Study On The Thermodynamic Characteristics And Kinetic Mechanism Of Hydrate Based CO₂/CH₄ Separation Process

Shale gas is one type of unconventional natural gases.The main component of shale gas is methane（CH₄）.Shale gas exploration by super critical carbon dioxide（CO₂）fracturing can not only enhance the shale gas recovery but also geologically storage CO₂.However,the recovered shale gas are mainly CO₂/CH₄ gas mixtures.The CO₂ gas should be efficiently removed from the CO₂/CH₄ gas mixtures in order to improve the quality of the shale gas.Hydrate-based CO₂/CH₄ separation process is a promising method with considerable merits such as mild operating conditions,simple process flow,low operating cost,less environmental impact,etc.Gas hydrates are solid inclusion compounds formed by water molecules and small gas molecules（such as CH₄,CO₂,etc.）at high-pressure and low temperature conditions.The hydrate-based gas separation process（HBGSP）is a new method on the basis of the fact that the priority for different gas molecules to form gas hydrates with water molecules differs at certain conditions.At present,domestic and foreign research activities of the hydrate-based CO₂ removal from CO₂/CH₄ gas mixtures have developed from the early pure water systems to solution systems containing thermodynamic promoters.Although phase equilibrium conditions for gas hydrates formed from CO₂/CH₄ gas mixtures have been shifted to even milder conditions,key parameters（for example,the rate of hydrate formation,gas storage capacity,CO₂selectivity,etc.）for the one stage HBGSP still need further improvements to meet the industrial demands.In view of the abovementioned issues and in order to further reduce the energy penalty and reaction time for hydrate formation,enhance gas hydrate storage capacity,and improve the CO₂ selectivity of the hydrates,this study is mainly focused on the thermodynamic characteristics of the mixed CO₂/CH₄ hydrates in various reaction systems and kinetic characteristics as well as reaction mechanism for the separation of CO₂/CH₄ gas mixtures via the HBGSP.The purpose of this study is to develop a low energy consumption,fast,and efficient hydrate-based reaction system or enhanced method so as to offer theoretical guidance and technical supports for potential applications of the hydrate-based CO₂ capture from shale gas at an industrial level.The main research work are highlighted as follows:（1）The phase equilibria of CO₂/CH₄ hydrates formed in various concentrations of tetrahydrofuran（THF）or tetra-n-butyl phosphonium bromide（TBPB）solutions were measured and determined by an experimental setup for gas hydrate formation/dissociation,which was independently designed and built.We found that both THF and TBPB can significantly reduce the phase equilibrium pressures of the CO₂/CH₄ mixed hydrates.The hydrate structures turned to be thermodynamically more stable as the increase of THF or TBPB concentrations（within the stoichiometric concentration）.（2）Kinetic characteristics and CO₂ separation efficiency for CO₂ removal from the CO₂/CH₄ gas mixtures by forming s I hydrates or THF-s II hydrates were studied in various reaction systems based on the results obtained from the thermodynamic measurements.A higher initial driving force for hydrate formation resulted in an enhanced rate of hydrate formation at the initial period of hydrate growth but a dramatically reduction of CO₂ separation efficiency.The CO₂ separation factor obtained from the pure water systems and the 1.0 mol%THF solution systems decreased from8.8±2.0 and 3.5±0.7 to 1.7±0.2 and 1.9±0.1,respectively.The normalized gas consumption（21.5±2.0 mmol gas/mol water）,CO₂ recovery（52.3±2.4%）,and CO₂separation factor（8.8±2.0）obtained in the pure water systems were all higher than that obtained in the 1.0 mol%THF solution systems,which were 12.9±0.3 mmol gas/mol water,49.9±2.9%,and 3.5±0.7,respectively.Although the operating pressure was reduced and the hydrate formation rate as well as the entire reaction time was improved in THF-s II hydrates,the CO₂ selectivity was compromised comparing with s I hydrates.（3）Due to the poor CO₂ selectivity of THF-s II hydrates,TBPB semiclathrate hydrate was employed to separate the CO₂/CH₄ gas mixtures in a stirred tank reactor.The highest hydrate formation rate（8.6±0.3 mmol gas/mol water/h）and the optimum reaction time（<2h）were found in the TBPB solutions at the stoichiometric concentration（33.2 wt%）.The CO₂ selectivity factor was improved to as high as31.1±3.3.Moreover,the operating temperature was further improved to near ambient temperature（284.2 K）.These results show a great potential for the application of TBPB semiclathrate hydrate for efficient separation of the CO₂/CH₄ gas mixtures.（4）Based on the self-designed high-pressure morphology apparatus,a"three-step method"was proposed to investigate the crystal growth morphology and kinetic characteristics of TBPB and tetra-n-butyl ammonium bromide（TBAB）semiclathrate hydrates under static conditions.We found that the rapid lateral growth of TBPB hydrate crystals resulted in a blockdown of the gas/liquid interface by the dense hydrate film,which significantly hindered the diffusion of gas molecules towards the liquid phase.While the needle-like or column shaped TBAB hydrate crystals formed near the gas/liquid interface preferred to drop down into the deep bottom of the reactor,leaving the gas/liquid interface free for gas molecules diffusing.Furthermore,the highly porous hydrate structures caused by the sedimentation phenomenon increases the channels for gas/liquid mass transfer.As a result,the gas storage capacity is enhanced in TBAB semiclathrate hydrates.（5）Considering the huge mass transfer resistance and high energy penalty in the stirred tank reactor,novel adsorption-hydration systems in the fixed bed reactor packed with water saturated zeolite 13X or coal particles were employed to enhance the CO₂/CH₄ separation.Compared with the stirred tank reactor,the hydrate formation rate and gas storage capacity obtained in the fixed bed reactor were greatly improved due to the synergistic effect of adsorption and hydrate formation.The highest gas storage capacity reached up to 51.1±0.9 mmol of gas/mol of water.We also found that the performance of CO₂ separation from the CO₂/CH₄ gas mixtures can be improved by adjusting the fixed bed height or water saturations of the fixed beds.（6）A novel method was proposed to evaluate the evolution of the molar ratio of CH₄ and CO₂ in the solid hydrate phase during the hydrate-based CO₂/CH₄ separation process,which is usually challenging in laboratory experiments.It is based on the Raman signals obtained from the self-built in situ Raman spectroscopy experimental system combined with the"Indirect hard modeling"algorithm.With this method,the temporal evolution of molar ratios of CH₄ and CO₂ in the vapor phase,the water-rich liquid phase before hydrate formation,and in the solid hydrate phase during hydrate formation process was investigated.It was found that the gas dissolute stage contributes～72%of the total increasement of the molar ratio of CH₄ and CO₂ in the vapor phase;the hydrate content in the hydrate gel/slurry increases with the decrease of CH₄compositions in the feed gas mixtures;the molar ratio of CH₄ and CO₂ in the solid hydrate phase increased rapidly at the initial stage of hydrate formation,indicating that the hydrate clusters favor CH₄ molecules at initial times of hydrate growth;and the CO₂separation factor obtained in various states always follows the order that the water-rich liquid phase>the hydrate gel>the solid hydrate phase.