A Novel & Efficient Phase-Change CO2 Capture Solvent: From Computer-Aided Molecular Design to Optimal Process Design and Pilot Plant Testing
by Panos Seferlis
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. They consist of miscible solute-solvent mixtures which undergo a liquid-liquid phase separation. This behaviour results in the formation of a CO2-lean phase which may be separated through a non-thermal approach.
The identification of phase-change solvents has been based entirely on lab and pilot-scale experiments. 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, the design of phase-change solvents through optimization-based Computer-Aided Molecular Design (CAMD) using multi-objective optimization is proposed. CAMD has been previously applied for the design of conventional CO2 capture solvents with very promising results. CAMD approach is significantly extended by introducing additional solvent design criteria that account for liquid-liquid phase separation and incorporating an automated, holistic sustainability assessment framework within CAMD. 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. 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, the impacts of considering sustainability indices during solvent design are also investigated.
The identified novel phase-change solvent performance is further enhanced by the consideration of systematic structural and operating modifications imposed on a reference absorption/desorption flowsheet. Such modifications are realized with the help of a rigorous and flexible model that can represent the phase-change behavior and includes stream redistribution options that aim to enhance the main process driving forces. An aqueous N-methylcyclohexylamine (MCA) solution and a novel solvent identified by CAMD are employed in an effort to exploit the solvent’s phase separation behavior towards the reduction of the total process cost and energy requirements. The achieved regeneration energy for the novel solvent drops below 2.5 GJ/ton CO2, which is much lower than the 4 GJ/ton CO2 required for conventional solvents such as Monoethanolamne (MEA). Finally, the identified novel phase-change solvent mixture has been tested in a pilot-plant unit.