Molecular behavior of hybrid gas hydrate nucleation: separation of soluble H 2 S from mixed gas.

Soluble H 2 S 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 H 2 S is still largely unclear. Molecular dynamics simulations were performed to reveal the effects of different initial mixed H 2 S/CH 4 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 H 2 S 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 H 2 S concentration as a potential nucleation location. When the initial proportion of mixed gas approaches, H 2 S 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 × 10 24 to 9.438 × 10 26 nuclei cm -3 s -1 . The present high subcooling and H 2 S concentration provided a high driving force to promote mixed hydrate nucleation and growth. The proportion of cages occupied by H 2 S increased with increasing initial H 2 S proportion, but the largest enrichment factor of 1.38 occurred at 10% initial H 2 S/CH 4 mixed gas.