Some of the heavier oils will be cracked at a later stage. The fractions with very high molecular weight settle out at the bottom. These are used as boiler fuel and asphalt. Heated crude oil enters some way from the bottom. The cracked mixture passes to the fractionating column.
Catalyst is passed to the fluid bed unit.
Products such as ethene, propene and heptane are separated from fractionating tower. These two types of polymer are important, and they produce such a wide range of everyday materials. Some hydrocarbon molecules are very reactive and can be joined together, or polymerised. The reactive units are those in which carbon atoms are joined by a double bond. Ethene, shown below, is an example.