Application of Computational Fluid Dynamics to Decompression Modeling of Carbon Dioxide Pipelines

Abstract

Pressurized liquefied gases such as carbon dioxide are transported at a pressure above their saturation pressure. Therefore, if a pipeline transporting this substance ruptures, a large decrease in pressure occurs, causing the flashing of the fluid. Computational tools that predict how fast the depressurization occurs (decompression models) are of paramount importance to assess the consequences of potential pipeline rupture scenarios. Some of the main challenges when modeling this expansion process include: capturing the choked flow at the exit plane, which initiates the propagation of a decompression wave through the fluid; and addressing the phase transition that results in a multiphase flow. The main objective of this research is to develop a 2-D full-bore rupture decompression model to simulate the transient depressurization of a pipeline transporting pure liquefied COv2, using ANSYS Fluent as CFD software. The scope of this work focuses on incorporating non-equilibrium phase transition while addressing the calculation of properties for the metastable liquid region. Additionally, the scope includes the comparison of the CFD model predictions when implementing the Peng-Robinson (PR) EoS, and correlations based on the Span-Wagner (SW) EoS to calculate thermodynamic properties of the liquid phase. When comparing the CFD model results with the experimental pressure-time curves and average decompression wave speed, the best prediction of the pressure plateaus for both PR and SW approaches are obtained using small values of the mass transfer coefficient in the source terms (C = 8 s^ -1 for PR, and C = 7 s ^-1 for SW), which highlights the importance of incorporating non-equilibrium phase transition when modeling a rapid COv2 decompression. On the other side, a more accurate prediction of the arrival of the decompression wave front at various locations along the pipeline is obtained when implementing correlations based on data from the SW EoS, in comparison to the CFD model incorporating the PR equation. In general, the thermodynamic approach is deemed to have a predominant effect on the arrival of the decompression wave front at different locations along the computational domain, while the mass transfer coefficient (C) governs the phase transition and the pressure plateau representing this phenomenon.