In-situ Raman Research On Growth And Dissociation Of CH₄/CH₄-C₂H₆ Hydrate

In-situ microscopic Raman spectroscopy can provide important information on hydrate structure and composition changes in hydrate formation/decomposition,providing a basis for explaining hydrate formation/decomposition mechanism and dynamic behaviors.In this thesis,three sets of high pressure optical cells?HPOC?used to form hydrate have been designed and manufactured for in-situ Raman spectroscopy measurement of hydrate.Crystal structure characteristics as well as its transformation,filling of gas molecules in the hydrate cages and other fundamental issues have been studied and a series of scientific discoveries have been achieved.?1?With respect to requirements on different experimental studies,three sets of HPOCs with sapphire windows were designed and manufactured to form hydrate and matching in-situ Raman measurement system was established.In the first cell,Raman laser traverses horizontally through sapphire window and water,then focuses on the hydrate film formed in the surface of gas bubble suspended in water system.This cell has been used to study the lateral and vertical growth of hydrate film in this work.In the second cell,Raman laser traverses vertically through sapphire window and the gas phase above hydrate,then focuses on the interface between hydrate sample and gas phase.This cell has been used to study the structure transition and filling of gas molecules in the hydrate cages during hydrate dissociation in this work.In the third one,Raman laser traverses horizontally through sapphire window and then focuses on the different heights of the hydrate sample near the interface between hydrate sample and sapphire window.This cell was designed to study molecules diffusion and migration through hydrate layer.?2?.With respect to the occurrence of miscible growth?some polyhedral structures appeared on the main body of uniform polycrystalline hydrate film?in structure II?sII?hydrate film formed from mixed gases?CH₄+C₂H₆?with greater than CH₄ mole fraction of 0.8,we used the first set of HPOC to determine the structure of polyhedral structures appeared on the background of uniform polycrystalline hydrate film formed by gas mixtures with high methane mole fraction of 0.81⁰.82.The collected Raman spectra show that the bright polyhedral structures were separated s II polyhedral hydrate crystals and dark ones were holes.The hole was mainly gas with a small amount of structure I?s I?hydrate initially,and was filled with s II hydrate after a certain time.But the relative content of methane and ethane was different from that of the main hydrate film,and the methane content was higher than that of the main body.These phenomena indicate that the gas composition in the front of the hydrate film changes obviously during the lateral growth of the film,that is,the methane concentration increases dramatically.This kind of change leads to the decrease in hydrate formation driving force and the appearance of gas holes.?3?CH₄ hydrate film thickening growth on the surface of bubble suspended in water was systematically studied using the first set of HPOC and in-situ Raman.The Raman measurement method on thickness of hydrate film was proposed.We found the Raman spectra collected in thickening growth could be sorted into three types.In the first type of Raman spectra,only free methane gas peak exists;in the second ones,both the peaks indicating CH₄ molecules in free gas and those enclosed in large and small cavities of hydrate exist where the peak in small cavities is overlaid by that in free gas;in the third ones,only peaks indicating the CH₄ molecules in large and small cavities of hydrate exist.It has been shown that the first type of peaks weakens while the second and third ones strengthen with the elapsed time,indicating the thickening of hydrate film and the decrease in the gaps on the surface of the film.In addition,it was found that the hydrate film has the characteristics of non-uniform growth and thickening rate is faster at higher subcooling by analyzing the film thickness measured.?4?CH₄-C₂H₆ double hydrate film thickening growth on the surface of bubble suspended in water was systematically studied using the first set of HPOC and in-situ Raman.It has been shown that,for hydrate film with s II formed by 0.95 CH₄ + 0.05 C₂H₆ mixture,only the free methane gas peak could be observed initially,and then the methaneethane double hydrate peaks appeared and became stronger and stronger with the elapsed time.It indicates that the probability of the gas signal on the surface of the shell detected by Raman laser decreases during the thickening growth of hydrate film.Additionally,it was found that the film thickening rate was lower at higher subcooling by analyzing the film thickness.This phenomenon might be attributed to the complex and heterogeneous morphology of these gas hydrate films.We guess the film thickened along the dendritic hydrate at first and subsequently in the mode of growth of the mass granular crystals.For sI hydrate film formed by 0.35 CH₄ + 0.65 C₂H₆,rules on the change of Raman peak spectra with elapsed time is similar to that of the former,i.e.,the intensity of gas peak decreased and those of hydrate increased with the elapsed time.However,unlike the former case,the film thickening rate increased with increasing subcooling in this case.The reason might be the thickening growth mode was similar to that of CH₄ hydrate film,i.e.,only growth of the mass granular crystals occurred.?5?The hydrate structure type and dissociation behavior for pure methane and methane-ethane hydrates at temperatures below the ice point?268.15 K-273.15 K?and atmospheric pressure were investigated using in situ Raman spectroscopic analysis in the second set of HPOC.It was found that the self-preservation effect of sI methane hydrate is significant at lower temperatures?268.15 to 270.15 K?.However,it weakened at higher temperatures?271.15 K and 272.15 K?.The self-preservation effect for methane-ethane double hydrate was observed at temperatures lower than 271.15 K.The structure transition from sI to s II occurred during the methane-ethane double hydrate decomposition process,which was clearly identified by the shift in peak positions and the change in relative peak intensities at temperatures from 269.15 K to 271.15 K.Further investigation showed that the selectivity for self-preservation of small cages over large cages leads to the structure transition.Below 268.15 K,the self-protection effects of both large and small cages were very strong,and no structural transition phenomenon occurred.