Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal

Methane hydrate exists in huge amounts in certain locations, in sea sediments and the geological structures below them, at low temperature and high pressure. Production methods are in development to produce the methane to a floating platform. There it can be reformed to produce hydrogen and carbon dioxide, in an endothermic process. Some of the methane can be burned to provide heat energy to develop all needed power on the platform and to support the reforming process. After separation, the hydrogen is the valuable and transportable product. All carbon dioxide produced on the platform can be separated from other gases and then sequestered in the sea as carbon dioxide hydrate. In this way, hydrogen is made available without the release of carbon dioxide to the atmosphere, and the hydrogen could be an enabling step toward a world hydrogen economy.

Kinetic mechanisms of methane hydrate replacement and carbon dioxide hydrate reorganization

Molecular insights into methane hydrate growth in the presence of wax molecules

Molecular Monte Carlo simulations are used to compute the three-phase (hydrate-liquid water-gas) equilibrium lines of methane and carbon dioxide hydrates, using the Transferable Potentials for Phase Equilibria model for carbon dioxide, the united atom optimized potential for liquid simulations model for methane, and the TIP4P/Ice and TIP4P/2005 models for water. The three-phase equilibrium temperatures have been computed for pressures between 50 and 4000 bar via free-energy calculations. The computed results are as expected for methane hydrates but deviate from the direct-coexistence molecular dynamics (MD) studies for carbon dioxide hydrates. At pressures higher than 1000 bar, both the methane and carbon dioxide hydrates dissociate at lower temperatures than expected from experiments and MD studies. The dissociation enthalpy is found to be largely independent on water models, and its values are measured to be 7.6 and 6.0 kJ/mol of water for methane hydrates and carbon dioxide hydrates, respectively. We evaluate the effect of systematic errors on the determination of chemical potentials and show that systematic errors of 0.1 kJ/mol in the chemical potential of water correspond to deviations of 5 K in the three-phase equilibrium temperatures.

The dissociation processes of methane and carbon dioxide hydrates were investigated by molecular dynamics simulation. The simulations were performed with 368 water molecules and 64 gas molecules using NPT ensembles. The TraPPE (single-site) and 5-site models were adopted for methane molecules. The EPM2 (3-site) and SPC/E models were used for carbon dioxide and water molecules, respectively. The simulations were carried out at 270 K and 5.0 MPa for hydrate stabilisation. Then, temperature was increased up to 370 K. The temperature increasing rates were 0.1–20 TK/s. The gas hydrates dissociated during increasing temperature or at 370 K. The potential models of methane molecule did not much influence the dissociation process of methane hydrate. The mechanisms of dissociation process were analysed with the coordination numbers and mean square displacements. It was found that the water cages break down first, then the gas molecules escape from the water cages. The methane hydrate was more stable than the carbon dioxide hydrate at the calculated conditions.

Hydrate that is exposed to fluid phases which are undersaturated with respect to equilibrium with the hydrate will dissociate due to gradients in chemical potential. Kinetic rates of methane hydrate dissociation towards pure water and seawater is important relative to hydrate reservoirs that are partly exposed towards the ocean floor. Corresponding results for carbon dioxide hydrate is important relative to hydrate sealing effects related to storage of carbon dioxide in cold aquifers. In this work we apply a phase field theory to the prediction of carbon dioxide hydrate and methane hydrate dissociation towards pure water at various conditions, some of which are inside and some which are outside the stability regions of the hydrates with respect to temperature and pressure. As expected from the differences in water solubility the methane hydrate dissolves significantly slower towards pure water than carbon dioxide hydrate.

The results of visual studies of the growth of methane and carbon dioxide hydrates from highly dilute aqueous solutions of acids and alkalis, as well as from the same solutions with the addition of 0.1 wt % sodium dodecyl sulfate, are presented. It was found that in addition to the growth of hydrate films at the water–gas interface (for solutions without sodium dodecyl sulfate) and the growth of a loose mass of hydrate on the walls of the reactor in the case of solutions with sodium dodecyl sulfate, there is also growth of the hydrate film on the free walls of the reactor. We speculate what these hydrate films form from the wetting water films located on the walls. In addition, growth of relatively large hydrate agglomerates on the reactor walls was observed. They look like "growing directly from the wall". Presumably, this is due to the possibility of film transfer of water between the formed hydrate films and the walls of the reactor. Possible features of the hydrate formation process caused by the formation of hydrates on wetting films are also discussed.

A series of nanoscale molecular dynamics simulations are carried out at 270 K and 500 bar to explore the nucleation of hydrates using a mixture of water, methane and amino acids. Different concentrations of phenylalanine and tryptophan are used to study the nucleation and growth of methane hydrates. It is found that both amino acids promote the nucleation and growth of methane hydrates at lower concentrations. The formation of different types of rings, partially and fully formed cages formed between water molecules are analyzed. Hydrogen bond analysis revealed a decrease in the number of hydrogen bonds formed between water molecules in the presence of amino acids. Each molecule of phenylalanine and tryptophan forms five to six hydrogen bonds with water molecules. The formation of hydrate cages is analyzed using the F4 structural order parameter, which indicated the co-existence of the hydrate cages as well as the liquid phase. The analysis of the radial distribution function (RDF) obtained for different concentrations of phenylalanine and tryptophan confirmed the encapsulation of methane molecules inside various water cages. The studies revealed that, at high concentrations of phenylalanine and tryptophan, the amino acids act as an inhibitor delaying both the nucleation and the growth of the hydrates. The formation of the cages and the encapsulation of methane molecules are further supported using the mean square displacement of methane molecules.

During the formation of gas hydrate crystals, stable isotope fractionation of the guest molecules occurs. For methane and ethane, hydrogen stable isotope fractionation has been reported in the range of a few per mil, but for carbon stable isotope fractionation, there is no difference in 13C, or the difference is below the detection limit. As for carbon dioxide in natural gas hydrates, little is known about stable isotope fractionation of carbon dioxide. In this study, we report the stable isotope fractionation during formation of synthetic carbon dioxide hydrates under various temperature conditions. Fine ice powder and pure carbon dioxide gas were introduced into a pressure cell and adjusted to be above the equilibrium pressure of the hydrate and below the liquefaction pressure of the carbon dioxide at each temperature. After the formation of carbon dioxide hydrate, the residual gas was collected and the hydrate was recovered under the liquid nitrogen temperature to obtain the hydrate-bound gas. The carbon stable isotope ratios of carbon dioxide in the hydrate and residual gases were determined using a stable isotope mass spectrometer. Over a wide range from 243 K to 278 K, the 13C of residual gas was always larger than the hydrate phase, indicating that the carbon dioxide hydrate preferentially enclathrated lighter molecules. This trend is consistent with the results of a previous study. These results suggest that the equilibrium pressure of 13CO2 hydrate is slightly higher than that of 12CO2 hydrate.

Dissociation of methane and carbon dioxide hydrates: Synergistic effects

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Role of induction time on carbon dioxide and methane gas hydrate kinetics

The effect of kinetic hydrate inhibitors (KHIs) on the growth of methane hydrate in the gas–liquid phase separation state is studied at the molecular level. The simulation results show that the kinetic inhibitors, named PVP and PVP-A, show good inhibitory effects on the growth of methane hydrate under the gas–liquid phase separation state, and the initial position of the kinetic hydrate inhibitors has a major effect on the growth of methane hydrates. In addition, inhibitors at different locations exhibit different inhibition performances. When the inhibitor molecules are located at the gas–liquid phase interface, increasing the contact area between the groups of the inhibitor molecules and methane is beneficial to enhance the inhibitory performance of the inhibitors. When inhibitor molecules are located at the solid–liquid phase interface, the inhibitor molecules adsorbed on the surface of the hydrate nucleus and decreased the direct contact of hydrate nucleus with the surrounding water and methane molecules, which would delay the growth of hydrate nucleus. Moreover, the increase of hydrate surface curvature and the Gibbs–Thomson effect caused by inhibitors can also reduce the growth rate of methane hydrate.

Hydrate phase transition kinetics from Phase Field Theory with implicit hydrodynamics and heat transport

Natural gas hydrates represent a valid opportunity to counteract two of the most serious issues that are affecting humanity this century: climate change and the need for new energy sources, due to the fast and constant increase in the population worldwide. The energy that might be produced with methane contained in hydrates is greater than any amount of energy producible with known conventional energy sources; being widespread in all oceans, they would greatly reduce problems and conflicts associated with the monopoly of energy sources. The possibility of extracting methane and simultaneously performing the permanent storage of carbon dioxide makes hydrate an almost carbon-neutral energy source. The main topic of scientific research is to improve the recovery of technologies and guest species replacement strategies in order to make the use of gas hydrates economically advantageous. In the present paper, an experimental study on how salt can alter the formation process of both methane and carbon dioxide hydrate was carried out. The pressure–temperature conditions existing between the two respective equilibrium curves are directly proportional to the effectiveness of the replacement process and thus its feasibility. Eighteen formation tests were realized at three different salinity values: 0, 30 and 37 g/L. Results show that, as the salinity degree increases, the space between CO2 and CH4 formation curves grows. A further aspect highlighted by the tests is how the carbon dioxide formation process tends to assume a very similar trend in all experiments, while curves obtained during methane tests show a similar trend but with some significant differences. Moreover, this tendency became more pronounced with the increase in the salinity degree.