Fractional distillation
The fractionating tower

Lets see how the process of fractional distillation works.

The diagram on the right shows an internal view of a fractionating tower. The blue arrow reveals the path that gaseous compounds are forced to take as they move upwards.

Crude oil enters the tower at approximately 375oC. At this temperature the oil becomes a hot mixture of liquid and gas. As this hot mixture enters the tower the liquid and gaseous compounds quickly separate. Liquids sink to the bottom of the tower, where they are collected, while the gaseous compounds rise upwards. As seen in the diagram above, the temperature in the tower gradually decreases as the gases rise. Different gas compounds condense, at different temperatures, as they move up the tower and are collected.

The tower contains many horizontal trays with hundreds of bubble caps. These bubble caps prevent the easy rise of gases through the column and force the rising gas mixture to bubble through condensed liquid in the tray. The bubble caps move up and down as gas pressure increases.

The animation on the left shows how the bubble caps work, while the arrow reveals one of the paths that the hot gas mixture is forced to take as it rises.


Compounds, in the rising gas mixture, that have the same boiling point as the condensed liquid in the tray will condense and be collected. As the gas mixture moves up the tower compounds with lower boiling temperatures will be collected.
Not only is the hot gas mixture separated by boiling temperature but also separated by molecular size. As we discovered earlier, alkanes are symmetrical molecules that are held together by weak intermolecular forces called dispersion forces. The strength of these relatively weak forces depends on the size of the molecule. Since the boiling temperature depends on the strength of the intermolecular forces, separating the molecules in order of boiling point will also separate them in order of size as well. As seen in the diagram above, compounds that condense further up the tower have relatively lower molecular mass than those that condense lower down the tower. In fact the hydrocarbons that are found at the bottom of the tower are so large that they are often used as bitumen for lining road surfaces.
The diagram on the left shows the hot mixture of gas rising through the trays in the tower. Identify the pure liquid in the tray and give reasons why you selected this compound.
The diagram on the left shows the hot mixture of gas rising through the trays in the tower. What is the composition of the gas mixture above the tray? Explain.
The fuel tanker contains octane, which is a major component of petrol and rolls along the road lined with bitumen. Describe where, on the fractionating tower would we expect to find octane and bitumen. Give reasons.
Compound "A" is expected to condense higher up the fractionating tower than compound "B". Why?
Continue with cracking