Analysis of boil-off rate problem in Liquefied Natural Gas (LNG) receiving terminals.

Liquefied natural gas (LNG) has an increasingly important role in the global natural gas market. Global demand for natural gas will grow over the coming years. LNG is transported by ships to unloading points at the storage terminals. During the LNG storage processes some part of LNG evaporates into gas phase. Evaporated LNG is called Boil-off gas (BOG). LNG is stored at cryogenic temperatures. Heat flow has an impact on evaporation process. It indicates there is continuous boil-off of small fraction or portion of LNG due to warming during storage process. This boil-off gas is generated primarily due to heat flow from the ambient air through tank insulation, unloading and recirculation-line insulation. Vaporization process causes changes in the composition of stored Liquefied Natural Gas. Increased vaporization process may negatively affect the stability and safety of the LNG storage process. Rate of vaporization (boil off rate) should be precisely determined. For these reasons different calculation models to determine the LNG boil-off rate are shown in this paper, also there are presented some boil off rate calculation results for different Liquefied Natural Gas compositions. Obtained results show that Boil-off rate is higher for LNG composition which contains nitrogen. Due to lower bubble temperature nitrogen evaporates first from the LNG, it causes significant LNG density drop in surface layer in storage tank. Difference of densities in surface and bottom layer of stored LNG may cause the stratification process and consequently affect the stability of storage process (possibility of roll-over phenomenon).