Azeotrope vapor-pressure curve

Since an azeotrope by definition has either a higher or a lower vapor pressure than that of any of the components, the azeotropic vapor pressure curve will always lie above or below the curves of the components. This is indicated schematically in Figure 1 where A and B are vapor pressure curves of the components and C is the vapor pressure of the azeotrope. If curve C crosses either A or B, the azeotropic vapor pressure is no longer greater or less than any of the components and the system will become nonazeotropic at the point of intersection. On the other hand, if the azeotropic curve is parallel to the other curves the system will be azeotropic up to the critical pressure. 

Since an azeotrope by definition has either a higher or a lower vapor pressure than that of any of the components, the azeotropic vapor pressure curve will always lie above or below the curves of the components. This is indicated schematically in Figure 1 where A and B are vapor... 

Figure 2. Azeotropic vapor pressure curves of methanol-methyl ethyl ketone, methanol-acetone, water-n-propanol, and water-ethanol...

Pressure has a marked effect on the azeotropic composition and vapor-liquid equilibrium diagrams of alcohol-ketone systems (J). This is due to the fact that the slopes of the vapor pressure curves of alcohols are appreciably greater than for ketones it results in an unusually larger change in the relative boiling points of the components of an alcohol-ketone system with change in pressure. 

Lecat (2) has considered the use of the vapor pressure curves of azeotropes to indicate the pressure at which a system would become nonazeotropic. However, he plotted in the conventional manner and could obtain the curves only by detailed experimental work. 

It has been found that the vapor pressure curves of azeotropes are straight lines when plotted on a Cox chart which permits determination of the complete vapor pressure curve from the data at two pressures. 

Figure 1. Schematic Diagram of Vapor Pressure Curves of Binary Azeotropes...

In cases where only the normal azeotropic boiling point is known, it is possible to predict the effect of pressure on the system by drawing the azeotrope curve through the normal boiling point with a slope equal to the average slopes of the component vapor pressure curves. This procedure will permit a fairly accurate prediction of whether the azeotrope will cease to exist below the critical pressure. 

Another use for this set of curves is for estimating the azeotropic boiling point and composition at pressures other than atmospheric. Consider the azeotrope methanol-benzene. Since the vapor pressure curves of methanol and benzene are known, the difference in boiling point, A, can be obtained at any pressure. From this value of A and the C-A curve for methanol-hydrocarbons the azeotropic concentration C at that pressure can be determined. For example, the effect of pressure on the methanol-benzene azeotrope is shown in Table I. 

Solubility. H bonding plays several roles in determining solubility, just as it influenced the vapor pressure curves and azeotropic behavior. Actually, the same classification can be used for solubility. Thus, in the same order as in Table 2-X, the six types of mixtures vv ould be expected to have mutual solubilities that were (1) low, (2) high, (3) intermediate, depending on relative strength of bonds broken and formed, (4 and 5) intermediate, (6) not determined by H bonds variable depending on polarity and dispersion forces. 

If the vapor pressures of the two pure constituents are close together, then any appreciable positive deviation from Raoult s law will lead to a maximum in the total vapor pressure curve similarly, a negative deviation will, in the same circumstances, be associated with a minimum in the curve. In any event, even if the vapor pressures of the pure constituents are appreciably different, marked positive or negative deviations can lead to a maximum or a minimum, respectively, in the total vapor pressure curve. Such maxima and minima are the cause of the formation of the familiar constant boiling mixtures or azeotropic mixtures. A liquid mixture having the composition represented by a maximum or a minimum will distil without change of composition, for the proportions of the two constituents are then the same in the liquid and vapor phases. That this must be the case will be shown in the next section. 

Example 1.9 shows that, for certain systems, deviations from Raoult s law can cause a maximum or a minimum in the vapor pressure to exist at a certain temperature and composition. At constant pressure, the boiling point or bubble point temperature curve could have a maximum or a minimum. Liquid mixtures whose vapor pressure curve or surface exhibits a maximum or a minimum are said to form azeotropes. The composition at which the azeotrope occurs is the azeotropic composition. Binaries are likely to form azeotropes if they deviate from Raoult s law and if their boiling points are not too far apart (within about 8°C). Azeotropes caused by positive deviations from Raoult s law are minimum boiling, that is, the azeotrope boils at a... 

Attempts to improve benzene purity by changing the pressure in hopes of altering the relative volatilities will not be very successful, as shown in Figure 2. The slopes of the vapor pressure curves of benzene and aliphatics are essentially parallel. Thus, the relative volatilities are almost unaffected, and the compositions of the binary azeotropes will not change much with change in pressure. 

Figure 1. Schematic of vapor pressure curves of binary azeotropes...

Two-pressure distillation is especially promising when the azeotrope-forming components have very different heats of evaporation. The thus resulting differing slopes of the vapor-pressure curves leads to a pressure dependence of different azeotropic compositions. 

Azeotropes What we have discussed so far in this section is only valid for mixtures of components that behave indifferently toward each other in both their liquid and vapor states. However, the hquid state often exhibits differing behavior. If the two components are highly compatible, the stronger interaction of the particles in the liquid mixture relative to the pure state hinders the transition to the vapor phase. The partial pressures of the components are smaller than in the case of indifferent behavior and the vapor pressure curves show a negative deviation from Raoult s law. Compared to the behavior of indifferent substances, the curves appear more or less distorted. As long as the disturbance is small, the behavior can be described in a similar manner as before. 

Fig. 2-16. Determination of the boiling point and the composition of the azeotropic mixture. Po,B Vapor-pressure curve of the substance to be purified...

As mentioned before, azeotropic behavior always occurs if the compounds to be separated have identical vapor pressures (Bancroft point). Since the slope of the vapor pressure curve, following the Clausius-Clapeyron equation, depends on the value of the enthalpy of vaporization, a low boiler may become the high boiler with rising temperature, if the enthalpy of vaporization is smaller than the one for the second compound. This is shown for ethanol and benzene in Figure 5.55. While benzene is the low boiler at low temperatures, the opposite becomes true at higher temperatures, since the molar enthalpy of vaporization of the polar component ethanol is larger than the molar enthalpy of vaporization of benzene (see Appendix A). 

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