Improving design and operation at LNG regasification terminals through a corrected storage tank model

Abstract

The simulation of liquefied natural gas (LNG) storage tanks is often based on several problematic assumptions, for instance, estimation of boil-off gas (BOG) generation using boil-off rate, vapor-liquid equilibrium in the tank, static liquid level, the use of only lateral area for heat loss calculations, and etcetera. Some of these assumptions are built into selected commercial simulators, creating further challenges in simulating the behavior of LNG tanks. The present study highlights these challenges in the context of a commonly used process simulator, Aspen HYSYS, and provides analytical and intuitive solutions to those problems. The resultant model is validated against an established first-principle model and then exploited for finding improved LNG regasification terminal design and operation strategies. Tank aspect ratio (AR) was studied in relation to plant capacity, recirculation rate, and recirculation line length. An aspect ratio of 1 consistently results in minimum BOG generation, in contrast to the value of 0.5 frequently cited in the literature. During the planned/unplanned shutdown of regasification terminal, higher liquid level in the tank decreases evaporation and thus BOG generation. Minimum recirculation rate that prevents 2-phase flow is found to minimize BOG generation and compressor duty.