Flow Assurance plays a key role in both design and operation of pipelines for anthropogenic CO2 transport to Carbon Capture & Storage (CCS) injection wells. Proper modelling of thermodynamic properties of CO2 streams containing impurities, as well as of the relevant fluid dynamics, are of vital importance for a safe, reliable, and cost-effective design and for a flexible operation of CCS pipelines.
The present paper addresses two important technical challenging issues related to both the thermodynamics and thermo-fluid dynamics of anthropogenic CO2, namely the choice of the most accurate Equation of State (EoS) for the calculation of key thermodynamic properties (such as Vapour-Liquid Equilibrium (VLE) and density) and the modelling of the expansion wave of the mixture.
In literature, it is in fact well established that while the thermodynamic properties of pure CO2 are modelled with good accuracy using the Span-Wagner EoS, there is not a “reference” EoS to define the thermodynamic state of CO2-rich mixtures. To fill this gap, an extensive testing of different and common Equations of State (implemented in the commercial and recognized code Multiflash) against high-quality experimental data related to CCS CO2-rich mixtures, is presented in this work, focusing on the quantitative estimation of their accuracy in predicting the Vapour-Liquid Equilibrium (VLE) and density of the fluids. The accuracy of the tested EoS is evaluated through quantitative parameters such as Average Deviation (AD) and WAM (Weighted Arithmetic Mean) of density and bubble point deviations. The results obtained show that, based on experimental data available in literature, GERG-2008 and CSMA EoS are generally more accurate than cubic EoS in predicting both bubble points and density, presenting a WAM of around 1%.
Concerning the modelling of the expansion curve, necessary to evaluate the arrest of a running ductile fracture, the development and validation of a new numerical tool (named CO2RIDER – CO2-RIchDEcompRession) are presented. The code, based on REFPROP v10.0 (by NIST) for iso-entropic p-T flash calculations, models the expansion wave by assuming a one-dimensional isentropic decompression while the local fluid velocity is calculated by the compatibility equation proposed by Picard. The results predicted by CO2RIDER are in very good agreement with experimental data found in literature for rapid decompressions of CO2-rich mixtures. Generally, among the different tested Equations of State implemented in REFPROP and depending on the specific composition, GERG-2008 or PR (Peng-Robinson) predict more accurately the plateau characterizing the two-phase transition, which corresponds to the fluid saturation pressure that is a fundamental parameter to be estimated for the ductile fracture control. A discussion on how the choice of the EoS influences the predicted plateau is finally reported.