Debutanization tower control scheme

First of all, I work in the field of process engineering, not in automatic control; let me share my understanding (the terminology related to automatic control might not be accurate): 1. The control mechanism here is a temperature-flow cascade control system, which is easy to understand – the temperature signal is converted into a flow signal for regulation purposes. The flow signal is simply introduced from the controller to the subsequent calculator； 2. The value obtained after the summation operation is fed into the flow controller. 3. Level and flow are controlled in a cascade manner. Taking these points into account, it can be understood that the lower control valve performs temperature-flow cascade control, where the flow refers to the liquefied gas flow exiting the device; while the upper control valve carries out level-flow cascade control, with the flow representing the total liquid flow leaving the reflux tank. By using this control scheme, it is possible to keep both the liquid level in the reflux tank and the temperature inside the tower within the desired ranges. However, in my opinion, although the logic behind this complex control circuit makes sense theoretically, practical operation may encounter issues such as large fluctuations, delays, and improper PID parameter settings, which can make it difficult to utilize this complex control circuit effectively

Is it then unnecessary to add the amount of liquefied gas delivered outside with the amount that returns?

It should be simple cascade loop control