GCSE OCR Gateway Organic Chemistry C6.2j Crude Oil: fractionation

C6.2j To be able to describe the separation of crude oil by fractional distillation and to name the fractions produced

Crude Oil Fractionation

Crude oil contains a mixture of hydrocarbons.

Hydrocarbons are molecules formed from just two elements hydrogen and carbon.

When crude oil is heated to a high temperature say over 400^oC the oil vaporises, all the hydrocarbons enter the gaseous phase.

If this hot mixture is gradually allowed to cool as it rises up a long stainless steel column the different hydrocarbons will start to condense.

These hydrocarbons start to condense because the further they are along the column from the source of heat the cooler it is. 

These columns, which can be many meters high, are called fractionating towers or fractionating columns.  They are the characteristic sight you see when looking at an oil refinery.

The arrangement of the fractionating tower means that at a given distance from the heat source the column will be at a specific temperature lower than 400^oC.

At this particular temperature say it is 120^oC. a particular group of hydrocarbons will condense because the temperature is not high enough for them to stay in the vapour state but the rest of the hydrocarbon molecules (smaller molecules) will remain in the vapour state and pass along and up the column.

At the 120^oC point on the column, a bubble plate is placed to accelerate separation of the more volatile smaller molecules of hydrocarbon.

In fact at several points or temperatures along the fractionating column are these bubble plates to facilitate the separation of the different molecules or fractions of the crude oil.

A bubble plate is a tray of bubble caps.

The picture below shows the different primary fractions from the initial fractionation of crude oil.

Please note that fractions are NOT pure compounds but mixtures of hydrocarbons molecules of different lengths.