Fractional distillation of petroleum

Petroleum is a mixture of substances with different boiling points. Fractional distillation separates it into useful groups of substances, called fractions, according to these differences.

Petroleum is heated so that much of it vaporises. The vapours enter a fractionating column, which is hot at the bottom and cooler towards the top. As the vapours rise, they cool and condense back into liquids at different levels.

Fractions with higher boiling points condense lower in the column, where it is hotter. Fractions with lower boiling points rise further and condense higher up, where it is cooler. The fractions are collected separately at their respective levels.

Properties of petroleum fractions

Petroleum fractions are mixtures of hydrocarbons with similar boiling points. The hydrocarbons in different fractions have different chain lengths, and their physical properties vary with those lengths.

Moving from the bottom to the top of a fractionating column:

• Hydrocarbon chain length decreases.
• Boiling point decreases.
• Volatility increases: the fractions vaporise more readily.
• Viscosity decreases: the fractions flow more easily.

Shorter hydrocarbon molecules generally have weaker attractions between them than longer molecules. Less energy is therefore needed to separate the molecules into a gas, so shorter-chain fractions have lower boiling points and vaporise more readily. They are also less viscous, so they flow more easily.

Uses of petroleum fractions

Petroleum is separated into fractions, each of which has particular uses. Some fractions are used as fuels, while others provide materials for making chemicals or other products.

• Refinery gas is used for heating and cooking.
• Gasoline (petrol) is used as fuel for cars.
• Naphtha is used as a chemical feedstock: a starting material for making other chemicals.
• Kerosene (paraffin) is used as jet fuel.
• Diesel oil (gas oil) is used as fuel for diesel engines.
• Fuel oil is used as fuel for ships and home heating systems.
• Lubricating oil is used to make lubricants, waxes and polishes.
• Bitumen is used to make road surfaces.