Experimental and simulation study of CO2 and H2S solubility in propylene carbonate, imidazolium-based ionic liquids and their mixtures

Experimental and simulation study of CO2 and H2S solubility in propylene carbonate, imidazolium-based ionic liquids and their mixtures

J. Chem. Thermodynamics 142 (2020) 106017 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locat...

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J. Chem. Thermodynamics 142 (2020) 106017

Contents lists available at ScienceDirect

J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct

Experimental and simulation study of CO2 and H2S solubility in propylene carbonate, imidazolium-based ionic liquids and their mixtures Zhijun Zhao a,b,⇑, Ying Huang c, Zhaohuan Zhang d, Weiyang Fei b, Mingsheng Luo a, Yongsheng Zhao e,⇑ a

College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Process & System Department, China HuanQiu Contracting & Engineering Corporation, Beijing 100012, China d National Institute Of Clean And Low-Carbon Energy, Beijing 102211, China e Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080, USA b c

a r t i c l e

i n f o

Article history: Received 2 November 2018 Received in revised form 22 November 2019 Accepted 24 November 2019 Available online 4 December 2019 Keywords: CO2 H2S Absorption Propylene carbonate (PC) Ionic liquids Simulation

a b s t r a c t Currently, solvent-based absorption is known as an effective technology to capture acid gases, e.g., CO2 or H2S. In this work, the constant-volume method is used to determine CO2 and H2S solubility in propylene carbonate (PC), imidazolium-based ionic liquids (ILs): 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF4], 1-hexyl-3-methylimidazolium tetrafluoroborate [Hmim][BF4], and 1-octyl-3methylimidazolium tetrafluoroborate [Omim][BF4], and their mixtures at temperature from 303.15 to 333.15 K under pressures up to about 1 MPa. Besides, the quantum chemistry-based COSMO-RS models are used to predict the vapor pressure and the solubility in these solvents. The experimental results illustrate that adding [Omim][BF4] into PC can improve the CO2 and H2S solubility and the H2S/CO2 selectivity compared with the pure PC, which increases with the increasing [Omim][BF4] mass fraction in the mixtures. The simulation with COSMO-RS shows the vapor pressures of PC in the mixtures decreases with the increasing [Omim][BF4] mass fraction. Moreover, the prediction results show that COSMO-RS-Lei model is in much better agreement than COSMO-RS (ADF 2005) for predicting both CO2 and H2S solubility and the prediction accuracy of CO2 is better than that of H2S. Finally, the mixtures of PC and [Omim][BF4] may be used as promising physical solvents to capture CO2 and H2S selectively with high partial pressures because they connect the advantages of organic solvents and ILs. Ó 2019 Elsevier Ltd.

1. Introduction The acid gases of CO2 and H2S are impurities in many gas streams such as natural gas, flue gas, synthesis gas and refinery streams [1,2]. The presence of CO2 reduces the fuel value of natural gas while the presence of H2S increases the toxicity and causes corrosion in transmission pipelines and process equipment in presence of moisture [3–6]. Therefore, it is important to remove these acid gases before being used in industrial application because of operational, economic, and environmental factors [7,8].

⇑ Corresponding authors at: College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China (Z. Zhao); Department of Chemical Engineering, University of California, Santa Barbara, California 93106-5080, USA (Y. Zhao). E-mail addresses: [email protected] (Z. Zhao), [email protected] (Y. Zhao). https://doi.org/10.1016/j.jct.2019.106017 0021-9614/Ó 2019 Elsevier Ltd.

The commercial technologies for acid gas separation from gas streams are membrane technology, adsorption, gas-liquid absorption and so no. Among these methods, the most powerful and efficient technique is gas-liquid absorption which includes chemical and physical absorption [9,10]. The traditional aqueous solutions of alkanolamines or its mixture such as MEA can be used as the chemical solvent to capture H2S or CO2 which is mature and economical. However, this kind of solvent faced several issues and challenges such as the corrosion, degradation, and loss of amine (due to its volatility and thermal instability) during recycling operations [7,11,12]. Moreover, the evaporation of amine solution into the atmosphere will lead to environmental problems and increase the cost of additional solvent/absorbent [6]. The physical absorption solvents such as N-methyl-2-pyrrolidone (NMP) [13], poly (ethylene glycol) dimethyl ether [14], sulfolane [15] propylene carbonate (PC) [16], methanol [17,18] can capture the acid gas at high pressure. And the gas can be easily regenerated by decreasing pressure and/or increasing temperature, therefore less energy is

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required than in the case of chemical solvents [19]. However, the disadvantage is that the acid gases should better be cooled before capture because the physical solvents possess the best capacity at low temperatures, which will decrease the thermal efficiency and thus increases the overall cost [20]. Besides, some other shortages such as high viscosity at low temperature, relative low absorption capacity (PC), toxicity (methanol) also limited their commercial application [4,21]. Recently, the ionic liquids (ILs) have been emerging as potential and promising absorbents for capturing acid gases because of their remarkable characteristics such as negligible volatility, high thermal stability, wide liquid range, and environmentally friendly [22–24]. For example, Brennecke’s group [25] investigated the solubility of CO2 in 10 imidazolium-based ILs and the results illustrated the changing of anion plays more role than the cation in affecting the CO2 solubility. Jalili’s group [5] studied the solubility H2S and CO2 of in the IL 1-ethyl-3-methylimidazolium tris(penta fluoroethyl)trifluorophosphate ([Emim][eFAP]), and the results showed that at fixed temperature and pressure, the amount of dissolved H2S is more than twice the amount of CO2. Jalili’s group [9] also studied the solubility of CO2 and H2S in 1-octyl-3methylimidazolium hexafluorophosphate [Omim][PF6] and the results showed the gas solubility increased with increasing pressure and decreased with increasing temperature. The results also showed the solubility of CO2 and H2S increased with the increasing number of carbons in the alkyl substituent of the methylimidazolium cation ring. Our previous work showed that the PC, has some advantages of economics, energy-saving, and relatively high selectivity of H2S/ CO2 [7]. However, it also has some disadvantages such as the relatively low absorption capacity [21]. As for the pure ILs, the high cost and viscosity have great influence on their practical applications [26,27]. Moreover, the cost of the imidazolium-based ILs is relatively economical compared with other kinds of functional ILs. For these reasons, the imidazolium-based ILs of [Bmim][BF4], [Hmim][BF4], and [Omim][BF4] were chosen for H2S and CO2 capture. The chemical structures for the propylene carbonate (PC), and the cations and anion of the imidazolium-based ILs are shown in Fig. 1. Therefore, in order to achieve the economics, minimized solvent loss, and relatively high gas capacity and H2S/CO2 selectivity, the imidazolium-based IL [Omim][BF4] was added into PC to make mixtures as absorption solvent which connect the advantages of organic solvent and the ILs. Then, for the purpose of getting the useful thermodynamic data in predicting the future industrial application, the H2S and CO2 solubility and the selectivity of H2S/ CO2 in these solvents were investigated under isothermal conditions at T = 303.15, 313.15, 323.15 and 333.15 K with the pressures up to about 1.0 MPa. In addition, COSMO-RS (conductor-like screening for real solvents) was taken to investigate the vapor pressure of PC in their mixtures. Moreover, two COSMO-RS models were used to predict the solubility of H2S and CO2 and the predicted results were compared with the experimental data. 2. Experiment section 2.1. Materials. The specifications and sources of chemicals used in this work are represented in Table 1. Before experiments, the ILs of 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF4], 1-hexyl-3-methylimidazolium tetrafluoroborate [Hmim][BF4], and 1-octyl-3-methylimidazolium tetrafluoroborate [Omim][BF4] were dried for 24 h under high vacuum at 333.15–338.15 K to remove volatile impurities and traces of water. The water content after treatment (before solubility experiments) was less than 400 ppm

Fig. 1. The chemical structure of propylene carbonate (PC), the cations and anion of the imidazolium-based ILs.

as determined by Karl Fischer titration (KLS701). Propylene carbonate (PC) and the gases were directly used without further purification. 2.2. Apparatus and experimental procedures The H2S and CO2 solubility were performed respectively at 303.19 K, 313.15 K, 323.15 K, and 333.15 K in the high-pressure equilibrium vessel under the pressure up to about 1.0 MPa with the apparatus in Fig.2. More details about the apparatus could be seen in the previous work [7,10]. In a typical run, the air in the two chambers should be evacuated down to about 0 kPa before introducing about 20 mL solvent. Next, the pure CO2 or H2S from its gas cylinder was fed into the gas reservoir to a given pressure. Then, the needle valve was turned on to allow the gases to be introduced to the equilibrium vessel. The absorption equilibrium can be reached when the pressures kept constant over 20 min. Under the same temperature and pressure, each experimental run was repeated at least three times to ensure the reproducibility of the experimental data. Next, the vapor–liquid phase equilibrium data were calculated from pressure balance. Finally, the amount of CO2 or H2S absorbed in the solvent was calculated using the equation of state by virtue of the difference between the initial and final equilibrium pressure of two chambers. The standard uncertainties of experimental temperature T, pressure P, mass m, and volumes V are 0.1 K, 0.001 MPa, 0.0001 g, and 0.05 mL, respectively while the expanded uncertainties of the H2S/CO2 solubility x1 in mole fraction in the liquid phase are calculated with 0.95 level of confidence. 2.3. Viscosity measurements The viscosities of these ILs were measured using an automated falling ball microviscometer (AMVn, Anton Paar, Austria) at about

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Z. Zhao et al. / J. Chem. Thermodynamics 142 (2020) 106017 Table 1 The chemicals used in this work. Chemical name CO2 H2S PC [Bmim][BF4] [Hmim][BF4] [Omim][BF4] a b c

Supplier BeiWen Gas in Beijing YS Special Gases Science and Technology co. Aladdin-in Shanghai Lanzhou Greenchem ILs, LICP, CAS, China Lanzhou Greenchem ILs, LICP, CAS, China Lanzhou Greenchem ILs, LICP, CAS, China

Purity in mole/mass fraction a

0.9999 0.999a 0.999b 0.99b 0.99b 0.99b

Purification method

Water content in mass fractionc

None None None under vacuum 24 h under vacuum 24 h under vacuum 24 h

None None None <400 ppm <400 ppm <400 ppm

The purity of the gases with mole fraction was provided by the supplier. The purity of the chemicals with mass fraction was provided by the supplier. Determined by Karl Fisher titration.

Fig. 2. Schematic diagram of the apparatus [10]. 1, H2S/CO2 gas cylinder; 2, buffer cell; 3, water; 4, water bath; 5, equilibrium cell; 6, waste container; 7, vacuum connector; 8, digital pressure transducer; 9, digital temperature transducer; 10, vent valve; 11, magnetic stirrer; 12, liquid injector; 13, computer.

0.1 MPa in an argon-filled glovebox. The relative expanded uncertainty Ur(g) was 0.142 (0.95 confidence level). The calibration was conducted with ultrapure water and viscosity standard oils (No. H117, Anton Paar, Austria). The experimental temperature ranged from 303.15 to 333.15 K with precision of ±0.1 K, which was controlled by a built-in precise Peltier thermostat. Experimental pressure was regulated by programmable logic controller in the glovebox, and the standard uncertainty u(p) is 0.1 kPa. 2.4. COSMO-RS model The COSMO-RS model developed by Klamt[28], which combines an electrostatic theory of locally interacting molecular surface charges with a statistical thermodynamics methodology, has also been widely used as an efficient priori tool to predict the thermodynamic properties (e.g., phase equilibrium, vapor pressure, gas

solubility, and activity coefficient) of the binary and ternary mixture systems. In this work, the vapor pressure was predicted by the COSMO-RS model. In addition, the predictive COSMO-RS model for ILs (ADF 2005) and the COSMO-RS-Lei model improved by Lei et al. (Beijing University of Chemical Technology) were used to predict the solubility of H2S and CO2 in the experimental work. Then, the predicted result was compared with the experimental data. As for the binary (CO2/H2S + IL) or ternary (CO2/H2S + IL + PC) systems, the gas-liquid equilibrium (GLE) at low and medium pressures is described as

  yi Pi £i ðT; P; yi Þ ¼ xi ci Psi £si T; Psi

ð1Þ

where yi and xi are the mole fractions of CO2/H2S in gas and liquid phases, respectively; £i ðT; P; yi Þis the gas-phase fugacity coefficient of CO2 calculated by the Span and Wagner equation of state [29] and the Peng-Robinson (PR) equation of state[30] while that of

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H2S calculated only by the Peng-Robinson (PR) equation of state, respectively. The results in Table S15 showed the average relative deviation percent (ARD%) of xi (the mole fractions of CO2 in liquid phases) computed by the Span and Wagner equation of state is little smaller than that by Peng-Robinson equation, which means the gas-phase fugacity computed by the Span and Wagner equation of state is better; P and T are the system pressure and temperature, respectively; P si is the saturated vapor pressure of CO2 and H2S, which can be obtained by the extrapolated Antoine equation   [31,32]; £si T; Psi is the fugacity coefficient of pure CO2 and H2S at saturated vapor pressure Psi and temperature T; and ci is the activity coefficient of CO2/H2S in the liquid phase, which can be calculated using the COSMO-RS model. Notably, the gas phase can be regarded as a pure solute component (i.e., yi = 1) due to the negligible vapor pressure of ILs and relatively high boiling point of PC. In this work, the IL molecular was described as a cation and an anion. Therefore, the binary mixture of IL and solute was considered as a hypothetical ternary system, which consists of cation, anion, and solute. A transform between activity coefficient of solute i in the binary and ternary mixture can be calculated by

cbin ¼ i

ctern xtern i i xbin i

¼

ctern i 2  xbin i

ð2Þ

bin where cbin are the activity coefficient and mole fraction of i and xi solute i in the binary system of (CO2/H2S + IL), respectively; ctern i and xtern are the corresponding values in the hypothetical ternary i system of (CO2/H2S + cation + anion). Similarly, for the ternary system of solute + IL + PC was considered as a hypothetical quaternary system. More details about the COSMO-RS calculation procedure can be seen in the User’s Manual (see https://www.scm.com/doc/ Tutorials/COSMO-RS/Ionic_Liquids.html).

3. Results and discussion 3.1. The reliability of apparatus The xexp represents the experimental solubility in this work, xref represents the solubility data in reference, and the relative deviation (RD) can be estimated by Eq. (3).

RD ¼ ðxexp  xref Þ=xexp

ð3Þ

The results of Fig.3 showed the maximum RD for the H2S solubility in [Bmim][BF4] at 303.15 K, 313.15 K, 323.15 K and 333.15 K is 9.02%, 6.56%, 7.11%, 1.92%, compared with the reference [33] respectively, which showed the error of the measurement of gas solubility is in the allowable range. The results in Fig.4 showed the maximum RD results for the CO2 solubility data in PC in reference [34] and reference [35] at 313.15 K is 8.80%, 8.64%, respectively, compared with the CO2 solubility data in PC in this work, which showed the error of the measurement of gas solubility is in the allowable range. The RD of the solubility data in reference [36] is 20.33% at lower pressure while the RD decreased to about 10% at higher pressure. As for the RD of the solubility data in reference [37] is 18.00% with the reason maybe that the temperature is 312.75 K which is little lower that the temperature of 313.15 K in this work. These results show that there may be relatively high deviations in reference [36] and reference [37]. 3.2. H2S solubility in pure PC and imidazolium ILs The solubility of H2S in pure PC and pure imidazolium-based ILs of [Bmim][BF4], [Hmim][BF4], and [Omim][BF4] was investigated at

temperatures of (303.15, 313.15, 323.15, and 333.15) K and pressures up to about 1.0 MPa. The results were illustrated in Tables S1–4 (shown in ESIy) and Fig.5, where PE is the Equilibrium Partial Pressure of H2S, x is the mole fraction of H2S in the solvent. The results in Fig.5 show that the H2S solubility in PC and these imidazolium ILs decreases with increasing temperature and increases with pressure which indicates that the absorption of H2S in these solvents belongs to the physical process. Besides, the results also show that the H2S solubility behavior in PC exhibits a similar linear trend while that in imidazolium ILs exhibits similar exponential trend under the experimental conditions, which agreed with other reports [7,33]. In addition, the H2S solubility in these imidazolium ILs can be increased by increasing the alkyl chain length on the cation, in which the solubility is in the order of: [Bmim][BF4] < [Hmim][BF4] < [Omim][BF4]. The solubilities of H2S in PC in this work were compared with the data in other references [34,36,38]. The result of the maximum RD in Fig.6 for the H2S solubility in PC at 313.15 K is 11.11% compared with the reference [36], which showed that the accuracy of the measurement of gas solubility is in the allowable range. While the results of Fig. 7 and 8 at 323.15 K and 333.15 K showed the maximum RD compared with the reference [34] and [38] is 20.07% and 28.08%, respectively. The deviations at 323.15 K is 20.07%. One of the reasons may be that the purity of H2S we used is with a mole fraction of 0.999 while the H2S used in reference [34] is 0.995. The other reason may be that we calculated the mole fraction from pressure balance without considering the solvent partial pressure while the authors calculated the mole fraction by the difference between the equilibrium total pressure and the solvent partial pressure. In addition, it should be noted that the standard uncertainty of u(p) in our wok is 1 kPa while that is 3.5 kPa in their work. The deviations at 333.15 K (28.08%) maybe two reasons. Firstly, we did not introduce other gases though the experimental process while the methane gas was added to maintain a total pressure when the partial pressure of H2S was below approximately 200 kPa in their work. Secondly, the computed method is different. We calculated the mole fraction from pressure balance while the author analyzed the liquid sample by firstly neutralization with 1.0 mol dm3 NaOH, then titrated with 0.1 mol dm3 iodine solution and back-titrated with Na2S2O3 in reference [38]. So, the deviation maybe caused by introducing the methane and the two steps of titration. The maximum RD results in Fig.9 for the H2S solubility in [Hmim][BF4] at 303.15 K, 313.15 K, 323.15 K and 333.15 K is 2.45%, 0.60%, 1.27%, 8.32% compared with the reference [39], respectively. Besides, the results of solubilities of H2S in [Bmim] [BF4] in this work compared with the reference [33] is also acceptable. The results in Fig. 3 and Figs.6–9 also showed the solubility data of H2S in this work is smaller than the relative references in general, but the error of the measured data is in the allowable range. The reasons of the systematic differences may be as follows. Firstly, the purity of H2S. The purity of H2S we used is a mole fraction of 0.999 while the H2S used in references [33,34], and [38] are all 0.995. Secondly, the computed method is different. We calculated the mole fraction from pressure balance without considering the solvent partial pressure while the authors in reference [34] calculated the mole fraction by the difference between the equilibrium total pressure and the solvent partial pressure and the authors in reference [38] analyzed the liquid sample by the two steps of titration.

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Fig. 3. Comparison of experimental and reported values of H2S in [Bmim][BF4] at different temperatures. j, this work;

, Ref. [33].

solubility data of H2S in the mixtures of [Omim][BF4] and PC with different mass fraction (w = 0–1.0, w1,w2 represents the mass fraction of PC and [Omim][BF4], respectively) in the mixtures were measured at temperatures of (303.15, 313.15, 323.15, and 333.15) K and pressures up to about 1.0 MPa (Tables S5–7y and Fig.10). The results show that H2S solubility in PC, [Omim][BF4], and their mixtures increases as pressure increases, decreases as temperature increases, and follows the order of: (w2 = 0) < (w2 = 0.2) < (w2 = 0.5) < (w2 = 0.8) < (w2 = 1.0). The comparison of H2S solubility in these mixtures with that in [Hmim][BF4], [Hmim][PF6] [39], Sulfolane, and NMP [34] at 323.15 K under the pressure of 1000 kPa was illustrated in Fig. 11, which showed the H2S solubility in the NMP is highest, followed by [Omim][BF4]. The H2S solubility in these solvents followed the order: PC < 20%(wt)[Omim][BF4] < Sulfolane < 50%(wt)[Omim][BF4] < [Hmim][PF6] < 50%(wt)[Omim][BF4] < [Hmim][BF4] < [Omim][BF4] < NMP. Fig. 4. Comparison of experimental and reported values of CO2 in PC at 313.15 K. j, this work; , Ref. [34]; , Ref. [35]; , Ref. [36]; , Ref. [37].

3.3. Effect of different mass fraction of the mixture on H2S absorption capacity The results in 3.2 show the H2S solubility in [Omim][BF4] is highest, which was chosen to make mixtures with PC. Then, the

3.4. Effect of different mass fraction of the mixture on CO2 absorption capacity The results in 3.3 show the H2S solubility in PC can be enhanced by adding [Omim][BF4]. Then, in order to find the effective and economic solvent which can capture H2S and CO2 together, the solubility data of CO2 in the same mass fraction mixtures of [Omim][BF4] and PC were measured under the same experimental

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Fig. 5. Comparison of the H2S solubility in PC and imidazolium ILs at different temperatures.

Fig. 6. Comparison of experimental and reported values of H2S in PC at 313.15 K. j, this work; , Ref. [36].

conditions like that of H2S. The comparison of CO2 solubility in [Omim][BF4] at 313.15 K in this work with that in the references [40] and [41] have also been added, which showed that the calcu-

Fig. 7. Comparison of experimental and reported values of H2S in PC at 323.15 K. j, this work; , Ref. [34].

lated maximum RD between the two references and our data is 10.00% and 11.29%, respectively. These results showed that the accuracy of the measurement of gas solubility is in the allowable range (Fig. 12).

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Fig. 8. Comparison of experimental and reported values of H2S in PC at 333.15 K. j, this work; , Ref [38].

The results in Figs. 4 and 12 also showed the solubility data of CO2 in this work is larger than the relative references in general, but the error of the measured data is in the allowable range

(mostly smaller or about 10%). There are maybe two reasons for the systematic differences. Firstly, the purity of PC and CO2. The purity of CO2 in this work is a mole fraction of 0.999 while the CO2 used in reference [34] is 0.997. The PC in this work is a mass fraction of 0.999 while the PC used in references [35,36], and [37] is 0.99, 0.98 and >0.99 respectively. Secondly, the computed method is different. We calculated the mole fraction from pressure balance without considering the solvent partial pressure while the authors in reference [34] calculated the mole fraction by the difference between the equilibrium total pressure and the authors in reference [40] calculated the CO2 dissolved in [Omim][BF4] by subtracting the CO2 in the vapor from the total CO2 supplied to the cell. The results (Tables S8–12y and Fig. 13) show that CO2 solubility in PC, [Omim][BF4], and their mixtures increases as pressure increases, but decreases as temperature increases and follows the order of: (w2 = 0) < (w2 = 0.2) < (w2 = 0.5) < (w2 = 0.8) < (w2 = 1.0).The difference between H2S and CO2 solubility in these mixtures is that the H2S solubility is higher than that of CO2 for a fixed mass fraction mixture under the same temperature and pressure. The comparison of CO2 solubility in these mixtures with that in [Bmim][NTF2] [42], [Omim][NTF2] [43], sulfolane, and NMP [34] at 323.15 K under the pressure of 1000 kPa was illustrated in Fig. 14, which showed the CO2 solubility in the sulfolane is smallest and that in NMP is between PC and 20%(wt)[Omim][BF4]. The CO2 solubility in all these solvents followed the order: Sulfolane <

Fig. 9. Comparison of experimental and reported values of H2S in [Hmim][BF4] at different temperatures. j, this work;

, Ref. [39].

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Fig. 10. Comparison of the H2S solubility in the pure [Omim][BF4] and the mixture of PC and [Omim][BF4] at different temperatures.

Fig. 11. Comparison of the H2S solubility in the mixture and other physical solvents at 323.15 K. j, PC, this work; , 20% (wt) [Omim][BF4], this work; , 50%(wt) [Omim][BF4], this work; , 80%(wt) [Omim][BF4], this work; , [Omim][BF4] this work; , [Hmim][BF4], [39]; ◆, [Hmim][PF6], [39]; , NMP, [34]; , Sulfolane, [34].

Fig. 12. Comparison of experimental and reported values of CO2 in [Omim][BF4] at 313.15 K. j, this work; , Ref. [40]; , Ref [41].

Z. Zhao et al. / J. Chem. Thermodynamics 142 (2020) 106017

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Fig. 13. Comparison of the CO2 solubility in the pure [Omim][BF4] and the mixture of PC and [Omim][BF4] at different temperatures.

PC < NMP < 20%(wt)[Omim][BF4] < 50%(wt)[Omim][BF4] < 80%(wt) [Omim][BF4] < [Omim][BF4] < [Bmim][NTF2] < [Omim][NTF2]. 3.5. H2S/CO2 selectivity for the mixtures In practical separation processes involve gas mixture such as natural gas containing CO2 and H2S, the selectivity of H2S/CO2 is necessary and important. Therefore, the solubility data of H2S and CO2 and the H2S/CO2 selectivity in the mixtures (w = 0–1.0, w1, w2 represents the mass fraction of PC and [Omim][BF4], respectively) were calculated at T = 303.15, 313.15, 323.15, and 333.15 K, and the pressures up to about 1 MPa. The data in Table 2 showed the ideal selectivity of H2S/CO2 in these solvents generally decreases with the increasing temperature. In addition, the result also showed the selectivity of H2S/ CO2 in [Omim][BF4] is high than that in PC under the same experimental conditions. Moreover, the selectivity of H2S/CO2 increases with the increasing mass fraction of [Omim][BF4] in the mixtures at fixed temperature and follows the order of: (w2 = 0) < (w2 = 0. 2) < (w2 = 0.5) < (w2 = 0.8) < (w2 = 1.0). Fig. 14. Comparison of the CO2 solubility in the mixture and other physical solvents at 323.15 K. j, PC, this work; , 20% (wt)[Omim][BF4], this work; , 50%(wt) [Omim][BF4], this work; , 80%(wt) [Omim][BF4], this work; , [Omim][BF4] this work; , [Bmim][NTF2], [42]; ◆,[Omim][NTF2], [43]; , NMP, [34]; , Sulfolane, [34].

3.6. The viscosity for the mixtures The viscosity of PC, [OMIM][BF4] and their mixtures with different mass fractions at different temperature were measured with

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Table 2 The comparison of solubility and selectivity for H2S/CO2 in the mixtures. Mixtures PC 20 wt%[Omim][BF4] 50 wt%[Omim][BF4] 80 wt%[Omim][BF4] [Omim][BF4] PC 20 wt%[Omim][BF4] 50 wt%[Omim][BF4] 80 wt%[Omim][BF4] [Omim][BF4] PC 20 wt%[Omim][BF4] 50 wt%[Omim][BF4] 80 wt%[Omim][BF4] [Omim][BF4] PC 20 wt%[Omim][BF4] 50 wt%[Omim][BF4] 80 wt%[Omim][BF4] [Omim][BF4]

+ 80 wt%PC + 50 wt%PC + 20 wt%PC

+ 80 wt%PC + 50 wt%PC + 20 wt%PC

+ 80 wt%PC + 50 wt%PC + 20 wt%PC

+ 80 wt%PC + 50 wt%PC + 20 wt%PC

T(K)

P(kPa)

H2S Solubility (xi)

Ue(xi)

P(kPa)

CO2 Solubility (xi)

Ue(xi)

H2S/CO2 Selectivity

303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15

1001 1010 1007 1010 1012 1010 1007 1011 1017 1015 1009 1012 1018 1018 1023 1019 1019 1027 1030 1010

0.3485 0.3941 0.4528 0.5122 0.5735 0.2783 0.3284 0.3929 0.4591 0.5121 0.2263 0.2766 0.3358 0.4029 0.4497 0.1855 0.2418 0.2950 0.3573 0.4136

0.0092 0.0118 0.0136 0.0156 0.0172 0.0075 0.0098 0.0116 0.0137 0.0156 0.0067 0.0081 0.0099 0.0122 0.0132 0.0057 0.0072 0.0085 0.0106 0.0124

1019 1009 1027 1017 1021 1021 1011 1028 1019 1026 1024 1015 1005 1036 1030 1020 1019 1037 1018 1023

0.1222 0.1339 0.1521 0.1686 0.1879 0.1023 0.1146 0.1327 0.1523 0.1692 0.0914 0.1022 0.1163 0.1377 0.1520 0.0808 0.0901 0.1074 0.1230 0.1414

0.0036 0.0039 0.0046 0.0051 0.0056 0.0031 0.0033 0.0039 0.0046 0.0050 0.0027 0.0030 0.0035 0.0041 0.0045 0.0025 0.0028 0.0032 0.0039 0.0042

2.8519 2.9432 2.9770 3.0380 3.0522 2.7204 2.8656 2.9608 3.0144 3.0266 2.4759 2.7065 2.8874 2.9259 2.9586 2.2958 2.6837 2.7467 2.9049 2.9250

Standard uncertainties u are u(T) = 0.1 K, u(p) = 0.001 MPa, u(v) = 0.05 mL, u(m1) = 0.0001 g, u(m2) = 0.0001 g; the expanded uncertainty Ue is Ue(xi), which was calculated with a level of confidence of 0.95. (The data are taken from the corresponding Tables S1–S12).

the method in 2.3. The experimental results in Table S13 showed the viscosity of the mixture increased with the increasing of the [OMIM][BF4] mass fraction at fixed temperatures while the viscosity of the mixture decreased with the temperatures increasing at fixed [OMIM][BF4] mass fraction. The Fig. 15 shows that the PC viscosity data in the references [44–46] is very near which means the relative deviation between these references is small. The viscosity data in reference [47] is higher than the data in the references [44–46] while the data in reference [48] is smaller than that. The experimental results in Fig.15 also illustrated the PC viscosity in this work is in relatively good agreement with the Pires’s work [48]. The results in Table S14 showed the relative deviation (RD) of current viscosity data in this work deviate by up to 9.92% (at 313.15 K),11.66% (at 323.15 K) respectively compared with the majority literature data in reference [48], while the RD deviation decreased to 2.59% (at 313.15 K), 1.00% (at 323.15 K) compared with the majority literature data in reference [47]. So, the accuracy of the measurement of the PC viscosity data in this work is in the allowable range.

However, the RD between data of Anouti’s work [49] and this work (or Pires’s work) is over 50% at 323.15 K, which showed there maybe high relative deviations in Anouti’s work. 3.7. Analysis of vapor pressures of the mixtures computed by COSMO-RS The vapor pressure of the solvent is an important parameter in an industrial H2S/CO2 capture process, especially the regeneration process in which the high vapor pressure will lead to the solvent loss because of high temperature. The solvent loss will decrease the efficiency of capturing process and increase the economic cost. Hence, the data of PC vapor pressures of the mixtures were predicted by the COSMO-RS model [50,51]. The geometric optimizations of PC, and the cation and anion of [Bmim][BF4] [Hmim] [BF4], [Omim][BF4] were performed at the B3LYP/6-31++G** theoretical level using Gaussian 09 programs package[52]. The final optimized structures with COSMO surfaces were shown in Fig. 16 and more details about the method could be seen in our previous work [10,53]. The results in Table 3 illustrated that the PC vapor pressures in these mixtures decrease with the increasing of [Omim][BF4] mass fraction, and is in the order of : (w2 = 0.2) < (w2 = 0.5) < (w2 = 0.8), which is consistent with Qi’s work [54]. Meanwhile, compared to the mass fraction, the temperature shows little effect on influencing the PC vapor pressures in the mixtures. 3.8. Prediction of H2S/CO2 solubility in the solvents computed by the COSMO-RS models

Fig. 15. Comparison of the viscosities of PC in this work with other references at different temperatures. , this work; , [44]; , [45]; , [46]; , [47]; , [48]; , [49].

Usually, the gas solubility prediction can be good complementarity for the experimental results. Then, the solubility of H2S/ CO2 in the pure PC, [Omim][BF4], and their mixtures with the different mass fraction at temperatures from 303.15 to 333.15 K under the pressures up to about 1 MPa was predicted with COSMO-RS (ADF2005) model and COSMO-RS-Lei [55] model. In total, 400 data points for H2S solubility while 200 data points for CO2 were concerned, and the detailed experimental data and predicted values by the COSMO-RS models (ADF2005 and COSMORS-Lei) are listed in Table S15 in supporting information. The data in Tables 4 and 5 are the predicted solubility values of H2S and CO2 by COSMO-RS model (ADF2005) and COSMO-RS-Lei model (the

11

Z. Zhao et al. / J. Chem. Thermodynamics 142 (2020) 106017

Fig. 16. The optimized structures with COSMO surfaces in this work (Red part means positive COSMO charge density, and the blue part means negative COSMO charge density). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 3 The decrease rate of the vapor pressures of the mixtures in comparison with pure PC at different temperatures and various contents. Mixtures

303.15 K

313.15 K

323.15 K

333.15 K

[Omim][BF4] (w2 = 0.2) + PC (w1 = 0.8) [Omim][BF4] (w2 = 0.5) + PC (w1 = 0.5) [Omim][BF4] (w2 = 0.8) + PC (w1 = 0.2)

11.63% 40.62% 72.95%

12.40% 40.90% 73.24%

13.10% 41.15% 73.50%

13.72% 41.37% 73.74%

gas-phase fugacity coefficient calculated by the Peng-Robinson (PR) equation of state). The results illustrated the predictions of COSMO-RS-Lei model are in rough agreement with the experimen-

tal data, with ARDs of 78.26% and 27.37% for H2S and CO2 respectively while the predictions of COSMO-RS model (ADF2005) are relatively poor with the corresponding ARDs as high as 91.90%

Table 4 Comparison of the H2S solubility in Solvents between experimental data and predicted results by COSMO-RS (ADF2005) model and COSMO-RS-Lei model.a Solvents

T range (K)

PC 20 wt%[Omim][BF4] + 80 wt%PC 50 wt%[Omim][BF4] + 50 wt%PC 80 wt%[Omim][BF4] + 20 wt%PC [Omim][BF4] a

ARD ¼ N1

PN

i¼1 d

303.15–333.15 303.15–333.15 303.15–333.15 303.15–333.15 303.15–333.15

P range (MPa)

0–1.019 0–1.019 0–1.027 0–1.030 0–1.023 Total

No. of data points

80 80 80 80 80 400

ARDs ADF 2005

COSMO-RS-Lei

1.1021 0.8631 0.9408 0.8845 0.8047 0.9190

0.9890 0.7501 0.8025 0.7285 0.6431 0.7826

ycal yexp i i e. yexp i

Table 5 Comparison of the CO2 solubility in Solvents between experimental data and predicted results by COSMO-RS (ADF2005) model and COSMO-RS-Lei model.a Solvents

T range (K)

PC 20 wt%[Omim][BF4] + 80 wt%PC 50 wt%[Omim][BF4] + 50 wt%PC 80 wt%[Omim][BF4] + 20 wt%PC [Omim][BF4] a

ARD ¼ N1

PN

i¼1 d

ycal yexp i i e. yexp i

303.15–333.15 303.15–333.15 303.15–333.15 303.15–333.15 303.15–333.15

P range (MPa)

0–1.024 0–1.019 0–1.037 0–1.036 0–1.030 Total

No. of data points

40 40 40 40 40 200

ARDs ADF 2005

COSMO-RS-Lei

0.5055 0.5610 0.6314 0.6312 0.4814 0.5621

0.3088 0.3244 0.3339 0.2614 0.1401 0.2737

12

Z. Zhao et al. / J. Chem. Thermodynamics 142 (2020) 106017

and 56.21%, respectively. The results of these two predicted models also show the predicted data of CO2 is better than that of H2S in these solvents. The reason may be, in the binary H2S + IL or / H2S + PC or ternary CO2/H2S + IL + PC systems, strong hydrogen bonds were formed between H2S and the solvent molecules during the absorption process because of the active nature of H2S. 4. Conclusion In this work, the constant-volume method was used to investigate the H2S and CO2 solubility and the H2S/CO2 selectivity in propylene carbonate (PC), imidazolium-based ILs, and their mixtures. The results showed the H2S solubility in pure solvents is in the order of: PC < [Bmim][BF4] < [Hmim][BF4] < [Omim][BF4] under the same experimental conditions. The experimental results showed that adding [Omim][BF4] into PC can improve the H2S and CO2 solubility and the H2S/CO2 selectivity compared with the pure PC, which increases with the increasing mass fraction of [Omim][BF4] in the mixtures. Secondly, the simulation with COSMO-RS shows the vapor pressures of PC in the mixtures decreases with the increasing mass fraction of [Omim][BF4]. Thirdly, the predicted results from COSMO-RS-Lei model show ARDs of 78.26% and 27.37% for H2S and CO2 respectively, which is better than that from COSMO-RS model (ADF2005) with the corresponding ARDs as high as 91.90% and 56.21%, respectively. The experimental and predicted results in this work are based on the industrial parameters of the commercial process of the H2S and CO2 capture, which can offer new insights and present some benefits for the absorbent optimization and its large-scale production in the future. CRediT authorship contribution statement Zhijun Zhao: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Ying Huang: Data curation, Software. Zhaohuan Zhang: Investigation, Validation. Weiyang Fei: Supervision. Mingsheng Luo: Writing - review & editing. Yongsheng Zhao: Software, Methodology, Writing review & editing. Acknowledgments This work was supported by Beijing Natural Science Foundation (No. 2164062) and State Key Laboratory of Chemical Engineering (No. SKL-ChE-15A01). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jct.2019.106017. References [1] T.N.G. Borhani, M. Afkhamipour, A. Azarpour, V. Akbari, S.H. Emadi, Z.A. Manan, Modeling study on CO2 and H2S simultaneous removal using MDEA solution, J. Ind. Eng. Chem. 34 (2016) 344–355. [2] S.J. Poormohammadian, A. Lashanizadegan, M.K. Salooki, Modelling VLE data of CO2 and H2S in aqueous solutions of N-methyldiethanolamine based on nonrandom mixing rules, Int. J. Greenh. Gas Control 42 (2015) 87–97. [3] M. Nematpour, A.H. Jalili, C. Ghotbi, D. Rashtchian, Solubility of CO2 and H2S in the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, J. Nat. Gas Sci. Eng. 30 (2016) 583–591. [4] K. Huang, X.M. Zhang, Y. Xu, Y.T. Wu, X.B. Hu, Protic ionic liquids for the selective absorption of H2S from CO2: Thermodynamic analysis, AIChE J. 60 (2014) 4232–4240. [5] A.H. Jalili, M. Shokouhi, G. Maurer, M. Hosseini-Jenab, Solubility of CO2 and H2S in the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate, J. Chem. Thermodyn. 67 (2013) 55–62.

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JCT 2018-904