Conventional caprolactam technology is based on the key intermediate cyclohexanone, which is usually produced by the oxidation of cyclohexane, but can also be made from phenol or toluene. Separately, hydroxylamine sulphate is manufactured by the oxidation of ammonia to nitrous oxide followed by hydrogenation in the presence of sulphuric acid. The hydroxylamine sulphate is then reacted with the cyclohexanone to produce cyclohexanone oxime. This is followed by a Beckmann rearrangement using oleum to yield caprolactam.
A disadvantage of existing technology is that large amounts of ammonium sulphate - up to 4.5 tonnes/tonne of caprolactam - are produced. Much development work is concentrating on reducing or even eliminating this byproduct. For example, DSM's HPO-plus process has substantially reduced this byproduct to 1.5 tonnes/tonne of caprolactam.
A more recent approach, originally developed by EniChem (now Syndial) and commercialised by Sumitomo in 2003 at its Ehime plant in Japan, completely eliminates the production of ammonium sulphate. The chemical reaction in this case is a so-called ammoximation reaction, whereby cyclohexane is reacted with ammonia and hydrogen peroxide at around 90oC in the presence of a titanosilicate catalyst. Significantly, since no hydroxylamine plant is needed, this leads to reduced capital investment. However, hydrogen peroxide is expensive and must be manufactured on a large scale to provide sensible scale economies and transfer pricing.
Toray in Japan has bypassed the need for the cyclohexanone or oximation steps by commercialising a photochemical process to convert cyclohexane into cyclohexanone oxime in the presence of nitrosyl chloride and hydrogen chloride. This process provides substantial capital cost savings, with the elimination of both cyclohexanone, hydroxylamine and oximation plants. However, the process requires access to low-cost power to be truly cost effective. Large scale photochemical reactors are difficult to design and require constant cleaning to remove tar-like reaction residues.
The mid-1990s saw considerable research and development activity to devise a process to manufacture caprolactam from butadiene or adiponitrile. DSM, working initially with DuPont and then later with Shell, developed a process using butadiene and carbon monoxide feedstocks to make caprolactam without ammonium sulphate production. Called Altam, the process employs four steps - carbonylation, hydroformylation, reductive amination and cyclisation. DSM claims cost reductions of 25-30%, simplified plant operations and lower energy consumption.
In the late 1990s, BASF and DuPont investigated the feasibility of investing in a butadiene to adiponitrile/HMDA/caprolactam process in China. Rhodia, meanwhile, developed its own alternative approach to caprolactam manufacture called Capucine .
Both the BASF and Rhodia processes involve the hydrogenation of adiponitrile to make 6-aminocapronitrile with an HMDA coproduct, using different operating conditions and catalysts. Adiponitrile can be manufactured from butadiene and hydrogen cyanide, and by electrolysis from acrylonitrile.
(Source: ECN 20 December 2004)
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Caprolactam
Uses and Outlook
The growth in caprolactam is very dependent on the nylon 6 business which has suffered from poor profitability. Around 70% of the caprolactam is used in making nylon 6 fibres, which are made into textile, carpet and industrial yarns. This market is mature and been in decline in the western world at the expense of growth in Asia. The nylon 6 fibres sector also sees competition from other materials such as nylon 6,6 and polyester. As a result, this sector is only expected to grow at 1%/year.
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