A non-equilibrium approach to modelling the weathering of stored Liquefied Natural Gas (LNG)

Abstract

A model is proposed to predict the weathering of LNG stored in containment tanks . It dispenses with a standard approximation where the temperature of the generated vapour within the tank is assumed to be the same as that of the stored LNG. Instead , it treats the heat influx from the surroundings into the vapour and liquid phases separately and allows for the heat transfer between the two phases. The model was validated only by comparing with the compositional data, as no reliable measurements of vapour temperature are available. The simulation results indicate that the temperature of the vapour phase will be higher than that of the LNG, by approximately 8 0 C over a period of one year, providing the heat transfer from the vapour is by conduction only; thus supporting circumstantial industrial findings. The effect on the Boil- off Gas (BOG) is considerable and the results indicate that the BOG rate will decrease by as much as 25% for particular scenarios. This has important consequence s for weathering models used in industry , which currently assume isothermal conditions within the containment tanks . In the initial stages of weathering , the nitrogen content of LNG will have a marked effect on the rate of BOG generation. The lowest BOG rate is observed when the LNG contains approximately 1.4- 1.5% of nitrogen.