Carbon dioxide capture with aqueous amino acids: Mechanistic study of amino acid regeneration by guanidine crystallization and process intensification

CO₂ capture from powerplant-generated flue gas via a phase-changing process involving absorption with aqueous amino acids (e.g., glycine or sarcosine) and bicarbonate crystallization with bis-iminoguanidines (e.g., glyoxal-bis-iminoguanidine or GBIG) is investigated in this paper. This process is of high interest due to its potential to decrease the energy penalty for CO₂ capture by significantly reducing the solvent regeneration energy typically associated with aqueous amine solvents. A critical step in the proposed CO₂ capture mechanism is the regeneration of the amino acid by removal of protons and bicarbonate ions from solution through crystallization of GBIGH₂²⁺ bicarbonate salt. Here, we investigated the thermodynamics and kinetics of glycine regeneration by crystallization of GBIGH₂²⁺(HCO₃^–)₂(H₂O)₂. A theoretical model was developed and compared to experimental data to simulate and predict the glycine regeneration and determine its reaction mechanism. This combined experimental and theoretical study led to the conclusion that, while the GBIGH₂²⁺ bicarbonate crystallization step provides most of the thermodynamic driving force for the glycine regeneration, the rate-limiting step is the protonation of GBIG prior to crystallization. The CO₂ loading and amino acid regeneration steps were combined into a single, intensified process using a bubble column reactor. The CO₂ loading capacity of GBIG was experimentally determined to be roughly 1.36 mol CO₂ per mol GBIG. These results provide the fundamental basis for developing an effective carbon capture technology with phase-changing amino acid/guanidine absorbents.