The Industrial Oxidation of KA Oil to Adipic Acid

This chapter describes the main features of the industrial process for the synthesis of adipic acid by means of oxidation of the KA Oil mixture, with special focus on the reaction mechanism, reactor technology, safety aspects and materials. Aspects here examined include the various technical solutions which have been implemented during the last 60 years, with the aim of both improving the process performance and decreasing its environmental impact. 

Adipic acid is best known in the history of chemistry for its role in the invention of nylon 6,6 by W. H. Carothers. However, today, adipic acid is also an important link in the chemistry of intermediates because of its characteristics, price and market availability. 

Although most of this product (about 60%) is used in the manufacture of polyamide 6,6, which is formed by a reaction with hexamethylenediamine, the growth in the use of adipic acid during the last 20 years is also attributable to the expansion of polyurethane and polyester resins. After nylon, these product sectors, together with PVC plasticizers and synthetic lubricants, consume most of the extra-nylon adipic acid produced today. 

The expansion of these sectors and the development of new markets have strongly contributed to the rise in the worldwide capacity to produce adipic acid, which in 2010 reached about 3.2 million tons. However, as we can see in Fig. 13.1, throughout the years, the geographical distribution of world capacity has undergone some changes. In fact, even though Europe and the United States still maintain their dominant position, in the last three years, the production capacity in China has recorded a huge increase. The newest adipic acid production plants in the world have been built in China by local manufacturers. 

The production cycle of adipic acid from hydrocarbons — benzene or cyclohexane — is a two-stage process (i) the production of the raw material for the synthesis of adipic acid, consisting of cyclohexanol alone or, more often, a mixture of cyclohexanol and cyclohexanone (KA oil), and (ii) the production of adipic acid by nitric acid oxidation in the presence of copper and vanadium catalysts (see Figs. 13.2 and 13.3). 

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Hessel et al. described a full process design vision for the manufacturing of adipic acid on an industrial scale [66,67]. Currently, commercial production of adipic acid comprises two reaction steps. The first step involves the selective oxidation of cyclohexane to KA oil, a mixture of cyclohexanone and cyclohexanol. The second step consists of oxidation of the KA oil to adipic acid with an excess of nitric acid in the presence of copper or vanadium catalysts. In contrast, Hessel et al. suggest that a new and direct route toward adipic add based on the direct oxidation of cyclohexene with hydrogen peroxide would be much more cost effective. This process is based on the protocol described by Noyori and coworkers and utilizes a... 

Adipic acid (qv) has a wide variety of commercial uses besides the manufacture of nylon-6,6, and thus is a common industrial chemical. Many routes to its manufacture have been developed over the years but most processes in commercial use proceed through a two-step oxidation of cyclohexane [110-83-8] or one of its derivatives. In the first step, cyclohexane is oxidized with air at elevated temperatures usually in the presence of a suitable catalyst to produce a mixture of cyclohexanone [108-94-1] and cyclohexanol [108-93-0] commonly abbreviated KA (ketone—alcohol) or KA oil ... 

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