A dispersion-entrainment model for liquid-liquid extraction column performance prediction

Abstract

Tracer measurements of axial dispersion parameters, in general, cannot be directly and simply translated to conditions of mass transfer in liquid-liquid extraction columns. A very important additional contribution to axial mixing in the continuous phase must be considered when mass transfer between phases occurs—entrainment in the drop boundary layer and wake of continuous phase which does NOT have the same solute concentration as the bulk of the continuous phase. Applying the same model, i.e. plug flow with axial dispersion, to the continuous phase in mass transfer and non-mass transfer conditions results in greater values (by as much as 200%) for the axial mixing parameters in the mass transfer situation. A combined dispersion-entrainment model of the continuous phase flow is presented to explain the differences above and to provide a means for utilizing tracer measured parameters under mass transfer conditions. The 'forward mixing' model of the dispersed phase is retained. The results confirm that low entrained volumes of solute-enriched (or depleted) continuous phase by the drops can account for the large differences between concentration profile, and tracer, measured axial mixing parameter values.