proces dynamics & controls.pdf

8/14/2019 Proces Dynamics & Controls.pdf

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Process Dynamics HHS, 8- Sep, 2011

Introduction to PD&C

In the chemical industry, the design of a controlsystem is essential to achieve:

1. Safety

2. Environmental protection3. Equipment protection

4. Smooth operation

5. Product quality

6. Profit

7. Monitoring and diagnosis


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Why?

Most chemical and biological engineers are notprocess control experts, but they need to

understand the basics!

Production engineering: responsible for day-to-dayoperations. Need to understand process controls to

deal with daily issues.

Process engineer: longer term view towardcontinuous process improvementtheseimprovements often involve new or modified processcontrols.


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Specialists

Some chemical and biological engineers becomeprocess control engineers focus on improving

process control systems and implementingstandard systems

Even fewer become process control specialists

implement solutions for specialized, nonstandard,control situations.


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Basic Control System

Oil heater that uses steam


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A balloon by itself indicates a discrete stand-alone instrument, such as a:

Transmitter

Sensor

Alarm

P & ID General Instrument Symbols:


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#1 we need a sensor (and we cannot measureeverything, like entropy or fugacity)


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#2 we need a controller (e.g., PID controller)


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#3 we need something to control (and we cannotcontrol everything like steam T and P)


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Introduction

Process Dynamics:

Study of the transient behavior of processes

Process Control

the use of process dynamics for the improvement ofprocess operation and performance

or

the use of process dynamics to alleviate the effect of

undesirable (unstable) process behaviors


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Ref. book

Focus is on information needed by process andproduction engineers (i.e., not specialists)

Reference book:1. Chemical Process Control: An Introduction to Theory and Practice, George Stephanopoulos2. Process dynamics and control, Modeling for control and prediction, Brain Roffel and BenBetlem


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Introduction

What do we mean by process?

A process, P, is an operation that takes an INPUT or a DISTURBANCEand givesan OUTPUT

INPUT: (u) Something that you can manipulate

DISTURBANCE: (d) Something that comes as a result of some outsidephenomenon

OUTPUT: (y) An observable quantity that we want to regulate

u

d

yP

Information Flow


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Examples

Stirred tank heater

M

Tin, w

Q

T, w

TinwQ

TProcess

Inputs Output


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Control

What is control?

To regulate of a process output despite the effect of disturbancese.g.

Driving a car

Controlling the temperature of a chemical reactor Reducing vibrations in a flexible structure

To stabilize unstable processes e.g.

Riding a bike

Flight of an airplane

Operation of a nuclear plant


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Benefits of Control

Economic Benefits

Quality (waste reduction)

Variance reduction (consistency)

Savings in energy, materials, manpower

Operability, safety (stability)

Performance

Efficiency

Accuracy robotics

Reliability Stabilizability

bicycle

aircraft

nuclear reactor


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Control

A controller is a system designed to regulate a given process Process typically obeys physical and chemical conservation laws

Controller obeys laws of mathematics and logic (sometimesintelligent)

e.g. - Riding a bike (human controller)

- Driving a car

- Automatic control (computer programmed to control)

Process

Controller

What is a controller?


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Block representations

Block diagrams are models of the physical systems

Process

System PhysicalBoundary Transfer offundamental

quantities

Mass, Energy and Momentum

Input variables Output variables

Physical

OperationAbstract


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Control

A controlled processis a system which is comprised of two interactingsystems:

e.g. Most controlled systems are feedback controlled systems

The controller is designed to provide regulation of process outputs in thepresence of disturbances

Process

Controller

OutputsDisturbances

Action Observation

monitorintervene


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What is required?

What is required to control a process?

1. Process Understanding Required measurements

Required actuators

Understand design limitations

2. Process Instrumentation Appropriate sensor and actuator selection

Integration in control system

Communication and computer architecture3. Process Control

Appropriate control strategy


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Types of Process Controls

1. Manual control (open loop)

2. On/Off control (closed feedback loop)

3. PID control (closed feedback loop)

4. Dynamic model-based control (closed feedforward

loop)1. Empirical models (e.g., cold shower)

2. Phenomenological models


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Classical Control 20/9

Control is meant to provide regulation of process outputs about areference/set point, r, despite inherent disturbances

The deviation of the plant output, e=(r-y), from its intended

reference/setpoint is used to make appropriate adjustments in the plantinput, u

ProcessController

Classical Feedback Control System

d

yur e+

-


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Control Nomenclature

Identification of all process variables

Inputs (affect process) Outputs (result of process)

Inputs

Disturbance variables Variables affecting process that are due to external forces

Manipulated variables Things that we can directly affect


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Control

Process is a combination of sensors and actuators

Controller is a computer (or operator) that performs the required

manipulations

e.g. Classical feedback control loop

yr e

AC P

M

d

Computer Actuator

Process

Sensor

-

+


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Positive & negative control


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Examples

Stirred-Tank Heater

Q T, w

Heater

TCThermocouple

Tin, w

Tin, w

ye

AC P

M

Controller

Tank

-

+

Heater

Thermocouple

TR


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Examples

Measure Ti, adjust Q

A PC

MTi

Qi

Q

Q

+

+

Feedforward Control

Control System Development


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Define Objectives

Develop a processmodel

Design controllerbased on model

Test bySimulation

Implement and Tune

MonitorPerformance

Control development is usually carried out following theseimportant steps

Often an iterative process, based on performance we maydecide to retune, redesign or remodel a given control system


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Objectives

What are we trying to control?

Process modeling

What do we need? Mechanistic and/or empirical

Controller design

How do we use the knowledge of process behavior to

reach our process control objectives? What variables should we measure?

What variables should we control?

What are the best manipulated variables?

What is the best controller structure?


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Implement and tune the controlled process

Test by simulation

incorporate control strategy to the process hardware tune and re-tune

Monitor performance periodic retuning and redesign is often necessary based

on sensitivity of process or market demands

statistical methods can be used to monitor performance


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Process Modeling

Motivation:

Develop understanding of process

a mathematical hypothesis of process mechanisms Match observed process behavior

useful in design, optimization and control of process

Control:

Interested in description of process dynamics Dynamic model is used to predict how process responds to given

input

Tells us how to react


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Process Modeling

What kind of model do we need?

Dynamic vs. Steady-state

Steady-state Variables not a function of time

useful for design calculation

Dynamic

Variables are a function of time Control requires dynamic model

Process Modeling


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Process Modeling

What kind of model do we need?

Experimental vs Theoretical

Experimental Derived from tests performed on actual process

Simpler model forms Easier to manipulate

Theoretical Application of fundamental laws of physics and chemistry

more complex but provides understanding

Required in design stages

Process Modeling


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g

Dynamic vs. Steady-state

Step change in input to observe

Starting at steady-state, we made a step change

The system oscillates and finds a new steady-state

Dynamics describe the transitory behavior

0 50 100 150 200 250 30040

45

50

55

60

65

Output

Time

Steady-State 1

Steady-State 2

Process Modeling


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Process Modeling

Empirical vs. Mechanistic models Empirical Models

only local representation of the process

(no extrapolation) model only as good as the data

Mechanistic Models Rely on our understanding of a process

Derived from first principles Observing laws of conservation of

Mass

Energy

Momentum Useful for simulation and exploration of new operating conditions

May contain unknown constants that must be estimated

Process Modeling


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Process Modeling

Empirical vs Mechanistic models

Empirical models

do not rely on underlying mechanisms Fit specific function to match process

Mathematical French curve

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 00 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1

1 . 1

1 . 2

1 . 3

Output

T i m e

Process Modeling


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g

Linear vs Nonlinear Linear

basis for most industrial control

simpler model form, easy to identify

easy to design controller

poor prediction, adequate control

Nonlinear

reality more complex and difficult to identify

need state-of-the-art controller design techniques to do the job

better prediction and control

Process Modeling


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g

In existing processes, we really on Dynamic models obtained from experiments

Usually of an empirical nature

LinearIn new applications (or difficult problems)

Focus on mechanistic modeling

Dynamic models derived from theory Nonlinear


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Process Modeling


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g

Modeling procedure

Check model consistency do we have more unknowns than equations

Determine unknown constants e.g. friction coefficients, fluid density and viscosity

Solve model equations typically nonlinear ordinary (or partial) differential equations

initial value problems

Check the validity of the model compare to process behavior

Process Control and Modeling


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In designing a controller, we must Define control objectives

Develop a process model

Design controller based on model Test through simulation

Implement to real process

Tune and monitor

y

d

Process

Model

Controllerur e

Design

Implementation

E l


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Example

T

L

T

wi, Ti

wc, Tci

wc, Tco

wo, To

h

Variables wi, wo: Tank inlet and outlet mass flows Ti, To: Tank inlet and outlet temperatures wc: Cooling jacket mass flow

Pc: Position of cooling jacket inlet valve Po: Position of tank outlet valve Tci, Tco: Cooling jacket inlet and outlet

temperatures h: Tank liquid level

Po

Pc

Example


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Example

Variables Inputs OutputsDisturbances Manipulated Measured Unmeasured Control

wiTiTciwch

woToPc

Po

Task: Classify the variables



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y p

Define Objectives

Develop a processmodel

Design controllerbased on model

Test bySimulation

Implement and Tune

MonitorPerformance

Control development is usually carried out following theseimportant steps

Often an iterative process, based on performance we maydecide to retune, redesign or remodel a given control system

The task of a process control system


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The task of a process control system

Monitoring certain variables that indicate process

conditions at any time (measurements)

Making rational decisionsregarding what corrective

action is needed (current state vs. desired state)

Inducing changes in the appropriate process

variables to improve process conditions (valves tomanipulate)

once more...

According to what rationale does a

feedback control system work?

On off control: the simplest one


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On-off control: the simplest one

The control variable is manipulated according to:

proces dynamics & controls.pdf

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