Remove harmful impurities in feed gas and substances that may solidify during cryogenic process. Such as hydrogen sulfide, carbon dioxide, water, heavy hydrocarbon and mercury. The feed gas treated by different types of LNG plants is different, so the treatment methods and processes are also different. Acid gas is generally H2S, CO2, cos, RSH and other gas phase impurities. The removal of acid gas is often called desulfurization and decarbonization, or traditionally desulfurization. When purifying natural gas, H2S and CO2 can be removed at the same time, because these two components can be removed together in alcohol amine method and molecular sieve adsorption purification. 2.3.2 selection of desulfurization method In the natural gas liquefaction unit, there are three common purification methods, namely alcohol amine method, hot potash method (benfied) and sulfonol amine method. Mercury: the presence of mercury can seriously corrode aluminum equipment. When Mercury (including elemental mercury, mercury ions and organic mercury compounds) exists, aluminum will react with water to produce white powdered corrosion products, which will seriously damage the properties of aluminum. A very small amount of mercury is enough to cause serious damage to aluminum equipment, and Mercury will also cause environmental pollution and harm to personnel during maintenance. Therefore, the content of mercury should be strictly limited. Mercury removal is based on the reaction of mercury and sulfur in a catalytic reactor. Heavy hydrocarbon: refers to hydrocarbons above C5 +. In hydrocarbons, when the molecular weight changes from small to large, its boiling point changes from low to high. Therefore, in the cycle of condensing natural gas, heavy hydrocarbons are always condensed first. If the heavy hydrocarbon is not separated first, or separated after condensation, the heavy hydrocarbon may freeze and block the equipment. Heavy hydrocarbons are partially removed by molecular sieve and other adsorbents during dehydration, and the rest are separated by cryogenic separation. Cos: it can be hydrated by a very small amount of water to form H2S and CO2, causing corrosion to the equipment. Easy to mix with recovered propane. It is usually removed together with H2S and CO2 during deacidification. Helium: natural gas is the main source of helium and should be separated and utilized. It has high utilization value by the combination of membrane separation and cryogenic separation. Nitrogen: the increase of its content will make natural gas liquefaction more difficult. The final flash method is generally used for selective removal from LNG. The main component of natural gas is methane (CH4), and its standard boiling point is 111k (- 162 ℃). The density of liquid methane at the standard boiling point is 426kg / m3, and the density of gaseous methane at the standard state is 0.717kg/m3, with a difference of about 600 times. The great difference in volume is the main reason for liquefied natural gas storage and transportation.