CHAPTER-1.pptx

CHAPTER-1
Introduction To
process Dynamics
and Control
By: Tafere A.

Outline
 INTRODUCTION
 Process control
 Example of controlled process
 Classification of variables in chemical
process
 Elements of control system
 Summary

Course Description
 This course combines the mathematical, physical
and chemical concepts for application to process
simulation and control.
 This is an introductory part for process control
design and analysis.
 Whenever appropriate, MATLAB is used to
demonstrate the behavior of the control system.

Process dynamics model
The two main subjects of this course are process
dynamics model and statical model.
 A static model is one,
which is developed
based on the steady state
information, in which;
- Nothing changes with
time.
 Are typically
represented with
algebraic equation.
A dynamic mode;
variables change
with times (transient
process behavior).
described by
differential
equations.
So, control systems are needed to handle such changes in
the process (variable change).

Introduction
 In any industrial plant the aim is to produce standard and high quality
products and sell them at prices which make profit.
 These purposes can be achieved in a successfully designed and
controlled processes.
 The primary objective of process control is to maintain a process at
the desired operating conditions, safely and efficiently, while
satisfying environmental and product quality requirements.
 Proper application of process control can actually improve the safety
and profitability of a process.
 The subject of process control is concerned with how to achieve
these goals.

Objectives
 The Operational Objectives process controls are:-
1. Safety
2. Production Specifications
3. Environmental Regulations
4. Operational Constraints
5. Economics
 Optimization: Combination of several variables together
with most suitable conditions.
 Optimum conditions are important for;
 Continuity
 Quality
 Economics of processes.
“main variable”

What is a process?
A process denotes an operation or
series of operations on fluid or solid
materials during which the
materials are placed in a more
useful state.
A process is an operation that uses
resources to transform inputs into
outputs.
 It is the resource that provides the
energy into the process for the
transformation to occur.

Con’t…
 The objective of a process is to convert certain raw
materials (input feedstock) into desired products
(output) using available sources of energy in the most
economical way.
 Many external and internal conditions affect the
performance of a process.
 These conditions may be expressed in terms of process
variables such as temperature, pressure, flow, liquid level,
weight, volume etc.

What is control ?
 The term Control means;
 measuring the value of the controlled variable and applying the
control signal to the system to correct or limit deviation of the
measured value from a desired value.
 Methods to force parameters in the environment to have specific
values.
 It is very important in process industry to use control to operate
processes in such a way that energy and raw materials are utilized safely,
efficiently and economically.
 Chemical engineers need to master this subject in order to be able to:-
 Design and operate the plants efficiently.

Why is Control necessary?
 Control - is necessary because during its operation, a
chemical plant must satisfy several requirements imposed by
its designers and the general technical, economic, and social
conditions in the presence of ever changing external
influences (disturbances).
 Such requirements are the following:
- Safety
- Production specifications
- Environmental regulations
- Operational constraints
- Economics

Safety
 The safe operation of a chemical process is a primary
requirement for the well-being of the people in the
plant and for its continued contribution to the
economic development.
 Thus the operating pressures, temperatures,
concentration of chemicals and so on should always
be within allowable limits.

Production specifications
 A plant should produce the desired amounts and quality of
the final products.
 For example; we may require the production of 2 million
pounds of ethylene per day, of 99.5% purity.
 Therefore, a control system is needed to ensure that
the production level and the purity specifications
are satisfied.
 Product certification procedures (e.g., ISO9000) are
used to guarantee product quality and place a large
emphasis on process control.
 http://www.iso.ch/iso/en/ISOOnline.openerpage

Environmental regulations
 Various federal and state laws may specify that the
temperatures, concentrations of chemicals and flow
rates of the effluents from a plant be within certain
limits.
 Such regulations exist, for example on the amounts of
SO2 that a plant can eject to the atmosphere, and on
the quality of the water returned to a river or lake.

Operational constraints
 The various types of equipment's used in a chemical plant
have constraints inherent to their operation.
 Constraints should be satisfied throughout the
operation of a plant.
 For example;
- Pumps must maintain a certain net positive suction head;
- Tanks should not overflow or go dry
- Distillation columns should not be flooded
- The temperature in a catalytic reactor should not exceed an
upper limit since the catalyst will be destroyed.
 Control systems are needed to satisfy these operational
constraints.

Economics
 The operation of a plant must conform with the market conditions, that
is, the availability of raw materials and the demand of the final
products.
 Furthermore it should be as economical as possible in its utilization of;
- Raw materials
- Energy
- Capital and
- Human labor.
 Consequently, it is required that the operating conditions are
controlled at given optimum levels of minimum operating cost,
maximum profit and so on.

Why is control necessary?
 All this requirements need for continuous monitoring
for the operation of a chemical plant and external
intervention (control);
 To guarantee the satisfaction of the operational
objectives.
 To optimize operations.
To interfere with situations when an unusual or
dangerous situation occurs.

How is control done
 A process Control is accomplished through a rational
arrangement or Can be controlled either by (human beings
manually) or by necessary instrumentation (automatically).
I. Equipment (measuring devices, valves, controllers,
computers) and;
II. Human intervention (plant designers, plant
operators), which together constitute a control
system.

What is Process control
 Process control: is the action of monitoring; changing of the process
parameters, technology and equipment’s based on the results of
process output.
 A process control is the act of controlling a final control element to
change the manipulated variable to maintain the process variable at
a desired set point.
 In process control, the basic objective is to regulate the value of some
quantity of interest at some desired value regardless of external
Influences.
 To regulate means to maintain that quantity at some desired value (reference
value or set point) regardless of external influences.

Con’t…
Process control: Required to maintain safe operations,
quality products and business viability (Profit).
Safety;
- The primary purpose of process control system.
- Personnel safety, environmental safety and equipment safety.
Quality
- Process control systems are central to maintaining product
quality.
Profit
 When safety and quality concerns are met, process
control objectives can be focused on profit.

What are the basic elements of
process control?
 The process itself, the sensor that measures the
process value, the final control element that changes
the manipulated variable and the controller.
The process
 Processes have a dynamic behavior that is determined
by physical properties which cannot be altered
without making a physical change to the process.

Sensors
 Measure the value of the process output called
Process Variable (PV) such as temperature, pressure,
mass, flow and level.
Final Control Element
 The physical device that receives commands from
the controller that manipulate the resource.
Controller
 Provides the signal to the final element.

 The Manipulated Variable (MV) – is the measure of
resource being fed into the process, for instance how
much thermal energy.

 A Final Control Element (FCE) – is the device that
changes the value of the manipulated variable.
 The Controller Output (CO) – is the signal from the
controller to the final control element.
 The Process Variable (PV) – is a measure of the
process output that changes in response to changes in the
manipulated variable.
 The Set Point (SP) – is the value we wish to maintain
the process variable at.

Why do we need process control?
 Structure of chemical process plant is very complex.
 Any chemical plant consist of various process units which are inter
connected with one another in systematic manner.
 Main objective of any plant is;
 To convert certain raw materials into desired product using
available sources of energy.
 Other objective:-
 Safety, product specification , environmental regulations,
operation constraints, economics.
 These all parameters are control by arrangement of various
equipment like measuring devices, valves, controller.

 Process control technology is the tool that enables manufacturers
to keep their operations running within specified limits and to set
more precise limits to maximize profitability, ensure quality
and safety.
Importance of process control are;
– To Reduce variability/maintain product quality
– To help processes operate efficiently/resourcefully
– To ensure the safe operation of processes (loss prevention)
– To meet environmental regulations (discharge).
– To meet operational constraints inherent to the operation of
equipment's used in a chemical plant (over flow, dry, flooded).
– Economics (Minimum operating cost and maximum profits).
NEEDS (IMPORTANCE) OF PROCESS CONTROL

Con’t …
 Increase productivity/Production rates.
 Increase stability
 Optimize the performance
 Minimize the influence of External Disturbance.
 Safety
 Reduce energy
So, the primary objective of process control is;
 To maintain a process at the desired operating
conditions, safely and efficiently, while satisfying
environmental and product quality requirements.

 How does a control
system fulfill the
above needs?

 A control system can meet the above mentioned
process operation (operational objectives of a
process control) by any combination of the
following:
i. Reduce (Suppressing) the influence of external
disturbances.
ii. Promote (Ensuring) the stability of a chemical
process, and
iii. Enhance (Optimizing) the performance of a
chemical process.

1. Suppress the influence of external
disturbances
 Suppressing the influence of the external disturbances on a process
is;
 The most common objective of a controller in a chemical
plant.
 Such disturbances denote the effect that the surroundings
(external world) have on a reactor, separator, heat exchanger,
compressor, etc., and;
 To introduce a control mechanism that will make the proper
changes on the process to cancel the negative impact that
disturbances may have on the desired operation of a chemical
plant.

Objectives: Achieve Set-point
T = Ts
h = hs
 Fi - is flow rate (𝑓𝑡3
/min)
 Ti- is the inlet temperature (OF), of entering liquid into the
tank.
 Fs - is the steam mass flowrate in lb/min used to heat the liquid
 F, T = the flow rate outgoing liquid and temperature of the
stream leaving the tank.
 The tank is considered to be well-stirred, (temperature of
liquid in the tank is uniform and is equal to the temperature of
the effluent).
 After reaching steady-state from start-up, Possible disturbances
include:
• Changes in the feed flowrate, Fi
• Changes in feed temperature Ti
• Changes in ambient temperature
 How to achieve the objective?
 Consider the tank heater system shown in
Figure;
Suppressing the effect
of disturbances
Cause changes in F, T.

Con’t…
The control Objectives of the stirred tank heater
are to:
1. To keep the effluent temperature T at a desired
value Ts and
2. To keep the volume of the liquid in the tank at a
desired value Vs.
=> Control action is needed to keep T and V at the
desired values.

1. To maintain the temperature of effluent ‘T’ at
desired temperature ‘Ts’
 The operation of the heater is
disturbed by external factors
such as changes in the feed flow
rate Fi and temperature Ti.
 Or; If nothing changed, then
after attaining T=Ts and V=Vs,
We could leave the system
alone without any
supervision and control.

 A thermocouple measures the
temperature T of the liquid in
the tank.
 Then T is compared with the
desired value Ts, yielding a
deviation, ε = Ts – T
 The value of the deviation ε is
sent to a control mechanism,
which decides what must be
done in order for the
temperature T to return back
to the desired value T.

 If ε > 0, which implies that T < Ts,
the controller opens the steam
valve, so that more heat can be
supplied.
 On the contrary, the controller
closes the steam valve when ε <
0, or T> Ts.
 It is clear that when T = Ts (i.e., ε
= 0), the controller does
nothing.

A possible control configuration
 This control system is called Feedback control because:
• The variable of direct importance (T in this case) is measured
• Corrective action is taken after the effect of the disturbance has been felt
by the system.
 The desired value Ts is called the Set Point
(decided or set based on process
requirement).
 The temperature of the liquid, T, is measured.
 T is compared with the desired value Ts.
 The difference called the error (e = Ts –T)
is sent to a controller .
The controller takes a corrective
action based on the error.

Conclusion;
 Measure T
 Compare measured T with Ts
 Compute error:
e = Ts - T
e > 0; Ts > T (increase Fst)
e < 0; Ts < T (reduce Fst)
Fig. Feedback ,control in a stirred tank heater

Alternative configuration for the
temperature control
 In Feedforward control:
 Feed forward control does not
wait until the effects of the
disturbances has been felt by the
system, but acts appropriately
before the external disturbance
affects the system anticipating
what its effect will be.
 We realize that we can use a
different control arrangement to
maintain T= Ts, when 𝑻𝒊 ,
changes.

Example-2:
Objective:-
 To keep the volume at its set
point or;
 The liquid level hs we
measure the level of the liquid
in the tank and we open or
close the effluent flow rate.
Therefore;
- h is controlled output
- F is manipulated variable
- Fi and Ti are disturbance
inputs.

II. To maintain the height of liquid ‘h’ in the
tank at desired level ‘hs’
 In Figure we see a control action to keep
h = hs when Ti or Fi, changes. So that
tank will not overflow or go dry.
 A level measuring device measures
the height h of the liquid in the
tank.
 Then, h is compared with the
desired value 𝒉𝒔 , yielding a
deviation;
 ε = h - ℎ𝑠.

II. To maintain the height of liquid ‘h’ in the tank
at desired level ‘hs’
 The value of the deviation ε is sent
to a control mechanism which
decides what must be done in
order for the height h to return
back to the desired value hs.
 It may open or close the valve
that affects the effluent flow rate
F.

II. To maintain the height of liquid ‘h’ in the tank
at desired level ‘hs’
 If ε > 0, which implies that h < hs, the
controller opens the steam valve so
that more heat can be supplied.
 On the contrary, the controller
closes the steam valve when ε < 0
or T>Ts.
 It is clear that when T = Ts (i.e., ε =
0), the controller does nothing.

2. Ensure the Stability of a Process
Consider the behavior of the variable x
shown:
• At time t = to the constant value of x is
disturbed by some external factors,
• As the time progresses the value of x
returns to its initial value to stays there.
If x is a process variable like
temperature, pressure, concentration,
flow-rate, etc.,
• This means;
– the process is stable or self-regulating
– needs no external intervention for its
stabilization
– no control mechanism is needed to
force x to return to its initial value.
Fig1. Response of a stable system
– x returns to steady-state
without an intervention in a
self-regulating process.

2. Ensure the Stability of a Process
• In contrast to the above behavior; the variable y
shown, does not return to, its initial value after it is
disturbed by external influences.
• Processes whose variables follow the pattern
indicated by y , (curves a, b, c) are called unstable
processes and require external control for the
stabilization of their behavior.
• Un Stability process. A process is said to be
unstable if its output becomes larger and larger
(either positively or negatively) as time increases.
Examples:
– The explosion of a hydrocarbon fuel with air is
such an unstable system.
– Riding a bicycle is an attempt to stabilize an
unstable system and we attain that by pedaling,
steering and leaning our body right or left.
Fig 2. Response of a
unstable system
– y never returns to
steady-state in three
different unstable
processes (A, B, C)

Example:
Controlling the Operation of an Unstable Reactor
 Consider a continuous stirred
tank reactor (CSTR) where an
irreversible exothermic reaction
A → B takes place.
 The reaction mixture is cooled
by a coolant medium that flows
through a jacket around the
reactor.

 The heat removed by the
coolant is a linear function of
the temperature T (curve B).
 At steady state, the heat
produced by the reaction
should be equal to the heat
removed by the coolant, thus
yielding the steady states Pl,
P2, P3 at the intersection of
the curves A and B.
 The steady states p1 and P3
are called stable while the P2 is
unstable.
 To understand the concept of
stability let us consider the
steady state P2.

 Assume that we are able to
start the reactor at the
temperature T2, and the
concentration CA that
corresponds to this
temperature.
 Consider that the
temperature of the feed Ti
increases.
 This will cause an increase in
the temperature of the
reacting mixture, say T2’

 At T2’, the heat released by
the reaction (Q2’) is more
than the heat removed by
the coolant, Q2”.
 Thus leading to higher
temperatures in the reactor
and consequently to
increased rates of reaction.
 Increased rates of reaction
produce larger amounts of
heat released by the
exothermic reaction which in
turn lead to higher
temperatures and so on.

 Therefore, we see that an increase in Ti takes the reactor
temperature away from the steady state P2 and that the
temperature will eventually reach the value of the steady state
P3.
Figure 1. Dynamic response of a CSTR: (a) and (b) indicate the instability of the
middle steady state.
(a) (b)

 Sometimes we would like to operate the CSTR at the
middle unstable steady state for the following reasons:
– The low temperature steady state 𝐏𝟏 causes very low yields
because the temperature 𝐓𝟏 is very low.
– The high temperature steady state 𝐏𝟑 may be very high
causing unsafe conditions, destroying the catalyst for a
catalytic reactor, or degrading the product B, etc.
 In such cases we need a controller which will ensure the
stability of the operation at the middle steady state.

 Question: The reader should suggest a control
mechanism to stabilize the operation of the reactor
at the unstable steady state P2.
This Question demonstrates very vividly the deed for
stabilizing the operation of a system using some type
of control in the presence of external disturbances
that tend to take the system away from the desired
point.

3. Optimize the Performance of a Chemical
Process
 Optimization: is a major requirement to achieve maximum profit.
 Safety and the satisfaction of the production specifications are the two main
operational objectives for a chemical plant.
 Once these are achieved, the next goal is how to make the operation of the
plant more profitable.
 Conditions that affect the operation of the plant do not remain the same, it is
clear that we would like to be able to change the operation of the plant (flow
rates, pressures, concentrations, temperatures) in such a way that an economic
objective (profit) is always maximized.
 This task is undertaken by the automatic controllers of the plant and its
human Operators.

Applications
 Process industries
• Petroleum
• Chemical
• Steel
• Power
• Food
 Goods manufacturing
• Automobile parts
• Refrigerators
• Electronic equipment's like, T.V and Radio

Con’t…
 Transport system
• Railways
• Airplanes
• Free missiles
• Ships
 Power machines
• Machine Tools
• Compressors and Pumps
• Prime movers
• Electrical power – Supply Units

Classification of variables in chemical
process
 A process variable is a condition of the process fluid that
can change the manufacturing process in some way.
Common process variables include:
 Pressure, Flow, Level, Temperature, Density, pH (acidity or
alkalinity), Liquid interface (the relative amounts of different
liquids that are combined in a vessel), concentration, Mass and
Conductivity.

Con’t…
The variables (flow rates, temperatures, pressures,
concentrations, etc.) associated with a chemical
process are classified into two:
1. Input Variable (manipulated and disturbances): -
– This variable shows the effect of the surroundings on
the chemical process.
– Normally refers to those factors that influence the
process.
2. Output variables (measured and unmeasured)
– Which denote the effect of process on the surroundings.
 Are Called controlled variables.

 For the CSTR reactor:
 We have:
– input variables: CAi , Ti, Fi, Tci , Fc, F
– output variables: CA, T, F, Tco , V

Con’t …
 F can be considered either as input or
output.
 If there is a control valve on the
effluent stream the variable F is an
input.
 Otherwise, F is an output variable.

The input variables can be further classified into
the following categories:
Manipulated (or adjustable) variables
 If their values can be adjusted freely by the human operator or a
control mechanism.
 Typically flow rates of streams entering process that we can
change in order to control the plant.
Disturbances (load) variables
 If their values are not the result of adjustment by an human operator
or a control system.
 It is an undesired change in one of the factors that can affect the
process variable.
 These are also called "load“ variables and represent input variables
that can cause the controlled variables to deviate from their respective
set points.

The output variables are also classified into:
Measured/ Controlled output variables:
 if their values are known by directly measuring them.
Example; Flow rates, compositions, temperatures, levels, and
pressures in the process that we will try to control, either
trying to hold them as constant as possible or trying to make
them follow some desired value .
Unmeasured output variables:
 Variables in the process that are not controlled/ measured
directly.

Example -2:
 Suppose that the inlet stream in the CSTR system
comes from an upstream unit over which we have
no control.
 Then, CAi , Fi, Ti are disturbances.
 If the coolant flow-rate is controlled by a control
valve, then
– Fc is a manipulated variable, while
– Tci is a disturbance.
 If the flowrate of the effluent stream is controlled
by a valve, then F is a manipulated variable,
otherwise it is an output variable.
 With respect to the output variables we have the
following: T, F, Tco, and V are measured outputs
 The concentration CA can be measured variable if
an analyzer (gas chromatograph, infrared
spectrometer, etc.) is attached to the effluent
stream.

Example:
For the tank heater system:
 Disturbance inputs: Fi and Ti
– Manipulated inputs can be: Fst, F
– Measured outputs can be: V, T

Con’t…
 Disturbances, based on their direct
measurability or not, can be further classified
into two categories:
– Measured e.g. the disturbances Fi and Ti of the
stirred tank heater and
– unmeasured disturbances e.g. the feed composition
for a distillation column, extraction unit, reactors
and the like.

Con’t
 Error: In process controls, error is defined as:
Error = set point - process variable.
Set point: The set point is where you would like a
controlled process variable to be.
 Set point variable - is the one that is set by operator,
master controller or computer as a desired value for a
controlled variable.
 It is also called reference value.

Con’t …
Set point; is a value for a process variable that is desired to be
maintained.
- For example, if a process temperature needs to kept
within 5 °C of 100 °C, then the set point is 100 °C.
 A temperature sensor can be used to maintain the
temperature at set point.
 If the temperature reading is 110 °C, then the controller
determines that the process is above set point and signals the
fuel valve of the burner to close slightly until the process cools
to 100 °C.

Example :-
 Input variable – 𝑇𝑖, 𝐹𝑖, 𝐹𝑠𝑡, 𝑇𝑠𝑡
 Output variable – F, T
 Set point – 𝑇𝑠
 These input variables are adjusted
dynamically to keep the controlled
variables at their set-points.
 Manipulated – 𝐹𝑠𝑡
 Disturbance – 𝑇𝑖, 𝐹𝑖, 𝐹𝑠𝑡, 𝑇𝑠𝑡

Examples of controlled processes
1. Controlling the temperature of a water stream by
controlling the amount of steam added to the shell of a
heat exchanger.

Con’t…
2. Operating a jacketed reactor isothermally by
controlling the coolant that flows through the jacket of
a jacketed reactor.

Con’t…
3. Controlling the height of fluid in a tank to ensure
that it does not overflow.

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