The crude oil arrives at the oil refinery from the oil production offshore platform subsea pipeline landfall point, or from oil tankers.
It will have most of the gas, water, and sand/grit removed by the production platform facilities before being processed at the crude oil refinery.
On arrival at the refinery it is again subjected to dewatering and salts removal before being heated to around 700ᴼF and fed into an atmospheric distillation column. This is a vertical column with trays situated at specific heights inside the column.
As the crude oil vapors boil off and pass upwards, the lower boiling point factions such as diesel, kerosene, and naphtha are drawn off at the trays at various temperatures for further refining.
Meanwhile the higher boiling point crude oil residue has dropped to the bottom of the distillation column. It is from these “bottoms” that the heavy fuel oil is drawn off, literally the bottom of the barrel. If no further processing is required it is stored in tanks ready for distribution.
Note: Heavy Fuel Oil can sometimes have addition of lighter fuel oil to reduce excessive viscosity. But it is always advisable to have samples of bunker oil checked to ensure no other “blending” has taken place i.e. addition of lube oil or addition of used motor oil!