GATE 2024 Chemical Engineering, Q23 · Process Control
The correct answer is D, positive, negative.
Question
Consider the control structure for the overhead section of a distillation column shown in the figure. The composition controller (CC) controls the heavy key impurity in the distillate by adjusting the setpoint of the reflux flow controller in a cascade arrangement, while the pressure controller (PC) manipulates a valve on the overhead vapor/condenser duty. The sign of the controller gain for the pressure controller (PC) and that for the composition controller (CC) are, respectively,
- A. negative, negative
- B. negative, positive
- C. positive, positive
- D. positive, negative
Official answer: positive, negative
Why
GATE 2024 official key: (D). Tracing the air-to-open/air-to-close valve actions and the direction of each measured variable's response to its manipulated variable in the figure gives a positive gain for PC and a negative gain for CC.
Explanation cross-checked for consistency. Final answer matches the official IIT answer key.