Research On The Precooling Law Of LNG Unloading Arm Pipeline

Before liquefied natural gas(LNG)is transported from the LNG ship to the LNG terminal,the LNG unloading arm pipeline needs to be pre-cooled from normal temperature to a low temperature below minus 150℃(123K)to discharge LNG at minus163℃(110K).The pre-cooling of the LNG unloading arm pipeline usually uses liquid nitrogen as the pre-cooling medium,but during the pre-cooling process,the liquid nitrogen will undergo a phase change,and this phase change will have a great impact on the temperature change of the pipeline.Therefore,when the cooling medium liquid nitrogen undergoes a phase change,the following research is carried out on the precooling temperature change law of the LNG unloading arm pipeline and the temperature change law under the influence of related factors:(1)The low-temperature characteristics of the LNG unloading arm pipeline material and the pre-cooling medium were analyzed,and the relationship between the heat transfer characteristic parameters and the temperature was fitted;the heat transfer mode between the pipe wall and the internal fluid and the external environment during the pre-cooling process was analyzed,the heat transfer equation of the LNG unloading arm pipeline is established.(2)For the phase transition process of liquid nitrogen in the tube,considering the relationship between pressure and saturation temperature,the Antoine equation is used to correct the Lee model,which improves the model’s ability to transport components and mass transfer between liquid nitrogen and liquid phases under the influence of pressure.The simulation effect is compared with the experimental value to verify the rationality of the model.(3)Write user-defined programs according to the liquid nitrogen phase change pipeline precooling modelmodel,load these program source items into ANSYS Fluent for secondary development,and simulate the pre-cooling process of the phase transition of liquid nitrogen in the LNG unloading arm pipeline.The calculation results show that during the pre-cooling process,the temperature of the pipe increases along the axis,the temperature difference of the radial section becomes smaller,and the temperature inside the elbow is lower than the temperature outside.After 140 min of pre-cooling,the maximum temperature of the pipe wall surface is minus 151.977℃(121.173K),which meets the pre-cooling requirements of the LNG unloading arm.(4)The influence of parameters such as the flow rate of liquid nitrogen at the inlet,the initial temperature of the fluid,the pressure in the pipe,and the pipe diameter on the pipe wall temperature during the pre-cooling of the LNG unloading arm was considered.The lower the pressure,the smaller the pipe diameter,the more heat exchange between the fluid in the pipe and the pipe wall,and the greater the temperature drop rate of the pipe wall.By establishing a liquid nitrogen phase change pipeline pre-cooling model,the precooling process of the LNG unloading arm was simulated and calculated,and the pipeline pre-cooling law was obtained,which lays a foundation for further optimization of the LNG unloading arm pre-cooling process.