Study On The Characteristics Of Coplanar Double Cage Structure Formation Mechanism And Phase Equilibrium Of S-? CO₂ Hydrate

Gas hydrates,which have non-stoichiometric solid inclusion bodies,are crystalline solid structures consisting of water and small gas molecules.Water molecules act as host and small gas molecules are guest.Since 1980's,scientists around the world began to carry out the investigation and research on the basis applications of gas hydrate.As an effective way to solve the problems of greenhouse effect,energy security and environmental pollution,hydrate technology can be applied in water resources treatment,environmental protection,climate,oil and gas storage and transportation,petrochemical industry,biochemical separation,biological engineering and biotechnology.However,many issues impede the extensive use of hydrate technology,such as severe formation conditions,long induction period and low hydrate conversion rate etc.Adding additives to the hydrate system can effectively reduce formation pressure,increase the generating temperature,shorten the induction time and improve conversion rate,which has remarkable effects on kinetics and thermodynamics conditions during hydrate formation.Based on the experimental and simulation methods,CP/TBAB/TBAC/TBPB have been selected as promoter of gas hydrate.This work studied the hydrate formation process in CO₂+water and promoter+water+CO₂ systems by experimental method and the cage formation process in cyclopentane coupling water molecules and cyclopentane+water+CO₂ molecules systems based on first principle theory.Firstly,in order to understand the nucleation mechanism and cage formation process of cyclopentane hydrate,as well as the structure and stability of cyclopentane hydrate cage,ab initio calculations was adopted to simulate the hydrate formation process in cyclopentane+water system,and the effect of CP(cyclopentane)molecule on the adsorption of gas molecules was studied.Based on density functional theory,the simulations using the GGA-PBE exchange correlation functions and the triple numerical plus polarization(TNP)basis set to optimize the structures of pure water(H₂O)_n(n=2-6),binary CP·(H₂O)_n(n=1-28).The optimized structure,average length of the hydrogen bond,stabilization energy,interaction energy were further calculated,and the adsorption of cyclopentane on guest molecules(CH₄,H₂ and CO₂)was also studied.The results showed that the CP ring favors to be parallel to the ring of water molecules.The cage precursor was a hexagonal ring of water molecules,which gradually developed into the intact 5¹²6⁴cage consisting of 28 water molecules.The stabilization energies and interaction energies increased with the increase of water molecules,thus the cage structure is thermodynamically feasible.In addition,it is found that the sharp turn of the stabilization energy is the result of fluctuation of the length or distance,and is dominated by the distance between the plane of the CP ring and the quasi-planar water face.Due to the small size of guest molecules,they tend to be adsorbed on the pentagonal face,and the guest molecules have little influence on the stability of the clathrate cage.Secondly,the nucleation and growth processes for the face-sharing double cages in cyclopentane+water+CO₂ system were simulated through ab initio calculations.With the same parameter setting as before,the structures of ternary CP·CO₂·(H₂O)_n(n=5-43)were optimized,and the optimized structure,average hydrogen bond length,stabilization energy and interaction energy were further calculated.Combined with the formation process of hydrate cage in the cyclopentane+water system,the influence of cyclopentane molecules on the nucleation and growth process of hydrate was revealed.The results showed that the clathrate cages tend to occur one by one,the 5¹² cage is thermodynamically feasible in the early stage of nucleation,and then the 5¹²6⁴ cage.In addition,the formation mechanism of the large cage and the small cage is not the same,the small cage follows the layer-separated mechanism,while the large cage follows the ring-expansion mechanism of triangular?quadrilateral?pentagonal.In addition,the results also showed that the water-water,water-carbon dioxide and water-CP interactions play a predominant role in the formation of the cages,while carbon dioxide-CP interactions have little effect on the formation process.Finally,the effect of cyclopentane on the phase equilibrium condition of carbon dioxide hydrate was experimentally studied by constant volume heating method,and the phase equilibrium data of CO₂+water and promoter+water+CO₂ systems were obtained.The results showed that the formation pressure of carbon dioxide hydrate decreased and the formation temperature increased after the additive solution with certain mass concentration was added.However,the same kind of additives with different concentrations have different effects on the equilibrium condition of carbon dioxide hydrate phase.With the increase of additive solution concentration,the temperature difference under the same pressure gradually decreases.In addition,the phase equilibrium condition of carbon dioxide hydrate was obviously improved after the addition of cyclopentane.The simulation results also show that the carbon dioxide hydrate in the cyclopentane system is more thermodynamically feasible and requires lower stabilization energy.