Methane refrigeration methane refrigeration is not a strict one-component refrigeration. The refrigerant contains a small amount of other components such as H2CO, which may change during the phase change process. However, since the methane coolant has the lowest cooling temperature and is expensive in cooling, its sensible heat can be sufficiently recovered by means of a cold box having high heat transfer efficiency, so that the returned gas phase refrigerant not only overheats but also has a high degree of superheat.
Hybrid refrigeration mixed cooling is between the above two, its cooling temperature is high to low, but the proportion of high temperature load is relatively large. Thus, if the mixed refrigerant is vaporized, it should immediately return to the compressor compression from the viewpoint of more favorable heat exchange and lower power consumption. However, the gas phase in the dew point state is unstable, and phase transition is liable to occur. The mixed refrigerant is a mixture of several components, and once condensation occurs, the heavy components are first liquefied and precipitated; the remaining gas phase, the group distribution ratio (the amount fraction of the substance) becomes light.
The mixing ratio of the mixed refrigerant group is an important parameter of the mixed refrigeration. From the compression condensation of the refrigerant to the vaporization and cooling, all of them are related to it. It can be seen that maintaining the stability of the mixed refrigerant components is to ensure the stable operation of the hybrid refrigeration system. A key factor. This is also a major feature of hybrid refrigeration that is different from traditional one-component refrigeration. Therefore, the mixed refrigerant gas phase must be subjected to a certain degree of superheat before returning to each section of the suction tank, which is a requirement for component stability and a requirement for smooth operation of the system.
The temperature at which the mixed refrigerant returns to the gas phase is theoretically as long as it is higher than its dew point. However, in actual operation, there is a certain degree of fluctuation in the operation of the system or the heat exchanger, so the superheat of the mixed refrigerant must have a suitable value, which should be at least equal to the fluctuation range of the heat load. Under normal conditions, the design margin of the system or equipment is generally 10%. This design margin is determined based on the fluctuation of the system during normal operation. According to this standard, under the condition that other conditions remain unchanged, the mixed refrigerants in several sets of ethylene plants in China currently overheat the temperature in the entire cooling channel by 10% from the dew point temperature, and their corresponding temperatures are overheated. The value is calculated by simulation.
The improved minimum flow line or stone wall line of the quenching line returns the hot gas of the outlet. After cyclic compression, the inlet/outlet temperature of the compressor may exceed the mechanical tolerance. Therefore, it is necessary to introduce liquid refrigerant for spray cooling. (In the case of single-component refrigeration, except that the liquid phase in the tank is cooled by mixing). The quenching line <6> is the result.
Conclusion With the development of refrigeration technology from single component to mixed refrigeration, the return of compressed gas phase refrigerant has evolved from excessive thermal overheating to moderate overheating. The fluctuation of superheat is also strictly limited by laissez-faire, and the superheat has become a refrigeration system. A very important design and operating parameter. So far, several sets of hybrid refrigeration ethylene plants have been put into operation in China, and the design of equipment and systems related to superheating of gas-phase refrigerants has become more and more mature; and there are now many sets of super-large petrochemical complexes (0.8 ~1Mt/a ethylene) is under intense design and construction. It is estimated that it will be put into use in the next two to three years, and the refrigeration system is one of their common core processes, so the design technology of superheat is timely developed and perfected. The meaning of reality is self-evident.
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