Study On Small Natural Gas Liquefaction Process Adopting Liquid Nitrogen

Liquefied natural gas (LNG) is a clean, efficient, convenient, safe energy. Its consumption is growing fastest in the world’s high-quality energy because of its high calorific value, less pollution, convenient transportation and storage. There are lots of scattered and remote natural gas resources in China which haven’t been utilized for limits of economic factors. The development of small-scale natural gas liquefaction device is to solve this problem, it has a great advantage in exploiting the scattered fields because of its small size, can be skid-mounted, highly mobile. In this thesis, the author gives a detailed study on liquefaction technology of small-sized skid-mounted LNG devices.In this paper, the domestic and international development trends of small-scale natural gas liquefaction plants are described. The author chooses the PR equation to calculate the phase equilibrium properties of LNG liquefaction. and LKP equation to calculate the amount of natural gas and refrigerants like enthalpy and entropy. This thesis makes comparative studies about the advantages and disadvantages of cascade liquefaction process, the liquefaction process with expander, and mixed refrigerant liquefaction process. In order to overcome the low product yield, a complex cooling system equipment, and large-scale compressor power consumption, the author design a natural gas liquefaction process with distillation adopting the cold energy of liquid nitrogen on the basis of summing up the advantages and disadvantages of various processes. The liquefied natural gas (LNG) and the liquefied petrol gas (LPG) can be get at the same time in this technology.The process system model was established adpoting HYSYS. The separation temperature under different pressures were analyzed, and the parameters in two columns influenced by the operating pressure were also discussed, the operating pressure of the two columns were determined. For a gas source, the liquefaction process was calculated, each node parameters which are temperature, pressure, enthalpy, entropy, flow, and the mole fraction of each component can be obtained. And the performance of distillation columns and the plate-fin heat exchangers has been described. The process performance influenced by natural gas temperature, pressure and the changes of mole fraction were studied. Finally, the economy between the use of liquid nitrogen cold gas liquefaction process and mixed refrigerant liquefaction process is compared.