Every hydrocarbon in crude oil has its own boiling point. A refinery uses this to sort the mixture into parts called fractions, in a process called fractional distillation.
Longer chains, as you have seen, boil at higher temperatures: they touch their neighbours along more of their length, so more small pulls hold them together. The refinery heats crude oil to about 400 °C, so most of it turns to vapour, and the vapour flows into the bottom of a tall tower called a fractionating column. The column is hottest at the bottom and coolest at the top.
The vapour rises through the column and cools as it goes. Each hydrocarbon turns back into a liquid, or condenses, once it reaches a height where the column is cooler than its boiling point, and trays at that height collect it. Small molecules rise all the way to the top. The biggest molecules stay liquid even at 400 °C, and they drain out of the bottom.
| Fraction | Carbons per molecule | Used for |
|---|---|---|
| refinery gases | 1 to 4 | bottled gas for heating and cooking |
| petrol | 5 to 12 | fuel for cars |
| kerosene | 9 to 16 | fuel for jet aircraft |
| diesel | 12 to 20 | fuel for lorries, buses and trains |
| lubricating oil | 20 to 50 | engine oil and grease |
| bitumen | 35 and more | road surfaces and roofing |
Each fraction is a mixture of molecules that boil over a range of temperatures. Molecules with the same number of carbons can boil at different temperatures (a branched chain boils lower than a straight one), so the carbon counts of neighbouring fractions overlap. Going down the column, the fractions get thicker, darker and harder to set alight. Refineries blend fractions too: petrol at the pump also holds some butane, so lesson 4 counted 4 to 12 carbons in petrol.
Cracking long chains
Crude oil holds more big molecules than people need, and fewer of the small ones that make petrol. Refineries break long chains into shorter ones by heating them to about 500 °C with a catalyst. This is called cracking. Decane, with 10 carbons, can crack into octane and ethene: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. Cracking makes petrol-sized alkanes, and alkenes such as ethene, the starting point for poly(ethene).