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

2. Advanced Controls Technology 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. Advanced Controls Technology 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. Advanced Controls Technology 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. Advanced Controls Technology 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. Advanced Controls Technology 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. Advanced Controls Technology 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. Advanced Controls Technology Modes of Process Operation Maximize efficiency for a given throughput: Optimal operation is T 1, T 2, F 1, F 2, etc. Maximize throughput: Optimal operation is T 1, T 2, F 1, F 2, etc. Throughput is a degree of freedom. F2F2 F1F1 T2T2 T1T1

9. Advanced Controls Technology Modes of Process Operation Maximize efficiency for a given throughput: Optimal operation is T 1, T 2, F 1, F 2, etc. Maximize throughput: Optimal operation is T 1, T 2, F 1, F 2, etc. Throughput is a degree of freedom. F2F2 F1F1 T2T2 T1T1 Design

10. Advanced Controls Technology Modes of Process Operation Maximize efficiency for a given throughput: Optimal operation is T 1, T 2, F 1, F 2, etc. Maximize throughput: Optimal operation is T 1, T 2, F 1, F 2, etc. Throughput is a degree of freedom. F2F2 F1F1 T2T2 T1T1 Operate

11. Advanced Controls Technology 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. Advanced Controls Technology Economic Process Operating Points Disturbance 1: Optimal operation is T 1, T 2, F 1, F 2, etc. Disturbance 2: Optimal operation is T 1, T 2, F 1, F 2, etc. F2F2 F1F1 T2T2 T1T1

13. Advanced Controls Technology 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. Advanced Controls Technology Control Variables for Economic Operation Control expected active constraints locally. Identify “self optimizing” control variables for the remaining unconstrained degrees of freedom, e.g. CV i = f ( T 1, T 2 ) F2F2 F1F1 T2T2 T1T1

15. Advanced Controls Technology Esterification Process Process production rate set at the process feeds

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

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

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

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

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

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

22. Advanced Controls Technology Extraction Process The primary disturbance is the aqueous feed composition, x F. Aqueous Acid Feed, F FC Organic Feed, S FC Raffinate, R Extract, E ILC x F is variable Desire the extract composition, x E, constant

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

24. Advanced Controls Technology Extraction Process F S FC R E ILC F S FC R E ILC Steady state performance of each strategy for holding x E constant: Strategy I: Strategy II: Strategy IStrategy II FC

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

26. Advanced Controls Technology 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. Advanced Controls Technology International Symposium on Advanced Control of Chemical Processes ADCHEM 2009