McCabe-Thiele plots provide a simple, graphical tool for the analysis of binary distillations, specifically the determination of the number of trays required.
Many real world applications are too complex for the McCabe-Thiele method however it provides a great tool for learning the basic thermodynamics of tray distillation, as well as understanding the impact of reflux rate, feed composition, product composition and vapor-liquid equilibrium on distillation column design.
The McCabe-Thiele method is a very simplistic model of distillation and subject to the following assumptions:
The mixture is binary i.e. there are only two components.
he heat of vaporization of the two components are equal (When one mole of the heavier component is vaporised, one mole of the lighter component is condensed).
Other thermodynamics effects are negligible, e.g. heat of dissolution is negligible.
100% tray efficiency. Trays calculated using the McCabe-Thiele method are called “theoretical trays”. In practice fluids do not reach equilibrium on each tray, and so the “tray efficiency” is used to determine the number of actual trays required to make a given separation.
If you plotted a graph between distillate purity and number of trays,👇you’ll observe that,
1. For higher distillate purity, the trays will begin to double, and the column height also increases drastically.
2. So say, at 99% purity, it can be 12 trays. But to get a small increment of 0.999%, i.e., 99.999% purity, the number of trays double to 24.
Important Point: Beyond a point, the purity achieved will not justify the increasing costs of a taller column.
