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Upon arrival at the refinery, the oil undergoes a series of treatments, which go together under the name of "refining", necessary to obtain all those products - LPG, gasoline, diesel and bitumen just to name a few - that pass through the gates and the dock of the refinery.
The first and perhaps the most important process that oil undergoes during refining is primary distillation. A sort of boiling, which takes place in the largest column of the refinery called topping.
This is what happens
Once inside the column, the lighter parts of the oil, in the form of vapor, quickly migrate upwards, followed by the heavier ones that are unable to go up the entire column and therefore stop at gradually lower heights. Then there is a part of the oil, about half, which is so heavy that it falls directly on the bottom of the topping. In this way the gases, the lighter compounds, are collected at the top of the column, which are partly used as fuel in the refinery itself and partly sold as LPG.
Below, gasoline, kerosene and diesel are collected. The bottom of the topping is instead sent to a new column, called Vacuum; all the products that come out of the vacuum pass through a series of systems (Visbreaking and Thermal Cracking) called conversion, which transform them, breaking up the long chains of carbon atoms, into lighter and more valuable products, that is gasoline and diesel.
However, what is not converted is used for the production of bitumen. Finally, gasoline and diesel oils, coming from topping and conversion plants, undergo some treatments that improve their characteristics, for example the sulfur content, that of benzene and the octane number, and make them suitable for being sold on the market.
Petroleum, whose name derives from the Latin (petra oleum) that is stone oil, is a sort of dense and oily soup, with a characteristic odor and a variable color from yellow-brown to blackish, a mixture of compounds, hydrocarbons, they would say chemicals to underline the fact that they are made up of hydrogen and carbon, characterized by very variable weights and dimensions. These compounds are the result of the ability of carbon to form long linear chains, branched or closed in a ring, of atoms directly connected to each other. What you have found scattered on this page are just some of the compounds, with complex and extravagant shapes, that can be found by browsing inside the barrel. Then, based on the shape and how the atoms bond with each other, the hydrocarbon compounds follow different paths within the refinery, but all of them are very long and complex.
The barrel is, together with the ton, the unit of measurement commonly used in the oil industry. It corresponds to about 160 liters as this size was the wooden barrels used, at the dawn of the oil industry, in Pennsylvania for the collection of crude oil.
A refinery can have a simple structure or an extremely complex and sophisticated one. As complexity increases, the refinery's ability to obtain a greater quantity of valuable products, such as gasoline and diesel, from the same barrel of crude oil, to the detriment of those with a lower economic value such as fuel oil.