Vapor-Liquid Equilibrium And Density Characteristics Investigation Of Mixture With Methane As The Main Component

The investigation on vapor-liquid equilibrium and density of the mixtures(methane+non-hydrocarbon components)in natural gas is of great significance for separation,purification,storage and transportation of natural gas.For the vapor-liquid equilibria of CH₄ and non-hydrocarbon components(N₂,CO₂),many research had been carried out by the previous researchers,but for binary mixtures composed of methane and other non-hydrocarbon components in natural gas,the research on vapor-liquid equilibrium and density was still scarce,especially for ternary mixtures with methane as the main component.Therefore,study on vapor-liquid equilibrium of binary mixtures(CH₄+Ar?CH₄+H₂S and CH₄+H₂)and ternary mixtures(H₂+N₂+CH₄?He+N₂+CH₄?N₂+CH₄+CO₂?N₂+Ar+CH₄?CH₄+CO₂+H₂S?CH₄+C₂H₆+CO₂and CH₄+CO₂+C₃H₈)were carried out in this paper.On thebasis of the above research,the density characteristics of several binary mixtures were studied detailedly.The main conclusions were as follows:(1)Research on vapor-liquid equilibrium of the binary mixturesFor the binary mixture CH₄+Ar,PR equation,combined with vdW mixing rule/G^E mixing rule(HV,MHV1,LCVM)and three activity coefficient models(Wilson,NRTL,UNIQUAC)were used to correlate the VLE.During the correlation,the interaction parameters depending on terperature or not in the mixing rule were considered.Finally,50 kinds of calculating methods were used to systematically and detailedly correlate the vapor-liquid equilibrium of the mixtures in a wide temperature range,and the calculated results were compared with experimental data from the literatures.When the interaction parameters were independent of the temperature,the accuracy of G^E mixing rule was better than that of vdW mixing rule;when the parameters in activity coefficient model were fixed,the accuracy of HV,MHV1 and LCVM mixing rule was equivalent.When the interaction parameters in the mixing rule were dependent of temperature,three temperature correlated functions were expressed as A+B*ln(T),A+B/T and A+B*T.The results showed that the accuracy of vdW and G^E mixing rule was equal in the whole temperature range,and the extrapolation effect was good;when the interaction parameters were expressed as A+B*ln(T)was applied to PR+HV+UNIQUAC model,the deviations of predicting result were relatively smaller.For binary mixtures CH₄+H₂S and CH₄+H₂,PR equation combined with vdW mixing rule,G^E mixing rule(HV,MHV1,LCVM)and three activity coefficient models(Wilson,NRTL,UNIQUAC)were adopted,and the interaction parameters in the mixing rule were divided into four cases:((1)the interaction parameters were considered as variables independent of temperature;(2)within the whole calculated temperature ranges,the interaction parameters were only considered as one varible value;(3)expressed in the form of A+B*ln(T);(4)expressed in the form of A+B*ln(T)).Finally,40 calculation methods were formed to systematically and detailedly calculate the vapor-liquid phase equilibrium of the two binary mixtures,and the results were compared with the experimental data from the literatures.The results showed that for the binary mixture CH₄+H₂S,within the selected temperature range,the correlated results of the selected models showed that the liquid phase lines were in good agreement with the experimental data,and the vapor lines were in good agreement with the experimental data when it is lower than 273 K;and when the temperature was higher than 273K,the vapor lines correlated by these models had large devations compared with the experimental data;for the binary mixture CH₄+H₂,in general,when the mixture pressure was lower than 20 MPa,the correlation results of the selected models were in good agreement with the experimental data,but when the mixture pressure was higher than 20 MPa,the correlated results of the selected models had great deviation from the experimental data,and these models were difficult to predict the vapor-liquid equilibrium of the mixture in this situation.(2)Investigation on vapor-liquid equilibrium of ternary mixturesFor seven ternary mixtures(H₂+N₂+CH₄,He+N₂+CH₄,N₂+CH₄+CO₂,N₂+Ar+CH₄,CH₄+CO₂+H₂S,CH₄+C₂H₆+CO₂ and CH₄+CO₂+C₃H₈),PR+MHV1+NRTL and SRK+MHV1+NRTL were used to correlate the VLE of these mixtures,and the corrlated results were compared with the experimental data from the literatures.The correlation results showed that the deviations in calculating pressures of PR+MHV1+NRTL and SRK+MHV1+NRTL were less than 5%for the ternary mixtures H₂+N₂+CH₄,He+N₂+CH₄ and N₂+Ar+CH₄;and less than0.5%for the ternary mixture N₂+CH₄+CO₂,except for some temperatures.For the ternary mixture H₂+N₂+CH₄,the calculation deviations of vapor phase compositions of the two models were less than 0.04.It can be clearly observed in the ternary phase diagram that the calculated values deviated far from the experimental data.For the ternary mixture H₂+N₂+CH₄,the correlated accuracy for the two models was poor;for the ternary mixtures He+N₂+CH₄,N₂+CH₄+CO₂,N₂+Ar+CH₄,the calculation deviations of vapor phase compositions of the two models were less than 0.01(except for individual temperature points).In the ternary phase diagram,the calculated values were in good agreement with the experimental data.For the ternary mixtures CH₄+CO₂+H₂S,CH₄+C₂H₆+CO₂,the calculated deviations of pressure were about 1%,and the calculated deviations of the vapor phase composition were within 0.02.In the ternary phase diagram,the calculated value deviated from the experimental data obviously.For the ternary mixture CH₄+CO₂+C₃H₈,the calculated accuracys of the two models were fluctuated at 230 and 270 K.At 270 K,the calculated deviations of pressure used by the two models were about 0.5%,and the calculation deviations of vapor phase compositions were within 0.003.The calculated values in the ternary phase diagram were in good agreement with the experimental data.At 230 K,the calculated deviations of pressure and vapor phase composition were large.To sum up,in the calculation of vapor liquid equilibrium of ternary mixtures,for the same mixture,the calculation accuracy of the two models was slightly different at different temperatures.For ternary mixture N₂+Ar+CH₄,correlation effect of the two models was the most satisfactory.For the mixture containing more than one alkane such as CH₄,it can be observed in the ternary phase diagram that the correlation effect of the two models was not very good.(3)Research on density of pure substances and binary mixturesThe density of pure substance and binary mixtures was studied.Seven models(PR,VTPR1,VTPR2,PT,TPR1,TPR2 and NCEOS equation)were used to predict the saturated vapor and liquid densities of pure CH₄,Ar,H₂S and H₂.For the densities of binary mixtures CH₄+Ar,CH₄+H₂S and CH₄+H₂ in vapor-liquid equilibrium,the PR+vdW model with the interaction parameters in the whole temperature region as one variable value was used to correlate.The correlation results showed that,for pure substances,the liquid phase density deviations were the largest for the PR model,and the other models were in good agreement with the results in the literatures.For binary mixture systems,the gas phase densities obtained by using the PR+vdW model in vapor-liquid equilibrium had a good consistency compared with the Refrpop9.0 data;but for the liquid phase densities in vapor-liquid equilibrium,the calculation results for the liquid phase densities of the 3 kinds of binary mixtures revealed unsatisfactory results.