Ternary mass transfer in liquid-liquid extraction

Abstract

Isothermal interphase mass transfer measurements were carried with the partially miscible Type I system glycero (1)-water (2)-acetone (3) in a modified batch Lewis extraction cell. Three types of experiments were performed: (A) in which the acetone-rich phase was saturated and the initial glycerol-rich phase composition lay on the extension of the corresponding tie-line, (B) in which the glycerol-rich phase was saturated and the initial acetone-rich phase composition lay on the extension of the corresponding tie-line and (C) in which initially both the acetone-rich and glycerol-rich phase were unsaturated. The equilibration trajectory was monitored in both phases by sampling at suitable intervals. In experiments A and B rectilinear equilibration trajectories were obtained while in the experiment C the approach towards equilibrium was highly curvilinear in the composition space. A theoretical model, based on irreversible thermodynamics and allowing for diffusional coupling between species transfers, is developed to simulate the three types of experiments above. The matrix of equilibration rate constants, in either fluid phase, is re-constructed by determining the eigenvectors and eigenvalues from experimental observations. It is found that both matrices of rate constants in the acetone-rich and glycerol-rich phases have extremely large off-diagonal elements. It is stressed in the discussions that the experimental results cannot be explained, even qualitatively, without inclusion of these off-diagonal contributions. Some interesting mass transfer phenomena in the region of the plait point are pointed out and the need for a fundamental irreversible thermodynamics approach to interphase mass transfer is emphasised. It is concluded that rigorous mass transfer formulations, allowing for diffusional coupling, will be required to be incorporated into design of extractors involving highly non-ideal liquid mixtures.