An Industrial and Academic Perspective on Plantwide Control

1. An Industrial and Academic Perspective on Plantwide Control James J. Downs Eastman Chemical Company Sigurd Skogestad Norwegian University of Science and Technology

2. Background Importance of plantwide control Industrial – academic partnerships in plantwide control Role of plantwide control in the field of systems engineering and chemical process control Linkage of plantwide control to chemical process design

3. Traditional Control Design Issues in Industry • Segregation of the process design function and the process control function • Difficulty in quantifying the cost / benefit tradeoff of controllability and operability ideas • Late involvement of process control expertise into the design process

4. What plantwide control issues face the chemical industry today? Fewer new designs, more operation of existing facilities in new ways. Less advanced control capability in house, more reliance upon contracted resources. Operators are more accountable for understanding their processes and their control systems.

5. Additional Comments • Control design priorities are (1) robustness, (2) disturbance rejection, and (3) economics. • Migration toward "time efficient" solutions. • Control strategy changes may become more difficult as time progresses due to training and documentation requirements.

6. Important Relationships for Plantwide Control Development Partnerships with process design – elimination of control problems at the source, understanding design intent Partnerships with academia – capability to transfer new technology and ideas into practice Partnerships with operations – understanding the economic drivers and process needs.

7. Plantwide Control Decisions • How to control the process material and energy balances • Where to set the process production rate • What controlled variables indicate stable operation and good economic performance

8. Modes of Process Operation Maximize efficiency for a given throughput: Optimal operation is T1 , T2 , F1 , F2 , etc. F2 F1 Maximize throughput: Optimal operation is T1 , T2 , F1 , F2 , etc. Throughput is a degree of freedom. T2 T1

9. Modes of Process Operation Design Maximize efficiency for a given throughput: Optimal operation is T1 , T2 , F1 , F2 , etc. F2 F1 Maximize throughput: Optimal operation is T1 , T2 , F1 , F2 , etc. Throughput is a degree of freedom. T2 T1

10. Modes of Process Operation Maximize efficiency for a given throughput: Optimal operation is T1 , T2 , F1 , F2 , etc. F2 F1 Maximize throughput: Optimal operation is T1 , T2 , F1 , F2 , etc. Throughput is a degree of freedom. T2 T1 Operate

11. Plantwide Control Considerations Steady state analysis of where the plant should operate for the expected set of disturbances to determine what the process constraints will be to determine what variables are indicative of the optimum operating point Selection of the throughput manipulator “near” expected plant bottlenecks dynamically acceptable

12. Economic Process Operating Points Disturbance 1: Optimal operation is T1 , T2 , F1 , F2 , etc. F2 F1 Disturbance 2: Optimal operation is T1 , T2 , F1 , F2 , etc. T2 T1

13. Plantwide Control Concepts • Setting the process production rate “near” the process bottleneck • Controlling known active constraints locally • Developing measurement combinations that imply nearness to economic optimal operation

14. Control Variables for Economic Operation Control expected active constraints locally. F2 F1 T2 Identify “self optimizing” control variables for the remaining unconstrained degrees of freedom, e.g. CVi = f ( T1 , T2 ) T1

15. Esterification Process Process production rate set at the process feeds

16. Esterification Process Disturbances propagate downstream Extractor is the process bottleneck

17. Esterification Process Process production rate set at the distillate of the first column Extractor is the process bottleneck

18. Esterification Process Disturbances entering this loop may grow Extractor is the process bottleneck

19. Esterification Process Process production rate set at the extractor feed Extractor feed set to its maximum using local extractor measurements

20. Esterification Process • Near economic optimum operation achieved .. • by relocating the throughput manipulator, • at maximum throughput, and • with active constraints held locally

21. ILC FC FC FC Extraction Process The economic optimum is when xE is constant Extract composition,xE Extract, E Aqueous Acid Feed, F Organic Feed, S Raffinate, R

22. ILC FC FC FC Extraction Process The primary disturbance is the aqueous feed composition, xF. xFis variable Desirethe extract composition,xE,constant Extract, E Aqueous Acid Feed, F Organic Feed, S Raffinate, R

23. FC FC FC FC ILC ILC Extraction Process Strategy I – Interface level controlled by manipulating the aqueous feed Strategy II – Interface level controlled by manipulating the raffinate flow E E F F FC FC S S R R Throughput set by the flow of S

24. FC FC FC FC ILC ILC Extraction Process Steady state performance of each strategy for holding xE constant: Strategy I: Strategy II: E F E F FC FC S S R R Strategy I Strategy II

25. FC FC ILC Extraction Process Steady state analysis indicates the holding the combination, [ F - R ], constant will result in xE being constant, that is, Strategy IV: [ F - R ]Target F FY R E F FC S R Strategy IV

26. Final Thoughts • Include process economic notions into the plantwide control design procedure – allow the base level control strategy to do most of the economic work. • Consider the ‘maximum production rate’ condition as the likely operating point. • Understand process disturbances and plan for variability propagation to harmless locations.

27. ADCHEM 2009 International Symposium on Advanced Control of Chemical Processes