Modeling And Dynamic Optimization Of Recondensation Process At LNG Receiving Terminals

Liquefied natural gas (LNG) has become an increasingly important global energy and itsannual world trade growth speed reaches nearly12%. These years, China’s LNG terminals inthe coastal areas are flourishing. Due to its cryogenic-temperature feature, LNG isunavoidably vaporized in terminals generating boiled-off gas (BOG) in LNG storage tanks,equipment and liquid-filled pipe lines. BOG handling system is one of the most importantfacilities at liquefied natural gas storing and receiving terminals, because its operation needsfor sufficient care under various situations to avoid failure may cause BOG loss and/or severeaccidents. BOG recondensation system has been widely used to liquefy and recover BOG atLNG receiving terminals saving30%-60%energy than compressing BOG directly todistribution pipeline system. However, the typical experiencing operation problems atrecondensation systems are BOG recondenser level instability, high energy consumption andoperational difficulties when BOG load and natural gas pipeline network load fluctuateslargely. This study focuses on typical BOG recondensing process in order to giveenergy-saving chance and guide the receiving terminal optimal operation. This research mayprovide guidance for LNG terminal practical operation and be expected to be a reference todesign improvement for other BOG recondensation processes.Pro-II is an excellent tool for study LNG process, with the choice of SRK equation tocalculate phase equilibrium and LKP equation (Lee-Kesler-Ploker) to calculate enthalpy andentropy. BOG multi-stage compression and condensation process model is established basedon Pro-II simulation. After in-depth analysis and research of process parameters optimization,energy-saving potential and operating flexibility improvement of BOG multi-stagerecondensation process, two-stage compression and condensation process is suggest to handleBOG in general LNG receiving terminal.It obviously saves energy by more than30%withmoderate investment and its BOG load capacity is improved by11.9%. In view of highenergy consumption and poor flexibility in boil-off gas (BOG) recondensation operation, anoptimization only of the control logic to BOG recondensation process, without any processchange, has been made. The optimized control system maintains BOG recondenser pressurevia the condensing LNG flow and liquid level via bypass LNG flow, which is a simpler andmore effective system. Dynamic simulation is employed to examine optimized system’soperation reliability and energy minimization. The result shows that optimized control systemhas advantage on maintaining liquid level within recondenser. BOG compressors energy isreduced by24％, total process energy decreases84kW, and the operation stability is wellimproved.