Research On The Design Of Dynamic Process Of BOG Multi-stage Compression And Recondensation At LNG Receiving Terminal

As the rapid development of LNG (Liquefied Natural Gas) industry in China, multipleLNG receiving terminals are under construction and expansion and its scale is becominglarger and larger. BOG (Boil off Gas) processing system is the most complicated and crucialsection in the terminal, its stability account for the whole terminal’s safety. Influenced byvarious factors, such as ship unloading, heat leakage and the asymmetry of downstream gasconsuming, the BOG production and LNG output load fluctuate from time to time. Thisfluctuation largely increases the difficulty of the design and operation of BOG processingsystem. Furthermore, there exist operational problems, such as high energy consumption,instability of the recondenser and inflexible, in present BOG processing system. On the basisof making full use of original system, this research designed and optimized the multi-stageBOG processing system for the terminals’ expansion to improve its performance.The mathematic model for calculating BOG production was established according to itsinfluencing factors to research the fluctuation. Thermodynamic analysis was performed forthe system to find out optimization opportunities. Then the multi-stage system was proposedfor system energy consumption is most heavily affected by recondenser’s operating pressure.Dynamic simulation model was established for original and multi-stage system. Under thepremise of high stability and sufficient safety margin, the minimization of system energyconsumption was chosen as the objective function to optimize the operating and controlparameters. The result shows that the second stage recondenser’s pressure is kept at885kPa,the first stage recondenser’s pressure is set at305kPa and the second stage compressor’sposition is set at25%. And the PID control system is employed. Compared with originalsystem,1247kW (19.7%) of system energy consumption was reduced. About308.5kWh ofpower could be saved every year, and this could bring2.363million yuan of economic benefit.Through one typical day’s operation, the recondensers’ level changes within the best range(50%-60%) and its pressure change range less than the safety margin (200kPa) of highpressure pump. Moreover, these two parameters could recover fast. Multi-stage systemdecreases the system energy consumption as well as improves system dynamic performance.