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Crude oil is converted to gasoline through a relatively simple refining process. The transformation begins with the extraction of oil from the ground, after which it is usually loaded into large container ships that deliver it to refineries all over the world. As any viewer of news footage has seen, crude oil emerges as a thick black substance, which does not resemble the clear and free-flowing gas used in motor vehicles. This is because crude oil is actually a mixture of hydrocarbons.
As the prehistoric plants and animals that make up crude oil broke down, they formed hydrocarbons consisting of variously sized chains and structures. Each hydrocarbon has a unique application, which the refinery process aims to maximize. The use for each depends on the number of carbon atoms in its structure. Gasoline, for example, has eight carbons, while light gases like propane have only three. Hydrocarbons have a lot of energy, when they can be disentangled, and the refining process accomplishes this.
The most important part of the refining process is known as fractional distillation. Because the hydrocarbons all have different boiling points, they can be separated by heating. The crude oil is heated in a boiler to temperatures up to 1112°F (600°C), which coverts all the hydrocarbons into a vapor. As they cool below their boiling points, they precipitate out as liquids.
The vapor is routed through a distillation column. At the bottom, the hydrocarbons with the highest boiling points are caught first on a screen that pulls out the residual, or coke, which is often flashed or burned for energy. The vapor moves up the column, and as it cools, screens along the way catch the various hydrocarbons, such as diesel, kerosene, gasoline, naphtha, and the light gases.
All of these outputs must be treated for impurities before they can be shipped. A sulfuric acid column removes particles, unsaturated hydrocarbons, oxygen compounds, and nitrogen compounds. Then, the liquid is passed through an absorption column that removes water, and it is treated to remove sulfur. After this process, the various crude products can be shipped to their end destinations through a large network.
Gasoline comprises almost half the output of a barrel of crude oil, although the chains that make up this hydrocarbon do not make up half a barrel. This difference is resolved through chemical refining, which allows refineries to build up or break down hydrocarbon chains to get different products. Chemical refining outputs are changed depending upon the demand, which is frequently heaviest for gasoline.
When hydrocarbons are broken down into small components, it is called cracking. Cracking can be accomplished by introducing heat to the hydrocarbons or by using a chemical catalyst, like hydrogen gas. When hydrocarbons are combined to form longer chains, it is known as unification. Unification most commonly uses platinum as a catalyst to combine small carbon chains, producing hydrogen gas as a byproduct. The hydrogen gas can be used for cracking or sold. Hydrocarbons are also chemically altered in a a process called alkylation, which combines compounds of a low molecular weight with a catalyst and introduces the mixture to the hydrocarbons being altered.
The process whereby crude oil is turned into gasoline is carried on at high volume all over the world. Most refineries are extremely efficient, using every hydrocarbon chain separated during the distillation process and tweaking the output as needed to adjust for market demands. The supply of crude oil is known to be limited, however, raising questions about the longevity of the future of refining. In addition, most of the world is heavily dependent upon oil from one highly unstable source: the Middle East.