Caprolactam Extraction in a Pulsed Disc and Doughnut Column with a Benign Mixed Solvent

Abstract

Benign solvents were selected and characterised on labscale as alternative for organic solvents like benzene, toluene, or chlorinated hydrocarbons for the extraction of caprolactam from the Beckmann-rearrangement mixture. The performance of the best candidate solvent was evaluated in a 4 cm diameter pilot scale pulsed disc and doughnut extraction column. Non-polar solvents are unfavourable for caprolactam extraction, because of its low partition ratio. These trends are opposite for polar solvents. As alternative solvents, mixtures containing a polar solvent (40 % heptanol) and an alkane (methylcyclohexane or heptane) were proposed, based on a high capacity and a low mutual solvent solubility. Relative to benzene an equal amount of mixed solvent is needed for the forward and back-extraction for a benchmark industrial process lay-out. The number of theoretical stages NTS for the mixed solvents and benzene is 5 and 9 in the forward and 10 and 5 in the back-extraction, respectively. The solvent mixture heptane/ heptanol was finally selected as best alternative solvent based on its beneficial physical properties (see Fig.).Pilot plant experiments with the mixed solvent were performed to determine the hydrau- Figure. Weight fraction of caprolactam in the raffinate phase as a function of the number of theoretical stages NTS for the mixed solvent (heptane/40 % heptanol) and benzene in forward extraction (a) and back extraction (b) at 40 C.