A Simplified Overall Kinetic Model for Cyclohexanone Oximation by Hydroxylamine Salt

Abstract

Cyclohexanone oxime (ONEOX) is the precursor of ε-caprolactam, a monomer used within the nylon-6 industry. This work studies the production of ONEOX from cyclohexanone (ONE) oximation with hydroxylammonium sulfate (HAS). The reaction involves two liquid phases: HAS is present in the aqueous phase, and ONEOX and ONE are present in the organic phase. The influence of pH, the interfacial area between phases, and the concentration of reagents on the oximation rate have been studied, taking into account the effect of these variables on the proposed model for the overall rate. Oximation runs have been carried out in a batch reactor at low agitation speed to control the interfacial area between phases. The pH ranged from 3 to 5.5, and the temperature was set at 353 or 358 K. The contact model proposed for the two liquid phases was based on the two-film theory, and it was assumed that the oximation reaction takes place in the aqueous film. Because of the low solubility of organic species in the aqueous phase (salting-out effect), the concentration of salts was considered negligible in the organic phase and the concentration of hydroxylamine was assumed to be constant in the aqueous film. The rate expression obtained was first order for ONE and 0.5 for HAS, using a pseudokinetic constant that included the transport coefficient of ONE in the aqueous film, and the pH effect. It was found that this apparent kinetic constant shows a slight increase when the pH is increased within the studied range. The kinetic model proposed predicts well the overall oximation rate under the experimental conditions studied. This model includes quantitatively the influence of the main variables: temperature (353−358 K), pH (3−5), and concentration of reagents