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an introduction
© Copyright 1997-2023 by M.T. Tham and R.C. Costello
Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapour pressure characteristics of liquid mixtures.

Vapour Pressure and Boiling

The vapour pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface. Here are some important points regarding vapour pressure:
arrored.gif (1174 bytes) energy input raises vapour pressure
arrored.gif (1174 bytes) vapour pressure is related to boiling
arrored.gif (1174 bytes) a liquid is said to ‘boil’ when its vapour pressure equals the surrounding pressure
arrored.gif (1174 bytes) the ease with which a liquid boils depends on its volatility
arrored.gif (1174 bytes) liquids with high vapour pressures (volatile liquids) will boil at lower temperatures
arrored.gif (1174 bytes) the vapour pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture
arrored.gif (1174 bytes) distillation occurs because of the differences in the volatility of the components in the liquid mixture

The Boiling Point Diagram

The boiling point diagram shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture containing 2 components (A and B) - a binary mixture. This has the following boiling point diagram.

bpt.gif (2784 bytes) The boiling point of A is that at which the mole fraction of A is 1. The boiling point of B is that at which the mole fraction of A is 0. In this example, A is the more volatile component and therefore has a lower boiling point than B. The upper curve in the diagram is called the dew-point curve while the lower one is called the bubble-point curve.

The dew-point is the temperature at which the saturated vapour starts to condense.

The bubble-point is the temperature at which the liquid starts to boil.

The region above the dew-point curve shows the equilibrium composition of the superheated vapour while the region below the bubble-point curve shows the equilibrium composition of the subcooled liquid.

For example, when a subcooled liquid with mole fraction of A=0.4 (point A) is heated, its concentration remains constant until it reaches the bubble-point (point B), when it starts to boil. The vapours evolved during the boiling has the equilibrium composition given by point C, approximately 0.8 mole fraction A. This is approximately 50% richer in A than the original liquid.

This difference between liquid and vapour compositions is the basis for distillation operations.

Relative Volatility

Relative volatility is a measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. The relative volatility of component ‘i’ with respect to component ‘j’ is defined as


yi = mole fraction of component ‘i’ in the vapour

xi = mole fraction of component ‘i’ in the liquid

Thus if the relative volatility between 2 components is very close to one, it is an indication that they have very similar vapour pressure characteristics. This means that they have very similar boiling points and therefore, it will be difficult to separate the two components via distillation.