Salting-Out Assisted Liquid-Liquid Extraction (SALLE) for the separation of morpholine from aqueous stream: Phase equilibrium, optimization and modeling

Abstract

In the present study, enhanced liquid-liquid extraction i.e., salting-out assisted liquid-liquid extraction (SALLE) was explored for the separation of morpholine from the aqueous stream. The salting out capacities of two kosmotropes K2CO3 and KCl with solvents (1-decanol and toluene) was investigated within its solubility limit. A Taguchi optimization technique was investigated to obtain the optimum combination of parameters employing a mixed array design (23×22). The results demonstrated that the salting-out effect of K2CO3 in combination with 1-decanol was more pronounced than KCl and thereby the tie-line data was generated for this quaternary system comprising water + morpholine + K2CO3 + 1-decanol at various temperatures (293.15–313.15 K). As the CO32− anion shows the more negative value of Gibb's free energy of hydration, ΔGhyd than Cl− revealed the better salting-out ability of CO32− ion. In other terms, the B-coefficients of the constituent anions (CO32− and Cl−) have been discussed further, which is related to the salt kosmotropicity, revealed the interactions between water molecules and electrolyte ions and their effect on the water lattice around the solute particles. Also, the critical concentration of K2CO3 required to form two phases is reduced with a rise in temperature. Further, the process parameters influencing the extraction efficiency, such as tie-line length, (TLL) of the quaternary system was also evaluated and data fitted to the Eisen-Joffe model to perform the consistency test in respect of the overall concentration of salt to water in each phase. The Eisen-Joffe salt parameter, Ks which signifies the reliability of the tie-line data, was obtained in the range of 1.71–5.93 and the tie-line data was also accurately predicted by the NRTL activity coefficient model. The applicability of the SALLE was finally investigated in the continuous counter-current extraction column indicating the SALLE to show good performance on the separation efficiency of morpholine.