The reduction in CO₂ 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 CO₂, being responsible for approximately 35% of total global CO₂ emissions¹ 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 CO₂ in aqueous amine solutions are among the most mature and accepted technologies for CO₂ capture from power plants². The objective of this work is to provide a model-based platform for assessing alternative designs and solvents for post-combustion CO₂ capture. In this talk, we report on the modelling of an absorption/desorption process using an aqueous solution of ammonia. The mechanism for the absorption of CO₂ is complex and special emphasis is placed on capturing the thermodynamic and kinetic behaviour of the solvent/CO₂/N₂ system. The models thus derived are incorporated within a process model, which is used to investigate the performance of the process. To account for the hydrogen-bonding interactions that are typical of ammonia and water, the statistical associating fluid theory (SAFT)³ is used. This is a molecular approach, specifically suited to associating fluids. In this contribution we use the SAFT approach for potentials of variable range (SAFT-VR⁴). The SAFT formalism is used to represent some of the equilibrium reactions present in the system, thereby simplifying the kinetic model. The molecules are represented by 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 determine values of the pure component and parameters and of the binary interaction parameters for all the pairs of the compounds in the system. These are obtained by fitting to experimental data and are used in a predictive manner to obtain ternary and quarternary phase behaviour. We also estimate reaction rate constants for the kinetic model. We also present a model of an absorption process, implemented in the gPROMS⁵ software package. The overall process model is used to assess the performance of the system under different operating scenarios. The sensitivity of the process to several design variables is studied and provides some insight into the key decisions in developing such processes.

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

4 Gil-Villegas, A., Galindo, A., Whitehead, P. J., Mills, S. J. & Jackson, G., J. Chem. Phys. 106 (10), 8 March 1997

5 Process Systems Enterprise (PSE) Ltd.