Dual nature of methanol as a thermodynamic inhibitor and kinetic promoter of methane hydrate formation in a wide concentration range

Methanol (MeOH) is a common thermodynamic inhibitor of gas hydrates. However, its impact on the kinetics of methane hydrate formation has not been systematically investigated. We have thoroughly examined the effect of MeOH in a wide concentration range on the nucleation and growth kinetics of methane hydrate. The onset temperature To and subcooling Delta To of methane hydrate were determined under ramp cooling of 1 K/h. The mean To is not affected by methanol in the 0-2.5 mass% range but decreases within 2.5-50 mass%. At the same time, it was revealed that Delta To for 1-40 mass% MeOH is significantly smaller than that of water, making methanol a hydrate nucleation promoter within the concentration range. The relationship between Delta To and the methanol concentration shows a minimum of 0.53 +/- 0.36 K (at 20 mass%). Adding 1-30 mass% of methanol reduces the stochasticity of the methane hydrate nucleation events. The hydrate nucleation rate increases by an order of magnitude at 1.5 K subcooling when 5 % of MeOH is added to water. Besides, methanol enhances the methane hydrate growth at 1-40 mass%, with the maximum rate occurring at 10 mass% MeOH. Despite the excellent promoting properties of methanol, extrapolation suggests that MeOH may inhibit both nucleation and growth of methane hydrate in more concentrated solutions (>= 50 mass%). Methanol has a dual nature as a thermodynamic inhibitor and a kinetic promoter of methane hydrate formation. The kinetic promotion function of methanol is observed within a specific range (1-40 mass%), but in more concentrated systems, it seems to be reversed.