(79d) Towards the Integrated Design of Processes and Solvent Blends for CO2 Capture

Fossil fuel combustion for power generation accounts for approximately 55% of total global CO₂ emissions. The flue gas streams of these processes have high flow rates, and are relatively dilute in terms of CO₂; the CO₂ concentration rarely exceeds 15%vol¹. This presents a significant capture challenge and there is an ongoing interest in improvements in solvent performance². The objective of this work is to provide a unified systems approach and model-based platform for assessing alternative solvents and solvent blends for post-combustion CO₂ capture from large fixed point emission sources. In developing this methodology, we combine state-of-the-art thermodynamics with rigorous process simulation tools and techniques. To account for the non-idealities that are typical of amines and water, the statistical associating fluid theory for potentials of variable range (SAFT-VR)^3,4, is used. This is a molecular approach, specifically suited to associating fluids. The SAFT formalism is used to represent some of the equilibrium reactions characterising the system, thereby simplifying the description of the chemical reactions⁵. A validated⁶ rate-based model of an absorber/stripper system for the chemisorption of acid gas in aqueous solvent solutions is implemented in the gPROMS⁷ software package. An important feature of the model is that, owing to the mass transfer limited nature of this process, the reaction kinetics are incorporated in the thermodynamic model and as a consequence empirical enhancement factor concepts are avoided. This aspect of our model gives us greater freedom from requiring experimental data; only binary mixture phase equilibrium data is required. Our model does not consider reaction products explicitly; as these are accounted for at the level of the thermodynamic model proposed for the fluid as reacted aggregates. We implicitly assume both phase and reaction equilibrium at the phase interface, and this allows us to capture the behaviour of the process with good accuracy. We consider a model flue gas comprising N₂ , CO₂ and H₂O. The industrial benchmark solvent for CO₂ capture is generally considered to be an aqueous mixture of monoethanolamine (MEA)⁸ owing to its high reaction rates and selectivity. However, MEA-based processes suffer from a number of significant disadvantages associated with the regeneration of the MEA solvent. Recently, blends of 2-amino-2-methyl-1-propanol (AMP) and ammonia (NH3) have been shown to be particularly promising novel solvent blends for CO₂ capture applications⁹ owing to a greater capacity to absorb CO₂, lower energy of regeneration, and a significantly improved resistance to degradation problems when compared to MEA solvents. Cost-optimal process and solvent-blend design are identified via a constrained, multi-parametric optimisation approach. The objective function is the minimisation of total process cost (both CAPEX and OPEX) per unit of CO₂ captured. The design variables are the flowrate and state of the lean solvent and inlet gas streams as well as column geometry. Key performance indicators such as CO₂ emissions, solvent losses to the environment as well as the minimisation of energy required for solvent regeneration and also minimisation of CO₂ emissions to the atmosphere are included in the optimality criteria.

1. IPCC, 2005: IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the IPCC, Cambridge University Press, Cambridge, United Kingdom and New York, USA 2. Wolsky, A. M., Daniels, E. J. and Jody, B. J., Environ. Prog., 13, 214 (1994) 3. Chapman, W.G., Gubbins, K.E., Jackson, G. & Radosz, M., Ind. Eng. Chem. Res. 29, 1709-1721., 1990 4. Gil-Villegas, A., Galindo, A., Whitehead, P. J., Mills, S. J. & Jackson, G., J. Chem. Phys. 106 (10), 1997 5. Mac Dowell, N., Llovell, F., Adjiman, C. S., Jackson, G and Galindo, A., Ind. Eng. Chem. Res., 49(4), 1883-1899, 2010 6. Mac Dowell, N., Galindo, A., Jackson, G and Adjiman, C. S., ESCAPE-20 7. Process Systems Enterprise (PSE) Ltd. http://www.psenterprise.com/index.html 8. Rao, A. B. and Rubin, E. S., Environ. Sci. Technol., 36(20), 4467-4475, 2002 9. Choi, W. B., Min, B. M., Shon, B. H., Seo, J. B. and Oh, K. J., J. Ind. Eng. Chem., 15(5) 635-640, 2009