Azeotrope / Azeotropic Mixture
Very large deviations from ideality lead to a special class of mixtures known as azeotropes, azeotropic mixtures, or constant-boiling mixtures. Azeotrope is a special class of liquid mixture that boils at a constant temperature at a certain composition. At this condition, it behaves as if it was one component with one constant boiling point.
A boiling liquid mixture at the azeotropic composition produces
a vapour of exactly the same composition, and the liquid does not change its
composition as it evaporates.
Two types of azeotropes are known: minimum-boiling and maximum-boiling (less common). One of the best known minimum-boiling azeotrope is the ethanol-water system which at 1 atm occurs at 89.4 mole percent ethanol and 78.2 oC. Other examples are:
carbon-disulfide - acetone (61.0 mole% CS2, 39.25 oC, 1 atm)
benzene - water (29.6 mole% H2O, 69.25 oC, 1 atm)
hydrochloric acid - water (11.1 mole% HCl, 110 oC, 1 atm)
acetone - chloroform (65.5 mole% chloroform, 64.5 oC, 1 atm)
[ For more examples, see Perry's Chemical Engineers' Handbook, 6th Ed., pp. 13-59 and 13-60 ]
When the positive deviations from ideality are sufficiently large, the mixture is said to form a minimum-boiling azeotrope. The Figures below show the constant temperature phase diagram (left) and constant pressure phase diagram plus equilibrium curve (right) for a minimum-boiling azeotropic mixture of carbon disulfide (CS2) and acetone. The characteristic of such mixture is that the total pressure goes through a maximum (constant temperature phase diagram), and therefore the temperature goes through a minimum (constant pressure phase diagram), shown as point L.
At point L, the concentration in the vapour phase is the same as the concentration in the liquid phase ( y = x ), and a = 1.0. This concentration is known as the azeotropic composition (0.61 mole fraction CS2). At this point, the mixture boils at a constant temperature (39.25 oC under 1 atm) and without change in composition. On the equilibrium diagram, it can be seen that at this point, the equilibrium curve crossed the 45o diagonal.
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It occurs when the negative deviations are very large, and the total pressure curve in this case passes through a minimum, giving rise to a maximum in the temperature (i.e. boiling point). The Figures below show the constant temperature phase diagram (left) and constant pressure phase diagram plus equilibrium curve (right) for a maximum-boiling azeotrope mixture of acetone and chloroform.
The azeotropic composition is 0.345 mole fraction acetone. Point L in the Figures is now a minimum on the constant temperature phase diagram, and a maximum (64.5 oC, under 1 atm) on the constant pressure phase diagram. These azeotropes are less common than the minimum type.
Separation of Azeotropic Mixtures
Azeotropic mixtures cannot be easily separated by ordinary distillation methods. For example, in the case of ethanol-water, one cannot recover more than 89.4 mole% ethanol using ordinary distillation, as the mixture becomes azeotropic at this point. Other separation techniques (such as azeotropic distillation) must be used. Often the equipment and set-up used is unique for each mixture.
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