Study On Carbon Dioxide Separation From Biogas Base On Clathrate Hydrate Technology

CO₂ separation from biogas by hydrate formation is a new decarbonization technology of biogas.The features of CO₂ hydrate form more easily than CH₄ hydrate in the same temperature and pressure conditions,which make CO₂ gas enrich in hydrate phase and CH₄ enrich in the gas phase to achieve decarburization.It has the advantages of simple process,low investment costs and pollution-free.However,high phase equilibrium conditions,low hydrate formation rate and selectivity are the main problems for gas separation by hydrate formation,which make industrial application difficulty.This paper aims at improving hydration separation efficiency and reducing energy consumptions by using thermodynamics and dynamics additives.In this paper,the influence of [BMIm]BF₄ to the phase equilibrium condition of CO₂/CH₄ mixture gas forming hydrate is measured.The phase equilibrium condition of CO₂/CH₄ mixture gas forming hydrate in 5 wt% TBAB solution with different concentration of [BMIm]BF₄ is also measured.The experimental results show that the addition of [BMIm]BF₄ into water has weak promotion in the phase equilibrium conditions of CO₂/CH₄ mixture gas.Adding TBAB and [BMIm]BF₄ into water lower the hydrate phase equilibrium conditions of CO₂/CH₄ mixture gas,which can be used to CO₂/CH₄ mixture gas separation.The effect of adding [BMIm]BF₄ in the TBAB solution for CO₂/CH₄ mixture gas seperation is researched.The hydration rate,the amount of hydrate and the CH₄ concentration of residual gas phase are increased by adding [BMIm] BF₄,which make the effect of hydrate separation better.The highest CH₄ concentration of residual gas phase is up to 84.0 mol% and the biggest CO₂ separation factor achieve maximum value when 0.12 wt% [BMIm] BF₄ is added at 278 K and 3.0 MPa,which make the effect of hydrate separation best.Neither low nor high temperature is advantage for hydrate separation.The effect of different initial gas liquid ratio on CH₄ concentration in the residual gas phase is very significant for CO₂/CH₄ gas mixture forming hydrate.When the initial gas liquid ratio was 3.61,CH₄ concentration in residual gas phase is up to 92.8%.In the case of increasing pressure by adding solution,CH₄ concentration in residual gas phase separation and CO₂ separation factor have been improved after hydraye seperation.CH₄ concentration in residual gas phase is 97.0 mol%,the CH₄ recovery is 88.3% and CO₂ separation factor is 24.9 after secondary hydrate separation.In CO₂/CH₄ + TBPO system,CH₄ concentration in residual gas phase is higher with low initial gas liquid ratio after hydrate separation.When the initial gas liquid volume ratio is 2.23,the highest CH₄ concentration in residual gas phase is up to 89.65 mol% and the maximum CO₂ separation factor is 10.62 after first hydrate separation.CH₄ recovery rate is 0.91 and CO₂ separation factor is 17.01 by adding 5 wt % TBPO.The solution is added into the vessel when pressure equilibrium is reached,which make the rest of the gas mixture to continue hydration reaction.In this case,more CO₂ in gas mixtures form hydrate makes CH₄ concentration in the residual gas phase increased.The highest CH₄ concentration in residual gas phase can reach 93.30 mol% after hydrate seperation.Spray is adopted to improve the CO₂/CH₄ mixture gas separation by forming hydrate.The effect of CO₂ separation is improved by using TBAB solution.The experimental results show that low temperature and low gas liquid volume ratio is advantageous to the gas separation.In CO₂/CH₄ + TBAB system,the best effect of hydrate separation though spray device is abtained when the equilibrium pressure is 3.0 MPa and the operating temperature is 277.5 K.CH₄ concentration are 82.8 mol% highest after standing balance.The separation effect spray device is close to the seperation effect of stirring device under the same temperature and pressure conditions for 85.6% CH₄ recovery rate and 10.98 CO₂ separation factor.The process of biogas decarbonizing based on hydrate formation is established by Aspen Plus 7.1.The energy consumptions of the main equipments in the process are analysed by simulation.The energy cost of biogas hydrate separation technology is 0.390 kWh·（kg-1biogas）though simulation analysis.The main energy consuming equipments in the process are compressed gas equipment and mixing equipment,which account for 48.43% and 39.03% of the total energy consumption respectively.The compression power consumption can be reduced by using multi-stage compression.The energy consumption of hydrate separation technology is higher compared with PSA,membrane separation and washing process;similar with cryogenic separation process;lower than chemical absorption process 0.582 kWh·（kg-1biogas）.If the static hydration technology can be achieved to separate biogas,the energy consumption of hydration can be reduced to 0.204-0.223 kWh·（kg-1biogas）,which is close to the traditional separation process and can be industrialized application.