Novel continuous multistage extraction column based on phase transition of critical-solution mixtures

Abstract

A novel continuous extraction technique is described, namely the PTE (phase-transition extraction) column. The PTE column is based on the use of partially miscible liquid solvents that have a critical solution temperature. In the column, the conventional mixing and settling sections are replaced by heating and cooling sections. The countercurrent feed and solvent streams passing those sections are heated and cooled across their coexistence curve and thereby undergo phase transitions which alternate between states of two distinct liquid phases and a single homogeneous phase. The operation and mass-transfer performance of the PTE column were studied in single-stage and three-stage laboratory-scale columns. Continuous operation with countercurrent flow of the solvents was shown to be feasible and complete mixing of the solvents in the mixing sections was achieved without the use of any mechanical agitation. The experiments also indicated that each heating-cooling stage acts as one theoretical stage, regardless of the number of stages in the column. This suggests that a large number of heating and cooling stages can be assembled into a tall PTE column without loss of efficiency. Furthermore, the PTE column has a design advantage in that it operates without any moving parts. Tests showed that systems with a high emulsifying tendency are handled in the PTE column without forming emulsions. A basic theoretical model was developed to descibe the flow pattern of the solvents in the PTE column. The model predicts the existence of back-flow streams between the mixing and settling sections of each stage. These streams affect the heat consumption but have a negligible impact on column efficiency. The model was consistent with the experimental measurements. The PTE column could provide significant advantages for difficult separation processes such as: separations that require a large number of theoretical stages, separations of large molecules that can be damaged by high shear stresses and separations of easily emulsifiable systems.