Design and analysis of multi-stage expander processes for liquefying natural gas

Abstract

Multi-stage expander refrigeration cycles were proposed and analyzed in order to develop an efficient natural gas liquefaction process. The proposed dual and cascade expander processes have high efficiency and the potential for larger liquefaction capacity and are suitable for small-scale and offshore natural gas liquefaction systems. While refrigeration cycles of conventional expander processes use pure nitrogen or methane as a refrigerant, the proposed refrigeration cycles use one or more mixtures as refrigerants. Since mixed refrigerants are used, the efficiency of the proposed multi-stage expander processes becomes higher than that of conventional expander processes. However, the proposed liquefaction processes are different from the single mixed refrigerant (SMR) and dual mixed refrigerant (DMR) processes. The proposed processes use mixed refrigerants as a form of gas, while the SMR and DMR processes use mixed refrigerants as a form of gas, liquid- or two-phase flow. Thus, expanders can be employed instead of Joule-Thomson (J-T) valves for refrigerant expansion. Expanders generate useful work, which is supplied to the compressor, while the high-pressure refrigerant is expanded in expanders to reduce its temperature. Various expander refrigeration cycles are presented to confirm their feasibility and estimate the performance of the proposed process. The specific work, composite curves and exergy analysis data are investigated to evaluate the performance of the proposed processes. A lower specific work was achieved to 1,590 kJ/kg in the dual expander process, and 1,460 kJ/kg in the cascade expander process. In addition, the results of exergy analysis revealed that cycle compressors with associated after-coolers and companders are main contributors to total exergy losses in proposed expander processes.