| Gas hydrate technology has good applications and development prospects in many fields such as energy,environment,and chemical industry.Hydrate-related applications include hydrogen storage,solidified natural gas storage and transportation,carbon dioxide sequestration,seawater desalination,and gas separation.Hydrate growth kinetics is closely related to hydrate technology applications,but the slow hydrate growth rate severely restricts the development of hydrate-related applications.Gas hydrate formation is a kinetic process of multi-component and multi-phase interaction.The nucleation and growth of hydrate mainly occur at the gas-liquid-solid interface,and the growth rate of hydrate is primarily affected by the mass transfer rate of gas and water molecules to the hydrate growth interface.However,the kinetics of hydrate growth at the interface and the mass transfer characteristics of host and guest molecules are still unclear.Therefore,based on the visualization methods such as microfocus X-ray CT and visual reactor,this thesis develops in-situ observation methods for hydrate growth in different systems.The growth behavior and its control mechanism are analyzed through the real-time tracking and quantitative characterization of the gas hydrate growth process.This thesis aims to provide theoretical support for hydrate growth to guide the development of hydrate-related technologies.First,the interfacial growth kinetics of hydrate formation from a single ice sphere and a water droplet is studied,which realizes the microscopic visualization and quantitative tracking of the hydrate growth process at the gas-water and gas-ice interface.The microscopic morphologies of the hydrate growth,the thickness of the hydrate shell,and the inner and outer diameters of the hydrate shell at different times are captured.It is found that the nucleation of hydrates on the surface of ice spheres is random and heterogeneous.The initial stage of hydrate formation is controlled by intrinsic kinetics,and the later stage is controlled by gas diffusion rate.For hydrate formation from ice spheres,the thickness of the formed hydrate shell is approximately 40～60μm.The formed hydrates are porous,and the porosity ranges from 10%to 20%.The growth of hydrate on the surface of water droplets is also divided into two stages:the initial reaction-controlled stage and later mass-transfer-controlled stage.The thickness of the hydrate shell in the transition period between these two stages is about 40μm.In the mass-transfer-controlled stage,there is a capillary permeation phenomenon during hydrate growth,the outward water permeation leading to the formation of hydrate bulges on the outer surface of the hydrate shell.The mass transfer of water molecules to the hydrate-gas interface is the main factor affecting the growth of hydrates.Combined with intrinsic kinetics and mass transfer theory,kinetic models of hydrate growth on the ice sphere and water droplet surfaces are established,respectively.The reaction rate constants for the growth of hydrates on ice spheres and water droplets and the effective diffusion coefficients for gas and water molecules in the hydrates are obtained,respectively.In the temperature range of-15～-1°C,the reaction rate constant for xenon hydrate formation on the ice surface ranges from 2.7×10-8 to 1.76×10-7 m/s,and the measured activation energy of hydrate formation is 71.1 k J/mol.In the temperature range of 2～15°C,the reaction rate constant for xenon hydrate formation from water droplets ranges from 5.3×10-7 to 1.65×10-6 m/s,and the corresponding activation energy of hydrate formation is 56.6 k J/mol.In the temperature range of 2～10°C,the effective diffusion coefficient of xenon in hydrates is estimated to be in the range of 1.3×10-14～1.9×10-13 m2/s,with that of the interstitial water in the range of 2.9×10-12～3.8×10-11 m2/s.Aiming at the mass transfer limitation of hydrate film at the gas-water interface,the effects of sodium dodecyl sulfate（SDS）and small molecular alcohols such as methanol,ethanol,and ethylene glycol on the formation of gas hydrate are studied.It is found that SDS and small alcohols can both inhibit the formation of hydrate film at the gas-liquid interface,and promote the formation of a porous hydrate layer on the reactor wall,thereby increasing the hydrate growth rate and water-to-hydrate conversion rate.It is found that the initial crystal nucleus position in the SDS solution would affect the subsequent hydrate growth mode.The presence of hydrate crystals at the gas-liquid-solid contact line is the precondition for the upward growth of hydrate on the reactor wall.It is proposed that the lower interfacial tension between water and hydrate is an essential reason for hydrate growth on the reactor wall.The conversion of water to porous hydrate occurs after the SDS solution is absorbed to the reactor wall.The porosity of the formed hydrate layer is 10%to 40%,which decreases with the increase of the subcooling.At low concentrations（≤2 wt%）,small-molecule alcohols can enhance mass transfer and promote hydrate growth;at higher concentrations（>5 wt%）,small-molecule alcohols compete with water molecules to form hydrogen bonds,reducing the chemical potential of water and inhibiting the formation of hydrate films.It is revealed that adding 1wt%～2 wt%alcohols in water could enhance mass transfer and promote the rapid formation of hydrate.The growth of gas hydrate crystals in water solution is carried out,and the influencing factors of the controllable and stable growth of gas hydrate crystals in water solution are revealed.It is found that the volume of gas hydrate crystals in water solution increases linearly with time,and the growth rate of hydrate crystal volume does not increase with the increase of hydrate crystal surface area.It is also found that increasing the hydrate formation pressure or decreasing the temperature can increase the hydrate growth rate.In addition,increasing the SDS concentration can also enhance the gas mass transfer in water.With the increase of driving force,the morphology of hydrate crystals changes from cube to hopper and then to dendrite.The influence of hydrate growth driving force and crystal size on hydrate crystal morphology instability is elucidated,and the relationship between hydrate growth driving force and hydrate growth morphology is established.A gas-diffusion-controlled growth mode of gas hydrate crystals in water solution is revealed. |
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