In-situ synthesis of useful polyamines for CO2 capture from piperazine

Concentrated piperazine (PZ) is considered to be a promising solvent for use in post-combustion CO₂ capture from coal-derived flue gas due to its fast rate of CO₂ absorption, high CO₂ capacity, and low degradation rates. However, this solvent has a limited solubility window at conditions corresponding to the operating temperature of a CO2 capture plant, limiting its use in commercial CO₂ capture systems. Blending PZ with a triamine analogue of PZ, such as aminoethylpiperazine (AEP), can widen the solubility window while maintaining many of the properties that make concentrated PZ attractive. With the exception of AEP, many of the triamine PZ analogues are expensive and are not commercially available. They can, however, be made in-situ in the regeneration section of the CO₂ capture plant by the selective reaction of PZ with an amine that forms an oxazolidinone (either a primary or secondary alkanolamine) as its initial thermal degradation byproduct. The data suggests that the structure of the oxazolidinone is a strong function of reactivity with PZ. Increasing steric hindrance - such as increasing the chain length between the amino group or hydroxyl group or by adding alkyl functional groups to the carbon chain between the hydroxyl and amino groups - can reduce reaction rates by a factor of three to five. Secondary amines that form oxazolidinones react four times as fast as primary oxazolidinone-forming amines. Piperazine and solvent alkalinity losses are reduced significantly (∼0.2%/week) once the oxazolidinone-forming amine is completely consumed, indicating that the synthesized amine blend is thermally stable.