Answer. The crude oil is refined in the refineries. Refining process is the separation of crude oil mixture into various useful products
(fractions). It is carried out by a process called fractional distillation. The principle of fractional distillation is based upon separation of substances depending upon their boiling points. The substances having low boiling points boil out first leaving behind others. Then next fraction of slightly higher boiling point boils out. This process remain continue until a residue is left behind. The vapours of each fraction are collected and condensed separately. The fractional distillation of petroleum is carried out in a tall fractionating tower as shown in figure below:
The crude oil is heated in a furnace up to a temperature of 400°C under high pressure. Then vapours are passed through a fractionating column from near its bottom as shown in figure. Hot vapours rise up in the column and gradually cool down and condense. Such that vapours of higher boiling point fraction (350—400°G) condense first in the lower part of the tower, while vapours of medium and lower boiling point fractions rise upwards in the tower and condense gradually with respect to their boiling points at different levels. In this way crude oil is separated into six hydrocarbon fractions as discussed below. Each fraction has its specific boiling range, composition and uses.