Large Lng Storage Tank Vapor Space Simulation

Natural gas is favored in many countries because of its characteristic of high quality and pollution-free to the environment. Liquefied natural gas has some advantages in long distance transportation and storage of natural gas. Now, production and trade of LNG has become an important factor that influence accommodate pattern of natural gas (in the world). LNG industry is being developed rapidly in our country, and many great progresses have been made in each tache of LNG chain. LNG terminals have been beginning built in coast to receive import LNG form foreign.LNG tank is main equipment of LNG terminal, and it normal operation is the key to keep LNG terminal in normal working. LNG is cryonic liquid with a temperature of -162 ℃. During storage, some part of LNG vapor and inner tank pressure increase due to heat leak of tank. For keeping tank pressure in design range, some of boil off gas need to be released form tank through boil off gas treatment system in order to control tank pressure. The aim of this paper is to study evaporation of LNG liquid and simulate dynamic process of vapor space.Composition and thermodynamics character of LNG vapor and liquid vary as liquid's vaporizing, so LNG physical characteristic must be calculated in the simulate process. According to property of LNG, SRK equation has been chosen to calculate the phase equilibrium properties; LKP equation has been chosen to calculate the thermodynamics character, and it is comparatively accurate in calculating thermodynamic characteristic.The heat leak of tank during storage has been analyzed. For different type tank, heat transmission through tank roof, sides and bottom has been defined and described in general formula.A conclusion has been drawn through analyses of the evaporation inner tank liquid, which is the evaporation of liquid occurred at and near surface during storage and boil-off gas and up liquid gets equilibrium during storage. On the base of analyses of evaporation, dynamic process of vapor space has been described during storage and dynamic model of large LNG tank's vapor space has been established.At last, simulate calculation of actual LNG tank has been done, and influence on inner pressure under diversified condition has been studied on the base of analyzes of result. Some guidance to LNG storage tank operation and equipment specification has been put forward.