Why does reducing the nitrogen liquefied gas reflux valve in the lower column of an air separation unit decrease the amount of air entering the column, thereby lowering the pressure in the upper column?

Is it because the heat exchange area in the main cooler decreases and the volume of the lower column shrinks, that the amount of air entering the column is reduced? Then why does the pressure in the lower column increase while the pressure in the upper column decreases?

Your question should consider the lower column as an independent production unit; the opening degree of this reflux valve depends on the liquid level height on the lower side of the main cooler column. According to the original design concept, this valve should be fully open during normal operation, as the U-shaped liquid seal installed during actual installation is used to control the return flow from the lower column by regulating the amount of liquid nitrogen taken from the upper column. The general operating concept for open-loop systems is that even when a U-shaped liquid seal is available, control is still required through the valve opening degree; it’s just that kind of concept. The purpose of this liquid seal arrangement is to ensure that no liquid accumulates on the main cooling nitrogen side, thereby maintaining the maximum heat exchange area. Reducing the reflux valve slightly will cause the liquid level on the nitrogen side of the main cooler to rise (with the valve leading to the upper tower remaining unchanged). This results in a decrease in the heat exchange area, a reduction in the amount of liquid that is condensed, and a decrease in the upward flow of gas. As a consequence, the amount of gas entering the tower decreases, leading to an increase in the pressure inside the tower. Also, reduce the valve for the lower reflux liquid, as an insufficient supply of lower reflux liquid can lead to changes in resistance and purity. To understand the entire device, it is essential to adhere to the design values specified for normal production; without these design values, understanding of its principles becomes impossible. For reference. Oh, there’s one more important point to note: the driving force for gas to enter the tower is the condensation of the gas, which reduces its volume by about 800 times. It is the continuous supply of this gas that enables the air compressor to function properly by providing the necessary gas material. For this to happen, certain conditions must be met, namely regarding pressure and flow rate.

Reducing the reflux lowers the gas phase volume on the nitrogen side of the main cooler; as a result, the amount of gas entering the main cooler, and thus the amount of air entering the tower, decreases； As the heat exchange on the main cooling nitrogen side decreases, the evaporation rate in the upper column drops, and consequently the pressure in the upper column also decreases.

By reducing the liquid nitrogen reflux valve in the lower column, the amount of nitrogen at the top of the column that condenses into liquid nitrogen decreases, resulting in an increase in the pressure within the lower column. Similarly, less condensation occurs in the condensing evaporator; as a result, the amount of vapor entering the upper column decreases, and the pressure in that column drops.