Studies of Intensified Small-scale Processes for Liquid-Liquid Separations in Spent Nuclear Fuel Reprocessing

Abstract

The main contribution of the thesis is to study and develop small-scale processes for ionic liquid-based extractions that can intensify the liquid-liquid separations in the spent nuclear fuel reprocessing cycle. The industrial application of small scale processes requires that their hydrodynamics and mass transfer behaviour are well characterised and predicted. In addition, modelling methodologies are proposed to evaluate the applicability of the small scale extractors in reprocessing the large volumes of nuclear waste used in industrial scale. The first part of the work involves the study of the hydrodynamic behaviour of two-phase (ionic liquid-aqueous) flows. Flow pattern formations within channels have been identified for a wide range of operating conditions and were found to be strongly affected by channel size and material, fluid properties, and flow rates. The main patterns observed were plug flow, annular flow, and drop flow. Subsequently, the work focused on the investigation of the plug flow which has been found to enhance mass transfer because of circulation patterns that develop within the phases. Plug flow was thoroughly investigated in various channel sizes of different material mainly for TBP/ionic liquid (30% v/v) mixtures-nitric acid solutions, relevant to spent nuclear fuel reprocessing. Several hydrodynamic characteristics, such as plug length, plug velocity, film thickness, and pressure drop have been investigated for different ionic liquids, channel sizes, and phase flow rates. Results have been compared with literature, and new (or modified) correlations have been proposed for estimating the plug length, film thickness, and pressure drop. Furthermore, circulation patterns and mixing characteristics within aqueous plugs were investigated by means of μ-PIV (micro Particle Image Velocimetry). The mixing within a plug was locally quantified by the non-dimensional circulation time and the results were correlated with the mass transfer performance. Mixing within the plug was found to be affected by several parameters, but the most decisive one was the size of the channel; mixing was enhanced by decreasing the channel diameter. The last stage of the experimental part of this research involves studies of the extraction of dioxouranium(VI) ions from nitric acid solutions into TBP/IL mixtures (30%, v/v), relevant to spent nuclear fuel reprocessing in channels with sizes ranging from 0.5 to 2 mm ID. The effects of ionic liquid type, initial nitric acid concentration, and residence time on the extraction performance of the contactor were studied. Experimental mass transfer coefficients Abstract 2 were compared against predictive models derived from the literature and good agreement was found with those for liquid-liquid contactors. Experimental results were also compared with extraction units already in operation in spent nuclear reprocessing plants. It was found that comparable amount of spent nuclear fuel (1045 tonnes per year) can be reprocessed and extraction of dioxouranium(VI) >99% can be achieved ...

Studies of plug formation in microchannel liquid–liquid flows using advanced particle image velocimetry techniques

“…This method, based on the BW contrast measurements between the particles shadow and the optical background, is less complex than conventional µPIV as it does not require laser lights coupled with suitable fluorescent particles, while it can be combined with high speed cameras for high frequency image acquisition. Tsaoulidis et al (2014) used an image acquisition system based on bright field PIV principles to visualize the recirculation patterns in fully formed liquid plugs in small channels. The technique has not been applied before to visualize the high speed plug formation in the inlet zone of a microchannel.…”

Computational fluid dynamics (CFD) assisted design and prototyping of the immiscible drop separation section for an intensified perstraction system

“…mini and microchannels) [16,17]. hazardous substances, less space requirement and the increase of the specific area and the mass transfer [18,19]. Driven by the significant increase of the mass and energy transfer velocity, the chemical and biochemical industry has shown an increasing interest in the application of microsystems for the miniaturization of conventional operation units.…”

“…[21]. Despite its importance, the behavior of immiscible fluids through microchannels and its corresponding separation technologies have not been widely explored [18,22,23].…”

Continuous Reactive Precipitation in a Coiled Flow Inverter: Inert Particle Tracking, Modular Design, and Production of Uniform CaCO3 Particles

“…Curved microchannels and capillaries provide enhanced heat and mass transport due to their controllable flow patterns and high surface to volume ratio. Thus, researchers have long investigated these devices for process intensification (PI). − Planar designs with a rectangular channel cross section are commonly utilized for two-phase flow applications with different flow patterns. Slug flow (segmented or Taylor flow) is one of the most investigated flow pattern in microreactors as it provides a wide operating window with a well-defined and stable flow profile. − Several experimental and numerical studies are available for the characterization of the slug flow profile in straight horizontal, inclined, and vertical tubes either with circular or rectangular cross section. − Furthermore, the heat and mass transfer characterization along with mixing behavior of a gas–liquid (G–L) slug flow were also investigated in microchannels. − These studies revealed that the enhanced heat and mass transfer can be achieved by utilizing G–L slug flow in microfluidics with a slight increase in pressure drop.…”