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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