Estimation method for dry ice formation under triple point through tank depressurization

• Efficient CO2 transport in high-density using low-pressure tank near triple point. • Preventing dry ice formation to avoid equipment damage and ensure safety. • Equations predict phase changes from heat input and CO2 release rates. • Tests validated the theory by visualizing phase changes during depressurization. • Supports efficient CO2 ships, including safety systems for carbon neutrality. To achieve a carbon–neutral society, efficient CO 2 transportation is necessary. Ship transportation is a potential candidate for long-distance transportation, wherein storing CO 2 in a high-density, low-pressure state in tanks is desirable. Storing liquid CO 2 near the triple point is regarded as an effective approach. However, the solidification of the liquid when the tank pressure reduces for any reason is cited as a risk. Dry ice formation should be prevented because it can hinder CO 2 unloading, cause pipeline blockages, and damage equipment. To establish high-performance, efficient CO 2 transportation ships that store CO 2 at or near the triple point, understanding the phase changes within tanks during depressurization is crucial. This study first derived theoretical equations that can be applied straightforwardly in practical situations, based on mass, volume, and energy conservation laws to calculate the solid and liquid phase changes caused by heat input and CO 2 release outside the tank. To verify these equations, experiments were conducted to visualize the solid and liquid phase changes inside a tank at a triple point by venting CO 2 gas at a constant flow rate. The experiments were performed in a vacuum chamber under controlled heat input conditions. The validity and effectiveness of the theory were demonstrated by comparing the derived phase-change prediction equations with the experimental results.