Selection of mixed solvent for crude caprolactam extraction

Abstract

Most of the world’s caprolactam production involves synthesizing caprolactam from cyclohexanone via the Beckmann rearrangement. Due to the presence of both polar and nonpolar impurities in crude caprolactam, purification traditionally includes forward extraction with a nonpolar solvent, such as trichloroethylene, followed by back-extraction with water and then multistage distillation. This study investigates mixed solvents containing 1-octanol or 1-heptanol combined with cyclohexane or n-heptane as safer, non-carcinogenic alternatives to trichloroethylene in caprolactam extraction. Cyclohexane and n-heptane were selected to adjust the solubility of the alcohols in the aqueous phase. Optimal alcohol concentrations in these mixed solvents were estimated through modeling based on Hansen solubility parameters and experimentally validated using a real crude caprolactam sample (70 wt% caprolactam in water). A 40 wt% 1-octanol in heptane mixture demonstrated a caprolactam distribution coefficient closely matching that of trichloroethylene. Liquid-liquid equilibrium data and key physical properties influencing mass transport (density, viscosity, and interfacial tension) for the chosen mixed solvents were experimentally determined and used to design Karr extraction columns for both forward and back-extraction processes. Literature correlations were applied to predict hydraulic characteristics, including Sauter drop diameter and dispersed phase hold-up. The model was further validated through performance trials with an actual extraction unit using trichloroethylene in caprolactam refining. A mixture of 60 wt% 1-octanol in cyclohexane was identified as optimal, offering a balance between column height, maximum caprolactam content in extract and mixed solvent consumption.