2.2.1. Vapour/liquid equilibrium
The vapour/liquid equilibrium is defined by the equality of
the chemical potentials (μi) for component (i) in the two phases.
μ
Vapour
i
= μ
Liquid
i (4)
Substituting Eq. (2) into Eq. (4) yields the definition of the
equilibrium relation. The vapour/liquid equilibrium constant
corresponds to Henry’s law coefficient (KH) and is defined by
Eq. (5).
KH
i−W ( atm l/mol) = exp
−(μ
◦−Vapour
i
− μ
◦−Liquid
i )
RT
= fi
aif
◦
i
(5)
where subscript i–W indicates that the Henry’s law constant
(KH
i−W) is for solute (i) in solvent (water). Relevant Henry’s law
constants in anaerobic digesters are shown in Table 3.
Furthermore solvent water also exhibits a vapour/liquid equilibrium.
When the water activity (aW) is unity, the solvent phase