Computational and experimental study of solid phase formation during the decompression of high-pressure CO2 pipelines

Decompression of CO2 pipelines is studied both experimentally and numerically to provide a partially validated model as the basis for the prediction of the hazards associated with CO2 solid formation. The pipeline decompression experiments, performed using a fully instrumented 36.7 m long and 50 mm internal diameter test pipe up to a maximum pressure of 45 bar, incorporating discharge orifice diameters of 4 and 6 mm, reveal the stabilisation of pressure and temperature near the CO2 triple point. Also, video recordings of the decompression flow in the reinforced transparent section of the steel pipe show that initial stratification of the constituent liquid and vapour phases is followed by rapid CO2 solid formation and accumulation in the pipe. To aid the prediction of hazards associated with solids formation in pipelines, a homogeneous equilibrium pipeline decompression model is developed accounting for the pertinent physical properties of CO2 in the liquid, vapour and solid states. The model is validated against the experimental data, showing ability to accurately predict the measured pressure and temperature variations with time along the pipe, as well as the time and amount of the solid CO2 formed upon decompression across the triple point.