Methane hydrate formation and dissociation : roles of promoters

Roles of promoters become necessary in methane hydrate formation for the attempt of gas storage application, but some additives might cause unwanted results in gas recovery. The effects of amino acids (l-leucine and l-valine) and surfactants, sodium dodecyl sulfate (SDS), and methyl ester sulfonate (MES) was investigated at 8 MPa and 2 to 4 °C. L-leucine system enhanced hydrate formation because of its surface activity and surface adsorption at interfaces. A noticeable methane uptake yield was also observed in the l-valine solution. Its influences might be from the nature of l-valine that has non-polar, aliphatic hydrophobic side chain resulting in lower surface activity or surface adsorption at gas/liquid interfaces. Moreover, synergism between MES and SDS was observed with fast kinetics and high gas uptake because of micelle formation to lower the interfacial tension in the solution. At the same scale of the concentration, 0.23 wt% (CMC-SDS), amino acids cannot achieve methane hydrate formation compared to surfactants. The comparison of promoting an effect of each promoter at the optimum concentrations was exhibited by using 0.5 wt% l-leucine, 0.7 wt% l-valine, 0.23 wt% and 0.12 wt%. Although every promotor was possible to achieve high methane uptake, its pros and cons were clearly indicated. The formation with the amino acids was found with much slower formation rate. However, dissociated gas from the surfactants caused a lot of foam that could against the gas release, taking for a longer time to complete the gas recovery, and losing the amount of surfactant. Unlike the surfactants, methane could be recovered from the hydrate formed with the amino acids at a faster rate and reusability.