(112b) Effect of the Impurities On Thermophysical and Tranport Properties of CO2 for Transportation

Carbon dioxide (CO2) obtained from different carbon-capture processes usually contains small amounts of other gases, such as N2, H2, CO, CH4, H2S, NO2 and SO2, as well as traces of liquid water. The presence of these impurities may lead to challenging issues, as they can importantly modify the conditions of transport of supercritical CO2. Usually, the gaseous CO2-rich stream is generally compressed to be transported as liquid or dense-supercritical phase in order to avoid two-phase flow. However, the inclusion of substances other than CO2 can severely change the phase diagram of carbon dioxide, producing changes on the density of the system, the bubble point pressure, the supercritical region limits or the viscosity, hence affecting the compression needs. Therefore, it is of great interest to have accurate equations of state (EoSs) for evaluating those effects. Empirical correlations and cubic equations of state have been commonly proposed to model those mixtures, achieving a good degree of success. However, these equations do not fully consider some important effects, such as the hydrogen-bonding interactions of water and other associating compounds, or the quadrupole moment of CO2, leading to some inaccuracies and a lack of extrapolation capability. In this regard, molecular-based EoSs can become an excellent platform so as to account for these interactions and provide a complete thermodynamic description of CO2 mixtures with a high degree of accuracy. The goal of this work is to identify the most important effects of CO2-impurities in its thermodynamic and transport properties using the refined molecular-based soft-SAFT EoS [1]. Soft-SAFT considers the molecular effects due to repulsive and attractive interactions, chain formation and hydrogen bonding and builds molecular models for each molecule based on their physical properties and chemical structures, finding a characteristic set of molecular parameters for each compound. We will analyze the effect of the inclusion of other gases and water in the density, solubility and pressure conditions of the CO2 system. A specific treatment for the critical region of CO2 is used for an accurate calculation around this area [2]. Finally, the effect of these compounds on the stream viscosity is also studied by using the Free Volume Theory (FVT) coupled with the soft-SAFT equation [3].

[1] Blas, F. J.; Vega, L. F. Mol. Phys. 1997, 92, 135-150. [2] Llovell, F.; Pàmies, J. C.; Vega, L. F. J. Chem. Phys. 2004, 121, 10715-10724. [3] Llovell, F.; Marcos, R.M.; Vega, L. F. J. Phys. Chem. B 2013 (submitted).