Procedure

 

Below is the general procedure for the McCabe-Thiele Method. Follow the links for more specific information on any topic. The graphical implementation of this procedure is shown in the example. To implement this procedure for any specific system, please visit the interactive example.

 

1. Create the x-y diagram of the two-component system and add the x=y (diagonal) line.

2. Specify the purity of the top and bottoms products, as well as the feed plate condition (xD,xB,and zF, respectively).

3. Draw the operating lines by meeting one of the following:
  • Slope of the q-line and the rectifying section known: Draw the two known operating lines from the known slopes. The stripping section line can be drawn between the points where the known operating lines intersect and the point where XBmeets the diagonal line. On the graph to the right, the rectifying section operating line is red (upper section), the q-line is blue and the stripping section is green (lower section)
  • Information about the reflux ratio is known:With the use a limiting condition, there will be two possibilities: only the slope of the q-line will be necessary, or no further information will be necessary.

4. Draw the stages, shown in purple, starting at xD on the diagonal line, move horizontal to the equilibrium line, and then vertically back to x-y line (see graph, and the example below.)


5. Continue process toward xB, switching the lower operating line after crossing the q-line, estimating the last stage if necessary. The optimum feed plate is one which crosses the q-line. In the below example, there are four stages, and the optimum feed plate is number three.

6. This procedure finds the theoretical number of plates, if all equipment was 100% efficient. In reality, an efficiency factor should be specified to represent a real system. One efficiency factor is the overall efficiency, where the theoretical plates are a percentage of the real plates:

efficiency

Example

This is an example of how the procedure for designing a binary distillation column with the McCabe-Thiele Method. On the right is summary of each step.

Summary of Steps
  1. Draw x-y diagram and x=y line
  2. Find the outlet and feed conditions known
  3. Draw q-line
  4. Draw upper operating line
  5. Connect lower operating line
  6. Step stages from know end conditions, and count stages

 

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