Lesson 2 - Fractional Distillation

Because crude oil is a mixture of different hydrocarbons, we can separate the mixture.

Each of the different hydrocarbons in the crude oil has a different boiling point and so will boil/condense at a different temperature.

The separation of crude oil works through a process called 'fractional distillation'.

See diagram below...

1. Crude oil is heated strongly to around 350-400^oC. This causes ALL of the different alkane hydrocarbons to boil and turn into a gas.

2. All the gases go into the bottom of the fractionating column and rise up the column (hot gases rise!)

3. Because the column gets cooler as you get higher up, the different alkane hydrocarbons will condense when the temperature falls below their boiling point (gas > liquid).

4. At the very top of the column, it is around room temperature but the alkanes are so small that their boiling point is too low to turn into liquid. They are tapped off the top as gases.

Because it is impossible to separate the column into one level for each different alkane, groups of alkanes are 'tapped off' (with similar boiling points) together. Each of these smaller groups/mixtures is called a 'fraction'.... which is why we call the process fractional distillation.

Each fraction has different properties.....

• Boiling points increase as the alkanes get larger - the reason for this is because the 'intermolecular forces' between the alkane molecules get stronger as the molecule gets larger.
• Viscosity increase as the alkanes get larger - viscosity is a measure of how 'thick or runny' the alkane is. When the molecules get larger, they tangle together more and so become harder to 'pour' and 'thicker'.
• Flammability decreases as the alkanes get larger. The alkanes become harder to set alight because the molecules are a lot bigger and the chains are harder to break apart. They also need more oxygen to burn.

The uses of different fractions are;