2013-Improvement of the CO2 Absorption Performance Using Ionic Liquid
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Articlethe CO2 Absorption Performance Using Ionic Liquid [NH2emim][BF4] and [emim][BF4]/[bmim][BF4] Mixtures
Mei Wang,†,‡,§ Liqi Zhang,*,† Linxia Gao,§ Kewu Pi,§ Junying Zhang,† and Chuguang Zheng†
†
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ‡ School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, People’s Republic of China § School of Chemical and Environmental Engineering, Hubei University of Technology, Wuhan 430068, People’s Republic of China ABSTRACT: Functional ionic liquids (ILs) have potential advantages in reducing CO2 emissions when applied for CO2 absorption. However, the higher viscosity of functional ILs greatly affects the CO2 absorption separation process. To improve the absorption performance, a functional IL [NH2e-mim][BF4] was synthesized and mixed with low-viscosity conventional ILs [emim][BF4]/[bmim][BF4] based on their respective advantages in terms of CO2 reduction, and the CO2 absorption effect and regeneration performance of the binary ILs were investigated. {[NH2e-mim][BF4] + [emim][BF4]/[bmim][BF4]} showed better performance when the mole fraction of [NH2e-mim][BF4] (X[NH2e‑mim][BF4]) was 0.4, and the CO2 absorption performance reduced with the increase in the temperature. Density and viscosity of the binary rose with the increase of X[NH2e‑mim][BF4] and decrease in temperature. The optimal regeneration temperature was approximately 353.15 K when X[NH2e‑mim][BF4] was 0.4 in 0.1 MPa. During five cycles of absorption and regeneration, CO2 absorption capacity of {[NH2e-mim][BF4] + [emim][BF4]/ [bmim][BF4]} was maintained at 75−85% of the first absorption capacity. Moreover, the quality and density changed slightly, and viscosity showed a 5−10% increase in each loop. Thus, the mixed system had an effective regeneration performance.
1. INTRODUCTION Global warming is caused by greenhouse gas emissions, particularly by a large amount of CO2 released into the atmosphere by burning fossil fuel, and this issue continues to receive worldwide attention.1−4 Therefore, economically viable CO2 capture for large-scale reduction is becoming increasingly important. Currently, amine-based scrubbing (e.g., monoethanolamine, N-methyldiethanolamine, etc.) is widely used for post-combustion CO2 capture.5,6 The method has high CO2 absorption efficiency but is limited by regeneration difficulties and high energy consumption. Furthermore, amino is easily oxidized, which leads to the decrease in absorption efficiency. Organic solvents are volatile and cause equipment corrosion as well as add to environmental pollution.7−9 A series of studies showed that ionic liquids (ILs) have efficient CO2 absorption performance, low vapor pressure, adjustable structure, and other relevant characteristics. Therefore, the application of ILs in CO2 emission reduction has gained significant attention.10−16 Conventional and functional ILs are applicable for CO2 capture. In comparison to conventional ILs, functional ILs have a faster absorption rate and higher absorption capacity, and the use of functional ILs facilitates large-scale industrial application.17−19 However, the viscosity of functional ILs is higher, which largely affects the gas−liquid mass transfer, and the CO2 absorption separation process is impeded. In addition, regeneration of functional ILs becomes more difficult and requires more energy than that of conventional ILs.20−23 To improve the CO2 absorption performance of functional ILs, low-viscosity conventional ILs and functional ILs are mixed on the basis of their respective advantages in terms of CO2
© 2012 American Chemical Society
reduction. The mixture is expected to result in a faster absorption rate, higher absorption capacity, and better regeneration performance to meet the demands of large-scale industrial CO2 reduction. In 2002, Bates et al.24 synthesized an amine-functionalized IL [NH2p-bmim][BF4] that could dissolve 7.4 wt % CO2. Brett, Gurkan, and Zhang, among others,25−32 found that aminefunctionalized ILs have advantages in CO2 absorption. The present study selected an amine-functional IL [NH2e-mim][BF4] that has a shorter carbon chain substituted in the cation and mixed it with a series of conventional imidazole ILs that have low viscosity and an effective CO2 absorption performance. Subsequently, the performance of the mixed systems in terms of CO2 trapping was discussed. Previous studies by our group showed that imidazole ILs, which have anions [BF4] and [CH3CO2] can be dissolved with [NH2e-mim][BF4], whereas anion [Tf2N] imidazole ILs are suspended in the mixtures. Moreover, the mixtures of anion [BF4] or [Tf2N] conventional ILs and [NH2e-mim][BF4] showed better CO2 absorption capacity compared to the single IL. CO2 absorption capacity dropped after anion [CH3CO2] ILs were mixed with [NH2emim][BF4]. Hence, the mixtures of binary ILs that are composed of [NH2e-mim][BF4] and [emim][BF4]/[bmim][BF4] will be discussed in terms of their CO2 absorption and regeneration performances.
Mei Wang,†,‡,§ Liqi Zhang,*,† Linxia Gao,§ Kewu Pi,§ Junying Zhang,† and Chuguang Zheng†
†
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ‡ School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, People’s Republic of China § School of Chemical and Environmental Engineering, Hubei University of Technology, Wuhan 430068, People’s Republic of China ABSTRACT: Functional ionic liquids (ILs) have potential advantages in reducing CO2 emissions when applied for CO2 absorption. However, the higher viscosity of functional ILs greatly affects the CO2 absorption separation process. To improve the absorption performance, a functional IL [NH2e-mim][BF4] was synthesized and mixed with low-viscosity conventional ILs [emim][BF4]/[bmim][BF4] based on their respective advantages in terms of CO2 reduction, and the CO2 absorption effect and regeneration performance of the binary ILs were investigated. {[NH2e-mim][BF4] + [emim][BF4]/[bmim][BF4]} showed better performance when the mole fraction of [NH2e-mim][BF4] (X[NH2e‑mim][BF4]) was 0.4, and the CO2 absorption performance reduced with the increase in the temperature. Density and viscosity of the binary rose with the increase of X[NH2e‑mim][BF4] and decrease in temperature. The optimal regeneration temperature was approximately 353.15 K when X[NH2e‑mim][BF4] was 0.4 in 0.1 MPa. During five cycles of absorption and regeneration, CO2 absorption capacity of {[NH2e-mim][BF4] + [emim][BF4]/ [bmim][BF4]} was maintained at 75−85% of the first absorption capacity. Moreover, the quality and density changed slightly, and viscosity showed a 5−10% increase in each loop. Thus, the mixed system had an effective regeneration performance.
1. INTRODUCTION Global warming is caused by greenhouse gas emissions, particularly by a large amount of CO2 released into the atmosphere by burning fossil fuel, and this issue continues to receive worldwide attention.1−4 Therefore, economically viable CO2 capture for large-scale reduction is becoming increasingly important. Currently, amine-based scrubbing (e.g., monoethanolamine, N-methyldiethanolamine, etc.) is widely used for post-combustion CO2 capture.5,6 The method has high CO2 absorption efficiency but is limited by regeneration difficulties and high energy consumption. Furthermore, amino is easily oxidized, which leads to the decrease in absorption efficiency. Organic solvents are volatile and cause equipment corrosion as well as add to environmental pollution.7−9 A series of studies showed that ionic liquids (ILs) have efficient CO2 absorption performance, low vapor pressure, adjustable structure, and other relevant characteristics. Therefore, the application of ILs in CO2 emission reduction has gained significant attention.10−16 Conventional and functional ILs are applicable for CO2 capture. In comparison to conventional ILs, functional ILs have a faster absorption rate and higher absorption capacity, and the use of functional ILs facilitates large-scale industrial application.17−19 However, the viscosity of functional ILs is higher, which largely affects the gas−liquid mass transfer, and the CO2 absorption separation process is impeded. In addition, regeneration of functional ILs becomes more difficult and requires more energy than that of conventional ILs.20−23 To improve the CO2 absorption performance of functional ILs, low-viscosity conventional ILs and functional ILs are mixed on the basis of their respective advantages in terms of CO2
© 2012 American Chemical Society
reduction. The mixture is expected to result in a faster absorption rate, higher absorption capacity, and better regeneration performance to meet the demands of large-scale industrial CO2 reduction. In 2002, Bates et al.24 synthesized an amine-functionalized IL [NH2p-bmim][BF4] that could dissolve 7.4 wt % CO2. Brett, Gurkan, and Zhang, among others,25−32 found that aminefunctionalized ILs have advantages in CO2 absorption. The present study selected an amine-functional IL [NH2e-mim][BF4] that has a shorter carbon chain substituted in the cation and mixed it with a series of conventional imidazole ILs that have low viscosity and an effective CO2 absorption performance. Subsequently, the performance of the mixed systems in terms of CO2 trapping was discussed. Previous studies by our group showed that imidazole ILs, which have anions [BF4] and [CH3CO2] can be dissolved with [NH2e-mim][BF4], whereas anion [Tf2N] imidazole ILs are suspended in the mixtures. Moreover, the mixtures of anion [BF4] or [Tf2N] conventional ILs and [NH2e-mim][BF4] showed better CO2 absorption capacity compared to the single IL. CO2 absorption capacity dropped after anion [CH3CO2] ILs were mixed with [NH2emim][BF4]. Hence, the mixtures of binary ILs that are composed of [NH2e-mim][BF4] and [emim][BF4]/[bmim][BF4] will be discussed in terms of their CO2 absorption and regeneration performances.
