INTERACTION OF PROCESS DESIGN AND CONTROL Ref Seider

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

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

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. 3

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

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. 5

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. 6

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. 7

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, which are determined using a process model according to: ND = NVariables - NEquations Degrees of freedom Number of variables Number of equations ND = Nmanipulated + NExternally Defined NManipulated = NVariables - Nexternally defined- NEquations 8

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

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

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

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 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). 12

EXAMPLE 1: CONTROL OF CSTR (Cont’d) Selection of manipulated variables. ¶ Fi should 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). • Fo should be selected since it directly and rapidly affects h (Guidelines 6, 7 and 8). 13

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

PART TWO: Plantwide Control System design Ref: Seider, Seader and Lewin, Chapter 20 15

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. 16

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. 17

EXAMPLE 2: ACYCLIC PROCESS Select V-7 for On-demand product flow Select V-1 for fixed feed 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. 18

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. 19

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

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 21

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 22

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. 23

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

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

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

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

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

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

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

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 31

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. 32

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

SUMMARY ¶ Outlined qualitative approach for unit-byunit control structure selection ¶ Outlined qualitative approach for plantwide control structure selection 34