Review Of Binary Distillation – McCabe-Thiele By Computer

For many years the McCabe-Thiele graphical method (1) has been the accepted procedure for determining  the number of theoretical trays in a binary distillation when constant molal overflow prevails throughout the column.  For close boiling separations such as in an ethane-ethylene or propane-propylene splitter, constant molal overflow is very closely attained.  However, in the rectifying sections of these columns, relative volatilities are close to one.  As a result, the equilibrium curve and upper operating line are very close together,  especially in the region of the overhead product composition x_D. This situation makes it very difficult to graphically step-off theoretical stages, which can be great in number in this region.

The difficulties described above can be circumvented if one is able to solve the McCabe-Thiele method analytically or by numerical methods on the computer or on a PC.  Here a BASIC computer program is  presented for doing just that.  This program should be especially useful, for example, to a process engineer, working on an ethylene plant proposal, who needs to make a quick size (height) estimate for a C₂ or C₃ splitter column.  Use of the program is illustrated for a specific example involving a C₃ splitter.

Finally, a brief discussion is presented for estimating the effect of errors in relative volatility on the calculation of the number of theoretical stages.

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