With worldwide LNG demand increasing rapidly, LNG liquefaction plants and liquefaction processes are a highly valued industry. Recently, there has been an increase in research and development of LNG-FPSO technologies for 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, and butane and/or pentane have been added to the refrigerant to increase the efficiency of the cycle. In the proposed cycle, a distillation column is used to separate mixed refrigerant into HK (Heavy Key) and LK (Light Key) components. HK refrigerant is used for pre-cooling the natural gas, and the natural gas is liquefied and partially sub-cooled by LK refrigerant in the cryogenic range of the heat exchanger. Since the proposed liquefaction cycle has a simple structure with high compactness and power efficiency, 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) in order to meet a number of challenges, including the demand of greater efficiency and larger capacity. 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.