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Methane hydrate phase stability with lower mole fractions of tetrahydrofuran (THF) and tert-butylamine (t-BuNH2)
Fluid Phase Equilibria 315 (2012) 126–130
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Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Short communication
Methane hydrate phase stability with lower mole fractions of tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ) Vangala Dhanunjana Chari, Deepala V.S.G.K. Sharma, Pinnelli S.R. Prasad ∗ Gas Hydrates Division, National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad 500007, India
a r t i c l e
i n f o
Article history: Received 9 September 2011 Received in revised form 3 November 2011 Accepted 13 November 2011 Available online 20 November 2011 Keywords: Clathrate hydrates Tetrahydrofuran tert-Butyl amine Methane hydrate formation and dissociation
a b s t r a c t A systematic study on methane hydrate formation and dissociation was carried out in presence of liquid hydrocarbon promoters namely tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ). Mixed hydrates with THF/t-BuNH2 (mole fraction x = 0.05) were of sII symmetry. The hydrate formation/dissociation of mixed hydrates with x = 0.033 and 0.0056 were complex with co-existence of sII and sI structures. We observed the hydrate formation in two steps, hydrates in the first stage (∼T = 295 K) were mostly with sII and in the second stage (∼T = 275 K) were with sI structure, corroborating the literature. However, dissociation pattern of the hydrates formed in second stage significantly differ from pure methane hydrates and the above two liquid hydrocarbons show considerable promoter effect for sI hydrates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gas hydrates (clathrates) are the non-stoichiometric inclusion compounds encaging a small, normally apolar (guest) gaseous molecule in the framework of hydrogen-bonded, ice-like host molecules, and exist as a stable solid phase at high gas pressures and/or low temperatures. Three distinct structural families, structures I, II, and H are known, showing distinct size and shape of polyhedral cages that capture the guest molecules according to the structures. The amount of gas stored in this form is extremely high and depends on available vacant cages in hydrogen-bonded water network. There are five types of hydrate cages commonly found of which the increasing in size is: pentagonal dodecahedron (512 -cage), dodecahedron (43 56 63 -cage), tetrakaidecahedron (512 62 -cage), hexakaidecahedron (512 64 -cage), and icosahedron (512 68 -cage). Three common unit cells (sI, sII and sH) of gas hydrates are known to form from a few types of hydrate cages depending largely on the size and physical properties of the guest species. For example, sI can host small molecules such as methane, ethane and carbon dioxide, while sII can host larger molecules such as propane and isobutane. The cubic sI unit cell contains 46 H2 O molecules (two 12-hedra (512 ) and six 14-hedra (512 62 )) where 512 used to indicate that the polyhedron contains 12 five-member ring faces. The cubic sII cell contains 136 water molecules (eight large (512 64 ) and 16 small (512 ) cages). The sH hydrate consists of
∗ Corresponding author. E-mail address: [email protected] (P.S.R. Prasad). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.11.012
three different cages: three 512 -cages, two 43 56 63 -cages and one 512 68 -cage [1]. Gas hydrates have been a particular concern of the oil and gas industry because the operating conditions of oil and gas production pipelines may be constructive for the formation of gas hydrate, resulting in blockage of pipelines [2–4]. However, the studies of gas hydrate have greatly evolved because of not only the concern in production pipelines but also the great potential of hydrates as a source of natural gas as there are massive deposits both under the permafrost and in the sediment of the continental margins. Gas hydrate also represents an attractive way of storing large quantities of gas, such as hydrogen [5–9], natural gas [4,10–12], and carbon dioxide [4,13]. Extensive efforts have been carried out to develop efficient storage techniques in both scientific and industrial fields; although up to date there has been little effort to understand the physical properties of gas hydrates formed with multi-components and natural gas. Methane hydrates normally are stable under high pressure and subzero temperatures; thus not convenient for storage/transportation applications. The gas separation processes by hydrate formation also often demands operable pressure and temperature conditions [4,14–17]. Thermodynamic promoter/inhibitors such as THF/methanol are useful to overcome some of the operational difficulties. The disadvantage with such multi-component hydrate forming systems is the promoter molecules themselves are the “guests” in hydrate systems and thus the overall storage capacity for fuel gas may be reduced. However, the advantage is that the hydrate formation may occur at lower pressures and higher temperatures compared to pure (methane) hydrates.
V. Dhanunjana Chari et al. / Fluid Phase Equilibria 315 (2012) 126–130 Table 1 Details of the samples utilized along with their purity and suppliers. Sample
Purity
Supplier
Tetrahydrofuran (THF)
98%
t-Butyl amine (tBA) Water
98% Doubly distilled and deionised water
Qualigens Fine Chemicals, India Sigma Aldrich, Germany
In this paper we present methane hydrate formation and dissociation characteristics under identical experimental conditions in the presence of tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ) molecules with molar concentrations less than ideal stoichiometry (i.e., 0.033 and 0.0056 mole fractions). Under these conditions mixed hydrates are formed with complex co-existing structures. Our objective of this work is to probe the behavior of methane hydrates forming in second stage with sI like characteristic at lower molar concentrations of promoter molecules. The unit cell structure of THF hydrates is sII with eight 512 64 cages populated by THF molecules and sixteen 512 cages are vacant and could possibly be occupied by methane molecules in mixed hydrates [18]. The unit cell structure for t-BuNH2 hydrate is sVI with 16 (43 59 62 73 ) cages occupied by t-BuNH2 and 12 (44 54 ) cages are vacant [19]. Recent studies have demonstrated that the sVI structure of t-BuNH2 clathrates is highly unstable upon pressurizing with suitable gas (CH4 or H2 ) to fill the vacant 44 54 cages [20,21]. Experimental studies by Kim et al. [21] and Prasad et al. [7,20] clearly established the structure of mixed (with more than one guest molecules) hydrate system as sII. 2. Experimental method 2.1. Materials Aqueous solutions were prepared following the gravimetric method using an Metler Toledo (AB104-S) accurate analytical balance. Consequently, uncertainties on mole fractions are estimated to be below 0.01. The source and purity of various liquid promoters used in this study were tabulated in Table 1. 2.2. Experimental apparatus Briefly, the main part of the apparatus is a SS-316 cylindrical vessel, which can withstand pressures up to 10 MPa. The volume of vessel is 100 ml. A stirrer with variable speed was installed in the vessel to agitate the fluids and hydrate crystals inside it. All the experiments were conducted with a fixed speed of 500 rpm. Cold fluid (water + glycol mixture) was circulated around the vessel with the help of Lab Companion (RW-0525G) circulator, to maintain the temperature inside it at a desired level. A platinum resistance thermometer (Pt100) inserted into the vessel was used to measure temperatures and check for equality of temperatures within temperature measurement uncertainties, which is estimated to be less than 0.2 K. The pressure in the vessel was measured with a WIKA pressure transducer (WIKA, type A-10 for pressure range 0–16 MPa). The vessel containing aqueous solution (approximately 40% by volume of the vessel) was immersed into the temperature controlled bath and the gas was supplied from a cylinder to desired level using Teledyne ISCO Syringe pump (Model 100DX). Note that the vessel was evacuated before introducing any aqueous solution and gas. After obtaining temperature and pressure stability (far enough from the hydrate formation region), the valve inline connecting the vessel and the ISCO pump/cylinder was closed. Subsequently, temperature was slowly decreased to form the hydrate
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and hydrate formation in the vessel was detected by pressure drop. The temperature was then increased in steps of 1.0 K. At each step, temperature was kept constant with sufficient time to achieve equilibrium state in the vessel. In this way, a pressure temperature diagram was obtained for each experimental run, from which we determined the hydrate formation and dissociation pattern. If 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 phase equilibria of fluids in the vessel. Consequently, the point at which the slope of pressure–temperature data plot changes sharply is considered to be the point at which all hydrate crystals have dissociated. Micro-Raman measurements were carried out using 514.5 nm (wavelength) from an air cooled argon ion (Spectra Physics) laser as excitation source. The scattered light was dispersed through a JYT64000 triple monochromator system, attached to a liquid nitrogen cooled CCD detector. The Raman measurements were repeatable up to a precision of 0.5 cm−1 . The scattered light was detected (in back scattered geometry) using a Jobin Yvon confocal microanalysis system attached with an Olympus microscopy (Model U-LH 100/3). The laser focal spot size is around 2 m and the Raman spectrum is collected from such spot in confocal geometry. The temperature dependence measurements were performed by using the Linkam FTIR-600 stage, which was placed under the confocal microscope of the Raman system. The temperature stability of the stage was better than ±0.2 K. 3. Results and discussion The phase equilibrium studies aimed to probe the equilibrium pressure and temperatures under which three phases like liquid hydrate (H), water (Lw ) and vapor (V) have discernible boundary. The promoters like THF [22] or t-BuNH2 [23] are used in hydrate formation process to alleviate the hydrate formation/dissociation processes [24]. Doubly distilled, de-ionized and degassed water was used for hydrate formations. The reactor was filled with 30–40 ml of water/stock solution with required mole fraction of promoter molecules. The reactor was then pressurized with methane gas and the stirrer was kept on (@500 rpm) for the entire experimental run. The sample preparative conditions and the amount of methane consumed from the vapor phase due to hydrate formation are shown in Table 2. In Figs. 1 and 2, we show pressure variations as a function of temperature during the hydrate formation and dissociations. Typical rate of cooling from point a to b, b to c and c to d, respectively, is 0.1, 0.002 and 0.3 K/min (see Fig. 1B). It is evident that methane hydrate formation occurred at higher temperatures in the presence of promoter molecules and the number of methane molecules (‘n’ moles) in the reactor was estimated from: n=
PV ZRT
where the compressibility factor Z was referred from Perry’s Chemical Engineers’ Handbook (Methane Z Compressibility Factor Data, p. 172), ‘V’ the volume of the gas phase in the reactor and ‘R’ gas constant. Pressure (P) and temperature (T) were logged at a fixed time interval as the hydrate formation/dissociation progresses. The temperature was held constant for about 15 h (overnight) for the maximum hydrate formation in each case (b–c region). Fig. 1 shows the pressure–temperature (P–T) trajectory of methane hydrate formation/dissociation with different mole fractions of THF, i.e. x = 0 (Fig. 1A), x = 0.060 (Fig. 1B), x = 0.033 (Fig. 1C) and x = 0.0056 (Fig. 1D). In Table 2 we show the amount of water and the promoters like THF or t-BuNH2 used in each experimental run.
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Table 2 Table describing the preparative compositions of methane hydrates with aqueous solutions of THF and t-BuNH2 . The total methane consumed during the experimental runs and calculated hydrate yield is also tabulated. Sample
Water THF THF THF t-BuNH2 t-BuNH2 t-BuNH2
Preparation
Total CH4 consumed (g)
Water (g)
Promoter (g)
Mole fraction (x)
30 31 35 39 29 35 39
– 7.974 4.43 0.886 7.656 4.873 0.886
– 0.060 0.033 0.0056 0.061 0.033 0.0056
The solid lines shows the phase boundary behavior computed from CSMGEM programme for sI CH4 hydrates and the phase boundary curve for sII is only a guide-to-eye. It is well known from the literature that pure methane hydrate crystallizes into sI structure, where as with addition of THF (x ∼ 0.0556) the resultant structure is sII [24–26]. Seo et al. [24], in their earlier studies reported that mixed hydrates formed in aqueous THF solution (x = 0.0335) were more complex. Two stage hydrate formation was reported with sII and sI structures having dissociation point around (298.05 K, 4.7 MPa) and (276.35 K, 3.68 MPa) [24]. In the present study the pressure reduction in first stage around 295 K was predominate because of the formation/dissociation of mixed (THF + CH4 ) hydrates more similar to the hydrates with ideal stoichiometry (x = 0.0556). In Table 2, the methane consumption and hydrate yield are tabulated using the experimental data. The methane consumption during the hydrate
1.65 1.55 1.12 1.09 1.40 1.65 1.58
Consumed CH4 during hydrate formation of sI (g)
sII (g)
1.65 – 0.27 0.92 – 0.89 1.45
– 1.55 0.85 0.17 1.40 0.76 0.13
% Yield
39.28 49.77 37.41 20.67 48.52 37.71 29.21
formation was estimated by converting the gas into number of moles using the pressure–temperature data with the help of real gas equation. In case of co-existing hydrate phases the fraction of sI hydrates were estimated from the available excess water in the reactor. The hydrate yield is computed from the observed methane gas consumed from the experiments to the expected values with stiochiometric compositions. Our results (see Fig. 1B and C) broadly corroborate the literature data. Further, Seo et al. [24] observed a pressure drop at lower temperatures and they interpreted it as pure CH4 hydrate (sI) formation. The present results deviate at this stage (see Fig. 1C). The sI phase boundary computed from CSMGEM has no overlap with the observed P–T trajectory (see Fig. 1B–D), indicating that the hydrates formed during this stage may not be pure CH4 hydrates. The hydrate formation/dissociation will certainly be influenced by the rate of cooling/heating and stirring, so we
Fig. 1. Formation and dissociation of methane hydrate (MH) with water (A), and aqueous solutions of tetrahydrofuran (THF) mole fraction x = 0.060 (B); x = 0.033 (C); x = 0.0056 (D). Filled and open symbols, respectively, represent the observed behavior during cooling and warming cycles. The phase boundary curves for sI and sII are shown by the solid lines. The symbols ‘+ and ×’ in these figures are the hydrate dissociation points reported in the literature Refs. [24,25]. The THF mole fractions for the data reported by Susilo et al. (Ref. [25], ×) are x = 0.056 (B) and x = 0.01 (D). The data points plotted in from Seo et al. (Ref. [24], +) in Fig. 1B and C are with THF mole fractions x = 0.05 and x = 0.033.
V. Dhanunjana Chari et al. / Fluid Phase Equilibria 315 (2012) 126–130
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Fig. 2. Formation and dissociation of methane hydrate (MH) with water (A), and aqueous solutions of tert-butylamine (t-BuNH2 ) (mole fraction (x) = 0.061 (B); x = 0.033 (C); x = 0.0056 (D). Filled and open symbols, respectively, represent the observed behavior during cooling and warming cycles. The phase boundary curves for sI and sII are shown by the solid lines. The symbols ‘× and +’ in these figures are the hydrate dissociation points reported in the literature Refs. [23,27]. The data points plotted from Liang et al. (Ref. [23], ×) in Fig. 2B and D are with (t-BuNH2 ) mole fractions x = 0.056 and x = 0.01 and the data points plotted from Kim et al. (Ref. [27], +) in Fig. 2B–D are with (t-BuNH2 ) mole fractions x = 0.056, x = 0.02 and x = 0.003.
maintained all these parameters constant throughout the experiments and also conducted a run with pure CH4 for guidance (see Fig. 1A). On the other hand addition of THF/t-BuNH2 , shifted the dissociation of hydrates to right of sI phase boundary curve indicating that they could have ‘promotion effect’ for sI hydrates. It is well documented that the mixed hydrates of CH4 with THF/t-BuNH2 are of sII structure with 512 64 cages occupied by the larger hydrocarbons and 512 cages by the methane molecules and the apparent phase boundary curve shifted to the right of sI. In Fig. 1B we plotted the dissociation points for mixed hydrates from the literature for comparison. The symbols (×, +) shown in Fig. 1B are from Seo et al. [24], Susilo et al. [25] and the THF concentration in these studies are x = 0.05 and x = 0.056, respectively. It is also reported in the literature that the excess of THF (x ≥ 0.056) has no effect on the phase boundary while at lower concentrations it is shifted towards sI phase boundary [24,25]. The experiments with THF x = 0.033 (see Fig. 1C) has shown two stage mixed hydrate formation in agreement with Seo et al. [24]. The ‘+’ symbols used in the figure indicate the dissociation points reported by Seo et al. [24] for sII and sI hydrate fractions with THF x = 0.03. At much lower concentrations of THF, the more preferred structure of mixed hydrates is more complex and co-existing sI and sII. A two stage hydrate formation could be more probable with lower concentrations. The P–T trajectory shown in Fig. 1D are with THF x = 0.0056 and for comparison we also plotted the data from Susilo et al. [25], with THF x = 0.01. Those authors have reported that the phase boundary shifted marginally (∼4◦ ) to the left of stiochiometric composition. As clearly seen in Fig. 1D the dissociation curve for mixed hydrates formed with THF
(x ∼ 0.0056) shifted right of sI phase boundary curve and stretched towards sII phase boundary curve probably due to the presence of co-existing sI + sII hydrates. Earlier micro-Raman studies from our laboratory indicated the admixture of sI and sII mixed hydrates with THF x = 0.044; x = 0.0295 and an unusual structural transformation occurred in the mixed hydrates with THF (x ∼ 0.0146) [26]. Another promoter, t-BuNH2 is used with similar molar concentrations as that of THF. The P–T trajectories are shown in Fig. 2. For completeness we show the P, T behavior of pure CH4 hydrate also. It is necessary to recall that the stable structure for t-BuNH2 clathrate is sVI and it transformed in to sII (similar to THF clathrates) upon populating the vacant small cages [20]. The dissociation curve for mixed hydrates with t-BuNH2 shifted right of sI phase boundary, indicating hydrate promotion, but this is 6◦ less compare to THF (see Fig. 1B). The hydrate dissociation points reported in literature [23,27] are also plotted in Fig. 2B–D using symbols ‘× and +’. In Fig. 2B, the P–T trajectory is for t-BuNH2 with x = 0.061 and is compared with the literature data with x = 0.056 [23,27]. There is reasonably good agreement with all these results. Further decrease in molar concentration of t-BuNH2 (x = 0.33) as shown in Fig. 2C, resulted in two stage hydrate formation (the symbols ‘+’; Ref. [27]; for x = 0.02); similar to Fig. 1C. Hydrates formed with 0.0056 mole fraction of t-BuNH2 are comparable with P–T trajectory of pure CH4 hydrates (shown in Fig. 2D). The hydrate formation is mostly in single step (∼279 K) and methane consumption is ∼1.45 g (see Table 2). The experimental data reported by Kim et al. (Ref. [27]; x = 0.003; ‘+’) and Liang et al. (Ref. [23]; x = 0.01; ‘×’) are also plotted in Fig. 2D. Interestingly the P–T trajectory for the molar composition
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intermediate mole fractions (x = 0.033 for THF and for t-BuNH2 ) a two stage hydrate formation in both sII and sI structures are prominently observed. However, at much lower mole fractions (x = 0.0056 for THF and t-BuNH2 ) methane hydrates are dominated by sI structure. Noticeably the dissociation curve shifted to the right of pure methane hydrate phase boundary curve by around 2◦ indicating the hydrate promotion affect. However, the mixed hydrates with THF with x = 0.0056 are dominated by sII. Acknowledgments The authors acknowledge the research grants from DST, MOES, MoP & G (NHGP) and DRDO – ASL through sponsored projects. Authors sincerely thank the Director of the National Geophysical Research Institute, Hyderabad, for his encouragement, and permission to publish this paper. Fig. 3. The Raman spectra of enclathrated methane (solid line) and t-BuNH2 (dotted line) molecules in the characteristic CH region. The concentration of t-BuNH2 are x = 0.093 (a); x = 0.033 (b); and x = 0.0056 (c). The Raman spectra in b and c are with CH4 as co-guest in binary t-BuNH2 + CH4 hydrates while (a) is without CH4 .
is clearly shifted to the right of sI phase boundary curve indicating the promotion effect for methane hydrates at such lower molar concentrations of t-BuNH2 . In Fig. 3, the Raman spectra for mixed hydrates with different molar compositions of t-BuNH2 are shown. Earlier studies [28] of the H2 O + t-BuNH2 system have shown that the Raman intensities and band positions strongly depend on the molar concentration of t-BuNH2 . On the other hand, the stronger symmetric stretching bands for CH3 groups have two components at 2913 and 2926 cm−1 (see Fig. 3a) with in the vicinity of the characteristic methane modes for the clathrate hydrates. When aqueous t-BuNH2 (x = 0.033) mixture pressurized with methane, new bands around 2905 and 2915 cm−1 are clearly observed with positions and line widths different from CH3 groups of t-BuNH2 (Fig. 3b) [20]. The Raman shift and linewidth for this mode could not be attributed to the free methane gas, but instead it is due to methane gas trapped in 512 and 512 64 cages of sII. The intensity of 2915 cm−1 mode is more than that of 2905 cm−1 and this could be due to more number of 512 64 cages in unit cell for sII. The mixed hydrates with lower concentration of t-BuNH2 (x = 0.0056) show features in the Raman spectrum typical to that of sI structure with Raman bands around 2905 and 2916 cm−1 which indicates that methane molecules are entrapped in 512 and 512 62 cages (Fig. 3c). The Raman intensity ratios along with observed P, T trajectory indicates that mixed hydrates are dominated by sI hydrates. 4. Conclusions Tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ) are the thermodynamic promoters for the methane hydrates. The mixed hydrates formed with methane and THF/t-BuNH2 in sII stoichiometry dissociates at much higher temperatures (THF – 18◦ and t-BuNH2 – 12◦ ) compared to pure methane hydrate. At
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Contents lists available at SciVerse ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Short communication
Methane hydrate phase stability with lower mole fractions of tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ) Vangala Dhanunjana Chari, Deepala V.S.G.K. Sharma, Pinnelli S.R. Prasad ∗ Gas Hydrates Division, National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad 500007, India
a r t i c l e
i n f o
Article history: Received 9 September 2011 Received in revised form 3 November 2011 Accepted 13 November 2011 Available online 20 November 2011 Keywords: Clathrate hydrates Tetrahydrofuran tert-Butyl amine Methane hydrate formation and dissociation
a b s t r a c t A systematic study on methane hydrate formation and dissociation was carried out in presence of liquid hydrocarbon promoters namely tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ). Mixed hydrates with THF/t-BuNH2 (mole fraction x = 0.05) were of sII symmetry. The hydrate formation/dissociation of mixed hydrates with x = 0.033 and 0.0056 were complex with co-existence of sII and sI structures. We observed the hydrate formation in two steps, hydrates in the first stage (∼T = 295 K) were mostly with sII and in the second stage (∼T = 275 K) were with sI structure, corroborating the literature. However, dissociation pattern of the hydrates formed in second stage significantly differ from pure methane hydrates and the above two liquid hydrocarbons show considerable promoter effect for sI hydrates. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Gas hydrates (clathrates) are the non-stoichiometric inclusion compounds encaging a small, normally apolar (guest) gaseous molecule in the framework of hydrogen-bonded, ice-like host molecules, and exist as a stable solid phase at high gas pressures and/or low temperatures. Three distinct structural families, structures I, II, and H are known, showing distinct size and shape of polyhedral cages that capture the guest molecules according to the structures. The amount of gas stored in this form is extremely high and depends on available vacant cages in hydrogen-bonded water network. There are five types of hydrate cages commonly found of which the increasing in size is: pentagonal dodecahedron (512 -cage), dodecahedron (43 56 63 -cage), tetrakaidecahedron (512 62 -cage), hexakaidecahedron (512 64 -cage), and icosahedron (512 68 -cage). Three common unit cells (sI, sII and sH) of gas hydrates are known to form from a few types of hydrate cages depending largely on the size and physical properties of the guest species. For example, sI can host small molecules such as methane, ethane and carbon dioxide, while sII can host larger molecules such as propane and isobutane. The cubic sI unit cell contains 46 H2 O molecules (two 12-hedra (512 ) and six 14-hedra (512 62 )) where 512 used to indicate that the polyhedron contains 12 five-member ring faces. The cubic sII cell contains 136 water molecules (eight large (512 64 ) and 16 small (512 ) cages). The sH hydrate consists of
∗ Corresponding author. E-mail address: [email protected] (P.S.R. Prasad). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.11.012
three different cages: three 512 -cages, two 43 56 63 -cages and one 512 68 -cage [1]. Gas hydrates have been a particular concern of the oil and gas industry because the operating conditions of oil and gas production pipelines may be constructive for the formation of gas hydrate, resulting in blockage of pipelines [2–4]. However, the studies of gas hydrate have greatly evolved because of not only the concern in production pipelines but also the great potential of hydrates as a source of natural gas as there are massive deposits both under the permafrost and in the sediment of the continental margins. Gas hydrate also represents an attractive way of storing large quantities of gas, such as hydrogen [5–9], natural gas [4,10–12], and carbon dioxide [4,13]. Extensive efforts have been carried out to develop efficient storage techniques in both scientific and industrial fields; although up to date there has been little effort to understand the physical properties of gas hydrates formed with multi-components and natural gas. Methane hydrates normally are stable under high pressure and subzero temperatures; thus not convenient for storage/transportation applications. The gas separation processes by hydrate formation also often demands operable pressure and temperature conditions [4,14–17]. Thermodynamic promoter/inhibitors such as THF/methanol are useful to overcome some of the operational difficulties. The disadvantage with such multi-component hydrate forming systems is the promoter molecules themselves are the “guests” in hydrate systems and thus the overall storage capacity for fuel gas may be reduced. However, the advantage is that the hydrate formation may occur at lower pressures and higher temperatures compared to pure (methane) hydrates.
V. Dhanunjana Chari et al. / Fluid Phase Equilibria 315 (2012) 126–130 Table 1 Details of the samples utilized along with their purity and suppliers. Sample
Purity
Supplier
Tetrahydrofuran (THF)
98%
t-Butyl amine (tBA) Water
98% Doubly distilled and deionised water
Qualigens Fine Chemicals, India Sigma Aldrich, Germany
In this paper we present methane hydrate formation and dissociation characteristics under identical experimental conditions in the presence of tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ) molecules with molar concentrations less than ideal stoichiometry (i.e., 0.033 and 0.0056 mole fractions). Under these conditions mixed hydrates are formed with complex co-existing structures. Our objective of this work is to probe the behavior of methane hydrates forming in second stage with sI like characteristic at lower molar concentrations of promoter molecules. The unit cell structure of THF hydrates is sII with eight 512 64 cages populated by THF molecules and sixteen 512 cages are vacant and could possibly be occupied by methane molecules in mixed hydrates [18]. The unit cell structure for t-BuNH2 hydrate is sVI with 16 (43 59 62 73 ) cages occupied by t-BuNH2 and 12 (44 54 ) cages are vacant [19]. Recent studies have demonstrated that the sVI structure of t-BuNH2 clathrates is highly unstable upon pressurizing with suitable gas (CH4 or H2 ) to fill the vacant 44 54 cages [20,21]. Experimental studies by Kim et al. [21] and Prasad et al. [7,20] clearly established the structure of mixed (with more than one guest molecules) hydrate system as sII. 2. Experimental method 2.1. Materials Aqueous solutions were prepared following the gravimetric method using an Metler Toledo (AB104-S) accurate analytical balance. Consequently, uncertainties on mole fractions are estimated to be below 0.01. The source and purity of various liquid promoters used in this study were tabulated in Table 1. 2.2. Experimental apparatus Briefly, the main part of the apparatus is a SS-316 cylindrical vessel, which can withstand pressures up to 10 MPa. The volume of vessel is 100 ml. A stirrer with variable speed was installed in the vessel to agitate the fluids and hydrate crystals inside it. All the experiments were conducted with a fixed speed of 500 rpm. Cold fluid (water + glycol mixture) was circulated around the vessel with the help of Lab Companion (RW-0525G) circulator, to maintain the temperature inside it at a desired level. A platinum resistance thermometer (Pt100) inserted into the vessel was used to measure temperatures and check for equality of temperatures within temperature measurement uncertainties, which is estimated to be less than 0.2 K. The pressure in the vessel was measured with a WIKA pressure transducer (WIKA, type A-10 for pressure range 0–16 MPa). The vessel containing aqueous solution (approximately 40% by volume of the vessel) was immersed into the temperature controlled bath and the gas was supplied from a cylinder to desired level using Teledyne ISCO Syringe pump (Model 100DX). Note that the vessel was evacuated before introducing any aqueous solution and gas. After obtaining temperature and pressure stability (far enough from the hydrate formation region), the valve inline connecting the vessel and the ISCO pump/cylinder was closed. Subsequently, temperature was slowly decreased to form the hydrate
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and hydrate formation in the vessel was detected by pressure drop. The temperature was then increased in steps of 1.0 K. At each step, temperature was kept constant with sufficient time to achieve equilibrium state in the vessel. In this way, a pressure temperature diagram was obtained for each experimental run, from which we determined the hydrate formation and dissociation pattern. If 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 phase equilibria of fluids in the vessel. Consequently, the point at which the slope of pressure–temperature data plot changes sharply is considered to be the point at which all hydrate crystals have dissociated. Micro-Raman measurements were carried out using 514.5 nm (wavelength) from an air cooled argon ion (Spectra Physics) laser as excitation source. The scattered light was dispersed through a JYT64000 triple monochromator system, attached to a liquid nitrogen cooled CCD detector. The Raman measurements were repeatable up to a precision of 0.5 cm−1 . The scattered light was detected (in back scattered geometry) using a Jobin Yvon confocal microanalysis system attached with an Olympus microscopy (Model U-LH 100/3). The laser focal spot size is around 2 m and the Raman spectrum is collected from such spot in confocal geometry. The temperature dependence measurements were performed by using the Linkam FTIR-600 stage, which was placed under the confocal microscope of the Raman system. The temperature stability of the stage was better than ±0.2 K. 3. Results and discussion The phase equilibrium studies aimed to probe the equilibrium pressure and temperatures under which three phases like liquid hydrate (H), water (Lw ) and vapor (V) have discernible boundary. The promoters like THF [22] or t-BuNH2 [23] are used in hydrate formation process to alleviate the hydrate formation/dissociation processes [24]. Doubly distilled, de-ionized and degassed water was used for hydrate formations. The reactor was filled with 30–40 ml of water/stock solution with required mole fraction of promoter molecules. The reactor was then pressurized with methane gas and the stirrer was kept on (@500 rpm) for the entire experimental run. The sample preparative conditions and the amount of methane consumed from the vapor phase due to hydrate formation are shown in Table 2. In Figs. 1 and 2, we show pressure variations as a function of temperature during the hydrate formation and dissociations. Typical rate of cooling from point a to b, b to c and c to d, respectively, is 0.1, 0.002 and 0.3 K/min (see Fig. 1B). It is evident that methane hydrate formation occurred at higher temperatures in the presence of promoter molecules and the number of methane molecules (‘n’ moles) in the reactor was estimated from: n=
PV ZRT
where the compressibility factor Z was referred from Perry’s Chemical Engineers’ Handbook (Methane Z Compressibility Factor Data, p. 172), ‘V’ the volume of the gas phase in the reactor and ‘R’ gas constant. Pressure (P) and temperature (T) were logged at a fixed time interval as the hydrate formation/dissociation progresses. The temperature was held constant for about 15 h (overnight) for the maximum hydrate formation in each case (b–c region). Fig. 1 shows the pressure–temperature (P–T) trajectory of methane hydrate formation/dissociation with different mole fractions of THF, i.e. x = 0 (Fig. 1A), x = 0.060 (Fig. 1B), x = 0.033 (Fig. 1C) and x = 0.0056 (Fig. 1D). In Table 2 we show the amount of water and the promoters like THF or t-BuNH2 used in each experimental run.
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Table 2 Table describing the preparative compositions of methane hydrates with aqueous solutions of THF and t-BuNH2 . The total methane consumed during the experimental runs and calculated hydrate yield is also tabulated. Sample
Water THF THF THF t-BuNH2 t-BuNH2 t-BuNH2
Preparation
Total CH4 consumed (g)
Water (g)
Promoter (g)
Mole fraction (x)
30 31 35 39 29 35 39
– 7.974 4.43 0.886 7.656 4.873 0.886
– 0.060 0.033 0.0056 0.061 0.033 0.0056
The solid lines shows the phase boundary behavior computed from CSMGEM programme for sI CH4 hydrates and the phase boundary curve for sII is only a guide-to-eye. It is well known from the literature that pure methane hydrate crystallizes into sI structure, where as with addition of THF (x ∼ 0.0556) the resultant structure is sII [24–26]. Seo et al. [24], in their earlier studies reported that mixed hydrates formed in aqueous THF solution (x = 0.0335) were more complex. Two stage hydrate formation was reported with sII and sI structures having dissociation point around (298.05 K, 4.7 MPa) and (276.35 K, 3.68 MPa) [24]. In the present study the pressure reduction in first stage around 295 K was predominate because of the formation/dissociation of mixed (THF + CH4 ) hydrates more similar to the hydrates with ideal stoichiometry (x = 0.0556). In Table 2, the methane consumption and hydrate yield are tabulated using the experimental data. The methane consumption during the hydrate
1.65 1.55 1.12 1.09 1.40 1.65 1.58
Consumed CH4 during hydrate formation of sI (g)
sII (g)
1.65 – 0.27 0.92 – 0.89 1.45
– 1.55 0.85 0.17 1.40 0.76 0.13
% Yield
39.28 49.77 37.41 20.67 48.52 37.71 29.21
formation was estimated by converting the gas into number of moles using the pressure–temperature data with the help of real gas equation. In case of co-existing hydrate phases the fraction of sI hydrates were estimated from the available excess water in the reactor. The hydrate yield is computed from the observed methane gas consumed from the experiments to the expected values with stiochiometric compositions. Our results (see Fig. 1B and C) broadly corroborate the literature data. Further, Seo et al. [24] observed a pressure drop at lower temperatures and they interpreted it as pure CH4 hydrate (sI) formation. The present results deviate at this stage (see Fig. 1C). The sI phase boundary computed from CSMGEM has no overlap with the observed P–T trajectory (see Fig. 1B–D), indicating that the hydrates formed during this stage may not be pure CH4 hydrates. The hydrate formation/dissociation will certainly be influenced by the rate of cooling/heating and stirring, so we
Fig. 1. Formation and dissociation of methane hydrate (MH) with water (A), and aqueous solutions of tetrahydrofuran (THF) mole fraction x = 0.060 (B); x = 0.033 (C); x = 0.0056 (D). Filled and open symbols, respectively, represent the observed behavior during cooling and warming cycles. The phase boundary curves for sI and sII are shown by the solid lines. The symbols ‘+ and ×’ in these figures are the hydrate dissociation points reported in the literature Refs. [24,25]. The THF mole fractions for the data reported by Susilo et al. (Ref. [25], ×) are x = 0.056 (B) and x = 0.01 (D). The data points plotted in from Seo et al. (Ref. [24], +) in Fig. 1B and C are with THF mole fractions x = 0.05 and x = 0.033.
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Fig. 2. Formation and dissociation of methane hydrate (MH) with water (A), and aqueous solutions of tert-butylamine (t-BuNH2 ) (mole fraction (x) = 0.061 (B); x = 0.033 (C); x = 0.0056 (D). Filled and open symbols, respectively, represent the observed behavior during cooling and warming cycles. The phase boundary curves for sI and sII are shown by the solid lines. The symbols ‘× and +’ in these figures are the hydrate dissociation points reported in the literature Refs. [23,27]. The data points plotted from Liang et al. (Ref. [23], ×) in Fig. 2B and D are with (t-BuNH2 ) mole fractions x = 0.056 and x = 0.01 and the data points plotted from Kim et al. (Ref. [27], +) in Fig. 2B–D are with (t-BuNH2 ) mole fractions x = 0.056, x = 0.02 and x = 0.003.
maintained all these parameters constant throughout the experiments and also conducted a run with pure CH4 for guidance (see Fig. 1A). On the other hand addition of THF/t-BuNH2 , shifted the dissociation of hydrates to right of sI phase boundary curve indicating that they could have ‘promotion effect’ for sI hydrates. It is well documented that the mixed hydrates of CH4 with THF/t-BuNH2 are of sII structure with 512 64 cages occupied by the larger hydrocarbons and 512 cages by the methane molecules and the apparent phase boundary curve shifted to the right of sI. In Fig. 1B we plotted the dissociation points for mixed hydrates from the literature for comparison. The symbols (×, +) shown in Fig. 1B are from Seo et al. [24], Susilo et al. [25] and the THF concentration in these studies are x = 0.05 and x = 0.056, respectively. It is also reported in the literature that the excess of THF (x ≥ 0.056) has no effect on the phase boundary while at lower concentrations it is shifted towards sI phase boundary [24,25]. The experiments with THF x = 0.033 (see Fig. 1C) has shown two stage mixed hydrate formation in agreement with Seo et al. [24]. The ‘+’ symbols used in the figure indicate the dissociation points reported by Seo et al. [24] for sII and sI hydrate fractions with THF x = 0.03. At much lower concentrations of THF, the more preferred structure of mixed hydrates is more complex and co-existing sI and sII. A two stage hydrate formation could be more probable with lower concentrations. The P–T trajectory shown in Fig. 1D are with THF x = 0.0056 and for comparison we also plotted the data from Susilo et al. [25], with THF x = 0.01. Those authors have reported that the phase boundary shifted marginally (∼4◦ ) to the left of stiochiometric composition. As clearly seen in Fig. 1D the dissociation curve for mixed hydrates formed with THF
(x ∼ 0.0056) shifted right of sI phase boundary curve and stretched towards sII phase boundary curve probably due to the presence of co-existing sI + sII hydrates. Earlier micro-Raman studies from our laboratory indicated the admixture of sI and sII mixed hydrates with THF x = 0.044; x = 0.0295 and an unusual structural transformation occurred in the mixed hydrates with THF (x ∼ 0.0146) [26]. Another promoter, t-BuNH2 is used with similar molar concentrations as that of THF. The P–T trajectories are shown in Fig. 2. For completeness we show the P, T behavior of pure CH4 hydrate also. It is necessary to recall that the stable structure for t-BuNH2 clathrate is sVI and it transformed in to sII (similar to THF clathrates) upon populating the vacant small cages [20]. The dissociation curve for mixed hydrates with t-BuNH2 shifted right of sI phase boundary, indicating hydrate promotion, but this is 6◦ less compare to THF (see Fig. 1B). The hydrate dissociation points reported in literature [23,27] are also plotted in Fig. 2B–D using symbols ‘× and +’. In Fig. 2B, the P–T trajectory is for t-BuNH2 with x = 0.061 and is compared with the literature data with x = 0.056 [23,27]. There is reasonably good agreement with all these results. Further decrease in molar concentration of t-BuNH2 (x = 0.33) as shown in Fig. 2C, resulted in two stage hydrate formation (the symbols ‘+’; Ref. [27]; for x = 0.02); similar to Fig. 1C. Hydrates formed with 0.0056 mole fraction of t-BuNH2 are comparable with P–T trajectory of pure CH4 hydrates (shown in Fig. 2D). The hydrate formation is mostly in single step (∼279 K) and methane consumption is ∼1.45 g (see Table 2). The experimental data reported by Kim et al. (Ref. [27]; x = 0.003; ‘+’) and Liang et al. (Ref. [23]; x = 0.01; ‘×’) are also plotted in Fig. 2D. Interestingly the P–T trajectory for the molar composition
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intermediate mole fractions (x = 0.033 for THF and for t-BuNH2 ) a two stage hydrate formation in both sII and sI structures are prominently observed. However, at much lower mole fractions (x = 0.0056 for THF and t-BuNH2 ) methane hydrates are dominated by sI structure. Noticeably the dissociation curve shifted to the right of pure methane hydrate phase boundary curve by around 2◦ indicating the hydrate promotion affect. However, the mixed hydrates with THF with x = 0.0056 are dominated by sII. Acknowledgments The authors acknowledge the research grants from DST, MOES, MoP & G (NHGP) and DRDO – ASL through sponsored projects. Authors sincerely thank the Director of the National Geophysical Research Institute, Hyderabad, for his encouragement, and permission to publish this paper. Fig. 3. The Raman spectra of enclathrated methane (solid line) and t-BuNH2 (dotted line) molecules in the characteristic CH region. The concentration of t-BuNH2 are x = 0.093 (a); x = 0.033 (b); and x = 0.0056 (c). The Raman spectra in b and c are with CH4 as co-guest in binary t-BuNH2 + CH4 hydrates while (a) is without CH4 .
is clearly shifted to the right of sI phase boundary curve indicating the promotion effect for methane hydrates at such lower molar concentrations of t-BuNH2 . In Fig. 3, the Raman spectra for mixed hydrates with different molar compositions of t-BuNH2 are shown. Earlier studies [28] of the H2 O + t-BuNH2 system have shown that the Raman intensities and band positions strongly depend on the molar concentration of t-BuNH2 . On the other hand, the stronger symmetric stretching bands for CH3 groups have two components at 2913 and 2926 cm−1 (see Fig. 3a) with in the vicinity of the characteristic methane modes for the clathrate hydrates. When aqueous t-BuNH2 (x = 0.033) mixture pressurized with methane, new bands around 2905 and 2915 cm−1 are clearly observed with positions and line widths different from CH3 groups of t-BuNH2 (Fig. 3b) [20]. The Raman shift and linewidth for this mode could not be attributed to the free methane gas, but instead it is due to methane gas trapped in 512 and 512 64 cages of sII. The intensity of 2915 cm−1 mode is more than that of 2905 cm−1 and this could be due to more number of 512 64 cages in unit cell for sII. The mixed hydrates with lower concentration of t-BuNH2 (x = 0.0056) show features in the Raman spectrum typical to that of sI structure with Raman bands around 2905 and 2916 cm−1 which indicates that methane molecules are entrapped in 512 and 512 62 cages (Fig. 3c). The Raman intensity ratios along with observed P, T trajectory indicates that mixed hydrates are dominated by sI hydrates. 4. Conclusions Tetrahydrofuran (THF) and tert-butylamine (t-BuNH2 ) are the thermodynamic promoters for the methane hydrates. The mixed hydrates formed with methane and THF/t-BuNH2 in sII stoichiometry dissociates at much higher temperatures (THF – 18◦ and t-BuNH2 – 12◦ ) compared to pure methane hydrate. At
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