°0 0.2 0.4 0.6 0.8 1.0 XC$2 ' ^ frac*'on C$2 ,n ltyuid

Fia. 2-2. Equilibrium yfx data for CCI4-CS2 mixtures.

Fig. 2-2 for the same conditions. In this curve the composition yi = £2 is plotted as ordinate with composition xx as abscissa, « as ordinate with xa as abscissa, and so forth. This particular relation is very useful in distillation calculations. It does not give so much information as Fig. 2-1, owing to the elimination of temperature. However, in most distillation calculations it is desired to make a given separation between the components and the temperatures are allowed

Fig. 2-3. Temperature-composition diagram.

to adjust themselves accordingly. On Fig. 2-2 the 45° line represents a vapor of the same composition as the liquid. If the temperature is important, this variable can be plotted vs. the liquid composition on the same figure.

The curves given in Figs. 2-1 and 2-2 are termed the normal type. However, there are several other common types of curves. In Fig. 2-3 temperature-composition diagrams for constant total pressure are given for four different types of binary mixtures, and in Fig. 2-4 the corresponding vapor-liquid diagrams are given for the four same mixtures.

Type I is normal, i.e., the composition of the equilibrium vapor is always richer in the same component than the composition of the liquid, thus by repeated operations it is possible to obtain complete separation.

In type II the temperature-composition diagram passes through a minimum, and the vapor-liquid composition diagram crosses the diagonal. Thus there are mixtures that have lower boiling points than either of the pure components at the same pressure. In other words, the mixture is the minimum boiling-point type. When such temperature-composition diagrams are encountered, the vapor-liquid composition curve will always cross the 45° line. In the region below this intersection with the diagonal, the equilibrium vapor is richer in one

x,Mol fraction in liquid Fig. 2-4. Vapor-liquid equilibrium curves.

component than the liquid; above this intersection, the vapor is poorer in this component than the corresponding liquid from which it came. Thus the volatilities have reversed. Where the vapor-liquid curve crosses the 45° line, the vapor has the same composition as the liquid and operations based on producing an equilibrium vapor from this liquid would not be able to separate mixtures of this composition. This particular composition is called a constant boiling mixture, or azeotropic mixture, sincejtwill vaporize without any change in composition and, therefore; without any change in temperature during the evaporation.

Type TITTs the reverse of type II. In this case there are mixtures that have boiling points higher than either of the pure components at the same pressure. It will be noted in Fig. 2-4 that the curve of type

III also crosses the 45° line but curve II cuts the 45° line with the slope less than 1 while curve III crosses the 45° line with the slope greater than 1. Curve III is of the maximum boiling-point type, and the particular composition at which the curve crosses the 45° line is called a maximum constant boiling mixture or a maximum boiling azeotrope.

The curve of type IV is similar to that of type II except that, for a considerable range of composition, the temperature of the liquid phase is constant. This curve type is characteristic of a partly miscible liquid system. In the immiscible region, two liquid phases are present and the phase rule indicates that the boiling temperature of the mixture must be constant. In the diagram, the over-all composition of the liquid is plotted as x regardless of whether one or two phases are present. There is no single liquid phase that has a composition equal to the value given in the two-phase region. The y,x data for this system are given in Fig. 2-4. In this case the y,x curve crosses the diagonal in the two-phase region; thus at this intersection the composition of the vapor is the same as that of the combined liquid phases. Such a mixture can be evaporated to dryness at constant pressure without change in composition or temperature. The mixture of this particular composition is termed a pseudo-azeotrope. This terminology is sometimes applied to any two-phase mixture, but the original usage of the term azeotrope by Wade and Merriman (Ref. 50) implied that the liquid could be evaporated to dryness without change in composition. Only the two-phase mixture corresponding to the intersection of the y,x curve with the y = x line can be evaporated without changing composition.

In other cases the y,x curve may not cross the diagonal in the two-phase region, and such mixtures do not form pseudo-azeotropes, but they may form, and usually do, true azeotropes in one of their singlephase regions.

Literature Data. The vapor-liquid equilibria for a large number of mixtures have been experimentally determined, and Table 2-1 lists some of the more reliable and useful determinations.

The data given in the table represent a large amount of experimental effort, and owing to the difficulties of making such determinations a number of investigators have tried to develop theoretical and empirical methods of predicting such vapor-liquid equilibria from the physical properties of the pure components. While certain correlations have been developed by this method, reliable experimental determinations are still to be preferred to any such calculated values.

Table 2-1. Vapor-Liquid Equilibrium Data


Constant T OTP

Technique *



760 mm.

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