Fractional distillation takes place in a fractionating column. The crude oil is first vapourised by heating, then passed into the fractionating column. The column is hotter at the bottom than at the top , and gases pass up the column through a series of bubble caps. Eventually, the gases reach a temperature that is lower than their boiling points, where the vapour condenses into a liquid. The liquid fractions are then tapped off into storage containers.
Short chained hydrocarbons with lower boiling points condense near the top of the column, whereas longer chained hydrocarbons with higher boiling points condense nearer to the bottom. Gases that do not condense are passed through an outlet at the top of the column as 'petroleum gas'. The residue from the process is bitumen, which is removed from the bottom of the column.
The fractions obtained from crude oil can be used as fuels or can be processed further to make petrochemicals- chemicals made from natural gas or oil. The fractions can also be distilled further to form pure liquids in other fractionating columns, operating over a narrower range of temperatures. Bitumen can be used to surface roads or as roof covering.
Effect of Chain Length
As the chain length increases, the boiling point increases because the intermolecular forces between the molecules get stronger. In a longer chained alkane, there are more points of contact between the molecules, leading to more van der Waals' forces between the molecules. It takes more energy, and therefore a higher temperature, to separate the molecules.
Effect of Branching
Isomers of alkanes have the same molecular mass. A branched isomer has a lower boiling point than the unbranched isomer. In a branched alkane, there are fewer points of contact between molecules, leading to fewer van der Waals' forces between the molecules. Also, branched molecules cannot get as close to each other as unbranched ones, thus decreasing the intermolecular forces between the molecules. Therefore, less energy is required to separate the molecules.