Oil refineries
Liquids, gases and pressure

We can change the boiling point of a liquid by changing the pressure around it. Reducing the pressure, reduces the boiling point and increasing the pressure, increases the boiling point.

Photo of vacuum still
A vacuum still at Fawley.

We use this in at least two ways on the refinery.

  • in vacuum distillation - we reduce the pressure to boil the heavier fractions at a temperature way below their normal boiling points.
  • to liquify gases to make LPG - we increase the pressure to turn propane and butane gases into liquids, making them easier to store, transport and use in the home.

So why does the boiling point of a liquid change with pressure?

To understand this, we need to look at what happens when we heat a liquid.

All of matter is made of particles. In a liquid, the particles move are moving about randomly. Some of these particles are moving fast enough to break free of the surface. They form a vapour above the liquid.

As we heat the liquid, more particles can break free producing more and more vapour. This vapour will produce a pressure above the liquid. (In our picture, there is a lid of the container and you can think of the vapour pushing up on this lid).

Eventually, the pressure from the vapour will equal atmospheric pressure. This is the boiling point and, as we continue heating it, the liquid will turn into a gas. I.e. the liquid starts to boil when its vapour pressure equals the surrounding pressure.

Graphic of particles in a liquid

If we reduce the pressure of the surroundings, then the vapour pressure will equal the surrounding pressure at a lower temperature. Therefore, we have reduced the boiling point.

You can think of it in this way: if the surrounding pressure is lower, it is easier for the liquid molecules to escape from the surface so it boils more easily. If the surrounding pressure is higher, the liquid molecules will find it more difficult to escape, so they will stay in the liquid until a higher temperature is reached.

Gas BP °C State at
20°C
As LPG
propane - 42 gas pressure: 
BP:
state:
14 atm
38°C
liquid
butane - 0.5 gas pressure: 
BP:
state:
2.6 atm
38°C
liquid

Increasing the pressure

We get the reverse effect if we raise the pressure. The particles find it more difficult to leave the surface of the liquid. And the liquid has to be hotter before its vapour pressure matches the higher surrounding pressure.

Therefore, the boiling point of a liquid increases at higher pressure.

If we raise the boiling point of a gas from below room temperature to above it, then the gas will turn into a liquid. Looking at this another way, we can pressurise a gas to liquify it. This is what happens with propane and butane to make Liquified Petroleum Gas (LPG).

Propane has a boiling point of -42°C. This means it is normally a gas. However, if we increase the pressure to 14 times atmospheric pressure, we can raise it boiling point to 38°C. This means it is a liquid (even on a hot day). Once it has been liquified, it can be put into sealed containers which keep it liquid by keeping it at the higher pressure. It is now Liquified Petroleum gas (or LPG).

Similarly, butane has a boiling point of -0.5°C which can be raised to 38°C by increasing its pressure to 2.6 atmospheres.

So, by compressing the gases, we get them to turn from a gas to a liquid at room temperature.