Depressurization of CO2-N2 and CO2-He in a pipe: Experiments and modelling of pressure and temperature dynamics

To design and operate safe and efficient CO2-transportation systems for CO2 capture and storage (CCS), engineers need simulation tools properly accounting for the fluid and thermodynamics of CO2. As the transportation systems evolve into networks, it becomes important that these tools also account for impurities in the CO2, which may significantly affect the thermophysical properties, directly impacting system design and safety. Tube depressurization experiments provide crucial data to develop and validate models describing transient multi-phase multicomponent flow in pipes. In this work, we perform experiments in a new facility with dense and fast instrumentation for both pressure and temperature. One experiment is for CO2 with 1.8 mol % N-2, and one has 1.92 mol % He, both starting from 12 MPa and 25 degrees C. In order to quantify the effect of impurities, the experiments are compared to results for pure CO2 and analysed on the background of simulations. We employ a homogeneous equilibrium model (HEM) augmented in this work to account for the appearance of solid CO2 in CO2 mixtures. We observe that the moderate amounts of impurities significantly influence both pressure and temperature dynamics. In particular, the 'pressure plateau', a key quantity for the assessment of running-ductile fracture, increases as much as 4 MPa for CO2-He compared to pure CO2. A further insight is that models must account for solid CO2 in order to capture the correct temperature development as the pressure decreases towards atmospheric conditions.