Distillation columns are one of the most often used unit operations for separation and purification in the process industries. They can also be some of the most complex to operate and control, because they involve two-phase, multi-stage, counter-current mass and heat transfer (each tray or segment of packing is a theoretical equilibrium stage). The greater the number of trays, the longer the time constants related to composition changes. For a two-product distillation column (top and bottom product), there are typically five degrees of control freedom (control valves):
- Reflux flow
- Top product flow
- Reboiler heat input flow
- Bottom product flow
- Pressure control valve (the specific location depends upon how the pressure is controlled).