| LNG(abbreviation for Liquefied Natural Gas)refers to liquid which turns from natural gas exploited from oil gas fields after a series of purifications including dehydration,depickling and removal of heavy hydrocarbon,which followed by a series of processes of compression,condensation and liquefaction.The temperature of LNG is-162? while its volume is about 1/625 of natural gas under normal pressure.LNG tank occupies an important position in the LNG receiving terminals;LNG tank is not only the station's core equipment,but also the main facility for LNG storage.In the process of LNG storage,the aging of LNG and mixture of new and old LNG will result in many problems such as evaporation,stratification and tumbling in the tanks.This paper studies the large-scale LNG tank focusing on the static evaporation inside it as well as the influencing characteristic and evolution of stratification and tumbling of LNG in order to provide suggestions for secure storage and reasonable loading and unloading.Software is used to construct a numerical model and carry out numerical calculation for static evaporation inside the confined tank under natural conditions.Within the allowable range of maximum pressure capacity,the results are as followed:(1)There is an optimal filling rate for confined LNG tanks at which tanks acquire the longest secure storage time.A greater or less filling rate would lead to a decrease of secure storage time with the increase or decrease of the initial filling rate.As for a LNG tank with a volume of 160000 m3,the optimal filling rate is 78.35% while its secure storage time is 24.22 d.(2)The higher the ambient temperature and the larger the thermal conductivity of insulation layer of tanks,the faster the pressure rises inside the tanks so as to a shorter secure storage time.(3)A larger nitrogen content of LNG leads to a faster rising of pressure inside the tank as well as a shorter secure storage time.(4)Tanks of smaller volume have the higher pressure capacity,larger filling rate and longer secure storage time.However,many factors have contributed to the development of large-scale tanks.Based on assumption of Boussinesp,software is used to develop the numerical model of stratification and tumbling of LNG inside the tank and then the mixing rule under the condition of stratification is calculated.The results are as followed:(1)With equivalent diameter of tanks and density difference,the tumbling of three layers with a longer duration tends to be faster and more violent than that of two layers.(2)With equivalent diameter of tanks and density difference,a thicker layer will lead to a faster and more violent tumbling with a longer duration.(3)With equivalent diameter of tanks and density difference,a larger difference in density between the layers leads to a faster and more violent tumbling with a longer duration.Software is used to combine the static evaporation rule with stratification and tumbling,and to construct the numerical simulation of gas phase in the stratification and tumbling to study the movement of gas phase inner the LNG tank,the results are as followed:(1)Following the stratification,the process of tumbling can be divided into four periods: occurrence of interfacial disturbance,growth of disturbance,violent tumbling with increasing pressure and smooth recovery.(2)The tumbling inside the tank makes LNG to evaporate at a high speed which sharply increases the pressure inner the tank.When the pressure reaches the maximum working pressure of tank,the relief value would open to release the excessive gas in case of overpressure.(3)The evaporation rate of LNG when tumbling occurs is 10 to 50 times or more than it under normal condition.For the conditions of this paper(the volume of tank is 160000m3,the filling rate is 70%,thickness of top heavy LNG is 1m and the density difference in stratified layers is 1kg/m3),the average evaporation rate is 32.3 times of the static rate,and the relief value would open about 16 hours late after the tumbling. |
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