Study On Cryogenic Separation Technology Of CO₂-C₂H₆ Azeotrope

Cryogenic distillation technology is a new approach of CO₂ flooding associated gas recovery in recent years,which has been widely used in the fields of associated gas decarburization,hydrocarbon removal,liquefaction.Compared with the traditional natural gas treatment technology,the cryogenic distillation technology condenses light hydrocarbons into qualified products by taking advantage of different boiling point temperatures of hydrocarbon components in associated gas,and the generated CO₂ as a liquid product can be directly used in CO₂-EOR technology,saving the extra capital investment of liquefied CO₂.However,with the promotion of CO₂-EOR-the third oil recovery technology,the content of CO₂ in associated gas increases year by year,resulting in the formation of CO₂-C₂H₆ azeotrope in the cryogenic distillation separation process,and the corresponding purity of carbon dioxide and ethane products can not meet the requirements.Therefore,the study on the azeotrope characteristics and the separation process of CO₂-C₂H₆ azeotrope in cryogenic distillation process can not only helpful to realize the double recovery of high purity of CO₂ and C₂H₆ products,but also deepen the understanding of the formation and separation characteristics of CO₂-C₂H₆ azeotrope under cryogenic conditions.Based on in-depth investigation of the current situation at home and abroad,this paper has done the following works:Firstly,the phase equilibrium characteristics of pure CO₂,CO₂-C₂H₆ azeotrope and n-C₅H₁₂-CO₂-C₂H₆ ternary system were studied.Based on the collected experimental data,the vapor liquid fugacity balance principles of cubic equation of states(vd W,RK,SRK,PR)combined with mixing rules(vd W,Margles,CVD)were established.Considering the influence of pressure and temperature on the calculation results,the method of average absolute deviation was adopted to select the best thermodynamic model.Considering the influence of binary interaction parameters,the optimization of binary interaction parameters was carried out by using the method of programming solution.The calculated results were verified with the Aspen Plus data regression system.The results show that the accuracy of SRK equation of state in predicting the vapor-liquid equilibrium property of CO₂ pure substance is the highest.The accuracy of PR equation of state combined with margles mixing rule to predict the vapor liquid equilibrium characteristics of CO₂-C₂H₆ azeotropic system is the highest,and the average absolute error is only 0.1841.The accuracy of SRK equation of state combined with margles mixing rule to predict the vapor-liquid equilibrium property of n-C₅H₁₂-CO₂-C₂H₆ ternary system is the highest,and the average absolute deviation is only 0.9192.The results show that the cubic equation of states are feasible to simulate the separation process of CO₂-C₂H₆azeotrope.Secondly,the characteristics and separation methods of CO₂-C₂H₆ azeotrope were studied.By analyzing the T-xy phase diagram of CO₂-C₂H₆ system,it can be seen that with the decrease of rectification temperature,CO₂-C₂H₆ system appeared an azeotropic point,namely CO₂-C₂H₆azeotrope.Then the applicability of pressure swing distillation,extractive distillation and azeotropic distillation for the separation of CO₂-C₂H₆ azeotrope system was analyzed.The results showed that pressure swing distillation could increase the relative volatility of carbon dioxide and ethane,but could not completely break the CO₂-C₂H₆ azeotropic point,indicating the inapplicability of pressure swing distillation.When ethane homologue was added into CO₂-C₂H₆ azeotrope system,it showed extraction effect instead of azeotropy,and reduced the freezing and plugging temperature of carbon dioxide,indicating the applicability of extractive distillation.The molar amount of ethane homologue decreased with the increase of the number of light hydrocarbon C atoms and the increase of the boiling point.Thirdly,the design of CO₂-C₂H₆ azeotrope extractive distillation process was studied.According to the different types of additives,the processes of pure component additives,mixed component additives and recycle additives were designed respectively and optimized including total stage number,feed inlet location,additive inlet location,and additives molar flow with the help of sensitivity analysis.Double recovery of 95%carbon dioxide and 96%ethane was achieved respectively.Total annual cost is the only valid indicator to be used to find the optimum cryogenic extractive distillation process.Compared with the current running cryogenic distillation process,the purity of ethane product has been improved by 24.3%and the total annual cost has been reduced by 13.79%.The results show that the cryogenic extractive distillation process designed for processing CO₂ flooding associated gas is effective and efficient.Finally,the dynamic simulation of the extractive distillation process of CO₂-C₂H₆azeotrope was studied.According to the process parameters of steady-state simulation,two dynamic control structures of fixed reflux ratio and reflux/feed ratio were set up,and the dynamic simulation of the extractive distillation process of CO₂-C₂H₆ azeotrope was successfully realized.Taking dynamic response time and product temperature,molar flow and purity as evaluation criteria,two dynamic control structures were compared by changing the feed molar flow and the CO₂ content in the feed.The results show that the fixed reflux ratio control structure is the optimal dynamic control structure.The controllable range of the fixed reflux ratio control structure is given by dynamic simulation:CO₂ content is 7%?12%,and with the increase of CO₂ content,the minimum allowable value of feed flow increases.