Modeling of CO2 Decompression across the Triple Point

The formation of significant quantities of solid CO2 as a result of surpassing its triple point during rapid decompression of CO2 pipelines employed as part of the carbon capture and sequestration (CCS) chain can present serious operational and safety challenges. In this paper, the development, testing and validation of a rigorous computational fluid dynamics (CFD) flow model for predicting solid CO2 formation during decompression is presented. Multiphase flow is modeled by assuming homogeneous equilibrium, and the pertinent thermodynamic data are computed using real-fluid equations of state. The flow model is validated against pressure and temperature data recorded during the decompression of an extensively instrumented 144 m long, 150 mm i.d. CO2 pipe initially at 5.25 °C and 153.3 bar. For the conditions tested, the simulated results indicate CO2 solid mass fractions as high as 35% at the rupture plane, whose magnitude gradually decreases with distance toward the pipe’s intact end.