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Refining Crude Oil

What is a refinery?

Petroleum refineries convert crude oil and other liquids into many petroleum products that people use every day. Most refineries focus on producing transportation fuels. On average, U.S. refineries produce, from a 42-gallon barrel of crude oil, about 19 to 20 gallons of motor gasoline, 11 to 12 gallons of distillate fuel, most of which is sold as diesel fuel, and 4 gallons of jet fuel. More than a dozen other petroleum products are also produced in refineries. Petroleum refineries produce liquids the petrochemical industry uses to make a variety of chemicals and plastics.

A photo of the Pascagoula Refinery in Mississippi
A night photo of the Pascagoula Refinery, Mississippi

Source: Stock photography (copyrighted)

Refineries operate 24/7

A refinery runs 24 hours a day, 365 days a year and requires a large number of employees. A refinery can occupy as much land as several hundred football fields.

Last updated: May 23, 2019

How crude oil is refined into petroleum products

Petroleum refineries change crude oil into petroleum products for use as fuels for transportation, heating, paving roads, and generating electricity and as feedstocks for making chemicals.

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, while more complex refineries may have vacuum distillation units.

Diagram of a refinery distillation column and major products produced.

Fluid catalytic cracking distillation unit
Richmond Refinery, Fluid Catalytic Cracking Distillation Column.

Source: Chevron (copyrighted).

Refining workers overlooking a refinery
Caltex, Star Petroleum Refinery, Refining workers overlook refinery

Source: Chevron (copyrighted).

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, while 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. This is where 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 to add value rather than splitting molecules.

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 other locations across the country.

Last reviewed: May 16, 2018

Petroleum refineries process crude oil into many different petroleum products. The physical characteristics of crude oil determine how the refineries turn it into the highest value products.

Not all crude oil is the same

The physical characteristics of crude oil determine how refineries process it. In simple terms, crude oils are classified by density and sulfur content. Less dense (lighter) crude oils generally have a higher share of light hydrocarbons. Refineries can produce high-value products such as gasoline, diesel fuel, and jet fuel from light crude oil with simple distillation. When refineries use simple distillation on denser (heavier) crude oils, it produces low-value products. Heavy crude oils require additional, more expensive processing to produce high-value products. Some crude oils also have a high sulfur content, which is an undesirable characteristic in both processing and product quality.

Refineries use more than just crude oil

In addition to crude oil, refineries and blending facilities add other oils and liquids during processing to produce the finished products that are sold to consumers. These other oils and liquids include liquids that condense in natural gas wells (called lease condensates); natural gas plant liquids from natural gas processing; liquefied gases from the refinery itself; and unfinished oils that are produced by partially refining crude oil, such as naphthas and lighter oils, kerosene and light gas oils, heavy gas oils, and residuum. Residuum is a residue from crude oil that remains after distilling off all but the heaviest components.

Refineries and blending facilities combine various gasoline blending components and fuel ethanol to produce the finished motor gasoline that is sold for use in the United States. They add biodiesel to petroleum diesel fuel to make biodiesel fuel.

Refining output is larger than input

The total volume of products refineries produce (output) is greater than the volume of crude oil that refineries process (input) because most of the products they make have a lower density than the crude oil they process. This increase in volume is called processing gain. The average processing gain at U.S. refineries was about 6.5% in 2017. In 2017, U.S. refineries produced an average of 44.73 gallons of refined products for every 42-gallon barrel of crude oil they refined.

Did you know?

Gasoline accounts for the largest share of total petroleum products produced each year in U.S. petroleum refineries.

Petroleum products produced from
one 42-gallon barrel of oil input
at U.S. refineries, 2017

Product Gallons
Finished motor gasoline 19.53
Distillate fuel oil 12.18
Kerosene-type jet fuel   4.16
Petroleum coke   2.23
Still gas   1.68
Hydrocarbon gas liquids   1.55
Residual fuel oil   1.05
Asphalt and road oil   0.80
Naptha for feedstocks   0.46
Lubricants   0.42
Other oils for feedstocks   0.29
Miscellaneous products   0.25
Special napthas   0.08
Finished aviation gasoline   0.04
Kerosene <0.01
Waxes <0.01
Total 44.73
Processing gain   2.73

Source: U.S. Energy Information Administration,
Petroleum Supply Annual, August 2018

Last updated: September 28, 2018

Top 10 U.S. refineries operable capacity*

As of January 1, 2019





Barrels per calendar day
1 Saudi Aramco Motiva Enterprises LLC Texas Port Arthur 607,000
2 Marathon Petroleum Corp Marathon Petroleum Co LP Texas Galveston Bay 585,000
3 Marathon Petroleum Corp Marathon Petroleum Co LP Louisiana Garyville 564,000
4 ExxonMobil Corp ExxonMobil Refining and Supply Co Texas Baytown 560,500
5 ExxonMobil Corp ExxonMobil Refining and Supply Co Louisiana Baton Rouge 502,500
6 BP PLC BP Products North America Inc Indiana Whiting 430,000
7 PDV America Inc Citgo Petroleum Corp Louisiana Lake Charles 418,000
8 ExxonMobil Corp ExxonMobil Refining and Supply Co Texas Beaumont 369,024
9 Marathon Petroleum Corp Tesoro Refining and Marketing Co California Carson 363,000
10 Chevron Corp Chevron USA Inc Mississippi Pascagoula 356,440

*Includes only refineries with atmospheric crude oil distillation capacity.
Source: Refinery Capacity Report

See full list of refineries

Last updated: June 21, 2019