Comparison of mass transfer coefficients and desorption rates of CO2 absorption into aqueous MEA + ionic liquids solution
Graphical abstract
Introduction
Carbon dioxide (CO2) is known as the main greenhouse gas which causes the global warming nowadays (Ma et al., 2013, Godini and Mowla, 2008). Therefore, the exploitation of economical and practical capture technology is becoming a hot issue in the field of greenhouse gas emission (Ober et al., 2012, Shiflett et al., 2010, Holst et al., 2009, Liu et al., 2013, Kavoshi et al., 2013). The commercial way on CO2 capture depends on chemical absorption with the most widely used alkanolamines (Moioli and Pellegrini, 2015), such as monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA). The alkanolamines are known as good absorption performance for the separation of CO2 from flue gas and crude natural gas (Bidart et al., 2011). Among these alkanolamines, MEA is the most common absorbent for CO2 capture due to its advantages of low cost, high capacity and reasonable thermal stability. However, aqueous MEA solution has also many drawbacks, such as high solvent loss, corrosion, high energy demand for regeneration and instability to oxidant (Gutowski and Maginn, 2008, Wang et al., 2010, Privalova et al., 2013).
As a potential absorbent, room temperature ionic liquids (RTILs) have attracted more and more attention compared to alkanolamines solvents owing to their advantages, such as negligible volatility, low energy consumption for regeneration, superior thermal stability and good performance of CO2 absorption (Wang et al., 2010, Privalova et al., 2013, Galán Sánchez et al., 2011, Bara et al., 2009, Kühne et al., 2008, Zhang et al., 2013). In general, the absorption of CO2 into conventional ionic liquids is a physical process. Different with alkanolamines, the physical absorption of CO2 in ionic liquids largely depends on its solubility and operating conditions, and owns low value at ambient pressure and temperature (Zhang et al., 2008). Therefore, there is a growing trend for research amino-functionalized ionic liquids (NH2-RTIL), because both physical and chemical absorption takes place during the process of absorption that lead to 2:1 molar ratio of NH2-IL to CO2 (Gutowski and Maginn, 2008, Wang et al., 2010, Galán Sánchez et al., 2011). However, it had been reported that the pure task-specific ionic liquids (TSILs) had high viscosities and their viscosities increased dramatically after contacting with CO2, which would lead to many drawbacks in industrial application (Gutowski and Maginn, 2008).
Recently, it had been reported that the mixture of alkanolamine and ionic liquids had good performance for CO2 capture even at very low pressure (Bara et al., 2009, Kühne et al., 2008). Alkanolamine can be dissolved and reacted with CO2 in ionic liquids readily. Meanwhile, CO2 can be released easily from these mixtures by heating solution. It overcomes the deficiency of high viscosities and volatile, heavy corrosion caused by other methods on CO2 capture. The CO2 absorption into aqueous MEA and ionic liquids solution has been reported at high pressure in many papers (Kühne et al., 2008, Shin et al., 2008, Shariati and Peters, 2004). For instance, Shiflett et al. (2010) studied the absorption process of CO2 using 1-butyl-3-methylimidazolium acetate at atmospheric pressure. Ahmady and Zhao discussed the absorption performance of CO2 into several blends combined by alkanolamine and ionic liquids at wide range of carbon dioxide partial pressures, and the activation energy of the mixture of 1-buthyl-3-methylimidazolium tetrafluoroborate and MDEA solution has been proved to be reduced by compared to pure MDEA solution (Ahmady et al., 2012, Zhao et al., 2010). And it was reported that ionic liquids toke a role of catalyst in the process of CO2 absorption into the blend of MDEA and [Bmim][BF4] in their paper. Meanwhile, the study of absorbing CO2 with the mixtures of MEA and imidazolium-based ionic liquids in packed column has not been reported at present. Since MEA is used widely in industry, also in order to check catalytic action of ionic liquids and design column for CO2 absorption process, it is necessary to study the mass transfer coefficient of CO2 absorption into aqueous MEA + ionic liquids solution. Therefore, 1-ethyl-3-methylimidazolium bromine ([Emim][Br]) and 1-butyl-3-methylimidazolium bromine ([Bmim][Br]) were added into aqueous MEA solution, respectively. And the volumetric overall mass transfer coefficients () of CO2 absorption into these solution were calculated in this article. It will be helpful to learn the action of ionic liquids in the process of CO2 absorption. The effect of temperature, liquid flow rate and CO2 flow rate on the mass transfer of CO2 absorption into aqueous MEA + ionic liquids([Emim][Br], [Bmim][Br]) solution was discussed, respectively. Meanwhile, in order to determine the capacity of CO2 absorption and the effect of ionic liquids on CO2 desorption, the performance of CO2 desorption from different rich solution was studied at different temperatures.
Section snippets
CO2 absorption in aqueous MEA solution
The mechanism of CO2 absorption into aqueous MEA solution was described by Danckwerts (Danckwerts, 1979, Sada et al., 1976). In this process, Eq. (1) represents that CO2 is dissolved in aqueous MEA solution, it is a physical process accompanied with heat emission. As shown in Reactions (2) and (3), the reaction between CO2 and MEA is consisted of two-step
Chemicals
Carbon dioxide (≥99.9%) and nitrogen (≥99.9%) were supplied by Oxygen Co., Ltd. of WISCO, China. Bromobutane and bromomethyl were supplied by Sinopharm Chemical Reagent Co., Ltd. N-Methylimidazole was purchased from Hubei Hongyuan pharmaceutical company. The 1-buthyl-3-methylimidazolium bromide ([Bmim][Br]) and 1-ethyl-3-methylimidazolium bromide ([Emim][Br]) were prepared using the similar procedure in literatures (Dzyuba and Bartsch, 2001, Burrell et al., 2007, MacFarlane et al., 2001).
Experimental setup and operative description
The
Physical constant
Table 1 showed the physicochemical data of MEA + ionic liquids solution with ionic liquids mass fraction from 20% to 30% at 303–323 K. The results showed that the density and viscosity of aqueous MEA + ionic liquids solution increased with the increase of ionic liquid mass percentage ([Emim][Br], [Bmim][Br]), but decreased with the increase of temperature. Compared to the viscosity of lean solution, it was found that the viscosity of rich solution increased slightly. This was explained that
Conclusion
The value increased as absorption temperature increased in aqueous MEA + ionic liquids solution, but started to decline in aqueous MEA solution as temperature reached at 313 K. And then the value increased as liquid flow rate increased. The inert gas flow had little effect on the value.
It was found that the mixture of high concentration [Emim][Br] and MEA had superior performance on CO2 absorption. Aqueous 5% MEA + 20% [Emim][Br] solution had slight higher absorption efficiency and
Acknowledgments
The authors are grateful for the assistance from National Science and Technology Support Program of China (No. 2012BAC02B04), and the National Natural Science Foundation of China (No. E041104).
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First author.