Dissertation - Cheng-Ju Hsieh - Examination of Aqueous Electrolyte Solutions Modeling with Association Electrolyte Non-Random Two-Liquid Model

The industrial and natural processes highly involve aqueous electrolyte solutions. Those electrolyte species play critical roles in the physical and chemical properties of the solutions. Water desalination, produced water treatment, and salt brine mining are classic examples of the scientific study of aqueous electrolyte solutions. On the other hand, organic electrolyte solubility is studied in pharmaceutical processes, and aqueous electrolyte solutions such as carbonate, ammonia, or amine-based solutions are usually used to capture carbon dioxide. The process conditions are unlimited so that people cannot measure all the experimental data. Hence, a rigorous thermodynamic model becomes the cornerstone to estimate the required properties of solutions to simulate and design the processes. The association electrolyte non-random two-liquid model (association eNRTL) is known to have the potential to capture the nonideality of aqueous electrolyte solutions by describing association behaviors, including ion-pairing, ion-hydration, and self-association of water, which is a weakness of the original eNRTL model. The first part of this study examines aqueous electrolyte solutions’ properties, such as mean ionic activity coefficient and osmotic coefficient, at 298 K by regressing literature data to identify model parameters. The association eNRTL model wonderfully captures their thermodynamic properties up to the saturated point and suggests the association term is the dominant term. The electrolytes in this work involve 130 electrolyte species, including 42 cation species and 17 anion species, and the mean relative deviation is generally less than 10%. The second part of this study shows the potential of the association eNRTL model to predict the thermodynamic properties of aqueous mixed-electrolyte solutions only with parameters identified from aqueous single-electrolyte solutions. The examined data includes data with fixed ionic strength and varied electrolyte species fraction (Harned’s rule), data with varied ionic strength and fixed electrolyte species fraction, and infinite dilution of HCl in other aqueous electrolyte solutions with varied ionic strength. Since the association term is the dominant term, the association behaviors in the aqueous mixed-electrolyte solutions can be described. The association eNRTL model is found to capture the data trend well and is expected to be applied in industrial process design compared to the original eNRTL model and the Pitzer model. The third part of this study chooses CO2 absorption by K2CO3 as an industrial application example. The previous thermodynamic modeling by the eNRTL model does not investigate the enthalpy of CO2 absorption and vapor-liquid-equilibrium at high K2CO3 concentration. This work expands the thermodynamic modeling by the association eNRTL model with a temperature range of 273.15 – 473.15 K and up to 60 wt% K2CO3. The association eNRTL model is shown to be a competitive model for the CO2 absorption process by capturing the heat of absorption and vapor-liquid-equilibrium compared to the original eNRTL model.