Refining crude oil – the refining process – U.S. Energy Information Administration (EIA)

What does an oil refinery do

how crude oil is refined into petroleum products

Oil refineries transform crude oil into petroleum products for use as fuels for transportation, heating, road paving, and electricity generation, and as feedstock to make chemicals.

refining breaks down crude oil into its various components, which are then selectively reconfigured into new products. oil refineries are complex and expensive industrial facilities. all refineries have three basic steps:

  • separation
  • conversion
  • treatment


Modern separation involves channeling 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.

Inside distillation units, liquids and vapors are separated into petroleum components called fractions based on their boiling points. heavy fractions are at the bottom and light fractions are at the top.

The lighter fractions, including gasoline and liquefied refinery gases, evaporate and rise to the top of the distillation tower, where they condense back into liquids.

Medium weight liquids, including kerosene and distillates, remain in the middle of the distillation tower.

Heavier liquids, called gas oils, separate further down the distillation tower, while heavier fractions with higher boiling points settle to the bottom of the tower.


After distillation, the heavy, lower value distillation fractions can be processed into lighter, higher value products such as gasoline. this is where the 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/hydrocracking units.

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Cracking is not the only way to convert crude oil. other refinery processes rearrange molecules to add value rather than break them down.

Alkylation, for example, produces gasoline components by combining some of the gaseous byproducts of cracking. the process, which is essentially reverse cracking, takes place in a series of large horizontal vessels and tall, skinny towers.

reforming uses heat, moderate pressure, and catalysts to convert naphtha, a light fraction of relatively low value, into high-octane gasoline components.


The final touches are given 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 gasoline blending.


Both incoming crude oil and outgoing final products are temporarily stored in large tanks in a tank farm near the refinery. pipelines, trains and trucks transport the final products from the storage tanks to other places in the country.

last revised: February 23, 2022

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