Nonideality

In most distillation systems, the predominant nonideality occurs in the liquid phase because of molecular interactions. Equation (2.4) contains yv the liquidphase activity coefficient of the yth component.

When chemically dissimilar components are mixed together (for example, oil molecules and water molecules), there can be repulsion or attraction between dissimilar molecules. If the molecules repel each other, they exert a higher partial pressure than if they were ideal. In this case the activity coefficients are greater than unity (called a "positive deviation" from Raoulfs law). If the molecules attract each other, they exert a lower partial pressure than if they were ideal. Activity coefficients are less than unity (negative deviations).

Activity coefficients are usually calculated from experimental data. Figure 2.15 sketches typical activity coefficient data as a function of the light component composition ¿Cj. Empirical equations (Van Laar, Margules, Wilson, etc.) are used to correlate activity coefficient data.

Azeotropes occur in a number of nonideal systems. An azeotrope exists when the liquid and vapor compositions are the same (xj = yy). There are several types of azeotropes. Figures 2.16, 2.17, and 2.18 sketch typical phase diagrams for these.

Negative deviations (attraction) can give a higher temperature boiling mixture than the boiling point of the heavier component. This can lead to formation of a "maximum-boiling" azeotrope (Figure 2.16). Positive deviations (repulsion) can give a lower temperature boiling mixture than the boiling point of the light component. A modest amount of repulsion can lead to the formation of a minimum boiling azeotrope (Figure 2.17). If the repulsion is very strong, the system may break into two-liquid phases with different compositions in each liquid phase. This is called a "heterogeneous" azeotrope (Figure 2.18).

Maximum Boiling Point Phase Diagram

FIGURE 2.15

Typical activity coefficients as functions of light component composition

FIGURE 2.15

Typical activity coefficients as functions of light component composition

Azeotrope Break
x and y

FIGURE 2.16

Typical homogeneous "maximum boiling" azeotrope

FIGURE 2.16

Typical homogeneous "maximum boiling" azeotrope

Homogeneous Azeotropic
x and y

FIGURE 2.17

Homogeneous "minimum boiling" azeotrope

FIGURE 2.17

Homogeneous "minimum boiling" azeotrope

A detailed discussion of azeotropes is beyond the scope of this brief introduction. The control engineer should be aware that the existence of azeotropes imposes restrictions on the operation and performance of a distillation column.

2.4 GRAPHICAL SOLUTION TECHNIQUES

The equations describing a binary distillation column can be solved graphically using the famous McCabe—Thiele diagrams. These techniques are very useful in gaining an appreciation of the effects of various design and operating parameters.

Heterogeneous Azeotrope

FIGURE 2.18

Heterogeneous azeotropes

FIGURE 2.18

Heterogeneous azeotropes

These effects are usually apparent on the diagrams, so one gets a picture of the process.

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