(58g) Towards the Development of a Solvent Screening Tool for CO2 Capture Using Molecular Thermodynamics

In the context of sustainable development and clean energy production, one of the most important alternatives to mitigate anthropogenic CO₂ emissions is to capture and separate CO₂ (CCS) from diluted sources, such as gases emitted from fossil fuel combustion and other industrial processes. In this regards, and given some of the limitations of current absorption processes, the development of new alkanolamines/amines is a topic which has attracted a great deal research interest. One of the major challenges when evaluating the techno-economic performance of new amines and blends is the lack of experimental data on the thermophysical properties, required for a reliable process design and simulation. Having a trustworthy modeling tool to fill the gap would be of high value. We have developed and implemented a robust theoretical framework for the description of key thermophysical properties of aqueous solutions of single and mixed alkanolamines/amines at relevant gas separation process conditions. The approach is based the combination of soft-SAFT [1], a molecular-based equation of state, with the Free-Volume Theory and the Density Gradient Theory for the integrated modeling of phase behavior, enthalpies, densities, viscosities and interfacial tensions. We present and discuss here the application of this procedure to investigate the absorption of CO₂ in five selected amines, differing in families and molecular structures: primary monoethanolamine (MEA), as a benchmark, secondary diethanolamine (DEA), tertiary methyldiethanolamine (MDEA), sterically hindered 2-amino-2-methyl-1-propanol (AMP) and cyclic piperazine (PZ). The study is performed in a systematic manner, starting from the development of the models for the pure amines [2], the description of their thermophysical properties, the properties of the aqueous mixtures [3] and the absorption of CO₂ in aqueous solutions of single and blended amines at conditions of relevance for post-combustion CO₂ capture applications. A scheme of implicit reactions is used to describe the formation of carbamate and/or bicarbonate products resulting from the chemical reactions between CO₂ and the examined amines. This procedure eliminates the need to specify the detailed equilibrium reactions and significantly reduces the number of parameters required to represent the absorption process. A maximum of two adjustable model parameters (one of which with a linear temperature dependency), optimized for a fixed amine concentration, suffice to represent the absorption of CO₂ in aqueous solutions of single amines over a broad range of temperatures (298 413 K) and CO₂ partial pressures (0.1 1000 kPa). The extrapolation capability of the model is tested by predicting the absorption of CO₂ in aqueous solutions of single amines for different amines concentrations, in reasonable good quantitative agreement with experimental data. The predictive capability of the model is evaluated for predicting the absorption of CO₂ in blends. The models are finally used to assess the CO₂ capture performance of selected amine systems in terms of solvent cyclic capacity and regeneration energy. These results demonstrate the feasibility of the developed approach as a reliable platform for the screening of amine solvents as function of key process parameters and as a valuable tool for process modeling and optimization, being also applicable to model other absorbents or processes.

This work has been funded by the Gas Research Center, The Petroleum Institute, Khalifa University of Science and Technology, through projects GRC17001 and GRC18003.

[1] F.J. Blas, L.F. Vega, Mol. Phys. 92, 135–150 (1997). F.J. Blas, L.F. Vega, Ind. Eng. Chem. Res. 37, 660–674 (1998).

[2] J.O. Lloret, L.F. Vega, F. Llovell, J. CO2 Util. 21, 521–33 (2018)

[3] L.M.C. Pereira, F. Llovell, L.F. Vega, Applied Energy, in press (2018)