(524c) Molecular Design of Optimum CO2 Capture Solvents: From Conceptual Screening to SAFT-Based Validation

The wide adoption of chemical absorption/desorption systems for post-combustion CO₂ capture in industry is currently challenged by the high energy penalty in solvent regeneration and the environmental impacts associated with solvents and their derivatives. Intense research efforts reported in recent years are predominantly based on lab and pilot-scale experiments to select solvents which may potentially improve the overall performance of absorption/desorption CO₂ capture. However, this is very challenging due to a) the highly non-ideal solvent-CO₂-water chemical interactions, b) the countless combinations of potential capture solvent and blend candidates and c) the need for combined consideration of numerous thermodynamic, kinetic and sustainability properties as performance criteria prior to selecting solvents with optimum capture features. Computer-aided molecular design methods (CAMD) can help address these challenges and have been successful in supporting the synthesis of molecules with desired physical, chemical and environmental properties in non-CO₂ separations [1]. Despite extensive developments in CAMD methods, few recent works have reported their utilization in the design of CO₂ capture solvents or mixtures for chemical and physical absorption using the Statistical Associating Fluid Theory with potentials of Variable Range (SAFT-VR) [2, 3] and for physical absorption using the Perturbed Chain Polar Statistical Associating Fluid Theory (PCP-SAFT) [4]. While these approaches enable an accurate and reliable determination of solvent-process vapour-liquid equilibria, the set of few solvents screened to date will be further expanded as research efforts extend the rigorous predictive capabilities of SAFT-based models towards additional molecular structures. On the other hand, few CAMD approaches have also been reported [5, 6, 7] that use less accurate, but well-established group contribution methods to screen a much wider set of molecular structures by approximating as solvent performance criteria mainly properties relevant to thermodynamics and toxicity. 

In our current work, we propose the use of an optimization-based CAMD method to select post-combustion CO₂ capture solvents of optimum performance in molecular and mixture properties associated with thermodynamics, kinetics and sustainability. The problem is first approached in a fast screening stage where solvent structures are evaluated based on the simultaneous consideration of important pure component properties. For the first time numerous properties are considered as performance criteria reflecting solvent characteristics based on thermodynamic (e.g., vapour pressure, CO₂ solubility etc.), kinetic (solvent basicity, steric hindrance etc.) and sustainability (e.g., health and safety hazard, life cycle assessment etc.) behaviour. The simultaneous consideration of properties selected to capture the molecular chemistry effects on the absorption/desorption process compensates for the initial utilization of simpler models and ensures the selection of fewer but more effective solvents. A few highly-performing solvents are further evaluated using the SAFT-VR and SAFT-γ equations of state to predict accurately the very non-ideal solvent-water-CO₂ mixture vapour-liquid equilibrium behaviour. Different functionalities of the employed CAMD method are used both to design optimum, novel molecular structures and to screen a dataset of commercially available amine solvents suitable for CO₂ capture. The obtained results reveal interesting structure-property trade-offs and point to commercial molecules, which have very recently been considered or have yet to be employed as capture solvents.

Acknowledgements

Funding from the European Commission under grant FP7-ENERGY-2011-1-282789-CAPSOL is gratefully acknowledged.

Cited References

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