Nucleation kinetics of glycine promoted concentrated potassium carbonate solvents for carbon dioxide absorption

Glycine promoted concentrated potassium carbonate (K₂CO₃) solvents are considered to have potential for use in carbon dioxide (CO₂) capture processes. Understanding their precipitation behaviour, particularly nucleation kinetics, becomes important if the solvents are used as absorbents at concentrations of 40 wt% or greater. In this work, liquid-solid equilibrium of the quaternary solvent system glycine-potassium carbonate-potassium bicarbonate-water was investigated in an Optimax workstation 1001 at different temperatures. The results were interpreted using a regressed Electrolyte Non Random Two Liquids (ENRTL) thermodynamic model in Aspen Plus^TM within an average deviation of 9.2%. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), high performance liquid chromatography (HPLC) and total nitrogen elemental analysis, the precipitates confirmed that only kalicinite (KHCO₃), and no glycine related products such as glycine solids and carbamates precipitated under the temperatures investigated. The metastable zone width (MSZW) of the solvent systems with different concentrations of glycine at different constant cooling rates was measured using focused beam reflectance measurement (FBRM), and four methods including Nývlt relation, Self-Nývlt relation, the CNT theory and the pseudo induction time method were used to evaluate nucleation kinetics. It was found that the introduction of glycine significantly increases the MSZW, resulting in fast nucleation kinetics. However, at a high concentration of glycine (1.36 mol/kg water) promoted 40 wt% K₂CO₃ with 0.4 CO₂ loading, the maximum MSZW was achieved at an intermediate cooling rate (35 °C/h) rather than the highest cooling rate (60 °C/h), this may result from dual competing effects between strong ionic strength within the solvent system and superstation generated by cooling rates. Correspondingly, these traditional nucleation analysis methods cannot explain the nucleation kinetic behaviour for this specific case, and other theories are required to explain the obscured behaviour of the quaternary solvent system.