Oil refineries
Bubble traps
Graphic of bubble trays
Bubble trays hold the bubble caps

When crude oil emerges from the furnace it is in two parts - a liquid and vapour. Both are mixtures but the vapour contains a higher proportion of the more volatile (easy to boil) hydrocarbons.

When this vapour mixture meets the first bubble cap tray it condenses to form a liquid mixture. At first, this condensate is the same as the original mixture of hydrocarbons.

However, more hot vapour rises up through the bubble caps and makes some hydrocarbons in this liquid boil. This produces a vapour mixture that has a greater proportion of the more volatile hydrocarbons.

This vapour mixture rises further up the column, where some condenses on the next bubble cap tray. The process repeats at each of the forty trays up the column.

There are about 40 bubble cap trays in an atmospheric still. Each one forces rising vapours to mix with falling liquids in a seething foam.

There is an overflow weir to allow some liquid to fall back down to be evaporated again. Some of the trays have outlets from which the liquid fractions can be tapped off.

Each tray is cooler than the one below it. So the mixture on each tray has more and more of the volatile hydrocarbons as we go up the tower.

The most volatile ones are gases at room temperature. They are collected off the top of the tower.

The bubble caps make up a bubble cap tray (or bubble trap).

They allow hot vapour to bubble up through the tray whilst supporting the liquid in the tray.

Graphic of bubble trap
A bubble cap forces the vapours and liquid to mix.