非共沸混合制冷剂低温制冷系统优化及换热特性研究

发布时间：2019-05-11 23:50

【摘要】：在天然气液化工业领域中,广泛采用非共沸混合工质低温制冷技术,其中,中小型天然气液化流程的冷箱主要采用板翅式换热器。冷箱内工质换热的温度滑移较大,可达到200°C左右。在其低温蒸发段,混合制冷工质干度低,流速慢,尤其在天然气液化负荷较低的工况下,换热器的换热性能下降。然而,这种工况下板翅式换热器的换热、工质液相积存等问题的理论和实验研究仍然欠缺。为了研究低温低干度混合工质在板翅式换热器中的换热性能,建立具有实际参考价值的理论预测模型,本文搭建了一套以板翅式换热器为低温段回热装置、以类似于天然气液化流程所用的混合工质(N2+CH4+C2H4+C3H8+iC4H10)为制冷剂、以普通商用涡旋压缩机驱动的低温制冷系统,在获取最低-160°C低温的前提下,对低干度低流速混合工质在板翅式换热器中的换热性能进行了研究;同时,整理了国内外学者对混合工质冷凝及沸腾换热关联式的研究,基于传热单元模型对板翅式换热器的总传热系数进行了理论预测,并与实验结果进行了对比分析,同时,基于实验数据对部分关联式进行了修正。在此基础上,对应用于天然气液化流程的板翅式换热器的优化设计进行改进,采用遗传算法,以单位熵增数为目标函数,冷热流压降为约束条件对板翅式换热器中通道和翅片参数进行寻优,并提出板翅式换热器设计的改进方向。混合工质的传热特性与流动特性密切相关,工质在系统中的液相积存不仅影响换热器的换热性能,还会影响混合工质的循环浓度,系统的总体性能也因此改变。因此,本文针对混合工质低温制冷循环中的降温特性、回热器中的冷热流温差变化、换热器负荷变化以及液相积存等问题进行了分析讨论。另外,基于混合工质一维竖直流动方程建立了低温制冷系统中混合工质液相积存的数学模型,分析讨论了低温制冷循环中混合工质的液相积存特性,以及混合工质循环浓度随降温过程的变化规律。天然气液化流程中混合工质的循环浓度对系统效率和稳定运行起着至关重要的作用,然而,在实际运行过程中很难将运行循环浓度调节至与优化浓度一致。通过对SMR液化流程的优化发现,在以系统(火用)效率为目标函数的前提下对混合工质进行寻优,可以得到差异较大的不同混合工质循环浓度,同时,系统在获取较高(大于0.4)的(火用)效率同时,换热器中温差匹配均匀合理。另外,在不同的预冷温度、系统高低压的情况下,优化的循环浓度均能使系统保持高效运行,这说明混合工质的浓度配比在天然气液化流程中存在较强的鲁棒性;另外,本文提出了SMR液化流程的调节方案,在实际工程应用中具有一定的参考价值。
[Abstract]:In the field of natural gas liquefaction industry, non-azeotropic mixture cryogenic refrigeration technology is widely used, in which plate-finned heat exchanger is mainly used in the cold box of small and medium-sized natural gas liquefaction process. The temperature slip of working fluid heat transfer in cold box is large, which can reach about 200 掳C. In the low temperature evaporation section, the dryness of the mixed refrigerating fluid is low and the flow rate is slow, especially under the condition of low liquefaction load of natural gas, the heat transfer performance of the heat exchanger decreases. However, the theoretical and experimental research on the heat transfer and liquid phase accumulation of plate-wing heat exchanger under this condition is still lacking. In order to study the heat transfer performance of low temperature and low dryness mixed working fluid in plate-wing heat exchanger and establish a theoretical prediction model with practical reference value, a set of plate-wing heat exchanger is built in this paper. Using a mixture (N2 CH4 C2H4 C3H8 iC4H10) similar to that used in a natural gas liquefaction process as a refrigerant and a cryogenic refrigeration system driven by a commercial scroll compressor, on the premise of obtaining a minimum temperature of-160C, The heat transfer performance of low dryness and low velocity mixed working fluid in plate-finned heat exchanger was studied. At the same time, the correlation of condensation and boiling heat transfer of mixed working fluid is studied by scholars at home and abroad. Based on the heat transfer unit model, the total heat transfer coefficient of plate-finned heat exchanger is predicted theoretically, and the results are compared with the experimental results. Based on the experimental data, part of the correlation is modified. On this basis, the optimal design of plate-wing heat exchanger applied to natural gas liquefaction process is improved, and the genetic algorithm is adopted, and the unit entropy increase is taken as the objective function. The pressure drop of cold and hot flow is used as the constraint condition to optimize the channel and finned parameters of the plate-finned heat exchanger, and the improvement direction of the design of the plate-finned heat exchanger is put forward. The heat transfer characteristics of the mixed working fluid are closely related to the flow characteristics. The liquid phase accumulation of the working fluid in the system not only affects the heat transfer performance of the heat exchanger, but also affects the circulating concentration of the mixed working fluid, and the overall performance of the system is also changed. Therefore, the cooling characteristics in the low temperature refrigeration cycle of mixed working fluid, the temperature difference of cold and heat flow in the regenerator, the load change of the heat exchanger and the liquid phase accumulation are analyzed and discussed in this paper. In addition, based on the one-dimensional vertical flow equation of mixed working fluid, the mathematical model of liquid phase accumulation of mixed working fluid in low temperature refrigeration system is established, and the liquid phase accumulation characteristics of mixed working fluid in low temperature refrigeration cycle are analyzed and discussed. And the variation of the concentration of mixed working fluid cycle with the cooling process. The circulating concentration of mixed working fluid in natural gas liquefaction process plays an important role in the efficiency and stable operation of the system. However, it is difficult to adjust the operating cycle concentration to be consistent with the optimized concentration in the actual operation process. Through the optimization of SMR liquefaction process, it is found that the cycle concentration of different mixtures can be obtained by optimizing the mixture under the premise of system (exergy) efficiency as the objective function. At the same time, the temperature difference matching in the heat exchanger is uniform and reasonable when the exergy efficiency is higher than 0.4. In addition, under the condition of different pre-cooling temperature and high and low pressure of the system, the optimized cycle concentration can keep the system running efficiently, which indicates that the concentration ratio of the mixture has strong robustness in the natural gas liquefaction process. In addition, this paper puts forward the adjustment scheme of SMR liquefaction process, which has certain reference value in practical engineering application.
【学位授予单位】：华南理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TB657

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