4470 Lecture 19 2012

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Integration of Design & Control

CHEN 4470 Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 19 Integration of Design and Control I

March 27, 2012

Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

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Outline

Part I: This Lecture

Provide motivation for handling flowsheetcontrollability and resiliency as an integral part ofthe design process

Outline qualitative approach for control variableselection and unit by unit control structuring

Part II Next Lecture

Introduce a qualitative plantwide control structureselection method

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Importance of Early-Stage Decision

Motivation 1:2

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Process Design Stages and Tools

Motivation 2:2

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Process Objectives

Classification of Variables

Degree of Freedom (DOF) Analysis

Unit by Unit Control

Basic Control Concepts 1:7

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Process Objectives

The design of a control system for a chemical plant isguided by the objective to maximize profits bytransforming raw materials into useful products whilesatisfying:

Product specifications: quality, rate.

Safety

Operational constraints

Environmental regulations - on air and water quality as well

as waste disposal.


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Classification of Variables

Variables that effect and are affected by the processshould 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 aredefined, one selects which of the outputs should be

controlled variables.


Process Outputs

Manipulatedvariables

Disturbances

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Selection of Controlled Variables

Rule 1: Select variables that are not self-regulating.

Rule 2: Select output variables that would exceed theequipment and operating constraints without control.

Rule 3: Select output variables that are a directmeasure 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 favorablestatic and dynamic responses to the available controlvariables.


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Selection of Manipulated Variables

Rule 6: Select inputs that significantly affect thecontrolled variables.

Rule 7: Select inputs that rapidly affect the controlled

variables.

Rule 8: The manipulated variables should affect thecontrolled variables directly rather than indirectly.

Rule 9: Avoid recycling disturbances.


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Selection of Measured Variables

Rule 10: Reliable, accurate measurements areessential for good control.

Rule 11: Select measurement points that are

sufficiently sensitive.

Rule 12: Select measurement points that minimizetime delays and time constants.


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Degree of Freedom Analysis

Before selecting the controlled and manipulatedvariables, one must determine the number of variablespermissible. The number of manipulated variablescannot exceed the degrees of freedom, which are

determined using a process model according to:


ND = NVariables - NEquations

ND = Nmanipulated + NExternallyDefined

Degrees offreedom Number ofvariables Number ofequations

NManipulated = NVariables - Nexternally defined- NEquations

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Classification of Variables ND = NVariables = 10

Externally defined (disturbances):CAi,

Ti,Tco

Example 1: CSTR Control 1:6

T, CA

Ti, CAi

c

T, CA

Tc

hTco

o

i

h, T, CA, CAi, Ti,

Fi, Fo, Fc, Tc, Tco

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Balance Equations


T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

oi FFdtdhA

TCrAhCFCFhCdtdA AAoAiiA ,

csA

oii

TTUAHTCrAh

CpTFCpTFThdt

dCpA

,

csccccoccc

cc TTUATCpFTCpFdt

dTCpV

Overall Mass Balance

Component Balance

Energy Balance Mixture

Energy Balance Coolant

NEquations = 4

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Degrees of Freedom NManipulated = NVariables NExt. Defined NEquations NManipulated = 10 3 4

NManipulated

= 3


T, CA

Ti, CAi

c

T, CA

Tc

hTco

o

i

Thus we need to select 3variables to be controlled.Each must be controlled by

varying one or more of theremaining variables

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Selection of Controlled Variables

CA should be selected since it directly affects theproduct quality (Rule 3).

T should be selected because it must be regulatedproperly to avoid safety problems (Rule 2) and becauseit interacts with CA (Rule 4).

h must be selected as a controlled output because it is

non-self-regulating (Rule 1).


T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

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Selection of Manipulated Variables

Fishould 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).


T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

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Suggested Control Structure


T, CA

Ti, CAi

c

T, CA

Tc

hTco

o

i

TC

CCLC

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Summary

Part I: This Lecture

Provide motivation for handling flowsheetcontrollability and resiliency as an integral part ofthe design process

Outline qualitative approach for control variableselection

Part II Next Lecture

Introduce a qualitative plantwide control structureselection method

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Next Lecture March 29

Integration of design and control Part II (Plantwide)

SSLW 322-340

Milliken Plant Visit March 29

Directions and official excuse placed on website

Leave from Ross Hall at 1:00 PM

No open shoes/sandals/shorts/sleeveless shirts

Limited jewelry 3:00 EST Arrive at plant

3:15 EST Introductions / Safety Review

3:45 EST High level plant tour

4:30 EST Discussion

5:30 EST Wrap up with dinner

Other Business

Documents

4470 Lecture 19 2012