Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates

This paper reports an experimental study on the effects of surfactant additives on the formation of a clathrate hydrate in a quiescent methane/liquid-water system, which was initially composed of a 300-cm3cm3 aqueous phase and an ∼640∼640-cm3cm3 methane-gas phase, then successively provided with methane such that the system pressure was held constant. The surfactants used in the present study were three sodium alkyl sulfates appreciably different in the alkyl chain length—they were sodium dodecyl sulfate (abbreviated as SDS), sodium tetradecyl sulfate (abbreviated as STS) and sodium hexadecyl sulfate (abbreviated as SHS). For each surfactant added to water up to, at most, 1.82–3.75 times the solubility, we performed visual observations of hydrate formation simultaneously with the measurements of methane uptake due to the hydrate formation. The qualitative hydrate-formation behavior thus observed was almost the same irrespective of the species as well as the initial concentration of the surfactant used; i.e., thick, highly porous hydrate layers were formed and grew on the horizontal gas/liquid interface and also on the test-chamber wall above the level of the gas/liquid interface. In each experimental operation, hydrate formation continued for a limited time (from ∼6∼6 to ∼25h) and then practically ceased, leaving only a small proportion (typically 15% or less) of the aqueous solution unconverted into hydrate crystals. The variations in the time-averaged rate of hydrate formation (as measured by the rate of methane uptake) and the final water-to-hydrate conversion ratio with the initial concentration of each surfactant were investigated. Moreover, we examined the promotion of hydrate formation with the aid of a water-cooled cold plate, a steel-made flat-plate-type heat sink, vertically dipped into the aqueous phase across the gas/liquid interface.