Petroleum
Overview
Petroleum (peh-TRO-lee-yum) is a mixture, not a compound. Mixtures differ from compounds in a number of important ways. The parts that make up a mixture are not chemically combined with each other, as they are in a compound. Also, mixtures have no definite composition, but consist of varying amounts of the substances of which they are formed.
Key Facts
Other Names:
Crude oil; oil
Formula:
Not applicable
Elements:
Carbon, hydrogen, oxygen, other elements
Compound Type:
Not applicable
State:
Liquid
Molecular Weight:
Not applicable
Melting Point:
Not applicable
Boiling Point:
Not applicable
Solubility:
Not miscible with water; miscible with most organic solvents
Petroleum is a very complex mixture of hydrocarbons-compounds that consist of carbon and hydrogen only. Small amounts of other organic compounds containing oxygen, sulfur, nitrogen, phosphorus, and other elements are also present. The hydrocarbons that make up petroleum include the alkanes, alkenes, alkynes, cyclic hydrocarbons, and aromatic hydrocarbons. Alkanes are hydrocarbons in which carbon and hydrogen are bonded to each other by single bonds. In alkenes, at least one double bond is present. In alkynes, carbon and hydrogen atoms are bonded by at least one triple bond. In cyclic hydrocarbons, the carbon atoms are arranged in a ring. In aromatic hydrocarbons, the carbon atoms are joined to each other in a ring that has alternate single and double bonds. Many of the hydrocarbons in petroleum also have side chains that also consist of carbon and hydrogen atoms.
The physical properties of petroleum vary somewhat, depending on the source from which it comes and its composition. Generally, it is a thick, oily liquid that is dark yellow to brown to greenish-black in color, with a strong, unpleasant odor.
How It Is Made
Scientists believe that petroleum was formed about 300 million years ago. When microscopic plants and animals that lived in the ocean died and sank to the bottom, they were gradually covered and compressed with more layers of organic material along with sand and mud. As the mud grew thicker, it created pressure on the organic material, causing it to become increasingly warmer. The heat and pressure caused the organic matter to decay in the absence of oxygen, converting it into petroleum and natural gas.
Over time, the primitive oceans dried up. The sand and mud that had accumulated on the ocean floors changed into rock. The natural gas and liquid petroleum that had formed on the ocean floor was trapped in the rock. It flowed through cracks in the rock until it reached porous rock that acted like a sponge and soaked up the petroleum and natural gas. These fossil fuels remain trapped in the porous rock by non-porous layers of rock that act like caps or seals on the porous rocks.
Geologists have discovered a number of ways of finding these oil- and gas-soaked reservoirs. They measure changes in the Earth's magnetic field and use electronic "sniffers" that can detect hydrocarbons trapped in rock. Once oil is found, prospectors extract it by drilling into the porous rock. The pressure of gases within the porous rock pushes petroleum up to the surface, where it can be captured and stored.
Liquid petroleum has no commercial value as it comes from the earth. After it is captured from wells, therefore, it has to be refined, or separated into useful components. Refining is a process by which petroleum is heated to high temperatures in a tall cylindrical tower. The many different hydrocarbons in petroleum boil off and rise upward in the tower. The higher they rise, the more they cool off.
Each hydrocarbon eventually reaches a height at which it changes back to a liquid. Traps are inserted at various heights in the tower to catch each hydrocarbon as it changes back to a liquid. One level of traps, for example, catches hydrocarbons that condense to a liquid between about 40°C and 170°C (100°F and 340°F). This "fraction" is called the gasoline or petrol fraction of petroleum. Another level of traps catches hydrocarbons that condense between temperatures of about 170°C and 250°C (340°F and 480°F). This group of hydrocarbons is called the kerosene fraction. The other major fractions obtained from petroleum are the diesel and fuel oil fractions. Anything that boils above about 400°C (750°F) is referred to as the residual oil fraction. In many cases, each of the fractions obtained by this process can be further refined to separate it into even smaller fractions.
Interesting Facts
- The United States uses by far more petroleum than any other nation in the world. It uses about 1,000 billion liters (250 billion gallons) every year. About half that amount is imported from other countries, primarily in the Middle East.
- The average fuel economy of automobiles produced in the United States has decreased steadily since 1985.
Common Uses and Potential Hazards
By far the most common use of petroleum is as gasoline for automobiles and trucks. About 90 percent of the petroleum used in the United States fuels vehicles powered by internal combustion engines. Diesel fuel is used to power trucks, buses, trains, and some automobiles. Heating oil warms buildings and powers industrial boilers. Utilities use residual fuel oil to generate electricity. Jet fuel, produced from kerosene, powers airplanes. Some airplanes use aviation gasoline, which has a higher octane number than automobile gasoline. Octane number is a measure of a fuel's efficiency.
Kerosene is used to power lamps and heaters. Power plants and factories use petroleum coke, a solid fuel made of petroleum, as a fuel. Asphalt and tar, solid materials left over after the fractionation of petroleum, are components of paved roads. Petrolatum, another solid component of petroleum, is used as a lubricant and moisturizer. Paraffin wax, also obtained as a by-product of petroleum distillation, is an ingredient in candles, candy, polishes, and matches. Other forms of petroleum are used as lubricating oils for engines or as solvents in paints.
A second very important use of petroleum is in the manufacture of plastics and other chemicals. The number of chemical compounds obtained from petroleum and used as raw materials in chemical reactions is almost endless. It includes compounds such as methane, ethane, propane, butane, ethene (ethylene), propene (propylene), butene (butylene), benzene, methanol (methyl alcohol), ethanol (ethyl alcohol), phenol, xylene, naphthalene, and anthracene, to mention only a few.
There are many drawbacks associated with using petroleum as a fuel. One of the most important is environmental pollution. Burning petroleum products releases greenhouse gases-gases that tend to accumulate in the atmosphere and contribute to a gradual warming of the Earth's annual average temperature. Other pollutants produced when petroleum products are burned include sulfur oxides and oxides of nitrogen, compounds that contribute to the development of acid rain, and carbon monoxide and ozone, pollutants that damage plant and animal life as well as physical structures, like buildings and statues.
The production and transportation of petroleum products also poses environmental problems. Oil spills that occur during any phase of petroleum production and use may kill aquatic life and pollute an area for years. The worst oil spill in North America, for example, occurred during the wreck of the oil tanker Exxon Valdez in 1989, when 40,000 tons of crude oil were dumped into Alaska's Prince William Sound. The worst oil spill in the world occurred in 1978 as a result of the wreck of the oil tanker Amoco Cadiz off the coast of Brittany, France. More than 220,000 tons of oil were released during that accident.
Words to Know
A process of separating two or more substances by boiling the mixture of which they are composed and condensing the vapors produced at different temperatures. A compound that contains hydrogen and carbon atoms. Able to be mixed; especially applies to the mixing of one liquid with another. A collection of two or more elements and/or compounds with no definite composition.For Further Information
American Petroleum Institute. http://api-ec.api.org/newsplashpage/index.cfm (accessed on December 22, 2005).
Hyne, Norman J. Nontechnical Guide to Petroleum Geology, Exploration, Drilling and Production, 2nd ed. Tulsa, OK: Pennwell Books, 2001.
Leffler, William L. Petroleum Refining in Nontechnical Language. Tulsa, OK: Pennwell Books, 2000.
"Oil and Gas-Energy for the World." Institute of Petroleum. http://www.energyinst.org.uk/education/oilandgas/energy.htm (accessed on December 22, 2005).
"Petroleum." U.S. Energy Information Administration. http://www.eia.doe.gov/oil_gas/petroleum/info_glance/petroleum.html (accessed on December 22, 2005).
"Petroleum Refining Processes." OSHA Technical Manual. Washington, DC: Occupational Safety and Health Administration, 1999. Available online at http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html#1 (accessed on December 22, 2005).
See Also
Methane; Petrolatum; Propane
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