Analysis of Gas Source for the Replacement of CH4 with

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Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Analysis of Gas Source for the Replacement of CH4 with CO2 in Gas Hydrate Production from the Perspective of Dissociation Enthalpy Shicai Sun,*,†,‡ Yuchao Hao,† and Jianrui Zhao† †

Shandong Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation, College of Civil Engineering and Architecture, Shandong University of Science and Technology, Qingdao 266590, China ‡ Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China ABSTRACT: The dissociation enthalpies of CH4 hydrate, CO2 hydrate, and CO2−N2 mixture gas hydrate in the simulated submarine environment were calculated using their phase equilibrium data and the Clausius−Clapeyron equation. The results show that the dissociation enthalpy is related to the hydrate stability; that is, the ions in salt solution or seawater can reduce the dissociation enthalpy, while the effect of sediments on the dissociation enthalpy depends on the particle size in that the dissociation enthalpy is lower in sediments with smaller particle size. From the perspective of dissociation enthalpy, flue gas from a power plant can be directly used as the CO2 gas source for the replacement of methane in natural gas hydrate production, with the addition of a trace of tetrabutyl ammonium bromide (TBAB) additive without the use of pure CO2, so as to meet the required heat of CH4 hydrate exploitation and save energy for separating CO2 in flue gas.

1. INTRODUCTION Natural gas hydrate contains much natural gas which is regarded as one of the most promising new energies, so that many countries are focusing on the development of natural gas hydrate.1 Because of special physical and chemical properties, the traditional mining method of oil and gas cannot be applied to the production of gas from natural gas hydrate. Generally, natural gas hydrate is dissociated by external energy and then the released gas is transported to the ground by the traditional method.2,3 How to ensure safety is the most important issue during natural gas hydrate exploitation, so the input or output of energy must be effectively controlled within a certain range to prevent excessive gas being instantaneously released from gas hydrate that can cause accidents or geological disasters.4,5 In recent years, many scholars have proposed a method of CO2 replacement for gas production from gas hydrate due to a lower equilibrium pressure for CO2 hydrate formation. When CO2 gas goes into a natural gas hydrate reservoir, the heat (enthalpy) released from CO2 hydrate formation leads to the dissociation of natural gas hydrate. The advantage of this method is that it can use industrial emissions of CO2 to replace high-quality natural gas, and CO2 as solid hydrate is stored in the deficit reservoir to reinforce the reservoirs and avoid geological disaster.6−10 Hydrate formation/dissociation enthalpy is the heat released or absorbed during the hydrate formation/dissociation process. The values of the formation and dissociation enthalpy are almost equal, so no distinction between them is done in this © XXXX American Chemical Society

paper and we call both the dissociation enthalpy of hydrate. However, the dissociation enthalpies of hydrates formed by different guest gases may vary greatly. Therefore, it is necessary to analyze the CO2 gas source for the replacement of CH4 with CO2 in the natural gas hydrate production from the perspective of dissociation enthalpy. If the selected CO2 gas source more easily forms hydrate and the corresponding dissociation enthalpy is greater, the input of external energy can be greatly reduced to improve energy efficiency. At present, there are two main methods of determining the dissociation enthalpy of hydrate: direct measurement and indirect measurement.1 The direct measurement method includes the calorimetric technique, differential thermal analysis (DTA), and differential scanning calorimetry (DSC), etc. The indirect measurement method is to calculate the dissociation enthalpy using the Clapeyron equation or the Clausius−Clapeyron equation. Since the gas hydrate formation pressure is relatively high and the purity of hydrate is not easy to determine, the dissociation enthalpies of some hydrates are difficult to directly measure. However, hydrate phase equilibrium is relatively easy to measure; therefore, the indirect measurement method is more convenient. In contrast, the accuracy of the Clapeyron equation is higher than that of the Clausius−Clapeyron equation, but more parameters have to be provided for the Clapeyron and Received: October 3, 2017 Accepted: February 7, 2018

A

DOI: 10.1021/acs.jced.7b00872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Experimental Materials material

purity

CH4 CO2 CO2−N2 NaCl MgCl2 CaCl2 THF TBAB silica sand natural seawater natural sea sand deionized water

CH4, 0.9999 (molar fraction) CO2, 0.9999 (molar fraction) each gas >0.99 (molar fraction) NaCl, 0.995 (mass fraction) MgCl2·6H2O, 0.995 (mass fraction) CaCl2, 0.990 (mass fraction) THF, 0.990 (mass fraction) TBAB, 0.990 (mass fraction) SiO2, 0.999 (mass fraction) salinity = 0.035 (mass fraction)

concentration

supplier Qingdao Ruifeng Gas Co., Ltd.

xCO2 = 0.101, 0.180, 0.251, molar fraction yNaCl = 0.5, 1.0, 2.0 mol·L−1 yMgCl2 = 1.0 mol·L−1 yCaCl2 = 1.0 mol·L−1 yTHF = 0.004, 0.012, 0.042, molar fraction yTBAB = 0.004, 0.012, 042, molar fraction

Sinopharm Chemical Reagent Co., Ltd.

Lingshou BaoLei Mineral Processing Co., Ltd. taken from the offshore area of Qingdao, China laboratory-made

Table 2. Specification of Silica Sands and Sea Sand percentage/% size/μm

no. 1

no. 2

no. 3

no. 4

no. 5

no. 6

no. 7

no. 8

seas and

2000

0 0 0 6.807 93.193 0

0 0 3.032 48.932 48.036 0

0 0 12.410 61.918 25.672 0

0 0.010 82.637 17.353 0 0

1.589 43.635 54.776 0 0 0

6.457 75.523 17.659 0.361 0 0

7.690 91.519 0.791 0 0 0

81.712 18.288 0 0 0 0

0 0 77.827 22.173 0 0

some parameters are even difficult to obtain,11,12 such as the solubility, etc. These parameters for the sediment and its mixture with the solution can only be measured by experiments, which is clearly unrealistic. In fact, the accuracy of the Clausius−Clapeyron equation can basically meet the practical engineering application. Natural gas hydrates mainly occur in marine sediments and their phase change processes are constrained by sediments and pore water, but there are few reports on their influences on the dissociation enthalpy. Handa and Stupin thought that the dissociation enthalpy of CH4 hydrate-bearing mesoporous silica glass obtained by differential scanning calorimetry (DSC) was depressed.13 Similarly, Seshradi et al.14 presented a lower dissociation enthalpy of propane hydrate bearing mesoporous silica obtained by the Clausius−Clapeyron equation. Anderson et al.15 proposed that the dissociation enthalpy of CH4 hydrate was conservative and not a strong function of pore size. Zhang et al.16 found that the dissociation enthalpy of CH4 hydrate bearing sediments with different physical properties and water saturation were apparently higher than the bulk value. We have measured the equilibrium data of CH4 hydrates, CO2 hydrates, and CO2−N2 hydrates in various sediments, salt solutions, chemical reagent solutions, and their mixtures.17−20 In this paper, the dissociation enthalpies of CH4 hydrate, CO2 hydrate, and CO2−N2 hydrate are calculated using these equilibrium data and the Clausius−Clapeyron equation and analyze the selection and then the CO2 gas source for the replacement of CH4 in natural gas hydrate production are analyzed from the perspective of dissociation enthalpy. This work is helpful for the high efficient exploitation of natural gas hydrate resources.

literature.17−20 The materials used in the experiments include various silica sands, natural sea sand, CH4, CO2, CO2−N2 mixture, TBAB, tetrahydrofuran (THF), NaCl, MgCl2, CaCl2, and distilled water, as shown in Table 1. The chemicals were delivered by the suppliers and used without any further treatment. Three kinds of molar fraction of CO2−N2 mixture were prepared: xCO2 = 0.101, 0.180, 0.251. Three kinds of molar concentration of THF and TBAB aqueous solutions (yTHF/yTBAB = 0.004, 0.012, 0.042), three kinds of concentration of NaCl aqueous solutions ((0.5, 1.0, 2.0) mol·L−1), one kind of concentration of MgCl2 aqueous solution (1.0 mol· L−1), and one kind of concentration of CaCl2 aqueous solution (1.0 mol·L−1) were used in the experiments. All of the aqueous solutions were prepared by the gravimetric method. The mass uncertainty of analytical balance is ±0.0001 g. Consequently, uncertainties on the basis of molar fraction are estimated to be less than ±0.001. The gas was provided by Qingdao Ruifeng Gas Co., Ltd. and used without any further purification. The certified uncertainties for CH4, CO2, and CO2−N2 were ±0.0001, ± 0.0001, and ±0.001, respectively. Before experiments, the particle size of silica sand and natural sea sand were measured by a Malvern MS2000 laser particle size analyzer, as shown in Table 2. The certified uncertainty for particle size distribution was ±0.001.

3. THE CLAUSIUS−CLAPEYRON EQUATION The Clausius−Clapeyron equation is the simplified Clapeyron equation by assuming (1) the molar volume of hydrate and water are equal and the volume change could be approximated by the molar volume of gas; (2) the slope of the three-phase equilibrium curve is not changed with pressure and temperature. The dissociation enthalpy of gas hydrates can be calculated using the Clausius−Clapeyron equation from the univariant slope of the phase equilibrium boundary21−25

2. EXPERIMENTS The equilibrium data of CH4 hydrate, CO2 hydrate, and CO2− N2 mixture hydrate in different systems were determined by the isometric multistep heating dissociation method. The detailed experimental setup and experimental process are detailed in the B

DOI: 10.1021/acs.jced.7b00872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data d ln(p) ΔH =− d(1/T ) zR

Article

the dissociation enthalpy of CH4 hydrate using a high pressure DSC and did not find any temperature dependence. Anderson11,12 presented that the dissociation enthalpies of CH4 and CO2 hydrate calculated by the Clapeyron equation were also independent of temperature. It can be seen from Figure 5−Figure 8 that the dissociation enthalpies of hydrates in different systems decrease with the increase of temperature. This phenomenon may be related to the assumptions of eq 1 as described above. In addition, as can be seen that the slope values of the ln p − 1/T curves in the same or similar system are nearly equal while the dissociation enthalpies vary due to the different ranges of the measured temperature and pressure, which results into different gas compression factors. According to the theory of hydrate phase equilibrium, the smaller the particle size of silica sand is, the worse the hydrate stability is that indicates that less energy is required for hydrate dissociation. So the dissociation enthalpy is smaller correspondingly. Similarly, the stability of hydrate in silica sand-salt solution is poorer than that of hydrate in silica sand−pure water so that the dissociation enthalpy of the former is smaller. Moreover, the dissociation enthalpy is reduced with the increase of the concentration of solution. However, it can be seen from Figure 5 that the dissociation enthalpy of CH4 hydrate is larger in silica sand of small particle size, which is due to the different accuracy of Clausius−Clapeyron equation within the different temperature−pressure range. Sloan and Fleyfel21 and Gupta27 concluded that hydrate dissociation enthalpies calculated by the Clausius−Clapeyron equation agree with the experimental values in the low-pressure range, while the errors are big in the high-pressure range. Considering the shortcomings of the experimental method and the Clapeyron equation, the results obtained by the Clausius− Clapeyron equation are acceptable from the perspective of engineering applications. It can be seen from Figure 6 and Figure 7 that the dissociation enthalpies of CO2 hydrate in NaCl−no. 2 silica sand are basically consistent with those in the corresponding concentration of NaCl solution. Similarly, the dissociation enthalpies of CO2−N2 gas mixture hydrate in sea sand−seawater are consistent with those in the seawater. According to the hydrate phase equilibrium, the hydrate stability in these systems is mainly affected by the ions in NaCl solution or seawater, while the effect of silica sand or sea sand is not obvious.17−20 Additionally, since the guest gas and solution used in the corresponding systems are the same, the hydrate structure and cage occupancy of guest molecules are also the same. So the difference of the dissociation enthalpies of hydrate in the corresponding systems shows that the ions affect the dissociation enthalpy of hydrate. 4.3. Discussions on CO2 Gas Source. From Figure 5 to Figure 7 and Table 3 it can be seen that the average value of the dissociation enthalpies of CH4 hydrate in silica sand, silica sand−salt solution, and seawater is 51.2 ± 0.99−65.6 ± 0.96 kJ· mol−1; the average value of the dissociation enthalpies of CO2 hydrate in salt solution, seawater, and silica sand-salt solution is 57.1 ± 2.50−66.7 ± 5.29 kJ·mol−1. These test systems are used to simulate the occurrence conditions of CH4 hydrate in the natural environment. Obviously, the dissociation enthalpy of CO2 hydrate is close to that of CH4 hydrate. If pure CO2 gas is selected for the replacement of CH4 in natural gas hydrate production, the formation enthalpy of pure CO2 hydrate just offset the dissociation enthalpy of CH4 hydrate, but cannot provides excess heat to increase the temperature of CH4 hydrate reservoir to break it down. The main components of

(1)

where p and T are the equilibrium pressure and temperature of gas hydrate, ΔH is the dissociation enthalpy of gas hydrate, z is the compressibility factor, which can be determined by the Peng−Robinson (PR) equation of state, and R is the universal gas constant.26

4. RESULTS AND DISCUSSIONS 4.1. ln p − 1/T. As described above, ln p and 1/T must satisfy the linear relationship when the dissociation enthalpy is calculated using the Clausius−Clapeyron equation and hydrate phase equilibrium data. Figure 1−Figure 4 show the ln p − 1/T

Figure 1. ln p vs 1/T for CH4 hydrate in various systems.

relationship of CH4 hydrates in silica sands/silica sand-salt solutions/seawater, CO2 hydrates in salt solutions/silica sandsalt solutions/seawater, CO2−N2 hydrates in H2O/seawater/ sea sand/seawater−sea sand and CO2−N2 hydrate in TBAB/ THF solutions, respectively. It can be seen that the ln p is proportional to the 1/T in all of the experiment systems so that the values of these slopes can be obtained by fitting the linear relationships, as shown in Table 3. In general, the effects of different particle-sizes of silica sand or different concentrations of salt solution on the hydrate phase equilibrium are different, but the trends are close to the same as long as the physical and chemical properties of the sediment samples or salt solutions are the same or similar.17−20 Therefore, the ln p − 1/T linear curves in the same or similar medium are basically parallel. The silica sands of no. 1−no. 4 and natural sea sand show no significant effect on hydrate equilibrium, so the ln p − 1/T linear curves of the mixtures of these sediments with salt solutions coincide with the corresponding ones of salt solutions, as shown in Figure 1−Figure 3. Both THF and TBAB significantly affect hydrate phase equilibrium, that is, the pressure required for hydrate formation is reduced or the temperature is increased. However, since their physical and chemical properties are quite different, so the degrees of their impact on hydrate phase equilibrium are different so as to cause nonparallel ln p − 1/T curves, as shown in Figure 4. 4.2. Calculation of Dissociation Enthalpy. Using the values of ln p − 1/T curve slope and eq 1, the dissociation enthalpies of hydrates under the various conditions in different systems can be calculated as shown in Table 3 and Figure 5 to Figure 8. The compression factor z of the gas is obtained using the Peng−Robinson (PR) equation. Gupta et al.27 measured C

DOI: 10.1021/acs.jced.7b00872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 3. Dissociation Enthalpies of CH4 Hydrate, CO2, CO2−N2 Hydrate in Various Systems Calculated by the ClausiusClapeyron Equation

a

system

T/K

p/MPa

slope

ΔHa

average

CH4−no. 1 CH4−no. 2 CH4−no. 3 CH4−no. 4 CH4−no. 5 CH4−no. 6 CH4−no. 7 CH4−no. 8 CH4−0.5NaCl−no. 2 CH4−1.0NaCl−no. 2 CH4-2.0NaCl−no. 2 CH4-2.0NaCl−no. 7 CH4-1.0MgCl2−no. 6 CH4-1.0CaCl2−no. 5 CH4−seawater CO2−no. 2.0NaCl CO2−no. 1.0NaCl CO2−no. 0.5NaCl CO2−2.0NaCl−no. 2 CO2−no. 1.0NaCl−no. 2 CO2−no. 0.5NaCl−no. 2 CO2 + seawater 0.101CO2−N2-H2O 0.180CO2−N2−H2O 0.251CO2−N2−H2O 0.180CO2−N2-0.004TBAB 0.180CO2−N2−0.012TBAB 0.180CO2−N2−0.042TBAB 0.180CO2−N2−0.004THF 0.180CO2−N2−0.012THF 0.180CO2−N2−0.042THF 0.101CO2−N2−seawater 0.180CO2−N2−seawater 0.251CO2−N2-seawater 0.101CO2−N2‑sea sand−H2O 0.180CO2−N2−sea sand−H2O 0.251CO2−N2−sea sand−H2O 0.101CO2−N2−seawater−sea sand 0.180CO2−N2−seawater−sea sand 0.251CO2−N2−seawater−sea sand

281.6−284.3 281.2−284.2 274.8−282.8 275.1−282.7 285.9−289.7 285.3−289.7 284.4−289.5 284.6−288.9 286.0−289.7 282.5−288.3 280.6−286.3 279.7−285.6 279.6−283.8 279.5−284.6 287.4−289.2 272.9−276.8 275.4−279.5 274.5−278.9 273.7−276.8 275.6−279.8 276.8−280.1 278.0−281.6 273.4−276.8 273.5−277.4 273.6−278.4 279.6−288.6 284.3−288.6 286.8−288.3 278.9−286.8 277.4−287.6 284.4−289.6 271.7−275.2 271.6−275.5 271.5−275.4 273.6−277.0 273.8−277.8 273.8−278.6 272.2−275.4 272.2−275.8 272.0−276.0

6.1−8.0 5.9−7.8 3.3−6.7 3.2−6.8 10.1−15.8 9.9−16.1 9.2−16.0 9.6−15.7 10.7−16.2 8.7−16.8 9.0−17.4 9.0−17.4 9.3−14.8 8.3−14.0 14.1−17.4 2.1−3.6 2.1−3.6 1.7−2.9 2.3−3.5 2.2−3.7 2.3−3.3 2.5−4.2 12.0−17.5 7.2−12.9 5.3−10.4 1.3−14.6 1.2−5.8 1.9−3.6 3.3−11.0 0.6−4.7 1.0−3.6 11.7−17.7 6.9−12.3 5.0−8.5 12.2−17.9 7.6−13.9 5.5−10.7 12.7−18.3 7.4−12.9 5.1−9.5

−8.36652 −7.7727 −6.85818 −7.62331 −9.83079 −9.28791 −8.85351 −9.2504 −9.31324 −9.33269 −9.1929 −8.92566 −8.78697 −8.33304 −8.785 −10080.2 −9779.78 −9278.28 −10201.7 −9471.02 −8665.92 −11128.1 −8434.19 −11343.28 −10739.51 −21145.89 −29829.29 −35251.88 −12217.71 −16011.6 −19764.77 −8926.29 −11353.93 −10287.83 −8484.46 −11258.97 −10440.27 −8406.39 −11468.74 −11285.19

59.0−60.9 54.9−56.8 49.4−52.7 54.8−58.7 64.8−67.1 61.2−63.5 58.3−61.0 60.8−63.3 59.5−59.7 59.8−61.6 59.0−59.8 57.2−57.9 54.6−56.6 51.8−55.1 57.7−57.8 59.0−70.3 58.5−68.6 60.5−67.7 60.4−69.9 55.5−66.0 54.0−61.6 60.7−75.7 66.3−67.7 86.1−87.6 79.1−82.7 164.0−173.3 236.8−245.1 288.0−284.1 94.6−98.2 127.9−132.2 159.4−162.7 67.1−67.9 85.9−87.7 76.5−79.3 66.7−68.2 85.5−86.8 76.8−80.1 66.0−67.6 88.3−86.7 83.3−86.9

59.9 ± 0.70 56.0 ± 0.66 51.2 ± 0.99 57.0 ± 1.18 65.6 ± 0.96 62.0 ± 0.92 59.1 ± 1.01 61.7 ± 1.00 59.2 ± 0.43 59.8 ± 1.14 58.5 ± 1.00 56.8 ± 0.97 55.1 ± 0.87 53.2 ± 1.44 57.7 ± 0.08 64.5 ± 4.22 63.4 ± 3.82 64.4 ± 2.95 65.8 ± 3.85 60.6 ± 3.91 57.1 ± 2.50 66. Seven ±5.29 66.8 ± 0.47 86.6 ± 0.57 80.7 ± 1.21 167.9 ± 3.68 240.7 ± 2.81 286.1 ± 1.47 96.1 ± 1.38 130.6 ± 1.55 160.9 ± 1.17 67.3 ± 0.34 86.8 ± 0.66 77.9 ± 0.97 67.3 ± 0.51 85.9 ± 0.47 78.5 ± 1.2 66.7 ± 0.58 87.4 ± 0.55 85.0 ± 1.3

The unit of dissociation enthalpies ΔH is kJ·mol−1 (CH4), kJ·mol−1 (CO2), or kJ·mol−1 (CO2−N2).

the flue gas discharged from the power plant are CO2, N2, and O2. Since the phase equilibria of N2 hydrate and O2 hydrate are basically the same, the flue gas is usually regarded as a binary mixture of CO2 and N2. Table 3 and Figure 7 show that the dissociation enthalpy of CO2−N2 mixture gas hydrate in a simulated submarine environment and the dissociation enthalpies in pure water, seawater, sea sand, and seawater− sea sand system are between 66.7 ± 0.58 kJ·mol−1 and 87.4 ± 0.55 kJ·mol−1, slightly bigger than that of pure CO2 hydrate. If CH4 hydrate is directly replaced by the flue gas, the dissociation enthalpy of CO2−N2 mixture gas hydrate can only satisfy the enthalpy change required for the dissociation of CH4 hydrate, not providing excess energy to heat the reservoir. Conversely, compared with pure CO2 hydrate, the CO2−N2 mixture gas hydrate requires higher pressure and lower temperature, which results in the difficulty of displacement extraction.

Figure 2. ln p vs 1/T for CO2 hydrate in various systems.

D

DOI: 10.1021/acs.jced.7b00872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 6. Dissociation enthalpy of CO2 hydrate in various systems. Figure 3. ln p vs 1/T for CO2−N2 hydrate in H2O/seawater/sea sand.

Figure 7. Dissociation enthalpy of CO2−N2 hydrate in H2O/ seawater/sea sand.

Figure 4. ln p vs 1/T for CO2−N2 hydrate in TBAB and THF solution.

replacement of CH4 with CO2 in natural gas hydrate production. In this paper, the dissociation enthalpies of CO2−N2 mixture gas hydrate in different concentrations of TBAB and THF solution are discussed, as shown in Table 3 and Figure 8. It can be seen that the trace of TBAB or THF greatly increases the dissociation enthalpy of CO2−N2 mixture gas hydrate. A higher concentration of TBAB or THF solution not only increases the stability of CO2−N2 mixture gas hydrate but also increases the corresponding dissociation enthalpy. In addition, the effect of TBAB solution on the dissociation enthalpy is more insignificant than that of THF solution with the same concentration. For example, the dissociation enthalpy in TBAB solution with a mass fraction of 0.004 is consistent with the enthalpy in THF solution with a mass fraction of 0.042. The dissociation enthalpy of CO2−N2 mixture gas hydrate in THF or TBAB solution with a mass fraction of 0.004−0.042 is 96.1 ± 1.38−160.9 ± 1.17 kJ·mol−1 and 167.9 ± 3.68−286.1 ± 1.47 kJ·mol−1, respectively. It is clear that the addition of the trace of THF or TBAB not only satisfies the enthalpy change required for CH4 hydrate dissociation but also provides excess heat to enhance the temperature of CH4 hydrate reservoir so as to reduce the input of external energy. As a simple example, the equilibrium temperature, pressure, and dissociation enthalpy of CH4 hydrate in the seabed

Figure 5. Dissociation enthalpy of CH4 hydrate in various samples.

As we all know, TBAB as an environmentally friendly additive also forms a semiclathrate hydrate (TBAB hydrate) with water molecules. Although THF can also significantly reduce the formation pressure of hydrate, THF is volatile and may bring a negative impact on the environment.28−30 Relatively, TBAB is more suitable as an additive used for the E

DOI: 10.1021/acs.jced.7b00872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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5. CONCLUSIONS The replacement of CH4 with CO2 in natural gas hydrate production is a win−win solution for both CO2 sequestration and CH4 hydrate production. In this paper, the dissociation enthalpies of CH4 hydrate, CO2 hydrate, and CO2−N2 mixture gas hydrate were calculated using their phase equilibrium data and the Clausius−Clapeyron equation. On the basis of the dissociation enthalpies, the CO2 gas source used for the replacement of CH4 in CH4 hydrate production can directly employ the flue gas (mainly CO2−N2) together with a trace of TBAB, so that the energy required for CH4 hydrate dissociation can be met and the energy for CO2 separation can be saved. Therefore, the efficiency of the CH4 hydrate production can be improved greatly.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 532 86057593. Fax: +86 532 86057957. E-mail: [email protected].

Figure 8. Dissociation enthalpy of CO2−N2 hydrate in TBAB and THF solution.

ORCID

Shicai Sun: 0000-0002-9636-2959

environment of a temperature of 278.15 K and a depth of 1400 m are about 287.4 K, 14.1 MPa, and 57.7 kJ·mol−1, respectively, as shown in Table 3. If the traditional method of steam injection is employed to develop the CH4 hydrate, the required heat contains three parts supposing the pressure in the submarine environment remains constant and the heat is completely consumed by CH4 hydrate dissociation (i.e., regardless of heat loss): (1) the sensible heat for enhancing the temperature of CH4 hydrate reservoir to the equilibrium temperature; (2) the latent heat for the enthalpy change of CH4 hydrate dissociation; (3) the sensible heat for a certain degree of superheat for CH4 hydrate dissociation. Assuming that the input of external heat makes the temperature of the CH4 hydrate reservoir reach 300 K to break it down, the total sensible heat and dissociation enthalpy are 5.44 kJ·mol−1 and 57.7 kJ·mol−1 (i.e., the total required heat is 63.14 kJ·mol−1 or expressed as 46.43 × 104 kJ·m−3 CH4 hydrate specific heat capacity cp = 0.258 kJ·mol−1·K−1), respectively, for 1 mol CH4 hydrate dissociation from the ambient temperature 278.15 K to dissociation temperature 300 K. If the selected gas is pure CO2 for the replacement of CH4 in CH4 hydrate production, the released heat of 1 mol CO2 hydrate formation is 66.7 kJ·mol−1 which can only meet the required heat of 1 mol CH4 hydrate dissociation. Similarly, if the selected gas is a nonadditive CO2− N2 mixture gas, the released heat of 1 mol CO2−N2 (xCO2 = 0.180) mixture gas hydrate formation is 86.8 kJ·mol−1 or expressed as 72.94 × 104 kJ·m−3 which can also only meet the required heat of 1.58 m3 CH4 hydrate dissociation. However, if CO2−N2−TBAB (xCO2 = 0.180, xTBAB = 0.042) is used for displacement extraction, the heat released from 1 mol CO2−N2 mixture gas hydrate formation is 286.1 kJ·mol−1 or expressed as 185.18 × 104 kJ·m−3 which can meet the required heat of 4 m3 CH4 hydrate dissociation. Therefore, the flue gas can be directly used as the CO2 gas source for the replacement of CH4 in CH4 hydrate production, and the trace of an additive such as environmentally friendly TBAB can be added, which not only can improve the temperature−pressure condition of hydrate formation but also save energy for separating CO2 in flue gas and enhance the energy efficiency. If heat loss is considered in the process, more external energy has to be input to the reservoir, while the heat loss of the replacement method is relatively smaller.

Funding

This work was funded by the National Natural Science Foundation of China (51376114), Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (no. Y707kf1001) and Natural Science Foundation of Shandong Province (ZR2014JL033). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathratehydrates of Natural Gases, 3rd ed.; CRC Press: New York, 2008. (2) Holder, G. D.; Kamath, V. A.; Godbole, S. P. The potential of natural gas hydrates as an energy resource. Annu. Rev. Energy 1984, 9, 427−45. (3) Makogon, Y. F. Natural gas hydrates−A promising source of energy. J. Nat. Gas Sci. Eng. 2010, 2, 49−59. (4) Brown, H. E.; Holbrook, W. S.; Hornbach, M. J.; Nealon, J. Slide structure and role of gas hydrate at the northern boundary of the Storegga Slide, offshore Norway. Mar. Geol. 2006, 229, 179−186. (5) Wang, S. Y.; Zheng, W.; Lu, X. B.; Li, Q. P. The effects of gas hydrate dissociation on the stability of pipeline in seabed, The 19th International Offshore and Polar Engineering Conference, Osaka, Japan, July 21−26, 2009. (6) Yuan, Q.; Sun, C. Y.; Liu, B.; Wang, X.; Ma, Z. W.; Ma, Q. L.; Yang, L. Y.; Chen, G. J.; Li, Q. P.; Li, S.; Zhang, K. Methane recovery from natural gas hydrate in porous sediment using pressurized liquid CO2. Energy Convers. Manage. 2013, 67, 257−264. (7) Nago, A.; Nieto, A. Natural gas production from methane hydrate deposits using CO2 clathrate sequestration: state-of-art review and new technical approaches. J. Geol. Res. 2011, 2011, 239−397. (8) Espinoza, D. N.; Santamarina, J. C. P-wave monitoring of hydrate-bearing sand during CH4−CO2 replacement. Int. J. Greenhouse Gas Control 2011, 5, 1032−1038. (9) Ota, M.; Morohashi, K.; Abe, Y.; Watanabe, M.; Smith, R. L., Jr.; Inomata, H. Replacement of CH4 in the hydrate by use of liquid CO2. Energy Convers. Manage. 2005, 46, 1680−1691. (10) Ohgaki, K.; Takano, K.; Sangawa, H.; Sangawa, H.; Matsubara, T.; Nakano, S. Methane exploitation by carbon dioxide from gas hydrates − phase equilibria for C02−CH4 mixed hydrate system. J. Chem. Eng. Jpn. 1996, 29, 478−483. (11) Anderson, G. K. Enthalphy of dissociation and hydration number of carbon-dioxide hydrate from the Clapeyron equation. J. Chem. Thermodyn. 2003, 35, 1171−1183. F

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(12) Anderson, G. K. Enthalpy of dissociation and hydration number of methane hydrate from the Clapeyron equation. J. Chem. Thermodyn. 2004, 36, 1119−1127. (13) Handa, Y. P.; Stupin, D. Thermodynamic properties and dissociation characteristics of methane and propane hydrates in 70-Åradius silica gel pores. J. Phys. Chem. 1992, 96, 8599−8603. (14) Seshadri, K.; Wilder, J. W.; Smith, D. H. Measurements of equilibrium pressures and temperatures for propane hydrate in silica gels with different pore-size distributions. J. Phys. Chem. B 2001, 105, 2627−2631. (15) Anderson, R.; Llamedo, M.; Tohidi, B.; Burgass, R. W. Experimental measurement of methane and carbon dioxide clathrate hydrate equilibria in mesoporous silica. J. Phys. Chem. B 2003, 107, 3507−3514. (16) Zhang, Y.; Li, X. S.; Wang, Y.; Chen, Z. Y.; Yan, K. F. Decomposition conditions of methane hydrate in marine sediments from South China Sea. Fluid Phase Equilib. 2016, 413, 110−115. (17) Sun, S. C.; Liu, C. L.; Ye, Y. G. Phase equilibrium condition of marine carbon dioxide hydrate. J. Chem. Thermodyn. 2013, 57, 256− 260. (18) Sun, S. C.; Liu, C. L.; Meng, Q. G. Hydrate phase equilibrium of binary guest-mixtures containing CO2 and N2 in various systems. J. Chem. Thermodyn. 2015, 84, 1−6. (19) Sun, S. C.; Ye, Y. G.; Liu, C. L.; Xiang, F.k.; Ma, Y. P-T stability conditions of methane hydrate in sediment from South China Sea. J. Nat. Gas Chem. 2011, 20, 531−536. (20) Sun, S. C.; Ye, Y. G.; Liu, C. L.; Tan, Y. Z.; Xiang, F.k.; Ma, Y. Stable Conditions for Methane Hydrate in Quartz Sand. J. Nat. Gas Chem. 2011, 69, 1135−1140. (21) Sloan, E. D.; Fleyfel, F. Hydrate dissociation enthalpy and guest size. Fluid Phase Equilib. 1992, 76, 123−140. (22) Chen, Z. Y.; Li, Q. P.; Yan, Z. Y.; Yan, K. F.; Zeng, Z. Y.; Li, X. S. Phase equilibrium and dissociation enthalpies for cyclopentane + methane hydrates in NaCl aqueous solutions. J. Chem. Eng. Data 2010, 55, 4444−4449. (23) Sabil, K. M.; Witkamp, G. J.; Peters, C. J. Estimations of enthalpies of dissociation of simple and mixed carbon dioxide hydrates from phase equilibrium data. Fluid Phase Equilib. 2010, 290, 109−114. (24) Lv, Q. N.; Li, X. S.; Chen, Z. Y.; Feng, J. C. Phase equilibrium and dissociation enthalpies for hydrates of various water-insoluble organic promoters with methane. J. Chem. Eng. Data 2013, 58, 3249− 3253. (25) Sabil, K. M.; Nashed, O.; Lal, B.; Ismail, L.; Japper-Jaafar, A. Experimental investigation on the dissociation conditions of methane hydrate in the presence of imidazolium-based ionic liquids. J. Chem. Thermodyn. 2015, 84, 7−13. (26) Peng, D. Y.; Robinson, D. B. A new two-constant equation of state. Ind. Eng. Chem. Fundam. 1976, 15, 59−64. (27) Gupta, A.; Lachance, J.; Sloan, E. D.; Koh, C. A. Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry. Chem. Eng. Sci. 2008, 63, 5848−5853. (28) Mohammadi, A. H.; Eslamimanesh, A.; Belandria, V.; Richon, D. Phase equilibria of semiclathrate hydrates of CO2, N2, CH4, or H2 + Tetra-n-butylammonium Bromide aqueous solution. J. Chem. Eng. Data 2011, 56, 3855−3865. (29) Mohammadi, A. H.; Eslamimanesh, A.; Belandria, V.; Richon, D.; Naidoo, P.; Ramjugernath, D. Phase equilibrium measurements for semi-clathrate hydrates of the (CO2 + N2 + tetra-n-butylammonium bromide) aqueous solution system. J. Chem. Thermodyn. 2012, 46, 57− 61. (30) Ye, N.; Zhang, P. Equilibrium data and morphology of Tetra-nbutyl Ammonium Bromide semiclathrate hydrate with carbon dioxide. J. Chem. Eng. Data 2012, 57, 1557−1562.

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DOI: 10.1021/acs.jced.7b00872 J. Chem. Eng. Data XXXX, XXX, XXX−XXX