Highlighting diffusional coupling effects in ternary liquid extraction and comparisons with distillation

Abstract

Liquid extraction processes involve the separation of mixtures containing three or more species whose compositions are close to the binodal curve; this proximity causes the diffusion equilibration process to be strongly influenced by phase equilibrium thermodynamics. Due to thermodynamic factors, the interphase transfer flux of any component is influenced by the driving force of all the constituent species in the mixture, i.e. the diffusion process is strongly coupled. The transient diffusion equilibration process within spherical droplets dispersed within a continuous liquid phase is quantified by the classic Geddes model, used in combination with the Maxwell-Stefan diffusion formulation. For 13 different partially miscible ternary liquid mixtures, the equilibration trajectories in composition space are found to be curvilinear in shape. In all cases, the component Murphree efficiencies, Ei, are unequal to one another. The separations achieved are significantly different from those predicted by a simpler model that ignores coupling effects. In ternary distillation, the existence of azeotropes creates boundaries in composition space, whose crossings are disallowed in equilibrium-stage calculations. The application of the Geddes model for transient diffusion inside vapor bubbles yields curvilinear trajectories that demonstrate the possibility of boundary crossing; such crossings are in conformity with published experimental data.