Going in depth


The Three Stages of Refining

Crude oil needs to be processed before it can be used (See Close-Up: "Why Crude Oil Needs to be Refined"). Three major types of operation are performed to refine the oil into finished products: separation, conversion and treating.

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A crude oil separator on one of the artificial islands on the Kashagan field in Kazakhstan. © ROUSSEL MARC - TOTAL


In the first step, molecules are separated through atmospheric distillation (i.e. at normal atmospheric pressure), according to their molecular weight. During the process, which is also known as topping (refining)Another name for atmospheric distillation. , the oil is heated at the bottom of a 60-meter distillation column at a temperature of 350 to 400°C, causing it to vaporize. The vapors rise inside the column while the heaviest molecules, or residuals, remain at the bottom, without vaporizing. As the vapors rise, the molecules condense into liquids at different temperatures in the column. Only gases reach the top, where the temperature has dropped to 150°C. The liquids, which are become increasingly light the higher they are found in the column, are collected on trays located at different heights of the column. Each tray collects a different petroleum cut (fraction) A product obtained through the fractional distillation of oil. , also known as a petroleum cut, with highly viscous preservation (hydrocarbons)The final phase in petroleum system formation, after a deposit has accumulated... like asphaltAt ambient temperature, a sticky, black and highly viscous or solid mixture of heavy hydrocarbons... (bitumen) at the bottom and gases at the top.  

The heavy residuals left over after atmospheric distillation still contain many products of medium density. The residuals are transferred to another column where they undergo a second distillation to recover middle distillates like heavy fuel oilHeavy fuel oil is high-viscosity liquid fuel, used in big diesel ship engines or as a thermal power plant fuel.... and dieselDiesel is the name of an internal combustion engine that works by compression-ignition... .


There are still many too heavy hydrocarbonOrganic compound consisting of carbon and hydrogen. Hydrocarbons are the principal constituents of crude oil, natural gas and petroleum products. molecules remaining after the separation process. To meet demand for lighter products, the heavy molecules are “cracked” into two or more lighter ones.

The conversion process, which is carried out at 500°C, is also known as catalytic cracking because it uses a substance called a catalyst to speed up the chemical reaction. This process converts 75% of the heavy products into gas, gasoline and diesel. The yield can be increased further by adding hydrogenThe simplest and lightest atom, the most abundant element in the universe. , a process called hydrocrackingRefining process that converts heavy hydrocarbons into lighter, low-sulfur products in the presence of hydrogen. , or by using deep conversion to remove carbon.

The more complex the operation, the more it costs and the more energy it uses. The refining industry’s ongoing objective is to find a balance between yield and the cost of conversion.


TreatingTreating is all of the refining processes intended to remove unwanted compounds (contaminants)... involves removing or significantly reducing molecules that are corrosive or cause air pollution, especially sulfur. European Union sulfur emission standards are very stringent. Since January 1, 2009, gasoline and diesel sold in Europe cannot contain more than 10 parts per million (ppm)Dimensionless quantity representing 10-6 (1 to 1 million). This proportion can be used for both weight (mass) and volume... , or 10 milligrams per kilogram, of sulfur. The purpose of these measures is to improve air quality and optimize the effectiveness of catalytic converters used to treat exhaust gas. For diesel, desulfurizationOperation that removes most of the sulfur from flue gas, smoke, sour natural gas and refined petroleum products... , or sulfur removal, is performed at 370°C, at a pressure of 60 bar. The hydrogen used in the process combines with the sulfur to form hydrogen sulfide (H2S), which is then treated to remove the sulfur, a substance used in industry.

Kerosene, butane and propane are washed in a caustic soda (sodium hydroxide) solution to remove thiols, also known as mercaptans. This process called sweetening.

The octane rating measures a fuel’s resistance to detonation, which causes engine knocking.

Treating Automotive Fuels

Automotive fuels must also be treated to increase their octane rating, which is a measure of a fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... 's resistance to detonation, based on a scale of 0 to 100. (Engine knocking occurs when the fuel in an internal combustion engine ignites spontaneously with no input from the spark plug.) If the octane rating isn’t high enough, the engine will eventually be irreversibly damaged. To avoid this, it is necessary to boost the octane rating to 95 or 98.

The process used to produce high-octane products is called catalytic reforming. The chemical reactions during catalytic reforming, which uses platinum as a catalyst, occur at 500°C and a pressure of 10 bar. They convert some of the naphthenic hydrocarbons (saturated cyclic hydrocarbons) into aromatic hydrocarbons (unsaturated cyclic hydrocarbons), which have a much higher octane rating. Other chemical reactions, such as alkylationAlkylation is one of the main reactions used in the petrochemical industry. It increases the number of carbon atoms... , also improve the octane rating.

Refined Petroleum Products and Their Uses

Each refined petroleum product obtained from crude oilOil that has not been refined. has a specific use:


Source :

(1) Europa