In this Ph.D. research study, a non-equilibrium thermodynamic model and a non-equilibrium multilayer thermodynamic model are developed to assess the thermal performance of liquefied natural gas (LNG) storage tanks by reducing the computation time compared with computational fluid dynamics (CFD) methods.

The non-equilibrium thermodynamic model incorporates a resistance-capacitance network to evaluate the thermal performance of vertical and horizontal LNG storage tanks. This model is validated against two sets of experimental data, and compared with the equilibrium thermodynamic model. The results indicate that, under stationary conditions, the equilibrium and non-equilibrium thermodynamic models predict the pressure and temperature profiles in the vertical and horizontal LNG storage tanks with good accuracy. However, the non-equilibrium thermodynamic model is the only model that can predict the thermal performance of the storage tanks under dynamic operating conditions such as sudden pressure changes due to the vapor return to a tank or LNG dispensing to heavy-duty trucks. Also, the results highlight that the LNG holding time in horizontal storage tanks is longer than that in vertical storage tanks under dynamic operating conditions.

The non-equilibrium multilayer thermodynamic model is developed to study the thermal stratification and rollover in LNG storage tanks. This model divides the liquid and vapor domains in the tanks into multi-layers, considers the boundary layer formation along the tank vertical walls and, uses adaptive mesh to accommodate the changes in the LNG level over time. This model is verified against experimental data available in the literature to predict the thermal stratification and rollover in cryogenic storage tanks. The parametric study indicates that in atmospheric LNG storage tanks, the amount of heat transfer to the tank and the tank aspect ratio (i.e., diameter to height ratio) do not affect the rollover start time. However, they affect the LNG evaporation rate. The rollover start time is directly affected by the ratio between the amount of fresh LNG loaded to the tank (cargo) and the amount of LNG left in the tank (heel) before loading the cargo. This work suggests that the cargo to heel ratio should be monitored regularly to prevent the rollover in LNG storage tanks.