ARTICLE pubs.acs.org/IECR
Phase Equilibria of Natural Gas Hydrates in the Presence of Methanol, Ethylene Glycol, and NaCl Aqueous Solutions Jong-Won Lee† and Seong-Pil Kang‡,* †
Department of Environmental Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Republic of Korea ‡ Climate Change Research Division, Korea Institute of Energy Research, 102 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea ABSTRACT: The phase equilibria of natural gas hydrate were measured using natural gas distributed in the Korean domestic grid. In addition, the hydrate-phase equilibria in the presence of organic inhibitors of methanol and ethylene glycol at various concentrations and a salt (NaCl) were also investigated at 266.2290.0 K and 0.998.86 MPa depending on the type and the concentration of the inhibitors. Experimental results showed that the hydrate-phase equilibria were shifted to the inhibition region in accordance with the increased inhibitor concentration. In the presence of a constant concentration of NaCl (3.5 wt %), the additional inhibition caused by the salt was found to be the same for either methanol or ethylene glycol at any concentration. Comparison of experimental temperature suppression caused by the addition of an inhibitor (both with and without the presence of NaCl) with the calculated results by two prediction methods showed good agreement except for the high concentrations of methanol and ethylene glycol.
’ INTRODUCTION Gas hydrates are solid crystalline structures which can be formed by a reaction between water (host) and low molecular-weight guest species under specific conditions (generally, low temperature and high pressure). Once the hydrate crystal is formed, it is stabilized by entrapping either carbon dioxide, nitrogen, or a hydrocarbon gas (e.g., methane, ethane, or propane) inside cavities in the three-dimensional lattice structure of hydrogen-bonded water.1 Because the gas hydrate can hold a large amount of gas in a unit volume of solid phase, it can be applied either to CO2 sequestration or the transportation and storage of natural gas.2,3 However, in the oil and natural gas industries, gas hydrates are known as hazardous materials which cause “blockage problems,” leading to serious operational and safety problems.4 When there are water contents in the pipeline and they come into contact with a specific oil or natural gas component, a gas hydrate can form and grow until the pipeline may become entirely plugged. Thus, research on hydrates has typically focused on identifying the hydrate-phase equilibria in order to prevent hydrate formation in oil and natural gas pipelines. However, the best pipeline conditions for avoiding hydrate formation require higher temperature or lower pressure than the hydrate-phase stability region, which means lower energy density through the pipelines. Therefore, adding a hydrate inhibitor to the system is a more common way of preventing hydrate formation. The addition of a hydrate inhibitor dilutes water molecules and decreases the chemical potential of aqueous water, allowing the hydrate-phase equilibrium PT curve to move into the inhibition region, marked by higher pressure at specific temperature or lower temperature at a given pressure.5 Hydrophilic substances like alcohols and organic compounds like glycols have emerged as the most efficient and cost-effective hydrate inhibitors. However, when oil and gas transportation is associated with saline water, r 2011 American Chemical Society
electrolyte components such as NaCl and KCl should be taken into account as natural inhibitors as well.6 Many researchers have reported hydrate-phase equilibria, including some through the use of thermodynamic inhibitors. Recently, Mohammadi and Richon reported hydrate-phase equilibria in the presence of ethylene glycol or methanol in aqueous solutions.7 Maekawa measured equilibrium conditions of CO2 hydrates in the presence various alcohols, glycols, and glycerol in aqueous solutions.8 Mohammadi and Richon also reported phase equilibria of H2S hydrates when the hydrate formed from aqueous solutions including NaCl, KCl, CaCl2, and ethanol.9 These experimental data suggest that inhibition of hydrate systems can be achieved with various inhibitors. In 1939, Hammerschmidt reported the following equation for predicting hydrate suppression of typical natural gases in contact with dilute aqueous solutions of antifreeze agents such as methanol: ΔT ¼
1297W Mð100 WÞ
ð1Þ
where M is the molecular weight of the antifreeze agent, W is the weight percent of the antifreeze agent in the solution, and ΔT is the hydrate suppression in Kelvin.10 The correlation depends only on the type and amount of inhibitor present in the aqueous solution, regardless of pressure, natural gas composition, and hydrate structure. Since Hammerschmidt reported this equation, many researchers have attempted to formulate a similar equation for predicting hydrate phase equilibria or inhibition effect in the Received: January 26, 2011 Accepted: June 1, 2011 Revised: May 27, 2011 Published: June 01, 2011 8750
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Table 1. Dry-Based Natural Gas Composition gas
composition (%)
gas
composition (%)
CH4
89.86
n-C4H10
0.49
C2H6
6.40
n-C5H12
0.02
C3H8
2.71
N2
0.04
iso-C4H10
0.48
Figure 2. Phase equilibria for natural gas hydrates in the presence of ethylene glycol at various concentrations; b, 0 wt %; 2, 10 wt %; 9, 20 wt %; (, 30 wt %; O, 40 wt %; 0, 50 wt %. Solid lines indicate calculated results from CSMGem (ref 1) at various ethylene glycol concentrations. The symbol () represents experimental results obtained in the preliminary study compared with reported values (dashed line) in the previous literature (refs 14 and 15).
Figure 1. Phase equilibria for natural gas hydrates in the presence of methanol at various concentrations; b, 0 wt %; 2, 10 wt %; 9, 20 wt %; (, 30 wt %. Solid lines indicate calculated results from CSMGem (ref 1) at various methanol concentrations. The symbol () represents experimental results obtained in the preliminary study compared with reported values (dashed line) in the previous literature (refs 14 and 15).
presence of various inhibitors. In 2006, Najibi et al.11 used the freezing point depression of aqueous solutions to create the following equation for estimating the safety margin in the presence of salt and/or organic inhibitors: ΔT ¼ 0:6825ΔT f
ð2Þ
where ΔT and ΔTf are the hydrate suppression and freezing point depression expressed in Kelvin. In addition, Li et al. adapted the statistical associating fluid theory (SAFT) equation of state to predict hydrate formation in the presence of methanol, glycerol, and ethylene glycol.12 Since collecting experimental data for a variety of hydrate systems in the presence of numerous inhibitors is impractical, these predicting equations can be extremely useful for selecting optimal operational conditions or evaluating safety in engineering fields. Attempts to develop a model equation and collect experimental data for hydrate-phase equilibria in the presence of any inhibitor has thus far focused primarily on simple hydrate systems such as CH4, CO2, or H2S, because of their analytical simplicity. Accordingly, there has been insufficient experimental data collected for binary, ternary, or more complex hydrate systems. In this report, we measured hydrate-phase equilibria in the presence of various inhibitors, using synthesized natural gas with the same composition as that which is distributed in the Korean grid. We also measured phase equilibria for natural gas hydrates both with and without the presence of NaCl. The phase equilibrium data obtained in this study is useful for evaluating how to inhibit natural gas hydrates in the engineering fields.
These data can also be used for identifying molecular behaviors of various gas components in the presence of an inhibitor.
’ EXPERIMENTAL METHODS The synthesized natural gas used in this study was supplied by Rigas (Korea) and had a UHP grade. Table 1 summarizes the dry-based gas composition, which simulates the natural gas composition distributed in the Korean domestic natural gas grid. HPLC grade water with a minimum purity of 99.99 mol % was supplied by Sigma-Aldrich Chemical Co. Methanol, ethylene glycol, and NaCl used as inhibitors in aqueous solutions were also purchased from Sigma-Aldrich Chemical Co. with nominal purity greater than 99.8, 99, and 99.5 mol %, respectively. These materials were used without further purification or treatment. Phase equilibria were measured with a specially constructed high-pressure vessel. The reactor has the same shape and dimensions as that used in our previous report,13 but it is equipped with a mechanical stirrer on the lid for mixing liquid phase. To measure an equilibrium point, approximately 200 cm3 of water was first charged in a high-pressure cell made of 316 stainless steel (maximum working pressure is 15 MPa and internal volume is about 350 cm3). Then, the cell was purged by natural gas to remove the remaining air from the system, pressurized to the desired pressure, and cooled to 263.0 K at a cooling rate of 1.5 K/h. An external circulator with accuracy of (0.1 K (RW-40G, Jeio Tech Korea) was used for cooling and controlling temperatures. When the pressure drop caused by hydrate formation reached a steady-state condition, the cell was heated stepwise by 0.1 K and kept for 1 h to reach thermal equilibrium. During the experiments, the temperature and pressure of the high-pressure reactor was recorded using a data acquisition system with an interval of 10 min. In the data acquisition system, temperature and pressure sensors with accuracies of (0.1 K and (0.01 MPa were used. The equilibrium pressure and temperature of the natural gas hydrate were determined by tracing the P-T profiles from the hydrate 8751
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formation to dissociation. The experimental method used in this study was similar to those described by Sloan and Koh.1 To verify reproducibility and accuracy of the apparatus, we measured phase equilibria of the pure CH4 hydrate in our preliminary experiments, which showed good agreement with reported values in the literature within (0.1% of maximum deviation.14,15
’ RESULTS AND DISCUSSION Alcohols and glycols were selected from a variety of inhibitors to be investigated by being added to the natural gas hydrate systems. Specifically, methanol and ethylene glycol, the lightest chemicals in each group, were used to measure the hydratephase equilibria of natural gas. Figures 1 and 2 show the phase equilibria for the natural gas hydrates mixed with methanol of 1030 wt % and ethylene glycol of 1050 wt %, respectively. The phase equilibria in the presence of both inhibitors are also summarized in Tables 2 and 3. For comparison, the figures also include the phase equilibria for natural gas hydrates without any inhibitor, pure CH4 hydrate,14,15 and some calculated results from CSMGem.1 The figures show that the equilibrium curve for the natural gas hydrate has a lot milder conditions (approximately 911 K depending on formation pressures) compared with that of pure CH4 hydrate. Such milder conditions are attributed to the addition of C2 and heavier components in the natural gas because those components have milder formation conditions when they act as a single guest for hydrate formation. Moreover, addition of C2 and heavier components also changes the crystal structure of formed hydrate from cubic sI (pure CH4 hydrate) to cubic sII in order to accommodate larger hydrocarbon molecules. However, when an inhibitor is added to the natural gas system, the equilibrium curves shift to the inhibition region, and when the concentration of the inhibitor is increased, the equilibrium curves move further into the inhibited region. Comparing the inhibition effect of methanol with that of ethylene glycol, an addition of 10 wt % methanol to the system causes temperature suppression of about 5.0 K, while ethylene glycol of the same concentration only suppresses
hydrate formation by about 4.0 K. In addition, it should be noted that the calculated values were found to reasonably agree with the experimental values for 1030 wt % addition, while the difference between two values were significant when higher concentrations (40 and 50 wt % addition of ethylene glycol) were used in some oil and gas fields. Therefore, when multicomponent natural gas hydrates formed in the presence of an inhibitor in aqueous solutions (especially at higher inhibitor concentrations) are dealt with, a modified prediction model is necessary to accurately simulate temperature suppression by the inhibitor. Figures 3 and 4 present the phase equilibria in the presence of both a salt and an organic inhibitor, and the results are also summarized in Tables 4 and 5. Hydrate-phase equilibria in the presence of various concentrations of the inhibitors are measured in mixtures with a salt of constant 3.5 wt % concentration. For comparison, the phase equilibria without the salt are also plotted in the figures in addition to calculated results from CSMGem. The figures show that the hydrate-phase equilibria of natural gas in the presence of the inhibitors further shift to the inhibition region with the addition of 3.5 wt % NaCl. The additional inhibition caused by the 3.5 wt % NaCl is found to be almost constant (approximately 2.83.2 K) for either methanol or ethylene glycol at any concentration. This means that a salt at a given concentration can inhibit hydrate formation without any
Table 2. Experimental Phase Equilibria for Natural Gas Hydrates in the Presence of Methanol Aqueous Solutions pure water
10 wt % MeOH
20 wt % MeOH
30 wt % MeOH
T (K) P (MPa) T (K) P (MPa) T (K) P (MPa) T (K) P (MPa) 281.4
1.71
277.8
2.08
274.5
2.21
273.1
3.80
283.3
2.18
283.0
4.05
279.0
4.10
276.0
6.26
288.3
4.17
285.9
6.16
281.5
6.07
276.4
7.08
290.1
5.29
287.2
7.76
283.1
8.17
277.0
8.50
291.4
6.61
Figure 3. Phase equilibria for natural gas hydrates in the presence of NaCl and methanol at various concentrations; b, (pure water þ3.5 wt % NaCl); 2, (pure water þ3.5 wt % NaCl þ 10 wt % methanol); 9, (pure water þ3.5 wt % NaCl þ 20 wt % methanol); (, (pure water þ3.5 wt % NaCl þ 30 wt % methanol). Blank symbols represent hydrate-phase equilibria without the salt at corresponding methanol concentrations. Solid and dashed lines indicate calculations obtained by CSMGem (ref 1).
Table 3. Experimental Phase Equilibria for Natural Gas Hydrates in the Presence of Ethylene Glycol Aqueous Solutions pure water
10 wt % MEG
20 wt % MEG
30 wt % MEG
40 wt % MEG
50 wt % MEG
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
P (MPa)
T (K)
P (MPa)
281.4
1.71
275.7
1.52
271.1
1.09
266.2
0.99
261.2
1.28
264.6
4.04
283.3
2.18
280.1
2.48
277.0
2.27
270.5
1.70
265.7
2.23
267.6
6.09
288.3 290.1
4.17 5.29
284.5 288.0
4.26 7.08
281.9 284.6
4.02 5.97
278.6 281.6
4.48 6.55
268.0 272.2
2.88 4.83
268.9 270.2
7.32 9.21
291.4
6.61
289.2
8.22
287.7
9.20
283.7
8.64
275.8
8.15
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Table 4. Experimental Phase Equilibria for Natural Gas Hydrates in the Presence of Methanol and NaCl Aqueous Solutions (pure water þ3.5 wt % NaCl)
10 wt % MeOH
20 wt % MeOH
30 wt % MeOH
þ 3.5 wt % NaCl þ 3.5 wt % NaCl þ 3.5 wt % NaCl
T (K) P (MPa) T (K) P (MPa) T (K) P (MPa) T (K) P (MPa)
Figure 4. Phase equilibria for natural gas hydrates in the presence of NaCl and ethylene glycol at various concentrations; b, (pure water þ3.5 wt % NaCl); 2, (pure water þ3.5 wt % NaCl þ 10 wt % ethylene glycol); 9, (pure water þ3.5 wt % NaCl þ 20 wt % ethylene glycol); (, (pure water þ3.5 wt % NaCl þ 30 wt % ethylene glycol). Blank symbols represent hydrate-phase equilibria without the salt at corresponding ethylene glycol concentrations. Solid and dashed lines indicate calculations obtained by CSMGem (ref 1).
273.0
1.01
273.4
1.78
269.6
1.65
267.9
2.57
275.0
1.28
276.9
2.80
272.4
2.56
271.5
4.05
278.0 280.0
1.81 2.30
279.0 280.8
3.56 4.56
275.9 277.6
4.10 5.26
272.9 274.3
5.23 6.99
283.0
3.30
282.9
5.97
279.2
7.06
285.0
4.26
284.8
8.09
286.0
4.86
288.0
6.44
290.0
8.86
Table 5. Experimental Phase Equilibria for Natural Gas Hydrates in the Presence of Ethylene Glycol and NaCl Aqueous Solutions (pure water
interaction with an organic inhibitor. However, it should be noted that temperature suppressions of the hydrate formation are 2.3 and 3.8 K for 30 wt % methanol and 30 wt % ethylene glycol, respectively. Such deviations can be attributed to some interactions or incorporation of two inhibitors, which requires further investigation to identify clearly. To investigate the inhibition effects of a variety of inhibitors on gas hydrate systems, we measured suppressed temperatures of hydrate formation in the presence of inhibitors at various concentrations. The experimental values are summarized in Table 6, along with calculated values generated by two predicting methods.10,11 Although Najibi et al. used a thermodynamic model to calculate the freezing point depression (ΔTf), we treated ΔTf as a colligative property which can be determined by molality and a solvent constant for simplification. It should be also noted that the calculations obtained by Hammerschmidt’s equation is a simple summation of each component when the systems include two or more inhibitors because Hammerschmidt did not expand the equation to systems that include mixed inhibitors. As seen in the table, experimental temperature suppressions of ethylene glycol are larger than the calculated values for all concentrations, while those of methanol are slightly larger or less than the calculated values. Such differences can be partially attributed to a heavier molecular weight of ethylene glycol than methanol. Because a molecule of ethylene glycol has two hydroxyl groups being capable of forming hydrogen bonds with multiple water molecules, it can be said that the molecule has good affinity with water molecules so as to keep water molecule from forming hydrate cages for hold gaseous guests. The reported prediction models, dependent only on the molecular weight and concentration of an inhibitor, cannot help showing inherent deviations because it does not take such chemical or physical properties of an inhibitor into account. Therefore, the experimental measurements obtained in this study can provide some offsets to values predicted by the existing models. After the 3.5 wt % salt is added to the aqueous system,
þ3.5 wt % NaCl)
10 wt % MEG þ 20 wt % MEG þ 30 wt % MEG þ 3.5 wt % NaCl
3.5 wt % NaCl
3.5 wt % NaCl
T (K) P (MPa) T (K) P (MPa) T (K) P (MPa) T (K) P (MPa) 273.0
1.01
273.9
1.73
268.9
1.42
266.8
1.83
275.0
1.28
275.4
2.10
271.3
1.88
271.3
3.08
278.0
1.81
279.9
3.55
276.3
3.27
273.9
4.07
280.0
2.30
282.9
5.24
279.3
4.80
277.2
5.91
283.0
3.30
284.4
6.73
281.5
6.90
278.8
7.66
285.0 286.0
4.26 4.86
286.3
8.96
282.5
8.33
279.5
9.16
288.0
6.44
290.0
8.86
the temperature suppressions obtained experimentally show greater inhibition effects regardless of concentration and inhibitor type. This difference can be attributed to the smaller value calculated for the “empty” system, which contained only 3.5 wt % NaCl. Temperature suppression is calculated to be about 1.6 K in the prediction methods, while it is found to be around 3.0 K in the experiments. However, even though the temperature difference of approximately 1.4 K between the calculated and experimental values for the “empty” system is taken into account, the results still show significant difference for the hydrate system including both ethylene glycol and NaCl. As a whole, the phase equilibria of the natural gas hydrate are shifted to milder formation conditions than those of pure CH4 hydrate. This shift is due to the presence of C2 or heavier components in the natural gas. When an inhibitor is added to the system, the hydrate-phase equilibria drastically shift to the inhibition region, in accordance with the increase of inhibitor concentration. Guest species incorporated with the physical properties of an inhibitor are thought to affect the inhibition of hydrate-phase equilibria. The hydroxyl group in methanol and ethylene glycol can form a strong hydrogen bond with water 8753
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Table 6. Temperature Suppression of Natural Gas Hydrate Formation at Various Inhibitor Concentrations inhibitor methanol
ethylene glycol
methanol þ 3.5 wt % NaCl
ethylene glycol þ 3.5 wt % NaCl
wt % (mol %)
ΔTa (K)
10 (5.9)
5.0
20 (12.3) 30 (19.4)
9.09.5 15.015.7
ΔTb (K)
ΔTc (K)
4.5
4.4
10.1 17.4
9.9 17.0 2.3
10 (3.1)
4.04.2
2.3
20 (6.8)
5.96.2
5.2
5.1
30 (11.1)
10.010.9
9.0
8.8
40 (16.2)
17.017.7
13.9
13.6
50 (22.5)
22.923.2
20.9
20.5
0
2.93.1
1.6d
10
7.98.1
6.1d
6.3
20
12.012.5
11.7d
12.4
30
17.218.0
18.9d
20.2
0
2.93.1
1.6d
1.6
10
7.17.4
3.9d
4.1
20 30
9.09.4 13.814.7
6.8d 10.5d
7.3 11.5
1.6
a
Experimental values in this study. b Calculation by Hammerschmidt10 equation. c Calculation by Najibi11 equation with freezing point depression obtained by molality and solvent constant. d Simple summation of two results calculated for each inhibitor.
molecules so as to prevent water molecules from forming hydrogen-bonded cages to capture gaseous guests. Accordingly, the stronger the affinity of an inhibitor with water molecules is (or the larger the inhibitor concentration is), the more hydrate formation is affected. Although methanol can suppress hydrate formation more efficiently than ethylene glycol at the same concentrations, physical properties of an inhibitor can be critical in an engineering field. In particular, considering that methanol can cause problems in desalination operations and water management,16 physical properties of an inhibitor as well as cost (economic efficiency) should be taken into account before selecting an optimum inhibitor for flow assurance. However, the inhibition effect of adding a salt is found to be virtually identical to that in the pure CH4 hydrate system. Unlike an organic inhibitor, a salt does not directly affect the hostguest interaction; instead, it dilutes the host (water) species by the “salt-out” effect. Therefore, when the same salt is added at the same concentration, the hydrate-phase equilibria are consistently inhibited, regardless of guest components. However, some deviations found at higher concentrations of methanol and ethylene glycol lead us to conclude that further investigations are necessary to explain additional factors affecting the inhibition effect. Moreover, large deviations of temperature suppression between the calculated and experimental values lead us to conclude that a modified prediction model may be necessary to secure safety in streams when multicomponents of natural gas are involved and a high concentration of an inhibitor is used in an engineering field.
’ CONCLUSIONS This report presents measurements of the hydrate-phase equilibria in the presence of methanol and ethylene glycol using
the same composition of natural gas used in the Korean domestic grid. In addition, equilibrium data in the presence of both methanol (or ethylene glycol) and NaCl are also measured with various inhibitor concentrations. In accordance with previous studies, the inhibition effect of methanol was found to be larger than that of ethylene glycol, because of its smaller molecular weight. However, when considered on the molar basis, adding ethylene glycol can inhibit hydrate formation slightly more than methanol at the similar molar fraction. Adding 3.5 wt % salts to the aqueous system results in additional inhibition of about 3.0 K, which remains virtually constant regardless of the inhibitor species and concentrations except for some deviations at the highest concentration of an inhibitor. Such a constant inhibition effect suggests that no chemical interactions exist between an organic inhibitor and a salt when both compounds are added to the system. To quantitatively investigate how the hydrate inhibition is affected by the presence of an inhibitor, temperature suppressions caused by the addition of an inhibitor (both with and without the presence of NaCl) were measured experimentally and compared with the calculated results of two prediction methods. Because the experimental suppressions were found to show larger deviations from the calculations especially in higher concentrations of an inhibitor, further research is necessary to evaluate the phase equilibria in the presence of high concentrations of an inhibitor and to obtain a modified prediction model.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: þ82-42-860-3475. Fax: þ82-42860-3134.
’ ACKNOWLEDGMENT This work was supported by the Energy Efficiency & Resources Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Knowledge Economy (No.2010201030001A). This work was also supported by the Korea National Oil Corporation Grant (Experimental Study on Gas Hydrates Formation/ Dissociation). ’ REFERENCES (1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (2) Gudmundsson, J. S.; Parlaktuna, M.; Khokhar, A. A. Storing Natural Gas as Frozen Hydrate. SPE Prod. Facil. 1994, 9, 69–73. (3) Kang, S. P.; Lee, H. Recovery of CO2 from Flue Gas Using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements. Environ. Sci. Technol. 2000, 34, 4397–4400. (4) Hammerschmidt, E. G. Formation of Gas Hydrates in Natural Gas Transmission Lines. Ind. Eng. Chem. 1934, 26, 851–855. (5) Yasuda, K.; Takeya, S.; Sakashita, M.; Yamawaki, H.; Ohmura, R. Binary EthanolMethane Clathrate Hydrate Formation in the System CH4C2H5OHH2O: Confirmation of Structure II Hydrate Formation. J. Phys. Chem. C 2009, 113, 12598–12601. (6) Masoudi, R.; Tohidi, B.; Danesh, A.; Todd, A. C.; Anderson, R.; Burgass, R. W.; Yang, J. Measurement and Prediction of Gas Hydrate and Hydrated Salt Equilibria in Aqueous Ethylene Glycol and Electrolyte Solutions. Chem. Eng. Sci. 2005, 60, 4213–4224. (7) Mohammadi, A. H.; Richon, D. Gas Hydrate Phase Equilibrium in the Presence of Ethylene Glycol or Methanol Aqueous Solution. Ind. Eng. Chem. Res. 2010, 49, 8865–8869. 8754
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(8) Maekawa, T. Equilibrium Conditions for Carbon Dioxide Hydrates in the Presence of Aqueous Solutions of Alcohols, Glycols, and Glycerol. J. Chem. Eng. Data 2010, 55, 1280–1284. (9) Mohammadi, A. H.; Richon, D. Phase Equilibria of Hydrogen Sulfide Clathrate Hydrates in the Presence of Methanol, Ethanol, NaCl, KCl, or CaCl2 Aqueous Solutions. Ind. Eng. Chem. Res. 2009, 48, 7847– 7851. (10) Hammerschmidt, E. G. Gas Hydrate Formations, A Further Study on Their Prevention and Elimination from Natural Gas Pipe Lines. Gas 1939, 15, 30–34. (11) Najibi, H.; Mohammadi, A. H.; Tohidi, B. Estimating the Hydrate Safety Margin in the Presence of Salt and/or Organic Inhibitor Using Freezing Point Depression Data of Aqueous Solutions. Ind. Eng. Chem. Res. 2006, 45, 4441–4446. (12) Li, X.-S.; Wu, H.-J.; Englezos, P. Prediction of Gas Hydrate Formation Conditions in the Presence of Methanol, Glycerol, Ethylene Glycol, and Triethylene Glycol with the Statistical Associating Fluid Theory Equation of State. Ind. Eng. Chem. Res. 2006, 45, 2131–2137. (13) Kang, S. P.; Lee, J.-W.; Ryu, H.-J. Phase Behavior of Methane and Carbon Dioxide Hydrates in Meso- and Macrosized Porous Media. Fluid Phase Equilib. 2008, 274, 68–72. (14) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of Carbon Dioxide and Methane Mixtures. J. Chem. Eng. Data 1991, 36, 68–71. (15) Deaton, W. M.; Frost, E. M. Gas Hydrates and Their Relation to the Operation of Natural Gas Pipe Lines. U.S. Bureau Mines Monogr. 8 1946, 101. (16) Mokhatab, S.; Wilkens, R. J.; Leontaritis, K. J. A Review of Strategies for Solving Gas-Hydrate Problems in Subsea Pipelines. Energy Sour., Part A 2007, 29, 39–45.
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