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.