essential process control for chemical engineers

Essential Process Control for Chemical Engineers 1 st edition © 2017 Dr.. ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS 5 CONTENTS... ESSENTIAL PROCESS CONTROL 9.3 Solution of differe

DR BRUCE POSTLETHWAITE

ESSENTIAL PROCESS

CONTROL FOR

CHEMICAL ENGINEERS

Essential Process Control for Chemical Engineers

1 st edition

© 2017 Dr Bruce Postlethwaite & bookboon.com

ISBN 978-87-403-1655-1

Peer reviewed by Dr Iain Burns, Senior Lecturer, Director of Teaching, University of Strathclyde

ESSENTIAL PROCESS CONTROL

CONTENTS

2.2 Factors to be considered in selecting an instrument 13

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ESSENTIAL PROCESS CONTROL

FOR CHEMICAL ENGINEERS

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CONTENTS

ESSENTIAL PROCESS CONTROL

9.3 Solution of differential equations using Laplace transforms 91

10.3 Simplifying expressions through deviation variables 110 10.4 Procedure for simplifying and solving a non-linear model 112 10.5 Putting it all together – a reactant balance for a CSTR 112

12.3 Analysis of proportional control of a first-order process 140 12.4 Example of a first order process under proportional control 142 12.5 Example of a second-order process under proportional control 145 12.6 Analysis of integral control of a first-order process 148

ESSENTIAL PROCESS CONTROL

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CONTENTS

13.1 What needs to be done to tune a PID Controller? 149 13.2 How do you decide what is a good controller performance? 150

15.3 How to determine the number of controlled variables 185

16.1 The effect of technology on process plant control rooms 194

The use of software for teaching process control at

ESSENTIAL PROCESS CONTROL

FOREWORD

his book is based on the course notes from the introductory process control class at Strathclyde University in Glasgow, Scotland his course is itself based on the IChemE model-curriculum for chemical engineers and covers the material that ALL chemical engineers are supposed know he IChemE curriculum was drawn up by a team of industrialists and academics, led by Professor Jon Love, in response to a recognised need for chemical engineers

to be taught a more industrially relevant course

his book isn’t a traditional academic textbook in that there are no references anywhere

in the text he main reason for this is that the material has been gathered from many diferent sources after a working lifetime of teaching in the area and trying to identify an original source is impossible I have included a bibliography for readers who wish to look further into the subject

I hope students and teachers ind this book useful A major new part of the course at Strathclyde University (where I teach) has been the introduction of new process control learning software called PISim, and this is described in the appendix PISim will be commercially released in late Autum 2017

ESSENTIAL PROCESS CONTROL

FOR CHEMICAL ENGINEERS

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INTRODUCTION

1 INTRODUCTION

MAIN LEARNING POINTS

• Why process control is necessary

Process control is concerned with making sure that processes do what they are supposed

to in a safe and economical way his isn’t an easy task as most processes are subject to

many inputs called disturbances that constantly cause the controlled variables to move away from their desired values (or setpoints) To prevent this other process inputs called manipulations have to be moved to restore the process to the desired state

Process control is concerned with the overall system A control engineer has to know about the

instruments used to measure process quantities, the valves and other inal control elements that allow control systems to adjust the process, communications to transmit information around, the control algorithms that decide how to respond to the information coming from the process, and inally the control engineer needs to understand how the process

itself behaves: not just its steady-state behaviour but more importantly its dynamic response.

Control engineering is now an area which ofers big career opportunities for chemical engineers he area used to be dominated by electrical/electronic engineers as the major challenges were in the hardware his has changed Sophisticated modern control systems allow much more complicated, process related, control schemes and now a major requirement for a control engineer is that they have a good understanding of the process 1.1 WHY DO WE NEED CONTROL?

Figure 1 – a pressure trace from a SCADA system

ESSENTIAL PROCESS CONTROL

• In real chemical plants, steady-state doesn’t exist hings are always changing Temperatures move up and down, levels get lower and higher, etc (see igure 1)

• All processes are subject to disturbances hese are inputs to the process that change

in a way that is beyond the reach of the local control system A rainstorm on the outside of a distillation column will cool the column and require action to be taken to increase the heat input Raw material variations are another common disturbance Actions of other control systems can also cause disturbances to the process of interest – if a control system upstream or downstream of a process reduces

a lowrate its efects will cascade throughout the rest of the process

• he control system needs to actively regulate against the efects of these disturbances

It does this by either measuring the disturbances directly (where this is possible and

economic) or by measuring their efects on the controlled variables of the process It then makes adjustments to other inputs to the process called manipulated variables

to try to reduce or eliminate the efects of the disturbances When controllers are

holding controlled variables at ixed setpoints they are said to be in regulator or disturbance rejection mode.

• Process don’t suddenly start at their lowsheet conditions, they don’t shut down

on their own and don’t change production rate, etc without active intervention from control systems When these major changes are being made to a process, the

controllers will be acting in a setpoint tracking or servo mode In servo mode, a

controller will be trying to make the controlled variable track a moving setpoint

• Control systems also have a major part to play in process safety he basic control system will usually ensure that the process stays within acceptable limits and will

be equipped with alarms to warn operators of any problems Interlocks may also

be present in the basic system hese are used to lock particular inputs when other conditions are in existence For example, the access doors to a kiln may be locked

by a control system if the internal temperature is dangerously high In extreme

circumstances, special control systems (called safety instrumented systems or SIS ) that

are separate from the normal process control system may come into play hese may be local to a particular piece of equipment, for example a high-temperature trip on a pump motor; or may have a process or plant-wide focus, for example an emergency shutdown system

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INTRODUCTION

• Finally, good control saves money Plants are normally operated close to constraints (e.g the acceptable product quality) Poor control means more variability and this means that the mean value of a controlled variable needs to be held further from the constraint than is necessary with good control Figure 2 shows a simple example – the tops quality from a distillation column his may have a limit on the lowest acceptable composition and it’s the responsibility of the control system

to hold the composition about this limit If the control is poor and there’s lots of variability, then it will be necessary to set an average value of composition much higher than if good control is used his higher average composition will lead to increased relux going down the column and hence more vapour having to be generated by the reboiler, which increases steam costs

Figure 2 – The advantage of good control

ESSENTIAL PROCESS CONTROL

2 INSTRUMENTATION

MAIN LEARNING POINTS

• Factors involved in selecting instrumentation

• Techniques for temperature, pressure, low and level measurements

he instruments on a chemical plant are the devices used to monitor the important variables that allow the condition of the process to be determined

2.1 WHAT IS AN INSTRUMENT?

Transducers or sensors are the primary sensing elements hey are devices that convert some

physical quantity that we want to measure (e.g temperature, pressure, etc) into some sort

of signal that can be processed further For example, a thermocouple converts a temperature diference into a voltage; a piezo resistive pressure sensor converts a pressure into a change

in electrical resistance

360°

thinking.

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FOR CHEMICAL ENGINEERS

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INSTRUMENTATION

Signal conditioning is the signal processing that is applied to the output of the transducer

Sometimes this could simply be ampliication, but often more complicated things like

linearisation are required (ideally, the output of a device should change linearly with changes

in the quantity being measured) Modern instruments such as Coriolis lowmeters have very complicated signal processing build into them to detect the phase shifts in the motion of the sensing elements

A transmitter is a device that converts the output from signal conditioning into a signal

that is compatible with the communication system being used in the plant here are many

diferent standards in use ranging from 4-20mA analogue signals up to digital Fieldbus

systems – these will be discussed later

An instrument is a device that contains at least one but usually more, and often all of

the above (transducer, signal conditioning and transmitter) An instrument is a complete measurement package that senses the quantity to be measured and presents that measurement

in a form suitable for use (e.g a simple instrument might be a Bourdon gauge for pressure measurement – the transducer, a helical metal tube, distorts with pressure and drives the gauge needle directly; a more typical instrument for modern chemical plants might be a packaged RTD (resistance temperature device) – it will include the RTD, signal conditioning, and a transmitter)

2.2 FACTORS TO BE CONSIDERED IN SELECTING AN INSTRUMENT

2.2.1 RANGE

he range of an instrument is range of the measured quantity over which the instrument

will give a reliable output he range is always the same or bigger than the span of an

instrument While an instrument with a large range might seem to be always desirable this

isn’t usually the case in practice he sensitivity (change in output vs change in measurement)

of transducers drops signiicantly in large range devices leading to reduced accuracy

2.2.2 SPAN

he span of an instrument is an adjustable parameter (there will be a button, screw or software link on the instrument that will allow the adjustment) he span is the distance the measured quantity has to move to drive the instrument output from its minimum value

to its maximum (remember that instrument outputs match communication standards which

have ixed maximum and minimum values) By adjusting the span, the instrument’s sensitivity

(output change vs input change) can be altered – large spans will lead to lower sensitivities

ESSENTIAL PROCESS CONTROL

2.2.3 ACCURACY AND PRECISION

All measurement instruments are subject to random error – if you take repeated measurements

of a ixed quantity you will always get a scatter of values about a mean An accurate

instrument is one where the mean is centred close to the actual value of the quantity being measured An instrument can be accurate, but still have a signiicant amount of error on an individual measurement – the accurate mean can only be obtained by taking many repeat

measurements A precise instrument is one which, when measuring a constant quantity,

returns output values which are very close to one another – the scatter between readings

is small It is possible for an instrument to be precise (high repeatability in measurement) but not accurate (with the mean some distance away from the true value of the quantity being measured) An ideal instrument is one which is both precise and accurate

Figure 3 – accuracy and precision

2.2.4 REPEATABILITY AND DRIFT

In most instruments repeatability and precision mean the same thing However, some sensors sufer from hysteresis In these sensors the measurement is afected by what the variable being

measured was doing prior to the measurement It is most prevalent in systems which involve some sort of mechanical element in their sensing For example, bourdon pressure sensors usually exhibit hysteresis – the measurement they produce will be diferent if the pressure was rising or falling immediately prior to the measurement

Drift is a medium to long term efect that causes some instruments to lose mainly accuracy, but also possibly precision For example, corrosion of a thermocouple will alter its thermoelectric properties and hence the voltage produced at a particular temperature