Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and micelle-forming surfactants

This paper reports an experimental study in which we intended to obtain a better understanding of the possible role of surfactant micelles on the formation of a clathrate hydrate in a quiescent methane/liquid-water system. The experiments were performed using a laboratory-scale, isobaric hydrate-forming reactor, which was initially composed of a 300-cm3 aqueous phase and a ∼640-cm3 methane-gas phase, then successively provided with methane such that the system pressure was held constant at 3.9 or 4.0 MPa. The surfactants used in this study were lithium dodecyl sulfate (LDS), dodecylbenzene sulfonic acid (DBSA), and sodium oleate (SO), which have sufficiently low Krafft points and hence, unlike sodium alkyl sulfates (sodium dodecyl sulfate and its homologues), allow the micelle formation under such typical hydrate-forming conditions as those used in this study (∼275 K in temperature and 3.9−4.0 MPa in pressure). Significant increases in the rate of hydrate formation and the final water-to-hydrate conversion ratio were simultaneously observed by the addition of LDS to the aqueous phase up to concentrations in the range from ∼0.6 to ∼1.6 times the relevant critical micelle concentration (CMC). Neither the rate of hydrate formation nor the final water-to-hydrate conversion ratio exhibited any appreciable change in the above concentration range. Similar observations were obtained by the addition of DBSA to the aqueous phase up to the concentrations that ranged from ∼0.5 to ∼2.9 times the relevant CMC. Based on these observations, we have concluded that micelles of LDS and DBSA have no practical effect on hydrate formation. No substantial promotion of hydrate formation was detected by the addition of SO to the aqueous phase up to concentrations that ranged from ∼0.8 to ∼4.2 times the relevant CMC.

► Discusses the effects of LDS, DBSA and SO on hydrate formation with no stirring. ► Shows no practical effect of micelle formation on hydrate formation. ► Shows DBSA to be more effective than SDS for promoting hydrate formation.