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PSE and PROCESS CONTROL

Sigurd Skogestad

Department of Chemical EngineeringNorwegian University of Science and Tecnology (NTNU)Trondheim, Norway

PSE Education SessionAMIDIQ 2012, Mexico, May 2012

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PROCESS CONTROL

TheoryLeft side of brain = logical

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PROCESS CONTROL

Control structures + PractiseRight side of brain = creative

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PROCESS CONTROL

Theory & practiseCombine both sides!

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Process control course.Four main elements:

1. PROCESS– Process dynamics: Step responses, simulation– Process control structures: Flowsheet (P&ID*). PID tuning

2. CONTROL– theory: Feedback idea, block diagrams, stability, transfer functions

(Laplace), feedforward/cascade/frequency response, identification, multivariable control (MPC)

3. PRACTISE– Laboratory– Simulation (Aspen, Hysys/Unisim..)

4. SYSTEMS– Modelling principles, Solution. State space models, linearization

(ABCD), optimization

*P&ID: 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

1. ”Process control” crash course (3 weeks)

– Focus on process control structures (P&ID)2. Standard process control course (11 weeks)

– Focus on theory

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Crash course process control

Sigurd SkogestadInstitutt for kjemisk prosessteknologi

Rom K4-211skoge@ntnu.no

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

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Why control?

• Until now: Design of process. Assume steady-state

• Now: Operation

time

Actual value(dynamic)Steady-state (average)

“Disturbances” (d’s)

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Example: Control of shower temperature

MVs, CVs and control

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LCH

Hs

flow in

flow out

OUTFLOW: INPUT FOR CONTROLINFLOW: DISTURBANCE

CLASSIFICATION OF VARIABLES

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BLOCK DIAGRAMS

Controller(brain)

Measurementdevice

ys

Desired valueSetpoint

ys-ym

error

ym

measured output

FEEDBACK (measure output):

Process(shower)

uinput (MV)

youtput (CV)

d

Controller(brain)

Process(shower)

Measurementdevice

FEEDFORWARD (measure disturbance):

dm

measured disturbance

d

uinput (MV)

youtput (CV)

•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 (P&ID) (flowsheet)

TC2nd 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 (Δp) C: composition X: quality H: enthalpy/energy

Ts

(setpoint CV)T(measured CV) MV (could be valve)

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CC

LV

Typical distillation control: Two-point composition controlLV-configuration with inner T-loop

TCTs

xB

CC xD

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Process dynamics (response)

• “Things take time”• Step response (step in u):

– k = Δy(∞)/ Δu – process gain– - process time constant (63%)– - process time delay

• Time constant Often equal to residence time = V[m3]/q[m3/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 variablesMain 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)

• From step response

– k = Δy(∞)/ Δu – process gain

– - process time constant (63%)

– - process time delay

• Proposed SIMC controller tunings

k = Δy(∞)/ Δu

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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