Petroleum refineries convert (refine) crude oil into petroleum products for use as fuels for transportation, heating, paving roads, 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:
1. Separation
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.
2. Conversion
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. 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.
3. Treatment
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.
4. Storage
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.
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 (API gravity) and sulfur content. Less dense (lighter) crude oils (with higher API gravity) generally have more 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 (with lower API gravity), they produce 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
-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.
Refineries and blending facilities combine various gasoline blending components and fuel ethanol to produce the finished motor gasoline sold in the United States. They may also add other biofuels to petroleum fuels to make blends of biomass-based diesel, jet fuel, and heating oil.
Refining output is larger than input. The total volume of products that 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 6.3% in 2022. In 2022, U.S. refineries produced an average of about 45 gallons of refined products for every 42-gallon barrel of crude oil they refined.