Advanced thermodynamic and processing modelling integration for amine scrubbing in post-combustion CO~2~ capture

The reduction in CO~2~ emissions from anthropogenic sources has become a topic of widespread interest over the past number of years. As the power generation sector is by far the largest stationary-point-source of CO~2~, being responsible for approximately 35% of total global CO~2~ emissions^1^ this question has special relevance for this industry. As the inclusion of carbon capture facilities incurs a significant energy penalty on the efficiency coal-fired power-stations, there is a strong requirement for the improvement of these systems in terms of the minimisation of operation and maintenance costs, capital costs and the maximisation of efficiency and flexibility. This last issue has relevance for start-up times and ramp-rates. Post-combustion capture methods based on the chemisorption of CO~2~ in aqueous amine solutions are among the most mature and accepted technologies for CO2 capture from power plants^2^. However, amines are complex, associating solvents requiring a sophisticated thermodynamic model, capable of modelling the hydrogen bonding interactions that occur in these systems. One such model is provided by the statistical associating fluid theory (SAFT^3^). This is a molecular approach, specifically suited to hydrogen-bonding, chain-like fluids. In this contribution we use the SAFT approach for potentials of variable range (SAFT-VR^4^) to model the thermodynamics and phase equilibria of a number of amines including ammonia and monoethanolamine. The molecules are modelled as homonuclear chains of tangentially bonded square-well segments of variable range, and a number of short-ranged off-centre attractive square-well sites are used to mediate the anisotropic effects due to association in the fluids. We also determine values of the binary parameters for mixtures and then use these parameters to predict the phase equilibria of amine+water, amine+carbon dioxide as well as water+carbon dioxide mixtures. We then consider the phase equilibria of the ternary mixtures of amine+water+carbon dioxide and finally that of quaternary mixtures of amine+water+carbon dioxide+nitrogen. A good quantitative understanding of the phase behaviour of these quaternary mixtures is essential for accurate modelling of absorption processes for carbon dioxide capture. 

1. Steeneveldt, R., Berger, B. & Torp, T.A., ChERD, 84(A9): 739-763, 2006
2. Rao, A.B.; Rubin, E.S., 2002. A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control. Environ. Sci. Technol. 36, 4467-4475
3. Chapman, W.G., Gubbins, K.E., Jackson, G. & Radosz, M., Ind. Eng. Chem. Res., 1990. 29, 1709-1721
3. Gil-Villegas, A., Galindo, A., Whitehead, P. J., Mills, S. J. & Jackson, G., J. Chem. Phys. 106 (10), 8 March 199