Elsevier

Fluid Phase Equilibria

Volume 524, 1 December 2020, 112743
Fluid Phase Equilibria

The contribution of aqueous l-arginine salts to equilibrium carbon dioxide absorption in a co-promoter role at high pressure

https://doi.org/10.1016/j.fluid.2020.112743Get rights and content

Abstract

Salts of the basic amino acid l-arginine are potential co-promoters and blending components for the conventional solvents used in carbon dioxide-based separations. This study defines the contribution of arginine salts to equilibrium carbon dioxide absorption at co-promoter concentrations (less than 1 M) and high pressure. Experimentally determined carbon dioxide loadings in aqueous potassium and sodium salt solutions of l-arginine show a positive relationship with pressure (range: 110–4110 kPa); but a negative behaviour with an increase in temperature (range: 303.15–363.15K) and solvents concentrations (range: 0.25–0.75 M). The experimental results are correlated by the Kent-Eisenberg and the explicit models, with an average absolute deviation of 10.72% and 5.03%, respectively. The regressed parameters of both models allow satisfactory estimation of the carbon dioxide loadings in arginine salt solutions at other process conditions. Comparison with piperazine and monoethanolamine shows that l-arginine salts have a better carbon dioxide absorption capacity at high pressure.

Introduction

Carbon capture processes are being positively considered for upgrading the natural gas, industrial syngas, hydrogen and bio-gas at a pre-combustion stage. The carbon dioxide content is separated, improving the calorific content of the fuel gas and reducing the incidence of corrosion in gas transmission/storage systems. Technical options include absorption, adsorption, membranes, hydrates and cryogenic separations, which mostly operate at high pressure [1]. Of which, absorption via chemical solvents shows promising potential, because of their efficient separation performance combined with operational simplicity and easy scale-up from conventional post-combustion carbon capture [2,3]. These solvent-based absorption systems remove carbon dioxide in the absorber at low temperature using a weak base, where, aqueous alkanolamines are conventionally used. Carbon dioxide is then separated in the stripping section of the absorption unit at a higher temperature than absorber and the recovered solvent is recycled.

Solvents such as tertiary amines (e.g. N-methyl-diethanolamine), sterically hindered amines (like 2-amino-2-methyl-1-propanol) and ionic liquids show a high capacity for carbon dioxide absorption [4]. This is highly desirable in separation systems, where the solvent can carry more carbon dioxide in each cycle. However, they exhibit a slow rate of carbon dioxide absorption [5]. This necessitates that a co-promoter should be added to the solvent, to primarily improve its kinetics of the absorption/desorption [6] and secondarily add to the absorption capacity of the first solvent [7]. This reduces the equipment size for the absorption and desorption columns, resulting in capital cost savings. Piperazine is one sort of a promising co-promoter for the carbon dioxide separation, both in low and high-pressure operation [8]. It has been widely studied and have improved the reactive kinetics for various carbon dioxide capture solvents that include N-methyl-diethanolamine [9], N-(2-aminoethyl)-ethanolamine [10], N, N-Diethyl-ethanolamine [11], 2-Amino-2-ethyl-1,3-propanediol [12] and l-lysine [13]. However, the co-promoter is toxic, corrosive in high concentrations and have high volatility, which limits its suitability in ever-evolving standards of safe operation [14]. This has instigated the recent research to develop many other options for high performance like tune-able phase change solvents [[15], [16], [17]] and biological enzyme-based co-promoters [[18], [19], [20]].

Recently, l-arginine has received immense importance as a potential co-promoter for carbon dioxide capture. Being biologically sourced, the amino acid is inherently safe for the environment (in case of leakage) and have very low volatility. Upon activation by a strong base (usually potassium or sodium hydroxide), small quantities of arginine salts increase the reaction rates tremendously [21,22]. Shen et al. [23,24] have studied the effect of l-arginine on the potassium carbonate solutions at absorber conditions. Mahmud et al. [25] and Iswanto et al. [26] studied the reaction kinetics of carbon dioxide in l-arginine mixed with MDEA. Chemat et al. [27] showed that l-arginine is an effective activator/co-promoter for ionic liquids in carbon dioxide separations. Previously, Holst et al. [28] studied the kinetic performance of arginine in pure potassium salt of arginine with good results. All studies show that l-arginine exhibits a near second-order zwitterion behaviour. This means that apart from improving the rate of carbon dioxide absorption, the arginine can itself absorb high amounts of carbon dioxide [29]. Its structure (shown in Fig. 1) strengthens this postulate further, as the presence of four nitrogen-based (amine) bonds points to a possibility of capturing four carbon dioxide molecules per arginine molecule. However, this postulate is not experimentally established.

l-arginine is a basic α-amino acid, having a molecular weight of 174.2 g/mol and chemical formula of C6H14N4O2. It contains an α-amino group, an α-carboxylic acid group, and a side chain consisting of a 3-carbon aliphatic straight-chain ending in a guanidino group. At physiological pH, the carboxylic acid is deprotonated (COO), the amino group is protonated (NH3+) and the guanidino group is also protonated to give the guanidinium form (C(NH2)2+), making l-arginine a charged, aliphatic amino acid [30]. The molecule has three pKa values of 1.97, 9.05 and 11.94 at 298.15K [31].

When used in low concentrations, l-arginine salts significantly increases the rate of carbon dioxide absorption and is a good co-promoter for post-combustion carbon capture, as discussed above. Despite these kinetic studies, the open information about the vapour-liquid equilibrium of carbon dioxide loaded aqueous pure arginine salts is quite limited and restricted to low pressure – low loading region.

Munoz et al. [32] studied the absorption capacities of various amino acids including potassium salt of l-arginine. Based on their experiments, l-arginine and other basic amino acids showed the highest carbon dioxide absorption capacity. This was attributed to their lateral chains having a strong basic character, which abstraction of the hydrogen from the CO2–amino acid complex (V–B) to give the carbamate moiety (VI–B) is easily accomplished by the lateral guanidinium or amine group.

Shen et al. [33] investigated the carbon dioxide solubility in potassium arginate solutions at low partial pressures of carbon dioxide (<15 kPa). The salt showed similar behaviour as of other conventional carbon dioxide solvents with gas loadings as high as 0.45 mol CO2/mole of potassium arginate. The tested concentrations varied between 0.26 and 1.23 M solutions. The gas absorption rates were found superior to monoethanolamine. NMR showed that the main product upon reaction with carbon dioxide was carbamate of arginine, which means that arginine is not fully dissociated in the water. This was attributed to its low solubility in water. This puts a serious limitation for the use of arginine salts as a main/sole solvent for carbon dioxide separation. However, they can still be used as a co-promoter due to its very fast kinetics of carbamate formation, by reacting directly with carbon dioxide.

The study of Sistla and Khanna [34] offers a very direct comparison of the performance of various amino acids groups in the absorption of carbon dioxide. Using them as anions of [bmim] cation, several ionic liquids were created and tested for carbon dioxide solubility up to 10 bars. Although the amino acids were not activated by a strong base, carbon dioxide loading values around 0.8 mol CO2/mole of Arginine based IL were reported. Interestingly, this arginine based IL ([bmim][ARG]) showed the highest carbon dioxide solubility among the tested amino acids.

Recently, Haghtalab and Gholami [29] have analysed the performance of l-arginine as a co-solvent with di-isopropyl amine (DIPA) and diethanolamine (DEA) at high pressures (1–38 bars) and a range of operational temperatures (328.15–363.15K). The l-arginine was used in varying concentrations of 5–15 wt%, while the total solvent concentration was fixed at 40%. It is interesting to note that the addition of 10% arginine did improve the equilibrium of carbon dioxide solubility in blends, establishing it as a good co-promoter at high pressures. However, the study did not discuss the individual absorption capacity of arginine salts at high pressure.

The effectiveness of l-arginine salts in promoting the rate of reaction and overall equilibrium solubility as a co-solvent/blend (with ionic liquids and/or aqueous alkanolamines) is well understood for both low and high-pressure regions. However, their contribution to improving the overall carbon dioxide absorption is not defined at high pressure. It is therefore imperative that the carbon dioxide solubility in pure arginine salts is investigated at high pressure, similar to the work of Shen et al. [33] for low pressure. This will help to identify the contribution of arginine salts in capturing carbon dioxide within a solvent blend. In addition, the VLE information in the high-pressure region will help to define transport properties and process design with confidence for carbon dioxide separation, relevant to fuel gas upgrading.

Therefore, this study presents experimental measurements of equilibrium carbon dioxide solubility in 0.25, 0.50 M and 0.75 M potassium and sodium salt of l-arginine solutions. The solvent concentrations are based on the arginine's performance as a co-promoter as established by Refs. [29,34]. The tested range of temperature (303.15–363.15K) and pressure (110–4110 kPa) allows a good overall interpretation of results and are in good coherence with the literature at low pressure. Moreover, experimental findings of this study are satisfactorily correlated by the Kent-Eisenberg and the explicit models [35]. Finally, the absorption capacity of arginine salts for carbon dioxide is compared to that of aqueous piperazine (PZ) and monoethanolamine (MEA) solutions.

Section snippets

Materials and methods

All chemicals were sourced from Merck Malaysia (Pvt.) Ltd (Kuala Lumpur, Malaysia) whereas, the gases (carbon dioxide and nitrogen) were provided by Gas Walker (Pvt.) Ltd (Kuala Lumpur, Malaysia). Detailed information on materials used in this study is provided in Table 1. All chemicals were used without further purification.

The solutions were prepared by first weighing the alkali (either sodium hydroxide or potassium hydroxide) followed by separately weighing the equimolar quantity of l

Kent-Eisenberg model for CO2 loaded aqueous arginine salt solutions

The comprehensive reaction mechanism for the absorption of carbon dioxide in aqueous amino acid salt solutions is shown in equations (4)–(9) [40].RRNH2+kaRRNH+H+RRNCOO+H2OkbRRNH+HCO3CO2+H2OkcHCO3+H+HCO3kdH++CO32H2OkeH++OHBOHkfB++OH

The equilibrium constants, kakf, for reactions 4–9 are given below.ka=[RRNH][H+][RRNH2+];kb=[RRNH][HCO3][RRNCOO];kc=[HCO3][H+][CO2];kd=[CO32][H+][HCO3];ke=[OH][H+];kf=[OH][B+][BOH]

The material balance of species for the reactions given

Experimental results

Table 5, Table 6 present the experimental results of equilibrium carbon dioxide loadings (moles CO2/mole of arginine salt) in aqueous solutions of sodium arginate and potassium arginate, respectively.

The VLE data for both sodium arginate and potassium arginate exhibit common behaviour as seen with other amino-acid salts, aqueous alkanolamines and other absorption solvents [43]. The increase of temperature results in a reduction of carbon dioxide loading values as the gas molecules become more

Conclusions

In this study, the experimental measurement of carbon dioxide solubility is reported in the sodium and potassium salts of l-arginine. The solubility has been studied for a wide range of temperature (303.15–363.15K), amino acid concentrations (0.25–0.75 M) and pressure (110–4110 kPa). The carbon dioxide loadings in both potassium and sodium salt solutions of l-arginine have a positive relationship with pressure. Contrarily, they have a negative relationship with temperature and solution

CRediT authorship contribution statement

Humbul Suleman: Investigation, Formal analysis, Conceptualization, Methodology, Software, Writing - original draft. Abdulhalim Shah Maulud: Supervision, Project administration, Resources, Funding acquisition. Afaf Syalsabila: Validation, Writing - original draft. Muhammad Zubair Shahid: Visualization, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the Universiti Teknologi PETRONAS for financial support under Yayasan UTP (YUTP) grant (Cost Centre: 0153AA-E69) and CO2 Research Centre (CO2RES), for providing the technical support to complete this study.

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