LNG Storage Tank And Dynamic Analysis For Its Boil Off Gas Space

LNG storage tank is essential for liquefied gas to transport and store the LNG.LNG needs low temperature when it shores and the temperature of storage is about-162℃. Due to the storage tank can’t be fully adiabatic and the heat from outside could transfer to the inner tank during the storage of LNG, which leading inner tank pressure increases continuously. In this paper, take a 200 L LNG storage tank as a research object, analyze evaporation of LNG liquid and use Visual C++ to simulate the dynamic model of LNG storage tank’s vapor space based on the four-zone model.As the liquid of LNG vaporizes, the composition and physical characteristic of LNG vapor and liquid have changed. Through the analyses of the liquid’s vaporizing, the conclusion is that the evaporation of liquid occurred at and near surface and space of boil off gas with liquid surface keeps equilibrium. In this paper, SRK equation has been used to describe gas and liquid equilibrium state. LKP equation has been used to calculate the density and enthalpy and achieve its process through programming language. To LNG properties, high precious calculation expressions has been matched.25-point two-dimension-al search method has been selected to solve the parameters of hypothermal gas.When the state parameters of boil off gas was calculated, if(P,T) was to be calculated(ρ,h) must be known. The problems can’t be solved only by simple iterative algorithm method. However, 25-point method can solve this problem. In this paper, 25-point method was used to calculate state parameters.In order to validate the correction of the four-zone model, firstly choose a 15 L LNG storage tank to have an experiment on different filling ratio. The times of the experiment is four and get the data of pressure of time in gas-phase and time of storage. The data from the experiment and from the calculation has been compared. The results indicated that data from model and data from test were matched well. The conclusion is that the model in this paper is basically correct.At last, four-zone model was used to simulate dynamic analysis for its boil off gas space. Through the simulation results we concludes that with the increasing of pressure in boil off gas space storage time becomes longer; The temperature in boil off gas space is higher that that in liquefied space, but there is not make difference between them. When the filling ratio is about 75%, the storage tank has the longest safety storage time. The original pressure is higher, when the late period speed of pressure increasing becomes slower; When the LNG storage tanks has the same filling ratio and heat loss, different sizes tanks has the similar storage regular. Storage time is in direct proportion to the inner diameter. When the methane add some heavy components, the speed of pressure increasing reduces, safety storage time becomes longer, when adds some light components can weaken the influence of heavy components. The heat loss of storage tank has the most important impact towards the pressure of boil off gas. These storage regular of LNG tank have guiding meaning to the storage of LNG tank.