Abstract
In this paper the effect of channel size on the mass transfer characteristics of liquid–liquid plug flow was investigated for capillaries with internal diameter ranging from 0.5 to 2mm. The extraction of {UO2}2+ ions from nitric acid solutions into TBP/IL mixtures, relevant to spent nuclear fuel reprocessing, was studied for different residence times, dispersed phase fractions, and mixture velocities. It was found that extraction efficiencies increased as the channel size decreased. For a given channel length and for all channel sizes, an increase in mixture velocity decreased the extraction efficiency. The overall mass transfer coefficients (kLα) for all channels varied between 0.049 and 0.29s−1 and decreased as the channel size increased. The evolution of the kLα along the extraction channel showed a decreasing trend for all the channel sizes. The experimentally obtained mass transfer coefficients were compared with existing models for liquid–liquid and gas–liquid segmented flows from the literature. The results showed good agreement with the empirical correlation proposed for a liquid–liquid system. A finite element model was developed that solved the velocity and concentration fields in the channel for both phases considering a unit cell (one plug and one slug) with periodic boundary conditions at the inlet and the outlet. The model used experimental data for the geometric characteristics of the plug flow and predicted reasonably well the experimentally measured extraction efficiencies (with mean relative error of 11%).
Highlights
Two-phase, liquid–liquid systems find many applications in the process and chemical industries including solvent extraction, catalysis, polymerization, and nitration [1]
Equilibrium extractions were performed at room temperature with equal volumes of TBP/[C4mim][NTf2] (30%, v/v) and HNO3/dioxouranium(VI) to estimate the distribution coefficient (KU) (Eq (14))
In the model by Vandu et al [19] (Eq (7)), which was evaluated against data from different channel sizes (ID = 1–3 mm) and unit cell lengths (LUC = 5–60 mm), the only contribution on the mass transfer coefficient is by the film
Summary
Two-phase, liquid–liquid systems find many applications in the process and chemical industries including solvent extraction, catalysis, polymerization, and nitration [1]. Solvent extraction in particular is one of the main processes in reprocessing of the spent fuel from a nuclear reactor. Uranium(VI) (and plutonium(IV)) are recovered from nitric acid solutions of the spent nuclear fuel through the PUREX process, with mixtures of organic solvents with tributylphosphate (TBP) as extractant [2]. Ionic liquids are salts composed from ions that have low melting points (below 100 °C), while many of them are liquid even at room temperature [4]. They have very good physical characteristics such as, negligible vapour pressure and high thermal stability in normal operating conditions, while their high resistance to radiation makes them suitable for spent nuclear fuel reprocessing.
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