Petroleum refineries convert (refine) crude oil into petroleum products for use as fuels for transportation, heating, paving roads, and generating electricity and as feedstocks for making chemicals.
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Refining breaks crude oil down into its various components, which are then selectively reconfigured into new products. Petroleum refineries are complex and expensive industrial facilities. All refineries have three basic steps:
Modern separation involves piping crude oil through hot furnaces. The resulting liquids and vapors are discharged into distillation units. All refineries have atmospheric distillation units, but more complex refineries may have vacuum distillation units.
Inside the distillation units, the liquids and vapors separate into petroleum components, called fractions, according to their boiling points. Heavy fractions are on the bottom and light fractions are on the top.
The lightest fractions, including gasoline and liquefied refinery gases, vaporize and rise to the top of the distillation tower, where they condense back to liquids.
Medium weight liquids, including kerosene and distillates, stay in the middle of the distillation tower.
Heavier liquids, called gas oils, separate lower down in the distillation tower, and the heaviest fractions with the highest boiling points settle at the bottom of the tower.
After distillation, heavy, lower-value distillation fractions can be processed further into lighter, higher-value products such as gasoline. At this point in the process, fractions from the distillation units are transformed into streams (intermediate components) that eventually become finished products.
The most widely used conversion method is called cracking because it uses heat, pressure, catalysts, and sometimes hydrogen to crack heavy hydrocarbon molecules into lighter ones. A cracking unit consists of one or more tall, thick-walled, rocket-shaped reactors and a network of furnaces, heat exchangers, and other vessels. Complex refineries may have one or more types of crackers, including fluid catalytic cracking units and hydrocracking/hydrocracker units.
Cracking is not the only form of crude oil conversion. Other refinery processes rearrange molecules rather than splitting molecules to add value.
Fluid catalytic cracking distillation unit
Source: Chevron (copyrighted)
Refining workers overlooking a refinery
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Source: Chevron (copyrighted)
Alkylation, for example, makes gasoline components by combining some of the gaseous byproducts of cracking. The process, which essentially is cracking in reverse, takes place in a series of large, horizontal vessels and tall, skinny towers.
Reforming uses heat, moderate pressure, and catalysts to turn naphtha, a light, relatively low-value fraction, into high-octane gasoline components.
The finishing touches occur during the final treatment. To make gasoline, refinery technicians carefully combine a variety of streams from the processing units. Octane level, vapor pressure ratings, and other special considerations determine the gasoline blend.
Both incoming crude oil and the outgoing final products are stored temporarily in large tanks on a tank farm near the refinery. Pipelines, trains, and trucks carry the final products from the storage tanks to locations across the country.
Last updated: February 22,
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, 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 fraction, also known as a petroleum cut, with highly viscous hydrocarbons like (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 oil and .
There are still many too heavy hydrocarbon 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 , a process called , 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.
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, , gasoline and diesel sold in Europe cannot contain more than 10 , or 10 milligrams per kilogram, of sulfur1. The purpose of these measures is to improve air quality and optimize the effectiveness of catalytic converters used to treat exhaust gas. For diesel, , 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.
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