Basic Study On Gas Separation Technology On Methane/ Carbon Dioxide By Hydrate Formation

With the increasing demand for natural gas energy,biogas is one of important renewable energy sources and it has attracted more and more attention.Based on gas hydrate separation technology,CH₄ in the biogas could be concentrated effectively by hydrate method.In order to enhance biogas separation effect,it is important to select efficient and environmental hydrate additives.Addition of four imidazole ionic liquids and two biological surfactants in solution were studied as hydrate additives to anaylize their influence on hydrate formation thermodynamics and kinetics.At the same time,combination of tetrabutylammonium bromide(TBAB)with ionic liquids and biological surfactants were studied on gas hydrate formation.The hydrate thermodynamic and kinetic models were set up based on experiment data.At the same time,biogas hydrate separation experiments were carried out at different experimental conditions.Because TBAB has better thermodynamic promotion effect on hydrate formation pressure,multistage hydrate separation for oil shale retort were studied.So the application of hydrate based separation technology was broden.Main study in this work included the following:(1)The themodynamic influence of four imidazole ionic liquids and two biological surfactants on methane and carbon dioxide hydrate were studied respectively.Themodynamic influence on hydrate formation of anion and cation in ionic liquid were compared.Simulation software COSMO-RS was used to study influence of hydrogen energy between ionic liquds and water molecules on water activity.So influence of ionic liquids on gas hydrate formation pressure could be explained ulteriorly.Hydrate thermodynamic formation pressure experiment data has consistency with simulation data of COSMO-RS.Thermodynamics inhibition effect ability of four imidazole ionic liquids on hydrate formation pressure from strong to weak was[BMIm]Br>[BMIm]BF₄>[BMIm]PF₆>[OMIm]BF₄.(2)The themodynamic calculation model of pure gas CH₄ and CO₂ hydrate formation pressure in ionic liquid solution was built.Van der Waals-Platteeuw hydrate model and Chen-Guo hydrate model were used to calculate fugacity of hydrate phase component,NRTL activity coefficient model and Pitzer-Mayorga-Zavitsas activity coefficient model were used to calculate water activity in ionic liquid solution.Two hydrate models were combined with two activity coefficient models respectively.Meanwhile,Krichevsky-Kasarnovsky model was used to calculate influence of pure gas solubility on water activity in aqueous phase.Through the comparision of experimental and model values,combination of Chen-Guo hydrate model and Pitzer-Mayorga-Zavitsas activity coefficient model could decrease hydrate formation predicted deviation to 1%and it has better predictability.Based on Chen-Guo hydrate model,Pitzer-Mayorga-Zavitsas activity coefficient calculation model,two-characteristic parameter correlation osmotic coefficient calculation model of TBAB and TBAB semi-clathrate hydrate model were combined,calculation model of biogas hydrate formation pressure in ionic liquids and TBAB solution was built and the acquired deviation of experiment and model values was within 5%.(3)Solubility of CO₂ in four imidazole ionic liquid solutions under different experiment conditions of concentration,temperature and pressure were studied though drainage method.So the relationship between CO₂ solubility in ionic liquid solution and gas hydrate reaction was realized.Through the study of CO₂ gas solubility experiments in ionic liquid solutions,the order of CO₂ solubility values in ionic liquid solutions was[BMIm]BF₄>[OMIm]BF₄>[BMIm]PF₆>[BMIm]Br.The surface tention of surfactants solution under different temperatures and pressures were measured.Addition of rhamnolipid could make water surface tension decrease from 74.21 m N/m to34.65m N/m.Addition of sophorolipid could make water surface tension decreased from74.21 m N/m to 57.05m N/m.Relationship between change of surface tention with hydrate kinetic and biogas hydrate separation could be obtained.Lower surface tension of hydrate reaction aqueous solution was favor of mixture gases hydrate separation.(4)CH₄ and CO₂ hydrate formation kinetic in ionic liquid and surfactant solutions under constant temperature and volume condition were studied.The experiment results indicated that four ionic liquids have kinetic inhibition effect on CH₄ hydrate formation.Compared with pure water,addition of ionic liquid could decreased gas hydrate storage and hydrate formation rate.Ionic liquids have promotion effect on CO₂ hydrate formation.Compared with pure water,gas hydrate storage increased.Two biological surfactants have kinetic promotion effect on pure gas hydrate formation.For CO₂hydrate formation experiments in surfactants solution of constant temperature and constant volume,calculation model of CO₂ gas storage in hydrate phase was studied based on Chen-Guo hydrate formation kinetic model.(5)Biogas hydrate separation in ionic liquids and surfactants solutions were studied from four aspects of additive concentration,temperature,feeding gas pressure and gas liquid ratio respectively.TBAB was combined with ionic liquids and surfactants to study synergistic effect on biogas hydrate separation respectively.Through comparison of CH₄ composition in remaining gas,CH₄ recovery and gas hydrate ratio,better hydrate separation experiment conditions were determined.From study of biogas hydate separation in ionic liquid solution,CH₄ composition in remaining gas could be enhanced sixteen percent points.Biogas hydrate separation results in ionic liquid solutions from good to bad was[BMIm]PF₆>[BMIm]Br>[BMIm]BF₄>[OMIm]BF₄.Compared with pure water,addition of ionic liquid promoted CO₂ solubleness.For biogas hydrate separation in ionic liquid solutions,CO₂ hydrate ratio increased and CH₄hydrate ratio decreased.For biogas hydrate separation in surfactants solution,Compared with pure water,kinetic promotion effect was obvious and hydrate time was decreased to one fifth of value in pure water.Sophorolipid has better promotion effect than rhamnolipid on biogas separation results.For biogas hydrate separation in TBAB solutions combined with ionic liquids and surfactans respectively.CH₄ recovery increased and its value was above ninety percent.(6)Calculation model of biogas hydrate separation in ionic liquid under constant temperature and pressure condition was built.Based on Chen-Guo hydrate model,Pitzer-Mayorga-Zavitsas activity coefficient model was used to calculate water activity in ionic liquid solution,mass balance of hydrate reaction was also calculated.Biogas hydrate separation results under different ionic liquid concentration,gas liquid ratio,temperature and feeding gas pressure were obtained.For biogas hydrate separation in[BMIm]Br solution,mean calculation and experiment values deviation of equilibrium pressre was 1.16%,mean calculation and experiment values deviation of CH₄composition was 2.24%.For hydrate separation in[BMIm]BF₄ solution,mean calculation and experiment values deviation of equilibrium pressre was 3.11%,mean calculation and experiment values deviation of CH₄composition was 2.23%.Calculation accuracy of biogas hydrate separation model was better.(7)Oil shale retort was one of important sources of fuel gas.The hydrate separation of oil shale retort in pure water was completed in the laboratory.Single-stage hydrate separation for high ethane retort was studied.Hydrocarbon gases in retort(CH₄+C₂H₆+C₃H₈)could be enthanced from intial 79.12mol%to 90.0mol%.Hydrocarbon gases recovery could arrived at 43.30%.It was very difficult for conventional oil shale retort to form hydrate in pure water.Because TBAB could decreased hydrate formation pressure obviousely.So TBAB was selsected as thermodynamic promoter for oil shale retort multistage hydrate separation.For per stage of retort multistage hydrate separation,hydrate separation conditions would be optimized systematically.CH₄ in retort could be enhanced from 15.0mol%to92.01mol%after six levels hydrate separation.CH₄ recovery could arrive at 11%.Experiment results indicated that hydrate separation technology could be used as one of effective methods to obtain CH₄ from oil shale retort.