Better and more intelligently formulated CO₂ absorbents

Project target:

Improving the understanding of the phenomena of the CO₂ absorption process, including the reaction mechanisms. Develop predictive equilibrium models for CO₂ absorption based on a mechanistic understanding in conjunction with a minimum of laboratory measurements.

Technical content:

The project is divided into two main parts: i) modeling of equilibria in gas using the extended UNIQUAC system combined with determination of selected equilibria and ii) determination of carbamate formation and hydrolysis constants in conjunction with predictive model development for determination of these constants.

Teknical advantages:

A successful project will allow faster identification of improved CO₂ capture solvents.

R&D challenges:

Prediction of carbamate formation and hydrolysis constants has been attempted using theoretical modelling, however, ambiguous results have been obtained so far . Some correlations have have been successfully made on the basis of pKa . Furthermore, few methods are available in the literature for accurate determination of Keq values for the carbamate reaction. Hence limited information is available in the literature for our endeavour.

Results achieved to date:

A new analytical method (wet chemical) for speciation of carbonated amine solution was developed. The method employs easily available laboratory techniques . The method was then applied to determination of carbonated alkylamine solution samples with increasing alkyl-carbon chain length: propyl-, butyl- and pentylamine.
Figure 1: KHYDa vs. Loading. Loading = ratio of initial [NaHCO3 ]init/ [amine]init.

The effects of molecular structure on amine reactivity towards carbamate formation and carbamate stability was examined . Linear relationships for primary alkylamine carbamate equilibrium constants and decomposition constants vs 15N NMR chemical shift have been established opening for estimation of these equilibrium constants for primary alkylamines .

A CO₂ absorption equilibrium model has been developed for primary & tertiary amine solvents based on limited CO₂ solubility- and limited NMR data measurements as model input values. The CO₂ absorption equilibrium can be described using species concentration values. The CO₂ absorption equilibrium model predicts full speciation of the amine solvent .

notes:
1 a) EF da Silva, HF Svendsen, Ind. Eng. Chem. Res., 45, 2497 (2006); b) EF da Silva, HF Svendsen, Int. J. Greenhouse Gas Contr., 1, 151 (2007); c) EF da Silva, Energy Procedia, 4, 164 (2011); d) S Gangarapu, ATM Marcelis, H Zuilhof, ChemPhysChem, 13, 3973 (2012).
2 a) RJ Littel, GF Versteeg, WPM van Swaaij, Chem. Eng. Sci., 47, 2027 (1992); b) RJ Littel, GF Versteeg, WPM van Swaaij, ibid., 2037; c) W Conway, X Wang, D Fernandes, R Burns, G Lawrance, G Puxty, M Maeder, Environ. Sci. Technol., 47, 1163-1169 (2013).
3 G.L. Samarakoon S.P.A. ‘Post combustion CO₂ capture using aqueous amine solvents: Investigations on molecular structure-activity relationships for amine solvents’. PhD Thesis Telemark University College 2015.
4 C. Perinu, B. Arstad, A. M. Bouzga, K.-J. Jens,13C and 15N NMR Characterization of Amine Nucleophilic Reactivity and Solvent Effects in CO₂ Capture, Journal of Physical Chemistry B (2014), 118(34), 10167-10174.
5 C. Perinu, G. Saramakoon, B. Arstad, K.-J. Jens, Application of 15N-NMR spectroscopy to analysis of amine based CO₂ capture solvents, Energy Procedia, (2014), 63, 1144-1150.
6 K.J. Jens, D.A. Eimer, B.Arstad, Z.Idris, C.Perinu, G.Samarakoon, J.A. Svendsen, Attempts to Predict Absorption Equilibria, 3rd Post Combustion Capture Conference (PCCC3), Regina, Canada, 2015.