Microscopic insights on clathrate hydrate growth from non-equilibrium molecular dynamics simulations

Impact of hydroxyl group in surfactant structure on methane hydrate formation, pelletization, and dissociation for advanced transportable methane pellets

This study investigates the impact of hydroxyl functionalization on biosurfactants for methane hydrate formation, emphasizing their potential in sustainable gas storage. Sodium oleate (SO) and hydroxylated sodium oleate (HSO), derived from oleic and ricinoleic acids, respectively, were synthesized and evaluated as eco-friendly promoters. HSO outperformed SO and conventional surfactants, such as sodium dodecyl sulfate (SDS), particularly at low concentrations (50 ppm). Methane hydrate formation with HSO achieved an impressive conversion efficiency of 94.95 % at 50 ppm, surpassing SDS. HSO significantly enhanced storage capacity up to 161.86 v/v, exceeding the 157.90 v/v capacity of SDS. The optimized balance of hydrophilic and hydrophobic properties in HSO enhanced gas-water interactions, enabling rapid hydrate crystallization and stabilization. Furthermore, HSO exhibited superior performance in saline environments, achieving higher methane consumption and water-to-hydrate conversion rates compared to SO and SDS. This highlights the advantages of using seawater as a medium for methane hydrate formation, as it reduces operational costs and enhances sustainability. Methane hydrate pellet formation experiments revealed that HSO led to a faster formation rate and higher conversion degree. The resulting pellets were more stable and exhibited greater methane storage capacity. In long-term stability tests, HSO-based pellets retained more methane than SO-based pellets after 15 days at -5 °C. Additionally, HSO demonstrated excellent thermal stability in both pure and saline water, remaining structurally intact at elevated temperatures. These findings highlight the potential of molecularly tailored biosurfactants, such as HSO, as green and efficient alternatives to conventional surfactants for methane storage and transportation. This advancement aligns with global sustainability goals and supports the broader adoption of hydrate-based solidified methane technology.

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.

Comparing brute force to transition path sampling for gas hydrate nucleation with a flat interface: comments on time reversal symmetry

Fluid to solid nucleation is often investigated with the rare event method transition path sampling (TPS). I claim that the inherent irreversibility of solid nucleation, even at stationary conditions, calls into question TPS's applicability for determining solid nucleation mechanisms, especially for pre-critical behavior. Even when applied to a phenomenon which displays time reversal asymmetry like solid nucleation, TPS is a good means of exploring phase space and giving trends in post-critical structure, and its ability to facilitate nucleation rate and free energy calculations remains outstanding. Forward-only splitting and ratcheting methods such as forward flux sampling are more attractive for understanding nucleation mechanisms as they do not require time reversal symmetry, but at low driving forces may suffer from the same limitations as brute force: they may never make it to the first ratchet. Here I briefly summarize the TPS method and gas hydrate nucleation simulation literature, focusing on topics within both to facilitate a comparison of brute force hydrate nucleation to transition path sampling of hydrate nucleation. Perhaps anecdotally, the brute force technique results in more crystalline trajectories despite having higher driving forces than TPS. I maintain this difference is because of the inherent irreversibility of hydrate nucleation, meaning its pre-critical behavior cannot accurately be determined by the melting trajectories that comprise approximately half of the configurations in TPS's path ensemble.