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Creating efficient novel aqueous two-phase systems: Salting-out effect and high solubility of salt
Fluid Phase Equilibria 490 (2019) 77e85
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Creating efficient novel aqueous two-phase systems: Salting-out effect and high solubility of salt Chuhan Fu a, Wenli Song b, Conghua Yi b, Shaoqu Xie a, * a b
The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, USA School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou, 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 December 2018 Received in revised form 28 February 2019 Accepted 4 March 2019 Available online 7 March 2019
In light of the advantages of liquid-liquid extraction, it is important to reduce the mutual solubility of extractant and solvent in the liquid feed. Highly efficient novel aqueous two-phase systems composed of 2-propanol and water were created by the introduction of highly soluble salts at high salt concentrations. The water þ 2-propanol þ K2HPO4/K4P2O7 systems are recommended for the extraction of the valueadded chemicals since they lead to trace amounts of 2-propanol in the aqueous phase and salt in the organic phase. The e-NRTL-RK model was employed to correlate the aqueous two-phase systems (organic phase and aqueous phase) with high accuracy. With the roles of the salting-out effects and the solubility of the salt in mind, we are able to exploit the highly efficient novel aqueous two-phase systems composed of 2-propanol, water, and other highly soluble salts. © 2019 Elsevier B.V. All rights reserved.
Keywords: Salting-out effect Aqueous two-phase system 2-Propanol Recovery Highly soluble salts
1. Introduction The traditional aqueous two-phase systems are composed of two polymers and water, or one polymer, one inorganic salt and water [1], which were widely used for separation of cells, membranes, enzymes, viruses, proteins, nucleic acids, and other biomolecules [2]. Recently the novel aqueous two-phase systems composed of a small organic molecule, water, and a salt/sugar are on the rise because of the flourishing production of bio-based chemicals and biofuels from renewable resources [3e6]. For example, when methanol, ethanol [7], or other hydrophilic organic solvent is mixed with water and an inorganic salt at an appropriate concentration, there is a liquid-liquid phase splitting, thus forming a new type of aqueous two-phase system. Compared with the traditional extraction and other separation techniques, the novel aqueous two-phase system is featured with the advantages of mild operating conditions, large handling capacity and easy continuous operation on any scale. It is generally believed that the salting-out effects of the inorganic salts induce the liquid-liquid phase splitting [8e10]. Due to inefficiency of common salts to create phase separation between
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Xie). https://doi.org/10.1016/j.fluid.2019.03.002 0378-3812/© 2019 Elsevier B.V. All rights reserved.
water and a strong polar solvent [11,12], i.e. methanol or ethanol, it is indispensable to turn to the highly efficient aqueous two-phase system to avoid the solvent loss. The employment of propanol or butanol þ water þ salt system, especially the 2-propanol þ water þ salt system is the most popular method used for the separation of metal ions from industrial and waste solutions [13,14]. Liquid-liquid equilibria (LLE) for the 2-propanol þ water þ ordinary salt (easily soluble) systems were widely studied. These salts included Na2SO3 [15], NaCl [15e19], KCl [18], K2SO4 [20], Li2SO4 [21], MgSO4/ZnSO4 [22], Na2CO3 [23], C2K2O4 [24], Na2S2O3 [25]. After the addition of these salts into the aqueous 2-propanol solution, only a portion of 2-propanol was salted out due to the limited salting-out effects. LLE for 2-propanol þ water þ highly soluble salt systems were also well investigated at different temperatures. These highly soluble salts (highly soluble) included KF [26], CsCl [27], (NH4)2SO4 [28], K2CO3 [23,29], LiCl [30], KCl þ CsCl [31], Na2C6H6O7 [32], K2HPO4 [33]. However, these 2-propanol þ water þ highly soluble salt systems were either created at low salt concentrations or showed mixed solvation effects [34,35] which featured these systems with high salt contents of the organic phase and high solvents contents of the aqueous phase. As a result, the losses of 2propanol and the salt during the salting-out extraction cannot be avoided, suggesting that this unit operation burdens the whole separation units with extra costs. In this work, the highly efficient aqueous two-phase systems
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C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
composed of 2-propanol þ water þ highly soluble salts systems were investigated at low and high salt concentrations. The 2propanol recovery and the water removal were used to evaluate the efficiency of the aqueous two-phase systems. Tie-line data were correlated by eNRTL-RK model which represented the reality of the experimental results. Last but not least, the salting-out effects of the highly soluble salts on 2-propanol were also discussed and compared to other results. 2. Experimental 2.1. Materials Tripotassium phosphate trihydrate (K3PO4$3H2O) was purchased from Guangzhou Chemical Reagent Factory. Potassium carbonate (K2CO3) was supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. Dipotassium hydrogen phosphate trihydrate (K2HPO4$3H2O) was purchased from Guanghua Chemical Plants Co. Ltd. Pyrophosphate potassium trihydrate (K4P2O7$3H2O) was purchased from Tianjin Damao Chemical Plants Co. Ltd. Potassium chloride (KCl) was purchased from Shanghai Aladdin Bio-Chem Technology Co.,LTD. The HPLC-grade 2-propanol was obtained from Guanghua Chemical Plants Co. Ltd. All the chemicals were used as received and their purities were shown in Table 1. Deionized water was prepared throughout the Ultrapure Water system in our lab. The purities of the salts were confirmed by the flame atomic absorption spectrometry (FAAS) in our previous studies [7,8,36]. Herein, flame atomic absorption spectrometry was used for purity assessment again. Anhydrous salt content was determined by FAAS. The purity of 2-propanol was stated by the supplier and confirmed by gas chromatography (GC) analysis (GC-TCD). 2.2. General procedure for the salting-out experiments 20 mL headspace bottles with tan PTFE/silicone septa and aluminium caps were used to perform the salting-out experiments [36]. 2-Propanol, water, and salt were successively added to the headspace bottles in terms of different known initial salt concentrations. The controlled initial salt concentration (CI) has the following expression,
msalt ðgÞ 1000 CI ¼ msalt ðgÞ þ mwater ðgÞ
(1)
where msalt is the mass of an anhydrous salt, and mwater is the mass of water in the whole system. For the better phase splitting, the mass ratio of 2-propanol to the deionized water was controlled under 4:6. Then the headspace bottles were sealed. For the determination of the mutual solubility data, the headspace bottles were shaken for 1 h at room temperature to enable the ternary system to move sufficiently and settled at a controlled
temperature of 298.15 K in a water bath for 24 h. The effect of temperature on system 2-methyl-2-propanol þ H2O þ K2CO3 was insignificantly [29], thus the equilibria were established at 298.15 K [27]. 2.3. Analysis After the equilibrium, 0.6 mL of the 2-propanol-rich and waterrich phases were withdrawn by microsyringe for gas chromatography (GC) analysis. GC (Techcomp GC7900, China) was equipped with a 2 m(L) 3 mm (ID) 5 mm (OD) Porapak Q 80e100 mesh packed column and a thermal conductivity detector (TCD). The water and 2-propanol contents of two phases were determined by the area normalization method [37,38]. After the liquid-liquid phase splitting, the salt content of the 2-propanol-rich phase was determined by flame atomic absorption spectrometry (FAAS) at the wavelength of 766.5 nm. The salt concentrations of the aqueous phase were computed by difference. The titration method was also used for the determination of potassium carbonate in the organic phase using automatic potentiometric titrator (905 Titrando, Metrohm Corp., Switzerland) [29]. The results showed a good agreement with those using FAAS. The phosphates of the organic were also analyzed using a Basic IC 792 ion chromatograph. The results also showed a good agreement with those using FAAS. The salt content in the materials was measured at the subsensitive resonance line (404.5 nm) of potassium by flame atomic absorption spectrometry which adopted an external standard method [39]. Samples were pretreated with nitric acid (water: nitric acid ¼ 1:1, V:V) and cesium nitrate (10 g/L). Cesium nitrate was used as an ionization buffer. KCl was dried at 500 C for 6 h and used to prepare the K standard solutions. The potassium salts were then dissolved in the deionized water to confirm the salt content. These results verified that K2HPO4$3H2O, K3PO4$3H2O, and K4P2O7$3H2O were stable at room temperature. 3. Results and discussion 3.1. Liquid-liquid equilibria (LLE) data As the simplest example of a secondary alcohol, 2-propanol is miscible in water but not miscible with saline solutions due to the salting-out effect. After the addition of a salt to a 2-propanol aqueous solution, the 2-propanol was separated into a distinct layer (an organic phase) with a certain amount of water and salt. Most of salt and water retained in water and form the aqueous phase. As a result, the aqueous two-phase system formed for the salting-out extraction of some extractable substrates. First of all, the LLE data for the water þ 2-propanol þ salt ternary system at 298.15 K were investigated in great detail. The composition of each phase as a function of initial salt concentration was
Table 1 Chemicals in this study. Reagents
CAS number
Supplier
Purity (mass fraction)
Anhydrous salt content (mass fraction)
2-Propanol K2HPO4$3H2O K3PO4$3H2O K2CO3 K4P2O7$3H2O KCl Deionized water
67-63-0 16788-57-1 7778-53-2 584-08-7 7320-34-5 7447-40-7 7732-18-5
Guanghua Chemical Plants Co. Ltd. Guanghua Chemical Plants Co. Ltd. Guangzhou Chemical Reagent Factory Shanghai Lingfeng Chemical Reagent Co., Ltd Tianjin Damao Chemical Plants Co. Ltd. Shanghai Aladdin Bio-Chem Technology Co.,LTD. Ultrapure Water system in our lab
0.997 0.99 0.99 0.99 0.99 0.998 （Electrical conductivity ＜1.5 104 S m1）
e 0.756 0.789 0.991 0.851 e e
Note: The purities of K2HPO4$3H2O, K3PO4$3H2O, K2CO3, K4P2O7$3H2O were stated by the supplier and confirmed by FAAS. Herein KCl was used as the standard. The purity of 2-propanol was stated by the supplier and confirmed by GC-TCD.
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85 Table 2 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K2CO3) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500
0.6000 0.4622 0.3787 0.3092 0.2493 0.2010 0.1576 0.1217 0.0907 0.0662
0.4000 0.5251 0.6158 0.6887 0.7499 0.7987 0.8423 0.8782 0.9093 0.9338
0.0000 0.0127 0.0054 0.0021 0.0008 0.0003 0.0001 0.0001 0.0001 0.0001
0.6000 0.7580 0.7492 0.7238 0.6936 0.6568 0.6174 0.5759 0.5328 0.4879
0.4000 0.0824 0.0411 0.0238 0.0116 0.0059 0.0032 0.0012 0.0003 0.0001
0.0000 0.1596 0.2096 0.2524 0.2947 0.3373 0.3794 0.4229 0.4669 0.5120
2-propanol
aqueous phase K2CO3
water
2-propanol
K2CO3
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript
i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
Table 3 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K2HPO4) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500 550 600
0.6000 0.5331 0.4784 0.4226 0.3674 0.3150 0.2641 0.2229 0.1741 0.1353 0.1047 0.0759
0.4000 0.4501 0.5119 0.5719 0.6298 0.6838 0.7355 0.7769 0.8259 0.8646 0.8953 0.9241
0.0000 0.0168 0.0097 0.0054 0.0028 0.0013 0.0004 0.0002 0.0001 0.0001 0.0000 0.0000
0.6000 0.7114 0.6990 0.6805 0.6566 0.6280 0.5960 0.5591 0.5220 0.4812 0.4377 0.3929
0.4000 0.0462 0.0328 0.0220 0.0141 0.0083 0.0046 0.0021 0.0010 0.0003 0.0001 0.0000
0.0000 0.2425 0.2681 0.2974 0.3293 0.3637 0.3994 0.4388 0.4769 0.5184 0.5622 0.6071
2-propanol
aqueous phase K2HPO4
water
2-propanol
K2HPO4
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript
i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
Table 4 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K3PO4) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500
0.6000 0.4919 0.4234 0.3512 0.2901 0.2319 0.1841 0.1450 0.1088 0.0810
0.4000 0.4963 0.5711 0.6462 0.7086 0.7678 0.8152 0.8546 0.8909 0.9187
0.0000 0.0118 0.0055 0.0026 0.0014 0.0003 0.0007 0.0003 0.0003 0.0003
0.6000 0.7562 0.7369 0.7166 0.6875 0.6543 0.6168 0.5758 0.5334 0.4888
0.4000 0.0549 0.0329 0.0187 0.0099 0.0049 0.0022 0.0008 0.0002 0.0001
0.0000 0.1890 0.2302 0.2647 0.3026 0.3408 0.3810 0.4234 0.4663 0.5111
2-propanol
aqueous phase K3PO4
water
2-propanol
K3PO4
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript
i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
79
Table 5 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K4P2O7) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500 550
0.6000 0.5197 0.4595 0.3994 0.3329 0.2751 0.2229 0.1702 0.1249 0.0904 0.0627
0.4000 0.4681 0.5341 0.5976 0.6662 0.7246 0.7770 0.8298 0.8751 0.9095 0.9373
0.0000 0.0122 0.0064 0.0030 0.0010 0.0003 0.0001 0.0001 0.0000 0.0000 0.0000
0.6000 0.7282 0.7128 0.6913 0.6687 0.6386 0.6042 0.5681 0.5285 0.4857 0.4410
0.4000 0.0326 0.0229 0.0150 0.0089 0.0047 0.0023 0.0009 0.0003 0.0001 0.0000
0.0000 0.2392 0.2643 0.2936 0.3223 0.3567 0.3936 0.4311 0.4712 0.5142 0.5590
2-propanol
aqueous phase K4P2O7
water
2-propanol
K4P2O7
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
listed in Tables 2e5. The highly soluble salts showed excellent phase splitting ability in the 2-propanol aqueous solutions. LLE data for the water þ 2-propanol þ K2CO3 system (Table 2) shows that the mutual solubility of water and 2-propanol was influenced greatly by the initial salt concentration. With an increasing initial salt concentration (from 100 g/kg to nearing saturation condition), there were sharp decreases in the water content of the organic phase, the 2-propanol content of the aqueous phase and the salt content of the organic phase. By contrast with the control sample, the 2-propanol aqueous solution was very sensitive to the salting-out effect. The ideal aqueous twophase system is to avoid the salt residue in the organic phase and the 2-propanol residue in the aqueous phase. However, in comparison to the low K2CO3 initial concentrations, high K2CO3 initial concentrations, especially the concentration of 500 g/kg made the aqueous two-phase system to be in a near-ideal state. At the concentration of 500 g/kg, only 0.0001 (mass fraction) of salt and 0.0001 (mass fraction) of 2-propanol were detected in the organic phase and aqueous phase respectively. The water content of the organic phase was 0.4622 (mass fraction) at 100 g/kg, whereas it decreased to 0.0662 (mass fraction) at 500 g/kg due to the enhanced salting-out effect. Consequently, the solubility of the salt in the organic phase was reduced with the reduction in the water content of the organic phase. To understand the roles of other highly soluble salts on the separation of 2-propanol from water, the tie-line data of (water þ 2propanol þ K2HPO4/K3PO4/K4P2O7) ternary system were shown in Tables 3e5 respectively. It can be seen that the water contents of the organic phase were lower than 0.10 (mass fraction) under the nearing salt-saturation condition. Interestingly the 2-propanol content of the aqueous phase and the salt content of the organic phase were not detected at the initial K2HPO4 and K4P2O7 concentrations of 600 g/kg and 550 g/kg respectively. In comparison to K2HPO4 and K4P2O7, K3PO4 showed a little weaker desalination of the organic phase when the water content of organic phase was kept below 0.10 (mass fraction). In spite of the similar water content of the organic phase, the water þ 2-propanol þ K2HPO4/K4P2O7 system was superior to the water þ 2-propanol þ K3PO4 in the application of the highly efficient aqueous two-phase systems to extract some value-added chemicals. The water þ 2propanol þ K2HPO4 system is slightly alkaline whereas the water þ 2-propanol þ K4P2O7 system is strongly alkaline. Thus the
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C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
investigations along this direction are required for the high recovery of a target value-added chemical. Although the contents of 2-propanol, water and salt in both phases are different, comparison with published data for validation of results is needed, as shown in Fig. 1. In order to test the reliability of the results, the experimental data of 2-propanol-water-K2CO3/ K2HPO4 system are compared with the reference data [29,33]. Taking the mass fraction of salt as the abscissa and the mass fraction of 2-propanol as the ordinate, the literature data and the results of this experiment were compared, and the results were shown in the insert figures of Fig. 1. The experimental results in our study are in good agreement with the literature data, which proves that the experimental results have a high degree of accuracy. On the other hand, with the comparison of phase diagram results with other lower soluble salts, e.g. Refs. [15,17,21,23,24], the broader twophase regions were observed for the highly soluble salts induced aqueous two-phase systems, which demonstrated that the solubility of salt is very important for creating efficient novel aqueous two-phase systems. 3.2. Separation of 2-propanol and water For the water þ 2-propanol þ salt system, the efficiency of the aqueous two-phase system itself is specifically manifested in the recovery of 2-propanol into the organic phase and the removal of water into the aqueous phase, which is a foundation for enabling and managing the applications of the aqueous two-phase system to the extraction of other chemicals. In general, the 2-propanol recovery and the water removal were expressed in Eq. (2) and (3) respectively,
Recovery ¼
0:4m0 m1 u21 0:4m0
Water removal ¼
(2)
0:6m0 m2 u12 0:6m0
(3)
where m1 is the mass of the aqueous phase, m2 is the mass of the organic phase, m0 is the mass of the initial 2-propanol þ water system before the addition of an inorganic electrolyte, u21 is the 2propanol content of the aqueous phase, and u12 is the water content of the organic phase.
K2CO3 this study K2HPO4 this study K3PO4 this study K4P2O7 this study C2K2O4[24] Na2SO3[15] NaCl[17] Li2SO4[21] Na2CO3[23]
Mass fraction of 2-propanol
1.0
0.8
0.6
3.3. Theoretical calculation of the liquid-liquid equilibria Several models, such as UNIQUAC and NRTL can be applied to correlate the liquid-liquid equilibria so that the reality of the experimental results can be seen [42]. According to the large ionic species in the ternary system, we used e-NRTL model [43] to correlate the LLE of our systems. The data regression was analyzed using Aspen Plus V9 with the Unsymmetric e-NRTL property method (eNRTL-RK). The e-NRTL model for the excess Gibbs free energy is built up from a long range interaction contribution (gex*,pdh) and a short range interaction contribution (gex*,lc), as can be seen in Eq. (4).
gex* g ex*; pdh gex*; lc ¼ þ RT RT RT
(4)
where the notation “*” denotes the unsymmetric convention, gex is the excess Gibbs energy of electrolyte systems, gex,lc is the contribution from the local composition (lc) interactions, gex,pdh is the contribution arising from long-range ioneion interactions using Pitzer-Debye-Huckel (pdh) equations, R is the universal gas constant (8.314 J mol1 K1), T is the temperature (K), and the notation “*” denotes the unsymmetric convention [43]. Accordingly Eq. (4) leads to the following expression for activity coefficients,
K2CO3 this study K2CO3[29]
0.4
After the calculation, the 2-propanol recovery and the water removal as a function of initial salt concentration are presented in Fig. 2. The recovery results show that with increasing addition of an inorganic electrolyte, the recovery of 2-propanol increased gradually except in the cases when the concentrations of K2HPO4 and K4P2O7 were 100 g/kg. This anomaly resulted from the higher water contents of the organic phase thus less water retained in the aqueous phase. As a result, the recovery of 2-propanol increased correspondingly. Although lower salt concentrations achieved considerable recovery of 2-propanol, the salting-out extraction system deviated from the high efficiency, especially for the removal of water, as shown in Fig. 2(b). The lower water content of the organic phase is recommended because it favors the separation of the solvents after the salting-out extraction of some value-added chemicals. An increase in the initial salt concentration improved the dehydration of the organic phase to give purer solvents [41]. The highest conceentration of each salt achieved the highest dehydration ratio. However, the liquid-liquidsolid equilibria for the ternary systems should be avoided due to the additional unit for separating the solid phase. The nearingsaturation conditions, such as at the K2CO3, K3PO4, K4P2O7, and K2HPO4 concentrations of 500 g/kg, 500 g/kg, 550 g/kg, 600 g/kg were therefore chosen as the optimal conditions. At these salt concentrations, the recovery of 2-propanol reached almost 100.00%. At the same time, more than 94% of water was retained in the aqueous phase. The efficiency of the salts used on removing the water was in the following order K2CO3＞K3PO4＞K4P2O7＞K2HPO4 at the same initial salt concentration.
pdh þ ln glc lnðgi Þ ¼ ln gi i
K2HPO4 this study K2HPO4[33]
(5)
where gi is the activity coefficient of the component i in the mixture. The pdh equations for the long-range contribution are used to express the excess Gibbs free energy,
0.2
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Mass fraction of salt Fig. 1. Phase diagram of 2-propanolesalt aqueous two-phase system.
0.7
! X gex*; pdh 1000 1=2 4A∅ Ix 1=2 ¼ ln 1 þ rI x xk Ms RT r
(6)
k
where Ix is the ionic strength parameter, Ms is the molecular weight
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
81
1.0 1.00
(a)
0.99
0.8
Recovery
0.97
K2CO3
0.96
K2HPO4 K3PO4
0.95
K4P2O7
0.94
Water removal
0.98
0.7 0.6
K2CO3
0.5
K2HPO4
0.4
K3PO4 K4P2O7
0.3
0.93 0.92
(b)
0.9
100
200
300
400
500
600
0.2
100
200
300
CI (g/kg)
400
500
600
CI (g/kg)
Fig. 2. 2-Propanol recovery (a) and removal of water (b) as a function of the initial salt concentration.
of the solvent s, and Af is the Debye-Huckel constant for osmotic coefficients, as shown in the following,
X 1 Z 2i xi Ix ¼ 2 3 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 e 2pd0 NA pffiffiffiffiffiffiffiffiffi A∅ ¼ 3 1000 DkT
(8)
For Eq. (6)e(8), i refer to ions, Zi is the absolute value of the charge on the species of i, x is the mole fraction, and r is the closest distance parameter. Moreover, NA is Avogadro's number equal to 6.0232 10 23 mol-1, k is Boltzmann constant, e is the electronic charge, d0 is the mixed solvent density and D is the dielectric constant, respectively. The densities of the solvents are from Aspen Plus. Accordingly Eq. (6) leads to the following expression for activity coefficients,
(14)
1=2
3=2
Z 2i I x 2I x þ 1=2 1 þ rI x
aij ¼ cij þ dij ðT 273:15 KÞ
(15)
Gji ¼ exp atji
(16)
Where t is the dimensionless interaction parameter, i and j refer to different species, a, b, c, d, e are parameters and a is the nonrandomness factor that can be set as a fixed value. But e and d were set to 0 in this study. The calculated excess Gibbs energy values from Eq. (16) can be used to calculate both the overall excess Gibbs energy for the short range interaction contribution (Eq. (17)) and the activity coefficients for each component.
gex*;lc ¼ xm ðxcm þ xam Þtca;m þ xc xmc Zc tm;ca þ xa xma Za tm;ca RT þ xc Zc tm;ca þ Gcm tca;m xa Za tm;ca þ Gma tca;m
2 ! 1000 1=2 4 2Z 2i 1=2 pdh* ¼ ln gi A∅ ln 1 þ rI x Ms r
(17)
3 5
(9) lnglc* c ¼
The short range interaction contribution is based upon the NRTL model (NRTL local interaction contribution),
xcm þ xam þ xmm ¼ 1 xma þ xca ¼ 1
!
(7)
Tref T bij T þ eij þ ln tij ¼ aij þ Tref T 273:15 K T
x2m tcm Gcm
Za xa tma xm Gma ðxc Gcm þ xa Gam þ xm Þ2 ðxc þ xm Gma Þ2 Zc xm tmc Gmc Zc tmc Gcm tcm þ ðxa þ xm Gmc Þ
(18)
(10) (11)
lnglc* a ¼
x2m tam Gam ðxc Gcm þ xa Gam þ xm Þ2
Zc xc tmc xm Gmc ðxa þ xm Gmc Þ2
þ
Za xm tma Gma ðxc þ xm Gma Þ
Za tma Gam tam xmc þ xac ¼ 1
where c is short for cation, a is short for anion, and m is short for solvent (molecule, water or alcohol). G, t and g are energy parameters [43] and given by,
tji ¼
gji gii RT
(19)
(12) lnglc m ¼ xcm tcm þ xam tam þ
Zc xc Gmc tmc xa ðxa þ Gmc xm Þ2
xc xm Gcm tcm ðxc Gcm þ xa Gam þ xm Þ2
(13)
þ
Za xa Gma tma xc
ðxc þ Gma xm Þ2 xa xm Gam tam
ðxc Gcm þ xa Gam þ xm Þ2 (20)
where xij is given by,
82
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
xim ¼
xi Gim ðxa Gam þ xc Gcm þ xm Þ
(21)
xac ¼
xa xa þ xm Gmc;ac
(22)
xca ¼
xc xc þ xm Gma;ca
(23)
3.4. Investigating the salting-out effects using setschenow-based equation
Fortunately, there are following relationships between different
t parameters,
tam ¼ tcm ¼ tac;m
(24)
tmc:ac ¼ tma;ca ¼ tm;ca
(25)
The mixed solvent (water and alcohol) was used, thus the Born correction that uses the dielectric constants for the long range interactions was adapted to calculate the unsymmetric pdh formula,
DGborn RT
¼
reported in Table 7 are shown in Fig. 3. It is concluded that the LLE data for the aqueous two-phase systems composed of 2-propanol, water, and highly soluble salts can be reproduced with excellent accuracy by using the e-NRTL-RK model.
Ne2 1 1 xi Z 2i 2 10 2kT εd εu i ri
(26)
where εw is the dielectric constant of water; and ri is the Born radius of segment species i. The detailed expression for the activity coefficient of segment species i can be found on reference [43]. For the correlation of the tie-line data, the equilibria chemistry included in this analysis is provided in Table 6. We only regressed water - ion pair and 2-propanol - ion pair because the equilibria constant is very large. The undissociated salt could be negligible so that water - undissociated salt and 2-propanol - undissociated salt parameters were kept at zero. The equilibria constant is calculated by,
lnðKeqÞ ¼ A
(27)
where the concentration basis for Keq is mole fraction. Furthermore, the dielectric constants of 2-propanol and water at 25 C were set as 18 and 80.4. There are six interaction parameters (tmca, tcam, twca, tcaw, tmw, and twm) and three non-randomness factors (awca, amw, and amca) for the eNRTL-RK model. But in this study, the amw was set to 0.3 and awca was set to 0.2 [43,44], and m and w represent 2-propanol and water respectively. Other parameters were obtained from the regression of the LLE data. The default objective function in our study was the maximum likelihood objective function. The obtained binary parameters or electrolyte pair parameters using the Maximum likelihood objective function are reported in Table 7. It should be noted that, different non-randomness factors for alcohol and electrolytes in fitting the experimental LLE data obtained in this work are different. Using the parameters reported in Table 7, we could successfully correlate the tie-lines data by the generalized e-NRTL-RK model. Moreover, to show the reliability of the e-NRTL-RK model in correlating the tie-lines data, experimental phase diagram and estimated phase diagram using the parameters
Many salts can induce salting-out phenomena [45] thus the quantitation of the salting-out effect can reflect the nature of the phase splitting phenomena involved in the “salting out” of organic small molecules by inorganic electrolytes. Dating back to 1925, the salting-out of proteins was quantitated by the Setschenow-base equation which is a simple equation derived to describe the relationship between the solubility of inorganic solutes and the concentration of inorganic electrolytes [46]. Molar concentration [47e49], molal concentration [50,51] and mole fraction were widely employed to express salt concentrations. After the fitting of the above-mentioned variants, the empirical Setschenow saltingout constants obtained can quantitatively represent the salting effects. Similar to the salt-induced precipitation, the salting-out of 2propanol was based on the electrolyte-nonelectrolyte interaction in the aqueous saline solution in which 2-propanol was less soluble at a high salt concentration. After the fitting of the logarithm of the solubility of 2-propanol and the salt content by using the empirical Setschenow equation, the salting-out effect on the aqueous 2-propanol solution then can be quantitated [10,52]. The absolute values of the slopes for the Setschenow equations are salting-out constants (also called Setschenow constants) which represent the salting-out effects clearly. Herein the 2-propanol solubility (s21) has the units of g per 100 g water in the saline solution and is expressed by,
s21 ¼
u21 100 u11
(28)
where u21 is the mass fraction of 2-propanol in the saline solution, and u11 is the mass fraction of water in the saline solution. The salt concentration is expressed by molality which has the units of mol per 1 kg water and is expressed by,
bB ¼
u31 1000 Ms u11
(29)
where u31 is the mass fraction of a salt in the saline solution, and Ms is the molar mass of a salt. As reported previously, the liquid-liquid equilibria data from the literature together with our results were fitted through the following Setschenow equation,
lns21 ¼ PbB þ Q
(30)
Where P and Q are constants, especially P is also called the Setschenow constant. The slopes whose absolute values give the salting out effect, together with the corresponding intercepts and the coefficients of determination R2, are shown in Table 8. On the basis of our experiments and previous studies of the salting-out of
Table 6 Chemistry for the data regression. Reaction no.
Specification type
Stoichiometry
Equilibrium constant (A)
1 2 3 4
Equilibrium Equilibrium Equilibrium Equilibrium
K2CO3#2 KþþCO23 K2HPO4#2 KþþHPO24 þ 3K3PO4#3 K þPO4 þ 4K4P2O7#4 K þP2O7
46 38 90 50
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
83
Table 7 Values of parameters for the six interaction parameters of e-NRTL-RK model for 2-propanol (m) þ salt (ca) þ water (w) systems at T ¼ 298.15 Ka. Species
Water(i)þ2-propsanol(j) Water(i)þK2CO3(j) 2-propanol(i)þ K2CO3 (j) Water(i)þ K2HPO4 (j) 2-propanol(i)þ K2HPO4 (j) Water(i)þ K3PO4(j) 2-propanol(i)þ K3PO4 (j) Water(i)þ K4P2O7 (j) 2-propanol(i)þ K4P2O7 (j) a
Binary parameters or electrolyte pair parameters aij
bij
cij
aji
bji
cji
1.219 3.140 19.834 43.609 39.831 20 10.368 13.446 16.550
850.087 2115.072 400.296 9978 6832.53 3654.88 9412.01 430.149 513.844
0.3 0.2 0.0511 0.2 0.0497 0.2 0.0248 0.2 0.0525
1.154 20 18.737 16.007 39.642 18.501 19.708 4.417 10.772
1257.1 7895.2688 2155.091 6579.56 5863.27 7349.23 7397.13 3206.3 2278.33
0.3 0.2 0.0511 0.2 0.0497 0.2 0.0248 0.2 0.0525
Standard uncertainties u are u(T) ¼ 0.05 K.
2-propanol, the natural logarithm of the solubility of 2-propanol was plotted against the molality of salt in aqueous saline solutions (Fig. 4). Both the coefficients of determination R2 and the
setschenow-based plots verified that the salting-out effect could be satisfactorily characterized by Eq. (30). Different salts show different salting-out behavior, suggesting that the salt type is one of
Fig. 3. Experimental and estimated phase diagram for 2-propanol þ salt þ water systems at T ¼ 298.15 K. Experimental tie-lines (symbol) and the calculated ones (lines). At the K4P2O7 concentration of 550 g/kg, the content of the salt in the organic phase and the 2-propanol content of the aqueous phase were too low to be detected so that the LLE data were not included in this Figure.
84
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
Table 8 Constants P, Q and the coefficients of determination R2 after the regression by using Setschenow equation. salt
P
Q
R2
No.a
Reference
K4P2O7 K3PO4 K2HPO4 K2CO3 CsCl KF Na2SO3 LiCl Na2C6H6O7 MgSO4 C2K2O4 Na2S2O3 ZnSO4 NaCl Li2SO4
2.5475 1.7363 1.0166 0.987 0.2093 0.1934 1.4254 0.5451 0.6719 1.2086 1.8581 0.7089 1.0559 0.6541 1.3521
4.0242 4.0065 3.7228 3.6836 3.9039 3.2108 4.5881 12.006 3.8875 4.5485 5.2041 3.7771 4.3568 5.6093 4.5121
0.9998 0.9998 0.9973 0.9918 0.9481 0.9838 0.9694 0.9368 0.999 1.0000 0.9984 0.993 1.0000 0.991 0.9899
9 9 10 9 4 7 3 3 3 3 4 5 3 8 5
This This This This [27] [26] [15] [30] [32] [22] [24] [25] [22] [19] [21]
a
work work work work
Number of data points in the regression.
4
K4P2O7 K3PO4 K2HPO4 K2CO3 CsCl KF Na2SO3 LiCl Na2C6H6O7 MgSO4 C2K2O4 Na2S2O3 ZnSO4 NaCl Li2SO4
3 2
ln (s21)
1 0 -1 -2 -3 -4 -5 -6
0
2
4
6
8
10
12
14
16
18
Molality of salt (mol/kg) Fig. 4. Relationship between the solubility of 2-propanol and the molality of salt in aqueous saline solutions. Except the K4P2O7, K3PO4, K2HPO4, and K2CO3 data were from our study, other data were from Refs. [15,19,21,22,24e27,30,32].
the important factors for the salting-out effect. K4P2O7 shows the greatest absolute value that represents the strongest salting-out ability of the salt for 2-propanol in the studied system, followed by C2K2O4 with the value of 1.8581. It seems that C2K2O4 is also favorable for the 2-propanol separation. However, the solubility of it limited the further ionic hydration [53]. By contrast, K3PO4 shows the similar salting-out effects whereas the solubility of K3PO4 is much greater than that of C2K2O4. As a result, the highly efficient separation can be achieved. Similarly, K2CO3, K2HPO4 are also the alternative bases for the 2-propanol separation from aqueous solutions due to their high solubilities which remedy their salting-out effects, as shown in Fig. 4. With the roles of the salting-out effects and the solubility of the salt in mind, we are able to exploit the highly efficient aqueous two-phase systems composed of 2propanol, water, and highly soluble salts. After the data regression, the 2-propanol solubility in the aqueous K4P2O7, K3PO4, K2HPO4, or K2CO3 solutions is dependent on the salt concentration in the same phase. For the abovementioned order of the salts, the corresponding equations were obtained as Eq. (31) ~ (34),
lns21 ¼ 2:5475bB þ 4:0242
(31)
lns21 ¼ 1:7363bB þ 4:0065
(32)
lns21 ¼ 1:0166bB þ 3:7228
(33)
lns21 ¼ 0:987bB þ 3:6836
(34)
4. Conclusions The liquideliquid equilibrium for 2-propanol, highly soluble salts þ water ternary systems were studied at 298.15 K. It was shown that K4P2O7, K3PO4, K2HPO4, or K2CO3 had greater salting out effects on 2-propanol, especially under nearing saturation conditions which made the organic phase composed of water and 2propanol and the aqueous phase composed of water and salt. The water þ 2-propanol þ K2HPO4/K4P2O7 system was recommended for the extraction of the value-added chemicals from dilute aqueous solutions according to trace amount of 2-propanol in the aqueous phase and salt in the organic phase. The e-NRTL-RK model was used for the correlation of the aqueous two-phase systems composed of 2-propanol, water, and highly soluble salts with high accuracy. After the fitting of the logarithm of the solubility of 2-propanol and the salt content by using the empirical Setschenow equation satisfactorily, the salting-out effect on the aqueous 2-propanol solution were quantitated. According to the Setschenow constant, K4P2O7 showed the strongest salting-out ability of the salt for 2propanol in the studied system. With the roles of the salting-out effects and the solubility of the salt in mind, we are able to exploit the highly efficient aqueous two-phase systems composed of 2-propanol, water, and highly soluble salts. Acknowledgments This work was funded by the Fundamental Research Funds for the Central Universities of China (2015ZM169), the International S&T Cooperation Program of China (2013DFA41670), and the National High-tech R&D Program (863 Program) (863 Program) (No. 2012AA021202). References [1] P.K. Wan, J.C.W. Lan, P.W. Chen, J.S. Tan, H.S. Ng, Recovery of intracellular ectoine from Halomonas salina cells with poly(propylene) glycol/salt aqueous biphasic system, J. Taiwan Inst. Chem. Eng. 82 (2018) 28e32, https://doi.org/ 10.1016/j.jtice.2017.11.003. [2] R. Hatti-Kaul, Aqueous two-phase systems: a general overview, Appl. Biochem. Biotechnol. Part B Mol. Biotechnol. 19 (2001) 269e277, https://doi.org/ 10.1385/MB:19:3:269. [3] J.Y. Dai, Y.Q. Sun, Z.L. Xiu, Separation of bio-based chemicals from fermentation broths by salting-out extraction, Eng. Life Sci. 14 (2014) 108e117, https:// doi.org/10.1002/elsc.201200210. [4] J.Y. Dai, C.J. Liu, Z.L. Xiu, Sugaring-out extraction of 2,3-butanediol from fermentation broths, Process Biochem. 50 (2015) 1951e1957, https://doi.org/ 10.1016/j.procbio.2015.08.004. [5] H. Fu, X. Wang, Y. Sun, L. Yan, J. Shen, J. Wang, S.T. Yang, Z. Xiu, Effects of salting-out and salting-out extraction on the separation of butyric acid, Separ. Purif. Technol. 180 (2017) 44e50, https://doi.org/10.1016/ j.seppur.2017.02.042. [6] H.S. Wu, Y.J. Wang, Salting-out effect on recovery of 1,3-propanediol from fermentation broth, Ind. Eng. Chem. Res. 51 (2012) 10930e10935, https:// doi.org/10.1021/ie300404t. [7] S. Xie, W. Song, C. Fu, C. Yi, X. Qiu, Separation of acetone: from a water miscible system to an efficient aqueous two-phase system, Separ. Purif. Technol. 192 (2018) 55e61, https://doi.org/10.1016/j.seppur.2017.09.056. [8] S. Xie, Y. Zhang, Y. Zhou, Z. Wang, C. Yi, X. Qiu, Salting-out of bio-based 2,3-
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Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Creating efficient novel aqueous two-phase systems: Salting-out effect and high solubility of salt Chuhan Fu a, Wenli Song b, Conghua Yi b, Shaoqu Xie a, * a b
The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, USA School of Chemistry & Chemical Engineering, South China University of Technology, No. 381 Wushan Road, Guangzhou, 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 December 2018 Received in revised form 28 February 2019 Accepted 4 March 2019 Available online 7 March 2019
In light of the advantages of liquid-liquid extraction, it is important to reduce the mutual solubility of extractant and solvent in the liquid feed. Highly efficient novel aqueous two-phase systems composed of 2-propanol and water were created by the introduction of highly soluble salts at high salt concentrations. The water þ 2-propanol þ K2HPO4/K4P2O7 systems are recommended for the extraction of the valueadded chemicals since they lead to trace amounts of 2-propanol in the aqueous phase and salt in the organic phase. The e-NRTL-RK model was employed to correlate the aqueous two-phase systems (organic phase and aqueous phase) with high accuracy. With the roles of the salting-out effects and the solubility of the salt in mind, we are able to exploit the highly efficient novel aqueous two-phase systems composed of 2-propanol, water, and other highly soluble salts. © 2019 Elsevier B.V. All rights reserved.
Keywords: Salting-out effect Aqueous two-phase system 2-Propanol Recovery Highly soluble salts
1. Introduction The traditional aqueous two-phase systems are composed of two polymers and water, or one polymer, one inorganic salt and water [1], which were widely used for separation of cells, membranes, enzymes, viruses, proteins, nucleic acids, and other biomolecules [2]. Recently the novel aqueous two-phase systems composed of a small organic molecule, water, and a salt/sugar are on the rise because of the flourishing production of bio-based chemicals and biofuels from renewable resources [3e6]. For example, when methanol, ethanol [7], or other hydrophilic organic solvent is mixed with water and an inorganic salt at an appropriate concentration, there is a liquid-liquid phase splitting, thus forming a new type of aqueous two-phase system. Compared with the traditional extraction and other separation techniques, the novel aqueous two-phase system is featured with the advantages of mild operating conditions, large handling capacity and easy continuous operation on any scale. It is generally believed that the salting-out effects of the inorganic salts induce the liquid-liquid phase splitting [8e10]. Due to inefficiency of common salts to create phase separation between
* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Xie). https://doi.org/10.1016/j.fluid.2019.03.002 0378-3812/© 2019 Elsevier B.V. All rights reserved.
water and a strong polar solvent [11,12], i.e. methanol or ethanol, it is indispensable to turn to the highly efficient aqueous two-phase system to avoid the solvent loss. The employment of propanol or butanol þ water þ salt system, especially the 2-propanol þ water þ salt system is the most popular method used for the separation of metal ions from industrial and waste solutions [13,14]. Liquid-liquid equilibria (LLE) for the 2-propanol þ water þ ordinary salt (easily soluble) systems were widely studied. These salts included Na2SO3 [15], NaCl [15e19], KCl [18], K2SO4 [20], Li2SO4 [21], MgSO4/ZnSO4 [22], Na2CO3 [23], C2K2O4 [24], Na2S2O3 [25]. After the addition of these salts into the aqueous 2-propanol solution, only a portion of 2-propanol was salted out due to the limited salting-out effects. LLE for 2-propanol þ water þ highly soluble salt systems were also well investigated at different temperatures. These highly soluble salts (highly soluble) included KF [26], CsCl [27], (NH4)2SO4 [28], K2CO3 [23,29], LiCl [30], KCl þ CsCl [31], Na2C6H6O7 [32], K2HPO4 [33]. However, these 2-propanol þ water þ highly soluble salt systems were either created at low salt concentrations or showed mixed solvation effects [34,35] which featured these systems with high salt contents of the organic phase and high solvents contents of the aqueous phase. As a result, the losses of 2propanol and the salt during the salting-out extraction cannot be avoided, suggesting that this unit operation burdens the whole separation units with extra costs. In this work, the highly efficient aqueous two-phase systems
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C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
composed of 2-propanol þ water þ highly soluble salts systems were investigated at low and high salt concentrations. The 2propanol recovery and the water removal were used to evaluate the efficiency of the aqueous two-phase systems. Tie-line data were correlated by eNRTL-RK model which represented the reality of the experimental results. Last but not least, the salting-out effects of the highly soluble salts on 2-propanol were also discussed and compared to other results. 2. Experimental 2.1. Materials Tripotassium phosphate trihydrate (K3PO4$3H2O) was purchased from Guangzhou Chemical Reagent Factory. Potassium carbonate (K2CO3) was supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. Dipotassium hydrogen phosphate trihydrate (K2HPO4$3H2O) was purchased from Guanghua Chemical Plants Co. Ltd. Pyrophosphate potassium trihydrate (K4P2O7$3H2O) was purchased from Tianjin Damao Chemical Plants Co. Ltd. Potassium chloride (KCl) was purchased from Shanghai Aladdin Bio-Chem Technology Co.,LTD. The HPLC-grade 2-propanol was obtained from Guanghua Chemical Plants Co. Ltd. All the chemicals were used as received and their purities were shown in Table 1. Deionized water was prepared throughout the Ultrapure Water system in our lab. The purities of the salts were confirmed by the flame atomic absorption spectrometry (FAAS) in our previous studies [7,8,36]. Herein, flame atomic absorption spectrometry was used for purity assessment again. Anhydrous salt content was determined by FAAS. The purity of 2-propanol was stated by the supplier and confirmed by gas chromatography (GC) analysis (GC-TCD). 2.2. General procedure for the salting-out experiments 20 mL headspace bottles with tan PTFE/silicone septa and aluminium caps were used to perform the salting-out experiments [36]. 2-Propanol, water, and salt were successively added to the headspace bottles in terms of different known initial salt concentrations. The controlled initial salt concentration (CI) has the following expression,
msalt ðgÞ 1000 CI ¼ msalt ðgÞ þ mwater ðgÞ
(1)
where msalt is the mass of an anhydrous salt, and mwater is the mass of water in the whole system. For the better phase splitting, the mass ratio of 2-propanol to the deionized water was controlled under 4:6. Then the headspace bottles were sealed. For the determination of the mutual solubility data, the headspace bottles were shaken for 1 h at room temperature to enable the ternary system to move sufficiently and settled at a controlled
temperature of 298.15 K in a water bath for 24 h. The effect of temperature on system 2-methyl-2-propanol þ H2O þ K2CO3 was insignificantly [29], thus the equilibria were established at 298.15 K [27]. 2.3. Analysis After the equilibrium, 0.6 mL of the 2-propanol-rich and waterrich phases were withdrawn by microsyringe for gas chromatography (GC) analysis. GC (Techcomp GC7900, China) was equipped with a 2 m(L) 3 mm (ID) 5 mm (OD) Porapak Q 80e100 mesh packed column and a thermal conductivity detector (TCD). The water and 2-propanol contents of two phases were determined by the area normalization method [37,38]. After the liquid-liquid phase splitting, the salt content of the 2-propanol-rich phase was determined by flame atomic absorption spectrometry (FAAS) at the wavelength of 766.5 nm. The salt concentrations of the aqueous phase were computed by difference. The titration method was also used for the determination of potassium carbonate in the organic phase using automatic potentiometric titrator (905 Titrando, Metrohm Corp., Switzerland) [29]. The results showed a good agreement with those using FAAS. The phosphates of the organic were also analyzed using a Basic IC 792 ion chromatograph. The results also showed a good agreement with those using FAAS. The salt content in the materials was measured at the subsensitive resonance line (404.5 nm) of potassium by flame atomic absorption spectrometry which adopted an external standard method [39]. Samples were pretreated with nitric acid (water: nitric acid ¼ 1:1, V:V) and cesium nitrate (10 g/L). Cesium nitrate was used as an ionization buffer. KCl was dried at 500 C for 6 h and used to prepare the K standard solutions. The potassium salts were then dissolved in the deionized water to confirm the salt content. These results verified that K2HPO4$3H2O, K3PO4$3H2O, and K4P2O7$3H2O were stable at room temperature. 3. Results and discussion 3.1. Liquid-liquid equilibria (LLE) data As the simplest example of a secondary alcohol, 2-propanol is miscible in water but not miscible with saline solutions due to the salting-out effect. After the addition of a salt to a 2-propanol aqueous solution, the 2-propanol was separated into a distinct layer (an organic phase) with a certain amount of water and salt. Most of salt and water retained in water and form the aqueous phase. As a result, the aqueous two-phase system formed for the salting-out extraction of some extractable substrates. First of all, the LLE data for the water þ 2-propanol þ salt ternary system at 298.15 K were investigated in great detail. The composition of each phase as a function of initial salt concentration was
Table 1 Chemicals in this study. Reagents
CAS number
Supplier
Purity (mass fraction)
Anhydrous salt content (mass fraction)
2-Propanol K2HPO4$3H2O K3PO4$3H2O K2CO3 K4P2O7$3H2O KCl Deionized water
67-63-0 16788-57-1 7778-53-2 584-08-7 7320-34-5 7447-40-7 7732-18-5
Guanghua Chemical Plants Co. Ltd. Guanghua Chemical Plants Co. Ltd. Guangzhou Chemical Reagent Factory Shanghai Lingfeng Chemical Reagent Co., Ltd Tianjin Damao Chemical Plants Co. Ltd. Shanghai Aladdin Bio-Chem Technology Co.,LTD. Ultrapure Water system in our lab
0.997 0.99 0.99 0.99 0.99 0.998 （Electrical conductivity ＜1.5 104 S m1）
e 0.756 0.789 0.991 0.851 e e
Note: The purities of K2HPO4$3H2O, K3PO4$3H2O, K2CO3, K4P2O7$3H2O were stated by the supplier and confirmed by FAAS. Herein KCl was used as the standard. The purity of 2-propanol was stated by the supplier and confirmed by GC-TCD.
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85 Table 2 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K2CO3) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500
0.6000 0.4622 0.3787 0.3092 0.2493 0.2010 0.1576 0.1217 0.0907 0.0662
0.4000 0.5251 0.6158 0.6887 0.7499 0.7987 0.8423 0.8782 0.9093 0.9338
0.0000 0.0127 0.0054 0.0021 0.0008 0.0003 0.0001 0.0001 0.0001 0.0001
0.6000 0.7580 0.7492 0.7238 0.6936 0.6568 0.6174 0.5759 0.5328 0.4879
0.4000 0.0824 0.0411 0.0238 0.0116 0.0059 0.0032 0.0012 0.0003 0.0001
0.0000 0.1596 0.2096 0.2524 0.2947 0.3373 0.3794 0.4229 0.4669 0.5120
2-propanol
aqueous phase K2CO3
water
2-propanol
K2CO3
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript
i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
Table 3 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K2HPO4) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500 550 600
0.6000 0.5331 0.4784 0.4226 0.3674 0.3150 0.2641 0.2229 0.1741 0.1353 0.1047 0.0759
0.4000 0.4501 0.5119 0.5719 0.6298 0.6838 0.7355 0.7769 0.8259 0.8646 0.8953 0.9241
0.0000 0.0168 0.0097 0.0054 0.0028 0.0013 0.0004 0.0002 0.0001 0.0001 0.0000 0.0000
0.6000 0.7114 0.6990 0.6805 0.6566 0.6280 0.5960 0.5591 0.5220 0.4812 0.4377 0.3929
0.4000 0.0462 0.0328 0.0220 0.0141 0.0083 0.0046 0.0021 0.0010 0.0003 0.0001 0.0000
0.0000 0.2425 0.2681 0.2974 0.3293 0.3637 0.3994 0.4388 0.4769 0.5184 0.5622 0.6071
2-propanol
aqueous phase K2HPO4
water
2-propanol
K2HPO4
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript
i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
Table 4 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K3PO4) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500
0.6000 0.4919 0.4234 0.3512 0.2901 0.2319 0.1841 0.1450 0.1088 0.0810
0.4000 0.4963 0.5711 0.6462 0.7086 0.7678 0.8152 0.8546 0.8909 0.9187
0.0000 0.0118 0.0055 0.0026 0.0014 0.0003 0.0007 0.0003 0.0003 0.0003
0.6000 0.7562 0.7369 0.7166 0.6875 0.6543 0.6168 0.5758 0.5334 0.4888
0.4000 0.0549 0.0329 0.0187 0.0099 0.0049 0.0022 0.0008 0.0002 0.0001
0.0000 0.1890 0.2302 0.2647 0.3026 0.3408 0.3810 0.4234 0.4663 0.5111
2-propanol
aqueous phase K3PO4
water
2-propanol
K3PO4
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript
i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
79
Table 5 Experimental Tie-line Data on Mass Fractions for (Water þ 2-Propanol þ K4P2O7) Ternary System at 298.15 K and p ¼ 0.1 MPaa. CI
organic phase
(g$kg1)
water
u12
u22
u32
u11
u21
u31
control 100 150 200 250 300 350 400 450 500 550
0.6000 0.5197 0.4595 0.3994 0.3329 0.2751 0.2229 0.1702 0.1249 0.0904 0.0627
0.4000 0.4681 0.5341 0.5976 0.6662 0.7246 0.7770 0.8298 0.8751 0.9095 0.9373
0.0000 0.0122 0.0064 0.0030 0.0010 0.0003 0.0001 0.0001 0.0000 0.0000 0.0000
0.6000 0.7282 0.7128 0.6913 0.6687 0.6386 0.6042 0.5681 0.5285 0.4857 0.4410
0.4000 0.0326 0.0229 0.0150 0.0089 0.0047 0.0023 0.0009 0.0003 0.0001 0.0000
0.0000 0.2392 0.2643 0.2936 0.3223 0.3567 0.3936 0.4311 0.4712 0.5142 0.5590
2-propanol
aqueous phase K4P2O7
water
2-propanol
K4P2O7
uij ¼ mass fraction of one component in the aqueous or organic phase (subscript i ¼ 1,2,3 represent water, 2-propanol, and salt, respectively; subscript j ¼ 1,2 represent the aqueous phase and organic phase, respectively). a Standard uncertainties u are u (uwater) ¼ 0.003, u (u2-propanol) ¼ 0.003, u (u32) ¼ 0.00003, u (u32) ¼ 0.003, u(T) ¼ 0.05 K, u(p) ¼ 0.0015 MPa.
listed in Tables 2e5. The highly soluble salts showed excellent phase splitting ability in the 2-propanol aqueous solutions. LLE data for the water þ 2-propanol þ K2CO3 system (Table 2) shows that the mutual solubility of water and 2-propanol was influenced greatly by the initial salt concentration. With an increasing initial salt concentration (from 100 g/kg to nearing saturation condition), there were sharp decreases in the water content of the organic phase, the 2-propanol content of the aqueous phase and the salt content of the organic phase. By contrast with the control sample, the 2-propanol aqueous solution was very sensitive to the salting-out effect. The ideal aqueous twophase system is to avoid the salt residue in the organic phase and the 2-propanol residue in the aqueous phase. However, in comparison to the low K2CO3 initial concentrations, high K2CO3 initial concentrations, especially the concentration of 500 g/kg made the aqueous two-phase system to be in a near-ideal state. At the concentration of 500 g/kg, only 0.0001 (mass fraction) of salt and 0.0001 (mass fraction) of 2-propanol were detected in the organic phase and aqueous phase respectively. The water content of the organic phase was 0.4622 (mass fraction) at 100 g/kg, whereas it decreased to 0.0662 (mass fraction) at 500 g/kg due to the enhanced salting-out effect. Consequently, the solubility of the salt in the organic phase was reduced with the reduction in the water content of the organic phase. To understand the roles of other highly soluble salts on the separation of 2-propanol from water, the tie-line data of (water þ 2propanol þ K2HPO4/K3PO4/K4P2O7) ternary system were shown in Tables 3e5 respectively. It can be seen that the water contents of the organic phase were lower than 0.10 (mass fraction) under the nearing salt-saturation condition. Interestingly the 2-propanol content of the aqueous phase and the salt content of the organic phase were not detected at the initial K2HPO4 and K4P2O7 concentrations of 600 g/kg and 550 g/kg respectively. In comparison to K2HPO4 and K4P2O7, K3PO4 showed a little weaker desalination of the organic phase when the water content of organic phase was kept below 0.10 (mass fraction). In spite of the similar water content of the organic phase, the water þ 2-propanol þ K2HPO4/K4P2O7 system was superior to the water þ 2-propanol þ K3PO4 in the application of the highly efficient aqueous two-phase systems to extract some value-added chemicals. The water þ 2propanol þ K2HPO4 system is slightly alkaline whereas the water þ 2-propanol þ K4P2O7 system is strongly alkaline. Thus the
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C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
investigations along this direction are required for the high recovery of a target value-added chemical. Although the contents of 2-propanol, water and salt in both phases are different, comparison with published data for validation of results is needed, as shown in Fig. 1. In order to test the reliability of the results, the experimental data of 2-propanol-water-K2CO3/ K2HPO4 system are compared with the reference data [29,33]. Taking the mass fraction of salt as the abscissa and the mass fraction of 2-propanol as the ordinate, the literature data and the results of this experiment were compared, and the results were shown in the insert figures of Fig. 1. The experimental results in our study are in good agreement with the literature data, which proves that the experimental results have a high degree of accuracy. On the other hand, with the comparison of phase diagram results with other lower soluble salts, e.g. Refs. [15,17,21,23,24], the broader twophase regions were observed for the highly soluble salts induced aqueous two-phase systems, which demonstrated that the solubility of salt is very important for creating efficient novel aqueous two-phase systems. 3.2. Separation of 2-propanol and water For the water þ 2-propanol þ salt system, the efficiency of the aqueous two-phase system itself is specifically manifested in the recovery of 2-propanol into the organic phase and the removal of water into the aqueous phase, which is a foundation for enabling and managing the applications of the aqueous two-phase system to the extraction of other chemicals. In general, the 2-propanol recovery and the water removal were expressed in Eq. (2) and (3) respectively,
Recovery ¼
0:4m0 m1 u21 0:4m0
Water removal ¼
(2)
0:6m0 m2 u12 0:6m0
(3)
where m1 is the mass of the aqueous phase, m2 is the mass of the organic phase, m0 is the mass of the initial 2-propanol þ water system before the addition of an inorganic electrolyte, u21 is the 2propanol content of the aqueous phase, and u12 is the water content of the organic phase.
K2CO3 this study K2HPO4 this study K3PO4 this study K4P2O7 this study C2K2O4[24] Na2SO3[15] NaCl[17] Li2SO4[21] Na2CO3[23]
Mass fraction of 2-propanol
1.0
0.8
0.6
3.3. Theoretical calculation of the liquid-liquid equilibria Several models, such as UNIQUAC and NRTL can be applied to correlate the liquid-liquid equilibria so that the reality of the experimental results can be seen [42]. According to the large ionic species in the ternary system, we used e-NRTL model [43] to correlate the LLE of our systems. The data regression was analyzed using Aspen Plus V9 with the Unsymmetric e-NRTL property method (eNRTL-RK). The e-NRTL model for the excess Gibbs free energy is built up from a long range interaction contribution (gex*,pdh) and a short range interaction contribution (gex*,lc), as can be seen in Eq. (4).
gex* g ex*; pdh gex*; lc ¼ þ RT RT RT
(4)
where the notation “*” denotes the unsymmetric convention, gex is the excess Gibbs energy of electrolyte systems, gex,lc is the contribution from the local composition (lc) interactions, gex,pdh is the contribution arising from long-range ioneion interactions using Pitzer-Debye-Huckel (pdh) equations, R is the universal gas constant (8.314 J mol1 K1), T is the temperature (K), and the notation “*” denotes the unsymmetric convention [43]. Accordingly Eq. (4) leads to the following expression for activity coefficients,
K2CO3 this study K2CO3[29]
0.4
After the calculation, the 2-propanol recovery and the water removal as a function of initial salt concentration are presented in Fig. 2. The recovery results show that with increasing addition of an inorganic electrolyte, the recovery of 2-propanol increased gradually except in the cases when the concentrations of K2HPO4 and K4P2O7 were 100 g/kg. This anomaly resulted from the higher water contents of the organic phase thus less water retained in the aqueous phase. As a result, the recovery of 2-propanol increased correspondingly. Although lower salt concentrations achieved considerable recovery of 2-propanol, the salting-out extraction system deviated from the high efficiency, especially for the removal of water, as shown in Fig. 2(b). The lower water content of the organic phase is recommended because it favors the separation of the solvents after the salting-out extraction of some value-added chemicals. An increase in the initial salt concentration improved the dehydration of the organic phase to give purer solvents [41]. The highest conceentration of each salt achieved the highest dehydration ratio. However, the liquid-liquidsolid equilibria for the ternary systems should be avoided due to the additional unit for separating the solid phase. The nearingsaturation conditions, such as at the K2CO3, K3PO4, K4P2O7, and K2HPO4 concentrations of 500 g/kg, 500 g/kg, 550 g/kg, 600 g/kg were therefore chosen as the optimal conditions. At these salt concentrations, the recovery of 2-propanol reached almost 100.00%. At the same time, more than 94% of water was retained in the aqueous phase. The efficiency of the salts used on removing the water was in the following order K2CO3＞K3PO4＞K4P2O7＞K2HPO4 at the same initial salt concentration.
pdh þ ln glc lnðgi Þ ¼ ln gi i
K2HPO4 this study K2HPO4[33]
(5)
where gi is the activity coefficient of the component i in the mixture. The pdh equations for the long-range contribution are used to express the excess Gibbs free energy,
0.2
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Mass fraction of salt Fig. 1. Phase diagram of 2-propanolesalt aqueous two-phase system.
0.7
! X gex*; pdh 1000 1=2 4A∅ Ix 1=2 ¼ ln 1 þ rI x xk Ms RT r
(6)
k
where Ix is the ionic strength parameter, Ms is the molecular weight
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
81
1.0 1.00
(a)
0.99
0.8
Recovery
0.97
K2CO3
0.96
K2HPO4 K3PO4
0.95
K4P2O7
0.94
Water removal
0.98
0.7 0.6
K2CO3
0.5
K2HPO4
0.4
K3PO4 K4P2O7
0.3
0.93 0.92
(b)
0.9
100
200
300
400
500
600
0.2
100
200
300
CI (g/kg)
400
500
600
CI (g/kg)
Fig. 2. 2-Propanol recovery (a) and removal of water (b) as a function of the initial salt concentration.
of the solvent s, and Af is the Debye-Huckel constant for osmotic coefficients, as shown in the following,
X 1 Z 2i xi Ix ¼ 2 3 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 e 2pd0 NA pffiffiffiffiffiffiffiffiffi A∅ ¼ 3 1000 DkT
(8)
For Eq. (6)e(8), i refer to ions, Zi is the absolute value of the charge on the species of i, x is the mole fraction, and r is the closest distance parameter. Moreover, NA is Avogadro's number equal to 6.0232 10 23 mol-1, k is Boltzmann constant, e is the electronic charge, d0 is the mixed solvent density and D is the dielectric constant, respectively. The densities of the solvents are from Aspen Plus. Accordingly Eq. (6) leads to the following expression for activity coefficients,
(14)
1=2
3=2
Z 2i I x 2I x þ 1=2 1 þ rI x
aij ¼ cij þ dij ðT 273:15 KÞ
(15)
Gji ¼ exp atji
(16)
Where t is the dimensionless interaction parameter, i and j refer to different species, a, b, c, d, e are parameters and a is the nonrandomness factor that can be set as a fixed value. But e and d were set to 0 in this study. The calculated excess Gibbs energy values from Eq. (16) can be used to calculate both the overall excess Gibbs energy for the short range interaction contribution (Eq. (17)) and the activity coefficients for each component.
gex*;lc ¼ xm ðxcm þ xam Þtca;m þ xc xmc Zc tm;ca þ xa xma Za tm;ca RT þ xc Zc tm;ca þ Gcm tca;m xa Za tm;ca þ Gma tca;m
2 ! 1000 1=2 4 2Z 2i 1=2 pdh* ¼ ln gi A∅ ln 1 þ rI x Ms r
(17)
3 5
(9) lnglc* c ¼
The short range interaction contribution is based upon the NRTL model (NRTL local interaction contribution),
xcm þ xam þ xmm ¼ 1 xma þ xca ¼ 1
!
(7)
Tref T bij T þ eij þ ln tij ¼ aij þ Tref T 273:15 K T
x2m tcm Gcm
Za xa tma xm Gma ðxc Gcm þ xa Gam þ xm Þ2 ðxc þ xm Gma Þ2 Zc xm tmc Gmc Zc tmc Gcm tcm þ ðxa þ xm Gmc Þ
(18)
(10) (11)
lnglc* a ¼
x2m tam Gam ðxc Gcm þ xa Gam þ xm Þ2
Zc xc tmc xm Gmc ðxa þ xm Gmc Þ2
þ
Za xm tma Gma ðxc þ xm Gma Þ
Za tma Gam tam xmc þ xac ¼ 1
where c is short for cation, a is short for anion, and m is short for solvent (molecule, water or alcohol). G, t and g are energy parameters [43] and given by,
tji ¼
gji gii RT
(19)
(12) lnglc m ¼ xcm tcm þ xam tam þ
Zc xc Gmc tmc xa ðxa þ Gmc xm Þ2
xc xm Gcm tcm ðxc Gcm þ xa Gam þ xm Þ2
(13)
þ
Za xa Gma tma xc
ðxc þ Gma xm Þ2 xa xm Gam tam
ðxc Gcm þ xa Gam þ xm Þ2 (20)
where xij is given by,
82
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
xim ¼
xi Gim ðxa Gam þ xc Gcm þ xm Þ
(21)
xac ¼
xa xa þ xm Gmc;ac
(22)
xca ¼
xc xc þ xm Gma;ca
(23)
3.4. Investigating the salting-out effects using setschenow-based equation
Fortunately, there are following relationships between different
t parameters,
tam ¼ tcm ¼ tac;m
(24)
tmc:ac ¼ tma;ca ¼ tm;ca
(25)
The mixed solvent (water and alcohol) was used, thus the Born correction that uses the dielectric constants for the long range interactions was adapted to calculate the unsymmetric pdh formula,
DGborn RT
¼
reported in Table 7 are shown in Fig. 3. It is concluded that the LLE data for the aqueous two-phase systems composed of 2-propanol, water, and highly soluble salts can be reproduced with excellent accuracy by using the e-NRTL-RK model.
Ne2 1 1 xi Z 2i 2 10 2kT εd εu i ri
(26)
where εw is the dielectric constant of water; and ri is the Born radius of segment species i. The detailed expression for the activity coefficient of segment species i can be found on reference [43]. For the correlation of the tie-line data, the equilibria chemistry included in this analysis is provided in Table 6. We only regressed water - ion pair and 2-propanol - ion pair because the equilibria constant is very large. The undissociated salt could be negligible so that water - undissociated salt and 2-propanol - undissociated salt parameters were kept at zero. The equilibria constant is calculated by,
lnðKeqÞ ¼ A
(27)
where the concentration basis for Keq is mole fraction. Furthermore, the dielectric constants of 2-propanol and water at 25 C were set as 18 and 80.4. There are six interaction parameters (tmca, tcam, twca, tcaw, tmw, and twm) and three non-randomness factors (awca, amw, and amca) for the eNRTL-RK model. But in this study, the amw was set to 0.3 and awca was set to 0.2 [43,44], and m and w represent 2-propanol and water respectively. Other parameters were obtained from the regression of the LLE data. The default objective function in our study was the maximum likelihood objective function. The obtained binary parameters or electrolyte pair parameters using the Maximum likelihood objective function are reported in Table 7. It should be noted that, different non-randomness factors for alcohol and electrolytes in fitting the experimental LLE data obtained in this work are different. Using the parameters reported in Table 7, we could successfully correlate the tie-lines data by the generalized e-NRTL-RK model. Moreover, to show the reliability of the e-NRTL-RK model in correlating the tie-lines data, experimental phase diagram and estimated phase diagram using the parameters
Many salts can induce salting-out phenomena [45] thus the quantitation of the salting-out effect can reflect the nature of the phase splitting phenomena involved in the “salting out” of organic small molecules by inorganic electrolytes. Dating back to 1925, the salting-out of proteins was quantitated by the Setschenow-base equation which is a simple equation derived to describe the relationship between the solubility of inorganic solutes and the concentration of inorganic electrolytes [46]. Molar concentration [47e49], molal concentration [50,51] and mole fraction were widely employed to express salt concentrations. After the fitting of the above-mentioned variants, the empirical Setschenow saltingout constants obtained can quantitatively represent the salting effects. Similar to the salt-induced precipitation, the salting-out of 2propanol was based on the electrolyte-nonelectrolyte interaction in the aqueous saline solution in which 2-propanol was less soluble at a high salt concentration. After the fitting of the logarithm of the solubility of 2-propanol and the salt content by using the empirical Setschenow equation, the salting-out effect on the aqueous 2-propanol solution then can be quantitated [10,52]. The absolute values of the slopes for the Setschenow equations are salting-out constants (also called Setschenow constants) which represent the salting-out effects clearly. Herein the 2-propanol solubility (s21) has the units of g per 100 g water in the saline solution and is expressed by,
s21 ¼
u21 100 u11
(28)
where u21 is the mass fraction of 2-propanol in the saline solution, and u11 is the mass fraction of water in the saline solution. The salt concentration is expressed by molality which has the units of mol per 1 kg water and is expressed by,
bB ¼
u31 1000 Ms u11
(29)
where u31 is the mass fraction of a salt in the saline solution, and Ms is the molar mass of a salt. As reported previously, the liquid-liquid equilibria data from the literature together with our results were fitted through the following Setschenow equation,
lns21 ¼ PbB þ Q
(30)
Where P and Q are constants, especially P is also called the Setschenow constant. The slopes whose absolute values give the salting out effect, together with the corresponding intercepts and the coefficients of determination R2, are shown in Table 8. On the basis of our experiments and previous studies of the salting-out of
Table 6 Chemistry for the data regression. Reaction no.
Specification type
Stoichiometry
Equilibrium constant (A)
1 2 3 4
Equilibrium Equilibrium Equilibrium Equilibrium
K2CO3#2 KþþCO23 K2HPO4#2 KþþHPO24 þ 3K3PO4#3 K þPO4 þ 4K4P2O7#4 K þP2O7
46 38 90 50
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
83
Table 7 Values of parameters for the six interaction parameters of e-NRTL-RK model for 2-propanol (m) þ salt (ca) þ water (w) systems at T ¼ 298.15 Ka. Species
Water(i)þ2-propsanol(j) Water(i)þK2CO3(j) 2-propanol(i)þ K2CO3 (j) Water(i)þ K2HPO4 (j) 2-propanol(i)þ K2HPO4 (j) Water(i)þ K3PO4(j) 2-propanol(i)þ K3PO4 (j) Water(i)þ K4P2O7 (j) 2-propanol(i)þ K4P2O7 (j) a
Binary parameters or electrolyte pair parameters aij
bij
cij
aji
bji
cji
1.219 3.140 19.834 43.609 39.831 20 10.368 13.446 16.550
850.087 2115.072 400.296 9978 6832.53 3654.88 9412.01 430.149 513.844
0.3 0.2 0.0511 0.2 0.0497 0.2 0.0248 0.2 0.0525
1.154 20 18.737 16.007 39.642 18.501 19.708 4.417 10.772
1257.1 7895.2688 2155.091 6579.56 5863.27 7349.23 7397.13 3206.3 2278.33
0.3 0.2 0.0511 0.2 0.0497 0.2 0.0248 0.2 0.0525
Standard uncertainties u are u(T) ¼ 0.05 K.
2-propanol, the natural logarithm of the solubility of 2-propanol was plotted against the molality of salt in aqueous saline solutions (Fig. 4). Both the coefficients of determination R2 and the
setschenow-based plots verified that the salting-out effect could be satisfactorily characterized by Eq. (30). Different salts show different salting-out behavior, suggesting that the salt type is one of
Fig. 3. Experimental and estimated phase diagram for 2-propanol þ salt þ water systems at T ¼ 298.15 K. Experimental tie-lines (symbol) and the calculated ones (lines). At the K4P2O7 concentration of 550 g/kg, the content of the salt in the organic phase and the 2-propanol content of the aqueous phase were too low to be detected so that the LLE data were not included in this Figure.
84
C. Fu et al. / Fluid Phase Equilibria 490 (2019) 77e85
Table 8 Constants P, Q and the coefficients of determination R2 after the regression by using Setschenow equation. salt
P
Q
R2
No.a
Reference
K4P2O7 K3PO4 K2HPO4 K2CO3 CsCl KF Na2SO3 LiCl Na2C6H6O7 MgSO4 C2K2O4 Na2S2O3 ZnSO4 NaCl Li2SO4
2.5475 1.7363 1.0166 0.987 0.2093 0.1934 1.4254 0.5451 0.6719 1.2086 1.8581 0.7089 1.0559 0.6541 1.3521
4.0242 4.0065 3.7228 3.6836 3.9039 3.2108 4.5881 12.006 3.8875 4.5485 5.2041 3.7771 4.3568 5.6093 4.5121
0.9998 0.9998 0.9973 0.9918 0.9481 0.9838 0.9694 0.9368 0.999 1.0000 0.9984 0.993 1.0000 0.991 0.9899
9 9 10 9 4 7 3 3 3 3 4 5 3 8 5
This This This This [27] [26] [15] [30] [32] [22] [24] [25] [22] [19] [21]
a
work work work work
Number of data points in the regression.
4
K4P2O7 K3PO4 K2HPO4 K2CO3 CsCl KF Na2SO3 LiCl Na2C6H6O7 MgSO4 C2K2O4 Na2S2O3 ZnSO4 NaCl Li2SO4
3 2
ln (s21)
1 0 -1 -2 -3 -4 -5 -6
0
2
4
6
8
10
12
14
16
18
Molality of salt (mol/kg) Fig. 4. Relationship between the solubility of 2-propanol and the molality of salt in aqueous saline solutions. Except the K4P2O7, K3PO4, K2HPO4, and K2CO3 data were from our study, other data were from Refs. [15,19,21,22,24e27,30,32].
the important factors for the salting-out effect. K4P2O7 shows the greatest absolute value that represents the strongest salting-out ability of the salt for 2-propanol in the studied system, followed by C2K2O4 with the value of 1.8581. It seems that C2K2O4 is also favorable for the 2-propanol separation. However, the solubility of it limited the further ionic hydration [53]. By contrast, K3PO4 shows the similar salting-out effects whereas the solubility of K3PO4 is much greater than that of C2K2O4. As a result, the highly efficient separation can be achieved. Similarly, K2CO3, K2HPO4 are also the alternative bases for the 2-propanol separation from aqueous solutions due to their high solubilities which remedy their salting-out effects, as shown in Fig. 4. With the roles of the salting-out effects and the solubility of the salt in mind, we are able to exploit the highly efficient aqueous two-phase systems composed of 2propanol, water, and highly soluble salts. After the data regression, the 2-propanol solubility in the aqueous K4P2O7, K3PO4, K2HPO4, or K2CO3 solutions is dependent on the salt concentration in the same phase. For the abovementioned order of the salts, the corresponding equations were obtained as Eq. (31) ~ (34),
lns21 ¼ 2:5475bB þ 4:0242
(31)
lns21 ¼ 1:7363bB þ 4:0065
(32)
lns21 ¼ 1:0166bB þ 3:7228
(33)
lns21 ¼ 0:987bB þ 3:6836
(34)
4. Conclusions The liquideliquid equilibrium for 2-propanol, highly soluble salts þ water ternary systems were studied at 298.15 K. It was shown that K4P2O7, K3PO4, K2HPO4, or K2CO3 had greater salting out effects on 2-propanol, especially under nearing saturation conditions which made the organic phase composed of water and 2propanol and the aqueous phase composed of water and salt. The water þ 2-propanol þ K2HPO4/K4P2O7 system was recommended for the extraction of the value-added chemicals from dilute aqueous solutions according to trace amount of 2-propanol in the aqueous phase and salt in the organic phase. The e-NRTL-RK model was used for the correlation of the aqueous two-phase systems composed of 2-propanol, water, and highly soluble salts with high accuracy. After the fitting of the logarithm of the solubility of 2-propanol and the salt content by using the empirical Setschenow equation satisfactorily, the salting-out effect on the aqueous 2-propanol solution were quantitated. According to the Setschenow constant, K4P2O7 showed the strongest salting-out ability of the salt for 2propanol in the studied system. With the roles of the salting-out effects and the solubility of the salt in mind, we are able to exploit the highly efficient aqueous two-phase systems composed of 2-propanol, water, and highly soluble salts. Acknowledgments This work was funded by the Fundamental Research Funds for the Central Universities of China (2015ZM169), the International S&T Cooperation Program of China (2013DFA41670), and the National High-tech R&D Program (863 Program) (863 Program) (No. 2012AA021202). References [1] P.K. Wan, J.C.W. Lan, P.W. Chen, J.S. Tan, H.S. Ng, Recovery of intracellular ectoine from Halomonas salina cells with poly(propylene) glycol/salt aqueous biphasic system, J. Taiwan Inst. Chem. Eng. 82 (2018) 28e32, https://doi.org/ 10.1016/j.jtice.2017.11.003. [2] R. Hatti-Kaul, Aqueous two-phase systems: a general overview, Appl. Biochem. Biotechnol. Part B Mol. Biotechnol. 19 (2001) 269e277, https://doi.org/ 10.1385/MB:19:3:269. [3] J.Y. Dai, Y.Q. Sun, Z.L. Xiu, Separation of bio-based chemicals from fermentation broths by salting-out extraction, Eng. Life Sci. 14 (2014) 108e117, https:// doi.org/10.1002/elsc.201200210. [4] J.Y. Dai, C.J. Liu, Z.L. Xiu, Sugaring-out extraction of 2,3-butanediol from fermentation broths, Process Biochem. 50 (2015) 1951e1957, https://doi.org/ 10.1016/j.procbio.2015.08.004. [5] H. Fu, X. Wang, Y. Sun, L. Yan, J. Shen, J. Wang, S.T. Yang, Z. Xiu, Effects of salting-out and salting-out extraction on the separation of butyric acid, Separ. Purif. Technol. 180 (2017) 44e50, https://doi.org/10.1016/ j.seppur.2017.02.042. [6] H.S. Wu, Y.J. Wang, Salting-out effect on recovery of 1,3-propanediol from fermentation broth, Ind. Eng. Chem. Res. 51 (2012) 10930e10935, https:// doi.org/10.1021/ie300404t. [7] S. Xie, W. Song, C. Fu, C. Yi, X. Qiu, Separation of acetone: from a water miscible system to an efficient aqueous two-phase system, Separ. Purif. Technol. 192 (2018) 55e61, https://doi.org/10.1016/j.seppur.2017.09.056. [8] S. Xie, Y. Zhang, Y. Zhou, Z. Wang, C. Yi, X. Qiu, Salting-out of bio-based 2,3-
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