Raman spectroscopic determination of hydration number of methane hydrates

• Published in: AIChE journal Vol. 45; no. 12; pp. 2641 - 2645
• Main Authors: Uchida, Tsutomu, Hirano, Takashi, Ebinuma, Takao, Narita, Hideo, Gohara, Kazutoshi, Mae, Shinji, Matsumoto, Ryo
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01.12.1999; Wiley Subscription Services; American Institute of Chemical Engineers
• Subjects: 03 NATURAL GAS; Condensed matter: electronic structure, electrical, magnetic, and optical properties; CRYSTALLOGRAPHY; Exact sciences and technology; GAS HYDRATES; Hydrates; Infrared and raman spectra and scattering; METHANE; NATURAL GAS; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Organic compounds, polymers; Physics; STOICHIOMETRY; THERMODYNAMICS; Methane; Hydrocarbons; Gas hydrates; Raman spectroscopy; Natural gas; Organic compounds
• ISSN: 0001-1541; 1547-5905
• DOI: 10.1002/aic.690451220

Hydration numbers of methane hydrates with various water/gas ratios of the sample were measured by Raman spectroscopy. Artificial methane hydrates were formed at temperatures ranging from 273.2 to 278.4 K, and pressures from 3.0 to 7.0 MPa, with a stirring rate of approximately 500 rpm. Under such nonequilibrium conditions, the obtained hydrate samples had a water/gas ratio ranging from 6.4 to 17.4, which was determined by mass measurements. The spectroscopic analysis revealed, however, that the crystallographic hydration number was almost constant at 6.2±0.2, which corresponds to the hydration number estimated from the thermodynamic model. The same measurements were carried out on natural gas hydrate samples obtained from the deep‐sea floor of Blake Ridge. The coincidence of the hydration numbers between natural and artificial samples indicated that the crystallographic hydration number is independent of the formation conditions and the large variation in the water/gas ratio of samples is described by the ratio between pure hydrate and free water.