- Email: [email protected]
Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride
Fluid Phase Equilibria xxx (2015) 1e5
Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride Jing Cai a, b, Chun-Gang Xu a, b, Ya-Fei Hu a, b, c, Ya-Long Ding a, b, c, Xiao-Sen Li a, b, * a
Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, PR China c University of Chinese Academy of Sciences, Beijing 100083, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 August 2015 Received in revised form 22 September 2015 Accepted 28 September 2015 Available online xxx
The experimental study on phase equilibrium conditions for semi-clathrate hydrates formed in the systems of CH4/N2 gas mixture and tetra-butyl-ammonium fluoride (TBAF) with different ratios (0.210, 0.293 and 0.500 mol%) to water was conducted in this paper. The experiments were carried out under the conditions of 281.15e291.15 K and 0.30e3.70 MPa. The structures of the hydrates were characterized using in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. At a certain given pressure, the equilibrium temperature of the semi-clathrate hydrates formed in the systems with 0.500 mol% TBAF is significantly higher than that of semi-clathrate hydrates formed the systems with 0.210 or 0.293 mol% TBAF, and even higher than that of structure II hydrates formed in the systems with THF-SDS. Moreover, TBAF has more positive influence on enhancing dissociation enthalpies of the hydrates than THF. Due to the quite weak signal of NeN triple bond vibration or/and seldom N2 molecules encaged into the cavities, only CH4 molecules could be determined clearly in the mixed hydrates. In addition, the crystal morphology of the semi-clathrate hydrates is affected by the ratio of TBAF to water. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrate Phase equilibrium TBAF Semi-clathrate Dissociation enthalpy
1. Introduction Gas Hydrate is a kind of inclusion crystalline compound, which consists of host water molecules and guest molecules such as lightcarbon molecules, dioxide carbon (CO2), hydrogen (H2), nitrogen (N2) cyclopentane (CP), tetrahydrofuran (THF), etc., under the high pressure and/or low temperature [1]. During the hydrate formation, the water molecules construct the various cavities via hydrogen bonds, while the guest molecules encage into the certain cavities via van der Waals forces. The hydrate technology which is environmental friendly and relatively simply has been widely studied in various applications, such as natural gas storage and transportation, gas separation and desalination. Especially, the hydrate-based gas separation has been vastly investigated into CO2 capture from fuel gas (CO2/H2) [2] and flue gas (CO2/N2) [3], and CH4 recovery from biogas (CH4/CO2) [4] and coal mine methane gas mixture (CH4/N2) [3,5e10].
* Corresponding author. Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail address: [email protected] (X.-S. Li).
However, the hydrate-based gas separation is still not suitable for commercial applications owing to the low gas uptake, slow hydrate formation rate and poor separation efficiency. For this purpose, hydrate formation additives are injected into the hydrate formation systems to accelerate the hydrate formation and enhance the gas separation efficiency. Generally, hydrate formation additives are categorized into thermodynamic additives such as tetra-nbutylammonium (TBAB) [7], tetrahydrofuran (THF) [3,8] and cyclopentane (CP) [11], and kinetic additives such as sodium sodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) [3,12]. The thermodynamic additives take part in constructing the hydrate structures, and furthermore, moderating the hydrate formation conditions. For instance, THF and CP molecules encage into the hydrate cavities and make the hydrates present for structure II (sII). Nevertheless, TBAB molecules participates in building the hydrates structure and making the hydrates be semiclathrate (sc) structure. The kinetic promoters have positive influence on increasing the gas/liquid interface, further improving the gas dissolving in the water, and further enhance the gas hydrate formation rate. In our previous work, the combined additives including thermodynamic and kinetic promoter were applied in CH4 recovery from the coal-bed methane (CBM) gas mixture [3,13].
http://dx.doi.org/10.1016/j.fluid.2015.09.054 0378-3812/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
2
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
It was proven that the combined additive containing 1.0 mol% THF and 500 ppm SDS was an appreciate hydrate additive for recovering CH4 from the CBM gas mixture. However, due to the volatile properties of THF, the out entrainment of THF from the releasing gas leaded to the serious loss of THF during the hydrate dissociation. Consequently, the concentration of THF in the recycled reaction solution might be lower than 1.0 mol%, which would bring about the negative effect on the hydrate formation rate and CH4 separation efficiency for the next gas separation process. In order to maintain the additives and warrant the required separation efficiency in the recycled reaction solution, it is necessary to exploit and evaluate other additives for the CH4 recovery from CBM gas mixture via clathrate hydrates. Quaternary ammonium salts (QAS) has been proven to form the semi-clathrate hydrates and have a positive effect on moderating the hydrate equilibrium conditions [7,9,10,14e18]. The structures of the hydrate containing QAS are different from the three typical hydrates (sI, sII and sH) [1]. During the hydrates formation, the anions (e.g., F, Br, Cl, NO-3) take part in building up the cavities of the hydrates together with water molecules while TBAþ are partially encaged into the cavities [19e21]. It is possible that the equilibrium conditions of the CH4/N2/QAS semi-clathrate hydrates are milder than those of the CH4/N2/THF sII hydrates in the process of recovering CH4 from the CBM gas mixture. For developing the application of hydrate-based CH4 separation technology from the CBM gas mixture, this paper represents the basic phase equilibrium data of the semi-clathrate hydrates obtained in the systems consisted of CH4/N2 gas mixture and TBAF with different concentrations. Moreover, the hydrate structures of the TBAF/CH4/N2 hydrates are characterized by in-situ Raman spectrometer and the dissociation enthalpies of these hydrates are calculated by the ClausiuseClapeyron equation based on the data of the hydrate phase equilibrium. 2. Experimental section 2.1. Materials The gas mixture with composition of 50 mol% CH4 and 50 mol% N2 was supplied by Foshan Huate Special Gas Co., Ltd. Such gas mixture was adopted to simulate the typical CBM gas mixture collected from the coal mine [3]. TBAF (75% in water) was supplied by Aladdin Industrial Co., Ltd. The deionized water was produced by an ultrapure water system with a resistivity of 18.25 mU$cm1, and used in all experimental runs. 2.2. Apparatus The experimental apparatus to perform the hydrate equilibrium conditions are illustrated in Fig. 1. For Fig. 1, it shows the semi-batch experimental apparatus with stirring, SHD-I, employed to perform the equilibrium experiments to determine equilibrium conditions of the hydrates containing TBAF. The experimental apparatus is comprised of supply vessel (SV) with 1350 ml and hydrate crystallizer (CR) with 336 ml. SV and CR are both made by 316 stainless steel and immersed into the water bath with the controlled temperature. Especially, two visible windows are located on the front and back of the CR and used to observe the hydrate formation process. In the CR, the gas and liquid solution are mixed thoroughly by a magnetic stirring at the speed of 500 rpm. The maximum operation pressure of the SV and CR are 25 MPa, and the pressures of the SV and CR are measured using two Setra smart pressure transducers (model SS2, Boxborough, MA) with an uncertainty of 0.02 MPa. The temperatures in the CR are measured with Pt1000 thermocouples (JM6081) with an accuracy of 0.1 K. All above data of
Fig. 1. Schematic of experimental apparatus.
pressures and temperature are acquired by a data acquisition system, and automatically recorded by a personal computer (PC). In addition, the hydrate structures are measured by another apparatus combining with an in-situ Raman spectrometer, which was described in our previous work [22]. 2.3. Measurement procedure The isochoric temperature method is employed to measure the hydrate formation equilibrium conditions, and the corresponding conditions are determined according to the temperature point of the last hydrate crystal dissociating. The solution of 180 ml is added into the hydrate formation CR. Subsequently, the CR is evacuated to air free. Then, under the setting temperature, the pressure is introduced to an operation pressure which is 2e3 MPa higher than the expected pressure to form the hydrates. During the hydrate formation, the content in the CR is mixed by a magnetic stirrer. Once the hydrates are formed, the temperature begins to be increased gradually to the point of the last hydrate crystal dissociation. The point when the last hydrate crystal disappearing completely is defined as the hydrate phase equilibrium point correspondingly. After that, the temperature is lowed to the formation point again as same as the above mentioned one, and the same process of hydrate dissociation are repeated three times. The Raman spectrometer (Horiba, LabRAM HR) with a single Mono Chromator of 1800 grooves/mm grating and a multichannel air-cooled CCD (charge coupled device) detector is employed to determine the structures of the CH4/N2 mixed hydrates containing TBAF. A 532 nm incident laser beam was used. The Ar-ion laser was focused on the sample by a 1 microscope objective. The spectroscopic data are detected by a CCD detector with an energy resolution of 100 mW and recorded with a 10 s integration time over 2e5 scans. The silicon (Si) crystal standard of 520.7 cm1 is employed to calibrate the subtractive spectrograph. 3. Results and discussion 3.1. Phase equilibrium measurements The reliability and accuracy of the experimental study were
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
Where a, b, x and n are the mole number of methane, nitrogen, the additive and water in hydrate crystalline, respectively. The hydrate dissociation enthalpies are calculated by ClausiuseClapeyron equation based on the phase equilibrium data [24e26]. dlnP/d(1/T)¼ DdissHm/ (zR) Where P and T are the absolution pressure and temperature of hydrate equilibrium; z is the compressibility factor of the CH4/N2 35 30
P (MPa)
25 20 15 10 5
274
276
278
280
282
284
286
288
290
System
T/K
P/MPa
DdissHm/kJ$mol1
0.210 mol% TBAF/CH4/N2
281.55 282.55 283.25 284.05 283.25 283.95 285.35 286.95 286.35 287.35 289.45 290.35 281.35 284.45 286.35 290.75
0.81 1.47 1.95 2.69 0.72 0.86 1.87 3.20 0.30 0.52 1.21 2.14 0.95 1.66 2.20 3.68
312.0003 307.8131 304.8769 300.4785 314.3773 313.5742 307.4090 299.8683 324.2049 322.8434 318.7492 313.3884 95.25780 93.97559 93.06668 90.85228
0.293 mol% TBAF/CH4/N2
0.500 mol% TBAF/CH4/N2
1.000 mol% THF-SDS/CH4/N2
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
aCH4 þ bN2 þ xTBAF þ nH2 04aCH4 $bN2 $xTBAF$nH2 0
0 272
Table 1 Three-Phase (L-V-H) Equilibrium Data and the Dissociation Enthalpies of CH4/N2 mixed hydrates containing TBAF with various ratios.
P (MPa)
examined by measuring the equilibrium data of the CH4/N2 binary hydrates in pure water system. The experimental equilibrium conditions obtained and the equilibrium data referred to the literature are shown in Fig. 2. It can be found that the data are in agreement with literature data [23]. It illustrates that the validity of experimental apparatus and the method are feasible. The equilibrium conditions of the CH4/N2 hydrates in the systems with various ratios of TBAF to water are listed in Table 1. The ratios of TBAF to water adopted in our measurements are 0.210, 0.293 and 0.500 mol%, respectively. Meanwhile, the curve of the measured equilibrium pressures against temperatures is also plotted in Fig. 3. In comparison with Fig. 2, it can be clearly observed that the equilibrium conditions of the CH4/N2 mixed hydrates are drastically moderated by adding hydrate additive TBAF. Moreover, the equilibrium pressures decrease or the equilibrium temperatures increase with increase of the concentrations of TBAF. The high concentration of TBAF is benefit for moderating the equilibrium conditions of the CH4/N2 mixed hydrates. Considering the possibility of TBAF in the CH4 recovery from the CBM gas mixture, the equilibrium data of the THF/CH4/N2 sII hydrates, which was adopted to concentrate CH4 in the CBM gas mixture in our previous work [3], are also plotted in Fig. 3. The figure illustrates that the equilibrium conditions of TBAF/CH4/N2 hydrates formed in the systems with 0.500 mol% TBAF can be moderated more drastically in comparison with those of the THF/CH4/N2 sII hydrates formed in the system by addition of THF-SDS. However, the systems with the TBAF of 0.210, 0.293 mol% fail to perform the same promotion effect on moderating the equilibrium conditions as that of 0.500 mol%. Therefore, the systems with 0.500 mol% TBAF have a potential to be adopted for recovering CH4 from the CBM gas mixture. The model of TBAF/CH4/N2 hydrates formation and dissociation can be described as following:
3
292
T (K) Fig. 2. Equilibrium conditions of the CH4/N2 binary hydrates in pure water in this work and previous work; -, 50 mol% CH4/N2 in this work; :, 50.25 mol% CH4/N2 in Ref. [23].
0.0 280
282
284
286
288
290
292
T (K) Fig. 3. Equilibrium conditions of the CH4/N2 mixed hydrates containing TBAF with various ratios in comparison with that containing THF; ,, 0.210 mol% TBAF; B, 0.293 mol% TBAF; *, 0.500 mol% TBAF; 7, 1.000 mol% THF-SDS.
gas mixture at each temperature and pressure, which is calculated by the SRK equation; R is the universal gas constant with the value of 8.3145 J$mol1 K1; DdissHm is the dissociation enthalpy of the hydrates. Based on the equilibrium data listed in Table 1, the curves of natural logarithms of pressure (ln P) versus reciprocal of temperature (1/T) are plotted, as shown in Fig. 4. It can be observed that the good linear relationships are shown between ln P and 1/T in different systems. Therefore, the dissociation enthalpies of the hydrate can be determined by the slopes (k) of the lines in Fig. 4 and calculated in terms of ClausiuseClapeyron equation. The values of the dissociation enthalpies are tabulated in Table 1. It can be found TBAF has more positive influence on enhancing the dissociation enthalpies of the hydrate than THF, and the equilibrium condition of the CH4/N2 mixed hydrates can be moderated by adding TBAF. Moreover, the dissociation enthalpies of the TBAF/CH4/N2 mixed hydrates are increased with increase of the concentration of TBAF. Especially, the highest dissociation enthalpy of the TBAF/CH4/N2 mixed hydrates is 324.2049 kJ$mol1 under the equilibrium condition of 286.35 K and 0.30 MPa with the concentration of 0.500 mol%. Therefore, the TBAF is a better thermodynamic hydrate promoter to replace THF, and the TBAF of 0.500 mol% can be adopted to recover CH4 from the CBM gas mixture.
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
4
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
2.5
1.5
Intensity (a. u.)
1.0 0.5
lnP
A
R-square=0.98285 R-square=0.97235 R-square=0.99008 R-square=0.98279
2.0
0.0 -0.5
II
-1.0 -1.5 -2.0 0.00340
I 0.00344
0.00348
0.00352
0.00356
0.00360
1000
1500
-1
1/T (K )
2000
2500
3000
3500
4000
-1
Raman shift (cm )
Fig. 4. Natural logarithms of pressures (ln P) versus reciprocals of temperature (1/T) for the CH4/N2 mixed hydrates in different systems; ,, 0.210 mol% TBAF, k ¼ 38170.82503, r ¼ 135.41017; B, 0.293 mol% TBAF, k ¼ 38381.60381, r ¼ 134.99055; *, 0.500 mol% TBAF, k ¼ 39224.33017, r ¼ 135.79604; △, 1.000 mol% THF-SDS, k ¼ 11689.18914, r ¼ 41.55315.
B
*
V
In-situ Raman spectroscopy is employed to characterize the hydrate structures of the TBAF/CH4/N2 hydrates and determine the existence of gas molecules in the mixed hydrates under the condition of 276.15 K and 2.50 MPa. The Raman spectra for the semiclathrate hydrates in the CH4/N2 and TBAF systems are shown in Fig. 5. Fig. 5 (A) shows the Raman spectra for the CH4/N2 gas mixture in gas phase and the Raman spectra for TBAF aqueous solution. Fig. 5(B) shows the mixed hydrates formed in the systems within different ratios of TBAF to water. As shown in Fig. 5(A), the peaks at around 2320 cm1 and 2915 cm1 are ascribed to NeN triple bond vibration of N2 molecules and CeH vibration of CH4 molecules, respectively [22,27]. Additionally, the Raman peaks appearing at around 700e1600 cm1 and 2600e3000 cm1 correspond to TBAF molecules in aqueous phase, as shown in Fig. 5(A) [28]. From Fig. 5(B), these typical Raman peaks of TBAF molecules are clearly observed from the Raman spectra for the mixed hydrates at around 700e1600 cm1 and 2600e3000 cm1 in this work. Moreover, no obvious shift can be found for indicating the difference of the concentrations of TBAF. As shown in Fig. 5, the Raman peaks at around 2915 cm1 corresponding to CeH vibration of CH4 molecules. It illustrates that CH4 molecules are encaged into the semiclathrate hydrates [20,22,28]. However, no remarkable Raman peaks corresponding to N2 molecules are found in semi-clathrate hydrates [27]. On the one hand, it demonstrates that the most of the encaged molecules are CH4 molecules rather than N2 molecules in the mixed hydrates because of that the equilibrium conditions of CH4 hydrates are significantly milder than those of N2 hydrates [23]. On the other hand, the signals of NeN triple bond vibration are too weak to be detected using Raman spectrometer. In addition, the broad peaks at around 3000e3600 cm1 are ascribed to OeH vibration of host water lattice, and the peak shape is affected by the crystallinity of the mixed hydrates in some extent. Generally, the larger crystallinity of the mixed hydrates leads to the sharper peak shape. Therefore, as shown in Fig. 5, the crystallinity of the mixed hydrates decrease with increase of the concentrations of TBAF. Such phenomenon can also be observed from Fig. 6. Fig. 6 shows the crystal morphologies of the TBAF/CH4/N2 and mixed hydrates obtained with different concentrations of TBAF after a 10 h
Intensity (a. u.)
* 3.2. Raman spectroscopic measurements
IV
*
III
1000
1500
2000
2500
3000
3500
4000
-1
Raman shift (cm ) Fig. 5. Raman spectra for CH4/N2 in gas phase and for the semi-clathrate hydrates obtained in various TBAF under 276.15 K and 2.50 MPa. A: I, CH4/N2 in gas phase; II, 0.500 mol% TBAF pure aqueous solution; B: III, 0.210 mol% TBAF; IV, 0.293 mol% TBAF; V, 0.500 mol% TBAF. The inverted star represents the peaks ascribed to CeH vibrations of CH4.
reaction. It is confirmed that crystal size of the mixed hydrates is changed with the concentrations of TBAF to water. Moreover, the crystal morphology of signal crystal is consistent with that reported by S. Hashimoto et al. [29]. 4. Conclusion The three phase (L-V-H) equilibrium conditions are measured for the systems of CH4/N2 gas mixture in the presence of TBAF in the conditions of 281.15e291.15 K and 0.30e3.70 MPa, and the structures of the CH4/N2 mixed hydrates containing TBAF are characterized by in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. The experimental results illustrate that the phase equilibrium conditions of the semi-clathrate hydrates formed in the system with 0.500 mol% TBAF is the mildest condition. The dissociation enthalpies of the CH4/N2 mixed hydrates containing TBAF, determined by ClausiuseClapeyron equation on the basis of the equilibrium pressures and temperatures, are significantly higher than those obtained in the systems within THF-SDS. In addition, TBAF/ CH4 semi-clathrate hydrates, instead of TBAF/CH4/N2 semiclathrate hydrates, are formed in the CH4/N2 and TBAF systems at
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
5
Fig. 6. Photos of hydrate crystals for the TBAF/CH4 semi-clathrate hydrates formed under 276.15 K and 2.50 MPa after 10 h; I, 0.210 mol% TBAF; II, 0.293 mol% TBAF; III, 0.500 mol% TBAF. Each label corresponds to that of Fig. 5. The rim that looks yellow is the seal material part for connecting the quartz windows and the stainless steel. The single crystals can be observed in the aqueous solution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
276.15 K and 2.50 MPa. Moreover, the concentrations of TBAF have significant effects on the crystal morphology and crystallinity of the semi-clathrate hydrates. Acknowledgement This work was supported by National Science Fund for Distinguished Young Scholars of China (51225603), National Natural Science Fund (51376184), International S&T Cooperation Program of China (2015DFA61790) and Open Fund of HUST (HKDGH2013001), which are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. Englezos, Ind. Eng. Chem. Res. 32 (1993) 1251e1274. X.-S. Li, C.-G. Xu, Z.-Y. Chen, H.-J. Wu, Energy 36 (2011) 1394e1403. X.-S. Li, J. Cai, Z.-Y. Chen, C.-G. Xu, Energ Fuel 26 (2012) 1144e1151. Z.M. Xia, Z.Y. Chen, X.S. Li, Y. Zhang, K.F. Yan, Q.N. Lv, C.G. Xu, J. Cai, J. Chem. Eng. Data 57 (2012) 3290e3295. C.J. Bibler, J.S. Marshall, R.C. Pilcher, Int. J. Coal Geol. 35 (1998) 283e310. P.L. Guo, M. Chem, Ind. Eng. Process 27 (2008) 963e967. Q. Sun, X. Guo, A. Liu, B. Liu, Y. Huo, G. Chen, Ind. Eng. Chem. Res. 50 (2010) 2284e2288. B.Y. Zhang, Q. Wu, Energ Fuel 24 (2010) 2530e2535. D.L. Zhong, P. Englezos, Energ Fuel 26 (2012) 2098e2106. D.L. Zhong, Y. Ye, C. Yang, Y. Bian, K. Ding, Ind. Eng. Chem. Res. 51 (2012) 14806e14813.
[11] D.-L. Zhong, N. Daraboina, P. Englezos, Fuel 106 (2013) 425e430. [12] H. Ganji, M. Manteghian, K.S. Zadeh, M.R. Omidkhah, H.R. Mofrad, Fuel 86 (2007) 434e441. [13] J. Cai, C. Xu, C. Chen, Z. Chen, X. Li 61 (2014) 812e816. [14] W. Shimada, T. Ebinuma, H. Oyama, Y. Kamata, S. Takeya, T. Uchida, J. Nagao, H. Narita, Jpn. J. Appl. Phys. 2 (42) (2003) 129e131. [15] V.Y. Komarov, T.V. Rodionova, I.S. Terekhova, N.V. Kuratieva, J. Incl. Phenom. Macro 59 (2007) 11e15. [16] S.F. Li, S.S. Fan, J.Q. Wang, X.M. Lang, Y.H. Wang, J. Chem. Eng. Data 55 (2010) 3212e3215. [17] Z.X. Liao, X.Q. Guo, Y.Y. Zhao, Y.W. Wang, Q. Sun, A.X. Liu, C.Y. Sun, G.J. Chen, Ind. Eng. Chem. Res. 52 (2013) 18440e18446. [18] A. Mohammadi, M. Manteghian, A.H. Mohammadi, Fluid Phase Equilib. 381 (2014) 102e107. [19] T. Makino, T. Yamamoto, K. Nagata, H. Sakamoto, S. Hashimoto, T. Sugahara, K. Ohgaki, J. Chem. Eng. Data 55 (2010) 839e841. [20] J. Sakamoto, S. Hashimoto, T. Tsuda, T. Sugahara, Y. Inoue, K. Ohgaki, Chem. Eng. Sci. 63 (2008) 5789e5794. [21] K. Sato, H. Tokutomi, R. Ohmura, Fluid Phase Equilib. 337 (2013) 115e118. [22] C.G. Xu, J. Cai, F.H. Lin, Z.Y. Chen, X.S. Li, Energy 79 (2015) 111e116. [23] D.B.R. Jaysukh Jhaveri, Can. J. Chem. Eng. 43 (1965) 75e78. [24] A.T. Trueba, L.J. Rovetto, L.J. Florusse, M.C. Kroon, C.J. Peters, Fluid Phase Equilib. 307 (2011) 6e10. [25] P. Babu, T. Yang, H.P. Veluswamy, R. Kumar, P. Linga, J. Chem. Thermodyn. 61 (2013) 58e63. [26] Q.N. Lv, X.S. Li, Z.Y. Chen, J.C. Feng, J. Chem. Eng. Data 58 (2013) 3249e3253. [27] E. Kim, Y.K. Jin, Y. Seo, Fluid Phase Equilib. 393 (2015) 85e90. [28] S. Lee, Y. Lee, S. Park, Y. Kim, J.D. Lee, Y. Seo, J. Phys. Chem. B 116 (2012) 9075e9081. [29] S. Hashimoto, S. Murayama, T. Sugahara, H. Sato, K. Ohgaki, Chem. Eng. Sci. 61 (2006) 7884e7888.
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride Jing Cai a, b, Chun-Gang Xu a, b, Ya-Fei Hu a, b, c, Ya-Long Ding a, b, c, Xiao-Sen Li a, b, * a
Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, PR China c University of Chinese Academy of Sciences, Beijing 100083, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 August 2015 Received in revised form 22 September 2015 Accepted 28 September 2015 Available online xxx
The experimental study on phase equilibrium conditions for semi-clathrate hydrates formed in the systems of CH4/N2 gas mixture and tetra-butyl-ammonium fluoride (TBAF) with different ratios (0.210, 0.293 and 0.500 mol%) to water was conducted in this paper. The experiments were carried out under the conditions of 281.15e291.15 K and 0.30e3.70 MPa. The structures of the hydrates were characterized using in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. At a certain given pressure, the equilibrium temperature of the semi-clathrate hydrates formed in the systems with 0.500 mol% TBAF is significantly higher than that of semi-clathrate hydrates formed the systems with 0.210 or 0.293 mol% TBAF, and even higher than that of structure II hydrates formed in the systems with THF-SDS. Moreover, TBAF has more positive influence on enhancing dissociation enthalpies of the hydrates than THF. Due to the quite weak signal of NeN triple bond vibration or/and seldom N2 molecules encaged into the cavities, only CH4 molecules could be determined clearly in the mixed hydrates. In addition, the crystal morphology of the semi-clathrate hydrates is affected by the ratio of TBAF to water. © 2015 Elsevier B.V. All rights reserved.
Keywords: Hydrate Phase equilibrium TBAF Semi-clathrate Dissociation enthalpy
1. Introduction Gas Hydrate is a kind of inclusion crystalline compound, which consists of host water molecules and guest molecules such as lightcarbon molecules, dioxide carbon (CO2), hydrogen (H2), nitrogen (N2) cyclopentane (CP), tetrahydrofuran (THF), etc., under the high pressure and/or low temperature [1]. During the hydrate formation, the water molecules construct the various cavities via hydrogen bonds, while the guest molecules encage into the certain cavities via van der Waals forces. The hydrate technology which is environmental friendly and relatively simply has been widely studied in various applications, such as natural gas storage and transportation, gas separation and desalination. Especially, the hydrate-based gas separation has been vastly investigated into CO2 capture from fuel gas (CO2/H2) [2] and flue gas (CO2/N2) [3], and CH4 recovery from biogas (CH4/CO2) [4] and coal mine methane gas mixture (CH4/N2) [3,5e10].
* Corresponding author. Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China. E-mail address: [email protected] (X.-S. Li).
However, the hydrate-based gas separation is still not suitable for commercial applications owing to the low gas uptake, slow hydrate formation rate and poor separation efficiency. For this purpose, hydrate formation additives are injected into the hydrate formation systems to accelerate the hydrate formation and enhance the gas separation efficiency. Generally, hydrate formation additives are categorized into thermodynamic additives such as tetra-nbutylammonium (TBAB) [7], tetrahydrofuran (THF) [3,8] and cyclopentane (CP) [11], and kinetic additives such as sodium sodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) [3,12]. The thermodynamic additives take part in constructing the hydrate structures, and furthermore, moderating the hydrate formation conditions. For instance, THF and CP molecules encage into the hydrate cavities and make the hydrates present for structure II (sII). Nevertheless, TBAB molecules participates in building the hydrates structure and making the hydrates be semiclathrate (sc) structure. The kinetic promoters have positive influence on increasing the gas/liquid interface, further improving the gas dissolving in the water, and further enhance the gas hydrate formation rate. In our previous work, the combined additives including thermodynamic and kinetic promoter were applied in CH4 recovery from the coal-bed methane (CBM) gas mixture [3,13].
http://dx.doi.org/10.1016/j.fluid.2015.09.054 0378-3812/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
2
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
It was proven that the combined additive containing 1.0 mol% THF and 500 ppm SDS was an appreciate hydrate additive for recovering CH4 from the CBM gas mixture. However, due to the volatile properties of THF, the out entrainment of THF from the releasing gas leaded to the serious loss of THF during the hydrate dissociation. Consequently, the concentration of THF in the recycled reaction solution might be lower than 1.0 mol%, which would bring about the negative effect on the hydrate formation rate and CH4 separation efficiency for the next gas separation process. In order to maintain the additives and warrant the required separation efficiency in the recycled reaction solution, it is necessary to exploit and evaluate other additives for the CH4 recovery from CBM gas mixture via clathrate hydrates. Quaternary ammonium salts (QAS) has been proven to form the semi-clathrate hydrates and have a positive effect on moderating the hydrate equilibrium conditions [7,9,10,14e18]. The structures of the hydrate containing QAS are different from the three typical hydrates (sI, sII and sH) [1]. During the hydrates formation, the anions (e.g., F, Br, Cl, NO-3) take part in building up the cavities of the hydrates together with water molecules while TBAþ are partially encaged into the cavities [19e21]. It is possible that the equilibrium conditions of the CH4/N2/QAS semi-clathrate hydrates are milder than those of the CH4/N2/THF sII hydrates in the process of recovering CH4 from the CBM gas mixture. For developing the application of hydrate-based CH4 separation technology from the CBM gas mixture, this paper represents the basic phase equilibrium data of the semi-clathrate hydrates obtained in the systems consisted of CH4/N2 gas mixture and TBAF with different concentrations. Moreover, the hydrate structures of the TBAF/CH4/N2 hydrates are characterized by in-situ Raman spectrometer and the dissociation enthalpies of these hydrates are calculated by the ClausiuseClapeyron equation based on the data of the hydrate phase equilibrium. 2. Experimental section 2.1. Materials The gas mixture with composition of 50 mol% CH4 and 50 mol% N2 was supplied by Foshan Huate Special Gas Co., Ltd. Such gas mixture was adopted to simulate the typical CBM gas mixture collected from the coal mine [3]. TBAF (75% in water) was supplied by Aladdin Industrial Co., Ltd. The deionized water was produced by an ultrapure water system with a resistivity of 18.25 mU$cm1, and used in all experimental runs. 2.2. Apparatus The experimental apparatus to perform the hydrate equilibrium conditions are illustrated in Fig. 1. For Fig. 1, it shows the semi-batch experimental apparatus with stirring, SHD-I, employed to perform the equilibrium experiments to determine equilibrium conditions of the hydrates containing TBAF. The experimental apparatus is comprised of supply vessel (SV) with 1350 ml and hydrate crystallizer (CR) with 336 ml. SV and CR are both made by 316 stainless steel and immersed into the water bath with the controlled temperature. Especially, two visible windows are located on the front and back of the CR and used to observe the hydrate formation process. In the CR, the gas and liquid solution are mixed thoroughly by a magnetic stirring at the speed of 500 rpm. The maximum operation pressure of the SV and CR are 25 MPa, and the pressures of the SV and CR are measured using two Setra smart pressure transducers (model SS2, Boxborough, MA) with an uncertainty of 0.02 MPa. The temperatures in the CR are measured with Pt1000 thermocouples (JM6081) with an accuracy of 0.1 K. All above data of
Fig. 1. Schematic of experimental apparatus.
pressures and temperature are acquired by a data acquisition system, and automatically recorded by a personal computer (PC). In addition, the hydrate structures are measured by another apparatus combining with an in-situ Raman spectrometer, which was described in our previous work [22]. 2.3. Measurement procedure The isochoric temperature method is employed to measure the hydrate formation equilibrium conditions, and the corresponding conditions are determined according to the temperature point of the last hydrate crystal dissociating. The solution of 180 ml is added into the hydrate formation CR. Subsequently, the CR is evacuated to air free. Then, under the setting temperature, the pressure is introduced to an operation pressure which is 2e3 MPa higher than the expected pressure to form the hydrates. During the hydrate formation, the content in the CR is mixed by a magnetic stirrer. Once the hydrates are formed, the temperature begins to be increased gradually to the point of the last hydrate crystal dissociation. The point when the last hydrate crystal disappearing completely is defined as the hydrate phase equilibrium point correspondingly. After that, the temperature is lowed to the formation point again as same as the above mentioned one, and the same process of hydrate dissociation are repeated three times. The Raman spectrometer (Horiba, LabRAM HR) with a single Mono Chromator of 1800 grooves/mm grating and a multichannel air-cooled CCD (charge coupled device) detector is employed to determine the structures of the CH4/N2 mixed hydrates containing TBAF. A 532 nm incident laser beam was used. The Ar-ion laser was focused on the sample by a 1 microscope objective. The spectroscopic data are detected by a CCD detector with an energy resolution of 100 mW and recorded with a 10 s integration time over 2e5 scans. The silicon (Si) crystal standard of 520.7 cm1 is employed to calibrate the subtractive spectrograph. 3. Results and discussion 3.1. Phase equilibrium measurements The reliability and accuracy of the experimental study were
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
Where a, b, x and n are the mole number of methane, nitrogen, the additive and water in hydrate crystalline, respectively. The hydrate dissociation enthalpies are calculated by ClausiuseClapeyron equation based on the phase equilibrium data [24e26]. dlnP/d(1/T)¼ DdissHm/ (zR) Where P and T are the absolution pressure and temperature of hydrate equilibrium; z is the compressibility factor of the CH4/N2 35 30
P (MPa)
25 20 15 10 5
274
276
278
280
282
284
286
288
290
System
T/K
P/MPa
DdissHm/kJ$mol1
0.210 mol% TBAF/CH4/N2
281.55 282.55 283.25 284.05 283.25 283.95 285.35 286.95 286.35 287.35 289.45 290.35 281.35 284.45 286.35 290.75
0.81 1.47 1.95 2.69 0.72 0.86 1.87 3.20 0.30 0.52 1.21 2.14 0.95 1.66 2.20 3.68
312.0003 307.8131 304.8769 300.4785 314.3773 313.5742 307.4090 299.8683 324.2049 322.8434 318.7492 313.3884 95.25780 93.97559 93.06668 90.85228
0.293 mol% TBAF/CH4/N2
0.500 mol% TBAF/CH4/N2
1.000 mol% THF-SDS/CH4/N2
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
aCH4 þ bN2 þ xTBAF þ nH2 04aCH4 $bN2 $xTBAF$nH2 0
0 272
Table 1 Three-Phase (L-V-H) Equilibrium Data and the Dissociation Enthalpies of CH4/N2 mixed hydrates containing TBAF with various ratios.
P (MPa)
examined by measuring the equilibrium data of the CH4/N2 binary hydrates in pure water system. The experimental equilibrium conditions obtained and the equilibrium data referred to the literature are shown in Fig. 2. It can be found that the data are in agreement with literature data [23]. It illustrates that the validity of experimental apparatus and the method are feasible. The equilibrium conditions of the CH4/N2 hydrates in the systems with various ratios of TBAF to water are listed in Table 1. The ratios of TBAF to water adopted in our measurements are 0.210, 0.293 and 0.500 mol%, respectively. Meanwhile, the curve of the measured equilibrium pressures against temperatures is also plotted in Fig. 3. In comparison with Fig. 2, it can be clearly observed that the equilibrium conditions of the CH4/N2 mixed hydrates are drastically moderated by adding hydrate additive TBAF. Moreover, the equilibrium pressures decrease or the equilibrium temperatures increase with increase of the concentrations of TBAF. The high concentration of TBAF is benefit for moderating the equilibrium conditions of the CH4/N2 mixed hydrates. Considering the possibility of TBAF in the CH4 recovery from the CBM gas mixture, the equilibrium data of the THF/CH4/N2 sII hydrates, which was adopted to concentrate CH4 in the CBM gas mixture in our previous work [3], are also plotted in Fig. 3. The figure illustrates that the equilibrium conditions of TBAF/CH4/N2 hydrates formed in the systems with 0.500 mol% TBAF can be moderated more drastically in comparison with those of the THF/CH4/N2 sII hydrates formed in the system by addition of THF-SDS. However, the systems with the TBAF of 0.210, 0.293 mol% fail to perform the same promotion effect on moderating the equilibrium conditions as that of 0.500 mol%. Therefore, the systems with 0.500 mol% TBAF have a potential to be adopted for recovering CH4 from the CBM gas mixture. The model of TBAF/CH4/N2 hydrates formation and dissociation can be described as following:
3
292
T (K) Fig. 2. Equilibrium conditions of the CH4/N2 binary hydrates in pure water in this work and previous work; -, 50 mol% CH4/N2 in this work; :, 50.25 mol% CH4/N2 in Ref. [23].
0.0 280
282
284
286
288
290
292
T (K) Fig. 3. Equilibrium conditions of the CH4/N2 mixed hydrates containing TBAF with various ratios in comparison with that containing THF; ,, 0.210 mol% TBAF; B, 0.293 mol% TBAF; *, 0.500 mol% TBAF; 7, 1.000 mol% THF-SDS.
gas mixture at each temperature and pressure, which is calculated by the SRK equation; R is the universal gas constant with the value of 8.3145 J$mol1 K1; DdissHm is the dissociation enthalpy of the hydrates. Based on the equilibrium data listed in Table 1, the curves of natural logarithms of pressure (ln P) versus reciprocal of temperature (1/T) are plotted, as shown in Fig. 4. It can be observed that the good linear relationships are shown between ln P and 1/T in different systems. Therefore, the dissociation enthalpies of the hydrate can be determined by the slopes (k) of the lines in Fig. 4 and calculated in terms of ClausiuseClapeyron equation. The values of the dissociation enthalpies are tabulated in Table 1. It can be found TBAF has more positive influence on enhancing the dissociation enthalpies of the hydrate than THF, and the equilibrium condition of the CH4/N2 mixed hydrates can be moderated by adding TBAF. Moreover, the dissociation enthalpies of the TBAF/CH4/N2 mixed hydrates are increased with increase of the concentration of TBAF. Especially, the highest dissociation enthalpy of the TBAF/CH4/N2 mixed hydrates is 324.2049 kJ$mol1 under the equilibrium condition of 286.35 K and 0.30 MPa with the concentration of 0.500 mol%. Therefore, the TBAF is a better thermodynamic hydrate promoter to replace THF, and the TBAF of 0.500 mol% can be adopted to recover CH4 from the CBM gas mixture.
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
4
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
2.5
1.5
Intensity (a. u.)
1.0 0.5
lnP
A
R-square=0.98285 R-square=0.97235 R-square=0.99008 R-square=0.98279
2.0
0.0 -0.5
II
-1.0 -1.5 -2.0 0.00340
I 0.00344
0.00348
0.00352
0.00356
0.00360
1000
1500
-1
1/T (K )
2000
2500
3000
3500
4000
-1
Raman shift (cm )
Fig. 4. Natural logarithms of pressures (ln P) versus reciprocals of temperature (1/T) for the CH4/N2 mixed hydrates in different systems; ,, 0.210 mol% TBAF, k ¼ 38170.82503, r ¼ 135.41017; B, 0.293 mol% TBAF, k ¼ 38381.60381, r ¼ 134.99055; *, 0.500 mol% TBAF, k ¼ 39224.33017, r ¼ 135.79604; △, 1.000 mol% THF-SDS, k ¼ 11689.18914, r ¼ 41.55315.
B
*
V
In-situ Raman spectroscopy is employed to characterize the hydrate structures of the TBAF/CH4/N2 hydrates and determine the existence of gas molecules in the mixed hydrates under the condition of 276.15 K and 2.50 MPa. The Raman spectra for the semiclathrate hydrates in the CH4/N2 and TBAF systems are shown in Fig. 5. Fig. 5 (A) shows the Raman spectra for the CH4/N2 gas mixture in gas phase and the Raman spectra for TBAF aqueous solution. Fig. 5(B) shows the mixed hydrates formed in the systems within different ratios of TBAF to water. As shown in Fig. 5(A), the peaks at around 2320 cm1 and 2915 cm1 are ascribed to NeN triple bond vibration of N2 molecules and CeH vibration of CH4 molecules, respectively [22,27]. Additionally, the Raman peaks appearing at around 700e1600 cm1 and 2600e3000 cm1 correspond to TBAF molecules in aqueous phase, as shown in Fig. 5(A) [28]. From Fig. 5(B), these typical Raman peaks of TBAF molecules are clearly observed from the Raman spectra for the mixed hydrates at around 700e1600 cm1 and 2600e3000 cm1 in this work. Moreover, no obvious shift can be found for indicating the difference of the concentrations of TBAF. As shown in Fig. 5, the Raman peaks at around 2915 cm1 corresponding to CeH vibration of CH4 molecules. It illustrates that CH4 molecules are encaged into the semiclathrate hydrates [20,22,28]. However, no remarkable Raman peaks corresponding to N2 molecules are found in semi-clathrate hydrates [27]. On the one hand, it demonstrates that the most of the encaged molecules are CH4 molecules rather than N2 molecules in the mixed hydrates because of that the equilibrium conditions of CH4 hydrates are significantly milder than those of N2 hydrates [23]. On the other hand, the signals of NeN triple bond vibration are too weak to be detected using Raman spectrometer. In addition, the broad peaks at around 3000e3600 cm1 are ascribed to OeH vibration of host water lattice, and the peak shape is affected by the crystallinity of the mixed hydrates in some extent. Generally, the larger crystallinity of the mixed hydrates leads to the sharper peak shape. Therefore, as shown in Fig. 5, the crystallinity of the mixed hydrates decrease with increase of the concentrations of TBAF. Such phenomenon can also be observed from Fig. 6. Fig. 6 shows the crystal morphologies of the TBAF/CH4/N2 and mixed hydrates obtained with different concentrations of TBAF after a 10 h
Intensity (a. u.)
* 3.2. Raman spectroscopic measurements
IV
*
III
1000
1500
2000
2500
3000
3500
4000
-1
Raman shift (cm ) Fig. 5. Raman spectra for CH4/N2 in gas phase and for the semi-clathrate hydrates obtained in various TBAF under 276.15 K and 2.50 MPa. A: I, CH4/N2 in gas phase; II, 0.500 mol% TBAF pure aqueous solution; B: III, 0.210 mol% TBAF; IV, 0.293 mol% TBAF; V, 0.500 mol% TBAF. The inverted star represents the peaks ascribed to CeH vibrations of CH4.
reaction. It is confirmed that crystal size of the mixed hydrates is changed with the concentrations of TBAF to water. Moreover, the crystal morphology of signal crystal is consistent with that reported by S. Hashimoto et al. [29]. 4. Conclusion The three phase (L-V-H) equilibrium conditions are measured for the systems of CH4/N2 gas mixture in the presence of TBAF in the conditions of 281.15e291.15 K and 0.30e3.70 MPa, and the structures of the CH4/N2 mixed hydrates containing TBAF are characterized by in-situ Raman spectroscopy at 276.15 K and 2.50 MPa. The experimental results illustrate that the phase equilibrium conditions of the semi-clathrate hydrates formed in the system with 0.500 mol% TBAF is the mildest condition. The dissociation enthalpies of the CH4/N2 mixed hydrates containing TBAF, determined by ClausiuseClapeyron equation on the basis of the equilibrium pressures and temperatures, are significantly higher than those obtained in the systems within THF-SDS. In addition, TBAF/ CH4 semi-clathrate hydrates, instead of TBAF/CH4/N2 semiclathrate hydrates, are formed in the CH4/N2 and TBAF systems at
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054
J. Cai et al. / Fluid Phase Equilibria xxx (2015) 1e5
5
Fig. 6. Photos of hydrate crystals for the TBAF/CH4 semi-clathrate hydrates formed under 276.15 K and 2.50 MPa after 10 h; I, 0.210 mol% TBAF; II, 0.293 mol% TBAF; III, 0.500 mol% TBAF. Each label corresponds to that of Fig. 5. The rim that looks yellow is the seal material part for connecting the quartz windows and the stainless steel. The single crystals can be observed in the aqueous solution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
276.15 K and 2.50 MPa. Moreover, the concentrations of TBAF have significant effects on the crystal morphology and crystallinity of the semi-clathrate hydrates. Acknowledgement This work was supported by National Science Fund for Distinguished Young Scholars of China (51225603), National Natural Science Fund (51376184), International S&T Cooperation Program of China (2015DFA61790) and Open Fund of HUST (HKDGH2013001), which are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. Englezos, Ind. Eng. Chem. Res. 32 (1993) 1251e1274. X.-S. Li, C.-G. Xu, Z.-Y. Chen, H.-J. Wu, Energy 36 (2011) 1394e1403. X.-S. Li, J. Cai, Z.-Y. Chen, C.-G. Xu, Energ Fuel 26 (2012) 1144e1151. Z.M. Xia, Z.Y. Chen, X.S. Li, Y. Zhang, K.F. Yan, Q.N. Lv, C.G. Xu, J. Cai, J. Chem. Eng. Data 57 (2012) 3290e3295. C.J. Bibler, J.S. Marshall, R.C. Pilcher, Int. J. Coal Geol. 35 (1998) 283e310. P.L. Guo, M. Chem, Ind. Eng. Process 27 (2008) 963e967. Q. Sun, X. Guo, A. Liu, B. Liu, Y. Huo, G. Chen, Ind. Eng. Chem. Res. 50 (2010) 2284e2288. B.Y. Zhang, Q. Wu, Energ Fuel 24 (2010) 2530e2535. D.L. Zhong, P. Englezos, Energ Fuel 26 (2012) 2098e2106. D.L. Zhong, Y. Ye, C. Yang, Y. Bian, K. Ding, Ind. Eng. Chem. Res. 51 (2012) 14806e14813.
[11] D.-L. Zhong, N. Daraboina, P. Englezos, Fuel 106 (2013) 425e430. [12] H. Ganji, M. Manteghian, K.S. Zadeh, M.R. Omidkhah, H.R. Mofrad, Fuel 86 (2007) 434e441. [13] J. Cai, C. Xu, C. Chen, Z. Chen, X. Li 61 (2014) 812e816. [14] W. Shimada, T. Ebinuma, H. Oyama, Y. Kamata, S. Takeya, T. Uchida, J. Nagao, H. Narita, Jpn. J. Appl. Phys. 2 (42) (2003) 129e131. [15] V.Y. Komarov, T.V. Rodionova, I.S. Terekhova, N.V. Kuratieva, J. Incl. Phenom. Macro 59 (2007) 11e15. [16] S.F. Li, S.S. Fan, J.Q. Wang, X.M. Lang, Y.H. Wang, J. Chem. Eng. Data 55 (2010) 3212e3215. [17] Z.X. Liao, X.Q. Guo, Y.Y. Zhao, Y.W. Wang, Q. Sun, A.X. Liu, C.Y. Sun, G.J. Chen, Ind. Eng. Chem. Res. 52 (2013) 18440e18446. [18] A. Mohammadi, M. Manteghian, A.H. Mohammadi, Fluid Phase Equilib. 381 (2014) 102e107. [19] T. Makino, T. Yamamoto, K. Nagata, H. Sakamoto, S. Hashimoto, T. Sugahara, K. Ohgaki, J. Chem. Eng. Data 55 (2010) 839e841. [20] J. Sakamoto, S. Hashimoto, T. Tsuda, T. Sugahara, Y. Inoue, K. Ohgaki, Chem. Eng. Sci. 63 (2008) 5789e5794. [21] K. Sato, H. Tokutomi, R. Ohmura, Fluid Phase Equilib. 337 (2013) 115e118. [22] C.G. Xu, J. Cai, F.H. Lin, Z.Y. Chen, X.S. Li, Energy 79 (2015) 111e116. [23] D.B.R. Jaysukh Jhaveri, Can. J. Chem. Eng. 43 (1965) 75e78. [24] A.T. Trueba, L.J. Rovetto, L.J. Florusse, M.C. Kroon, C.J. Peters, Fluid Phase Equilib. 307 (2011) 6e10. [25] P. Babu, T. Yang, H.P. Veluswamy, R. Kumar, P. Linga, J. Chem. Thermodyn. 61 (2013) 58e63. [26] Q.N. Lv, X.S. Li, Z.Y. Chen, J.C. Feng, J. Chem. Eng. Data 58 (2013) 3249e3253. [27] E. Kim, Y.K. Jin, Y. Seo, Fluid Phase Equilib. 393 (2015) 85e90. [28] S. Lee, Y. Lee, S. Park, Y. Kim, J.D. Lee, Y. Seo, J. Phys. Chem. B 116 (2012) 9075e9081. [29] S. Hashimoto, S. Murayama, T. Sugahara, H. Sato, K. Ohgaki, Chem. Eng. Sci. 61 (2006) 7884e7888.
Please cite this article in press as: J. Cai, et al., Phase equilibrium and Raman spectroscopic studies of semi-clathrate hydrates for methane, nitrogen and tetra-butyl-ammonium fluoride, Fluid Phase Equilibria (2015), http://dx.doi.org/10.1016/j.fluid.2015.09.054