Design And Evaluation Of Absorption Process For CO₂ Capture With Ionic Liquids

The phase equilibrium data of CO₂/ionic liquid system is an important theoretical basis for the designing the capture of CO₂ with ionic liquid.It is of great significant to establish complete and comprehensive phase equilibrium database of CO₂/ionic liquid system.A gas-liquid equilibrium database and phase equilibrium experimental data processing software based on the constant volume method was built in this work.After that,the vapor-liquid equilibrium data of the CO₂/[Emim][Tf₂N]system was determined to develop a process for the absorption and capture of CO₂ with ionic liquids.The key parameters of the process were also analyzed,as well as the properties of the ionic liquids.The resesearch contents are shown as follows:（1）The ionic liquid database development and the CO₂ capture technologies has been systematically reviewed.Particular,the CO₂ capture technologies with ionic liquid was carefully evaluated.In addition,the design philosophy,organizational structure,and both of their advantages and disadvantages were discussed in detail.Three kinds of classified CO₂ capture technologies,as known as i.e.,pre-combustion capture,oxygen-enriched combustion capture,and post-combustion capture,were illustrated clearly in this paper.Subsequently,the characteristics,strengths,limitations and the industrial application status of these three technologies were proposed.The development of these technologies opened a new horizon on CO₂ capture towards theoretical research and engineering application;（2）To complete the information of CO₂/ionic liquid system phase equilibrium data which is scattered in various types of papers imperfectly,it developed CO₂/ionic liquid system phase equilibrium database with the data collected between 2007 and2017 in related papers.The relational database management system ACCESS 2016was used as the database development platform,the structured query language SQL and the language VBA were realized the functions of querying and controlling the forms,respectively.The CO₂/Ionic Liquid System Phase Equilibrium Database contains nearly 3,000 physical properties data.This database can be used to query critical properties of ionic liquids,CO₂ solubility data in ionic liquids,saturated vapor pressure,liquid density,viscosity,surface tension,thermal conductivity and constant pressure heat capacity of ionic liquid,etc,and also it includes various physical properties measurement methods,literature information,etc.（3）In order to further enrich the information of CO₂/ionic liquid phase equilibrium database and solve the data processing problems of constant volume method for measuring the solubility of gas in ionic liquid.A set of gas/liquid system equilibrium calculation software was developed.It includes the module of gas solubility calculation and gas dissolution thermodynamic calculation.Visual Basic 6.0programming language was adopted to write the user interface and main body while Matrix VB,a matrix function library embedded in VB,was used to realize he graphic image processing and data fitting.The solubility data of CO₂ in ionic liquid[Emim][Tf₂N]at the conditions of temperature T（303.15 K,313.15 K,323.15 K）and pressure of 0-600 kPa was obtained by this software with the calculated and plotted data.（4）Using the established CO₂/ionic liquid system phase equilibrium database,the Aspen simulation and evaluation study of ionic liquid capture of CO₂ in flue gas from coal-fired power plants was carried out.Firstly,the ionic liquid[Emim][Tf₂N]was selected as the absorbent.The data of thermal conductivity,constant pressure heat capacity,vapor pressure,viscosity,density and surface tension were retrieved from the phase equilibrium database and the parameter regression based on these data were performed;Then,by using the NRTL-RK model,the gas-liquid phase equilibrium data of[Emim][Tf₂N]absorbed CO₂ was correlated.The Henry’s coefficient of dissolution of CO₂ in ionic liquid and the binary interaction parameters of NRTL were obtained.Finally,using the established full-process model,the sensitivity analysis of parameters such as absorption tower operating pressure and absorption tray number was carried out,and the absorption process was optimized based on the temperature distribution in the absorption tower.The process simulation results showed that the system had the lowest energy consumption,when the absorption pressure was 20 bar and the number of plates was 15.Per ton of CO₂ collected after process optimization only required2.169 GJ of electrical energy.While the energy consumption of traditional MEA regeneration was in the range of 3.59³.7 GJ/t CO₂,which was better than the traditional process.Using the designability of ionic liquids,a new concept of"american ionic liquids"was introduced,and the influence of the nature of the ionic liquids on the energy consumption of the process and the amount of solvent circulation were discussed.The results showed that the density was half of the reference ionic liquid,solvent circulation and energy consumption increased by 61.1%and 59.7%,respectively,and the density was twice than that of the reference,the amount of ionic liquid solvent circulation and energy consumption were reduced by 49.8%and 46.7%,respectively.Compared with the reference ionic liquid[Emim][Tf₂N],the constant pressure heat capacity was half of the reference ionic liquid,the solvent circulation amount and energy consumption increase by 3.7%and 3.5%respectively.The constant pressure heat capacity was twice than that of the reference substance,and the solvent circulation amount and energy consumption were reduced by 2.0%respectively,compared with the reference ionic liquid[Emim][Tf₂N].This study can provide important data foundation for related research.The developed CO₂ capture process using ionic liquid as absorbent has a good application prospect.Simulation and evaluation results will provide guidance for the process optimization and the engineering application.