The distillation of partially miscible liquids

Consider a pair of liquids that are partially miscible and form a low-boiling azeotrope. This combination is quite common because both properties reflect the tendency of the two kinds of molecule to avoid each other. There are two possibilities: one in which the liquids become fully miscible before they boil; the other in which boiling occurs before mixing is complete. Figure 6.26 shows the phase diagram for two components that become fully miscible before they boil. Distillation of a mixture of composition a1leads to a vapour of com positionb1, which condenses to the completely miscible single-phase solution at b2. Phase separation occurs only when this distillate is cooled to a point in the two-phase liquid region, such as b3. This description applies only to the first drop of distillate. If distillation continues, the composition of the remaining liquid changes. In the end, when the whole sample has evaporated and condensed, the composition is back to a1. Figure 6.27 shows the second possibility, in which there is no upper critical solution temperature. The distillate obtained from a liquid initially of composition a1has com positionb3and is a two-phase mixture. One phase has composition b3 ′and the other has composition b3″. The behaviour of a system of composition represented by the isopleth ein Fig. 6.27 is interesting. A system at e1 forms two phases, which persist (but with changing proportions) up to the boiling point at e2. The vapour of this mixture has the same composition as the liquid (the liquid is an azeotrope). Similarly, condensing a vapour of composition e3 gives a two-phase liquid of the same overall composition. At a fixed temperature, the mixture vaporizes and condenses like a single substance.

Fig. 6.26 The temperature–composition diagram for a binary system in which the upper critical temperature is less than the boiling point at all compositions. The mixture forms a low-boiling azeotrope.

Fig. 6.27 The temperature–composition diagram for a binary system in which boiling occurs before the two liquids are fully miscible.

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Units

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption