Residence time model for assessing liquid-liquid extraction systems

A model based on the residence time of solvent in the extraction system 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 solution being extracted. This approach was tested on two commercially available continuous extraction systems. The difference in their performance was found to be related to the level of agitation of the two contactor vessels: the stirred system was approximately six times more efficient than the simple column continuous extractor. The stirred system achieved a near equilibrium analyte distribution between the solvent and sample and could be described accurately in terms of residence time theory. This was in marked contrast to its unmixed counterpart where the analyte distribution between solvent and sample reached 14% of its equilibrium value during its residence in the contactor. The effect of dead volume of solvent within the extraction assembly on the extraction rate was also apparent; its main effect was to delay the extraction process.