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Control in a Chemical Engineering Course
Copyright © IFAC Trends in Control and Measurement Education, Swansea, UK, 1988
CONTROL IN A CHEMICAL ENGINEERING COURSE C. McDermott* and A. Johnson** *University of Binningham, Edgbaston, Binllingham B15 2 IT, UK **Technical University Delft, Prins Bemhardlaan 6, 2628 BW Delft, The Netherlands
Abstract. The requirements of a course in control for Chemical Engineers are discussed and the place of traditional control theory in such a course questloned, A s yllabus for a short course is presented which should provide a basic level of ucderstanding for the non specialist, The lecture, tutorial and practical content is chosen to maintain interest in the subject and motivate the student towards further study. KeY.lOrds. Control teaching; control theory; chemical plant; flow level temperature; unit operations control,
single loop, offers little towards preparing the chemical engineer for this kind of e xperience, We begin by taking a closer look at the complete course to tr y to state speciflc objectives to be met by the course a nd then go on to suggest the content of a course which meets these
INTRODUCTION The time available for the teaching of control in a course other than one for control engineers, although varying greatly between departments and engineering disciplines, is necessarily limited, The wide range of subjects to be covered and th e depth of treatment required for each makes thlS particularly so in the case of chemical engineering. Clearly the material to be included in the control course must be carefully selected and the following is an attempt to identify the needs of the chemical engineer as far as a knowledge of control is concerned, in order to ass i st with ma king that selection. A course is then suggested which contains the topics identified and has a duration of appro ximately one term.
requirements.
OBJECTIVES Most chemical engineering courses are inevitably oriented towards an analysis of the steady state operating condition of the process, especially during the early stages, This is reflected in the fact that the the basic defining equations, that is mass and energy balance equations,
equilibrium relationships and so on, are usually presented in their steady state form, The effect is that students beginning the control course think mainl y in terms of the steady state, with the result that the earl y part of the course must be de voted to imparting an appreciation of dynamic beha viour. A basic requirement then would be that the initial teaching should use the dynam i c form of such equations so that the relationship between equipment size, stream flowrates and the form and duration of the response is recognised at an early stage,
The chemical engineer comes face to face with serious plant control problems when he first enters the control room, the nerve centre of the plant. He will need to understand the meanlng of the displays and how these relate to plant operation in whatever form they may be; that is control co~puter schematic, faceplate, trace, gauge or other. He ma y well be tal d that no changes may be made to control loop parameters, with the possible exception of limited adjustments to certain set points, as that i s the job of the instrument engineer. He will, however, need to to be able to recognlse plant malfunction from the record or plot and, from an intuitive understanding of how the loop operates, decide whether this is a sticking valve, leaking pipe flange or some other cause,
The disturbances to which a plant is subject need to be considered ln some detail and those characteristics which ensure the natural suppression of :he disturbance recognised, The characterisation of d,sturbances which are not naturally suppressed and the design and sizing of eq uipment to take advantage of natural suppression should be familiar,
As a design engineer he will be concerned to size equipment for ease of control and decide on the overall plant control strategy. He will thus need to decide upon the principal loops for product quallty control, those necessary for the maintenance of stable operation and be able to liase with the professional control engineer to produce a robust reliable plant control system,
Having identitied those process outputs which need to be regulated and the sources of the major disturbances, the selection of the form of the control to be used follows. The choice of manual control should be considered in the case where information would be available to the operator but not from plant instrumentation, for example upstream plant operation variations or "eather changes or forecasts. The case for auto~atic control and whether this should be
The traditional control course, with its emphasis on transfer functions, frequency response diagra~s and detailed analysis of the TCI'IE-F
69
70
C. McDermott and A. Johnson
feedforward, feedback or so.e co~bination the two and finally the means of implementation ie. pneu.atic, electrical or control computer must be decided, the choice being based on the site conditions prevailing, number of loops required and econo.ic factors. The characterisation of good control and the .easures used in the design of optimal control systems, the system response to load and setpolnt changes, the behaviour of the loop as a filter for the various forms of disturbance and the presence of limits to loop stability should all form part of the chemical engineers background knowledge. A basic component in an y control course has always been the use of the transfer function and the mathematics of the closed loop. It is doubtful, in a course of this type, whether the benefit obtained justifies the time and effort involved. The attraction of the use of material such as frequency response diagrams, root locus plots and the closed loop transfer function lies in the insight it offers into loop behaViour and the control actions required. In courses for control and electrical engineers such material would be recognised as sufficiently basic to allow enough time fo r the necessary understanding to be aquired. Its in clusi on in a Chemical Engineeri ng course would mean displacing discussion of topics of Immediate relevance such as strategies for th e control of particular unit operations. In fact experience with past students has indicated that they are deterred from further study of control by the difficulty of traditional theo r y and its apparent remoteness from the realIties of
COURSE A suggested course, consisting of a total of thirty hours of lecture, tutorial and practical sessions, is proposed which goes some way towards meeting the above requirements. Of the available hours, eighteen are allocated for lectures and si x each for tutorial and practical classes. Lectures
1. The use of the unsteady state material or energy balance to pred ict the response of various simple process elements (tank level, hea ter etc. ) to step, ramp and other disturbances. Support with bench or simulation e>:periments.
2. The idea of a linear ( e xpo nential or RC) lag and the significance of the steady state gain and time constant. Methods of Identifying these from experimental observations. 3 . The steady state. Use of b l ock diagram representation for the combination ( multiplIcation ) of a sequence of stages, each represented in terms of the st eady state gain. Extension to the closed loo P! dimensionless open
loop gain and calculation of offset . 4. Nature of disturbances affecting a process. Dominant frequencies and the behaviour of the process as a filter. Natural disturbance suppression by the process and cases needing suppression by control actIon.
process control.
5. Types of control namely
All that can really be ju stif i ed here isthe basic idea of proportional feedback control, supported quantitatively perhaps in terms of a simple linear differential equation, and the qualitative introduction of int egral and derivative terms with sufficient mathematical treatment to demonstrate their effect on stability. It is desirable, of course, that some e xp erience is obtained of the tuning of a three term controller using say the Ziegler Nichols or
Manual - where compensation is needed when abnormal conditions are anticipated.
reaction curve method and this can easily be included in a laboratory session.
6. Simple state-space representation of the controlled system by means of a single first order linear ODE. Demonstration of offset fer proportional control and its e limina tion using the int egral term.
As indicated abo ve the difficulty ariSIng from a course concentrat i ng primarily on linear control
theory is that little reference is likely to be made to the process to be controlled. The basic strategies used for th e control of individual operations rel y on a sound understanding of pr incipl es governing the operation cf the process. In the cas e of distillation, for e xample, an analysis of the steady state behaviour leads to the conclusion that product COMposition can be controll ed by var yi ng product flowrate. The problems of loop tuning are l ess serious than those of the most appropriate choice of coupled var iabl es. The effect of variations in the flowrate of the chosen stream
On other process variables then needs to be predicted before the methods of linear control theory can be applied to produce the necessary compensation. It is at this stage that the necessity arises for the chemical engineer to liaise with the control engineer to produce a practical solution. This requires of the chemical engineer a kno wledge of control engineering terminology and an appreCiation of the techniques available .
Feedback - to correct for errors produced by unmonitored variables. Feedforward - to correct for changes in measured values of specific inputs and in combination with feedback.
7. Characterisation of system response. Integral SSQ or absolute de viation, decay ratio and damping, period and recovery time. 8. Tuning the three term cont ro lle r, the meaning of int egral and deri va tive action times.
Ziegler-Nlchois loop tuning and reaction curve methods 9. Means of implementation of control action. The case for single loop electrical and pneumatic de vices. Computer control , supervisory and DDC together with considerations of reliability and economics. 10. Multiloop control: ratio, cascade and split range, with e xamples of control of reaction, pH and a furnace 11. Interacting control loop s demonstrated using the e xample of dual control of flow and composition in a s impl e mixer. Simulation of the degradation of qualit y of control for the case of simultaneous control of temperature and stack oxygen in a furnace.
Control in a Chemical Engineering Course 12. The control valve. Construction of the valve and actuator, valve characteristic and valve sizing. 13. Applications to control of flow, temperature, pressure and other single loops. 14. Detailed consideration of a typical unit operation, for example the control of a heat exchanger. The strategies available require loops considered In the previous lecture with applications of cascade and ratio control. Also the use of the valve characteristic to compensate for the nonlinear response. 15. The process model. linear and nonlinear models and techniques available for compensation for nonlinearity and time delay. 16. State space and s-domain representation of multiloop systems. Block diagram representation for calculation of system response using transfer function representation of dynamic elements. 17. Use of the complex model with commercial Simulation and system deSign packages such as SIMNON or SYMBOL, available from Cambridge Control Limited. For simple Single loop systems the CONCADE package from the Department of Engineering, University of Manchester Cdn be used. 18. Review of more recent approaches to control such as optimal control, sampled data methods, self tuning and adaptive control.
duplication of many basic items of equipment. In a large department, laboratory accomodation and equipment is not always available on the scale needed to give every student sufficient hands on experience. In any case, the duplication of elementary facilities would represent an inefficient use of scarce resources.An effective substitute for some experimental work has been found In the use of a number of low cost personal computers and, as long as the hardware IS not displaced completely, the experience gained still proves to be valuable. The computers have been programmed to simulate various items of process equipment such as a storage tank for level control, an electrical heater for temperature and an evaporator for composition. Other less elementary applications include sampled data control of temperature, mulivariable control of a flash drum and control of pH to demonstrate nonl i neari ty. Although, with sufficient programming skill, it would be possible to produce s i mulations offering a choice of controlled and manipulated variables, thiS has not been undertaken. The simple versions referred to above are easy to produce and provide valuable e x perience at the early stages of the study of process control. The investigation of alternative control schemes has been made possible at Bi rmingham by the provision by IBM UK Ltd. of the IBM ACS IAdvanced Control System). This commercial computer control system gives students the opportunity to appl y control methods using industrial equipment on a realistic real time simulation.
Tutorials
Three two hour tutorials illustrating lecture material as follows : 1. Unsteady state balances on a heater and pure capacity tank to determine the transfer function. Evaporator feedback control specified In terms of steady state gains. Determination of offset under proportional onl y control. 2. Calculation of controller gains, integral and derivative times for first and second order processes producing quadratic characteristic equations. Decay ratio and period resulting from above calculated parameters. Design of second order system producing response with the required characteristics. 3. Cascaded loops: comparison of predicted response with and without the slave loop. Response of interacting loops in a simple case and comparison with those of the individual loops. Calculation of decoupler gain and the response of the decoupled loops.
Practic.ls
The nature and content of the practical or laboratory sessions accompanying such a course are inevitably dictated by the facilities available within the institution concerned. The requirement of such practical work is that it should be possible for all students on the course to carry out appropriate experimental work at each stage. This means extensive
The three practical sessions suggested are as follows, using SUitable equipment where possible and computer simulations otherwise: 1. For a single stage process in the open loop, to identify the response to step, ramp and sinus iod al Inputs. This Information to be used, where appropriate, to determine the time constant . With the loop closed and using proportional only control, observe the steady state response (offset) of pure capacity and self regulating processes when subject to load and set point changes. 2. Study of the effect on quality of control and loop stability of introducing integral and d~rivative terms into the controller. Application of the Zlegler Nichols reaction curve and loop tuning methods for determination of controller settings. 3. Use of the response of a second order system to calculate controller settings to give prescribed decay ratio and period of the response. Comparison of the response of two interacting loops with their individual responses. Comparison of the response of a cascade control system with that where the primary loop acts directly on the manipulated variable. In each session proviSIon should be made for access to ACS or similar terminals and to experimental equipment for as many students as possible.
71
C. McDermott and A. Johnson
72 DISCUSSION
Obviously no short course can do more than scratch the surface of control technology in whatever area it is applied. !n this attempt the absence of true chemical engineering applications is .till evident. The dtfftculty being that most such industrtal examples Involve problems which are outside the scope of an elementary course. Any realistic example does, however, involve numerous simple single loops and the behaviour of such loops needs to be appreciated. It is hoped that the above helps in producing such an appreciation and at the same time gives some hint of the interest and satisfaction which can accompany a further study of control. The primary object of the control course should be to encourage a genuine interest in pr OCESS control. If such an interest could be generated in the majority o f chemical e n gineers instead ef a specialist minority, plant effic i ency and operab i lity should be signiflcantly improved.
CONTROL IN A CHEMICAL ENGINEERING COURSE C. McDermott* and A. Johnson** *University of Binningham, Edgbaston, Binllingham B15 2 IT, UK **Technical University Delft, Prins Bemhardlaan 6, 2628 BW Delft, The Netherlands
Abstract. The requirements of a course in control for Chemical Engineers are discussed and the place of traditional control theory in such a course questloned, A s yllabus for a short course is presented which should provide a basic level of ucderstanding for the non specialist, The lecture, tutorial and practical content is chosen to maintain interest in the subject and motivate the student towards further study. KeY.lOrds. Control teaching; control theory; chemical plant; flow level temperature; unit operations control,
single loop, offers little towards preparing the chemical engineer for this kind of e xperience, We begin by taking a closer look at the complete course to tr y to state speciflc objectives to be met by the course a nd then go on to suggest the content of a course which meets these
INTRODUCTION The time available for the teaching of control in a course other than one for control engineers, although varying greatly between departments and engineering disciplines, is necessarily limited, The wide range of subjects to be covered and th e depth of treatment required for each makes thlS particularly so in the case of chemical engineering. Clearly the material to be included in the control course must be carefully selected and the following is an attempt to identify the needs of the chemical engineer as far as a knowledge of control is concerned, in order to ass i st with ma king that selection. A course is then suggested which contains the topics identified and has a duration of appro ximately one term.
requirements.
OBJECTIVES Most chemical engineering courses are inevitably oriented towards an analysis of the steady state operating condition of the process, especially during the early stages, This is reflected in the fact that the the basic defining equations, that is mass and energy balance equations,
equilibrium relationships and so on, are usually presented in their steady state form, The effect is that students beginning the control course think mainl y in terms of the steady state, with the result that the earl y part of the course must be de voted to imparting an appreciation of dynamic beha viour. A basic requirement then would be that the initial teaching should use the dynam i c form of such equations so that the relationship between equipment size, stream flowrates and the form and duration of the response is recognised at an early stage,
The chemical engineer comes face to face with serious plant control problems when he first enters the control room, the nerve centre of the plant. He will need to understand the meanlng of the displays and how these relate to plant operation in whatever form they may be; that is control co~puter schematic, faceplate, trace, gauge or other. He ma y well be tal d that no changes may be made to control loop parameters, with the possible exception of limited adjustments to certain set points, as that i s the job of the instrument engineer. He will, however, need to to be able to recognlse plant malfunction from the record or plot and, from an intuitive understanding of how the loop operates, decide whether this is a sticking valve, leaking pipe flange or some other cause,
The disturbances to which a plant is subject need to be considered ln some detail and those characteristics which ensure the natural suppression of :he disturbance recognised, The characterisation of d,sturbances which are not naturally suppressed and the design and sizing of eq uipment to take advantage of natural suppression should be familiar,
As a design engineer he will be concerned to size equipment for ease of control and decide on the overall plant control strategy. He will thus need to decide upon the principal loops for product quallty control, those necessary for the maintenance of stable operation and be able to liase with the professional control engineer to produce a robust reliable plant control system,
Having identitied those process outputs which need to be regulated and the sources of the major disturbances, the selection of the form of the control to be used follows. The choice of manual control should be considered in the case where information would be available to the operator but not from plant instrumentation, for example upstream plant operation variations or "eather changes or forecasts. The case for auto~atic control and whether this should be
The traditional control course, with its emphasis on transfer functions, frequency response diagra~s and detailed analysis of the TCI'IE-F
69
70
C. McDermott and A. Johnson
feedforward, feedback or so.e co~bination the two and finally the means of implementation ie. pneu.atic, electrical or control computer must be decided, the choice being based on the site conditions prevailing, number of loops required and econo.ic factors. The characterisation of good control and the .easures used in the design of optimal control systems, the system response to load and setpolnt changes, the behaviour of the loop as a filter for the various forms of disturbance and the presence of limits to loop stability should all form part of the chemical engineers background knowledge. A basic component in an y control course has always been the use of the transfer function and the mathematics of the closed loop. It is doubtful, in a course of this type, whether the benefit obtained justifies the time and effort involved. The attraction of the use of material such as frequency response diagrams, root locus plots and the closed loop transfer function lies in the insight it offers into loop behaViour and the control actions required. In courses for control and electrical engineers such material would be recognised as sufficiently basic to allow enough time fo r the necessary understanding to be aquired. Its in clusi on in a Chemical Engineeri ng course would mean displacing discussion of topics of Immediate relevance such as strategies for th e control of particular unit operations. In fact experience with past students has indicated that they are deterred from further study of control by the difficulty of traditional theo r y and its apparent remoteness from the realIties of
COURSE A suggested course, consisting of a total of thirty hours of lecture, tutorial and practical sessions, is proposed which goes some way towards meeting the above requirements. Of the available hours, eighteen are allocated for lectures and si x each for tutorial and practical classes. Lectures
1. The use of the unsteady state material or energy balance to pred ict the response of various simple process elements (tank level, hea ter etc. ) to step, ramp and other disturbances. Support with bench or simulation e>:periments.
2. The idea of a linear ( e xpo nential or RC) lag and the significance of the steady state gain and time constant. Methods of Identifying these from experimental observations. 3 . The steady state. Use of b l ock diagram representation for the combination ( multiplIcation ) of a sequence of stages, each represented in terms of the st eady state gain. Extension to the closed loo P! dimensionless open
loop gain and calculation of offset . 4. Nature of disturbances affecting a process. Dominant frequencies and the behaviour of the process as a filter. Natural disturbance suppression by the process and cases needing suppression by control actIon.
process control.
5. Types of control namely
All that can really be ju stif i ed here isthe basic idea of proportional feedback control, supported quantitatively perhaps in terms of a simple linear differential equation, and the qualitative introduction of int egral and derivative terms with sufficient mathematical treatment to demonstrate their effect on stability. It is desirable, of course, that some e xp erience is obtained of the tuning of a three term controller using say the Ziegler Nichols or
Manual - where compensation is needed when abnormal conditions are anticipated.
reaction curve method and this can easily be included in a laboratory session.
6. Simple state-space representation of the controlled system by means of a single first order linear ODE. Demonstration of offset fer proportional control and its e limina tion using the int egral term.
As indicated abo ve the difficulty ariSIng from a course concentrat i ng primarily on linear control
theory is that little reference is likely to be made to the process to be controlled. The basic strategies used for th e control of individual operations rel y on a sound understanding of pr incipl es governing the operation cf the process. In the cas e of distillation, for e xample, an analysis of the steady state behaviour leads to the conclusion that product COMposition can be controll ed by var yi ng product flowrate. The problems of loop tuning are l ess serious than those of the most appropriate choice of coupled var iabl es. The effect of variations in the flowrate of the chosen stream
On other process variables then needs to be predicted before the methods of linear control theory can be applied to produce the necessary compensation. It is at this stage that the necessity arises for the chemical engineer to liaise with the control engineer to produce a practical solution. This requires of the chemical engineer a kno wledge of control engineering terminology and an appreCiation of the techniques available .
Feedback - to correct for errors produced by unmonitored variables. Feedforward - to correct for changes in measured values of specific inputs and in combination with feedback.
7. Characterisation of system response. Integral SSQ or absolute de viation, decay ratio and damping, period and recovery time. 8. Tuning the three term cont ro lle r, the meaning of int egral and deri va tive action times.
Ziegler-Nlchois loop tuning and reaction curve methods 9. Means of implementation of control action. The case for single loop electrical and pneumatic de vices. Computer control , supervisory and DDC together with considerations of reliability and economics. 10. Multiloop control: ratio, cascade and split range, with e xamples of control of reaction, pH and a furnace 11. Interacting control loop s demonstrated using the e xample of dual control of flow and composition in a s impl e mixer. Simulation of the degradation of qualit y of control for the case of simultaneous control of temperature and stack oxygen in a furnace.
Control in a Chemical Engineering Course 12. The control valve. Construction of the valve and actuator, valve characteristic and valve sizing. 13. Applications to control of flow, temperature, pressure and other single loops. 14. Detailed consideration of a typical unit operation, for example the control of a heat exchanger. The strategies available require loops considered In the previous lecture with applications of cascade and ratio control. Also the use of the valve characteristic to compensate for the nonlinear response. 15. The process model. linear and nonlinear models and techniques available for compensation for nonlinearity and time delay. 16. State space and s-domain representation of multiloop systems. Block diagram representation for calculation of system response using transfer function representation of dynamic elements. 17. Use of the complex model with commercial Simulation and system deSign packages such as SIMNON or SYMBOL, available from Cambridge Control Limited. For simple Single loop systems the CONCADE package from the Department of Engineering, University of Manchester Cdn be used. 18. Review of more recent approaches to control such as optimal control, sampled data methods, self tuning and adaptive control.
duplication of many basic items of equipment. In a large department, laboratory accomodation and equipment is not always available on the scale needed to give every student sufficient hands on experience. In any case, the duplication of elementary facilities would represent an inefficient use of scarce resources.An effective substitute for some experimental work has been found In the use of a number of low cost personal computers and, as long as the hardware IS not displaced completely, the experience gained still proves to be valuable. The computers have been programmed to simulate various items of process equipment such as a storage tank for level control, an electrical heater for temperature and an evaporator for composition. Other less elementary applications include sampled data control of temperature, mulivariable control of a flash drum and control of pH to demonstrate nonl i neari ty. Although, with sufficient programming skill, it would be possible to produce s i mulations offering a choice of controlled and manipulated variables, thiS has not been undertaken. The simple versions referred to above are easy to produce and provide valuable e x perience at the early stages of the study of process control. The investigation of alternative control schemes has been made possible at Bi rmingham by the provision by IBM UK Ltd. of the IBM ACS IAdvanced Control System). This commercial computer control system gives students the opportunity to appl y control methods using industrial equipment on a realistic real time simulation.
Tutorials
Three two hour tutorials illustrating lecture material as follows : 1. Unsteady state balances on a heater and pure capacity tank to determine the transfer function. Evaporator feedback control specified In terms of steady state gains. Determination of offset under proportional onl y control. 2. Calculation of controller gains, integral and derivative times for first and second order processes producing quadratic characteristic equations. Decay ratio and period resulting from above calculated parameters. Design of second order system producing response with the required characteristics. 3. Cascaded loops: comparison of predicted response with and without the slave loop. Response of interacting loops in a simple case and comparison with those of the individual loops. Calculation of decoupler gain and the response of the decoupled loops.
Practic.ls
The nature and content of the practical or laboratory sessions accompanying such a course are inevitably dictated by the facilities available within the institution concerned. The requirement of such practical work is that it should be possible for all students on the course to carry out appropriate experimental work at each stage. This means extensive
The three practical sessions suggested are as follows, using SUitable equipment where possible and computer simulations otherwise: 1. For a single stage process in the open loop, to identify the response to step, ramp and sinus iod al Inputs. This Information to be used, where appropriate, to determine the time constant . With the loop closed and using proportional only control, observe the steady state response (offset) of pure capacity and self regulating processes when subject to load and set point changes. 2. Study of the effect on quality of control and loop stability of introducing integral and d~rivative terms into the controller. Application of the Zlegler Nichols reaction curve and loop tuning methods for determination of controller settings. 3. Use of the response of a second order system to calculate controller settings to give prescribed decay ratio and period of the response. Comparison of the response of two interacting loops with their individual responses. Comparison of the response of a cascade control system with that where the primary loop acts directly on the manipulated variable. In each session proviSIon should be made for access to ACS or similar terminals and to experimental equipment for as many students as possible.
71
C. McDermott and A. Johnson
72 DISCUSSION
Obviously no short course can do more than scratch the surface of control technology in whatever area it is applied. !n this attempt the absence of true chemical engineering applications is .till evident. The dtfftculty being that most such industrtal examples Involve problems which are outside the scope of an elementary course. Any realistic example does, however, involve numerous simple single loops and the behaviour of such loops needs to be appreciated. It is hoped that the above helps in producing such an appreciation and at the same time gives some hint of the interest and satisfaction which can accompany a further study of control. The primary object of the control course should be to encourage a genuine interest in pr OCESS control. If such an interest could be generated in the majority o f chemical e n gineers instead ef a specialist minority, plant effic i ency and operab i lity should be signiflcantly improved.