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Srinath Chowdary Velaga Huge deposits of carbon is trapped in the form of methane gas hydrates, these methane gas hydrates represent a potential energy source that could possibly last for thousands of years. Gas hydrate reservoirs are receiving increased attention as potential locations for CO2 sequestration, with CO2 replacing the methane that is recovered as an energy source. In this scenario it is very important to correctly characterize the cage occupancies of CO2 to correctly assess the sequestration potential as well as the methane recoverability. In order to predict accurate cage occupancies, the guest-host interaction potential must be represented properly. Earlier, these potential parameters were obtained by fitting to experimental data and these fitted parameters do not match with those obtained by second virial coefficient or gas viscosity data. Ab initio quantum mechanical calculations provide an independent means to directly obtain accurate intermolecular potentials. A potential energy surface (PES) between H2O and CO2 was computed at the MP2/aug-cc-pVTZ level and corrected for basis set superposition error (BSSE), an error caused due to the lower basis set, by using 0.361 of the full counterpoise and 0.639 of the uncorrected energy correction. Intermolecular potentials were obtained by fitting Exponential-6 and Lennard-Jones 6-12 models to the ab initio PES, correcting for many-body interactions. Reference parameters for structure I carbon dioxide hydrate has been calculate with this site-site ab initio intermolecular potentials as ∆ = 1204 3 J/mol and ∆ = 1189 12 J/mol. The pure CO2 hydrate equilibrium pressure was predicted with an average absolute deviation of less than 2% from the experimental data. Predictions of the small cage occupancy ranged from 22-38% and the hydration number for the CO2 hydrate was calculated to be above 7.0, whereas the large cage is more than 98% occupied. Cell potential parameters, the potential well depths and volumes of negative energy, have been found for carbon dioxide hydrate system from the center-well solution. The Langmuir constants are computed from the ab initio site-site intermolecular potentials. These Cell potential parameters can be used to predict the mixed hydrate properties for carbon dioxide with other guest molecule.

Phase equilibrium and cage occupancy calculations of carbon dioxide hydrates using ab initio intermolecular potentials