Petroleum or crude oil is composed of long chain hydrocarbons. These complex hydrocarbons are broken down into simpler molecules by the breaking of carbon to carbon bonds resulting in smaller, more useful alkanes and alkenes. Cracking the long chained alkanes produced from fractional distillation is necessary to produce ethene, propene and octane.
Alkanes are molecules where the hydrogen and carbon atoms share single bonds. An alkene is an unsaturated hydrocarbon that has at least one carbon to carbon double bond. When the process of hydrocarbon cracking is applied, long chain hydrocarbons become shorter ones. When crude oil is processed, it's components have various boiling temperatures.
During the fractional distillation process, the liquid is heated with high pressure steam. The mixture boils forming gases. These enter a long column called the fractional distillation column. The vapor rises in the column, and eventually cools to form a liquid. The substance with the lowest boiling point condenses at the highest point in the column. Trays collect the separated liquid fractions. They are cooled further and placed in storage tanks. These components will further be chemically processed.
Only 40 percent of crude oil is gasoline. Some of the other fractions from the distillation column are used when making gasoline, increasing the yield per barrel of crude oil. Cracking is done to the naphtha fraction or the gas oil fraction from fractional distillation of petroleum or crude oil. These fractions are re-vaporized before cracking. When C15 H32 is cracked, it forms ethene, propene and octane. The octane is one of the molecules in gasoline. Catalysts are used in these types of reactions when they occur at lower temperatures and pressures.