5. Clathrate Hydrate Formation
Angela Carstensen, Minjas Zugik,
Jefferson Creeka and Carolyn A. Koh*
King’s College London,
Department of Chemistry,
Strand, London WC2R 2LS
*Corresponding Author: email@example.com,
tel. +44 (0)207 848 2380,
fax +44 (0)207 848 2810.
aChevron Petroleum Technology,
Houston, TX 77082,
The formation of tetrahydrofuran (THF) hydrate and methane hydrate has been studied using in situ Raman spectroscopy. The effects of the addition of 1 wt.% and of 5 wt.% methanol, a thermodynamic inhibitor, and of 0.5 wt.% poly-N-vinyl pyrrolidone, a kinetic inhibitor, on THF hydrate formation have been examined. The addition of methanol was found to have no significant effect on the hydrate growth period. However, the appearance of the ring-breathing mode of THF was found to be significantly different for the THF hydrate sample grown in the presence of methanol compared to that grown in its absence. This could be indicative of the perturbations on host and/or guest molecules due to the presence of methanol, although these underlying structural reasons are currently under further investigation. In contrast, the hydrate spectrum was unaffected by the presence of poly-N-vinyl pyrrolidone even though the inhibitor retarded the growth of THF hydrate. The ratio of large-to-small cages occupied by methane in methane hydrate was found to remain at around 2:1 in the presence of poly-N-vinyl pyrrolidone, until the temperature was significantly reduced (by at least a further 2 oC of sub-cooling), at which stage the usual 3:1 large cage : small cage ratio is obtained.
Clathrate hydrates  are solid inclusion compounds composed of polyhedral water cavities, which incorporate guest molecules, including a wide variety of relatively small gas molecules as well as some liquids, such as tetrahydrofuran (THF). The two most common types of clathrate hydrate are structure I (e.g. formed by methane guests) and structure II (e.g. formed by propane, a methane/propane mixture, or THF guests). Both structure I hydrate and structure II hydrate contain dodecahedral cavities (consisting of twelve pentagonal faces), which can incorporate guest molecules of up to 5.8 Å in diameter. Structure I hydrate also contains larger tetrakaidecahedral cavities (consisting of twelve pentagonal and two hexagonal faces), which can accommodate guest molecules of up to 6.1 Å diameter. Structure II hydrate contains even larger hexakaidecahedral cavities (twelve pentagonal and four hexagonal faces) which can enclathrate guest molecules of up to 6.6 Å in diameter.
Gas hydrates can form in gas and oil subsea transmission and production pipelines when the temperature and pressure conditions of the fluid within the pipeline are within the hydrate stable region . This can lead to blockage of the pipelines and severe economic and safety risks to the industry . Methane hydrate has also been shown to occur naturally in sediments in the deep ocean continental margins . Since these methane hydrate deposits are so extensive (1 m3 of methane hydrate contains the equivalent of around 156 m3 of methane under standard conditions), they present an attractive energy resource for the future.
THF forms structure II hydrate which is the same structure as that of natural gas hydrates. THF hydrate (see figure 1 ) is, therefore, a reasonable model of industrial gas hydrates. The advantage of studying THF hydrate as a model system is that it can be prepared at temperatures below ca. 277 K and at atmospheric pressure. In contrast, gas hydrates require high pressure conditions. In order to control the formation of gas hydrates in pipelines or develop the technology required for extracting methane from oceanic sediments, further understanding of the structural, phase equilibria and kinetic properties of these compounds is needed . The study of THF hydrate and gas hydrate formation and inhibition is important industrially, as well as from a fundamental scientific perspective. One particularly useful and efficient technique for studying these properties of gas hydrates is Raman spectroscopy.
Figure 1. A model of THF hydrate, where the positions of the THF guest molecules have been arbitrarily inserted into the large hexacaidecahedral cavities .
Previous vibrational spectroscopic studies of clathrate hydrates have been largely concerned with the mid-infrared spectra of the water host lattice and guest molecules recorded at cryogenic temperatures [5-7]. The bandwidth and shape of the O-D stretching mode of isotopically isolated HOD molecules of the water lattice has been used to characterize the different hydrate structures . There are four different O…O distances [1,5] in the hydrate unit cell: 2.79Å, 2.75Å, 2.81Å, 2.77Å in structure I hydrate and 2.75Å, 2.76Å, 2.77Å, 2.78Å in structure II hydrate. Therefore, an O-D stretching band of full width at half height (FWHH) of around 80 cm-1 is assigned to a clathrate hydrate structure (the band shape is asymmetric for structure I and Gaussian for structure II), whereas a FWHH of around 20 cm-1 is assigned to ice Ih .
Vibrational spectroscopy provides a useful probe for studying the structural characteristics of hydrate samples because the vibrational motions of water molecules and guest molecules are different for structure I hydrate, structure II hydrate, a mixture of liquid water and hydrate former, and ice. However, the difficult sampling techniques for infra-red spectroscopy involving either epitaxially growing thin films by molecular beam deposition, or condensation of polar guest molecules onto an ice surface, or preparation of adamantane pellets at cryogenic temperatures have limited the use of this technique for hydrate studies. Raman spectroscopy, on the other hand, is far more convenient for hydrate studies (the requisite vibrational modes to be studied are also Raman active), in terms of simple sample preparation and the scope for in situ studies at temperature and pressure conditions close to industry operating conditions.
Previous Raman studies of THF hydrate have shown that the position of the ether C-O-C antisymmetric stretching band at 1073 cm-1 is much closer to the gas phase value (1080 cm-1) than that observed for an amorphous low temperature condensate with water (doublet at 1052 cm-1 and 1034 cm-1) . The THF ring-breathing mode varies with temperature, with a low frequency shoulder appearing at temperatures below 100 K, which is attributed to a reduction in the rapid reorientational motion of THF in the large cage .
The effect of pressure on the Raman spectrum of THF hydrate shows that the bandwidth of the O-D stretching mode decreases by 38.5 cm-1 per GPa at 240 K, and above 0.26 GPa bands characteristic of ice II appear. Further increases in pressure result in degradation of the hydrate structure corresponding to the appearance of bands due to ice V and loss of THF from the material .
Recently, a few Raman spectroscopic studies have been performed on gas hydrates at high pressure and low temperature to measure gas hydrate phase equilibria and establish the guest occupancy of the large and small cavities of sI hydrate and sII hydrate [10-12]. In particular, the n1 symmetric C-H stretch of methane is indicative of the type of hydrate structure formed since its frequency depends on the type of cavity it occupies. For structure I hydrate, bands at 2915 cm-1 and 2905 cm-1 are assigned to CH4 occupying small cavities and large cavities, respectively; while structure II hydrate (from a CH4/THF-d8 mixture) gives bands at 2914 cm-1 and 2904 cm-1 assigned to CH4 occupying the small and large cavities, respectively . The relative intensities of the n1(C-H) peaks due to methane occupying the large and small cavities depend directly on the relative occupancies of these cavities. The ratio of large : small cavities occupied by methane is 3:1 in structure I hydrate and 1:2 in structure II hydrate .
Although a few Raman spectroscopic studies have been recently reported on gas hydrate formation , the Raman spectroscopic work reported so far on the effect of inhibitors on hydrate formation has been limited. The work presented in this paper is focussed on in situ Raman spectroscopic studies of THF hydrate (structure II hydrate) and methane hydrate (structure I hydrate) formation without and with the presence of hydrate inhibitors. The effect of both thermodynamic and kinetic inhibitors on the hydrate formation process and Raman spectral data are examined. A thermodynamic inhibitor, such as methanol, operates by shifting the equilibrium conditions required for hydrate formation to lower temperatures and/or higher pressures. On the other hand, a kinetic inhibitor retards hydrate formation. These experiments were performed over a wide frequency range, with the purpose of obtaining both kinetic information and further understanding the interactions between inhibitor and guest molecules and between inhibitor and host molecules.
In situ Raman spectra were recorded at atmospheric pressure and ca. 5.5 MPa for THF and methane hydrates respectively and 298 K to 273.8 K. The laser was focused in the variable temperature and high-pressure cells through optical windows and the scattered light was collected with the same lens. The instrument was calibrated using a mercury lamp at 5460.7 Å. A cooling bath and a constant pressure pump were used to maintain constant temperature and pressure, respectively. The temperature of the cells was measured with thermocouples inside the cell inserted directly into the aqueous phase. In the case of the methane hydrate experiments the cell was stirred at all times except during collection of spectra. In the case of the THF hydrate experiments, the THF/water solution (1:17 mole fraction) was pre-cooled to the desired temperature and spectra were recorded every minute in the low frequency region and in the ring-breathing mode region. After about two minutes from the start of the experiment, the solution was probed with a syringe tip to induce nucleation.
Results and Discussion
The formation of THF hydrate and methane hydrate has been studied using in situ Raman spectroscopy to obtain information on the crystal growth period of these materials. In the case of THF hydrate, both water lattice modes and guest modes were monitored during hydrate crystallisation, while in the case of methane hydrate only the guest modes were monitored during the growth process. The effects of both a thermodynamic inhibitor and a kinetic inhibitor on the hydrate growth period and the guest/host Raman spectral features have been studied. The following section gives a summary of these results for uninhibited and inhibited THF hydrate and methane hydrate.
Figure 2 shows the Raman spectrum of a solid THF hydrate sample prepared from a THF/water solution containing 5 wt.% D2O (with respect to H2O). The most important features for investigating hydrate formation are: the host water lattice mode ca. 200 to 300 cm-1, the O-D stretching band, and the THF ring-breathing band at 916 cm-1. The water lattice vibrational mode of THF hydrate is similar to that of ice Ih, and its presence indicates whether the sample has solidified. The O-D stretching band occurs at around 2450 cm-1 and its FWHH value is used to confirm whether liquid water, hydrate, or ice is present.
Figure 2. The complete Raman spectrum of THF hydrate (containing 5 wt.% D2O with respect to H2O).
Figure 3. The ring-breathing mode and anti-symmetric stretching mode of liquid THF, an aqueous THF solution, and THF hydrate.
Figure 3 shows the ring-breathing mode, the symmetric C-C-C stretching mode, and the anti-symmetric stretching mode of liquid THF, an aqueous THF solution, and THF hydrate. The THF ring-breathing band at 913 cm-1 of liquid THF consists of at least three overlapped bands that correspond to normal modes that have the same energy (detailed normal coordinate analysis can be found in ref. ). This accidental degeneracy is a consequence of the rapid pseudorotational motion of the THF molecule . Therefore, the vibrational energy levels of the THF molecules attain higher symmetry than C2 symmetry on the Raman scattering time scale. In an aqueous solution of THF and water, THF molecules become hydrogen-bonded to water molecules and this raises the barrier for pseudorotation. This relaxes the degeneracy and the band splits into two components with a shoulder (at around 893 cm-1) on the low frequency side of the ring-breathing band (see figure 3). THF molecules incorporated within hydrate cages are free to pseudorotate and therefore the two bands merge again at 919 cm-1 (this is slightly shifted from the position of the band observed for liquid THF). Consequently, this signal can be used to follow the changes in the vibrational energy levels of THF molecules during the hydrate formation process.
Thus, the appearance of the ring-breathing mode is similar for liquid THF and for THF hydrate. Recording spectra at regular intervals during hydrate growth from a THF/water mixture, a gradual change from spectra corresponding to THF molecules hydrogen-bonded to water to spectra corresponding to ‘free’ THF molecules is observed (see figure 4). The time from the start of nucleation to the time at which the spectra no longer change is taken as the hydrate growth period.
Figure 4. In situ Raman spectra during THF hydrate formation at around 274 K in the ring-breathing mode region (spikes are cosmic ray artifacts).
Recording the low frequency region during THF hydrate formation (see figure 5), the water lattice modes are seen to appear gradually from the foot of the Rayleigh line that dominates the spectrum of the THF/water solution and from the LA mode (this mode is clearly seen in the Raman spectrum of the solution) of the hydrogen-bonded water molecules. Again, the time from the start of nucleation until the time at which the spectra no longer change is taken as the time for the conversion of liquid water to the hydrate water lattice.
Figure 5. In situ Raman spectra during THF hydrate formation at around 274 K in the low frequency region (spikes are cosmic ray artifacts).
Figures 6 and 7 are contour diagrams of the in situ Raman spectra given in figures 4 and 5 normal to the intensity axis (the different colours in these projections represent different intensities, however intensity is in arbitrary units, therefore the important feature in these projections is the shape of the contour bands). The vertical axis is time in minutes. In both the ring-breathing mode and low frequency regions, the spectra start to change after about 12 minutes from the start of nucleation. Both regions show that the hydrate growth period is completed after about 18 minutes.
Figure 6. In situ Raman spectra during THF hydrate formation at around 274 K in the low frequency region projected along the intensity axis.
Figure 7. In situ Raman spectra during THF hydrate formation at around 274 K in the ring-breathing region projected along the intensity axis.
After each in situ experiment, the O-D stretching band was examined to check whether any ice was present. Figure 8 shows the comparison between the n(O-D) mode for ice and that for THF hydrate. The FWHH is clearly shown to be significantly larger for THF hydrate compared to that for ice. The position of the n(O-D) peak maximum is also slightly shifted in the hydrate spectrum compared to the ice spectrum.
Figure 8. n(O-D) band of ice and THF hydrate at around 268 K.
Two additives, methanol and poly-N-vinyl pyrrolidone (PVP) representing proto-typical examples of a thermodynamic inhibitor and a kinetic inhibitor, respectively, were selected for study. Figure 9 shows that the growth velocity of THF hydrate does not change significantly in the presence of 1 wt.% methanol. The same observation was made with the addition of 5 wt.% methanol. This inhibitor only shifts the thermodynamic equilibrium to lower formation temperatures (or higher pressures), rather than modifying the crystal growth process.
Figure 9. Projected view of the ring-breathing region during THF hydrate formation in the presence of 1 wt.% methanol at 274 K
PVP, on the other hand, significantly retards THF hydrate growth (see figure 10). This may be explained by PVP molecules being adsorbed onto the hydrate surface [15-17], and therefore one would expect this to affect the crystal growth process.
Figure 10. Projected view of the ring-breathing region during THF hydrate formation in the presence of 0.5 wt.% PVP at around 274 K.
The n(O-D) band for THF hydrate grown in the presence of both methanol and PVP does not differ significantly from that of THF hydrate grown without inhibitor (see figure 11). The FWHH values for this band are 66 cm-1 and 62 cm-1 for THF hydrate grown in the presence of methanol and PVP, respectively. This confirms that structure II hydrate is obtained for THF hydrate grown without and with the presence of both inhibitors. However, a closer look at the ring-breathing region of these hydrates grown with inhibitors (figure 12) reveals that in the case of methanol, the additional shoulder at lower frequency does not disappear completely even after 24 hours. In contrast, in the presence of PVP, the low frequency shoulder of the THF hydrate ring-breathing mode completely disappears within 40 minutes of the experimental period.
Figure 11. The n(O-D) band of THF hydrate formed in the presence of 1 wt.% methanol and 0.5 wt.% PVP at around 274 K.
Figure 12. Ring-breathing band of THF hydrate grown at around 274 K in the presence of 1 wt.% methanol and 0.5 wt.% PVP.
This observation may be attributed to one of two effects: there may be still some liquid THF hydrogen-bonded to water molecules trapped between the crystallites, although the low frequency spectra suggest that all the liquid water has been converted to hydrate. Alternatively some THF molecules may remain hydrogen-bonded to the hydrate lattice and therefore retain their molecular symmetry. However, the molecular basis underlying these results remains to be fully established and is currently being probed in detail.
The n1 C-H symmetric stretch of methane was recorded during methane hydrate formation without and with the addition of PVP. The integrated band intensity ratio of methane in a large cage to methane in a small cage increases to approximately 3:1 during methane hydrate formation which is similar to the results reported by Sloan et al. . Figure 13 shows the symmetric C-H stretching mode during methane hydrate formation in the presence of the kinetic hydrate inhibitor, PVP (0.5 wt.%). The integrated band intensity ratio of methane in a large cage to methane in a small cage for methane hydrate grown in the presence of PVP remains below 2:1 (see figure 14) until the system is cooled below 273 K, at which stage the usual 3:1 ratio was obtained.
Figure 13. In situ Raman spectra of the symmetric C-H stretching mode during (green traces) and after (red trace) methane hydrate formation in the presence of the kinetic inhibitor, PVP.
Figure 14. Integrated band intensity ratios of methane in a large cage to methane in a small cage (C-HLC /C-HSC) for methane hydrate grown in the presence of the kinetic inhibitor, PVP, at around 275 K, except where indicated.
The formation of THF hydrate has been investigated by observing changes in the Raman bands due to both the THF guest molecules and the water host lattice from an aqueous THF solution. Solidification of the water framework coincides with the complete conversion from THF molecules hydrogen-bonded to water to ‘free’ THF molecules. The same method was applied to aqueous THF solutions containing methanol and PVP inhibitors. Methanol does not affect the THF hydrate growth period. In contrast, PVP increases the growth period significantly. This may be explained by the adsorption of PVP molecules onto the crystal surface, thereby disrupting the THF-water-hydrate adsorption equilibrium. It was also found that THF hydrate grown in the presence of methanol contains a non-negligible concentration of hydrogen-bonded THF molecules, which is not observed with PVP inhibited hydrate. Finally, the presence of PVP was found to modify the crystal growth process of methane hydrate.
The financial support of the Gas Research Institute, DeepStar Industry Consortium, and Chevron Petroleum Technology are gratefully acknowledged.
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Received 16th November 2001, accepted 1st December 2001.
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