Preservation of methane gas in the form of hydrates: Use of mixed hydrates

https://doi.org/10.1016/j.jngse.2015.04.030Get rights and content

Highlights

  • Methane gas storage in self-preservation window.

  • Mixed hydrates with large amount of CH4 has no self-preservation.

  • Mixture of sI & sII hydrates formed in lower mole fractions of 51264 guests.

  • Delayed release of CH4 gas in hydrate mixtures is due to melting of sII component.

Abstract

The release of methane gas was compared in pure CH4 (sI) and mixed (sII) hydrates (with C4H8O – tetrahydrofuran and C3H8 – propane) having methane as dominant constituent. We report absence of the self preservation effect in mixed hydrates, having stiochiometric composition (sII) of larger guest molecules, and they populate the 51264 cages. Their dissociation behaviour is in accordance with the respective phase boundary curve. While a partial methane gas release was observed at 270 K, particularly, in dilute systems. Further, excess gas release was at T > 270 K and complete methane release is governed by the thermodynamic stability of mixed hydrates.

Introduction

Adaptability of gas hydrate based technology for natural gas storage and transportation demands a stringent pressure and temperature conditions for its stability. Its transportation through a network of pipelines is particularly useful for long haul, while it becomes uneconomical for mid or short distances (Gudmundson and Borrehaug, 1996). The liquefied natural gas (LNG) or compressed natural gas (CNG) transportation methods, although, preferentially used in these sectors have inherent operational difficulties, such as need of cryo-temperature and high pressures respectively (Thomas and Dawe, 2003). Thus, these techniques require skilled man-power for sustained and safer operations. Adoption of gas hydrate based methodology in this sector is attractive, but the limitations are mostly technology driven (Nakoryakov and Misyura, 2013, Mimachi et al., 2014). The hydrate conversion is quite inefficient and time consuming without an agitator and therefore some special reactors are being employed. Another approach is to add of some kinetic promoters or surfactants to the hydrate forming (gas + water) system; and such experiments are still at laboratory scale (Ganji et al., 2007). Additionally, usage of certain porous materials have shown some attractive features in hydrate conversion process (Prasad et al., 2014, Chari et al., 2013a, Chari et al., 2013b, Kang and Lee, 2010, Kang et al., 2013, Linga et al., 2007).

Clathrate hydrates, or gas hydrates, are the crystalline ice-like inclusion compounds consisting of hydrogen bonded water molecules forming open cages of different sizes. Four essential conditions often required for its stable existence are (i) enough supply of guest (suitably sized hydrocarbons or other) and (ii) host (water) molecules; and simultaneous presence of (iii) moderately high pressure, (iv) lower temperature conditions. Such stringent requirements are often fulfilled in certain geological locations around the globe, both under the permafrost and the oceanic sediments. Typical pressure and temperatures for the methane hydrates stability are 2.5 MPa and 273 K; and required pressure increases exponentially at higher temperatures (Sloan and Koh, 2008). On the other hand gas hydrates encasing methane molecules (dominant constituent in natural gas hydrates) can be preserved for a longer duration even at ambient pressure in the temperature window 240–270 K, and this is most popularly known as “anomalous (or self-) preservation effect” (Stern et al., 2001, Stern et al., 2003). The metastable nature of methane hydrates (MH) has been a subject of immense interest; however, a precise molecular mechanism responsible for this effect has not emerged. For example, is it an exclusive property for methane hydrates (sI - Pm3n) alone or is it true for all other sI or sII (Fd3m) hydrates also. Will the mixed hydrates (sII), with CH4 as dominant fraction, possess this exceptional property? How can one model the mechanism for this unique property without ambiguities? Nevertheless unexpected longevity of the methane hydrates at ambient pressure found important applications, particularly, in gas storage and transportation of natural gas hydrates, which are dominated by the methane gas (Takeya et al., 2012, Mimachi et al., 2014). There are several factors that control the rate of hydrate dissociation, such as experimental temperature (Stern et al., 2001, Stern et al., 2003) and pressure (Komai et al., 2004). Further, composition of the feed gas (Stern et al., 2003, Takeya and Ripmeester, 2010, Uchida et al., 2011a, Uchida et al., 2011b), hydrate granular size (Takeya et al., 2005, Mimachi et al., 2014) and co-existence of certain additives or electrolytes (Zhang and Rogers, 2008, Takeya et al., 2012, Sato et al., 2013), and presence of fine glass beads (Hachikubo et al., 2011) in hydrate forming systems also influence the dissociation process. The effect of temperature on the dissociation process is exhaustively studied in the past by Stern et al. (2001) and they reported significantly slower at atmospheric pressure in a temperature window 241–271 K (generally known as self-preservation window), with two minima at 249 and 269 K. Nakoryakov and Misyura (2013) have reported that the thermal dissociation rate is significantly lower in natural hydrates compare to synthetic analogues in self preservation window despite of structural similarity.

Takeya and Ripmeester (2010) have shown that the self-preservation property is not an exclusive for MH alone, but the hydrates with certain other guest molecules like CH3F, CF4 and CO2 forming sI structure and O2, N2, Ar and Kr forming sII can also show this phenomenon. These authors further argued that the interactions between the guest molecules and H2O molecules in ice play a critical role in self-preservation phenomenon. Uchida et al., 2011a, Uchida et al., 2011b have summarized earlier kinetic models on this unusual self-preservation phenomena and have concluded that the ice shield, around hydrate grains, below 240 K is mostly granular type and the retardation in gas diffusion is governed by slower hydrate dissociation rather than gas diffusion. However, in the self-preservation temperature region, i.e., 240 K < T < 270 K, the morphology of ice shield is plate-like with stacking faults and hydrate dissociation is governed by the molecular gas diffusion through the ice layers. Further, the ice layer also stabilizes the hydrate structure vindicating the importance of gas-H2O interactions in this phenomenon. Takeya et al. (2011) have measured the average thickness of ice layer around hydrate particles as 100 μm after preserving them for 24 h at 253 K. Although it is difficult to understand the effect of gas-H2O interactions on the self-preservation of gas hydrates quantitatively, they become very important when we consider the natural gas hydrates as storage and transportation materials. Zhang and Rogers (2008) claimed that the 0.04% of the stored gas was evolved during 256 h at 268 K from the hydrates formed with gas mixture of (CH4 + C2H6 + C3H8) at atmospheric pressure. According to these authors, ice shielding is not the primary mechanism for this phenomena and the enhancement of preservation by usage of additives may be of practical possibility. Recently Kida et al. (2011) proposed that the “direct measurements of the dissociation behaviours of pure methane and ethane hydrates trapped in sintered tetrahydrofuran hydrate through a temperature ramping method showed that the tetrahydrofuran hydrate controls dissociation of the gas hydrates under thermodynamic instability at temperatures above the melting point of ice”. The sample preparation procedure adopted by Kida et al. (2011) involved several steps, such as mixing-up of fine powders of independently prepared MH and THF hydrates at low temperatures. Later they were pelletized using oil pressure of 6 MPa and 263 K. Further prior to depressurization experiments, the pellets were again broken into 1–2 mm chips and were soaked for about 30 min under methane gas pressure of 6 MPa and 263 K. Thus, the possibility of mixture of both sI (MH) and sII (THF + CH4) hydrates cannot be excluded in such experiments. Thus it was not clear whether the delayed dissociation of hydrates was due to mixed hydrates or due to anomalous preservation nature of sI hydrates. In order to gain further understanding we compared the dissociation behaviour of methane hydrates (MH), after depressurization, in its pure (sI) and mixed (with THF – 0.052, 0.021 & 0.011 and C3H8 gas – 0.13, 0.051 & 0.017 mol fractions) form. The THF is highly soluble in water and readily form sII hydrates, while propane is less soluble and also form sII hydrates because of its larger molecular size.

Section snippets

Experimental

Experimental procedure followed for gas hydrate synthesis has already been described earlier (Chari et al., 2011, Chari et al., 2013a, Sharma et al., 2014). Briefly, the main part was an SS-316 cylindrical vessel, which can withstand gas pressures up to 15 MPa, and volume of the vessel was 400 mL. A cold fluid (water + glycol mixture) was circulated around the vessel with the help of a circulator to bring and maintain temperature inside the cell at a desired level. A platinum resistance

Results and discussion

Table 1, shows the mole fractions of feed guest molecules and the amount of gas liberated during the dissociation of gas hydrates. The gas release below 273 K is mostly due to the dissociation of sI component, while that above is because of sII hydrate dissociation. The hydration number for pure methane hydrates has been computed as 5.94 (Chari et al., 2014) and that for C3H8 + CH4 mixtures is estimated from CSMGEM program (Sloan and Koh, 2008). The hydration number for THF + CH4 hydrates was

Conclusions

In summary, our experimental results show that pure methane hydrates exist in metastable state in a temperature range 250–268 K upon depressurization to atmospheric pressure. The hydrate dissociation is rapid in a smaller temperature range, i.e., 268–270.8 K. When the cumulative gas pressure exceeds 2.35 MPa, further hydrate dissociation is governed by the thermodynamic stability conditions of methane hydrates. On the other hand, it is possible to store CH4 gas in the form of mixed hydrates and

Acknowledgements

Author sincerely thanks the Director of the National Geophysical Research Institute, Hyderabad, for his encouragement, and permission to publish this paper. Partial financial support from DST (India) and DGH-NGHP (India) is acknowledged. This is a contribution to GEOSCAPE Project of NGRI under the 12th Five Year Scientific Program of CSIR.

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