How Oil Refining Works

By: Craig Freudenrich, Ph.D.

Crude Oil

­Crude oil is the term for "unprocessed" oil, the stuff that comes out of the ground. It is also known as petroleum. Crude oil is a fossil fuel, meaning that it was made natural­ly from decaying plants and animals living in ancient seas millions of years ago -- most places you can find crude oil were once sea beds. Crude oils vary in color, from clear to tar-black, and in viscosity, from water to almost solid.

Crude oils are such a useful starting point for so many different substances because they contain hydrocarbons. Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings.


There are two things that make hydrocarbons exciting to chemists:

  • Hydrocarbons contain a lot of energy. Many of the things derived from crude oil like gasoline, diesel fuel, paraffin wax and so on take advantage of this energy.
  • Hydrocarbons can take on many different forms. The smallest hydrocarbon is methane (CH4), which is a gas that is a lighter than air. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar. By chemically cross-linking hydrocarbon chains you can get everything from synthetic rubber to nylon to the plastic in tupperware. Hydrocarbon chains are very versatile!

The major classes of hydrocarbons in crude oils include:

  • Paraffins general formula: CnH2n+2 (n is a whole number, usually from 1 to 20) straight- or branched-chain molecules can be gasses or liquids at room temperature depending upon the molecule examples: methane, ethane, propane, butane, isobutane, pentane, hexane
  • Aromatics general formula: C6H5 - Y (Y is a longer, straight molecule that connects to the benzene ring) ringed structures with one or more rings rings contain six carbon atoms, with alternating double and single bonds between the carbons typically liquids examples: benzene, napthalene
  • Napthenes or Cycloalkanes general formula: CnH2n (n is a whole number usually from 1 to 20) ringed structures with one or more rings rings contain only single bonds between the carbon atoms typically liquids at room temperature examples: cyclohexane, methyl cyclopentane
  • Other hydrocarbons Alkenes general formula: CnH2n (n is a whole number, usually from 1 to 20) linear or branched chain molecules containing one carbon-carbon double-bond can be liquid or gas examples: ethylene, butene, isobutene Dienes and Alkynes general formula: CnH2n-2 (n is a whole number, usually from 1 to 20) linear or branched chain molecules containing two carbon-carbon double-bonds can be liquid or gas examples: acetylene, butadienes

­To see examples of the structures of these types of hydrocarbons, see the OSHA Technical Manual and this page on the Refining of Petroleum.

Now that we know what's in crude oil, let's see what we can make from it.

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