1. introduction 2. influence of process design 3. control degrees … · 2014-09-17 · process...
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Process Unit Control System Design
1. Introduction
2. Influence of process design
3. Control degrees of freedom
4. Selection of control system variables
5. Process safety
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Introduction
Control system requirements
» Safe operation
» Satisfy environmental regulations
» Stable plant operation
» Satisfy specifications on product quality and production rate
» Economic plant operation
Control system design
» Select controlled, manipulated and measured variables
» Specify control structure and controller type
» Select controller tuning parameters
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Multivariable Control Strategies
Multivariable processes » Multiple input and output variables
» Requires multivariable control strategy
Multiloop (decentralized) control » Manipulated inputs and controlled outputs are paired
together
» A controller is designed for each input/output pair
» Simple to design and often provides satisfactory performance
Multivariable control » Manipulated inputs and controlled outputs are not
explicitly paired
» All the inputs are used simultaneously to control all the outputs
» Often necessary for processes with slow dynamics, strong variable interactions, and/or variable constraints
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Influence of Process Design
Process design
» Invariably performed prior to control system
design
» Based on steady-state analysis
» May produce processes that are very difficult to
control dynamically
Integrated process design and control
» Dynamic operability considered during the
process design stage
» Currently a popular research topic
» Starting to be practiced in industry
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Heat Integration
• Increase interactions between columns
• Lose second column reboiler duty as a
manipulated variable
• Increase interactions between units
• Introduces positive feedback
• Lose hot oil flow as a manipulated variable
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Control Degrees of Freedom
Process degrees of freedom (NF) » NF = NV - NE
» NV = number of process variables
» NE = number of independent model equations
Control degrees of freedom (NFC) » NFC = number of process variables that can be
independently controlled
» NFC = NF - ND
» ND = number of disturbance variables
» NFC usually can be calculated as the number of independent material and energy flows that can be manipulated
Feedback control usually does not change NFC
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Distillation Column Example
Manipulated variables: B, D, R, qc, qh
NFC = 5
Controlled variables: xB, xD, hD, hB, P
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Selection of Control System Variables
Output variables » Measured variables – variables that can be
measured
» Controlled variables – usually a subset of the measured variables
Input variables » Manipulated variables – inputs that can be
manipulated
» Disturbance variables – inputs that cannot be manipulated and are determined outside the control system
Basic requirement » Number of manipulated variables must be greater
than or equal to the number of controlled variables
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Selection of Controlled Outputs
All outputs that are not self-regulating must be controlled » Liquid level in a tank with outlet pump
Select outputs that must be maintained within equipment and operating constraints » Polymer reactor temperature
Select outputs that directly represent product quality or strongly affect product quality » Distillation column overhead product composition
Select outputs that strongly interact with other output variables » Steam header pressure in steam generation unit
Select outputs that have favorably steady-state and dynamic characteristics » Large steady-state gains, small time constants, and
small time delays
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Selection of Manipulated Inputs
Select inputs that have large effects on controlled variables
» Large steady-state gains and large input ranges
Select inputs that rapidly affect the controlled outputs
» Small time constants and small time delays
Select inputs that directly (rather than indirectly) affect the controlled outputs
» Condenser duty rather than reboiler duty for column overhead composition control
Avoid selecting inputs that recycle disturbances
» Flow rate of column product stream recycled to chemical reactor
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Selection of Measured Variables
Select measurements that are reliable and accurate
» Stream temperature rather than composition
Select measurements that exhibit sensitivity to the manipulated inputs
» High purity distillation column tray temperature
Select measurements that minimize time constants and time delays
» Distillation column product composition measurement with gas chromatograph
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Evaporator Example
NFC = 3
Controlled outputs: h, xB, P
Manipulated inputs: B (for h), D (for P), Ps (for xB)
Measured variables: h, xB, P, B, D, Ps
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Evaporator Control System
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Process Safety
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Alarms and Interlocks