Probing Nucleation Mechanisms of Gas Hydrates via Molecular Simulations
Date
2023-11-02
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Abstract
Gas hydrates are ice-like solids where guest species can be encapsulated in water cages. Gas hydrates have received considerable attention because of the vast natural methane hydrates and various promising hydrate-based technologies. Relevant to the applications, a fundamental molecular-level understanding of nucleation behavior and mechanisms is necessary to support ongoing developments associated with gas hydrates. With the primary goal to provide significant insights into the mechanistic details of gas hydrate formation at the molecular level, this thesis investigates the behavior of gas hydrate nucleation and growth using Molecular Dynamic (MD) simulations along with Markov State Models (MSMs). The investigation of the nucleation behavior of mixed CH<sub>4</sub>/H<sub>2</sub>S hydrates within bulk aqueous solutions reveals the roles of different guest species in the hydrate formation process and the impacts of guest compositions, temperature, and different physical setups on the hydrate nucleation behavior. CH<sub>4</sub> species has been consistently observed enriched in the hydrate phase relative to the aqueous solution, while increasing H<sub>2</sub>S composition can significantly enhance the rate of nucleation and growth of the hydrate through enhancement of the solubility of guests in solution. The investigation of the nucleation mechanisms of gas hydrates in water nanodroplets with pure-guest and mixed-guest species reveals key factors affecting hydrate nucleation behavior, including guest species, guest compositions, size of the nanodroplet, and temperature. The effects of temperature and the size of the water nanodroplet on the location of the initial hydrate nucleus have been explored. Utilizing water nanodroplet systems, the possible origins of the different effects of temperature control schemes on the behavior of hydrate formation have been addressed. It is found that the finite size of the surroundings is the origin of the influence of the temperature control schemes on the hydrate nucleation rates. Novel structural analyses, based on applying MSMs to data from gas hydrate simulations, have been used to identify and characterize a general transition network describing hydrate formation. This transition network of cage formation and the corresponding thermodynamic behavior confirm and validate that hydrate nucleation is essentially an ordering-in-stages process, where the early-stage behavior leading to a hydrate nucleus is dominated by entropic aspects.
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Keywords
Molecular simulation, Gas hydrate, Markov state model, nucleation mechanism
Citation
Wang, L. (2023). Probing nucleation mechanisms of gas hydrates via molecular simulations (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.