Modeling ionic liquids with ePC-SAFT ─ properties and gas solubilities

Abstract: Global warming is now widely recognized as being the biggest global issue facing human beings. Mitigating CO2 emission from fossil-fueled power plants as well as from transports has become an urgent and worldwide research topic, in which CO2 separation is often needed. Technologies have been developed and commercialized, whereas the cost is still high. Developing new technologies for CO2 separation is one focus research area. Ionic liquids (ILs) are promising absorbents for CO2 separation due to their very low vapor pressure, high solubility and selectivity for CO2 as well as low energy usage for solvent regeneration. To develop IL-based technologies, thermodynamic properties (density, heat capacity, gas solubility, etc.), viscosity, and surface tension of ILs are the prerequisites. As the number of ILs that can be theoretically synthesized is up to an order of 1018, determining all the properties experimentally is impractical, not to mention the time-consuming with high cost. It is desirable to develop theoretical tools to predict the thermodynamics and transport properties of ILs and IL-containing mixtures in a wide temperature and pressure range. In our previous work, the framework of ion-specific electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) has been developed with reliable results. However, the work is still limited to the imidazolium-based ILs, and the model performance for other commonly used ILs is still unclear. Meanwhile, it has been pointed out that the model with the parameters fitted to the experimental data may result in pitfalls, and further validation is needed. In this thesis, the ion-specific ePC-SAFT was further developed and extended to the ILs which are composed of the IL-cations ([Cnmim]+ , [Cnpy]+ , [Cnmpy] + , [Cnmpyr]+ , and [THTDP]+ ) and the IL-anions ([Tf2N]- , [PF6] - , [BF4] - , [tfo]- , [DCA]- , [SCN]- , [C1SO4] - , [C2SO4] - , [eFAP]- , Cl- , [Ac]- , and Br- ). Before modeling the properties, a method and scheme were developed to investigate the pitfall when modeling IL with ePC-SAFT. All 96 ILs considered in the thesis were covered. The investigation shows that for most ILs (86 of 96 ILs), the additional fictitious critical temperature is low enough not to affect the calculations at a normal temperature range, and after further phase equilibrium calculation, only one IL ([C8mpy][BF4]) may generate a risk of pitfall occurrence at the temperature and pressure of interest for CO2 separation. The parameters for [Cnmpy]+ may need to be modified in future work. The prediction of the derivative properties (isobaric heat capacity, isochoric heat capacity, speed of sound, isentropic compressibility coefficient, isothermal compressibility coefficient, thermal expansion coefficient, thermal pressure coefficient, and internal pressure) combined with the comparison to the available experimental data shows that ePC-SAFT can provide reliable results for most ILs. ePC-SAFT was used to predict the CO2 solubilities in 46 ILs, and the prediction agrees well with the experimental data in a wide temperature and pressure range for 36 ILs. The addition of an ion-specific binary ii parameter between IL-ion and CO2 can further improve the model performance significantly for the 10 ILs with relatively poor model performance. ePC-SAFT can also provide a reliable prediction for the solubility of other pure gases (CH4, CO, H2, N2, and O2). To further verify the model performance on the viscosity of ILs, ePC-SAFT coupled with the free volume theory (FVT) (i.e., ePC-SAFT-FVT) was studied. Two strategies were applied to adjust the FVT parameters, i.e., molecular-based approach with parameters for each IL (strategy 1) and IL-cation molecular-weight linearized parameters for the ILs in the same homologous series (strategy 2). The comparison with the available experimental viscosities for 96 ILs shows that the strategy 1 can provide reliable results for 89 ILs in a wide temperature and pressure range, while strategy 2 can provide almost similar reliable results as strategy 1. ePC-SAFT-FVT can be further used to predict the viscosity of ILmixtures reliably. The model ePC-SAFT-DGT, i.e., the coupling of ePC-SAFT with the density gradient theory (DGT), was further developed and used to model the interfacial properties of ILs. The comparison with the available experimental surface tensions for 82 ILs shows that the model can represent the surface tension reliably, and the use of the anion-specific influence parameters linearized with the molecular weights of IL-cations allows predicting the surface tension of the ILs in the same homologous series. The density profile on the vaporliquid interface can be further predicted with the influence parameter adjusted by the surface tension. In summary, the ion-specific ePC-SAFT is a suitable tool for IL-systems, which can be highly recommended to be applied in industrial design and optimization.