AS AQA Chemistry - Unit 1: Fractional Distillation, Cracking

Petroleum

Petroleum/Crude Oil - complex mixture of hydrocarbons (mainly alkanes.) It comes from the remains of sea creatures and plants. Compression caused high pressures and temperatures, and along with the absence of air, converted it into oil and gas.

Alkanes are the homologous series of sautrated (Single Bonds) hydrocarbons (Only containing Hydrogen and Carbon atoms) with the general formula CnH2n+2. They're important as fuels because they burn in oxygen with highly exothermic reactions.

Crude oil is able to be separated by fractional distillation because the complex mixture of hydrocarbons have different boiling points.

Fractional Distillation

Fractional Distillation is a physical process. Energy is needed only to separate the molecules from each other, to overcome the VDW's forces.

Complex mixture of hydrocarbons in crude oil is separated into less complicated mixtures, or fractions.

The fractioning column is the long vertical tower used.

(http://mrsmaineswiki.wikispaces.com/file/view/gcsechem_27.gif/31979815/gcsechem_27.gif)

The crude oil is heated and the vapour/liquid mixture passed into the tower.

The temperature gradient in the column allows the separation of the petroleum mixture into fractions depending on the boiling points of the hydrocarbons present.

To avoid decomposition of the residue because of the high temperatures, it is further distilled under reduced pressure (vacuum distillation).