(77d) In-Situ Synthesis of Useful Polyamines for CO2 Capture From Piperazine and MEA Derivatives

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, concentrated PZ (at operating temperatures associated with CO₂ capture) has a limited solubility window and as a result a narrow operating window.  This problem limits 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⁽¹⁾.  However, these amines are expensive and can add to the solvent cost.

Monoethanolamine (MEA) can react with PZ at stripper conditions to selectively form AEP as well as diaminoethylpiperazine (DAEP), the tetramine analogue of AEP⁽²⁾.  The synthesis of AEP was studied at 150 ^oC with an initial loading of 0.3 mol CO₂ / mol alkalinity with two blends: 7 m PZ/2 m MEA and 6 m PZ/4 m MEA.  A combined AEP/DAEP yield of 68% was obtained with 89% of the total alkalinity present as PZ, AEP, or DAEP over 840 hours for the 7 m PZ/2 m MEA system.   A combined AEP/DAEP yield of 52% was obtained with 77% of the total alkalinity present as PZ, AEP, or DAEP for the 6 m PZ/4 m MEA system.  Ureas appear to be the greatest contributor to alkalinity loss. MEA degradation products and formates do not appear to significantly contribute to alkalinity loss.

Analogues of MEA can also react with PZ to form triamines and tetramines.  These include the hindered amine aminomethylpropanol (AMP) as well as monoisopropylamine (MIPA), methylaminoethanol (MAE), and monopropanolamine (MPA).  Rates of formation of the triamine and tetramine products between these amines and PZ at 135 and 150 ^oC will be presented.

References:

(1)    Rochelle GT et al. "CO₂ Capture by Aqueous Absorption, Fourth Quarterly Progress Report 2011." Luminant Carbon Management Program.  The University of Texas at Austin. 2012.

(2)    Davis JD. Thermal Degradation of Aqueous Amines Used for Carbon Dioxide Capture. The University of Texas at Austin. Ph.D. Dissertation. 2009.