During operation, if the deviations from design specifications are excessive, then the column may no longer be able handle the separation task. To overcome the problems associated with the feed, some column are designed to have multiple feed points when the feed is expected to containing varying amounts of components. As the reflux ratio is increased, the gradient of operating line for the rectification section moves towards a maximum value of 1. Physically, what this means is that more and more liquid that is rich in the more volatile components are being recycled back into the column. Separation then becomes better and thus less trays are needed to achieve the same degree of separation.
Minimum trays are required under total reflux conditions, i.e. there is no withdrawal of distillate.
On the other hand, as reflux is decreased, the operating line for the rectification section moves towards the equilibrium line. The ' pinch ' between operating and equilibrium lines becomes more pronounced and more and more trays are required. This is easy to verify using the McCabe-Thiele method . The limiting condition occurs at minimum reflux ration , when an infinite number of trays will be required to effect separation. Although it provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid buildup on trays.