Thermodynamic Properties Of CO₂ Mixtures During Cryogenic Separation

It is important to understand thermodynamic properties of binary and ternary mixture containing CO₂ when the CO₂ is separated by cryogenic method,especially,the vapor-liquid equilibrium properties,which is of great significance to improve the separation efficiency and separation quality of CO₂.The equation of state and mixing rules used in the field of gas separation are selected through a large number of literatures,and the calculation and analysis of vapor-liquid equilibrium are carried out by using the mathematical models of different EOS and mixing rules in CO₂-N₂ binary mixture.The results show that PR+VDW?Peng-Robinson+Vander Waals?model or SRK+VDW?Redlich-Kong-Soave?model are most consistent with the experimental values,and the components and pressure deviations are both within 10%,and the former model is better.Therefore,the mathematical model of PR+VDW is used to calculate and analyze the vapor-liquid equilibrium of CO₂ mixtures.In terms of thermodynamic characteristics,it is pointed out that PR equation is the optimal model for calculating the density characteristics of CO₂ mixtures.The density characteristics of binary mixtures of different concentrations of CO₂-N₂,CO₂-O₂ and CO₂-Ar at 213.15K,223.15K and 303.15K are calculated and analyzed.The results show that,in the critical region,small pressure disturbance will cause large density change of CO₂ mixture.The presence of N₂,O₂ and Ar will reduce the density of CO₂ mixture,and the density deviation will increase with the increase of non-CO₂ concentration.In addition,the compression factor and isobaric specific heat capacity of CO₂ mixture are calculated.The analysis provides necessary physical laws for CCS?Carbon Capture and Storage?technological process.The vapor-liquid equilibrium characteristics of CO₂-O₂ and CO₂-N₂ binary mixtures are calculated and analyzed.It is pointed out that when the binary interaction coefficient is regarded as a constant,the deviation between the calculated value of bubble point line and the experimental value in literature increases with the increase of pressure.For example,the maximum deviation of CO₂-N₂ binary mixture at 240K could reach 35.649%.In view of the large deviation of PR+VDW model under high pressure,the binary interaction coefficient is correlated with the function of reduced temperature and eccentricity factor by using the experimental data in the literature,which improves the accuracy of the model.Especially under 10-16 MPa high pressure,the deviation of phase equilibrium calculation is reduced to about 0.8%,compared with that in the literature,which is over 1.3%.Finally,the vapor-liquid equilibrium characteristics of CO₂-O₂-N₂,CO₂-O₂-Ar and CO₂-O₂-SO₂ ternary mixtures are calculated and analyzed in the range of 223K-263K and2-8MPa.The maximum deviation of CO₂ molar fraction in CO₂-N₂ and CO₂-O₂ binary mixtures is only 0.0002.It can be concluded that the effect of O₂ and N₂ on the vapor-liquid equilibrium of CO₂-containing mixtures is the same under this working condition.When studying the vapor-liquid equilibrium of CO₂-containing mixtures,O₂ and N₂ can be regarded as one substance.For CO₂-O₂-Ar ternary mixture,the liquid phase fraction of CO₂ increases with the increase of temperature and the decrease of pressure.For CO₂-O₂-SO₂ ternary mixture,there is a maximum liquid phase fraction of CO₂.At the same temperature,the liquid phase fraction of CO₂ increases first and then decreases with the increase of pressure,and when the temperature increases,the liquid phase fraction of CO₂ increases.The peak value of the rate decreases and the corresponding peak pressure increases.