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.

Modeling oceanic sedimentary methane hydrate growth through molecular dynamics simulation

The crystallization process of methane hydrates in a confined geometry resembling seabed porous silica sedimentary conditions has been studied using molecular dynamics simulations. With this objective in mind, a fully atomistic quartz silica slit pore has been designed, and the temperature stability of a methane hydrate crystalline seed in the presence of water and guest molecule methane has been analyzed. NaCl ion pairs have been added in different concentrations, simulating salinity conditions up to values higher than average oceanic conditions. The structure obtained when the hydrate crystallizes inside the pore is discussed, paying special attention to the presence of ionic doping inside the hydrate and the subsequent induced structural distortion. The shift in the hydrate stability conditions due to the increasing water salinity is discussed and compared with the case of unconfined hydrate, concluding that the influence of the confinement geometry and pore hydrophilicity produces a larger deviation in the confined hydrate phase equilibria.

Understanding the effect of moderate concentration SDS on CO2 hydrates growth in the presence of THF

Hypothesis Additives like Tetrahydrofuran (THF) and Sodium Dodecylsulfate (SDS) improve Carbon Dioxide (CO2) hydrates thermal stability and growth rate when used separately. It has been hypothesised that combining them could improve the kinetics of growth and the thermodynamic stability of CO2 hydrates. Simulations and Experiments We exploit atomistic molecular dynamics simulations to investigate the combined impact of THF and SDS under different temperatures and concentrations. The simulation insights are verified experimentally using pendant drop tensiometry conducted at ambient pressures and high-pressure differential scanning calorimetry. Findings Our simulations revealed that the combination of both additives is synergistic at low temperatures but antagonistic at temperatures above 274.1 K due to the aggregation of SDS molecules induced by THF molecules. These aggregates effectively remove THF and CO2 from the hydrate-liquid interface, thereby reducing the driving force for hydrates growth. Experiments revealed that the critical micelle concentration of SDS in water decreases by 20% upon the addition of THF. Further experiments in the presence of THF showed that only small amounts of SDS are sufficient to increase the CO2 storage efficiency by over 40% compared to results obtained without promoters. Overall, our results provide microscopic insights into the mechanisms of THF and SDS promoters on CO2 hydrates, useful for determining the optimal conditions for hydrate growth.

Solubility of carbon dioxide in water: Some useful results for hydrate nucleation

In this paper, the solubility of carbon dioxide (CO2) in water along the isobar of 400 bar is determined by computer simulations using the well-known TIP4P/Ice force field for water and the TraPPE model for CO2. In particular, the solubility of CO2 in water when in contact with the CO2 liquid phase and the solubility of CO2 in water when in contact with the hydrate have been determined. The solubility of CO2 in a liquid-liquid system decreases as the temperature increases. The solubility of CO2 in a hydrate-liquid system increases with temperature. The two curves intersect at a certain temperature that determines the dissociation temperature of the hydrate at 400 bar (T3). We compare the predictions with T3 obtained using the direct coexistence technique in a previous work. The results of both methods agree, and we suggest 290(2) K as the value of T3 for this system using the same cutoff distance for dispersive interactions. We also propose a novel and alternative route to evaluate the change in chemical potential for the formation of hydrates along the isobar. The new approach is based on the use of the solubility curve of CO2 when the aqueous solution is in contact with the hydrate phase. It considers rigorously the non-ideality of the aqueous solution of CO2, providing reliable values for the driving force for nucleation of hydrates in good agreement with other thermodynamic routes used. It is shown that the driving force for hydrate nucleation at 400 bar is larger for the methane hydrate than for the carbon dioxide hydrate when compared at the same supercooling. We have also analyzed and discussed the effect of the cutoff distance of dispersive interactions and the occupancy of CO2 on the driving force for nucleation of the hydrate.

Phase equilibria molecular simulations of hydrogen hydrates via the direct phase coexistence approach

We report the three-phase (hydrate-liquid water-vapor) equilibrium conditions of the hydrogen-water binary system calculated with molecular dynamics simulations via the direct phase coexistence approach. A significant improvement of ∼10.5 K is obtained in the current study, over earlier simulation attempts, by using a combination of modifications related to the hydrogen model that include (i) hydrogen Lennard-Jones parameters that are a function of temperature and (ii) the water-guest energy interaction parameters optimized further by using the Lorentz-Berthelot combining rules, based on an improved description of the solubility of hydrogen in water.

Assessing the effect of a liquid water layer on the adsorption of hydrate anti-agglomerants using molecular simulations

We have performed Molecular Dynamics simulations to study the adsorption of ten hydrate anti-agglomerants onto a mixed methane-propane sII hydrate surface covered by layers of liquid water of various thickness. As a general trend, we found that the more liquid water is present on the hydrate surface the less favorable the adsorption becomes, even though there are considerable differences between the individual molecules, indicating that the presence and thickness of this liquid water layer is a crucial parameter for anti-agglomerant adsorption studies. Additionally, we found that there exists an optimal thickness of the liquid water layer favoring hydrate growth due to the presence of both liquid water and hydrate-forming guest molecules. For all other cases of liquid water layer thickness, hydrate growth is slower due to the limited availability of hydrate-forming guests close to the hydrate formation front. Finally, we investigated the connection between the thickness of the liquid water layer and the degree of subcooling, and found a very good agreement between our Molecular Dynamics simulations and theoretical predictions.

Phase Diagrams for sII Clathrate Hydrates of CO2 from First-Principles Thermodynamics

Clathrate hydrates are crystalline solid compounds consisting of a water caged framework and guest molecules such as CH₄, C₂H₆, and CO₂. Understanding the phase equilibrium conditions of hydrates is significantly important for the industrial exploitation and experimental synthesis of hydrates. Based on the correct description of the intermolecular noncovalent interactions of clathrate hydrates with vdW-DF2, we studied the crystal structures and the chemical potential phase diagrams of sII hydrates encapsulated with CO₂ molecules to provide a deep understanding of the stability mechanism of hydrates. Under the given p-T conditions, the partially occupied hydrates (136H₂O·1CO₂ and 136H₂O·16CO₂) and fully occupied hydrates (136H₂O·24CO₂) are thermodynamically stable, and the equilibrium temperature decreases as the relative CO₂ chemical potential increases at the same pressure. We expect that the present study may provide vital information on the stability conditions of CO₂ hydrates and trigger new experiments to establish an effective replacement strategy for CO₂/CH₄.

A novel hybrid method for the calculation of methane hydrate-water interfacial tension along the three-phase (hydrate-liquid water-vapor) equilibrium line

We use a novel hybrid method to explore the temperature dependence of the solid-liquid interfacial tension of a system that consists of solid methane hydrate and liquid water. The calculated values along the three-phase (hydrate-liquid water-vapor) equilibrium line are obtained through the combination of available experimental measurements and computational results that are based on approaches at the atomistic scale, including molecular dynamics and Monte Carlo. An extensive comparison with available experimental and computational studies is performed, and a critical assessment and re-evaluation of previously reported data is presented.

Solubility of Carbon Dioxide, Hydrogen Sulfide, Methane, and Nitrogen in Monoethylene Glycol; Experiments and Molecular Simulation

Knowledge on the solubility of gases, especially carbon dioxide (CO₂), in monoethylene glycol (MEG) is relevant for a number of industrial applications such as separation processes and gas hydrate prevention. In this study, the solubility of CO₂ in MEG was measured experimentally at temperatures of 333.15, 353.15, and 373.15 K. Experimental data were used to validate Monte Carlo (MC) simulations. Continuous fractional component MC simulations in the osmotic ensemble were performed to compute the solubility of CO₂ in MEG at the same temperatures and at pressures up to 10 bar. MC simulations were also used to study the solubility of methane (CH₄), hydrogen sulfide (H₂S), and nitrogen (N₂) in MEG at 373.15 K. Solubilities from experiments and simulations are in good agreement at low pressures, but deviations were observed at high pressures. Henry coefficients were also computed using MC simulations and compared to experimental values. The order of solubilities of the gases in MEG at 373.15 K was computed as H₂S > CO₂ > CH₄ > N₂. Force field modifications may be required to improve the prediction of solubilities of gases in MEG at high pressures and low temperatures.

Size dependence of the dissociation process of spherical hydrate particles via microsecond molecular dynamics simulations

The dissociation process of spherical sII mixed methane–propane hydrate particles in liquid hydrocarbon was investigated via microsecond-long molecular dynamics simulations.

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.

Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates

The cage occupancy plays a crucial role in the thermodynamic stability of clathrate hydrates and is an important quantity for understanding the CO₂-CH₄ replacement phenomenon. In this work, the occupancy isotherms of pure CH₄, pure CO₂, and their mixture in sI and sII hydrates are studied by GCMC + MD simulations. The adsorption of CH₄ and CO₂ + CH₄ in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated occupancy ratio θ_L/θ_S and the abundance ratio of CO₂ to CH₄ vary with the temperature and pressure, which provide the prerequisite information for the prediction of CH₄ recovery yield at different conditions in the CO₂-CH₄ gas exchange process. The phase equilibria of clathrate hydrates of pure gases and mixtures are explored and the corresponding heat of dissociation and hydration numbers are determined. The current investigation provides new perspectives to understand the mechanism behind the gas adsorption behavior of clathrate hydrates.

Monte Carlo simulations of the separation of a binary gas mixture (CH4 + CO2) using hydrates

The current study employs Grand Canonical Monte Carlo simulations in order to calculate the process efficiency of separating CH₄ + CO₂ gas mixtures by utilizing structure sI clathrate hydrates.