Discussion on Decompression Path and Wave Speed of Supercritical and Dense CO2 with Impurities

Decompression of dense and supercritical CO2 containing impurities was investigated using an equilibrium isentropic model coupled with NIST REFPROP 10 and GERG-2008. The pressure plateau was determined from the intersection of the isentropic path and the bubble line. The adopted numerical settings, ΔP = 0.5 kPa and 500 pressure points, changed the plateau pressure by less than 1%. Validation included 18 binary phase-equilibrium points, seven shock-tube cases, and four large-scale tests. The mean absolute relative deviations were 1.25%, 4.54% and 1.75%, respectively. For the shock-tube cases, the deviation range was −10.14% to 8.19%; six cases had R2 ≥ 0.95 and RMSE ≤ 0.53 MPa, whereas the H2-containing case gave RMSE = 1.42 MPa. At 15 MPa and 40 °C, 3 mol% of CH4, Ar, N2, O2, CO or H2 increased the bubble pressure at 20 °C by 0.76–2.06 MPa relative to pure CO2, following the order H2 > CO > O2 > N2 > Ar > CH4. The plateau pressure increased with impurity concentration and initial temperature, whereas initial pressure mainly shifted the isentropic path. These results provide a basis for impurity screening and fracture-control assessment in CO2 pipelines.