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Transporting stranded gas as hydrates
When natural gas is discovered far from a pipeline and when the quantity of gas is insufficient to justify a liquefaction facility, it may be possible to transport gas by conversion into hydrates. The concept is that the hydrate concentrates gas by a factor of about 164, without the cost of compressing and transporting gas at high pressure.
Using hydrates for shipping
In shipping, preservation of hydrated gas is vital to prevent losses. Recent measurements have shown that only a mild amount of refrigeration (e.g., to 20°F) will prevent rapid hydrate dissociation, with rates that are several orders of magnitude less than values interpolated from lower or higher temperatures. The cause for this anomalous hydrate self-preservation effect is most likely uncertain because of an outer ice barrier that prevents inner-particle dissociation. The ice protective shell happens because the water from the melted hydrate surface, caused by endothermic hydrate melting, freezes.
Laboratory measurements and calculations indicated that hydrated-gas transportation is economical for distances that are greater than approximately 400 km. British Gas Ltd. has a pilot facility to convert gas to hydrate. The Japanese National Marine Research Institute began a similar project, and there will be further investigation of transportation economics compared to liquefied natural gas.
A schematic diagram of stranded-gas-hydrate formation, transportation, and dissociation system is shown in Fig. 1. In the first block, gas and seawater are combined to form hydrates on a floating production, storage, and offloading (FPSO) vessel. The second block is concerned with transportation and storage, and the third deals with hydrate dissociation and integration into the gas distribution system at a port. There are three factors to this approach becoming an economic process:
- Initial formation of large amounts of gas hydrates to prevent high-pressure recompression on recycle
- Reproducible, rapid conversion of water to hydrate so that a minimal amount of water is transported
- Transportation of hydrated gas to port with small amounts of refrigeration and dissociation. This is perhaps one of the more innovative emerging techniques for gas transportation.
- Stern, L.A., Circone, S., and Kirby, S.H. 2001. Anomalous Preservation of Pure Methane Hydrate at 1 atm. J. of Physical Chem. 105: 1756.
- Gudmundsson, J.S., Hveding, F., and Borrehaug, A. 1995. Transport of Natural Gas as Frozen Hydrate. Proc., Fifth Intl. Offshore and Polar Engineering Conference, The Hague, 11-15 June, 282.
- Shirota, H. et al. 2002. Hydrate Transportation of Stranded Gas. Proc., Fourth Intl. Hydrate Conference, Yokohama, Japan, 972.
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