Page 43 - Distillation theory

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1.9 Conclusion 17

2 3

Figure 1.15. Liquid–liquid phase diagram for six-component mix-
1 4 ture (points indicate the compositions of equilibrium liquid phases
of binary constituents).

6 5

We can see that the phase diagrams differ by the number of one- and two-
phase liquid regions Reg . In Fig. 1.13e, a system with a three-phase liquid
L−L
region Reg is shown among others.
L−L−L
The most common types of phase diagrams are those represented in Figs. 1.13a
and b with one two-phase region. In Figs. 1.14a and b, the examples of liquid–
liquid phase diagrams for four-component mixtures with one two-phase region
are shown.
For a clear representation of liquid–liquid equilibrium in multicomponent sys-
tems with one two-phase liquid region Reg , it is possible to use the graph in
L−L
Fig. 1.15. From Fig. 1.15, it is clear that component 1 is a heteroforming agent (in
practice, it is water that plays this role in most cases). Components 1–2, 1–3, and
1–4 form two liquid phases. The rest of the components do not form liquid phases
between each other. In such a way, the description of liquid–liquid phase diagrams
for multicomponent mixtures with one two-phase region Reg L−L is rather simple.

1.9. Conclusion

The variety of possible compositions of a multicomponent mixture can be repre-
sented as a multidimensional simplex. Liquid–vapor phase equilibrium is a factor
determining the distillation process results. Depending on the nonideality degree,
the following types of mixtures – differing in their behavior during the distillation
process – can be distinguished: ideal, nonideal zeotropic, mixtures with a tangent
azeotrope, azeotropic mixtures, and heteroazeotropic mixtures. Residue curves
and their bundles characterize phase behavior of mixtures in each point of the
concentration simplex. Residue curve bundles define the possible cases of mix-
ture separation in one of the distillation-limiting modes, in particular, the infinite
reflux mode. Residue curve bundles split the concentration simplex into distilla-
∞
tion regions Reg , separated by the lines, surfaces, or hypersurfaces.
Bondsbetweenthestationarypoints(pointsofthecomponentsandazeotropes)
and distillation subregions Reg sub are the structural elements of the distillation
regions. Residue curve bundle structure of multicomponent mixtures can be de-
scribed with the help of a structural matrix that reflects the bonds available be-
tween the stationary points.
Lines, surfaces, and hypersurfaces of equal phase equilibrium coefficients of
two components split the concentration simplex into regions of identical order of
ijk
components Reg (K i > K j > K k ) that define the possible causes of separation
ord
under the finite reflux mode.