Dewatering caprolactam solutions by direct osmosis using osmotic pressure of concentrated aqueous ammonium sulphate solutions

The concept of dewatering CPL containing solutions by using the osmotic pressure of solutions of concentrated aqueous AS has been investigated in order to reduce evaporation costs. Thermodynamic calculations were made to determine the iso-osmotic pressure lines of these aqueous flows and to predict the operating window of this application of direct osmosis. Based on thermodynamics it is shown that an aqueous CPL solution can be concentrated by osmosis with the help of a saturated AS solution just up to 57 wt% CPL. Commercial available composite flat-sheet and tubular type reverse osmosis membranes were tested on pilot plant scale for their possible use in an osmosis unit. The experiments were performed to validate the calculated iso-osmotic conditions and to obtain global information on the water fluxes through the membranes. The experimental results of equilibrium experiments are in good agreement with the outcome of the thermodynamic predictions. However, the experimental determined fluxes for the water transport from the CPL containing solution to the AS containing solution are rather low (0.67 and 0.57 kg m–2 hr–1). The consequence of these low water fluxes is that very large membrane areas will be required, resulting in high capital costs for dewatering CPL solutions. For commercial application of direct osmosis as a method for dewatering CPL solutions a further increase of the flux of membranes and/or a reduction of the investments seems necessary.