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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. Thermodynamics 41 (2009) 779–782
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Methane hydrate phase equilibrium in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions: Experimental measurements and predictions of dissociation conditions Amir H. Mohammadi a, Ilyas Kraouti b, Dominique Richon a,* a b
Mines ParisTech, CEP/TEP – Centre énergétique et procédés, CNRS FRE 2861, 35 Rue Saint Honoré, 77305 Fontainebleau, France Département Mesures Physiques, Institut Universitaire de Technologie, Université de Paris-Sud (XI), Plateau de Moulon, 91400 Orsay, France
a r t i c l e
i n f o
Article history: Received 3 July 2008 Received in revised form 12 January 2009 Accepted 16 January 2009 Available online 27 January 2009 Keywords: Gas hydrate Methane NaBr, KBr, CaBr2, K2CO3, and MgCl2 Experimental data Prediction
a b s t r a c t In this communication, experimental data for dissociation conditions of methane hydrates in the presence of 0.05 and 0.1 mass fractions NaBr, KBr, K2CO3, and MgCl2 aqueous solutions and in the presence of 0.05 and 0.15 mass fractions CaBr2 aqueous solutions are reported. The experimental data were generated using an isochoric pressure-search method. The new experimental dissociation data for methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution are compared with some selected experimental data from the literature and the agreements are generally found acceptable. Some of new data are finally compared with the predictions of a correlation, which is generally used in the absence of experimental data, and acceptable agreements between the experimental and predicted data are observed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Gas hydrates are solid ice-like compounds formed through a combination of water and suitably sized guest molecules under low temperatures and elevated pressures commonly occur in petroleum industry [1–3]. The formation of gas hydrates can cause equipment blockage, operational problems, and safety concerns in hydrocarbon production, transportation, and processing [1–3]. For pipelines carrying a cocktail of multiphase fluids including petroleum fluids and formation water with various concentrations of salts, saline water may provide the required gas hydrate formation inhibition [1]. On the other hand, addition of some specific salts in drilling muds can inhibit formation of gas hydrates in drilling operations [1]. Reliable experimental data for gas hydrate phase equilibrium in the presence/absence of salt aqueous solutions are therefore necessary to avoid formation of gas hydrates. Although many experimental data have been reported for phase equilibria of gas hydrates in the presence of NaCl, KCl, and CaCl2 aqueous solutions [1,3], information for gas hydrates phase equilibria in the presence of other salts aqueous solutions is limited [1]. In this work, we report experimental dissociation data for methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions, which have been measured based on
* Corresponding author. Tel.: +33 1 64 69 49 65; fax: +33 1 64 69 49 68. E-mail address: [email protected] (D. Richon). 0021-9614/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2009.01.004
our previous experimental work [2,3] that takes advantage of an isochoric pressure-search method [4]. Table 1 summarizes the experiments carried out in terms of type of salt, salt concentration in the aqueous solution, and dissociation temperature ranges. The experimental data on dissociation conditions of methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution are successfully compared with some selected experimental data from the literature [5], which demonstrates the reliability of the experimental technique and the new experimental data reported in this work. Some of experimental dissociation data are finally compared with the predictions of a general correlation [6] and acceptable agreements between the experimental and the predicted data are generally found.
2. Experimental section Purities and suppliers of materials are provided in table 2. A description of the experimental setup used in this study is given elsewhere [2,3]. Briefly, the main part of the apparatus is a cylindrical vessel, which can withstand pressures higher than 40 MPa. The vessel has a volume of 57.5 cm3 with two sapphire windows. A magnetic stirrer ensures sufficient agitation to facilitate reaching equilibrium. The vessel was immersed inside a temperature controlled bath to maintain the temperatures of study. Two platinum resistance thermometers (Pt100) inserted into the vessel were used to measure temperature and check for equality of
780
A.H. Mohammadi et al. / J. Chem. Thermodynamics 41 (2009) 779–782
TABLE 1 Dissociation temperature (T) ranges studied in this work for methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions (w1 = mass fraction of salt in aqueous solution). Salt
w1
T/K
NaBr
0.05 0.1
272.5 to 285.0 274.0 to 284.5
KBr
0.05 0.1
273.1 to 285.5 272.8 to 283.7
CaBr2
0.05 0.15
273.6 to 284.8 272.9 to 282.8
K2CO3
0.05 0.1
273.4 to 284.5 274.1 to 284.1
MgCl2
0.05 0.1
273.3 to 283.7 272.9 to 282.0
T/Ka
p/MPab 0.05 mass fraction NaBr
272.5 274.3 276.7 279.9 282.2 285.0
2.85 3.37 4.29 5.90 7.51 10.12 0.1 mass fraction NaBr
274.0 275.7 276.9 279.1 280.5 282.0 284.5
3.65 4.48 5.05 6.29 7.29 8.58 11.32 0.05 mass fraction KBr
TABLE 2 Purities and suppliers of materials.a Chemical
Supplier
Purity
Methane NaBr KBr CaBr2 K2CO3 MgCl2
Messer Griesheim Sigma–Aldrich Sigma Aldrich Sigma–Aldrich Sigma
99.995 (volume)% P99% P99% 98% 99% 99%
a
TABLE 3 Experimental dissociation data for methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions.
Deionized water was used in all experiments.
temperatures within temperature measurement uncertainties, which is estimated to be less than 0.1 K. This temperature uncertainty estimation comes from careful calibration against a 25 X reference platinum resistance thermometer. The pressure in the vessel was measured with a DRUCK pressure transducer (Druck, type PTX611 for pressures up to 20 MPa). Pressure measurement accuracies are estimated to be better than 5 kPa. The hydrate dissociation points were measured with isochoric pressure-search procedure [4]. The vessel containing the aqueous solution (50% by volume of the vessel was filled by the aqueous solution) was immersed into the temperature controlled bath, and the gas was supplied from a high pressure cylinder through a pressure regulating valve into the partially evacuated vessel. After getting temperature and pressure stability (far enough from the hydrate formation region), the valve between the vessel and the cylinder was closed. Subsequently, the temperature was slowly decreased to form the hydrate. Hydrate formation in the vessel was detected by pressure drop. The temperature was then increased with steps of 0.1 K. At every temperature step, the temperature was kept constant with sufficient time to achieve a steady equilibrium state in the vessel. In this way, a pressure–temperature diagram was obtained for each experimental run, from which we determined the hydrate dissociation point. If the temperature is increased in the hydrate forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. If the temperature is increased outside the hydrate region, only a smaller increase in the pressure is observed as a result of the change in the phase equilibria of the fluids in the vessel [7]. Consequently, the point at which the slope of pressure–temperature data plots changes sharply is considered to be the point at which all hydrate crystals have dissociated and hence reported as the dissociation point. 3. Results and discussion All dissociation data measured in this work are reported in table 3 and are plotted in figures 1 to 5. A semi-logarithmic scale has
273.1 275.9 278.4 280.2 282.6 285.5
2.89 3.80 4.88 5.87 7.57 10.42 0.1 mass fraction KBr
272.8 275.0 277.8 279.5 281.4 283.7
3.15 3.98 5.19 6.20 7.59 9.80 0.05 mass fraction CaBr2
273.6 276.4 279.3 282.0 284.8
3.01 4.00 5.35 7.14 9.62 0.15 mass fraction CaBr2
272.9 275.2 277.5 279.4 282.8
3.68 4.68 5.92 7.29 10.68 0.05 mass fraction MgCl2
273.3 276.3 278.9 281.1 283.7
3.30 4.57 5.94 7.45 10.11 0.1 mass fraction MgCl2
272.9 275.4 277.7 280.1 282.0
4.17 5.70 7.29 9.50 11.90 0.05 mass fraction K2CO3
273.4 277.1 279.6 282.0 284.5
2.95 4.28 5.56 7.19 9.55 0.1 mass fraction K2CO3
274.1 276.3 278.8 281.0 284.1
3.61 4.53 5.89 7.44 10.63
a Uncertainty on temperatures through calibrated platinum resistance thermometers is estimated to be less than 0.1 K. b Uncertainty on pressures through calibrated pressure transducer is estimated to be less than 5 kPa.
781
A.H. Mohammadi et al. / J. Chem. Thermodynamics 41 (2009) 779–782
100
p /MPa
been used in these figures to show the data consistency, as the logarithm of hydrate dissociation pressure versus temperature has approximately linear behavior. It should be mentioned that the graphical data of Maekawa and Imai [8] for 10 mass% of NaBr in aqueous solution have not been shown in figure 2. The experimental data on dissociation conditions of methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution are compared with some selected experimental data from the literature [5]. The acceptable agreements between the experimental data measured in this work and the experimental data reported in literature [5] demonstrate the reliability of the experimental technique and new data reported in this work. However, the experimental data reported in reference [5] apparently show unusual behavior at higher concentration of MgCl2 in aqueous phase. Figures 2 to 4 also show predictions of a general correlation [6] for hydrate disso-
10
1 271
273
275
277
279
281
283
285
287
T /K
100
10
1 271
100
273
275
277
279
281
283
285
T /K FIGURE 1. Experimental dissociation conditions of methane hydrates in the presence of MgCl2 aqueous solutions. Symbols are for experimental dissociation conditions: 4, 0.05 mass fraction MgCl2 aqueous solution, this work; s, 0.1 mass fraction MgCl2 aqueous solution, this work; d, 0.1 mass fraction MgCl2 aqueous solution, Literature data [5]. The general correlation [6] has not been developed for MgCl2 aqueous solution.
p /MPa
p /MPa
FIGURE 3. Experimental and predicted dissociation conditions of methane hydrates in the presence of KBr aqueous solutions. Symbols are for experimental dissociation conditions: s, 0.05 mass fraction KBr aqueous solution, this work; 4, 0.1 mass fraction KBr aqueous solution, this work; solid lines, predictions of hydrate dissociation conditions using general correlation [6]. Error band: 0.2 K.
10
1 271
273
275
277
279
281
283
285
287
T /K FIGURE 4. Experimental and predicted dissociation conditions of methane hydrates in the presence of K2CO3 aqueous solutions. Symbols are for experimental dissociation conditions: s, 0.05 mass fraction K2CO3 aqueous solution, this work; 4, 0.1 mass fraction K2CO3 aqueous solution, this work; solid lines, predictions of hydrate dissociation conditions using general correlation [6]. Error band: 0.4 K.
100
p/ MPa
ciation conditions. It should be mentioned that this general correlation [6] has not been developed for MgCl2 and CaBr2 aqueous solutions and therefore no comparisons have been made for these two salts. Briefly, the following equation has been used for predicting hydrate dissociation temperature of a fluid in the presence of inhibitor, T, from hydrate suppression temperature (or suppression of hydrate dissociation temperature) (DT):
10
T ¼ T 0 DT; 1
271
273
275
277
279
281
283
285
287
T /K FIGURE 2. Experimental and predicted dissociation conditions of methane hydrates in the presence of NaBr aqueous solutions. Symbols are for experimental dissociation conditions: 4, 0.05 mass fraction NaBr aqueous solution, this work; s, 0.1 mass fraction NaBr aqueous solution, this work; Solid lines, predictions of hydrate dissociation conditions using general correlation [6]. Error band: 0.2 K. The graphical data of Maekawa and Imai [8] at 10 mass% of NaBr in aqueous solution have not been shown in this figure.
ð1Þ
where T0 stands for hydrate dissociation temperature of the same fluid system in the presence of distilled water. In the above equation, DT is calculated using the following equation [6]:
DT=K ¼ c1 w1 þ c2 w21 þ c3 w31 fc4 lnðp=kPaÞ þ c5 g½c6 fðp0 =kPaÞ 1000g þ 1; ð2Þ where w1, p, and p0 are the concentration of the inhibitor in the aqueous phase (in mass fraction), the pressure of the system, and the dissociation pressure of fluid in the presence of distilled water
782
A.H. Mohammadi et al. / J. Chem. Thermodynamics 41 (2009) 779–782
100
than 6% deviations for dissociation pressures), which are acceptable.
p/MPa
4. Conclusions
10
1
272
274
276
278
T/K
280
282
284
286
FIGURE 5. Experimental dissociation conditions of methane hydrates in the presence of CaBr2 aqueous solutions. Symbols are for experimental dissociation conditions: s, 0.05 mass fraction CaBr2 aqueous solution, this work; 4, 0.15 mass fraction CaBr2 aqueous solution, this work. The general correlation [6] has not been developed for CaBr2 aqueous solution.
TABLE 4 Constants ci in equation (2) for NaBr, KBr, and K2CO3 [6] (the general correlation [6] has not been developed for MgCl2 and CaBr2 aqueous solutions). Salt
c1
c2
c3
c4 102
c5
c6 105
NaBr KBr K2CO3
41.9 34.06 18.37
65 7.8 57
1.098 0.822 2.551
2.529 3.014 6.917
0.303 0.3486 1.101
2.46 2.30 2.71
at 273.15 K. The constants ci are given in the original manuscript for various inhibitors [6]. These constants for NaBr, KBr, and K2CO3 are reported in table 4. In equation (1), T0 can be calculated at any given pressure by using an appropriate predictive method such as the HWHYD thermodynamic model [9], which is capable of predicting different scenarios in hydrate phase equilibrium calculations. A detailed description of this model is given elsewhere [10,11]. The model [9] is briefly based on the equality of fugacity concept, which uses the Valderrama modification of the Patel–Teja equation of state [12] and non-density dependent mixing rules [13] for modeling the fluid phases and the van der Waals and Platteeuw theory [14] is used for modeling the hydrate phase. This model, however, has not been developed for the salt systems studied in this work. As can be observed in figures 2-4, the predicted data show generally less than 0.5 K deviations from the experimental data (less
Experimental data on dissociation conditions of methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions with different concentrations of single salt and at various temperatures (table 1) were reported in this work. An isochoric pressure-search method [2–4] was used for performing all the measurements. The experimental data on dissociation conditions of methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution were compared with some selected experimental data from the literature [5]. The acceptable agreements between the experimental data measured in this work and the experimental data reported in literature [5] helped to confirm the reliability of the experimental technique and the data generated in this work. Some experimental data were finally compared with the predictions of a general correlation [6] and acceptable agreements were found between the experimental and predicted data. Acknowledgement Ilyas Kraouti would like to thank the CEP-TEP laboratory for the opportunity to work as a trainee. References [1] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, third ed., CRC Press, Taylor & Francis Group, Boca Raton, 2007. [2] W. Afzal, A.H. Mohammadi, D. Richon, J. Chem. Eng. Data 52 (2007) 2053– 2055. [3] A.H. Mohammadi, W. Afzal, D. Richon, J. Chem. Thermodyn. 40 (2008) 1693– 1697. [4] B. Tohidi, R.W. Burgass, A. Danesh, K.K. Østergaard, A.C. Todd, Ann. N.Y. Acad. Sci. 912 (2000) 924–931. [5] S.P. Kang, M.K. Chun, H. Lee, Fluid Phase Equilib. 147 (1998) 229–238. [6] K.K. Østergaard, R. Masoudi, B. Tohidi, A. Danesh, A.C. Todd, J. Pet. Sci. Eng. 48 (2005) 70–80. [7] R. Ohmura, S. Takeya, T. Uchida, T. Ebinuma, Ind. Eng. Chem. Res. 43 (2004) 4964–4966. [8] T. Maekawa, N. Imai, Ann. N.Y. Acad. Sci. (2000) 932–939. [9] Heriot-Watt University Hydrate model:
JCT 08-239
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Methane hydrate phase equilibrium in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions: Experimental measurements and predictions of dissociation conditions Amir H. Mohammadi a, Ilyas Kraouti b, Dominique Richon a,* a b
Mines ParisTech, CEP/TEP – Centre énergétique et procédés, CNRS FRE 2861, 35 Rue Saint Honoré, 77305 Fontainebleau, France Département Mesures Physiques, Institut Universitaire de Technologie, Université de Paris-Sud (XI), Plateau de Moulon, 91400 Orsay, France
a r t i c l e
i n f o
Article history: Received 3 July 2008 Received in revised form 12 January 2009 Accepted 16 January 2009 Available online 27 January 2009 Keywords: Gas hydrate Methane NaBr, KBr, CaBr2, K2CO3, and MgCl2 Experimental data Prediction
a b s t r a c t In this communication, experimental data for dissociation conditions of methane hydrates in the presence of 0.05 and 0.1 mass fractions NaBr, KBr, K2CO3, and MgCl2 aqueous solutions and in the presence of 0.05 and 0.15 mass fractions CaBr2 aqueous solutions are reported. The experimental data were generated using an isochoric pressure-search method. The new experimental dissociation data for methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution are compared with some selected experimental data from the literature and the agreements are generally found acceptable. Some of new data are finally compared with the predictions of a correlation, which is generally used in the absence of experimental data, and acceptable agreements between the experimental and predicted data are observed. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Gas hydrates are solid ice-like compounds formed through a combination of water and suitably sized guest molecules under low temperatures and elevated pressures commonly occur in petroleum industry [1–3]. The formation of gas hydrates can cause equipment blockage, operational problems, and safety concerns in hydrocarbon production, transportation, and processing [1–3]. For pipelines carrying a cocktail of multiphase fluids including petroleum fluids and formation water with various concentrations of salts, saline water may provide the required gas hydrate formation inhibition [1]. On the other hand, addition of some specific salts in drilling muds can inhibit formation of gas hydrates in drilling operations [1]. Reliable experimental data for gas hydrate phase equilibrium in the presence/absence of salt aqueous solutions are therefore necessary to avoid formation of gas hydrates. Although many experimental data have been reported for phase equilibria of gas hydrates in the presence of NaCl, KCl, and CaCl2 aqueous solutions [1,3], information for gas hydrates phase equilibria in the presence of other salts aqueous solutions is limited [1]. In this work, we report experimental dissociation data for methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions, which have been measured based on
* Corresponding author. Tel.: +33 1 64 69 49 65; fax: +33 1 64 69 49 68. E-mail address: [email protected] (D. Richon). 0021-9614/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2009.01.004
our previous experimental work [2,3] that takes advantage of an isochoric pressure-search method [4]. Table 1 summarizes the experiments carried out in terms of type of salt, salt concentration in the aqueous solution, and dissociation temperature ranges. The experimental data on dissociation conditions of methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution are successfully compared with some selected experimental data from the literature [5], which demonstrates the reliability of the experimental technique and the new experimental data reported in this work. Some of experimental dissociation data are finally compared with the predictions of a general correlation [6] and acceptable agreements between the experimental and the predicted data are generally found.
2. Experimental section Purities and suppliers of materials are provided in table 2. A description of the experimental setup used in this study is given elsewhere [2,3]. Briefly, the main part of the apparatus is a cylindrical vessel, which can withstand pressures higher than 40 MPa. The vessel has a volume of 57.5 cm3 with two sapphire windows. A magnetic stirrer ensures sufficient agitation to facilitate reaching equilibrium. The vessel was immersed inside a temperature controlled bath to maintain the temperatures of study. Two platinum resistance thermometers (Pt100) inserted into the vessel were used to measure temperature and check for equality of
780
A.H. Mohammadi et al. / J. Chem. Thermodynamics 41 (2009) 779–782
TABLE 1 Dissociation temperature (T) ranges studied in this work for methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions (w1 = mass fraction of salt in aqueous solution). Salt
w1
T/K
NaBr
0.05 0.1
272.5 to 285.0 274.0 to 284.5
KBr
0.05 0.1
273.1 to 285.5 272.8 to 283.7
CaBr2
0.05 0.15
273.6 to 284.8 272.9 to 282.8
K2CO3
0.05 0.1
273.4 to 284.5 274.1 to 284.1
MgCl2
0.05 0.1
273.3 to 283.7 272.9 to 282.0
T/Ka
p/MPab 0.05 mass fraction NaBr
272.5 274.3 276.7 279.9 282.2 285.0
2.85 3.37 4.29 5.90 7.51 10.12 0.1 mass fraction NaBr
274.0 275.7 276.9 279.1 280.5 282.0 284.5
3.65 4.48 5.05 6.29 7.29 8.58 11.32 0.05 mass fraction KBr
TABLE 2 Purities and suppliers of materials.a Chemical
Supplier
Purity
Methane NaBr KBr CaBr2 K2CO3 MgCl2
Messer Griesheim Sigma–Aldrich Sigma Aldrich Sigma–Aldrich Sigma
99.995 (volume)% P99% P99% 98% 99% 99%
a
TABLE 3 Experimental dissociation data for methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions.
Deionized water was used in all experiments.
temperatures within temperature measurement uncertainties, which is estimated to be less than 0.1 K. This temperature uncertainty estimation comes from careful calibration against a 25 X reference platinum resistance thermometer. The pressure in the vessel was measured with a DRUCK pressure transducer (Druck, type PTX611 for pressures up to 20 MPa). Pressure measurement accuracies are estimated to be better than 5 kPa. The hydrate dissociation points were measured with isochoric pressure-search procedure [4]. The vessel containing the aqueous solution (50% by volume of the vessel was filled by the aqueous solution) was immersed into the temperature controlled bath, and the gas was supplied from a high pressure cylinder through a pressure regulating valve into the partially evacuated vessel. After getting temperature and pressure stability (far enough from the hydrate formation region), the valve between the vessel and the cylinder was closed. Subsequently, the temperature was slowly decreased to form the hydrate. Hydrate formation in the vessel was detected by pressure drop. The temperature was then increased with steps of 0.1 K. At every temperature step, the temperature was kept constant with sufficient time to achieve a steady equilibrium state in the vessel. In this way, a pressure–temperature diagram was obtained for each experimental run, from which we determined the hydrate dissociation point. If the temperature is increased in the hydrate forming region, hydrate crystals partially dissociate, thereby substantially increasing the pressure. If the temperature is increased outside the hydrate region, only a smaller increase in the pressure is observed as a result of the change in the phase equilibria of the fluids in the vessel [7]. Consequently, the point at which the slope of pressure–temperature data plots changes sharply is considered to be the point at which all hydrate crystals have dissociated and hence reported as the dissociation point. 3. Results and discussion All dissociation data measured in this work are reported in table 3 and are plotted in figures 1 to 5. A semi-logarithmic scale has
273.1 275.9 278.4 280.2 282.6 285.5
2.89 3.80 4.88 5.87 7.57 10.42 0.1 mass fraction KBr
272.8 275.0 277.8 279.5 281.4 283.7
3.15 3.98 5.19 6.20 7.59 9.80 0.05 mass fraction CaBr2
273.6 276.4 279.3 282.0 284.8
3.01 4.00 5.35 7.14 9.62 0.15 mass fraction CaBr2
272.9 275.2 277.5 279.4 282.8
3.68 4.68 5.92 7.29 10.68 0.05 mass fraction MgCl2
273.3 276.3 278.9 281.1 283.7
3.30 4.57 5.94 7.45 10.11 0.1 mass fraction MgCl2
272.9 275.4 277.7 280.1 282.0
4.17 5.70 7.29 9.50 11.90 0.05 mass fraction K2CO3
273.4 277.1 279.6 282.0 284.5
2.95 4.28 5.56 7.19 9.55 0.1 mass fraction K2CO3
274.1 276.3 278.8 281.0 284.1
3.61 4.53 5.89 7.44 10.63
a Uncertainty on temperatures through calibrated platinum resistance thermometers is estimated to be less than 0.1 K. b Uncertainty on pressures through calibrated pressure transducer is estimated to be less than 5 kPa.
781
A.H. Mohammadi et al. / J. Chem. Thermodynamics 41 (2009) 779–782
100
p /MPa
been used in these figures to show the data consistency, as the logarithm of hydrate dissociation pressure versus temperature has approximately linear behavior. It should be mentioned that the graphical data of Maekawa and Imai [8] for 10 mass% of NaBr in aqueous solution have not been shown in figure 2. The experimental data on dissociation conditions of methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution are compared with some selected experimental data from the literature [5]. The acceptable agreements between the experimental data measured in this work and the experimental data reported in literature [5] demonstrate the reliability of the experimental technique and new data reported in this work. However, the experimental data reported in reference [5] apparently show unusual behavior at higher concentration of MgCl2 in aqueous phase. Figures 2 to 4 also show predictions of a general correlation [6] for hydrate disso-
10
1 271
273
275
277
279
281
283
285
287
T /K
100
10
1 271
100
273
275
277
279
281
283
285
T /K FIGURE 1. Experimental dissociation conditions of methane hydrates in the presence of MgCl2 aqueous solutions. Symbols are for experimental dissociation conditions: 4, 0.05 mass fraction MgCl2 aqueous solution, this work; s, 0.1 mass fraction MgCl2 aqueous solution, this work; d, 0.1 mass fraction MgCl2 aqueous solution, Literature data [5]. The general correlation [6] has not been developed for MgCl2 aqueous solution.
p /MPa
p /MPa
FIGURE 3. Experimental and predicted dissociation conditions of methane hydrates in the presence of KBr aqueous solutions. Symbols are for experimental dissociation conditions: s, 0.05 mass fraction KBr aqueous solution, this work; 4, 0.1 mass fraction KBr aqueous solution, this work; solid lines, predictions of hydrate dissociation conditions using general correlation [6]. Error band: 0.2 K.
10
1 271
273
275
277
279
281
283
285
287
T /K FIGURE 4. Experimental and predicted dissociation conditions of methane hydrates in the presence of K2CO3 aqueous solutions. Symbols are for experimental dissociation conditions: s, 0.05 mass fraction K2CO3 aqueous solution, this work; 4, 0.1 mass fraction K2CO3 aqueous solution, this work; solid lines, predictions of hydrate dissociation conditions using general correlation [6]. Error band: 0.4 K.
100
p/ MPa
ciation conditions. It should be mentioned that this general correlation [6] has not been developed for MgCl2 and CaBr2 aqueous solutions and therefore no comparisons have been made for these two salts. Briefly, the following equation has been used for predicting hydrate dissociation temperature of a fluid in the presence of inhibitor, T, from hydrate suppression temperature (or suppression of hydrate dissociation temperature) (DT):
10
T ¼ T 0 DT; 1
271
273
275
277
279
281
283
285
287
T /K FIGURE 2. Experimental and predicted dissociation conditions of methane hydrates in the presence of NaBr aqueous solutions. Symbols are for experimental dissociation conditions: 4, 0.05 mass fraction NaBr aqueous solution, this work; s, 0.1 mass fraction NaBr aqueous solution, this work; Solid lines, predictions of hydrate dissociation conditions using general correlation [6]. Error band: 0.2 K. The graphical data of Maekawa and Imai [8] at 10 mass% of NaBr in aqueous solution have not been shown in this figure.
ð1Þ
where T0 stands for hydrate dissociation temperature of the same fluid system in the presence of distilled water. In the above equation, DT is calculated using the following equation [6]:
DT=K ¼ c1 w1 þ c2 w21 þ c3 w31 fc4 lnðp=kPaÞ þ c5 g½c6 fðp0 =kPaÞ 1000g þ 1; ð2Þ where w1, p, and p0 are the concentration of the inhibitor in the aqueous phase (in mass fraction), the pressure of the system, and the dissociation pressure of fluid in the presence of distilled water
782
A.H. Mohammadi et al. / J. Chem. Thermodynamics 41 (2009) 779–782
100
than 6% deviations for dissociation pressures), which are acceptable.
p/MPa
4. Conclusions
10
1
272
274
276
278
T/K
280
282
284
286
FIGURE 5. Experimental dissociation conditions of methane hydrates in the presence of CaBr2 aqueous solutions. Symbols are for experimental dissociation conditions: s, 0.05 mass fraction CaBr2 aqueous solution, this work; 4, 0.15 mass fraction CaBr2 aqueous solution, this work. The general correlation [6] has not been developed for CaBr2 aqueous solution.
TABLE 4 Constants ci in equation (2) for NaBr, KBr, and K2CO3 [6] (the general correlation [6] has not been developed for MgCl2 and CaBr2 aqueous solutions). Salt
c1
c2
c3
c4 102
c5
c6 105
NaBr KBr K2CO3
41.9 34.06 18.37
65 7.8 57
1.098 0.822 2.551
2.529 3.014 6.917
0.303 0.3486 1.101
2.46 2.30 2.71
at 273.15 K. The constants ci are given in the original manuscript for various inhibitors [6]. These constants for NaBr, KBr, and K2CO3 are reported in table 4. In equation (1), T0 can be calculated at any given pressure by using an appropriate predictive method such as the HWHYD thermodynamic model [9], which is capable of predicting different scenarios in hydrate phase equilibrium calculations. A detailed description of this model is given elsewhere [10,11]. The model [9] is briefly based on the equality of fugacity concept, which uses the Valderrama modification of the Patel–Teja equation of state [12] and non-density dependent mixing rules [13] for modeling the fluid phases and the van der Waals and Platteeuw theory [14] is used for modeling the hydrate phase. This model, however, has not been developed for the salt systems studied in this work. As can be observed in figures 2-4, the predicted data show generally less than 0.5 K deviations from the experimental data (less
Experimental data on dissociation conditions of methane hydrates in the presence of NaBr, KBr, CaBr2, K2CO3, and MgCl2 aqueous solutions with different concentrations of single salt and at various temperatures (table 1) were reported in this work. An isochoric pressure-search method [2–4] was used for performing all the measurements. The experimental data on dissociation conditions of methane hydrates in the presence of 0.1 mass fraction MgCl2 aqueous solution were compared with some selected experimental data from the literature [5]. The acceptable agreements between the experimental data measured in this work and the experimental data reported in literature [5] helped to confirm the reliability of the experimental technique and the data generated in this work. Some experimental data were finally compared with the predictions of a general correlation [6] and acceptable agreements were found between the experimental and predicted data. Acknowledgement Ilyas Kraouti would like to thank the CEP-TEP laboratory for the opportunity to work as a trainee. References [1] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, third ed., CRC Press, Taylor & Francis Group, Boca Raton, 2007. [2] W. Afzal, A.H. Mohammadi, D. Richon, J. Chem. Eng. Data 52 (2007) 2053– 2055. [3] A.H. Mohammadi, W. Afzal, D. Richon, J. Chem. Thermodyn. 40 (2008) 1693– 1697. [4] B. Tohidi, R.W. Burgass, A. Danesh, K.K. Østergaard, A.C. Todd, Ann. N.Y. Acad. Sci. 912 (2000) 924–931. [5] S.P. Kang, M.K. Chun, H. Lee, Fluid Phase Equilib. 147 (1998) 229–238. [6] K.K. Østergaard, R. Masoudi, B. Tohidi, A. Danesh, A.C. Todd, J. Pet. Sci. Eng. 48 (2005) 70–80. [7] R. Ohmura, S. Takeya, T. Uchida, T. Ebinuma, Ind. Eng. Chem. Res. 43 (2004) 4964–4966. [8] T. Maekawa, N. Imai, Ann. N.Y. Acad. Sci. (2000) 932–939. [9] Heriot-Watt University Hydrate model:
JCT 08-239