Acetylene

Description

Acetylene (100% purity) is odourless, but commercial purity has a distinctive garlic-like odour and is very soluble in alcohol and almost miscible with ethane. Acetylene is a flammable gas and kept under pressure in gas cylinders. Under certain conditions, acetylene can react with copper, silver, and mercury to form acetylides, compounds which can act as ignition sources. Brasses containing less than 65% copper in the alloy and certain nickel alloys are suitable for acetylene. Acetylene is not compatible with strong oxidisers such as chlorine, bromine pentafluoride, oxygen, oxygen difluoride and nitrogen trifluoride, brass metal, calcium hypochlorite, heavy metals such as copper, silver, mercury, and their salts, bromine, chlorine, iodine, fluorine, sodium hydride, caesium hydride, ozone, perchloric acid, and potassium.

Chemical Properties

Acetylene (100% purity) is odorless but commercial purity has a distinctive garlic-like odor. It is very soluble in alcohol and almost miscible with ethane. Acetylene is a flammable gas and kept under pressure in gas cylinders. Under certain conditions, acetylene can react with copper, silver, and mercury to form acetylides, compounds that can act as ignition sources. Brasses contain a form acetylides, compounds that can act as ignition sources. Brasses containing less than 65% copper in the alloy and certain nickel alloys are suitable for acetylene. Acetylene is not compatible with strong oxidizers such as chlorine, bromine pentafl uoride, oxygen, oxygen difl uoride, and nitrogen trifl uoride, brass metal, calcium hypochlorite, heavy metals such as copper, silver, mercury, and their salts, bromine, chlorine, iodine, fl uorine, sodium hydride, cesium hydride, ozone, perchloric acid, or potassium.

Chemical Properties

Acetylene is an extremely flammable, colorless, compressed gas. It has a faint ethereal odor when pure; a garlic-like odor when contaminated

Physical properties

Acetylene, which is the simplest alkyne hydrocarbon, exists as a colorless, flammable, unstable gas with a distinctive pleasant odor (acetylene prepared from calcium carbide has a garlic smell resulting from traces of phosphine produced in this process). The term acetylenes is used generically in the petroleum industry to denote chemicals based on the carbon-carbon triple bond.

History

Acetylene was discovered in 1836 by Edmund Davy (1785-1857) who produced the gas while trying to make potassium metal from potassium carbide (K₂C₂). In 1859, Marcel Morren in France produced acetylene by running an electric arc between carbon electrodes in the presence of hydrogen. Morren called the gas produced carbonized hydrogen. Three years later, Pierre Eugène-Marcelin Berthelot (1827-1907) repeated Morren’s experiment and identified carbonized hydrogen as acetylene.

Uses

About 80% of acetylene production is used as a closed-system manufacturing intermediate for the production of other chemicals. The other chemicals synthesized from acetylene include vinyl chloride monomer, N-vinylcarbazole, 1,4- butanediol, vinyl ethers, N-vinyl-2-pyrrolidone, vinyl fluoride, N-vinylcaprolactam, and vinyl esters. The other use of acetylene as oxyacetylene torches for metal cutting and welding is about 20%.

Uses

Acetylene is used in oxyacetylene flame forwelding and cutting metals; as an illuminant;as a fuel; for purifying copper, silver, andother metals; and in the manufacture ofacetic acid, acetaldehyde, and acetylides. Itis formed when calcium carbide reacts withwater. It is also obtained from cracking ofpetroleum naphtha fractions.

Uses

Illuminant, oxyacetylene welding, cutting, and soldering metals, signalling; pptg metals, particularly Cu; manufacture of acetaldehyde, acetic acid; fuel for motor boats.

Production Methods

Commercially, acetylene is produced from the pyrolysis of naphtha in a two-stage cracking process. Both acetylene and ethylene are end products. The ratio of the two products can be changed by varying the naphtha feed rate. Acetylene also has been produced by a submerged-flame process from crude oil. In essence, gasification of the crude oil occurs by means of the flame, which is supported by oxygen beneath the surface of the oil. Combustion and cracking of the oil take place at the boundaries of the flame. The composition of the cracked gas includes about 6.3% acetylene and 6.7% ethylene. Thus, further separation and purification are required. Several years ago when procedures were developed for the safe handling of acetylene on a large scale, J. W. Reppe worked out a series of reactions that later became known as “Reppe chemistry.” These reactions were particularly important to the manufacture of many high polymers and other synthetic products. Reppe and his associates were able to effect synthesis of chemicals that had been commercially unavailable. An example is the synthesis of cyclooctatetraene by heating a solution of acetylene under pressure in tetrahydrofuran in the presence of a nickel cyanide catalyst. In another reaction, acrylic acid was produced from CO and H2O in the presence of a nickel catalyst: C2H2 + CO + H2O → CH2:CH·COOH. These two reactions are representative of a much larger number of reactions, both those that are straight-chain only, and those involving ring closure.

Definition

A gaseous alkyne. Traditionally ethyne has found use in oxy-acetylene welding torches, since its combustion with oxygen produces a flame of very high temperature. It is also important in the organic chemicals industry for the production of chloroethene (vinyl chloride), which is the starting material for the production of polyvinyl chloride (PVC), and for the production of other vinyl compounds. Until recently, ethyne was manufactured by the synthesis and subsequent hydrolysis of calcium dicarbide, a very expensive procedure. Modern methods increasingly employ the cracking of alkanes.

Production Methods

The traditional method of producing acetylene is from reacting lime, calcium oxide (CaO), with coke to produce calcium carbide (CaC₂). The calcium carbide is then combined with water to produce acetylene:
2CaO_(s) + 5C_(s)→2CaC_2(g) + CO_2(g)
CaC_2(s) + 2H₂O_(l)→ C₂H_2(g) + Ca(OH)_2(aq)
Several processes for producing acetylene from natural gas and other petroleum products developed in the 1920s. Thermal cracking of methane involves heating methane to approximately 600℃ in an environment deficient in oxygen to prevent combustion of all the methane. Combustion of part of the methane mix increases the temperature to approximately 1,500℃, causing the remaining methane to crack according the reaction: 2CH_4(g) → C2H_2(g) + 3H_2(g). In addition to methane, ethane, propane, ethylene, and other hydrocarbons can be used as feed gases to produce acetylene.

Reactions

Acetylene reacts (1) with chlorine, to form acetylene tetrachloride C2H2Cl4 or CHCl2·CHCl2 or acetylene dichloride C2H2Cl2 or CHCl:CHCl, (2) with bromine, to form acetylene tetrabromide C2H2Br4 or CHBr2·CHBr2 or acetylene dibromide C2H2Br2 or CHBr:CHBr, (3) with hydrogen chloride (bromide, iodide), to form ethylene monochloride CH2:CHCl (monobromide, monoiodide), and 1,1-dichloroethane, ethylidene chloride CH3·CHCl2 (dibromide, diiodide), (4) with H2O in the presence of a catalyzer, e.g., mercuric sulfate HgO4S, to form acetaldehyde CH3·CHO, (5) with hydrogen, in the presence of a catalyzer, e.g., finely divided nickel heated, to form ethylene C2H4 or ethane C2H6, (6) with metals, such as copper or nickel, when moist, also lead or zinc, when moist and unpurified. Tin is not attacked. Sodium yields, upon heating, the compounds C2HNa and C2Na2. (7) With ammoniocuprous (or silver) salt solution, to form cuprous (or silver) acetylide C2Cu2, dark red precipitate, explosive when dry, and yielding acetylene upon treatment with acid, (8) with mercuric chloride solution, to form trichloromercuric acetaldehyde C(HgCl)3·CHO, precipitate, which yields with HCl acetaldehyde plus mercuric chloride.

General Description

A colorless gas with a faint garlic-like odor. Easily ignited and burns with a sooty flame. Gas is lighter than air. Flame may flash back to the source of a leak very easily. Under prolonged exposure to fire or heat the containers may rupture violently and rocket.

Air & Water Reactions

Highly flammable. Slightly soluble in water. Reacts with water to form toxic ammonia fumes.

Reactivity Profile

Acetylene reacts with alkali metals, forming Hydrogen gas. Acetylene can react explosively with bromine [Von Schwartz 1918. p.142 ]. Acetylene forms a sensitive acetylide when passed into an aqueous solution of mercuric nitrate, [Mellor 4:933. 1946-47]. An Acetylene torch used to drill through a plow frame, which was filled with hydrogen gas, produced an explosion [NIOSH, June 1998]. Acetylene reacts with silver, copper and lead to form sensitive, explosive salts. Since Acetylene is endothermic and effectively a reducing agent, it's reaction with oxidants can be very violent (examples: calcium hypochlorite, nitric acid, nitrogen oxide, ozone, trifluoromethyl hypofluorite, etc.). Contact of very cold liquefied gas with water may result in vigorous or violent boiling of the product and extremely rapid vaporization, due to the large temperature differences involved. If the water is hot, there is the possibility that a liquid "superheat" explosion may occur. Pressures may build to dangerous levels if liquid gas contacts water in a closed container [Handling Chemicals Safely 1980]. Acetylene and ammonia can form explosive silver salts in contact with Ag. (Renner, Hermann, Gunther Schlamp. "Silver, Silver Compounds, and Silver Alloys." Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA. 2001.).

Hazard

The LEL of acetylene is reached well before asphyxiation can occur, and the danger of explosion is reached before any other health hazard is present. When fighting fires involving acetylene containers, the fire should be extinguished before closing the valve to the container. This is because the acetylene has such a wide flammable range that it can burn inside the container. Acetylene is incompatible with bromine, chlorine, fluorine, copper, silver, mercury, and their compounds. Acetylene has a four-digit UN identification number of 1001. The NFPA 704 designation is health 1, flammability 4, and reactivity 3. Reactivity is reduced to 2 when the acetylene is dissolved in acetone.

Health Hazard

Prolonged periods of exposure to acetylene cause symptoms including headaches, respiratory diffi culty, ringing in ears, shortness of breath, wheezing, dizziness, drowsiness, unconsciousness, nausea, vomiting, and depression of all the senses. The skin of a victim of overexposure may have a blue color. Currently, there are no known adverse health effects associated with chronic exposure to the components of this compressed gas. Lack of suffi cient oxygen may cause serious injury or death. The target organs include the kidneys, CNS, liver, respiratory system, and eyes.

Health Hazard

Acetylene is an asphyxiate like other hydrocarbongases. Exposure to its atmosphere cancause death from suffocation or asphyxiationby exclusion of oxygen. It is nontoxicbut narcotic at high concentrations. An exposureto 20% concentration in air may produceheadache and dyspnea in humans; whileinhaling a 50% acetylene in air for 5 minutesmay be fatal (NIOSH 1986).

Health Hazard

Headache, dizziness and loss of consciousness may occur. Death from ``smothering'' may occur if oxygen content of the air is severely reduced by dilution with Acetylene.

Fire Hazard

Behavior in Fire: May explode in fire

Flammability and Explosibility

Acetylene is a highly flammable gas and forms explosive mixtures with air over an unusually wide range of concentrations (2 to 80%). Acetylene can polymerize exothermically, leading to deflagration. With a very high positive free energy of formation, acetylene is thermodynamically unstable and is sensitive to shock and pressure. Its stability is enhanced by the presence of small amounts of other compounds such as methane, and acetylene in cylinders is relatively safe to handle because it is dissolved in acetone. Acetylene fires can be fought with carbon dioxide, dry chemical, and halon extinguishers; firefighting is greatly facilitated by shutting off the gas supply.

Industrial uses

Acetylene is a colorless, flammable gas with a garlic-like odor. Under compressed conditions, it is highly explosive; however, it can be safely compressed and stored in high-pressure cylinders if the cylinders are lined with absorbent material soaked with acetone. Users are cautioned not to discharge acetylene at pressures exceeding 15 psig (103 kPa), as noted by the red line on acetylene pressure gauges.
With its intense heat and controllability, the oxyacetylene flame can be used for many different welding and cutting operations including hardfacing, brazing, beveling, gouging, and scarfing. The heating capability of acetylene also can be utilized in the bending, straightening, forming, hardening, softening, and strengthening of metals.

Safety Profile

Mtldly toxic by inhalation. Human systemic effects by inhalation: headache and dyspnea. Narcotic in high concentration. In general industrial practice, acetylene does not constitute a serious toxic hazard. It is a very dangerous fire hazard when exposed to heat, flame, or oxidizers. Moderate explosion hazard when exposed to heat or flame or by spontaneous chemical reaction. At high pressures and moderate temperatures, and in the absence of air, acetylene has been known to decompose explosively. Reacts with copper to form the explosive copper acetylide. Incompatible with brass, copper salts, copper carbide, powdered Co, Hg, Hg salts, K, Ag and Ag salts, RbH, CsH, halogens, HNO3, NaH, oxidants. Acetylene + halide + UV can explode. Molten Kignites in C2H2 and then explodes. C2H2 reacts vigorously with trifluoromethyl hypo fluorite. With O2, C2H2 can detonate very powerfully. See ACETYLIDES. When ignited, it burns with an intensely hot flame; can react vigorously with oxidizing materials. When mixed with O2 in proportions of 40% or more, acetylene acts as a narcotic and has been used in anesthesia. Acetylene O2 in the air to a level that wd not support life. However, the presence of impurities in commercial acetylene may result in the production of symptoms before an asphyxiant concentration is reached. Thus: 10% in air produces a slight intoxication, 20% produces a staggering gait, 30% produces general incoordination, 33% leads to unconsciousness in 7 minutes, up to 80% produces complete anesthesia, increased blood pressure, narcosis, and stimulated respiration. symptoms, and (in hgh concentration) semi-asphyxia and brief loss of consciousness have all been reported. See ARGON for a dmussion of simple asphyxiants. To fight fire, use CO2, water spray, or dry chemical. Stop flow of gas

Potential Exposure

Acetylene can be burned in air or oxygen and is used for brazing, welding, cutting, metallizing, hardening, flame scarfing; and local heating in metallurgy. The flame is also used in the glass industry. Chemically, acetylene is used in the manufacture of vinyl chloride, acrylinitrile, synthetic rubber; vinyl acetate; trichloroethylene, acrylate, butyrolactone, 1,4-butanediol, vinyl alkyl ethers, pyrrolidone, and other substances

storage

Acetylene should be kept stored in a cool, dry place in a tightly sealed container, and should only be used in a well-ventilated area. Cylinders should be separated from oxygen and other oxidizers by a minimum of 20 ft or by a barrier of non-combustible material at least 5 ft high, having a fi re resistance rating of at least 30 min. Storage in excess of 2500 cu ft is prohibited in buildings with other occupancies. Cylinders should be stored upright with a valve protection cap in place and fi rmly secured to prevent falling or being knocked over. The cylinders should be protected from physical damage and avoid dragging, rolling, sliding, or dropping the cylinder. During transport, workers should use a suitable hand truck for cylinder movement. Care should be taken to label “No Smoking” or “Open Flames” signs in the storage or use areas. There should be no sources of ignition. All electrical equipment should be explosion-proof in the storage and use areas.

Shipping

UN1001 Acetylene, dissolved, Hazard Class: 2.1; Labels: 2.1-Flammable gas. Cylinders must be transported in a secure upright position, in a well-ventilated truck. Protect cylinder and labels from physical damage. The owner of the compressed gas cylinder is the only entity allowed by federal law (49CFR) to transport and refill them. It is a violation of transportation regulations to refill compressed gas cylinders without the express written permission of the owner

Purification Methods

If very impure, acetylene should be purified by successive passage through spiral wash bottles containing, in this order, saturated aqueous NaHSO4, H2O, 0.2M iodine in aqueous KI (two bottles), sodium thiosulfate solution (two bottles), alkaline sodium hydrosulfite with sodium anthraquinone-2-sulfonate as indicator (two bottles), and 10% aqueous KOH solution (two bottles). The gas is then passed through a Dry-Ice trap and two drying tubes, the first containing CaCl2, and the second, Dehydrite [Mg(ClO4)2] [Conn et al. J Am Chem Soc 61 1868 1939]. Acetone vapour can be removed from acetylene by passage through H2O, then conc H2SO4, or by passage through two gas traps at -65o and -80o, conc H2SO4 and a soda lime tower, a tower of 1-mesh Al2O3 then through H2SO4 [Reichert & Nieuwland Org Synth Coll Vol I 229 1941, Wiley Org Synth Coll Vol III 853 1955, Jones & Whiting Org Synth Coll Vol IV 793 1963]. Sometimes it contains acetone and air. These can be removed by a series of bulb-to-bulb distillations, e.g. a train consisting of a conc H2SO4 trap and a cold EtOH trap (-73o), or passage through H2O and H2SO4, then over KOH and CaCl2. [See Brandsma Preparative Acetylenic Chemistry, 1st Edn Elsevier 1971 p15, for pK, ISBN 0444409475, 2nd Edn Elsevier 1988, ISBN 0444429603, and below for sodium acetylide.] It is also available commercially as 10ppm in helium, and several concentrations in N2 for instrument calibration. [Beilstein 1 IV 939.] Sodium acetylide [1066-26-8] M 48.0, is prepared by dissolving Na (23g) in liquid NH3 (1L) and bubbling acetylene until the blue color is discharged (ca 30minutes) and evaporated to dryness [Saunders Org Synth Coll Vol III 416 1955], and is available commercially as a suspension in xylene/light mineral oil. [See entry in “Metal-organic Compounds”, Chapter 5.]

Toxicity evaluation

Acetylene is released to the environment through various industrial waste streams of industries. Because of the vapor pressure of acetylene (4.04×10⁴mmHg at 25°C), it exists in the environment exclusively in the form of gas. The gaseous phase of acetylene is degraded in the environment with photochemically induced hydroxyl radicals; the halflife for this photochemical degradation is approximately 20 days. The estimated Koc of acetylene is 38, and based upon this Koc value, acetylene is expected to possess high mobility if released to soil. Based on the Henry’s law constant of 0.022 atm-m3mol
^-1, derived from vapor pressure 4.04×10⁴mmHg and water solubility 1200 mg l
1, volatilization from moist soil is the major fate process for acetylene. In soil, biodegradation is not expected to be an important fate process for acetylene, as suggested by 0% biochemical oxygen demand (BOD) in 28 days. Acetylene is not anticipated to be adsorbed by suspended solids and sediments if released to water because of its Koc value. Removal of acetylene from water is expected to be through the volatilization process. The estimated bioconcentration factor (BCF) of 3 for acetylene suggests that the potential for bioaccumulation of acetylene in aquatic organism is low.

Incompatibilities

The substance may polymerize due to heating. The substance decomposes on heating and increasing pressure, causing a fire and explosion hazard. The substance is a strong reducing agent and reacts violently with oxidants and with fluorine or chlorine under influence of light, causing fire and explosion hazard. Reacts with copper, silver, and mercury or their salts, forming shock-sensitive compounds (acetylides). The content of lines carrying acetylene must not exceed 63% copper. May form explosive mixture with air. Forms shock-sensitive mixture with copper and copper salts; mercury and mercury salts; and silver and silver salts. Reacts with brass, bromine, cesium hydride, chlorine, cobalt, cuprous acetylize; fluorine, iodine, mercuric nitrate; nitric acid, potassium, rubidium hydride; trifluoromethyl hypofluorite; and sodium hydride.

Waste Disposal

Return refillable compressed gas cylinders to supplier. Consult with environmental regulatory agencies for guidance on acceptable disposal practices. Generators of waste containing this contaminant (≥100 kg/mo) must conform with EPA regulations governing storage, transportation, treatment, and waste disposal. Incineration.