Extraction Kinetics and Hydrodynamics in a Miniature Annular Centrifugal Contactor

Annular centrifugal contactors have become attractive equipment for the separation step in the recycling process of spent nuclear fuel due to their high efficiency, high throughput, and rapid operational properties relative to other recycling equipment such as mixer-settlers and pulsed columns. Various sizes of contactors ranging from laboratory scale to commercial scale have been developed for different needs and process scales. As in all solvent extraction equipment, the efficiency of a single stage will depend on residence time and interfacial area. A characteristic to centrifugal contactors is the free interface of their mixing zone causing the holdup volume to change depending on the operational parameters. To successfully simulate and model the separation process in a centrifugal contactor, certain aspects of the hydrodynamics must be known. In this work, the holdup volume of the dispersed phase in the mixing zone of the lab-scale Robatel BXP012 centrifugal contactor was investigated by two approaches. First, residence time distribution (RTD) measurements of both the aqueous and the organic phases by the method of a pulse input was performed. The aqueous phase was found to have a longer residence time suggesting a larger holdup. The interfacial mass transfer kinetics for the extraction of dysprosium ions from a nitric acid medium with both cyanex 572 and HEH[EHP] was studied with the use of a Nitsch cell. A forward extraction rate law was suggested having a first order and half order dependence on the metal ion in the aqueous phase and the extracting ligand in the organic phase, respectively. Mass transfer coefficients were obtained from both the Nitsch cell and a similar centrifugal contactor and the method of a chemical reaction was applied to the Robatel BXP012 to estimate its organic holdup volume. The holdup volume in the contactor mixing zone was found to increase with total flow rate ranging from 0.1 ml to about 0.5 ml when a constant mass transfer coefficient is use