Spectroscopic Observations and Thermodynamic Calculations on Clathrate Hydrates of Mixed Gas Containing Methane and Ethane:  Determination of Structure, Composition and Cage Occupancy

Abstract

Clathrate hydrates of methane-ethane mixed gases have two crystal structures depending on their composition. To study their compositions and cage occupancies and how their structure is determined, we synthesized hydrate samples from methane-ethane mixtures. Analysis of the samples using X-ray diffraction, Raman spectroscopy, and gas chromatography revealed their structures, compositions, and cage occupancies. Experimentally, hydrate structure II existed in samples formed when the gas equilibrated with hydrates was approximately 2% C 2 H 6 (molar fraction) whereas both structures I (sI) and II (sII) coexisted for 12 to 22% C 2 H 6 . The structures below 2% and above 22% of C 2 H 6 existed only as sI hydrates. Volume ratios of both structures were obtained from the ratio of the peak intensities of the C-C stretching peaks in the Raman spectra. In the transition zone containing both structures, the volume ratio of the sII structure gradually decreased with increasing C 2 H 6 concentration. Cage occupancies of guest molecules in the hydrate cages were determined by the relative intensity ratio of Raman spectra. C 2 H 6 molecules occupied only large cages in both structures, whereas CH 4 molecules occupied the remaining cages. The experiments agree with the structure and molecular distributions that were predicted by minimizing the Gibbs free energy of the sample. This model calculation provides insight into the structural transition mechanism.

On the thermodynamic stability and structural transition of clathrate hydrates

“…The structure II clathrate hydrate is the stable phase in the space between the two surfaces and the structure I is more stable elsewhere. Now we can further optimize the LJ parameter sets for the two guest species such that they give the experimentally observed composition range, 78%-98%, in which structure II forms at 263.2 K. 13 Given in Table III are the free energies obtained from those doubly optimized LJ parameters. The free energy of larger cage occupancy for structure II is lower than that for structure I for both guest species.…”

“…The seemingly gradual transformation from structure I to II with increasing the ethane mole fraction observed by experiments 12,13 needs to be examined carefully. Coexistence of four phases in a certain range of the mole fraction of the gas mixture should be due to a kinetic factor, because it violates the Gibbs phase rule and therefore cannot be a true equilibrium state.…”

“…10,11 The transition has been accounted for by minimizing the relevant Gibbs free energy of model systems with parameters optimized to reproduce thermodynamic properties of clathrate hydrates. 12,13 Here, we show that the structural transition can be accounted for on the basis of the vdWP theory with the free energy of cage occupancy evaluated from intermolecular interactions only. Our method treats a clathrate hydrate of a gas mixture as an open system with respect to the guest species and therefore is a more advantageous way to examine its thermodynamic stability and cage occupancy.…”

Support of Gulf of Mexico Hydrate Research Consortium: Activities to Support Establishment of a Sea Floor Monitoring Station Project

“…Currently, nuclear magnetic resonance (NMR) 13,14 , Raman [15][16][17][18][19][20][21] and Fourier transform infrared (FT-IR) spectroscopy [22][23][24] , along with X-ray diffraction 20,25 , gas chromatography 25 , and neutron diffraction 14 have provided a wealth of information on gas hydrates. In addition to fundamental studies, a great need exists for the capability to monitor in-situ hydrate formation and dissociation in both environmental and industrial settings.…”

“…The conversion from raw voltages to concentrations can be done on the topside. For each sensor node on the mooring we should expect to parse and store at least 14 channels (this could be expanded later to 25 There is a fully operational mode. Also, individual components can be selectively shut down to reduce power consumption.…”