The Research In The Pre-cooling Of Large LNG Tank

The large-scale LNG tank is not only a core equipment in LNG receiving terminal,but also a main equipment to store LNG.Before the tank being put into production,the LNG storage tank cooling-down operation is the most significant and difficult link in the debugging work of LNG tank.The successful completion of pre-cooling process takes an important part in safety and efficiency of station operation.Take the16×10~4m~3 LNG storage tank as a study object in this paper,it studies pre-cooling parameters and the variation of physical field during the pre-cooling process by combining the mathematical theory analysis with numerical simulation methods.The main contents and conclusions are as follows,Analyze the thermodynamics during the LNG tank pre-cooling process,and build mathematical model.It studies pre-cooling parameters during the pre-cooling process,and conclusions are as follows:(1)As the precooling rates increase,the precooling time will be shorten and the amount of pre-cooling medium will reduce.And the optimum cooling rate is4.9K/h,corresponding to precooling time 39h and pre-cooling medium LNG 609.11t.The gas in tank,9%Ni steel,aluminum ceiling,insulation layer,heat leakage of precooling medium dosage is respective 20%,31%,2.6%,38.6%and 7.8%.(2)The quantity of pre-cooling medium that tank required for heat leakage is about 5%to 12%of the total amout.What's more,the maximum heat leakage is at the bottom of the tank.So the bottom of the tank should be the insulation target.(3)Calculate the pre-cooling parameters of different volumes of tank and find that the dosage of Liquid Nitrogen is twice of the LNG,but the cost of Liquid Nitrogen is half of the LNG.Therefore,it is more cost savings for LNG storage tank to precool with Liquid Nitrogen.By analyzing the insulation properties of tank insulating layer,conclusions are as follows,(1)The insulating layer needs the most amount of the pre-cooling medium.The mass of the insulating layer is one of reasons to affect it cool down.(2)Considering both cold loss rate and the preventing the LNG from vaporizing The thickness of tank insulating layer varying from 0.38m to 0.66m,the loss rate of heat preservation and evaporation loss rate decreased by only 1.5%and 0.0048%.Therefore,Decreasing the thickness of tank insulating layer not only can decrease the consumption of pre-cooling medium,and reduce the cost of pre-cooling process,but also can reduce the cost of tank construction.LNG storage tank pre-cooling can be divided into the low temperature gas pre-cooling and cryogenic liquid pre-cooling.Take a numerical simulation of the LNG storage tank cryogenic gas pre-cooling,establish a 16×10~4m~3 LNG tank model,then simulate gas pre-cooling of LNG storage tank by FLUENT software.According to the simulation results,the reference flow rate of inlet BOG at different temperatures and regularity of temperature field were obtained.The maximum temperature gradient is on the tank wall,the maximum temperature drop is located at the tank wall where close to the bottom of the tank,and the gas temperature drop inside the tank is greater than the temperature drop of inner tank.When taking a numerical simulation of the LNG storage tank cryogenic liquid pre-cooling,for the question of phase change about LNG pre-cooling,analys the theory of phase change,edit the UDF program.According to the simulation results,LNG went into the tank by sprays,it can't fully cover the entire tank after vaporizing.The minimum temperature and pressure are in LNG inlets.The direction of gas flow related to the inlets flow and the exit position.It is obvious that the fluid disturbance and tank wall blocking have an effect on the temperature field in the tank.So under the cooling rate within the specified requirements,enlarging the LNG flow is conducive to the temperature field balance rapidly,and improve the efficiency of pre-cooling process.