ANS:A - total

In the McCabe-Thiele method of theoretical plate calculation for a distillation column, the operating lines of the stripping and rectifying sections coincide with the diagonal at total reflux. Explanation:

• The McCabe-Thiele method is a graphical technique used to analyze binary distillation processes and determine the number of theoretical plates required for separation.
• In this method, the stripping section operating line represents the equilibrium relationship between the vapor and liquid phases leaving the bottom of the column, while the rectifying section operating line represents the equilibrium relationship between the vapor and liquid phases leaving the top of the column.
• At total reflux, the reflux ratio is infinite, meaning that all the condensed vapor (distillate) is returned to the column as reflux, and no product is withdrawn.
• At total reflux, the compositions of the liquid and vapor leaving the top and bottom of the column are identical to those of the feed. As a result, the operating lines of both the stripping and rectifying sections coincide with the diagonal line on the McCabe-Thiele diagram.
• At total reflux, the intersection of the stripping section operating line with the diagonal represents the composition of the liquid leaving the bottom of the column, while the intersection of the rectifying section operating line with the diagonal represents the composition of the vapor leaving the top of the column.
• Total reflux serves as a reference point for analyzing the distillation process and determining the minimum number of theoretical plates required for separation.