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Industrial countercurrent chromatography separations based on a cascade of centrifugal mixer-settler extractors
Journal of Chromatography A, 1572 (2018) 212–216
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Short communication
Industrial countercurrent chromatography separations based on a cascade of centrifugal mixer-settler extractors Artak E. Kostanyan ∗ , Andrey A. Erastov Kurnakov Institute of General & Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 31, Moscow 119991, Russia
a r t i c l e
i n f o
Article history: Received 10 July 2018 Received in revised form 10 August 2018 Accepted 18 August 2018 Available online 20 August 2018 Keywords: Counter-current chromatography Solvent extraction Centrifugal mixer-settler extractors Rare-earth metal separation
a b s t r a c t In hydrometallurgy, traditional extraction technologies, in particular, for isolation and purification of rareearth metals include a number of processing steps using up to hundreds of mixer-settler extractors. These technologies could be greatly simplified by using the methods of countercurrent chromatography (CCC) separation. However, the current CCC equipment cannot process large volumes of feed material formed during the industrial production of these metals. In this paper, the cascade of centrifugal mixer-settler extractors assembled as a multi-stage unit is suggested for industrial application of CCC and discussed. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Separation processes largely determine the quality of the products in the production of organic and inorganic materials. An urgent problem is the production of pure and ultrapure substances. To address this issue, new, highly efficient processes of separation and purification of substances are being developed. Countercurrent chromatography (CCC) and centrifugal partition chromatography (CPC) separation processes, collectively called countercurrent chromatography (CCC) [1–13], combine the features of solvent extraction and partition chromatography [14,15]. Both separation methods are based on the different distribution of compounds of a mixture between two immiscible liquids, and the process of separation is controlled by the rates of interphase mass transfer and longitudinal mixing. These methods differ in the scale of the apparatus and in the mode of the separation process. Countercurrent extraction processes are usually carried out in the columns of large diameter or in a cascade of mixer settlers under steady state conditions. In CCC, to hold one of the phases stationary in a chromatography column, centrifugal devices of complex construction are used: in hydrostatic devices, a cascade of chambers is placed in a conventional centrifuge; in hydrodynamic devices, a tube in a spiral shape is wound in one or several layers onto the
∗ Corresponding author. E-mail address: [email protected] (A.E. Kostanyan). https://doi.org/10.1016/j.chroma.2018.08.039 0021-9673/© 2018 Elsevier B.V. All rights reserved.
drum of a planetary centrifuge. A distinctive feature of chromatographic separations is non-steady state mass transfer where the sample is introduced into the column for a relatively short time. In CCC devices, the interphase mass transfer is enhanced due to the high degree of contact between the two phases in the alternating centrifugal force field of the coil planet centrifuge. In partition cells of CPC chromatographs with Z-type cells, a sufficient interfacial area is not provided to achieve rapid mass transfer. For example, the separation efficiency of the FCPC Kromaton Technologies apparatus (Annonay, France), containing 1320 partition cells, was equal to only 120 theoretical plates [16]. However, with newest twincell-design, up to 900 theoretical plates have been calculated for a 30 ml Kromaton FCPC rotor [17]. The complexity of centrifugal chromatographic devices imposes restrictions on their scale, and limits the productivity of the separation processes. Technical limitations of the larger instruments do not allow expanding the scope of application of CCC technologies, for example, in hydrometallurgy for separation of rare-earth metals. The purpose of this paper is to discuss the possibilities of using currently available countercurrent solvent extraction equipment, in particular, a cascade of centrifugal mixer-settler extractors, for conducting chromatographic separation processes including conventional CCC separation and more novel elution modes such as multiple dual mode [18–21] and closed-loop recycling [22–28] CCC and promote the application of CCC in the field of hydrometallurgy.
A.E. Kostanyan, A.A. Erastov / J. Chromatogr. A 1572 (2018) 212–216
2. Countercurrent chromatography separations in a cascade of centrifugal mixer-settler extractors As mentioned above, a disadvantage of chromatographic apparatuses is relatively low throughput when addressing the needs of large volume industries, such as hydrometallurgy. They are incapable of throughput levels achievable by conventional solvent extraction equipment. The performance of mixer-settler extractors is several orders of magnitude higher than that of chromatographs and they are able to handle large volumes of dilute liquors. Multistage cascades of mixer-settler extractors are widely used in hydrometallurgy. A plant producing rare-earth elements may contain hundreds of stages of mixer-settlers [29]. Extractors can be assembled as a multi-stage unit, providing the required number of stages. This does not require inter-stage pumps. A mixer-settler extractor basically consists of two – mixing and settling – chambers. In principle, CCC separations can be carried out in cascades of conventional mixer–settlers. However, in conventional extractors, the separation of the liquid phases occurs in the gravitational field, which causes a large volume of settling
213
chambers. In the case of chromatography, this will enhance the broadening of chromatographic bands as they move along the cascade. Centrifugal extractors are free of this disadvantage; the volumes of mixing and settling chambers are approximately equal. In the mixing chamber – a stirred tank (TsENTREK, Russia [30]; Rousselet Robatel, France (www.rousselet.com – www.rousseletrobatel.com)) or the annular zone between the outside of a hollow rotor and the inside of the outer housing [31–33] – an intense mixing of two immiscible liquids occurs. In the settling chamber (inside the hollow rotor), the two phases are efficient and fast separated by centrifugal forces. The use of the cascade of centrifugal mixer-settlers (CCMS) as a chromatography plant offers a variety of options for performing industrial CCC separation processes. Fig. 1 shows the cascade of centrifugal mixer-settler extractors of the TsENTREK type operating in CCC separation modes. The stirrer 1 and the hollow rotor 2 (centrifugal separator) are mounted on a common shaft. Due to the large interfacial area created by the intense mixing, and the efficient centrifugal phase separation, each stage in the cascade can be considered as an equilibrium (theoretical) stage. In the case when the
Fig. 1. Cascade of centrifugal mixer-settler extractors operating in CCC separation modes: A – conventional isocratic elution mode; B – CCMS CCC separation in the closed-loop with a long recycling pipe; C, D – Craig’s counter-current distribution method.
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A.E. Kostanyan, A.A. Erastov / J. Chromatogr. A 1572 (2018) 212–216
number of stages available is insufficient to achieve the required separation in the conventional elution mode, the efficiency of the cascade can be increased many times using closed-loop recycling and cyclic elution modes [26–28,34,35]. In addition, all the diversity of CCC methods, such as different modifications of multiple dual mode, multi-dimensional CCC [36], gradient elution and pH-zone refining CCC [37], combination of two [38] and more different stationary phases, etc. can be performed in the cascade of centrifugal mixer-settler extractors. Fig. 1B shows the scheme of the CCMS CCC separation in the closed-loop with a long recycling pipe. As was shown earlier [26], this configuration makes it possible to significantly reduce the number of actual stages required for the separation. Furthermore, it allows the simultaneous separation and concentration of the target components from complex mixtures [39]. To retain the stationary phase in the cascade, in conventional isocratic single pass (Fig. 1A) and closed-loop recycling (Fig. 1B) elution modes, this phase is recycled between the mixing and separation chambers in each stage of the cascade. In cyclic dual-mode processes (Fig. 1C,D), light and heavy phases flow in an alternating sequence along the cascade in opposite directions: during the heavy phase flow period, the light phase remains in the stages of the cascade, whereas a volume of the heavy phase equal to its volume in a stage of the cascade is fed into one end and discharged out of the other end of the cascade; in the second half of the cycle, the process is repeated with the heavy phase stationary and the light phase flowing in the opposite direction. This process can be carried out in two versions: with continuous or discrete phase supply to the cascade during their flow periods. In the latter case, the volume of mobile phase on each stage moves stepwise to the adjacent stage during its flow period, and the separation process in the cascade corresponds to Craig’s counter-current distribution method. This process is illustrated in Fig. 1C, D. 3. Experimental 3.1. Apparatus To proof the principle of CCMS CCC, experiments were carried out with two serially connected centrifugal mixer-settler extractors of the TsENTREK type (laboratory model EC33 F from “NIKIMITAtomstroi”, Russia: the volume of the mixing chamber – 90 mL, the volume of the separation chamber – 40 mL) [30]. As noted above, the agitator 1 in the mixing chamber and the hollow rotor 2, which is the phase separation chamber, are fixed to the common shaft (Fig. 1A). 3.2. Two-phase solvent systems To evaluate the performance of the CCMS CCC device, the biphasic solvent system hexane / isopropanol / water with volume ratio of 1:1:1 was applied. 3.3. Experimental procedures As stationary phase retention is of paramount importance for successful performance of the operating schemes A and B (Fig. 1), the retention of the stationary phase in conventional isocratic single pass (Fig. 1A) elution mode was experimentally studied. Two versions were tested: the lower phase is retained in the extractors by circulating it between the mixing and separation chambers; the upper phase is held by circulation between the chambers. Experiments were carried out as follows: the extractors were first filled with the stationary (lower or upper) phase, and then the rotor rotation was set up at the speed needed for the experiment while the mobile (upper or lower) phase was pumped through the device at
Table 1 The results of experiments on stationary phase retention. Solvent system
Rotation speed (rpm)
F (mL/min)
Sf (%) Upper phase mobile Lower phase mobile
Hexaneisopropanolwater
1350 1600 1900 2150 2150 2150 2150 2150
12 12 12 12 6 12 18 24
70 73 75 74 83 81 83 80
30 41 45 60 58 55 55 44
a given flow rate. The displaced stationary phase volume was collected and measured. The influence of the rotor rotation speed and mobile phase flow rate (F) on stationary phase retention (Sf ) was studied. Two experiments were conducted to test the chromatographic behavior of caffeine and coumarin. 4. Results and discussion The results of experiments on stationary phase retention are shown in Table 1. As seen, the stationary phase retention and its dependence on the rotor rotation speed and the mobile phase flow rate are different for lower and upper stationary phase conditions. This is probably due to the design features of the tested extractors, in particular the system for outputting phase flows from the separation chamber. If there is further technical development, the CCMS CCC equipment can be improved (the design of the chambers and the outlet ducts can be substantially simplified) to implement the operating schemes A and B (Fig. 1). The necessity of further development of the design of centrifugal extractors is shown in [40–42]. To implement the process schemes C and D at the existing countercurrent extraction plants, it will only be necessary to modify the system for supplying the phase flows to the plant. Fig. 2 shows the elution curves of caffeine and coumarin (lower phase mobile) obtained in the cascade of two extractors. The equation of a peak eluted from two serially connected equilibrium stages can be presented as: X = 4a2 t exp(−2at),
a=
1 1 − Sf + Sf KD
(1)
where t = (F/V) is the dimensionless time, F is the volumetric flow rate of the mobile phase, V is the volume of the two stages, is the actual time; X = x/¯x is the dimensionless concentration of a solute in the mobile phase, x¯ = Q/V is the average concentration of the solute in the instrument, Q is the amount of the solute in the sample injected. In Fig. 2 the experimental chromatograms are compared with those simulated by Eq. (1). The partition coefficient (KD ) of the solutes (caffeine, KD = 0.21; coumarin, KD = 0.86) were determined experimentally on the controlled-cycle pulsed liquid–liquid chromatography installation [35]. As shown in Fig. 2, both solutes are eluted according to their distribution constants. The results of experiments presented above demonstrate that the cascade of centrifugal extractors can indeed operate as a countercurrent chromatography machine. 5. Conclusion and future work The on CCMS based CCC can be used not only for the industrial separation and purification of natural products, but it can also find wide application in the production of inorganic materials. The development of CCMS CCC technology due to its advantages,
A.E. Kostanyan, A.A. Erastov / J. Chromatogr. A 1572 (2018) 212–216
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Fig. 2. Comparison of theoretical (1) and experimental (2) chromatograms of caffeine (A) and coumarin (B), obtained in a cascade of two extractors of the TsENTREK type (C), operating in the isocratic elution mode (Fig. 1A). Process parameters: Sf = 0.6; A – KD = 0.21; B – KD = 0.86.
such as the ability to handle large volumes of dilute liquors; fewer geometry, solvent system and phase holdups restrictions and high throughput compared with conventional CCC; ease of scaling (the eluting profiles of solutes can be predicted from the partition coefficients) and operating (the operation of the cascade can be stopped and resumed without interfering with the separation process); high recovery of target compounds in a single processing step by using optimal operating mode (traditional solvent extraction methods require several process steps) can make this technology one of the most appropriate commercial technologies for chromatographic separation of rare-earths. The chromatographic operation modes of the cascade of centrifugal mixer-settlers have still to be experimentally investigated. Investigations are to be done to estimate the optimal operating conditions and technical development of the CCMS CCC separation technology. The development of CCMS CCC instrumentation can enable the application of CCC methods on an industrial scale in the production of organic and inorganic materials.
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[25] J. Meng, Z. Yang, J. Liang, H. Zhou, S. Wu, Multi-channel recycling counter-current chromatography for natural product isolation: tensions as examples, J. Chromatogr. A 1327 (2014) 27–38. [26] A.E. Kostanyan, A.A. Erastov, Theoretical study of closed-loop recycling liquid-liquid chromatography and experimental verification of the theory, J. Chromatogr. A 1462 (2016) 55–62. [27] A.E. Kostanyan, Modeling of preparative closed-loop recycling liquid-liquid chromatography with specified duration of sample loading, J. Chromatogr. A 1471 (2016) 94–101. [28] A.E. Kostanyan, Theoretical study of separation and concentration of solutes by closed-loop recycling liquid-liquid chromatography with multiple sample injection, J. Chromatogr. A 1506 (2017) 82–93. [29] F. Xie, T.A. Zhang, D. Dreisinger, F. Doyle, A critical review on solvent extraction of rare earths from aqueous solutions, Miner. Eng. 56 (2014) 10–28. [30] G.I. Kuznetsov, A.A. Pushkov, A.V. Kosogorov, TsENTREKS Centrifugal Extractors, RKhTU im. D.I.Mendeleeva, Moscow, 2000 [in Russian]. [31] W. Xu, S. Wang, G. Dai, K. Tang, P. Zhang, B. Xiong, Y. Liu, Construction and application of a model on the resolution of tropic acid enantiomers by enantioselective liquid-liquid extraction in centrifugal contactor separators, Process. Biochem. 59 (2017) 150–158. [32] M. Ilmi, A. Kloekhorst, J.G.M. Winkelman, G.J.W. Euverink, C. Hidayat, H.J. Heeres, Process intensification of catalytic liquid-liquid solid processes: continuous biodiesel production using an immobilized lipase in a centrifugal contactor separator, Chem. Eng. J. 321 (2017) 76–85. [33] X. Jing, J. Wanga, H. Cao, P. Ning, Z. Sun, Rapid selective extraction of V(V) from leaching solution using annular centrifugal contactors and stripping for NH4 VO3 , Sep. Purif. Technol. 187 (2017) 407–414. [34] A.E. Kostanyan, Controlled-cycle counter-current chromatography, J. Chromatogr. A 1211 (2008) 55–59. [35] A.E. Kostanyan, A.A. Voshkin, N.V. Kodin, Controlled-cycle pulsed liquid–liquid chromatography. A modified version of Craig’s counter-current distribution, J. Chromatogr. A 1218 (2011) 6135–6143. [36] J. Meng, Z. Yang, J. Liang, H. Zhou, S. Wu, Comprehensive multi-channel multi-dimensional counter-current chromatography for separation of tanshinones from Salvia miltiorrhiza Bunge, J. Chromatogr. A 1323 (2014) 73–81. [37] Y. Ito, pH-zone-refining counter-current chromatography: origin, mechanism, procedure and applications, J. Chromatogr. A 1271 (2013) 71–85. [38] G. D’Orazio, S. Fanali, Combination of two different stationary phases for on-line pre-concentration and separation of basic drugs by using nano-liquid chromatography, J. Chromatogr. A 1285 (2013) 118–123. [39] A. Kostanyan, M. Martynova, A. Erastov, V. Belova, Simultaneous concentration and separation of target compounds from multicomponent mixtures by closed-loop recycling countercurrent chromatography, J. Chromatogr. A 1560 (2018) 26–34. [40] B. Schuur, A.J. Hallett, J.G.M. Winkelman, J.G. de Vries, H.J. Heeres, Scalable enantioseparation of amino acid derivatives using continuous liquid-liquid extraction in a cascade of centrifugal contactor separators, Org. Process Res. Dev. 13 (2009) 911–914. [41] K.E. Wardle, Liquid–liquid mixing studies in annular centrifugal contactors comparing stationary mixing vane options, Solvent Extr. Ion Exch. 00 (2015) 1–20. [42] K.E. Wardle, U.S. Pat. US9,744,476 (2017).
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Short communication
Industrial countercurrent chromatography separations based on a cascade of centrifugal mixer-settler extractors Artak E. Kostanyan ∗ , Andrey A. Erastov Kurnakov Institute of General & Inorganic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 31, Moscow 119991, Russia
a r t i c l e
i n f o
Article history: Received 10 July 2018 Received in revised form 10 August 2018 Accepted 18 August 2018 Available online 20 August 2018 Keywords: Counter-current chromatography Solvent extraction Centrifugal mixer-settler extractors Rare-earth metal separation
a b s t r a c t In hydrometallurgy, traditional extraction technologies, in particular, for isolation and purification of rareearth metals include a number of processing steps using up to hundreds of mixer-settler extractors. These technologies could be greatly simplified by using the methods of countercurrent chromatography (CCC) separation. However, the current CCC equipment cannot process large volumes of feed material formed during the industrial production of these metals. In this paper, the cascade of centrifugal mixer-settler extractors assembled as a multi-stage unit is suggested for industrial application of CCC and discussed. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Separation processes largely determine the quality of the products in the production of organic and inorganic materials. An urgent problem is the production of pure and ultrapure substances. To address this issue, new, highly efficient processes of separation and purification of substances are being developed. Countercurrent chromatography (CCC) and centrifugal partition chromatography (CPC) separation processes, collectively called countercurrent chromatography (CCC) [1–13], combine the features of solvent extraction and partition chromatography [14,15]. Both separation methods are based on the different distribution of compounds of a mixture between two immiscible liquids, and the process of separation is controlled by the rates of interphase mass transfer and longitudinal mixing. These methods differ in the scale of the apparatus and in the mode of the separation process. Countercurrent extraction processes are usually carried out in the columns of large diameter or in a cascade of mixer settlers under steady state conditions. In CCC, to hold one of the phases stationary in a chromatography column, centrifugal devices of complex construction are used: in hydrostatic devices, a cascade of chambers is placed in a conventional centrifuge; in hydrodynamic devices, a tube in a spiral shape is wound in one or several layers onto the
∗ Corresponding author. E-mail address: [email protected] (A.E. Kostanyan). https://doi.org/10.1016/j.chroma.2018.08.039 0021-9673/© 2018 Elsevier B.V. All rights reserved.
drum of a planetary centrifuge. A distinctive feature of chromatographic separations is non-steady state mass transfer where the sample is introduced into the column for a relatively short time. In CCC devices, the interphase mass transfer is enhanced due to the high degree of contact between the two phases in the alternating centrifugal force field of the coil planet centrifuge. In partition cells of CPC chromatographs with Z-type cells, a sufficient interfacial area is not provided to achieve rapid mass transfer. For example, the separation efficiency of the FCPC Kromaton Technologies apparatus (Annonay, France), containing 1320 partition cells, was equal to only 120 theoretical plates [16]. However, with newest twincell-design, up to 900 theoretical plates have been calculated for a 30 ml Kromaton FCPC rotor [17]. The complexity of centrifugal chromatographic devices imposes restrictions on their scale, and limits the productivity of the separation processes. Technical limitations of the larger instruments do not allow expanding the scope of application of CCC technologies, for example, in hydrometallurgy for separation of rare-earth metals. The purpose of this paper is to discuss the possibilities of using currently available countercurrent solvent extraction equipment, in particular, a cascade of centrifugal mixer-settler extractors, for conducting chromatographic separation processes including conventional CCC separation and more novel elution modes such as multiple dual mode [18–21] and closed-loop recycling [22–28] CCC and promote the application of CCC in the field of hydrometallurgy.
A.E. Kostanyan, A.A. Erastov / J. Chromatogr. A 1572 (2018) 212–216
2. Countercurrent chromatography separations in a cascade of centrifugal mixer-settler extractors As mentioned above, a disadvantage of chromatographic apparatuses is relatively low throughput when addressing the needs of large volume industries, such as hydrometallurgy. They are incapable of throughput levels achievable by conventional solvent extraction equipment. The performance of mixer-settler extractors is several orders of magnitude higher than that of chromatographs and they are able to handle large volumes of dilute liquors. Multistage cascades of mixer-settler extractors are widely used in hydrometallurgy. A plant producing rare-earth elements may contain hundreds of stages of mixer-settlers [29]. Extractors can be assembled as a multi-stage unit, providing the required number of stages. This does not require inter-stage pumps. A mixer-settler extractor basically consists of two – mixing and settling – chambers. In principle, CCC separations can be carried out in cascades of conventional mixer–settlers. However, in conventional extractors, the separation of the liquid phases occurs in the gravitational field, which causes a large volume of settling
213
chambers. In the case of chromatography, this will enhance the broadening of chromatographic bands as they move along the cascade. Centrifugal extractors are free of this disadvantage; the volumes of mixing and settling chambers are approximately equal. In the mixing chamber – a stirred tank (TsENTREK, Russia [30]; Rousselet Robatel, France (www.rousselet.com – www.rousseletrobatel.com)) or the annular zone between the outside of a hollow rotor and the inside of the outer housing [31–33] – an intense mixing of two immiscible liquids occurs. In the settling chamber (inside the hollow rotor), the two phases are efficient and fast separated by centrifugal forces. The use of the cascade of centrifugal mixer-settlers (CCMS) as a chromatography plant offers a variety of options for performing industrial CCC separation processes. Fig. 1 shows the cascade of centrifugal mixer-settler extractors of the TsENTREK type operating in CCC separation modes. The stirrer 1 and the hollow rotor 2 (centrifugal separator) are mounted on a common shaft. Due to the large interfacial area created by the intense mixing, and the efficient centrifugal phase separation, each stage in the cascade can be considered as an equilibrium (theoretical) stage. In the case when the
Fig. 1. Cascade of centrifugal mixer-settler extractors operating in CCC separation modes: A – conventional isocratic elution mode; B – CCMS CCC separation in the closed-loop with a long recycling pipe; C, D – Craig’s counter-current distribution method.
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number of stages available is insufficient to achieve the required separation in the conventional elution mode, the efficiency of the cascade can be increased many times using closed-loop recycling and cyclic elution modes [26–28,34,35]. In addition, all the diversity of CCC methods, such as different modifications of multiple dual mode, multi-dimensional CCC [36], gradient elution and pH-zone refining CCC [37], combination of two [38] and more different stationary phases, etc. can be performed in the cascade of centrifugal mixer-settler extractors. Fig. 1B shows the scheme of the CCMS CCC separation in the closed-loop with a long recycling pipe. As was shown earlier [26], this configuration makes it possible to significantly reduce the number of actual stages required for the separation. Furthermore, it allows the simultaneous separation and concentration of the target components from complex mixtures [39]. To retain the stationary phase in the cascade, in conventional isocratic single pass (Fig. 1A) and closed-loop recycling (Fig. 1B) elution modes, this phase is recycled between the mixing and separation chambers in each stage of the cascade. In cyclic dual-mode processes (Fig. 1C,D), light and heavy phases flow in an alternating sequence along the cascade in opposite directions: during the heavy phase flow period, the light phase remains in the stages of the cascade, whereas a volume of the heavy phase equal to its volume in a stage of the cascade is fed into one end and discharged out of the other end of the cascade; in the second half of the cycle, the process is repeated with the heavy phase stationary and the light phase flowing in the opposite direction. This process can be carried out in two versions: with continuous or discrete phase supply to the cascade during their flow periods. In the latter case, the volume of mobile phase on each stage moves stepwise to the adjacent stage during its flow period, and the separation process in the cascade corresponds to Craig’s counter-current distribution method. This process is illustrated in Fig. 1C, D. 3. Experimental 3.1. Apparatus To proof the principle of CCMS CCC, experiments were carried out with two serially connected centrifugal mixer-settler extractors of the TsENTREK type (laboratory model EC33 F from “NIKIMITAtomstroi”, Russia: the volume of the mixing chamber – 90 mL, the volume of the separation chamber – 40 mL) [30]. As noted above, the agitator 1 in the mixing chamber and the hollow rotor 2, which is the phase separation chamber, are fixed to the common shaft (Fig. 1A). 3.2. Two-phase solvent systems To evaluate the performance of the CCMS CCC device, the biphasic solvent system hexane / isopropanol / water with volume ratio of 1:1:1 was applied. 3.3. Experimental procedures As stationary phase retention is of paramount importance for successful performance of the operating schemes A and B (Fig. 1), the retention of the stationary phase in conventional isocratic single pass (Fig. 1A) elution mode was experimentally studied. Two versions were tested: the lower phase is retained in the extractors by circulating it between the mixing and separation chambers; the upper phase is held by circulation between the chambers. Experiments were carried out as follows: the extractors were first filled with the stationary (lower or upper) phase, and then the rotor rotation was set up at the speed needed for the experiment while the mobile (upper or lower) phase was pumped through the device at
Table 1 The results of experiments on stationary phase retention. Solvent system
Rotation speed (rpm)
F (mL/min)
Sf (%) Upper phase mobile Lower phase mobile
Hexaneisopropanolwater
1350 1600 1900 2150 2150 2150 2150 2150
12 12 12 12 6 12 18 24
70 73 75 74 83 81 83 80
30 41 45 60 58 55 55 44
a given flow rate. The displaced stationary phase volume was collected and measured. The influence of the rotor rotation speed and mobile phase flow rate (F) on stationary phase retention (Sf ) was studied. Two experiments were conducted to test the chromatographic behavior of caffeine and coumarin. 4. Results and discussion The results of experiments on stationary phase retention are shown in Table 1. As seen, the stationary phase retention and its dependence on the rotor rotation speed and the mobile phase flow rate are different for lower and upper stationary phase conditions. This is probably due to the design features of the tested extractors, in particular the system for outputting phase flows from the separation chamber. If there is further technical development, the CCMS CCC equipment can be improved (the design of the chambers and the outlet ducts can be substantially simplified) to implement the operating schemes A and B (Fig. 1). The necessity of further development of the design of centrifugal extractors is shown in [40–42]. To implement the process schemes C and D at the existing countercurrent extraction plants, it will only be necessary to modify the system for supplying the phase flows to the plant. Fig. 2 shows the elution curves of caffeine and coumarin (lower phase mobile) obtained in the cascade of two extractors. The equation of a peak eluted from two serially connected equilibrium stages can be presented as: X = 4a2 t exp(−2at),
a=
1 1 − Sf + Sf KD
(1)
where t = (F/V) is the dimensionless time, F is the volumetric flow rate of the mobile phase, V is the volume of the two stages, is the actual time; X = x/¯x is the dimensionless concentration of a solute in the mobile phase, x¯ = Q/V is the average concentration of the solute in the instrument, Q is the amount of the solute in the sample injected. In Fig. 2 the experimental chromatograms are compared with those simulated by Eq. (1). The partition coefficient (KD ) of the solutes (caffeine, KD = 0.21; coumarin, KD = 0.86) were determined experimentally on the controlled-cycle pulsed liquid–liquid chromatography installation [35]. As shown in Fig. 2, both solutes are eluted according to their distribution constants. The results of experiments presented above demonstrate that the cascade of centrifugal extractors can indeed operate as a countercurrent chromatography machine. 5. Conclusion and future work The on CCMS based CCC can be used not only for the industrial separation and purification of natural products, but it can also find wide application in the production of inorganic materials. The development of CCMS CCC technology due to its advantages,
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215
Fig. 2. Comparison of theoretical (1) and experimental (2) chromatograms of caffeine (A) and coumarin (B), obtained in a cascade of two extractors of the TsENTREK type (C), operating in the isocratic elution mode (Fig. 1A). Process parameters: Sf = 0.6; A – KD = 0.21; B – KD = 0.86.
such as the ability to handle large volumes of dilute liquors; fewer geometry, solvent system and phase holdups restrictions and high throughput compared with conventional CCC; ease of scaling (the eluting profiles of solutes can be predicted from the partition coefficients) and operating (the operation of the cascade can be stopped and resumed without interfering with the separation process); high recovery of target compounds in a single processing step by using optimal operating mode (traditional solvent extraction methods require several process steps) can make this technology one of the most appropriate commercial technologies for chromatographic separation of rare-earths. The chromatographic operation modes of the cascade of centrifugal mixer-settlers have still to be experimentally investigated. Investigations are to be done to estimate the optimal operating conditions and technical development of the CCMS CCC separation technology. The development of CCMS CCC instrumentation can enable the application of CCC methods on an industrial scale in the production of organic and inorganic materials.
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