Simulation And Optimization Of Liquefaction Process In A Large-scale Load-type LNG Plant

Liquefied natural gas(LNG)is a clean source of power,providing a variety of power supplies for all the countries around the world.However,LNG plants are energy intensive,and the selection of liquefaction process for LNG plant is very important.There is little research on the process of large-scale-base-load LNG plant in China,the process of large-scale-base-load LNG plant includes DMRC process developed by Shell Company of the Netherlands,MFC process developed by Linde Company of Germany,C3/MRC process and AP-X process developed by APCI Company of the United States.This paper lists the application of these four liquefaction processes in LNG plant,compares the advantages and disadvantages of these processes,and finally chooses AP-X process for Djibouti 2000×104Nm3/d large-scale-base-load LNG plant.AP-X process is widely used in large-scale-base-load LNG plant,with advantage of mature technology and low power consumption.The three refrigeration cycles of AP-X process include propane precooling cycle,mixed refrigerant liquefaction cycle and nitrogen expansion sub-cooling cycle.The three refrigeration cycles cool natural gas from ambient temperature to-30℃,-105℃ and-154℃ according to the temperature gradient,and LNG is formed.Then LNG is cooled to-158.3℃ after J-T valve depressurization and cooling,and the rate of LNG production is 95.53%.By using HYSYS to simulate and optimize the AP-X process,the unit power consumption of the optimized AP-X process is reduced to 5.18kW-h/kmol LNG,which is 15.64%lower than that of the base case with is 6.14kW·h/kmol,and 5.8%lower than that of a typical LNG plant of 5.5-6kW·h/kmol.The results show that the optimized AP-X process is one of the most effective liquefaction processes so far and is suitable for large-scale-base-load LNG plant.The analysis of exergy shows that the exergy loss of compressor and cryogenic heat exchanger accounts for 31.1%and 27.9%of the total exergy loss of the whole process.Plate-fin heat exchanger is selected for propane precooling refrigerator and nitrogen expansion sub-cooling refrigerator,spiral wound heat exchanger for mixed refrigerant liquefaction refrigerator,centrifugal compressor for propane compressor,mixed refrigerant compressor and nitrogen compressor,and gas turbine for driving equipment of compressor.