Modeling and numerical simulation of LNG behavior in a cryogenic tank

The evaporation process of Liquefied Natural Gas (LNG) in LNG vessel tanks is closely related to heat infiltration through the walls of the insulation and also to the sloshing of the liquid in the tank. Most of the models developed give predictions assuming a constant evaporation rate or BOR Design. Real evaporation depends on many more physical parameters because LNG is a multi-component mixture consisted essentially of methane, stored at its boiling point of about -161 deg. C at atmospheric pressure. A detailed zero-dimensional (0D) dynamic tool that enables one to evaluate the thermal transfers and thermodynamic properties in LNG carrier tanks has been developed. The model is used to investigate the variation of the LNG and BOG quantity, composition and thermodynamic properties during typical voyage profiles of a case study LNG carrier or Operational BOR concept. The principle of the operational Boil-Off Rate (Operational BOR in this paper) is different from the design BOR. The design BOR is a benchmark of the thermal performance of the insulation under standard environmental conditions in a stationary configuration, translated into daily evaporation rate for pure methane. Conversely, the Operational BOR which is an unsteady calculation of the thermal and thermodynamic state of LNG, is designed to measure the amount of boil-off gas produced during the navigation taking into account the actual environmental conditions. A correlations studies carried out in the model gave results with relative errors between 2% and 5% of natural BOG between observation and simulation. (author)