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1. Introduction
Natural gas is one of the most effective means of coping with today’s energy crisis. It has many applications for domestic purposes such as heating, electric generation in power plants and powering vehicles. The demand of natural gas has increased 5 times in the last few decades [1]. This demand is due to the fact that natural gas is easily transportable and environmentally friendly. Chen et al. [2] predicted the temperature and pressure changes
in a lique fi ed natural gas cryogenic tank . The properties and composition of LNG fuel were simulated inside the tank as a func-tion of time. Boil off is de fined as the gas being released from the liquid. Boil off of LNG in these LNG tank s usually takes place at LNG stations and can cause excessive pressure build up in LNG tanks. Boil-off is caused by heat added to the LNG fuel during storage and the fi lling processes. Heat can leak through the shell of the tank, and be added to the LNG fuel during operation. They stated that the boil-off of the LNG is mainly due to the heat gained by the tank from outside ambient temperature. Also, the heat leakage into the tank leads to an increase of the vapor pressure.

1 引言

天然气是一种应对今天的能源危机的最有效的手段。它有如加热国内许多的应用目的，在发电厂的发电和驱动汽车。天然气的需求量增加了5倍，在过去的几十年里，[ 1 ]。这种需求是由于天然气易于运输和环境友好。Chen等人。[ 2 ]预测的温度和压力的变化在液化天然气低温储罐Wi-Fi版。模拟舱内的作为时间的函数和液化天然气燃料组合物的性质。蒸发是定义为气体从液体释放。汽化LNG在这些LNG储罐的LNG站通常发生在和会导致过度的压力在LNG储罐的建立。蒸发是造成热添加到液化天然气燃料在储存和填充过程。热泄漏通过罐的外壳，并在操作过程中添加到液化天然气燃料。他们指出，液化天然气的断开的主要原因是获得的坦克从外界环境温度的热。同时，热漏入罐导致蒸汽压力的增加。

Natural gas is stored in a liquefi ed state at a temperature as low as 16.2 C in order to decrease its volume and to facilitate trans-portation. In its liquid state, the density of natural gas is 60 0 times more than Compressed Natural Gas (CNG) at room temperature and atmospheric pressure [2]. Compressed natural gas is typically stored at pressures up to 24.821 MPa in cylindrical steel tanks. In order to reduce boil-off, Lique fied Natural Gas (LNG) should be stored in special tanks which have multi-layered insulation that minimize heat leakage. There are three different classifications for liquefied na tu ra l g as (L NG ) s to rage ta nk s [3]: single containment, double containment, and full containment. A single containment tank is either a single tank or a tank consisting of an inner and an outer tank such that only the inner tank is capable of storing the LNG. A double containment tank is defined as having an inner and outer tank that is both capable of independently containing the LNG. A full contain-ment tank is defined as a double tank in which the outer tank of a full ontainment tank is capable of both containing the liquid cryogen and of controlled venting of the vapor of the cryogen after a leak.

Tanks are additionally classified by th e e leva ti on s f ro m t he ground level: above-ground type, in-ground type and under-ground type. The type of tank treated in this study is an above-ground full containment type tank. Generally, LNG storage tanks are composed of three parts: inner tank, outer concrete wall and roof. According to the shape of the inner tank, there are two types of LNG storage tank: 9%-Ni type and membrane type[4]. The 9%-Ni type has a self-supporting inner tank which endures the thermal contraction of LNG tempera-ture, and本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。