(426c) An Integrated Process and Solvent Design Platform for CO2 Capture From Low Pressure Gas

Fossil fuel combustion for power generation accounts for approximately 55% of total global CO₂ emissions. Further the flue gas streams of these processes typically have high flow-rates, but are at the same time dilute in terms of CO₂, the concentration rarely exceeds 15%vol¹. Consequently there is an ongoing interest in improvements in solvents² with a view to reducing the energy penalty associated with CO₂ capture. The objective of this work is to provide a unified systems approach, model-based platform for assessing alternative solvents and solvent blends for post-combustion CO₂ capture from large fixed point emission sources. In developing this technique, 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 present in the system, thereby simplifying the description of the chemical reactions. The molecules are represented as homonuclear chains of 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. A rate-based model of an absorber/stripper system for the chemisorption of acid gas in aqueous solvent solutions is implemented in the gPROMS⁵ software package. An important feature of the model is that for mass transport dominated processes of this kind the reaction kinetics are incorporated in the thermodynamic model and as such 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; again these are accounted for at the level of the thermodynamic model proposed for the fluid. We implicitly assume reaction equilibrium at the interface, and this allows us to capture the behaviour of the process with good accuracy. We consider a model flue gas of N₂, CO₂ and H₂O. The performance of the absorber model is validated against published pilot plant data⁶ for an MEA based absorber. We validate our model for its ability to predict temperature, gas- and liquid-phase composition profiles. The relative error in predicted temperature profiles is 0.17%, and the absolute error in composition profiles is 0.0025 and 0.002 for the gas phase CO₂ composition and the liquid phase CO₂ loading (xCO₂/xMEA) respectively. We then investigate the impact of blends of solvents on the column performance. The blends investigated include varying compositions of NH₃, ethanol, MEA and alkylamines. Scenario based optimisation studies are carried out. Various design variables are studied including the variation in the flowrate and composition 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 minimisation of energy required for solvent regeneration and theminimisation 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., 1990. 29, 1709-1721

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

5.Process Systems Enterprise (PSE) Ltd. http://www.psenterprise.com/index.html

6.Tontiwachwuthikul, P., Meisen, A. and Lim, C. J., Chem. Eng. Sci., 47, 2, 381-390, 1992