Is acetylene obsolete? The chemical industry left carbide in the 1960s welding, cutting and heating kept it, for reasons of flame physics

"Isn't acetylene obsolete?" is the most damaging question in the industrial-gas market, and it is usually asked by someone who has read that the chemical industry abandoned acetylene decades ago. That part is true. What is not true is that the gas in a welder's cylinder went with it. The record shows two separate markets with two separate fates: the chemical feedstock market left acetylene for ethylene in the 1960s on cost; the fuel-gas market kept it, because no cheaper gas does what its flame does. This page sets out the evidence for both.

For most of the twentieth century there were really two acetylene businesses using the same molecule. One burned it — first for lighting, then from 1903 for welding and from 1905 for cutting. The other used it as a building block for chemicals: acetaldehyde and acetic acid, vinyl chloride for PVC, vinyl acetate, acrylonitrile, chloroprene for neoprene, trichloroethylene, and in wartime Germany the butadiene for Buna synthetic rubber. The two were sized very differently at different times.

The German 1942 figure is the wartime anomaly that most people remember: a country without oil made its synthetic rubber, plastics and solvents from carbide, and four-fifths of its acetylene went into chemicals. In peacetime and outside Germany the fuel market was the larger part of the business until, in Miller's account, chemical use caught up and overtook it after 1936.

The economic argument was put plainly in 1965. As compiled by S. A. Miller, where acetylene and ethylene competed for the same product, the acetylene route was usually shorter, but ethylene was much cheaper; and the plants making acetylene from hydrocarbons — arc, regenerative and partial-combustion processes developed in the 1930s and 1940s — were no more able than carbide plants to produce acetylene at less than twice the cost of ethylene. As long as ethylene was scarce, the short route won; once naphtha crackers made ethylene cheap and plentiful, it lost. Acetaldehyde moved to ethylene oxidation, vinyl chloride to ethylene dichloride, acrylonitrile to propylene ammoxidation, and even chloroprene, long an acetylene preserve, began to move to butadiene.

The turn can be dated precisely in Britain. Miller's book went to press with an addendum reporting news from The Times and the Financial Times of 9 June 1965: British Geon would build a plant making acetylene from naphtha for 135,000 tonnes a year of vinyl chloride at Barry in South Wales, and the Distillers Company would cease to manufacture calcium carbide at Kenfig — the largest carbide plant in Britain — in 1966. The main text of the same book had just argued that carbide remained competitive. For most of the world's chemical industry, that is where carbide acetylene ended.

Propane and natural gas took much of the preheating and heavy cutting market, where long preheat times matter less than fuel cost per metre; the trade-offs are set out on acetylene vs LPG cutting. Miller listed oxygen-cutting among the uses where propane, butane and natural gas were already competitors in the 1960s. They compete there still. They do not weld steel.

If acetylene's fuel market had collapsed with its chemical market, the dissolved acetylene tonnages would show it. They do not. In the international carbide table for 1948, 1953 and 1957, compiled from von Haken, Klöckner and Lang, every country that reports dissolved acetylene shows it as a distinct and growing trade: about 100,000 tonnes in the United States in 1953, 18,500 in West Germany, 14,800 in Great Britain (15,300 in 1954 and in 1958), 8,790 in France and 1,000 in India. Dissolved acetylene — the cylinder trade — is structurally independent of feedstock acetylene, and was never built on it. The Indian side of that story is on acetylene in India.

One consequence of the two industries is worth knowing. Acetylene made from hydrocarbons is free of phosphine, arsine and hydrogen sulphide, as CGA's commodity specification G-1.1 (1972) noted; carbide acetylene carries them and must be purified. The fuel-gas trade is overwhelmingly carbide-based, so purification is part of the product. IS 308:2024 limits phosphine to 0.01 percent and hydrogen sulphide to 0.02 percent by volume in the cylinder.