A case study of processing and optimization of BOG gas treatment in an LNG terminal in Qingdao

The BOG (Boil-Off Gas) processing system is one of the key issues relating directly to energy consumption and safe & steady operation in an LNG terminal. Therefore, we introduced four processing systems for BOG treatment: direct condensation, re-condensation, indirect heat exchange reliquefaction, and cooling storage reliquefaction. On this basis, we applied the HYSYS to build different LNG terminal models, simulated and compared the commonly used re-condensation process and the direct condensation in terms of process and energy consumption. And we also analyzed the impacts of the flow rate, pressure of send-out NG and re-condenser pressure on the energy-saving results of BOG processing. Therefore, we recommended a further innovative measure for the re-condensation BOG process, i. e., to precool the BOG before it enters the re-condenser, which can save the energy consumption by 18.2%. Meanwhile, we also presented an optimal operating scheme of mixing the above two processes for an LNG terminal in Qingdao. This can keep the LNG flow rate steadily entering the re-condenser, and the uncondensed BOG can be compressed to mix with the liquefied LNG, thus the BOG will be saved instead of entering the flare venting system, meanwhile, the flow rate fluctuation at the inlet of the re-condenser will be reduced to make the whole BOG processing more stable and cost-effective as a result.