Distillation of partially miscible and immiscible liquids

If the pressure on the system shown in Fig. 15.4(a) is lowered, the boiling points will all be shifted downward. At a low enough pressure, the boiling point curves will intersect the liquid-liquid solubility curves. The result is shown in Fig. 15.4(b), which represents schematically the system water-n-butanol under 1 atm pressure. 

If a two-phase system in the composition range between/and h is heated, then at t liquids/and g are present, and vapor h appears. The system at t is invariant. Since the vapor is richer in butanol than the original overall composition, the butanol-rich layer evaporates preferentially, leaving liquid/and vapor h. As the temperature rises, the liquid is depleted in butanol finally only vapor remains. 

The point h has the azeotropic property a system of this composition distills unchanged. It cannot be separated into its components by distillation. 

The distillation of immiscible substances is most easily discussed from a different standpoint. Consider two immiscible liquids in equilibrium with vapor at a specified temperature (Fig. 15.5). The barrier only keeps the liquids apart since they are immiscible, removing the barrier would not change anything. The total vapor pressure is the sum of the 

Equation (15.3) can be applied to the steam distillation of liquids. Some liquids that decompose if distilled in the ordinary way can be steam distilled if they have fair volatility near the boiling point of water. In the laboratory, steam is passed through the liquid to be steam distilled. Since the vapor pressure is greater than that of either component, it follows that the boiling point is below the boiling points of both liquids. Furthermore, the boiling point is an invariant temperature so long as the two liquid phases and the vapor are present. 

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