Experiments and modelling of CO₂ vessel depressurization: Flash boiling, heat transfer and critical flow through nozzles

Transport of CO₂ in pressurized vessels has become increasingly important in carbon capture and storage systems. Several operational scenarios require depressurization of these vessels, which poses safety concerns: dry ice can clog pipes and valves, and low temperatures can cause steel embrittlement. This work presents a new, densely instrumented experimental facility for studying CO₂ vessel depressurization. To our knowledge, it is the first to measure the choke pressure and mass flux simultaneously, together with vessel and wall temperatures, for detailed model validation. Nine experiments are reported, with nominal initial conditions of 120 bar and temperatures of 15 and 25 ◦C, with and without a riser tube that draws liquid instead of gas, and with outlet nozzle diameters of 8.0, 6.5 and 4.5 mm. Significant dry-ice formation was observed in the experiments without a riser. We also observed high heat transfer in the part of the vessel wall wetted by liquid CO₂. A model accounting for critical flow through the nozzle, flash boiling inside the vessel and wall-to-fluid heat transfer is presented and compared with the data. Good agreement was obtained: the mass of dry ice was predicted to within 2 %, and without a riser tube the predicted mass flux deviated by 3.9 %. With a riser tube, the deviation grew from 9 % for the largest nozzle to 24 % for the smallest, which we attribute to a larger degree of non-equilibrium in small nozzles. Together, the data and the validated model support the safe design and operation of CO₂-transport systems.