MATHEMATICAL MODEL OF LIQUEFIED NATURAL GAS EVAPORATION AND ANALYSIS OF ORIGINAL COMPOSITION EFFECT ON EVAPORATION SPEED

Subject of Research. The paper proposes a model for predicting evaporation of liquefied natural gas (LNG) stored in tanks of regasification terminals. The model is a combination of the strict thermodynamic LNG vapor-liquid equilibrium and heat transfer realistic models. Analysis of the initial composition effect and the stock of LNG, the initial content of nitrogen and the value of the outside air temperature on the LNG evaporation rate is carried out. Method. A numerical experiment using a mathematical model of the LNG evaporation process was based on the LNG composition data and its storage time in aboveground tanks. The proposed method provided a number of advantages compared to previously developed models: the heat input to the low-temperature part of the LNG storage tank was calculated taking into account the outdoor temperature and the LNG composition; the evaporation coefficient was not an input parameter, but was calculated as part of the simulation; the LNG density was calculated by experimental correlation. Main Results. An important parameter in assessing the LNG loss from its evaporation is the nitrogen content in the liquid, which evaporates at a higher rate than methane, the main LNG component. Analysis of the initial composition effect on the LNG evaporation rate has shown that the growth of the nitrogen content in the initial mixture causes noticeable decrease in the evaporation rate due to the nitrogen priority evaporation and, as a consequence, the growth of latent mixture heat vaporization. Analysis of the initial LNG supply value at the evaporation rate has shown that a large degree of the tank filling causes an outstripping reduction in losses from evaporation. The result is an earlier reduction in the amount of boil-off gas (BOG, stripping gas) for more filled tank. Analysis of the outdoor temperature effect on the rate of LNG evaporation has shown that the ambient temperature change of 1 °C leads to the decrease in the BOG amount by 0.2 %. Practical Relevance. The proposed mathematical model of the LNG evaporation process reduces BOG formation during long-term storage, maintain the original LNG quality, optimizes the operational parameters of regasification terminals, and provides the consumers with reliable supply of high-quality natural gas.