Hydrate growth at the interface between water and pure or mixed CO2/CH4 gases: Influence of pressure, temperature, gas composition and water-soluble surfactants

Abstract

International audienceThe morphology and growth of gas hydrate at the interface between an aqueous solution and gaseous mixtures of CO2 and CH4 are observed by means of a simple experimental procedure, in which hydrate formation is triggered at the top of a sessile water drop by contact with another piece of gas hydrate and the ensuing hydrate growth is video-monitored. The aqueous solution is either pure water or a solution of a nonionic or anionic surfactant at low concentration (in the 100-1000ppmw range). In agreement with previously published data, hydrates formed from pure water and aqueous solutions of non-ionic surfactant grow rapidly as a low-permeable polycrystalline crust along the water/gas interface, which then inhibits further growth in a direction perpendicular to the interface. Lateral growth rates increase strongly with subcooling and CO2 content in the gas mixture. Similar lateral growth rates, but varying morphologies, are observed with the non-ionic surfactants tested. In contrast, the two anionic surfactants tested, sodium dodecyl sulfate (SDS) and dioctyl sodium sulfosuccinate (AOT), promote in the presence of CH4 (but not in the presence of CO2) a rapid and full conversion of the water drop into hydrate through a 'capillary-driven' growth process. Insights are given into this process, which is observed with AOT for an unprecedented low concentration of 100ppmw

Infrared spectroscopy on the role of surfactants during methane hydrate formation

“…AOT is considered a double-tailed surfactant, which requires lower dosage compared to SDS for the same decrease in surface tension and activity at the interface between water and hydrate. 41,42 This already indicates that at the same concentration the accelerating effect on the gas hydrate formation should be higher for AOT, as observed.…”

Influence of the carbon chain length of a sulfate-based surfactant on the formation of CO2, CH4 and CO2–CH4 gas hydrates

“…As in the study of Daniel-David et al (2015), we observed two different growth patterns. One for the systems with pure CH 4 and the 25:75 gas mixture, and the other for those with pure CO 2 and the 75:25 gas mixture.…”

“…in a recent article byDaniel-David et al (2015). In our experiments, cell pressure and cell temperature were the same as those reached by the system just before the THF injection in the experiments conducted in the reactor cell, namely P = 2.8 MPa, 3.4 MPa, 4.3 MPa and 5.1 MPa for the CO 2 , the 75:25 and 25:75 gas mixtures, and the CH 4 respectively, and T = T targ (= 275 K).…”

The inhibition of methane hydrate formation by water alignment underneath surface adsorption of surfactants

“…In the aspect of fundamentals, the aforementioned context reflects a lack of a universal understanding of the microscopic pathway of surfactant effects, especially in low concentration regime. Indeed, the fact that current literature neglects the importance of interfacial water when discussing the influence of surfactants (on gas hydrate formation) is of major shortcoming because the interface between gas and water has been observed to be the starting points of gas hydrate nucleation [39][40][41] . In other words, local water molecules at gas-water interface involve directly in the nucleation of gas hydrate whereby they undergo structural rearrangement and transformation.…”