Prediction model of LNG weathering using net mass and heat transfer

In an liquefied natural gas (LNG) storage tank, the composition of LNG changes with time owing to the boil-off gas (BOG) generation, and it is a crucial characteristic in designing a BOG management system. Until recently, the vapor phase temperature and the heat exchange at the interface have been utilized to predict the non-equilibrium thermodynamic behavior of LNG and BOG. However, the measured vapor phase temperature cannot represent the entire vapor phase owing to thermal stratification, and it is difficult to quantify the heat transfer at the interface rigorously in a large-scale tank. Thus, this paper proposes a prediction model of LNG weathering, which excludes vapor phase modeling. To facilitate this approach, the net mass transfer was calculated using the statistical rate theory, which converts a temperature term into pressure, and the net heat transfer at the interface was also applied. To verify the proposed model, we performed a long-term (50 days) experiment in a large-scale tank (50 m3) containing nitrogen and hydrocarbons, under isobaric operation. The simulation results showed good agreement with the measured data. In addition, the applicable range for the net expression equations was studied by measuring the temperature difference between the two phases.