Once referred to as ‘the mother of organic synthesis industry’, acetylene is a key base material in organic synthesis. Between 1960 and 1970, when worldwide acetylene production peaked, it served as the primary feedstock for a wide variety of commodity and specialty chemicals.
Advances in olefins technology, concerns about acetylene safety, and a fundamental loss of cost competitiveness began to limit the importance of acetylene, but with oil prices showing a high degree of volatility and significant rises in recent years this has once again brought acetylene into the limelight.
Acetylene is a versatile tool in today’s speciality and fine chemical industries, due to its high and well controllable reactivity.
Based on the characteristic features of acetylene’s C-C-triplebond, vinylations as well as ethynylations can in many cases be achieved effectively with only minor synthetic effort.
Therefore acetylene is used to contribute to the build-up of complex molecular structures, used for example in the production of perfume components, vitamins, polymer-additives, solvents for construction industries, surface-active compounds, and much more.
Approximately 80% of the annual acetylene production of the world is used for chemical synthesis, as the gas has become increasingly prominent as a raw material for a whole series of organic compounds - among them acetaldehyde, acetic acid, and acetic anhydride.
The remaining 20% of acetylene production is principally used for oxyacetylene cutting, heat treating, and welding. Combustion of acetylene with oxygen produces a flame of over 3300°C (6000°F) and acetylene is also used for carburization of steel when the object is too large to fit into a furnace.
Although acetylene was once a major raw material for the production of synthetic chemicals such as vinyl chloride, vinyl acetate, acrylonitrile and acetaldehyde, since the mid 1960’s it began to be replaced by ethylene produced from low-cost petroleum.
The classical commercial route to acetylene production is by calcium carbide, in which lime is reduced by carbon in an electric furnace to yield calcium carbide. The calcium carbide is than hydrolyzed to produce acetylene.
There are two methods of producing acetylene from calcium carbide, based on the type of generator used, wet and dry. For large scale plants, dry generation design is more common. Both processes yield acetylene of 99.6% volume purity after a light scrubbing.
However, calcium carbide technology suffers from high energy costs, which have led to its decline since 1960s.
Partial oxidation or combustion processes meanwhile, consist of those in which necessary energy needed to supply the reaction heat is provided by burning a portion of hydrocarbon feed or by combustion of residual gas.
Carbon feedstock can come from a variety of materials including natural gas, ethane, natural gas liquids, naptha and liquefied petroleum gases (LPG).
An electric arc discharge is used to supply the necessary energy to convert hydrocarbons to acetylene at a very high flux and short reaction time. Feedstocks to the furnace can be gaseous or liquid hydrocarbons, or solid in the form of coal.
Safety & handling
Acetylene is a simple asphyxiant and anaesthetic, with no harmful effects from chronic exposure to acetylene at high concentrations. Pure acetylene is a colourless, highly flammable gas with an agreeable ethereal odour, an odour which in terms of the commercial purity grade is distinctively garlic-like.
Acetylene can be safely stored and used in cylinders filled with a porous material and containing a solvent (acetone) into which the acetylene has been dissolved.
When not dissolved in a solvent, free acetylene as it is known, can begin to dissociate or decompose at pressures above 15 psig.
The products of dissociation are carbon, in the form of lampblack, and hydrogen, while considerable amounts of heat are generated by dissociation, which may produce explosions of great violence.
Acetylene gas is fundamentally unstable and may explode, it is therefore shipped in special cylinders designed to keep the gas dissolved.
The cylinders are packed with porous materials (such as kapok fibre or diatomaceous earth) and then filled with acetone to half its total filling weight. When acetylene is introduced into the cylinders it dissolves into the acetone.
Outlook & future prospects
The market place for acetylene is highly competitive, with an increasing number of suppliers and reduced usage in welding because of the trend towards using flameless welding processes.
In addition, many offshore and steel manufacturing companies have moved from European (Western) countries to Asian and Eastern European countries, leading to a decline in demand in the former and a corresponding increase in the latter two markets.
During the 1970s and 1980s, mostly due to fairly low oil prices, acetylene lost most of its attractiveness to ethylene, propylene and other olefins. Olefins from naphtha became the cheaper alternative, but with rising oil prices in recent years (though it is around $ 36 per barrel as of 1st January 2009, rather low when compared to mid 2008) acetylene chemistry seems to have staged a comeback.
The high price for crude oil causes higher production costs to the disadvantage of olefins. Acetylene is easily available from natural gas however, which is found in huge amounts in Iran, Russia and the Middle East and Asia and can be easily produced in a one-step-process from natural gas by high-temperature pyrolysis and quenching.
It has to be pointed out, that acetylene is the only hydrocarbon which can be produced directly from natural gas in large amounts.
Olefins are generally produced from oil products, especially from naphtha. Therefore, the price of olefins has risen in the past and is expected to remain on the higher side in the medium term. The production of acetylene is especially competitive in regions where natural gas is easily and cheaply available, like the Middle East and other oil producing countries, where natural gas even has to be flared-off sometimes.
This relation could turn the acetylene into an economic alternative for the production of compounds, which have lost their competitiveness due to production via the olefin and petrol routes. In the future, the acetylene chemistry might even become the only way of production for some products.
The first step though, to use natural gas increasingly for the production of acetylene is the replacement of the carbide route for the production of acetylene. This technology involves a rather high energy consumption.