen availability to aquatic
life decreases during the summer months when the
biological processes which consume oxygen are
most active. Summer water temperatures of 25 to
30C are typical for many surface waters in the
U.S. Henry's Law constant for oxygen in water is
61.2 mg/L-atm at 5C and 40.2 mg/L-atm at 25C.
What is the solubility of oxygen at 5C and at
25C?
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Solution O2 Solubility Example
At 50C the solubility is
At 250C the solubility is
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Air Stripping Example
An air stripping tower, similar to that shown, is
to be used to remove dissolved carbon dioxide gas
from a groundwater supply. If the tower
lowers the level to twice the equilibrium
concentration, what amount of dissolved gas will
remain in the water after treatment? The partial
pressure of carbon dioxide in the atmosphere is
10-3.7 atm.
Example 4.2 from Ray
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Solution Air Stripping Example
Henry's Law constant for carbon dioxide 1.14L/L
Divide by RT, i.e. 1.14/(0.083x288) 0.048
10-1.3 mol L-1atm-1 The equilibrium solubility
is then
Answer 0.9 mg/L CO2
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Two-film partitioning in gas-liquid
ys
Bulk liquid
Cs
Bulk liquid
Bulk gas
ys
Cb
Cs
Cb
yb
yb
ys
Cs
Bulk gas
ys
Cs
Absorption
Stripping
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Mass transfer calculations
Henrys law applies because of equilibrium at
interface ys HCs
Ficks law applies flux is proportional to the
driving force JA kL(Cb Cs) kg(ys yb)
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Relationship of transfer coefficients
Another approach is that all the resistance to
transfer is in the liquid phase, so yb
HCs
Alternatively all the resistance to transfer
could be in the gas phase, so ys HCb
Mass flux based on water phase, substitutions,
and applying Henrys law to liquid concentrations
leads to 1/KLa 1/kLa
1/Hkga Similarly 1/KGa H/kLa 1/kga