Modelling choked flow for CO2 from the dense phase to below the triple point

The modelling of the choked flow for CO2 undergoing expansion from the dense phase state to below its triple point is presented. The choked flow properties, including mass flux, fluid phase composition, pressure, speed of sound and solids mass fraction are determined for the range of stagnation conditions relevant to the pipeline transportation of CO2 following its accidental failure. The data indicate that the decompression of the dense phase CO2 results in the stagnation of the flow at its triple point. At pressures below the triple point, the solid/vapour mixture is shown to be released at a mass flux of ca. 2000 kg/(m2s). For a 50 mm diameter puncture in a pipeline transporting CO2 at its critical point (304.2 K, 73.83 bar), the above translates into a discharge rate of ca. 1 kg/s of solid CO2. Under such circumstances, in the case of buried pipelines, the accumulation of the slow subliming solid CO2 within the resulting crater may significantly modify the subsequent dispersion behaviour of the migrating CO2 cloud. As such, the phase transition behaviour of CO2 around its triple point must be accounted for when performing safety assessment of CO2 pipelines being considered as part of the carbon capture and sequestration chain.