| Due to the huge amounts of energy sources and the plug formed in the oil and gas pipeline,gas hydrate has been drawn extensive concern and study in the world.However,the formation and inhibition mechanism of gas hydrates remains poorly understood.The study of the mechanism of the formation and inhibition of methane hydrate is not only significant for the scientific theory,but also for the development of new and improved inhibitors,which is critical for the exploitation of gas hydrate in marine sediment and the control of gas hydrates in offshore oil-gas pipelines.By combining Raman spectroscopy and multivariate curve resolution,we study the structure of methane hydration shell and short alcohols hydration shell,and the results are applied to discuss the controversy on the existence of clathrate-like hydrate precursors and the inhibition mechanism of alcohols on the hydrate inhibition when the in low concentration alcohols containing system.What is more,no study reports the process of the formation of methane hydrate in inhibitor containing system.On the basis of in-situ quantitative Raman Spectroscopy technology combined with the technique of in-situ high pressure observation,we investigate the kinetic process of methane hydrate growth under the control of mass transfer in pure water,methanol and PVP10(polyvinyl pyrrolidone,molecular weight 10000)containing system.The discussions and conclusions of this paper are listed as following:(1)By studying the dependence of temperature and NaCl concentration on the Raman cross section of water OH stretching band,we re-evaluated the effect of temperature and NaCl concentration on the Raman quantitative factor in the NaCl-CH4-H2O system.The new established methane Raman quantitiave factor can be used to quantitatively determine the concentration of dissolved methane for seawater or geofluids samples.The results show that the dependence of Raman quantitative factor on temperature and NaCl concentration was totally caused by the variation of the Raman cross section of water OH stretching band resulting from the rearrangement of the H-bond network.The following equation for the Raman cross section with temperature and NaCl concentration can be used to calculate the Raman quantitative factor under different temperatures and NaCl concentration:?(mNaCl,T,30 MPa)/s(Pure water,20 ?,30 MPa)= f(T,mNaCl)= a(T-20)+ b where a = 0.000089 × mNaCl1/2-0.001165,R2 = 0.991;b = 0.0355 × mNaCl + 0.9989,R2 = 1.000.(2)The Raman-MCR was applied to study the properties of the hydrophobic hydration such as methane,methanol,ethanol and n-propanol and PVP10 within wide temperature and pressure range,then the methanism of methane hydrate nucleation and the effect of low concentration alcohols and PVP10 on the formation of methane hydrate were discussed from the molecular level.The results indicate that the hydration-shells of methane and alcohols are more tetrahedral structure than pure liquid water and have more strong hydrogen bond than bulk water.What is more,the associated hydration-shell tetrahedrality crossover(relative to bulk water)was observed and quantified by combing experimental and molecular dynamics results.The enhanced tetrahedrality of water around oily chains decreases and then transforms to a more disordered structure with increasing chain length,and the associated crossover length scale decreases with increasing temperature.The crossover temperatures of methane,methanol,ethanol and n-propanol were around 112±10 oC,164±10 oC,127±10 oC and 90±10 oC,respectively.This clathrate-like or ice-like structure of hydrophobic hydration shell will disappear when the temperature is higher than these crossover temperatures.The structure of the hydration shell of PVP10 is much more similar to that of bulk water without the existence of clathrate-like or ice-like structure.What is more,dangling OH was found in the hydration shell of PVP10.(3)The tetrahedrality of the structure of methane hydration shell is much smaller than that of solid methane hydrate,so the structure of methane hydration shell is much more similar with that of bulk water.The local structuring nucleation hypothesis is believed to be true in the aqueous solution.(4)The existence of the clathrate-like or ice-like structure of alcohols hydration shell like methanol,ethanol and propanol indicates that these alcohols strengthen the local structure of liquid water.Due to this hydrophobic effect,hydrophobic group induced clathrate-like or ice-like structure can increase the concentration of local dissolved methane around these alcohols molecules and act as seeds(nucleation sites).Because of these two effects,alcohols will increase the nucleation rate,not behaves as inhibitors when the concentration of alcohols dissolved in liquid water is higher than 20wt% resulting from the distribution of hydrogen bond network by OH group.(5)The temperature dependence of diffusive transfer coefficient on the temperature,methanol and PVP10 concentration within an optical capillary cell was observed and determined by the least-squares method via time-dependent Raman spectroscopy.The results indicate that the diffusion coefficient of methane in 5wt% methanol aqueous solution is lower than that of methane in pure water at the same condition.The diffusion coefficient of dissolved methane in 2wt%(20000 ppm)PVP10 aqueous solution increased linearly with increasing temperature within 25 oC.The relationship between the concentration of PVP10 in pure water and the diffusion of methane in solution is quite complicated.The presence of PVP10 increase the diffusion coefficient of methane in the aqueous solution when the concentration of PVP is lower than 0.1wt%.A contrary trend was found when the concentration of PVP10 is larger than 0.1wt%.(6)To study the effect of mass transfer on the time dependent process of methane hydrate growth in a system containing PVP10 inhibitor,a high-pressure in-situ noninvasive technique was applied to measure the distribution of dissolved methane in the solution around a growing crystal.The growth rate of methane hydrate from pure water,5wt% methanol aqueous solution and 2wt% PVP10 aqueous solution at different temperatures and pressures was measured.The results showed that the growth rate of methane hydrate was totally controlled by the methane flux which was supplied through diffusion in the aqueous solution from the gas CH4-aqueous solution interface.The concentration gradient of dissolved methane in aqueous solution and the diffusion coefficient determined the mass transfer coefficient kd.The presence of methanol both increased the concentration of methane in gas CH4-aqueous solution interface and methane hydrate-aqueous solution interface resulting from the existence of the clathratelike structure of alcohols hydration shells.PVP10 has negligible effect on the solubility of methane in gas CH4-aqueous solution interface.This can be explained by the similarity of the structure of hydration shell of PVP10 and bulk water.The solubility of methane in hydrate-aqueous solution interface is higher in 2wt% PVP10 aqueous solution than that in pure water system.The Raman spectroscopy of methane hydrate forming from 2wt% PVP10 aqueous solution shows that PVP10 absorb on the surface of the lattice of methane hydrate when methane hydrate forming from 2wt% PVP10 aqueous solution.This is thought to cause the increased solubility of methane in the hydrate-aqueous solution interface.Because of the perturbation for the structure of local water,the addition of low concentration of alcohols such as methanol will result in the decrease of mass transfer of components to the growing crystal surface but the increase of the rate of the crystal surface reaction.Due to the absorption of PVP10 on the surface of crystal surface,the presence of 2wt% PVP10 both inhibition the process of mass transfer and the crystal surface reaction.What is more,the existence of the dangling OH around the hydration of PVP10 is believed to be correlated to the PVP inhibition mechanism on gas hydrate. |
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