The McCabe-Thiele method can be used for a variety of special cases. Figure 5-14 illustrates the situation if two feeds are used.

Figure 5-14: Distillation column with two feeds (40).

See that each feed has its own feed line. The juncture of each with the rectifying and stripping mass balances, respectively, are connected with the intermediate section mass balance. In essence the sequence is to use the rectifying mass balance from X _D = Y _D to the first juncture, then the intermediate mass balance to the second juncture, and finally the stripping mass balance to X _B = Y _B.

A second special case is shown in Figure 5-15. Here a sidestream L _S (composition of X _S) is removed from the column. In this situation the rectifying mass balance line is located as described earlier.

Figure 5-15: Distillation with sidestream (40).

Next, a vertical line is erected at X _S. The intercept of this line with the rectifying mass balance is then connected to the intercept of the stripping mass balance line with the column feed line. Stage determination uses the rectifying mass balance to X _S, the intermediate mass balance to the feed line, and the stripping mass balance to X _B = Y _B. The crosshatched lines indicate the portions of the relevant mass balances that are not used.

A more rigorous design than the McCabe-Thiele would be one...