Fractional Distillation

The properties of hydrocarbons

Boiling point- The temperature at which the liquid boils or the gas condenses. Short chain hydrocarbons have low boiling points, long chain hydrocarbons have high boiling points.

Volatility- The tendency to turn into a gas. Short chain hydrocarbons turn into gas easily, long chain hydrocarbons are the opposite.

Viscosity- How easily it flows. Short chains have a low viscosity (runny). Long chains have a high viscosity.

Flammability- How easily it burns. Short chains are very flammable, longs chains are not very flammable (and produce smoky flames).

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Fractional Distillation

  • Crude oil is separated into fractions at refineries using fractional distillation.

The crude oil is vaporised and fed into a fractioning column. this is hot at the bottom and cooler at the top.Inside the column there are many trays with holes to allow gases through. The vapours move up the column getting cooler as they go up. The hydrcarbons condense to liquids when they reach the level that is at their boiling point. Different liquids collect on the tays at different levels and there are outlets to collect the fractions.

  • Hydrocarbons with small molecules and low boiling points collect at the top.  
  • Hydrocarbons with big molecules and high boiling points collect at the bottom. (
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