| The understanding of the thermodynamic behavior of electrolyte solutions is important in many natural and industrial processes. Much experimental work has been done on measuring the thermodynamic properties of these types of solutions, but for engineering purposes, an accurate model is needed from which various properties can be calculated as functions of temperature, pressure, and composition. The subject of this dissertation is the modeling of electrolyte solutions over wide ranges of temperature, pressure, and composition with a simple Helmholtz energy equation of state. The equation we have developed is an expression for the Helmholtz energy as a function of temperature, volume, and number of moles of each component in the system. The equation consists of four terms that account for various interactions between the molecules in solution: A Peng-Robinson term for the short-range interactions between uncharged species, a Born term for the charging free energy of ions, a mean spherical approximation (MSA) term for the long-range electrostatic interactions between ions, and an association term for the formation of ion-pairs. The equation of state was applied to 138 aqueous strong electrolyte solutions at 25°C and 1 bar using three adjustable parameters for each salt and to seven aqueous salt solutions from 0--300°C and from 1--120 bar using simple inverse temperature dependences for two adjustable parameters plus a single binary interaction parameter. The equation gives very accurate correlations of activity coefficients, osmotic coefficients, densities, and free energies of hydration for these systems. Our equation was also applied to high-temperature NaCl solutions from 500--1000°C, where NaCl is predominantly ion-paired. We are able to correlate the available phase equilibria data in this temperature range with reasonable accuracy. We are also able to use our equation of state to calculate extents of salt ionization as well as vapor pressures, densities, and equilibrium vapor and liquid compositions in partially dissociated electrolyte solutions. For aqueous salt mixtures, and nonaqueous salt solutions, we are able to calculate osmotic coefficients and vapor pressures with a high degree of accuracy from our equation of state using only a single adjustable binary interaction parameter. |
