Li, Qi; (2018) Experimental and numerical investigations of ionic liquid-aqueous microchannel extractions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The thesis presents investigations on the process intensification behaviour in small scale separators operating in plug (segmented) flow. The continuous extraction behaviour, as well as the hydrodynamic characteristics of the liquid-liquid flow in microchannels is numerically and experimentally studied. The interphase mass transfer process using the extractant species Europium (III) as tracer was observed and quantified. The Eu(III) microfluidic extraction from nitric acid solutions was carried out in 0.2 mm and 0.5 mm channels using an ionic liquid solution (0.2M n-octyl(phenyl)-N,Ndiisobutylcarbamoylmethyphosphine oxide (CMPO) -1.2M Tributylphosphate) (TBP)/1-butyl- 3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide; ([C4min][NTf2]) as the extraction phase. Distribution and mass transfer coefficients were found to have maximum values at nitric acid concentration of 1M. Mass transfer coefficients were higher in the small channel, where recirculation within the plugs and interfacial area are large, compared to the large channel for the same mixture velocities and phase flow rates. Within the same channel, mass transfer coefficients decreased with increasing residence time indicating that significant mass transfer takes place at the channel inlet where the two phases come into contact. The experimental results were compared with previous correlations on mass transfer coefficients in plug flows. To better characterize the microfluidic flow, bright field Micro-Particle Image Velocimetry and high speed imaging were employed to measure velocity profiles and to obtain the geometric characterstics of the plug flow for the 1M HNO3 solution that was used in the extraction experiments. Correlations regarding film thickness, plug velocity and plug length are developed based dimensionless parameters. It was found that the liquid film surrounding the plug is largely affected by the changes in the front cap for the range of Capillary numbers studied (0.0224< Ca <0.299), while the droplet volume is highly dependent on the channel diameter as well as the mixture velocity. The volume-of-fluid (VOF) method is then used to model the velocities and pressure distribution in the plug flow in the channel and shows good agreement with experimental results and previous models. These features will help in optimizing the microfluidic plug flow for mixing, as well as mass and heat mass transfer enhancement. The mass transfer profiles of the extractant species Eu(III) are also studied using Laser Induced Fluorescence. Recirculation patterns appear in the dispersed aqueous phase from the plug formation stage, and 30-50 % of the mass transfer occurs during plug formation, where new interfaces are formed and mixing is enhanced from the recirculation pattern, especially at high Umix. After experiencing convection and normal diffusion extraction equilibrium achieved, the fluorescent signal in the ionic liquid phase is very strong as Eu(III) transfers into it. The correlations proposed on the hydrodynamics and observations of the mass transfer characteristics during plug flow will contribute to the development of microfluidic devices.

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