Development of a simulation model of a LNG Tank integrating interfacial mass transfer through CFD

Abstract

Liquefied Natural Gas (LNG) is stored in a LNG fueled vessel which contains fuel at cryogenic temperatures (-160 $^\circ C$). The tank holds the fuel in both gas and liquid phases under various pressures, maintaining a pseudo-steady state. However, constant heat intake from the walls cause the LNG to evaporate, forming boil-off gas which increases the pressure of the tank. This process is amplified during the fueling process and the dynamic condition hinders the prediction of quantifying the amount of evaporation. As the evaporation of LNG or the condensation of boil-off gas is directly related to multiphase and multicomponent systems, it is essential to understand the mass and heat transfer theories along with the thermodynamics that occur between the two phases in order to optimize tank designs or operating conditions. This paper studies the guideline of designs and operating conditions of an LNG tank through 3D CFD process simulation. The modeling of evaporation and condensation include setting interfacial transport, mass transfer correlations and vapor-liquid equilibria across the surface. A fueling process of a 2-dimensional simplified tank is simulated to observe the boil-off effects. This is achieved by analyzing the mass transfer rates, mass and heat transfer coefficients across the liquid-gas interface. A 3-dimensional simplified tank is also designed to observe the effects of velocity of the surface on boil-off gas formation, and the mass transfer rates are observed with respect to time and location. For comparison, the fueling process is simulated with varying the flowrate of the inlet and the mass transfer rates are analyzed.