Finding the minimum reflux ratio required for the separation allows for an actual reflux value to be chosen on an economic basis and the extra costs incurred compared to the datum set by minimum reflux. For an ethanol-water system the minimum reflux is calculated from a graphical construction. The gradient of the line that is tangential to the equilibrium curve between the mole fraction of ethanol in the feed xf = 0.065 and the mole fraction of ethanol in the distillate xd = 0.779 is proportional to the minimum reflux ratio Rmin, figure 5.3