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INTERACTION OF PROCESS DESIGN AND CONTROL

INTERACTION OF PROCESS DESIGN AND CONTROL. Ref: Seider, Seader and Lewin (2004), Chapter 20. PART ONE: CLASSIFICATION OF VARIABLES, DOF ANALYSIS & UNIT-BY-UNIT CONTROL. Ref: Seider, Seader and Lewin (2004), Chapter 20. PROCESS OBJECTIVES.

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INTERACTION OF PROCESS DESIGN AND CONTROL

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  1. INTERACTION OF PROCESS DESIGN AND CONTROL • Ref: Seider, Seader and Lewin (2004), Chapter 20

  2. PART ONE: CLASSIFICATION OF VARIABLES,DOF ANALYSIS & UNIT-BY-UNIT CONTROL Ref: Seider, Seader and Lewin (2004), Chapter 20

  3. 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.

  4. Manipulated • variables • Outputs • Disturbances CLASSIFICATION OF VARIABLES • Variables that effect and are affected by the process should be categorized as either control (manipulated) variables, disturbances and outputs. • Process • 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.

  5. 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.

  6. 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.

  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.

  8. Number of variables • Number of equations • Degrees of freedom DEGREES OF FREEDOM ANALYSIS • Before selecting the controlled and manipulated variables for a control system, one must determine the number of variables permissible. The number of manipulated variables cannot exceed the degrees of freedom, whichare determined using a process model according to: • ND = NVariables - NEquations • ND = Nmanipulated + NExternallyDefined • NManipulated = NVariables - Nexternally defined- NEquations

  9. EXAMPLE 1: CONTROL OF CSTR • Number of variables. • Nvariables = • 10 • Externally defined (disturbances) : CAi , Ti , and TCO

  10. NEquations = • 4 EXAMPLE 1: CONTROL OF CSTR (Cont’d) • Material and energy balances:

  11. EXAMPLE 1: CONTROL OF CSTR (Cont’d) • NManipulated = NVariables - Next. defined- Nequations • = 10 • - 3 • - 4 • = 3

  12. EXAMPLE 1: CONTROL OF CSTR (Cont’d) • Selection of controlled variables. • CA should be selected since it directly affects the product quality (Rule 3). • T should be selectedbecause it must be regulated properly to avoid safety problems (Rule 2) and because it interacts with CA (Rule 4). • hmust be selected as a controlled output because it is non-self-regulating (Rule 1).

  13. EXAMPLE 1: CONTROL OF CSTR (Cont’d) • Selection of manipulated variables. • Fishould be selected since it directly and rapidly affects CA (Guidelines 6, 7 and 8). • Fc should be selected since it directly and rapidly affects T(Guidelines 6, 7 and 8). • Foshould be selected since it directly and rapidly affects h (Guidelines 6, 7 and 8).

  14. EXAMPLE 1: CONTROL OF CSTR (Cont’d) • This suggests the following control configuration: • Can you think of alternatives or improvements ?

  15. PART TWO: Plantwide Control System designRef: Seider, Seader and Lewin, Chapter 20

  16. PLANTWIDE CONTROL DESIGN • Luyben et al. (1999) suggest a method for the conceptual design of plant-wide control systems, which consists of the following steps: • Step 1:Establish the control objectives. • Step 2: Determine the control degrees of freedom. Simply stated – the number of control valves – with additions if necessary. • Step 3: Establish the energy management system. Regulation of exothermic or endothermic reactors, and placement of controllers to attenuate temperature disturbances. • Step 4: Set the production rate. • Step 5:Control the product quality and handle safety, environmental, and operational constraints.

  17. PLANTWIDE CONTROL DESIGN (Cont’d) • Step 6:Fix a flow rate in every recycle loop and control vapor and liquid inventories (vessel pressures and levels). • Step 7: Check component balances. Establish control to prevent the accumulation of individual chemical species in the process. • Step 8:Control the individual process units.Use remaining DOFs to improve local control, but only after resolving more important plant-wide issues. • Step 9: Optimize economics and improve dynamic controllability. Add nice-to-have options with any remaining DOFs.

  18. Select V-7 for On-demand product flow Select V-1 for fixed feed EXAMPLE 2: ACYCLIC PROCESS Steps 1 & 2:Establish the control objectives and DOFs: • Maintain a constant production rate • Achieve constant composition in the liquid effluent from the flash drum. • Keep the conversion of the plant at its highest permissible value.

  19. EXAMPLE 2: ACYCLIC PROCESS (Cont’d) Step 3:Establish energy management system. • Need to control reactor temperature: Use V-2. • Need to control reactor feed temperature: Use V-3.

  20. EXAMPLE 2: ACYCLIC PROCESS (Cont’d) Step 4:Set the production rate. • For on-demand product: Use V-7.

  21. EXAMPLE 2: ACYCLIC PROCESS (Cont’d) Step 5:Control product quality, and meet safety, environmental, and operational constraints. • To regulate V-100 pressure: Use V-5 • To regulate V-100 temperature: Use V-6

  22. EXAMPLE 2: ACYCLIC PROCESS (Cont’d) Step 6:Fix recycle flow rates and vapor and liquid inventories • Need to control vapor inventory in V-100: Use V-5 (already installed) • Need to control liquid inventory in V-100: Use V-4 • Need to control liquid inventory in R-100: Use V-1

  23. EXAMPLE 2: ACYCLIC PROCESS (Cont’d) Step 7:Check component balances. (N/A) Step 8:Control the individual process units (N/A) Step 9:Optimization • Install composition controller, cascaded with TC of reactor.

  24. Select V-1 for fixed feed EXAMPLE 2 (Class): ACYCLIC PROCESS Try your hand at designing a plant-wide control system for fixed feed rate.

  25. EXAMPLE 2 (Class): ACYCLIC PROCESS Possible solution.

  26. EXAMPLE 3: CYCLIC PROCESS The above control system for (fixed feed) has an inherent problem? Can you see what it is?

  27. EXAMPLE 3: CYCLIC PROCESS (Cont’d) The above control system for (fixed feed) has an inherent problem? Can you see what it is?

  28. EXAMPLE 3: CYCLIC PROCESS (Cont’d) Steps 1 & 2:Establish the control objectives and DOFs: • Maintain the production rate at a specified level. • Keep the conversion of the plant at its highest permissible value.

  29. EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 3:Establish energy management system. • Need to control reactor temperature: Use V-2.

  30. EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 4:Set the production rate. • For fixed feed: Use V-1.

  31. EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 5:Control product quality, and meet safety, environmental, and operational constraints. • To regulate V-100 pressure: Use V-4 • To regulate V-100 temperature: Use V-5

  32. EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 6:Fix recycle flow rates and vapor and liquid inventories • Need to control recycle flow rate: Use V-6 • Need to control vapor inventory in V-100: Use V-4 (already installed) • Need to control liquid inventory in V-100: Use V-3 • Need to control liquid inventory in R-100: Cascade to FC on V-1.

  33. EXAMPLE 3: CYCLIC PROCESS (Cont’d) Steps 7, 8 and 9:Improvements • Install composition controller, cascaded with TC of reactor.

  34. SUMMARY • Outlined qualitative approach for unit-by-unit control structure selection • Outlined qualitative approach for plantwide control structure selection

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