Comparative microfluidic screening of amino acid salt solutions for post-combustion CO2 capture
Alexander P. Hallenbeck, Adefemi Egbebi, Kevin P. Resnik, David Hopkinson, Shelley L. Anna and John R. Kitchin, “Comparative microfluidic screening of amino acid salt solutions for post-combustion CO2 capture”, International Journal of Greenhouse Gas Control 43, 189-197 (2015). https://doi.org/10.1016/j.ijggc.2015.10.026
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Note on the source text: the extracted text is readable but two-column interleaved, and the figures (absorption curves and Raman spectra) are not available, so results are taken from the prose and tables.
Question
Aqueous 30 wt% monoethanolamine (MEA) is the benchmark solvent for post-combustion CO2 capture, but it is volatile, toxic, corrosive, degrades oxidatively and is costly to regenerate. Aqueous salts of amino acids are attractive alternatives: once the amine group is deprotonated with a metal hydroxide it can react with CO2 like an alkanolamine, while the ionic solution has very low volatility. Amino acids with basic side chains carry extra nitrogen that could raise the CO2 loading, yet lysine had appeared in only one earlier study. The paper asks how the potassium salt of lysine compares with MEA and with the salts of glycine, taurine and proline, and whether a microfluidic segmented-flow experiment can serve as a fast, low-volume screening tool whose trends carry over to a larger reactor (Hallenbeck et al. 2015).
Approach
Potassium salts were made by neutralizing each amino acid with an equimolar amount of KOH, and concentrations were checked by titration (0.50 M for all solutions except 0.53 M potassium lysinate). In a commercial all-glass microreactor with a Y-junction and serpentine channel, pure CO2 at a constant 1.7 atm was segmented into gas plugs by the flowing liquid at ambient temperature. A 500 Hz high-speed camera tracked the length of an individual plug along the channel, and the plug volume was computed as a cylinder with hemispherical end caps (a 121 micrometer plug diameter, allowing for the wall film in a 125 micrometer channel). The initial plug length was found by extrapolating an exponential fit back to the junction, and the ideal gas law converted volume loss into absorbed CO2, with a stated precision of 0.02 M. Physical solubility was subtracted by approximating it with that of water. As a macroscale check, equilibrium capacities of lysinate and MEA were measured by breakthrough in a 300 mL stirred reactor at 40 C, and solution viscosities and Raman spectra (532 nm excitation, before and after the microfluidic run) were recorded (Hallenbeck et al. 2015).
Key findings
In the microfluidic experiments the maximum reacted loading was 0.70 mol CO2 per mol absorbent for lysinate, against 0.46 for MEA, 0.43 for both glycinate and taurinate, and 0.34 for prolinate. Lysinate thus exceeded MEA by more than 50%, but doubling the number of primary amines relative to glycine gave less than a doubling of uptake, which the authors attribute to protonation of the side-chain amine (pKa 10.53) as the solution pH (initially 11.58) falls during absorption. The low prolinate loading suggests its secondary amine does not shift the products toward bicarbonate, and steric hindrance may contribute. The time to reach 90% of the maximum loading ranged from 0.30 s (MEA) to 0.47 s (lysinate), yet lysinate had the highest initial absorption flux over the first 0.02 s, 0.52 mol m-2 s-1, versus 0.48 for MEA and 0.25 to 0.35 for the other salts. These fluxes are far larger than wetted-wall values, so the authors caution that absolute rates should only be compared within one setup.
In the stirred reactor at equal 0.53 M concentration, lysinate absorbed 1.32 mol CO2/mol versus 0.70 for MEA, more than 90% higher, reproducing the microfluidic ranking. Both values exceeded the microfluidic ones, which the authors read as a sign that the microfluidic plug shrinkage is partly mass-transfer limited. On a constant water fraction basis the ranking reversed: 1.48 M lysinate (70 wt% water) took up 1.68 mmol/g solution against 2.63 mmol/g for 30 wt% MEA, because lysine is much heavier. The viscosities of these two solutions were nearly identical (about 3.1 mPa s near 293 K and 1.7 mPa s near 313 K). Raman spectra showed loss of the N-H stretch near 3310 to 3320 cm-1 for the primary amines and new bands assigned to carbamate, bicarbonate and carbonate; the carbamate band of lysinate appeared at 1130 cm-1 rather than 1162 cm-1 (MEA) or 1175 cm-1 (glycinate), possibly indicating a weaker carbamate that favors bicarbonate and carbonate. The prolinate spectrum changed too much to interpret reliably (Hallenbeck et al. 2015).
Significance
The work shows that tracking dissolving gas plugs in segmented microfluidic flow can rank CO2 solvents by capacity using less than a millilitre of solution and much less time than stirred-reactor or wetted-wall testing, which matters for novel solvents that are expensive to make at litre scale. It identifies potassium lysinate as a promising, overlooked amino acid salt on a molar basis, while stressing that molecular weight must be considered when choosing the basis of comparison, and that regeneration, thermal properties and precipitation at high concentration still need study (Hallenbeck et al. 2015).