Refining Crude Oil

How Crude Oil Is Refined into Petroleum Products

The world uses gasoline and petroleum products to move merchandise and people, help make plastics, and do many other things. At a refinery, different parts of the crude oil are separated into useable petroleum products. Today, some refineries turn more than half of every 42-gallon barrel of crude oil into gasoline.

How does this transformation take place? Essentially, refining breaks crude oil down into its various components, which then are selectively reconfigured into new products.

All refineries perform three basic steps:

1. Separation
2. Conversion
3. Treatment

Source: Adapted from Chevron

Separation

Heavy petroleum components or "fractions" are on the bottom; light fractions are on the top. This difference in weights allows the separation of the various petrochemicals. Modern separation involves piping oil through hot furnaces. The resulting liquids and vapors are discharged into distillation towers.

Inside the towers, the liquids and vapors separate into fractions according to weight and boiling point.

The lightest fractions, including gasoline and liquid petroleum gas (LPG), vaporize and rise to the top of the tower, where they condense back to liquids.

Medium weight liquids, including kerosene and diesel oil distillates, stay in the middle.

Heavier liquids, called gas oils, separate lower down, while the heaviest fractions with the highest boiling points settle at the bottom.

Fluid Catalytic Cracking Distillation Column

Photo courtesy of Chevron.

Refining Workers Overlooking a Refinery

Photo courtesy of Chevron.

Conversion

Cracking and rearranging molecules takes a heavy, low-valued feedstock — often itself the output from an earlier process — and change it into lighter, higher-valued output such as gasoline. This is where refining's fanciest footwork takes place — where fractions from the distillation towers are transformed into streams (intermediate components) that eventually become finished products.

The most widely used conversion method is called cracking because it uses heat and pressure to "crack" heavy hydrocarbon molecules into lighter ones. A cracking unit consists of one or more tall, thick-walled, bullet-shaped reactors and a network of furnaces, heat exchangers, and other vessels.

Cracking and coking are not the only forms of conversion. Other refinery processes, instead of splitting molecules, rearrange them 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 that loom above other refinery structures.

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

Treatment

The finishing touches occur during the final treatment. To make gasoline, refinery technicians carefully combine a variety of streams from the processing units. Among the variables that determine the blend are octane level, vapor pressure ratings and special considerations, such as whether the gasoline will be used at high altitudes.

Storage

Both the incoming crude oil and the outgoing final products need to be stored. These liquids are stored in large tanks on a tank farm near the refinery. Pipelines then carry the final products from the tank farm to other tanks all across the country.

All of these activities are required to make the gasoline that powers our cars, the diesel fuel that brings our food to market, and the jet fuel that flies our planes. These provide us with the energy we need to get from place to place quickly and comfortably.

Tank Farm Near a Refinery

Photo courtesy of Chevron.

Refineries process crude oil into different petroleum products, such as gasoline, jet fuel, asphalt, and others. The most basic refining process separates crude oil into its various components. Crude oil is heated and put into a distillation tower (a still) where different hydrocarbon components are boiled off and recovered as they condense at different temperatures.

Not All Crude Oil Is Created Equal

The physical characteristics of crude oils can be different. In simple terms, crude oils are classified by their density and sulfur content. Less dense ("lighter") crudes generally have a higher share of light hydrocarbons from which higher-value products such as gasoline, jet fuel, and diesel can be recovered with simple distillation. The denser ("heavier") crude oils produce a greater share of lower-valued products with simple distillation and require additional processing to produce the desired range of products. Some crude oils also have a higher sulfur content, an undesirable characteristic with respect to both processing and product quality.

Refineries Use More Than Just Crude Oil

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In addition to crude oil, refineries and blending facilities use and add other oils and liquids to produce finished products for sale to consumers. These include liquids that condense in gas wells (called "lease condensates"), natural gas plant liquids from natural gas processing, and unfinished oils that are produced by partial refining of 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 after distilling off all but the heaviest components).

Blending facilities add oxygenates, such as ethanol, and various "blending components" to produce finished motor gasoline. Blenders also add relatively small, but increasing, amounts of "biodiesel" (made from vegetable oils or animal fats) to diesel fuel and heating oil.

Output Is Larger than Input

Petroleum refining results in output greater than the input because of changes in the overall density of the refined products relative to that of the input oils. These changes result in an increase in the volume of products produced that is called processing gain. U.S. processing gain averaged about 6.3% from 1997 through 2011. In 2011, about 44.98 gallons of refined products were produced for every 42 gallon barrel of oil input into U.S. refineries.

Gasoline Accounts for Almost Half of All the Petroleum Products We Produce

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Petroleum Products Produced from
One 42-Gallon Barrel of Oil Input
to U.S. Refineries, 2011

Source: U.S. Energy Information Administration, Petroleum Supply Annual 2011, Volume 1, Table 23.

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U.S. Refineries* Operable Capacity

(2011 data)

*Only Refineries with Atmospheric Crude Oil Distillation Capacity

Source: Refinery Capacity Data by individual refinery as of January 1, 2012