About a century ago it became clear that distillation of crude oil alone could not satisfy the rapidly growing demand for petrol.
Early in the history of petroleum refining, it was found that higher-boiling hydrocarbons could be broken down or 'cracked' into lower-boiling ones by subjecting them to prolonged high temperatures. High pressures are needed, unless a catalyst is used.
THERMAL CRACKING gives mainly low-octane gasoline components; mainly straight chain alkanes.
CATALYTIC CRACKING gives higher-octane aromatics, branched alkanes, aromatics and alkenes.
A third type of cracking, HYDROCRACKING, consists of cracking in the presence of hydrogen. Hydrocracking is the most flexible type of cracking because the amount of hydrogen you add add determines the products. You can make large amounts of petrol (or furnace oil), as required.
The petrol fraction made by this method tends to be lower octane because it contains less alkenes and aromatics.
Back to top