This Demonstration shows how to determine the number of equilibrium stages needed for a two-component (methanol/water) separation in a counter-current distillation column. It also shows the optimal location to feed the binary mixture into the column. Check "view process flow diagram" to show the x-y diagram on the left and the process flow diagram on the right. When that box is not checked, a larger x-y diagram is shown. The column feed has a methanol mole fraction of 0.5 and a quality of 0.5. The condenser is a complete condenser. Set the external reflux ratio L/D, as well as the methanol mole fractions in the distillate x_D and bottoms x_B streams with sliders. When the "view process flow diagram" box is not checked, you can move the cursor over the x-y diagram to see the curve labels. The number of stages is determined by the stepping off stages starting at the top of the column; the reboiler is an equilibrium stage. The number of equilibrium stages depends on the compositions of the exiting streams and the reflux ratio.