Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using HFC-32 and sodium dodecyl sulfate

This paper deals with the effects of a surfactant additive on the formation of a clathrate hydrate in a quiescent guest-gas/liquid-water system. The paper first presents our strong suspicion against the existing hypothesis that the surfactant-micelle formation in the liquid-water phase promotes the hydrate formation. It is pointed out that the Krafft point for sodium dodecyl sulfate (SDS), a popular anionic surfactant often used in previous hydrate-forming experiments, is presumably higher than the system temperatures set in these experiments and hence that no micelles may have formed in these experiments. The paper then describes our experimental observations of the hydrate formation from a hydrofluorocarbon gas, HFC-32 (CH2F2), to show how the hydrate formation behavior is affected by the addition of SDS to the water when brought into contact with HFC-32. In each experiment, HFC-32 gas was continuously supplied to a rectangular chamber partially filled with a quiescent pool of water (pure water or an aqueous SDS solution) to compensate for the gas consumption due to the hydrate formation, thereby maintaining a constant pressure inside the chamber. The present experiments featured the following characteristics: (a) detailed visual observations along horizontal axes through large side windows in the test chamber, and (b) surface tension measurements of the aqueous SDS solutions with the aid of a pendant-drop device inserted in the same chamber to determine the SDS-in-water solubility, which seems to have been misunderstood as the critical micelle concentration (CMC) in some previous studies, under the hydrate-forming conditions. The former revealed that the addition of SDS to the pool-forming water results in the formation of thick, highly porous hydrate layers not only on the liquid-pool surface but also on the chamber walls above the level of the pool surface, leaving the bulk of the liquid pool free from hydrate crystals. The latter led to an important finding that the SDS concentration at which the rate of the hydrate formation peaks is slightly lower than the solubility (the false CMC). An excessive addition of SDS beyond the solubility was found to cause a decrease in the rate of hydrate formation but an increase in the final level of the water-to-hydrate conversion. (c) 2005 Elsevier Ltd. All rights reserved.