Fractional Distillation

Fractional Distillation Column

How does fractionating work?

• Column is hottest at bottom and coolest at top
• Vapours cool as they rise up the column
• Vapours condense when they reach a part of the column that is below their boiling points
• Liquid falls onto tray and is piped away
• The vapours with the lowest boiling points do not condense at all and leave the top of the column as a mixture of gases

Trends in hydrocarbons

• The larger a molecule is, the higher its boiling point will be
• This is because the molecules are held together by intermolecular forces.
• The larger a molecule is, the more intermolecular forces there will be and so more energy is needed to break them up
• The longer a hydrocarbon chain is, the more intermolecular forces there will be and hence the more viscous the substance will be
• The lower a molecules boiling point is, the easier it will be to ignite. Hence, shorter chain hydrocarbons are easier to ignite.

Different fractions have different uses

• Fuels: These come from fractions near the top of the column. Fuels need to have a low boiling point and low viscosity
• Lubricating oils: Need to be highly viscous and hence are made from fractions near the bottom of the column. Bitumen is used in asphalt on the road.