Study On CO₂ Removal Process Of Associated Gas Based On Azeotropic Distillation And Extractive Distillation

Low-temperature distillation separation technology is a new CO₂-EOR associated gas treatment technology emerging in recent years,which has been widely used in associated gas decarbonization,dehydrocarbon,liquefaction and other aspects.Compared with the traditional natural gas treatment technology,cryogenic distillation technology utilizes the different boiling point temperature of hydrocarbon components in associated gas to condense light hydrocarbons into qualified products in turn,and the generated CO₂ as liquid product can be directly used for CO₂-EOR technology,saving the additional capital investment of liquefied CO₂.However,with the promotion of CO₂-EOR third oil recovery technology,the content of CO₂ in associated gas increases year by year,leading to the phenomenon of the generation of CO₂-C₂H₆ azeotropes in the process of low-temperature distillation and separation,and the purity of corresponding CO₂ and C₂H₆ products cannot meet the requirements.Therefore,the study on the azeotropic properties of CO₂-C₂H₆ azeotropes in low-temperature distillation technology and the low-temperature decarbonization technology of natural gas not only contributes to the realization of high-purity double recovery of CO₂ and C₂H₆ products,but also deepen people’s understanding of the formation and separation characteristics of CO₂-C₂H₆ azeotropes at low temperatures.Based on in-depth investigation of the current situation at home and abroad,this paper has done the following work:First,in order to remove CO₂ from natural gas by azeotropic distillation method,the CO₂-C₂H₆ azeotropic system was verified by using isothermal gas-liquid equilibrium data of CH₄+C₂H₆+CO₂ binary system obtained from previous experiments,and these data were tested by thermodynamic consistency.It is found that the AAD（y₁）and AAD（p）values of PR+Margles mixed model are less than 0.0045 and 0.19K,respectively,which are superior to the mixed model of RK+Margles and SRK+Margles.Then,the design of the new decarbonization process of low temperature natural gas distillation was carried out.According to the phase equilibrium analysis of CO₂-C₂H₆ azeotrope system,the azeotrope interval of CO₂-C₂H₆was determined in combination with the actual simulation situation,so that CO₂-C₂H₆could form a stable azeotrope.Using Aspen Hysys to analyze the optimal feed temperature,feed pressure and feed flow of the feed and additive C₂H₆,analyze the influence of pressure and temperature on the composition of CO₂-C₂H₆ azeotropic compounds,and determine the appropriate feed and C₂H₆additive physical parameters.The minimum theoretical plate number and minimum reflux ratio were determined by simple calculation of the distillation column.The physical property parameters in the column were determined according to simulation to ensure the formation of CO₂-C₂H₆ azeotrope.The capacity of the column was determined according to the feed flow rate.Change the content of CO₂ in the feed（50%,60%,70%,80%,90%）,test the applicability and feasibility of natural gas decarbonization process.It is found that when the temperature of the bottom of the tower is determined,changing the operating pressure and the feed flow of azeotropic agent can make the bottom of the tower form stable azeotropic logistics,and the top of the tower can get high purity natural gas,so as to realize the research and development of the new technology of natural gas decarbonization.Then,using C₂H₆ homologue as extraction agent can break the characteristics of CO₂-C₂H₆ azeotropic,extractive distillation method combined with feed fractionation method was adopted to recover high purity C₂H₆,propane,butane,pentane products,and obtain liquid high pressure purity of 95%CO₂products,which can be directly reinjection of CO₂ flooding process.In this paper,two different methods of system energy internal integration are proposed.It is suggested in this work that the feed should be preheated in two different ways.One is that the feed should be preheated completely to absorb the internal energy of the system.The other keeps the appropriate portion of the feed at its original temperature and the remainder heated with a hotter bottom product.The annual total cost and CO₂ emission of the two preheating processes were compared with the basic process.The results show that the energy integration effect of the preheating process of feed separation is better than that of the full preheating process.The annual total cost is reduced by 24%,and the CO₂ emission rate is reduced by15.4%.Finally,according to the principle of similarity and compatibility,the C2H6 homologue is used to break the CO₂-C₂H₆ azeotrope and remove the CO₂ in the associated gas.D-optimum mixing design was used to optimize the solvent ratio of CO₂-C₂H₆ azeotropic separation in extractive distillation.The effects of C₃H₈,i-C₄H₁₀,n-C₄H₁₀,i-C₅H₁₂,n-C₅H₁₂components on the total energy consumption of extractive distillation process were studied.he significance of the model and residual analysis were carried out through variance analysis to verify the reliability and stability of regression model.The interaction between components was analyzed through the contour map and the three-dimensional response surface,and finally the optimal ratio of extractant was obtained through the component optimization and verification of the optimal energy consumption.The prediction model is highly consistent with the experimental values（R-squared value=99.87%）.Under the limitation of typical composition of NGL on the extraction agent mixture,the optimal extraction agent composition with energy consumption was found.