Kinetics of alkali-catalyzed condensation of impurities in the cyclohexanone purification process

Abstract

In the cyclohexanone purification process, some impurities, such as pentanal, hexanal, and 2-cyclohexen-1-one, must be removed in order to ensure good quality of nylon fibers in the caprolactam polymerization step. To do this, an industrial common practice is to add a homogeneous basic catalyst (such as sodium hydroxide, NaOH) to promote the condensation of these impurities with cyclohexanone because the condensation products are easily separated by distillation. In this study, a kinetic model for the catalytic condensation of each impurity was developed, including variables such as temperature, impurity concentration, and catalyst concentration. In order to fulfill this purpose, runs were carried out in a batch reactor containing 70 g of cyclohexanone and different contents of impurities. NaOH was used as the catalyst (CNaOH values ranging from 2.5 to 30.0 mmol/kg). Runs were carried out by a nonisothermal procedure; the reaction temperature was changed from 298 to 423 K, and several temperature ramps were applied. All of the experiments were conducted at a pressure of 10 bar to ensure that all of the volatile compounds remained in the liquid phase. The products of the condensation reaction of each impurity with cyclohexanone were identified and quantified by gas chromatography/mass spectrometry. The reaction products found were as follows: 2-(1-pentenyl)cyclohexanone (A1) and 2-pentylidenecyclohexanone (A2), in which both isomers were lumped together and quantified as A; 2-(1-hexen-1-yl)cyclohexanone (B1) and 2-hexylidenecyclohexanone (B2), in which these isomers were lumped together and quantified as B; [1,1′-bicyclohexyl]-2,2′-dione (C1) and [1,1′-bicyclohexyl]-2,3′-dione (C2), in which both were lumped together as C. The kinetic parameters were estimated by data fitting. The estimated activation energies of impurity elimination were 3.47 kJ/mol for pentanal, 3.99 kJ/mol for hexanal, and 24.23 kJ/mol for 2-cyclohexen-1-one. This kinetic model reproduced the experimental results quite well. Moreover, experimental data from isothermal experiments were also reasonably well predicted with the model