(554a) Modelling and Design of MEA-Based CO2 Capture Processes: Combining Advanced Thermodynamics and Rate-Based Models

The primary source of CO ₂ emissions worldwide is fossil fuel combustion in the power and industry sectors, accounting for approximately 60% to total global emissions. Within this grouping, coal is currently the dominant fuel in the power sector; it is the preferred fuel for ~ 40% of electricity generation¹. The current method of choice for large-scale CO₂ capture is amine-based chemisorption; typically this is carried out in packed columns, with the solvent of choice being a primary alkanolamine - monoethanolamine (MEA). The objective of this work is to provide a model-based platform for assessing alternative designs, modes of operation and solvent composition for post-combustion CO₂ capture. We combine state-of-the-art thermodynamics with rigorous process simulation tools and techniques to facilitate this. 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)²,³ is used. This is a molecular approach, specifically suited to associating fluids. The SAFT formalism is used to represent the key 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 gPROMS⁴ software package. The important features of the model are that the reaction equilibria are incorporated in the thermodynamic model and as such enhancement factor concepts are avoided. Though the model does not explicitly consider reaction products, these are accounted for at the level of the thermodynamic model proposed for the fluid. For mass transport dominated reactions of this type one can assume reaction equilibrium at the interface. This allows us to capture the behaviour of the process with good accuracy. We consider a model flue gas of N₂, CO₂ and H₂ O. The performance of the absorber model is validated against published pilot plant data⁵. We validate our model against 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. Scenario based optimisation studies are carried out. Various design variables are studied including a 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 the minimisation of CO₂ emissions to 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

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

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

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