Aqueous Amino Acid Salt Solutions Research Articles

Direct air capture of CO2 using green amino acid salts

Direct air capture (DAC) of CO2 using liquid sorbent technology is gaining attention as a promising approach in tackling the looming climate crisis. Despite technical advancements, critical aspects such as contactor selection, energy efficiency, sustainability, and environmental compatibility still pose uncertainties. In this study, various green amino acid salts performances in a DAC system were explored using non-porous hollow fiber membrane contactors (HFMCs). Two DAC absorption and desorption apparatuses were developed. For the DAC-absorption unit, the thermodynamic and kinetic behavior of five types of aqueous amino acid salt solutions were evaluated in long-term operations. High absorption stability for most of the solutions in different solvent loadings (up to 80% CO2 solvent loaded) were observed and potassium glycinate (GlyK) was selected as the most suitable candidate for DAC. To enhance the CO2 separation efficiency, parametric analysis on air and solvent flow rates, solvent temperature and concentration were conducted using GlyK. Vacuum low-temperature desorption experiments were carried out with GlyK to evaluate the CO2 removal efficiency over a range of solvent temperatures and concentrations, CO2 loadings, vacuum pressures, and vacuum/sweep gas mode. The results successfully quantified the effect of each operational parameter under various conditions on CO2 removal in a DAC system. Finally, to investigate the impact of membrane characteristics on DAC absorption–desorption performance, a developed and validated model was used. Taken all together, hybrid technology of membrane modules and green amino acid salts is shown to be a viable pathway towards a sustainable DAC process.

Thermodynamic modelling of carbon dioxide solubility in aqueous amino acid salt solutions and their blends with alkanolamines

In this study, two thermodynamic approaches, namely explicit model and modified Kent-Eisenberg model are presented for the accurate determination of carbon dioxide solubility in aqueous solutions of amino acid salts with different levels of complexity. Both models were further extended to the correlation of carbon dioxide solubility in aqueous blends of amino acid salts with alkanolamines. Firstly, the explicit model consists of a single mathematical equation. Its structure is computationally simple and derived from equilibrium thermodynamics theory. Secondly, the modified Kent-Eisenberg model utilizes detailed reaction mechanism for development of a polynomial equation, that is solved by combining all non-idealities in two correction factors. Both models suitably determined the thermodynamics of carbon dioxide loaded aqueous amino acid salt solutions and their blends with alkanolamines. The results were in good agreement with experimental data for representative amino acid salt solutions (potassium and/or sodium salts of lysine, glycine, proline, sarcosine, serine, threonine, alanine, phenylalanine, amino-butyric acid, glutamine and asparagine) and their blends with alkanolamines (2-amino-2-methyl-1-propanol and piperazine) for range of process parameters. For carbon dioxide solubility in aqueous amino acid salt solutions, the AARE% for explicit and modified Kent-Eisenberg model was 13.85% and 13.84%, respectively. For carbon dioxide solubility in aqueous blends of amino acid salt and alkanolamine solutions, the AARE% for explicit and modified Kent-Eisenberg model was 18.25% and 17.25%, respectively. Both models use a small number of adjustable parameters. This indicates that generated parameters are able to accurately predict the carbon dioxide solubility at other process conditions, with minimum computational intricacy.

Physicochemical Properties of Aqueous Potassium Salts of Basic Amino Acids as Absorbents for CO2 Capture

Aqueous amino acid salt (AAS) solutions could be attractive absorbents for CO2 removal from flue gases. Density, viscosity, and solubility of N2O in aqueous potassium salts of lysine, histidine, and arginine were measured over the temperature range from (293 to 353) K and molality concentration range from (0.26 to 3.6) mol·kg–1. Experimental results were correlated well with empirical correlations. The ion specific parameters (hAA–), based on the Weisenberger and Schumpe model, were fitted using the solubility data of N2O in the aforementioned AAS systems, valid up to 328 K. The models using temperature dependence of hAA– predict well within 2.5% AAD. The physical solubility of CO2 in the aqueous AAS solutions was also estimated.

Equilibrium solubility of carbon dioxide in aqueous potassium l-asparaginate and potassium l-glutaminate solutions

Aqueous amino acid salt solutions are promising alternatives to aqueous alkanolamine absorbents for CO2 capture. In this study, the absorption of CO2 in two amino acid salt systems: aqueous potassium l-asparaginate (KAsn) and aqueous potassium l-glutaminate (KGln) were investigated. Using a vapor–liquid equilibrium apparatus, the solubility of CO2 in aqueous KAsn and KGln solutions were determined at temperatures 313.2, 333.2, and 353.2K and pressures up to 950kPa. The amino acid salt concentration in the solutions was varied from 8.5 to 34.0wt% for KAsn and 9.2 to 36.8wt% for KGln. The studied amino acid salt systems were found to have appreciable CO2 absorption capacities, which were comparable with those of commonly used alkanolamines. A modified Kent–Eisenberg model was applied to correlate CO2 solubility (expressed as mole CO2 per mole amine) in the solution with CO2 partial pressure, temperature, and amino acid salt concentration. The model satisfactorily predicted the equilibrium data. The obtained average absolute deviations between the experimental and the calculated values of equilibrium CO2 partial pressures were 12.8% and 10.3%, for aqueous KAsn and KGln systems, respectively.

Thermophysical property characterization of aqueous amino acid salt solutions containing α-aminobutyric acid

In this study, density, electrical conductivity, refractive index and viscosity of aqueous potassium and sodium salt solutions of α-aminobutyric acid were presented. Measurements were done over the temperature range (303.15 to 343.15)K and atmospheric pressure for salt compositions from x1=0.009 to 0.062. A modified Graber et al. equation was used to correlate the density, electrical conductivity, and refractive index with temperature and composition leading to average absolute deviations (AAD) between the predicted and calculated values of 0.04%, 0.7%, and 0.01%, respectively. The viscosity data were represented as a function of temperature and composition via Vogel–Tamman–Fulcher (VTF) type equation at an AAD of 0.6%.

Reactive Absorption of Carbon Dioxide in l-Prolinate Salt Solutions

Aqueous amino acid salt solutions are seen as more sustainable alternative CO2 solvents compared to conventional alkanolamines. The absorption of CO2 into aqueous solutions of potassium l-prolinate, over the temperature range of 290–303 K, was studied using a stirred cell. To compare the effect of potassium versus sodium as counterion, the absorption rates of CO2 in sodium l-prolinate solutions were also determined. The amino acid salt concentration was varied between 0.5 and 3 mol dm–3. Physicochemical properties such as density, viscosity, and physical solubility of N2O, required in the interpretation of absorption rate experiments, were determined separately. The obtained experimental reactive absorption fluxes were interpreted, using the pseudo-first-order approach, into intrinsic reaction kinetics. The potassium-based solvent showed, on average, a 32% higher reactivity toward CO2 than the sodium equivalent. The kinetic data were correlated in a power-law reaction rate expression. The reaction order with respect to the amino acid was found to be between 1.40 and 1.44 for both l-prolinate salts, and the second order kinetic rate constant, k2, was calculated for the potassium salt to be 93.7 × 103 dm3·mol–1·s–1 at 298 K with an activation energy of 43.3 kJ·mol–1. l-Prolinate salts show higher chemical reactivity toward CO2 than most of the amino-alcohols and amino acid salts.

Carbon dioxide absorption characteristics of aqueous amino acid salt solutions

In the present study, the aqueous potassium salts of 16 common amino acids and some blends with piperazine (PZ) were experimentally screened. Critical solution concentration and surface tension were measured to assess the aqueous solutions as CO2 absorbents for membrane-gas absorption (MGA). Cyclic CO2 absorption and desorption were conducted on the prepared absorbent solution, at 40°C with a continuous feed of 15kPa CO2 (N2 balance) and at 80°C with a N2 feed, respectively. The cyclic CO2 absorption performances were evaluated by computing and comparing net cyclic capacity as well as initial rates of absorption and desorption. From the experimental results, it was found that (i) longer distance between amino group-CO2-binding site-and other functional group and (ii) bulkier functional group of amino acid, would result in higher net cyclic capacity as well as faster CO2 desorption. Of the 16 amino acids, the alanine (ALA), serine (SER), and α-aminobutyric acid (AABA) salts had relatively fast initial rates of absorption and desorption, resulting in high net cyclic capacity. A small amount of PZ added as a rate promoter to the ALA, SER, and AABA salts increased net cyclic capacity by more than 25%. This result indicates that such salts could be energy-efficient alternates for monoethanolamine (MEA). Due to their high surface tension values, thus lower membrane pore wettability, ALA and SER salt solutions with PZ addition could be utilized as CO2 absorbing liquids in membrane contactors.

Precipitation regime for selected amino acid salts for CO2 capture from flue gases

The tendency of alkaline (sodium, potassium and lithium) salts of taurine, β-alanine, sarcosine and L-proline to form precipitates under varying operational conditions of CO2 absorption has been investigated. CO2 absorption experiments have been performed at 293.15 and 313.15 K, at partial pressures relevant for capturing CO2 from flue gas conditions and amino acid salt concentrations up to the saturation concentration. In addition to this ‘window of operation’ for the (non-) precipitation regime, the identity of precipitates formed was revealed and relevant properties like density of the saturated aqueous amino acid salt solutions at 293.15 K have been determined.

Physiochemical Properties of Several Aqueous Potassium Amino Acid Salts

Aqueous amino acid salt solutions might be an attractive alternative over traditional alkanolamines in the removal of CO2 from flue gases especially because of their oxidative stability and negligible volatility. The density and viscosity of the aqueous solutions of the potassium salts of the following amino acids have been measured: β-alanine, 6-aminohexanoic acid, l-arginine, l-aspartic acid, l-glutamic Acid, dl-methionine, l-phenylalanine, l-proline, and sarcosine. Data are reported in the temperature range of (25 to 60) °C and concentration range of (0.25 to 3.5) mol·L−1. The data could be correlated accurately with relatively simple relations. The physical solubility of N2O in the same amino acid salt solutions was also measured at a temperature of 298 K and a concentration of 0.5 mol·L−1 of the amino acid salt. Using the N2O/CO2 analogy on one hand and Schumpe’s method on the other, the physical solubility of CO2 in these solutions was estimated. Both methods yielded results very close to each other.

Diffusivities in Aqueous Solutions of the Potassium Salt of Amino Acids

Diffusion coefficients of the potassium salt of taurine, sarcosine, l-proline, and glycine have been determined in aqueous solutions using the Taylor dispersion technique over a temperature range from (293 to 368) K and salt concentrations ranging from (0 to 3.0) mol·dm−3. The diffusion coefficients of absorbed N2O in aqueous solutions of the potassium salt of taurine, sarcosine, l-proline, and glycine have been determined over a temperature range from (293 to 333) K and salt concentrations ranging from (1.0 to 3.0) mol·dm−3. The experimental results have been compared to estimated values from a modified Stokes–Einstein relation and available literature data. The diffusivities can be estimated with sufficient accuracy using a modified Stokes–Einstein relation. The diffusion coefficients of CO2 in the aqueous amino acid salt solutions can be estimated by means of the N2O analogy.

New absorption liquids for the removal of CO 2 from dilute gas streams using membrane contactors

A new absorption liquid based on amino acid salts has been studied for CO 2 removal in membrane gas–liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyolefin microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace–Young equation. The performance of the new absorption liquid in the removal of CO 2 was studied in a single fiber membrane contactor over a wide range of partial pressures of CO 2 in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO 2 in the liquid flowing through the hollow fiber was developed. The numerical model gives a good prediction of the CO 2 absorption flux across the membrane for the absorption of CO 2 in the aqueous amino acid salt solutions flowing through the hollow fiber.

Density, Viscosity, Solubility, and Diffusivity of N2O in Aqueous Amino Acid Salt Solutions

Solubility and diffusivity of N2O in aqueous solutions of potassium taurate are reported over a wide range of concentration and temperature. Also, the solubility of N2O in aqueous potassium glycinate solution is reported at 295 K. The ion specific constants, based on the model of Schumpe,5,6 are reported for taurate and glycinate anions. A modified Stokes−Einstein relationship is proposed for the diffusivity of N2O in aqueous potassium taurate solution in the temperature range 293−308 K.