The various Marine Fuels with quality in accordance with ISO8217 are produced from crude oil by means of fractional distillation in refineries.
In the process, crude oil is gradually heated to just below the temperature at which it would decompose.
If during the heating process, the boiling point of a particular component of the crude oil – a “fraction” – is exceeded, it passes into the gas phase. Fractions with smaller molecules evaporate at lower temperatures than those with long-chain (and therefore larger) molecules. The individual fractions are then cooled to below their boiling point again, and thus liquefied.
The fractions separated from the crude oil in this way are called distillates. The remaining residue in the fractional distillation, which does not pass into the gas phase, is referred to as residual fuel or Heavy Fuel Oil.