Integration of Design & Control

1. Integration of Design & Control CHEN 4470 – Process Design Practice Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn 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. Motivation 1:2 • Importance of Early-Stage Decision

4. Motivation 2:2 • Process Design Stages and Tools

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

6. Basic Control Concepts 2:7 • 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.

7. Manipulated • variables • Outputs • Disturbances Basic Control Concepts 3:7 • 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. • Process

8. Basic Control Concepts 4:7 • 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.

9. Basic Control Concepts 5:7 • 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.

10. Basic Control Concepts 6:7 • 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.

11. Number of variables • Number of equations • Degrees of freedom Basic Control Concepts 7:7 • 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: • ND = NVariables - NEquations • ND = Nmanipulated + NExternallyDefined • NManipulated = NVariables - Nexternally defined- NEquations

12. Example 1: CSTR Control 1:6 • Classification of Variables • ND = NVariables = 10 • Externally defined (disturbances): CAi, Ti, Tco h, T, CA, CAi, Ti, Fi, Fo, Fc, Tc, Tco

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

14. Example 1: CSTR Control 3:6 • Degrees of Freedom • NManipulated = NVariables – NExt. Defined – NEquations • NManipulated = 10 – 3 – 4 • NManipulated = 3 Thus we need to select 3 variables to be controlled. Each must be controlled by varying one or more of the remaining variables

15. Example 1: CSTR Control 4:6 • Selection of Controlled Variables • CA 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 CA (Rule 4). • h must be selected as a controlled output because it is non-self-regulating (Rule 1).

16. Example 1: CSTR Control 5:6 • Selection of Manipulated Variables • Fi should be selected as it directly and rapidly affects CA (Rules 6, 7 and 8). • Fc should be selected as it directly and rapidly affects T (Rules 6, 7 and 8). • Fo should be selected as it directly and rapidly affects h (Rules 6, 7 and 8).

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

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. Other Business • 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

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

21. Other Business • 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