On the phase behaviour of CCS fluid systems

Abstract

This study investigated the phase behaviour of CO₂-rich mixtures containing co-capture impurities at low temperatures, which is critical for carbon capture, utilisation, and storage (CCUS) applications, particularly in offshore CO₂ transport by ship at temperatures as low as 223.15 K. Due to limited data on CO₂-rich systems at these conditions, the study conducted constant composition expansion and isochoric experiments to assess the effects of impurities - such as Methane (CH4), Oxygen (O2), Argon (Ar), Nitrogen (N2), Hydrogen (H2), Carbon monoxide (CO), and Dimethyl ether (DME) on the phase behaviour of CO2-streams at low temperatures. Temperature and pressure measurement uncertainties were 0.14 K and 0.03 MPa, respectively. The study findings show that non-condensable impurities generally raise bubble point and dew point pressures, with hydrogen having the most significant impact due to its high volatility, followed by nitrogen. In contrast, the relatively low volatility of DME results in negative deviations of the CO₂ stream from pure CO₂ phase behaviour. Model validation revealed that the Peng–Robinson equation of state (EoS), with adjusted binary interaction parameters (BIPs), provided the most accurate predictions, with average absolute deviations (AAD) below 5% for all datasets. The study also collected new bubble and dew point data for seven CO₂-rich multicomponent systems across temperatures from 238.15 K to near their critical points. Even at low impurity levels, bubble point pressures increased significantly at lower temperatures, while dew point pressure effects only became substantial at higher impurity concentrations. Both the Peng-Robinson and MFHEA EoS models predicted the data well, with AADs below 3.4% for dew and bubble points. These insights are essential for optimising CCUS transport and storage, as impurity effects necessitate careful temperature and pressure management to ensure safe and efficient CO₂ handling in pipelines and storage vessels.