Carbon dioxide solubility in aqueous potassium salt solutions of l-proline and dl-α-aminobutyric acid at high pressures
Introduction
The leading technology today for post-combustion CO2 capture is based on the reversible chemical absorption of CO2 in amine-based solvents. The process most commonly utilises aqueous solutions of alkanolamines particularly monoethanolamine (MEA). The wide use of MEA as CO2 absorbent is due to its fast CO2 reaction kinetics, low solvent cost, high alkalinity, and other desirable characteristics. However, the use of MEA also entails a number of process limitations and other disadvantages. Aqueous MEA is corrosive at high concentrations, forms degradation products due to long exposure with CO2 and O2, and exhibits high vapour pressure causing significant solvent losses during processing. Also, it has a high heat of reaction with CO2, which leads to high energy requirement during solvent regeneration [1], [2]. Thus, development of new and better solvents than aqueous MEA has always been necessary.
Amino acid salts are proposed as suitable alternatives to aqueous alkanolamine solvents [3], [4], [5], [6]. Due to the presence of amino functional groups in their molecules, amino acid salts, like alkanolamines, have fast reactivity and appreciable absorption capacity for CO2 [4], [7], [8], [9], [10], [11]. The reaction kinetics of potassium salts of some amino acids were found to be even faster than MEA’s [12]. Additional advantages of aqueous amino acid salt solutions include low volatility and resistance to oxidative degradation [13]. They also have surface tension and viscosity that are similar to those of water, which makes their application more practical [14], [15], [16], [17]. It has also been demonstrated that the utilisation of this type of solvents would lead to a more environment-friendly and energy-effective separation process for CO2 capture [18].
In the present work, we investigated the CO2 absorption capacities of two amino acid salts that are potential CO2 absorbents—the potassium salts of l-proline and dl-α-aminobutyric acid. Proline is a secondary amino acid with a distinctive cyclic structure, which includes an amine group while dl-α-aminobutyric acid is a sterically-hindered amino acid having an ethyl group substituent. The potassium salt of l-proline (KPr) is proposed by several authors as a candidate absorbent for CO2 capture due to its fast reaction rate and high absorption capacity for CO2 [10], [12], [19]. van Holst et al. [12] examined the absorption kinetics of CO2 in potassium salts of several amino acids including 6-aminohexanoic acid, β-alanine, l-arginine, l-glutamic acid, dl-methionine, l-proline, and sarcosine at T = 298 K. They reported that the potassium salt of l-proline (and sarcosine) has relatively high apparent rate constant and low pKa making it a promising CO2 absorbent. In a more recent study, Paul and Thomsen [10], used a zwitterionic mechanism, which generally applies for the reaction of CO2 with primary and secondary alkanolamines, to describe the kinetics of reaction of CO2 with KPr. They found that the overall reaction rate constant for the latter is much higher than that of potassium threonate (KThr) and potassium taurate (KTau). The solvent has also been shown to have higher absorption capacity compared with the benchmark alkanolamine, MEA, and (vapour + liquid) equilibrium (VLE) data for low CO2 partial pressures (up to 30 kPa) have been reported [5], [19]. On the other hand, the potassium salt of dl-α-aminobutyric acid (KAABA) has been found to have a significantly higher net cyclic capacity compared with MEA and potassium salts of some other amino acids [5]. The results of the study suggest that KAABA could be an energy-efficient alternative to MEA. However, except for the results of the (absorption + desorption) experiments at 15 kPa presented by Song et al. [5], no other VLE data has been reported for CO2 absorption in KAABA.
The objective of this work was to provide VLE data for the absorption of CO2 in aqueous KPr and aqueous KAABA solutions at high pressures. The solubility of CO2 in the solvents was measured at temperatures (313.2, 333.2, and 353.2) K and pressures up to 1000 kPa. In addition, a modified Kent–Eisenberg model [20] was used to represent the correlation of the measured CO2 solubility (, expressed as mole CO2 per mole amine) with CO2 partial pressure, temperature, and amino acid salt (AAS) concentration.
Section snippets
Chemicals
The aqueous potassium salt solutions of l-proline (Pr, mass fraction purity > 0.99), dl-α-aminobutyric acid (AABA, purity > 99 wt%), and sarcosine (Sr, mass fraction purity > 0.98) were prepared by adding an equimolar amount of KOH (pellets, mass fraction purity > 0.85) to the amino acid in a volumetric flask with deionised-distilled water. The description and sources of all chemicals used, including those used in the validation experiments (Sr and MEA), are listed in table 1. They were used without
Validation of experimental method
To validate the experimental method used in this work, CO2 solubility in 15.3 wt% MEA solution at T = 313.2 K and 42.4 wt% potassium sarcosinate solution (KSr) at T = 353.2 K were measured. The VLE values obtained are given in TABLE 2, TABLE 3 along with those reported by Lee et al. [24] and Park et al. [25] for CO2–MEA–H2O and Kang et al. [26] for CO2–KSr–H2O systems, respectively. Graphical representations of the results are also provided in figure 1 showing that the experimental values are in
Modelling
In this work, we used a modified form of the Kent–Eisenberg model proposed by Li and Shen [20] to correlate the CO2 loading in the aqueous amino acid salt solution with temperature, CO2 partial pressure, and AAS concentration. This model allows, with simplicity, the calculation of CO2 solubility in amine solvents using equilibrium constant expressions derived from equilibrium reactions, which are assumed to occur between CO2 and the amine. It has been shown that the absorption of CO2 in aqueous
Conclusions
The CO2 solubility in aqueous potassium salts of l-proline and dl-α-aminobutyric acid were measured at temperatures (313.2, 333.2, and 353.2) K and CO2 partial pressures up to 1000 kPa for amino acid salt concentrations (0.5, 1.0, and 2.0) M. Results show that CO2 loadings in the solvents increase with increasing CO2 partial pressure and decrease as temperature increases. It was also found that the absorption capacities of the amino acid salt systems studied are in the same order of magnitude as
Acknowledgement
This research was supported by Grant MOST-103-2221-E-033-067-MY3 from the Ministry of Science and Technology of the Republic of China. We also thank Chung Yuan Christian University for financial support.
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