Precipitating amino acid solutions

Aqueous amino acid salt solutions that precipitate due to CO₂ absorption are a relatively novel and promising class of solvents. Although the presence of solids in the process stream requires adequate measures related to slurry handling and crystallization control, precipitation also increases the solvent CO₂ capacity and the energy performance of the stripper. By using an additional solidliquid separation step to concentrate the slurry before it is sent to the stripper, known as the DECAB+ process, the energy performance can be further improved. Most of the current experimental work on these systems focuses on the amino acids taurine, α- and β-alanine, sarcosine, glycine, and proline, neutralized with equimolar amounts of potassium hydroxide. Investigated properties include the CO₂ concentration at which precipitation is first observed, which is closely related to the solubility of the amino acid, the composition of the precipitate, and the CO₂ absorption rate. The precipitate typically contains pure amino acids and/or bicarbonate, depending on the amino acid solubility. Shortcut process simulations have been used to estimate the energy performance of absorption processes using precipitating amino acids, so far focusing on 4M KOH + taurine (K-tau) and 4M KOH +α-alanine (K-α-ala) solutions. Specific regeneration duties below 2GJ/ton CO₂ seem possible for the DECAB+ process when using K-α-ala and lean vapor compressor (LVC). Compared to a conventional process using 30wt % monoethanolamine and LVC, this corresponds to a reduction of 35% in the regeneration duty. Future research should focus on more-detailed process simulations, based on a predictive thermodynamic model that consistently describes the vaporliquidsolid equilibria of the system. Demonstration of the DECAB+ process on a pilot scale is also highly recommended, with a special focus on the crystallization and slurry handling process steps and units.