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Fractional distillation

Fractional distillation is the separation of a mixture of compounds by their boiling point, by heating to high enough temperatures.

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Fractional Distillation in a Laboratory

Apparatus

Method

As an example, consider the distillation of a mixture of ethanol and water. Ethanol boils at 80°C whilst water boils at 100°C. So by gently heating the mixture, the ethanol will boil off first.

The apparatus is assembled as in the diagram. The mixture is put into the round bottomed flask along with a few anti bumping granules , and the fractionating column is fitted into the top. As the mixture boils, vapour rises up the column. The vapour condenses on the glass platforms inside the column, and runs back down into the liquid below. Only the most volatile of the vapours stays in gaseous form all the way to the top. The vapour at the top of the column will be almost pure ethanol. This then passes into the condenser, which cools it down until it liquefies. The process continues until all the ethanol boils out of the mixture. This point can be recognised by the sharp rise in temperature shown on the thermometer, from the boiling point of ethanol to the boiling point of water.

Industrial uses of Fractional Distillation

The most important industrial application of fractional distillation is the distillation of crude oil. The process is similar in principle to the laboratory method described above except for scale and the fact that crude oil has many different compounds mixed together. The fractionating column has outlets at regular intervals up the column which allow the different fractions to run out at different temperatures, with the highly volatile gases coming out the topmost outlet graduating to the less volatile road tar, (bitumen) coming out at the bottom.



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