Systematic approaches for design of ionic liquids and their mixtures for enhanced carbon capture purpose

Post-combustion capture using amine-based solvents has been considered as the most viable technology for carbon capture, to mitigate industrial carbon dioxide (CO2) emissions; but the solvents show a number of shortcomings. Recently, ionic liquids (ILs) are suggested as possible alternative to amine-based solvents, for they can be molecularly engineered to match various target thermophysical properties. This work focused on the development of systematic approaches to design IL-based solvents for carbon capture purpose. The first focus of this work is to develop an insight-based based visual approach to determine potential IL solvents as substitute to conventional carbon capture solvents. This approach allows visualisation of high-dimensional problem to be visualised in two or three dimensions, and assist designers without mathematical programming background in IL design. Following that, a mathematical optimisation approach to design optimal IL solvent for CO2 capture purpose was developed as second focus of this thesis. This has been done by formulating the IL solvent design problem as mixed integer non-linear programming (MINLP) optimisation problem. The abovementioned approaches were developed to design task-specific ILs with high CO2 absorption capacity as substitute to common carbon capture solvents. However, studies show that such ILs are relatively more expensive and have higher viscosities. To reduce the cost and viscosity of solvent, task-specific IL can be mixed with conventional IL, ensuring CO2 solubility remains high, while viscosity and cost are acceptable. Hence, the previously developed visual approach was extended to design pure ILs and IL mixtures, specifically to capture CO2. In order to ensure the designed IL is performing at its optimum (highest CO2 solubility in this case), the operating conditions of the carbon capture process shall be considered because they will affect the thermophysical properties and CO2 solubility of ILs. Therefore, the forth focus of this work will be incorporation of operating temperature and pressure into design of IL solvents. Similarly, the design problem was formulated as MINLP problem and solved using mathematical optimisation approach, where operating temperature and pressure were defined as variables through disjunctive programming. Replacing solvent for carbon capture system to IL-based solvent or installing carbon capture system will affect the overall process, as this will affect the utilities consumption of carbon capture system. Therefore, process design has been integrated with IL design in this thesis, to study how the solvent substitution affects the entire process, and followed by retrofitting of the entire process including carbon capture system accordingly. The design problem was formulated and solved as MINLP problem. Finally, this thesis concludes with possible extensions and future works in this area of research work.