Direct osmosis membrane process to de-water aqueous caprolactam with concentrated aqueous ammonium sulphate

A new concept of de-watering aqueous caprolactam (aq. CPL) solutions via water permeable membranes by employing the osmotic pressure of solutions of concentrated aqueous ammonium sulphate (aq. AS) has been investigated in order to reduce the energy costs associated with evaporation of water. Introduction of this technology will increase the sustainability of the overall caprolactam production process. Initially, the applicability of this technology was demonstrated by calculating the thermodynamic equilibria of various aq. CPL and aq. AS solutions (iso-osmotic conditions). Based on these results, an operating window could be defined to de-water aq. CPL with concentrated aq. AS solutions. The technical feasibility of this new application was proven by experiments with two commercially available flat-sheet membranes (Filmtec SW-30 and Toray UTC-80) and one tubular type (PCI AFC-99) composite membrane. The results of these experiments were in good agreement with the thermodynamic calculations. The determined average water fluxes of the tested flat-sheet composite membranes are in the range 0.57–0.67 kg m−2 h−1 (0.16–0.19 g m−2 s−1). Although these values are inline with data found in literature of other direct osmosis applications, for commercial application of direct osmosis as a technology for de-watering CPL solutions, a further increase of the flux of membranes and/or reduction of the investments seems necessary.