(599d) Dynamic Modelling of an LNG Storage Tank in a Regasification Terminal

Liquefied natural gas (LNG) is a convenient medium for storage and transport of natural gas, by virtue of its relative compactness. On the downside, LNG storage is a cryogenic process, which inevitably suffers from heat leak. This in turn generates boil-off gas, which is conventionally recycled to the storage tank via an energy-intensive re-liquefaction process. Generation of boil-off gas is particularly intensive at regasification terminals carrying large LNG inventories, especially due to the presence of recirculation flows which maintain cryogenic temperature in loading, unloading, and transfer lines. The present work introduces a comprehensive dynamic model for LNG storage tanks at regasification terminals, a semi-analytical solution to the model, and several case studies on minimization of boil-off gas generation¹.

The necessity for the present model arises from the observation that LNG storage tank modelling almost invariably assume thermal and mass equilibrium between the vapor- and liquid-phases, in contradiction with measured temperature data from our industrial partner. A case in point is the influential work of Miana et al. and , wherein the vapor- and liquid-phases are treated as a flashing mixture at each time step². This approximation is expected to be appropriate for marine cargo tanks, wherein surface wetting results in greatly increase heat- and mass-transfer area, but not for the relatively still vapor-liquid interface in land-based LNG storage tanks. A second concern relevant to the aforementioned work concerns the use of an arbitrary, constant boil-off rate, which is expected to vary circumstantially. Others works e.g. Migliore et al. have eliminated the use of constant boil-off rate³, but retain the equilibrium assumption. Note also that both the temperature and the mass flow rate of the boil-off gas are needed to accurately estimate the compressor duty of the re-liquefaction process.

To date, we were only able to identify two studies which incorporate heat leak arising from recirculation flows. The first study, published by Querol et al., is a simple but comprehensive attempt at estimating heat leak into LNG storage tanks⁴. The steady-state model presented therein incorporates heat leaks from various heat-transfer areas and dissipation of shaft work, and forms an excellent framework for a more detailed dynamic model. The second study, published by Park et al., incorporates the same features, albeit using a dynamic process simulator⁵. Both studies appear to capture the main features of LNG storage tank modelling in regasification terminals, but lack the details on the heat- and mass-transfer processes, and thus possess limited predictive value.

The present study consolidates the best features of the aforementioned studies in the form of a single model which accounts for the sources of heat leak indicated by Querol et al. and the heat-transfer mechanics of Migliore et al., while also accounting for vapor-liquid disequilibrium via vapor-liquid interface heat transfer and the dynamic effects introduced by changing liquid level. The model possesses a neat partial solution in the form of a set of algebraic equations for the special case of nitrogen-free LNG. The model is subsequently used to study minimization of boil-off generation as a function of tank aspect ratio and recirculation line properties for a representative case study. The insights gleaned from this model should prove valuable in future designs of LNG regasification terminals.

References

1. Effendy S, Khan MS, Farooq S, Karimi IA. Dynamic Modelling and Optimization of an LNG Storage Tank in a Regasification Terminal with Semi-Analytical Solutions for N 2-Free LNG. Computers & Chemical Engineering. 2017.

2. Miana M, Del Hoyo R, Rodrigálvarez V, Valdés JR, Llorens R. Calculation Models for Prediction of Liquefied Natural Gas (LNG) Ageing During Ship Transportation. Applied energy. 2010;87(5):1687-1700.

3. Migliore C, Tubilleja C, Vesovic V. Weathering Prediction Model for Stored Liquefied Natural Gas (LNG). J. Nat. Gas. Sci. Eng. 2015;26:570-580.

4. Querol E, Gonzalez-Regueral B, García-Torrent J, García-Martínez M. Boil Off Gas (BOG) Management in Spanish Liquid Natural Gas (LNG) Terminals. Applied energy. 2010;87(11):3384-3392.

5. Park C, Lee C-J, Lim Y, Lee S, Han C. Optimization of Recirculation Operating in Liquefied Natural Gas Receiving Terminal. Journal of the Taiwan Institute of Chemical Engineers. 2010;41(4):482-491.