Cracking is used to convert components of crude oil, for which there is insufficient demand, into smaller molecules, for which a demand exists. Cracking may be carried out:

  • At a very high temperature with steam (steam cracking)
  • At a high temperature with a catalyst (catalytic cracking)
  • Using hydrogen and a high pressure (hydrocracking) The choice partly depends on the products required
Cracking of Oil Products

Steam Cracking

The feedstock is vaporised by pre-heating, mixed with steam, and passed into a furnace. The resulting mixture of products depends on the feedstock, the temperature used (between 700 and 900 °C), and the time spent in the furnace ("residence time"), which ranges from 0.1 and 0.5s.

All feedstock produces significant quantities of ethene, a particularly useful chemical, but naphtha and gas oil also generate a high proportion of raw pyrolysis gas (RPG), and this is favoured by lower temperatures and shorter residence times.

RPG is a mixture of C5 to C8 hydrocarbons, including several aromatic compounds, including benzene. The composition of RPG includes:

  • benzene
  • methyl benzene
  • 1,2- 1,3- & 1,4- dimethylbenzene
  • aliphatic (non-aromatic) hydrocarbons in the C5 to C10 range, including both alkanes and alkenes

Naphtha cracking results in about 30% each of ethene and RPG, and significant quantities of other products.

Processing RPG

The RPG is reacted with hydrogen, and this results in:

  • The hydrogenation of mono-alkenes to saturated alkanes, for example:

  • The desulphurisation of the gas mixture (any sulphur present will react with the hydrogen)
  • No hydrogenation of any benzene to cyclohexane under these conditions (because of the stability of the benzene ring)

The benzene and other aromatics are removed by solvent extraction, as these will dissolve in polar solvents whereas the other hydrocarbons will not.

Desulphurisation produces hydrogen sulphide, which can be removed by dissolving in a base. The hydrogen sulphide can be recovered and used in other processes (see sulphuric acid section).

Separating Benzene

The separation of pure benzene from its feedstock is difficult as it forms azeotropic mixtures (i.e. constant boiling mixtures) with many C6 - C8 alkanes. Distillation will always produce a mixture, not the pure compounds. Benzene cannot therefore be separated from these alkanes by fractional distillation and various other techniques have to be employed, usually solvent extraction. Click for details of separation methods used.

Question

Why is the removal of sulphur important to achieve?

diagram: Naphtha cracking products

  back to top