DR BRUCE POSTLETHWAITE ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS Essential Process Control for Chemical Engineers 1st 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 FOR CHEMICAL ENGINEERS CONTENTS CONTENTS Foreword Introduction Main Learning points 1.1 Why we need control? Instrumentation 12 Main learning points 12 2.1 What is an instrument? 12 2.2 Factors to be considered in selecting an instrument 13 2.3 Instruments for temperature measurement 17 2.4 Pressure measurement 20 2.5 Flow measurement 23 2.6 Level measurement 27 2.7 Chemical composition 30 www.sylvania.com We not reinvent the wheel we reinvent light Fascinating lighting offers an ininite spectrum of possibilities: Innovative technologies and new markets provide both opportunities and challenges An environment in which your expertise is in high demand Enjoy the supportive working atmosphere within our global group and beneit from international career paths Implement sustainable ideas in close cooperation with other specialists and contribute to inluencing our future Come and join us in reinventing light every day Light is OSRAM ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS CONTENTS Communication signals 32 Main learning points 32 3.1 Types of communication signal 32 Final control elements 39 Main Learning points 39 4.1 Control valves 39 4.2 Control valve sizing 41 Diagrams for process control systems 48 Main learning points 48 5.1 Process flow diagrams (PFDs) 48 5.2 Piping and instrumentation diagrams (P&IDs) 49 Inputs and outputs in control systems 55 Main learning points 55 6.1 Process inputs 55 6.2 Process outputs 56 6.3 Processes in control engineering 57 6.4 An example of variables and processes 58 Introduction to feedback control 59 Main learning points 59 7.1 Feedback control and block diagrams 59 7.2 Positive and negative feedback 61 7.3 Control loop problems 61 7.4 Direction of control action 64 7.5 Controller hardware 66 Introduction to steady-state and dynamic response 70 Main learning points 70 8.1 Steady-state gain 70 8.2 Dynamic response 73 ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS CONTENTS Dynamic modelling 87 Main learning points 87 9.1 Laplace transforms 88 9.2 Derivation of basic transforms 88 9.3 Solution of differential equations using Laplace transforms 91 9.4 Transfer functions 93 9.5 Block Diagrams 94 9.6 Block diagram algebra 95 9.7 Solutions of responses for high-order systems 95 9.8 Forming dynamic models 100 10 Analytical solution of real world models 106 Main learning points 106 10.1 Types of non-linearity 106 10.2 Linearisation of non-linear equations 107 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 11 PID Controller algorithm 117 Main learning points 117 11.1 Really simple feedback controller – on-off 118 11.2 Proportional-integral-derivative (PID) control 119 11.3 Proportional only control 121 11.4 Integral only control 126 11.5 Derivative action 127 11.6 Proportional-Intergral (PI) control 130 11.7 PID control response 131 11.8 Other forms of PID algorithm 133 12 Control system analysis 137 Main learning points 137 12.1 Analysis of a typical feedback control system 137 12.2 The PID algorithm as a transfer function 139 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 FOR CHEMICAL ENGINEERS CONTENTS 13 Controller tuning 149 Main learning points 149 13.1 What needs to be done to tune a PID Controller? 149 13.2 How you decide what is a good controller performance? 150 13.3 Some methods of controller tuning 154 13.4 Control loop health monitoring 161 13.5 Control loop diagnostics 162 14 More advanced single-loop control arrangements 163 Main learning points 163 14.1 Cascade control 163 14.2 Selective or autioneering control 167 14.3 Override control 169 14.4 Ratio control 172 14.5 Feedforward control 173 15 Design of control systems 181 Main learning points 181 15.1 Control envelope 181 15.2 Multivariable processes 184 15.3 How to determine the number of controlled variables 185 15.4 Plantwide mass balance control 191 16 Control system architecture 194 Main learning points 194 16.1 The effect of technology on process plant control rooms 194 16.2 Human factors in control room displays 197 16.3 Distributed control systems 200 16.4 Safety Instrumented Systems 201 17 Bibliography 202 Acknowledgements 203 Appendix 204 The use of software for teaching process control at Strathclyde University 204 ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS FOREWORD 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 INTRODUCTION INTRODUCTION MAIN LEARNING POINTS • Why process control is necessary Process control is concerned with making sure that processes 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 – a pressure trace from a SCADA system ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS INTRODUCTION • 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 10