Distillation towers residue curves

Hence, Eq. (7.23) approximates the operating lines at total reflux and, because i and h are dimensionless variables and Eq. (7.19) is identical in form, the residue curves approximate the operating lines of a distillation tower operating at total reflux. 

Ultimately, when J = oo, L /G y L /G x operating curves both coincide with the 45 line on the xy plot, the A points are at infinity on the Hxy plot, and the number of trays required is the minimum value, This is shown in Fig. 9.23. The condition can be realized practically by returning all the distillate to the top tray as reflux and reboiling all the residue, whereupon the feed to the tower must be stopped. 

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