Designing Distillation Columns: Stage Balances and Operating Lines

This lecture covers the design of distillation columns, focusing on the stage-by-stage balance in the stripping section. The instructor explains the constant molar flow assumption, where liquid and vapor flow rates below the feed remain unchanged. The lecture emphasizes the importance of equilibrium stages, where streams leaving the column are in equilibrium, while those entering are not. The instructor introduces the concept of the reboiler as an additional equilibrium stage, highlighting that the total number of stages includes the reboiler. The operating line below the feed is derived, showing the relationship between liquid and vapor compositions. The boilup ratio is defined, linking it to the operating line and mass balance equations. The graphical representation of the McCabe-Thiele diagram is introduced, illustrating how to determine compositions at various stages. The lecture concludes with a discussion on the staircase method for calculating compositions in both the top and bottom sections of the distillation column, reinforcing the foundational concepts of distillation design.