천연가스 액화를 위한 초저온 캐스케이드 공정에 관한 연구

Abstract

Abstract

Refrigeration technology is widely used in the various cryogenic applications including liquid-fuel rockets, MRI (Magnetic Resonance Imaging), HTS (High Temperature Superconductor), and natural gas liquefaction according to the development of new refrigeration cycles, compressors, and cryogenic heat exchangers. Among the applications, natural gas liquefaction is highlighted for the world's future energy economy, because of its relative cleanness and large reserves in comparison with other fossil fuels. Natural gas is a mixture of methane, ethane, propane, butane and other hydrocarbons. Fraction of methane in natural gas is about 80%. Natural gas is delivered from gas wells to end users largely in two ways; PNG (Pipeline Natural Gas) system and LNG (Liquefied Natural Gas) system. LNG transportation requires the liquefaction of natural gas at -162℃. The liquefaction plant is a combination of pre-processing, fractionation, liquefaction and storage facilities. It takes the highest position in the value chain of natural gas industry. In this study, a simulation was carried out focusing on the cascade process, which is applied to the natural gas liquefaction. The simulation was consisted of propane, ethylene, and methane cycles. The effects of an inter-cooler and of a liquid-gas heat exchanger were first researched with the Phillips Optimized Cascade Process using HYSYS software. Afterward, the Phillips Optimized Cascade Process was modified to include an expander in the methane cycle and the ethylene cycle. The main conclusions of this study are : In the cascade process with an inter-cooler, an optimum performance of liquefaction ratio and a specific energy can be obtained by the small compression work and high COP (Coefficient Of Performance). The result shows that a medium pressure and a bypass ratio for each cycle are: 600 kPa and 23% for the propane cycle, 1,400 kPa and 13.1% for the ethylene cycle, and 679 kPa and 23.5% for the methane cycle. When the liquid-gas heat exchanger is applied with the increasing of the refrigerant bypass rate, the compression work decreases due to the sub-cooling effect. The number of the optimal compression stages is four, four, and five for the propane, ethylene, and methane cycles, respectively. This stages show the same number in the cascade process with an inter-cooler and in the Phillips Cascade Process with an inter-cooler. In comparison with the basic cascade process, the results of the 4-4-5 stage of the inter-cooler cascade process show 27.6% less compression work, 6.3% less refrigeration capacity, 27.6% less specific energy, and 29.5% higher COP. In comparison with the Phillips Cascade Process, the 4-4-5-stage of the inter-cooler cascade process has 5.8% less compression work, 1.9% less refrigeration capacity, 5.8% less specific energy, and 4.1% higher COP. It is found that the compression-expansion process using the expander allows a part of the heat duty to shift from the low temperature region to the high temperature region. The total power consumption of the expanded cascade process is about 7% less than the conventional Phillips Cascade Process due to the reduced refrigerant mass flow. It is possible to remove the propane cycle from the Phillips Cascade Process by splitting the ethylene cycle. One of two streams in this cycle is used as refrigerant in a Reverse Brayton Cycle. Although there are disadvantages of relatively large compression work, low COP, and low specific energy, the new cascade process is an attractive alternative, because it provides extra compactness and simplicity by the use of only two pure refrigerants without the propane cycle in the conventional process.