dynamic simulation of gas purging inliquefied CO₂ carrier tanks

Addressing the safety and economic challenges associated with the gas replacement process during the first loading test of the cargo system for the world’s first-purpose-built liquefied CO₂ carrier, a dynamic mass-transfer and thermalcoupled model was developed using Aspen HYSYS for two 3800 m³ Type C cargo tanks. The Peng-Robinson equation of state was employed to accurately describe the CO₂-N₂ vapor-liquid equilibrium. Integrated with a PID control strategy, the model maintained a stable tank pressure of 0.500 ± 0.005 barg. The simulation results reveal a three-stage characteristic of the purging process: a nitrogen-dominated period (0-4 h, with CO₂ concentration rising rapidly at 10%/h), a mixed transition period (4-20 h, where concentration progression lagged due to gravity-induced stratification), and a CO₂ saturation period (20-30 h, where purging efficiency decayed to 28% of the initial value). Utilizing the continuous purge method, a final CO₂ concentration of 95.3% was achieved within 30 hours, with a total nitrogen consumption of 69.91 tonnes. This study provides theoretical support and technical reference for the design and performance optimization of the tank gas purging process in LCO₂ carriers.