New tank cleaning systems have been developed to reduce the environmental impact of the ships and these make it possible to collect oil residues that can be treated on land rather than disposed of at sea. In order to obtain end products from oil, it is necessary to separate the mix of hydrocarbons contained in crude oil. This process, called fractionating distillation or, more commonly, refining, is carried out in an industrial complex called a refinery.

Crude oil is fed into a furnace and brought to a temperature of around 400°C that changes its physical state from liquid to vapour. The oil vapour is then injected into a fractionating column or refining tower.

In the refining tower gasses pass through a series of filter plates, rise to the top and cool down. At different temperatures they condense and return to a liquid state. When they fall back down deposits are formed on the plates which gives rise to the separation of the different fractions of hydrocarbons.

Fuel oils, lubricants, paraffin, wax and bitumen condense at the lowest level of the tower. At between 350° and 250° gas oil, used for diesel engines and domestic heating, condenses; at between 250° and 160° kerosene, an oily combustible used to fuel jet aeroplanes and heating plants; at between 160° and 70° naphtha, a liquid substance used as a combustible and, as a raw material, in the production of plastic materials, pharmaceuticals, pesticides, fertilisers.