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48 2 Basic Properties of Gases

The drag coefficients are geometry dependent and mostly determined experi-
mentally. The drag coefficients of spheres at different Reynolds number are shown
Fig. 2.4. More complicated cases can be found in most fluid dynamics books.

2.3 Gas–Liquid Interfacial Behavior

2.3.1 Solubility and Henry’s Law

According to International Union of Pure and Applied Chemistry (IUPAC), solu-
bility is the analytical composition of a saturated solution expressed as a proportion
of a designated solute in a designated solvent. The most widely used solvent is
liquid, which can be a pure substance or a mixture. The extent of the solubility of a
substance in a specific solvent is measured as the saturation concentration, where
adding more solute does not increase the concentration of the solution and begin to
precipitate the excess amount of solute. Solubility may be stated in units of con-
3
centration ðC i ; kg/m Þ, mol fraction (x i , mol/mol) and other units. The solubility of
a substance depends on many physical and chemical properties of the solute and the
solvent such as temperature, pressure, and the pH.
Consider a process shown in Fig. 2.5, where gas i is mixed with another
insoluble gas, and gas i is soluble. The gas molecules will enter the liquid phase and
become part of the liquid mixture. Given enough time, the system reaches equi-
librium state.
The Henry’s law governs the equilibrium state,

P i ¼ Hx i ð2:78Þ
where P i ¼ partial pressure of gas i in the gas phase above the liquid, x i ¼ the
equilibrium concentration of gas i in the liquid phase, and H ¼ Henry’s law con-
stant with a unit that is determined by that of P i =x i . When the partial pressure is in

Fig. 2.5 Absorption of gas
i into liquid and Henry’s Law
Solute
Gas i with a partial pressure
of p i

x i Solvent