A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence (original) (raw)
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Journal of Natural Gas Science and Engineering, 2017
In the present work, the thermodynamic, structural and dynamical properties of methane hydrate system were predicted by molecular dynamic simulations. Having knowledge of how methane/water system undergoes thermodynamic, structural and dynamical changes is practical, when methane hydrate is formed. So the aim of this work is to investigate the differences in the properties between two systems; methane/water and methane/water/hydrate systems. The results showed the thermodynamic properties of methane/water/hydrate system are lower than that of another one implying the hydrate structure is more stable and decreases the energy surface of system. The potential energy, density and MSD profiles were determined to distinguish the transition position and width of the interface of hydrate clathrate. Also, the diffusivity reduction proves that the molecular structure varied from liquid-like to the solid-like. A procedure was used to calculate the water/hydrate surface tension that is consistent with the previously reported. Finally, the comparison of oxygen-oxygen, carbon-carbon and carbon-oxygen radial distribution functions indicated that the heights of peaks increase and become narrow in methane hydrate system confirming the regular arrangement of methane and water molecules in the hydrate phase.
The Journal of Chemical Physics, 2006
We have obtained the excess chemical potential of methane in water, over a broad range of temperatures, from computer simulation. The methane molecules are described as simple Lennard-Jones interaction sites, while water is modeled by the recently proposed TIP4P/2005 model. We have observed that the experimental values of the chemical potential are not reproduced when using the Lorentz-Berthelot combining rules. However, we also noticed that the deviation is systematic, suggesting that this may be corrected. In fact, by introducing positive deviations from the energetic Lorentz-Berthelot rule to account indirectly for the polarization methane-water energy, we are able to describe accurately the excess chemical potential of methane in water. Thus, by using a model capable of describing accurately the density of pure water in a wide range of temperatures and by deviating from the Lorentz-Berthelot combining rules, it is possible to reproduce the properties of methane in water at infinite dilution. In addition, we have applied this methane-water potential to the study of the solid methane hydrate structure, commonly denoted as sI, and find that the model describes the experimental value of the unit cell of the hydrate with an error of about 0.2%. Moreover, we have considered the effect of the amount of methane contained in the hydrate. In doing so, we determine that the presence of methane increases slightly the value of the unit cell and decreases slightly the compressibility of the structure. We also note that the presence of methane increases greatly the range of pressures where the sI hydrate is mechanically stable.
Calculation of Liquid Water−Hydrate−Methane Vapor Phase Equilibria from Molecular Simulations
The Journal of Physical Chemistry B, 2010
Monte Carlo simulation methods for determining fluid-and crystal-phase chemical potentials are used for the first time to calculate liquid water-methane hydrate-methane vapor phase equilibria from knowledge of atomistic interaction potentials alone. The water and methane molecules are modeled using the TIP4P/ice potential and a united-atom Lennard-Jones potential, respectively. The equilibrium calculation method for this system has three components, (i) thermodynamic integration from a supercritical ideal gas to obtain the fluid-phase chemical potentials, (ii) calculation of the chemical potential of the zero-occupancy hydrate system using thermodynamic integration from an Einstein crystal reference state, and (iii) thermodynamic integration to obtain the water and guest molecules' chemical potentials as a function of the hydrate occupancy. The three-phase equilibrium curve is calculated for pressures ranging from 20 to 500 bar and is shown to follow the Clapeyron behavior, in agreement with experiment; coexistence temperatures differ from the latter by 4-16 K in the pressure range studied. The enthalpy of dissociation extracted from the calculated P-T curve is within 2% of the experimental value at corresponding conditions. While computationally intensive, simulations such as these are essential to map the thermodynamically stable conditions for hydrate systems.
Phase Diagram of Methane and Carbon Dioxide Hydrates Computed by Monte Carlo Simulations
The Journal of Physical Chemistry B
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 TraPPE model for carbon dioxide, the OPLS-UA 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 behave as expected for methane hydrates, but deviates from direct coexistence Molecular Dynamics studies for carbon dioxide hydrates. At pressures higher than 1000 bar, both methane hydrates and carbon dioxide hydrates dissociate at lower temperatures than expeted from experiments and Molecular Dynamics studies. The dissociation enthalpy is found to be largely independent of water models. The dissociation enthalpy is measured to be 7.6 kJ/mol of water for methane hydrates, and 6.0 kJ/mol of water for carbon dioxide hydrates. We evaluate the effect of systematic errors in 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.
Molecular dynamics simulations of methane clathrate hydrate and methane/water mixtures
The melting of structure I methane clathrate hydrate has been investigated using NVT molecular dynamics simulations, for a number of potential energy models for water and methane. The equilibrated hydrate crystal has been heated carefully from 270 K, in steps of 5 K, until a well de® ned phase instability appears. At a density of 0± 92 g cm Õ $ , an upper bound for the mechanical stability of the methane hydrate lattice over a timescale of 11 nanoseconds is 330 K. Finite size eOE ects have been investigated by simulating systems of 1 and 8 units cells of methane hydrate. The properties of the melted system upon cooling are examined.
Determining the three-phase coexistence line in methane hydrates using computer simulations
The Journal of Chemical Physics, 2010
Molecular dynamics simulations have been performed to estimate the three-phase ͑solid hydrate-liquid water-gaseous methane͒ coexistence line for the water-methane binary mixture. The temperature at which the three phases are in equilibrium was determined for three different pressures, namely, 40, 100, and 400 bar by using direct coexistence simulations. In the simulations water was described by using either TIP4P, TIP4P/2005, or TIP4P/Ice models and methane was described as simple Lennard-Jones interaction site. Lorentz-Berthelot combining rules were used to obtain the parameters of the cross interactions. For the TIP4P/2005 model positive deviations from the energetic Lorentz-Berthelot rule were also considered to indirectly account for the polarization of methane when introduced in liquid water. To locate the three-phase coexistence point, two different global compositions were used, which yielded ͑to within statistical uncertainty͒ the same predictions for the three-phase coexistence temperatures, although with a somewhat different time evolution. The three-phase coexistence temperatures obtained at different pressures when using the TIP4P/Ice model of water were in agreement with the experimental results. The main reason for this is that the TIP4P/Ice model reproduces the melting point of ice I h . Physics 133, 064507-1 064507-2 M. M. Conde and C. Vega J. Chem. Phys. 133, 064507 ͑2010͒ 064507-3 Computer simulations of methane hydrates J. Chem. Phys. 133, 064507 ͑2010͒ 064507-4 M. M. Conde and C. Vega J. Chem. Phys. 133, 064507 ͑2010͒ 064507-5 Computer simulations of methane hydrates J. Chem. Phys. 133, 064507 ͑2010͒ 064507-6 M. M. Conde and C. Vega J. Chem. Phys. 133, 064507 ͑2010͒ 064507-7 Computer simulations of methane hydrates J. Chem. Phys. 133, 064507 ͑2010͒ 064507-8 M. M. Conde and C. Vega J. Chem. Phys. 133, 064507 ͑2010͒ 064507-9 Computer simulations of methane hydrates J. Chem. Phys. 133, 064507 ͑2010͒
Fluid Phase Equilibria, 1999
Hydrate phase equilibrium conditions were measured with a Cailletet apparatus in the pressure range 2-14 MPa. The investigated 1,4-dioxane concentrations were 1, 2, 5, 7, 10, 20 and 30 mol% relative to water. The results show that adding 1,4-dioxane up to concentrations of 6 mol%, about the stoichiometric ratio of large Ž. sII cages to water 1r17 , reduced the equilibrium pressure of hydrate formation. Adding 1,4-dioxane beyond 6 mol% caused a slow increase of the equilibrium pressures. The hydrate phase equilibria data were modeled as equilibrium between a liquid phase of water and 1,4-dioxane, with a small amount of methane, and a sII hydrate of 1,4-dioxane and methane. The chemical potential of the hydrate phase was described using the van der Waals and Platteeuw theory. Activity coefficients of the liquid phase were calculated by a van Laar relation, based on literature 1,4-dioxane q water VLE data. The predicted equilibrium pressures calculated were within 5% of the data up to a concentration of 20 mol% 1,4-dioxane relative to water.
Journal of Engineering Thermophysics, 2010
The properties of methane + ethane and methane + propane hydrates of cubic structures sI and sII are theoretically investigated. It is shown that the composition of the formed binary hydrate strongly depends on the percentage of a heavier guest in gas phase. For instance, for a 1% molar ethane concentration in gas phase, even at a low pressure, ethane occupies 60% large cavities in the hydrate sII, and as the pressure grows to 100 atm, it occupies 80% large cavities at a low temperature. The tendency remains the same at a temperature of higher than the ice melting point, but the methane concentration in the hydrate decreases to 30%. In the structure sI, the influence of the component composition of the gas mixture on that of the formed hydrate is less evident. However, in this case, calculation showed also that for a 1% molar ethane concentration in gas phase, ethane molecules occupy from 8 to 10% large hydrate cavities, depending on the pressure. This work is concerned with modeling phase transitions between cubic structures sI and sII of methane + ethane binary hydrates in view of incomplete occupation of cavities in the hydrate by guest molecules. For an ethane concentration under 2% in the gas mixture, the structure sII becomes more thermodynamically stable than the structure sI. However, as the ethane concentration grows to 20% in the equilibrium ice-gas-hydrate and to 40% in the equilibrium water-gas-hydrate, the structure sI becomes more thermodynamically stable. Hence, for low ethane concentrations in a gas mixture, the structure sI can be formed only as a metastable phase. Phase equilibria of methane hydrate sI and mixed methane + propane hydrate sII are considered, depending on the gas phase composition. Similar results are obtained for this equilibrium; this can evidence simultaneous formation of hydrates sI and sII at low propane concentrations.