(245b) Speed of Sound and Density of (Carbon Dioxide + Nonane) and (Carbon Dioxide + Methylbenzene) at Temperatures between (283 and 473) K and Pressures up to 390 Mpa

Understanding the thermophysical properties for mixtures of CO₂ and hydrocarbons at reservoir conditions is very important for the correct design and optimization of CO₂-enhanced oil recovery and carbon storage in depleted oil or gas fields. In this paper, we present a comprehensive thermodynamic study of two prototype systems: (carbon dioxide + nonane) and (carbon dioxide + methybenzene). The experimental study comprised highly-accurate measurements of the compressed-fluid densities and speeds of sound at temperatures from 283 K to 473 K, with pressures up to 68 MPa for density and 390 MPa for sound speed. Measurements were carried out over the full composition range and are characterized by small uncertainties: 0.05 % for density and 0.1 % for sound speed. The two mixtures studied here exemplify aliphatic and aromatic hydrocarbons.

Densities were measured using a vibrating U-tube denimeter which was carefully calibrated over the full working ranges of temperature and pressure using helium and water as the reference fluids. Validation measurements carried out on the pure substances verify the uncertainty analysis.

The speed of sound was measured using a dual-path pulse-echo apparatus operating at a frequency of 5 MHz. The difference between the lengths of the two acoustic paths at a reference temperature and pressure was obtained by calibration with water. The pathlength difference at other conditions was calculated from the known thermal expansivity and compressibility of the fused-quartz spacer tubes.

Mixtures were prepared gravimetrically in a variable-volume cell. They were then compressed to a pressure above their bubble point and homogenized prior to filling the apparatus. Thereafter, the mixture in the variable volume cell was always maintained in the single-phase region. However, flashing occurred when the mixture was initially introduced into the measurement apparatus. Therefore, the sound-speed apparatus was equipped with a re-circulation pump to permit homogenization in situ. The density apparatus, having a small hold-up volume, was instead flushed under pressure to expel the initial fluid charge.

The experimental results are used to examine the predictive capability of several leading thermodynamic models, including the Predictive Peng-Robinson (PPR-78) equation of state [1], a version of the Statistical Associating Fluid Theory for potentials of the Mie form (SAFT-γ Mie) [2] and the GERG-2008 equation of state [3]. The first two of these are group contribution approaches and can be applied to complex multi-component systems. The GERG model was developed primarily for natural gas systems but can be applied to liquid and super-critical mixtures. PPR-78 and SAFT-γ Mie both give a good account of the literature vapor-liquid equilibrium data and predict the densities to within (2 to 4) %. However, SAFT-γ Mie is much more accurate for derivative properties and is the only one capable of describing the sound-speed data. The GERG-2008 model is the most accurate for the pure substances but perform poorly for the mixtures, probably because it lacks binary-specific departure functions for these systems.

References

1. Jaubert J-N, Privat R. Relationship between the binary interaction parameters (k_ij) of the Peng-Robinson and those of the Soave-Redlich-Kwong equations of state: Application to the definition of the PR2SRK model. Fluid Phase Equilib. 2010;295:26-37
2. Dufal S, Papaioannou V, Sadeqzadeh M, Pogiatzis T, Chremos A, Adjiman CS, et al. Prediction of Thermodynamic Properties and Phase Behavior of Fluids and Mixtures with the SAFT-gamma Mie Group-Contribution Equation of State. J Chem Eng Data. 2014;59:3272-88
3. Kunz O, Wagner W. The GERG-2008 wide-range equation of state for natural gases and other mixtures: an expansion of GERG-2004. J Chem Eng Data. 2012;57:3032–91.