Plotting McCabe-Thiele Diagrams in Microsoft Excel for Non-Ideal Systems

McCabe-Thiele diagrams are an indispensable aid to teaching distillation, as they graphically demonstrate key concepts (number of stages, feed location, minimum reflux ratio, etc.) related to operability of a distillation column. Their drawback is that they are difficult to draw freehand, and this difficulty is aggravated when the vapor-liquid equilibrium data for the system is represented by a set of equilibrium data points, as then the necessary first step is to construct the equilibrium curve. In this paper, a method is presented to automate the process of fitting binary equilibrium data to a smooth curve (using a cubic B-spline algorithm implemented via Visual Basic for Applications, or VBA) and then drawing the McCabe-Thiele diagram in Microsoft Excel. In this way, the effect of changes to the operating conditions can be easily demonstrated. Furthermore, the method will locate the azeotrope if the system has one. The goals of this paper are to provide instructors a quick, automated method of generating a McCabe-Thiele diagram for a nonideal binary system to facilitate classroom instruction, to aid students in learning about and manipulating these diagrams, and to demonstrate how to integrate VBA calculations (including the cubic B-splines) into an Excel worksheet.