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.