Crude Oil

What is Crude Oil?

Crude oil is a complex mixture of hydrocarbons, mainly compounds made up of hydrogen and carbon atoms. It is formed over millions of years from the remains of ancient marine plants and animals (biomass) that were buried under layers of sediment. Over time, heat and pressure transformed this biomass into crude oil.

Non-renewable resource: Crude oil is a finite resource because it takes millions of years to form. Once used, it cannot be replaced on a human timescale.

Composition of Crude Oil

Crude oil contains hydrocarbons with chains of different lengths. Most of these hydrocarbons are alkanes, which are saturated hydrocarbons with single bonds only. The hydrocarbons range from small molecules like methane (1 carbon) to very large molecules with many carbon atoms.

Besides hydrocarbons, crude oil also contains impurities such as sulphur compounds, nitrogen compounds, and metals. These impurities can affect how crude oil is processed and used.

Fractional Distillation

Crude oil is separated into useful parts called fractions by a process called fractional distillation. This works because different hydrocarbons have different boiling points.

The crude oil is heated until it vaporises and then passed into a tall fractionating column. Inside the column, the temperature is hotter at the bottom and cooler at the top. Hydrocarbons condense back into liquids at different heights depending on their boiling points:

• Longer hydrocarbon chains have higher boiling points and condense near the bottom.
• Shorter chains have lower boiling points and condense near the top.

Each fraction collected contains hydrocarbons with similar chain lengths and boiling points, making them suitable for specific uses.

For instance, petrol (gasoline) is collected near the top of the column because it has a low boiling point, while bitumen, which is thick and sticky, condenses near the bottom.

Example: Hydrocarbons with boiling points below 1006C rise to the top and condense there, while those with boiling points above 3506C condense near the bottom.

Uses of Crude Oil Fractions

The fractions obtained from crude oil have many important uses:

• Fuel: Many fractions are used as fuels:
  • Petrol (for cars)
  • Diesel (for trucks and some cars)
  • Kerosene (for jet fuel and heating)
  • Liquefied petroleum gas (LPG) (for heating and cooking)
• Feedstock for petrochemicals: Some fractions are raw materials for the chemical industry. They are used to make plastics, solvents, detergents, and medicines.
• Lubricants: Certain fractions are used to make oils that reduce friction in engines and machinery.
• Bitumen: The heaviest fraction is bitumen, used for road surfacing and roofing.

Learning Example: Fractional Distillation

Imagine crude oil is heated and passed into a fractionating column. Hydrocarbons with boiling points below 1006C rise to the top and condense there, while those with boiling points above 3506C condense near the bottom.

If petrol has a boiling range of about 406C2056C, it will be collected near the top. Diesel, with a boiling range of 2506C3506C, condenses lower down. Bitumen, boiling above 3506C, collects at the bottom.