Article pubs.acs.org/EF
Screening Amino Acid Salts as Rate Promoters in Potassium Carbonate Solvent for Carbon Dioxide Absorption Guoping Hu, Kathryn H. Smith, Yue Wu, Sandra E. Kentish, and Geoff W. Stevens* Peter Cook Centre for Carbon Capture and Storage Research (PCC), Particulate Fluids Processing Centre (PFPC), Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia ABSTRACT: Potassium carbonate shows promise as a solvent for carbon capture due to its low cost and low environmental impact. However, improving the absorption kinetics of potassium carbonate solvent is crucial for reducing the capital cost of absorption equipment required to build the carbon dioxide capture plant. In this study, a series of amino acid salts were screened as reactants with carbon dioxide using the stopped flow technique. The amino acids investigated in this study were 2piperazinecarboxylic acid, asparagine, aspartic acid, glycine, leucine, lysine, proline, sarcosine, serine, and valine. Furthermore, proline, sarcosine, glycine, leucine, and lysine were tested as rate promoters in potassium carbonate solvent for carbon dioxide absorption using a wetted wall column. Results showed that the amino group in the anions of the amino acid salt is the major species reacting with carbon dioxide. Therefore, the promoting effect of amino acid salts is sensitive to changes in pH values due to changes in species distribution of the amino acid salts with pH. Sarcosine and proline are the most effective promoters among the amino acid salts tested in this study with comparable promoting performance at higher pH values (≥12.5) but with sarcosine more effective at lower pH values (12.0), while the promoting performance of MEA was comparable with that of proline and slightly poorer than that of sarcosine at low pH (15 MΩ cm−1) was used to prepare all solutions. CO2 and nitrogen (N2) mixtures were purchased from BOC Australia (Preston, VIC, Australia). Compressed air pretreated with a gas generator (Parker Filtration & Separation Division, Balston, US) to remove water and oil residues was used as the driving gas for the stopped flow equipment. 2.2. Stopped Flow Experiments. A SX.20 stopped flow spectrometer from Applied Photophysics Ltd. (United Kingdom) with an optical path length of 10 mm was used to screen the reaction kinetics between amino acids and carbon dioxide. The reactions were studied by mixing both reactants (CO2 solution and amino acid salt solution) in a reaction cell using two separate glass syringes. The CO2 solution was prepared by bubbling CO2 gas in water at 22 °C, and the CO2 concentration was calculated as described in the previous literature.27 The concentration of amino acid salts used in the stopped flow experiments was lower than 0.02 M, and the pH value of the solution was adjusted using 0.4 M sulfuric acid and potassium hydroxide. The temperature of the reaction cell was controlled with a water jacket using an external bath controller (W15, Grant Instrument, Cambridge, UK, ± 0.1 °C). The distribution of different amino acid salt ions was calculated based on the pKa values of the different amino acid salts. The pKa values (refer to Table 1) were obtained by titration
k H2O
amino acid
pKa1
2-piperazinecarboxylic acid asparagine aspartic acid glycine leucine proline lysine sarcosine serine valine
9.61 8.88 9.96 9.81 9.73 10.80 9.46 10.27 9.24 9.77
pKa1
10.74
(2)
OH− + CO2 ⇐ ⇒ HCO3−
(3)
HCO3− + OH− ↔ H 2O + CO32 −
(4)
′ kobs
⇐ ⇒ R 2HC(R1NCOO−)COO− + H 2O
The reaction rate for amino acid promoted carbonate solutions can be written as eq 6, in which k′obs is the corrected reaction rate constant, kH2O is the reaction constant between water and CO2, kOH− is the reaction rate constant between OH− and CO2, kAAS is the reaction rate constant between amino acid salts and CO2, n is the reaction order between amino acid salts and CO2, [AAS] and [CO2] are the concentration of amino acid salts and CO2, respectively. The reaction rate constant between H2O + CO2 and OH− + CO2 was obtained from the previous literature.20
′ + k H2O + k OH−[OH−])[CO2 ] r = (kobs = (k H2O + k OH−[OH−] + kAAS[OH−][AAS]n )[CO2 ]
(6)
2.3. Wetted Wall Column Experiments. The wetted wall column (Figure 1) used in this study is the same as described in our previous research.20 In the wetted wall column, the liquid film forming on the outer surface of the central column flows countercurrently with the gas flowing from the bottom of the reaction chamber. The pressure of the reaction chamber was kept at 1 atm during all experiments, and the absorption rate of the CO2 was calculated by monitoring the amount of CO2 entering and exiting the reaction chamber. The overall gas phase mass transfer coefficient (KG, mol Pa−1 s−1 m−2) of CO2 was calculated using eq 7, and the enhancement factor was calculated using eq 8,20 in which P (Pa) is the pressure, VCO2 (m3) is the volume of CO2 absorbed, A (m2) is the contact area for gas absorption, R (m3 Pa K−1) is the gas constant, T (K) is the temperature, kg (mol Pa−1 m−2 s−1) is the gas mass transfer coefficient, HCO2 (Pa−1 m3 mol−1) is the Henry constant, E is the enhancement factor, and kol (m s−1) is the physical mass transfer coefficient. The P*CO2 was obtained from the literature.30 The effect of the addition of amino acid on the equilibrium partial pressure of CO2 was assumed to be negligible due to the high CO2 partial pressure used in the present work. The HCO2 was calculated following the method given by Astarita.31 The kol was calculated following using eq 9,32 in which DCO2 (m2 s−1) is the
pKa2
12.5−13.0 9.31 9.26 10.36 8.95 9.73
H 2O ↔ H+ + OH−
R 2HC(R1NH)COO− + CO2 + OH−
323 K pKa2
(1)
k OH
Table 1. pKa Values of Amino Acid Salts at 298 K in Diluted Solutions 298 K
H 2O + CO2 ⇐⇒ HCO3− + H+
10.10
experimentally using a Metrohom 890 dosimeter using Tiamo 2.2 software for data analysis with a water bath for keeping temperature constant. The experimental pKa values of sarcosine (10.27) and proline (10.80) at 298 K are consistent with the previous literature values reported as 10.21 (sarcosine)28 and 10.64 (proline).19,29 The reaction rates were calculated by monitoring the pH via measuring the absorbance of the solution after mixing, as reported in our previous research.20,23 The reaction was monitored over a time B
DOI: 10.1021/acs.energyfuels.7b00157 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Wetted wall column used in this research (adapted from Thee34). diffusivity of CO2,33 ρ (kg m−3) is the density of the solvents, Γ (kg m−1 s−1) is the mass rate per unit width, δ (m) is the film thickness, L (m) is the column height, and the gas mass transfer resistance is small. KG =
PVCO2 ART (P − P*)
(7)
HCO2 1 1 = + KG kg Ek lo
(8)
⎛ 6DCO2 Γ ⎞1/2 k lo = ⎜ ⎟ ⎝ πρδL ⎠
(9)
3. RESULTS AND DISCUSSION 3.1. Speciation and Reaction Kinetics of Amino Acid Salts with CO2. The amino acids can transform from cations to zwitterions to anions as the pH increases from low to neutral to high values, respectively (Figures 2 and 3). Species
Figure 3. Distribution of valine ionic species at various pH values.
species reacting with CO2, which agrees with previous research for glycine22 and histidine.23 The reaction kinetics between the amino group in the anions of the amino acid salt and CO2 was investigated at pH conditions near the pKa of each amino acid (7.9−10.7), and the pseudo-first-order reaction constants are shown in Figure 5 as a function of the anion concentration. In this figure, the concentration of only the active reacting species (i.e., the anion concentration) is used to compare the kinetic performance of the different amino acids. The reaction rate constants follow the order of proline > glycine > sarcosine ≈ valine ≈ aspartic acid > leucine ≈ 2-piperazinecarboxylic acid > lysine# > histidine ≈ serine > lysine > asparagine. The fast reaction rate between proline and CO2 agrees with the work of van Holst,19 in which the CO2 absorption kinetics using potassium salts of amino acids was investigated. It can be concluded that proline shows the fastest reaction rate among the amino acids under the conditions studied here. 3.2. Promotion Performance of Amino Acid Salts in Potassium Carbonate Solvent. As the stopped flow experiments described above were performed at low temper-
Figure 2. Transformation of different species of amino acid salts with pH.
distribution was calculated based on the titrated pKa values shown in Table 1. It can be seen that the percentage of anions is negligible at neutral pH conditions (6.0−8.0), while the anions become the principal species present at high pH (>10.0). The effect of amino acid speciation on the reaction kinetics with CO2 was investigated over a range of pH values at similar amino acid salt concentrations (5 mM). As shown in Figure 4, the observed pseudo-first-order reaction rate constant (k′obs) between the amino acid salts and CO2 at neutral pH (7.3 ± 0.2) was small compared with that at high pH values, indicating that the anionic species of the amino acid salts is the major C
DOI: 10.1021/acs.energyfuels.7b00157 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 6. Enhancement factors using 30 wt % potassium carbonate solvents with and without amino acid salts (0.5 M) in a WWC at pH of 12.5 and temperature of 323 K.
Figure 4. Reaction rate between CO2 and amino acid salt solutions (∼5 mM) at neutral (7.3 ± 0.2) and basic pH values at 298 K (lysine: pH ∼ pKa1, lysine*: pH ∼ pKa2).
As shown in Figure 6, the enhancement factors under the same pH conditions were increased by adding amino acid salts into 30 wt % potassium carbonate solvents. Among the amino acid salts examined, glycine had the least effect with an increase in absorption rate of ∼3 times, leucine and lysine showed moderate promoting effect with ∼4 times increase in absorption rate, while proline and sarcosine showed the fastest promoting effect on the absorption rate (∼6 times increase). These results differ from the stopped flow experiments in which glycine was faster than sarcosine. This could be attributed to the reaction order between CO2 and the amino acids. The reaction order between glycine and carbon dioxide has been reported to be 1,22 but for the reaction between sarcosine and CO224 the reaction order is between 1.3 and 1.6. This means that as the concentration of amino acids increases, the reaction rate between glycinate and CO2 increases linearly regardless of the effect of ionic strength while that of sarcosinate increases with concentration to the power of 1.3−1.6. These results are consistent with the previous literature.24 Thus, it can be
ature (298 K) and concentration (5 mM), further investigation of absorption kinetics is needed to determine the performance in industrial potassium carbonate solvents at higher temperature. The preliminary screening results showed that proline had the fastest reaction rate with CO2. Additionally, sarcosine has been reported19,24 to have fast reaction kinetics under higher concentrations and temperatures, while glycine, leucine, and lysine are all economically favorable as promoters as they are produced as common nutrients. Therefore, the promoting performance of these five amino acids (proline, sarcosine, glycine, leucine, and lysine) was further tested in the wetted wall column (WWC) using 30 wt % potassium carbonate solvents with CO2 loading 12.0) conditions in 30 wt % potassium carbonate solvents at 323 K. This agrees with the previous literature that reports the first-order reaction rate constants at 323 K between MEA and CO2 (2.7 × 104 M−1 s−1)36 are much lower than that of sarcosine with CO2 (3.1 × 105 M−1 s−1)37 and proline with CO2 (3.2 × 105 M−1 s−1).38 However, at lower pH values, the promoting effects of proline, sarcosine, and MEA became similar to the sarcosine promoted potassium carbonate solvent having slightly faster CO2 absorption due to the changing speciation of the amino acids. In industrial processes, the pH of the solvent along the absorber length can vary from 10.5−11.5 (depending on conditions such as the concentration of K2CO3 and CO2 in solution), which is important for the transformation of amino acids as shown by the results of this study. This again highlights the importance of solution pH when determining the promoting performance of rate promoters for both laboratory research and industrial CO 2 capture applications.
Figure 7. Enhancement factors using 30 wt % potassium carbonate solvents with and without amino acid salts (0.5 M) in a WWC over a range of pH values at 323 K.
As shown in Figure 7, the addition of amino acid salts increased the enhancement factor dramatically at high pH (≥12) conditions, particularly in comparison with the unpromoted potassium carbonate solvents. The promoting effects of amino acid salts are clearly sensitive to pH values, which is attributed to the reaction mechanism between CO2 and amino acid salts (only the amino group in the anions of the amino acid salt reacts with CO2) as discussed in Section 3.1. Additionally, sarcosine showed higher promoting performance at lower pH (12.5). This could be due to the lower pKa value of sarcosine (refer to Section 3.1). These results are important for interpreting the promotion effects of amino acids for industrial CO2 capture conditions as promoted potassium carbonate solutions generally operate at pH values above 12. 3.4. Comparison of Amino Acids and Monoethanolamine (MEA) as Rate Promoters for CO2 Absorption in Potassium Carbonate Solvent. Monoethanolamine (MEA) is a widely used solvent for CO2 absorption, and it has also been used as a rate promoter in potassium carbonate solvents.35 Therefore, the promoting effects of MEA (Figure 8) in 30 wt % K2CO3 were also measured using the WWC and compared with the promoting performance of proline and sarcosine under similar experimental conditions.
4. CONCLUSIONS The amino group in the anions of the amino acid salt was shown via stopped flow experiments to be the major species reacting with carbon dioxide. The promoting effects of these salts in 30 wt % potassium carbonate solvents were found to be sensitive to pH due to the variation in pKa values and the corresponding anion reaction rate with carbon dioxide. Sarcosine and proline were found to be the most effective rate promoters among the amino acid salts tested in this study. Comparable performance was observed for these two promoters at high pH (>12.5), while at lower pH (12.0), while the promoting performance of MEA was comparable to that of proline and slightly poorer than that of sarcosine at low pH (
