There could be numerous reasons depending on how your tower is controlled. Depending on your situation some could be:
* Increased reboiler Q
* Trying to control a tight spec on both the top and bottom product. No decoupling in an advanced control scheme.
* Targetting an impossible bottoms spec, i.e., too low a vapour pressure on the bottoms product.
* Lower pressure (lowering separation efficiency)
* Inaccurate overhead product analyser or predictor
* Inaccurate bottoms analyser or predictor
* Controls reacting improperly because tower is entering a flooded state.
