Abstract: In a fractional distillation process in which it is desired to maintain a desired overhead product composition and bottoms product composition, a set point for the flow rate of the external reflux to the fractional distillation column is increased or decreased by an incremental amount or is left constant based on whether the concentration by the heavy key component in the overhead product stream and the concentration of the light key component in the bottoms product stream are above a high limit, below a low limit, between the high limit and low limit or some combination thereof and also based on the relationship between a set point for the flow rate of the external reflux and a high limit for the flow rate of the external reflux. At the same time, a temperature set point for the fractional distillation column is increased or decreased by an incremental amount or is left constant depending on the same factors.