|This is another treatment that is used to produce molecules that will raise the octane number of petrol. This process changes the shape of molecules rather than their size (as in cracking which makes smaller molecules).
The starting material is naphtha, another fraction from the distillation of crude oil. In this process straight chain alkanes are converted to cycloalkanes and then to aromatic hydrocarbons.
The naphtha is preheated to about 500oC with hydrogen. It is fed into reactor vessels at about 20 times atmospheric pressure in the presence of the catalyst. Rearrangement reactions take place producing aromatic and other hydrocarbon molecules with high anti-knock vlaues.
Before the mixture is fed into the reactors, it is passed through a Hydrofiner. Sulphur and sulphur compounds, which would damage the catalyst, are removed by reacting with the hydrogen forming hydrogen sulphide.