Extension of PC-SAFT to model inhomogeneous and transport properties

Abstract: CO2 separation plays an important role in greenhouse gas emission mitigation, in bio-fuel production via biomass gasification as well as in biogas upgrading. The current CO2 separation technologies are energy intensive, and new cost-effective CO2 separation technologies are needed. Ionic liquids (ILs) are promising absolvents for CO2 separation due to very low vapor pressurehigh solubility and selectivity for CO2 as well as low energy consumption for solvent regeneration. However, the absorption capacity of CO2 by ILs at low pressure is not high enough for practical application, and the capacity can be increased by incorporating functional groups such as amine group in ILs (task-specific ILs). In the last decades, a huge number of ILs has been synthesized in order to improve the IL performance for CO2 separation. Another drawback of using IL for CO2separation is the high viscosities, and using supported ILs has been proved to be a promising solution. Using supported ILs can take advantage of the high selectivity of gas in IL, and the high surface area of materials can reduce the impact of viscosity and improve the gas transfer, and hence increase the absorption rate. Considering the potential industrial applications and scientific interests, studying the thermophysical and transport properties of ILs and IL-containing mixtures both in bulk phase and in porous materials is crucial. The work of this thesis aims to develop tools for the estimation of important bulk and inhomogeneous properties of fluids related to CO2 separation using ILs. A first step of this thesis was to develop a density functional theory (DFT) based on perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EoS) to describe the properties of inhomogeneous fluids in the pores of materials, which can be later used to describe the gas absorption on the materials supported with ILs. Compared to the molecular simulation results, the developed DFT model can rigorously provide the micro-structure of inhomogeneous fluids, and the model results are in good agreement with the simulation data for various “model” systems. The developed DFT model was further extended to model the gas adsorption on porous materials by assuming that the material has single-size pores with simple shape (e.g. slit or cylinder). It was found that the model with the parameters fitted to pure-gas adsorption at one temperature can be used to predict the pure- and mixed-gas adsorption isotherms at other temperatures with satisfied accuracy. As many porous materials have a wide pore size distribution (PSD), the influence of the PSD on the DFT model performance was investigated. In general, PSD can improve the accuracy of model results. In the modeling of transport properties, the viscosity models (friction theory (FT) and free volume theory (FVT)) were combined with electrolyte PC-SAFT (ePC-SAFT) to represent the viscosities of pure ILs and IL/CO2 mixtures. The viscosity model parameters of FT and FVT were obtained by fitting to the experimental viscosity data of ILs and linearized with the molecular weight of the IL-cation. It was found that FT can provide accurate results for both pure and binary systems, and FVT can provide satisfied results with few parameters. Hence FVT can be recommended for general estimation of viscosity containing ILs, and FT can be used for accurate calculation. Finally, ePC-SAFT was extended to describe the thermodynamic derivative properties such as heat capacities, isothermal and isentropic compressibilities, thermal pressure coefficient, speed of sound, thermal expansion coefficient and internal pressure. It is shown that the model with the parameters obtained from the easily-accessible experimental data (pure-IL density) can be used to predict the thermodynamic derivative properties over a wide range of temperature and pressure.