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Dioxouranium(VI) extraction in microchannels using ionic liquids
Chemical Engineering Journal 227 (2013) 151–157
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Dioxouranium(VI) extraction in microchannels using ionic liquids Dimitrios Tsaoulidis a, Valentina Dore a, Panagiota Angeli a,⇑, Natalia V. Plechkova b, Kenneth R. Seddon b a b
Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK QUILL Research Centre, School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, UK
h i g h l i g h t s " Continuous extraction of uranium(VI) from nitric acid solutions in microchannels by using ionic liquids. " Mass transfer coefficients in the microchannel system during segmented flow. " The effects of nitric acid concentration on the extraction efficiency.
a r t i c l e
i n f o
Article history: Available online 5 September 2012 Keywords: Liquid–liquid extraction Ionic liquids Microchannel Dioxouranium(VI) nitrate UV–Vis spectroscopy
a b s t r a c t The extraction of uranium(VI) from aqueous nitric acid solutions by tributylphosphate {TBP; 30%(v/v)} dissolved in the ionic liquid 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide was investigated. The experiments were performed in a Teflon microchannel of 0.5 mm internal diameter, while the dioxouranium(VI) concentrations in the aqueous and the ionic liquid phases were determined by UV–Vis spectroscopy. The effects of initial nitric acid concentration (0.01–3 M), residence time, and phase flow rate ratio were studied. It was found that, with increasing nitric acid concentration, the percentage of dioxouranium(VI) extracted decreased and then increased again, while the extraction efficiency followed a slightly different trend. Overall mass transfer coefficients varied between 0.049 s1 and 0.312 s1. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nuclear energy from fission can provide substantial amounts of carbon free electricity and process heat [1,2]. However, one of the main concerns in the use of the nuclear power generation is the management of the irradiated nuclear fuel from the reactor (spent fuel) which can remain toxic for thousands of years. Efficient spent fuel reprocessing through a series of operations can separate uranium to be reused as reactor fuel and reduce the volume and toxicity of the rest of the spent fuel for storage or disposal. In most commercial applications uranium is extracted from aqueous nitric acid solutions of the spent fuel using extractants such as tributylphosphate {TBP; O = P(OC4H9)3} in diluents of large aliphatic chain hydrocarbons (i.e. kerosene, dodecane) [3]. The need to use volatile organic compounds (VOCs) as solvents, or the subsequent generation of VOCs, introduces safety risks [4]. Recently, ionic liquids (ILs) have been suggested as alternatives to organic solvents because of their negligible volatility and flammability under common industrial conditions, which reduce solvent loss and make them inherently safe and environmentally friendly [5,6]. Ionic liquids are ⇑ Corresponding author. Tel.: +44 (0) 20 7679 3832; fax: +44 (0) 20 7383 2348. E-mail address: [email protected] (P. Angeli). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.08.064
salts with low melting points (below 100 °C) composed exclusively of ions [6]. The ability to tune the properties of ionic liquids by the choice of the anion and the cation and optimise them for a particular application has expanded significantly their use in synthesis, catalysis and separations in recent years [7,8]. Their high resistance to radiation (much higher than the commonly used TBP/kerosene mixtures) makes ionic liquids particularly suitable for extractions in spent fuel reprocessing [9–11]. Hydrophobic ionic liquids are promising candidates for use in extraction applications with aqueous solutions [12]. The hydrophobicity of the ionic liquids depends on the alkyl chain length of the associated cation and on the anion. However, even ionic liquids that are immiscible with water can absorb small amounts of water [13]. The absorbed water affects the viscosity of the ionic liquids. Recently, the bis{(trifluoromethyl)sulfonyl}amide anion, [N(SO2CF3)2] (abbreviated to bistriflamide, [NTf2]), has become a popular choice for synthesising hydrophobic ionic liquids that are chemically and thermally more robust and of lower viscosity compared to the majority of ionic liquids [14]. Ionic liquids are in general non-coordinating and in the absence of an extractant, do not extract metal ions efficiently from the aqueous phase. To enable extractions in spent fuel reprocessing, many investigators have dissolved known extracting moieties such as CMPO
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Nomenclature cross sectional area (m2) internal diameter of the microchannel (m) volumetric flow rate of fluid (m3 s-1) distribution coefficient of dioxouranium(VI) (–) extraction (%) extraction efficiency (%) initial concentration of dioxouranium(VI) in aqueous phase (mol L-1) [U]aq,fin final concentration of dioxouranium(VI) in aqueous phase (mol L-1) [U]aq,eq concentration of dioxouranium(VI) in aqueous phase at equilibrium (mol L-1) [HNO3]aq,init initial concentration of nitric acid (mol L-1) kLa overall volumetric mass transfer coefficient (s1)
A ID Q DU E Eeff [U]aq,init
{octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide} [15,16], TTA (2-thenolyltrifluoroacetone) [17], TBP [18] or HDEHP {bis(2-ethylhexyl)phosphoric acid} [19] in the ionic liquids. The use of quaternary ammonium bistriflamide ionic liquids gave, in some cases, very high partition coefficients for fUO2 g2þ [20]. Studies of uranium extraction from aqueous nitric acid solutions to ionic liquids suggest more than one mechanism for the extraction, i.e. cation exchange, anion exchange and/or solvation, depending on the nature of extractant, concentration of the counter anion, structure of the ionic liquid, and the aqueous phase composition [21,22]. This is in contrast to traditional solvents, such as dodecane, where the apparent stoicheiometry of uranium(VI) extraction by TBP is [23]: fUO2 g2þ aq þ 2½NO3 aq þ 2TBPorg $ UO2 ðNO3 Þ2 2TBPorg
which suggests that the extracted dioxouranium(VI) moiety forms a complex with two nitrate ions and two TBP ligands. This is considered as disadvantageous for their use in nuclear processing because of losses of ionic liquid components into the aqueous phase, although there are cases where ionic liquids behave as conventional solvents [19]. All these results are from equilibrium studies, and no reported cases of continuous extraction systems exist. The high costs of some ionic liquids have discouraged their extensive use in continuous large-scale systems [24]. This can be overcome by operating in microchannel extractors which require smaller amounts of solvent. The reduction in solvent volume is compensated by the high efficiencies achieved, because of the thin fluidic films formed in the confined spaces of the small channels, which can significantly reduce mass transfer resistances. In an earlier study of ionic liquids in microchannel flows, much higher yields were found than in intensely mixed batch processes for biosynthesis [25]. In two-phase microchannel flows, mass transfer rates greatly depend on the flow patterns that form. A common pattern is segmented or plug flow, where one phase forms drops with sizes larger than the channel diameter (plugs) that separate the continuous phase into segments (slugs) [26]. It has been found that overall mass transfer rates depend on plug geometry. In addition, depending on wall wetting properties, a thin film may form, between the plugs and the wall, which increases the specific interfacial area available for mass transfer. Moreover, the mass transfer is intensified through internal circulation within the slugs or the plugs, caused by the shear between the continuous phase/wall surface and the slug or plug axis, which enhances diffusive penetration. [27–31]. To increase throughput for industrial scale applications, scaleup of the micro-units is essential. In micro-chemical systems,
Greek letters specific interfacial area (m2 m-3) residence time (s)
a s
Subscripts IL ionic liquid org organic aq aqueous eq equilibrium init initial fin final eff efficiency sat saturated mix mixture
scale-up is achieved by numbering-up, i.e. the multiple, parallel repetitions of the microchannels or micro-processing units. Both internal and external numbering-up have been suggested for two-phase flow systems [32]. In this work, the extraction of dioxouranium(VI) ions from aqueous nitric acid solutions into TBP/[C4mim][NTf2] mixtures, relevant to spent fuel reprocessing, was studied in microchannel separators. Experiments were carried out during plug flow, and the effects on dioxouranium(VI) extraction of initial nitric acid concentration, residence time and flow rate ratio of the two phases were investigated. 2. Experimental set-up, procedure and materials 2.1. Experimental set-up A schematic of the experimental set-up used for the extraction of fUO2 g2þ from aqueous nitric acid solutions by TBP dissolved in the ionic liquid [C4mim][NTf2] is depicted in Fig. 1. It comprises five main sections: the fluids delivery section in the mixing zone, the flow visualisation section, the pressure drop measurement section, the online separation section and the dioxouranium(VI) ion detection section. The solutions were introduced into the mixing zone separately by a double high precision pump (KdScientific). The uncertainties in the flow rates were estimated to be ±1%. A T-junction inlet (made of Teflon) with all the branches having the same ID (0.5 mm) was used for bringing the fluids together. In the T-junction the two liquids entered into the mixing zone perpendicularly to each other, with the ionic liquid solution injected parallel to the test channel axis. The test microchannel was made of Teflon with 0.5 mm ID and length from 10 cm to 30 cm. The internal diameter of the microchannels was measured with a microscope. The flow visualisation section comprised a high-speed camera (Photron APX) connected to a computer for data storage and a light source. Images were acquired at a distance of 8 cm downstream from the inlet. For the pressure drop measurements, a differential pressure meter Comark C9555 (range: 0–±200 kPa, accuracy ±0.2%) was used, connected to two pressure ports before and after the microchannel, as illustrated in Fig. 1. At the end of the reaction zone, a flow splitter was connected to the main channel for the separation of the two liquid phases. The splitter had two side channels made of stainless steel and of PTFE that have different wettabilities for the two liquids (similar to [33]). With this configuration, pure ionic liquid phase was obtained from the PTFE outlet and a mixture of ionic liquid and aqueous solution from the steel outlet. The pure ionic liquid phase was used for the
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P: Syringe pumps
1: IL inlet
R1: HNO3 outlet
A: Mixing zone
2: HNO3 inlet
R2: IL outlet
C: Microchannel
B: Pressure ports FS: Flow splitter Fig. 1. Schematic of the experimental set-up.
uranium(VI) measurements with (USB2000+, from Ocean Optics).
a
UV–Vis
spectrometer
2.2. Materials The ionic liquid used for this work was prepared at the QUILL research centre by a standard route [14]. The HNO3/U(VI) solutions were prepared in the UCL laboratory from aqueous nitric acid (65%) solutions of general purpose grade by Fisher Scientific and dioxouranium(VI) nitrate hexahydrate, UO2(NO3)26H2O ex Sigma–Aldrich. Tributylphosphate (TBP) was obtained from Sigma–Aldrich. 2.3. Procedure 2.3.1. Extraction of fUO2 g2þ at equilibrium All equilibrium extractions were carried out at room temperature with 1:1 ionic liquid to aqueous phase ratio. The ionic liquid in its pure state absorbs a small amount of water depending on the initial nitric acid concentration that can vary from 17000 to 25000 ppm for nitric acid concentrations from 0.01 M to 3 M respectively [34]. In the experiments a solution of 30% (v/v) TBP in [C4mim][ NTf2] was prepared and first equilibrated with the nitric acid solution at the required concentration by mechanical shaking for varying lengths of time. Saturation was confirmed by measuring the viscosity for both pure and saturated ionic liquids with a digital Rheometer DV-III Ultra (Brookfield), since the viscosity does not change over time. It was found that saturation of the ionic liquid was established after being in contact with the nitric acid solution for 20 min. Then, the two phases were separated and the pre-equilibrated ionic liquid phase was brought into contact with a fresh aqueous phase with the same nitric acid concentration, which also contained 0.05 M uranium (VI) added in the form of dioxouranium(VI) nitrate hexahydrate. Mechanical shaking was applied for 3 h before the two phases were separated. The dioxouranium(VI) concentration was measured in the aqueous phase before and after the equilibration, and was determined by UV–Vis spectroscopy. The distribution coefficient (DU) is calculated from the following equation:
DU ¼
½Uaq;init ½Uaq;eq ½Uaq;eq
ð1Þ
The concentration of nitric acid varied from 0.01 M to 3 M while the initial concentration of U(VI) in the aqueous phase was kept constant at 0.05 M (±4%). All equilibrium experiments were performed at least five times and the deviation between the measurements was between 4.8% and 15.1%.
2.3.2. Continuous extraction of fUO2 g2þ in the microchannel For the continuous extraction, the two phases TBP/ [C4mim][NTf2] (30% v/v) and HNO3/U(VI) were introduced into the test channel via the T-junction at a total volumetric flow rate Qmix = 14.6 cm3 h-1 and flow ratios of ionic liquid to aqueous phase from 0.25 to 1. Saturated ionic liquid was used, which was obtained as described in Section 2.3.1. After the online separation, the dioxouranium(VI) concentration in the ionic liquid phase was determined by UV–Vis spectroscopy. The percentage of dioxouranium(VI) extracted (%E) in the ionic liquid phase is found from Eq. (2). Extraction experiments were repeated several times with the deviation of the measurements varying from 5.2% to 9.9%. The extraction efficiency (%Eeff) of the system, which is the ratio of the amount of the species transferred to the maximum amount transferable, is calculated from Eq. (3) and the deviation of the measurements was less than 7%. In addition, the overall volumetric mass transfer coefficient (kLa) was determined from Eq. (4) with a deviation of 6.0% to 14.1%.
%E ¼
½Uaq;init ½Uaq;fin ½Uaq;init
%Eeff ¼
kL a ¼
1
s
ð2Þ
½Uaq;fin ½Uaq;init ½Uaq;eq ½Uaq;init
ln
½Uaq;eq ½Uaq;init ½Uaq;eq ½Uaq;fin
ð3Þ ! ð4Þ
The extraction measurements were carried out during plug flow with the water forming the plugs and the ionic liquid the continuous phase. The size of the water plugs at the different flow ratios was determined from the high speed camera images (deviation between measurements was less than 4%).
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3. Results and discussion 3.1. Dioxouranium(VI) extraction at equilibrium The distribution coefficient of dioxouranium(VI), DU was determined as a function of initial nitric acid concentration for TBP/[C4mim][NTf2] (30% v/v) and is shown in Table 1. As can be seen, the distribution coefficient is high at low nitric acid concentrations (0.01 M), decreases to its lowest level at [HNO3]aq,init = 1 M, and is followed by an increase up to concentration of 3 M. Similar trends were found by other investigators [3,34] for [C4mim][NTf2], even though some experimental details, i.e. dioxouranium(VI) concentration, temperature, and experimental procedure for phase preparation were not the same. The dioxouranium(VI) concentration used in this work is 0.05 M, while Giridhar et al. [3] and Billard et al. [34] used concentrations of 4.2 104 M and 103 M (or 102 M) respectively. Giridhar et al. [3] found distribution coefficient values DU of 15.3 and 0.7 at initial nitric acid concentrations 0.01 M and 1 M, respectively. At high initial nitric acid concentrations ([HNO3]aq,init), the extraction of fUO2 g2þ by the TBP/ [C4mim][NTf2] mixture has been attributed to an ion exchange mechanism in which ½UO2 ðNO3 Þ3 anions are exchanged with the anion of the ionic liquid i.e. ½NTf 2 [3,34]. By contrast, at low [HNO3]aq,init values, the extraction mechanism strongly depends on the cationic part of the ionic liquid [35]. The UV–Vis aqueous phase absorption spectra of the dioxouranium(VI) solutions before and after the extraction are depicted in Fig. 2. The initial dioxouranium(VI) solutions show characteristic absorption bands similar to those described in the literature for dioxouranium(VI) ions in aqueous solutions [36]. As can be seen, the shifts in the absorption peaks with nitric acid concentration are negligible, i.e. the peak at 414 nm with 0.01 M nitric acid has shifted to 415 nm and to 416 nm as the nitric acid concentration is increased to 1 M and 3 M, respectively. This slight shift is probably due to the formation of dioxouranium(VI) nitrate complexes in the acid solution [36].
Fig. 2. UV–Vis aqueous-phase spectra of dioxouranium(VI) (a) before and (b) after the extraction with different nitric acid concentrations.
Fig. 3. Dioxouranium(VI) extraction (%E), as a function of initial nitric acid concentration into TBP/[C4mim][NTf2] (30% v/v) in a 10 cm microchannel. Table 2 Extraction efficiency of dioxouranium(VI) (%Eeff), as a function of nitric acid concentration into TBP/[C4mim][NTf2] (30% v/v) in a 10 cm microchannel.
3.2. Effects of initial nitric acid concentration [HNO3]aq,init on dioxouranium(VI) extraction in microchannels
%Eeff at [HNO3]aq,init
The extraction of dioxouranium(VI) (%E) from the nitric acid solutions into TBP/[C4mim][NTf2] (30% v/v) in the 0.5 mm ID microchannel is plotted in Fig. 3. The total volumetric flow rate was Qmix = 14.6 cm3 h-1 and the ratio of ionic liquid to aqueous phase was 1:1. The length of the microchannel was 10 cm. As can be seen, with increasing nitric acid concentration the extraction of dioxouranium(VI) decreases until 1 M and then increases again. The highest extraction (64%) was achieved at the lowest nitric acid concentration, i.e. [HNO3]aq,init = 0.01 M. The extraction efficiency (%Eeff), however, as can be seen from Table 2 does not follow the same trend as the dioxouranium(VI) extraction (%E). Within the 10 cm of the reactor (4.84 s residence time), the amount extracted reaches 78% of the equilibrium value for nitric acid concentration of 0.01 M and is followed by a reduction for 0.1 M, while it is slightly increased for 1 M to 61%. The initial decrease from 0.01 M to 0.1 M is expected since the driving force (concentration difference from equilibrium, Table 1) is reduced. Table 1 Comparison of the dioxouranium(VI) distribution coefficient between a nitric acid aqueous solution and TBP/[C4mim][NTf2] (30% v/v) as a function of initial nitric acid concentration. DU at [HNO3]aq,init 0.01 M 4.55
0.1 M 1.63
1M 0.96
2M 2.13
3M 2.98
0.01 M 78%
0.1 M 58%
1M 61%
2M 56%
3M 58%
However, the increase from 0.1 M to 1 M is not expected from the equilibrium data (Table 1), which show a decrease in driving force with an increase in nitric acid concentration. At nitric acid concentrations over 1 M, the extraction efficiency decreases for 2 M and then slightly increases to 58% for 3 M. The initial decrease from 1 M to 2 M is not consistent with the equilibrium data, while the subsequent increase from 2 M to 3 M follows the same trend as DU. This difference in the trend between the extraction percentage and the extraction efficiency as a function of the initial nitric acid concentration, at concentration close to 1 M, can be attributed to the change of the extraction mechanism. It has been proposed [34] that the mechanism of uranium(VI) extraction changes with nitric acid concentration from cation exchange at low concentrations to solvation at high ones. Because of the change of mechanism, different dioxouranium(VI) complexes form that may have different mass transfer rates and can therefore affect the extraction efficiency. 3.3. Effects of residence time on dioxouranium(VI) extraction in microchannel To study the effects of residence time on dioxouranium(VI) extraction in the microchannel, experiments were conducted for
D. Tsaoulidis et al. / Chemical Engineering Journal 227 (2013) 151–157
Fig. 4. Dioxouranium(VI) extraction (%E) as a function of residence time from different nitric acid solutions into TBP/[C4mim][NTf2] (30% v/v). Lines correspond to dioxouranium(VI) %extraction at equilibrium at three different initial nitric acid concentrations: [HNO3]aq,init = 3 M (solid), [HNO3]aq,init = 2 M (dotted), and [HNO3]aq,init = 0.01 M (dashed). Symbols correspond to initial nitric acid concentration [HNO3]aq,init: (e) 3 M, (–) 2 M, and () 0.01 M.
155
different channel lengths, namely 10 cm, 20 cm and 30 cm at the same total volumetric flow rate, Qmix = 14.6 cm3 h-1, and ionic liquid to aqueous phase flow rate ratio of 1:1. The initial nitric acid concentration was varied from 0.01 M to 3 M. Under these conditions, the pressure drop of the two-phase flow along the microchannels was also measured and was found to vary from 5.45 kPa to 17 kPa. It should be pointed out that the viscosity of the saturated ionic liquid was 0.041 kg m-1s-1. In continuous extractors, the residence time is defined as the volume of the reactor divided by the total volumetric flow rate of the two phases. As can be seen in Fig. 4, the extraction of dioxouranium(VI) increases by increasing the residence time. At initial nitric acid concentration of 0.01 M, the extraction of dioxouranium(VI) reaches 95% of the equilibrium value in 9.68 s, while in the cases of [HNO3]aq,init = 2 M and [HNO3]aq,init = 3 M within 14.52 s it reaches 85% and 91% of the equilibrium value, respectively. This is in agreement with the results shown in Table 2 for the 10 cm channel, where concentrations closer to equilibrium were achieved for the [HNO3]aq,init = 0.01 M compared to 2 M and 3 M. In Fig. 5, the UV–Vis spectra of the dioxouranium(VI) species extracted in by TBP/[C4mim][NTf2] (30% v/v) are shown at different residence times and at equilibrium (3 h). As the residence time is increased, the absorption spectra approach the equilibrium limit. There is a slight shift of the most intense peaks compared to the absorption spectra of the dioxouranium(VI) species in the nitric acid solutions (see Fig. 2). A shift to longer wavelengths in the absorption peaks of dioxouranium(VI) ions in ionic liquids with TBP and CMPO was also reported by Visser et al. [37] and was attributed to the chemical environment in the ionic liquid which favours the formation of a dioxouranium(VI) nitric acid complex and thus causes a red-shift. 3.4. Effects of flow ratio on dioxouranium(VI) extraction in microchannel
Fig. 5. Absorption spectra of dioxouranium(VI) in TBP/[C4mim][NTf2] (30% v/v) extracted from a 3 M nitric acid solution at different residence times in the microchannel and at equilibrium after 3 h of mechanical shaking.
The effect of the phase flow rate ratio on the dioxouranium(VI) extraction in the microchannel is shown in Fig. 6. For these experiments the total volumetric flow rate (Qmix) was kept constant at 14.6 cm3 h-1, while the ratio of ionic liquid to the aqueous phase was varied from 0.25 to 1. According to the stoicheiometry of the reaction, even in the case of a 0.25 phase ratio, TBP is still in an
Fig. 6. Dioxouranium(VI) extraction (%E) as a function of phase flow rate ratio from nitric acid solutions into TBP/[C4mim][NTf2] (30% v/v). Horizontal lines correspond to dioxouranium(VI) extraction at equilibrium at [HNO3]aq,init = 3 M (dotted) and [HNO3]aq,init = 1 M (solid). Symbols: (–) [HNO3]aq,init = 1 M, () [HNO3]aq,init = 3 M.
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lated are much greater than those of conventional contactors and comparable with mass transfer coefficients found by Kashid et al. [39] for plug flow systems. In Table 3, the overall volumetric mass transfer coefficients as a function of ionic liquid to aqueous phase ratio at initial nitric acid concentrations of 1 M and 3 M in a 10 cm microchannel are shown. As the ratio decreases from 1 to 0.25, kLa also decreases. As was discussed above, a decrease in the phase ratio reduces the interfacial area available for mass transfer (Fig. 6) and increases the circulation time inside the plug, which leads to less efficient mixing. As a result, the rate of mass transfer is decreased. 4. Conclusions
Fig. 7. Volumetric mass transfer coefficient (kLa) as a function of initial nitric acid concentration at different residence times, s, for a phase flow rate ratio of 1.
Table 3 Volumetric mass transfer coefficient (kLa) as a function of IL to aqueous phase ratio at initial nitric acid concentration of 1 M and 3 M in a 10 cm microchannel. IL to aqueous flow rate ratio
1 0.75 0.5 0.25
kLa at [HNO3]aq,
1 init/s
1M
3M
0.196 0.163 0.156 0.108
0.182 0.143 0.112 0.049
excess for the extraction of dioxouranium(VI) from the nitric acid solution. The length of the microchannel was 10 cm. The corresponding flow patterns were recorded and two indicative pictures for the lowest and highest ratio i.e. 0.25 and 1 are shown in Fig. 6, for initial nitric acid concentrations 1 M and 3 M. The length of the plugs varied from 1.61 mm to 3.65 mm as the phase ratio decreased from 1 to 0.25. It is evident, that by decreasing the ratio from 1 to 0.25, the dioxouranium(VI) %extraction also decreases for both cases, which can be attributed to the decrease in interfacial area available for mass transfer. Moreover, within the bigger plugs, the recirculation within the plugs is less efficient [38], and the mixing time depends on diffusion, which could easily reach several minutes, much longer than the residence time in the microchannel (4.84 s). For an initial nitric acid concentration of 3 M, the decrease is more obvious than for 1 M maybe as a result of the change of the extraction mechanism; it is possible that the mass transfer at 3 M is more dependent on the mixing of the transferred species within a phase and the kinetics of the complex formation than on the interfacial area. 3.5. Volumetric mass transfer coefficient in microchannel The volumetric mass transfer coefficients (kLa) were determined from Eq. (4) in the microchannel during plug flow at different initial nitric acid concentrations and residence times at a flow rate ratio of ionic liquid to aqueous phase equal to 1:1. As can be seen in Fig. 7, for each nitric acid concentration the mass transfer coefficients were similar for the different channel lengths used, indicating that the flow was already developed from the 10 cm length. There is a small difference at a nitric acid concentration of 0.01 M. The overall volumetric mass transfer coefficient decreases with increasing nitric acid concentration for a given length of the microchannel (i.e. same residence time). The values calcu-
The extraction of dioxouranium(VI) from nitric acid solutions by TBP, 30%(v/v) dissolved in the ionic liquid [C4mim][NTf2] in a microchannel of 0.5 mm internal diameter for various initial nitric acid concentrations, residence times and phase ratios were studied and the results were compared with those obtained from equilibrium experiments. The variation of extraction of dioxouranium(VI) with the initial nitric acid concentration was similar to the trend of the equilibrium values. With increasing initial aqueous nitric acid concentration from 0.01 M to 1 M, the extraction of dioxouranium(VI) decreased, while with further increase to 3 M the extraction increased. However, the extraction efficiency followed a slightly different trend, which may be due to the change of extraction mechanism from cation transfer to solvation and anion transfer as the nitric acid concentration increases. Extraction of dioxouranium(VI) increased by increasing the residence time and reached 95% of the equilibrium value in 9.68 s at nitric acid concentration of 0.01 M, while at nitric acid concentrations of 2 M and 3 M, it reached 85% and 91% in 14.52 s, respectively. Overall volumetric mass transfer coefficients decreased from 0.312 s1 to 0.132 s1 with increasing nitric acid concentration at an ionic liquid to aqueous phase flow rate ratio of 1. These values agreed with results obtained in other plug flow microchannel extraction systems. At a smaller phase ratio of 0.25, the mass transfer coefficients decreased. References [1] S. Ansolabehere, J. Deutch, M. Driscoll, P.E. Gray, J.P. Holdren, P.L. Joskow, R.K. Lester, E.J. Moniz, N.E. Todreas, The Future of Nuclear Power: An Interdisciplinary MIT Study, Massachusetts Institute of Technology, Boston, 2003, http://web.mit.edu/nuclearpower/pdf/nuclearpower-full.pdf. [2] J.M. Deutch, C.W. Forsberg, A.C. Kadak, M.S. Kazimi, E.J. Moniz, J.E. Parsons, Update of the MIT 2003 Future of Nuclear Power Study: An Interdisciplinary MIT Study, Massachusetts Institute of Technology, Boston, 2009, http:// web.mit.edu/nuclearpower/pdf/nuclearpower-update2009.pdf. [3] P. Giridhar, K.A. Venkatesan, S. Subramaniam, T.G. Srinivasan, P.R.V. Rao, Extraction of uranium(VI) by 1.1 M tri-n-butylphosphate/ionic liquid and the feasibility of recovery by direct electrodeposition from organic phase, J. Alloys Compd. 448 (2008) 104–108. [4] N. Stern, The Economics of Climate Change: The Stern Review, Cambridge University Press, Cambridge, 2007. [5] K.R. Seddon, Ionic liquids for clean technology, J. Chem. Technol. Biotechnol. 68 (4) (1997) 351–356. [6] M. Freemantle, An Introduction to Ionic Liquids, RSC Publications, Cambridge, UK, 2010. [7] A. Stark, K.R. Seddon, Ionic Liquids, in Kirk-Othmer Encyclopaedia of Chemical Technology, 5th ed., in: A. Seidel (Ed.), vol. 26, John Wiley & Sons, Inc., Hoboken, New Jersey, 2007, pp. 836–920. [8] N.V. Plechkova, K.R. Seddon, Applications of ionic liquids in the chemical industry, Chem. Soc. Rev. 37 (2008) 123–150. [9] D. Allen, G. Baston, A.E. Bradley, T. Gorman, A. Haile, I. Hamblett, J.E. Hatter, M.J.F. Healey, B. Hodgson, R. Lewin, K.V. Lovell, B. Newton, W.R. Pitner, D.W. Rooney, D. Sanders, K.R. Seddon, H.E. Sims, R.C. Thied, An investigation of the radiochemical stability of ionic liquids, Green Chem. 4 (2) (2002) 152–158. [10] G.M.N. Baston, A.E. Bradley, T. Gorman, I. Hamblett, C. Hardacre, J.E. Hatter, M.J.F. Healy, B. Hodgson, R. Lewin, K.V. Lovell, G.W.A. Newton, M. Nieuwenhuyzen, W.R. Pitner, D.W. Rooney, D. Sanders, K.R. Seddon, H.E. Sims, R.C. Thied, Ionic Liquids for the Nuclear Industry: A Radiochemical, Structural, and Electrochemical Investigation, in: R.D. Rogers, K.R. Seddon
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Dioxouranium(VI) extraction in microchannels using ionic liquids Dimitrios Tsaoulidis a, Valentina Dore a, Panagiota Angeli a,⇑, Natalia V. Plechkova b, Kenneth R. Seddon b a b
Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK QUILL Research Centre, School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, UK
h i g h l i g h t s " Continuous extraction of uranium(VI) from nitric acid solutions in microchannels by using ionic liquids. " Mass transfer coefficients in the microchannel system during segmented flow. " The effects of nitric acid concentration on the extraction efficiency.
a r t i c l e
i n f o
Article history: Available online 5 September 2012 Keywords: Liquid–liquid extraction Ionic liquids Microchannel Dioxouranium(VI) nitrate UV–Vis spectroscopy
a b s t r a c t The extraction of uranium(VI) from aqueous nitric acid solutions by tributylphosphate {TBP; 30%(v/v)} dissolved in the ionic liquid 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide was investigated. The experiments were performed in a Teflon microchannel of 0.5 mm internal diameter, while the dioxouranium(VI) concentrations in the aqueous and the ionic liquid phases were determined by UV–Vis spectroscopy. The effects of initial nitric acid concentration (0.01–3 M), residence time, and phase flow rate ratio were studied. It was found that, with increasing nitric acid concentration, the percentage of dioxouranium(VI) extracted decreased and then increased again, while the extraction efficiency followed a slightly different trend. Overall mass transfer coefficients varied between 0.049 s1 and 0.312 s1. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Nuclear energy from fission can provide substantial amounts of carbon free electricity and process heat [1,2]. However, one of the main concerns in the use of the nuclear power generation is the management of the irradiated nuclear fuel from the reactor (spent fuel) which can remain toxic for thousands of years. Efficient spent fuel reprocessing through a series of operations can separate uranium to be reused as reactor fuel and reduce the volume and toxicity of the rest of the spent fuel for storage or disposal. In most commercial applications uranium is extracted from aqueous nitric acid solutions of the spent fuel using extractants such as tributylphosphate {TBP; O = P(OC4H9)3} in diluents of large aliphatic chain hydrocarbons (i.e. kerosene, dodecane) [3]. The need to use volatile organic compounds (VOCs) as solvents, or the subsequent generation of VOCs, introduces safety risks [4]. Recently, ionic liquids (ILs) have been suggested as alternatives to organic solvents because of their negligible volatility and flammability under common industrial conditions, which reduce solvent loss and make them inherently safe and environmentally friendly [5,6]. Ionic liquids are ⇑ Corresponding author. Tel.: +44 (0) 20 7679 3832; fax: +44 (0) 20 7383 2348. E-mail address: [email protected] (P. Angeli). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.08.064
salts with low melting points (below 100 °C) composed exclusively of ions [6]. The ability to tune the properties of ionic liquids by the choice of the anion and the cation and optimise them for a particular application has expanded significantly their use in synthesis, catalysis and separations in recent years [7,8]. Their high resistance to radiation (much higher than the commonly used TBP/kerosene mixtures) makes ionic liquids particularly suitable for extractions in spent fuel reprocessing [9–11]. Hydrophobic ionic liquids are promising candidates for use in extraction applications with aqueous solutions [12]. The hydrophobicity of the ionic liquids depends on the alkyl chain length of the associated cation and on the anion. However, even ionic liquids that are immiscible with water can absorb small amounts of water [13]. The absorbed water affects the viscosity of the ionic liquids. Recently, the bis{(trifluoromethyl)sulfonyl}amide anion, [N(SO2CF3)2] (abbreviated to bistriflamide, [NTf2]), has become a popular choice for synthesising hydrophobic ionic liquids that are chemically and thermally more robust and of lower viscosity compared to the majority of ionic liquids [14]. Ionic liquids are in general non-coordinating and in the absence of an extractant, do not extract metal ions efficiently from the aqueous phase. To enable extractions in spent fuel reprocessing, many investigators have dissolved known extracting moieties such as CMPO
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Nomenclature cross sectional area (m2) internal diameter of the microchannel (m) volumetric flow rate of fluid (m3 s-1) distribution coefficient of dioxouranium(VI) (–) extraction (%) extraction efficiency (%) initial concentration of dioxouranium(VI) in aqueous phase (mol L-1) [U]aq,fin final concentration of dioxouranium(VI) in aqueous phase (mol L-1) [U]aq,eq concentration of dioxouranium(VI) in aqueous phase at equilibrium (mol L-1) [HNO3]aq,init initial concentration of nitric acid (mol L-1) kLa overall volumetric mass transfer coefficient (s1)
A ID Q DU E Eeff [U]aq,init
{octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide} [15,16], TTA (2-thenolyltrifluoroacetone) [17], TBP [18] or HDEHP {bis(2-ethylhexyl)phosphoric acid} [19] in the ionic liquids. The use of quaternary ammonium bistriflamide ionic liquids gave, in some cases, very high partition coefficients for fUO2 g2þ [20]. Studies of uranium extraction from aqueous nitric acid solutions to ionic liquids suggest more than one mechanism for the extraction, i.e. cation exchange, anion exchange and/or solvation, depending on the nature of extractant, concentration of the counter anion, structure of the ionic liquid, and the aqueous phase composition [21,22]. This is in contrast to traditional solvents, such as dodecane, where the apparent stoicheiometry of uranium(VI) extraction by TBP is [23]: fUO2 g2þ aq þ 2½NO3 aq þ 2TBPorg $ UO2 ðNO3 Þ2 2TBPorg
which suggests that the extracted dioxouranium(VI) moiety forms a complex with two nitrate ions and two TBP ligands. This is considered as disadvantageous for their use in nuclear processing because of losses of ionic liquid components into the aqueous phase, although there are cases where ionic liquids behave as conventional solvents [19]. All these results are from equilibrium studies, and no reported cases of continuous extraction systems exist. The high costs of some ionic liquids have discouraged their extensive use in continuous large-scale systems [24]. This can be overcome by operating in microchannel extractors which require smaller amounts of solvent. The reduction in solvent volume is compensated by the high efficiencies achieved, because of the thin fluidic films formed in the confined spaces of the small channels, which can significantly reduce mass transfer resistances. In an earlier study of ionic liquids in microchannel flows, much higher yields were found than in intensely mixed batch processes for biosynthesis [25]. In two-phase microchannel flows, mass transfer rates greatly depend on the flow patterns that form. A common pattern is segmented or plug flow, where one phase forms drops with sizes larger than the channel diameter (plugs) that separate the continuous phase into segments (slugs) [26]. It has been found that overall mass transfer rates depend on plug geometry. In addition, depending on wall wetting properties, a thin film may form, between the plugs and the wall, which increases the specific interfacial area available for mass transfer. Moreover, the mass transfer is intensified through internal circulation within the slugs or the plugs, caused by the shear between the continuous phase/wall surface and the slug or plug axis, which enhances diffusive penetration. [27–31]. To increase throughput for industrial scale applications, scaleup of the micro-units is essential. In micro-chemical systems,
Greek letters specific interfacial area (m2 m-3) residence time (s)
a s
Subscripts IL ionic liquid org organic aq aqueous eq equilibrium init initial fin final eff efficiency sat saturated mix mixture
scale-up is achieved by numbering-up, i.e. the multiple, parallel repetitions of the microchannels or micro-processing units. Both internal and external numbering-up have been suggested for two-phase flow systems [32]. In this work, the extraction of dioxouranium(VI) ions from aqueous nitric acid solutions into TBP/[C4mim][NTf2] mixtures, relevant to spent fuel reprocessing, was studied in microchannel separators. Experiments were carried out during plug flow, and the effects on dioxouranium(VI) extraction of initial nitric acid concentration, residence time and flow rate ratio of the two phases were investigated. 2. Experimental set-up, procedure and materials 2.1. Experimental set-up A schematic of the experimental set-up used for the extraction of fUO2 g2þ from aqueous nitric acid solutions by TBP dissolved in the ionic liquid [C4mim][NTf2] is depicted in Fig. 1. It comprises five main sections: the fluids delivery section in the mixing zone, the flow visualisation section, the pressure drop measurement section, the online separation section and the dioxouranium(VI) ion detection section. The solutions were introduced into the mixing zone separately by a double high precision pump (KdScientific). The uncertainties in the flow rates were estimated to be ±1%. A T-junction inlet (made of Teflon) with all the branches having the same ID (0.5 mm) was used for bringing the fluids together. In the T-junction the two liquids entered into the mixing zone perpendicularly to each other, with the ionic liquid solution injected parallel to the test channel axis. The test microchannel was made of Teflon with 0.5 mm ID and length from 10 cm to 30 cm. The internal diameter of the microchannels was measured with a microscope. The flow visualisation section comprised a high-speed camera (Photron APX) connected to a computer for data storage and a light source. Images were acquired at a distance of 8 cm downstream from the inlet. For the pressure drop measurements, a differential pressure meter Comark C9555 (range: 0–±200 kPa, accuracy ±0.2%) was used, connected to two pressure ports before and after the microchannel, as illustrated in Fig. 1. At the end of the reaction zone, a flow splitter was connected to the main channel for the separation of the two liquid phases. The splitter had two side channels made of stainless steel and of PTFE that have different wettabilities for the two liquids (similar to [33]). With this configuration, pure ionic liquid phase was obtained from the PTFE outlet and a mixture of ionic liquid and aqueous solution from the steel outlet. The pure ionic liquid phase was used for the
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P: Syringe pumps
1: IL inlet
R1: HNO3 outlet
A: Mixing zone
2: HNO3 inlet
R2: IL outlet
C: Microchannel
B: Pressure ports FS: Flow splitter Fig. 1. Schematic of the experimental set-up.
uranium(VI) measurements with (USB2000+, from Ocean Optics).
a
UV–Vis
spectrometer
2.2. Materials The ionic liquid used for this work was prepared at the QUILL research centre by a standard route [14]. The HNO3/U(VI) solutions were prepared in the UCL laboratory from aqueous nitric acid (65%) solutions of general purpose grade by Fisher Scientific and dioxouranium(VI) nitrate hexahydrate, UO2(NO3)26H2O ex Sigma–Aldrich. Tributylphosphate (TBP) was obtained from Sigma–Aldrich. 2.3. Procedure 2.3.1. Extraction of fUO2 g2þ at equilibrium All equilibrium extractions were carried out at room temperature with 1:1 ionic liquid to aqueous phase ratio. The ionic liquid in its pure state absorbs a small amount of water depending on the initial nitric acid concentration that can vary from 17000 to 25000 ppm for nitric acid concentrations from 0.01 M to 3 M respectively [34]. In the experiments a solution of 30% (v/v) TBP in [C4mim][ NTf2] was prepared and first equilibrated with the nitric acid solution at the required concentration by mechanical shaking for varying lengths of time. Saturation was confirmed by measuring the viscosity for both pure and saturated ionic liquids with a digital Rheometer DV-III Ultra (Brookfield), since the viscosity does not change over time. It was found that saturation of the ionic liquid was established after being in contact with the nitric acid solution for 20 min. Then, the two phases were separated and the pre-equilibrated ionic liquid phase was brought into contact with a fresh aqueous phase with the same nitric acid concentration, which also contained 0.05 M uranium (VI) added in the form of dioxouranium(VI) nitrate hexahydrate. Mechanical shaking was applied for 3 h before the two phases were separated. The dioxouranium(VI) concentration was measured in the aqueous phase before and after the equilibration, and was determined by UV–Vis spectroscopy. The distribution coefficient (DU) is calculated from the following equation:
DU ¼
½Uaq;init ½Uaq;eq ½Uaq;eq
ð1Þ
The concentration of nitric acid varied from 0.01 M to 3 M while the initial concentration of U(VI) in the aqueous phase was kept constant at 0.05 M (±4%). All equilibrium experiments were performed at least five times and the deviation between the measurements was between 4.8% and 15.1%.
2.3.2. Continuous extraction of fUO2 g2þ in the microchannel For the continuous extraction, the two phases TBP/ [C4mim][NTf2] (30% v/v) and HNO3/U(VI) were introduced into the test channel via the T-junction at a total volumetric flow rate Qmix = 14.6 cm3 h-1 and flow ratios of ionic liquid to aqueous phase from 0.25 to 1. Saturated ionic liquid was used, which was obtained as described in Section 2.3.1. After the online separation, the dioxouranium(VI) concentration in the ionic liquid phase was determined by UV–Vis spectroscopy. The percentage of dioxouranium(VI) extracted (%E) in the ionic liquid phase is found from Eq. (2). Extraction experiments were repeated several times with the deviation of the measurements varying from 5.2% to 9.9%. The extraction efficiency (%Eeff) of the system, which is the ratio of the amount of the species transferred to the maximum amount transferable, is calculated from Eq. (3) and the deviation of the measurements was less than 7%. In addition, the overall volumetric mass transfer coefficient (kLa) was determined from Eq. (4) with a deviation of 6.0% to 14.1%.
%E ¼
½Uaq;init ½Uaq;fin ½Uaq;init
%Eeff ¼
kL a ¼
1
s
ð2Þ
½Uaq;fin ½Uaq;init ½Uaq;eq ½Uaq;init
ln
½Uaq;eq ½Uaq;init ½Uaq;eq ½Uaq;fin
ð3Þ ! ð4Þ
The extraction measurements were carried out during plug flow with the water forming the plugs and the ionic liquid the continuous phase. The size of the water plugs at the different flow ratios was determined from the high speed camera images (deviation between measurements was less than 4%).
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3. Results and discussion 3.1. Dioxouranium(VI) extraction at equilibrium The distribution coefficient of dioxouranium(VI), DU was determined as a function of initial nitric acid concentration for TBP/[C4mim][NTf2] (30% v/v) and is shown in Table 1. As can be seen, the distribution coefficient is high at low nitric acid concentrations (0.01 M), decreases to its lowest level at [HNO3]aq,init = 1 M, and is followed by an increase up to concentration of 3 M. Similar trends were found by other investigators [3,34] for [C4mim][NTf2], even though some experimental details, i.e. dioxouranium(VI) concentration, temperature, and experimental procedure for phase preparation were not the same. The dioxouranium(VI) concentration used in this work is 0.05 M, while Giridhar et al. [3] and Billard et al. [34] used concentrations of 4.2 104 M and 103 M (or 102 M) respectively. Giridhar et al. [3] found distribution coefficient values DU of 15.3 and 0.7 at initial nitric acid concentrations 0.01 M and 1 M, respectively. At high initial nitric acid concentrations ([HNO3]aq,init), the extraction of fUO2 g2þ by the TBP/ [C4mim][NTf2] mixture has been attributed to an ion exchange mechanism in which ½UO2 ðNO3 Þ3 anions are exchanged with the anion of the ionic liquid i.e. ½NTf 2 [3,34]. By contrast, at low [HNO3]aq,init values, the extraction mechanism strongly depends on the cationic part of the ionic liquid [35]. The UV–Vis aqueous phase absorption spectra of the dioxouranium(VI) solutions before and after the extraction are depicted in Fig. 2. The initial dioxouranium(VI) solutions show characteristic absorption bands similar to those described in the literature for dioxouranium(VI) ions in aqueous solutions [36]. As can be seen, the shifts in the absorption peaks with nitric acid concentration are negligible, i.e. the peak at 414 nm with 0.01 M nitric acid has shifted to 415 nm and to 416 nm as the nitric acid concentration is increased to 1 M and 3 M, respectively. This slight shift is probably due to the formation of dioxouranium(VI) nitrate complexes in the acid solution [36].
Fig. 2. UV–Vis aqueous-phase spectra of dioxouranium(VI) (a) before and (b) after the extraction with different nitric acid concentrations.
Fig. 3. Dioxouranium(VI) extraction (%E), as a function of initial nitric acid concentration into TBP/[C4mim][NTf2] (30% v/v) in a 10 cm microchannel. Table 2 Extraction efficiency of dioxouranium(VI) (%Eeff), as a function of nitric acid concentration into TBP/[C4mim][NTf2] (30% v/v) in a 10 cm microchannel.
3.2. Effects of initial nitric acid concentration [HNO3]aq,init on dioxouranium(VI) extraction in microchannels
%Eeff at [HNO3]aq,init
The extraction of dioxouranium(VI) (%E) from the nitric acid solutions into TBP/[C4mim][NTf2] (30% v/v) in the 0.5 mm ID microchannel is plotted in Fig. 3. The total volumetric flow rate was Qmix = 14.6 cm3 h-1 and the ratio of ionic liquid to aqueous phase was 1:1. The length of the microchannel was 10 cm. As can be seen, with increasing nitric acid concentration the extraction of dioxouranium(VI) decreases until 1 M and then increases again. The highest extraction (64%) was achieved at the lowest nitric acid concentration, i.e. [HNO3]aq,init = 0.01 M. The extraction efficiency (%Eeff), however, as can be seen from Table 2 does not follow the same trend as the dioxouranium(VI) extraction (%E). Within the 10 cm of the reactor (4.84 s residence time), the amount extracted reaches 78% of the equilibrium value for nitric acid concentration of 0.01 M and is followed by a reduction for 0.1 M, while it is slightly increased for 1 M to 61%. The initial decrease from 0.01 M to 0.1 M is expected since the driving force (concentration difference from equilibrium, Table 1) is reduced. Table 1 Comparison of the dioxouranium(VI) distribution coefficient between a nitric acid aqueous solution and TBP/[C4mim][NTf2] (30% v/v) as a function of initial nitric acid concentration. DU at [HNO3]aq,init 0.01 M 4.55
0.1 M 1.63
1M 0.96
2M 2.13
3M 2.98
0.01 M 78%
0.1 M 58%
1M 61%
2M 56%
3M 58%
However, the increase from 0.1 M to 1 M is not expected from the equilibrium data (Table 1), which show a decrease in driving force with an increase in nitric acid concentration. At nitric acid concentrations over 1 M, the extraction efficiency decreases for 2 M and then slightly increases to 58% for 3 M. The initial decrease from 1 M to 2 M is not consistent with the equilibrium data, while the subsequent increase from 2 M to 3 M follows the same trend as DU. This difference in the trend between the extraction percentage and the extraction efficiency as a function of the initial nitric acid concentration, at concentration close to 1 M, can be attributed to the change of the extraction mechanism. It has been proposed [34] that the mechanism of uranium(VI) extraction changes with nitric acid concentration from cation exchange at low concentrations to solvation at high ones. Because of the change of mechanism, different dioxouranium(VI) complexes form that may have different mass transfer rates and can therefore affect the extraction efficiency. 3.3. Effects of residence time on dioxouranium(VI) extraction in microchannel To study the effects of residence time on dioxouranium(VI) extraction in the microchannel, experiments were conducted for
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Fig. 4. Dioxouranium(VI) extraction (%E) as a function of residence time from different nitric acid solutions into TBP/[C4mim][NTf2] (30% v/v). Lines correspond to dioxouranium(VI) %extraction at equilibrium at three different initial nitric acid concentrations: [HNO3]aq,init = 3 M (solid), [HNO3]aq,init = 2 M (dotted), and [HNO3]aq,init = 0.01 M (dashed). Symbols correspond to initial nitric acid concentration [HNO3]aq,init: (e) 3 M, (–) 2 M, and () 0.01 M.
155
different channel lengths, namely 10 cm, 20 cm and 30 cm at the same total volumetric flow rate, Qmix = 14.6 cm3 h-1, and ionic liquid to aqueous phase flow rate ratio of 1:1. The initial nitric acid concentration was varied from 0.01 M to 3 M. Under these conditions, the pressure drop of the two-phase flow along the microchannels was also measured and was found to vary from 5.45 kPa to 17 kPa. It should be pointed out that the viscosity of the saturated ionic liquid was 0.041 kg m-1s-1. In continuous extractors, the residence time is defined as the volume of the reactor divided by the total volumetric flow rate of the two phases. As can be seen in Fig. 4, the extraction of dioxouranium(VI) increases by increasing the residence time. At initial nitric acid concentration of 0.01 M, the extraction of dioxouranium(VI) reaches 95% of the equilibrium value in 9.68 s, while in the cases of [HNO3]aq,init = 2 M and [HNO3]aq,init = 3 M within 14.52 s it reaches 85% and 91% of the equilibrium value, respectively. This is in agreement with the results shown in Table 2 for the 10 cm channel, where concentrations closer to equilibrium were achieved for the [HNO3]aq,init = 0.01 M compared to 2 M and 3 M. In Fig. 5, the UV–Vis spectra of the dioxouranium(VI) species extracted in by TBP/[C4mim][NTf2] (30% v/v) are shown at different residence times and at equilibrium (3 h). As the residence time is increased, the absorption spectra approach the equilibrium limit. There is a slight shift of the most intense peaks compared to the absorption spectra of the dioxouranium(VI) species in the nitric acid solutions (see Fig. 2). A shift to longer wavelengths in the absorption peaks of dioxouranium(VI) ions in ionic liquids with TBP and CMPO was also reported by Visser et al. [37] and was attributed to the chemical environment in the ionic liquid which favours the formation of a dioxouranium(VI) nitric acid complex and thus causes a red-shift. 3.4. Effects of flow ratio on dioxouranium(VI) extraction in microchannel
Fig. 5. Absorption spectra of dioxouranium(VI) in TBP/[C4mim][NTf2] (30% v/v) extracted from a 3 M nitric acid solution at different residence times in the microchannel and at equilibrium after 3 h of mechanical shaking.
The effect of the phase flow rate ratio on the dioxouranium(VI) extraction in the microchannel is shown in Fig. 6. For these experiments the total volumetric flow rate (Qmix) was kept constant at 14.6 cm3 h-1, while the ratio of ionic liquid to the aqueous phase was varied from 0.25 to 1. According to the stoicheiometry of the reaction, even in the case of a 0.25 phase ratio, TBP is still in an
Fig. 6. Dioxouranium(VI) extraction (%E) as a function of phase flow rate ratio from nitric acid solutions into TBP/[C4mim][NTf2] (30% v/v). Horizontal lines correspond to dioxouranium(VI) extraction at equilibrium at [HNO3]aq,init = 3 M (dotted) and [HNO3]aq,init = 1 M (solid). Symbols: (–) [HNO3]aq,init = 1 M, () [HNO3]aq,init = 3 M.
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lated are much greater than those of conventional contactors and comparable with mass transfer coefficients found by Kashid et al. [39] for plug flow systems. In Table 3, the overall volumetric mass transfer coefficients as a function of ionic liquid to aqueous phase ratio at initial nitric acid concentrations of 1 M and 3 M in a 10 cm microchannel are shown. As the ratio decreases from 1 to 0.25, kLa also decreases. As was discussed above, a decrease in the phase ratio reduces the interfacial area available for mass transfer (Fig. 6) and increases the circulation time inside the plug, which leads to less efficient mixing. As a result, the rate of mass transfer is decreased. 4. Conclusions
Fig. 7. Volumetric mass transfer coefficient (kLa) as a function of initial nitric acid concentration at different residence times, s, for a phase flow rate ratio of 1.
Table 3 Volumetric mass transfer coefficient (kLa) as a function of IL to aqueous phase ratio at initial nitric acid concentration of 1 M and 3 M in a 10 cm microchannel. IL to aqueous flow rate ratio
1 0.75 0.5 0.25
kLa at [HNO3]aq,
1 init/s
1M
3M
0.196 0.163 0.156 0.108
0.182 0.143 0.112 0.049
excess for the extraction of dioxouranium(VI) from the nitric acid solution. The length of the microchannel was 10 cm. The corresponding flow patterns were recorded and two indicative pictures for the lowest and highest ratio i.e. 0.25 and 1 are shown in Fig. 6, for initial nitric acid concentrations 1 M and 3 M. The length of the plugs varied from 1.61 mm to 3.65 mm as the phase ratio decreased from 1 to 0.25. It is evident, that by decreasing the ratio from 1 to 0.25, the dioxouranium(VI) %extraction also decreases for both cases, which can be attributed to the decrease in interfacial area available for mass transfer. Moreover, within the bigger plugs, the recirculation within the plugs is less efficient [38], and the mixing time depends on diffusion, which could easily reach several minutes, much longer than the residence time in the microchannel (4.84 s). For an initial nitric acid concentration of 3 M, the decrease is more obvious than for 1 M maybe as a result of the change of the extraction mechanism; it is possible that the mass transfer at 3 M is more dependent on the mixing of the transferred species within a phase and the kinetics of the complex formation than on the interfacial area. 3.5. Volumetric mass transfer coefficient in microchannel The volumetric mass transfer coefficients (kLa) were determined from Eq. (4) in the microchannel during plug flow at different initial nitric acid concentrations and residence times at a flow rate ratio of ionic liquid to aqueous phase equal to 1:1. As can be seen in Fig. 7, for each nitric acid concentration the mass transfer coefficients were similar for the different channel lengths used, indicating that the flow was already developed from the 10 cm length. There is a small difference at a nitric acid concentration of 0.01 M. The overall volumetric mass transfer coefficient decreases with increasing nitric acid concentration for a given length of the microchannel (i.e. same residence time). The values calcu-
The extraction of dioxouranium(VI) from nitric acid solutions by TBP, 30%(v/v) dissolved in the ionic liquid [C4mim][NTf2] in a microchannel of 0.5 mm internal diameter for various initial nitric acid concentrations, residence times and phase ratios were studied and the results were compared with those obtained from equilibrium experiments. The variation of extraction of dioxouranium(VI) with the initial nitric acid concentration was similar to the trend of the equilibrium values. With increasing initial aqueous nitric acid concentration from 0.01 M to 1 M, the extraction of dioxouranium(VI) decreased, while with further increase to 3 M the extraction increased. However, the extraction efficiency followed a slightly different trend, which may be due to the change of extraction mechanism from cation transfer to solvation and anion transfer as the nitric acid concentration increases. Extraction of dioxouranium(VI) increased by increasing the residence time and reached 95% of the equilibrium value in 9.68 s at nitric acid concentration of 0.01 M, while at nitric acid concentrations of 2 M and 3 M, it reached 85% and 91% in 14.52 s, respectively. Overall volumetric mass transfer coefficients decreased from 0.312 s1 to 0.132 s1 with increasing nitric acid concentration at an ionic liquid to aqueous phase flow rate ratio of 1. These values agreed with results obtained in other plug flow microchannel extraction systems. At a smaller phase ratio of 0.25, the mass transfer coefficients decreased. References [1] S. Ansolabehere, J. Deutch, M. Driscoll, P.E. Gray, J.P. Holdren, P.L. Joskow, R.K. Lester, E.J. Moniz, N.E. Todreas, The Future of Nuclear Power: An Interdisciplinary MIT Study, Massachusetts Institute of Technology, Boston, 2003, http://web.mit.edu/nuclearpower/pdf/nuclearpower-full.pdf. [2] J.M. Deutch, C.W. Forsberg, A.C. Kadak, M.S. Kazimi, E.J. Moniz, J.E. Parsons, Update of the MIT 2003 Future of Nuclear Power Study: An Interdisciplinary MIT Study, Massachusetts Institute of Technology, Boston, 2009, http:// web.mit.edu/nuclearpower/pdf/nuclearpower-update2009.pdf. [3] P. Giridhar, K.A. Venkatesan, S. Subramaniam, T.G. Srinivasan, P.R.V. Rao, Extraction of uranium(VI) by 1.1 M tri-n-butylphosphate/ionic liquid and the feasibility of recovery by direct electrodeposition from organic phase, J. Alloys Compd. 448 (2008) 104–108. [4] N. Stern, The Economics of Climate Change: The Stern Review, Cambridge University Press, Cambridge, 2007. [5] K.R. Seddon, Ionic liquids for clean technology, J. Chem. Technol. Biotechnol. 68 (4) (1997) 351–356. [6] M. Freemantle, An Introduction to Ionic Liquids, RSC Publications, Cambridge, UK, 2010. [7] A. Stark, K.R. Seddon, Ionic Liquids, in Kirk-Othmer Encyclopaedia of Chemical Technology, 5th ed., in: A. Seidel (Ed.), vol. 26, John Wiley & Sons, Inc., Hoboken, New Jersey, 2007, pp. 836–920. [8] N.V. Plechkova, K.R. Seddon, Applications of ionic liquids in the chemical industry, Chem. Soc. Rev. 37 (2008) 123–150. [9] D. Allen, G. Baston, A.E. Bradley, T. Gorman, A. Haile, I. Hamblett, J.E. Hatter, M.J.F. Healey, B. Hodgson, R. Lewin, K.V. Lovell, B. Newton, W.R. Pitner, D.W. Rooney, D. Sanders, K.R. Seddon, H.E. Sims, R.C. Thied, An investigation of the radiochemical stability of ionic liquids, Green Chem. 4 (2) (2002) 152–158. [10] G.M.N. Baston, A.E. Bradley, T. Gorman, I. Hamblett, C. Hardacre, J.E. Hatter, M.J.F. Healy, B. Hodgson, R. Lewin, K.V. Lovell, G.W.A. Newton, M. Nieuwenhuyzen, W.R. Pitner, D.W. Rooney, D. Sanders, K.R. Seddon, H.E. Sims, R.C. Thied, Ionic Liquids for the Nuclear Industry: A Radiochemical, Structural, and Electrochemical Investigation, in: R.D. Rogers, K.R. Seddon
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