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Introduction to
Chemical Process Dynamics,
Instrumentation & Control
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Chapter Objectives
End of this chapter, you should be able to:
1. Understand the role of process dynamics and
control in industry
2. Understand general concepts
3. Classify variables
4. Understand the purpose of process control
5. Understand control aspects of complete
chemical plant
6. Understand hardware for process control system
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Role of process dynamics and control
in industry
Illustration with examples
Example 1 a simple process where dynamic response is
important
Example 2 use of a single feedback controller
Example 3 simple but typical chemical engineering plant
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Example 1 A gravity-flow tank
Under steady state conditions,
the flow rate out of the tank
must equal the flow rate into
the tank.
What would happen dynamically
if we changed Fo?
How will h(t) and F(t) vary
will time?
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Example 2 Heat Exchanger
We want to control the temperature of oil leaving the
heat exchanger.
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How to control?
A thermocouple is inserted in a thermowell in
the exit oil pipe.
Thermocouple wires are connected to a
temperature transmitter that converts the millivolt output into
a 4- to 20 mA signal.
This signal sent to a temperature controller.
The temperature controller opens the steam
valve if more steam is needed or closes it a
little if the temperature is too high.
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Components of control loop
A sensor
A transmitter
A controller
A final control element
Process control deal with:
What type of controller to be used?
How it should be tuned?
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Example 3 - A typical chemical plant
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Concepts of Process Control
Another simple
example:
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Block diagram
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Need for control
Performance requirements for process plants have become
increasingly difficult to satisfy.
Key factors for tightening product quality specifications:
Stronger competition
Rapidly changing economic conditions
Tough environmental and safety regulations
Modern plants are complex and highly integrated
It is difficult to prevent disturbances from propagating from
oneunit to other interconnected units.
Process control has become increasingly important due to
increased importance on safe and efficient plant operation.
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The term process dynamics refer to unsteady
state (or transient) behavior.
Dynamic studies provide us the behavior of
the process under unsteady-state conditions
Gain knowledge about the process
behavior.
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Objectives of Process Control
Maintain a process at the desired operating conditions, safely
and efficiently
Satisfy product quality and environmental requirements
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Process control applications
Large-scale integrated processing plants such as oil refineries
or ethylene plants require thousands of process variables such as
temperature, pressure, flow, level and compositions are measured
and controlled.
Large number of process variables, mainly flow rates, can be
manipulated.
Feedback control systems compare measurements with their desired
values and then adjust the manipulated variables accordingly.
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Representative process control problems
Foundation of process control is process understanding.
What is a process?
The conversion of feed materials to useful products using
chemical and physical operations PROCESS.
Common processes can be continuous, batch or semi-batch.
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Continuous Processes
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Tubular Heat Exchanger
Control problem: The exit temperature of the
process fluid is controlled by manipulating the
cooling water flow rate.
Disturbances: Variations in the inlet temperatures
and process fluid flow rate.
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Continuous stirred tank reactor
(CSTR)
Control problem: If the reaction is highly exothermic, it is
necessary to control the reactor temperature by
manipulating the flow rate of the coolant in a jacket or
cooling coil.
Disturbances: The feed conditions (composition, flow
rate, and temperature).
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Thermal cracking furnace
Control Problem: The furnace
temperature and amount of excess
air in the flue gas to be controlled by
manipulating the fuel flow rate and
the fuel/air ratio.
Disturbances: The crude oil
composition and the heating quality
of the fuel.
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Multi-component distillation column
Control Problem: Distillate
composition can be controlled
by adjusting the reflux flow rate
or the distillate flow rate.
Disturbances: The feed
conditions
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Process variables
Three important types: (Control Terminology)
1. Controlled variables - these are the variables which
quantify the performance or quality of the final
product, which are also called output variables.
2. Manipulated variables - these input variables are
adjusted dynamically to keep the controlled
variables at their set-points.
3. Disturbance variables - these are also called "load"
variables and represent input variables that can
cause the controlled variables to deviate from their
respective set points.
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Process variables
Specification of controlled variables, manipulated variables and
disturbance
variables is a critical step in developing a
control system
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Batch and semi-batch processes
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Control problems
Batch or semi-batch reactor: The reactor temperature is
controlled by manipulating a coolant flow rate.
Batch digester: The end point of the chemical reaction is
indicated by Kappa number, a measure of lignin content. It is
controlled to a desired value by adjusting the digester
temperature, pressure, and/or cycle time.
Plasma etcher: The unwanted material on a layer of a
microelectronics circuit is selectively removed by chemical
reactions. The temperature, pressure and flow rates of etching
gases to the reactor are controlled by adjusting electrical heaters
and control valves.
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Control problems
Kidney dialysis unit: The blood flow rate is maintained by a
pump, and ambient conditions, such as temperature of the unit, are
controlled by adjusting a flow rate.
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Control Terminology(2)
set-point change - implementing a change in the operating
conditions. The set-point signal is changed
and the manipulated variable is adjusted appropriately
to achieve the new operating conditions.
Also called servomechanism (or "servo") control.
disturbance change - the process transient behavior when a
disturbance enters, also called regulatory
control or load change.
A control system should be able to return each
controlled variable back to its set-point.
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Illustrative Example:
Blending system
Notation:
w1, w2 and w are mass
flow rates
x1, x2 and x are mass
fractions of component A
Assumptions:
w1 is constant
x2 = const. = 1 (stream 2 is pure A
Perfect mixing in the tank
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Blending system
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
Design Question
What value of is required to have ?spxx 2w
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Overall balance:
Component A balance:
(The overbars denote nominal steady-state design values)
At the design conditions, .
Substitute in Eq.1-2, and , then solve Eq. 1-2
for :
Equation 1-3 is the design equation for the blending
system.
spxx
spxx
1 20 (1-1)w w w
1 1 2 2 0 (1-2)w x w x wx
12 x
2w1
2 1 (1-3)1
SP
SP
x xw w
x
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If our assumptions are correct, then this value of will
keep at .
But what if conditions change?
Control Question. Suppose that the inlet concentration x1
changes with time. How can we ensure that x remains at
or near the set point ?
As a specific example, if and , then x > xSP.
2w
x spx
spx
11 xx 22 ww
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Some Possible Control Strategies
Method 1. Measure x and adjust w2.
Intuitively, if x is too high, we should reduce w2;
Manual control vs. automatic control
Proportional feedback control law
Kc is called the controller gain
w2(t) and x(t) denote variables that change with time t
The change in the flow rate, is proportional to
the deviation from the set point, xSP x(t).
2 2 (1-4)c SPw t w K x x t
2 2 ,w t w
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Control Method 1
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Method 2
Measure x1 and adjust w2
Thus, if x1 is greater than , we would decrease w2so that .
One approach: Consider Eq. (1-3) and replace and
with x1(t) and w2(t) to get a control law:
Because Eq. (1-3) applies only at steady state, it is not
clear how effective the control law in (1-5) will be for
transient conditions.
1x
22 ww
1x
2w
1
2 1 (1-5)1
SP
SP
x x tw t w
x
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Control Method 2
Method 3. Measure x1 and x, adjust w2.
This approach is a combination of Methods 1and 2.
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Control Method 4
Use a larger tank.
If a larger tank is used, fluctuations in x1 willtend to be
damped out due to the larger
capacitance of the tank contents.
However, a larger tank means an increasedcapital cost.
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Classification of Control Strategies
Table. 1.1 Control Strategies for the Blending
System
1 x w2 FB
2 x1 w2 FF
3 x1 and x w2 FF/FB
MethodMeasured
Variable
Manipulated
Variable Category
4 - - Design
change
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Feedback Control
Distinguishing feature: measure the controlled variable.
It is important to make a distinction between negative feedback
and positive feedback.
Engineering Usage vs. Social Sciences
Advantages:
Corrective action is taken regardless of the source of the
disturbances.
Reduces sensitivity of the controlled variable to disturbances
and changes in the process.
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Feedback Control
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
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Feedforward Control
Distinguishing feature:
Measure a disturbance variable
Advantage:
Correct for disturbance before it upsets the process.
Disadvantage:
Must be able to measure the disturbance
No corrective action for unmeasured disturbances
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Justification of Process Control
Specific Objectives of Control
Increased product throughput
Increased yield of higher valued products
Decreased energy consumption
Decreased pollution
Decreased off-spec product
Increased Safety
Extended life of equipment
Improved Operability
Decreased production labor
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Economic Incentives - Advanced
Control
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Hierarchy of process control activities
1. Measurement and Actuation
2. Safety, Envi ronment and Equipment Protection
3a. Regulatory Contro l
4. Rea l-T ime Optimization
5. P lann ing and Schedu ling
Proces s
3b. Multivariab le and Cons traint Contro l
(days-months)
(< 1 second)
(< 1 second)
(seconds-minutes)
(minutes-hours)
(hours-day s)
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Major steps in control system development
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Conclusions
You have been introduced to:
1. the role of process dynamics and control in
industry
2. general concepts of process control
3. classification of variables
4. the purpose of process control
5. control aspects of complete chemical plant
6. hardware for process control system
