Accidental release of liquid CO₂ from transport and storage

Abstract

An unintended release of liquid CO2 during transport by tanks or pipelines could result in instantaneous depressurization and rapid phase transition with explosive evaporation. Under certain conditions, it could lead to a boiling liquid expansion vapor explosion (BLEVE). This research project is concerned with the safety of CO2 transport and storage facilities. It is a continuation of the prior research in the USN laboratory to investigate the rapid phase changes and the evolution of the evaporation and blast waves. Further, to evaluate the hazards that arise during the accidental release of liquid CO2 into the environment. This experimental research expanded former studies to cover the effect of a diverging cross-sectional geometry on phase transition mechanism and expansion wave behavior during sudden depressurization. It investigated the effect of liquid volume fraction on the dynamic characteristics of rarefaction, evaporation, and blast wave in the existing setup. In addition, the research examined the phase transition mechanism during the CO2 release from the bottom of a rectangular duct.

This research presents a new installation comprising a vertical high-pressure conicalshaped vessel with a double-membrane release process. A slip-on flange separates the two membranes, constituting a medium-pressure (MP) section. The experimental rig is instrumented with pressure transducers, temperature thermocouples, and a highspeed video camera. The vapor/liquid CO2 could be depressurized by increasing or decreasing the pressure in the MP flange. Operational analyses based on the experimental results showed that increasing the pressure in the MP section is a more reliable method to run CO2 depressurization tests on this installation. In contrast to decreasing the pressure in the MP section technique, its operation was more controllable, ensuring instantaneous membranes’ rupture without reflecting waves affecting the evaporation process in the test section.

The dynamic evaporation characteristics were investigated by increasing pressure in the MP section. The results showed that the evaporation wave velocity increased downward with decreasing cross-sectional area and increased liquid volume fraction. The divergent cross-section led to a more significant fluid expansion as the rarefaction wave propagated toward a smaller area. Simultaneously, the expanded two-phase mixture propagated toward a constantly increased cross-sectional area. In addition, the evaporation wave upstream state properties after the isentropic expansion in the metastable region were resolved by applying the Span-Wanger equation of state. Comparing the obtained results with former conclusions of CO2 decompression from a constant cross-section rectangular duct demonstrated considerable differences in rarefaction and evaporation wave velocities. The waves' propagation speed was steady in the rectangular duct, but their velocities increased as they propagated towards a diminished cross-sectional area in the conical-shaped vessel. Besides, the liquid in the conical vessel had attained a higher degree of superheating at the metastable state.

Blastwave effects during CO2 depressurization in the conical vessel have been studied experimentally. Overpressures and multiphase flow evolution have been recorded during CO2 release through a polycarbonate tube with atmospheric conditions. Analysis of measured peak overpressures and calculated impulse indicated their increase as the liquid volume fraction increased. The results also demonstrated that the rapid liquid evaporation has substantially influenced the peak overpressure and impulse positive phases and durations. Additionally, the study examined the flying fragments' velocities and their kinetic energies based on the captured high-speed videos. Results also showed increased fragments' velocity and kinetic energy with increased liquid volume fraction. Fragment velocity and kinetic energy of about 121 m/s and 205 J, respectively, have been observed from a test with a liquid volume fraction of 73.6%. Such fragments involve a significant hazard.

A series of experiments were performed to release saturated liquid and vapor CO2 below the liquid level in a rectangular duct. The study characterized the rarefaction and evaporation wave pattern and the evolution of the expanded two-phase flow inside the duct after the diaphragm ruptured at the bottom. Pressure and temperature records were analyzed simultaneously with high-speed shadowgraph images to gain insight into the phase transition mechanism. The results were also compared with previous toprelease experiments. During bottom-release tests, the liquid evaporated two times faster than in the top-release, but it attained a lower degree of superheating due to instant liquid outflow from the bottom. Shadowgraphs showed that the vapor expansion had an insignificant influence on the evaporation onset. The flow measurements and the visualized wave pattern demonstrated a unique evaporation mechanism during bottom-release experiments. This different fluid expansion behavior could potentially lead to a container’s catastrophic failure and must be considered in risk analysis to prevent such accidental hazards.