(262q) Development of a Thermodynamic Model for Confined Fluids

Fluids confined in porous media play a significant role in many engineering applications. Modeling fluids in oil reservoirs, adsorption based separations, and heterogeneous catalysis require the accurate prediction of thermodynamic properties at a wide range of conditions. For confined fluids, this task involves accounting for the fluid-solid interactions induced by proximity to a solid wall. Most engineering models are only able to predict average properties of the system and fail to give information related to the heterogeneity of the confined fluid. Non-uniform behavior in such systems exists due to many effects, including the variation of the fluid properties with distance from the wall, and the presence of different sizes or geometries within the same porous space. Other than these microscopic effects, confined fluids could be subject to effects at a larger scale. For instance, gravity creates a molar distribution of the components in deep oil reservoirs along the depth of the formation. As a result, including the gravitational and confinement effects simultaneously will give a better representation of the properties of the components in these systems.

The objective of this work is to establish a general framework for determining equilibrium properties of fluids in confined media. The thermodynamic model utilizes an equation of state (EoS) to describe both the bulk and confined phases without introducing any additional parameters. On the other hand, the confinement effect could be represented for any solid, regardless of its shape or size, provided that an appropriate model to describe the solid-fluid interactions is available. Another goal is to find equilibrium conditions when several effects are acting on the system, whether they arise from the nature of the confined media or from other sources.

In this model, the variation of properties throughout the system takes place across regions that are defined depending on the effects present. Regions where confinement effects are important are further discretized into layers to capture local distribution within the confinement. For all elements, the volume is specified together with the temperature, and the total amount of each component. Thus, minimizing the Helmholtz energy determines the number of moles in each element; subsequently other properties are obtained. The Helmholtz energy accounts for internal interactions through EoS; in this work the volume translated Peng Robinson EoS is used. Additionally, the Helmholtz energy function includes an external contribution represented by an adsorption potential to account for fluid-solid interactions. The Steele 10-4-3 potential is used to investigate the confinement of light hydrocarbons in activated carbon. Specifically, the potential is applied to obtain local density and compositional profiles inside the activated carbon silt pores as well as to compute adsorption isotherms for a heterogeneous solid (different pore sizes). The prediction of local behavior is found to be comparable to classical Density Functional Theory (DFT) calculations. When computing the adsorbed amount, good agreement between experimental data and calculations is obtained. Another adsorption potential employed in this work is the Dubinin-Radushkevich-Astakhov (DRA) potential, which is utilized to predict confinement of binary and ternary mixtures of methane, nitrogen, carbon dioxide on activated carbon at various pressures and compositions. The computed isotherms illustrate the adsorption behavior of the studied systems including the preferential adsorption of carbon dioxide over the other two components. These DRA results are found to agree reasonably well with experimental data. Moreover, the ability of the framework to model systems where confinement as well as other effects are present is demonstrated through predicting the molar distribution of methane and carbon dioxide in a porous reservoir where gravitational effects are accounted for.