CRACKING AND REFORMING
Cracking is the process of further treatment of crude oil fractions in which bigger hydrocarbon fractions are broken down into smaller hydrocarbon fractions. It consists of heating the hydrocarbons in the C 15- and- above range under high pressure to decompose them to smaller molecules. It is economical to convert some of the higher boiling point hydrocarbon fractions into lower boiling point fractions because the hydrocarbons obtained from the fractional distillation of crude oil range from those with one carbon atom to those with more than sixty carbon atoms. It is only the low molecular mass and low boiling fractions that are useful as fuels and raw materials in the chemical industry.
There are two types of cracking – Thermal cracking and catalytic cracking.
1) Thermal cracking: In Thermal cracking of petroleum, the less volatile crude oil fractions are passed through a chamber heated to a very high temperature (above 1000°C). It involves vapourising oil fractions of long carbon chain by heating them to a high temperature and pressure. Long chain alkanes are converted into smaller chain alkanes, alkenes and hydrogen, with ethene, C2H4 being the predominant alkene present. Thermal cracking is not efficient because there is poor control over the cracking system to yield alkanes of desirable chain length because the carbon to carbon bonds of the alkanes in the heavy oil fractions are of the same strength.
2) Catalytic cracking: This involves heating the long chain hydrocarbons under atmospheric pressure in the presence of catalyst. Higher boiling-point fractions, particularly gas oil, are brought into contact with a finely silica-alumina catalyst at a temperature of 450°C to 550°C under a slight pressure. The catalyst speeds up the process at a reduced temperature and produces more molecules of branched chain and aromatic hydrocarbons. (The catalyst silica – alumina consist of finely divided platinum suspended on aluminium oxide) or Zeolite).
Catalytic cracking produces petrol mixture rich in branched chain hydrocarbon with an attendant increase in octane number more than thermal cracking.
Benefits of the cracking process
- It increases the yield of petrol from crude oil.
- It yields as by-products, large quantities of ethene, propene, butene, etc. These are used for making plastics, synthetic rubber, detergents and many important chemicals.
This is the making of structural changes to hydrocarbon to improve its antiknock properties. It involves heating the petroleum fraction in the presence of oxide of silicon as catalyst at a temperature of about 600°C and pressure of about 12_ atmosphere in order to increase the fraction’s octane number. Reforming is used to improve the quality of petrol by converting straight-chain hydrocarbons to branched chain and cyclic hydrocarbons which yield high performance fuels.
Cracking is a breaking down process, while reforming is an isomerisation process.