Physical Chemistry , 1st ed.

are limiting our systems to two components, these azeotropes are all binary
azeotropes, but in systems that have more than two components, there are also
ternary azeotropes, quaternary azeotropes, and so forth. Almost all real systems
have azeotropes in their liquid-vapor phase diagrams, and there is always only
one unique composition for an azeotrope for any set of components.
Fractional distillation for a system that has an azeotrope is similar to the
process illustrated in Figure 7.11. However, as the tie lines move from one
composition to another, eventually either a pure component is reached, or an
azeotrope is reached. If an azeotrope is reached, then there will be no further
change in the composition of the vapor,and no further separationof the two
components will take place by means of distillation. (There are other ways to
separate the components of an azeotrope, but not by straightforward distilla-
tion. Such is the conclusion of thermodynamics.)

Example 7.7

Using a temperature-composition phase diagram like Figure 7.14, predict the

general composition of the ultimate distillation product if a solution having

a mole fraction x 1 of 0.9 is distilled.

Solution

Refer to Figure 7.16. Using the tie lines to connect the vapor composition for

each liquid phase composition, we ultimately find ourselves at the minimum-

boiling azeotrope. Therefore, the azeotrope is our ultimate product and no

further separation can be performed using distillation.

As an additional example, what is the expected outcome if the solution has

an initial mole fraction x 1 of 0.1?

Example 7.8

Using a temperature-composition phase diagram like Figure 7.15, predict the

general composition of the ultimate distillation product if a solution having

a mole fraction x 1 of 0.5 is distilled.

7.4 Nonideal Two-Component Liquid Solutions 181

T(BP 2 )

T(BP 1 )

Temperature

0.5

x 1 , y 1

0.0 1.0

Initial composition

of liquid phase

Final composition

of liquid and vapor

phase: azeotrope

Figure 7.16 See Example 7.7. If one starts with a liquid having the composition indicated, the
minimum-boiling azeotrope is the ultimate product.