Fractions and Uses

Fractional Distillation

Fractional distillation is the process used to separate crude oil into different fractions based on their boiling points. Crude oil is heated until it vaporises, then the vapour rises up a fractionating column. The temperature decreases higher up the column, so different hydrocarbons condense at different levels where their boiling points are reached.

Each fraction is a mixture of hydrocarbons with similar chain lengths and boiling point ranges. The shorter the hydrocarbon chain, the lower the boiling point, so these condense near the top of the column. Longer chains have higher boiling points and condense near the bottom.

The main fractions collected include refinery gas, petrol, kerosene, diesel oil, fuel oil, and bitumen. Each fraction has specific uses depending on its properties.

For instance, hydrocarbons with boiling points between about 406C and 706C condense near the top and form the petrol fraction, while those boiling between 1506C and 3006C form the diesel fraction lower down.

Common Fractions

The main fractions obtained from fractional distillation of crude oil are:

• Refinery gas: The lightest fraction, consisting of very short hydrocarbons (1-4 carbon atoms). It is a gas at room temperature.
• Gasoline (petrol): Contains hydrocarbons with about 5-12 carbon atoms. It is a volatile liquid used mainly as fuel for cars.
• Kerosene: Hydrocarbons with 12-15 carbon atoms. It is used as jet fuel and for heating.
• Diesel oil: Hydrocarbons with 15-19 carbon atoms. Used as fuel for diesel engines in vehicles and some heating systems.
• Fuel oil: Heavier hydrocarbons with 20-30 carbon atoms. Used to power ships and for industrial heating.
• Bitumen: The heaviest fraction with very long chains (over 30 carbon atoms). It is a thick, sticky substance used for road surfacing and roofing.

Uses of Fractions

Each fraction has practical uses based on its properties:

• Refinery gas: Used for bottled gas (LPG) for heating and cooking.
• Petrol: The main fuel for cars and motorcycles.
• Kerosene: Used as jet fuel for aircraft and for heating in some homes.
• Diesel oil: Fuel for diesel engines in trucks, buses, and some cars.
• Fuel oil: Used in ships and for industrial heating systems.
• Bitumen: Used to surface roads and for roofing materials due to its waterproof and sticky nature.

For example, kerosene is chosen for jet engines because it burns cleanly and efficiently at high altitudes.

Properties of Fractions

The properties of each fraction depend on the length of the hydrocarbon chains they contain:

• Viscosity: This is a measure of how thick or runny a liquid is. Shorter chain fractions like petrol are less viscous (runny), while longer chain fractions like bitumen are very viscous (thick and sticky).
• Volatility: Volatility refers to how easily a liquid evaporates. Short chain fractions have high volatility and evaporate easily (e.g., petrol), whereas long chain fractions have low volatility (e.g., fuel oil, bitumen).
• Flammability: Short chain fractions are more flammable and ignite more easily, making them suitable for fuels like petrol. Longer chain fractions are less flammable and burn less easily.
• Carbon chain length: Shorter chains have fewer carbon atoms (around 1-12), while longer chains can have over 30 carbon atoms.

For example, refinery gas with very short chains is very volatile and flammable, making it ideal for cooking gas, while bitumen with very long chains is solid or semi-solid at room temperature and used for road surfaces.

The differences in these properties explain why different fractions are suited to different uses.

Example: If a fraction has hydrocarbons with carbon chains of length 10 to 15 atoms, it will have moderate viscosity and volatility, making it suitable for use as kerosene (jet fuel and heating).

For example, a fraction with hydrocarbons having boiling points between 406C and 706C is petrol, which is used as fuel for cars and motorcycles.