Development of advanced thermodynamic models for CO2 absorption: From numerical methods to process modelling
Creators: name: Graham, Edward John
Publisher: name: Imperial College London
Year: 2020

description: This thesis considers the development of predictive thermodynamic models for amine-based carbon capture processes, motivated by the imminent requirement for the reduction in anthropogenically produced carbon dioxide emissions. In the introduction, we show how the use of molecular-based equations of state, such as SAFT (Statistical Associating Fluid Theory), can be highly effective in this context. Due to the level of molecular detail captured in their theoretical development, one can reduce the reliance on experimental data by transferring their parameters based on sound physical arguments. In particular, the inherent chemical reactions in amine-based carbon dioxide absorption processes can be modelled by a physical association scheme, offering a vast simplification over the conventional treatments. In the following chapter a rate-based absorber model is presented to investigate the reactive capture of carbon dioxide CO2 using aqueous monoethanolamine (MEA) as a solvent. The SAFT-VR SW equation is used as the thermodynamic model. Due to the physical treatment of the reactions, the process model equations only needs to consider the apparent concentrations of the molecular species, while the reactions are implicit in the SAFT equation. With the assumption that the species diffuse as non-associated species, the rate of CO2 absorption is overpredicted, providing an upper bound on the solvent performance. A single parameter is adjusted to the pilot plant data, reflecting the reduction in mass transfer rate in the apparent CO2 in its aggregated form, which is found to be transferable over all of the pilot plant runs. The development of new models for the SAFT- Mie equation is then considered for improvement of the thermodynamic model. This is because the thermodynamic models developed for the SAFT-VR SW (used in the absorber) provide an inaccurate description of the liquid heat capacity and the heat of absorption of CO2. We consider a novel approach to the parameter estimation problem. It is shown that posing the parameter estimation as a multi-objective optimization problem offers numerous advantages over conventional (single-objective optimization) techniques. A robust and efficient algorithm that deals with multiple objective functions is tailored for this purpose. We consider objective functions that characterise the deviation between the SAFT model and experimental measurements for different thermodynamic property types. Using the multi-objective optimization technique we develop SAFT- Mie models for water where saturated liquid denisty, vapour pressure and isobaric heat capacity are treated as competing objectives. A single (non-spherical) model for water is chosen from the Pareto fronts obtained. Next, we develop SAFT- Mie (or SAFT-VR Mie) models for the CO2 + MEA + H2O mixture, with focus on developing models that provide a simultaneous accurate description of the vapour-liquid equilibria and the caloric properties. In comparison with the previous models developed for the SAFT-VR SW equation of state, the new models provide a better description of key thermodynamic properties in the chemisorption process, in particular the vapour pressure of CO2 above v the solvent mixture, the isobaric liquid heat capacity and the heat of absorption. We show that incorporating the new thermodynamic models in our process model for the absorber, we obtain a slightly better prediction of the column temperature profile. In the last chapter we derive a classical density functional theory (DFT) that incorporates the SAFT-VR Mie equation of state (SAFT-VR Mie MF DFT). The proposed method is applicable for a wide variety of fluids, including fluids/ fluid mixtures that consist of associating molecules and molecules of varying chain length. We derive a theory that is numerically tractable and show a novel implementation of the DFT equations in gPROMS. We show that the theory can be used to accurately predict the experimental interfacial tension for the SAFT models developed in this thesis, and the predicted density profiles in the intefacial region compare favourably with molecular simulations. The SAFT-VR Mie MF DFT approach developed in this chapter is used throughout the thesis for validation of the thermodynamic models.