Fractional distillation and petrochemicals
Crude oil is a complex mixture of hundreds of hydrocarbons. Fractional distillation at an oil refinery separates it into useful fractions — groups of hydrocarbons with similar boiling points.
How fractional distillation works
- Crude oil is heated until most of it evaporates.
- Vapours rise into a fractionating column that has a temperature gradient — hotter at the bottom, cooler at the top.
- As vapours rise, each fraction condenses at a different height where the temperature equals its boiling point.
- Long-chain fractions (high BP) condense low down (gas oil, fuel oil, bitumen).
- Short-chain fractions (low BP) rise higher (petrol, naphtha, refinery gases).
The main fractions (top to bottom)
| Fraction | Use | Carbon range |
|---|---|---|
| Refinery gases (LPG) | Domestic heating, cooking | C1–C4 |
| Petrol (gasoline) | Cars | C5–C10 |
| Naphtha | Petrochemical feedstock | C8–C12 |
| Kerosene | Aviation / paraffin | C10–C16 |
| Diesel oil | Diesel engines | C14–C20 |
| Fuel oil | Ships, power stations | C20–C30 |
| Bitumen | Roads, roofing | C30+ |
Why fractions have different uses
- Short chains: low BP, low viscosity, very flammable → great fuels for transport.
- Long chains: high BP, viscous, sticky → roofing, road surfacing.
Petrochemicals
Crude oil is a feedstock for the petrochemical industry. Many everyday materials are made from oil-derived compounds:
- Solvents (e.g. cleaning products).
- Lubricants.
- Polymers (plastics like polythene, PVC).
- Detergents.
Without crude oil, manufacturing many household chemicals would be vastly more difficult.
✦Worked example— Worked example — choosing a fraction
A student needs a fuel that is easily ignited at room temperature for a small camping stove. Which fraction is best?
Refinery gases (LPG, propane/butane) — small molecules, low BP, very flammable, easy to ignite. Petrol works but is liquid; LPG is gas/liquid under low pressure.
⚠Common mistakes
- Saying fractions are pure substances — each fraction is itself a mixture of similar hydrocarbons.
- Assuming all of crude oil is fuel — bitumen, lubricants, feedstocks too.
- Confusing distillation with cracking — distillation separates; cracking breaks larger molecules into smaller ones (C7.4).
- Mixing temperature gradient direction — top is cool, bottom is hot.
Links
Builds on C2.5 (states of matter), C7.1 (alkanes). Sets up C7.3 (combustion of fuels), C7.4 (cracking).
AI-generated · claude-opus-4-7 · v3-deep-chemistry