Study On The Synthesis And Structural Stability Of Hydrates In Mixed Gases And Confined Spaces

Natural gas hydrate is a kind of clean energy with rich reserves,and its reasonable exploitation and utilization can effectively alleviate the global energy crisis.Natural gas hydrates often contain multicomponent guest molecules and most of them coexist with porous media,which have significant impacts on the key basic physical properties of hydrates,such as microstructural characteristics,thermodynamic stability,and formation processes.However,studies on the influences of mixed guest molecules and confined space generated by porous media on the thermodynamic stability and formation mechanism of gas hydrate are relatively weak,which limit the exploration and development of natural gas hydrates,gas separation,storage and transportation and other hydrate-based technologies and applications.In response to the above problems,the structural evolution,thermodynamic stability and formation process of hydrates in methane-propane(trace)system,carbon dioxide atmosphere system,layered mineral-hydrotalcite system and porous biomass system are investigated by means of various experimental analysis methods and theoretical calculation methods in this paper,which are of great practical significance for the realization of the exploration and development of natural gas hydrate,gas separation,storage and transportation,as well as the storage of greenhouse gases and other practical applications.The details are as follows:(1)A gas mixture of methane and low-concentration propane is selected to explore the effect of trace propane on the hydrate properties.The neutron diffraction experiment confirms the formation of pure II-type(structure-II,s II)hydrate and the detailed distributions of methane and propane molecules in the hydrate are obtained.The calculation results of binding energy and growth rate prove that s II hydrate has a better thermodynamic stability,and its growth rate basically remains constant with the increase of propane concentration,while the growth rate of I-type(structure-I,s I)hydrate gradually decreases.That is,propane selectively hinders the growth of s I hydrate.This work explores the effects of trace amounts of propane on hydrate structure,thermodynamic stability,and growth process from both micro and macro perspectives.This is conducive to understanding the accumulation mechanism of s II hydrate and provides an important basis for the detection of s II hydrate and the separation,storage and transportation of gas.(2)By exploring the influence of carbon dioxide on the structural properties of s I methane hydrate,the feasibility of gas replacement of methane in hydrate is evaluated.First,the methane hydrate is tested with neutrons in methane and carbon dioxide atmospheres,respectively.It can be seen from the measurement results that when the ambient gas changes from methane to carbon dioxide,part of the methane in the large cages of hydrate is replaced by carbon dioxide without destroying the lattice structure of the hydrate.Then,through theoretical analysis,it is found that the intermediate state in which the large cages of hydrates are occupied by methane-carbon dioxide bi-molecules greatly reduces the diffusion barriers of methane and carbon dioxide in hydrates,it is therefore feasible to replace methane in hydrate with carbon dioxide in dynamics.This study is helpful to utilize hydrate resources without reducing the reservoir strength,and realize the sequestration of carbon dioxide at the same time.(3)The structure,stability,and growth mechanism of methane hydrate in the confined space of layered clay-hydrotalcite are studied.First,it is proved by X-ray diffraction that s I methane hydrate is formed in the non-interlayer space of hydrotalcite.The phase equilibrium diagram shows that the stability of the hydrate is reduced.Molecular dynamics simulations are then used to show that the confined space created by the hydrotalcite and its surface ions suppress the thermodynamic stability of the hydrate.Finally,Raman spectrums show that the presence of hydrotalcite affects the distribution of guest molecules in methane hydrate.This study explores the influence of hydrotalcite on the structural and thermodynamic stability properties of hydrate,and proposes a new growth mechanism for the hydrates in layered minerals,providing a certain reference for the natural gas storage in porous media.(4)By comparing the gas adsorption characteristics in biological materials-eggplant and agricultural wastes(corncob pith and sorghum stalk),and characterizing the unique gas adsorption behavior of corncob pith,the behavior characteristics of hydrates in cheap porous biomass and the potential application for natural gas storage and transportation in it are revealed.It is found by electron microscopy that the micro and nano porous structure of corncob pith increases the specific surface area and is conducive to full contact between water and methane fully contact,and its chemical composition provides nucleation sites for the formation of hydrate,accelerating the formation of hydrate.It can be seen from Nuclear Magnetic Resonance measurements that its porous structure reduces the water activity and further reduced the stability of hydrate.This work explores the formation rate and stability of hydrate in biomass,and provides some ideas for the storage and transportation of natural gas.