A comparative study on the enhanced formation of methane hydrate using CM-95 and CM-100 MWCNTs

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Abstract

1 m3 of methane hydrate can be decomposed into a maximum of 216 m3 of methane gas under standard conditions. Conversely, such a large volume of methane hydrate can be utilized to store and transport a large quantity of natural gas. When methane hydrate is formed artificially by simply reversing its process of natural generation, the amount of methane gas consumed owing to hydrate formation is fairly low which would be problematic for its massive synthesis and application. In this study, experiments are carried out with the goal of increasing the amount of gas consumed by adding two kinds (CM-95 and CM-100) of multi-walled carbon nanotubes (MWCNTs) to pure water, where the physical length of CM-95 is much shorter than CM-100. When the 0.004 wt.% CM-95 MWCNT solution is compared with pure water, the gas consumption rate almost triples indicating its effect in hydrate formation. Also, the CM-95 MWCNTs decreased the hydrate formation time to a greater extent than the CM-100 MWCNTs at a low subcooling temperature.

Introduction

Gas hydrates are formed by the physical binding between water molecules and gases such as methane, ethane, propane, carbon dioxide, etc., which are captured in the cavities of the water molecules at a specific temperature and pressure. These hydrates are solid containers similar to ice, whose crystal structure is composed of polyhedron cavities consisting of water molecules connected by hydrogen bonds, and is expressed as nm. For example, 51262 means 14 cavities with 12 pentagons and 2 hexagons. The following cavity types have so far been recognized: 512, 51262, 51264, 51268, and 435663. Structure I in Fig. 1 is a combination of 6 polyhedrons (51262) containing 14 facets with 2 polyhedrons (512) containing 12 facets, structure II in Fig. 2 is a combination of 16 polyhedrons (512) containing 12 facets with 8 polyhedrons (51264) containing 16 facets, and structure H in Fig. 3 is a combination of 3 polyhedrons (512) containing 12 facets with 2 polyhedrons (435663) containing 16 facets and 1 polyhedron (51268) containing 20 facets [1], [2]. Methane hydrate in Fig. 4 is a kind of gas hydrate formed by the physical binding between a water molecule and methane gas molecule captured in the cavities of the water molecules at specific temperatures and pressures, which belongs to structure I.

Naturally produced methane hydrate is widely dispersed in the continental slopes and continental shelves of the Pacific and Atlantic oceans, Antarctica, etc. The reserve of fossil fuels amounts to some 500 billion carbon tons and that of methane consists of 360 million carbon tons. The reserve of methane hydrate amounts to more than 1 trillion carbon tons, which is twice that of fossil fuels [3]. Therefore, methane hydrate, a particular kind of gas hydrate, is expected to replace fossil fuels as a new energy source in the 21st century.

Also, 1 m3 of methane hydrate can be decomposed into a maximum of 216 m3 of methane gas under standard conditions [4]. If this characteristic of hydrates is utilized in the opposite sense, natural gas can be fixed into water in the form of a hydrate solid. Therefore, hydrates are considered to be a great way to transport and store natural gas in large quantities. In particular, the transportation cost of methane hydrate is expected to be 18–24% lower than that of liquefied natural gas [5], [6].

The methane hydrate supply chain consists of three main parts, viz. the production, marine transportation and regasification processes. The production part can be located on land using loading facilities for large hydrate carriers. Transportation is performed by bulk carriers specially designed for dry hydrates, hydrate slurries, and pellet type hydrates. The regasification part of the frozen hydrate takes place at a receiving terminal located close to the market for natural gas [7].

However, when methane hydrate is formed artificially, the quantity of gas fixed in water may be relatively low, due to the slow reaction between water and methane gas. Therefore, many researchers have studied methods of enhancing the formation of methane hydrate for natural gas transport and storage. Lin et al. [8] investigated experimentally the effects of the anionic surfactant, sodium dodecyl sulfate (SDS), on the methane hydrate storage capacity and showed that the presence of SDS could enhance the formation process of methane hydrate. Cho and Lee [9] reported that the hydrate formation rate increased with increasing concentration of sodium dodecyl benzene sulphonic acid (DBS). Seo et al. [10] and Ryu et al. [11] examined the active roles of porous silica gels and nanosized materials when used as natural gas storage media.

In this study, comparative measurements are carried out on the hydrate formation time, gas consumption, and equilibria by adding two kinds of multi-walled carbon nanotubes (MWCNTs) to pure water to pure water, which are considered to be a potential material for the storage of methane gas [12].

Section snippets

Experimental apparatus

Fig. 5 shows a schematic diagram of the experimental apparatus used in this study. A 350 mL reactor (20) and 100 mL supply vessel (10) were manufactured with SUS316 to endure a pressure of over 10 MPa and salt erosion. The reactor was immersed in a constant temperature bath (water + ethylene glycol). Two copper-constantan thermocouples (9) were used to measure the temperatures of the gas and liquid in the reactor, and one pressure gauge (8) was installed to check the inside pressure of the reactor.

Measurements of gas consumption

150 mL of distilled water was poured into the reactor and cooled to 274.15 K and the experimental gas injected at the experimental pressure. The experiments were carried out for 12 h and the temperature was kept constant until the termination of the experiment. As the experimental gas reacts with distilled water to form hydrates, it is constantly replenished by the diaphragm and metering valves, maintaining the pressure at a constant preset value. At any instant, the number of moles of the gas

Conclusions

Comparative experiments were carried out with the goal of increasing the hydrate formation rate by adding two kinds of MWCNTs to pure water. The maximum rate of gas consumption was observed for the 0.004 wt.% solutions of CM-95 and CM-100 MWCNTs in pure water. Especially, the 0.004 wt.% CM-95 MWCNT solution's gas consumption rate exceeded three times that of pure water. Regardless of the type of additive used, the amount of gas consumed was the highest when the CM-95 MWCNTs were present in the

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. R01-2008-000-20575-0 and No. 2009-0092786) and the 2nd stage Brain Korea 21 program of the Ministry of Education and Human Resources Development.

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Communicated by W.J. Minkowycz.

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