Review pubs.acs.org/CR
Production of Acetylene and Acetylene-based Chemicals from Coal Harold Schobert* The EMS Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16803, United States 6. Conclusions Author Information Corresponding Author Notes Biography Acknowledgments References
1. INTRODUCTION Acetylene has been known since the mid-19th century, when it was discovered by Edmund Davy in Ireland and later studied extensively by Marcellin Berthelot in France. The triple bond in acetylene, and the fact that acetylene is thermodynamically unstable, give acetylene a very rich chemistry, with many applications. The accidental discovery of calcium carbide, produced from coal or coke and limestone in an attempt to prepare calcium metal, and the recognition of the reaction of calcium carbide with water to form acetylene, opened a route to the large-scale production of acetylene. In the 20th century, the two giants in the field of acetylene chemistry were Julius Nieuwland, at Notre Dame University in the United States, and Walter Reppe, at I.G. Farbenindustrie and BASF in Germany. Their extensive studies, along with contributions from many others, created a role for acetylene as a vital feedstock for the production of commodity chemicals. Acetylene, along with the byproduct tar from metallurgical coke plants, were the two pillars of the organic chemical industry to about the middle of the 20th century. The global availability of inexpensive petroleum after about 1950 led to the development of a variety of petrochemical processes, especially based on ethylene. This was a factor in the steady displacement of coal tar and acetylene by petroleum- or gas-derived feedstocks. As a feedstock for production of chemicals, acetylene increased in importance after the Second World War and peaked in the 1960s.1 In 1960, world acetylene production was 10 million metric tons.2 By the early 1990s, annual worldwide production had declined to about half that value,2 and currently only several hundred thousand metric tons are made, mostly from sources other than coal.
CONTENTS 1. Introduction 2. Production of Acetylene 2.1. Indirect Production via Calcium Carbide 2.2. Direct Production via Arc Plasma Reactions 2.2.1. Effects of Coal Composition 2.2.2. Effects of Reaction Conditions 2.2.3. Yields and Selectivity 2.2.4. Pilot-Scale Testing 2.3. Alternatives to the Arc-Plasma Process 2.4. Other Methods 3. Purification and Handling of Acetylene 3.1. Purification 3.2. Handling Acetylene 4. Acetylene as a Fuel 5. Conversion of Acetylene to Commodity Chemicals and Materials 5.1. Acetaldehyde 5.2. Acetic Acid and Acetic Anhydride 5.3. Acetylene Tetrabromide 5.4. Acrylic Acid 5.5. Acrylonitrile 5.6. Aromatic Hydrocarbons 5.7. Alkynes 5.8. 1,4-Butanediol 5.9. Carbon Black 5.10. Chlorinated Solvents 5.11. Ethanol 5.12. Ethylene 5.13. Heterocyclic Compounds 5.14. Isoprene and Chloroprene 5.15. Lewisite 5.16. Norbornadiene 5.17. Polyacetylene 5.18. Propargyl Alcohol 5.19. Resins 5.20. Vinyl Acetate 5.21. Vinyl Chloride 5.22. Vinyl Esters and Ethers 5.23. Vinyl Fluoride © XXXX American Chemical Society
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2. PRODUCTION OF ACETYLENE The field of coal liquefaction is divided into indirect processes (coal to synthesis gas to liquids) and direct processes (making liquids straight from the coal). Acetylene production from coal can be thought of in an analogous fashion. Indirect production of acetylene involves using coal, or coal-derived coke, to Special Issue: 2014 Chemicals from Coal, Alkynes, and Biofuels Received: May 21, 2013
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dx.doi.org/10.1021/cr400276u | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
Review
aggregates, rather than the batch operation of a traditional coke oven.13 Along the same line, lignite has been carbonized with calcium hydroxide or calcium carbonate at 900−1400 °C to form an aggregated solid that is then fed to an oxygen-thermal furnace for calcium carbide production, as discussed further below.14 Methods have been developed to take advantage of low-rank coal feedstocks for calcium carbide production, as alternatives to the more expensive anthracite or coke. One such method includes the partial combustion of some of the coal in the furnace charge.15 The most recent involves the conversion of low-rank coal to a “process middle coke” containing ≈87% carbon.16 This material is mixed with calcium oxide and heated electrically, but with the addition of some oxygen to the furnace. Combustion of a portion of the process middle coke reduces the electricity consumption and helps to heat the charge. Calcium carbide formation in a plasma reactor has also been demonstrated.17 In the context of modern chemical technology, calcium carbide production has the negative characteristics of being an energy-intensive, long-residence-time batch process involving the handling and feeding of solids. Furthermore, the severe reaction conditions result in a relatively short working life for an expensive electric furnace. Electricity consumption is approximately 3.3 kW·h/kg of calcium carbide produced.8,16,18,19 Despite these disadvantages, some 12 million metric tons of calcium carbide were produced in China in 2006.15 This represents by far the greatest share of worldwide production, which has been estimated to be >15 million metric tons/year.20 Growth rates in calcium carbide demand are forecast to be about 1−2% annually in the United States and western Europe but 8−10% in China.21 Commercial calcium carbide is available in various grades and particle sizes. The best-quality material produces 288 L of acetylene/kg of carbide.10 Acetylene is produced by the reaction of calcium carbide with water.1,2,6,11,22 Details of the generating equipment and its operation are provided in the early literature6,11 and have not undergone fundamental changes in the years since. The reaction can be effected in so-called wet or dry generators.8 The wet generator uses an excess of water, in at least a 6:1 ratio of water to carbide;23 more recent data suggest ratios in the range 7−9 t of water/t of calcium carbide.16 Calcium hydroxide is produced in a water slurry as byproduct, at a rate of 2.8 t of Ca(OH)2/t of acetylene.1 The dry generator is run with a near-stoichiometric quantity of water and produces a pourable powder of calcium hydroxide, any excess water being evaporated by the heat of reaction.2 The largest dry generators have an output of ≈3750 m3/h, equivalent to 32 000 t/year.2 The acetylene yield amounts to 1 t per 3.1 t of 80% pure calcium carbide.8 A portion of the byproduct calcium hydroxide could be recycled to carbide production16 or could have markets in the agricultural sector as fertilizer and in chemical industries as, for example, raw material for cement production. Recycle is limited to a maximum of ≈60%, to avoid accumulation of impurities in the furnace.8 Of the two types of generators, the wet system is considered to be safer to operate,24 presumably because the excess water helps dissipate the heat of reaction. The reaction of calcium carbide with water is highly exothermic. If no cooling were used, the heat of reaction could raise the temperature of the reacting mixture to 700 °C.23 Temperatures in this range could trigger further exothermic
produce calcium carbide and then making acetylene from the carbide. The direct process involves various ways of making acetylene directly from a coal feedstock, without an intermediate step of calcium carbide. 2.1. Indirect Production via Calcium Carbide
Calcium carbide belongs to the family of carbide compounds known as acetylenides, which can be considered to be salts of the C22− anion. Numerous elements are known to form acetylenides, including all of the alkali and alkaline earth elements, the metals of groups 11 and 12 of the periodic table, and some of the lanthanides.3 Calcium carbide is the only one to have achieved large-scale commercial production. Friedrich Wöhler discovered calcium carbide in 1862.3 Supposedly, the first synthesis of calcium carbide from calcium carbonate and coke was the unintended outcome of an attempt to obtain metallic calcium by carbothermic reduction of a calcium compound.4 Production of calcium carbide in an electric arc furnace goes back at least 120 years.5 Details of Willson’s discovery and other 19th-century work on calcium carbide and acetylene production are reviewed by Thompson.6 The importance of Willson’s work, relative to the earlier studies of Davy, Wohler, and Berthelot, is twofold: it was the first synthesis of calcium carbide that could be applied commercially, and the product was a nearly pure, crystalline material.7 The highly endothermic reaction of calcium oxide with metallurgical coke occurs in an electric furnace with three Söderberg electrodes at temperatures of 2000−2300 °C.1,8 Other carbon sources can be used; however, the two that appear to be most suitable are metallurgical coke and anthracite.9 The reaction can be written as CaO + 3C ⇄ CaC2 + CO
Production of 1000 metric tons (t) of calcium carbide requires 875 t of calcium oxide and 650 t of carbon, usually in the form of anthracite or coke.10 Calcium carbide formation begins at temperatures above 1600 °C; below this temperature, the reaction runs from right to left as written above, and carbon monoxide will decompose the carbide.3 Calcium carbide and calcium oxide form a eutectic at 1630 °C.9 At temperatures >2200 °C, calcium carbide will begin to react further with calcium oxide to produce calcium metal and carbon monoxide.3 The furnaces operate with 100−250 V alternating current and a current density in the electrodes of 1200 °C, reaction in a plasma offers a route to direct production of acetylene from coal. Compared to the indirect route, reaction of coal in a plasma or arc process offers the advantages of an overall simpler process scheme and less impact on the environment.35 Work on plasma conversion of coal to acetylene is continuing in China.36,37 The process has been scaled up to a 2-MW pilotplant reactor38 and then to a 5-MW reactor, for which a useful model is available.39 The 5-MW reactor is thought to be the largest arc-plasma pyrolysis reactor in the world.40 2.2.1. Effects of Coal Composition. Acetylene yield is inversely related to coal rank, with coals of higher volatile matter content (i.e., of lower rank) providing higher yields.27,33,41,42 It is proposed that acetylene formation is a two-step process, involving first the rapid thermal decomposition of the coal into volatiles and solid (char, coke, or soot) and then reaction of the volatiles in the plasma, accounting for the actual formation of acetylene.33,43 Rank also impacts the formation of byproduct coke. Tests with Chinese coals showed that anthracite had low tendency for formation of coke and adhesion of coke particles to the reactor walls, while bituminous coals had significant coking and adhesion.44 This same rank relationship might be expected to be observed at much lower temperatures and heating rates as well. Thermodynamic modeling indicates that two properties impact acetylene yield: the volatile matter content, which has a positive effect, and the oxygen content, which diminishes acetylene formation with concomitant increase in carbon monoxide yield.45 Both of these parameters decrease with increasing rank. Qualitatively, coals of high volatile matter but low oxygen content give the best yields of acetylene.46 Rather, a coal of about 80% carbon and 12% oxygen gives a better acetylene yield.45 Other work has suggested 25−44% volatile matter and 37% volatile matter should ideally contain 50% in an Ar/H2 mixture have been argued to reduce acetylene yield by reducing the bulk plasma temperature.51 The highest acetylene yield, 60% (corresponding to conversion of 74% of the carbon in the coal) was achieved with a British coal of 33.4% volatile matter in the 90:10 Ar/H2 atmosphere.57 A 40% conversion of the carbon in coal to acetylene was achieved by reaction in a 90:10 argon/hydrogen plasma.33,56 Nonetheless, it is argued that an abundance of hydrogen in the reactor atmosphere enhances conversion of coal to acetylene.59 Acetylene yield in a hydrogen atmosphere is 3 times greater than from a comparable reaction performed in argon.60 The importance of hydrogen is attributed to its role in retarding the decomposition of acetylene.47 These reports are consistent with a doubling of acetylene yield in a 90:10 argon/hydrogen plasma compared to the yield obtained in pure argon.56 The yields of acetylene, and corresponding energy consumptions, vary considerably among reports in the literature. Thermodynamic analysis suggests optimum temperatures for acetylene production at atmospheric pressure in the range 3200−4200 °C.55 In practice, reaction temperature >1400 °C is important.2 High temperature is critical; plasma treatment at temperatures
