HR: 0830h
AN: OS51B-0842    [PDF]
TI: Coexistance of two Different Methane Hydrate Phases at Moderate Pressure and Temperature Conditions
AU: Erzinger, J
EM: erzqgfz-potsdam.de
AF: AGU J”rg Erzinger, GeoForschungsZentrum Telegrafenberg, Potsdam, 14473 Germany
AB: For a better understanding of the formation and decomposition of natural gas hydrates, detailed data on their thermodynamical properties are an important prerequisite. Until now, it was generally accepted that small guest molecules such as methane and carbondioxid form structure I hydrates whereas large molecules such as propane form structure II hydrates. Expectedly, natural gas hydrates and synthesized methane hydrates grown under natural conditions, prefer structure I. The formation of structure II methane hydrates was only observed at very high pressures ( 100 MPa) by Chou et al. 1 In this contribution, results from Raman spectroscopic investigation on pure methane hydrates are presented. It will be shown, that a coexistence of structure I and structure II methane hydrates at pressures between 3.0 and 9.0 MPa and at temperatures between -15$\deg$C and +15$\deg$C is possible. However, structure II methane hydrate are present as a metastable phase in a kinetic inhibited equilibrium with the established structure I methane hydrate phase. A slight variation of the T-P-x-conditions initiates the transformation of the structure II methane hydrates into structure I hydrates. This (exothermic) process results in a re crystallization of the complete hydrate phase until a stable state is reached. It turned out, that the instability of the structure II methane hydrate phase is also the driving force for other processes, such as the exchange of CH4 with CO2: A change in the composition of the gaseous phase through the addition of CO2 induces an immediate replacement of methane with carbondioxid in the structure II hydrate. In contrast, the exchange of CH4 with CO2 in structure I hydrates is a very slow process. 1 Chou, I-M.; Sharma, A.; Burrus, R. C.; Shu, J.; Mao, H-k.; Hemley, R. J.; Goncharov, A. F.; Stern, L. A.; Kirby, S. H.; PNAS; vol. 97; no. 25;13484-13487 (2000)
DE: 0330 Geochemical cycles
DE: 1600 GLOBAL CHANGE (New category)
DE: 4806 Carbon cycling
DE: 4820 Gases
DE: 4825 Geochemistry
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting