Air stripping is a process used to remove volatile compounds from the liquid pha

Question

Air stripping is a process used to remove volatile compounds from the liquid phase. Consider a groundwater with 25 mg/L CO2 and 3.5 mg/L O2. An engineer designs a spray tower operating at 25oC into which 200 gpm of the groundwater is injected with spray nozzles at the top of the tower and 100 cfm of air is blown into the bottom of the tower. The tower is countercurrent since the water droplets fall down through the tower and the air rises up and exits near the top. Assume that the gas in the water equilibrates with the air and that the KH for O2 and CO2 to be 770 L*atm/mol and 29 L*atm/mol, respectively.

a) What is the final concentration of CO2 in the water?

b) What is the final concentration of O2 in the water?

c) What is the percent removal of CO2?

Explanation / Answer

(a): Given Henry's constant for CO2, K = 29 L*atm/mol

Partial pressure of CO2 in air, PCO2 = 3.95x10-4 atm

Applying Henry's Law

Caq = PCO2 / K = 3.95x10-4 atm /  29 L*atm/mol = 1.361x10-5 mol CO2/L = 1.361x10-5 mol x 44.0 g/mol/L

= 6.0x10-4 g/L = 0.6 mg/L (answer)

(b):

Given Henry's constant, K = 770 L*atm/mol

Partial pressure of O2 in air, PO2 = 0.209 atm

Applying Henry's Law

Caq = PO2 / K = 0.209 atm / 770 L*atm/mol = 2.7143x10-4 mol O2/L = 2.7143x10-4 mol x 32.0 g/mol/L

= 8.70x10-3 g/L = 8.7 mg/L (answer)

(c): initial concentration of CO2 = 25 mg/L

Final concentration of CO2 = 0.6 mg/L

Hence percent removal = [(25.0 - 0.6) / 25.0] x 100 = 97.6 % (answer)

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