Ethylene Glycol and Its Mixtures with Water and Electrolytes: Thermodynamic and Transport Properties

Abstract

A comprehensive thermodynamic model has been developed for calculating thermodynamic and transport properties of mixtures containing monoethylene glycol (MEG), water, and inorganic salts and gases. The model is based on the previously developed mixed-solvent electrolyte (MSE) framework, which has been designed for the simultaneous calculation of phase equilibria and speciation of electrolytes in aqueous, nonaqueous, and mixed solvents up to the saturation or pure solute limit. In the MSE framework, the standard-state properties of species are calculated from the Helgeson–Kirkham–Flowers equation of state, whereas the excess Gibbs energy includes a long-range electrostatic interaction term expressed by a Pitzer–Debye–Hückel equation, a virial coefficient-type term for interactions between ions and a short-range term for interactions involving neutral molecules. Model parameters have been established to reproduce the vapor pressures, solubilities of solids and gases, heat capacities, and densities for MEG + H2O + solute systems, where the solute is one or more of the following components: NaCl, KCl, CaCl2, Na2SO4, K2SO4, CaSO4, BaSO4, Na2CO3, K2CO3, NaHCO3, KHCO3, CaCO3, HCl, CO2, H2S, and O2. In particular, emphasis has been put on accurately representing the solubilities of mineral scales, which commonly appear in oil and gas environments. Additionally, the model predicts the pH of mixed-solvent solutions up to high MEG contents. On the basis of speciation obtained from the thermodynamic model, the electrical conductivity of the MEG + H2O + NaCl + NaHCO3 solutions is also calculated over wide ranges of solvent composition and salt concentration. Additionally, associated models have been established to compute the thermal conductivity, viscosity, and surface tension of aqueous MEG mixtures.