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• Liquefied Natural Gas (LNG)


  Natural gas that has been cooled to liquid form for ease and safety of non-pressurized storage or transport
• LNG process


  1. Feed gas delivery

  2. Pretreatment to remove impurities

  3. Heavier hydrocarbon removal

  4. Liquefaction using refrigerant

  5. Further cooling and subcooling

  6. Flashed vapors and boil off gas recycling
• LNG plants often consist of a number of parallel units called trains which treat and liquefy natural gas and then send the LNG to several storage tanks
• Liquefaction process


  • Key element of the LNG plant
  • Based on a refrigeration cycle where a refrigerant transports heat from the process side

  
• Liquefaction capacity of a train is determined by the liquefaction process, refrigerant used, size of compressor/driver, and heat exchangers
• Cooling curve


  Curve showing the relationship between cooling/heating of the process gas and the refrigerant

  
• LNG processes are designed to closely approach the cooling curve of the gas being liquefied using specially mixed multicomponent refrigerants

  
• Thermodynamic analysis of LNG processes


  1. Single stage cooler/condenser

  2. Removal of NGL, non-condensable gases, and impurities

  3. Self-liquefaction by compressing, cooling, and flashing the process stream

  4. Liquefaction by cooling the process stream using refrigerants

  
• Ideal cooling process can be analysed using material and energy balances, heat transfer equations, and Carnot refrigerator principles

  
• Real cooling processes are less efficient than ideal reversible processes due to factors like compressor friction, finite temperature differences, irreversible flashes, and heat loss

  
• Real cooling processes


  • Simple flash condensation
  • Modified Linde process

  
• For a simple flash condensation process, about 24% of the methane feed at 100 bar and 210 K can be liquefied at 4 bar and 131.4 K

  
• The total work required for the simple flash condensation process is 869.3 kJ/kg LNG or 110 MW for a 4 MTPA LNG unit