3.2.1 Fractional Distillation of Crude Oil

The Process of Fractional Distillation

The steps involved in the fractional distillation of crude oil are as follows:

1. Heating the Crude Oil

• Crude oil is heated to approximately 350°C. Most of the hydrocarbons are vaporised at this temperature, except for the largest hydrocarbons, which remain in liquid form due to their high boiling points.

2. Entering the Fractionating Column

• The vaporised crude oil enters the bottom of a fractionating column. This column has a temperature gradient: it is hottest at the bottom and gradually cools as you move up the column.

3. Separation by Boiling Point

• Larger hydrocarbons with higher boiling points condense back into liquids near the bottom of the column, where it is hotter.
• Smaller hydrocarbons with lower boiling points rise higher in the column and condense near the top, where the temperature is cooler.

4. Formation of Fractions

• As the hydrocarbons condense at different levels of the column, they are separated into fractions. Each fraction contains hydrocarbons with similar boiling points.
• Example: Heavy fuel oils with large molecules collect at the bottom, while lighter fractions like gasoline and naphtha collect higher up.

5. Hydrocarbons that Don't Condense

• The hydrocarbons with the lowest boiling points (e.g., methane, ethane) remain as gases and exit the top of the column without condensing.

Crude Oil Fractions

Each fraction produced by fractional distillation has different uses, depending on the size and type of hydrocarbons it contains. Some common fractions include:

• Refinery gases: Used as fuels for heating or cooking.
• Gasoline (petrol): Used as fuel for cars.
• Kerosene: Used in jet fuel and heating.
• Diesel: Used in diesel engines.
• Heavy fuel oil: Used in ships and for industrial heating.
• Bitumen: Used for road surfacing.

Why Fractional Distillation is Important

Fractional distillation is crucial because it allows crude oil, a complex mixture of hydrocarbons, to be separated into components that are useful for various applications. Without this process, crude oil would be far less valuable as it would be impossible to isolate the specific hydrocarbons needed for products like gasoline, diesel, and lubricants.