Plantwide Control

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INTERACTION OF PROCESS DESIGN AND CONTROL

Ref: Seider, Seader and Lewin (2004), Chapter 20

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

CONTROL

Ref: Seider, Seader and Lewin (2004), Chapter 20

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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

water quality as well as waste disposal.

PROCESS OBJECTIVES

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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

Manipulated variables

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

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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

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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

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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

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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

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EXAMPLE 1: CONTROL OF CSTR

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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)

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T, CA

Ti, CAi

Fc

T, CA

Tc

hTco

Fo

Fi

NManipulated = NVariables - Next. defined- Nequations

= 10 - 3 - 4 = 3


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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).


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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).


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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 ?


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PART TWO: PART TWO: Plantwide Control System designPlantwide Control System design

Ref: Seider, Seader and Lewin, Chapter 20

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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 after resolving 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.

Step 4: Set the production rate.

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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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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 fixed feed: Use V-1.

Step 4: Set the production rate.

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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

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

Outlined qualitative approach for plantwide control structure selection

SUMMARY

Plantwide Control

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