Acetylene

Background

Acetylene is a colorless, combustible gas with a distinctive odor. When acetylene is liquefied, compressed, heated, or mixed with air, it becomes highly explosive. As a result special precautions are required during its production and handling. The most common use of acetylene is as a raw material for the production of various organic chemicals including 1,4-butanediol, which is widely used in the preparation of polyurethane and polyester plastics. The second most common use is as the fuel component in oxy-acetylene welding and metal cutting. Some commercially useful acetylene compounds include acetylene black, which is used in certain dry-cell batteries, and acetylenic alcohols, which are used in the synthesis of vitamins.

Acetylene was discovered in 1836, when Edmund Davy was experimenting with potassium carbide. One of his chemical reactions produced a flammable gas, which is now known as acetylene. In 1859, Marcel Morren successfully generated acetylene when he used carbon electrodes to strike an electric arc in an atmosphere of hydrogen. The electric arc tore carbon atoms away from the electrodes and bonded them with hydrogen atoms to form acetylene molecules. He called this gas carbonized hydrogen.

By the late 1800s, a method had been developed for making acetylene by reacting calcium carbide with water. This generated a controlled flow of acetylene that could be combusted in air to produce a brilliant white light. Carbide lanterns were used by miners and carbide lamps were used for street illumination before the general availability of electric lights. In 1897, Georges Claude and A. Hess noted that acetylene gas could be safely stored by dissolving it in acetone. Nils Dalen used this new method in 1905 to develop long-burning, automated marine and railroad signal lights. In 1906, Dalen went on to develop an acetylene torch for welding and metal cutting.

In the 1920s, the German firm BASF developed a process for manufacturing acetylene from natural gas and petroleum-based hydrocarbons. The first plant went into operation in Germany in 1940. The technology came to the United States in the early 1950s and quickly became the primary method of producing acetylene.

Demand for acetylene grew as new processes were developed for converting it into useful plastics and chemicals. In the United States, demand peaked sometime between 1965 and 1970, then fell off sharply as new, lower-cost alternative conversion materials were discovered. Since the early 1980s, the demand for acetylene has grown slowly at a rate of about 2-4% per year.

In 1991, there were eight plants in the United States that produced acetylene. Together they produced a total of 352 million lb (160 million kg) of acetylene per year. Of this production, 66% was derived from natural gas and 15% from petroleum processing. Most acetylene from these two sources was used on or near the site where it was produced to make other organic chemicals. The remaining 19% came from calcium carbide. Some of the acetylene from this source was used to make organic chemicals, and the rest was used by regional industrial gas producers to fill pressurized cylinders for local welding and metal cutting customers.

In Western Europe, natural gas and petroleum were the principal sources of acetylene in 1991, while calcium carbide was the principal source in Eastern Europe and Japan.

Raw Materials

Acetylene is a hydrocarbon consisting of two carbon atoms and two hydrogen atoms. Its chemical symbol is C₂H₂. For commercial purposes, acetylene can be made from several different raw materials depending on the process used.

The simplest process reacts calcium carbide with water to produce acetylene gas and a calcium carbonate slurry, called hydrated lime. The chemical reaction may be written as CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂.

Other processes use natural gas, which is mostly methane, or a petroleum-based hydrocarbon such as crude oil, naphtha, or bunker C oil as raw materials. Coal can also be used. These processes use high temperature to convert the raw materials into a wide variety of gases, including hydrogen, carbon monoxide, carbon dioxide, acetylene, and others. The chemical reaction for converting methane into acetylene and hydrogen may be written 2 CH₄ → C₂H₂ + 3 H₂. The other gases are the products of combustion with oxygen. In order to separate the acetylene, it is dissolved in a solvent such as water, anhydrous ammonia, chilled methanol, or acetone, or several other solvents depending on the process.

The Manufacturing
Process

There are two basic conversion processes used to make acetylene. One is a chemical reaction process, which occurs at normal temperatures. The other is a thermal cracking process, which occurs at extremely high temperatures.

Here are typical sequences of operations used to convert various raw materials into acetylene by each of the two basic processes.

Chemical reaction process

Acetylene may be generated by the chemical reaction between calcium carbide and water. This reaction produces a considerable amount of heat, which must be removed to prevent the acetylene gas from exploding. There are several variations of this process in which either calcium carbide is added to water or water is added to calcium carbide. Both of these variations are called wet processes because an excess amount of water is used to absorb the heat of the reaction. A third variation, called a dry process, uses only a limited amount of water, which then evaporates as it absorbs the heat. The first variation is most commonly used in the United States and is described below.

1. Most high-capacity acetylene generators use a rotating screw conveyor to feed calcium carbide granules into the reaction chamber, which has been filled to a certain level with water. The granules measure about 0.08 in x 0.25 in (2 mm x 6 mm), which provides the right amount of exposed surfaces to allow a complete reaction. The feed rate is determined by the desired rate of gas flow and is controlled by a pressure switch in the chamber. If too much gas is being produced at one time, the pressure switch opens and cuts back the feed rate.
2. To ensure a complete reaction, the solution of calcium carbide granules and water is constantly agitated by a set of rotating paddles inside the reaction chamber. This also prevents any granules from floating on the surface where they could over-heat and ignite the acetylene
3. The acetylene gas bubbles to the surface and is drawn off under low pressure. As it leaves the reaction chamber, the gas is cooled by a spray of water. This water spray also adds water to the reaction chamber to keep the reaction going as new calcium carbide is added. After the gas is cooled, it passes through a flash arrester, which prevents any accidental ignition from equipment downstream of the chamber.
4. As the calcium carbide reacts with the water, it forms a slurry of calcium carbonate, which sinks to the bottom of the chamber. Periodically the reaction must be stopped to remove the built-up slurry. The slurry is drained from the chamber and pumped into a holding pond, where the calcium carbonate settles out and the water is drawn off. The thickened calcium carbonate is then dried and sold for use as an industrial waste water treatment agent, acid neutralizer, or soil conditioner for road construction.

Thermal cracking process

Acetylene may also be generated by raising the temperature of various hydrocarbons to the point where their atomic bonds break, or crack, in what is known as a thermal cracking process. After the hydrocarbon atoms break apart, they can be made to rebond to form different materials than the original raw materials. This process is widely used to convert oil or natural gas to a variety of chemicals.

There are several variations of this process depending on the raw materials used and the method for raising the temperature. Some cracking processes use an electric arc to heat the raw materials, while others use a combustion chamber that burns part of the hydrocarbons to provide a flame. Some acetylene is generated as a coproduct of the steam cracking process used to make ethylene. In the United States, the most common process uses a combustion chamber to heat and burn natural gas as described below.

1. Natural gas, which is mostly methane, is heated to about 1,200° F (650° C). Preheating the gas will cause it to self-ignite once it reaches the burner and requires less oxygen for combustion.
2. The heated gas passes through a narrow pipe, called a venturi, where oxygen is injected and mixed with the hot gas.
3. The mixture of hot gas and oxygen passes through a diffuser, which slows its velocity to the desired speed. This is critical. If the velocity is too high, the incoming gas will blow out the flame in the burner. If the velocity is too low, the flame can flash back and ignite the gas before it reaches the burner.
4. The gas mixture flows into the burner block, which contains more than 100 narrow channels. As the gas flows into each channel, it self-ignites and produces a flame which raises the gas temperature to about 2,730° F (1,500° C). A small amount of oxygen is added in the burner to stabilize the combustion.
5. The burning gas flows into the reaction space just beyond the burner where the high temperature cause about one-third of the methane to be converted into acetylene, while most of the rest of the methane is burned. The entire combustion process takes only a few milliseconds.
6. The flaming gas is quickly quenched with water sprays at the point where the conversion to acetylene is the greatest. The cooled gas contains a large amount of carbon monoxide and hydrogen, with lesser amounts of carbon soot, plus carbon dioxide, acetylene, methane, and other gases.
7. The gas passes through a water scrubber, which removes much of the carbon soot. The gas then passes through a second scrubber where it is sprayed with a solvent known as N-methylpyrrolidinone which absorbs the acetylene, but not the other gases.
8. The solvent is pumped into a separation tower where the acetylene is boiled out of the solvent and is drawn off at the top of the tower as a gas, while the solvent is drawn out of the bottom.

Storage and Handling

Because acetylene is highly explosive, it must be stored and handled with great care. When it is transported through pipelines, the pressure is kept very low and the length of the pipeline is very short. In most chemical production operations, the acetylene is transported only as far as an adjacent plant, or "over the fence" as they say in the chemical processing business.

When acetylene must be pressurized and stored for use in oxy-acetylene welding and metal cutting operations, special storage cylinders are used. The cylinders are filled with an absorbent material, like diatomaceous earth, and a small amount of acetone. The acetylene is pumped into the cylinders at a pressure of about 300 psi (2,070 kPa), where it is dissolved in the acetone. Once dissolved, it loses its explosive capability, making it safe to transport. When the cylinder valve is opened, the pressure drop causes some of the acetylene to vaporize into gas again and flow through the connecting hose to the welding or cutting torch.

Quality Control

Grade B acetylene may have a maximum of 2% impurities and is generally used for oxyacetylene welding and metal cutting. Acetylene produced by the chemical reaction process meets this standard. Grade A acetylene may have no more than 0.5% impurities and is generally used for chemical production processes. Acetylene produced by the thermal cracking process may meet this standard or may require further purification, depending on the specific process and raw materials.

The Future

The use of acetylene is expected to continue a gradual increase in the future as new applications are developed. One new application is the conversion of acetylene to ethylene for use in making a variety of polyethylene plastics. In the past, a small amount of acetylene had been generated and wasted as part of the steam cracking process used to make ethylene. A new catalyst developed by Phillips Petroleum allows most of this acetylene to be converted into ethylene for increased yields at a reduced overall cost.

Where to Learn More

Books

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

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

Other

Acetylene Pamphlet G-1. Compressed Gas Association, 1990.

Compressed Gas Association. http://www.cganet.com.

—ChrisCavette

Acetylene

OVERVIEW

Acetylene (uh-SET-ill-ene) is the simplest hydrocarbon, consisting of two carbon atoms joined to each other by a triple bond with their associated hydrogen atoms. It occurs as a colorless gas with a sweet odor when pure, but an unpleasant odor due to the presence of phosphine (PH₃) and/or arsine (AsH₃), with which it is often contaminated. Acetylene is a highly flammable gas that is also somewhat explosive. This property accounts for one of its most important uses, in torches that burn at very high temperatures.

KEY FACTS

OTHER NAMES:

Ethyne; ethine

FORMULA:

CH=CH

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Alkyne (unsaturated hydrocarbon; organic)

STATE:

Gas

MOLECULAR WEIGHT:

26.04 g/mol

MELTING POINT:

−80.8°C (−113°F)

BOILING POINT:

−84°C (−120°F)

SOLUBILITY:

Slightly soluble in water and alcohol; soluble in acetone and benzene

Acetylene was discovered by the British chemist Edmund Davy (1785–1857) in 1836. Davy obtained the gas accidentally when he combined water with potassium carbide (KCH₂) while attempting to make potassium metal. He noted that the gas burned with a bright flame and thought it might be used as a source of illumination. That application was impractical, however, because of the high cost of potassium carbide. When the German chemist Frederich Wöhler (1800–1882) discovered the far less expensive calcium carbide (CaC₂) in 1862, however, that problem was solved, and the demand for acetylene in lamps and other applications grew rapidly.

HOW IT IS MADE

The original method of making acetylene by the action of water on calcium carbide was abandoned when more efficient methods of synthesis became available. One of those methods, known as the Wulff process, involves the cracking of hydrocarbons from petroleum. In this process, liquid petroleum, which consists of many different kinds of hydrocarbons, is heated with very hot steam, causing the hydrocarbons to break apart ("crack") into smaller compounds. Acetylene is one of these compounds.

A later improvement on this procedure was developed by the German chemist Walter Reppe (1892–1969), sometimes called the Father of Acetylene Chemistry. Since Reppe worked at that time for the BASF chemical corporation, the process is also known as the BASF process of making acetylene. In this procedure, hydrocarbons from petroleum are treated with an oxidizing agent that causes them to break apart to form smaller compounds, one of which, again, is acetylene.

COMMON USES AND POTENTIAL HAZARDS

By far the most important use of acetylene is in the manufacture of other chemicals, such as vinyl chloride, vinylidene chloride, vinyl acetate, acrylonitrile, trichloroethylene and perchloroethylene, and the family of plastics known as the acrylates. (The name vinyl refers to the remnant of an acetylene molecule after one hydrogen atom has been removed: -CH₂=CH-.) About 20 percent of all the acetylene produced is used for torches that produce very hot flames. In one such torch, the oxyacetylene torch, acetylene gas and oxygen are mixed and ignited at the tip of the torch. The combination of gases burns at a temperature of 3,000°C to 3,500°C (5,500°F to 6,300°F). Oxyacetylene torches are used to cut through metal and to weld two metals to each other. They can be used in very cold climates and even under water.

Interesting Facts

Acetylene was used as a source of illumination beginning in the early 1900s. In one type of acetylene lamp, water was allowed to drop on a solid chunk of calcium carbide at a controlled rate. The acetylene produced then passed into an ignition chamber, where it burned with a brilliant white light. Acetylene lamps were used in a variety of settings, such as the source of light for bicycles and early motor cars and as a source of illumination for miners. Some small towns even used acetylene lamps as their major source of town lighting. Acetylene lamps continue to be a popular collectors item among antique dealers today.

Acetylene is both very flammable and explosive. Anyone who works with the compound or uses it in any form should know how to use the device that contains the gas. Acetylene also has the somewhat unusual chemical property of reacting with certain metals, such as copper and silver, to form highly explosive compounds known as acetylides. Lamps, torches, and other devices built to hold and dispense acetylene can not contain any of these metals. High concentrations of the gas also pose a health hazard to humans. It is classified as an asphyxiant, a gas that can produce disorientation, unconsciousness, and death when inhaled to excess.

Words to Know

OXIDATION

A chemical reaction in which oxygen reacts with some other substance or, alternatively, in which some substances loses electrons to another substance, the oxidizing agent.

SYNTHESIS

A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.

FOR FURTHER INFORMATION

"Acetylene-Properties, Purity and Packaging." http://www.c-f-c.com/specgas_products/acetylene.htm (accessed on September 16, 2005).

"The Environmental Impact of Acetylene Compared to Impact of Propane-Chemtane 2." Chemtane. http://www.chemtane2.com/environmental/cva_env_impact.html (accessed on September 16, 2005).

O'Hara, Kelly. "Chemistry Hall of Fame-1997-Acetylene." York University. http://www.chem.yorku.ca/hall_of_fame/essays97/acetylene/acetylen.htm (accessed on September 16, 2005).

"Oxy-acetylene Welding and Cutting." ESAB. http://www.esabna.com/EUWeb/OXY_handbook/589oxy2_1.htm (accessed on September 16, 2005).

See AlsoPolyvinyl chloride

a·cet·y·lene / əˈsetlən; -ˌēn/ • n. Chem. a colorless pungent-smelling hydrocarbon gas ( C₂H₂), which burns with a bright flame, used in welding and formerly in lighting.

acetylene See ethyne