Solubility of dilute SO2 in 1,4-dioxane, 15-crown-5 ether, polyethylene glycol 200, polyethylene glycol 300, and their binary mixtures at 308.15 K and 122.66 kPa

Fluid Phase Equilibria 344 (2013) 65–70

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Solubility of dilute SO2 in 1,4-dioxane, 15-crown-5 ether, polyethylene glycol 200, polyethylene glycol 300, and their binary mixtures at 308.15 K and 122.66 kPa Yanxia Niu a , Fei Gao b , Shaoyang Sun a , Jianbai Xiao a , Xionghui Wei a,∗ a b

Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China College of Chemical Engineering, Inner Mongolia University of Technology, Huhhot 010051, China

a r t i c l e

i n f o

Article history: Received 24 September 2012 Received in revised form 11 January 2013 Accepted 16 January 2013 Available online 26 January 2013 Keywords: Solubility Sulfur dioxide Polyethylene glycol Cyclic ether Henry’s law constant

a b s t r a c t This work reports the solubility data of dilute sulfur dioxide in 1,4-dioxane, 15-crown-5 ether, polyethylene glycol 200 (PEG 200), polyethylene glycol 300 (PEG 300), and their binary mixtures at 308.15 K and 122.66 kPa with the SO2 partial pressure ranging between (7.35 and 118 Pa). From the solubility data, Henry’s law constants of SO2 in pure solvents were calculated. The results show that the solubilities of SO2 in these four solvents increase linearly with the enhancement of the equilibrium partial pressure of SO2 in the gas phase within the studied regions. Furthermore, the absorption of SO2 in pure 1,4-dioxane, 15-crown-5 ether, PEG 200 are typical physical processes, and the solubility of SO2 in the three solvents decreases in the order: 1,4-dioxane > 15-crown-5 ether > PEG 200. But the SO2 absorption in PEG 300 is a physical process accompanied by a chemical process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Sulfur dioxide, mainly emitted from fossil-fuel combustion, is one of the main contributors to the atmospheric pollutants in industrial area. The disposal of SO2 emission has become a key issue of global concern. Flue gas desulfurization (FGD) techniques [1–3], including wet FGD, dry FGD, and semidry FGD processes, have been widely applied in industry. Nevertheless, these methods generate secondary pollutant and useless byproduct such as calcium sulfate. So further fundamental research is needed in order to improve the current FGD techniques. Recently, ionic liquids at room temperature (ILs) [4–6] have attracted much interest due to their advantages, such as good solubility and high selectivity to SO2 . However, the high viscosity and expense of ionic liquids prohibit their use in industry. Actually, one of the most attractive approaches for separating a gas from a gas mixture stream is by solvent absorption [7,8]. Generally, the solvent-based process can be broadly classified as chemical absorption, physical absorption, and mixed chemical/physical absorption. Many economical solvent-based processes are based on physical absorption, such as N-methyl-2-pyrrolidinone, dimethyl sulfoxide, N,N-dimethylformamide, propylene carbonate, etc. [9–11]. The

∗ Corresponding author. Tel.: +86 010 62751529; fax: +86 010 62670662. E-mail address: [email protected] (X. Wei). 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.01.008

solvent-based physical absorption is handled at low temperature as well as high pressure, and solvent can be regenerated by the pressure or temperature swing processes. Polyethylene glycols (PEG) [12,13], which have the favorable features of nontoxic, chemical stability, low vapor pressure, and low melting point, are important industrial solvents used for absorbing SO2 . In the previous work [14,15], the gas–liquid equilibrium (GLE) properties for absorption of dilute SO2 in PEG and PEG + water have been studied systematically. The results show that these solutions have good absorption and desorption properties. This work is a continuation of phase equilibrium studies on the SO2 absorption properties of organic solvents [16,17]. The GLE experiments were performed at a temperature of 308.15 K and a pressure of 122.66 kPa. New solubility data of SO2 in 1,4-dioxane, 15-crown-5 ether, polyethylene glycol 200, polyethylene glycol 300 and their binary mixtures were presented to show the effect of functional group on the solvent’s ability to dissolve SO2 , and at the same time offer some essential data for chemical engineering. 2. Experimental 2.1. Materials A certified standard gas SO2 in N2 (8000 ppmv) was supplied by Beijing Gas Center, Peking University (China); 1,4-dioxane, polyethylene glycol 200 (PEG 200) and polyethylene glycol 300 (PEG

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Y. Niu et al. / Fluid Phase Equilibria 344 (2013) 65–70

Table 1 Basic properties of 1,4-dioxane, 15-crown-5 ether, PEG 200, PEG 300. Component

DX 15-5 CE PEG 200 PEG 300 a

General formula

( CH2 CH2 O )n ( CH2 CH2 O )n HO( CH2 CH2 O )n H HO( CH2 CH2 O )n H

n

2 5 4.1 6.4

 (g cm−3 )

Mw

88.11 220.27 200 300

 (mPa s)

Expt.

Lit.

1.0281a 1.106a 1.1215a 1.1234a

1.02806 [18]

Expt. 1.1972a

Lit. 1.196 [19]

1.12098 [18]

40.7273a 61.7242a

48.157 [18]

Measurements at 298.15 K; n denotes the average number of repeating units.

300) were purchased from Beijing Reagent Company; 15-crown-5 ether (15-5 CE) was obtained from Application Chemical Research Institute, Xianju, Zhejiang province. All substances were used without further purification. Some properties of these substances are listed in Table 1.

The maximum relative error is obtained by

2.2. Apparatus and experimental procedures

3.1. Gas solubility in pure components

The solubility data for SO2 in 1,4-dioxane, 15-crown-5 ether, PEG 200, PEG 300, and their binary mixtures were obtained by using a dynamic analytic method reported in the previous work [14] at 308.15 K and 122.66 kPa with the SO2 partial pressure being between (7.35 and 118 Pa). The concentrations of SO2 in the gas phase were determined by a gas chromatograph (Agilent 6890N) equipped with a 2 m × 3.2 mm Porapak Q packed column, an FPD detector and an HP6890 workstation. To determine the relationship between the concentrations of SO2 and the response values of the gas chromatograph (GC), a calibration curve was constructed using an external standard method. According to Ref [20], the sulfur (IV) concentration in the liquid phase (CSO2 ) was determined. Each experiment was carried out at 308.15 K and 122.66 kPa.

The solubility experiments for dilute SO2 in pure 1,4-dioxane, 15-crown-5 ether, PEG 200 and PEG 300 were performed at 308.15 K and 122.66 kPa. The solubility data are listed in Table 2, and solubility curves are shown in Fig. 1. In Table 2, the ˚SO2 denotes the volume fraction of SO2 in the gas phase, CSO2 denotes the mass concentration of SO2 in the liquid phase, and pSO2 is the partial pressure of SO2 in an equilibrium state. The ˚SO2 can be calculated from

The temperature was maintained constant by a circulation water bath with ±0.02 K uncertainty, and the system pressure was determined by using a pressure gauge with an accuracy of ±0.1 kPa. The relative uncertainty of SO2 concentration in the liquid phase was estimated to be ±0.8%. The FPD detector of GC was calibrated with the various standard gas mixtures (SO2 + N2 ) before applied to determine the concentration of SO2 in the gas phase. The calibration results show that GC method presents high stability with ±1.0% uncertainty, and the calibration curves are linear by double logarithm method. Under given conditions (T = 308.15 K and Ptotal = 122.66 kPa), the calibration relation is found to be y = 0.4381х + 1.0642 with the correlation coefficient (r) being 0.9971. Here, x represents the logarithm of the SO2 peak area and y represents the logarithm of the volume fraction of SO2 in the gas phase. In this work, the maximum deviation value is defined as ¯ (y¯ − ymin )) y = max((ymax − y),

(1)

where ymax and ymin are the maximum and minimum measurements of SO2 concentration in the gas phase respectively, and y¯ is the average of eight measurement values. They are calculated by the formula below: ymax = max(y1,exp , y2,exp , . . . , y8,exp )

(2)

ymin = min(y1,exp , y2,exp , . . . , y8,exp )

(3)

 yi,exp

y − y y¯

(5)

3. Results and discussion

˚SO2 ≈

VSO2 VSO2 + VN2 + Vsolvent

=

VSO2

(6)

Vtotal

where VSO2 , VN2 and Vsolvent denote the partial volume of SO2 , N2 and solvent vapor in gas phase, respectively. The Vtotal denotes the total volume of gas liquid equilibrium (GLE) system. The pSO2 is calculated by pSO2 = Ptotal × ˚SO2

(7)

where Ptotal denotes the total pressure of the GLE system, and its uncertainty can be mainly attributed to the uncertainty of ˚SO2 . From Table 2 and Fig. 1, it can be seen that the equilibrium partial pressure of SO2 showed a linear response to the concentration of SO2 in the four solvents here, which indicates that the absorption

120

100

80

pSO2/Pa

2.3. Data treatment and sources of error

ı=

60

40

20

0 0

300

600

900

1200

CSO2/mg L

1500

1800

2100

-1

8

y¯ =

i=1

8

(4)

Fig. 1. Solubility of SO2 in 1,4-dioxane, 15-crown-5 ether, PEG 200 and PEG 300 at 308.15 K and 122.66 kPa: , 1,4-dioxane; , 15-crown-5 ether; , PEG 200; ♦, PEG 300; —, curve fitting.

Y. Niu et al. / Fluid Phase Equilibria 344 (2013) 65–70

67

Table 2 Volume fraction (˚SO2 ), mass concentration (CSO2 ), partial pressure (pSO2 ) in 1,4-dioxane, 15-crown-5 ether, PEG 200 and PEG 300 at 308.15 K and 122.66 kPa.

1,4-Dioxane 153 277 380 398 503 15-Crown-5 ether 176 312 475 555 PEG 200 170 295 431 PEG 300 174 266 417

CSO2 (mg L−1 )

pSO2 (Pa)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

314.4 510.8 700.4 710.4 905.0

18.8 34.0 46.6 48.8 61.7

574 648 799 964

1070 1278 1707 2010

70.4 79.5 98.0 118

234.4 388.0 604.2 723.1

21.6 38.2 58.2 68.1

611 751 804

850.2 992.2 1159

74.9 92.1 98.6

151.9 259.2 343.4

20.8 36.2 52.9

557 696 824

457.3 584.4 736.3

68.4 85.4 101

435.8 505.2 615.8

21.4 32.6 51.2

711 800 938

767.7 828.7 944.3

87.2 98.2 115

processes obey Henry’s law within the region of investigated pressure. Furthermore, it was also found that the solubility curves for SO2 in 1,4-dioxane, 15-crown-5 ether and PEG 200 approximately pass the coordinate zero, which illustrates the absorption of SO2 in the three liquids are typical physical processes at the studied pressure herein [21,22]. The solubility of SO2 in the three solvents decreases in the order: 1,4-dioxane > 15-crown-5 ether > PEG 200. However, the prolonged solubility curve for SO2 in PEG 300 can cross with the horizontal axis. This seemingly means that the absorption of SO2 in PEG 300 involves chemical and physical processes [23]. Therefore, the desorption experiments were also performed using N2 extraction method at the temperature of 65 ◦ C under a constant flow of 2 L min−1 for the systems of SO2 + 1,4-dioxane, SO2 + PEG 200 and SO2 + PEG 300. The corresponding desorption curves were given in Supplementary Data (Fig. 1). From which, the concentrations of SO2 in 1,4-dioxane and PEG 200 decrease dramatically to zero within 15 min. Yet the process of SO2 desorption from PEG 300 is divided into two steps. Within 20 min SO2 concentration in liquid reduces quickly, but the concentration changes slowly after that. Even the desorption time reaches 90 min, there are still 7% of SO2 remaining in PEG 300. The desorption results further supports the above analysis. These differences of solubility in the four solvents may result from the variance of their molecular structures. The four solvents can be classified as follows: (1) epoxide compound, with cyclic ether bond (two oxygen atoms in 1,4-dioxane, and five oxygen atoms in 15-crown-5 ether); (2) polyethylene glycols, with two hydroxyl groups and chain ether bond (with average 4.1 and 6.4 ether bond in PEG 200 and PEG 300, respectively). As a Lewis base with lone pair electrons on oxygen atom of ether group, the epoxide compound can form some molecular complex with another Lewis acid, and at the same time, the oxygen atoms of ether group were stretched outward [24], which may lead to better interaction with SO2 . For polyethylene glycols, the hydroxyl can form intramolecular hydrogen bond, which will result in a stronger interaction between the solvent molecules themselves, and the cooperativity effects of the intramolecular and intermolecular hydrogen bonds enables the formation of multimer hydrogen bonds in PEG [25,26]. Based on previous study [27], it is well known that the absorption process of a gas substance in physical solvent can be divided into two steps: firstly, the interaction of solvent molecules must be broken to provide some cavities which can accommodate the incoming molecules; secondly, the cavities formed in the first step are filled with the gas molecules and new interactions of the gas molecules with solvent molecules are

created. Because of the higher density of free O groups in PEG 300 than PEG 200, the former showed superior absorbing capacity for SO2 . 3.2. Henry’s law constant For a gas substance, its solubility in a liquid can be generally described in terms of Henry’s law, which is defined as [28]: H1 (T, P) ≡ lim

f1L

C1 →0 C1

≈

P1 C1

(8)

where H1 (T, P) is the Henry’s law constant, C1 is the concentration of gas dissolved in the liquid phase, f1 L is the fugacity of vapor in the liquid phase, and P1 is the pressure of the gas. Eq. (8) implies that the solubility of gas which behaves nearly ideally is linearly related to the pressure. As shown in Fig. 1, the experimental results for SO2 in 1,4-dioxane, 15-crown-5 ether, PEG 200 and PEG 300 can be reasonably explained by Henry’s law with the investigated pressure range. Therefore, those Henry’s law

120

100

pSO2/Pa

106 ˚SO2

80

60

40

20

0

0

300

600

900

1200

CSO2/mg· L

1500

1800

2100

-1

Fig. 2. Solubility of SO2 in the binary mixtures of 1,4-dioxane + 15-crown-5 ether at 308.15 K and 122.66 kPa. VFDX denotes the volume fraction of 1,4-dioxane in the mixtures: , VFDX = 0%; , VFDX = 20%; , VFDX = 30%; ♦, VFDX = 50%; 夽, VFDX = 100%.

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Y. Niu et al. / Fluid Phase Equilibria 344 (2013) 65–70

Table 3 Henry’s constants (H1 ) of SO2 in 1,4-dioxane, 15-crown-5 ether, PEG 200 and PEG 300 at 308.15 K and 122.66 kPa. Component

1,4-Dioxane

15-Crown-5 ether

PEG 200

PEG 300

100H1 (Pa mg−1 L)

5.76 ± 0.002

8.71 ± 0.003

14.1 ± 0.005

19.2 ± 0.007

Table 4 Volume fraction (˚SO2 ), mass concentration (CSO2 ), partial pressure (pSO2 ) in 1,4-dioxane + 15-crown-5 ether, at 308.15 K and 122.66 kPa. VFDX denotes the volume fraction of 1,4-dioxane in the mixtures. VFDX (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

VFDX (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

20 20 20 20 20 20 20 30 30 30

144 238 350 510 625 737 804 126 300 383

269.1 416.0 533.2 782.5 960.8 1167 1345 224.5 511.8 620.7

17.6 29.1 42.9 62.5 76.7 90.4 98.6 15.4 36.8 47.0

30 30 30 30 50 50 50 50 50 50

511 665 736 805 87.0 260 402 495 599 719

843.6 1156 1347 1476 79.24 386.3 696.7 884.9 1065 1403

62.6 81.5 90.3 98.7 10.7 31.9 49.3 60.7 73.4 88.2

Table 5 Volume fraction (˚SO2 ), mass concentration (CSO2 ), partial pressure (pSO2 ) in 1,4-dioxane + PEG 200, at 308.15 K and 122.66 kPa. VFDX denotes the volume fraction of 1,4-dioxane in the mixtures. VFDX (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

VFDX (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

20 20 20 20 20 20 20 30 30 30 30

147 272 382 470 607 713 815 151 275 441 620

163.4 298.8 401.2 529.9 655.4 817.2 927.8 227.8 361.5 559.6 789.1

18.1 33.4 46.8 57.7 74.4 87.5 99.9 18.5 33.7 54.1 76.1

30 30 30 50 50 50 50 50 50 50

711 803 906 159 263 396 480 616 788 872

936.0 1137 1315 264.1 421.0 658.7 804.0 1088 1403 1586

87.2 98.5 111 19.5 32.2 48.5 58.8 75.6 97.0 107

Table 6 Volume fraction (˚SO2 ), mass concentration (CSO2 ), partial pressure (pSO2 ) in 1,4-dioxane + PEG 300, at 308.15 K and 122.66 kPa. VFDX denotes the volume fraction of 1,4-dioxane in the mixtures. VFDX (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

VFDX (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

10 10 10 10 10 10 10 20 20 20 20 20 20 20

73.9 211 451 553 675 802 892 73.5 166 274 379 573 710 788

345.0 374.8 468.8 582.8 690.1 808.9 914.6 254.2 317.0 432.5 551.4 749.5 901.4 1012

9.06 25.8 55.3 67.8 82.8 98.4 109.4 9.01 20.3 33.6 46.4 70.3 87.0 96.7

30 30 30 30 30 30 30 50 50 50 50 50 50 50

131 282 418 587 739 809 875 67.8 193 342 513 580 714 863

523.3 610.8 721.4 939.4 1179 1271 1395 376.4 523.3 737.9 983.9 1015 1332 1659

16.1 34.6 51.2 72.0 90.7 99.3 107 8.31 23.7 42.0 62.97 71.2 87.5 106

Table 7 Volume fraction (˚SO2 ), mass concentration (CSO2 ), partial pressure (pSO2 ) in 15-crown-5 ether + PEG 300, at 308.15 K and 122.66 kPa. VF15-5 denotes the volume fraction of 15-crown-5 ether in the mixtures. VF15-5 (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

VF15-5 (%)

106 ˚SO2

CSO2 (mg L−1 )

pSO2 (Pa)

20 20 20 20 20 20 20 30 30 30

60.0 176 326 430 618 774 916 111 216 303

323.6 521.7 629.0 761.1 904.7 1060 1245 168.4 264.1 376.4

7.35 21.6 40.0 52.7 75.8 94.9 112 13.6 26.5 37.2

30 30 30 50 50 50 50 50 50 50

491 622 742 111 270 454 570 680 824

566.2 695.0 850.2 161.8 330.2 511.8 642.2 772.6 995.5

60.3 76.3 91.0 13.6 33.2 55.7 69.9 83.4 101

120

120

100

100

80

80

pSO2/Pa

pSO2/Pa

Y. Niu et al. / Fluid Phase Equilibria 344 (2013) 65–70

60

40

20

20

0

300

600

900

1200

CSO2/mg· L

1500

1800

2100

0

200

400

600

800

CSO2/mg· L

constants were acquired by calculating the linear slope of the solubility curves. And all these values are given in Table 3. 3.3. Gas solubility in binary mixtures A series of solubility experiments of SO2 in the binary mixtures of 1,4-dioxane + 15-crown-5 ether, 1,4-dioxane + PEG 200, 1,4-dioxane + PEG 300, and PEG 300 + 15-crown-5 ether were measured. The results were shown in Tables 4–7, and plotted in Figs. 2–5. In these tables and figures, the VFDX and VF15-5 denote the volume fraction of 1,4-dioxane and 15-crown-5 ether in their binary mixtures, respectively. From Figs. 2 and 3, it can be seen clearly that the solubility of SO2 in DX + 15-5 CE and DX + PEG 200 all increases with the volume fraction of DX raise, but this tendency is more marked for the DX + PEG 200 mixture. Figs. 4 and 5

120

1000

1200

-1

Fig. 5. Solubility of SO2 in the binary mixtures of 15-crown-5 ether + PEG 300 at 308.15 K and 122.66 kPa. VF15-5 denotes the volume fraction of 15-crown-5 ether in the mixtures: , VF15-5 = 0%; , VF15-5 = 20%; , VF15-5 = 30%; 夽, VF15-5 = 50%; ♦, VF15-5 = 100%.

show the SO2 solubility in 1,4-dioxane + PEG 300, and 15-crown5 ether + PEG 300, respectively. It can be seen that SO2 solubility does not increase monotonously with the concentration of DX raise when the partial pressure changes from lower to higher values, which is different from the case of 1,4-dioxane + 15-crown-5 ether and 1,4-dioxane + PEG 200. This may be attributed to the interaction between the components of liquid mixtures. According to the Ref. [29–31], PEG, with terminal hydroxyl groups, bears both donors and acceptors characteristics, while the O-atoms of cyclic ethers can only act as proton acceptors. Actually, the terminal hydroxyl groups can give head-to-tail connections and form O H· · ·O chains in PEG molecules. Adding cyclic ethers to PEG can perturb the Hbonds of the original head-to-tail connections and instructs new H-bonds between them. The viewpoint was also illustrated through the Hückel calculations on isolated molecules of the cyclic ethers by Ottani et al. [18,32]. So for PEG 300, the capability of absorbing SO2 was influenced by the cyclic ethers. 4. Conclusion

100

80

60

40

20

0

0

-1

Fig. 3. Solubility of SO2 in the binary mixtures of 1,4-dioxane + PEG 200 at 308.15 K and 122.66 kPa. VFDX denotes the volume fraction of 1,4-dioxane in the mixtures: , VFDX = 0%; , VFDX = 20%; , VFDX = 30%; ♦, VFDX = 50%; 夽, VFDX = 100%.

pSO2/Pa

60

40

0

69

0

300

600

900

1200

CSO2/mg· L

1500

1800

2100

This article presents the results of fundamental investigations on the solubility data of dilute SO2 in 1,4-dioxane, 15-crown-5 ether, PEG 200, PEG 300, and their binary mixtures, which were obtained at 308.15 K and 122.66 kPa. The curves of equilibrium partial pressure of SO2 versus dissolved SO2 concentration are linear, which indicates that the absorption of dilute SO2 in these solutions obeys Henry’s law within the investigated pressure range. Moreover, the absorption SO2 in 1,4-dioxane, 15-crown-5 ether and PEG 200 are typical physical processes, and the solubility of SO2 in the three solvents decreases in the order: 1,4-dioxane > 15-crown-5 ether > PEG 200. Yet the absorption SO2 in PEG 300 is physical process accompanied by a chemical process. Regarding these cases of SO2 in binary mixtures, the solubility trends are not always identical in all mixtures, which may derive from the interaction between solvent molecules.

-1

Fig. 4. Solubility of SO2 in the binary mixtures of 1,4-dioxane + PEG 300 at 308.15 K and 122.66 kPa. VFDX denotes the volume fraction of 1,4-dioxane in the mixtures: , VFDX = 0%; , VFDX = 10%; , VFDX = 20%; , VFDX = 30%; ♦, VFDX = 50%; 夽, VFDX = 100%.

Acknowledgment This work was supported by Boyuan Hengsheng HighTechnology Co., Ltd., Beijing, China.

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