The document discusses the McCabe-Thiele design method for distillation column design using vapor-liquid equilibrium (VLE) data. It explains that the McCabe-Thiele method uses a graphical approach to determine the theoretical number of stages required for a binary separation based on the VLE plot. Operating lines are drawn on the VLE diagram to define the mass balance relationships between the liquid and vapor phases. The operating line for the rectification section is constructed by drawing a line with slope R/(R+1) from the desired top product composition point, where R is the reflux ratio. The operating line for the stripping section has a slope of Ls/Vs, where Ls and Vs are the liquid and