introduction to chemical process dynamics, instrumentation & control
TRANSCRIPT
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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 one
unit 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 w2
so 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 1
and 2.
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Control Method 4
Use a larger tank.
• If a larger tank is used, fluctuations in x1 will
tend to be damped out due to the larger
capacitance of the tank contents.
• However, a larger tank means an increased
capital cost.
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Classification of Control Strategies
Table. 1.1 Control Strategies for the Blending
System
1 x w2FB
2 x1 w2FF
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