Dictionary: oxygen   (ŏk'sĭ-jən)

A nonmetallic element constituting 21 percent of the atmosphere by volume that occurs as a diatomic gas, O₂, and in many compounds such as water and iron ore. It combines with most elements, is essential for plant and animal respiration, and is required for nearly all combustion. Atomic number 8; atomic weight 15.9994; melting point −218.4°C; boiling point −183.0°C; gas density at 0°C 1.429 grams per liter; valence 2.

[French oxygène : Greek oxus, sharp, acid + French -gène, -gen.]

Background

Oxygen is one of the basic chemical elements. In its most common form, oxygen is a colorless gas found in air. It is one of the life-sustaining elements on Earth and is needed by all animals. Oxygen is also used in many industrial, commercial, medical, and scientific applications. It is used in blast furnaces to make steel, and is an important component in the production of many synthetic chemicals, including ammonia, alcohols, and various plastics. Oxygen and acetylene are combusted together to provide the very high temperatures needed for welding and metal cutting. When oxygen is cooled below -297° F (-183° C), it becomes a pale blue liquid that is used as a rocket fuel.

Oxygen is one of the most abundant chemical elements on Earth. About one-half of the earth's crust is made up of chemical compounds containing oxygen, and a fifth of our atmosphere is oxygen gas. The human body is about two-thirds oxygen. Although oxygen has been present since the beginning of scientific investigation, it wasn't discovered and recognized as a separate element until 1774 when Joseph Priestley of England isolated it by heating mercuric oxide in an inverted test tube with the focused rays of the sun. Priestley described his discovery to the French scientist Antoine Lavoisier, who experimented further and determined that it was one of the two main components of air. Lavoisier named the new gas oxygen using the Greek words oxys, meaning sour or acid, and genes, meaning producing or forming, because he believed it was an essential part of all acids.

In 1895, Karl Paul Gottfried von Linde of Germany and William Hampson of England independently developed a process for lowering the temperature of air until it liquefied. By carefully distillation of the liquid air, the various component gases could be boiled off one at a time and captured. This process quickly became the principal source of high quality oxygen, nitrogen, and argon.

In 1901, compressed oxygen gas was burned with acetylene gas in the first demonstration of oxy-acetylene welding. This technique became a common industrial method of welding and cutting metals.

The first use of liquid rocket propellants came in 1923 when Robert Goddard of the United States developed a rocket engine using gasoline as the fuel and liquid oxygen as the oxidizer. In 1926, he successfully flew a small liquid-fueled rocket a distance of 184 ft (56 m) at a speed of about 60 mph (97 kph).

After World War II, new technologies brought significant improvements to the air separation process used to produce oxygen. Production volumes and purity levels increased while costs decreased. In 1991, over 470 billion cubic feet (13.4 billion cubic meters) of oxygen were produced in the United States, making it the second-largest-volume industrial gas in use.

Worldwide the five largest oxygen-producing areas are Western Europe, Russia (formerly the USSR), the United States, Eastern Europe, and Japan.

Raw Materials

Oxygen can be produced from a number of materials, using several different methods. The most common natural method is photo-synthesis, in which plants use sunlight convert carbon dioxide in the air into oxygen. This offsets the respiration process, in which animals convert oxygen in the air back into carbon dioxide.

The most common commercial method for producing oxygen is the separation of air using either a cryogenic distillation process or a vacuum swing adsorption process. Nitrogen and argon are also produced by separating them from air.

Oxygen can also be produced as the result of a chemical reaction in which oxygen is freed from a chemical compound and becomes a gas. This method is used to generate limited quantities of oxygen for life support on submarines, aircraft, and spacecraft.

Hydrogen and oxygen can be generated by passing an electric current through water and collecting the two gases as they bubble off. Hydrogen forms at the negative terminal and oxygen at the positive terminal. This method is called electrolysis and produces very pure hydrogen and oxygen. It uses a large amount of electrical energy, however, and is not economical for large-volume production.

The Manufacturing
Process

Most commercial oxygen is produced using a variation of the cryogenic distillation process originally developed in 1895. This process produces oxygen that is 99+% pure. More recently, the more energy-efficient vacuum swing adsorption process has been used for a limited number of applications that do not require oxygen with more than 90-93% purity.

Here are the steps used to produce commercial-grade oxygen from air using the cryogenic distillation process.

Pretreating

Because this process utilizes an extremely cold cryogenic section to separate the air, all impurities that might solidify—such as water vapor, carbon dioxide, and certain heavy hydrocarbons—must first be removed to prevent them from freezing and plugging the cryogenic piping.

• The air is compressed to about 94 psi (650 kPa or 6.5 atm) in a multi-stage compressor. It then passes through a water-cooled aftercooler to condense any water vapor, and the condensed water is removed in a water separator.
• The air passes through a molecular sieve adsorber. The adsorber contains zeolite and silica gel-type adsorbents, which trap the carbon dioxide, heavier hydrocarbons, and any remaining traces of water vapor. Periodically the adsorber is flushed clean to remove the trapped impurities. This usually requires two adsorbers operating in parallel, so that one can continue to process the air-flow while the other one is flushed.

Separating

Air is separated into its major components—nitrogen, oxygen, and argon—through a distillation process known as fractional distillation. Sometimes this name is shortened to fractionation, and the vertical structures used to perform this separation are called fractionating columns. In the fractional distillation process, the components are gradually separated in several stages. At each stage the level of concentration, or fraction, of each component is increased until the separation is complete.

Because all distillation processes work on the principle of boiling a liquid to separate one or more of the components, a cryogenic section is required to provide the very low temperatures needed to liquefy the gas components.

• The pretreated air stream is split. A small portion of the air is diverted through a compressor, where its pressure is boosted. It is then cooled and allowed to expand to nearly atmospheric pressure. This expansion rapidly cools the air, which is injected into the cryogenic section to provide the required cold temperatures for operation.
• The main stream of air passes through one side of a pair of plate fin heat exchangers operating in series, while very cold oxygen and nitrogen from the cryogenic section pass through the other side. The incoming air stream is cooled, while the oxygen and nitrogen are warmed. In some operations, the air may be cooled by passing it through an expansion valve instead of the second heat exchanger. In either case, the temperature of the air is lowered to the point where the oxygen, which has the highest boiling point, starts to liquefy.
• The air stream—now part liquid and part gas—enters the base of the high-pressure fractionating column. As the air works its way up the column, it loses additional heat. The oxygen continues to liquefy, forming an oxygen-rich mixture in the bottom of the column, while most of the nitrogen and argon flow to the top as a vapor.
• The liquid oxygen mixture, called crude liquid oxygen, is drawn out of the bottom of the lower fractionating column and is cooled further in the subcooler. Part of this stream is allowed to expand to nearly atmospheric pressure and is fed into the low-pressure fractionating column. As the crude liquid oxygen works its way down the column, most of the remaining nitrogen and argon separate, leaving 99.5% pure oxygen at the bottom of the column.
• Meanwhile, the nitrogen/argon vapor from the top of the high-pressure column is cooled further in the subcooler. The mixed vapor is allowed to expand to nearly atmospheric pressure and is fed into the top of the low-pressure fractionating column. The nitrogen, which has the lowest boiling point, turns to gas first and flows out the top of the column as 99.995% pure nitrogen.
• The argon, which has a boiling point between the oxygen and the nitrogen, remains a vapor and begins to sink as the nitrogen boils off. As the argon vapor reaches a point about two-thirds the way down the column, the argon concentration reaches its maximum of about 7-12% and is drawn off into a third fractionating column, where it is further recirculated and refined. The final product is a stream of crude argon containing 93-96% argon, 2-5% oxygen, and the balance nitrogen with traces of other gases.

Purifying

The oxygen at the bottom of the low-pressure column is about 99.5% pure. Newer cryogenic distillation units are designed to recover more of the argon from the low-pressure column, and this improves the oxygen purity to about 99.8%.

• If higher purity is needed, one or more additional fractionating columns may be added in conjunction with the low-pressure column to further refine the oxygen product. In some cases, the oxygen may also be passed over a catalyst to oxidize any hydrocarbons. This process produces carbon dioxide and water vapor, which are then captured and removed.

Distributing

About 80-90% of the oxygen produced in the United States is distributed to the end users in gas pipelines from nearby air separation plants. In some parts of the country, an extensive network of pipelines serves many end users over an area of hundred of miles (kilometers). The gas is compressed to about 500 psi (3.4 MPa or 34 atm) and flows through pipes that are 4-12 in (10-30 cm) in diameter. Most of the remaining oxygen is distributed in insulated tank trailers or railroad tank cars as liquid oxygen.

• If the oxygen is to be liquefied, this process is usually done within the low-pressure fractionating column of the air separation plant. Nitrogen from the top of the low-pressure column is compressed, cooled, and expanded to liquefy the nitrogen. This liquid nitrogen stream is then fed back into the low-pressure column to provide the additional cooling required to liquefy the oxygen as it sinks to the bottom of the column.
• Because liquid oxygen has a high boiling point, it boils off rapidly and is rarely shipped farther than 500 mi (800 km). It is transported in large, insulated tanks. The tank body is constructed of two shells and the air is evacuated between the inner and outer shell to retard heat loss. The vacuum space is filled with a semisolid insulating material to further halt heat flow from the outside.

Quality Control

The Compressed Gas Association establishes grading standards for both gaseous oxygen and liquid oxygen based on the amount and type of impurities present. Gas grades are called Type I and range from A, which is 99.0% pure, to F, which is 99.995% pure. Liquid grades are called Type II and also range from A to F, although the types and amounts of allowable impurities in liquid grades are different than in gas grades. Type I Grade B and Grade C and Type II Grade C are 99.5% pure and are the most commonly produced grades of oxygen. They are used in steel making and in the manufacture of synthetic chemicals.

The operation of cryogenic distillation airseparation units is monitored by automatic instruments and often uses computer controls. As a result, their output is consistent in quality. Periodic sampling and analysis of the final product ensures that the standards of purity are being met.

The Future

In January 1998, the United States launched the Lunar Prospector satellite into orbit around the moon. Among its many tasks, this satellite will be scanning the surface of the moon for indications of water. Scientists hope that if sufficient quantities of water are found, it could be used to produce hydrogen and oxygen gases through electrolysis, using solar power to generate the electricity. The hydrogen could be used as a fuel, and the oxygen could be used to provide life support for lunar colonies. Another plan involves extracting oxygen from chemical compounds in the lunar soil using a solar-powered furnace for heat.

Where to Learn More

Books

Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook, 14th Edition. McGraw-Hill, 1997.

Handbook of Compressed Gases, 3rd edition. Compressed Gas Association, Inc., Van Nostrand Reinhold Co., Inc., 1990.

Heiserman, David L. Exploring Chemical Elements and Their Compounds. TAB Books, 1992.

Kent, James A., editor. Riegel's Handbook of Industrial Chemistry, 9th edition. International Thomson Publishing, 1997.

Kroschwitz, Jacqueline I., executive editor, and Mary Howe-Grant, editor. Encyclopedia of Chemical Technology, 4th edition. John Wiley and Sons, Inc., 1993.

Stwertka, Albert. A Guide to the Elements. Oxford University Press, 1996.

Periodicals

Allen, J.B. "Making Oxygen on the Moon," Popular Science (August 1995): 23.

Other

Air Products and Chemicals, Inc. http://www.airproducts.com/gases/oxgen.html.

http://www.intercorr.com/periodic/8.htm (This website contains a summary of the history, sources, properties, and uses of oxygen.)

[Article by: Chris Cavette]

A gaseous chemical element, O, atomic number 8, and atomic weight 15.9994. Oxygen is of great interest because it is the essential element both in the respiration process in most living cells and in combustion processes. It is the most abundant element in the Earth's crust. About one-fifth (by volume) of the air is oxygen.

Oxygen is separated from air by liquefaction and fractional distillation. The chief uses of oxygen in order of their importance are (1) smelting, refining, and fabrication of steel and other metals; (2) manufacture of chemical products by controlled oxidation; (3) rocket propulsion; (4) biological life support and medicine; and (5) mining, production, and fabrication of stone and glass products. See also Periodic table.

Uncombined gaseous oxygen usually exists in the form of diatomic molecules, O₂, but oxygen also exists in a unique triatomic form, O₃, called ozone. See also Ozone.

Under ordinary conditions oxygen is a colorless, odorless, and tasteless gas. It condenses to a pale blue liquid, in contrast to nitrogen, which is colorless in the liquid state. Oxygen is one of a small group of slightly paramagnetic gases, and it is the most paramagnetic of the group. Liquid oxygen is also slightly paramagnetic. Some data on oxygen and some properties of its ordinary form, O₂, are listed in the table. See also Paramagnetism.

Practically all chemical elements except the inert gases form compounds with oxygen. Most elements form oxides when heated in an atmosphere containing oxygen gas. Many elements form more than one oxide; for example, sulfur forms sulfur dioxide (SO₂) and sulfur trioxide (SO₃). Among the most abundant binary oxygen compounds are water, H₂O, and silica, SiO₂, the latter being the chief ingredient of sand. Among compounds containing more than two elements, the most abundant are the silicates, which constitute most of the rocks and soil. Other widely occurring compounds are calcium carbonate (limestone and marble), calcium sulfate (gypsum), aluminum oxide (bauxite), and the various oxides of iron which are mined as a source of iron. Several other metals are also mined in the form of their oxides. Hydrogen peroxide, H₂O₂, is an interesting compound used extensively for bleaching. See also Hydrogen peroxide; Oxidation-reduction; Oxide; Peroxide; Water desalination.

Oxygen is the most common of all chemical elements on earth, being found in water, air, and most mineral and organic substances, including most compounds in the human body. It combines with almost all other elements, and is so reactive that it was given the Greek name ‘oxygen’, meaning acid-forming. However, most of the compounds it forms are not acids. Its chemical reactions usually form heat (as in the animal body) and sometimes light (as in candles).

It has always been known that animals cannot live without air, but in 1674 Mayow showed that only one part of the air, about one-fifth, is essential for life, and named it ‘vital air’. A hundred years later Priestley isolated this part, oxygen; Lavoisier purified oxygen and its properties began to be studied.

Atmospheric air contains 21% oxygen, at a pressure of about 150 mm Hg varying with barometric pressure and to a small extent with humidity. It enters the lungs during breathing and is absorbed into the blood passively by diffusion, combining with haemoglobin and being carried in the bloodstream to all parts of the body. There it is used to metabolize or ‘burn’ foodstuffs in the cells, especially fats and carbohydrates, providing heat and creating new chemical compounds, water, and the waste product carbon dioxide. Tissues and organs vary in the length of time they can survive without oxygen, according to their provision for anaerobic metabolism. The brain cannot survive without oxygen; the cessation of breathing will cause unconsciousness in a few minutes, and death soon afterwards. Other tissues such as skeletal muscle can continue to work for a limited time, when glycogen stores are broken down without oxygen to provide energy; lactic acid is a by-product that leaks into the blood and makes it more acid, but can be recycled into carbohydrate stores in the liver.

In quiet breathing at rest we absorb about 0.2-0.3 litres/min of oxygen (depending on body size), but in vigorous exercise this can go up to over 2 litres/min. This increase is accomplished by increased breathing (which supplies oxygen to the lungs at a greater rate), increased cardiac output and flow of blood to the muscles, and greater extraction of oxygen from the blood by the muscles. If the oxygen supply to the muscles is inadequate then the anaerobic threshold is passed and anaerobic metabolism takes place, with production of lactic acid. After the exercise additional oxygen is needed to convert the lactic acid back to glycogen, and breathing remains enhanced while the oxygen debt is repaid.

The supply of oxygen to the body depends not on the percentage in the air breathed but on its tension or pressure. At high altitude, say 5000 metres above sea level, the percentage of oxygen is still 21%, but because atmospheric pressure is halved, the oxygen pressure is half that at sea level — 75 mm Hg rather than 150 mm Hg. A person may as a result suffer from hypoxia — a lack of oxygen.

High oxygen pressures can be harmful and cause oxygen poisoning, including lung damage and brain dysfunction. In nature high oxygen pressures only exist in deep water diving, and mankind has not had to evolve ways of combating them. Once scientists had purified oxygen it became possible to administer it to patients; this has life-saving possibilities, but care has to be taken not to exceed the toxic level.

Some compounds rich in oxygen, such as the pollutant ozone (itself a molecular form of oxygen), and hydrogen peroxide, can react with cells to produce strongly reactive forms of oxygen. Superoxide anions and unstable oxygen free radicals (such as hydroxyl and hydroperoxy radicals) can be toxic to cells, by way of excess lipid peroxidation. These are implicated, for example, in damage following the restoration of blood flow (reperfusion) after the blockage which causes heart attacks or strokes, and in a variety of other disease processes. However the body does have inherent enzymatic defences against free radical accumulation, and there are antioxidants, such as uric acid, ascorbate, and glutathione, which can inactivate them. Free radicals are likely to contribute also to the ageing process: the very substance by which we live may itself limit our lifespan. Thus oxygen, like most good things, can be dangerous in excess.

Mankind evolved to live close to sea level. Climbing mountains (causing hypoxia) and deep-sea diving (causing nitrogen narcosis or oxygen poisoning) can both be dangerous, in the absence of the right precautions.

— John Widdicombe

See also breathing; diving; free radicals; gases in the body; hypoxia; respiration.

An odourless, colourless gas that makes up about one-fifth of the atmosphere. It is essential for survival. A person completely starved of oxygen would die after only a few minutes. Artificial supplies have been used to improve athletic performance and to aid recovery after exertion. However, routine administration of supplemental oxygen for brief periods before, during, or after exercise has minimal benefits, unless the normal oxygen supply to the tissues is in some way restricted. Supplemental oxygen will benefit people exercising at high altitudes and those suffering from a medical condition (e.g. a disease of the heart, lungs, or blood) that reduces the ability to absorb and transport oxygen. The greatest benefits to a healthy person are gained when oxygen is continuously supplied during exercise, but this has little practical use because of the problems of administering the gas to an active exerciser. Oxygen inhaled under high pressure may be an effective therapy for some sports injuries (see hyperbaric oxygen therapy), but it should be administered only under medical supervision because of the danger of poisoning (see oxygen poisoning).

A tasteless, odorless, colorless gas essential for human respiration. Its atomic number is 8 and its atomic weight is 15.9994.

For more information on oxygen, visit Britannica.com.

An odourless, colourless gas that makes up about one-fifth of the atmosphere. It is essential for human survival. Attempts have been made to use oxygen as an ergogenic aid, but there is no evidence that it is beneficial before or after exercise. Oxygen taken during exhaustive exercise may boost performance, but it is of little practical use because of the problems of administering the gas to an active athlete.

An element, normally a gas, that makes up about one-fifth of the atmosphere of the Earth. Oxygen is usually found as a molecule made up of two atoms. Its symbol is O.

A chemical element, atomic number 8, atomic weight 15.999, symbol O. It is a colorless and odorless gas that makes up about 20% of the atmosphere. In combination with hydrogen, it forms water; by weight, 90% of water is oxygen. It is the most abundant of all the elements of nature. Large quantities of it are distributed throughout the solid matter of the earth, because the gas combines readily with many other elements. With carbon and hydrogen, oxygen forms the chemical basis of much organic material. Oxygen is essential in sustaining all kinds of life.

• o. analyzer — an instrument that measures the concentration of oxygen in a gas mixture.
• o. deficiency — significant cause of losses in cultivated finfish in enclosed dams, but also in rivers and estuaries, caused by lack of natural aeration of the water or to heavy algal blooms, bushfire ash deposits and overcast conditions leading to respiration rather than photosynthesis or a high concentration of organic matter and leading to the development of a bacterial bloom; a high temperature exacerbates the development.
• o. flux equation — a calculation that determines the rate at which oxygen is made available to tissues, based on cardiac output and arterial oxygen content.
• o.–hemoglobin dissociation curve — a graphic explanation of the release and acquisition of oxygen from and to the hemoglobin in the blood in varying circumstances of oxygen partial pressure in the environment.
• o. regulator — see reducing valve.
• o. saturation — the amount of oxygen bound to hemoglobin in the blood expressed as a percentage of the maximal binding capacity.
• o. saturation curve — graphical representation describing the relationship (usually curvilinear) between fraction of oxygen-binding sites (of a protein) that have oxygen bound to them and the partial pressure (concentration) of free oxygen.
• o. tank — the heavy metal cylinder in which medical gases are compressed at high pressure. Called also oxygen cylinder.
• o. tension — see tension (2).
• o. tent — an enclosed space or plastic canopy used for oxygen therapy, humidity therapy or aerosol therapy.
• o. therapy — supplemental oxygen administered for the purpose of relieving hypoxemia and preventing damage to the tissue cells as a result of oxygen lack (hypoxia). Companion animals are usually placed in a special cage with oxygen piped to it. A mask is used for short-term administration. Large animals can be supplied by a nasal tube taped in place to deliver oxygen into the pharynx.
• o. toxicity — tissue damage may occur with exposure to high concentrations of oxygen for long periods. See also retrolental fibroplasia.
• o.-transfer chain — a functional chain describing the transfer of oxygen from the external environment to the metabolizing tissue; includes uptake in the respiratory system, binding to hemoglobin, transport through the circulatory system, diffusion and dissociation in tissues and utilization in mitochondria, i.e. oxidatable substrates and enzymes.
• o. transport — process of transfer of oxygen around the body either attached to hemoglobin or myoglobin.

An element with atomic number 8; symbol: O. It is actually the most common element in the crusts and mantles of the inner planets and rocky moons, making up all silicate minerals. Along with hydrogen, carbon, and nitrogen, oxygen is essential to life.