INTERACTION OF PROCESS

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• INTERACTION OF PROCESS
• DESIGN AND CONTROL

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

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PART ONE CLASSIFICATION OF VARIABLES,DOF
ANALYSIS UNIT-BY-UNIT CONTROL
Ref Seider, Seader and Lewin (2004), Chapter 20
3
PROCESS OBJECTIVES

• 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.

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CLASSIFICATION OF VARIABLES

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

• Process

• 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.

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SELECTION OF CONTROLLED VARIABLES

• 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.

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SELECTION OF MANIPULATED VARIABLES

• 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.

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SELECTION OF MEASURED VARIABLES

• 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.

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DEGREES OF FREEDOM ANALYSIS

• 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

• NManipulated NVariables - Nexternally
  defined- NEquations

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EXAMPLE 1 CONTROL OF CSTR

• Number of variables.

• Nvariables

• 10

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

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EXAMPLE 1 CONTROL OF CSTR (Contd)

• Material and energy balances

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EXAMPLE 1 CONTROL OF CSTR (Contd)

• NManipulated NVariables - Next. defined-
  Nequations

• 10

• - 3

• - 4

• 3

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EXAMPLE 1 CONTROL OF CSTR (Contd)

• Selection of controlled variables.

• 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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EXAMPLE 1 CONTROL OF CSTR (Contd)

• Selection of manipulated variables.

• 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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EXAMPLE 1 CONTROL OF CSTR (Contd)

• This suggests the following control configuration

• Can you think of alternatives or improvements ?

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PART TWO Plantwide 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 (Contd)

• 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 8Control 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
Steps 1 2 Establish the control objectives and
DOFs

• 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.

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EXAMPLE 2 ACYCLIC PROCESS (Contd)
Step 3 Establish energy management system.

• Need to control reactor temperature Use V-2.

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

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EXAMPLE 2 ACYCLIC PROCESS (Contd)
Step 4 Set the production rate.

• For on-demand product Use V-7.

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EXAMPLE 2 ACYCLIC PROCESS (Contd)
Step 5 Control product quality, and meet safety,
environmental, and operational constraints.

• To regulate V-100 pressure Use V-5

• To regulate V-100 temperature Use V-6

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EXAMPLE 2 ACYCLIC PROCESS (Contd)
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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EXAMPLE 2 ACYCLIC PROCESS (Contd)
Step 7 Check component balances. (N/A)
Step 8 Control the individual process units (N/A)
Step 9 Optimization

• Install composition controller, cascaded with TC
  of reactor.

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EXAMPLE 2 (Class) ACYCLIC PROCESS
Try your hand at designing a plant-wide control
system for fixed feed rate.
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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 (Contd)
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 (Contd)
Steps 1 2 Establish the control objectives and
DOFs

• Maintain the production rate at a specified
  level.
• Keep the conversion of the plant at its highest
  permissible value.

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EXAMPLE 3 CYCLIC PROCESS (Contd)
Step 3 Establish energy management system.

• Need to control reactor temperature Use V-2.

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EXAMPLE 3 CYCLIC PROCESS (Contd)
Step 4 Set the production rate.

• For on-demand product Use V-7.

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EXAMPLE 3 CYCLIC PROCESS (Contd)
Step 5 Control product quality, and meet safety,
environmental, and operational constraints.

• To regulate V-100 pressure Use V-4

• To regulate V-100 temperature Use V-5

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EXAMPLE 3 CYCLIC PROCESS (Contd)
Step 6 Fix recycle flow rates and vapor and
liquid inventories

• Need to control recycle flow rate Use V-6

• 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.

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EXAMPLE 3 CYCLIC PROCESS (Contd)
Steps 7, 8 and 9 Improvements

• Install composition controller, cascaded with TC
  of reactor.

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SUMMARY

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