Absorption of CO2 in amino acid ionic liquid (AAIL) activated MDEA solutions
Introduction
The combustion of fossil fuels causes large emission of CO2 and high CO2 concentration in the atmosphere, which has destructively affected the environment through the so-called “greenhouse effect” (Knight and Harrison, 2012, Meinshausen et al., 2009, Rochelle, 2009). There is no doubt that carbon capture and sequestration (CCS) is one of the most promising solution to address this world-widely concerned environmental problem (MacDowell et al., 2010). However, it still remains to select out the proper methods to capture CO2 not only in terms of the capital and operation cost, but also in terms of the environmental impact.
In industries, the most attractive approach for the separation of a target compound from a mixture of gases is selective absorption into liquid (Rochelle, 2009). In the case of CO2 capture, aqueous solutions of alkanolamine or their mixtures, including monoethanolamine (MEA), diethanolamine (DEA) and N-methyldiethanolamine (MDEA) are extensively employed as solvents to trap CO2 from the flue gas in the industrial processes (Giuffrida et al., 2013, Nuchitprasittichai and Cremaschi, 2013, Rochelle, 2009). Since the absorption of CO2 in MDEA aqueous solution is quite slow, activators such as Piperazine (PZ) was adding into MDEA aqueous solution to overcome this shortcoming. However, there are many disadvantages in these processes for the use of alkanolamine absorbents, including volatile loss of alkanolamine in regeneration and intensive energy input to regenerate the absorbent and degradation of the alkanolamine to form corrosive byproducts. These disadvantages cause extra energy consumption, second pollution of alkanolamine emission and more equipment investment.
Ionic liquids (ILs), which are organic molten salts, have been proposed to be a new class of green solvents that exhibit great potential applications in many fields including catalysis (Welton, 1999), electrolysis (Armand et al., 2009), extraction (Huddleston et al., 1998), membrane separation (Fang et al., 2013, Scovazzo et al., 2002), gas absorption (Blanchard et al., 1999, Baj et al., 2012), etc., due to their unique properties such as extremely low vapour pressure, high thermal stability, designable structure, wide liquid temperature range and strong solubilization for many compounds (Wappel et al., 2010).
Anthony et al. (2002) first reported 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) as CO2-philic liquids with selectivity toward other inert gases (e.g. N2 and CH4). Subsequently, solubility of CO2 in a variety of other ILs was reported (Kamps et al., 2003). However, these ILs only enable physical interaction with CO2, resulting in limited CO2 absorption capacities. Taking into account the chemical interaction of amino group on CO2, Bates et al. (2002) proposed the concept of “task-specific ionic liquids” (TSILs) in which solubility of acid gases in ILs could be significantly enhanced by introducing functional group to the framework of ILs. The first reported TSIL for CO2 capture was [NH2-pbim][BF4] which could uptake CO2 by a stoichiometry of 2:1. Amino acid ionic liquid (AAIL) means the ionic liquid which anion is modified with amino functional group. Adding of AAIL, even of a very small concentration, into the MDEA aqueous solution causes a dramatic increase in the absorption rate of CO2 (Zhang et al., 2012); it is extremely low vapour pressure and high thermal stability can avoid wastage and second pollution during regeneration process. Moreover, it is large surface tension weakens foam, and enhance transfer process of CO2 absorbed in MDEA solution.
Inspired by the pioneering works, amino acid ILs, including tetrabutylphosphomium amino acids ILs (Zhang et al., 2006) and dual amino-functionalized phosphonium ILs (Zhang et al., 2009), were also prepared for CO2 absorption. The absorption mechanism of CO2 capture in amino acid based ILs are the same as the case of [NH2-pbim][BF4] as these two amine groups both attract one CO2 molecule. However, these amino acid ILs are usually of high viscosities (>200 cP) (Gurkan et al., 2010) and the liquid viscosity would increase dramatically during CO2 absorption due to the formation of complex hydrogen bond net (Gutowski and Maginn, 2008, Jiang et al., 2008). Practically, the high viscosity of TSILs strongly limits the transport of ILs and diffusion of CO2 into ILs when they are actually applied in industry. Thus, design and developing new kinds of TSILs become a very important and attractive work for the researchers.
A series of tetraalkylammonium and asymmetric tetraalkylammonium amino acid ILs of low viscosities for fast CO2 absorption were synthesized in our lab (Yu et al., 2009). The lowest viscosity was found for [N2224][l-Ala], with a value of 29 cP, in which CO2 absorption could reach equilibrium within 30 min. However, these ILs are still much more viscous than normal organic solvents or alkanolamine aqueous solutions and can be hardly industrialized. In addition, the cost of ILs is usually quite high. Both the two factors hinder the industrial application of ILs.
A worthwhile solution to these problems is to blend ILs with water and/or other organic solvents to form mixed absorbents (Bara et al., 2007), using amino acid ILs as activator to replace high volatile MEA and blending with MDEA and water may forms special hybrid solvents for CO2 capture (Zhang et al., 2010, Zhang et al., 2012). Noticeably, compared with other amino acid ILs when combined with MDEA and water, tetramethylammonium glycinate ([N1111][Gly]) is of low cost and superior CO2 absorption performance. The effects of the concentration of [N1111][Gly] and MDEA on the CO2 absorption rate and CO2 loading capacity has been systematically investigated in our previous works (Zhang et al., 2010, Zhang et al., 2012). Moreover, the regeneration performance of amino acid ionic liquid (AAIL) activated MDEA solutions has been studied to evaluate the influence of solution composition and regeneration temperature on the regeneration efficiencies and absorption rate of renewed solutions. It was found that in the thermal regeneration, most of the gas gives off before boiling of solution and the regeneration efficiencies of the solutions (30% MDEA+10% [N1111][Gly], 30% MDEA+15% [N1111][Gly] and 40% MDEA+10% [N1111][Gly]) regenerated at 378 K are all higher than 90% (Zhang et al., 2013). In the present work, the physical properties of the mixed absorbents (e.g. density, viscosity and surface tension), which determines the transfer performance, are discussed. And the equilibrium absorption of CO2 in the aqueous solutions of MDEA and [N1111][Gly] at different pressures and temperatures is studied in detail to find a feasible concentration and parameter combinations for industrial CO2 capture.
Section snippets
Mechanism
The mechanism for the reaction of MDEA, which is a tertiary amine, and CO2 has been proposed by Austgen et al. (1989) and Posey and Rochelle (1997):MDEA + CO2 + H2O → MDEAH+ + HCO3−This reaction is essentially a base-catalyzed CO2 hydrolysis and MDEA does not combine with CO2 directly. Therefore, the absorption of CO2 in MDEA aqueous is very slow though the absorption capacity could reach 1 molCO2/molMDEA.
In aqueous solutions, tetraalkylammonium amino acid IL can completely dissociate into hydrated
Materials and absorbents preparation
Analytical grade MDEA (purity >99.9%), tetramethylammonium hydroxide (purity >99.9%) and glycine (purity >99.9%) were supplied by Shanghai Bangcheng Chemical Co., Ltd. Distilled water had been further degassed by boiling before use. CO2 (purity >99.9%) was purchased from Nanjing Gas Supply Inc.
The amino acid IL was synthesized in the way described by Jiang et al. (2008). The aqueous solutions of IL + MDEA were prepared through weighting method. In the IL + MDEA aqueous solutions, the weight
Physical properties of the aqueous solutions
The physical properties of the absorbents are presented in Table 1.
It can be seen in Table 1 that the density increases slightly as the concentration of IL and MDEA rises because the density values of IL and MDEA are close to that of water. On the other hand, when the temperature grows, the density of every solution gradually decreases and the density difference between 25 °C and 70 °C is within 3%, which means that the concentration or temperature has little influence on the density of solution.
Conclusion
Since density values of IL and MDEA are close to that of water, the density increases slightly as the concentration of IL and MDEA rises. And the solution viscosity greatly increases as the concentration of IL or MDEA rises. For the solution 10% IL + 40% MDEA, the viscosity is over 7 mPa·s, and further increase in IL or MDEA concentration causes too higher viscosity to be applied in industry.
When Pe < 60 kPa, CO2 loads increase dramatically as Pe rises and the difference in the absorption capacities
Acknowledgements
The authors are grateful for the financial support from National Nature Science Foundation of China (No. 21376115 and 21076101) and Natural Science Foundation of Jiangsu Province (No. BK2011633 and 20131343).
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