1 ChE / MET Apr 12 Cascade Control: Ch 09 Ratio Control: Ch 10 Advanced control schemes

Tuning a Cascade System 2 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 Check secondary loop 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

3 In-Class Exercise: Tuning Cascade Controllers Select Jacketed Reactor Set T cooling inlet at 46 o C (normal operation temperature; sometimes it drops to 40 o C) Set output of controller at 50%. Desired T out set point is 86 o C (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 o C to 40 o C

4 In-Class Exercise: Tuning Cascade Controllers Select Cascade Jacketed Reactor Set T cooling inlet at 46 o C (again) Set output of controller (secondary) at 50%. Desired T out set point is 86 o C (as before) Note the secondary outlet temperature (69 o C) 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 o C setpoint changes Tune the primary loop for PI control; make 3 o C set point changes (2 nd -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 o C. How does response compare to single PI feedback loop?

Ratio Control Special type of feed forward control Blending/Reaction/Flocculation A and B must be in certain ratio to each other AB 5

Ratio Control Possible control system: 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. A B 6 FT FC FY FT FC FY

Ratio Control Possible cascade control systems: “wild” stream A B 7 FT FY FC Desired Ratio A B FT FY FC Desired Ratio This unit multiplies A by the desired ratio; so output = “wild” stream

Ratio Control Uses: 8 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

9 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 Independent MV PV Ratio set point Dependent MV Disturbance var. TC TC output Desired 2 – 5% excess O 2 Check TC tuning to disturbance & SP changes.

10 ChE / MET Apr 12 Feed Forward Control: Ch 11 Advanced control schemes

Feed Forward Control Suppose q i is primary disturbance 11 Heat Exchanger TC TT ? What is a drawback to this feedback control loop? ? Is there a potentially better way? Heat Exchanger TT FT FF What if Ti changes? FF must be done with FB control! steam

Feed Forward and Feedback Control 12 Heat Exchanger TT FT TY steam TC FF TY Block diagram:

Feed Forward Control No change; perfect compensation! Response to MFF

Feed Forward Control Examine FFC T.F For “perfect” FF control:

Feed Forward Control: FFC Identification Set by traditional means: 15 Model fit to FOPDT equation: FF GainLead/lag unit Dead time compensator { FFC ss } steady state FF control { FFC dyn } dynamic FF control Accounts for time differences in 2 legs Often ignored; if set term to 1 Eqn: p 379

Feed Forward Control: FFC Identification How to determine FOPDT models : With G c disconnected: Step change CO FB, say 5% Fit C(s) response to FOPDT Still in open loop: Step change Q, say 5 gpm Fit C(s) response to FOPDT lead time lag time

Lead/Lag or Dynamic Compensator Look at effect of these two to step change in input 17 Final Change from: Magnitude of step change, Initial response by the lead/lag, Exponential decay from lag, Output or response

Feed Forward Control Rule of Thumb: if lead-lag won’t help much; use FFCss 18 (p 389) In text: pp , useful comments if implementing FFC Compensates for disturbances before they affect the process 1. Requires measurement or estimation of the disturbance 2. Can improve the reliability of the feedback controller by reducing the deviation from set point 2. Does not compensate for unmeasured disturbances 3. Offers advantages for slow processes or processes with large deadtime. 3. Linear based correction; only as good as the models; performance decreases with nonlinear processes. No improvement using FFC with set point changes.

In-Class PS Exercise: Feed Forward Control 19 What is the G m, and what is the G D ? Determine FCC Tune PI controller to aggressive IMC Test PI Controller Test PI + FFCss only Test PI + FFC full For disturbance: T jacket in 50 o C – 60 o C – 50 o C

In-Class PS Exercise: Feed Forward Control 20 PI only PI + FFCss only PI + full FFC

21 ChE / MET 433