Analysis of three-phase equilibrium conditions for methane hydrate by isometric-isothermal molecular dynamics simulations

A Molecular Dynamics Study on Xe/Kr Separation Mechanisms Using Crystal Growth Method

The separation of xenon/krypton gas mixtures is a valuable but challenging endeavor in the gas industry due to their similar physical characteristics and closely sized molecules. To address this, we investigated the effectiveness of the hydrate-based gas separation method for mixed Xe-Kr gas via molecular dynamics (MD) simulations. The formation process of hydrates facilitates the encapsulation of guest molecules within hydrate cages, offering a potential strategy for gas separation. Higher temperatures and pressures are advantageous for accelerating the hydrate growth rate. The final occupancy of guest molecules and empty cages within 512, 51264, and all hydrate cages were thoroughly examined. An increase in the pressure and temperature enhanced the occupancy rates of Xe in both 512 and 51264 cages, whereas elevated pressure alone improved the occupancy of Kr in 51264 cages. However, the impact of temperature and pressure on Kr occupancy within 512 cages was found to be minimal. Elevated temperature and pressure resulted in a reduced occupancy of empty cages. Predominantly, 51264 cages were occupied by Xe, whereas Kr showed a propensity to occupy the 512 cages. With increasing simulated pressure, the final occupancy of Xe molecules in all cages rose from 0.37 to 0.41 for simulations at 260 K, while the final occupancy of empty cages decreased from 0.24 to 0.2.

Mesomorphology of clathrate hydrates from molecular ordering

Clathrate hydrates are crystals formed by guest molecules that stabilize cages of hydrogen-bonded water molecules. Whereas thermodynamic equilibrium is well described via the van der Waals and Platteeuw approach, the increasing concerns with global warming and energy transition require extending the knowledge to non-equilibrium conditions in multiphase, sheared systems, in a multiscale framework. Potential macro-applications concern the storage of carbon dioxide in the form of clathrates, and the reduction of hydrate inhibition additives currently required in hydrocarbon production. We evidence porous mesomorphologies as key to bridging the molecular scales to macro-applications of low solubility guests. We discuss the coupling of molecular ordering with the mesoscales, including (i) the emergence of porous patterns as a combined factor from the walk over the free energy landscape and 3D competitive nucleation and growth and (ii) the role of molecular attachment rates in crystallization-diffusion models that allow predicting the timescale of pore sealing. This is a perspective study that discusses the use of discrete models (molecular dynamics) to build continuum models (phase field models, crystallization laws, and transport phenomena) to predict multiscale manifestations at a feasible computational cost. Several advances in correlated fields (ice, polymers, alloys, and nanoparticles) are discussed in the scenario of clathrate hydrates, as well as the challenges and necessary developments to push the field forward.

The performance of OPC water model in prediction of the phase equilibria of methane hydrate

Molecular dynamics (MD) simulations were performed to determine the three-phase coexistence line of sI methane hydrates. The MD simulations were carried out at four different pressures (4, 10, 40 and 100 MPa) by using direct phase coexistence method. In current simulations, water was described by either TIP4P/Ice or OPC models and methane was described as a simple Lennard-Jones (LJ) interaction site. Lorentz-Berthelot combining rules were used to calculate the parameters of the cross interactions. For OPC model, positive deviations from the energetic Lorentz-Berthelot rule were also considered based on the solubility of methane in water. For TIP4P/Ice water model, the obtained three phase coexistence temperatures showed good agreement with experiment data at higher pressures, which is consistent with previous predictions. For OPC water model, simulations using the classic and the modified LB parameters both showed negative deviations to the experimental values. Our results also indicated that the deviation of the T3 prediction by OPC model not much correlated with the predicted melting point of ice. At 4 MPa, the modified OPC model showed outstanding prediction of hydrate equilibrium temperature, even better than the prediction by TIP4P/Ice. The relative higher accuracy in biomolecular MD of OPC model suggests that this model may have a better performance in hydrate MD simulations of biomolecule-based additives.

Molecular Dynamics Simulation Study on the Growth of Structure II Nitrogen Hydrate

Crystal growth of N₂ hydrate in a three-phase system consisting of N₂ hydrate, liquid water, and gaseous N₂ was performed by molecular dynamics simulation at 260 K. Pressure influence on hydrate growth was evaluated. The kinetic properties including the growth rates and cage occupancies of the newly formed hydrate and the diffusion coefficient and concentration of N₂ molecules in liquid phase were measured. The results showed that the growth of N₂ hydrate could be divided into two stages where N₂ molecules in gas phase had to dissolve in liquid phase and then form hydrate cages at the liquid-hydrate interface. The diffusion coefficient and concentration of N₂ in liquid phase increased linearly with increasing pressure. As the pressure rose from 50 to 100 MPa, the hydrate growth rate kept increasing from 0.11 to 0.62 cages·ns^-1·Å^-2 and then dropped down to around 0.40 cages·ns^-1·Å^-2 once the pressure surpassed 100 MPa. During the hydrate formation, the initial sII N₂ hydrate phase set in the system served as a template for the subsequent growth of N₂ hydrate so that no new crystal structure was found. Analysis on the cage occupancies revealed that the amount of cages occupied by two N₂ molecules increased evidently when the pressure was above 100 MPa, which slowed down the growth rate of hydrate cages. Additionally, a small fraction of defective cages including two N₂ molecules trapped in 5¹²6⁵ cages and three N₂ molecules trapped 5¹²6⁸ cages was observed during the hydrate growth.