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Phase equilibria and dissociation enthalpies for tetra-n-butylammonium chloride semiclathrate hydrates formed with CO2, CH4, and CO2 + CH4
Accepted Manuscript Phase Equilibria and Dissociation Enthalpies for Tetra-n-butylammonium Chloride Semiclathrate Hydrates Formed with CO2, CH4, and CO2 + CH4 Sheng-Lan Qing, Dong-Liang Zhong, Da-Tong Yi, Yi-Yu Lu, Zheng Li PII: DOI: Reference:
S0021-9614(17)30271-9 http://dx.doi.org/10.1016/j.jct.2017.07.039 YJCHT 5157
To appear in:
J. Chem. Thermodynamics
Received Date: Revised Date: Accepted Date:
4 May 2017 28 July 2017 29 July 2017
Please cite this article as: S-L. Qing, D-L. Zhong, D-T. Yi, Y-Y. Lu, Z. Li, Phase Equilibria and Dissociation Enthalpies for Tetra-n-butylammonium Chloride Semiclathrate Hydrates Formed with CO2, CH4, and CO2 + CH4, J. Chem. Thermodynamics (2017), doi: http://dx.doi.org/10.1016/j.jct.2017.07.039
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Phase Equilibria and Dissociation Enthalpies for Tetra-n-butylammonium Chloride Semiclathrate Hydrates Formed with CO2, CH4, and CO2 + CH4
Sheng-Lan Qing a, Dong-Liang Zhong a, b, *, Da-Tong Yi b, Yi-Yu Lu a, Zheng Li b
a
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China
b
College of Power Engineering, Chongqing University, Chongqing 400044, China
1
Abstract: In this work, a high pressure micro-differential scanning calorimeter (HP µ-DSC) was employed to measure the phase equilibrium data and dissociation enthalpies of tetra-n-butylammonium Chloride (TBAC) semiclathrate hydrate formed in the presence of CO2, CH4, and CO2 + CH4. The TBAC concentration varied from 1.0 mol% to 5.0 mol%, and the operating pressure changed from 1.0 MPa to 5.0 MPa. It was found that the phase boundary of TBAC semiclathrate hydrate formed at 3.3 mol% TBAC was lower than that obtained at 1.0 mol% and 5.0 mol% TBAC in the presence of CO2 + CH4, and the hydrate dissociation enthalpy obtained at 3.3 mol% TBAC was larger than that obtained at 1.0 mol% and 5.0 mol% TBAC. This result indicates that TBAC semiclathrate (TBAC·29.7H2O) formed at the stoichiometric concentration (3.3 mol%) was more stable than that formed at 1.0 mol% and 5.0 mol% TBAC in the presence of CO2 + CH4. In addition, pure TBAC semiclathrate hydrate and the mixed TBAC + CO2 + CH4 semiclathrate hydrate were found to coexist at 1.0 mol% TBAC. The hydrate dissociation enthalpy for the mixed TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC was found to increase with the increase of pressure. This is probably because a larger amount of the CO 2 + CH4 gas mixture was incorporated into the TBAC semiclathrate hydrate at a higher pressure.
Keywords: Semiclathrate hydrate; Phase equilibrium; DSC; Dissociation enthalpy; Thermodynamics
* Corresponding author. E-mail: [email protected] (D. L. Zhong) 2
1. Introduction Gas hydrates are ice like non-stoichiometric inclusion compounds formed at low temperature and high pressure conditions, in which small gas molecules like CH4 and CO2 are trapped in the hydrogen-bonded water cavities via Van der Waal’s forces. Structure I (sI), structure II (sII), structure H (sH), and semiclathrate hydrate are four common types of hydrates that have been identified so far [1, 2]. The naturally occurring gas hydrates stored in ocean sediments and permafrost regions have been considered as a potential energy resource for the 21st century as the amount of methane contained in gas hydrates is estimated to be two times larger than that in conventional fossil fuels (coal, oil, and natural gas) [3-6]. Besides, the hydrate-based technology has a great potential for the applications of gas storage and transportation [7, 8], desalination [9], and gas mixture separation [10-13] due to the large storage capacity and high separation efficiency of gas hydrates. The hydrate-based gas separation process for CO2 capture from shale gas, flue gas, and fuel gas has been intensively studied in recent years [14-20]. In order to save energy costs of the separation process, the attempt to shift hydrate formation conditions to higher temperatures or lower pressures has been made by adding thermodynamic promoters into the hydrate formation solutions. Tetrahydrofuran (THF), cyclopentane (CP), and cyclohexane (CH) are well-known thermodynamic promoters that have been widely used for different hydrate formation systems [21, 22]. Zhang et al. [23] presented the data for the equilibrium conditions of CP + H2 and CP + CO2 binary hydrates, which were measured using a high pressure micro-differential scanning calorimeter (HP µ-DSC). Lee et al. [24] reported the phase equilibrium data of gas hydrates formed from a CO2 + CH4 gas mixture in the presence of THF. We measured the phase equilibrium data of gas hydrates formed from a model shale gas (40 mol% CO2/CH4) at
various THF concentrations using an isochoric
pressure-search method, and found significant decrease in the equilibrium pressure at a given temperature [25]. Recently, quaternary ammonium salts (QAS) like tetra-n-butylammonium bromide (TBAB) and tetra-n-butylammonium fluoride (TBAF) have been employed to form semiclathrate hydrates for gas separation 3
[26-29]. It was found that the phase boundaries for semiclathrate hydrates were much lower than those obtained in the presence of THF. Understanding the phase behaviors of gas hydrates is of great importance to develop hydrate-based gas separation technologies. However, the phase behaviors of semiclathrate hydrate formed in the presence of QAS are not well known. In this work, a high pressure micro-differential scanning calorimeter (HP µ-DSC) was employed to investigate the thermodynamic phase behaviors of TBAC semiclathrate hydrate formed with different gases (CO2, CH4, and CO2 + CH4). The purpose of this work is to report the phase equilibrium data and dissociation enthalpies of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 with TBAC concentration changing between 1.0 mol% and
5.0 mol%, and reveal the phase
behaviors of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 at different TBAC concentrations and different operating pressures.
2. Experimental Section 2.1 Materials Table 1 shows the source, purification methods, and final purity of the materials used in this work. The pure CH4, CO2, and the CO2 + CH4 gas mixture (40 mol% CO2 and 60 mol% CH4) were supplied by Chongqing Jiarun Gas with a measurement accuracy of ± 0.01 mol%. Tetra-n-butyl ammonium chloride (TBAC) with a certified mass purity of 97% was purchased from Chongqing Oriental Chemical Co. The purity was determined by argentometric titration. Deionized water for all experimental runs was produced in the lab with a resistivity of 18.2 MΩ·cm. 2.2 Apparatus A high pressure micro-differential scanning calorimeter (HP µ-DSC VII evo, Setaram Inc., France) was utilized to measure the three-phase (H-Lw-V) equilibria of TBAC semiclathrate hydrate formed with CO2, CH4, and the CO2 + CH4 gas mixture. Fig. 1 shows the schematic diagram of HP µ-DSC that was connected with an external refrigerator and a data acquisition system. The HP µ-DSC is composed of a reference cell and a sample cell that were made of Hastelloy C276 to avoid contamination and 4
corrosion. The cells can be operated at a pressure up to 40 MPa with the internal volume fixed at 0.33 cm3. The operating temperature range for the calorimeter is between 228 K and 393 K while the temperature scanning rate can be set in the range of 0.001-1 K/min. The resolution of the HP µ-DSC is 0.02 µW. 2.3 Procedure Prior to experiments the HP µ-DSC sample cell was cleaned with deionized water and dried. Then the TBAC solution with a mass of 8.5-10.5 mg was injected into the sample cell. Afterwards, the sample cell was inserted into the DSC furnace and connected to the gas line. It was purged at least six times using the experimental gas to eliminate any air remaining in the cells and tubes. The desired pressure was achieved by adjusting the pressure regulator installed between the gas cylinder and the supply vessel (Fig. 1). During the experiments the valves between the supply vessel and the cells were kept open while the vent valves were closed. The dynamic and stepwise methods were adopted to investigate the phase behavior of gas hydrates and to measure the hydrate dissociation temperature at a given pressure [30, 31]. The first step is to crystallize gas hydrates in the sample cell. The system was cooled down from 293.15 K to 253.15 K at a rate of 0.3 K/min and was kept 3 h at 253.15 K to ensure the complete freezing of the liquid in the sample cell. Then the temperature was raised from 253.15 K to 293.15 K at a rate of 0.3 K/min. The dissociation enthalpies of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 were determined by integrating the DSC endothermic peak obtained in the hydrate dissociation process. For the stepwise method, the temperature was increased from 253.15 K to a temperature that is at most 0.6 K lower than the dissociation temperature using the same heating rate (0.3 K/min). The temperature was then increased in a heating rate of 0.1 K with the isothermal interval between each heating step was adjusted to 2 h. The step temperature (Tset) after the last endothermic melting signal was determined as the hydrate phase equilibrium temperature at the given pressure. Fig. 2 shows the typical heat-flow profile of TBAC semiclathrate hydrate formed from 40 mol% CO2/CH4 at 3.0 MPa and 5.0 mol% TBAC, which was obtained using the dynamic method. Fig. 3 illustrates the stepwise 5
method for measuring the phase equilibrium point of TBAC semiclathrate hydrate formed from 40 mol% CO2/CH4.
3. Results and Discussion 3.1 Validation of the stepwise method for phase equilibrium measurement of CH4 hydrate and CO2 hydrate The phase equilibrium data of pure CH4 hydrate and CO2 hydrate were measured by the stepwise method and compared with those reported in the literature. The phase equilibrium data were tabulated in Table S1 as supporting information. It can be seen from Fig. 4a that the phase equilibrium data of CH 4 hydrate measured by the stepwise method agree well with that predicted by CSMHYD [2] and are in good agreement with that reported by Mohammadi et al [32]. Similarly, it can be seen from Fig. 4b that the phase equilibrium data of CO2 hydrate measured by the stepwise method coincide with that calculated by CSMHYD [2] and agree with that reported in the literature [32, 33]. Therefore, the phase equilibrium comparison shown in Fig. 4 indicates that the stepwise method adopted in this work is able to produce accurate phase equilibrium data of gas hydrates. 3.2 Phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate Fig. 5 shows the phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC, which were measured by the stepwise method. The equilibrium temperatures, pressures, and hydrate dissociation enthalpies were summarized in Table 2. As seen in Fig. 5, the phase equilibrium data of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured in this work are in good agreement with those reported by Soyong Kim et al. [34] It is interesting to note that the phase boundaries of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate obtained in this work are slightly lower than those reported by Soyong Kim et al. [34] The small discrepancy may be caused by the different methods that were adopted for hydrate phase equilibrium measurement. We employed the calorimetric method (HP µ-DSC) in this work while the classical thermodynamic method (PVT) was used by Soyong Kim [34] to measure the hydrate phase equilibrium data. Generally, the HP µ-DSC 6
method is able to generate more accurate phase equilibrium data than the PVT method as the accuracy of phase equilibrium points determined in the PVT apparatus is usually influenced by the gas thermal expansion properties and the uncertainties of temperature and pressure measurement in the equilibrium cell. This analysis is also stated in the literature [35]. 3.3 Phase equilibria of TBAC + CO2 + CH4 semiclathrate hydrate Fig. 6 shows the phase equilibrium data of TBAC semiclathrate hydrate formed from CO2 + CH4 with TBAC concentration varying between 1.0 mol% and 5.0 mol%. The equilibrium temperatures, pressures, and hydrate dissociation enthalpies were given in Table 3. As seen in Fig. 6, comparing with the equilibrium pressure of the binary CO2 + CH4 gas hydrates formed in pure water at a given temperature [25, 36], the hydrate equilibrium pressure obtained in the presence of TBAC decreased significantly. This indicates that adding TBAC into the aqueous solution can shift the hydrate phase equilibrium conditions to lower pressures at a given temperature or higher temperatures at a given pressure. Therefore, the thermodynamic stability of TBAC semiclathrate hydrate formed from CO2 + CH4 is greatly improved as compared to the binary CO2 + CH4 gas hydrates formed in pure water. In addition, Fig. 6 shows that the phase equilibrium boundary obtained at 3.3 mol% TBAC is lower than that obtained at 1.0 mol% and 5.0 mol% TBAC. Note that 3.3 mol% corresponds to the stoichiometric concentration of TBAC semiclathrate hydrate (TBAC·29.7H2O), so the comparison of phase equilibrium boundaries indicates that TBAC + CO2 + CH4 semiclathrate hydrate formed at the stoichiometric concentration of TBAC·29.7H2O (3.3 mol%) is more stable than that formed at 1.0 mol% and 5.0 mol% TBAC. This result agrees with the hydrate dissociation enthalpies that were obtained from hydrate dissociation endothermic peaks. As seen in Table 3, the hydrate dissociation enthalpy obtained at 3.3 mol% TBAC is 290.8 ± 15.6 J/g, which is higher than that obtained at 1.0 mol% (247.2 ± 12.2 J/g) and 5.0 mol% (273 ± 9.9 J/g). 3.4 Phase behavior of TBAC + CO2 + CH4 semiclathrate hydrate at different TBAC concentrations Fig. 7 shows the dissociation thermograms of TBAC semiclathrate hydrate formed 7
at three different TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%) in the presence of 40 mol% CO2/CH4. The operating pressure was fixed at 3.0 MPa. As seen in Fig. 7, at 1.0 mol% TBAC three endothermic peaks were detected in the heating process. The endothermic peak occurring at 273 K represents ice melting. The onset temperature of the middle endothermic peak was found to be 279.8 K, which agrees with the phase equilibrium temperature of pure TBAC semiclathrate hydrate formed at 3.0 MPa. Therefore, it can be inferred that the middle endothermic peak stands for the dissociation of pure TBAC semiclathrate hydrate. The onset temperature of the third endothermic peak was found to be 286.1 K, which is close to the phase equilibrium temperature of TBAC + CO2 + CH4 semiclathrate hydrate determined by the stepwise method (Table 3). Therefore, the endothermic peaks detected indicate that the hydrates formed at 1.0 mol% TBAC are a mixture of pure TBAC semiclathratre hydrate and the TBAC + CO2 + CH4 semiclathrate hydrate. The dissociation thermogram obtained at 5.0 mol% TBAC shows that the ice melting peak became very small and the dissociation peak for pure TBAC semiclathrate hydrate disappeared, on the contrary, a large dissociation peak for the TBAC + CO2 + CH4 mixed semiclathrate hydrate was observed. This result indicates that most of the liquid sample was converted into the TBAC + CO2 + CH4 semiclathrate hydrate, and nearly no ice and TBAC semiclathrate hydrate formed at 5.0 mol% TBAC as compared to that obtained at 1.0 mol% TBAC. Interestingly, the dissociation thermogram obtained at the stoichiometric TBAC concentration (3.3 mol%) was different from that obtained at 1.0 mol% TBAC. As seen in Fig. 7, no ice melting peak was found during the heating process. There was only one endothermic peak observed for the TBAC + CO2 + CH4 mixed semiclathrate hydrate during the heating process, which was sharper and larger than that obtained at 1.0 mol% and 5.0 mol% TBAC. This result indicates that at 3.3 mol% TBAC the liquid sample was completely converted into the TBAC + CO2 + CH4 semiclathrate hydrate whereas pure TBAC semiclathrate and ice did not form. Since the amount of TBAC solution charged into the sample cell was almost the same for all experimental runs, the amount of water in 5.0 mol% TBAC solution would be relatively lower than 8
that in 3.3 mol% TBAC solution. As a result, the amount of hydrate formed at 3.3 mol% TBAC was higher than that formed at 5.0 mol% TBAC. This might be the reason why the endothermic peak of the mixed TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC was larger than that obtained at 5.0 mol% TBAC. 3.5 Phase behavior of TBAC + CO2 + CH4 semiclathrate hydrate at different operating pressures Fig. 8 shows the dissociation thermogram of TBAC + CO2 + CH4 semiclathrate hydrate formed at 5.0 mol% TBAC with the operating pressure varying between 1.0 MPa to 5.0 MPa. As seen in the figure, with the increase of operating pressure the ice melting peak shrinks while the dissociation peak for TBAC + CO2 + CH4 semiclathrate hydrate becomes broader. This indicates that with the pressure increasing from 1.0 MPa to 5.0 MPa the mass of ice formed from the liquid sample decreased, and the amount of TBAC solution converted into the TBAC + CO2 + CH4 mixed semiclathrate hydrate increased. This is probably because the driving force for hydrate crystallization was elevated, and thus more hydrate formed at a higher pressure. Interestingly, no significant change in the dissociation enthalpy of TBAC + CO2 + CH4 semiclathrate hydrate was observed at 5.0 mol% TBAC when increasing the operating pressure from 1.0 MPa to 5.0 MPa. The average value of the dissociation enthalpy obtained at 5.0 mol% TBAC is 273.0 ± 9.9 J/g. Fig. 9 exhibits the phase behavior of TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC as the operating pressure increased from 1.0 MPa to 5.0 MPa. No ice melting peak was observed at 273 K while the endothermic peak for the TBAC + CO2 + CH4 mixed semiclathrate hydrate can be clearly seen, indicating that all water in the 3.3 mol% TBAC solution may be converted into TBAC + CO2 + CH4 semiclathrate hydrate. Although the endothermic peak obtained at 1.0 MPa was sharper than that obtained at 3.0 MPa and 5.0 MPa, it became broader as pressure increased from 1.0 MPa to 5.0 MPa. The area of the endothermic peak and the dissociation enthalpy obtained according to the area of the endothermic peak were found to increase when increasing the pressure from 1.0 MPa to 5.0 MPa. The dissociation enthalpy obtained at 1.0 MPa, 3.0 MPa, and 5.0 MPa was found to be 9
274.3 J/g, 292.9 J/g, and 305.3 J/g, respectively. This is probably because a larger amount of the CO2 + CH4 gas mixture was incorporated into the TBAC semiclathrate hydrate at a higher pressure.
4. Conclusions In this work, the phase equilibrium data of TBAC semiclathrate hydrate formed in the presence of CO2, CH4, and CO2 + CH4 were measured at various TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%), and the corresponding hydrate dissociation enthalpies were reported. The phase behaviors of TBAC semiclathrate hydrate formed in the presence of CO2 + CH4 were elucidated when increasing TBAC concentration from 1.0 mol% to 5.0 mol% and increasing the operating pressure from 1.0 MPa to 5.0 MPa. The semiclathrate hydrate formed at 3.3 mol% TBAC was found to be more stable than that formed at 1.0 mol% and 5.0 mol% TBAC in the presence of CO2 + CH4. The semiclathrate hydrate formed at 1.0 mol% TBAC is a mixture of pure TBAC semiclathrate and the TBAC + CO2 + CH4 semiclathrate hydrate. The hydrate dissociation enthalpy for the mixed TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC was found to increase with the increase of pressure, while this trend was not clearly seen at 5.0 mol% TBAC.
Acknowledgements The financial support from the Natural Science Foundation of China (No. 51676021), the National Key Basic Research Program of China (No. 2014CB239206), and
the
Fundamental
Research
Funds
for
Central
Universities
(No.
106112017CDJQJ148804) is greatly appreciated.
References: (1) P. Englezos, Ind. Eng. Chem. Res. 32 (1993) 1251-1274. (2) E.D. Sloan, C. A. Koh, Clathrate Hydrates of Natural Gases. Crc Press (2008). (3) P. Englezos, J. Lee, Korean J. Chem. Eng. 22 (2005) 671-681. (4) Y.F. Makogon, S.A. Holditch, T.Y. Makogon, J. Petrol. Sci. Eng. 56 (2007) 10
14-31. (5) Z.R. Chong, S.H.B. Yang, P. Babu, P. Linga, X.S. Li, Appl. Energy 162 (2016) 1633-1652. (6) G. Li, X.S. Li, B. Yang, L.P. Duan, N.S. Huang, Y. Zhang, L.G. Tang, Appl. Energy 112 (2013) 1303-1310. (7) D.L. Zhong, S.Y. He, D.J. Sun, C. Yang, Energy Procedia 61 (2014) 1573-1576. (8) H.P. Veluswamy, A.J.H. Wong, P. Babu, R. Kumar, S. Kulprathipanja, P. Rangsunvigit, P. Linga, Chem. Eng. J. 290 (2016) 161-173. (9) K.C. Kang, P. Linga, K.N. Park, S.J. Choi, J.D. Lee, Desalination 353 (2014) 84-90. (10) X.S. Li, Z.M. Xia, Z.Y. Chen, K.F. Yan, G. Li, H.J. Wu, Industrial Eng. Chem. Research 49 (2010) 11614-11619. (11) M.J. Yang, Y.C. Song, L. Jiang, Y. Zhao, X. Ruan, Y. Zhang, S. Wang, Applied Energy 116 (2014) 26-40. (12) D.L. Zhong, P. Englezos, Energy & Fuels 26 (2012) 2098-2106. (13) D.L. Zhong, Z. Li, Y.Y. Lu, J.L. Wang, J. Yan, Applied Energy 158 (2015) 133-141. (14) Z. Li, D.L. Zhong, Y.Y. Lu, J. Yan, Z.L. Zou, Applied Energy, 199 (2017) 370-381. (15) P. Linga, R. Kumar, P. Englezos, Chemical Engineering Science 62 (2007) 4268-4276. (16) P. Linga, R. Kumar, P. Englezos, J. Hazard. Mater. 149 (2007) 625-629. (17) S.M. Kim, J.D. Lee, H.J. Lee, E.K. Lee, Y. Kim, Int. J. Hydrogen Energy, 36 (2011) 1115-1121. (18) P. Babu, R. Kumar, P. Linga, Energy 50 (2013) 364-373. (19) X.S. Li, C.G. Xu, Z.Y. Chen, and H.J. Wu, Energy, 35 (2010) 3902-3908. (20) M.Yang, W. Jing, J. Zhao, Z. Ling, Y. Song, Energy, 106 (2016) 546-553. (21) B. Tohidi, A. Danesh, A.C. Todd, R.W. Burgass, K.K. Østergaard, Fluid Phase Equilib. 138 (1997) 241-250. (22) D.L. Zhong, K. Ding, C. Yang, Y. Bian, J. Ji, J. Chem. Eng. Data 57 (2012) 11
3751-3755. (23) J.S. Zhang, J.W. Lee, J. Chem. Eng. Data 54 (2008) 659-661. (24) Y.J. Lee, T. Kawamura, Y. Yamamoto, J.H. Yoon, J. Chem. Eng. Data 57 (2012) 3543-3548. (25) D.L. Zhong, Z. Li, Y.Y. Lu, D.J. Sun, J. Chem. Eng. Data 59 (2014) 4110-4117. (26) H. Akiba, R. Ohmura, J. Chem. Thermodyn. 97 (2016) 83-87. (27) P. Meysel, L. Oellrich, P.R. Bishnoi, M.A. Clarke, J. Chem. Thermodyn. 43 (2011) 1475-1479. (28) J. Verrett, J.S. Renault-Crispo, P. Servio, Fluid Phase Equilib. 388 (2015) 160-168. (29) S. Muromachi, H. Hashimoto, T. Maekawa, S. Takeya, Y. Yamamoto, Fluid Phase Equilib. 413 (2016) 249-253. (30) L.P.S. Silva, D. Dalmazzone, M. Stambouli, P. Arpentinier, A. Trueba, W. Fürst, J. Chem. Thermodyn. 102 (2016) 293-302. (31) L.P.S. Silva, D. Dalmazzone, M. Stambouli, A.L. Lesort, P. Arpentinier, A. Trueba, W. Fürst, Fluid Phase Equilib. 413 (2016) 28-35. (32) A.H. Mohammadi, R. Anderson, B. Tohidi, AIChE Journal 51(2005) 2825-2833. (33) M. Wendland, H. Hasse, G. Maurer, J. Chem. Eng. Data 44 (1999) 901-906. (34) S. Kim, I.H. Baek, J.K. You, Y. Seo, Applied Energy. 140 (2015) 107-112. (35) W. Lin, D. Dalmazzone, W. Fürst, A. Delahaye, L. Fournaison, P. Clain, Fluid Phase Equilib. 372 (2014) 63-68. (36) S. Adisasmito, R.J. Frank, E.D. Sloan, J. Chem. Eng. Data 36 (1991) 68-71.
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Figure Captions: Fig. 1. Schematic of the HP µ-DSC setup (E: Sample cell; R: Reference cell). Fig. 2. The DSC endothermic peak obtained with the dynamic method. Fig. 3. Illustration of the stepwise method for hydrate phase equilibrium measurement. Fig. 4. Phase equilibria of CH4 hydrate and CO2 hydrate measured with different methods. (a) Comparison between the phase equilibrium data of CH4 hydrate measured by the stepwise method and that predicted by CSMHYD; (b) Comparison between the phase equilibrium data of CO2 hydrate measured by the stepwise method and that predicted by CSMHYD. Fig. 5. Phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured with different methods. Fig. 6. Phase equilibria of TBAC + CO2 + CH4 semiclathrate hydrate measured by HP DSC. Fig. 7. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%). The experimental pressure was fixed at 3.0 MPa. Fig. 8. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 5.0 mol%. Fig. 9. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 3.3 mol%.
Table Captions: Table 1. Purities and Suppliers of Materials. Table 2. Three-phase (H-Lw-V) equilibria and dissociation enthalpies of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured by HP µ-DSC. Table 3. Three-phase (H-Lw-V) equilibria and dissociation enthalpy of TBAC semiclathrate hydrate formed from CO2 + CH4. 13
Fig. 1. Schematic of the HP µ-DSC setup (E: Sample cell; R: Reference cell).
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Fig. 4. Phase equilibria of CH4 hydrate and CO2 hydrate measured with different methods. (a) Comparison between the phase equilibrium data of CH4 hydrate measured by the stepwise method and that predicted by CSMHYD; (b) Comparison between the phase equilibrium data of CO2 hydrate measured by the stepwise method and that predicted by CSMHYD.
17
12 3.3 mol% TBAC + CO2 [25] 10
3.3 mol% TBAC + CO2 (this work) 3.3 mol% TBAC + CH4 [25]
P (MPa)
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Fig. 5. Phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured with different methods.
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12 pure water (0 mol% TBAC) [25, 36] 1.0 mol% TBAC 3.3 mol% TBAC 5.0 mol% TBAC
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295
T (K)
Fig. 6. Phase equilibria of TBAC + CO2 + CH4 semiclathrate hydrate measured by HP µ-DSC.
19
5 1.0 mol% TBAC 3.3 mol% TBAC 5.0 mol% TBAC
Heat flow (mW)
3
0
-3
-5
TBAC semiclathrate hydrate ice melting
-8
-10 255
260
265
270
275
280
285
290
295
300
Temperature (K)
Fig. 7. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%). The experimental pressure was fixed at 3.0 MPa.
20
4
1 MPa 3 MPa 5 MPa
Heat flow (mW)
2
5.0 mol% TBAC + CO2 + CH4
0
-2
ice melting
TBAC+CO2+CH4 mixed
-4
-6 250
semiclathrate hydrate
255
260
265
270
275
280
285
290
295
300
Temperature (K)
Fig. 8. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 5.0 mol%.
21
4
Heat flow (mW)
3.3 mol% TBAC + CO2 + CH4
1 MPa 3 MPa 5 MPa
2
0
-2
-4
-6 250
255
260
265
270
275
280
285
290
295
300
Temperature (K)
Fig. 9. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 3.3 mol%.
22
Table 1. Purities and Suppliers of Materials a Material
Supplier
Purity
Analysis Method c
CO2, CH4, CO2 + CH4b
Chongqing Jiarun Gas
99.99% (mole fraction)
GC
Tetra-n-butyl Chongqing Oriental 97% Argentometric ammonium chloride Chemical (mass fraction) titration a Deionized water with a resistivity of 18.2 MΩ·cm was produced in the lab and used for all experiments. b The gas mixture consists of 40 mol% CO2 and 60 mol% CH4. The standard uncertainty in the gas mixture composition was 0.01 mol%. c The purity analysis was performed by the supplier.
23
Table 2. Three-phase (H-Lw-V) equilibria and dissociation enthalpies of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured by HP µ-DSC. a 3.3 mol% TBAC + CO2 T (K)
P (MPa)
292.2 293.0
2.05 3.53
Dissociation enthalpy (J/g) b 336.3 ± 0.7
3.3 mol% TBAC + CH4 T (K)
P (MPa)
Dissociation enthalpy (J/g) b
291.3 292.8 293.4
3.10 5.02 5.73
264.1 ± 6.2
a
Standard uncertainties u are u(T) = 0.1 K and u(P) = 6.4 kPa. The standard uncertainty for TBAC content is 0.01 mol%. Phase equilibria were measured by the stepwise method. b Dissociation enthalpies were measured by the dynamic method and reported in the form of average value ± standard deviation.
24
Table 3. Three-phase (H-Lw-V) equilibria and dissociation enthalpy of TBAC semiclathrate hydrate formed from CO2 + CH4 a. TBAC (mol%) 1.0
3.3
5.0
T (K)
P (MPa)
Dissociation enthalpy (J/g) b
285.9 289.5 291.2 292.5
1.00 3.02 5.02 7.00
247.2 ± 12.2
289.9 291.9 293.2 294.3
1.00 3.05 5.07 6.95
290.8 ± 15.6
289.0 291.3 292.6 293.7
1.00 2.99 5.01 7.02
273.0 ± 9.9
a
The CO2 + CH4 gas mixture is composed of 40 mol% CO2 and 60 mol% CH4. Standard uncertainties u are u(T) = 0.1 K and u(P) = 6.4 kPa. The standard uncertainty for TBAC content is 0.01 mol%. Phase equilibria were measured by the stepwise method. b Dissociation enthalpies were measured by the dynamic method and reported in the form of average value ± standard deviation.
25
Highlights
Highlights:
HP µ-DSC was used for hydrate phase equilibrium measurement.
Phase equilibria of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 were reported.
Dissociation enthalpies of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 were reported.
26
S0021-9614(17)30271-9 http://dx.doi.org/10.1016/j.jct.2017.07.039 YJCHT 5157
To appear in:
J. Chem. Thermodynamics
Received Date: Revised Date: Accepted Date:
4 May 2017 28 July 2017 29 July 2017
Please cite this article as: S-L. Qing, D-L. Zhong, D-T. Yi, Y-Y. Lu, Z. Li, Phase Equilibria and Dissociation Enthalpies for Tetra-n-butylammonium Chloride Semiclathrate Hydrates Formed with CO2, CH4, and CO2 + CH4, J. Chem. Thermodynamics (2017), doi: http://dx.doi.org/10.1016/j.jct.2017.07.039
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Phase Equilibria and Dissociation Enthalpies for Tetra-n-butylammonium Chloride Semiclathrate Hydrates Formed with CO2, CH4, and CO2 + CH4
Sheng-Lan Qing a, Dong-Liang Zhong a, b, *, Da-Tong Yi b, Yi-Yu Lu a, Zheng Li b
a
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China
b
College of Power Engineering, Chongqing University, Chongqing 400044, China
1
Abstract: In this work, a high pressure micro-differential scanning calorimeter (HP µ-DSC) was employed to measure the phase equilibrium data and dissociation enthalpies of tetra-n-butylammonium Chloride (TBAC) semiclathrate hydrate formed in the presence of CO2, CH4, and CO2 + CH4. The TBAC concentration varied from 1.0 mol% to 5.0 mol%, and the operating pressure changed from 1.0 MPa to 5.0 MPa. It was found that the phase boundary of TBAC semiclathrate hydrate formed at 3.3 mol% TBAC was lower than that obtained at 1.0 mol% and 5.0 mol% TBAC in the presence of CO2 + CH4, and the hydrate dissociation enthalpy obtained at 3.3 mol% TBAC was larger than that obtained at 1.0 mol% and 5.0 mol% TBAC. This result indicates that TBAC semiclathrate (TBAC·29.7H2O) formed at the stoichiometric concentration (3.3 mol%) was more stable than that formed at 1.0 mol% and 5.0 mol% TBAC in the presence of CO2 + CH4. In addition, pure TBAC semiclathrate hydrate and the mixed TBAC + CO2 + CH4 semiclathrate hydrate were found to coexist at 1.0 mol% TBAC. The hydrate dissociation enthalpy for the mixed TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC was found to increase with the increase of pressure. This is probably because a larger amount of the CO 2 + CH4 gas mixture was incorporated into the TBAC semiclathrate hydrate at a higher pressure.
Keywords: Semiclathrate hydrate; Phase equilibrium; DSC; Dissociation enthalpy; Thermodynamics
* Corresponding author. E-mail: [email protected] (D. L. Zhong) 2
1. Introduction Gas hydrates are ice like non-stoichiometric inclusion compounds formed at low temperature and high pressure conditions, in which small gas molecules like CH4 and CO2 are trapped in the hydrogen-bonded water cavities via Van der Waal’s forces. Structure I (sI), structure II (sII), structure H (sH), and semiclathrate hydrate are four common types of hydrates that have been identified so far [1, 2]. The naturally occurring gas hydrates stored in ocean sediments and permafrost regions have been considered as a potential energy resource for the 21st century as the amount of methane contained in gas hydrates is estimated to be two times larger than that in conventional fossil fuels (coal, oil, and natural gas) [3-6]. Besides, the hydrate-based technology has a great potential for the applications of gas storage and transportation [7, 8], desalination [9], and gas mixture separation [10-13] due to the large storage capacity and high separation efficiency of gas hydrates. The hydrate-based gas separation process for CO2 capture from shale gas, flue gas, and fuel gas has been intensively studied in recent years [14-20]. In order to save energy costs of the separation process, the attempt to shift hydrate formation conditions to higher temperatures or lower pressures has been made by adding thermodynamic promoters into the hydrate formation solutions. Tetrahydrofuran (THF), cyclopentane (CP), and cyclohexane (CH) are well-known thermodynamic promoters that have been widely used for different hydrate formation systems [21, 22]. Zhang et al. [23] presented the data for the equilibrium conditions of CP + H2 and CP + CO2 binary hydrates, which were measured using a high pressure micro-differential scanning calorimeter (HP µ-DSC). Lee et al. [24] reported the phase equilibrium data of gas hydrates formed from a CO2 + CH4 gas mixture in the presence of THF. We measured the phase equilibrium data of gas hydrates formed from a model shale gas (40 mol% CO2/CH4) at
various THF concentrations using an isochoric
pressure-search method, and found significant decrease in the equilibrium pressure at a given temperature [25]. Recently, quaternary ammonium salts (QAS) like tetra-n-butylammonium bromide (TBAB) and tetra-n-butylammonium fluoride (TBAF) have been employed to form semiclathrate hydrates for gas separation 3
[26-29]. It was found that the phase boundaries for semiclathrate hydrates were much lower than those obtained in the presence of THF. Understanding the phase behaviors of gas hydrates is of great importance to develop hydrate-based gas separation technologies. However, the phase behaviors of semiclathrate hydrate formed in the presence of QAS are not well known. In this work, a high pressure micro-differential scanning calorimeter (HP µ-DSC) was employed to investigate the thermodynamic phase behaviors of TBAC semiclathrate hydrate formed with different gases (CO2, CH4, and CO2 + CH4). The purpose of this work is to report the phase equilibrium data and dissociation enthalpies of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 with TBAC concentration changing between 1.0 mol% and
5.0 mol%, and reveal the phase
behaviors of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 at different TBAC concentrations and different operating pressures.
2. Experimental Section 2.1 Materials Table 1 shows the source, purification methods, and final purity of the materials used in this work. The pure CH4, CO2, and the CO2 + CH4 gas mixture (40 mol% CO2 and 60 mol% CH4) were supplied by Chongqing Jiarun Gas with a measurement accuracy of ± 0.01 mol%. Tetra-n-butyl ammonium chloride (TBAC) with a certified mass purity of 97% was purchased from Chongqing Oriental Chemical Co. The purity was determined by argentometric titration. Deionized water for all experimental runs was produced in the lab with a resistivity of 18.2 MΩ·cm. 2.2 Apparatus A high pressure micro-differential scanning calorimeter (HP µ-DSC VII evo, Setaram Inc., France) was utilized to measure the three-phase (H-Lw-V) equilibria of TBAC semiclathrate hydrate formed with CO2, CH4, and the CO2 + CH4 gas mixture. Fig. 1 shows the schematic diagram of HP µ-DSC that was connected with an external refrigerator and a data acquisition system. The HP µ-DSC is composed of a reference cell and a sample cell that were made of Hastelloy C276 to avoid contamination and 4
corrosion. The cells can be operated at a pressure up to 40 MPa with the internal volume fixed at 0.33 cm3. The operating temperature range for the calorimeter is between 228 K and 393 K while the temperature scanning rate can be set in the range of 0.001-1 K/min. The resolution of the HP µ-DSC is 0.02 µW. 2.3 Procedure Prior to experiments the HP µ-DSC sample cell was cleaned with deionized water and dried. Then the TBAC solution with a mass of 8.5-10.5 mg was injected into the sample cell. Afterwards, the sample cell was inserted into the DSC furnace and connected to the gas line. It was purged at least six times using the experimental gas to eliminate any air remaining in the cells and tubes. The desired pressure was achieved by adjusting the pressure regulator installed between the gas cylinder and the supply vessel (Fig. 1). During the experiments the valves between the supply vessel and the cells were kept open while the vent valves were closed. The dynamic and stepwise methods were adopted to investigate the phase behavior of gas hydrates and to measure the hydrate dissociation temperature at a given pressure [30, 31]. The first step is to crystallize gas hydrates in the sample cell. The system was cooled down from 293.15 K to 253.15 K at a rate of 0.3 K/min and was kept 3 h at 253.15 K to ensure the complete freezing of the liquid in the sample cell. Then the temperature was raised from 253.15 K to 293.15 K at a rate of 0.3 K/min. The dissociation enthalpies of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 were determined by integrating the DSC endothermic peak obtained in the hydrate dissociation process. For the stepwise method, the temperature was increased from 253.15 K to a temperature that is at most 0.6 K lower than the dissociation temperature using the same heating rate (0.3 K/min). The temperature was then increased in a heating rate of 0.1 K with the isothermal interval between each heating step was adjusted to 2 h. The step temperature (Tset) after the last endothermic melting signal was determined as the hydrate phase equilibrium temperature at the given pressure. Fig. 2 shows the typical heat-flow profile of TBAC semiclathrate hydrate formed from 40 mol% CO2/CH4 at 3.0 MPa and 5.0 mol% TBAC, which was obtained using the dynamic method. Fig. 3 illustrates the stepwise 5
method for measuring the phase equilibrium point of TBAC semiclathrate hydrate formed from 40 mol% CO2/CH4.
3. Results and Discussion 3.1 Validation of the stepwise method for phase equilibrium measurement of CH4 hydrate and CO2 hydrate The phase equilibrium data of pure CH4 hydrate and CO2 hydrate were measured by the stepwise method and compared with those reported in the literature. The phase equilibrium data were tabulated in Table S1 as supporting information. It can be seen from Fig. 4a that the phase equilibrium data of CH 4 hydrate measured by the stepwise method agree well with that predicted by CSMHYD [2] and are in good agreement with that reported by Mohammadi et al [32]. Similarly, it can be seen from Fig. 4b that the phase equilibrium data of CO2 hydrate measured by the stepwise method coincide with that calculated by CSMHYD [2] and agree with that reported in the literature [32, 33]. Therefore, the phase equilibrium comparison shown in Fig. 4 indicates that the stepwise method adopted in this work is able to produce accurate phase equilibrium data of gas hydrates. 3.2 Phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate Fig. 5 shows the phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC, which were measured by the stepwise method. The equilibrium temperatures, pressures, and hydrate dissociation enthalpies were summarized in Table 2. As seen in Fig. 5, the phase equilibrium data of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured in this work are in good agreement with those reported by Soyong Kim et al. [34] It is interesting to note that the phase boundaries of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate obtained in this work are slightly lower than those reported by Soyong Kim et al. [34] The small discrepancy may be caused by the different methods that were adopted for hydrate phase equilibrium measurement. We employed the calorimetric method (HP µ-DSC) in this work while the classical thermodynamic method (PVT) was used by Soyong Kim [34] to measure the hydrate phase equilibrium data. Generally, the HP µ-DSC 6
method is able to generate more accurate phase equilibrium data than the PVT method as the accuracy of phase equilibrium points determined in the PVT apparatus is usually influenced by the gas thermal expansion properties and the uncertainties of temperature and pressure measurement in the equilibrium cell. This analysis is also stated in the literature [35]. 3.3 Phase equilibria of TBAC + CO2 + CH4 semiclathrate hydrate Fig. 6 shows the phase equilibrium data of TBAC semiclathrate hydrate formed from CO2 + CH4 with TBAC concentration varying between 1.0 mol% and 5.0 mol%. The equilibrium temperatures, pressures, and hydrate dissociation enthalpies were given in Table 3. As seen in Fig. 6, comparing with the equilibrium pressure of the binary CO2 + CH4 gas hydrates formed in pure water at a given temperature [25, 36], the hydrate equilibrium pressure obtained in the presence of TBAC decreased significantly. This indicates that adding TBAC into the aqueous solution can shift the hydrate phase equilibrium conditions to lower pressures at a given temperature or higher temperatures at a given pressure. Therefore, the thermodynamic stability of TBAC semiclathrate hydrate formed from CO2 + CH4 is greatly improved as compared to the binary CO2 + CH4 gas hydrates formed in pure water. In addition, Fig. 6 shows that the phase equilibrium boundary obtained at 3.3 mol% TBAC is lower than that obtained at 1.0 mol% and 5.0 mol% TBAC. Note that 3.3 mol% corresponds to the stoichiometric concentration of TBAC semiclathrate hydrate (TBAC·29.7H2O), so the comparison of phase equilibrium boundaries indicates that TBAC + CO2 + CH4 semiclathrate hydrate formed at the stoichiometric concentration of TBAC·29.7H2O (3.3 mol%) is more stable than that formed at 1.0 mol% and 5.0 mol% TBAC. This result agrees with the hydrate dissociation enthalpies that were obtained from hydrate dissociation endothermic peaks. As seen in Table 3, the hydrate dissociation enthalpy obtained at 3.3 mol% TBAC is 290.8 ± 15.6 J/g, which is higher than that obtained at 1.0 mol% (247.2 ± 12.2 J/g) and 5.0 mol% (273 ± 9.9 J/g). 3.4 Phase behavior of TBAC + CO2 + CH4 semiclathrate hydrate at different TBAC concentrations Fig. 7 shows the dissociation thermograms of TBAC semiclathrate hydrate formed 7
at three different TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%) in the presence of 40 mol% CO2/CH4. The operating pressure was fixed at 3.0 MPa. As seen in Fig. 7, at 1.0 mol% TBAC three endothermic peaks were detected in the heating process. The endothermic peak occurring at 273 K represents ice melting. The onset temperature of the middle endothermic peak was found to be 279.8 K, which agrees with the phase equilibrium temperature of pure TBAC semiclathrate hydrate formed at 3.0 MPa. Therefore, it can be inferred that the middle endothermic peak stands for the dissociation of pure TBAC semiclathrate hydrate. The onset temperature of the third endothermic peak was found to be 286.1 K, which is close to the phase equilibrium temperature of TBAC + CO2 + CH4 semiclathrate hydrate determined by the stepwise method (Table 3). Therefore, the endothermic peaks detected indicate that the hydrates formed at 1.0 mol% TBAC are a mixture of pure TBAC semiclathratre hydrate and the TBAC + CO2 + CH4 semiclathrate hydrate. The dissociation thermogram obtained at 5.0 mol% TBAC shows that the ice melting peak became very small and the dissociation peak for pure TBAC semiclathrate hydrate disappeared, on the contrary, a large dissociation peak for the TBAC + CO2 + CH4 mixed semiclathrate hydrate was observed. This result indicates that most of the liquid sample was converted into the TBAC + CO2 + CH4 semiclathrate hydrate, and nearly no ice and TBAC semiclathrate hydrate formed at 5.0 mol% TBAC as compared to that obtained at 1.0 mol% TBAC. Interestingly, the dissociation thermogram obtained at the stoichiometric TBAC concentration (3.3 mol%) was different from that obtained at 1.0 mol% TBAC. As seen in Fig. 7, no ice melting peak was found during the heating process. There was only one endothermic peak observed for the TBAC + CO2 + CH4 mixed semiclathrate hydrate during the heating process, which was sharper and larger than that obtained at 1.0 mol% and 5.0 mol% TBAC. This result indicates that at 3.3 mol% TBAC the liquid sample was completely converted into the TBAC + CO2 + CH4 semiclathrate hydrate whereas pure TBAC semiclathrate and ice did not form. Since the amount of TBAC solution charged into the sample cell was almost the same for all experimental runs, the amount of water in 5.0 mol% TBAC solution would be relatively lower than 8
that in 3.3 mol% TBAC solution. As a result, the amount of hydrate formed at 3.3 mol% TBAC was higher than that formed at 5.0 mol% TBAC. This might be the reason why the endothermic peak of the mixed TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC was larger than that obtained at 5.0 mol% TBAC. 3.5 Phase behavior of TBAC + CO2 + CH4 semiclathrate hydrate at different operating pressures Fig. 8 shows the dissociation thermogram of TBAC + CO2 + CH4 semiclathrate hydrate formed at 5.0 mol% TBAC with the operating pressure varying between 1.0 MPa to 5.0 MPa. As seen in the figure, with the increase of operating pressure the ice melting peak shrinks while the dissociation peak for TBAC + CO2 + CH4 semiclathrate hydrate becomes broader. This indicates that with the pressure increasing from 1.0 MPa to 5.0 MPa the mass of ice formed from the liquid sample decreased, and the amount of TBAC solution converted into the TBAC + CO2 + CH4 mixed semiclathrate hydrate increased. This is probably because the driving force for hydrate crystallization was elevated, and thus more hydrate formed at a higher pressure. Interestingly, no significant change in the dissociation enthalpy of TBAC + CO2 + CH4 semiclathrate hydrate was observed at 5.0 mol% TBAC when increasing the operating pressure from 1.0 MPa to 5.0 MPa. The average value of the dissociation enthalpy obtained at 5.0 mol% TBAC is 273.0 ± 9.9 J/g. Fig. 9 exhibits the phase behavior of TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC as the operating pressure increased from 1.0 MPa to 5.0 MPa. No ice melting peak was observed at 273 K while the endothermic peak for the TBAC + CO2 + CH4 mixed semiclathrate hydrate can be clearly seen, indicating that all water in the 3.3 mol% TBAC solution may be converted into TBAC + CO2 + CH4 semiclathrate hydrate. Although the endothermic peak obtained at 1.0 MPa was sharper than that obtained at 3.0 MPa and 5.0 MPa, it became broader as pressure increased from 1.0 MPa to 5.0 MPa. The area of the endothermic peak and the dissociation enthalpy obtained according to the area of the endothermic peak were found to increase when increasing the pressure from 1.0 MPa to 5.0 MPa. The dissociation enthalpy obtained at 1.0 MPa, 3.0 MPa, and 5.0 MPa was found to be 9
274.3 J/g, 292.9 J/g, and 305.3 J/g, respectively. This is probably because a larger amount of the CO2 + CH4 gas mixture was incorporated into the TBAC semiclathrate hydrate at a higher pressure.
4. Conclusions In this work, the phase equilibrium data of TBAC semiclathrate hydrate formed in the presence of CO2, CH4, and CO2 + CH4 were measured at various TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%), and the corresponding hydrate dissociation enthalpies were reported. The phase behaviors of TBAC semiclathrate hydrate formed in the presence of CO2 + CH4 were elucidated when increasing TBAC concentration from 1.0 mol% to 5.0 mol% and increasing the operating pressure from 1.0 MPa to 5.0 MPa. The semiclathrate hydrate formed at 3.3 mol% TBAC was found to be more stable than that formed at 1.0 mol% and 5.0 mol% TBAC in the presence of CO2 + CH4. The semiclathrate hydrate formed at 1.0 mol% TBAC is a mixture of pure TBAC semiclathrate and the TBAC + CO2 + CH4 semiclathrate hydrate. The hydrate dissociation enthalpy for the mixed TBAC + CO2 + CH4 semiclathrate hydrate formed at 3.3 mol% TBAC was found to increase with the increase of pressure, while this trend was not clearly seen at 5.0 mol% TBAC.
Acknowledgements The financial support from the Natural Science Foundation of China (No. 51676021), the National Key Basic Research Program of China (No. 2014CB239206), and
the
Fundamental
Research
Funds
for
Central
Universities
(No.
106112017CDJQJ148804) is greatly appreciated.
References: (1) P. Englezos, Ind. Eng. Chem. Res. 32 (1993) 1251-1274. (2) E.D. Sloan, C. A. Koh, Clathrate Hydrates of Natural Gases. Crc Press (2008). (3) P. Englezos, J. Lee, Korean J. Chem. Eng. 22 (2005) 671-681. (4) Y.F. Makogon, S.A. Holditch, T.Y. Makogon, J. Petrol. Sci. Eng. 56 (2007) 10
14-31. (5) Z.R. Chong, S.H.B. Yang, P. Babu, P. Linga, X.S. Li, Appl. Energy 162 (2016) 1633-1652. (6) G. Li, X.S. Li, B. Yang, L.P. Duan, N.S. Huang, Y. Zhang, L.G. Tang, Appl. Energy 112 (2013) 1303-1310. (7) D.L. Zhong, S.Y. He, D.J. Sun, C. Yang, Energy Procedia 61 (2014) 1573-1576. (8) H.P. Veluswamy, A.J.H. Wong, P. Babu, R. Kumar, S. Kulprathipanja, P. Rangsunvigit, P. Linga, Chem. Eng. J. 290 (2016) 161-173. (9) K.C. Kang, P. Linga, K.N. Park, S.J. Choi, J.D. Lee, Desalination 353 (2014) 84-90. (10) X.S. Li, Z.M. Xia, Z.Y. Chen, K.F. Yan, G. Li, H.J. Wu, Industrial Eng. Chem. Research 49 (2010) 11614-11619. (11) M.J. Yang, Y.C. Song, L. Jiang, Y. Zhao, X. Ruan, Y. Zhang, S. Wang, Applied Energy 116 (2014) 26-40. (12) D.L. Zhong, P. Englezos, Energy & Fuels 26 (2012) 2098-2106. (13) D.L. Zhong, Z. Li, Y.Y. Lu, J.L. Wang, J. Yan, Applied Energy 158 (2015) 133-141. (14) Z. Li, D.L. Zhong, Y.Y. Lu, J. Yan, Z.L. Zou, Applied Energy, 199 (2017) 370-381. (15) P. Linga, R. Kumar, P. Englezos, Chemical Engineering Science 62 (2007) 4268-4276. (16) P. Linga, R. Kumar, P. Englezos, J. Hazard. Mater. 149 (2007) 625-629. (17) S.M. Kim, J.D. Lee, H.J. Lee, E.K. Lee, Y. Kim, Int. J. Hydrogen Energy, 36 (2011) 1115-1121. (18) P. Babu, R. Kumar, P. Linga, Energy 50 (2013) 364-373. (19) X.S. Li, C.G. Xu, Z.Y. Chen, and H.J. Wu, Energy, 35 (2010) 3902-3908. (20) M.Yang, W. Jing, J. Zhao, Z. Ling, Y. Song, Energy, 106 (2016) 546-553. (21) B. Tohidi, A. Danesh, A.C. Todd, R.W. Burgass, K.K. Østergaard, Fluid Phase Equilib. 138 (1997) 241-250. (22) D.L. Zhong, K. Ding, C. Yang, Y. Bian, J. Ji, J. Chem. Eng. Data 57 (2012) 11
3751-3755. (23) J.S. Zhang, J.W. Lee, J. Chem. Eng. Data 54 (2008) 659-661. (24) Y.J. Lee, T. Kawamura, Y. Yamamoto, J.H. Yoon, J. Chem. Eng. Data 57 (2012) 3543-3548. (25) D.L. Zhong, Z. Li, Y.Y. Lu, D.J. Sun, J. Chem. Eng. Data 59 (2014) 4110-4117. (26) H. Akiba, R. Ohmura, J. Chem. Thermodyn. 97 (2016) 83-87. (27) P. Meysel, L. Oellrich, P.R. Bishnoi, M.A. Clarke, J. Chem. Thermodyn. 43 (2011) 1475-1479. (28) J. Verrett, J.S. Renault-Crispo, P. Servio, Fluid Phase Equilib. 388 (2015) 160-168. (29) S. Muromachi, H. Hashimoto, T. Maekawa, S. Takeya, Y. Yamamoto, Fluid Phase Equilib. 413 (2016) 249-253. (30) L.P.S. Silva, D. Dalmazzone, M. Stambouli, P. Arpentinier, A. Trueba, W. Fürst, J. Chem. Thermodyn. 102 (2016) 293-302. (31) L.P.S. Silva, D. Dalmazzone, M. Stambouli, A.L. Lesort, P. Arpentinier, A. Trueba, W. Fürst, Fluid Phase Equilib. 413 (2016) 28-35. (32) A.H. Mohammadi, R. Anderson, B. Tohidi, AIChE Journal 51(2005) 2825-2833. (33) M. Wendland, H. Hasse, G. Maurer, J. Chem. Eng. Data 44 (1999) 901-906. (34) S. Kim, I.H. Baek, J.K. You, Y. Seo, Applied Energy. 140 (2015) 107-112. (35) W. Lin, D. Dalmazzone, W. Fürst, A. Delahaye, L. Fournaison, P. Clain, Fluid Phase Equilib. 372 (2014) 63-68. (36) S. Adisasmito, R.J. Frank, E.D. Sloan, J. Chem. Eng. Data 36 (1991) 68-71.
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Figure Captions: Fig. 1. Schematic of the HP µ-DSC setup (E: Sample cell; R: Reference cell). Fig. 2. The DSC endothermic peak obtained with the dynamic method. Fig. 3. Illustration of the stepwise method for hydrate phase equilibrium measurement. Fig. 4. Phase equilibria of CH4 hydrate and CO2 hydrate measured with different methods. (a) Comparison between the phase equilibrium data of CH4 hydrate measured by the stepwise method and that predicted by CSMHYD; (b) Comparison between the phase equilibrium data of CO2 hydrate measured by the stepwise method and that predicted by CSMHYD. Fig. 5. Phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured with different methods. Fig. 6. Phase equilibria of TBAC + CO2 + CH4 semiclathrate hydrate measured by HP DSC. Fig. 7. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%). The experimental pressure was fixed at 3.0 MPa. Fig. 8. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 5.0 mol%. Fig. 9. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 3.3 mol%.
Table Captions: Table 1. Purities and Suppliers of Materials. Table 2. Three-phase (H-Lw-V) equilibria and dissociation enthalpies of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured by HP µ-DSC. Table 3. Three-phase (H-Lw-V) equilibria and dissociation enthalpy of TBAC semiclathrate hydrate formed from CO2 + CH4. 13
Fig. 1. Schematic of the HP µ-DSC setup (E: Sample cell; R: Reference cell).
14
-3.5
Tonset
-4.0
Baseline
-4.5
Heat flow (mW)
Tend
-5.0 -5.5 -6.0 -6.5 -7.0 -7.5 274
276
278
280
282
284
286
288
290
292
294
296
Temperature (K)
Fig. 2. The DSC endothermic peak obtained with the dynamic method.
15
0.05
292.5
heat flow temperature 292.0
-0.05 291.5 -0.10
Tstep=291.3 K 291.0
0.1 K
-0.15
290.5
-0.20
-0.25
Temperature (K)
Heat flow (mW)
0.00
0
5
10
15
20
25
30
35
290.0 40
Time (h)
Fig. 3. Illustration of the stepwise method for hydrate phase equilibrium measurement.
16
11 10
Pressure (MPa)
9
(a) Predicted by CSMHYD [2] Mohammadi et al. [32] DSC stepwise method (this work)
8 7 6 5 4 3 277
278
279
280
281
282
283
284
285
286
287
Temperature (K)
6
(b)
Pressure (MPa)
5
4
Predicted by CSMHYD [2] Mohammadi et al. [32] Wendland et al. [33] DSC stepwise method (this work)
3
2
1
0 276
277
278
279
280
281
282
283
284
Temperature (K)
Fig. 4. Phase equilibria of CH4 hydrate and CO2 hydrate measured with different methods. (a) Comparison between the phase equilibrium data of CH4 hydrate measured by the stepwise method and that predicted by CSMHYD; (b) Comparison between the phase equilibrium data of CO2 hydrate measured by the stepwise method and that predicted by CSMHYD.
17
12 3.3 mol% TBAC + CO2 [25] 10
3.3 mol% TBAC + CO2 (this work) 3.3 mol% TBAC + CH4 [25]
P (MPa)
8
3.3 mol% TBAC + CH4 (this work)
6
4
2
0 289
290
291
292
293
294
295
296
T (K)
Fig. 5. Phase equilibria of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured with different methods.
18
12 pure water (0 mol% TBAC) [25, 36] 1.0 mol% TBAC 3.3 mol% TBAC 5.0 mol% TBAC
10
P (MPa)
8
6
4
2
0 275
277
279
281
283
285
287
289
291
293
295
T (K)
Fig. 6. Phase equilibria of TBAC + CO2 + CH4 semiclathrate hydrate measured by HP µ-DSC.
19
5 1.0 mol% TBAC 3.3 mol% TBAC 5.0 mol% TBAC
Heat flow (mW)
3
0
-3
-5
TBAC semiclathrate hydrate ice melting
-8
-10 255
260
265
270
275
280
285
290
295
300
Temperature (K)
Fig. 7. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three TBAC concentrations (1.0 mol%, 3.3 mol%, and 5.0 mol%). The experimental pressure was fixed at 3.0 MPa.
20
4
1 MPa 3 MPa 5 MPa
Heat flow (mW)
2
5.0 mol% TBAC + CO2 + CH4
0
-2
ice melting
TBAC+CO2+CH4 mixed
-4
-6 250
semiclathrate hydrate
255
260
265
270
275
280
285
290
295
300
Temperature (K)
Fig. 8. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 5.0 mol%.
21
4
Heat flow (mW)
3.3 mol% TBAC + CO2 + CH4
1 MPa 3 MPa 5 MPa
2
0
-2
-4
-6 250
255
260
265
270
275
280
285
290
295
300
Temperature (K)
Fig. 9. Dissociation thermograms of TBAC semiclathrate hydrate formed with CO2 + CH4 at three operating pressures (1.0 MPa, 3.0 MPa, and 5.0 MPa). The TBAC concentration was fixed at 3.3 mol%.
22
Table 1. Purities and Suppliers of Materials a Material
Supplier
Purity
Analysis Method c
CO2, CH4, CO2 + CH4b
Chongqing Jiarun Gas
99.99% (mole fraction)
GC
Tetra-n-butyl Chongqing Oriental 97% Argentometric ammonium chloride Chemical (mass fraction) titration a Deionized water with a resistivity of 18.2 MΩ·cm was produced in the lab and used for all experiments. b The gas mixture consists of 40 mol% CO2 and 60 mol% CH4. The standard uncertainty in the gas mixture composition was 0.01 mol%. c The purity analysis was performed by the supplier.
23
Table 2. Three-phase (H-Lw-V) equilibria and dissociation enthalpies of TBAC + CO2 and TBAC + CH4 semiclathrate hydrate measured by HP µ-DSC. a 3.3 mol% TBAC + CO2 T (K)
P (MPa)
292.2 293.0
2.05 3.53
Dissociation enthalpy (J/g) b 336.3 ± 0.7
3.3 mol% TBAC + CH4 T (K)
P (MPa)
Dissociation enthalpy (J/g) b
291.3 292.8 293.4
3.10 5.02 5.73
264.1 ± 6.2
a
Standard uncertainties u are u(T) = 0.1 K and u(P) = 6.4 kPa. The standard uncertainty for TBAC content is 0.01 mol%. Phase equilibria were measured by the stepwise method. b Dissociation enthalpies were measured by the dynamic method and reported in the form of average value ± standard deviation.
24
Table 3. Three-phase (H-Lw-V) equilibria and dissociation enthalpy of TBAC semiclathrate hydrate formed from CO2 + CH4 a. TBAC (mol%) 1.0
3.3
5.0
T (K)
P (MPa)
Dissociation enthalpy (J/g) b
285.9 289.5 291.2 292.5
1.00 3.02 5.02 7.00
247.2 ± 12.2
289.9 291.9 293.2 294.3
1.00 3.05 5.07 6.95
290.8 ± 15.6
289.0 291.3 292.6 293.7
1.00 2.99 5.01 7.02
273.0 ± 9.9
a
The CO2 + CH4 gas mixture is composed of 40 mol% CO2 and 60 mol% CH4. Standard uncertainties u are u(T) = 0.1 K and u(P) = 6.4 kPa. The standard uncertainty for TBAC content is 0.01 mol%. Phase equilibria were measured by the stepwise method. b Dissociation enthalpies were measured by the dynamic method and reported in the form of average value ± standard deviation.
25
Highlights
Highlights:
HP µ-DSC was used for hydrate phase equilibrium measurement.
Phase equilibria of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 were reported.
Dissociation enthalpies of TBAC semiclathrate hydrate formed with CO2, CH4, and CO2 + CH4 were reported.
26