Formation and dissociation behaviors of SF6 hydrates in the presence of a surfactant and an antifoaming agent for hydrate-based greenhouse gas (SF6) separation

Abstract

Sulfur hexafluoride (SF6), the most potent greenhouse gas, should be separated from gas mixtures for recycling and for mitigation of global warming. In this study, the formation and dissociation behaviors of SF6 hydrates in the presence of a surfactant (sodium dodecyl sulfate, SDS) and an antifoaming agent (antifoam A concentrate, AAC) were investigated, with a primary focus on kinetic, spectroscopic, and morphological analyses for hydrate-based SF6 separation. The optimum concentrations of SDS and AAC for SF6 hydrates were found to be 250 ppm and 1,500 ppm, respectively. The structure of SF6 hydrates in the presence of SDS and AAC was identified as structure II, indicating that SDS and AAC had no impact on hydrate structure. The formation behaviors of SF6 hydrates were thoroughly examined through gas uptake measurements, visual observation, and in-situ Raman spectroscopy. The addition of SDS 250 ppm significantly accelerated the formation rate of SF6 hydrate and the additional injection of AAC did not inhibit the promoting effect of SDS. Visual observation, temperature profiles, and volume of retrieved gas during the dissociation of SF6 hydrates clearly demonstrated that SDS also had a promoting effect on SF6 hydrate dissociation and its effect was slightly diminished with the addition of AAC, although AAC showed a powerful defoaming effect during the dissociation of SF6 hydrates. The experimental results obtained in this study will be very useful for accelerating the formation rate of SF6 hydrates using SDS and for solving the foaming problem using AAC in the design and operation of the gas hydrate-based SF6 separation process.