Fractional Distillation of Crude Oil

Section 1 of 10Crude oil and fractions

A mixture

• A mixture Crude oil is a mixture of many hydrocarbons, most of them alkanes with only single carbon-carbon bonds.

Fractions

• Fractions Each fraction is a group of hydrocarbons with similar chain lengths and so similar boiling points.

Why separate

• Why separate The crude mixture is of little use, but each fraction has its own applications as a fuel or raw material.

Section 2 of 10How the column works

1

Crude oil is heated until it vaporises and the vapour enters the fractionating column near the bottom.

2

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

3

Vapours rise up the column until they reach a height where the temperature is below their boiling point, then they condense.

4

Large molecules with high boiling points condense low down and are drawn off as thick liquids.

5

Small molecules with low boiling points rise to the top and are drawn off as gases or light liquids.

Section 3 of 10Fractions and their uses

Fraction	Chain length	Use
Refinery gases	Shortest	Heating and cooking fuel (LPG)
Petrol	Short	Fuel for cars
Kerosene	Medium	Aircraft fuel
Diesel	Longer	Fuel for lorries, trains and some cars
Fuel oil	Long	Ships and power stations
Bitumen	Longest	Road surfaces and roofing

Section 4 of 10Trends as chains get longer

1Stronger forces

• Stronger forces Longer molecules have stronger intermolecular forces between them.

2Higher boiling point

• Higher boiling point More energy is needed to separate the molecules.

3Less volatile and more viscous

• Less volatile and more viscous The liquid evaporates less easily and flows more slowly.

4Harder to ignite

• Harder to ignite The fraction is less flammable and burns with a smokier flame.

Section 5 of 10Approximate chain lengths in each fraction

Fraction	Carbon atoms per molecule	Boiling point range
Refinery gases	1 to 4	Below about 25 °C
Petrol	About 5 to 10	Low
Kerosene	About 10 to 16	Medium
Diesel	About 14 to 20	Higher
Fuel oil	About 20 to 70	High
Bitumen	More than about 70	Highest; a thick residue left at the bottom

Section 6 of 10Burning the fractions

Combustion of hydrocarbons

Complete combustion

• Complete combustion With plenty of oxygen a hydrocarbon forms carbon dioxide and water, for example C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Incomplete combustion

• Incomplete combustion With limited oxygen, carbon monoxide and carbon (soot) form as well.
  • Longer chains have a higher proportion of carbon, so they burn with a smokier flame.

Energy released

• Energy released All hydrocarbon fuels release energy when they burn, which is why they are used for transport, heating and electricity generation.

Section 7 of 10Matching supply to demand

Imbalance

• Imbalance Crude oil contains more long-chain hydrocarbons than are needed, while there is greater demand for short-chain fuels such as petrol.

Cracking

• Cracking Long molecules are broken into shorter, more useful ones by cracking, which is a separate process carried out after distillation.

Section 8 of 10Fractional distillation in the laboratory

1

Heat the mixture of liquids gently in a flask connected to a fractionating column.

2

Vapours rise and the column provides a temperature gradient, with the coolest region at the top.

3

Collect the liquid that comes off in separate containers at different temperature ranges.

4

Each container holds a fraction with a similar boiling point, and the fractions with the lowest boiling points come off first.

Section 9 of 10Key terms

Term	Meaning
Hydrocarbon	A compound containing only hydrogen and carbon atoms.
Fraction	A mixture of hydrocarbons with similar boiling points collected together.
Fractional distillation	Separating a mixture of liquids by their different boiling points in a column.
Volatile	Evaporating easily because of a low boiling point.
Viscosity	How thick a liquid is and how slowly it flows.

Section 10 of 10Exam technique

1

Thinking the hydrocarbons are separated chemically; the process is physical and uses boiling points.

2

Reversing the columnthe hottest part is at the bottom and the largest molecules are collected there.

3

Writing that fractions are pure compounds; each fraction is still a mixture.

4

Saying that boiling points rise because the molecules break apart; they rise because intermolecular forces get stronger.