Process Design and ControlDr. Eng. Rami Bechara

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Personal Information• 27 years old

• Degree in chemical engineering from Ecole Centrale Paris, 2011

• Doctorate from Université Lyon I, School of chemistry, 2015- Thesis realized in IFPEN and EPFL – Specialization in Process Engineering

• Thesis entitled: “Methodology for optimal process design: Application to sugarcane conversion processes”

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Syllabus• Theoretical model of Chemical processes• Laplace Transforms, Transfer Functions and State-Space

Models• Dynamic Behavior of First-Order and Second-Order Systems• Open-Loop and Closed-Loop Stability Analysis• PID Controller Design, Analysis and Tuning• Feed Forward, Cascade, Internal Model Control, Smith

Predictor and Multiloop Control• Overview of Advanced control: Model Predictive Control and

Optimization

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Summary

• Introduction to process control

• Feedback control

• Case studies

• Feedback control strategies - PID

• Future courses

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Introduction to Process control

• Objective: Obtain and maintain desired operating conditions

• Compositions, pressures, temperatures

• By manipulating selected variables

• Flow rates

• By virtue of control valves

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Control Strategies

• Feedback Control

•Mix of Feedback and Feedforward

• Dampening or Design modifications

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Control StrategiesFeedback control• Output signal is used to control input variable

• Advantages: Corrective action occurs regardless of disturbance• Present in most industrial systems

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Feedback control strategiesFeedback type• Positive feedback or direct acting • Controller output increases with error• Case: Flow rate ↗ if leak ↗• Vicious cycle: Boiler turned on if room T ↗• Not commonly encountered

• Negative feedback or reverse acting • Controller output decreases with error• Case: Boiler turned off if room T ↗• Most common

• Regulatory control or Disturbance rejection• Servo control: track changing set-point

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Case study IGravity Drained Tanks

9• Case for positive feedback & Disturbance rejection• Problem: How can disturbance changes be countered?

Case study IIHeat exchanger

• Case for negative feedback & set point tracking• How can new set points be achieved? 10

Feedback control strategiesON/OFF variables

• Equation

• ResponseDiagram

• Case for a thermostat not applicable to precise process control 11

0if,0if,

)(min

max

eCOeCO

tCO

Feedback control strategiesProportional (P) response

• Equation

• Response Diagram

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)()( 0 teKCOtCO CCO0 controller biasKC controller gain

KC > 0 KC < 0

Feedback control strategiesP response

• Advantages• Offset is reduced (↗ KC )• Rather simple tuning• Problems:• Greater Overshooting (↗ KC )• Offset cannot be eliminated

Introduction of integral control13

Feedback control strategiesProportional Integral (PI) response

• Equation

• Response Diagram (P vs. PI)

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t

IC tteteKCOtCO

00 d)(1)()(

I =integral time (>0)(also called reset time)

Integral action contribution

Feedback control strategiesPI vs. P

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Feedback control strategiesPI response

• Advantages• Offset can be eliminated

• Problems:• Harder tuning – 2 parameters• Oscillatory response Instability• Integral controller saturation Windup• Unable to counter fast deviations Introduction

of derivative action16

Feedback control strategiesPI Derivative (PID) response• Equation

• Response Diagram

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t

tetteteKCOtCO D

t

IC d

)(dd)(1)()(0

0

D (>0)derivative time

Derivative action contribution

Feedback control strategiesPID response

• Advantages:

• Oscillations dampened• Process response speeded up• Counters fast deviations

• Problems:

• Harder tuning – 3 parameters• Noise amplification - Unable to handle noisy

measurements

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P vs. PI vs. PID

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Feedback controlPID Uses

• P-only controller: used when steady state offsets can be tolerated-Liquid level loops

• PI – controller: used when offsets need to be eliminated and no need for fast response - Large proportion of feedback loops

• PID: used when need for fast response, and process signal is noise-free – Temperature control 20

Future courses

• Concerning feedback control• Different PID configurations• Strategies for parameter tuning• Controller Characterization• Selection of manipulated/measured variables

• Concerning other control strategies• Disadvantage of feedback: First must allow a deviation

or error to appear before it can take action• Solution: Inclusion of Feed Forward Control 21

References• Textbook: Seborg, Dale E., et al. Process dynamics and control.

John Wiley & Sons, 2010 – Chapter 7

• http://controlguru.com/: Dr. Douglas Cooper, Direct Chemical and Biochemical Engineering department, University of Connecticut

• Further Coursework: Barry Johnston. 10.450 Process Dynamics, Operations, and Control, Spring 2006. (Massachusetts Institute of Technology: MIT OpenCourseWare), http://ocw.mit.edu

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