Fractional distillation and Crude Oil

What is crude oil?

• crude oil is a mixture of hydrocarbons

• most of these hydrocarbons are alkanes

• it is a finite resource and will eventually run out

Why is crude oil seperated?

It contains many different hydrocarbons that all have different boiling points. Fractional distillation is used to separate them into fractions with similar boiling points.

What is fractional distillation?

• A process that is used to separate crude oil into fractions

• It works because different hydrocarbons have different boiling points
  
  

What is a fraction?

A fraction is a mixture of hydrocarbons that all have similar boiling points. Each fraction has different properties and uses.

How does fractional distillation work?

1. Crude oil is heated and vapourised

2. The vapours enter a fractionating column

3. The column is hot at the bottom and cooler at the top

4. Hydrocarbons condense at different temperatures

5. Different fractions are collected at different levels

Why do hydrocarbons condense at different heights?

• Different hydrocarbons have different boiling points

• High boiling point hydrocarbons condense lower down where it is hotter

• Lower boiling point hydrocarbons rise higher before condensing

What happens to hydrocarbons with lower boiling points

They need less energy to vapourise so that means they need to rise further up the fractionating column finally condensing at the top.

What happens to hydrocarbons with higher boiling points

They need more energy to vapourise so they condense lower down the column. Some may not vapourise and remain as residue at the bottom.

How do the properties change down the column

Going down the column:

• Boiling point increases

• Viscosity increases

• Colour becomes darker

• Flammability decreases

What is Viscosity?

A measure of how easily a liquid flows.

High Viscosity: thick and flows slowly

Low Viscosity: runny and flows easily

What happens to the size of hydrocarbon molecules down the column?

Molecules become larger going down the column.

Smaller molecules are found near the top.

Larger molecules are found near the bottom.

What are the main fractions of crude oil?

From top → bottom:

Refinery gases

Petrol

Kerosene

Diesel oil

Fuel oil

Bitumen

What are fractions used for?

Refinery gases → heating and cooking

Petrol → fuel for cars

Kerosene → jet fuel

Diesel oil → fuel for diesel engines

Fuel oil → ships and power stations

Bitumen → road surfaces and roofing