Natural gas liquefaction (LNG) process.
Liquefin process—Dry natural gas (A) enters the liquefaction train’s pre-cooling section (1) where it is cooled between –50°C and –80°C (–60°F and –110°F). The heat is exchanged with a mixed refrigerant in a bank of brazed aluminum plate-fin heat exchangers (PFHEs). The cooled feed stream is then sent outside the liquefaction train to fractionator (2) for condensate removal. The gas returns to the cryogenic exchanger and enters the cryogenic section (3) where it is liquefied by heat exchange in compact, energy-efficient PFHEs with a second mixed refrigerant and leaves the cryogenic exchanger as LNG (B).
Pre-cooling refrigerant system (4)—Using mixed refrigerant reduces the feed-gas temperature to a much lower level than can be achieved with propane refrigerant. This allows the pre-cooling power requirement to be balanced with that of the cryogenic section so that two identical drivers (5) operating at optimum efficiency can be used, thus lowering investment, maintenance and operating costs.
Cryogenic refrigeration system (6)—The mixed refrigerant gas entering the pre-cooling section is completely condensed by the time it leaves the cryogenic section without using separation equipment. After leaving the cryogenic section, the refrigerant is expanded (7) and re-enters the cryogenic section where the process gas and cryogenic refrigerant are condensed.
Using two mixed-refrigerant systems and modular PFHEs in a large single train has lower investment and operating costs than systems involving single-component refrigerants or multiple cooling trains feeding a common liquefaction exchanger. Because the precooling and liquefaction sections comprise several parallel modules, single trains of any size—e.g., 8 million tpy—can be built. Proven extensively, the PFHEs are available from several vendors, which has
a very positive effect on price and delivery time.
Economics: Detailed studies by international petroleum and E&C companies comparing conventional 4.5 to 8 MMtpy propane-plus-mixed refrigerant liquefaction trains with Liquefin LNG installations have shown a 15–20% specific investment cost advantage for the LIQUEFIN train.