Measurements of boil-off gas and stratification in cryogenic liquid nitrogen with implications for the storage and transport of liquefied natural gas

• Published in: Energy (Oxford) 2021-05, Vol.222, p.119853, Article 119853
• Main Authors: Perez, Fernando, Al Ghafri, Saif Z.S., Gallagher, Liam, Siahvashi, Arman, Ryu, Yonghee, Kim, Sungwoo, Kim, Sung Gyu, Johns, Michael L., May, Eric F.
• Format: Article
• Language: English
• Subjects: BOG Modelling; Boil-off gas; Cryogenic storage; Dynamic models; Heat flux; Heat transfer; Liquefied natural gas; Liquid nitrogen; Natural gas; New BOG Apparatus; Nitrogen; Predictions; Pressurization; Steady state; Storage tanks; Thermal stratification
• ISSN: 0360-5442; 1873-6785
• DOI: 10.1016/j.energy.2021.119853

The boil-off gas (BOG) produced from liquefied natural gas (LNG) mixtures in cryogenic storage tanks must be predicted reliably as a function of tank shape, heat ingress, thermal stratification, pressure, and liquid volume fraction. However, current methods of estimating BOG rates for large-scale tanks are entirely empirical and based on limited available data, with no models available for reliable predictions. This affects the ability of LNG carriers to optimise BOG compressor sizing. A new apparatus was developed to explore the effects of heat flux, liquid stratification, volume, and mixture composition on the measured boil-off rate. The apparatus is demonstrated using liquid nitrogen with BOG rates quantified as a function of various heat fluxes, pressures, and initial liquid volume fractions. Three distinct periods of boil-off were observed: the pressurisation, transient, and steady-state stages. The data are compared with the available literature and the predictions of a new dynamic model accounting for heat transfer from the super-heated vapour. Excellent agreement is observed between model predictions and the data measured during the pressurisation and steady-state stages. However, the model does not capture the BOG rate observed in the transient stage, suggesting liquid thermal stratification should be considered in future models for LNG boil-off. •Unique apparatus providing comprehensive sets of data for cryogenic fluid boil-off.•Boil-off-gas (BOG) measurements of LN2 conducted for various conditions.•In-depth analysis of pressure build-up, thermal stratification and BOG rates.•Three distinct periods of BOG: pressurisation, transient, and steady-state stages.•A new robust model to predict the dynamic weathering of cryogenic fluids.