FRACTIONAL DISTILLATION

Miscible liquids are much more difficult to separate. Mixtures of miscible liquids can be separated by fractional distillation. It will provide the boiling points of the liquids are not too close.

If we want to separate a mixture of ethanol and water. The diagram below is suitable for this process. The fractionating column is packed with glass bead. It provides a large surface area for vaporization and condensation of the liquid mixture.

Ethanol is more volatile than water, since it has a lower boiling point (78^oC). The vapor rises up the fractionating column when the mixture is heated. Because ethanol is more volatile, the vapor contains more ethanol. The hot vapor condenses upon touching the cold glass beads. There is a continuos rise of hot vapor up the fractionating column at the same time. Hot vapor will make the condensed vapor boils again. It will contain more and more ethanol as the vapor rises up to the fractionating column.

The above process is to be repeated many times before the vapor consists only pure ethanol. During the process the escaping vapor is measured by a thermometer of the fractionating column. The temperature will remain steady for some time and will then rise quickly and become pure ethanol.

When the ethanol has boiled off completely, the escaping vapor will consist of pure water only .

Generally, for fractional distillation to work best,the difference in boiling points of liquids in the mixture should be greater than 10C. The separation will not be complete if it is not.

Fractional distillation is used in industry to separate oxygen and nitrogen from liquid air. In whisky production it is used to increase ethanol.