How to Use the McCabe-Thiele Method for Fractional Distillation
This Demonstration shows how to use the McCabe–Thiele method to determine the number of equilibrium stages needed to separate a binary mixture (methanol and water) in a distillation column. First, select "plot points" to show the location of the feed, distillate and bottoms compositions on the x-y diagram. The methanol feed mole fraction is zF = 0.5, the desired methanol mole fraction in the distillate is xD = 0.85, and the desired methanol mole fraction in the bottoms is xB = 0.05. Next, select "draw operating lines" to set the feed quality with a slider and plot the operating lines for the feed, rectifying and stripping sections. The diagram on the right shows a representation of part of the distillation column and explains the equations used for the operating lines. Then, select "count stages" and use the slider to step off stages and draw lines for each stage; the diagram on the right highlights the corresponding location on the column. The partial reboiler is considered an equilibrium stage. The green horizontal lines on the x-y diagram result from a material balance between stages. The purple vertical lines correspond to vapor-liquid equilibrium (VLE) on each stage. Finally, select "optimal feed location" to determine where the feed should enter the column; the feed should enter above the stage where the rectifying and stripping sections meet. Mouse over the stage numbers on the x-y diagram to see which section the numbers correspond to. 

Download the CDF file to view the simulation using the free Wolfram CDF player.