Measurements and modelling of liquid-liquid equilibria
of ɛ-caprolactam + water + solvent + ammonium sulfate systems
G. van Bochove
M. Wijtkamp – Mentor
W. de Loos – Mentor
J. de Swaan Arons – Mentor
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Abstract
Ɛ-Caprolactam. the monomer of Nylon-6, is obtained from the production process as an aqueous solution of ammonium sulfate and ɛ -caprolactam. The crude Ɛ-Caprolactam is purified by extraction with an organic solvent. To be able to model this extraction process, thermodynamic modelling of liquid-liquid equilibria of Ɛ-Caprolactam + water + solvent + ammonium sulfate is required. Since the number of models in literature is scarce, a dedicated model for the description of liquid-liquid equilibria of mixed solvent electrolyte systems has to be developed. In this work liquid-liquid equilibria were experimentally determined for the systems Ɛ-Caprolactam + water + 1-heptanol + ammonium sulfate at 20°, 40° and 60°C. In addition to this experimental work, a program was developed to correlate experimental liquid-liquid equilibrium data of mixed solvent electrolyte systems. The program is based on an activity coefficient model, that consists of a nonelectrostatic contribution and an electrostatic contribution. The Non Random Two Liquid (NRTL) theory is used to describe the nonelectrostatic contribution. The statistical mechanical Mean Spherical Approximation (MSA) theory is used to account for the electrostatic interactions. The MSA is a physically more correct model than the Debye-Hückel model, since it takes into account ion size effects. The new NRTL-MSA activity coefficient model is compared with the existing electrolyte NRTL models of Chen and Liu. The experimental work showed that 1-heptanol may be a suitable solvent for the extraction of Ɛ-Caprolactam. The presence of ammonium sulfate in the system favours the dissolution of Ɛ-Caprolactam in the organic phase and salts out the aqueous phase. At increasing temperature, the solubility of Ɛ-Caprolactam in 1-heptanol increases to a greater extent than the solubility in water. The NRTL-MSA model, built up from an electrolyte NRTL contribution and an electrostatic MSA contribution without explicit hard sphere contribution produces promising results for the representation of liquid-liquid equilibria of mixed solvent electrolyte systems. For the system water + 2-propanoI + sodium chloride, an average deviation of 0.006 w/w could be reached. It might be concluded carefully that the NRTL-MSA performs better than the electrolyte NRTL. However, the model still needs more testing. It is also recommended to extend and apply the nonprimitive MSA to mixed solvent systems and compare it with the results of the NRTL-MSA model, presented here.