Impurity distribution behavior in caprolactam extraction with environmentally benign mixed solvents

In a previous study a solvent mixture of heptane containing 40 mass % heptanol was selected as an alternative in the industrial extraction of caprolactam to replace benzene, toluene, or chlorinated hydrocarbons. This work reports the equilibrium distribution ratio of caprolactam and four model impurities of organic nature, namely, cyclohexanone, aniline, n-methylcaprolactam, and cyclohexane-carboxamide, comparing the mixed solvents with toluene as a reference. The resulting phase equilibria were interpreted using the equilibrium stage model. Based on these calculations it was found that, compared to toluene, the co-extracted fraction of cyclohexanone and aniline was higher, that of n-methylcaprolactam was comparable, and that of cyclohexane-carboxamide was lower using the mixed solvent. Overall, the mixed solvent reduced the fraction of co-extracted impurities by almost 10 %.