Transcript CE444 – Chemical Process Control

CE-307 – CHEMICAL ENGINEERING
DESIGN
CE-307 – Chemical Engineering
Design
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Instructor – University at Buffalo
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Mattheos Koffas
Teaching Assistant
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Chin Giaw (Ryan) Lim
Course Information
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Lectures
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Office Hours
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Mondays, Wednesdays 5:00-6:20 pm
10 Capen
Mondays 6:30-8:00 pm 904 Furnas Hall
By appointment
TA office hours
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Friday 4:00- 6:00 pm, 903 Furnas Hall
ISBN: 0-471-00077-9
Course Grade
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Homework assignments
Mid-term Class Tests
Final examination
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Average will be set as C.
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30%
40%
30%
Note on Academic Integrity: Copying is not
allowed!
Homework
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10 homework sets will be handed during the
semester.
Almost all homework (with the exception of
1) will be handed on Wednesday and will be
due on Monday, 5:00 pm in class.
No homework will be accepted after 5:01 pm.
Course Outline
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Introduction
Process Dynamics
Laplace Transforms
Transfer Function
Block Diagrams
Dynamic Behavior of Typical Process Systems
Feedback Control
Stability of Control Loops
Course Objectives
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Review of basic process modeling.
Develop dynamic models for processes
and solve them.
Obtain a realistic understanding of
industrial process control practice.
Introduction to Process
Control
The continuous change of measurements in a
chemical or biological process leads to the
conclusion that processes are dynamic.
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Process dynamics refer to an unsteady-state or
transient behavior.
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Steady-state vs. unsteady-state behavior
i. Steady state: variables do not change with
time
 So far, your ChE curriculum has emphasized
steady-state or equilibrium situations.
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Process Dynamics
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Only with an understanding of transient
behavior of physical systems can an engineer
design good processes.
This is exactly what process control does: it
provides the expertise needed to design
plants that function well in a dynamic
environment.
Bottom Line: process control has a major
impact on profitability
Examples
Continuous processes with 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)
iv. Batch Processes- Batch reactor
i. Composition changes with time
Multidisciplinary Field
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Process control is used in many
engineering fields:
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Chemical
Electrical
Mechanical
Control
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The following definition of control will
be used in this course:
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To maintain desired conditions in a physical
system by adjusting selected variables in
the system.
What does a control system
do?
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As an example, consider the heating system
of a house.
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We need to maintain the house temperature at a certain
point.
This is done by circulating hot water through a heat
exchanger.
The temperature is determined by a thermostat that
compares the value of the room temperature to a desired
range.
If the temperature is in the desired range, the pump
halts water circulation.
The temperature can exceed the limits, because the
furnace and heat exchanger cannot respond immediately.
Common features in process
control cases
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There is always a specific value (or range) as a desired value
(referred to as set point) for the controlled variable.
The conditions of the system are measured; that is, all control
systems use sensors to measure the physical variables that are
to be maintained near the desired values.
There is always a control calculation, or algorithm , which uses
the measured and desired values to determine the correction to
the process operation.
The results of this calculation are implemented by adjusting
some item of equipment in the system, which is termed the final
control element.
Some more definitions
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Input: input does not necessarily refer to
material moving into the system. In Process
Control, input denotes the effect of the
surroundings on the chemical or biochemical
process.
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Output:denotes the effect of the process on
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Input variables cause the output variables.
the surroundings.
Example
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In the heated room example, what are:
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The Input variable
The Output variable
Important terms
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Controlled variable: it is the variable that needs to
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variable.
Set Point: it is the desired value of the controlled
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be maintained or controlled at some desired value or
range. Sometimes also referred to as process
variable. Thus the job of a control system is to
maintain the controlled variable at its set point.
Manipulated variable: is the variable used to
maintain the controlled variable at its set point.
Disturbance: any variable that causes the controlled
variable to deviate from its set point. Also referred to
as upset.
Example
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In the room heating example, what are
the:
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Controlled variable
Manipulated variable
Possible Disturbance variable(s)
Why is Control necessary?
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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
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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
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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.
Production specifications
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Product certification procedures (e.g.,
ISO9000) are used to guarantee
product quality and place a large
emphasis on process control.
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http://www.iso.ch/iso/en/ISOOnline.opener
page
Environmental regulations
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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
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The various types of equipments used in a chemical plant
have constraints inherent to their operation. Such
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
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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.
Thus, 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?
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All the previous requirements dictate
the need for continuous monitoring of
the operation of a chemical plant and
external intervention (control) to
guarantee the satisfaction of the
operational objectives.
How is control done
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Control is accomplished through a
rational arrangement of equipment
(measuring devices, valves, controllers,
computers) and human intervention
(plant designers, plant operators),
which together constitute a control
system.
Where is control
implemented?
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The short answer to this question is:
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Sensors, local indicators and valves are in the
process.
Displays of all plant variables and control
calculations are in a centralized facility.
What does control engineering
“engineer”?
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Most of the engineering decisions are
introduced in the following five topics:
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Process Design
Measurements
Final elements
Control structure
Control calculations
Process Control Design
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We want to design a process that we
can then control well and easily.
For example, we would like a chemical
plant to be more responsive.
By responsive we mean that the
controlled variable responds quickly to
adjustments in the manipulated
variable.
Measurements
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A key decision is the selection and
location of sensors, because one can
control only what is measured.
The engineer should select sensors that
measure important variables rapidly and
with sufficient accuracy.
Final elements
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We will typically consider control valves
as the final elements, with the
percentage opening of these valves
determined by a signal sent to the valve
from a controller.
Control structure
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The engineer must decide some very
basic issues in designing a control
system.
For example, which valve should be
manipulated to control which
measurement?
Control calculations
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After the variables and control structure have
been selected, equations are chosen that use
the measurement and desired values in
calculating the manipulated variable.
As we will see, we only need to develop a few
equations that we will then use to control
many different types of plants.
Duties of a Control Engineer
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Tuning controllers for performance and
reliability
Selecting the proper PID mode and/or
advanced PID options
Control loop troubleshooting
Multi-unit controller design
Documentation of process control
changes
Characteristics of Effective
Process Control Engineers
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Use their knowledge of the process to
guide process control applications
Have a fundamentally sound picture of
process dynamics and feedback control
Work effectively with the operators
Operator Acceptance
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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
Process Control and Optimization
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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 set-points
Optimization chooses the values for key setpoints such that the process operates at the
“best” economic conditions
Background needed for Process
Control
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To be successful in the practice of automatic process
control, the engineer must first be familiar with the
basic principles of thermodynamics, fluid flow, heat
transfer, separation process, reaction processes etc.
Another important tool for the study and practice of
process control is computer simulation. Many of the
equations developed to describe processes are
nonlinear in nature and consequently, the most exact
way to solve them is by numerical methods. The
computer simulation of process models is called
simulation.
Example
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Book example