Weathering prediction model for stored liquefied natural gas (LNG)

• A new model to predict the weathering of LNG in storage tanks is developed.
• The model was validated using real industrial data.
• The presence of N2 in LNG has a marked effect on the Boil-Off Gas generation.
• BOG, expressed in mass terms, will exhibit a minimum at around 1.5% nitrogen.
• 1 °C change in the ambient temperature will result in BOG change of 0.2%.

A model is proposed to predict the weathering of LNG stored in containment tanks, typically used in regasification terminals, due to the effects of heat ingress and Boil-off-Gas (BOG) release. The model integrates a rigorous thermodynamic model of LNG vapour–liquid equilibrium and a realistic heat transfer model. It provides a number of advances on previously developed models, in so far as: (i) heat ingress is calculated based on the outside temperature and LNG composition, that allows for daily or seasonal variation; (ii) Boil-off-Ratio is not an input parameter, but is calculated as part of the simulations and (iii) the LNG density is estimated using an accurate experimentally based correlation.The model was validated using real industry data and the agreement obtained in predicting the overall composition of weathered LNG, its density and the amount vaporized was within current industry requirements. The model was run in the predictive mode to explore the sensitivity of BOG to different scenarios. In the initial stages of weathering the nitrogen content of LNG will have a marked effect on BOG generation. Even the presence of 0.5% of nitrogen will lead to nearly a 7% decrease in BOG, making the initial BOG unmarketable. The high sensitivity is a result of preferential evaporation of nitrogen and increase in the direct differential molar latent heat. In the final stages of weathering the heavier hydrocarbons govern the dynamics of BOG which becomes a strong function of the initial composition and the level of LNG remaining in the storage tank.The change in ambient temperature of 1 °C will lead to a change in BOG of 0.2%, irrespective of the size of the tank and initial LNG composition.

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