piperazine, Na2SO4, and H2O: Temperature - American Chemical

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX-XXX

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Novel Aqueous Two-Phase System Consisting of 1,4-Bis(2hydroxypropyl)-piperazine, Na2SO4, and H2O: TemperatureDependent Equilibrium Data and Physical Properties Zhengzhi Zheng,†,‡ Feng Ding,† Yazhong Chen,† Xiangying Chen,† and Peng Cui*,† †

School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China ‡ School of Chemistry & Chemical Engineering, Anhui University, Hefei, Anhui 230601, P. R. China ABSTRACT: In this work, we have developed a novel aqueous twophase system (ATPS) composed of 1,4-bis(2-hydroxypropyl)piperazine (HPP), Na2SO4, and H2O, which can determine the liquid−liquid equilibrium and phase diagrams at the temperatures of 303.15−323.15 K. Simultaneously, the physical properties such as density, viscosity, refractive index, and surface tension have been determined for the top phases and bottom ones of the ATPS. The results indicate that temperature has no significant effect upon the biphasic region, implying a small enthalpy contribution during phase separation. However, the changes in temperature result in a decrease of tie line slope, because H2O molecules are transferred from the bottom phase to the top phase. In determining the physical properties of the system, viscosity and refraction index increase with the system components increasing, during the phase of both HPP and Na2SO4, while density and surface tension fall with the increase of system components in the top phase and decrease in the bottom one. ATPS would be a new method for removing the heat-stable salts (HSS) containing sulfuric acid root of absorbent for flue gas desulfurization, which is as HPP.

1. INTRODUCTION

desorbed by thermal desorption method for recycling, the production has been scaled to industrial application through years of work by our group.14−16 The impurities in flue gas components inevitably make the amine of the flue gas desulfurization agent change into the salt, which is not thermally desorbed. This is known as the heat-stable salts (HSS). Sulfate generated from SO2 is the main constituent, whose content is far higher than other components.17 HSS could not be removed by the thermal desorption method. As a result, the HSS would accumulate too much to lead the organic amine forming ammonium ionic, which makes the desulfurization agent lose the desulfurization effect. The main methods of HSS purification treatment are electrodialysis,18−20 reverse osmosis,21 electric adsorption separation process,22,23 ion exchange adsorption process,24−26 etc. Because of the complex equipment, the tanglesome process of these methods, the easily polluted membrane, the expensive electrode material, long regeneration time, and low desalination rate, the ion exchange adsorption process is mainly applied in many industrial applications at present. However, this method cannot take out nonionic impurities, and ion-exchange resin is polluted by organic matter easily.

The aqueous two-phase system (ATPS) is a new type of separation and enrichment technology governed by the mixing of two components in a water-based solution. It has shown great superiority to separation, purification, and recovery of metals,1,2 antibiotics,3 dyes,4 and proteins.5,6 The developed systems include two-polymer ATPS,7 polymer−salt ATPS,8,9 ionic liquid−salt ATPS,10,11 and micromolecule organic solvent−salt ATPS.12,13 The ATPS containing polymer has high viscosity in the polymer phase. It is unfavorable for mass transfer and phase separation in the separation process. Therefore, the ATPS containing micromolecule organic or two phases with low viscosity has a better prospect. In our previous work, 1,4-bis(2-hydroxypropyl)-piperazine (HPP), a kind of organic amine with two nitrogen atoms in molecules, as shown in Figure 1, has been developed as the main component of sustainable flue gas desulfurization agent. Because it has the advantages of high desulfurization efficiency and SO2

Received: March 17, 2017 Accepted: November 1, 2017

Figure 1. Molecular structure of 1,4-bis(2-hydroxypropyl)-piperazine (HPP). © XXXX American Chemical Society

A

DOI: 10.1021/acs.jced.7b00276 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Hz, −CH3).14 The two carbon atoms connected with hydroxyl are chiral centers which might be R or S. So HPP is a mixture of RR-, SS-, and RS- enantiomers. The percentage of RSenantiomer determined by the TBDPS (tertbutyldiphenylsilyl) reaction of hydroxyl and 1H NMR analysis is 54%. The raw materials including concentrated hydrochloric acid, methyl red, anhydrous Na2SO4, anhydrous ethanol, barium chloride, magnesium chloride, ammonia and ammonium chloride, sodium hydroxide, and chrome black T indicator were purchased from Sinopharm Chemical Reagent Co., Ltd., without further purification. Doubly distilled water degassed by boiling was used to prepare aqueous solutions for the two phase system. The descriptions for applied chemical samples are summarized in Table 1. 2.2. Obtaining Binodal Curves. The binodal curves were determined using the cloud point method.27 In the present work, the binodal curves were determined by using several binary solutions. In general, a salt solution of known concentration was titrated with the HPP or vice versa until the solution turned turbid at the controlled temperature. First, 10.00 mL of Na2SO4 solution with known mass fractions was prepared and put into a thermostatic glass flask with a jacket outside to observe whether the solution is transparent. The temperature of the solution was maintained within 0.01 K uncertainty through thermostatic water circulating in the jacket using KAICC temperature controller. HPP solid was gradually added into the flask, and after completely dissolving, the mixture was kept for 1.0 min at the desired temperature. With the increase of mass ratio in the system, the solution turned turbid, and thus the final mass of HPP and Na2SO4 was obtained. Distilled water was added into the system dropwise until the solution became clear again, so the quantity of water necessary for the system to remain homogeneous was noted. The mixture was kept for 3.0 min, until the stability of the system was confirmed. Each procedure was at least triply repeated to minimize the experimental error. Procedures were carried out enough times to obtain the necessary points for making the binodal curve. 2.3. Construction of Phase Diagrams. The volume ratio of two phases, e.g., top phase and bottom phase, depends on the amount of Na2SO4 and HPP in HPP−Na2SO4−H2O ATPS. In order to make the two phases pose similar volumes to benefit the analysis and control, the mass ratio of Na2SO4 and HPP was optimized to be 3:4. Na2SO4 and HPP were added into 100.0 mL of H2O under stirring for 1.0 h, and then the mixtures were kept static for over 3.0 h. The phases were separated with a syringe very carefully, and then the mass of each phase was weighed by an analytical balance (Mettler Toledo AL104) with an uncertainty

ATPS composed of HPP−Na2SO4−H2O would be developed in the study of removing heat stable salts (HSS), especially sulfate, from the absorbent. That is a novel potential method of two-phase extraction for the removal of HSS directly besides the ion exchange adsorption process and electrodialysis. The method is shown in Figure 2. Adding NaOH to thermal desorbed

Figure 2. Flue gas desulfurization process.

absorbent adjusts the pH value to form ATPS, and then separating bottom phase removes the HSS formed by the sulfuric acid root effectively. But little amount Na+ can be brought into the absorbent. The method can reduce the extra energy and liquid waste significantly, and the process requires less equipment and fewer instruments, comparing with the ion exchange adsorption process and electrodialysis. It is better treated for the HSS containing high concentration of sulfuric acid root in the absorbent. Thus, in this work, phase equilibrium data is experimentally determined for ATPS composed of HPP−Na2SO4−H2O at temperatures in the industrial used range of 303.15−323.15 K. The effect of temperature on the phase equilibrium data is discussed. And physical properties, such as density, viscosity, refraction index, and surface tension of the system, are determined to further develop an efficient HSS removal method.

2. EXPERIMENTAL SECTION 2.1. Materials. HPP was synthesized by the reaction of piperazine and propene oxide using the stoichiometric amounts of reactants in ethanol solution at 338.15 K.14 1H NMR (chloroform-d, chemical shift, ppm): 3.86−3.79 (m, 2H, HO− CH), 2.72−2.19 (m, 12H, N−CH2), 1.14−1.12 (d, 6H, J = 6.3 Table 1. Specifications of the Chemical Samples in This Study chemical name hydrochloric acid anhydrous sodium sulfate barium chloride anhydrous ethanol ammonia sodium hydroxide ammonium chloride chrome black T methyl red HPPb a

source a

SCRC SCRC SCRC SCRC SCRC SCRC SCRC SCRC SCRC synthesis

CAS

initial mass fraction purity

purification method

final mass fraction purity

analysis methods

7647-01-0 7757-82-6 10361-37-2 64-17-5 1336-21-6 1310-73-2 12125-02-9 1787-61-7 493-52-7 7672-76-6

≥36.5% ≥99.0% ≥99.0% ≥99.8% ∼25.0−28.0% ≥99.0% ≥99.0% -

none none none none none none none none none recrystallization

99.0%

GCc

SCRC: Sinopharm Chemical Reagent Co., Ltd. bHPP: 1,4-bis(2-hydroxypropyl)-piperazine. cGC: Gas chromatography. B



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TLL = [(w1t − w1b)2 + (w2 t − w2 b)2 ]0.5

of 0.0001 g. The sulfate concentration of the top phase was determined through the volumetric method, and that in the bottom phase was determined using the weight method.28 HPP content in the top phase and that in the bottom phase were determined by the potentiometric titration method.14 HPP, as an organic base, can combine with two protons in aqueous solution, possessing a pKa1 value of 3.80 and pKa2 value of 8.20. When the pH value of aqueous solution is over 9.50 or less than 2.60, the pH value of the solution rapidly changes by adding acid or alkali. Thus, the concentration of HPP in aqueous solution was determined using the following method. First, HCl solution was added into the aqueous solution of HPP to obtain the pH value of 2.60, and then NaOH solution was gradually added to reach the pH value of 9.50. The amount of NaOH was recorded, and the mass fraction of HPP was calculated using eq 1. w1 =

0.101 × C NaOH × V m

STL =

(2)

w1t − w1b w2 t − w2 b

(3)

where w1t, w2t, w1b, and w2b are mass fraction of HPP in the top phase, mass fraction of Na2SO4 in the top phase, mass fraction of HPP in the bottom phase, and mass fraction of Na2SO4 in the bottom phase, respectively. 2.4. Density, Refractive Index, Surface Tension, and Viscosity Measurements. The density ρ of aqueous solution was measured by using a 25.00 mL Gay-Lussac pycnometer with uncertainty of 0.01 mL. The pycnometer containing solution was immersed in a well-stirred constant temperature water bath with (DF-II, Hong Kai Instrument Factory, Jiangsu China) temperature fluctuation of 0.1 K. The bath temperature was measured with a digital thermometer (A1-708P, Xiamen Yu Dian Automation Technology Co., Ltd., China). Once the solutions a reached certain temperature, they were weighed by the analytical balance (Mettler Toledo AL104) immediately. The internal volume of the pycnometer was calibrated by measuring the densities of distilled water. The reported value is an average of three measurements. Refractive indexes of the aqueous solutions were measured with the help of a thermostatic Abbe refractometer (Shanghai Optical Instruments Factory). Before use, the refractometer was calibrated with double distilled deionized water at experimental temperature range. Each refractive index value reported was the average of at least three measurements. The viscosities of aqueous solutions were measured using an Ubbelohde capillary viscometer (Shanghai Shengli Glass Instrument Co., Ltd., China). The viscometer was suspended inside a thermostatic water bath with temperature fluctuation of 0.1 K. The bath temperature was measured with a digital thermometer. Pure water was used for calibration. Each viscosity value reported was the average of at least three measurements. The surface tension measurements were carried out using a bubble pressure meter (DMP-2C, Applied Physics Institute of Nanjing University, China), which uses the maximum bubble pressure method. The temperature of the solutions was controlled with 0.1 K uncertainty. The apparatus was calibrated by measuring the surface tensions of distilled water. Each surface tension value reported was an average of five measurements.

(1)

where w1 is the mass fraction of HPP, CNaOH is the concentrations of NaOH in mol·L−1, V is the volume of NaOH solution in mL, and m is the total mass of the solution. A standard calibration curve showing the volume of the NaOH solution and HPP solution volume was constructed to show the validity of the method, as shown in Figure 3 with a square of correlation coefficient over 0.999.

Figure 3. Calibration curve of HPP mass fraction and the NaOH solution volume.

The H2O content in each phase was calculated by subtracting weights of Na2SO4 and HPP from the total weight of the phase. The tie-line length (TLL) and the slope of the tie-line (STL) were calculated using eqs 2 and 3.

Figure 4. Phase diagram of HPP−Na2SO4−H2O at selected temperatures: ■, 303.15; ○, 308.15; ▲, 313.15; ▽, 318; ◆, 323.15 K (w1, mass fraction of HPP; w2, mass fraction of Na2SO4). C



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Table 2. Binodal Data as the Mass Fraction for the HPP(w1)−Na2SO4(w2)−H2O ATPS at Selected Temperatures and Pressure p = 0.1 MPaa

a

100w1

100w2

100w1

100w2

100w1

1.37 1.51 1.691 1.871 2.05 2.29 2.69 3.28 3.67 4.25

26.86 26.30 25.72 25.05 24.28 24.26 23.44 22.03 21.76 20.99

4.69 5.52 6.15 6.83 6.79 8.98 9.90 11.15 12.07 12.82

20.38 19.45 18.88 18.18 17.94 16.14 15.41 14.55 13.89 13.33

14.29 16.17 17.02 17.89 18.83 19.52 20.26 20.90 22.04 22.99

0.58 0.70 0.75 0.83 0.95 0.97 1.06 1.15 1.28 1.39

30.33 29.61 29.26 28.86 28.36 28.32 27.63 27.44 27.01 26.43

1.54 1.69 1.89 2.20 2.50 3.00 3.78 4.76 6.20 6.87

26.15 25.69 25.15 24.42 23.77 22.75 21.35 20.20 18.77 17.63

9.00 11.15 13.43 15.24 17.87 19.43 21.41 22.37 23.71 25.94

0.58 0.71 0.84 1.01 1.22 1.52 1.91 2.38 3.02

30.05 29.52 28.82 27.81 26.93 26.32 25.00 24.38 23.04

4.20 6.87 11.54 13.95 15.99 18.37 20.08 21.46 22.75

21.73 17.77 14.23 12.65 11.47 10.17 9.26 8.51 7.86

24.19 25.36 26.87 27.61 28.58 29.51 30.43 31.31 31.75

0.81 0.84 0.91 1.01 1.22 1.55 2.04 2.84 3.90

31.79 30.86 28.87 27.11 26.45 25.97 24.47 22.75 20.95

5.24 7.30 9.44 13.44 15.92 17.37 20.99 22.27 23.82

19.07 17.12 15.96 13.30 11.69 10.82 8.89 8.23 7.45

24.75 25.58 26.47 27.90 29.20 29.93 30.80 31.61 32.42

0.76 0.88 1.10 1.32 1.72 2.21 2.72 3.46 4.15

29.27 28.48 27.24 26.47 25.17 23.78 22.80 21.29 20.44

5.60 7.10 9.63 12.64 16.12 17.78 19.80 22.72 23.85

18.74 17.30 15.40 13.40 11.35 10.44 9.35 7.98 7.43

26.02 27.65 28.57 29.95 31.31 31.96 32.98 33.72 34.20

100w2

100w1

T = 303.15 K 12.36 23.67 11.19 24.30 10.73 24.88 10.23 25.55 9.65 26.12 9.28 27.04 8.88 27.51 8.55 28.13 7.93 28.86 7.48 29.54 T = 308.15 K 16.04 27.12 14.65 27.99 13.14 29.17 11.98 30.21 10.44 30.98 9.56 31.70 8.49 32.52 7.98 33.16 7.33 33.46 6.32 34.48 T = 313.15 K 7.17 32.64 6.65 33.21 6.01 33.83 5.71 34.58 5.31 35.82 4.98 36.71 4.66 37.68 4.39 38.21 4.21 38.97 T = 318.15 K 7.00 33.06 6.60 33.76 6.24 34.33 5.64 35.08 5.16 35.77 4.85 36.18 4.58 36.67 4.31 37.28 4.08 37.90 T = 323.15 K 6.48 35.49 5.84 36.64 5.43 37.25 4.96 37.68 4.53 38.76 4.26 39.32 4.02 40.24 3.78 40.71 3.58 40.80

100w2

100w1

100w2

100w1

100w2

7.14 6.82 6.58 6.27 6.02 5.65 5.43 5.20 4.96 4.67

30.10 30.53 31.13 31.60 32.05 32.45 33.03 33.60 34.49 35.07

4.49 4.33 4.17 4.03 3.88 3.76 3.60 3.43 3.23 3.04

35.62 36.18 36.89 37.77 38.49

2.92 2.75 2.61 2.42 2.32

5.81 5.47 5.01 4.66 4.39 4.17 3.92 3.73 3.63 3.37

35.288 36.40 37.43 38.25 39.02 39.74 40.80 41.24 41.73 42.68

3.15 2.89 2.66 2.47 2.32 2.22 2.12 2.02 1.92 1.79

43.64 43.85 44.70 45.66 46.25 46.87 47.19 47.75

1.70 1.64 1.53 1.41 1.31 1.25 1.19 1.12

3.97 3.81 3.62 3.40 3.08 2.87 2.67 2.56 2.40

39.34 39.63 40.62 41.82 43.15 44.49 45.78 45.89 46.87

2.32 2.27 2.05 2.00 1.81 1.69 1.59 1.54 1.44

47.02 46.69 46.79 47.22 47.32 47.63 47.75 48.17 48.37

1.39 1.31 1.26 1.20 1.16 1.11 1.07 1.02 0.98

3.89 3.68 3.53 3.30 3.16 3.03 2.92 2.78 2.65

38.54 39.22 39.56 40.34 41.12 42.06 43.01 43.96 44.97

2.50 2.36 2.30 2.20 2.11 2.04 1.92 1.71 1.56

45.73 46.45 47.13 47.68 48.44 48.82 49.69 50.54

1.48 1.39 1.29 1.21 1.13 1.05 0.98 0.91

3.35 3.06 2.90 2.72 2.59 2.47 2.31 2.22 2.17

42.21 43.63 44.14 45.24 46.29 46.94 47.16 47.54 48.03

2.03 1.85 1.71 1.47 1.46 1.36 1.31 1.25 1.16

48.33 48.40 49.09 49.45 49.78

1.11 1.07 1.01 0.98 0.94

Standard uncertainties are u(T) = 0.01 K, u(w) = 0.005, and u(p) = 5 kPa.

3. RESULTS AND DISCUSSION

data are shown in Table 2. The biphasic region of the HPP− Na2SO4−H2O systems varies with rising temperature not obviously (Figure 4b), which suggests tiny variation in the calorific capacity of the phases, and consequently, low enthalpic change is associated with the separation process. The formation

3.1. Binodal Curves. The diagrams in Figure 4 represent binodal curves for the ATPS composed of HPP−Na2SO4−H2O, obtained by the turbidimetric method at the temperatures of 303.15, 308.15, 313.15, 318.15, and 323.15 K. The experimental D



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Table 3. Equilibrium Data of the HPP, Na2SO4, and H2O at Selected Temperatures and Pressure p = 0.1 MPaa,b global composition TL 1 2 3 4 5 T = 308.15 K 1 2 3 4 5 T = 313.15 K 1 2 3 4 5 T = 318.15 K 1 2 3 4 5 1 2 3 4 5

m1, g

m2, g

V3, mL

top phase V, mL

100w1 T = 303.15 K 36.81 35.42 33.37 31.00 28.61

bottom phase

100w2

100w1

100w2

100TLL

STL

1.94 2.89 3.25 4.33 5.64

1.69 2.28 2.72 3.70 6.54

27.03 25.08 24.01 22.45 19.18

43.16 39.88 37.02 32.77 30.90

−1.40 −1.49 −1.48 −1.51 −1.63

30.00 29.00 27.00 25.00 24.00

22.50 21.75 20.25 18.75 18.00

100.0 100.0 100.0 100.0 100.0

133.8 133.2 129.0 127.5 126.4

34.00 32.00 28.00 26.00 24.00

25.50 24.00 21.00 19.50 18.00

100.0 100.0 100.0 100.0 100.0

138.9 135.5 129.6 128.1 127.5

43.64 41.73 37.75 35.24 31.87

2.63 2.82 3.29 3.49 3.61

1.19 1.37 1.85 2.99 3.32

30.45 29.33 26.25 24.21 21.22

50.75 48.29 42.61 38.33 33.54

−1.53 −1.52 −1.56 −1.56 −1.62

36.00 32.00 28.00 26.00 24.00

27.00 24.00 21.00 19.50 18.00

100.0 100.0 100.0 100.0 100.0

141.2 136.0 130.5 129.1 127.8

45.78 41.82 37.44 34.22 29.74

1.59 2.80 3.32 4.35 5.49

1.28 1.74 3.19 3.79 5.93

29.026 26.75 24.03 22.35 19.48

52.28 46.68 40.03 35.35 27.62

−1.62 −1.67 −1.65 −1.69 −1.70

38.00 34.00 28.00 26.00 24.00

28.50 25.50 21.00 19.50 18.00

100.0 100.0 100.0 100.0 100.0

143.0 141.1 131.2 130.5 129.0

2.01 2.30 3.73 4.27 5.98

1.06 1.34 2.74 3.89 5.76

29.69 27.81 24.04 22.05 20.79

54.36 50.36 40.93 36.36 30.38

−1.69 −1.70 −1.75 −1.78 −1.79

40.00 34.00 30.00 26.00 24.00

30.00 25.50 22.50 19.50 18.00

100.0 100.0 100.0 100.0 100.0

146.0 140.1 135.4 130.5 129.2

1.14 2.07 2.63 4.05 5.97

0.96 1.69 2.60 3.64 6.65

29.06 27.30 24.31 21.47 19.81

56.81 50.98 44.28 35.65 28.83

−1.77 −1.76 −1.78 −1.78 −1.82

47.85 44.76 38.27 35.61 32.28 T = 323.15 K 50.44 45.98 41.21 34.74 31.94

a

Standard uncertainties are u(T) = 0.01 K, u(m)=0.01g, u(V)=0.1 mL, u(w) = 0.005, and u(p) = 5 kPa. bm1, mass of HPP; m2, mass of Na2SO4; V3, volume of H2O; V, total volume of the solution; w1, mass fraction of HPP; w2, mass fraction of Na2SO4; TLL, tie-line length; STL, slope of the tieline.

K. Four tie lines corresponding to the system are determined and represented in Figure 5. As the results indicated, an exclusionary relationship between the HPP and Na2SO4 in the system is confirmed independent of the temperature. This kind of exclusion existed in the ATPS composed of light alcohols− salts or amphiphilic molecules−salts. Generally, the top phase is mainly composed of organic compounds and water, while the bottom phase is mainly composed of salt and water. The reliability of experimentally measured liquid−liquid equilibrium data was verified by applying the Othmer−Tobias correlation:31

of the biphasic region is ascribed to the salting out effect of Na2SO4 in HPP aqueous solution, like those ATPS composed of ethanol/ammonium sulfate, 2-propanol/ammonium sulfate,3 or 1-propanol/ammonium sulfate.29 HPP possesses both hydrophilic and hydrophobic groups in its molecular structure, so it can be regarded as amphiphilic, which has been confirmed by our previous work that showed that HPP had higher solubility in methanol or ethanol than in water.30 In terms of microcosmos, the “ion−dipole” interaction between salt ions and H2O molecules can produce the hydration of ions, and then the phase-forming slat, in this work, Na2SO4, can dissolve in the bottom phase. Meanwhile, the mentioned interaction above decreases the amount of free H2O molecules in the bottom phase, and it leads to the exclusion of HPP in the top phase. As a result, the top phase is mainly composed of HPP and H2O, and at the same time the bottom phase is mainly composed of Na2SO4 and H2O. 3.2. Liquid−Liquid Equilibrium Data. The equilibrium compositions of the ATPS composed of HPP−Na2SO4−H2O at different temperatures are presented in Table 3, where the data referring to the phase components are expressed in terms of mass fraction (w/w, %). It is known that the solubility of HPP and Na2SO4 in the water increases with the temperature increase; thus, four different phase equilibrium experiments are carried out in different concentrations at the temperatures of 303.15−323.15

⎛1 − w b ⎞ ⎛ 1 − w1t ⎞ 2 ⎟ a c ln⎜⎜ ln = + ⎟ ⎜ ⎟ t b ⎝ w1 ⎠ ⎝ w2 ⎠

(4)

where, a and c are intercept and slopes for the Othmer−Tobias equation and w1t and w2b are the mass fraction of HPP in the top phase and mass fraction of Na2SO4 in the bottom phase, respectively. The straight lines calculated by eq 4 are shown in Figure 6. The fitting parameters (a and c) and corresponding regression coefficients (R2) are listed in Table 4. The regression coefficients are very close to 1, which indicates the experimental liquid− liquid equilibrium data are highly consistent. E



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temperature. The TLL value is dependent on the difference in HPP and Na2SO4 concentrations in the top and bottom phases. TLL values for the studied ATPS are shown in Table 3 and Figure 5. From Table 3, it is found that the TLL is closely related with the global composition. With the decrease of HPP mass fraction in the global composition, all the TLL values decrease irrespective of the temperatures in the range from 303.15 to 323.15 K. According to Sousa’s report,32 the TLL values are associated with the selectivity of the system and the global composition data of the system. The shorter and lower the partitioning coefficients value for the target is, the less selective the extraction can be. Thus, if the ATPS was utilized for the removal of the HSS in the rich amine sulfate solution, it was strongly suggested to add HPP to rich amine sulfate aqueous solution to increase the HPP mass fraction first and then remove the HSS through the ATPS system. The addition of HPP for recovery of its SO2 absorption capacity and HSS removal should be combined together, followed by adjusting the pH values of the absorbent. The STL for the ATPS with different global compositions and temperature is also shown in Table 3. The results indicate that STL became more negative with the temperature increasing. This behavior is reasonable, since the STL variation is caused by the increase of hydrophobicity, which is intensified by the temperature increase and causes H2O molecules of the solvating HPP to migrate from the top phase to the bottom phase. The reduction of H2O molecules leads to an increase of Na2SO4 concentration in the bottom phase, a small dilution in the top phase, and, consequently, an increased STL. Thus, for all the tie lines, an increase in HPP mass fraction in global composition resulted in higher HPP mass fraction and lower Na2SO4 in the top phase, while for the bottom phase, the mass fraction of sulfate increases but the mass fraction of HPP decreases. This trend is similar to that of the polymer−salt ATPS, where the conformational entropy of the polymer increased with the increase of temperature due to the folding of the polymer chain.33 In ATPS of this work, the conformational entropy for the packing of HPP molecules related with molecular association and hydrogen bonding also can increase with the temperature increase, and, thus, a similar trend that H2O molecules transfer from the top phase to the bottom phase can be found when the temperature for the ATPS increases. At the same HPP mass fraction in global composition, the lower the TLS value, the higher the content of Na2SO4 in the bottom phase. That is the separation process that is desired. 3.3. Physical Properties. The physical properties of the top and bottom phases present distinct characteristics in relation to composition and temperature. This information is necessary for design and scaling up for production and extraction processes. Therefore, in Table 5, the experimental values of viscosity (η/ (mPa·s)), density (ρ/(kg·m−3)), surface tension (γ/(mN·m−1)), and refraction index (nD) are presented for the phases of the ATPS composed of HPP−Na2SO4−H2O at the temperatures of 303.15, 308.15, 313.15, 318.15, and 323.15 K. The density of the top phases of the ATPS composed of HPP− Na2SO4−H2O at 303.15 K varies from 1.0584 g·cm−3 to 1.0649 g·cm−3, and that of the bottom ones varies from 1.2696 g·cm−3 to 1.2018 g·cm−3. By means of data analysis, an increase in density with global compositions of the ATPS increasing is confirmed in the top phases, but in the bottom ones the density has declined. The density of the HPP solution is much smaller than that of the Na2SO4 solution.15 As shown in Table 3, with the decrease of HPP concentration in the top phase, the concentration of

Figure 5. Liquid−liquid equilibrium for systems composed of HPP− Na2SO4−H2O at temperatures (a) 303.15 K, (b) 308.15 K, (c) 313.15 K, (d) 318.15 K, and (e) 323.15 K (w1, mass fraction of HPP; w2, mass fraction of Na2SO4).

Figure 6. Othmer−Tobias plot of HPP−Na2SO4−H2O ternary system at selected temperatures: ■, 303.15 K; ☆, 308.15K; ▲, 313.15K; ○, 318.15K; ◆, 323.15K (w1t, mass fraction of HPP in the top phase; w2b, mass fraction of Na2SO4 in the bottom phase).

Table 4. Othmer−Tobias Equation Parameters Fitted from Liquid−Liquid Equilibrium Data T, K

a

c

R2

303.15 308.15 313.15 318.15 323.15

0.4055 0.5635 0.7610 0.7993 0.8927

1.0968 0.9657 0.7570 0.7451 0.6572

0.9828 0.9975 0.9994 0.9978 0.9979

The TLL is an important thermodynamic parameter, expressing the difference in intensive thermodynamic functions between the top and the bottom phases at constant pressure and F



Article

Table 5. Density (ρ), Refraction Index (nD), Viscosity (η), and Surface Tension (γ) for the Top and Bottom Phases of the System Composed of HPP−Na2SO4−H2O at Selected Temperatures and Pressure p = 0.1 MPaa top phase TL T = 303.15 K 1 2 3 4 5 T = 308.15 K 1 2 3 4 5 T = 313.15 K 1 2 3 4 5 T = 318.15 K 1 2 3 4 5 T = 323.15 K 1 2 3 4 5

ρ, g·cm

−3

bottom phase nD

η, mPa·s

γ, mN·m−1

1.2696 1.2521 1.2380 1.2018 1.1992

1.3731 1.3729 1.3720 1.3718 1.3717

2.41 2.39 2.34 2.17 2.13

49.9 49.8 49.0 48.7 48.6

45.1 45.4 46.2 46.6 46.8

1.2772 1.2671 1.2456 1.2178 1.2129

1.3760 1.3749 1.3709 1.3702 1.3690

2.33 2.21 1.99 1.93 1.87

47.7 47.5 47.2 47.0 46.8

8.44 6.65 4.90 4.30 3.47

43.6 44.1 44.8 45.1 45.4

1.2686 1.2564 1.2234 1.2093 1.1809

1.3749 1.3720 1.3705 1.3700 1.3696

1.96 1.81 1.74 1.65 1.61

47.5 46.6 46.4 46.1 45.8

1.4244 1.4190 1.4083 1.4030 1.3961

7.98 6.41 4.22 3.48 2.94

41.9 42.2 42.9 43.7 44.0

1.2810 1.2610 1.2175 1.1973 1.1743

1.3745 1.3728 1.3695 1.3680 1.3678

1.85 1.71 1.50 1.46 1.45

44.8 44.5 44.2 44.2 44.1

1.4236 1.4185 1.4120 1.4003 1.3979

6.63 5.67 4.35 3.10 2.74

41.6 42.5 43.0 43.4 43.6

1.2701 1.2490 1.2210 1.2010 1.1685

1.3738 1.3703 1.3686 1.3678 1.3674

1.69 1.50 1.39 1.36 1.33

43.7 43.2 43.0 42.6 42.1

nD

η, mPa·s

γ/mN·m

1.0584 1.0590 1.0611 1.0649 1.0685

1.4101 1.4070 1.4039 1.3992 1.3950

8.10 7.60 6.49 5.68 4.33

47.0 48.0 48.5 48.9 49.2

1.0563 1.0566 1.0616 1.0655 1.0671

1.4201 1.4140 1.4079 1.4022 1.3975

9.19 8.70 6.37 5.40 4.32

1.0510 1.0526 1.0545 1.0551 1.0656

1.4199 1.4139 1.4062 1.4020 1.3968

1.0470 1.0480 1.0516 1.0557 1.0620 1.0418 1.0441 1.0459 1.0530 1.0594

−1

−3

ρ, g·cm

a Standard uncertainties are u(T) = 0.1 K and u(p) = 5 kPa. Expanded uncertainties are uc(ρ) = 4 × 10−4 g·cm−3, uc(nD) = 0.0002, uc(η) = 0.06 mPa·s, and uc(γ) = 0.6 mN·m−1 (level of confidence = 0.95).

With the increase of global compositions of the ATPS, an increase in viscosity for the top phases and bottom phases is observed for all the temperatures. The viscosities of ATPS composed of HPP−Na2SO4−H2O are lower than those of ATPS mainly composed of PEG−salt.34 Lower viscosity values are ideal for the process of extracting the ATPS, since they are highly beneficial in industrial processes. At the same time, the maximum viscosity difference for the top phase and bottom phase reaches about 350%. According to the results of Da Silva et al.,33 the high difference in viscosity between phases is necessary in consideration of operation, since it increases the phase separation rate. It is found that at all temperatures the surface tensions of the top phases decrease with the decrease of global compositions of ATPS composed of HPP−Na2SO4−H2O, while those of the bottom ones increase, as shown in Table 5. The difference in surface tension is not so significant. Generally, increase in temperature results in lower surface tensions for both the top and the bottom phase. At the same time, at each temperature, the surface tension in the top phase increased with the w2, e.g., the mass fraction of Na2SO4. The surface tension of the bottom phases also shows a positive relation with w2.

Na2SO4 is increasing. As a result, the density of the top phase rises with the decline of HPP concentration, but the value of the increase is relatively small. The difference in densities for the ATPS benefits the kinetics of phase separation.34 The difference in densities for the top and bottom phases is larger than that of the ATPS composed of PEG−Na2SO4−H2O,35 which is better for phase formation. These trends are also observed at temperatures of 308.15, 313.15, 318.15, and 323.15 K. The refraction index of the ATPS composed of HPP− Na2SO4−H2O falls insignificantly with temperatures The top phase presents higher values than the bottom phase because of its rising density, which makes light propagation more difficult.36 The refractive indexes of the top phase and bottom phases fell with the decrease of the global concentration, due to the higher content of H2O in top and bottom phases. It can be seen from Table 5 that the viscosity of the top phase descends with the decrease of HPP concentration in the solution. This is because the viscosity of HPP solution ascends with the increase of the concentration or with the decline of temperature.15 The viscosity of sodium sulfate solution is not obvious with the change of concentration. In the ATPS of HPP− Na2SO4−H2O, the concentration of HPP is much higher than that of Na2SO4, so the viscosity of the top phase decreases with the decline of HPP concentration. It is observed that the HPPrich top phase is more viscous than the salt-rich bottom phase. G



Article

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4. CONCLUSIONS HPP, Na2SO4, and H2O can form ATPS; the top phase contains a lot of HPP and a small amount of Na2SO4, but the bottom phase contains a lot of Na2SO4 and a small amount of HPP. Liquid− liquid equilibrium data were obtained for the ATPS composed of HPP−Na2SO4−H2O at temperatures of 303.15, 308.15, 313.15, 318.15, and 323.15 K. The effect of the temperature on the systems did not present large variations in the biphasic region. There was an increase in the tie line slope because of the increased temperature. This leads to the conclusion that H2O molecules are transferred from the top phase to the bottom one. Values of viscosity, density, surface tension, and refraction index were determined experimentally for all systems studied. Viscosity, density, surface tension, and refraction index were influenced by concentration. Viscosity and refraction index increased with the system components increasing, during the phase of both HPP and Na2SO4, while density and surface tension increased with the system components rising up in the bottom phase and decreasing in the top phase. The ATPS of HPP−-Na2SO4−H2O could provide potential applications for separating matters being stable in alkaline conditions. Meanwhile, ATPS would be a new method for removing the HSS containing sulfuric acid root of absorbent for flue gas desulfurization, which is HPP.

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AUTHOR INFORMATION

Corresponding Author

*(P.C.) Tel.: +86-551-62902661. Fax: +86-551-62901450. Email: [email protected]. ORCID

Xiangying Chen: 0000-0002-0433-4759 Peng Cui: 0000-0003-2547-9516 Funding

Financial support was from the Ministry of Anhui Provincial Science and Technology (No. 08010202124). Notes

The authors declare no competing financial interest.

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piperazine, Na2SO4, and H2O: Temperature - American Chemical

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