(711c) Developing a Model for Carbon Dioxide Using the Modified Group Contribution-SAFT-VR with Dipole Theory
The statistical associating fluid theory (SAFT)  is a commonly used molecular-based equation of state that has been successfully applied to study a wide range of fluid systems. In recent work, the GC-SAFT-VR equation was developed, which combines the SAFT equation for potentials of variable range (VR)  with a group contribution (GC) approach  that allows for the description of hetero-segmented chain molecules. The GC-SAFT-VR approach has been shown to provide an excellent description of the phase behavior of pure associating and non-associating fluids and their mixtures, with minimal reliance on fitting the model parameters to experimental data [3-5].
Carbon dioxide (CO2) is central to a wide range of environmental and energy applications resulting in the importance of understanding the phase behavior and thermophysical properties of carbon dioxide containing systems. Due to the symmetry of CO2, it contains no dipole moment; however, a significant quadrupole moment is present. The presence of these multipoles can have an impact on the thermodynamics and phase equilibrium behavior and thus should be included in any molecular model of the molecule. In this work, a new model for CO2 was tested in which the quadrupole in CO2 is represented by two point dipoles oriented away from each other at 180°and described using the GC-SAFT-VR approach, which has been extended to handle dipolar square-well fluids by implementing the dipolar term from the SAFT-VR+D equation  into the GC-SAFT-VR theory. Isothermal-isobaric (NPT) and Gibbs ensemble Monte Carlo (GEMC) simulations have been performed to test the ability of the theory to capture the thermodynamics of the proposed model. Predictions for the thermodynamic properties and phase equilibrium of dipolar square-well dimer fluids are considered and compared with simulation data at various dipole strengths, temperatures, and interaction strengths.
With the ability of the theory to accurately describe the proposed model verified, CO2 parameters were obtained through fitting to experimental vapor pressure and saturated liquid density data. Results will be presented that show that the CO2 model provides a good description of the phase behavior of pure CO2 and its mixtures.
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