(664d) A Molecular-Based Screening Tool for Water-Free Amine Solvents for CO2 Capture

Currently, the chemical absorption of CO₂ using aqueous solutions of amines mainly, is the most mature technology for CO₂ capture and separation, that has been applied in many industrial processes. However, the high regeneration energy requirement is the major hurdle to the large-scale deployment of this technology. Among the various strategies present as a remedy for this issue, the utilization of water-lean or water-free solvents provide an attractive alternative. This is done through either partial (water-lean) or full (water-free) replacement of water with alternative physical solvents with lower heat capacity and evaporation enthalpy that, when combined with traditional alkanolamines, form physical-chemical absorbents. The accurate determination of CO₂ solubility in the utilized solvents is an integral factor in the design and simulation of carbon capture or acid gas treatment processes along with their techno-economic evaluation. However, the lack of experimental data on the thermophysical properties of new amine-based solvents or blends is a major obstacle for reliable process design and simulation. Therefore, devising a consistent and reliable thermodynamic model capable of accurately describing the phase equilibria, thermophysical and transport properties of CO₂ and amine-based systems is highly needed.

In this contribution, we demonstrate the capabilities of a robust molecular‑based framework for the description of key thermophysical properties of water-free and water-lean amine-based solvents for the efficient removal of CO₂ and other acid gases from industrial gas streams at relevant gas separation process conditions. The screening tool is built on the molecular-based equation of state (EoS) soft-SAFT [1] combined with the Free-Volume Theory for the integrated modeling of phase behavior, enthalpies, densities and viscosities. Solvents investigated in this work include mixtures of various alkanolamines with different physical solvents such as alcohols, glymes, glycols, polar aprotic solvents and ionic liquids (ILs). Molecular parameters of all the investigated substances are either taken from previous contributions or developed in this work and used here in a transferable manner for describing the solubility of CO₂ in the selected solvents at conditions of relevance for industrial applications.

The description of the chemisorption of CO₂ in the studied amine + physical solvent blends is done in a consistent manner with that for the chemisorption of CO₂ in aqueous amine reported in our previous contributions [2,3]. 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. The influence of the organic solvents on the chemical reactivity of amines with CO₂ in water-free solvents is investigated in detail. 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, optimized for a fixed amine concentration, suffice to represent the absorption of CO₂ in the studied solvents over a broad range of conditions, with high predictive and extrapolative capabilities. The acquired results from our the developed thermodynamic framework are compared to experimental data gathered from literature and predictions from the recommended thermodynamic models implemented in the Aspen Plus^® (V.10) process simulator and the model found to be capable of providing reliable solubility estimations over a broad range of pressure, temperature and compositions.

The collection of these results demonstrates that once the robust molecular model is developed and validated, it can be used as a predictive tool, combined with macroscopic thermodynamics, as a reliable platform for the screening of chemical solvents as function of key process parameters, and as a valuable tool for process modelling and optimization.

This work is partially funded by the ADNOC Gas Processing and its stakeholders through the Gas Research Center (project GRC2018-003) and Khalifa University of Science and Technology (RC2-2019-007).

References

[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 (2017).

[3] L.M.C. Pereira, F. Llovell, L.F. Vega, Applied Energy. 222, 687-703 (2018).