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Advanced Controls TechnologyAdvanced Controls Technology
An Industrial and Academic Perspective on Plantwide Control
James J. Downs
Eastman Chemical Company
Sigurd Skogestad
Norwegian University of Science and Technology
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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
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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
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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.
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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.
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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.
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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
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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.
F2F1
T2
T1
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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.
F2F1
T2
T1
Design
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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.
F2F1
T2
T1
Operate
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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
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Economic Process Operating Points
Disturbance 1:Optimal operation isT1 , T2 , F1 , F2 , etc.
Disturbance 2:Optimal operation isT1 , T2 , F1 , F2 , etc.
F2F1
T2
T1
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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
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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 )
F2F1
T2
T1
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LC
LC LC
LC
LC
FCDIST
DIST
EXT
LC
LCLC
LCLC LCLC
LCLC
LCLC
FCFCDIST
DIST
EXT
LCLC
Esterification Process
Process production rate set at the process feeds
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LC
LC LC
LC
LC
FCDIST
DIST
EXT
LC
LCLC
LCLC LCLC
LCLC
LCLC
FCFCDIST
DIST
EXT
LCLC
Esterification Process
Disturbances propagate
downstream
Extractor is the process bottleneck
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LC
LC LC
LC
LC
DIST
DIST
EXT
LC
FC
LCLC
LCLC LCLC
LCLC
LCLC
DIST
DIST
EXT
LCLC
FCFC
Esterification Process
Process production rate set at the
distillate of the first column
Extractor is the process bottleneck
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LC
LC LC
LC
LC
DIST
DIST
EXT
LC
FC
LCLC
LCLC LCLC
LCLC
LCLC
DIST
DIST
EXT
LCLC
FCFC
Esterification Process
Disturbances entering this loop may grow
Extractor is the process bottleneck
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LC
LC LC
LC
LC
DIST
DIST
EXT
LC
FC
LCLC
LCLC LCLC
LCLC
LCLC
DIST
DIST
EXT
LCLC
FCFC
Esterification Process
Process production rate set at the extractor feed
Extractor feed set to its maximum using local extractor measurements
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LC
LC LC
LC
LC
DIST
DIST
EXT
LC
FC
LCLC
LCLC LCLC
LCLC
LCLC
DIST
DIST
EXT
LCLC
FCFC
Esterification Process
Near economic optimum operation achieved ..
• by relocating the throughput manipulator,
• at maximum throughput, and
• with active constraints held locally
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Extraction Process
The economic optimum is when xE is constant
Aqueous Acid Feed, F
FC
Organic Feed, S FC
FC
Raffinate, R
Extract, E
ILC
Extract composition, xE
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Extraction Process
The primary disturbance is the aqueous feed composition, xF.
Aqueous Acid Feed, F
FC
Organic Feed, S FC
FC
Raffinate, R
Extract, E
ILCxF is variable Desire the extract
composition, xE , constant
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Extraction Process
F
S
FC
R
E
FC
ILC
F
S
FC
R
E
FC
FC
ILC
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
FC
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Extraction Process
F
S
FC
R
E
FC
ILC
F
S
FC
R
E
FC
ILC
Steady state performance of each strategy for holding xE constant:
Strategy I:
Strategy II:
9.0
F
E
x
x
5.0
F
E
x
x
Strategy I Strategy II
FCFC
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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
F
E
x
x
Strategy IV
F
S
FC
R
E
FC
FC
ILCFY
[ F - R ]Target
R
F
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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.
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International Symposium on Advanced Control of Chemical Processes
ADCHEM 2009