Factors affecting removal of compounds by aeration
Calculation of solubility of gases
Henrys’ Law is defined as:
Where pA is the partial pressure of any compound A in air (atm), H is the Henrys' constant which depends upon temperature and xA is the mol fraction of compound A in water. By definition
Since moles of oxygen in liquid solution are usually very less as compared to moles of water in liquid solution, therefore,
xA = ( ( C oncentration of compound A (in g/L)x 18) / ( M olecular W eight of compound A (in g/mol) x 10 3 ) )
xA = ( ( CA (in g/L) 18) ( MW (in g/mol) 103))
Putting in earlier equation
Where, CA is the solubility of compound A in water.
Variation of solubility of gases with temperature
Solubility of gases decreases with an increase in temperature. The change in Henrys’ constant with temperature can be computed using van’t Hoff type of equation:
Where, -ΔH is the Heat of absorption in kcal/kmol, R is the gas constant (=1.987 kcal/K-kmol), T is temperature in K and b is a dimensionless empirical constant.
Problem 3.3.1: The particle pressure of O2 in atmosphere is 0.21 atm. Find the concentration of O2 in water (in mg O2/ litre of water) at 20 oC & 5 oC. Given that for oxygen, Henry’s constant (H) is equal to 4.3×104 atm at 20 oC, ΔH=1.45 103 kcal/kmol, and b=7.11.
Solution: Given that: pA= 0.21 atm, H = 4.3×104 atm at 20 oC.
= 8.682 mg O2/ litre of water at 20 oC.
Now calculating Henrys’ constant at 5 oC
= 12.17 mg O2/ litre of water at 5 oC.