Bridging solution properties to gas hydrate nucleation through guest dynamics

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.

Mechanical properties of amorphous CO2 hydrates: insights from molecular simulations

Understanding physicochemical properties of amorphous gas hydrate systems is of great significance to reveal structural stabilities of polycrystalline gas hydrate systems. Furthermore, amorphous gas hydrates can occur ordinarily in the nucleation events of gas hydrate systems. Herein, the mechanical properties of amorphous carbon dioxide hydrates are examined by means of all-atom classical molecular dynamic simulations. Our molecular simulation results reveal that mechanical strengths of amorphous carbon dioxide hydrates are evidently governed by temperatures, confining pressures, and ratios of water to carbon dioxide molecules. Notably, under compressive loads, amorphous carbon dioxide hydrates firstly exhibit monotonic strain hardening, followed by an interesting distinct phenomenon characterized by a steady flow stress at further large deformation strains. Furthermore, structural evolutions of amorphous carbon dioxide hydrates are analyzed on the basis of the N-Hbond DOP order parameter. These important findings can not only contribute to our understanding of the structural stabilities of amorphous gas hydrate systems, but also help to develop fundamental understandings about grain boundaries of gas hydrate systems.

Understanding why constant energy or constant temperature may affect nucleation behavior in MD simulations: A study of gas hydrate nucleation

Molecular dynamics simulations have been widely used in exploring the nucleation behavior of many systems, including gas hydrates. Gas hydrates are ice-like solids in which gas molecules are trapped in water cages. During hydrate formation, a considerable amount of heat is released, and previous work has reported that the choice of temperature control scheme may affect the behavior of hydrate formation. The origins of this effect have remained an open question. To address this question, extensive NVE simulations and thermostatted (NPT and NVT) simulations with different temperature coupling strengths have been performed and compared for systems where a water nanodroplet is immersed in a H2S liquid. Detailed analysis of the hydrate structures and their mechanisms of formation has been carried out. Slower nucleation rates in NVE simulations in comparison to NPT simulations have been observed in agreement with previous studies. Probability distributions for various temperature measures along with their spatial distributions have been examined. Interestingly, a comparison of these temperature distributions reveals a small yet noticeable difference in the widths of the distributions for water. The somewhat reduced fluctuations in the temperature for the water species in the NVE simulations appear to be responsible for reducing the hydrate nucleation rate. We further conjecture that the NVE-impeded nucleation rate may be the result of the finite size of the surroundings (here the liquid H2S portion of the system). Additionally, a local spatial temperature gradient arising from the heat released during hydrate formation could not be detected.

Mixed Hydrate Nucleation: Molecular Mechanisms and Cage Structures

The molecular-level details of the formation of mixed gas hydrates remain elusive despite their significance for a variety of scientific and industrial applications. In this study, extensive molecular simulations have been performed to examine the behavior of CH₄/H₂S mixed hydrate nucleation utilizing two different simulation setups varying in compositions and temperatures. The observed behavior exhibits similar phenomenology across the various systems once differences in nucleation rates and guest uptake are accounted for. We find that CH₄ is always enriched in the hydrate phase while the aqueous phase is enriched in H₂S. Even with H₂S as a minor component (i.e., 10% mole fraction), the system can mirror the overall nucleation kinetics of pure H₂S hydrate systems with CH₄-dominant nuclei. Through analyses of cages and their transitions, nonstandard cages, particularly those with 12 faces (e.g., 5¹⁰6²), have been found to be key intermediate cage types in the early stage of nucleation. Additionally, we present previously unreported cage types comprising heptagonal faces (e.g., 5⁹6²7¹) as having a significant role in the early-stage gas hydrate structural transitions.

Molecular insight into carbon dioxide hydrate formation from saline solution

Carbon dioxide hydrate has been intensively investigated in recent years because of its potential use as gas and heat storage materials. To understand the hydrate formation mechanisms, the crystallization of CO₂ hydrate from NaCl solutions was simulated at a molecular level. The influence of temperature, pressure, salt concentration and CO₂ concentration on CO₂ hydrate formation was evaluated. Results showed that the amount of the newly formed hydrate cages pressure went through a fast linear growth period followed by a relatively stable period. Pressure had little effect on CO₂ hydrate formation and temperature had a significant influence. The linear growth rate was greatly reduced as the temperature dropped from 255 to 235 K. The salt ion pairs could inhibit CO₂ hydrate formation, suggesting that we should choose the lower salinity areas if we want to storage CO₂ as gas hydrates in the seabed sediments. The observations in this study can provide theoretical support for the micro mechanism of hydrate formation, and provide a theoretical reference for the technology of hydrate based CO₂ storage.

In the process of the carbon dioxide hydrate formation in NaCl solution, it could form 5¹², 5¹²6² and 5¹²6³ cages, and the 5¹²6² cage and 5¹² cage number ratio was slightly above 3 : 1.