Comparison and selection of process technology The process of natural gas liquefaction includes pretreatment (purification) of raw gas, liquefaction ( or LNG liquefaction plant), refrigerant circulation compression, product storage, loading and auxiliary system, etc. the main process includes purification of raw gas and liquefaction of purified gas.  Selection of natural gas purification process As raw gas, natural gas must be thoroughly purified before liquefaction.That is to remove the acid gas, water and impurities in the feed gas, such as H2S, CO2, H2O, Hg and aromatic hydrocarbons, so as to avoid blocking and corrosion of equipment and pipelines due to freezing at low temperature.Table 5.1-1 lists the pretreatment standards and the maximum content of impurities of feed gas in LNG plant. Maximum allowable impurity content of LNG feed gas Impurity  Content limit Basis H2O  ＜1ppmV A (it is allowed to exceed the solubility limit without limiting the yield) CO2 50～100ppmV B (limit solubility)  H2S  ＜4ppmV C (product technical requirements)  Total sulfur content  10～50mg/Nm3 C Hg ＜0.01μg/Nm3 A  Aromatic hydrocarbons ≤10ppmV A or B Total naphthenic hydrocarbon ≤10ppmV A or B               According to the data of feed gas, the water content in feed gas exceeds the standard and must be purified. A) Selection of deacidification process H2S and CO2 contained in natural gas are collectively referred to as acid gas. Their existence will cause metal corrosion and pollute the environment.In addition, if the CO2 content is too high, the calorific value of natural gas will be reduced.Therefore, the content of acid components in natural gas must be strictly controlled to meet the requirements of process and product quality. There are solvent absorption method, physical absorption method, oxidation-reduction method and molecular sieve adsorption method for removing acid gas from natural gas.At present, solvent absorption method is widely recognized and widely used.It is a desulfurization method based on reversible chemical reaction and using alkaline solvent as absorbent. The solvent reacts with acid components (mainly CO2) in feed gas to form compounds. The rich liquid absorbing acid gas can decompose and release acid gas under the condition of increasing temperature and reducing pressure, so as to realize the regeneration and utilization of solvent. In terms of energy consumption, treatment scale, investment and operation cost, MDEA amine liquid method is the most suitable process, so MDEA amine liquid method is selected for deacidification in this scheme. B) Selection of dehydration process Natural gas dehydration process generally includes low temperature dehydration, solid desiccant adsorption and solvent absorption. Freezing separation is mainly used to avoid hydrate of natural gas when the temperature is low, but its allowable low temperature is limited, which can not meet the requirements of natural gas liquefaction; solvent absorption usually includes concentrated acid (generally concentrated phosphoric acid and other organic acids), glycol (commonly used triethylene glycol), etc., but these methods have low dehydration depth, which can not be used in cryogenic unit; solid desiccant dehydration methods are silica gel method, molecular sieve method or the combination of the two methods. Natural gas liquefaction and dehydration must adopt solid adsorption method. Because molecular sieve has strong adsorption selectivity, high adsorption characteristics under low water vapor partial pressure, and can further remove residual acid gas at the same time, 4A molecular sieve is used as dehydration adsorbent in this scheme. C) Selection of mercury removal process At present, there are mainly two kinds of mercury removal processes: the HGSIV molecular sieve adsorption method of UOP company in the United States and the chemical reaction between mercury and sulfur by using sulfur impregnated activated carbon to produce mercury sulfide, which is adsorbed on the activated carbon. The former is high cost and suitable for the occasion with high mercury content; the latter is low cost and suitable for the occasion with low mercury content. On the one hand, the operation cost of HGSIV molecular sieve is very high; on the other hand, the mercury content in the feed gas of this unit is relatively low. Therefore, the use of sulfur impregnated activated carbon mercury removal process, the company has a successful experience.