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CE-307 CHEMICAL ENGINEERINGDESIGN
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CE-307 Chemical Engineering
Design Instructor University at Buffalo
Mattheos Koffas
Teaching Assistant
Chin Giaw (Ryan) Lim
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Course Information Lectures
Mondays, Wednesdays 5:00-6:20 pm
10 Capen
Office Hours Mondays 6:30-8:00 pm 904 Furnas Hall
By appointment
TA office hours Friday 4:00- 6:00 pm, 903 Furnas Hall
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SBN: 0-471-00077-9
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Course Grade Homework assignments 30%
Mid-term Class Tests 40%
Final examination 30%
Average will be set as C.
Note on Academic Integrity: Copying is notallowed!
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Homework 10 homework sets will be handed during the
semester.
Almost all homework (with the exception of1) will be handed on Wednesday and will bedue on Monday, 5:00 pm in class.
No homework will be accepted after 5:01 pm.
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Course Outline Introduction
Process Dynamics
Laplace Transforms
Transfer Function
Block Diagrams
Dynamic Behavior of Typical Process Systems Feedback Control
Stability of Control Loops
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Course Objectives Review of basic process modeling.
Develop dynamic models for processesand solve them.
Obtain a realistic understanding ofindustrial process control practice.
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Introduction to Process
Control The continuous change of measurements in a
chemical or biological process leads to theconclusion that processes are dynamic.
Process dynamics refer to an unsteady-state ortransient behavior.
Steady-state vs. unsteady-state behavior
i. Steady state: variables do not change withtime
So far, your ChE curriculum has emphasizedsteady-state or equilibrium situations.
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Process Dynamics 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 designplants that function well in a dynamic
environment.
Bottom Line: process control has a majorimpact on profitability
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Examples
Continuous processes with examples of transientbehavior:
i. Start up & shutdown
ii. Major disturbance: e.g., refinery duringstormy or hurricane conditions
iii. Equipment or instrument failure (e.g., pumpfailure)
iv. Batch Processes- Batch reactor
i. Composition changes with time
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Multidisciplinary Field
Process control is used in manyengineering fields: Chemical Electrical
Mechanical
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Control The following definition ofcontrol will
be used in this course:
To maintain desired conditions in aphysical system by adjusting selectedvariables in the system.
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What does a control system
do? As an example, consider the heating system
of a house. 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 thatcompares 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 thefurnace and heat exchanger cannot respondimmediately.
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Common features in process
control cases 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 areto be maintained near the desired values.
There is always a control calculation, or algorithm, which usesthe measured and desired values to determine the correction tothe process operation.
The results of this calculation are implemented by adjustingsome item of equipment in the system, which is termed the finalcontrol element.
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Some more definitions Input: input does not necessarily refer to
material moving into the system. In ProcessControl, input denotes the effect of thesurroundings on the chemical or biochemicalprocess.
Output:denotes the effect of the process onthe surroundings.
Input variables causethe output variables.
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Example In the heated room example, what are:
The Input variable
The Output variable
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Important terms Controlled variable: it is the variable that needs to
be maintained or controlled at some desired value orrange. Sometimes also referred to asprocessvariable.
Set Point:it is the desired value of the controlledvariable. Thus the job of a control system is tomaintain the controlled variable at its set point.
Manipulated variable:is the variable used tomaintain the controlled variable at its set point.
Disturbance: any variable that causes the controlledvariable to deviate from its set point. Also referred toas upset.
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Example In the room heating example, what are
the:
Controlled variable
Manipulated variable
Possible Disturbance variable(s)
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Why is Control necessary? Control is necessary because during its
operation, a chemical plant must satisfy
several requirements imposed by itsdesigners and the general technical,economic, and social conditions in the
presence of ever changing externalinfluences (disturbances). Suchrequirements are the following:
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Safety The safe operation of a chemical process is a
primary requirement for the well-being of the
people in the plant and for its continuedcontribution to the economic development.
Thus the operating pressures, temperatures,concentration of chemicals and so on shouldalways be within allowable limits.
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Production specifications A plant should produce the desired amounts
and quality of the final products.
For example, we may require the productionof 2 million pounds of ethylene per day, of99.5% purity. Therefore, a control system isneeded to ensure that the production leveland the purity specifications are satisfied.
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Production specifications Product certification procedures (e.g.,
ISO9000) are used to guarantee
product quality and place a largeemphasis on process control. http://www.iso.ch/iso/en/ISOOnline.opener
page
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Environmental regulations Various federal and state laws may specify
that the temperatures, concentrations of
chemicals and flow rates of the effluents froma plant be within certain limits.
Such regulations exist for example on theamounts of SO2 that a plant can eject to theatmosphere, and on the quality of the waterreturned to a river or lake.
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Operational constraints The various types of equipments used in a chemical plant
have constraints inherent to their operation. Suchconstraints should be satisfied throughout the operation ofa plant.
For example, pumps must maintain a certain net positivesuction head; tanks should not overflow or go dry;
distillation columns should not be flooded; the temperaturein a catalytic reactor should not exceed an upper limitsince the catalyst will be destroyed. Control systems areneeded to satisfy these operational constraints.
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Economics The operation of a plant must conform with
the market conditions, that is, the availabilityof raw materials and the demand of the finalproducts. Furthermore it should be aseconomical as possible in its utilization of rawmaterials, energy, capital and human labor.Thus, it is required that the operating
conditions are controlled at given optimumlevels of minimum operating cost, maximumprofit and so on.
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Why is control necessary?All the previous requirements dictate
the need for continuous monitoring of
the operation of a chemical plant andexternal intervention (control) toguarantee the satisfaction of the
operational objectives.
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How is control done
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 controlsystem.
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The short answer to this question is: Sensors, local indicators and valves are in the
process. Displays of all plant variables and control
calculations are in a centralized facility.
Where is control
implemented?
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What does control engineering
engineer? Most of the engineering decisions are
introduced in the following five topics:
Process Design
Measurements
Final elements
Control structure Control calculations
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Process Control Design We want to design a process that we
can then control well and easily.
For example, we would like a chemicalplant to be more responsive.
By responsive we mean that the
controlled variable responds quickly toadjustments in the manipulatedvariable.
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MeasurementsA 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 andwith sufficient accuracy.
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Final elements
We will typically consider control valves
as the final elements, with thepercentage opening of these valvesdetermined by a signal sent to the valvefrom a controller.
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Control structure
The engineer must decide some very
basic issues in designing a controlsystem.
For example, which valve should bemanipulated to control whichmeasurement?
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Control calculations
After the variables and control structure havebeen selected, equations are chosen that usethe measurement and desired values incalculating the manipulated variable.
As we will see, we only need to develop a fewequations that we will then use to controlmany different types of plants.
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Duties of a Control Engineer Tuning controllers for performance and
reliability
Selecting the proper PID mode and/oradvanced PID options
Control loop troubleshooting
Multi-unit controller design Documentation of process control
changes
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Characteristics of EffectiveProcess Control Engineers
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
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Operator Acceptance
A good relationship with the operators is aNECESSARY condition for the success of a
control engineer Build a relationship with the operators based
on mutual respect
Operators are a valuable source of plantexperience
A successful control project should make theoperators job easier, not harder
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Process Control and Optimization
Control and optimization are terms that aremany times erroneously interchanged
Control has to do with adjusting flow rates tomaintain the controlled variables of theprocess at specified set-points
Optimization chooses the values for key set-points such that the process operates at the
best economic conditions
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Background needed for Process
Control To be successful in the practice of automatic process
control, the engineer must first be familiar with thebasic principles of thermodynamics, fluid flow, heattransfer, separation process, reaction processes etc.
Another important tool for the study and practice ofprocess control is computer simulation. Many of theequations developed to describe processes arenonlinear in nature and consequently, the most exactway to solve them is by numerical methods. Thecomputer simulation of process models is calledsimulation.
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Example Book example