Lecture 01 IPC.ppt IPC PROCESS CONTROL BASICS

Instrumentation and Process Control
BS (Eng.) Chemical Engineering 6th Semester
Engr. Muhammad Suleman
Lecturer
1

Objective of IPC
• To impart knowledge of instruments and control in
order to design control systems for chemical
process industry.

3
• George Stephanopoulos. Chemical process control. Englewood Cliffs,
New Jersey: Prentice-Hall, 1984
• Donald R. Coughanowr and Steven E. LeBlanc. Process Systems
Analysis and Control. McGraw-Hill Science/Engineering/Math, 2008
• William L Luyben. Process modeling, simulation and control for
chemical engineers. 2nd
Edition, McGraw-Hill Higher Education, 1996
• Don Green and Robert Perry. Perry's Chemical Engineers' Handbook,
Eighth Edition McGraw-Hill, New York, 2007
• Dale E. Seborg, Thomas F. Edgar, and Duncan A. Mellichamp. Process
dynamics & control. Wiley. com, 2006.
• Lecture Notes/ Handouts
Text/ Reference Books

Introduction of Process Control
• Process control refers to the methods and tools used
to maintain desired outputs of a chemical process by
manipulating its inputs.
• Chemical industry comprised of various units
connecting with each other.
• For Example, Reactor, Distillation Column,
Pumps, Compressor, dryers, Mixers, etc.

Objective of a Chemical Plant
Raw Materials
Final Product
Chemical
Plant
(comprised of
unit operations
and unit
processes)
Economically
and safely

Process Control in our daily life
Bathroom water heater
Refrigerator
Traffic Light
Cruise Control
6

Place of Process Control in a typical Chemical Plant
7
Luyben (1996)

Need of a Control
For large and complex processes process automation becomes
more important from the viewpoint of:
Safety:
Equipment and Personnel
Production Specifications:
Quality and Quantity
Environmental Regulations:
Effluents
Operational Constraints:
Distillation columns (flooding, weeping); Tanks (overflow,
drying), Catalytic reactor (maximum temperature, pressure)
Economics:
Minimum operating cost, maximum profits 8

Process Dynamics
Refers to unsteady-state or transient behavior. Variables change with time
a) Continuous processes: Examples of transient behavior:
i. Start up & shutdown
ii. Major disturbance: e.g., refinery during stormy or hurricane conditions
iii.Equipment or instrument failure (e.g., pump failure)
b) Batch processes
i. Inherently unsteady-state operation
ii. Example: Batch reactor
iii.Composition changes with time
Other variables such as temperature could be constant.

Requirements of Industry
1. Safety must be ensured
2. Production specification must be met
such as Quantity and Quality of the
product
3. Environmental Requirements, such as
Concentration of chemicals, Sox,
Nox, Waste-water management.
4. Cost effectiveness

Operational Constraints
• There are certain operational constraints upon
them, we engineers have to make sure full
control, For Example,
1.Distillation Columns – Should not be flooded
2.Tanks – Should not be go dry or over flow
3.Reactor – Concentration should be
maintained
4.Heat Exchangers – Temperature should be
controlled

Issues to Encounter
1.Suppress the Influence of External Disturbances
2.Maintain the Stability of Chemical Process
3.Optimize the performance of Chemical Process

Shower System
Example : Block diagram of shower control system
Decision
delay
Brain to
muscle
delay
Valve
Flow
delay
Nerve to
brain
delay
Temperature
at finge
r
Desired
temperatur
e
Three Main Tasks in Process Control:
Measurement
Comparison
Adjustment

Input/Output Variables
Control Variables (CV):
•Variables which quantify the performance or
quality of the final product
•Also called output variables
Manipulated Variables (MV):
•Input variables adjusted dynamically to keep the
controlled variables at their set points
Disturbance Variables (DV):
•Input variables that can cause the controlled
variables to deviate from their respective set
points
•Also called load variables

Input/Output Variables
Control Variable
Gas
Flow
Temperatur
e
Manipulated Variable
Disturbance Variable
Water Flow
Tap Opening
Example 1.2 Bathroom Water Heater (Gieser)

Need for process control
Following set-point change:
•Implementing change in the operating conditions
•When the set point signal changes, the manipulated variable is adjusted
appropriately to achieve new operating conditions
•Also called servomechanics or servo-control. Suppressing disturbance change:
•The process transient behaviour when a disturbance enters
•Also called regulatory control or load change.
•A control system should be able to return each controlled variable to its set-point
despite a disturbance
• Controllers are used to enhance the stability of the process
13

Example Blending system
Notation:
•w1, w2 and w are mass flow rates
•x1, x2 and x are mass fractions of component A

Assumptions:
1. w1 is constant
2. x2 = constant = 1 (i.e., stream 2 is pure A)
3. Perfect mixing in the tank
Control Objective:
Keep x at a desired value (or “set point”) xsp, despite variations in x1(t).
Flow rate w2 can be adjusted for this purpose.
Terminology:
• Controlled variable (or “output variable”): x
• Manipulated variable (or “input variable”): w2
• Disturbance variable (or “load variable”): x1 15
15

Requirements
1. Suppressing External Disturbances
Objectives: Achieve Set-point
T = Ts
h = hs
After reaching steady-state
from start-up, disturbances in Fi
and Ti cause changes in F, T.
How to achieve the objective?
Stirred Tank Heater (Stephanopoulos, 1984)

Feed-back Control:
Distinguishing feature: measure the controlled variable (CV)
•Advantages:
 Corrective action is taken regardless of the source of the disturbance.
 Reduces sensitivity of the controlled variable to disturbances and
changes in the process.
•Disadvantages:
 No corrective action occurs until after the disturbance has upset the
process, that is, until after x differs from xsp.
 Very oscillatory responses, or even instability.

Controlling T in a Stirred Tank Heater
(feedbackward control)
 measure T
 compare measured T
with Ts
 Compute error:
e = Ts - T
e > 0; Ts > T (increase Fst)
e < 0; Ts < T (reduce Fst)
Feedback Control in a Stirred Tank Heater
(Stephanopoulos, 1984)

Example Temperature Feedback Control System

Lecture 01 IPC.ppt IPC PROCESS CONTROL BASICS

Specific objectives of control
1. Increase product throughput
2. Increase yield of higher valued products
3. Decrease energy consumption
4. Decrease pollution (e.g., amount of SO2 emission, or quality of
water returned to a river or a lake)
5. Decrease off-spec (i.e. off-specification) products
6. Increase safety (e.g., avoid development of explosive mixtures, or
maintain pressure below explosion)
7. Extend the life of equipment
8. Maintain operational constraints: (pump maintaining a certain net
positive suction head (NPSH), tanks do not overflow, distillation
columns do not flood)

Steps for Development of a Control Scheme
Steps:
-Control Objective
-Input and Output variables
(identify CV, MV, DV)
-Constraints (hard/soft)
-Operating Characteristics
(batch/continuous)
-Safety, environmental and
economic aspects
-Control structure (FB/FF)

Revisiting Example Shower System
Control Objective: to become clean, to
become refreshed, to be comfortable
(correct temp. and water velocity as it
contacts the body)
Input Variables: Manipulated input variables are
HW and CW valve positions, body position.
Disturbance inputs include a drop in water pressure
and changes to HW temp.
Output Variables: Measured output variables are
temp and flow rate of mixed stream as it contacts
your body.
Constraints: Min. and Max. valve positions on
both streams. Min. and Max. flow rates (hard
constraints). Mixed water temp (soft constraint).
Control Structure: Adjusting either valve affects
both temp and flow rate
Feedback / Feed-forward control

Feed-forward Control:
Distinguishing feature: measure a disturbance variable (DV)
•Advantage:
Correct for disturbance before it upsets the process.
•Disadvantage:
Must be able to measure the disturbance.
No corrective action for unmeasured disturbances.
25
25

Feedforward Temperature Control for Stirred Tank
Heater

2. Ensure the Stability of a Process
x (or y) can be T, CA, F; x is disturbed at t0
30
x returns to steady-state
without an intervention in a
self-regulating process
y never returns to steady-
state in three different
unstable processes (A, B, C)

Consider the behavior of the variable x shown in Figure. Notice that at time to the constant
value of x is disturbed by some external factors, but that as time progresses the value of x
returns to its initial value and stays there. If x is a process variable such as temperature,
pressure, concentration, or flow rate, we say that the process is stable or self-regulating and
needs no external intervention for its stabilization. It is clear that no control mechanism is
needed to force x to return to its initial value.
In contrast to the behavior described above, the variable y shown in Figure 1.6 does not
return to its initial value after it is disturbed by external influences. Processes whose
variables follow the pattern indi-cated by y in Figure 1.6 (curves A, B, C) are called
unstable processes and require external control for the stabilization of their behavior. 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.
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Description of stability of the Process

3. Optimization of the Performance of a Batch
Reactor
Optimization is a major requirement to achieve maximum profit.
A (feed) → B (desired) → C (undesired); endothermic reaction
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Scenarios:
Q(t) is given the largest value during
entire TR to favor A → B
Q(t) is given the smallest value during
entire TR to suppress B→ C
Optimization of Q(t) during TR
Steam
Condensate
Economic Objective
Maximize profit =
ʃ0
tR
f (A, B, steam) dt

Visualizing ‘Optimization’ in Chemical Plants
Case: Liquid can be pumped between two points by choosing different
pipe diameters (with right pumping system). The total cost of
transportation includes the pumping (and power) cost and piping cost.
33
Cost
/
year/
length
Pipe Diameter
Scenario One:
Pipe with smaller diameters are
cheaper but pumping cost
increases.
Scenario Two:
Pumping cost is small in a pipe
with large diameter but pipes
are expensive.
What is the ‘best’ pipe-
pump combination?
Peters and Timmerhaus (1991)

Optimization of Chemical Process
• Optimization of a chemical process is the process of
making changes to a chemical process to improve its
efficiency and reduce costs.
• This can include changes to the process itself, such as
reducing raw material costs or improving the efficiency of
the process, as well as changes to the machinery or
equipment used in the process.
• Optimizations may also include changes to the layout or
design of the process, as well as changes to the operating
conditions such as temperature, pressure, or composition of
reactants.

Controller
Final Control
Element
Measuring
Element
Process
Desired value
inputs outputs

input output
Input refers to a variable that causes an output.
•Driving example;
input: steering wheel position output: position
of the automobile
•Heated room example; input: fuel to the
furnace
output: room temperature

Input and output variables
of Chemical Plant
• Input Variables: • Temperature • Pressure •
Flow • Level • pH • Conductivity •
Composition
• Output Variables: • Valve Position • Pump
Speed • Heater Settings • Alarm Settings •
Cooling Control • Reactor Feed Rates •
Reactant Addition Rates
• Output variables can be either T,P,F,Level,
pH, Conductivity, Composition as well.

Types of Input Variables
1. Manipulated variables are variables that can
be controlled or adjusted in order to achieve a
certain desired outcome.
• They allow for precise adjustments to the process
that can result in greater efficiency and better
product quality.
• Examples of manipulated variables include
temperature, pressure, flow rate, and material
composition.

Types of Input variables
2.Disturbances are variables that are not under the direct
control of the process and may occur due to external factors.
• These variables can have a significant impact on the
process and can lead to inefficiencies or poor product
quality if not appropriately addressed.
• Examples of disturbances in chemical process control
include changes in feedstock composition, changes in raw
material properties, and changes in ambient temperature.

1. Measured output variables refer to variables
that can be measured by sensors or
instrumentation. Examples include temperature,
pressure, level, and flow.
2. Unmeasured output variables are variables
that cannot be measured directly by
instrumentation. Examples include product
quality, process efficiency, and energy
consumption.
Types of Output variables

1.What does a control system do?
Control: To maintain desired conditions in a
physical system by adjusting selected variables in
the system.
In control Systems:
• A specific value or range is used as a desired
value for the controlled variable
• The conditions of the system are measured
• Each system has a control calculation or algorithm
• The results of calculation are implemented by
final control element

1. Define Control Objective: what are the operational
objectives of a control system
2. Select Measurements: what variables must be
measured to monitor the performance of a chemical
plant
3. Select Manipulated Variables: what are the
manipulated variables to be used to control a
chemical process
4. Select the Control Configuration: After addressing
above 3 design elements, then decide which control
configuration is to select
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Design Elements in a Control

Control Objective- Quantitative or Qualitative?

Control Objective- Quantitative or
Qualitative?

2. Select Measurement
-- Primary and Secondary Measurement

3. Select Manipulated Variable

Variables, Load disturbance variables
For a binary distillation column, load disturbance
variables might include feed flow rate and feed
composition. Reflux, steam, cooling
water, distillate, and bottoms flow rates might be the
manipulated variables. Controlled variables might be
distillate product composition, bottoms product
composition, column pressure, base liquid level, and
reflux drum liquid level. The uncontrolled variables
would include the compositions and temperatures on
all the trays.
Note that one physical stream may be considered to
contain many variables:

Open Loop
Independent of the output or process variable. The
system does not use feedback to correct errors or
adjust the process. Instead, it relies on predefined
inputs or set points to achieve the desired output.
T, P, F, and concentration need to be controlled.
Open-loop systems do not measure these variables
during operation.
Control Action:
The controller sends a signal to the actuator (e.g.,
valve, heater, pump) based on a predefined input.
The system does not check whether the desired
output is achieved.
No Feedback: Do not use sensors to measure the
output and compare it to the set point.
Disturbances: Cannot compensate for disturbances
(e.g., changes in feed composition, ambient

Open Loop
Control Action:
Controller sends a signal to the actuator (e.g.,
valve, heater, pump) based on a predefined
input.
System does not check whether the desired
output is achieved.
No Feedback: Do not use sensors to measure
the output and compare it to the set point.
Disturbances: Cannot compensate for
disturbances (e.g., changes in feed
composition, ambient temperature, or
pressure).

Open Loop
Batch Reactor Heating:
Heater is turned on for a fixed duration to heat the
reactor contents to a desired temperature. System
does not measure the actual temperature during the
process.
Chemical Dosing:
Pump delivers a fixed volume of chemical into a
process stream based on a timer or predefined flow
rate. System does not adjust for variations in the
process stream.
Mixing Process:
Mixer operates at a fixed speed for a set time to
blend materials. System does not measure the
homogeneity of the mixture.
Drying Process:
Dryer operates at a fixed temperature and time

Closed Loop
•Also known as feedback control systems. Widely used
in chemical engineering processes to maintain precise
control over variables such as T,P,F, and concentration.
•Continuously monitor and adjust the process, ensuring
the desired output is achieved despite disturbances or
changes in operating conditions.
•Output (process variable) is measured and compared
to the desired set point. Difference between the set point
and the measured output (error) is used to adjust the
control action, ensuring the process variable converges to
the desired value.

Closed Loop
Feedback: Continuously measures the output using sensors (e.g.,
temperature sensors, pressure transmitters, flow meters). Measured
output is compared to the setpoint, and the error is calculated.
Controller:
(e.g., PID controller) processes the error and determines the
appropriate control action. Common control actions include
adjusting a valve position, heater power, or pump speed.
Actuator:
Actuator (e.g., control valve, heater, motor) implements the control
action to adjust the process.
Disturbance Rejection:
Can compensate for disturbances (e.g., changes in feed composition,
ambient temperature, or pressure).

Closed Loop
Process: The system being controlled (e.g., a reactor, distillation
column, or heat exchanger).
Sensor:
Measures the process variable (e.g., temperature, pressure, flow
rate).
Controller:
Computes the control action based on the error (e.g., PID controller).
Actuator:
Adjusts the process based on the controller's output (e.g., control
valve, heater).
Setpoint:
The desired value of the process variable.

Closed Loop
Temperature Control in a Reactor:
Temperature sensor measures the reactor temperature. Controller
compares the measured temperature to the set point and adjusts the
heater power to maintain the desired temperature.
Level Control in a Tank:
Level sensor measures the liquid level in the tank. Controller adjusts
the inlet or outlet flow rate to maintain the desired level.
Pressure Control in a Distillation Column:
Pressure transmitter measures the column pressure. Controller
adjusts the vent valve or reflux rate to maintain the desired pressure.
Flow Control in a Pipeline:
Flow meter measures the flow rate. Controller adjusts the control
valve to maintain the desired flow rate.

Comparison with Closed-Loop Control
Feature
Open-Loop Control
System
Closed-Loop Control
System
Feedback No feedback Uses feedback
Accuracy Less accurate More accurate
Complexity Simple More complex
Cost Lower Higher
Adaptability
Cannot adapt to
disturbances
Can adapt to disturbances
Stability Stable May become unstable

Select Control Configurations
Control Configurations include but
not limited to
1)Feedback control
2)Inferential control
3)Feedforward control
4)Cascade control
5)Splitrange control
6)Ratio control
7)others

What is the best control configuration for a
given chemical process control situation?

General Structure of Feedbackward
control configuration

General Structure of Feedforward

General Structure of Inferential

39
Designing Control Configurations in a Distillation
Column (Feedbackward control)
Define Control Objective:
95 % top product
Select Measurements:
composition of Distillate Select
Manipulated variables: Reflux
ratio
Select the Control
Configuration:
feedback control

40
Designing Configuration in a Distillation Column
(Feedforward Control)
95 % top product Select
Measurements: composition of
Feed
Select Manipulated variables:
Reflux ratio
Select the Control
Configuration:
feedback control

72
Classification of Variables
Input variables (sometime called as load variables or LV)
Further classified as disturbances and manipulated (or
Adjustable Variables or control variables)
Input variables
It denotes the effect of surroundings on the chemical process.
Manipulated Variable (Adjustable Variables)
These variables whose values can be adjusted freely by human
operator or controlled mechanism.

73
Disturbance Variable.
Are those variables whose values are not the result of
adjustment of human operator or control mechanism.

74
Output variables
It denotes the effect of the process on surroundings.
Classified as
Measured output Variable
If their values are known by directly measuring them.
Measured output Variable
If they are not be measured directly.

75
Further classified into measured and unmeasured variables
Often, manipulated variable effects output variable (measured)
known as controlled variable
When an output variable is chosen as a manipulated variable, it
becomes an input variable.
A manipulated variable is always an input variable.

76
Objective: h = hs (Controlled Variable or CV)
Scenario C.
Variabl
e
M.
Variabl
e
Input
Variabl
e
Output
Variabl
e
1 (shown) h F Fi h
2 h Fi F, h
Define Control Objective: what are the operational objectives of a control system
Select Measurements: what variables must be measured to monitor the performance of
a chemical plant
Select Manipulated Variables: what are the manipulated variables to be used to control
a chemical process
Select the Control Configuration: information structure for measured and controlled
variables. Configurations include feedback control, infrential control, feedforward
control
F
h
A

77
Input variables
Fi, Fst, Ti, (F)
Output variables
F, T, h
Control Objective
(a) T = Ts
(b) h = hs
F,
T
Fst
h
A
F,
T
h
A
Fst
Temperature and level control in a stirred
tank heater (Stephanopoulos, 1984)

78
Control Configurations in a Distillation
Column
95 % top product
Select Measurements:
composition of Distillate
Select Manipulated variables:
Reflux ratio
Select the Control
Configuration: feedback control

Feed Forward Control System
• The basic idea is shown in Fig. 1.8. The disturbance is detected as it enters the process
and an appropriate change is made in the manipulated variable such that the controlled
variable is held constant. Thus we begin to take corrective action as soon as a
disturbance entering the system is detected instead of waiting (as we do with feedback
control) for the disturbance to propagate all the way through the process before a
correction is made.
79

Feed Backward Control System
• The traditional way to control a process is to measure the variable that is to
be controlled, compare its value with the desired value (the setpoint to the
controller) and feed the difference (the error) into a feedback controller that
will change a manipulated variable to drive the controlled variable back to
the desired value. Information is thus “fed back” from the controlled
variable to a manipulated variable, as sketched in Fig. 1.7.
80

81
Feedforward Control Configuration in a
Distillation Column
Control xD

82
Inferential Control in a Distillation Column
Control Objective: xD
Unmeasured input =
f (secondary measurements)

• The process (chemical or physical)
• Measuring instruments and sensors (inputs, outputs)
what are the sensors for measuring T, P, F, h, x, etc?
• Transducers (converts measurements to current/ voltage)
• Transmission lines/ amplifier
• The controller (intelligence)
• The final control element
• Recording/ display
elements
Recall Process
Instrumentation
83
Hardware for a Process Control System

Lecture 01 IPC.ppt IPC PROCESS CONTROL BASICS

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