Reducing the high pressure of small air separation equipment with medium pressure expansion machine can reduce the energy consumption of air separation equipment. Here is a simple analysis of the influence of the temperature before the expansion machine on the high pressure.
Under the condition that the total cooling loss and the gas temperature before expansion are fixed, from the perspective of the heat balance of the total unit, increasing the high pressure can reduce the expansion gas; but from the perspective of ensuring the working condition of the cold section heat exchanger, with the increase of high pressure, it is required to increase the expansion amount and reduce the air flow into the cold section heat exchanger. Therefore, the selection of working pressure and expansion amount of the process should meet both requirements.
When the total cooling loss and working pressure of the process remain the same, increasing the temperature of expansion forward gas can increase the unit refrigerating capacity of the expansion machine. However, the temperature before expansion cannot be raised at will. The selection of the temperature not only ensures that the air entering the tower has a certain moisture content during the process thermal calculation, but also ensures the normal working condition of the cold section heat exchanger.
When the temperature increases before expansion, the expansion amount can decrease. But in order to ensure the normal working condition of cold section heat exchanger, it is required that the decreasing amplitude of expansion quantity is larger than that of the actual expansion quantity mentioned above. The main factor is to ensure the working condition of the cold section Heat Exchanger. Therefore, to further reduce the expansion, the only thing is to increase the high pressure.
Similarly, when the temperature drops before expansion, from the perspective of thermal balance of the unit, the expansion quantity is required to increase. In order to ensure the working condition of cold section heat exchanger, it is also required to increase expansion, and its amplitude is larger than the former. Therefore, only further reduce the high pressure. However, reducing the temperature before expansion has a certain limit to the decrease of high pressure. Because the high pressure pressure is too low, and the refrigerating capacity required to be provided is also certain, it is bound to require a large increase of expansion, at this time, it will cause the throttle air volume through the cold section heat exchanger is too small, there will be "zero temperature difference" or "negative temperature difference" in heat exchange, which damages the normal working condition; meanwhile, the temperature difference at the hot end of the heat exchanger in the heat section increases, and the incomplete cold loss of reheat increases, finally make the device unable to work normally.
Because there is a limit to the drop of high pressure, the temperature before expansion can not be considered as the lower the better. This temperature is not only limited by the above related factors, but also limited by the design requirements of the expansion machine. That is to say, liquefaction cannot occur after expansion, and take into account the adiabatic efficiency of the expansion machine and the enthalpy drop before and after the expansion machine.
