Fractional Distillation of Crude Oil

Crude Oil Composition

Crude oil is a complex mixture of hydrocarbons, which are compounds made up of hydrogen and carbon atoms only. These hydrocarbons vary in chain length, ranging from very short chains with a few carbon atoms to very long chains with many carbon atoms.

Because crude oil contains hydrocarbons of different sizes and structures, it serves as an important source of fuels (like petrol and diesel) and raw materials (feedstocks) for the chemical industry. These feedstocks are used to make products such as plastics, solvents, and other chemicals.

Fractional Distillation Process

Fractional distillation is the process used to separate crude oil into its different components, called fractions. This process takes advantage of the fact that hydrocarbons have different boiling points depending on their chain length.

The crude oil is first heated strongly in a furnace until it vaporises. The hot vapour then enters a tall fractionating column, which is cooler at the top and hotter at the bottom, creating a temperature gradient.

As the vapour rises up the column, it cools. Hydrocarbons with higher boiling points condense back into liquids lower down the column, while those with lower boiling points rise further before condensing. This means different fractions condense at different heights in the column, allowing them to be collected separately.

For example, long-chain hydrocarbons with high boiling points condense near the bottom, while short-chain hydrocarbons with low boiling points condense near the top.

For instance, if crude oil is heated to about 3506C, hydrocarbons with boiling points below this temperature vaporise and rise up the column. Those with boiling points above 3506C condense lower down. This separation based on boiling points allows the crude oil to be split into useful fractions.

Fractions Obtained

The main fractions collected from fractional distillation include:

• Gases: Very short-chain hydrocarbons that remain gaseous at room temperature (boiling points below 406C).
• Petrol (gasoline): Used mainly as fuel for cars (boiling range approx. 406C to 2006C).
• Kerosene: Used as jet fuel and for heating (boiling range approx. 1506C to 2506C).
• Diesel oil: Used as fuel for diesel engines in vehicles and some heating systems (boiling range approx. 2006C to 3506C).
• Bitumen: Very long-chain hydrocarbons used for road surfacing and roofing (boiling points above 3506C).

Each fraction has a characteristic boiling point range. Shorter chains have lower boiling points and longer chains have higher boiling points.

Importance of Fractional Distillation

Fractional distillation is essential because it separates crude oil into useful products that can be used directly or processed further. Without this process, crude oil would be difficult to use effectively.

The fractions obtained serve as the starting materials (feedstocks) for other chemical processes, such as cracking and refining (covered in other topics). These processes help produce fuels and chemicals that meet specific needs.

In summary, fractional distillation:

• Separates crude oil into fractions with similar boiling points.
• Provides useful fuels and raw materials for industry.
• Enables further processing to improve fuel quality and produce chemicals.

Example: Understanding Fractional Distillation

If crude oil is heated to 4006C, hydrocarbons with boiling points below 4006C vaporise and rise up the fractionating column. Hydrocarbons with boiling points above 4006C condense lower down.

For example, petrol has a boiling range of about 406C006C, so it will condense near the top of the column. Diesel, with a boiling range of about 2006C3506C, condenses further down. Bitumen, with a boiling point above 3506C, remains liquid near the bottom.

For instance, if the temperature at a certain height in the column is 1806C, hydrocarbons with boiling points around this temperature will condense there, such as kerosene.