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

Abstract

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.