(58h) Systematic Design of Phase-Change Solvents for Post-Combustion CO2 Capture Based on Advanced Thermodynamics and Holistic Sustainability Assessment

Solvent-based, post-combustion CO₂ capture in absorption/desorption processes is a mature technology, but its wider industrial adoption is severely challenged by high energy penalties in solvent regeneration and the environmental impacts associated with solvents and their derivatives. Phase-change solvents represent a class of materials promising to deliver significant energy reductions compared to conventional solvents (Wang and Li, 2015). They consist of miscible solute-solvent mixtures (e.g., amines in water) which undergo a liquid-liquid phase separation upon a change in the processing conditions, e.g. upon reaction of CO₂ with the solute or upon increasing the temperature after the reaction etc. This behaviour results in the formation of a CO₂-lean phase which may be separated through a non-thermal approach. The separation of the CO₂-lean phase reduces significantly the amount of CO₂-rich phase entering the desorber, while desorption may also take place at much lower temperatures than in the case of single-phase solvents. As a result, the required regeneration energy may drop below 2.5 GJ/ton CO₂, which is much lower than the 4 GJ/ton CO₂ required for conventional solvents such as Monoethanolamne (MEA).

Without exception, the few research efforts reported in recent years toward the identification of phase-change solvents have been based entirely on lab and pilot-scale experiments. This approach has resulted in the identification of very few phase-change solvent options that exhibit desirable regeneration energy reductions at the expense of very high experimental costs and without necessarily evaluating their sustainability performance. At the same time, it did not eliminate the existence of other options exhibiting similar or better behaviour. There are countless combinations of potential phase-change solvent and blend candidates and there is also a need for combined consideration of numerous thermodynamic, kinetic and sustainability properties as performance criteria prior to selecting phase-change solvents with optimum capture features.

To address these challenges, we approach for the first time the design of phase-change solvents through optimization-based Computer-Aided Molecular Design (CAMD). We have previously applied CAMD for the design of conventional CO₂ capture solvents with very promising results (Papadopoulos et al., 2016). However, the employed tools did not include indices and models for prediction of phase-change behaviour while data gaps in sustainability assessment could not be handled effectively. We now significantly extend this CAMD approach by introducing additional solvent design criteria that account for liquid-liquid phase separation, we incorporate an automated, holistic sustainability assessment framework which addresses data gaps within CAMD and we rigorously predict solvent-CO₂-water vapour-liquid-liquid equilibria using the group contribution SAFT-γ-Mie equation of state (EoS) (Papaioannou et al., 2014) for selected solvents. The proposed approach involves a screening stage where molecular structures are generated and evaluated during CAMD based on criteria and constraints such as CO₂ and water solubility in solvent, solvent vapour pressure, heat capacity, viscosity and basicity, to name but a few. Such properties capture the solvent effects on the process thermodynamic and reactivity performance, guiding CAMD toward useful options with the potential to exhibit liquid-liquid phase separation. In addition, we consider during CAMD numerous solvent properties associated with cradle-to-gate life-cycle assessment and safety, health and environmental hazard assessment. Such properties include cumulative energy demand for solvent production, global warming potential, mobility, fire or explosion, acute and chronic toxicity, water- and air-mediated effects, degradation in the environment and accumulation. To address the lack of models for the prediction of some of these models as well as the data gaps existing in several others we propose for the first time the use of a local lazy learning approach in the course of CAMD. This approach exploits structural similarities with a set of reference molecules to provide predictions for the desired properties. The employed CAMD method uses multi-objective optimization to identify few promising phase-change solvent options which are then evaluated through a rigorous equation of state. We employ the group-contribution SAFT-γ-Mie EoS (Papaioannou et al., 2014) to predict for the first time the vapour-liquid-liquid equilibrium behaviour of selected phase-change solvents. The employed EoS implicitly accounts for the phase and chemical equilibrium characteristics of solvent-water-CO₂ mixtures hence it predicts the mixture behaviour without the need to postulate reactions or reaction products. Being a group contribution approach, it does not require regression of pure component and mixture interaction parameters for the specific solvents. Solvents are represented as sets of functional groups for which parameters may be transferred from or to other molecules regardless of the application.

The proposed CAMD approach identifies phase-change solvents which exhibit favorable overall CO₂ capture performance, while we also consider solvent purchase price as an additional criterion for post-CAMD solvent analysis and selection. We investigate the impacts of considering sustainability indices during solvent design, as opposed to cases where sustainability is not considered. We find novel solvents, including cyclic and acyclic amines, for use as components in efficient phase-change mixtures and we also provide experimental verification for their suitability as CO₂ capture options. Known phase-change solvents such as N-Methylcyclo hexylamine (MCA) and N,N-Dimethyl cyclohexylamine (DMCA) (Zhang et al., 2012) are also designed during CAMD, hence verifying the validity of our approach.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the grant agreement 727503 - ROLINCAP – H2020-LCE-2016-2017/H2020-LCE-2016-RES-CCS-RIA.

Cited References

Papaioannou, V., Lafitte, T., Avendaño, C., Adjiman, C. S., Jackson, G., Müller, E. A., Galindo, A. 2014, Group contribution methodology based on the statistical associating fluid theory for heteronuclear molecules formed from Mie segments. Journal of Chemical Physics, 140, 054107–29

Papadopoulos, A.I., Badr, S., Chremos, A., Forte, E., Zarogiannis, T., Seferlis, P., Papadokonstantakis, S., Galindo, A., Jackson, G. and Adjiman, C.S., 2016. Computer-aided molecular design and selection of CO₂ capture solvents based on thermodynamics, reactivity and sustainability. Molecular Systems Design & Engineering, 1(3), 313-334.

Wang X., Li B., 2015, Chapter 1 - Phase-change solvents for CO₂ capture, In Novel Materials for Carbon Dioxide Mitigation Technology, edited by Fan Shi and Bryan Morreale, Elsevier, Amsterdam, 3-22.

Zhang J.F., Qiao Y., Agar D.W., 2012, Improvement of lipophilic-amine-based thermomorphic biphasic solvent for energy-efficient carbon capture, Energy Procedia, 4, 92-101.