RESIDENCE TIME MODEL FOR ASSESSING LIQUID-LIQUID-EXTRACTION SYSTEMS

A model based on the residence time of solvent in the extraction syste m may be used to describe the dynamic operation of a continuous liquid -liquid extractor. It is proposed that optimum performance occurs when the extract exiting from the contactor is near equilibrium with the s olution being extracted. This approach was tested on two commercially available continuous extraction systems. The difference in their perfo rmance was found to be related to the level of agitation of the two co ntactor vessels: the stirred system was approximately six times more e fficient than the simple column continuous extractor. The stirred syst em achieved a near equilibrium analyte distribution between the solven t and sample and could be described accurately in terms of residence t ime theory. This was in marked contrast to its unmixed counterpart whe re the analyte distribution between solvent and sample reached 14% of its equilibrium value during its residence in the contactor. The effec t of dead volume of solvent within the extraction assembly on the extr action rate was also apparent; its main effect was to delay the extrac tion process.