AIR STRIPPING The removal of volatile contaminants from water and contaminated soils

Air Stripping Tower Use

Air Stripping Towers

Some column internals Clockwise from top left: Packing, bubble caps, mist eliminator, sieve tray

Case Study: TCE Contaminated Site Remediation

Henry’s 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, C A = concentration of A, [mol/L] or [mg/L] K H =equilibrium constant (Henry's Law constant), [mol/L-atm] or [mg/L-atm] p A =partial pressure of A, [atm] See also

Example: Solubility of O 2 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 30  C 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 5  C and 40.2 mg/L-atm at 25  C. What is the solubility of oxygen at 5  C and at 25  C?

Solution O 2 Solubility Example At 5 0 C the solubility is: At 25 0 C the solubility is:

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 atm. Example 4.2 from Ray

Solution Air Stripping Example Henry's Law constant for carbon dioxide = 1.14L/L Divide by RT, i.e. 1.14/(0.083x288) = = mol L -1 atm -1 The equilibrium solubility is then: Answer = 0.9 mg/L CO 2

Two-film partitioning in gas-liquid ys*ys* ysys Cs*Cs* CsCs ys*ys* ysys Cs*Cs* CsCs ybyb CbCb ybyb CbCb Stripping Absorption Bulk gas Bulk liquid Bulk liquid Bulk gas

Mass transfer calculations Henry’s law applies because of equilibrium at interface: y s = HC s Fick’s law applies: flux is proportional to the driving force J A = k L (C b – Cs) – k g (y s – y b )

Relationship of transfer coefficients Alternatively all the resistance to transfer could be in the gas phase, so y s * = HC b Mass flux based on water phase, substitutions, and applying Henry’s law to liquid concentrations leads to 1/K L a = 1/k L a + 1/Hk g a Similarly 1/K G a = H/k L a + 1/k g a Another approach is that all the resistance to transfer is in the liquid phase, so y b = HC* s