Numerical Simulation of CO2 Flows in Pipes with Phase Transition across the Triple Point
The formation of solid CO2, commonly known as 'dry ice' resulting from the near isentropic expansion of CO2 to pressures below its triple point (5.18 bar) is of significant practical importance for the design and safe operation of various systems utilising high pressure CO2, including transportation pipelines and vessels, as well as cryogenic and cleaning devices. In the present study, a compressible flow Computational Fluid Dynamics (CFD) model is developed to predict the formation of dry ice during a transient decompression of CO2 pipelines. The model is based on the Homogeneous Equilibrium Mixture (HEM) assumption and utilizes an extended Peng Robinson equation of state to predict the physical properties of CO2 in vapour, liquid and solid states. To ensure hyperbolicity of the flow equations the frozen speed of sound model is applied to the solid liquid vapour mixtures at the thermodynamic triple point of CO2. The developed model is validated against the pressure and temperature measurements obtained in a full bore rupture test performed using a 144 m long 150 mm diameter pipeline, initially filled with dense phase CO2 at 153.3 bar and 5.25 oC. The results of simulation show that the total amount of dry ice found in the pipeline at the end of decompression process is ca 12 kg, which corresponds to 0.48% of the initial inventory of the pipe.