PSE and PROCESS CONTROL

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PSE and PROCESS CONTROL

• Sigurd Skogestad
• Department of Chemical Engineering
• Norwegian University of Science and Tecnology
  (NTNU)
• Trondheim, Norway
• PSE Education Session
• AMIDIQ 2012, Mexico, May 2012

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• Process control is an important subject in the
  chemical engineering education. In addition to
  covering feedback control, it is often the only
  course in the curriculum that includes aspects of
  process operation and process dynamics. It can
  also be a difficult course to teach, especially
  if this is the only course, because there are
  many topics and concepts that one would like to
  include, some of which are Control crash course
  (3 weeks) 1. Process operation Why do we need
  process control? 2. Classification of variables
  (inputs, outputs, disturbances, measurements) 3.
  Feedback versus feedforward control 4. Block
  diagram representation (information diagrams,
  causality) 5. Flowsheet representation (process
  instrumentation diagrams) 6. Single-loop control
  Pairing of input and outputs 7. More advanced
  control Ratio control, Cascade control, 8. The
  control hiearchy (optimization, advanced control,
  basic control) 9. Process dynamics (basics)
  first- and second order systems, time delay,
  identification 10. Process modelling balance
  principle 11. PID control and tuning 12.
  Simulation Control theory (10 weeks) 13. Laplace
  transforms, transfer functions 14. Closed-loop
  response, derivation of PID tuning rules 15. Pros
  and cons of high gain feedback. Stability. Change
  dynamics. Biological systems 16. Dynamic systems
  (theory). poles, zeros, state space,
  observability, controllability 17. Control
  systems (theory), frequency analysis, stability
  conditions, robustness 18. Controller
  implementation discrete control, windup,
  bumpless transfer 19. Identification (theory) 20.
  Multivariable control interactions, MPC I have
  listed the topics approximately in the order I
  teach them in my course. To make sure the
  students understand what the theory is going to
  be used for, I teach the 12 first topics as a
  3-week "crash course". Usually, some of the
  theoretical material (topics 13-20) has to be
  deleted, and the question is which? For example,
  can we omit Laplace transforms and frequency
  analysis? This is tempting, but it also makes it
  difficult to derive tuning rules analytically and
  to really understand feedback

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PROCESS CONTROL
Theory Left side of brain logical
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PROCESS CONTROL
Control structures Practise Right side of brain
creative
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PROCESS CONTROL
Theory practise Combine both sides!
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Process control course.Four main elements

• PROCESS
• Process dynamics Step responses, simulation
• Process control structures Flowsheet (PID).
  PID tuning
• CONTROL
• theory Feedback idea, block diagrams, stability,
  transfer functions (Laplace), feedforward/cascade/
  frequency response, identification, multivariable
  control (MPC)
• PRACTISE
• Laboratory
• Simulation (Aspen, Hysys/Unisim..)
• SYSTEMS
• Modelling principles, Solution. State space
  models, linearization (ABCD), optimization

PID Process and Instrumentation Diagrams
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Difficult course

• Many new concepts
• Inputs and outputs, causality
• Feedback
• Stability
• New mathematics
• Laplace
• Frequency analysis
• System theory (ABCD)
• And all of this combined with practise operation
  of real plants
• Too much for one course?

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I teach the course in two parts

• Process control crash course (3 weeks)
• Focus on process control structures (PID)
• Standard process control course (11 weeks)
• Focus on theory

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Crash course process control

• Sigurd Skogestad
• Institutt for kjemisk prosessteknologi
• Rom K4-211
• skoge_at_ntnu.no

• More information (literature, old exams, etc.)
• www.nt.ntnu.no/users/skoge/prosessregulering_lynk
  urs

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Why control?

• Until now Design of process. Assume steady-state
• Now Operation

Actual value(dynamic)
Steady-state (average)
time
Disturbances (ds)
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Example Control of shower temperature
MVs, CVs and control
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CLASSIFICATION OF VARIABLES
flow in
Hs
H
LC
flow out
OUTFLOW INPUT FOR CONTROL INFLOW DISTURBANCE
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BLOCK DIAGRAMS

• All lines Signals (information)
• Blocks controllers and process
• Do not confuse block diagram (lines are signals)
  with flowsheet (lines are flows) see below

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Most important control structures

1. Feedback control
2. Ratio control (special case of feedforward)
3. Cascade control

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Process and instrumentation diagram (PID)
(flowsheet)
Ts (setpoint CV)
T (measured CV)
TC
MV (could be valve)
2nd letter C controller I indicator
(measurement)
1st letter Controlled variable (CV). What we are
trying to control (keep constant) T
temperature F flow L level P pressure
DP differential pressure (?p) C composition
X quality H enthalpy/energy
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Typical distillation control Two-point
composition control LV-configuration with inner
T-loop
LV
CC
xD
Ts
TC
CC
xB
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Process dynamics (response)

• Things take time
• Step response (step in u)
• k ?y(8)/ ?u process gain
• ? - process time constant (63)
• ? - process time delay
• Time constant ? Often equal to residence time
  Vm3/qm3/s (but not always!)
• Can find ? (and k) from balance equations
• Rearrange to match standard form of 1st order
  linear differential equation

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Pairing of variables

• Main rule Pair close
• The response (from input to output) should be
  fast, large and in one direction. Avoid dead time
  and inverse responses!

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Model-based tuning (SIMC rule)
k ?y(8)/ ?u

• From step response
• k ?y(8)/ ?u process gain
• ? - process time constant (63)
• ? - process time delay
• Proposed SIMC controller tunings

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Process Control crash course (3 weeks)

• 1. Process operation Why do we need process
  control?
• 2. Classification of variables (inputs, outputs,
  disturbances, measurements)
• 3. Feedback versus feedforward control
• 4. Block diagram representation (information
  diagrams, causality)
• 5. Flowsheet representation (process
  instrumentation diagrams)
• 6. Single-loop control Pairing of input and
  outputs
• 7. More advanced control Ratio control, Cascade
  control,
• 8. The control hiearchy (optimization, advanced
  control, basic control)
• 9. Process dynamics (basics) first- and second
  order systems, time delay, identification
• 10. Process modelling balance principle
• 11. PID control and tuning
• 12. Simulation

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Control theory (11 weeks)standard course

• 13. Laplace transforms, transfer functions
• 14. Closed-loop response, derivation of PID
  tuning rules
• 15. Pros and cons of high gain feedback.
  Stability. Change dynamics. Biological systems
• 16. Dynamic systems (theory). poles, zeros, state
  space, observability, controllability
• 17. Control systems (theory), frequency analysis,
  stability conditions, robustness
• 18. Controller implementation discrete control,
  windup, bumpless transfer
• 19. Identification (theory)
• 20. Multivariable control interactions, MPC

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3. Practise

• LAB ?!!
• At least have demonstration
• SIMULATIONS ?!!
• Time consuming

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4. Systems engineering

• General modelling principles, DAE-system
• Solution of dynamic models (integration)
• Linearization, State space models (deviation
  variables)
• Optimization

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Conclusion Process systems engineering (PSE) and
process control

• Process control is a key course
• Engineers must know some control!
• Usually too little time to focus on systems
  issues
• Need advanced course to cover process systems
  aspects of process control