Hydrate Dissociation Equilibrium Conditions for Carbon Dioxide +

Article pubs.acs.org/jced

Hydrate Dissociation Equilibrium Conditions for Carbon Dioxide + Tetrahydrofuran Mao Wang, Zhi-Gao Sun,* Xiao-Han Qiu, Ming-Gui Zhu, Cheng-Hao Li, Ai-Jun Zhang, Juan Li, Cui-Min Li, and Hai-Feng Huang School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215009, China ABSTRACT: Hydrate equilibrium conditions (vapor + hydrate + liquid water + liquid tetrahydrofuran, THF) formed from carbon dioxide in aqueous solutions of THF is determined using a stepwise heating method under isochoric conditions in this work. THF is reported to form an SII hydrate. Hydrate phase equilibrium conditions are measured under the conditions of the THF mass concentration range of 0.0726−0.2503 and pressure range of 0.10−1.87 MPa. The experimental results present that the equilibrium pressures of the experimental systems of carbon dioxide + THF + H2O are obviously lower than that of the carbon dioxide + H2O system under test conditions. Furthermore, the experimental results also indicate that the carbon dioxide hydrate formation pressure decreased more apparently with more additives of THF.

1. INTRODUCTION Gas hydrate is a kind of molecular compound of gas and water, with solid crystals forming at certain conditions of pressure and temperature. The guest molecules, such as methane, propane, and so forth, are encaged in water cavities by hydrogen bonding. Guest molecules link with host (water) molecules through van der Waals forces. The structure of gas hydrate generally depends on the guest (gas) molecules and the temperature and pressure conditions. Gas hydrates involve three crystal structures of SI, SII, and SH.1 Gas hydrates have been considered for applications such as cold storage, gas storage and transportation, desalination, and carbon dioxide capture.2−6 Carbon dioxide hydrate can be used to store cold storage as the latent heat of carbon dioxide hydrate is about 500 kJ/kg. The latent heat of carbon dioxide hydrate is much higher, compared to ice. But carbon dioxide tends to form gas hydrate at lower temperatures or higher pressures (>1 MPa). Some additives were used to reduce carbon dioxide hydrate formation pressure.7−11 THF was reported to form SII hydrate under the conditions of 277.55 K at atmospheric pressure,4,12,13 which was a good low pressure material for hydrate formation. THF hydrate is also a good medium for air conditioning applications and hydrogen storage.14−20 THF can reduce the equilibrium pressure of carbon dioxide hydrate and promote hydrate formation. However, equilibrium hydrate dissociation conditions of carbon dioxide + THF + H2O are insufficient at present. Hydrate phase equilibrium data for carbon dioxide + THF + H2O system were tested in this work.

by Sun et al.21 Hydrate forms in the stainless steel cell, whose volume is about 300 cm3. The cell can work in the pressure range from 0 to 20 MPa. There are two glass windows in the wall of the cell. The process of hydrate formation or dissociation was watched from the glass windows during the experiments. The materials in the cell were agitated by a stirrer. The cell was put into a thermostatic bath to control experimental temperature. The thermostatic bath can work in the temperature range from 253 to 373 K, whose accuracy is ±0.05 K. The pressure was measured using an absolute

2. EXPERIMENTAL APPARATUS AND PROCEDURE Experimental Apparatus. The experimental setup is provided in Figure 1, which is described in the previous work

Received: October 1, 2016 Accepted: January 6, 2017 Published: January 17, 2017

© 2017 American Chemical Society

Figure 1. Schematic diagram of the experimental apparatus. APT, absolute pressure transducer; RTD, resistance temperature detector; V1−V8, valves.

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DOI: 10.1021/acs.jced.6b00848 J. Chem. Eng. Data 2017, 62, 812−815

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solutions of 7.26, 10.24, 19.05, and 25.03 mass % THF. The hydrate equilibrium data are tabulated in Table 2.

pressure transducer, whose measurement range is from 0 to 10 MPa and its accuracy was ±20 kPa. Pt100 thermometer was used to measure the experimental temperature, whose accuracy was ±0.1 K. The pressures and temperatures were recorded by an Agilent 34970A data collector in this work. Experimental Procedure. Table 1 provides the test chemicals used in this experiment. The test liquids, such as

Table 2. Three-Phase Equilibrium Temperatures and Pressures for Carbon Dioxide Hydrate at Different THF Mass Fractions w in Watera w

Table 1. Test Materials Used in This Work

a

component

purity

supplier

THF carbon dioxide water

≥99.0% 99.99%

Tianjin Damao Chemical Reagent Factory Suzhou Hongyun Gas Co. Distilled

T (K)

P (MPa)

w

T (K)

P (MPa)

0

276.45 278.15 279.05 280.25 280.85 281.55 282.05

1.81 2.25 2.51 2.94 3.19 3.55 3.83

0.0726

0.1024

277.15 278.35 279.65 281.75 282.75 285.25 286.95 288.05 280.75 282.45 283.35 287.45

0.12 0.21 0.29 0.49 0.63 1.07 1.45 1.85 0.28 0.47 0.59 1.44

0.1905

276.45 277.45 278.25 280.75 281.65 284.25 286.05 286.85 278.85 279.85 281.05 282.65 283.65 286.05 287.55 288.85

0.11 0.20 0.28 0.53 0.65 1.10 1.59 1.87 0.10 0.19 0.29 0.48 0.63 1.05 1.44 1.87

a

The purity of THF and carbon dioxide are the mass fraction and mole fraction, respectively.

distilled water and THF, were weighed using a CP225D balance. When the measurement range of the balance is from 0 to 80 g, its accuracy is ±0.01 mg. While more than 80 g of materials was weighted, its accuracy was ±0.1 mg. A stepwise heating method was used to measure hydrate equilibrium dissociation conditions in this work.21,22 The cell was rinsed three times using distilled water and dried. A vacuum pump was used to remove the air from the cell. The cell was loaded with THF and water. Then the cell was pressured using experimental gas to the desired pressure from the sample cell. The cell was put into the glycol water bath after the vent valve was closed. The temperature of the cell was slowly lowered to the point hydrates formed. When hydrate formed, the pressure dropped rapidly due to the encapsulation of carbon dioxide molecules within the hydrate cage or temperature rose due to the exothermic reaction during hydrate formation. The hydrate formed in the cell also may be observed from the windows visually. The speed of magnetic stirrer was about 500 rpm in this work. When a lot of hydrates were observed from the windows, the temperature of the cell was increased to dissociate all of the hydrate. The process of hydrate formation and dissociation was done again. The hydrate formation and dissociation twice is beneficial to hydrate formation in the following experimental procedure. Then the temperature of the cell was lowered to form hydrate. After the hydrate formed, the cell was heated in steps of 0.1 K to dissociate the hydrate. Sufficient time (3−5 h) was given for each step. While the temperature of the cell was raised, the pressure of the cell increased clearly at each step as some hydrate dissociated in the cell. Once all hydrate in the cell dissociated, a smaller pressure increase was observed at each step. The experimental data of temperatures and pressures were logged continuously, and the P−T curve of equilibrium conditions was plotted. The hydrate dissociation point was determined from the P−T curve, where a sharp slope changed was observed. It was also confirmed from the two lateral plexiglass windows of the cell.

0.2503

a

The overall errors of T and P are u(T) = 0.2 K and u(P) = 20 kPa, respectively.

Figure 2 shows the hydrate equilibrium data of carbon dioxide + H2O and carbon dioxide + THF + H2O with aqueous solutions of 19.05 mass % THF. The dissociation pressures of carbon dioxide + THF + H2O mixed hydrates were much lower than that of the simple hydrate of pure carbon dioxide at a certain temperature. It meant that THF was enclathrated in hydrate cages. THF molecules occupied large cages of SII hydrates described by Torre et al.4 It is possible that the system of carbon dioxide + THF+ H2O forms SII hydrates, compared to those of carbon dioxide + H2O that formed SI hydrate. The hydrate equilibrium pressure of the THF + carbon dioxide + H2O system increased with the temperature increase. Figure 2 presented that the pressure conditions of hydrate formation moved to lower pressures at any temperatures, due to the additives of THF. The higher temperature was, the larger difference of equilibrium pressure was in this work. Figure 2 also showed that the hydrate formed from THF + carbon dioxide was more stable than that from carbon dioxide. The phase equilibrium conditions in this work were in accordance with that from references as shown in Figure 2.1,23 Figure 3 presented the effect of the THF mass fraction on hydrate formation conditions for the carbon dioxide + THF + H2O system. As shown in Figure 3, the additives of THF improved hydrate formation conditions. The experimental results also indicated that the higher THF mass fraction was, the lower carbon dioxide hydrate formation pressure was when THF mass fraction was lower than 19.05 mass %. As an example, when T was 286.05 K, the hydrate equilibrium pressure was 1.05 MPa for carbon dioxide + THF (mass fraction, w = 0.1905) system, while the hydrate equilibrium pressure was 1.59 MPa for carbon dioxide + THF (mass fraction, w = 0.0726) system. One possible explanation was that

3. RESULTS AND DISCUSSION The hydrate phase equilibrium data of the carbon dioxide + THF + H2O system were obtained using stepwise heating method. THF was encapsulated into the large cages of SII hydrate. If THF occupied all large cages of SII hydrate, the mass ratio of THF and water was about 1:4.25 (19.05 mass % THF). In this work, experiments were conducted with aqueous 813

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Figure 2. Hydrate equilibrium conditions for carbon dioxide + THF + H2O.

Figure 3. Effect of the THF mass fraction on hydrate equilibrium conditions for carbon dioxide + THF + H2O.

dioxide hydrate formation pressure decreased more obviously with more THF additives in this work when the mass fraction of the aqueous solutions of THF was lowered than 19.05 mass %. Excessive THF did not further lower hydrate equilibrium pressures when the THF concentration was more than 19.05 mass %.

THF acted as a hydrate formation promoter, where carbon dioxide and THF were encaged in small cages and large cages of SII hydrate, respectively. Some physical techniques may be used to confirm how THF and carbon dioxide molecules enter the hydrate cages. The experimental results showed hydrate of carbon dioxide formed more easily with more additives of THF in this work. But when the mass fraction of the aqueous solutions of THF was more than 19.05 mass %, carbon dioxide hydrate formation pressure conditions did not lower further with more additives of THF. When THF occupied all large cages of SII hydrate, the mass fraction of THF was w = 0.1905. More THF did not take part in the hydrate formation procedure.

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AUTHOR INFORMATION

Corresponding Author

*Address: School of Environmental Science and Engineering, Suzhou University of Science and Technology, 1 Kerui Road, Suzhou 215009, P. R. China. E-mail address: [email protected]. ORCID

Zhi-Gao Sun: 0000-0002-9486-8825

4. CONCLUSIONS One SII formers, that is, THF, has been investigated in this work. Hydrate equilibrium data of the carbon dioxide + THF + H2O system have been studied and reported. The experimental results show that the systems of carbon dioxide + THF + H2O form SII hydrate, compared to forming SI hydrate with carbon dioxide + H2O. The test results also present that hydrate equilibrium pressure conditions of carbon dioxide reduce the greatly with the additives of THF. Moreover, the carbon

Funding

This authors would like to thank the financial support from Major Programs Foundation of Jiangsu Province Education Department (16KJA480001), Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, CAS (No. 2014DP173027), Jiangsu Key Laboratory of Intelligent Building Energy Efficiency (2014-07), the Priority Academic Program of Jiangsu Higher Education Institutions and the 814

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of TBAB, TBAB+NaCl and THF suitable for storage and transportation of natural gas. J. Nat. Gas Sci. Eng. 2016, 33, 509−517. (19) Mady, M. F.; Kelland, M. A. Tris(tert-heptyl)-N-alkyl-1ammonium bromidesPowerful THF hydrate crystal growth inhibitors and their synergism with poly-vinylcaprolactam kinetic gas hydrate inhibitor. Chem. Eng. Sci. 2016, 144, 275−282. (20) Sun, Q.; Chen, G.; Guo, X.; Liu, A. Experiments on the continuous separation of gas mixtures via dissolution and hydrate formation in the presence of THF. Fluid Phase Equilib. 2014, 361, 250−256. (21) Sun, Z. G.; Jiao, L. J.; Zhao, Z. G.; Wang, G. L.; Huang, H. F. Phase equilibrium conditions of semi-calthrate hydrates of (tetra-nbutyl ammonium chloride+Carbon dioxide). J. Chem. Thermodyn. 2014, 75, 116−118. (22) Hashimoto, S.; Sugahara, T.; Moritoki, M.; Sato, H.; Ohgaki, K. Thermodynamic stability of hydrogen + tetra-n-butyl ammonium bromide mixed gas hydrate in nonstoichiometric aqueous solutions. Chem. Eng. Sci. 2008, 63, 1092−1097. (23) Lee, Y. J.; Kawamura, T.; Yamamoto, Y.; Yoon, J. H. Phase equilibrium studies of tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 hydrates. J. Chem. Eng. Data 2012, 57, 3543−3548.

Graduate Innovation Training Foundation of SUST (SKCX15028). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.6b00848 J. Chem. Eng. Data 2017, 62, 812−815