1. CHE 185 – PROCESS CONTROL AND DYNAMICS
PID ENHANCEMENTS

2. Limitations of Convential PID Controllers
The performance of PID controllers can be substantially limited by:
Process nonlinearity
Measurement deadtime
Process constraints
there are several approaches for PID controllers to handle each of these problems

3. Inferential Control
Uses easily measured process variables (T, P, F) to infer more difficult to measure quantities such as compositions and molecular weight.
Can substantially reduce analyzer delay.
Can be much less expensive in terms of capital and operating costs.
Can provide measurements that are not available any other way

4. Inferential Control
Examples of variables that are not easy to measure directly
DENSITY
VAPOR PRESSURE
MELT INDEX
GAS COMPOSITION
MOLECULAR WEIGHT

5. Inferential Control
SECONDARY MEASUREMENTS ARE USED WITH THE FOLLOWING FOR INFERENTIAL CONTROL
PROCESS MODEL EQUATIONS
THERMODYNAMIC RELATIONSHIPS, I.E. LINKING TEMPERATURE TO CONCENTRATION
EMPIRICAL MODELING
ISOTHERMAL VISCOSITY VERSUS LIQUID COMPOSITION

6. INFERENTIAL CONTROL MEASURES A VARIABLE USING AN INDIRECT METHOD
USED WHEN
IT IS NOT PRACTICAL TO MEASURE THE TARGET VARIABLE
EXCESSIVE COST FOR CONTROL EQUIPMENT TO DIRECTLY MEASURE THE VARIABLE
EXCESSIVE DOWNTIME IN A TARGET VARIABLE SENSOR
THERE IS AN INFERENTIAL VARIABLE AVAILABLE

7. INFERENTIAL CONTROL CHARACTERISTICS OF THE INFERENTIAL VARIABLE
IT MUST BE CLOSELY RELATED TO THE TARGET VARIABLE
IT MUST NOT BE AFFECTED BY CHANGES IN THE PROCESS CONDITIONS
DYNAMICS ARE ADEQUATE FOR FEEDBACK CONTROL

8. INFERENTIAL CONTROL CORRECTIONS TO INFERENTIAL CONTROL VARIABLE
CAN USE A CASCADE CONTROL SOURCE
CAN BE MANUALLY ADJUSTED

9. INFERENTIAL CONTROL
example USING TEMPERATURE TO CONTROL COMPOSITION for isobaric flash

10. INFERENTIAL CONTROL
example USING TEMPERATURE TO CONTROL COMPOSITION for isobaric flash
CONTROLS COMPOSITION BASED ON FLASH TEMPERATURE
DIRECT CONTROLLED VARIABLE IS FLASH PRESSURE
LEVEL IS ALSO DIRECTLY CONTROLLED

11. INFERENTIAL CONTROL
example USING TEMPERATURE TO CONTROL COMPOSITION for isobaric flash
HOW IS THE TEMPERATURE SETTING CHECKED FOR THIS EXAMPLE?
MANUAL ANALYSIS CAN BE USED TO ADJUST
A FEED FORWARD SIGNAL FROM A PROCESS ANALYZER CAN ALSO BE USED (SEE SKETCH next slide)

12. INFERENTIAL CONTROL
IT IS ASSUMED THAT THE LAG TIME FOR THE ANALYZER LOOP IS LONGER THAN THAT FOR THE TEMPERATURE LOOP.
THIS ALSO WILL TAKE CARE OF ANY STEADY-STATE OFFSET FOR THE TEMPERATURE CONTROL

13. Inferential Temperature Control for Distillation Columns
Reboiler control based on tray temperature

14. Inferential Temperature Control for Distillation Columns
Choosing a Proper Tray Temperature Location
tray temperature used for inferential control should show strong sensitivity

15. Inferential Temperature Control for flow reactor
See example 13.2 in text

16. ARTIFICIAL NEURAL NETWORKS (ANN’s)
THESE ARE NON-LINEAR CONTROLLERS THAT ARE USED TO CONTROL NON-LINEAR PROCESSES
THE MODEL TAKES INPUT(S) FROM THE SYSTEM AND USES THESE WITH WEIGHTED FUNCTIONS, TO PROVIDE THE OUTPUT FOR THE CONTROLLER

17. ARTIFICIAL NEURAL NETWORKS (ANN’s)
THE WEIGHTING FUNCTIONS ARE REVISED OVER TIME TO OPTIMIZE THE OUTPUT
THE ANN IS TUNED BY THE SYSTEM AND ONLY APPLIES TO ONE SYSTEM.

18. ARTIFICIAL NEURAL NETWORKS (ANN’s)
Soft Sensors Based on Neural Networks
Neural network (NN) provides nonlinear correlation.
Weights are adjusted until NN agrees with plant data
NN-based soft sensors are used to infer NOx levels in the flue gas from power plants.

19. SCHEDULING CONTROLLER TUNING
THIS IS A METHOD TO COMPENSATE FOR PROCESS NON-LINEARITY THAT CAN AFFECT CONTROL RESPONSE
THE BASIC TECHNIQUE IS TO TUNE THE CONTROLLER BASED ON EMPIRICAL DATA
OPTIMUM TUNING DATA IS OBTAINED OVER A RANGE OF PROCESS SETTINGS.

20. SCHEDULING CONTROLLER TUNING
THE TUNING DATA IS THEN CONVERTED INTO Proportional, INTEGRAL AND DERIVATIVE RESET FUNCTIONS OF THE MANIPULATED VARIABLE.
THIS METHOD IS SIMILAR TO ANN EXCEPT IT ONLY LOOKS AT ONE INPUT VARIABLE AND RESULTS IN CLEARLY DEFINED FUNCTIONS

21. SCHEDULING CONTROLLER TUNING
Adjust tuning of heat exchanger control for various feed rates
Link tuning parameters to the flow rates

22. SCHEDULING CONTROLLER TUNING
Typical open loop response

23. SCHEDULING CONTROLLER TUNING
Close loop response with scheduling

24. SCHEDULING CONTROLLER TUNING
Close loop response with scheduling

25. SCHEDULING CONTROLLER TUNING
IMPLEMENTATION CAN TAKE THE FORM OF ADJUSTMENT OF PI GAIN AND INTEGRAL TIME USING THE TUNING FACTORS
For example using zeigler-nichols (equation ):

26. OVERRIDE/SELECT CONTROL
THIS METHOD EMPLOYS A SELECTION AMONG MULTIPLE INPUTS
IT CAN BE APPLIED TO ROUTINE CONTROL
IT CAN BE USED TO IMPLEMENT EMERGENCY CONTROL
UNDER NORMAL OPERATION A LOW SELECT OR A HIGH SELECT METHOD IS USED BY THE CONTROLLER TO ADJUST THE MANIPULATED VARIABLE

27. OVERRIDE/SELECT CONTROL
INPUT COMES FROM TWO OR MORE CONTROLLERS TO A SECOND IN A CASCADE CONFIGURATION
THE COMPARISON CONTROLLER CHOOSES THE LOWEST OR HIGHEST TO SEND TO THE ACTUATOR
CONSIDER A REACTOR WITH COOLING FOR TEMPERATURE CONTROL

28. OVERRIDE/SELECT CONTROL
THE LOW SELECTOR TAKES THE LOWER VALUE FROM THE COMPOSITION ANALYZER AND THE REACTOR TEMPERATURE SENSOR
THE LOWER VALUE IS SELECTED BECAUSE THIS ASSURES THE HIGHEST COOLING FLOW TO THE UNIT.

29. OVERRIDE/SELECT CONTROL
TEXT PROVIDES SEVERAL OTHER EXAMPLES BASED ON HIGH, LOW AND COMBINED SELECTION
NOTE THAT IT IS IMPORTANT FOR THE OPERATOR TO KNOW WHICH SIGNAL IS BEING USED BY THE CONTROLLER.
mAY BE USED FOR LOW AND HIGH LEVEL ALARM ACTIONS
ALERTS OPERATOR TO OUT-OF-RANGE AND INITIATES CORRECTION WITHIN THE LOOP
NOT INTENDED TO REPLACE SEPARATE HI-HI AND LO-LO ALARMS

30. COMPUTED MANIPULATED VARIABLE CONTROL
THESE ARE APPLIED MASS BALANCES, ENERGY BALANCES OR REACTION MODELS THAT ARE USED TO SPECIFY OPERATING SET POINTS.
CAN BE USED FOR COMPLICATED SYSTEMS THAT CAN BE CONVENIENTLY MODELED
TYPICALLY USED AS A SECONDARY SET POINT GENERATOR
May be linked to simulators

31. COMPUTED MANIPULATED VARIABLE CONTROL
Computed Reboiler Duty Control

32. COMPUTED MANIPULATED VARIABLE CONTROL
Internal Reflux Control