Page 75 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

2.3 Gas–Liquid Interfacial Behavior 49

Pa and x i in mol/mol, for example, the corresponding unit of H is [Pa/(mol in liquid/
mol of liquid)]. Common units of pressure are Pascal (Pa), atm, bar, and mmHg;
3
those of solute concentration can be g/g, mol/mol, mol/m , mol/L, and so on. Users
have to excuse unit conversion as needed.
In many engineering design practices, mole fraction of the air pollutant in the gas
phase (y i ) is used to quantify the concentration of a gas in a gas mixture; it is a
direct description of mass transfer between the two phases. In this case, the partial
pressure of the target gas is calculated using Eq. (2.41), P i ¼ y i P and the Henry’s
law equation becomes
Py i ¼ Hx i ð2:79Þ

A comprehensive compilation of Henry’ law constants was made by Sander
[15]. Based on the data therein, Henry’s law constants for some typical gases in
water at 25 °C or 298.15 K are summarized in Table 2.3.
Example 2.7: Gas solubility
Estimate the solubility of CO 2 in water in gram of CO 2 per kilogram of water under
standard condition (25 °C, 1 atm).
Solution
The Henry’s law constant for CO 2 in water at 20 °C and 1 atm is 29.41 atm/(mol of
gas per kg of water) (Table 2.3). Because air is an ideal gas and CO 2 takes
0.0314 % of the volume in air, the partial pressure of CO 2 is 0.0314 % of the
standard atmospheric pressure:

¼ 0:0314 % 1 atm ¼ 0:000314 atm
P CO 2
values into Eq. (2.78) gives
Substituting H and P CO 2
0:000314 atm
P CO 2
¼ ¼
x CO 2
H atm
29:41
mol of CO 2 per kg of water
¼ 1:068 10 5 mol of CO 2 =kg of water
The next step is to convert the unit into gram of CO 2 per g of water with the
molar weight of CO 2 being 44 g/mole.
44g
5
1:068 10 mol CO 2 gof CO
mol 4 2
¼ ¼ 4:698 10
x CO 2
kg of water kg of water
Liquid temperature affects the solubility of a gas and the consequent value of
Henry’s law constant. Usually, the higher temperature, the lower solubility, and the
greater Henry’s law constant. The Henry’s law constants of some typical air pol-
lutants at different temperatures are listed in Table 2.4. More solubility data can be
found in Perry’s Chemical Engineers’ Handbooks or similar publications.