Optimization of recirculation operating in liquefied natural gas receiving terminal

In a LNG (liquefied natural gas) receiving terminal, the recirculation flow rate which is an important process variable determines the temperature of the unloading pipe and affects the amount of BOG (boiled-off gas) flowing into the storage tank. Therefore the operation of the BOG compressor and HP compressor which handles the BOG in the storage tank affects the recirculation flow rate. The recirculation flow rate makes a trade-off between the operating costs of the BOG/HP (high pressure) compressor and the recirculation pump which sends the recirculation flow. This study was able to achieve the optimal recirculation flow rate taking into consideration the operating costs of the BOG/HP compressor and pump. Also the BOG handling method is different depending on the amount of send-out because the BOG is condensed by the send-out LNG. Therefore, it is proposed that optimized recirculation operations depend on the amount of LNG send-out. In order to predict the amount of BOG dependent on the recirculation flow rates, this study conducted a dynamic simulation of the unloading process in a LNG receiving terminal. A dynamic model that is able to reflect the influence of the valve operation was developed so that the change of the inner state that cannot be measured by sensors is predicted. Steady-state modeling and main pipeline modeling, branch pipeline modeling and boundary stream modeling were done using HYSYS Dynamics. Simulation of the developed dynamic model was done under the operating scenario based on normal operating procedures. Using the dynamic simulation, the operating costs of the BOG/HP compressor and pump were obtained through 29 recirculation flow rates. The optimal recirculation flow rate was obtained depending on the LNG sent-out rate. This study proposes optimized recirculation operations which control the recirculation flow rates and take into account the LNG send-out rate. The proposed method was compared to an existing method, and the cost-saving benefits were confirmed.