Introduction to Process Control

bustlingdivisionElectronics - Devices

Nov 15, 2013 (3 years and 8 months ago)

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Process Control

Spring, 1999

Professor: In
-
Beum Lee

Department of Chemical Engineering

Text and References


Text



Su whan Sung and In
-
Beum Lee, “PID Controllers and
Automatic Tuning” Ajin Press, 1998


References


Seborg D. E., T. F. Edgar, and D. A. Mellichamp,
“Process Dynamics and Control,”John Wiley & Sons,
New York, 1989


Stephanopoulos G., “Chemical Process Control
-
An
Introduction to Theory and Practice,”Prentice
-
Hall,
New Jersey, 1984.


Luyben W. L., “Process Modeling, Simulation and
Control for Chemical Engineers,” McGraw
-
Hill, New
York, 2nd Ed., 1990




Contents of the Lecture

1. Introduction to Process Control.

2. Mathematical Modeling of Chemical Processes.

3. Laplace Transforms.

4. The Transfer Function.

5. Dynamic Behavior of the Processes.

6. Feedback Controllers.

7. Closed
-
loop Control Systems.


Dynamic Behavior.


Stability.

8. Feedback Controller design and Tuning.

9. Process Identification and Controller Design.

1. Introduction to Process Control



Process Control
’ makes processes satisfy following
requirements.


Safety


Production specifications


Environmental regulations


Operational constraints


Economics




Mathematical model

of the process should be
known(Process Modeling & Identification).


1.1 Illustrative Examples


EX 1. Continuous stirred
-
tank heater


Question

;

Assume

that

inlet

temperature

changes

with

time
.

How

can

we

ensure

that

T

remains

at

or

near

the

set

point

T
R
?

Figure 1.1. Continuous stirred
-
tank heater.


Possible Strategies

1. Measure T and adjust Q .

2. Measure T
i

and adjust Q.

3. Measure T and adjust w.

4. Measure T
i

and adjust w.

5. Measure T and T
i

and adjust Q.

6. Measure T and T
i

and adjust w.

7. Place a heat exchanger on the inlet stream.

8. Use a large tank.


Classification


1 & 3;
Feedback

control


2 & 4;
Feedfoward

control


5 & 6; Feedfoward
-
Feedback control


7 & 8; Design change


EX 2. Furnace using electrical power



Question

;

Can

you

find

any

problem

in

controlling

the

following

furnace

process?

SCR unit
+
thermocouple
-
u(t)
y(t)
thermocouple
power
+
thermocouple
-
well insulated
SCR unit input
temperature
heating coil
Figure 1.2. Furnace using electrical power


EX 2. Furnace using electrical power



Question

;

Can

you

find

any

problem

in

controlling

the

following

furnace

process?



There is no way to
decrease the temperature
!


The power cannot be negative.


The heat loss is nearly zero.

SCR unit
+
thermocouple
-
u(t)
y(t)
thermocouple
power
+
thermocouple
-
well insulated
SCR unit input
temperature
heating coil
Figure 1.2. Furnace using electrical power

1.2 Classification of the variables


Input variables

; denote the effect of the surroundings on the chemical
process.

1. Manipulated variable (
MV
or Control variable)



; Its value can be adjusted freely by the human operator
or a control mechanism.


Ex) In heated tank, the amount of heat added(Q) or mass
flow rate(w).

2. Disturbance variable(
DV
)


; Its value is not the result of the adjustment by an
operator or a control system.


Ex) In heated tank, inlet temperature, because we can’t
usually control the temperature of inlet water.



Output variables

; denote the effect of the process oh the surroundings.

1. Measured output variable or Controlled variable(
CV
)


; Its value is known by directly measuring it.


Ex) In heated tank, outlet temperature.


2. Unmeasured output variables


; It is not or cannot be measured directly.


1.3 Classification of Control Strategies


Feedfoward Control


;

The

disturbance

variable

is

measured

and

the

measurement

is

used

to

manipulate

MV
.


Advantages


If all sources of the disturbances are known and these values
can be measured accurately.


Perfect Control !


Disadvantages


No corrective action for unmeasured disturbances.



In industrial applications, feedfoward control should be
used in combination with feedback control.


Feedback Control


;

The

process

variable

to

be

controlled

is

measured

and

used

to

adjust

another

process

variable

which

can

be

manipulated
.


Advantage


Corrective action is taken regardless of the source of the
disturbance.


Disadvantage


The controlled variable must be deviate from the set point
before corrective action is taken.


Not

Perfect Control!


Classification


Negative feedback
; The desirable situation where the corrective
action taken by the controller tends to move the controlled variable
toward set point.



Positive feedback
; The controller tends to make things worse by
forcing the controlled variable farther away from the set point.

1.4 Block Diagram


Schematic diagram


; Physical connection between the components of the control
system.



Figure 1.3. Schematic diagram of a temperature feedback control system
for a stirred
-
tank heater.
---
, Electrical instrument line; TT, temperature
transmitter;TC, temperature controller.


Block diagram


; Flow of information within the control system.



Each block represents a
dynamic

or
static

process
elements.



Dynamic elements
; variable which depends on time.



Static elements
; variable which is independent of time.


Figure 1.4. Block diagram for temperature feedback control system in Figure 3.

1.5 Control and Modeling Philosophies


Two approaches to design control system.

1. Traditional Approach

;

Control

strategy

and

system

hardware

are

selected

based

on

the

knowledge

of

process,

experience

and

insight
.

After

the

control

system

is

installed

in

the

plant,

the

controller

settings

are

adjusted,

that

is,

the

controller

is

tuned
.


2. Model
-
Based Approach


;

A

process

model

is

developed

and

based

on

the

model,

suitable

control

strategy

and

system

hardware

are

selected
.




The model
-
based approach is more advantageous.


Usage of the Process Model



It

can

be

used

as

the

basis

for

classical

controller

design

methods
.


It

can

be

incorporated

directly

in

the

control

law,

an

approach

that

now

is

the

starting

point

for

many

advanced

control

techniques
.


It

can

be

used

to

develop

a

computer

simulation

of

the

process

to

allow

exploration

of

alternative

control

strategies

and

to

calculate

preliminary

values

of

controller

setting
.

1.6 Analog and Digital Control


Analog controller

; Controller that has continuous input and output signals
.



Digital controller

;

Controller

which

involves

input

input

and

output

signals

that

change

only

at

discrete

instants

in

time,

the

so
-
called

sampling

instants
.


Merits

; increased flexibility and accuracy, and improved
monitoring of the plant through data acquisition, storage, and
analysis.

1.7 Economic Justification of Process Control


Justification based on


Safe operation


Satisfying environmental constraints


Economic benefit(ex.increased production level, reduced
raw material costs or enhanced production quality)


Extended equipment life

Example)


Figure 1.5. Production validity over time: (a) before improved control; (b) after.


The operating variable is % ethane.