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an introduction
© Copyright 1997-2023 by M.T. Tham and R.C. Costello
Distillation columns are designed based on the boiling point properties of the components in the mixtures being separated. Thus the sizes, particularly the height, of distillation columns are determined by the vapour liquid equilibrium (VLE) data for the mixtures.

Vapour-Liquid-Equilibrium (VLE) Curves

vle1.gif (3430 bytes) Constant pressure VLE data is obtained from boiling point diagrams. VLE data of binary mixtures is often presented as a plot, as shown in the figure on the right. The VLE plot expresses the bubble-point and the dew-point of a binary mixture at constant pressure. The curved line is called the equilibrium line and describes the compositions of the liquid and vapour in equilibrium at some fixed pressure.

This particular VLE plot shows a binary mixture that has a uniform vapour-liquid equilibrium that is relatively easy to separate. The next two VLE plots below on the other hand, shows non-ideal systems which will present more difficult separations. We can tell from the shapes of the curves and this will be explained further later on.

vle2.gif (3018 bytes)vle3.gif (3007 bytes)

The most intriguing VLE curves are generated by azeotropic systems. An azeotrope is a liquid mixture which when vaporised, produces the same composition as the liquid. The two VLE plots below, show two different azeotropic systems, one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines, and this are azeotropic points where the azeotropes occur. In other words azeotropic systems give rise to VLE plots where the equilibrium curves crosses the diagonals.

azeo1.gif (3671 bytes)

Note the shapes of the respective equilibrium lines in relation to the diagonal lines that bisect the VLE plots.

Both plots are however, obtained from homogenous azeotropic systems. An azeotrope that contains one liquid phase in contact with vapour is called a homogenous azeotrope. A homogenous azeotrope cannot be separated by conventional distillation. However, vacumn distillation may be used as the lower pressures can shift the azeotropic point.Alternatively, an additional substance may added to shift the azeotropic point to a more ‘favourable’ position.

arrored.gif (1174 bytes) When this additional component appears in appreciable amounts at the top of the column, the operation is called azeotropic distillation .
arrored.gif (1174 bytes) When the additional component appears mostly at the bottom of the column, the operation is called extractive distillation
azeo2.gif (1688 bytes) The VLE curve on the left is also generated by an azeotropic system, in this case a heterogenous azeotrope. Heterogenous azeotropes can be identified by the ‘flat’ portion on the equilibrium diagram.

They may be separated in 2 distillation columns since these substances usually form two liquid phases with widely differing compositions. The phases may be separated using settling tanks under appropriate conditions.

Next, we will look at how VLE plots/data are used to design distillation columns.