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Hydrate Equilibrium Measurements for CH4 and CO2/CH4 Mixture in the Presence of Single 2‑Methyl-2-propanol and 1,1-Dichloro-1fluoroethane Jing Qi, Yanhong Wang,* Shuanshi Fan, Xuemei Lang, Qi Li, Gang Li, and Jianbiao Chen

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Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China ABSTRACT: Hydrate phase equilibrium conditions of CO2/CH4 + 2-methyl-2-propanol (tBA) + water system, CH4 + 1,1-dichloro-1-fluoroethane (HCFC-141b) + water system, CO2 + HCFC-141b + water system, and CO2/CH4 + HCFC-141b + water system were measured. The moles fraction of CO2 in the mixture gases were 0.33 and 0.5, respectively. The mole fraction of HCFC-141b and tBA were 0.056. The measured temperature range is from 283.0 to 294.2 K. The phase equilibrium conditions were tested by an isochoric pressure-search method. Results showed that HCFC-141b has a stronger promotion effect on CH4 hydrate formation than on CO2 hydrate. The effect of CO2 content on the phase equilibrium conditions of CO2/CH4 mixture + HCFC-141b + water is weak. tBA also acts as a promoter for CO2/CH4 mixture gas. In the presence of tBA, the phase equilibrium conditions of the system with higher CO2 content moved to higher pressure and lower temperature region compared with the system with lower CO2 content.

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INTRODUCTION Gas hydrates are ice-like crystalline compounds which capture guest molecules in the cages formed by the hydrogen-bonding water molecules, and gas molecules like CH4 and CO2 interact with water molecules through van der Waals forces.1 Gas hydrates have already attracted wide attention due to its broad application, such as desalination,2,3 gas storage,4,5 and mixture gas separation.6−8 Using the hydrate-based separation, CO2 can be captured. Flue gas is the most researched gas.9−12 The other mixture gas is CO2/CH4 gas mixture, which is the main component of original natural gas or biogas.9,13,14 However, hydrate-based separation for CH4 and CO2 is challenging.9 On one hand, the formation conditions of CH4 and CO2 hydrates are harsh. On the other hand, because the difference between the hydrate phase equilibrium of CO2 and CH4 is relatively small, both CH4 and CO2 molecules may enter the hydrate crystal cavities during the hydrate formation. Therefore, hydrate promoters are used to shift the hydrate equilibrium conditions to the region where hydrates are easier to form or expand the gap between CH4 and CO2 hydrate phase equilibrium conditions.15−18 1,1-Dichloro-1-fluoroethane (HCFC-141b) is an air-conditioning refrigerant, which could easily form s-II hydrates at near atmospheric pressure.19,20 Wang et al.21 reported that HCFC-141b could significantly reduce the phase equilibrium pressure and improve the phase equilibrium temperature of CO2 hydrate. For example, at the temperature of 282 K the phase equilibrium pressure for CO2 + water system is 3.8 MPa,22 and the phase equilibrium pressure for CO2 + HCFC141b + water system is 0.18 MPa.21 As a good hydrate promoter, HCFC-141b should also reduce the phase equilibrium pressure and improve the phase equilibrium © XXXX American Chemical Society

temperature of CH4 hydrate and moderate CO2/CH4 mixture hydrate formation conditions so that the CO2/CH4 separation can be performed under milder conditions. 2-Methyl-2-propanol, also named tert-butanol (tBA), was reported to form s-II hydrates with the presence of gas, such as methane, carbon dioxide, and nitrogen with the inclusion of tBA in s-II large cages.23,24 With the presence of tBA, the hydrate phase equilibrium conditions of methane moved to the region where hydrates are easier to form, and the hydrate phase equilibrium conditions of CO2 moved to the region in which hydrates are more difficult to form.24 The effect of tBA on the formation of CH4 hydrates and CO2 hydrates was opposite, which might bring unexpected results in CO2/CH4 mixture separation based on hydrate technology. But the effect of tBA on the formation of CO2/CH4 mixture hydrate is not certain. It is necessary to measure the hydrate formation boundary of CO2/CH4 mixture in the presence of tBA. Though the components of biogas and landfill gas are CH4 and CO2, their contents are not same. In this study, the hydrate phase equilibrium conditions of two CO2/CH4 mixture (33/67 and 50/50, mole fraction) with HCFC-141b at the concentration of 5.6 mol % were measured. The 5.6 mol % HCFC-141b is the stoichiometric concentration of HCFC141b molecules and occupies the large cages of s-II hydrates. We also measure the hydrate formation boundary of CO2/CH4 mixture gas with tBA at the concentration of 5.6 mol %. These data will be used as essential for CO2/CH4 separation based on hydrate formation. Received: May 14, 2018 Accepted: July 17, 2018

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DOI: 10.1021/acs.jced.8b00401 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION Materials. Table 1 provided the suppliers and purities of chemicals used in this experiment. The mole fractions of tBA

until the desired pressure. Subsequently, the magnetic stirrer (about 600 rpm) and the thermostatic bath started. After the temperature and the pressure of the system were steady at room temperature, the system started to cool down with a rate of 3 K/h. An abrupt drop in pressure and suddenly rise in temperature indicated the formation of hydrate. After forming sufficient hydrate at a constant temperature (at least kept for 3 h), the system heat up with 0.1 K/h. When the temperature closed to the hydrate decomposition temperature, the increasing rate of temperature adjusted to 0.025 K/h. The data of temperature and pressure recorded during all experiments. The hydrate phase equilibrium point was the point at which the slope of the P−T diagram changes drastically.32

Table 1. Materials Used in the Experiments component

purity/mol %

CO2

99.99

CH4 50 mol % CO2/CH4 33 mol % CO2/CH4 tBA

99.99

HCFC-141b

99.9

water

98.0

supplier Guangzhou Zhuozheng Gas Industry Co. Foshan Kedi Gas Industry Co. Shengying Gas Industry Co. Shengying Gas Industry Co. JiangSu Yonghua Chemical Technology Co. Pujiang Jietong Environmental Protection Technology Co. deionized

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RESULTS AND DISCUSSION To verify the reliability of the experimental method and equipment, the phase equilibrium of hydrate formation conditions of HCFC-141b + CO2 were measured and compared with those reported in literature21 in Figure 2. The measured data listed in Table 2. In this work, the tested system of HCFC-141b + CO2 + H2O consists of three components (HCFC-141b, water, CO2) and four phases (gas,HCFC-141b,H2O, and hydrate). The free degree of the tested system is one according to Gibbs phase rule.21 Therefore, the phase equilibrium conditions of the measured system are independent of the HCFC-141b concentration. As Figure 2 showed, the experimental phase equilibrium data is consistent with that in literature,21 demonstrating that the experimental method and equipment is reliable. The measured hydrate phase equilibrium conditions of CH4 and CO2/CH4 mixture in the presence of HCFC-141b along with the hydrate phase equilibrium conditions of CH4, CO2, and CO2/CH4 mixture in pure water from literature were plotted in Figure 3 and listed in Table 3. The presence of HCFC-141b significantly decreased the equilibrium pressure of CH4 and CO2/CH4 mixture hydrates at any constant temperature, and a similar phenomenon was also observed for CO2 + HCFC-141b + water system.21 HCFC-141b forms s-II hydrates and occupies the large cages of s-II hydrates,19

and HCFC-141b solution used in this work are all 5.6% (wtBA = 23.02%, wHCFC‑141b = 36.4%, mass fraction), with an uncertainty of ±0.03%. The mixture gases in this work were 50 mol % CO2/CH4 and 33 mol % CO2/CH4. Experimental Apparatus. Figure 1 shows the schematic diagram of experimental apparatus used in this work, which has a detailed description in previous work.25−27 Major part of the device is a stirred high-pressure stainless steel cell with an effective volume of 300 cm3. Maximum working pressure of the cell is 20 MPa. The stainless steel cell was put in a water bath (Huber CC2-K20B) to control the temperature. Two built-in thermocouples (Westzh WZ-PT100) with ±0.1 K accuracy were installed to monitor temperature of gas and liquid phase in the reactor. A pressure sensor (Senex DG-1300) with an accuracy of ±0.01 MPa was used to measure the system pressure. The data of pressures and temperatures were recorded by data logger (Agilent 34970A) at 10 s intervals. Experimental Method. In this work, the isochoric pressure-search method was used to measure the phase equilibrium conditions of hydrate.28−31 First, the reactor was evacuated and was rinsed with distilled water. Then approximately 200 mL of experimental solution was inhaled into the reactor and the tested gas was injected into the cell

Figure 1. Schematic diagram of the experimental apparatus used for the measurement of gas hydrate equilibrium conditions. B

DOI: 10.1021/acs.jced.8b00401 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 3. Measured Hydrate Phase Equilibrium Data for CH4 and CO2/CH4 Mixture in HCFC-141b Solutiona gas components CH4

33 mol % CO2/CH4

50 mol % CO2/CH4

a

Figure 2. Hydrate phase equilibrium conditions for CO2 + HCFC141b + H2O system. Black □, ref 21; black ■, this work.

T/K

P/MPa 0.38 1.41

P/MPa

283.0 284.5 289.3 292.9 294.2 283.5 285.5 288.2 289.7 293.6 283.0 286.4 289.4 291.4 292.2

0.10 0.21 0.73 1.45 1.83 0.12 0.33 0.65 0.91 1.87 0.13 0.44 0.91 1.37 1.62

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.01 MPa.

that of CO2 hydrate. This ascribes to the cage occupancy competition between guest molecules. Because of the size of HCFC-141b molecule, HCFC-141b molecules only occupy the large cages of s-II hydrate. The size of methane molecules is small, and they are very suitable for small cages of hydrates. As for CO2 molecules, they are prone to occupy large cages because the small cavities are slightly tight for them.1 CO2 molecules and HCFC-141b molecules compete with each other to occupy large cages, and numerous small cages are left for CH4 molecules. Therefore, the hydrate equilibrium conditions of CH4 + HCFC-141b + water system are milder than that of CO2 + HCFC-141b + water system. In comparison to the hydrate phase equilibrium of CO2/CH4 mixture with those of CH4 and CO2 hydrates in the presence of HCFC-141b (Figure 3), both the hydrate phase equilibrium of the 33 mol % CO2/CH4 + HCFC-141b + H2O system and the 50 mol % CO2/CH4 + HCFC-1411b + H2O system are located between the phase equilibrium curves of CH4 + HCFC-141b + H2O system and CO2 + HCFC-141b + H2O system. The phase equilibrium conditions of these two systems are relatively close to the phase equilibrium of the CH4 + HCFC-141b + H2O system. This indicates that the content of CO2 in the mixture has a little effect on the hydrate phase equilibrium conditions of the CO2/CH4 mixture + HCFC141b + water system. Through the above experiments, we can find that HCFC-141b can be used as additive to moderate the CH4/CO2 hydrate formation conditions, which is the same as our expectation. But the effect of HCFC-141b on CH4/CO2 hydration separation needs further research. The phase equilibrium conditions for the hydrate formation of 33 mol % CO2/CH4 mixture and 50 mol % CO2/CH4 mixtures in tBA solution were measured and listed in Table 4. Experimental results are compared with the phase equilibrium data of CH4, CO2 in tBA solution,24 and CO2/CH4 mixture in pure water from literature33,34 in Figure 4. Seen from Figure 4, tBA + CO2/CH4 mixture hydrates confirmed the promotion effect that is signified by a pressure decrease at certain temperature, and the promotion effect of tBA on the hydrate formation of 33 mol % CO2/CH4 system was stronger than that of 50 mol % CO2/CH4 system. Seo et al.24 had reported that in the presence of tBA, CH4 or CO2 will form s-II hydrate by enclathrating tBA molecules in s-II large cages. Therefore,

Table 2. Measured Hydrate Phase Equilibrium Data for CO2 + HCFC-141b + Water Systema 287.4 288.8

T/K

a

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.01 MPa.

Figure 3. Hydrate phase equilibrium conditions for CH4, CO2, CO2/ CH4 mixture gas in HCFC-141b solution and in pure water. CH4 + H2O system: red ○, ref 22. CO2 + H2O system: black □, ref 22. CO2 + HCFC-141b + H2O system: black ■, ref 21. CH4 + HCFC-141b + H2O system: red ●, this work. (33 mol %) CO2/CH4 + HCFC-141b + H2O system: blue ▼, this work. (33 mol %) CO2/CH4 + H2O system: blue ▽, ref 26. (50 mol %) CO2/CH4 + HCFC1411b + H2O system: green ▲, this work. (50 mol %) CO2/CH4 + H2O system: green ◇, ref 33; green △, ref 34.

whereas CH4, CO2 forms s-I hydrate.1 Therefore, the addition of HCFC-141b in solution converted the CH4, CO2 hydrate structure from s-I to s-II. Thus, the gas hydrates formation conditions became mild. In addition, the promotion effect of HCFC-141b on the formation of CH4 hydrate is stronger than C

DOI: 10.1021/acs.jced.8b00401 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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hydrogen bonding with water molecules.35−37 Therefore, the test system with more CO2 has greater probability of hydrogen bonding between alcohol molecules and hydrates, which weaken the promotion effect of tBA on CO2/CH4 mixture gas hydrate. According to the experimental result, tBA may have the potential to separate CH4/CO2 as a hydration additive but the effect of tBA on hydrate selectivity of CH4/ CO2 still needs further study.

Table 4. Measured Hydrate Phase Equilibrium Data for CO2/CH4 + tBA + Water Systema gas components

T/K

P/MPa

33 mol % CO2/CH4

277.9 280.2 281.8 282.1 283.8 285.1 285.9 287.9 276.8 278.6 282.3 285.5 286.9

2.14 2.74 3.32 3.49 4.48 5.13 5.74 7.02 1.98 2.33 3.83 6.12 7.74

50 mol % CO2/CH4

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CONCLUSION This paper reported hydrate phase equilibrium of CH4 + HCFC-141b + water system, CO2/CH4 + HCFC-141b + H2O system, and CO2/CH4 + tBA + H2O system with the mole fraction of HCFC-141b and tBA was 5.6%. The result showed that HCFC-141b acts as a promoter for CH4 and CO2/CH4 mixture. The concentration of CO2 in mixture has little effect on the phase equilibrium conditions of CO2/CH4 mixture + HCFC-141b system. tBA also has promotion effect for CO2/ CH4 mixture gas hydrate, but the promotion effect on 50 mol % CO2/CH4 mixture was weaker than that on 33 mol % CO2/ CH4 mixture. The result demonstrated that the promotion effect of tBA on CO2/CH4 mixture gas depends on the composition of CO2/CH4.

a

Standard uncertainties are u(T) = 0.1 K, u(P) = 0.01 MPa.

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 86-20-22236581. E-mail: [email protected]. ORCID

Yanhong Wang: 0000-0002-0059-8159 Gang Li: 0000-0001-5197-3425 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2016YFC0304006 and 2017YFC0307302), National Natural Science Foundation of China (51576069), the Natural Science Foundation of Guangdong Province (2016A030313488), and PetroChina Innovation Foundation (2016D-5007-0210).

Figure 4. Hydrate phase equilibrium conditions for CH4, CO2, CO2/ CH4 mixture gas in tBA solution and in pure water. (33 mol %) CO2/ CH4+ tBA + H2O system: red ■, this work. (50 mol %) CO2/CH4 + tBA + H2O system: black ●, this work. CO2 + tBA + H2O system: blue ◁, ref 24. CH4 + tBA + H2O system: magenta ▷, ref 24. (33 mol %) CO2/CH4 + H2O system: red □, ref 26. (50 mol %) CO2/ CH4 + H2O system: black ◇, ref 33; black ○, ref 34.

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