Environmental Impact Assessment Of Ionic Liquid Gas Separation Process Based On Full Life Cycle

Ionic liquids（ILs）are emerging solvents in recent years.They are becoming a new type of absorbent to replace traditional solvents in the field of gas separation because of their excellent properties such as high solubility,low volatility and controllable structure.However,the environmental impact of a wide variety of ILs in different absorption processes is unknown,therefore,how to choose efficient and“green”ILs for specific application scenarios is a problem worth considering.To this end,a life cycle assessment（LCA）method based on process simulation was constructed in this thesis to quantitatively evaluate the environmental impact of two typical separation processes of ionic liquids used in natural gas dehydration and removal of chlorobenzene volatile organic compounds in gas.The specific research contents and main results are as follows:（1）The synthesis methods of eight typical imidazole-based ILs（[EMIM][BF₄],[EMIM][Tf₂N],[OMIM][PF₆],[OMIM][BF₄],[OMIM][Tf₂N],[BMIM][BF₄],[BMIM][PF₆]and[BMIM][Tf₂N]）were obtained through literature research.The environmental impact of the ILs synthesis processes was quantified by life cycle assessment software Sima Pro.And explored the role of anion and cation structures in their environmental impact results.The results show that in the process of synthesizing ionic liquids,the environmental impact generated mainly comes from three aspects,namely,the synthesis process of anion precursors,the synthesis process of cationic precursors and the effects of solvents involved in the purification process,and the overall contribution of these three to environmental impact is comparable.For ILs with different anion structures,the environmental impact results are greatly affected by the toxicity of the upstream material elements in the synthesis of anions.ILs with[BF₄]^-have a larger environmental impact in the air-related environmental impact category,while ILs with[PF₆]^-are concentrated in the water-related influence category due to the presence of phosphorus.For ILs with different cation structures,their impact on the environment gradually decreases with the extension of the alkane chain,i.e.environmental impact[EMIM]⁺>[BMIM]⁺>[OMIM]⁺,which is mainly related to the yield of target ILs.（2）To construct a full-life cycle environmental impact assessment model of ionic liquids for natural gas technology based on process simulation dehydration.Five kinds of ionic liquids（[EMIM][BF₄],[BMIM][BF₄],[OMIM][BF₄],[BMIM][PF₆]and[BMIM][Tf₂N]）natural gas dehydration processes were simulated by Aspen Plus.The process parameters of ILs were optimized by sensitivity analysis,and the dehydration performance of different ILs was compared.Subsequently,comprehensively consider the material and energy consumed by each IL to complete the separation task,and a systematic life cycle assessment of these dehydration processes was carried out by Sima Pro software.The results show that the dehydration performance of different ionic liquids natural gas dehydration processes shows[EMIM][BF₄]>[BMIM][BF₄]>[OMIM][BF₄]>[BMIM][PF₆]>[BMIM][Tf₂N].[EMIM][BF₄]is the preferred absorbent from the point of view of process energy consumption.Ranking the environmental impacts of these dehydration processes shows that[BMIM][PF₆]>[BMIM][Tf₂N]>[EMIM][BF₄]>[BMIM][BF₄]>[OMIM][BF₄],that is,the dehydration process using[BMIM][PF₆]as the absorbent caused the most serious environmental impact and was mainly concentrated in the environmental impact categories related to water and air.（3）The gas-liquid phase equilibrium data of three binary mixed systems（C₆H₅Cl+[EMIM][Tf₂N]/[BMIM][Tf₂N]/[BMIM][PF₆]）were experimentally determined under different temperature and molar ratio.Combined with the literature data,a corresponding UNIFAC model was established to obtain unknown group interaction parameters.The process model of ILs removal of chlorobenzene in waste gas was constructed by Aspen Plus software on this basis,and the process optimization design was carried out.Finally,from the perspective of life cycle,the environmental impact of the process of absorbing chlorobenzene by ILs was analyzed and compared.The results show that the experimental values of the saturated vapor pressures of the three systems and the predicted values of the UNIFAC model have a good match,which means that the interaction parameters between the groups obtained by fitting are reasonable.The optimized process parameters for ILs to absorb chlorobenzene obtained by sensitivity analysis were T_A=20°C,N_T=4,T_R=200°C,P_R=0.01 atm.The order of the absorption performance of ionic liquids for chlorobenzene is[OMIM][Tf₂N]>[BMIM][PF₆]>[BMIM][Tf₂N]>[BMIM][PF₆]>[OMIM][BF₄]>[EMIM][Tf₂N]>[BMIM][BF₄]>[EMIM][BF₄].From the perspective of environmental impact assessment,the environmental impact produced by each stage is quite different in the whole process,and a large number of environmental impacts are concentrated in the production stage of ILs.As for the effect of anion and cation structures on the environmental impact of ILs in this process,it is similar to the ILs synthesis stage.According to the results obtained from the research,it can provide support for the construction of a complete inventory of imidazole-based ILs product life cycle data and quantitative experience.As well as providing effective guidance for improving the production process of ILs and reducing the environmental impact of the synthesis process.Furthermore,ILs for dehydration of natural gas and absorption of chlorobenzene were screened from technical and environmental perspectives.In addition,the main stage of the environmental impact of ILs on the chlorobenzene absorption process were identified,which provided reliable data support for the development of green and recyclable processes in the future.