The distillation

[/center] 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:

energy input raises vapour pressure

vapour pressure is related to boiling

a liquid is said to ‘boil’ when its vapour pressure equals the surrounding pressure

the ease with which a liquid boils depends on its volatility

liquids with high vapour pressures (volatile liquids) will boil at lower temperatures

the vapour pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture

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 - a binary mixture. This has the following boiling point diagram.

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 , 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





[indent]y[sub]i[/sub] = mole fraction of component ‘i’ in the vapour

x[sub]i[/sub] = mole fraction of component ‘i’ in the liquid[/indent]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.









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

[font=Arial,Helvetica,sans-serif]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. [/font] 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.



The most intriguing VLE curves are generated by azeotropic systems. An azeotrope is a liquid mixture which when vaporized, 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.





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.











When this additional component appears in appreciable amounts at the top of the column, the operation is called azeotropic distillation.

When the additional component appears mostly at the bottom of the column, the operation is called extractive distillation 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.



As mentioned, distillation columns are designed using VLE data for the mixtures to be separated. The vapour-liquid equilibrium characteristics (indicated by the shape of the equilibrium curve) of the mixture will determine the number of stages, and hence the number of trays, required for the separation. This is illustrated clearly by applying the McCabe-Thiele method to design a binary column. McCABE-THIELE DESIGN METHOD The McCabe-Thiele approach is a graphical one, and uses the VLE plot to determine the theoretical number of stages required to effect the separation of a binary mixture. It assumes constant molar overflow and this implies that:

molal heats of vaporisation of the components are roughly the same

heat effects (heats of solution, heat losses to and from column, etc.) are negligible

for every mole of vapour condensed, 1 mole of liquid is vaporised The design procedure is simple. Given the VLE diagram of the binary mixture, operating lines are drawn first.

Operating lines define the mass balance relationships between the liquid and vapour phases in the column.

There is one operating line for the bottom (stripping) section of the column, and on for the top (rectification or enriching) section of the column.

Use of the constant molar overflow assumption also ensures the the operating lines are straight lines.

Operating Line for the Rectification Section



[left]The operating line for the rectification section is constructed as follows. First the desired top product composition is located on the VLE diagram, and a vertical line produced until it intersects the diagonal line that splits the VLE plot in half. A line with slope R/(R+1) is then drawn from this instersection point as shown in the