Morphology, Growth Kinetics, and Porous Structure of Surfactant-Promoted Gas Hydrates: Roles of Subcooling and Surfactant Formulation

Abstract

Transmission microscopy and microRaman imaging are used to investigate the morphologies, growth process, and porous structure of methane hydrate promoted by two prototypical surfactants, sodium dodecyl sulfate (SDS) and dioctyl sulfosuccinate sodium salt (AOT). Large ranges of surfactant concentrations and subcooling are considered from 100 to 1500 ppmw and 1 to 10 K. Gas hydrates are generated on the guest/water interface as hollow crystallites. These crystallites expand into the aqueous phase from their (open) basis on the interface until they detach from it to form a slurry and ultimately a porous structure, whose pores are filled with the aqueous phase. The morphologies and growth rates of those crystallites and the structure (porosity and pore sizes) of the porous medium are characterized as a function of subcooling and surfactant formulation. Under low subcooling (≈1 K), the hydrate crystallites are hollow cylinders, whose diameters decrease with increasing surfactant concentration. Under low-to-moderate subcooling (from 2 to 4 K), the crystallites are hollow spikes or cones, growing at a rate that increases with subcooling. Under high subcooling (>4 K), tiny (micronic or submicronic) crystallites are massively produced at the interface. The porosity and pore sizes of the resulting porous structure decrease with increasing subcooling, down to values in the range of 40–50% and a few microns for the highest subcooling investigated (≈10 K). Rather than altering the kinetics of hydrate formation, the surfactant prevents the aggregation of hydrate crystallites on the interface as an impermeable crust and/or at the rear of the interface, thus allowing the back-flow of water and the continuous production of hydrate crystallites. In other words, the surfactant ensures the flowability of the slurry of hydrate crystallites and its removal away from the interface. These phenomena are primarily driven by the interfacial properties of the water/gas/hydrate system.