In Situ Raman Study On The Structural Transition And Molecular Transfer Behavior During Gas Hydrate Decomposition

In situ Raman spectroscopy,as a non-destructive measurement method,is a powerful tool for hydrate research,which can provide important information of hydrate such as cage occupancy,guest molecule and structure.In this work,a series of fundenmental experiments are conducted that focus on the structural transition and the transfer behavior of gas molecule in structure I,II hyrates and water layer during the gas hydrate decomposition by using Raman spectroscopy,including the hydrate decomposition behavior below ice point,transfer of gas molecule in typical structure II hydrate,the effect of small molecule on CH₄ hydrate decomposition and the evaluation of stability of CO₂ hydrate.The results of these experiments can provide theoretical support for the hydrate exploitation,and have guiding significance for gas storage,gas separation and CO₂sequestration.The main achievements are summarized as follows.（1）Study on the dissociation behavior of methant-ethane hydrate below ice point.Besides of the self-preservation,structural transition is also an important feature during the decomposition of CH₄-C₂H₆ hydrate at atmospheric pressure and below ice point.The influences of hydrate-forming gas composition and temperature on the hydrate decomposition behaviors are examined in this work.The CH₄-C₂H₆ hydrates are found to transform from structure I to structure II over the CH₄ composition range of 50～68%.The structural transition occurs at the decomposition temperatures below 270.15 K.Furthermore,the hydrate dissociation is retarded upon decreasing the temperature.The above conclusion is important for understanding the hydrate decomposition mechanism,and is helpful for the hydrate exploitation by depressurization in permafrost zones.（2）Study on the transfer behavior of small molecular gas in structure II hydrate.The diffusion process of CH₄,CO₂ and H₂ molecules in a typical structure II,tetrahydrofuran hydrate,is observed by in situ Raman spectroscopy.It is found that H₂can penetrate a 5 mm tetrahydrofuran hydrate layer through migrating between the clathrate cages,while CH₄ and CO₂ molecules cannot penetrate the hydrate layer.The diffusion coefficient of H₂ molecule in the tetrahydrofuran hydrate is calculated by Fick’s first law,which is 6.1×10^-12 m²/s.The minimum pressure of H₂ to enter the tetrahydrofuran hydrate lattice and migrate between small cages is 2.9 MPa.The difference of diffusion behavior of gas molecules in structure II hydrate can be used for gas separation and purification.（3）Study on the effect of small molecular gas on the decomposition of CH₄ hydrate.The gas recovery can be significantly enhanced by adding small-molecule gases such as N₂or H₂to the injecting gas mixtures.To evaluate the independent effect of the small-molecule gas on the CH₄ hydrate decomposition,the depressurization of CH₄hydrate after the injection of H₂/N₂ is compared with the pure depressurization.The experimental results show that the addition of small molecule gases lead to the change of the hydrate phase equilibrium,which results in the increase of the hydrate decomposition pressure.Moreover,the content of small molecule gas and the hydrate decomposition pressure are positive correlation.The diffusion coefficients of H₂ and N₂in CH₄ hydrate are calculated by the Raman peak area data,which are 6.6×10^-10 and1.8×10^-10 m²/s respectively.The injection of H₂promotes the hydrate decomposition to a greater extent because of the higher diffusion coefficient.（4）CO₂ hydrate is not only one of the hydrate exploitation products,but also an effective medium to store the green house gases.The stability of CO₂ hydrate in marine environment is a significant index to evaluate the feasibility of these technologies.In this work,the dissolution rates of CO₂ hydrate in under-saturated water are quantified to evaluate the stabilities of CO₂ hydrate in different environments.The results show that the dissolution rates are related to the CO₂ solubility in different solutions.Meanwhile,there is a linear relationship between the dissolution rate of CO₂ hydrate and the water flow rate.