Because of the strict legislation for currently applied solvents in the industrial extraction of caprolactam, being benzene, toluene and chlorinated hydrocarbons, a need exists for alternative, environmentally benign solvents. An experimental screening procedure consisting of several steps was used to determine a suitable replacement solvent. (1) The distribution ratio of caprolactam was investigated at 293 K for an initial 10 mass% aqueous caprolactam solution; (2) the capacity and mutual solvent solubility were determined as function of the aqueous caprolactam concentration at equilibrium for solvents or solvent mixtures showing favourable capacities compared to benzene and toluene; (3) the equilibrium phase compositions for conditions covering the full industrial range were determined and correlated using the NRTL model for solvents or mixtures showing favourable characteristics compared to benzene and toluene; (4) the physical properties, being density and viscosity data of the separate phases and interfacial tension data of the liquid–liquid systems were determined and correlated for the remaining candidate solvents. It was found that the caprolactam distribution ratio increased with increasing solvent (mixture) polarity, characterized by a more polar active group (ether, ester, ketone and alcohol), decreasing carbon chain length of the polar solvent (C12–C6) or increasing polar solvent fraction in the mixture. Based on a high capacity, a low mutual solvent solubility and beneficial physical properties the solvent mixture of 40 mass% heptanol in heptane was finally selected as replacement solvent. Calculating the solvent to feed ratio (S/F) and the number of theoretical stages (NTS) needed to reach the industrially required raffinate concentrations, it was found that in the forward extraction the S/F is 3.0 for the mixed solvent and benzene, whereas the NTS is 5 and 9, respectively. In the back-extraction the S/F is 0.67 whereas the NTS is 10 and 5 for the solvent mixture and benzene, respectively.
- Mixed solvent
- Solvent selection