A distillation column to separate n-butane from n-pentane was recently put on line in a refinery. Apparently, there was a design error because the column did not make the desired separation, as shown below [Chem. Eng. Prog., 61(8), 79 (1965)]. It is proposed to add an intercondenser in the rectifying section to generate more reflux and an interreboiler in the stripping section to produce additional boilup. Show by use of a McCabe–Thiele diagram how this might improve the operation.

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Mol% nCs in distillate Mol% nC in bottoms Design Specification 0.26 0.16 Actual Operation 13.49 4.28