Methane hydrates are globally distributed in sediments along the continental margins and potentially contain more energy than all fossil fuel reserves. However, methane is also a potential greenhouse gas which could play a major role in global climate change. Understanding the stability of gas hydrates can help us to understand their role in the climate change. Three main factors affect the stability of hydrates: Temperature (T), Pressure (P) and composition. Hydrates become unstable when they are exposed to pressures and temperatures outside the hydrate stability zone (HSZ) in a process commonly called dissociation. However, hydrates can also become unstable even when the pressure and temperature are within the HSZ but the concentrations of the hydrate forming gases are below their fully-saturated levels in the surrounding water phase. This process can be described as dissolution. In situ observations of marine outcrops of gas hydrates indicate that these hydrates exposed to surrounding seawater are more stable than predicted using diffusion-controlled models based on the surrounding methane saturations.
Naturally-occurring hydrates may not be simple structure I (sI) methane hydrates and may contain higher hydrocarbons like ethane, propane etc., which forms structure II (sII) hydrates. Therefore, these mixed hydrates may act to stabilize the hydrates. In this work, the dissolution of sII hydrates in the presence of water has been studied using molecular dynamics (MD) simulations to understand if and how the presence of ethane and propane may stabilize the hydrate. Lattice constants for sII hydrates were calculated and compared to experimental values to validate the OPLS potentials used for the hydrocarbon guest molecules. The effect of higher hydrocarbons, such as ethane and propane, on the stability of gas hydrate was studied by changing the composition in the hydrate phase keeping the methane composition constant in the large cage and small cages. Also, the effect of methane composition was also studied by changing the methane occupancy in large and small cages. MD simulations reveal that the fully occupied hydrate is more stable than the presence of empty cages. The number of methane molecules moved into the liquid phase from the hydrate phase has been increased with the decrease in the large cage occupancy. No effect was found on the dissolution of sII hydrate by changing the small cage occupancy from 100% to 81.5%. The dissolution of sII hydrate was linear in the first few nanometers of the simulations and then observed an oscillatory behavior; this oscillatory behavior is due to the hydrate formation and dissociation at the hydrate-water layer interface.
The probable phase in the marine sediments can be a two phase hydrate (H)-Liquid water (Lw) thermodynamic equilibrium in the absence of vapor phase. Understanding the fate and transport of hydrocarbons and hydrocarbon mixtures in the deepsea and underlying sediments requires accurate determination of this two phase H-Lw thermodynamic equilibrium in the absence of a free gas phase. In addition to controlling hydrate formation directly from the aqueous phase, the H-Lw equilibrium also provides the aqueous solubility of dissolving hydrate. The two phase H-Lw thermodynamic model is based on the van der Waals and Platteeuw model and the Holder model. The Langmuir constants, an important term, in the van der Waals Platteeuw model were calculated from cell potential parameters obtained from ab initio intermolecular potentials and the experimental data, i.e. it does not rely on empirical fitting parameters. Variable reference parameters were used for each guest molecule instead of using single value for all the guests. The Pitzer model was used to calculate the activity of seawater. The solubilities of pure methane, ethane, and propane in water at H-Lw equilibrium are compared to the available experimental data and shown to be as accurate as the experimental data. The model predictions show that the ratio of large to small cage occupancy decreasing with increase in temperature or pressure. The prediction of the model shows that at the two phase H-L w equilibrium in the presence of electrolytes forms the hydrate at lower pressures compares to the pure water at a given temperature and dissolved hydrocarbon solubility. Thus the presence of electrolyte promotes the hydrate formation rather than acting as an inhibitor. This is in reverse to that in the three phase vapor-liquidwater-hydrate (VLwH) region where it is well known that salts act as an inhibitor to hydrate formation. Finally, the methane-ethane-propane ratio from the Macondo oil spill had been used as a typical thermogenic hydrocarbon mixture and hydrocarbon solubilities at H-Lw equilibrium under deepsea conditions have been presented for pure water and seawater.
|Advisor:||Anderson, Brian J.|
|Commitee:||Celik, Ismail, Popp, Brian, Stinespring, Charter, Turton, Richard|
|School:||West Virginia University|
|Department:||Statler College of Engineering and Mineral Resources|
|School Location:||United States -- West Virginia|
|Source:||DAI-B 75/08(E), Dissertation Abstracts International|
|Subjects:||Chemical Oceanography, Chemical engineering|
|Keywords:||Hydrate-liquid water equilibrium, Mixed gas hydrates, Molecular simulations|
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