McCabe – Thiele Method

Two graduate students from the Massachusetts Institute of Technology (MIT), Ernest W. Thiele and Warren L.McCabe presented the McCabe-Thiele method in the year 1925. For the binary distillation, this method is the preferred to be the most instructive and the simplest method. The fact that the mole fraction of one of the two components determines the composition at each equilibrium stage or at each theoretical tray forms the basis of this method.

Assumptions:

The assumptions which form the base for the McCabe-Thiele method is the assumption of the constant molar overflow which demands the feed component to have an equal amount of the molar heats of vaporization. A mole of vapor has to be condensed for every mole of the liquid that is vaporized.

Steps in McCabe-Thiele diagram:

There are several steps involved in the construction and the use of the McCabe-Thiele diagram. Drawing the equal sized horizontal and the vertical axes of a graph is the first step where the liquid phase’s lower-boiling feed component will be denoted by the x axis(the horizontal axis) and the vapor phase’s lower-boiling feed component will be denoted by the y axis(the vertical axis).

The second step is to make a straight line between the origin of the graph and the where the x and y becomes equal like 1.0 which shows the x=y line. For drawing the remaining lines accurately this 45 degree line is very helpful. Then the equilibrium line is drawn with the help of VLE data points in the lower boiling component which represents the vapor phase compositions in accordance with the each value in the liquid phase of the equilibrium. The vertical lines which protrudes from the x axis to the x=y line for the desire composition and for the feed of the corresponding bottoms product and for the top distillate product.

The third step is drawing the operating line of distillation column for the distillation column’s rectifying section. The rectifying operating line is need to be drawn at a downward slope starting from the meeting point of the distillate composition with the x=y line. In the operating line (Δy/Δx) of L / (D + L), the reflux’s molar flow rate is denoted as L and the distillate product’s molar flow rate is denoted as D.

The final step in the McCabe-Thiele method is drawing the blue q-line from the point x=y which has to intersect the rectifying operating line.

Questions:

• Who presented the McCabe-Thiele method?
• What is first step in the McCabe-Thiele method?