CHEE 319

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CHEE 319

• Process Dynamics and Control
• Winter 2009

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Outline - Module 1

• definitions and examples
• components of a control system
• why is control required?
• the control systems approach
• feedback control
• control objectives
• control benefits analysis

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Dynamics and Control

• Dynamics - time varying behaviour of a process
• Control - achieve desired conditions in a system
  by adjusting selected variables

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Example - Shower
Hot
Cold
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Example - Shower
Process Flow Diagram
FI
TI
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Example - Shower

• controlled variables - variables to be regulated
• total flow, temperature
• manipulated variables - variables that are
  adjusted
• Position of hot water tap
• Position of cold water tap

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Example Salt Tank Experiment
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Example - Tank Lab Experiment

• controlled variables - level- salt
  concentration - inferred from conductivity
• manipulated variables- concentrated salt flow
  rate (pump speed)- exit flow rate (pump speed)

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Components of the Shower Control System

• Desired Values or Setpoints - target values for
  flow and temperature
• Sensors to measure process conditions - skin and
  eyes
• Control Algorithm to determine corrective action
• fundamental knowledge and intuition
• Final Control Elements adjusted
• position of taps

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Example Shower
setpoint
control algo.
I want a hot shower
For hotter shower, turn HW tap clockwise
sensors
TI
final control elements
FI
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Automated Control Example Home Heating

• controlled variable
• temperature
• manipulated variable
• fuel flow to furnace
• electric current to heater

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Automated Control Example Home Heating

• controlled variable
• temperature
• manipulated variable
• fuel flow to furnace
• electric current to heater
• Talk with your neighbour about another automatic
  control situation. - What are the controlled
  and manipulated variables?

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On/off Control
Is on-off control good enough for all variables
in a chemical process? If the answer is yes,
the course is over!
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Control is necessary for

• rejecting disturbances e.g., when someone else
  brushes or flushes.
• adjust manipulated variables to restore and
  maintain controlled variables at their setpoints
  (feedback control)
• if we know the disturbance is coming we can act
  in advance to prevent deviations from setpoint
  (feedforward control)
• Disturbance rejection is sometimes called the
  load problem

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Control is necessary for ...

• following changes in setpoint
• move to new target values
• change to warmer temperature
• frequently for economic or environmental reasons
• grade changes to satisfy different customers
• summer vs. winter gasoline
• also known as the servo problem
• important in mechanical systems
• positioning of robot arms

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Outline - Module 1

• definitions and examples
• components of a control system
• why is control required?
• the control systems approach
• defn of feedback control
• control engineering
• control objectives
• control benefits analysis

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The Control Systems Approach

• Look at flow of information in the process
• Identify inputs and outputs
• Adjusting the inputs causes the outputs to
  change
• Design control system based on input-output
  behaviour

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The Control Systems Approach

• inputs - variables in a process that cause
  changes in other variables
• Valve positions, feed composition and temperature
• manipulated variables and disturbances
• outputs - variables that respond to inputs and
  are measured
• states - variables that are used to describe the
  operation of the process. Some states are
  measured outputs. Other states are variables that
  we are interested in but we dont have
  measurements

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The Control Systems Approach

• key issue - causality - cause and effect
• Changes in inputs cause outputs and states to
  respond
• manipulated variables are some of the inputs to a
  process
• controlled variables are some of the outputs in a
  process

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Systems Approach Salt Tank Experiment
water
brine soln
LI
effluent
AI

• inputs - concentrated salt flow, exit flow, inlet
  water flow
• outputs - level, salt concentration
• states - level, salt concentration, temperature,
  pressure
• input/output designation does not always match
  physical direction of flow

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Systems Approach Salt Tank Experiment
INPUTS
OUTPUTS
STATES
Salt Tank
brine flow
exit salt conc.

manipulated variable inputs
salt conc. level temperature pressure
exit flow
level

tap water flow (changes due topressure
fluctuations)
disturbance input
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Feedback Control

• Use measurements of process outputs and some sort
  of algorithm to determine changes to the inputs
  of the same process
• Feedback controllers act to reduce differences
  between the setpoint and the measured values of
  outputs. This is called negative feedback.

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Example Shower
setpoint
control algo.
I want a hot shower
For hotter shower, turn HW tap to right
sensors
final control elements
TI
FI
Feedback loop
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Process Control at an Industrial Plant
Sensors, local indicators, and valves in the
process
Central control room
Displays of hundreds of variables, calculations,
and commands to valves are in the centralized
control centre.
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Control Engineering addresses...

• Process Design
• controllability and operability.
• how long does it take for disturbances to have an
  effect on the output and how long does a problem
  persist after the disturbance goes away?
• How responsive is the output to changes in the
  inputs?
• Measurements
• selection and location of sensors.
• accuracy and speed of sensors.
• Are existing sensors good enough so that we can
  detect problems and remedy them?

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Control Engineering addresses...

• Final Control Elements
• location, type of variables to manipulate
• flexibility, speed of action
• Control Structure
• Which inputs should be used to control which
  outputs?
• We want to get to new setpoints quickly and take
  care of disturbances quickly.
• We want to make sure that the adjustments that
  our control system makes dont cause upsets
  elsewhere in the plant.

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Control Engineering addresses...

• Control Calculations
• algorithm to determine appropriate changes in
  manipulated variables in response to changes in
  measured variables and setpoint

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Outline - Module 1

• definitions and examples
• components of a control system
• why is control required?
• the systems approach
• defn of feedback control
• control engineering
• control objectives
• control benefits analysis

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Control Objectives

• Safety - paramount
• maintain proper operation to avoid dangerous
  situations
• emergency systems
• pressure relief valves
• automatic reactor shutdown systems
• Environmental Protection
• proper operation and containment
• prevent tanks from overflowing
• maintain low concentrations of undesirable
  compounds in effluent

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Control Objectives

• Equipment Protection
• proper operation and shutdown at limiting
  conditions
• prevent pumps from running dry
• prevent furnace tubes from getting too hot
• Smooth Operation
• for both inputs and outputs
• be a good neighbour
• minimize disturbances to integrated units

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Control Objectives

• Product Quality
• Maintain composition, physical properties and
  performance properties of products within
  customer specificationsAfter these objectives
  have been satisfied, we can focus on profit.

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Control Objectives

• Profit
• reduce costs
• improve efficiency
• consume fewer raw materials and less energy
• require less inventory
• increase revenues by achieving higher production
  rates or by making specialized products

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Control Objectives

• Monitoring and Diagnosis
• Collect useful information to improve plant
  operation
• Immediate and short-term
• Identify and potentially dangerous situations and
  take appropriate actions
• Provide timely information to operators,
  supervisors and plant engineers
• Longer-term - identify causes of poor plant
  operation and find solutions that will prevent
  future upsets and will reduce variability
• Operators, supervisors, plant engineers and
  managers use information provided by plant
  monitoring and control systems to plan process
  improvements

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EXAMPLE PROCESS FLASH SEPARATION
Vapor product
T6
P1
T5
Feed Methane Ethane (LK) Propane Butane Pentane
T1
P ? 1000 kPa T ? 298 K
T2
F1
T4
T3
L1
F2
F3
Liquid product
A1
Process fluid
Steam
L. Key
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth
operation 5. Product quality 6. High
profit 7. Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
High pressure in drum is dangerous
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth
operation 5. Product quality 6. High
profit 7. Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
To flare
Never release hydrocarbons to atmosphere
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
No flow could damage the pump
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Always keep the production rate smooth
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Achieve L.Key by adjusting the heating
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Use the least costly heating
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Give example
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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SEVEN CONTROL OBJECTIVES
Calculate plot key parameters, e.g., UA.
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
production rate 5. Product quality 6. High
profit 7. Monitoring diagnosis
UA
time
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SEVEN CONTROL OBJECTIVES
All seven must be achieved. Failure to do so
will lead to operation that is unprofitable or
worse, unsafe.
1. Safety 2. Environmental Protection 3.
Equipment protection 4. Smooth operation
5. Product quality 6. High profit 7.
Monitoring diagnosis
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Control Benefits Analysis

• Operating Window
• Region of feasible process operation
• Factors that determine limits on feasible
  operation
• Safety, environmental and equipment limits
• maximum safe pressure and temperature
• maximum pump throughput, tank size
• dont kill the catalyst
• keep microorganisms in bioreactor alive
• dont flood the distillation tower
• Product quality
• minimum octane rating and maximum vapour pressure
• upper and lower specification limits for polymer
  molecular weight
• Physical limitations
• mass fractions between 0 and 1
• Constraints are the frame around the operating
  window. We want to operate around the best
  possible point and stay inside the window.

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Operating Window - Example

• Boiler for producing steam
• Ensure temperature doesnt exceed metallurgical
  limits
• Ensure enough oxygen for complete combustion

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In defining operating window, we encounter...

• Soft Constraints - limits that can occasionally
  be violated for short periods of time
• e.g., product quality
• e.g., small deviations over equipment guidelines
• Hard Constraints - limits that cannot be exceeded
  under any circumstances
• maximum pressure in a vessel
• How close we can operate to a constraint depends
  on the variability of the process

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Process control reduces variability,
permittingoperation closer to operating limits
and providing an economic incentive for
implementing improved control schemes.
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Assessing Benefits of Process Control

• Compare operation before implementation of the
  new control strategy to operation with the new
  control strategy
• Sometimes benefits are calculated in dollars
• Sometimes benefits are calculated using other
  variables, such as yield improvement, efficiency,
  customer satisfaction, environmental compliance

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Control benefit analysis requires...

• Information about how profitability or
  performance varies with operating point
• Understanding of process variability with and
  without the new control strategy

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BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the
variability of key variables
Without feedback control
Composition ( H. Key)
Reflux valve
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BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the
variability of key variables
With feedback control
Composition ( H. Key)
Reflux valve
Variability is moved from controlled to
manipulated variable!
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Reducing Variability is Good, But ..

• How can a reduction in variability make or save
  money for a company?
• Talk to your neighbour about this
• How can we convince managers that they should
  retain or employ more control engineers?

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BENEFITS FROM PROCESS CONTROL
When we control a process, we reduce the
variability of key variables to achieve the seven
objectives.
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BENEFITS FROM PROCESS CONTROL
Process performance efficiency, yield,
production rate, etc. It measures performance
for a control objective.
Calculate the process performance using the
distribution, not just the average value of the
key variable!
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A
Example of Benefits of reduced variability for
chemical reactor
Goal Maximize conversion of feed ethane but do
not exceed 864C Which operation, A or B, is
better and explain why.
B
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Example of Benefits of reduced variability for
chemical reactor
Goal Maximize efficiency and keep O2gt0.50
mol Which operation, A or B, is better and
explain why.
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Control benefit analysis

• Collect representative data sets before and after
  control and construct histograms
• calculate average performance with control and
  without controlfor both data sets
• Fj - fraction of occurrences within jth bin of
  histogram
• Pj performance associated with jth bin
• Determine the difference in average performance
  due to new control system

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Look ahead...
Controller Design Procedure
Define Objectives
Characterize Process
Design Controller
Implement and Test