chap01 - a career in process control.ppt
TRANSCRIPT
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Chemical and Bio-Process Control
James B. Riggs
M. Nazmul Karim
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Chapter 1
Introduction
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A Career in Process Control
Requires that engineers use all of their
chemical engineering training (i.e., provides
an excellent technical profession that can
last an entire career)
Can become a technical Top Gun
Allows engineers to work on projects thatcan result in significant savings for their
companies (i.e., provides good visibility
within a company)
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A Career in Process Control
Provides professional mobility. There is a
shortage of experienced process control
engineers.
Is a well paid technical profession for
chemical engineers.
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Chemical Process Industries (CPI)
Hydrocarbon fuels
Chemical products
Pulp and paper products
Agrochemicals
Man-made fibers
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Bio-Process Industries
Use micro-organisms to produce useful
products
Pharmaceutical industry
Ethanol from grain industry
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Importance of Process Control for
the CPI PC directly affects the safety and reliability
of a process.
PC determines the quality of the products
produced by a process.
PC can affect how efficient a process is
operated.
Bottom Line: PC has a major impact on the
profitability of a company in the CPI.
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Safety and Reliability
The control system must provide safeoperation
Alarms, safety constraint control, start-up and
shutdown. A control system must be able to absorb a
variety of disturbances and keep the process
in a good operating region:Thunderstorms, feed composition upsets,
temporary loss of utilities (e.g., steam supply),
day to night variation in the ambient conditions
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Benefits of Improved Control
Time
Im
purity
Concentration
Limit
Old Controller
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Benefits of Improved Control
Time
Im
purity
Concentration
Limit
Time
Impurity
Concentration
Limit
Old Controller New Controller
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Better Control Means Products
with Reduced Variability For many cases, reduced variability
products are in high demand and have high
value added (e.g., feedstocks for polymers).
Product certification procedures (e.g., ISO
9000) are used to guarantee product quality
and place a large emphasis on processcontrol.
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Benefits of Improved Control
Time
Im
purity
Concentration
Limit
Time
Impurity
Concentration
Limit
Time
Impurity
Concentration
Limit
Old Controller New Controller
Improved Performance
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Maximizing the Profit of a Plant
Many times involves controlling against
constraints.
The closer that you are able to operate to
these constraints, the more profit you can
make. For example, maximizing the
product production rate usually involvingcontrolling the process against one or more
process constraints.
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Constraint Control Example
Consider a reactor temperature control
example for which at excessively high
temperatures the reactor will experience a
temperature runaway and explode.
But the higher the temperature the greater
the product yield.
Therefore, better reactor temperature
control allows safe operation at a higher
reactor temperature and thus more profit.
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Importance of Process Control for the
Bio-Process Industries
Improved product quality.
Faster and less expensive process validation.
Increased production rates.
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Driving a Car: An Everyday
Example of Process Control Control Objective (Setpoint): Maintain car in
proper lane.
Controlled variable- Location on the road Manipulated variable- Orientation of the front
wheels
Actuator- Drivers arms/steering wheel
Sensor- Drivers eyes
Controller- Driver
Disturbance- Curve in road
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Logic Flow Diagram for a
Feedback Control Loop
Controller Actuator Process
Sensor
CVSetpoint
Disturbance
+-
uce
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Temperature Control for a Heat
Exchanger: ChE Control Example
TT
Condensate
Steam
Feed
TCProduct
Stream
Setpoint
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Heat Exchanger Control
Controlled variable- Outlet temperature of
product stream
Manipulated variable- Steam flow
Actuator- Control valve on steam line
Sensor- Thermocouple on product stream
Disturbance- Changes in the inlet feed
temperature
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DO Control in a Bio-Reactor
Air
AC
Variable Speed
Air Compressor
AT
Setpoint
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DO Control
Controlled variable- the measured dissolvedO2 concentration
Manipulated variable- air flow rate to the
bio-reactor Actuator- variable speed air compressor
Sensor- ion-specific electrode in contact
with the broth in the bio-reactor Disturbance- Changes in the metabolism of
the microorganisms in the bio-reactor
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Logic Flow Diagram for a
Feedback Control Loop
Controller Actuator Process
Sensor
CVSetpoint
Disturbance
+-
uce
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Comparison of Driving a Car and
Control of a Heat Exchanger Actuator: Drivers arm and steering wheel
vs. Control valve
Controller: the driver vs. an electronic
controller
Sensor: the drivers eyes vs. thermocouple
Controlled variable: cars position on the
road vs. temperature of outlet stream
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The key feature of all feedbackcontrol
loops is that the measured value of the
controlled variable is compared withthe setpoint and this difference is used
to determine the control action taken.
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In-Class Exercise
Consider a person skiing down a mountain.
Identify the controller, the actuator, the
process, the sensor and the controlledvariable. Also, indicate the setpoint and
potential disturbances. Remember that the
process is affected by the actuator to changethe value of the controlled variable.
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Types of Feedback Controllers
On-Off Control- e.g., room thermostat
Manual Control- Used by operators and based on
more or less open loop responses
PID control- Most commonly used controller.Control action based on error from setpoint
(Chaps 6-8).
Advanced PID- Enhancements of PID: ratio,cascade, feedforward (Chaps 9-11).
Model-based Control- Uses model of the process
directly for control (Chap 13).
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Duties of a Control Engineer
Tuning controllers for performance and
reliability (Chap 7)
Selecting the proper PID mode and/oradvanced PID options (Chap 6, 10-12)
Control loop troubleshooting (Chap 2 & 8)
Multi-unit controller design (Chap 14)
Documentation of process control changes
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Characteristics of Effective
Process Control Engineers
Use their knowledge of the process to guide
their process control applications. They areprocess control engineers.
Have a fundamentally sound picture of
process dynamics and feedback control. Work effectively with the operators.
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Operator Acceptance
A good relationship with the operators is a
NECESSARY condition for the success of a
control engineer.
Build a relationship with the operators
based on mutual respect.
Operators are a valuable source of plant
experience.
A successful control project should make
the operators job easier, not harder.
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Process Control and
Optimization
Control and optimization are terms that are
many times erroneously interchanged.
Control has to do with adjusting flow rates
to maintain the controlled variables of the
process at specified setpoints.
Optimization chooses the values for keysetpoints such that the process operates at
the best economic conditions.
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Optimization and Control of a CSTR
TC
Feed
Product
TTSteam
FC
RSP
OptimizerRSP
FT
FV
CA0
CA,C
B, C
C
ABC
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Optimization Example
balances.
molefromcalculatedareandLikewise,
]/exp[
1
forSolving
]/exp[
:AonbalanceMole
11
0
110
CB
r
AA
A
rAAA
CC
Q
VRTEk
CC
C
VCRTEkCQCQ
CBA
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Economic Objective Function
AFACCBBAA VCQVCQVCQVCQ 0
VB
> VC, V
A, orV
AF
At low T, little formation of B
At high T, too much of B reacts to form C
Therefore, the exits an optimum reactor
temperature, T*
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Optimization Algorithm
1. Select initial guess for reactor
temperature
2. Evaluate CA, CB, and CC
3. Evaluate
4. Choose new reactor temperature and
return to 2 until T* identified.
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Graphical Solution of Optimum
Reactor Temperature, T*
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Process Optimization
Typical optimization objective function,
: = Product values-Feed costs-Utility costs
The steady-state solution of process models
is usually used to determine processoperating conditions which yields flow rates
of products, feed, and utilities.
Unit costs of feed and sale price of products
are combined with flows to yield
Optimization variables are adjusted until
is maximized (optimization solution).
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Generalized Optimization
Procedure
Numerical
Optimization
Algorithm
Process
Model
Economic
Parameters
Economic
Function
Evaluation
Optimization
Variables
Economic
Function
Value
Model
Results
Initial Estimate
of Optimization
Variables
Optimum
Operating
Conditions
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Optimization and Control of a CSTR
TC
Feed
Product
TTSteam
FC
RSP
OptimizerRSP
FT
FV
CA0
CA,C
B, C
C
ABC
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In-Class Exercise
Identify an example for which you use
optimization in your everyday life. List the
degrees of freedom (the things that you arefree to choose) and clearly define the
process and how you determine the
objective function.
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Overview of Course Material
Control loop hardware (Chap 2)
Dynamic modeling (Chap 3)
Transfer functions and idealized dynamic
behavior (Chap 4-6)
PID controls (Chap 7-10)
Advanced PID controls (Chap 12-14)
Control of MIMO processes (Chap 15-18)
F d t l U d t di d
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Fundamental Understanding and
Industrially Relevant Skills
Fundamental Understanding-
Laplace tranforms and transfer functions (Ch 4-5)
Idealized dynamic behavior (Ch 6)
Frequency response analysis (Ch 11)
Industrially Relevant Skills-
Control hardware and troubleshooting (Ch 2&10)
Controller Implementation and tuning (Ch 7-9)Advanced PID techniques (Ch 12-14)
MIMO control (Ch 15-18)
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Process Control Terminology
Important to be able to communicate with
operators, peers, and boss.
New terminology appears in bold in the text
New terminology is summarized at the end
of each chapter.
Review the terminology regularly in orderto keep up with it.
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Overall Course Objectives
Develop the skills necessary to function asan industrial process control engineer.
Skills
Tuning loops Control loop design
Control loop troubleshooting
Command of the terminology
Fundamental understanding Process dynamics
Feedback control
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Overview
All feedback control loops have acontroller, an actuator, a process, and a
sensor where the controller chooses control
action based upon the error from setpoint. Control has to do with adjusting flow rates
to maintain controlled variables at their
setpoints while for optimization thesetpoints for certain controllers are adjusted
to optimize the economic performance of
the plant.