Vacuum distillation

Crude oil is a mixture of hydrocarbons. They can be separated in a distillation tower. A tower that operates at atmospheric pressure leaves a residue of hydrocarbons with boiling points above 400°C and more than 70 carbon atoms in their chains.

Tea up a mountain
Trying to make tea up Everest. With thanks to Dave Richardson for the photo.

To separate these into smaller fractions at atmospheric pressure would need extremely high temperatures (and a taller tower). So we use the fact that the boiling point of a liquid is less at lower pressure. This is something that mountaineers know about.

They have always found that it is difficult to make a decent cup of tea up Mount Everest. This is because, up there, water boils at 72°C rather than 100°C. And this isn't hot enough to make tea (that tastes nice!). So why is the boiling point lower?

The boiling point is lower because the air pressure is less at the top of the mountain. And boiling point depends on air pressure. In fact, the lower the pressure, the lower the boiling point. It's even possible to boil water at room temperature by reducing the pressure to nearly a vacuum.

This is the principle that we use in the vacuum still. The heavier fractions have boiling points up to about 600°C. By reducing the pressure, we can lower their boiling points. It is more energy efficient to do this than to raise the temperature. Also, some of the hydrocarbons would decompose at the higher temperatures.

The vacuum still works on the same principle as the atmospheric still but with slightly different trays. Instead of bubble caps, the the trays are packed with metal rings. These allow the thick hydrocarbons to pass through without getting gummed up.

The hot, residue is taken from the bottom of the atmospheric still and passed into the vacuum still. The pressure is about a hundredth normal atmospheric pressure and this makes the fractions boil, allowing them to be separated.

But why does the boiling point decrease as we lower the pressure. To understand this, we need to look at what happens when we heat a liquid.

Vacuum distillation column
A vacuum still at Fawley.