With worldwide LNG demand increasing rapidly, LNG liquefaction plants and liquefaction processes are higher value-added industries. Recently, there has been an increase in research and development of LNG-FPSO technologies in offshore liquefied natural gas (LNG) service instead of land-based LNG plants. While onshore LNG facilities have traditionally focused on power efficiency as a key criterion for process design and equipment selection, offshore LNG would require not only power efficiency but also safety and compactness. A new natural gas liquefaction cycle is proposed in this paper. The structure of the new cycle is based on the SMR (Single Mixed Refrigerant) liquefaction cycle which has a very simple structure. The proposed liquefaction cycle has liquid-vapor separator to separate MR into HK (Heavy Key) and LK (Light Key) components, and each key is compressed separately after the main cryogenic heat exchanger, and then mixed again to make a single MR. The proposed cycle can be optimized using the temperature profiles in cryogenic heat exchanger and compressor power using each separated compressor. The proposed liquefaction cycle has a simple structure with high compactness and power efficiency, therefore the proposed cycle could be suitable for the LNG-FPSO liquefaction process.
Thermodynamic process for the liquefaction of natural gas has evolved since 1970's (Barron, 1985; Roberts, 2002; Andress, 2004; Flynn, 2005; Venkatarathnam, 2008; Chang, 2009, Lee, 2010) in order to meet a number of challenges, including the demand of greater efficiency and larger capacity. A liquefaction system is primarily composed of a series of compressors, coolers, expanders, and heat exchangers. Natural gas is cooled-down to LNG temperature in thermal contact with closed-cycle refrigerant(s). In order to reduce the input power for liquefaction, it is crucial to reduce entropy generation due to the temperature difference between hot stream (including feed gas and hot refrigerants) and cold refrigerants in the heat exchangers.