During the last decades, Liquefied Natural Gas (LNG) technologies have evolved to focus on improving the energy efficiency of the processes. The first two LNG plants were based on the Cascade Refrigeration Cycle with pure refrigerants. Today liquefaction technologies of two and three cooling cycles are based on mixed refrigerants or combinations of pure and mixed refrigerants cycles due to energy efficiency and low equipment sizing compared to one cooling cycle. Nevertheless, a clear difference between the available LNG processes has been the use of a precooling cycle based on pure refrigerant or mixed refrigerants. The advantages and disadvantages of using a pure refrigerant or a mixed refrigerant at the precooling cycle are discussed in this work. Power consumption of the compressor at precooling stage is evaluated in this work since compressors/drivers represent about 50% of the capital costs, and also about 40% of the total operating cost for a baseload LNG plant. The evaluation is carried out based on the theoretical model for a centrifugal compressor and using a Linde-Hampson refrigeration cycle. An energy balance around the heat exchanger, expansion valve and evaporator is performed in order to use the specific refrigerant effect as a parameter to compare the refrigerants. Also, the influence of refrigerants in the efficiency of the compressor is analyzed by the coefficient of performance, and their advantages and disadvantages regarding to start-up and operation of the systems is discussed. An important result of this work is to establish some thermodynamics criteria for the selecting of the suitable refrigerant for the precooling stage. Nevertheless for the final decision-making process regarding the selection of the precooling stage a proper model is required capable of taking into account all aspects that could affect the capital and operation costs.
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