Research On The Optimization And The Heat Transfer Characteristics Of The Mixed-Refrigerant Cryogenic System

Cryogenic technology using non-azeotropic mixed refrigerant is widely employed in liquefied natural gas(LNG) industry. And plate-fin heat exchangers(PFHEs) are mainly used in the medium- and small-size LNG systems. The temperature glide of the refrigerant in the cold-box is very large which can be approximately 200 °C. In the evaporating region with low temperature, the mixed refrigerant is very low in quality and flow velocity. However, researches about the heat transfer performance of low-temperature and low-quality mixed refrigerant in the PFHE, liquid hold-up of the mixed refrigerant, optimization of PFHE using mixed refrigerant, optimization of mixed refrigerant composition of a LNG system and so on are still not enough.In order to further study the heat transfer performance of the low-temperature and low-quality mixed refrigerant in the PFHE and establish the theoretical prediction model with practical reference value. A set of cryogenic cycle which used a PFHE as the low-temperature heat recovery unit and the compositions N2+CH4+C2H4+C3H8+iC4H10 as the mixed refrigerant was established. Meanwhile, five ordinary commercial vortex compressors were employed to achieve low temperature refrigeration in this system. In the experiments, the lowest refrigerant temperature of- 160 °C was obtained, based on which the heat transfer performance of the low-temperature and low-quality mixed refrigerant in the PFHE was studied. At the same time, some correlations of mixed refrigerant boiling and condensation have been compiled for predicting the heat transfer performance of the mixed refrigerant. Based on the heat transfer unit theory of the plate-fin heat exchanger, the overall heat transfer coefficient of the PFHE has been calculated and compared with the experimental results. And some correlations were modified by comparing the experimental data. Furthermore, the optimization design method of the PFHE was improved by introducing the modified model of heat transfer performance prediction. Genetic algorithm was employed and unit entropy generation was taken as the objection function and the pressure drop was the constraint. Some suggestion about the design of the PFHE have been proposed.There is closed relationship between the heat transfer characteristics and the flow characteristics of mixed refrigerant. The liquid hold-up of the mixed refrigerant not only affects the heat transfer performance of heat exchanger but also affect the circulated composition of the mixed refrigerant. Therefore, the overall performance of the system will be affected. In this paper, the temperature dropping characteristics, the temperature matching of the hot and cold flows in the recuperative heat exchanger, the heat exchanging load of the heat exchanger were discussed. Additionally, the liquid hold-up characteristics of the mixed refrigerant was studied based on the mathematic model of the mixed refrigerant liquid hold-up by establishing the one-dimensional vertical flowing function of the fluid. And the circulated composition of the mixed refrigerant changing trend during the temperature dropping process was discussed.The circulated composition of the mixed refrigerant plays an important role in a LNG plant which has direct effect in the efficiency and the operation stability of the system. However, it is very difficult to adjust to obtain the optimizing composition in the actual operation in a LNG process. In this paper, by optimizing the mixed refrigerant composition of the SMR liquefaction processes, many optimized compositions with obvious differences were obtained by using the system exergy efficiency as the objective function. Meanwhile, the system can be operated to obtain high exergy efficiencies(greater than 0.4) and the temperature difference in heat exchanger matching uniform and reasonable at the same time. Furthermore, all the optimized compositions can help the SMR system to obtain high efficiency even under different precooling temperature and high/low pressures. This indicated that the mixed refrigerant in the SMR system has strong robustness. Meanwhile, adjust method for a SMR system was proposed which has a certain reference value in the practical engineering applications.