NOVEL CONTINUOUS MULTISTAGE EXTRACTION COLUMN BASED ON PHASE-TRANSITION OF CRITICAL-SOLUTION MIXTURES

A novel continuous extraction technique is described, namely the PTE ( phase-transition extraction) column. The PTE column is based on the us e of partially miscible liquid solvents that have a critical solution temperature. In the column, the conventional mixing and settling secti ons are replaced by heating and cooling sections. The countercurrent f eed and solvent streams passing those sections are heated and cooled a cross their coexistence curve and thereby undergo phase transitions wh ich alternate between states of two distinct liquid phases and a singl e homogeneous phase. The operation and mass-transfer performance of th e PTE column were studied in single-stage and three-stage laboratory-s cale columns. Continuous operation with countercurrent flow of the sol vents was shown to be feasible and complete mixing of the solvents in the mixing sections was achieved without the use of any mechanical agi tation. 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 s tages can be assembled into a tall PTE column without loss of efficien cy. Furthermore, the PTE column has a design advantage in that it oper ates without any moving parts. Tests showed that systems with a high e mulsifying tendency are handled in the PTE column without forming emul sions. A basic theoretical model was developed to describe the flow pa ttern of the solvents in the PTE column. The model predicts the existe nce of back-flow streams between the mixing and settling sections of e ach stage. These streams affect the beat consumption but have a neglig ible impact on column efficiency. The model was consistent with the ex perimental measurements. The PTE column could provide significant adva ntages for difficult separation processes such as: separations that re quire a large number of theoretical stages, separations of large molec ules that can be damaged by high shear stresses and separations of eas ily emulsifiable systems. Copyright (C) 1997 Elsevier Science Ltd.