(545e) Intensified Liquid-Liquid Extractions for Nuclear Fuel Reprocessing – Hydrodynamics, Mass Transfer and Scale-up

Intensified
liquid-liquid extractions for nuclear fuel reprocessing – Hydrodynamics, mass
transfer and scale-up

Eduardo
Garciadiego Ortega, Dimitrios Tsaoulidis, Panagiota Angeli*

University
College London, WC1E 7JE, London, UK

Abstract

Reprocessing
spent nuclear fuel is an important step to make the risks associated with
nuclear energy more manageable and increase the sustainability of nuclear
energy. The PUREX process (Plutonium Uranium Redox EXtraction) is a
solvent-extraction-based process used to recover the fissionable and fertile
isotopes and reduce the volume and toxicity of the waste. It also reduces the
need for Uranium mining. The conventional contactors used in the PUREX process
are pulsed columns or batteries of mixer-settlers. Both types of equipment
require large solvent inventories, have wide residence time distributions,
viscosity limitations, and their flow fields are not easily characterised.

By
applying process intensification (PI) principles to the design of liquid-liquid
contactors, nuclear fuel reprocessing can be made inherently safer and more
sustainable. Segmented flow contactors are investigated in this work to achieve
these goals. This intensified contactor consists of small diameter channels
with both liquids flowing in segmented flow pattern (Fig. 1). It consists of a
sequence of plug-slug units, where the plugs are the aqueous phase, surrounded
by the organic phase slugs. High mass transfer rates and narrow residence times
are characteristic of these contactors. This is due to the regularity of the
flow pattern, short diffusion distances, large interfacial area, and intense
mixing within each phase.

Figure 1. Schematic of a segmented flow. The unit, plug, and slug
lengths are defined in the figure.

Although
the advantages of segmented flow contactors have been thoroughly validated, two
main hurdles limit their adoption in the PUREX and other industrial
liquid-liquid extraction processes. First, there are gaps in the quantitative
description of how design and operation variables affect the performance and
costs of these contactors. Second, a single channel cannot process sufficient
volume for a commercial application and the numbering-up of multiphase
processes is challenging. We have previously worked on a design strategy for the
two-phase numbering-up¹,
however, this strategy requires a two-phase pressure gradient correlation.
There are very few pressure gradient correlations for liquid-liquid flows in
small channels and none developed for the system of interest in this work.

This
work includes experimental work on segmented flow contactors for a wide range
of design and operation conditions. High-speed imaging is used to measure the
plug and slug lengths. These results are used to estimate the interfacial area,
an important parameter in mass transfer correlations. The pressure gradient is
measured with a differential pressure transducer. Mass transfer is studied
using a flow separator and UV-Vis spectroscopy. Fig. 2 shows a schematic of the
experimental setup used.

Figure 2. Schematic of the flow experimental setup used to measure
plug-slug units, pressure gradient and mass transfer.

We
will present results on the plug and slug sizes and specific interfacial area
obtained at a wide range of flow conditions. The internal diameter used goes up
to 4 mm, the mixture velocities include 1 to 4 cm s^-1 and
water-to-organic ratios span from 1:1 to 4:1. The pressure gradient is measured
and a new model is proposed from the data with very small dispersion. The mass
transfer studies are used to compare the effects of different design and
operation variables. Additionally, the pressure gradient measurements are used,
in conjunction with our numbering-up model, to design, build and test a
two-phase flow distributor. The flow distribution is measured for a range of
conditions for five and ten channels to demonstrate the versatility and
modularity of the distributor. This work sets a precedent of scaled-up intensified
processes using small channels.

References

1.        Garciadiego
Ortega, E., Tsaoulidis, D. & Angeli, P. Predictive model for the scale-out
of small channel two-phase flow contactors. Chem. Eng. J. (2018).
doi:10.1016/j.cej.2018.06.020