Kinetic and Thermodynamic Aspects of Clathrate Hydrate Nucleation and Growth

Abstract

In this study we present results of our investigations on simple CH4-hydrate and mixed hydrates during the initial steps of the hydrate formation process. In situ Raman spectroscopy, microscopic observation and in situ X-ray diffraction were used in our systematic studies. Although these techniques give only a limited view on the molecular level the combined results from the experiments reported here indicate that the labile cluster hypothesis can describe the initial hydrate formation process. Specifically, the guest molecules dissolve in the aqueous phase before they are encaged into single hydrate cavities which agglomerate to a solid phase. Results from Raman spectroscopic measurements suggest that the initially formed solid phase can be characterized by an excess of pentagonal dodecahedrons, whereas the formation of tetrakaidecahedrons or hexakaidecahedrons occurs as a subsequent step. At the time the tetrakaidecahedrons or hexakaidecahedrons are observed with Raman spectroscopy, corresponding X-ray diffraction experiments indicate the formation of a crystalline hydrate phase. Therefore, we assume that the solid phase formed at the very first state is not a hydrate phase in terms of a crystalline structure but some kind of an amorphous hydrate which transforms subsequently into a crystalline hydrate phase. Furthermore, the results suggest that the formation process and the properties of the resulting hydrate phase strongly depend on the properties of guest molecules.

From Microscale (400 μl) to Macroscale (425 L): Experimental Investigations of the CO2/N2‐CH4 Exchange in Gas Hydrates Simulating the Iġnik Sikumi Field Trial

“…The results indicate an enrichment of CO 2 in the secondary formed hydrate phase compared to the initial gas phase (23 mol% CO 2 and 77 mol% N 2 ). CO 2 is known as a good hydrate former, and the enrichment of CO 2 in the hydrate phase has already been reported in a different context elsewhere (Schicks & Luzi‐Helbing, ). Since N 2 is a small molecule and a comparably weak hydrate former (Sloan & Koh, ), the amount of N 2 encased in the hydrate phase is low compared to the amount of N 2 in the free gas phase.…”

Influence of temperature on methane hydrate formation

“…These amorphous clusters are deemed precursors of nucleation of clathrate hydrates 41 . Their specific formation pathways are significantly affected by the size and solubility of guest molecules 42 , a finding that has been confirmed with macroscopic measurement methods including Raman spectroscopy and X-ray diffraction 43 . Furthermore, using MD simulations, Walsh et al .…”

Communication: Structural interconversions between principal clathrate hydrate structures

“…For example, Schicks and Ripmeester 15 observed the coexistence of sI and sII hydrates formed from methane, ethane, and propane mixtures and Uchida et al 38 reported the coexistence of sI and sII hydrates formed from methane and propane mixtures. In recent work, Schicks and Luzi-Helbing 17 were also able to demonstrate the coexistences of sI and sH hydrates for a gas mixture of methane and FIG. 4.…”

“…A recent study reports the coexistence of sI and sH hydrates formed from a gas mixture of methane and 2-methylbutane. 17 Given these experimental observations, it would be interesting to identify the molecular mechanism of the possible structural a) Authors to whom correspondence should be addressed. Electronic addresses: liangshuai@ms.giec.ac.cn and pkusalik@ucalgary.ca interconversion and packing polymorphism between these crystalline phases; this is the focus of this study.…”