Insights into the porous structure of surfactant-promoted gas hydrate

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.

Growth Kinetics and Porous Structure of Surfactant-Promoted Gas Hydrate

Surfactants present in tiny amounts in the aqueous phase are known to be efficient gas hydrate promoters; yet, the promotion mechanisms are still not fully understood. Understanding and directing those mechanisms is key to the implementation of gas-hydrate-based applications such as gas storage and separation, secondary refrigeration or water treatment, and desalination. In this work, the growth at the water/gas interface and the porous structure of surfactant-promoted methane hydrate are observed by optical microscopy and Raman imaging in glass capillaries used as optical cells. Hollow crystals are continuously generated and expelled from the methane/water meniscus into the water or surfactant solution, where they ultimately form the skeleton of a porous medium filled with the solution. Unprecedented information is gathered over a range of scales from the molecular scale (crystal structure and cage filling) to the mesoscale (crystal morphologies, growth habits and pore sizes) and macroscale (rates and amounts of water and gas converted into hydrate and hydrate porosity). Following an early steady-state growth regime, a sudden order-of-magnitude increase of the conversion rate occurs, which is related to gaseous methane microbubbles being directly incorporated across the meniscus in the aqueous solution and later converted to methane hydrate. An assessment and comparison are made of the mechanisms and performance of two common anionic surfactants known to be efficient gas hydrate promoters, SDS (sodium dodecyl sulfate) and AOT (dioctylsulfosuccinate sodium or AerosolOcTyl). AOT provides a quicker but more limited conversion into hydrate than SDS, suggesting that it is more appropriate for continuous flow processes while SDS is better suited for gas storage applications. Raman spectra reveal that cage filling by methane of structure I methane hydrate is not affected by surfactants.