The reason why fully low-pressure air separation equipment can directly feed the expanded air into the upper column

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Why can full-low-pressure air separation units feed the expanded air directly into the upper column: The cooling capacity of full-low-pressure air separation units is primarily generated by expanders, and the operating pressure of such units corresponds to the operating pressure of the lower column, which is 0.55–0.65 MPa. After being expanded and cooled by the expander, the pressure of this pressurized gas drops to around 0.13 Ma, making it impossible to feed it into the lower column for distillation as in the medium-pressure process. If the expanded air only recovers heat in the heat exchanger without participating in distillation, the oxygen and nitrogen in this air cannot be extracted, which will inevitably affect the yield and extraction rate of oxygen and nitrogen. Since the reflux ratio in the distillation section of the upper column in full-low-pressure air separation units is significantly higher than the minimum value, it is possible to utilize the distillation potential of the excess reflux liquid. Therefore, the expanded air can be directly fed into the upper column for distillation, in order to recover oxygen and nitrogen from the expanded air and thereby increase the oxygen recovery rate. Since full-low-pressure air separation units feed the expanded air directly into the upper column, changes in the cooling capacity will lead to changes in the expansion volume, which inevitably affects the distillation process in the upper column. The close connection between refrigeration and distillation is the most prominent feature of all-low-pressure air separation units.