Simulation Study On Nucleation,Phase Equilibrium And Decomposition Mechanism Of Typical Gas Hydrate

Gas hydrates are crystalline inclusion compounds formed by water molecules and gas molecules.Gas hydrates have been recognized as a potential clean energy source.In addition,hydrate also plays a key role in gas storage,oil and gas transportation,refrigeration and cold storage.However,all these key technologies are inseparable from the study of hydrate nucleation and accumulation mechanism,phase stability conditions,spectral characteristics,and other basic physical properties.At present,there is no unified explanation for hydrate nucleation.The main reason is that nucleation is a super-fast process at molecular scale,and its small time and space scale makes experimental observation difficult.Therefore,computation simulation has become a powerful means to study this problem.In addition,the development of hydrate-based refrigeration and cold storage technology is closely related to the phase equilibrium conditions,which can provide theoretical basis for the selection of refrigerant by calculating the phase equilibrium conditions of hydrate.Moreover,in order to achieve controllable decomposition during hydrate extraction,using molecular dynamics simulation to study the action mechanism of additives is also a research hotspot in recent years.Based on this,this paper will deepen the understanding of hydrate nucleation and growth mechanism,phase equilibrium condition and decomposition mechanism at the atomic and molecular level,and provide valuable theoretical guidance for hydrate application in energy,environment,and other fields.Based on hydrate clusters,we use the dispersion-corrected density functional theory to study the fusion process of the water-cage clusters encapsulating CH₄ from bi-cage to tri-cage and discuss the nucleation mechanism of the early nucleation of CH₄ hydrate.The 4¹5¹⁰6² cages are more likely to appear at the early stages of CH₄ hydrate nucleation than other nonstandard cages.And tri-cages sharing three polygonal faces have higher stability than those sharing two polygonal faces.Moreover,the simulated Raman spectra illustrate the symmetric stretching vibrational frequencies of trapped CH₄ molecules in the tri-cage revealed a clear red-shift compared with those in the component mono-and bi-cages and follows the prediction of the“loose cage–tight cage”model.In this study,the water-cage fusion process in the early nucleation stage of methane hydrate was discussed,which provided a basis for finding the law of multi-cage fusion.Furthermore,the evolution of stabilities of amorphous precursors from mono-cage clusters CH₄（H₂O）_n（n=16-24）to deca-cage clusters（CH₄）₁₀（H₂O）_n（n=121-125）are systematically investigated by combining DFT with machine learning.The close-packed structures formed by the water-cage clusters are energetically favorable and the possible initial fusion pathways for water-cage clusters are proposed.The ¹³C NMR chemical shifts for encapsulated CH₄ molecules also exhibited regular changes.Machine learning can reproduce the DFT results well and the fitting results are universality,which can be used to predict the energy and NMR chemical shifts of such multi-cages with more water molecules.These results provide theoretical guidance and microscopic physical images for the nucleation of methane hydrate at the initial stage and the determination of the critical size of the hydrate crystal nucleus.Phase equilibrium is of great significance for hydrate generation,exploitation,transportation,and hydrate-based refrigeration and cold storage.Phase equilibrium conditions of fluoromethane（HFC）hydrate,which can be used for refrigeration and cold storage,are systematically studied by first-principles thermodynamics hydrates.We established the temperature and pressure phase diagrams of the HFC hydrates.It’s found that the C-H symmetric stretching vibration frequency shifts to blue and the ¹³C NMR chemical shift moves to negative with the increase of the number of F atoms in guest molecules.Our theoretical research reveals the optimal equilibrium conditions of HFC hydrates as refrigerants.Hydrate decomposition is a key step in the trial mine and utilization of hydrate deposits and plays an important role in the flow safety of deep-sea oil and gas transportation.The decomposition of C₃H₈ hydrate was studied with CH₃OH additive.By analyzing structural snapshots,radial distribution functions,density distributions,angle distributions,change of energies,mean square displacements,and diffusion coefficients of two comparative models,we found that encaging CH₃OH molecules in the 5¹² cages could enhance the diffusion behaviors of H₂O and C₃H₈ molecules and shorten the decomposition time of propane hydrate.The hydrogen bonds formed by the hydroxyl group of CH₃OH with the water-cage skeleton destroy the original hydrogen-bond balance of the hydrate.Our theoretical results could provide a useful guideline to understanding gas hydrate blocking wellbore in the production and transportation of oil and gas.