Molecular behavior of hybrid gas hydrate nucleation: separation of soluble H2S from mixed gas

Soluble H2S widely exists in natural gas or oil potentially corroding oil/gas pipelines. Furthermore, it can affect the hydrate formation condition, resulting in pipeline blockage; the nucleation mechanism from mixed gas including H2S is still largely unclear. Molecular dynamics simulations were performed to reveal the effects of different initial mixed H2S/CH4 compositions on the hydrate nucleation and growth process. The geometric details of the nanobubbles and gas composition in the nanobubbles were analyzed; the size of the nanobubbles was found to decrease from 3.4 nm to 1.4 nm. With the increase in the initial H2S proportion, the diameter of the nanobubbles decreased; more guest molecules were dissolved in the water, which improved the initial concentration of guest molecules in the water. A multi-site nucleation process was observed, and separate hydrate clusters could grow independently until the simulation box limited their growth due to high local H2S concentration as a potential nucleation location. When the initial proportion of mixed gas approaches, H2S preferred to occupy and stabilize the incipient cage. Moreover, 5(12), 4(1)5(10)6(2), and 5(12)6(2) cages accounted for approximately 95% of the first hydrate cage. Nucleation rates were shown to increase from 4.62 x 10(24) to 9.438 x 10(26) nuclei cm(-3) s(-1). The present high subcooling and H2S concentration provided a high driving force to promote mixed hydrate nucleation and growth. The proportion of cages occupied by H2S increased with increasing initial H2S proportion, but the largest enrichment factor of 1.38 occurred at 10% initial H2S/CH4 mixed gas.

Automated summary

This study investigates the effects of initial H2S/CH4 mixed gas on the nucleation and growth process of hydrates using molecular dynamics simulations. The results show that increasing the initial proportion of H2S can decrease the size of nanobubbles, increase the concentration of gas molecules in water, and promote mixed hydrate formation. The study also observes a multi-site nucleation process and the preferential occupation of H2S in hydrates.