Diffusion coefficients in carbon dioxide-[bmim][PF6] mixture at high pressure: Transport and thermodynamic modeling of experimental solubility

Ionic liquids are defined as molten salts which have melting point below 100°C. They have unique properties such as low vapor pressure, non-flammability and good thermal stability, that make them attractive as solvents. On the other hand, carbon dioxide, has been used as a solvent in both supercritical and liquid phases for decades. It can form a solvent system with ionic liquids to be used in different applications. A mass transfer process in one-direction starts with the introduction of carbon dioxide at high pressure to an ionic liquid containing system. Carbon dioxide dissolves in the liquid phase. However, the transport of the ionic liquid into the gas phase does not occur in measurable quantities. In this study, the objective is to determine the mutual diffusion coefficients of the carbon dioxide-ionic liquid, [bmim][PF6], system at temperatures of 313.15 K and 323.15 K and pressures of 5 MPa and 8 MPa. Experiments have been run to find time-dependent carbon dioxide solubilities in the ionic liquid and then, a transport model is fit to the experimental data in order to estimate the diffusion coefficients. The system is modeled as one-directional mass transfer from gas phase to liquid phase with a moving interface. The volume-of-fluid method is used to model the change in the position of the interface. To account for the variation in liquid density, an equation fit to the experimental density data is included in the transport model. The diffusivities at dilute concentration and at thermodynamic phase equilibrium are determined and compared with the literature values and those obtained from correlations.