CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 213
Note: In the case of a heterogeneous azeotrope, the composition of the vapour phase is
not identical with the composition of the equilibrium liquid phases.
- Ideal vapour phase, both components totally immiscible in the liquid phase
The overall pressure above a heterogeneous mixture is
p=ps 1 +ps 2 (7.56)
and the composition of the vapour phase is
y 1 =
ps 1
ps 1 +ps 2
. (7.57)
Note:The consequence of relation (7.56) is that a heterogeneous mixture of two very little
miscible substances boils at a temperature which is lower than the boiling point of any of
its components.
7.8.4 Three-component system containing two liquid phases.
Figure7.10shows a binodal line EC 1 F and a tie line EDF. The system of a global composition
given by point D split into two liquid phases whose composition is given [see7.4.4] by points
E and F on the binodal line. The amount of substance ratio of both liquid phases is given by
the lever rule [see7.4.5]. The compositions of both coexisting phases merge at point C 1 (the
critical point).
- Thedistribution coefficientis defined as the ratio of the molar fractions or concen-
trations of theithcomponent in one and the other liquid phases.
Kx,i=
x(i`^2 )
x(i`^1 )
or Kc,i=
c(i`^2 )
c(i`^1 )
, i= 1, 2 , 3. (7.58)
From (7.55) we also have
Kx,i=
γi(`^1 )
γi(`^2 )