Air sent to the upper column for expansion

The gas and liquid on the upper tray are saturated, so it is better that the expanded air sent to the upper tray is also saturated, even with a small amount of liquid. However, due to the limitation of the structure of the expander, the air can not reach the saturation state at the outlet of the expander, and no liquid can be produced, otherwise the expander will have a liquid hammer accident, causing damage to the expander, so there is a certain degree of superheat in the expansion air sent to the upper column.