Integration of Design & Control CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 12 – Integration of Design and Control I March 15, 2007 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel SSL Chapter 20

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

Motivation 1:2 Importance of Early-Stage Decision

Motivation 2:2 Process Design Stages and Tools

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

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.

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 Manipulated variables Outputs Disturbances

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.

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.

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.

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: Degrees of freedom ND = NVariables - NEquations Number of variables Number of equations ND = Nmanipulated + NExternally Defined NManipulated = NVariables - Nexternally defined- NEquations

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

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

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

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).

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).

Example 1: CSTR Control 6:6 Suggested Control Structure

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

Other Business Next Lecture – Tuesday March 20 Integration of design and control – Part II (Plantwide) No Lecture/Meetings Thursday March 22 Out of town, reschedule meetings for Monday if needed Individual Team Assignments Send short email (2-3 sentences) of what you would investigate and I’ll send the “official” problem statement Alternative Progress Report Due Dates #2: Friday March 23 (will be returned April 3) #3: Monday April 9 (will be returned April 12) Final: Friday April 20 @ 12:00 PM