Research On Key Issues Of The BOG Reliquefaction On LNG Carrier

In recent years, with the development of economy, more and more energy is consumed all around the world.The liquefied nature gas maritime trade is also becoming more and more frequently.The LNG ships as the main means of transport at sea are required higher performance. Because of the special chemical and physical characters of LNG,it is easily vaporized after heated.So the LNG wil evaporate into BOG(Boil off Gas) when the LNG ship at sea. And the pressure of LNG tank will increase,it can wreck the structure of the LNG tank and represent a risk to the maintenance system inside the cargo vessel. The traditional approach is to open the safety valve. The BOG is released into the air,burnt in the air or put it into drive system. Such tradition method is not only a waste of energy but also give rise to safety problems and environment pollution. Therefore,from the LNG ship transport economy and safety considerations.LNG ships can load BOG reliquefaction system.Under the condition that overviewing and analysing the study and status of development of LNG reliquefaction system as well as LNG marine reliquefaction system both domestic and overseas,we propose the principle of choosing reliquefaction system.The key issues of LNG ship BOG reliquefaction system would be studied by taking the propylene precool mixed refrigerant system as object of research.According to equilibrium theory and the definition of enthalpy-entropy.SRK equation is used to calculate the equilibrium of LNG system which precooled by propylene.LKP state equation is used to calculate the flow of enthalpy, entropy .And according to the known conditions on reliquefaction system, we performed a thermodynamic analysis and finally we know K=0.11985 W / m~2·K,LNG evaporation rateω=0.402% and BOG reliquefaction system handing capacity Q_m=1218 kg/ h.The process of BOG reliquefied equipment in LNG ship is simulated by ASPEN PLUS process software.The process model was constructed by the equipments as fllow: compressor, condenser, throttle valve, condenser, Gas-liquid separator, heat exchanger, mixer/splitter.Through the simulation and calculation of thermodynamics of the BOG reliquefied equipments of the whole process.The node pressure,temperature,the enthalpy,the entropy,the flow gas-liquid phase,the total flow,mole fraction of gas-liquid phases, the power consumption of compressor,refrigeraty capality.refrigeratoin coefficient and other parameter of the process were received.At the end of this paper,anlysised the influence of the outlet pressure of the BOG compressor, the outlet temperature of seawater heat exchanger,the outlet pressure of mixing refrigerant compressor over the process parameters through single-variable simulating.Obtained the laws of the main factors of reliquefaction system.To optimize the reliquefaction system processer and other further important reference wowrk.