When a mixture of two or more liquids is heated and boiled, the vapor has a different composition than the liquid. For example, if a10% mixture of ethanol in water is boiled, the vapor will contain over 50% ethanol. The vapor can be condensed and boiled again, which will result in an even higher concentration of ethanol. Distillation operates on this principle.
Clearly, repeated boiling and condensing is a clumsy process, however, this can be done as a continuous process in a distillation column. In the column, rising vapors will strip out the more volatile component, which will be gradually concentrated as the vapor climbs up the column.
The vapor/liquid equilibrium (VLE) relationship between ethanol and water is shown in Figure 2. A similar relationship exists between all compounds. From this type of data, it is a relatively simple task to calculate the design parameters using one of the classical methods, such as McCabe-Thiele.
The key to this separation is the relative volatility between the compounds to be separated. The higher the relative volatility, the easier the separation and vice versa. For a binary system, the mole fraction y of component a in the vapor in equilibrium with the mole fraction x in the liquid is calculated from the following equation.
Was this article helpful?