The Conversion Process of Hydrocarbon Hydrates into CO sub(2) Hydrates and Vice Versa: Thermodynamic Considerations

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2011-11, Vol.115 (46), p.13324-13331-13324-13331
• Main Authors: Schicks, J M, Luzi, M, Beeskow-Strauch, B
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Chemical potential; Confocal; Conversion; Diffraction; Exchange; Hydrates; Thermodynamics
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp109812v

Microscopy, confocal Raman spectroscopy and powder X-ray diffraction (PXRD) were used for in situ investigations of the CO sub(2)-hydrocarbon exchange process in gas hydrates and its driving forces. The study comprises the exposure of simple structure I CH sub(4) hydrate and mixed structure II CH sub(4)-C sub(2)H sub(6) and CH sub(4)-C sub(3)H sub(8) hydrates to gaseous CO sub(2) as well as the reverse reaction, i.e., the conversion of CO sub(2)-rich structure I hydrate into structure II mixed hydrate. In the case of CH sub(4)-C sub(3)H sub(8) hydrates, a conversion in the presence of gaseous CO sub(2) from a supposedly more stable structure II hydrate to a less stable structure I CO sub(2)-rich hydrate was observed. PXRD data show that the reverse process requires longer initiation times, and structural changes seem to be less complete. Generally, the exchange process can be described as a decomposition and reformation process, in terms of a rearrangement of molecules, and is primarily induced by the chemical potential gradient between hydrate phase and the provided gas phase. The results show furthermore the dependency of the conversion rate on the surface area of the hydrate phase, the thermodynamic stability of the original and resulting hydrate phase, as well as the mobility of guest molecules and formation kinetics of the resulting hydrate phase.