Promoting CH₄ Hydrate Formation Kinetics by Utilizing Synergistic Promotion Effects of Graphite Particles with Sodium Dodecyl Sulfate

Abstract

Solidified natural gas in the form of hydrates, ice-like crystalline compounds formed with water and gas, has advantages of high storage capacity, low explosion risk, and environment friendliness. However, practical applications of hydrates have been hindered by the uncertainty of nucleation and slow formation kinetics required for reliable and economic operation. Kinetic hydrate promoters (KHPs) such as surfactants and amino acids have been studied to enhance hydrate formation kinetics, but the rate and amount of gas uptake upon hydrate formation are still insufficient. Recently, graphite particles (GPs) were proposed as KHPs providing heterogeneous hydrate nucleation sites and allowing efficient heat removal for rapid hydrate growth. However, the combined use of GPs with conventional KHPs to achieve superior hydrate formation kinetics has not yet been investigated. Here, we report the synergistic promotion effect of GPs with sodium dodecyl sulfate (SDS) on CH4 hydrate formation kinetics. Under nonstirred conditions, while the use of GPs alone did not lead to hydrate formation, adding GPs to the systems with SDS and L-tryptophan (l-Trp) greatly increased the CH4 hydrate formation rate. The combined use of GPs and SDS rather than l-Trp achieved the fastest kinetics by enhanced mass transfer with the spontaneous formation and falling of GPs + hydrate aggregates. Under stirred conditions, the extensive surface area of GPs provides abundant heterogeneous nucleation sites and the combined use of GPs and SDS therefore resulted in a 350% increase in CH4 gas uptake rate upon hydrate formation compared to the pure water system. The synergistic effect observed between GPs and SDS was hypothesized by the reduced energy barrier for hydrate nucleation, supported by hydrate onset temperature and surface tension measurement data. GPs will be applicable to practical applications of hydrates by further revealing their role in hydrate formation and maximizing the synergistic promotion effects with other KHPs.