Nanothermodynamics and Molecular Simulations of Fluids in Porous Media

Abstract

The thermodynamic and transport properties of fluids confined to porous media are in this thesis investigated with nanothermodynamics, non-equilibrium thermodynamics, and molecular simulations. Non-equilibrium thermodynamics is applied to describe the non-isothermal transport of a two-phase fluid in a representative elementary volume (REV) of a porous medium. The thermodynamic variables of the REV are defined, and the entropy production and flux-force equations are derived. The thermodynamic variables of the REV are constructed from additive contributions, namely from the bulks phases, surfaces, and three-phase contact lines. There are three driving forces present, the thermal force, a chemical force, and a pressure force. For nanoporous systems, we found that we need to introduce the integral pressure. The integral pressure is a concept from nanothermodynamics and is different from the differential pressure, which is the normal pressure. We realized with this work, that to calculate the driving force in non-equilibrium conditions, namely the pressure gradient, we first needed to compose a procedure to calculate the pressure of a porous medium. We calculated the thermodynamic properties of fluids in nanoporous media using nanothermodynamics. The thermodynamic properties we calculated were for example the integral pressure, surface tension, entropy, and disjoining pressure. We calculated the transport coefficients of a single-phase fluid in a fcc lattice of solid spheres. We assumed that the integral pressure is constant when the system is in equilibrium and used this to calculate the integral pressure in a bulk fluid in equilibrium with the porous media. From this, we constructed an equation of state which relates the fluid density of the porous medium, temperature, and porosity to the integral pressure. The gradient in integral pressure is the driving force for fluid flow. Together with the mass flux and shear viscosity, we calculated the hydraulic conductivity and permeability of the system.