Crude oil principally consists of a mixture of carbon and hydrogen atoms. Referred to as a hydrocarbon, crude is primarily made up of paraffins, naphthenes, and aromatics in varying proportions depending on where it comes from. Crude oil also contains other elements, such as sulfur, nitrogen, oxygen, and small quantities of metals.
Oil refining starts by removing the salts and other impurities in crude input. Distillation is then employed to separate the crude into fractions, splitting the hydrocarbon molecules into smaller units. Crude oil is pre-heated to 355-370 ºC in pipe furnaces, where around 80% of it evaporates. Distillation proper takes place in tall towers, tens of meters tall, where gasoline and lighter fractions evaporate in the presence of heat and are drawn off in the upper section of the column.
Middle runnings – kerosene and gas oil (heating oil and diesel fuel) – which are heavier than gasoline, condense and are drawn off at the side of the column. Heavier fractions – in the form of heavy fuel oil, bitumen, and bottom oil – are drawn off at the bottom.
Subsequent processes, such as desulfurization, reforming, and cracking, modify the chemical structures of fractions and increase their value. Modern refineries feature numerous processing units. Neste Oil’s Porvoo refinery, for example, has around 40 and its Naantali refinery around 20.
A mixture of components
Oil refining produces a wide range of components. Some, such as butane and propane, are sold and used as such, as liquid gases. Aviation fuel is also sold as such. Most petroleum products are blends, however, containing a mix of components and additives. Motor gasoline, for example, typically includes more than 10 different components.