🔥 Bottom of the column
Hot. Hydrocarbons with large molecules and high boiling points condense here, low down, because they turn back into liquids easily.
Crude Oil and Fuels » Fractional Distillation and Petrochemicals
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.7.1.2
- How crude oil is separated into fractions
- How a fractionating column uses evaporation and condensation
- Which fuels come from crude oil
- Why crude oil is also a starting point for many useful materials
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseWhy crude oil needs separating
Crude oil is a mixture of many different hydrocarbons, mostly alkanes. Straight from the ground it is a thick, dark liquid that is not much use. A car cannot run on it, and a plastic factory cannot use it either. First it has to be split up.
The hydrocarbons are not chemically joined to each other, so they can be separated by a physical process. We use fractional distillation, which you met in the lesson on mixtures and separation techniques. This time the mixture is crude oil.
Key terms:
- Fraction: a group of hydrocarbons in crude oil with a similar number of carbon atoms, and so a similar boiling point range.
- Feedstock: a raw material that is used to make other useful substances.
- Petrochemical industry: the industry that turns fractions from crude oil into other useful materials.
How fractional distillation works
Those tall towers are fractionating columns: hot at the bottom, cooler at the top, so each fraction condenses at its own height
The separation happens in a tall fractionating column at an oil refinery. The column is very hot at the bottom and gets cooler towards the top. Follow the oil through these steps.
- The crude oil is heated until most of it evaporates and turns into a hot vapour.
- The vapour is fed into the bottom of the column.
- The vapour rises up the column, where it gets cooler and cooler.
- Each type of hydrocarbon condenses back into a liquid when it reaches a part of the column that is cooler than its boiling point.
- The liquids are drawn off at different heights as separate fractions.
❄ Top of the column
Cooler. Hydrocarbons with small molecules and low boiling points stay as gases all the way up and only condense at the top, or not at all.
So the fractions are sorted by boiling point, and the boiling point depends on the number of carbon atoms in the molecules. The detail of how boiling point changes with molecule size is covered in the lesson on properties of hydrocarbons.
Remember
Separation by fractional distillation depends on evaporation (liquid to gas) and then condensation (gas to liquid). Each fraction condenses where the column is just cooler than its boiling point.
Fuels from crude oil
Many of the fuels we depend on for our modern lifestyle come from crude oil. You need to know these five.
- Liquefied petroleum gases (LPG): small molecules that are gases at room temperature. They are squeezed into a liquid for storage, for example in gas cylinders for heating and cooking.
- Petrol: fuel for most cars.
- Kerosene: fuel for aircraft.
- Diesel oil: fuel for lorries, buses, trains and many cars.
- Heavy fuel oil: thick fuel for large ships and for some power stations.
LPG comes off near the top of the column. Heavy fuel oil is drawn off near the bottom. Petrol, kerosene and diesel are in between. You do not need to learn the names of any other fractions or fuels.
The petrochemical industry
Plastics, solvents, detergents - loads of everyday stuff on this shelf began life as feedstock from crude oil
Fuels are not the only use. Fractions can also be processed to make feedstock for the petrochemical industry. This industry makes many materials that modern life depends on:
🧪 Solvents
Liquids that dissolve other substances, used in paints and cleaning products.
⚙ Lubricants
Oils that help parts slide past each other, such as the oil in an engine.
📦 Polymers
Very large molecules that make plastics.
🧼 Detergents
Cleaning substances used in washing powders and washing-up liquids.
Because crude oil is so useful as a feedstock, burning all of it as fuel uses up a finite resource that we also need for making materials.
Why are there so many carbon compounds?
There is a vast array of natural and synthetic carbon compounds. This happens because carbon atoms can form families of similar compounds. The alkanes are one family: they all have similar chemical behaviour but different numbers of carbon atoms. You will meet more families later in this course.
Worked example
Question: Hydrocarbon X has a boiling point of 40 °C. Hydrocarbon Y has a boiling point of 350 °C. Which one is collected lower down the column?
Answer: Y. It has the higher boiling point, so it condenses back to a liquid sooner as it rises, where the column is still hot. X stays as a vapour for longer and condenses higher up.
Common mistakes
Saying that the oil is burned or that chemical reactions split it up. Fractional distillation is a physical process. Another mistake is saying that the small molecules are at the bottom. The column is hottest at the bottom, so the large molecules with high boiling points are collected there.
Exam-style question
Explain how fractional distillation separates crude oil into fractions. [4 marks]
Model answer
The crude oil is heated so that it evaporates. The vapour enters the bottom of the fractionating column, which is hot at the bottom and cooler at the top. The vapours rise up the column. Each hydrocarbon condenses when it reaches the part of the column that is below its boiling point. Hydrocarbons with higher boiling points condense lower down and those with lower boiling points condense higher up, so each fraction is collected at a different height.
Exam tip
Always use both words: evaporation and condensation. Say where each happens and link it to boiling points.