Topping: Atmospheric Distillation
The first step is that of the separation of the molecules by atmospheric distillation (ie at normal atmospheric pressure), in function of their molecular weights.
This process involves heating the oil to 350/400 ° C to cause it to evaporate. Heating is carried out at the base of a distillation tower 60 meters high. The crude vapors rise in the tower while the heaviest molecules, or heavy residues, remain at the base without evaporating. As the vapors rise, the molecules condense one after the other into liquids, to the gases which alone reach the top of the tower, where the temperature is no more than 150 ° C. At different levels of the tower are trays which make it possible to recover these liquids increasingly lighter. Each tray corresponds to a distillation fraction, also referred to as a petroleum fraction, from bitumens (highly viscous hydrocarbons) to gases.
The heavy residues from this distillation still contain many products of medium density. They are subjected, in another column, to a second distillation which makes it possible to recover more average products (heavy fuel oils and diesel oil).
A catalytic reforming unit for the increase of the octane number of gasolines, provided with hydrodesulfurization and de-ethanization units for liquefied petroleum gases.