Hydrocracking reactions

Hydrocracking is the hydrogenation of hydrocarbon and non-hydrocarbon compounds under the action of a catalyst, with alkanes and olefins undergoing cracking, isomerisation and a small number of cyclisation reactions, with the eventual conversion of polycyclic compounds into monocyclic compounds. Hydrocracking uses a bifunctional catalyst with both cracking and hydrogenation effects, so hydrocracking is essentially a catalytic cracking reaction carried out under hydrogen pressure.

Hydrocracking is a process in which hydrocarbon molecules are cracked and hydrogenated at the catalyst surface under high pressure to form The conversion process of smaller molecules also takes place with hydrodesulphurisation, denitrogenation and hydrogenation of unsaturated hydrocarbons. The chemical reactions include saturation, reduction, cracking and isomerisation. The direction and depth of reaction of hydrocarbons under hydrocracking conditions depends on factors such as the composition of the hydrocarbon, the performance of the catalyst and the operating conditions. During hydrocracking, hydrocarbon reactions follow the following pattern: increasing the reaction temperature intensifies C- C bond breaking, i.e. hydrocracking of alkanes, ring breaking of cycloalkanes and chain breaking of alkyl aromatics. If the reaction temperature is high and the hydrogen partial pressure is not high, this will also cause C-H bond breaking to produce olefins, hydrogen and aromatics. Increasing the reaction pressure favours saturation of the C-C bond; decreasing the reaction pressure favours dehydrogenation of alkanes to form olefins and cyclisation of olefins to form aromatics.

At lower pressures and higher temperatures, condensation reactions also occur until coke is produced. Hydrocracking catalysts have both a hydrogenation active centre and an acidic centre, which is a bifunctional catalyst. The acidic function is provided by the catalyst's carrier (silica-aluminium or zeolite), while the metal component of the catalyst (platinum or oxides of tungsten, molybdenum, nickel, etc.) provides the hydrogenation function. The result of hydrocracking hydrocarbons using a bifunctional catalyst in the hydrogenation process is largely related to the hydrogenation and acidic activity of the catalyst, and the proportional relationship between them. Hydrocracking catalysts are classified as having high hydrogenation activity and low acidity, and low hydrogenation activity and high acidity activity.