Oil, raw or unprocessed, is not very useful and has little use if it comes from the heart of the earth. Although sweet oil (low viscosity as well as low sulfur) was used unrefined in steam powered vehicles, its gases and other lighter solutions usually accumulated in the fuel tank and caused an explosion. Apart from the above, in order to use oil, in order to produce other products such as plastics, foams, etc., crude oil must be refined. Petroleum fuel products are used in a wide range of applications, ship fuel, jet fuel, gasoline and many more. Each of these materials has a different boiling point, so they can be separated by distillation. Because there is a high demand for lighter liquid components, in a modern oil refinery, heavy hydrocarbons and light gaseous components become more valuable materials during complex and energy-intensive processes.

Oil can be useful in many cases because it contains hydrocarbons of different weights and lengths, such as paraffin, aromatics, naphtha, alkenes, religions, and alkalis. Hydrocarbons are molecules of different lengths that are composed only of hydrogen and carbon, with different structures giving them different properties. The technique of oil refining is to separate and increase the purity of the components of the oil.
Once the components have been disassembled and purified, the lubricant or fuel can be consumed directly on the market. Combining smaller molecules such as isobutane and propylene or butylene can be done with processes such as alkalinization or dimerization to produce the desired octane fuel. Also, the octane degree of gasoline can be improved during the catalyst recovery process, during which hydrogen is separated from hydrocarbons and aromatic hydrocarbons are formed, which has a much higher octane degree. The intermediate products of the separator tower can be converted into lighter products during thermal cracking, hydrocracking or catalytic fluid cracking processes. The final step in gasoline production is the combination of different hydrocarbon materials with different octane degrees to achieve the desired product specifications.
Large refineries typically have the capacity to refine from hundreds of thousands to several hundred thousand barrels of oil per day. Due to the high capacity required, many refineries operate continuously for long periods of several months to several years.
Oil refineries have different complexities depending on the type of feed they are designed to produce, depending on the amount and type of products they produce. The amount of investment to build a refinery with an average complexity is between 18 to 19 thousand dollars per barrel of refining capacity, and with increasing degree of complexity sometimes requires up to 24 to 25 thousand dollars per barrel.