Estimating Boil-Off Gas Generation in Industrial Cryogenic Storage Tank during Unloading Operations Using CFD Models

Abstract

Unloading liquefied natural gas (LNG) from ships is a critical operation carried out at receiving terminals that act as a bridge between LNG exporters and end users. During unloading, a significant amount of boil-off gas (BOG) is generated, which if not managed effectively may compromise tank integrity and process safety. Computational fluid dynamics (CFD) models are commonly used to study the dynamics of BOG generation in LNG tanks. Existing models often rely on oversimplified geometries which restrict their ability to accurately represent key phenomena during unloading. In this work, we propose a unified CFD model that can be used to study top- and bottom-unloading as well as holding mode operation. The proposed axisymmetric 2D model uses a concentric inlet–outlet configuration which enables realistic 3D representation with high computational efficiency. Using this model, we compare the BOG generation during unloading against a baseline holding-mode operation. Our results demonstrate that top-unloading leads to a significant (up to 18×) increase in BOG generation due to flashing of the incoming LNG. Further, initial tank pressure is identified as the dominant controlling parameter; in contrast, the initial LNG level and density exhibit only a weak influence on the boil-off rate (BOR). A correlation has been established between tank pressure and BOR, with an 8.3% decrease in BOR for every 10 kPa increase in tank pressure. This correlation can serve as a practical tool for estimating BOR under various unloading conditions and provides recommendations to operators for managing BOG effectively, thereby minimizing flaring and energy consumption.