1. Integration of Design & Control CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 15 – Integration of Design and Control I February 28, 2013 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

2. Outline Part I: This Lecture  Provide motivation for handling flowsheet controllability and resiliency as an integral part of the design process  Outline qualitative approach for control variable selection and unit by unit control structuring Part II – Next Lecture  Introduce a qualitative plantwide control structure selection method

3. Importance of Early-Stage Decision Motivation 1:2

4. Process Design Stages and Tools Motivation 2:2

5. Process Objectives Classification of Variables Degree of Freedom (DOF) Analysis Unit by Unit Control Basic Control Concepts 1:7

6. Process Objectives –The design of a control system for a chemical plant is guided by the objective to maximize profits by transforming raw materials into useful products while satisfying: Product specifications: quality, rate. Safety Operational constraints Environmental regulations - on air and water quality as well as waste disposal. Basic Control Concepts 2:7

7. Classification of Variables –Variables that effect and are affected by the process should be categorized as either control (manipulated) variables, disturbances and outputs. –It is usually not possible to control all outputs (why?) –Thus, once the number of manipulated variables are defined, one selects which of the outputs should be controlled variables. Basic Control Concepts 3:7 Process Outputs Manipulated variables Disturbances

8. Selection of Controlled Variables –Rule 1: Select variables that are not self-regulating. –Rule 2: Select output variables that would exceed the equipment and operating constraints without control. –Rule 3: Select output variables that are a direct measure of the product quality or that strongly affect it. –Rule 4: Choose output variables that seriously interact with other controlled variables. –Rule 5: Choose output variables that have favorable static and dynamic responses to the available control variables. Basic Control Concepts 4:7

9. Selection of Manipulated Variables –Rule 6: Select inputs that significantly affect the controlled variables. –Rule 7: Select inputs that rapidly affect the controlled variables. –Rule 8: The manipulated variables should affect the controlled variables directly rather than indirectly. –Rule 9: Avoid recycling disturbances. Basic Control Concepts 5:7

10. Selection of Measured Variables –Rule 10: Reliable, accurate measurements are essential for good control. –Rule 11: Select measurement points that are sufficiently sensitive. –Rule 12: Select measurement points that minimize time delays and time constants. Basic Control Concepts 6:7

11. Degree of Freedom Analysis –Before selecting the controlled and manipulated variables, one must determine the number of variables permissible. The number of manipulated variables cannot exceed the degrees of freedom, which are determined using a process model according to: Basic Control Concepts 7:7 N D = N Variables - N Equations N D = N manipulated + N Externally Defined Degrees of freedom Number of variables Number of equations N Manipulated = N Variables - N externally defined - N Equations

12. Classification of Variables –ND = N Variables = 10 –Externally defined (disturbances): C Ai, T i, T co Example 1: CSTR Control 1:6 h, T, C A, C Ai, T i, F i, F o, F c, T c, T co

13. Balance Equations Example 1: CSTR Control 2:6 Overall Mass Balance Component Balance Energy Balance Mixture Energy Balance Coolant N Equations = 4

14. Degrees of Freedom –N Manipulated = N Variables – N Ext. Defined – N Equations –N Manipulated = 10 – 3 – 4 –N Manipulated = 3 Example 1: CSTR Control 3:6 Thus we need to select 3 variables to be controlled. Each must be controlled by varying one or more of the remaining variables

15. Selection of Controlled Variables –C A should be selected since it directly affects the product quality (Rule 3). –T should be selected because it must be regulated properly to avoid safety problems (Rule 2) and because it interacts with C A (Rule 4). –h must be selected as a controlled output because it is non-self-regulating (Rule 1). Example 1: CSTR Control 4:6

16. Selection of Manipulated Variables –F i should be selected as it directly and rapidly affects C A (Rules 6, 7 and 8). –F c should be selected as it directly and rapidly affects T (Rules 6, 7 and 8). –F o should be selected as it directly and rapidly affects h (Rules 6, 7 and 8). Example 1: CSTR Control 5:6

17. Suggested Control Structure Example 1: CSTR Control 6:6

18. Summary Part I: This Lecture  Provide motivation for handling flowsheet controllability and resiliency as an integral part of the design process  Outline qualitative approach for control variable selection Part II – Next Lecture  Introduce a qualitative plantwide control structure selection method

19. Evonik Site Visit – March 5 –What to bring? Photo ID (drivers license is preferred) –What not to bring? Cameras (including camera phones) Cell Phone use is only allowed in buildings Guns, Drugs or Alcohol (I hope this one is obvious) –What to wear? Long Pants Closed Shoes (no sandals or flip flops) Shirts with Sleeves (no tank tops) Safety Glasses Other Business

20. Evonik Site Visit – March 5 (Cont’d) –Tentative schedule Other Business

21. Next Lecture – March 7 –Integration of design and control – Part II (Plantwide) –SSLW 322-340 Progress Report 2 –Turn in Friday March 8 –Remember to fill out the team evaluation forms Other Business