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 xD. 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.