In an oil refinery a fractionating column like that shown separates crude oil into different useful hydrocarbon fractions - Leaving Cert Chemistry - Question 6 - 2016

The two main components of Liquid Petroleum Gas (LPG) are propane and butane.

The structural formula of propane (C₃H₈) is as follows:

   H   H   H
   |   |   |
H--C---C---C--H
   |   |   |
   H   H   H

Alternatively, butane (C₄H₁₀) can be represented as:

   H   H   H   H
   |   |   |   |
H--C---C---C---C--H
   |   |   |   |
   H   H   H   H

Small quantities of mercaptans, which are sulfur-containing compounds, are added to LPG to give it an odor. This is crucial for safety, as LPG is odorless in its natural state. The odor allows for easy detection of gas leaks, facilitating immediate action to prevent accidents.

The octane number of petrol is a measure of its ability to resist pre-ignition or knocking during combustion in an engine. A higher octane number indicates better resistance to knocking, which is beneficial for engine performance.

One way to increase the octane number of a fuel is by adding oxygenates or high-octane additives, such as ethanol or MTBE (methyl tert-butyl ether).

Fraction X is kerosene, which is commonly used as a fuel for jet aircraft and also serves as a central-heating oil.

The residue obtained at the base of the fractionating column is often used for road making, roofing, tar production, or in bitumen for waterproofing applications.