Measurement and modelling of the phase behaviour of (CO2 + diluents) mixtures in the context of CCS

Understanding the phase behaviour of (CO2 + diluent) mixtures is crucial in carbon capture, transport and storage (CCS) operations. For the design, optimisation and safe operation of these processes accurate equations of state... [ view full abstract ]

Understanding the phase behaviour of (CO₂ + diluent) mixtures is crucial in carbon capture, transport and storage (CCS) operations. For the design, optimisation and safe operation of these processes accurate equations of state (EOSs) are required to describe thermodynamic properties of the various mixtures of CO₂ with relevant substances, including H₂, N₂, CH₄, Ar, CO and O₂. A popular and potentially very-accurate approach is to use empirical multi-fluid multi-parameter EOS models. In order to adjust the interaction parameters in those models, reliable and accurate experimental phase equilibrium data are indispensable. Nevertheless, comprehensive reviews by Li et al. [1], Gernert et al. [2] and others, identify gaps and significant discrepancies in the literature reporting experimental VLE data for mixtures of CO₂ with substances relevant in CCS and flue-gas applications. Discrepancies between literature sources are particularly noticeable in the vicinity of critical points.

In the present study, new VLE measurements have been made on two relevant systems: (CO₂ + CH₄) and (CO₂ + CO). The measurements were carried out on isotherms at temperatures ranging from just above the triple-point to just below the critical point of CO₂ and at pressures from the vapor pressure of pure CO₂ up to approximately 15 MPa or, if lower, the mixture critical temperature. All measurements were performed using the low-temperature VLE apparatus described by Fandiño et al. [3], which is associated with low standard uncertainties of 0.006 K for temperature, 0.003 MPa for pressure and 10^-2x(1 - x) for mole fraction x.

The new VLE data, together with earlier results from our laboratory on (CO₂ + H₂) and (CO₂ + N₂) [3], have been compared with the predictions of available thermodynamic models, including GERG-2008 [4] and EOS-CG [2]. Additionally, the data have been correlated with the Peng-Robinson EOS [5], generally incorporating a single temperature-dependent binary interaction parameter k_ij (T). EOS-CG showed significantly better agreement, especially close to the critical point, than found with the Peng-Robinson model.

The new experimental VLE data help to fill important knowledge gaps relating to the thermophysical properties of mixtures of CO₂ with substances relevant in CCS and flue-gas applications.

[1] Li H, Jakobsen JP, Wilhelmsen Ø, Yan J. “PVTxy properties of CO₂ mixtures relevant for CO₂ capture, transport and storage: Review of available experimental data and theoretical models.” Applied Energy. 2011, 88(11): 3567-79.

[2] Gernert J, Span R. “EOS–CG: A Helmholtz energy mixture model for humid gases and CCS mixtures.” The Journal of Chemical Thermodynamics. 2016, 93: 274-93.

[3] Fandiño O, Trusler JPM and Vega-Maza D. “Phase behavior of (CO₂ + H₂) and (CO₂ + N₂) at temperatures between (218.15 and 303.15) K at pressures up to 15MPa.” International Journal of Greenhouse Gas Control. 2015, 36: 78-92.

[4] Kunz O and Wagner W. “The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion of GERG-2004.” Journal of Chemical & Engineering Data. 2012; 57(11): 3032-91.

[5] Peng, D.-Y. and D. B. Robinson "A new two-constant equation of state." Ind. Eng. Chem. Fundamentals. 1976, 15: 59−64.