When the expansion air is sent to the upper tower depends on the specific process of air separation. But the principle behind it is the same, which is to maximize the refrigeration capacity of the expander and allocate the cooling capacity reasonably.
Some air separation processes have a butterfly valve at the nitrogen outlet of the upper tower, which can be reduced to increase the pressure of the upper tower. At the same time, the expanded air first enters the subcooler, then enters the liquefier, and finally is sent to the switching heat exchanger for reheating. For such a process, it is not advisable to send gas to the upper tower too early, as sending gas to the upper tower will inevitably increase the pressure behind the expander, which is not conducive to exerting the refrigeration capacity of the expander. At the same time, as the subcooler and liquefier can fully recover the cooling capacity of the expansion air, there is no need to send gas to the upper tower too early. It is appropriate to stop one expander and then send gas to the upper tower after the main cooling liquid area has accumulated enough and normal distillation conditions have been established.
Some air separation processes are different. There is no butterfly valve at the nitrogen outlet of the upper tower, and at the same time, the expanded air is only sent to the liquefier without passing through the cooler. For such a process, gas can be sent to the upper tower at the beginning of liquid accumulation. Because this can fully utilize low-temperature gas to cool the upper tower, the cooling capacity of the gas discharged from the upper tower can also be fully recovered in the subcooler and liquefier. At the same time, the gas temperature returning to the cold end of the switching heat exchanger will not be too low, which is conducive to the self cleaning of the switching heat exchanger and will not cause the switching heat exchanger to become too cold. Moreover, the expansion air sent into the upper tower will slightly increase the pressure of the upper tower, which is also beneficial for the accumulation of liquid.
