Mass transfer and kinetics of CO2 absorption into aqueous monoethanolamine/1-hydroxyethy-3-methyl imidazolium glycinate solution
Introduction
The concept of carbon dioxide (CO2) capture and sequestration (CCS) has attracted considerable interest over the past two decades, as a result of the emerging problem of climate change [1]. One of the most potent techniques widely used in capturing CO2 from low pressure flue gas streams in power plants is chemical absorption using aqueous amine-based absorbents [2]. Alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) are the most commonly used absorbents in the past few years. The most advantage of MEA is high reactivity with CO2, but this process still suffers some disadvantages [3], [4], [5], [6], e.g., low CO2 capacity (0.5 mol CO2/mol absorbent), amine degradation and oxidation and high energy consumption. Thus the mixed solvents of amines with various additives are developed to overcome these existing drawbacks.
To date, ionic liquids (ILs) have received an advantage in various applications including in CO2 capture [7], [8]. More interestingly, introducing some special groups to the anion or the cation of ILs to obtain functionalized ILs can achieve excellent performance in CO2 capture due to the following characteristics [9], [10], [11], [12], [13], [14]: low volatility, low heat required for reaction, low thermal and oxidative degradation, high absorption capacity and easy to be regenerated.
To achieve excellent performance, the complexities of amine and ILs or functionalized ILs are proposed as new solvents for CO2 capture [15], [16], [17], [18]. It was found that CO2 solubilities in binary and ternary systems of guanidinium trifluoromethanesulfonate ([gua][OTf]) in MDEA and/or water were higher compared to other ILs, and decreased as the composition of [gua][OTf] in the systems increased [19]. Regeneration performance of [N1111][Gly] and MDEA aqueous solutions was performed, and It was found that the regeneration efficiency of 30 wt% MDEA + 15 wt% IL solution was a little higher than the other solution with different compositions [20]. In the other mixed solution, such as [N1111][Gly]/AMP system, the addition of [N1111][Gly] had greatly promoted the absorption of CO2 in AMP aqueous solution [21].
Besides, many works focus on kinetics analysis in order to investigate the mass transfer process. Our previous work found that [Bmim][BF4] had an active effect on the CO2 hydration and the values of enhancement factor (E) and the second-order reaction rate constant () for CO2 absorption into MEA/[Bmim][BF4] solution were higher than that of MEA solution [22]. Kinetics of DEA dispersion in [Hmim][Tf2N] for efficient CO2 capture was investigated by Iliuta et al., and the kinetic model used to describe the chemical reaction was based on the zwitterion mechanism followed by the removal of a proton by the amine and formation of immiscible solid carbamate crystals in the ionic liquid phase [23]. The kinetics region of absorption CO2 into aqueous [N1111][Gly] + AMP solution was found to be the fast pseudo-first order reaction regime and the activation energy of CO2 capture into [N1111][Gly] + AMP aqueous solution found to be 40.7 kJ mol−1 [21]. A summary of aqueous blend of amine and ILs or functionalized ILs for CO2 capture at low partial pressure were presented in Table 1, which indicated that the mixture seemed to be a suitable candidate for CO2 capture.
In our previous work, a dual functionalized ILs ([C2OHmim][Gly]) was designed and prepared based on the imidazolium ionic liquid with amino acid and hydroxyl group. Then the new hydrophilic amino acid ionic liquid was utilized to enhance the process of MEA for CO2 capture from low-pressure flue gas. The new blend of MEA and [C2OHmim][Gly] was proved to have an enhanced absorption capacity, a higher regeneration efficiency and a better resistance to O2 for the CO2 capture [26]. As a promising CO2-capturing solvent, although the absorption and desorption performance of CO2 capture into MEA/[C2OHmim][Gly] solution had been extensively studied, there is uncertainty about the mass transfer and the reaction kinetics of this new system. Herein, kinetics of the reaction of CO2 and MEA/[C2OHmim][Gly] in aqueous solutions was investigated using a double stirred-cell absorber with a defined gas/liquid interface. The kinetics model and parameters were measured in the presented work.
Section snippets
Chemicals
Monoethanolamine (MEA) with 99.0% purity was supplied by Shanghai Ling Feng Chemical Reagent Co. Ltd, China. The gas of CO2 (>99.99%), O2 (>99.99%) and N2 (>99.99%) supplied from the steel cylinder was purchased from Zhejiang Jin-gong Gas Co, China. The dual functionalized ionic liquid [C2OHmim][Gly] was prepared in our laboratory. No further purification was performed on the materials used.
Kinetic experiments
The experiments were carried out in a double stirred-cell absorber [22], which had a defined gas/liquid
Reaction mechanisms
The total reaction of CO2 absorption into aqueous blend of MEA/[C2OHmim][Gly] can be simplified as the sum of the reaction into CO2–MEA–H2O system and the reaction into CO2–[C2OHmim][Gly]–H2O system.
The chemical reaction of CO2 absorption into MEA solution was based on the formation of a zwitterion and follows by the removal of a proton by a base [27], [28], Bi. The reaction of CO2–MEA–H2O system follows a second order kinetic and is described by the overall reaction:
Physicochemical properties
The diffusion coefficient (D) and solubility (H) of CO2 in amine solution have been estimated or measured in other existing researches. These parameters of the new system are not easy to measure directly and are frequently estimated by the N2O analogy. The values of D and H of CO2 in MEA solution had been estimated in our previous work. D of CO2 in MEA solution could be calculated as follows [33], [34], [35]:
Conclusions
An aqueous blend of MEA/[C2OHmim][Gly] was proposed for CO2 capture into a double stirred-cell absorber. The reaction mechanism of CO2 capture into this new system was simplified as the sum of the CO2–MEA and CO2–[C2OHmim][Gly] reaction referring to the Zwitterion mechanism. Some important kinetic parameters, such as k2,mix, kov and E were measured and estimated in this work. The values of k2,mix and E into MEA/[C2OHmim][Gly] solution were all higher than that of MEA solution. They were
Acknowledgments
The work was sponsored by the National Natural Science Foundation of China (Nos. 21476203, 20976157), and supported by the Scientific Research Funds of Huaqiao University (15BS106).
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