Prediction of boil off rate in liquefied natural gas storage processes

Abstract

Liquefied Natural Gas (LNG) is stored in cryogenic tanks at receiving terminals. During the LNG storage some part of LNG evaporates into gas phase. Evaporated LNG is called Boil off Gas (BOG). Heat flow has an influence on evaporation process. It indicates continuous boil-off of small fraction of LNG due to heat flow from the ambient air through tank insulation. Vaporization process causes changes in the composition of stored LNG. 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 representing different sources of LNG. Obtained results show that Boil Off Rate is higher for LNG 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 (higher possibility of danger roll-over phenomenon).