Acetylene

Charles Pergiel

Extracted from various Wikipedia articles.

My comments in red.

September 3, 2011

General

A common propane/air flame burns at about 3,630 °F (2,000 °C), a propane/oxygen flame burns at about 4,530 °F (2,500 °C), and an acetylene/oxygen flame burns at about 6,330 °F (3,500 °C).

Chemical formula for acetylene is C2H2.

There is about 1700 kPa (250 psi) pressure in the tank when full. Acetylene when combined with oxygen burns at a temperature of 3200 °C to 3500 °C (5800 °F to 6300 °F), highest among commonly used gaseous fuels. As a fuel acetylene's primary disadvantage, in comparison to other fuels, is high cost.

Inherent Hazard

Samples of concentrated or pure acetylene can easily react in an addition-type reaction to form a number of products, typically benzene and/or vinylacetylene. This reaction is exothermic. (!) Consequently, acetylene can explode (!) with extreme violence (!) if the pressure of the gas exceeds about 200 kPa (29 psi) (! That is a low pressure, similar to what is in your car tires!) as a gas[16] or when in liquid or solid form. It is therefore shipped and stored dissolved in acetone (nail polish remover) (ordimethylformamide (DMF), contained in a metal cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling.

Acetylene gas is shipped in special cylinders designed to keep the gas dissolved. The cylinders are packed with porous materials (e.g. kapok fibre, diatomaceous earth, or (formerly) asbestos), then filled to around 50% capacity with acetone, as acetylene is acetone soluble. This method is necessary because above 207 kPa (30 lbf/in²) (absolute pressure) acetylene isunstable and may explode. (!)

As acetylene is unstable at a pressure roughly equivalent to 33 feet/10 meters underwater, water submerged cutting and welding is reserved for hydrogen rather than acetylene.

Production

Until the 1950s, when oil supplanted coal as the chief source of carbon (!), acetylene (and the aromatic fraction from coal tar) was the main source of organic chemicals in the chemical industry. It was prepared by the hydrolysis of calcium carbide (this means dropping the calcium carbide in water where it reacts spontaneously), a reaction discovered by Friedrich Wöhler in 1862 and still familiar to students:

CaC2 + 2H2O → Ca(OH)2 + C2H2

Calcium carbide production requires extremely high temperatures, ~2000 °C, necessitating the use of an electric arc furnace. In the US, this process was an important part of the late-19th century revolution in chemistry (!) enabled by the massivehydroelectric power project at Niagara Falls (!).

Calcium carbide is produced industrially in an electric arc furnace from a mixture of lime and coke at approximately 2000 °C. This method has not changed since its invention in 1888:

CaO + 3 C → CaC2 + CO

The high temperature required for this reaction is not practically achievable by traditional combustion, so the reaction is performed in an electric arc furnace with graphite electrodes.

Today acetylene is mainly manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from cracking of hydrocarbons. Approximately 400,000 tonnes are produced this way annually.

Consumption - Production of Other Substances

In 1881, the Russian chemist Mikhail Kucherov[10] described the hydration of acetylene to acetaldehyde using catalysts such as mercury(II) bromide. Before the advent of the Wacker process, this reaction was conducted on an industrial scale.[11]

The polymerization of acetylene with Ziegler-Natta catalysts produces polyacetylene films. Polyacetylene, a chain of CH centres with alternating single and double bonds, was the one of first discovered organic semiconductors. Its reaction withiodine produces a highly electrically conducting material. Although such materials are not useful, these discoveries led to the developments of organic semiconductors, as recognized by the Nobel Prize in Chemistry in 2000 to Alan J. Heeger, Alan G MacDiarmid, and Hideki Shirakawa. (Compared to the earlier chemical stuff, this happened yesterday.)

Consumption - Burning

In the early 20th Century acetylene was widely used for illumination, including street lighting in some towns.[12] Most early automobiles used carbide lamps before the adoption of electric headlights. Coal miners used carbide lamps, and cavers still do.

Acetylene is sometimes used for carburization (that is, hardening) of steel when the object is too large to fit into a furnace.[8]

Acetylene is used to volatilize carbon in radiocarbon dating. The carbonaceous material in an archeological sample is reacted with lithium metal in a small specialized research furnace to form lithium carbide (also known as lithium acetylide). The carbide can then be reacted with water, as usual, to form acetylene gas to be fed into mass spectrometer to sort out the isotopic ratio of carbon 14 to carbon 12.

Consumption - Welding

Acetylene is the primary fuel for oxy-fuel welding and is the fuel of choice for repair work and general cutting and welding.

Approximately 20 percent of acetylene is consumed for oxyacetylene gas welding and cutting due to the high temperature of the flame; combustion of acetylene with oxygen produces a flame of over 3600 K (3300 °C, 6000 °F), releasing 11.8 kJ/g. Oxyacetylene is the hottest burning common fuel gas.[8] Acetylene is the third hottest natural chemical flame after cyanogen at 4798 K (4525 °C, 8180 °F) and dicyanoacetylene's 5260 K (4990 °C, 9010 °F). (Cyano-what? Sounds like cyanide. I don’t think I would want to be around that stuff.)