Thermodynamic Modeling of Aqueous Nanobubble Dispersion

Abstract

Abstract The amount of gaseous species in water or brine can be greatly enhanced in the form of nanobubble (NB) dispersion. Aqueous NB dispersion has vast industrial applications, potentially in enhanced oil recovery and carbon dioxide (CO2) sequestration to control the mobility of gaseous species. Development of such NB technologies depends on a proper understanding of thermodynamic properties of aqueous NB dispersion. The objectives of this research are to analyze the thermodynamic stability of aqueous NB dispersion and to apply a thermodynamic equilibrium model to analyze experimental data. We first present a thermodynamic formulation for modeling aqueous NB dispersion, which clarifies that aqueous NB dispersion occurs in the aqueous phase that is supersaturated by the gaseous species in the system. That is, the gaseous species are present in two modes: dispersion of gas bubbles under capillary pressure, and molecule dispersion (supersaturation) in the external aqueous phase. Such a thermodynamic system is referred to as aqueous NB fluid in this research, and specified by (NC + 3) variables (e.g., temperature, total volume, components’ mole numbers, and capillary pressure), in which NC is the number of components. We then present a novel implementation of the GERG-2008 equation of state (EOS) in minimization of the Helmholtz free energy to solve for equilibrium properties of aqueous NB fluid. GERG-2008 was used in this research because it is suitable for modeling an aqueous phase that is supersaturated by gaseous species. The thermodynamic equilibrium model was applied to experimental data of aqueous NB fluid with nitrogen (N2) at pressures up to 277 bara (4019 psia) and 295.15 K (71.6°F). Application of the model to experimental data indicates that a large fraction (0.8 – 0.9) of the total amount of N2 is in the form of molecule dispersion, but such supersaturation of the aqueous phase is possible because of the presence of NB dispersion with capillary pressure. That is, NB dispersion can increase the gas content in aqueous NB fluid by enabling gas supersaturation in the aqueous phase as a thermodynamic system. Although experimental uncertainties resulted in a possible range of equilibrium properties for aqueous NB fluids at high pressures, the extrapolation of the calculation results to atmospheric pressure yielded a radius and a number density of bubbles within the range of data reported in the literature.