The Refining Process

What Is A Refinery?

A refinery is a factory. Just as a paper mill turns lumber into paper, a refinery takes crude oil and turns it into gasoline and hundreds of other useful products. A typical refinery costs billions of dollars to build and millions more to maintain. A refinery runs twenty-four hours a day, 365 days a year and requires a large number of employees to run. A refinery can occupy as much land as several hundred football fields. Workers ride bicycles to move from place to place inside the complex.

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: separation, conversion, and treatment.

Separation

Heavy petroleum fractions are on the bottom, light fractions are on the top. This 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 components or 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.)

Conversion

Cracking and rearranging molecules adds value to the products. 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’s, 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. Pipelines carry the final products from the tank farm near the refinery to other tanks all across the country.

Source: Energy Information Administration