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GATE 2024 CH – Question 23

Instrumentation and Process Control · Controller modes, control valves and tuning · 1 mark · Multiple choice

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. The sign of the controller gain for the pressure controller (PC) and that for the composition controller (CC) are, respectively,

a column with an overhead condenser. The pressure controller PC acts on an air-to-close (AC) valve at the condenser. The composition controller CC sets the reflux-to-distillate ratio L/D, which is multiplied by the distillate flow to give the setpoint of the reflux flow controller FC. The distillate flow is on an air-to-open (AO) valve. AC: air-to-close valve, AO: air-to-open valve.
  1. negative, negative
  2. negative, positive
  3. positive, positive
  4. positive, negative

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Correct answer: (D) positive, negative

Explanation

The gain of each controller has to make the loop a negative feedback loop. For the pressure loop, a rising pressure needs more cooling, that is a more open valve. The valve is air-to-close, so the controller output has to fall when the pressure rises. That is a reverse-acting controller, with a positive gain (the usual convention $u = K_c(SP - PV)$). For the composition loop, a rising impurity in the distillate needs more reflux, so the output (the L/D setpoint) has to rise when the impurity rises, a direct-acting controller with a negative gain. So the signs are positive for PC and negative for CC.