Control

10-Design and

Control

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin1

054402 Design and Analysis II

LECTURE 10: INTERACTION OF PROCESS

DESIGN AND CONTROL

Daniel R. Lewin

Department of Chemical Engineering

Technion, Haifa, IsraelRef: Seider, Seader and Lewin (2003), Chapter 20

10-Design and

Control


Provide motivation for handling flowsheet controllability and resiliency as an integral part of the design process

Outline qualitative approach for control variable selection

Introduce a qualitative plantwide control structure selection method

OUTLINE

10-Design and

Control


PART ONE: MOTIVATION

10-Design and

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IMPORTANCE OF EARLY STAGE-DESICISONS

10-Design and

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PROCESS DESIGN STAGES AND TOOLS

10-Design and

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PART TWO: CLASSIFICATION OF VARIABLES,DOF ANALYSIS & UNIT-BY-UNIT

CONTROL

Ref: Seider, Seader and Lewin (2003), Chapter 20

10-Design and

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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 waterquality as well as waste disposal.

PROCESS OBJECTIVES

10-Design and

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Variables that effect and are affected by the process should be categorized as either control (manipulated) variables, disturbances and outputs.

Process Outputs

Manipulatedvariables

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.

CLASSIFICATION OF VARIABLES

10-Design and

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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.

SELECTION OF CONTROLLED VARIABLES

10-Design and

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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.

SELECTION OF MANIPULATED VARIABLES

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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.

SELECTION OF MEASURED VARIABLES

10-Design and

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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

ND = Nmanipulated + NExternally Defined

Degrees of freedom

Number of variables

Number of equations

NManipulated = NVariables - Nexternally defined- NEquations

DEGREES OF FREEDOM ANALYSIS

10-Design and

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Number of variables.

T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

Nvariables =

Externally defined (disturbances) : CAi , Ti , and TCO

10

EXAMPLE 1: CONTROL OF CSTR

10-Design and

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Material and energy balances:

T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

oi FFdtdhA

TCrAhCFCFhCdtdA AAoAiiA ,

csA

oii

TTUAHTCrAh

CpTFCpTFThdtdCpA

,

csccccoccc

cc TTUATCpFTCpFdt

dTCpV

NEquations = 4

EXAMPLE 1: CONTROL OF CSTR (Cont’d)

10-Design and

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T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

NManipulated = NVariables - Next. defined- Nequations

= 10 - 3 - 4 = 3


10-Design and

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Selection of controlled variables.

T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

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).


10-Design and

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Selection of manipulated variables.

T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

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).


10-Design and

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This suggests the following control configuration:

T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

TC

CCLC

Can you think of alternatives or improvements ?


10-Design and

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PART THREE: Plantwide Control System design

Ref: Seider, Seader and Lewin, Chapter 20

10-Design and

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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.

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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 afterresolving more important plant-wide issues.

Step 9: Optimize economics and improve dynamic controllability. Add nice-to-have options with any remaining DOFs.

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EXAMPLE 2: ACYCLIC PROCESS

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.

Steps 1 & 2: Establish the control objectives and DOFs:

Select V-7 for On-demand product flow

Select V-1 for fixed feed

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EXAMPLE 2: ACYCLIC PROCESS (Cont’d)

Need to control reactor temperature: Use V-2.

Step 3: Establish energy management system.

Need to control reactor feed temperature: Use V-3.

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For on-demand product: Use V-7.


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To regulate V-100 pressure: Use V-5

Step 5: Control product quality, and meet safety, environmental, and operational constraints.

To regulate V-100 temperature: Use V-6

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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

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Step 7: Check component balances. (N/A)

Install composition controller, cascaded with TC of reactor.

Step 8: Control the individual process units (N/A)

Step 9: Optimization

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EXAMPLE 2 (Class): ACYCLIC PROCESS

Try your hand at designing a plant-wide control system for fixed feed rate.

Select V-1 for fixed feed

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EXAMPLE 2 (Class): ACYCLIC PROCESS

Possible solution.

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EXAMPLE 3: CYCLIC PROCESS

The above control system for (fixed feed) has an inherent problem?

Can you see what it is?

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EXAMPLE 3: CYCLIC PROCESS (Cont’d)

The above control system for (fixed feed) has an inherent problem?

Can you see what it is?

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0

0

0

Combined molar feed to the CSTR:

Molar material balance around the flash vessel:

Overall molar material balance:

F B

F B D B

F D

AA A 0 A

11 Rtotal total

R

dnkx c x F B kx c V

V dtR Ttotalc V n

Molar balance on CSTR:

A 0 A A 0 A1 1 R Ttotalx F B kx c V x F B kx n

0 A TBalance on A for perfect separation: F kx n

A 0 0

A

1

Tx F kn FB

xRearranging:

20

0

T

FB

kn F

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16.750

450150

208125

100100

4575

BF0

e.g., suppose knT = 200:


20

0

T

FB

kn F

A more general result uses the dimensionless, Damköhlernumber: Da = knT/F0 giving:

0

1

FB

Da

“Snowball” effect for Da 1

“Snowball” effect

10-Design and

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Maintain the production rate at a specified level.

Keep the conversion of the plant at its highest permissible value.

Steps 1 & 2: Establish the control objectives and DOFs:

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Need to control reactor temperature: Use V-2.

Step 3: Establish energy management system.

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For on-demand product: Use V-7.


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To regulate V-100 pressure: Use V-4

Step 5: Control product quality, and meet safety, environmental, and operational constraints.

To regulate V-100 temperature: Use V-5

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Step 6: Fix recycle flow rates and vapor and liquid inventories

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.

Need to control recycle flow rate: Use V-6

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Install composition controller, cascaded with TC of reactor.

Steps 7, 8 and 9: Improvements

10-Design and

Control


Provided motivation for handling flowsheet controllability and resiliency as an integral part of the design process

Outlined qualitative approach for unit-by-unit control structure selection

Outlined qualitative approach for plantwide control structure selection

SUMMARY

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