Nucleation Probability of Methane + Propane Mixed-Gas Hydrate Depending on Gas Composition

Gas hydrates show promise as gas storage and transportation media. But before using them in such industries, we should learn how to better control their formation. In particular, their nucleation is stochastic and requires either high supersaturation or long induction times. Here we study experimentally how the induction time varies with the gas composition using methane (CH4) + propane (C3H8) mixed-gas hydrate as a mimic of natural gas. We form CH4 + C3H8 mixed-gas hydrates with a composition ratio CH4:C3H8 ranging from 100:0 to 0:100 in a high-pressure vessel with a stirring system at a constant temperature of about 274.7 K. The induction time is determined as the time period from the start of stirring to the hydrate formation defined by either a sudden temperature rise or a pressure drop in the vessel. To obtain quantitative nucleation data, we repeated at least 10 experiments for each condition. Compared to that of pure CH4 hydrate, the induction time of CH4 + C3H8 mixed-gas hydrate is found to be significantly shorter when C3H8 is present at even 1% in the vapor. Then, we quantify the stochastic nucleation process as the composition-dependent nucleation frequency. As a result, we show that the nucleation frequency of CH4 + C3H8 mixed-gas hydrate exponentially decreases with increasing C3H8 in vapor. This behavior correlates well with the composition dependence of the empty-cage ratio in the gas hydrate. Based on these results, we propose a model for the nucleation of mixed-gas hydrates.