An Industrial and Academic Perspective on Plantwide Control

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Advanced Controls TechnologyAdvanced Controls Technology

James J. Downs

Eastman Chemical Company

Sigurd Skogestad

Norwegian University of Science and Technology

Background

• Importance of plantwide control

• Industrial – academic partnerships in plantwide control

• Role of plantwide control in the field of systems engineering and chemical process control

• Linkage of plantwide control to chemical process design

Traditional Control Design Issues in Industry

• Segregation of the process design function and the process control function

• Difficulty in quantifying the cost / benefit tradeoff of controllability and operability ideas

• Late involvement of process control expertise into the design process

What plantwide control issues face the chemical industry today?

• Fewer new designs, more operation of existing facilities in new ways.

• Less advanced control capability in house, more reliance upon contracted resources.

• Operators are more accountable for understanding their processes and their control systems.

Additional Comments

• Control design priorities are (1) robustness, (2) disturbance rejection, and (3) economics.

• Migration toward "time efficient" solutions.

• Control strategy changes may become more difficult as time progresses due to training and documentation requirements.

Important Relationships for Plantwide Control Development

• Partnerships with process design – elimination of control problems at the source, understanding design intent

• Partnerships with academia – capability to transfer new technology and ideas into practice

• Partnerships with operations – understanding the economic drivers and process needs.

Plantwide Control Decisions

• How to control the process material and energy balances

• Where to set the process production rate

• What controlled variables indicate stable operation and good economic performance

Modes of Process Operation

Maximize efficiency for a given throughput:Optimal operation isT1 , T2 , F1 , F2 , etc.

Maximize throughput:Optimal operation isT1 , T2 , F1 , F2 , etc.Throughput is a degree of freedom.

Design

Operate

Plantwide Control Considerations

• Steady state analysis of where the plant should operate for the expected set of disturbances

– to determine what the process constraints will be

– to determine what variables are indicative of the optimum operating point

• Selection of the throughput manipulator

– “near” expected plant bottlenecks

– dynamically acceptable

Economic Process Operating Points

Disturbance 1:Optimal operation isT1 , T2 , F1 , F2 , etc.

Disturbance 2:Optimal operation isT1 , T2 , F1 , F2 , etc.

Plantwide Control Concepts

• Setting the process production rate “near” the process bottleneck

• Controlling known active constraints locally

• Developing measurement combinations that imply nearness to economic optimal operation

Control Variables for Economic Operation

Control expected active constraints locally.

Identify “self optimizing” control variables for the remaining unconstrained degrees of freedom, e.g. CVi = f ( T1 , T2 )

FCDIST

LCLC LCLC

FCFCDIST

Esterification Process

Process production rate set at the process feeds

FCDIST

LCLC LCLC

FCFCDIST

Disturbances propagate

downstream

Extractor is the process bottleneck

LCLC LCLC

Process production rate set at the

distillate of the first column

LCLC LCLC

Disturbances entering this loop may grow

LCLC LCLC

Process production rate set at the extractor feed

Extractor feed set to its maximum using local extractor measurements

LCLC LCLC

Near economic optimum operation achieved ..

• by relocating the throughput manipulator,

• at maximum throughput, and

• with active constraints held locally

Extraction Process

The economic optimum is when xE is constant

Aqueous Acid Feed, F

Organic Feed, S FC

Raffinate, R

Extract, E

Extract composition, xE

Extraction Process

The primary disturbance is the aqueous feed composition, xF.

Aqueous Acid Feed, F

Organic Feed, S FC

Raffinate, R

Extract, E

ILCxF is variable Desire the extract

composition, xE , constant

Extraction Process

Strategy I – Interface level controlled by manipulating the

aqueous feed

Strategy II – Interface level controlled by manipulating the

raffinate flow

Throughput set by the flow of S

Extraction Process

Steady state performance of each strategy for holding xE constant:

Strategy I:

Strategy II:

Strategy I Strategy II

Extraction Process

Steady state analysis indicates the holding the combination, [ F - R ], constant will result in xE being constant, that is,

Strategy IV: 0.0

Strategy IV

[ F - R ]Target

Final Thoughts

• Include process economic notions into the plantwide control design procedure – allow the base level control strategy to do most of the economic work.

• Consider the ‘maximum production rate’ condition as the likely operating point.

• Understand process disturbances and plan for variability propagation to harmless locations.

International Symposium on Advanced Control of Chemical Processes

ADCHEM 2009

An Industrial and Academic Perspective on Plantwide Control

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