ANS:C - In Ponchon-Savarit method of no. of theoretical plate determination, the stripping and rectifying operating lines are vertical at total reflux.

The correct statement is: "Kremsor-Brown-Souder's equation is used to calculate the efficiency of a plate tower." Kremser-Brown-Souder equation, also known as the Kremser equation, is a widely used empirical correlation for estimating the efficiency of a distillation column tray. It relates the efficiency of the tray (𝜂η) to the vapor and liquid flow rates, as well as to the physical properties of the system. The other statements are incorrect:

• The slope of the stripping line can be greater than unity depending on the system and operating conditions.
• The fixed cost of a distillation column generally increases with an increase in reflux ratio due to the additional equipment required for higher reflux ratios.
• In the Ponchon-Savarit method, the stripping and rectifying operating lines intersect at the total reflux point, but they are not vertical lines. They have slopes determined by the equilibrium and operating conditions.