Numerical Analysis And Test Validation Of LNG Stratification And Roll-over In Storage Tanks

As a clean and efficient energy source,liquefied natural gas(LNG)has obvious advantages in terms of environmental protection and economical efficiency of energy utilization.China’s LNG demand and consumption have grown rapidly,at the same time,more and more LNG receiving stations have been built.As the core equipment of the LNG receiving station,the LNG storage tank is also the main equipment for LNG storage.When LNG is stored in a tank,it is prone to problems such as evaporation,stratification,and rollover due to aging or filling with new LNG.In severe cases,it can cause flammability and detonation.Therefore,the safe storage of LNG has become an important issue in the natural gas industrial chain.Based on the literature review at home and abroad,this dissertation analyzed the phenomena and mechanism of stratification and rolling in the LNG storage and transportation process.Applied the theory of heat and mass transfer and computer simulation to study the evolution rules and influencing factors of LNG stratification and rolling in storage tanks.The main contents are as follows:1、For the stratified 16×10~4m~3 LNG storage tank,selected two adjacent LNG layers with a height of 2m to establish a two-dimensional physical model and a mathematical model,and used the Fluent software to simulate the roll-over process of LNG.The simulation results show that the roll-over process can be defined into four stages:the roll-over stage at the tank wall,the roll-over transfer stage,the roll-over recovery stage,and the new stratification formation stage.2、Conducted numerical simulation of the roll-over process of the tank under different working conditions and finished comparative study of the impact of different factors on the roll-over.By setting different tank capacities,heat leakage locations,temperature differences,heat losses,and stratified heights,the LNG density distribution and mixing speed at different times in the tank were simulated.The start time,roll severity,and duration of LNG roll-over in the tank obtained can provide a certain reference for real-time monitoring and storage status determination of the tank in the project.3、A three-dimensional model of a 16×10~4m~3 storage tank was established,and dynamic grid technology was used to simulate the stratification of LNG in the storage tank during the filling process.The density distribution of LNG in the tank under three filling conditions was compared and analyzed.The corresponding relationship between the longitudinal density difference and each influencing factor was obtained,which provided a theoretical basis for the optimization of filling conditions.4、In response to the stratification phenomenon during the filling process of a petrochemical enterprise,a 3D physical model was established using the company’s 8×10~4m~3LNG tank as a prototype to simulate the filling process of the tank.The simulation results showed that LNG in the tank occurred stratification after the filling was completed.The simulation results are in good agreement with the actual data.Finally,the relationship between the filling rate,the density difference in the tank,and the pump work is compared and analyzed.It shows that when the filling rate is greater than 5m/s,the density difference is basically unchanged,but the pump work continues to increase.Therefore,when the unloading time permits,the preferable economical filling speed is 5 m/s.