ChE / MET 433

1. ChE / MET 433 Linearity, Windup, & PID 11 Apr 12Process Linearity, Integral Windup, PID Controllers

2. Quiz Solutions ChE / MET 433

3. Process Linearity • Test the Heat Exchanger process linearity by: • Starting Loop Pro trainer • Set %CO to 80% • Make steps down (say 10% down) to the %CO • Measure the response • Calculate the process gain

4. K = -1.09 K = -0.69 K = 0.-45 K = -0.33 K = -0.26 K = -0.15 Adaptive Control ?

5. Integral (Reset) Windup • “Windup” can occur if integral action present • Most modern controllers have anti-windup protection • If doesn’t have windup protection, set to manual when reach point of saturation, then switch back to auto, when drops below sat. level • IE: LoopPro Trainer, select Heat Exchanger • Set %CO to 90%; SP to 126; Kc to 1 %/deg C; Tau I to 1.0 min • Set Integral with Anti-Reset Windup ON • Change Set Point to 120 deg. C. (~10 min); then change back to 126 deg. C • Repeat with controller at ON: (Integral with Windup)

6. Integral (Reset) Windup

7. In-Class PID Controller Exercise • Tune the Heat Exchanger for a PID Controller: • Use the built in IMC, and choose Moderately Aggressive • Start Loop Pro trainer • Tune at the initial %CO and exit temperature • Compare PI with PID • Compare PID with PID with filter

8. ChE / MET 433 Advanced control schemes 11 Apr 12Cascade Control: Ch 9

9. Improve Feedback Control • Feedback control: • Disturbance must be measured before action taken • ~ 80% of control strategies are simple FB control • Reacts to disturbances that were not expected • We’ll look at: • Cascade Control (Master – Slave) • Ratio Control • Feed Forward

10. Cascade Control • Control w/ multiple loops • Used to better reject specific disturbances + - Take slow process: Split into 2 “processes” that can measure intermediate variable? + + - - Gp2 must be quicker responding than GP1. • Inner (2nd-dary) loop faster than primary loop • Outer loop is primary loop

11. Material Dryer Example MT % moisture MC steam Heat Exchger air blower T + -

12. Separate Gp into 2 blocks MT % moisture MC TC steam TT Heat Exchger air blower T + + - -

14. Problem Solving Exercise: Heat Exchanger TC Single feedback loop. Suppose known there will be steam pressure fluctuations… steam TT Hot water Heat Exchger T Design cascade system that measures (uses) the steam pressure in the HX shell. PT steam TT Hot water Heat Exchger T

15. Temperature Control of a Well-Mixed Reactor (CSTR) Ti Responds quicker to Ti changes than coolant temperature changes.

16. Temperature Control of a Well-Mixed Reactor (CSTR) Use Cascade Control to improve control. Ti If Tout (jacket) changes it is sensed and controlled before “seen” by primary T sensor. • Secondary Loop • Measures Tout (jacket) • Faster loop • SP by output primary loop • Primary Loop: • Measures controlled var. • SP by operator

17. Cascade Control Benefits: • Disturbances in secondary loop corrected by 2ndary loop controller • Flowrate loops are frequently cascaded with another control loop • Improves regulatory control, but doesn’t affect set point tracking • Can address different disturbances, as long as they impact the secondary loop before it significantly impacts the primary (outer loop). Challenges: • Secondary loop must be faster than primary loop • Bit more complex to tune • Requires additional sensor and controller

18. Cascade Control Examples Distillation Columns Objective: Regulate temperature (composition) at top and bottom of column

19. Furnace TP out Objective: Keep TP out at the set point Heat Exchanger T2 out Objective: Keep T2 out at the set point

20. In-Class Exercise: Cascade Control System Design • What affects flowrate? • Valve position • Height of liquid • P (delta P across valve) Design a cascade system to control level (note overhead P can’t be controlled)

21. In-Class Exercise: Cascade Control System Design Does this design reject P changes in the overhead vapor space?

22. Tuning a Cascade System • Both controllers in manual • Secondary controller set as P-only (could be PI, but this might slow sys) • Tune secondary controller for set point tracking • Checksecondaryloop for satisfactory set point tracking performance • Leave secondary controller in Auto • Tune primary controller for disturbance rejection (PI or PID) • Both controllers in Auto now • Verify acceptable performance

23. In-Class Exercise: Tuning Cascade Controllers • Select Jacketed Reactor • Set T cooling inlet at 46 oC(normal operation temperature; sometimes it drops to 40 oC) • Set output of controller at 50%. • Desired Tout set point is 86 oC(this is steady state temperature) • Tune the single loop PI control • Criteria: IMC aggressive tuning • Use doublet test with +/- 5 %CO • Test your tuning with disturbance from 46 oC to 40 oC

24. In-Class Exercise: Tuning Cascade Controllers • Select Cascade Jacketed Reactor • Set T cooling inlet at 46 oC (again) • Set output of controller (secondary) at 50%. • Desired Tout set point is 86 oC (as before) • Note the secondary outlet temperature (69 oC) is the SP of the secondary controller • Tune the secondary loop; use 5 %CO doublet open loop • Criteria: ITAE for set point tracking (P only) • Use doublet test with +/- 5 %CO • Test your tuning with 3 oCsetpoint changes • Tune the primary loop for PI control; make 3 oC set point changes (2nd-dary controller) • Note: MV = sp signal; and PV = T out of reactor • Criteria: IAE for aggressive tuning (PI) • Implement and with both controllers in Auto… change disturbance from 46 to 40 oC. • How does response compare to single PI feedback loop?

25. ChE / MET 433 Advanced control schemes 13 Apr 12Ratio Control: Ch 10

26. Ratio Control • Special type of feed forward control A B • Blending/Reaction/Flocculation • A and B must be in certain ratio to each other

27. Ratio Control Possible control system: FY FY FC FC FT FT B A • What if one stream could not be controlled? • i.e., suppose stream A was “wild”; or it came from an upstream process and couldn’t be controlled.

28. Ratio Control Possible cascade control systems: “wild” stream A FT Desired Ratio FC FY FT B “wild” stream A FT Desired Ratio This unit multiplies A by the desired ratio; so output = FY FC FT B

29. Ratio Control Uses: • Constant ratio between feed flowrate and steam in reboiler of distillation column • Constant reflux ratio • Ratio of reactants entering reactor • Ratio for blending two streams • Flocculent addition dependent on feed stream • Purge stream ratio • Fuel/air ratio in burner • Neutralization/pH

30. In-Class Exercise: Furnace Air/Fuel Ratio • Furnace Air/Fuel Ratio model • disturbance: liquid flowrate • “wild” stream: air flowrate • ratioed stream: fuel flowrate • Minimum Air/Fuel Ratio 10/1 • Fuel-rich undesired (enviro, econ, safety) • If air fails; fuel is shut down Check TC tuning to disturbance & SP changes. PV Desired 2 – 5% excess O2 Disturbance var. TC Dependent MV TC output Ratio set point Independent MV

31. ChE / MET 433 Advanced control schemes 16 Apr 12Feed Forward Control: Ch 11

32. Feed Forward Control steam Suppose qiis primary disturbance TC TT Heat Exchanger ? What is a drawback to this feedback control loop? ? Is there a potentially better way? steam What if Ti changes? FF Heat Exchanger FT TT FF must be done with FB control!

33. Feed Forward and Feedback Control TC steam TY FF TY FT TT Heat Exchanger Block diagram: + + + + + -

34. Feed Forward Control + + + + + - Response to MFF No change; perfect compensation!

35. Feed ForwardControl + + + + + - Examine FFC T.F. For “perfect” FF control: + +

36. Feed ForwardControl: FFC Identification Set by traditional means: Model fit to FOPDT equation: Dead time compensator FF Gain Lead/lagunit Often ignored; if set term to 1 Accounts for time differences in 2 legs { FFC ss } steady state FF control { FFC dyn } dynamic FF control

37. ChE / MET 433

38. Problem Solving Exercise: Heat Exchanger TC PC PT steam TT Hot water Heat Exchger T Draw the block diagram: what is the primary and what is the secondary loop? + + - -