The reason why the internal compressed air separation process can send the expanded air to the lower tower lies in the essential difference of pressure conditions and the optimization requirements of distillation conditions.
From the aspect of pressure, the operating pressure of the lower tower is usually 0.50~0.55 MPa (a). In the external compression process, the raw air pressure is about 0.6 MPa, which is pumped out through the middle section of the main heat exchanger and enters the expander. After expansion, the pressure drops to 0.40~0.45 MPa, which is lower than the pressure of the lower tower, and there is no differential pressure driving condition, so it can only be sent to the upper tower. While the internal compression process is equipped with an air booster, the high-pressure channel pressure of the main heat exchanger can reach 2.5~8.0 MPa, and the pressure at the extraction point of the expansion air is much higher than that of the external compression process. After isentropic expansion and cooling of the expander, the outlet pressure can still be maintained above 0.55 MPa, which is higher than the working pressure of the lower tower, and the fluid can flow smoothly into the lower tower depending on the pressure difference.
From the perspective of distillation, the expanded air is saturated air containing argon and oxygen, and its composition is consistent with that of the lower tower feed. Theoretically, it can be directly used as the lower tower feed to participate in distillation. If the expanded air is sent to the upper tower, its oxygen content is about 21%. Compared with the oxygen enriched liquid air and waste liquid nitrogen of the upper tower, it is an oxygen poor gas source. A large number of sent air will dilute the reflux liquid of the upper tower and reduce the oxygen extraction rate. If it is sent to the lower column, it can participate in the preliminary distillation first, so that the nitrogen components can be gradually separated in the upper and lower columns, avoiding direct interference with the distillation section of the upper column, which is conducive to improving the oxygen extraction rate.
In addition, the expanded air is in a low-temperature dry saturated state. After entering the lower tower, it combines with the main air to condense and distill. The heat balance can be kept stable by matching the heat load and expansion of the main heat exchanger through the process design.
