Alkanes from Crude Oil

Crude oil is a mixture of hydrocarbons, mostly alkanes, with chains from one to over fifty carbon atoms. It is formed from the remains of sea creatures over millions of years, so it is a finite, non-renewable resource.A mixture is not useful as it is, so crude oil is separated into fractions by fractional distillation. A fraction is a mixture of hydrocarbons with similar chain lengths and a similar range of boiling points.1. Crude oil is heated in a furnace to about 350 °C, so most of it vaporises.
2. The vapour passes into a tall fractionating column, which is hot at the bottom and cooler at the top (a temperature gradient).
3. Vapour rises through trays (bubble caps). When it reaches a level cooler than its boiling point, it condenses and is piped off.
4. Very long chains (the residue) never vaporise and are removed at the bottom as bitumen.
5. The smallest molecules stay as gases and leave at the top as refinery gases.Separation works because of London forces. Longer molecules have more electrons and more points of contact, so they have stronger London forces and higher boiling points. They condense low down, where it is hot.Main fractions, from top to bottom: refinery gases (C1-C4, fuel gas), gasoline (petrol, C5-C10), naphtha (feedstock for chemicals), kerosene (jet fuel), diesel (gas oil), fuel oil (ships, power stations) and bitumen (roads and roofs).Fractions with small molecules are more volatile, flow more easily and ignite more easily than fractions with large molecules.