The performance of ionic liquids and their mixtures ... - Semantic Scholar

Chemical Engineering Science 87 (2013) 270–276

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces

The performance of ionic liquids and their mixtures in inhibiting methane hydrate formation Anthony R. Richard, Hertanto Adidharma n Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071-3295, USA

H I G H L I G H T S c c c c c

EMIM-Cl could be more effective than MEG at high concentrations. The effectiveness of inhibitors containing EMIM-Cl increases with pressure. Synergistic effects are observed for EMIM-Cl and MEG at high pressures. Synergistic effects are observed for EMIM-Cl and EMIM-Br at high pressures. No synergistic effects are observed for EMIM-Cl and NaCl.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 July 2012 Received in revised form 9 October 2012 Accepted 15 October 2012 Available online 23 October 2012

The performance of 1-ethyl-3-methylimidazolium chloride (EMIM-Cl), one of novel ionic liquid inhibitors for gas hydrate, in inhibiting methane hydrate at low and high ionic liquid concentrations is investigated in a pressure range of 10–20 MPa. Experiments on methane hydrate dissociation conditions in the presence of mixed ionic liquid and conventional inhibitors, such as sodium chloride (NaCl) and monoethylene glycol (MEG), as well as a mixture containing two ionic liquids, EMIM-Cl and 1-ethyl-3-methylimidazolium bromide (EMIM-Br), are also performed to investigate any possible synergistic effects. It is observed that single component solutions of EMIM-Cl demonstrate a progressive increase in inhibition effect with increasing concentration, which may surpass the effectiveness of MEG at high concentrations. Although the thermodynamic inhibition performance of the mixtures of EMIM-Cl and MEG does not show any synergistic effects at low pressures, it does at higher pressures. The mixture of EMIM-Cl and EMIM-Br also shows a synergistic effect at higher pressures. Unlike MEG or NaCl, inhibitors containing EMIM-Cl or EMIM-Br demonstrate an increase in inhibition effectiveness as pressure increases. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Energy Gases Phase equilibria Solutions Hydrate inhibitor Ionic liquid

1. Introduction Clathrate hydrates, or gas hydrates, are solid crystalline structures that consist of water molecules, connected through hydrogen bonding, forming a cage-like structure that completely encapsulates another molecular species (Englezos, 1993). Hydrate formation is problematic in the oil and gas industry because blockages in transmission lines can lead to economic losses as well as safety and ecological risks (Koh, 2002). One common method to mitigate hydrate formation is to use hydrate inhibitors. There are two types of inhibitors that are currently in use: thermodynamic and kinetic. Thermodynamic inhibitors work by shifting the hydrate dissociation curve to higher pressures and

n

Corresponding author. Tel.: þ1 307 766 2500; fax: þ 1 307 766 6777. E-mail address: [email protected] (H. Adidharma).

0009-2509/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2012.10.021

lower temperatures, while kinetic inhibitors slow the formation of hydrates by altering formation geometry and influencing growth rates (Koh, 2002; Xiao et al., 2010). Since future gas recovery will inevitably occur in deeper waters where conditions are more favorable for hydrate formation, more effective hydrate inhibitors will be needed. Recently, a new class of dual function inhibitors was discovered, which are ionic liquids (ILs) that function both as thermodynamic and kinetic inhibitors for methane hydrate (Xiao and Adidharma, 2009; Xiao et al., 2010). The use of ILs as hydrate inhibitors has gained attention not only for hydrates of methane, but carbon dioxide as well (Chen et al., 2008; Li et al., 2011; Peng et al., 2010; Tumba et al., 2011). The thermodynamic inhibition performance of ILs is promising since ILs are a type of salt, and salt solutions have been extensively studied and shown to be effective thermodynamic inhibitors for hydrate formation (Atik et al., 2006; De Roo et al., 1983; Jager and Sloan, 2001; Kang et al., 1998; Kharrat

A.R. Richard, H. Adidharma / Chemical Engineering Science 87 (2013) 270–276

and Dalmazzone, 2003; Lafond et al., 2012; Maekawa and Imai, 2000; Maekawa et al., 1995; Mohammadi et al., 2008, 2009). In the previous study of ILs by Xiao et al., it was found that while 1ethyl-3-methylimidazolium chloride (EMIM-Cl) acted as a dual function inhibitor, it had the best thermodynamic inhibition performance among ILs investigated (Xiao et al., 2010). It is the purpose of this work to further examine the thermodynamic inhibition performance of this IL. Its kinetic inhibition performance will be studied further in a separate work. To perform well in inhibiting hydrate formation, especially at severe conditions, thermodynamic inhibitors are often used at high concentration (up to 50%) and/or mixed with other inhibitors (Jager et al., 2002; Koh, 2002; Lafond et al., 2012; Lee and Englezos, 2005; Masoudi and Tohidi, 2010; Masoudi et al., 2004; Mohammadi and Richon, 2009, 2010a, 2012b, 2012a, b; Ng et al., 1987; Szymczak et al., 2006). Past studies have examined several combinations of thermodynamic inhibitors, such as sodium chloride (NaCl)þethylene glycol (MEG) (Masoudi et al., 2004; Mohammadi and Richon, 2009, 2012a), NaCl þmethanol (MeOH) (Jager et al., 2002; Lafond et al., 2012; Masoudi and Tohidi, 2010; Mohammadi and Richon, 2009, 2012a), and MEG þMeOH (Mohammadi and Richon, 2010b), but studies addressing the effectiveness of inhibitor mixtures compared to the individual components are rare. This study presents new data and examines the effectiveness of more concentrated EMIM-Cl and its mixtures in inhibiting methane hydrate formation. The mixtures investigated are EMIM-Cl in conjunction with conventional thermodynamic inhibitors as well as with another IL. It is worth mentioning that the purpose of this work is not to seek immediate practical advantage that can be directly utilized in the field, but rather to provide a new horizon of the present knowledge on ionic liquid inhibitors. This work explores the thermodynamic inhibition performance of ionic liquid inhibitor at high concentrations and investigates the synergistic effects between ionic liquid and conventional inhibitors or other ionic liquid, all of which have never been studied and thus have never been understood.

2. Experimental 2.1. Materials The ionic liquids studied in this work are EMIM-Cl and 1ethyl-3-methylimidazolium bromide (EMIM-Br), the structures of which are shown in Table 1. EMIM-Cl is purchased from Acros with a purity of at least 97% and EMIM-Br is purchased from EMD with a purity of greater than 98%. The conventional inhibitors used are MEG and NaCl. The MEG is purchased from SigmaAldrich and has a purity of at least 99% and NaCl is also purchased from Sigma-Aldrich with a purity of 99þ%. For experiments with individual inhibitors, the concentrations of the ionic liquid are 5, 20, 30, and 40 wt%. For experiments with mixed inhibitors, i.e.,

271

EMIM-Clþother inhibitor combination, the concentration of 1:1 solutions used ranges from 10 to 30 wt%. Deionized water is used to prepare all of the sample solutions. Samples are weighed using a METTLER model AX205 analytical balance with an accuracy of 70.01 mg. The methane gas used for all tests is UHP grade purchased from United States Welding, Inc. and has a purity of 99.97%. 2.2. Apparatus and procedure For this study, a high-pressure SETARAM micro differential scanning calorimeter (HP mDSC) is used. The HP mDSC has a resolution of 0.4 mW and operates at pressures up to 40 MPa and temperatures from 228.15 K to 393.15 K. To control the temperature of the system, advanced Peltier cooling and heating principles are employed. Two high-pressure vessels are used for each experiment: one used for the sample solution and the other used for reference. The vessels are made of Hastelloy C276 and each has a volume of 0.5 mL. A MENSOR high-sensitive pressure transducer with an accuracy of 0.010% of full scale is used to measure the vessel pressure. One drop of fresh sample solution is loaded into the sample vessel for each experiment to rule out any history effects. Reference and sample vessels are then charged with methane and placed inside the calorimetric block of the HP mDSC. To determine the methane hydrate dissociation temperature, a nonisothermal mode is used, which consists of two steps: cooling and heating. First, the temperature is reduced at a rate of 0.6 K per minute from 298.15 K to 228.15 K, followed by an isothermal process for 10 min to ensure the formation of hydrates. Second, the temperature is then increased back to 298.15 K at a very slow rate of 0.01 K per minute in order to ensure that equilibrium condition could always be approximated during the dissociation process, which increases the accuracy of the results. The use of slow heating rates is important because mixing does not exist in DSC measurements. By using this slow rate, the repeatability of results using this DSC is excellent with no statistically significant variation between runs, as also indicated by Adidharma and Radosz (2012). A few tests are repeated to check the reproducibility of the data and insignificant variation has always been confirmed. Results reported are based on single tests. These tests are performed at pressures of approximately 10, 15, and 20 MPa. High pressure tests for particular systems are also performed at pressures up to approximately 38.5 MPa.

3. Results and discussion 3.1. Methane hydrate dissociation conditions in the presence of single inhibitors The dissociation conditions of methane hydrate in the presence of EMIM-Cl at concentrations of 5, 20, 30, and 40 wt% are

Table 1 Ionic liquids studied in this work. Symbol

Chemical name

EMIM-Cl

1-ethyl-3-methylimidazolium chloride

EMIM-Br

1-ethyl-3-methylimidazolium bromide

Chemical structure


measured at pressures P of approximately 10, 15, and 20 MPa. Methane hydrate dissociation condition is determined from the thermogram where the intersection of the returning part of the hydrate dissociation peak with the baseline represents the dissociation temperature T, as shown in Fig. 1 (Xiao et al., 2010). The pressure that corresponds to the intersection is the equilibrium pressure and is obtained from the pressure transducer data. The dissociation conditions of methane hydrate in the presence of EMIM-Cl performed in this study are listed in Table 2. These results as well as those for blank samples are also plotted in Fig. 2, which shows that the dissociation temperature decreases as the concentration of EMIM-Cl increases, as expected. The result for 10 wt% EMIM-Cl taken from Xiao et al. (2010) is also included. It can also be seen that the increment of the temperature shift from 30 to 40 wt% is larger than that from 20 to 30 wt%, and so on. This demonstrates a progressive increase in inhibition effectiveness as concentration increases. To further analyze the effectiveness of different inhibitors at a certain concentration, we calculate the average temperature shift T¯ as follows (Xiao et al., 2010): n 1X T¯ ¼ DT i ni¼1

ð1Þ

22

20

18

P (MPa)

272

Pure water 5 wt% EMIM-Cl 10 wt% EMIM-Cl 20 wt% EMIM-Cl 30 wt% EMIM-Cl 40 wt% EMIM-Cl

16

14

12

10 268

272

276

280

284

288

292

T (K) Fig. 2. Dissociation conditions of methane hydrate in the presence of EMIM-Cl at concentrations of 5, 10 (Xiao et al., 2010), 20, 30, and 40 wt%; dissociation conditions in the absence of EMIM-Cl (pure water (Gayet et al., 2005)) are included.

where n is the total number of pressure points and DTi is the temperature shift at a pressure point Pi calculated from:

0.2

Heat Flow / mW

Exo

ð2Þ

where T 0,Pi is the methane hydrate dissociation temperature for blank sample measured at pressure Pi, and T 1,Pi is the dissociation temperature for sample with inhibitor measured at the same pressure Pi. In this work, in using Eq. (1), dissociation temperatures are calculated at five pressure points, i.e., 11.1, 13.6, 15.6, 17.6, and 20.6 MPa, which are the same as those used by Xiao et al. (2010). Since experimental data for different inhibitors are usually available at different pressures, to perform the calculations, the following model for the dissociation temperature is used (Gayet et al., 2005):

0.0

-0.2

-0.4

-0.6 Furnace temperature / °C -0.8

DT i ¼ T 0,Pi T 1,Pi

14.75

15.00

15.25

15.50

15.75

16.00

Fig. 1. Example of hydrate dissociation peak in mDSC thermogram showing the intersection of the returning part of the peak with the baseline.

Table 2 Dissociation conditions of methane hydrate in the presence of EMIM-Cl. w (wt%)

T (K)

P (MPa)

5

285.7 289.0 291.4

10.1 14.8 19.9

20

283.3 286.7 288.2

10.0 14.9 19.8

30

280.5 282.8

10.0 14.8

285.3

19.8

40

273.6 276.3

10.1 14.9

278.4

19.9

T ¼

A ln P=P0 B

ð3Þ

where P is the pressure, P0 ¼ 0.101325 MPa, and A and B are the model parameters. The regression is performed for all data sets of different inhibitors compared in this work at various concentrations in the pressure range of 10–20 MPa. For the record, in all cases, the goodness of fit of Eq. (3) is excellent. Fig. 3 compares the average temperature shifts for MEG and EMIM-Cl along with second-order fit curves to show the trends of the temperature shifts; MEG data, from which the average temperature shifts are calculated, except at 5 wt%, which are obtained in this work, are taken from Mohammadi and Richon (2010b), Haghighi et al. (2009), and Robinson and Ng (1986). If the trends could be assumed correct, there could be a crossover point, beyond which the hydrate inhibition effectiveness for EMIM-Cl would be greater than that for MEG. To investigate this further, experiments for 50 and 60 wt% EMIM-Cl are then performed. At the pressure range of 10–20 MPa, the DSC cannot detect the dissociation of hydrate down to 228.15 K, which is the lowest temperature the DSC could reach. On the other hand, in our previous work, experiment for 60 wt% MEG at 10 MPa was successfully performed using the same apparatus (Adidharma and Radosz, 2012); the data point of this MEG is also included in Fig. 3. While these facts do not provide absolute proof, considering also that the degree of supercooling needed to form hydrates is


273

Table 3 Dissociation conditions of methane hydrate with mixed inhibitors. w (wt%)

T (K)

P (MPa)

284.9 288.0 289.7

10.0 14.8 19.7

20

281.9 285.6 287.2

9.7 14.8 19.8

30

279.6 281.6

9.9 14.9

284.0

19.8

280.4 282.9

9.9 14.8

284.6

19.9

281.9 285.6 287.2

9.7 14.8 19.8

EMIM-Cl þ MEG 1:1 mixture 10

EMIM-Cl þ NaCl 1:1 mixture 10

EMIM-Cl þ EMIM-Br 1:1 mixture 20

Fig. 3. Average temperature shifts T¯ for EMIM-Cl and MEG of different concentrations.

dependent on the mixture properties, they may support the prediction that EMIM-Cl would be more effective at inhibiting hydrate formation than MEG at inhibitor concentrations at or above approximately 60 wt%. 3.2. Methane hydrate dissociation conditions in the presence of mixtures containing EMIM-Cl and other inhibitors The dissociation conditions of methane hydrate are measured in the presence of mixed EMIM-Cl and MEG using 1:1 mixtures at concentrations of 10 wt% (5 wt% EMIM-Clþ5 wt% MEG), 20 wt% (10 wt% EMIM-Clþ10 wt% MEG), and 30 wt% (15 wt% EMIMClþ15 wt% MEG). Measurements are also performed on a mixture of 5 wt% EMIM-Clþ 5 wt% NaCl and a mixture of two ILs, i.e., 10 wt% EMIM-Cl þ10 wt% EMIM-Br. A 20 wt% mixture of two ILs is examined instead of 10 wt% since the dissociation conditions for 10 wt% EMIM-Cl and EMIM-Br are very similar (Xiao et al., 2010); for investigating the existence of synergistic effect, without adequate separation between dissociation conditions, even very small experimental error may potentially lead to an inaccurate conclusion. Methane hydrate dissociation condition is also determined from the thermogram using the same approach depicted in Fig. 1. Table 3 shows the dissociation conditions for these systems. To investigate any synergistic effects, DT i is used to compare the hydrate inhibition effect of a mixture to those of the individual components that are present in the mixture. This is accomplished by comparing DT i for the mixture, which is at a concentration of w, to the arithmetic mean of the temperature shifts for the individual components given by

DT a,i ¼

1 DT 1,i þ DT 2,i 2

ð4Þ

where DT 1,i and DT 2,i are the shifts of the first and second components at the same inhibitor concentration w, respectively. Therefore, all systems considered in the calculation have the same total concentration of inhibitor. Eq. (4) also implies that in a mixture of two non-interacting inhibitors, equal contribution of the individual inhibitors has been assumed, which is a sound assumption since the mixture is a 1:1 solution. Thus, if the different inhibitors in the mixture are not interacting to each other, the expected average temperature shift would be DT a,i . One may also suggest summing the effects of the individual inhibitors

Fig. 4. The effect of pressure on the temperature shift of 1:1 EMIM-Clþ MEG mixture and the arithmetic average of shifts of the individual components at 10 wt%.

of concentrations w/2 as a simple method for probing the synergistic effect of the inhibitors in a 1:1 mixture of concentration w. However, Fig. 2 demonstrates that even for single inhibitors, doubling the inhibitor concentration produces an effect that is greater than double that of the original concentration. For this reason, using such a method to probe whether or not a synergistic effect is present may provide misleading results. Fig. 4 shows the effect of pressure on the temperature shift of EMIM-ClþMEG solution and the arithmetic average of shifts of the individual components at 10 wt%. Note that the experimental data for 10 wt% EMIM-Cl and EMIM-ClþMEG have been extended to approximately 38 MPa to investigate a possible synergistic crossover at higher pressures, in which the temperature shift for the mixture exceeds the arithmetic shift average of the individual components. These additional data are listed in Table 4 and the needed data for 10 wt% MEG in this pressure range are taken from Haghighi et al. (2009). As shown in Fig. 4, this crossover is confirmed to occur near 35.3 MPa. Thus, although the thermodynamic inhibition performance of the mixtures of EMIM-Cl and

274


Table 4 Hydrate dissociation conditions for EMIM-Cl and EMIM-Cl þ MEG for pressures above 20 MPa. Inhibitor

T (K)

P (MPa)

EMIM-Cl

293.4 295.2

30.3 37.8

EMIM-Clþ MEG

292.8

29.7

294.8

38.5

Fig. 6. The effect of pressure on the temperature shift of 1:1 EMIM-Cl þ NaCl mixture and the arithmetic average of shifts of the individual components at 10 wt%. NaCl data are taken from Maekawa et al. (1995).

Fig. 5. The effect of pressure on the temperature shift of 1:1 EMIM-Cl þMEG mixture and the arithmetic average of shifts of the individual components at 20 and 30 wt%.

MEG does not show any synergistic effects at low pressures, it does at higher pressures. In other words, the adverse interactions between EMIM-Cl and MEG only happen at low pressures. At this stage, the reason for this crossover is still unknown. Fig. 5 shows a similar graph for samples containing 20 and 30 wt% EMIM-Cl þMEG. As shown in the figure, for 20 and 30 wt% EMIM-ClþMEG a synergistic crossover is also likely to occur. However, the crossover pressures predicted using Eqs. (3) and (4) are above 40 MPa and increase as concentration increases. Such pressures are beyond the operating range of the experimental apparatus and therefore the measurements are not performed. Figs. 6 and 7 also show similar graphs for samples containing EMIM-ClþNaCl and EMIM-ClþEMIM-Br, respectively. For the mixture of EMIM-ClþNaCl, the existence of a crossover is unlikely, as shown in Fig. 6. This may be due to strong adverse interactions between inhibitors, and the unique pressure effects experienced by each inhibitor. As can be observed in Fig. 7, the mixture of 20 wt% EMIM-Cl þEMIM-Br shows a synergistic effect at higher pressures, with a crossover occurring at approximately 17.7 MPa. The experimental results for samples with 20 wt% EMIM-Br, needed for comparison with samples containing EMIM-ClþEMIM-Br, are shown in Table 5. The seemingly scattered ‘data’ in Figs. 4–7 deserve some explanations. The ‘data’ are actually derived from the experimental data and Eq. (3) for the blank samples because no experimental data point has the corresponding data point of the blank sample measured at exactly the same pressure. The errors from Eq. (3) and taking the difference of two data points could produce larger errors. The solid lines in those figures, on the other hand, are derived only from Eq. (3) applied to both inhibited

Fig. 7. The effect of pressure on the temperature shift of 20 wt% 1:1 EMIMClþEMIM-Br mixture and the arithmetic average of shifts of the individual components.

Table 5 Dissociation conditions of methane hydrate in the presence of EMIM-Br. w (wt%)

T (K)

P (MPa)

20

284.7 287.6

10.1 15.0

289.6

20.0

systems and blank samples, which could be considered as smoothed data. Thus, they could better represent the trend. The cumulated errors of the ‘data’ with respect to the solid lines in Figs. 4–7 are still less than 70.4 K. To compare all of the systems examined that contain 10 wt% inhibitor, temperature shifts are plotted in Fig. 8 along with regression curves. It is interesting to note that the effectiveness


275

demonstrate an increase in inhibition effectiveness as pressure increases. At this stage, the reason for the different trends is still unknown.

Nomenclature A B n P P0 T T¯ T0,pi T1,pi w DTi DTa,I Fig. 8. Temperature shifts calculated from experimental hydrate dissociation conditions for systems with 10 wt% inhibitor with regression curve for each system. Shown are data for NaCl (Maekawa et al., 1995), MEG (Haghighi et al., 2009; Robinson and Ng, 1986), EMIM-Clþ NaCl (this work), EMIM-Cl þMEG (this work), EMIM-Cl (Xiao et al., 2010), and EMIM-Br (Xiao et al., 2010).

model parameter model parameter number of pressure points pressure, MPa model reference pressure, 0.101325 MPa temperature, K average temperature shift, K dissociation temperature for blank sample at pressure Pi dissociation temperature for sample with inhibitor at pressure Pi inhibitor concentration, wt% temperature shift at pressure Pi arithmetic mean of temperature shifts at Pi

Acknowledgments This work was partially funded by the Department of Education McNair Scholars Program and Wyoming NSF EPSCoR.

of inhibitors containing EMIM-Cl or EMIM-Br is increasing with pressure. This phenomenon does not exist for the conventional inhibitors examined in this work, such as MEG, the effectiveness of which is relatively constant, and NaCl which has a decrease in effectiveness with increasing pressure.

4. Conclusions Hydrate dissociation measurements for solutions containing high concentrations of EMIM-Cl up to 40 wt% are performed at approximately 10, 15, and 20 MPa in a high-pressure microdifferential scanning calorimeter. Experiments on methane hydrate dissociation conditions in the presence of mixed ionic liquid and conventional inhibitors, such as sodium chloride (NaCl) and monoethylene glycol (MEG), as well as a mixture containing two ionic liquids, EMIM-Cl and 1-ethyl-3-methylimidazolium bromide (EMIM-Br), are also performed to investigate any possible synergistic effects. For single inhibitor systems of EMIM-Cl, the results demonstrate a progressive increase in inhibition effectiveness as the inhibitor concentration increases. There is also an indication that EMIM-Cl would be more effective at inhibiting hydrate formation than MEG at inhibitor concentrations at or above approximately 60 wt%. We found that the effects of pressure and concentration on the behavior of mixed inhibitors containing EMIM-Cl are quite complex. For systems with 10 wt% EMIM-Cl þMEG, it is found that DT is greater than DT a at pressures above approximately 35.3 MPa, indicating the presence of a synergistic effect. At high concentrations, a synergistic effect for this system is likely to occur at higher pressures. For systems with 10 wt% EMIM-Cl þNaCl, no synergistic effect is observed. For systems with 20 wt% EMIMClþEMIM-Br, a synergistic effect is observed at pressures above approximately 17.7 MPa. Pressure is observed to have a unique effect on inhibition depending on the type of inhibitor and its concentration. Unlike MEG or NaCl, inhibitors containing EMIM-Cl or EMIM-Br

References Adidharma, H., Radosz, M., 2012. Hydrates in high inhibitor concentration systems. Gas Processors Association (GPA) Research Report RR-216. Atik, Z., Windmeier, C., Oellrich, L.R., 2006. Experimental gas hydrate dissociation pressures for pure methane in aqueous solutions of MgCl2 and CaCl2 and for a (methaneþethane) gas mixture in an aqueous solution of (NaClþ MgCl2). J. Chem. Eng. Data 51, 1862–1867. Chen, Q., Yu, Y., Zeng, P., Yang, W., Liang, Q., Peng, X., Liu, Y., Hu, Y., 2008. Effect of 1-butyl-3-methylimidazolium tetrafluoroborate on the formation rate of CO2 hydrate. J. Nat. Gas Chem. 17, 264–267. De Roo, J.L., Peters, C.J., Lichtenthaler, R.N., Diepen, G.A.M., 1983. Occurrence of methane hydrate in saturated and unsaturated solution of sodium chloride and water independence of temperature and pressure. AIChE J. 29, 651–657. Englezos, P., 1993. Clathrate hydrates. Ind. Eng. Chem. Res. 32, 1251–1274. Gayet, P., Dicharry, C., Marion, G., Graciaa, A., Lachaise, J., Nesterov, A., 2005. Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter. Chem. Eng. Sci. 60, 5751–5758. Haghighi, H., Chapoy, A., Burgess, R., Tohidi, B., 2009. Experimental and thermodynamic modelling of systems containing water and ethylene glycol: application to flow assurance and gas processing. Fluid Phase Equilibria 276, 24–30. Jager, M.D., Peters, C.J., Sloan, E.D., 2002. Experimental determination of methane hydrate stability in methanol and electrolyte solutions. Fluid Phase Equilibria 193, 17–28. Jager, M.D., Sloan, E.D., 2001. The effect of pressure on methane hydration in pure water and sodium chloride solutions. Fluid Phase Equilibria 185, 89–99. Kang, S.-P., Chun, M.K., Lee, H., 1998. Phase equilibria of methane and carbon dioxide hydrates in the aqueous MgCl2 solutions. Fluid Phase Equilibria 147, 229–238. Kharrat, M., Dalmazzone, D., 2003. Experimental determination of stability conditions of methane hydrate in aqueous calcium chloride solutions using high pressure differential scanning calorimetry. J. Chem. Thermodyn. 35, 1489–1505. Koh, C.A., 2002. Towards a fundamental understanding of natural gas hydrates. Chem. Soc. Rev. 31, 157–167. Lafond, P.G., Olcott, K.A., Sloan, E.D., Koh, C.A., Sum, A.K., 2012. Measurements of methane hydrate equilibrium in systems inhibited with NaCl and methanol. J. Chem. Thermodyn. 48, 1–6. Lee, J.D., Englezos, P., 2005. Enhancement of the performance of gas hydrate kinetic inhibitors with polyethylene oxide. Chem. Eng. Sci. 60, 5323–5330. Li, X.S., Liu, Y.J., Zeng, Z.Y., Chen, Z.Y., Li, G., Wu, H.J., 2011. Equilibrium hydrate formation conditions for the mixtures of methaneþ ionic liquidsþ water. J. Chem. Eng. Data 56, 119–123. Maekawa, T., Imai, N., 2000. Equilibrium conditions of methane and ethane hydrates in aqueous electrolyte solutions. Ann. NY Acad. Sci. 912, 932–939. Maekawa, T., Itoh, S., Sakata, S., Igari, S.I., Imai, N., 1995. Pressure and temperature conditions for methane hydrate dissociation in sodium chloride solutions. Geochem. J. 29, 325–329.

276


Masoudi, R., Tohidi, B., 2010. On modelling gas hydrate inhibition by salts and organic inhibitors. J. Pet. Sci. Eng. 74, 132–137. Masoudi, R., Tohidi, B., Anderson, R., Burgass, R.W., Yang, J., 2004. Experimental measurement and thermodynamic modelling of clathrate hydrate equilibria and salt solubility in aqueous ethylene glycol and electrolyte solutions. Fluid Phase Equilibria 219, 157–163. Mohammadi, A.H., Afzal, W., Richon, D., 2008. Gas hydrates of methane, ethane, propane, and carbon dioxide in the presence of single NaCl, KCl, and CaCl2 aqueous solutions: experimental measurements and predictions of dissociation conditions. J. Chem. Thermodyn. 40, 1693–1697. Mohammadi, A.H., Kraouti, I., Richon, D., 2009. Methane hydrate phase equilibrium in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions: experimental measurements and predictions of dissociation conditions. J. Chem. Thermodyn. 41, 779–782. Mohammadi, A.H., Richon, D., 2009. Methane hydrate phase equilibrium in the presence of salt (NaCl, KCl, or CaCl2)þ ethylene glycol or salt (NaCl, KCl, or CaCl2)þ methanol aqueous solution: experimental determination of dissociation condition. J. Chem. Thermodyn. 41, 1374–1377. Mohammadi, A.H., Richon, D., 2010a. Gas hydrate phase equilibrium in the presence of ethylene glycol or methanol aqueous solution. Ind. Eng. Chem. Res. 49, 8865–8869. Mohammadi, A.H., Richon, D., 2010b. Phase equilibria of methane hydrates in the presence of methanol and/or ethylene glycol aqueous solutions. Ind. Eng. Chem. Res. 49, 925–928. Mohammadi, A.H., Richon, D., 2012a. Phase equilibria of hydrogen sulfide and carbon dioxide simple hydrates in the presence of methanol,

(methanol þNaCl) and (ethylene glycol þNaCl) aqueous solutions. J. Chem. Thermodyn. 44, 26–30. Mohammadi, A.H., Richon, D., 2012b. Phase equilibria of propane and ethane simple hydrates in the presence of methanol or ethylene glycol aqueous solutions. Ind. Eng. Chem. Res. 51, 2804–2807. Ng, H.J., Chen, C.J., Saeterstad, T., 1987. Hydrate formation and inhibition in gas condensate and hydrocarbon liquid systems. Fluid Phase Equilibria 36, 99–106. Peng, X., Hu, Y., Liu, Y., Jin, C., Lin, H., 2010. Separation of ionic liquids from dilute aqueous solutions using the method based on CO2 hydrates. J. Nat. Gas Chem. 19, 81–85. Robinson, D.B., Ng, H.J., 1986. Hydrate formation and inhibition in gas or gas condensate streams. J. Can. Petrol. Technol. 25, 26–30. Szymczak, S., Sanders, K., Pakulski, M., Higgins, T., 2006. Chemical compromise: a thermodynamic and low-dose hydrate-inhibitor solution for hydrate control in the Gulf of Mexico. SPE Proj. Facilities Constr. 1, 1–5. Tumba, K., Reddy, P., Naidoo, P., Ramjugernath, D., Eslamimanesh, A., Mohammadi, A.H., Richon, D., 2011. Phase equilibria of methane and carbon dioxide clathrate hydrates in the presence of aqueous solutions of tributylmethylphosphonium methylsulfate ionic liquid. J. Chem. Eng. Data 56, 3620–3629. Xiao, C., Adidharma, H., 2009. Dual function inhibitors for methane hydrate. Chem. Eng. Sci. 64, 1522–1527. Xiao, C., Wibisono, N., Adidharma, H., 2010. Dialkylimidazolium halide ionic liquids as dual function inhibitors for methane hydrate. Chem. Eng. Sci. 65, 3080–3087.