Thermal stratification and rollover phenomena in liquefied natural gas tanks

Abstract

A non-equilibrium multilayer thermodynamic model is developed to predict the thermal stratification and rollover phenomena in liquefied natural gas (LNG) storage tanks. This model considers the boundary layer formation along the tank walls and uses an adaptive mesh to accommodate the changes in the LNG level in the tank over time. The accuracy of the model is verified against experiment data available in the literature to predict the thermal stratification and rollover in cryogenic storage tanks. A parametric study is conducted to investigate the key factors affecting the rollover start time. The results indicate that in an LNG storage tank at atmospheric pressure, the ratio between the amount of fresh LNG loaded to the tank (cargo) and the amount of LNG left in the tank (heel) prior to the loading of cargo, and the methane concentration in the cargo layer have direct effects on the rollover start time. The amount of heat transfer to the tank, the tank aspect ratio, and the nitrogen concentration in the heel or cargo layers affect the LNG evaporation rate. Our results indicate that the cargo to heel ratio should be determined with caution to prevent the rollover in LNG storage tanks.