Microflow visualization and electrical signature of tri-n-butyl phosphate/n-dodecane and nitric acid in a centrifugal contactor

Despite the common use of centrifugal contactors in solvent extraction applications, no direct, high-speed visualization of the underlying microflow structure has been published in the open literature. The system in this study is the most prevalent solvent extraction combination used in the nuclear reprocessing industry to extract metal ions from spent nuclear fuel from a nitric acid aqueous solution into an organic mixture of tributylphosphate and dodecane. This study has identified details of the dispersed phase state at all flow conditions of interest, for instance, under common operation conditions the aqueous phase is typically dispersed even when the aqueous flowrate is significantly greater than the organic flowrate. This discovery may have implications when the centrifugal contactor is operated in a stripping mode. Visual observations of the images and simultaneous electrical resistivity measurement of the fluid mixture allowed for the identification of the phase inversion point and hysteresis region captured in a flow regime hysteresis plot. It is shown that it is possible to operate the contactor within the hysteresis region in a stable manner, therefore a choice can be made for the desired dispersed phase. This discovery may have favorable implications for the improved operation of the contactor in either the extracting or stripping modes. This work provides new direct experimental evidence of fluid flow for validating theory, modeling and simulation efforts; there is currently a lack of microflow insight. For instance, existing volume-averaged theories in fluid mechanics do not capture the phase inversion phenomena and/or its hysteresis, which in turn, invalidates predictions of mass transfer.