CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 205
7.7 Liquid-gas equilibrium in mixtures
This problem concerns the solubility of gases in liquids in dependence of temperature, pressure,
and the liquid composition.
7.7.1 Basic concepts.
- Liquid-gas equilibrium
Let us have a multicomponent system formed by two coexisting phases, liquid and
gaseous. If the system temperature is higher than the critical temperature of any of
its components, we have to use the standard state of a pure hypothetical substance
at an infinite dilution for this component [see 6.5.3]. In such cases we speak about
liquid-gasequilibrium.
Note: If the system temperature is lower than the critical temperatures of all its com-
ponents, we may calculate phase equilibrium either as liquid-vapour equilibrium or as
liquid-gas equilibrium.- Gas solubility
Gas solubility is understood as an equilibrium concentration of the gaseous component
in the liquid phase of a given composition at a given temperature and pressure.
7.7.2 Henry’s law for a binary system
The relation
f 2 =KHx 2 , (7.40)
wheref 2 is the fugacity of component 2 in the gaseous phase andx 2 is its molar fraction in
the liquid, is called Henry’s law, and the constantKH=f(T, p) is calledHenry’s constant.
Component 2 is that which prevails in the gas, component 1 is that which prevails in the liquid.
- At larger deviations from Henry’s law, relation (7.40) is made more accurate by way of
introducing the activity coefficient
f 2 =KHx 2 γ 2 [x], (7.41)whereγ 2 [x]defined by relation (6.100) is a function of composition.