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Rolta Academy :Engineering Design Services

CHAPTER 6CONTROL

6.0 INTRODUCTIONControl of the processes in the plant is an essential part of the plant operation.There must be enough water in the boilers to act as a heat sink for the reactor butthere must not be water flowing out the top of the boilers towards the turbine. Thelevel of the boiler must be kept within a certain range. The heat transportpressure is another critical parameter that must be controlled. If it is too high thesystem will burst, if it is too low the water will boil. Either condition impairs theability of the heat transport system to cool the fuel.In this section we will look at the very basics of control. We will examine thefundamental control building blocks of proportional, integral and differential andtheir application to some simple systems.

6.1BASIC CONTROL PRINCIPLES

Consider a typical process control system. or a particular example let us look atan open tank, which supplies a process, say, a pump, at its output. The tank willre!uire a supply to maintain its level "and therefore the pump#s positive suctionhead$ at a fixed predetermined point. This predetermined level is referred to asthe set point "%&$ and it is also the controlled !uantity of the system.

Clearly whilst the inflow and outflow are in mass balance, the level will remainconstant. 'ny difference in the relative flows will cause the level to vary. (ow canwe effectively control this system to a constant level) We must first identify our variables. *bviously there could be a number of variables in any system, the twoin which we are most interested are+The controlled variable in our example this will be level.The manipulated variable - the inflow or outflow from the system.

If we look more closely at our sample system " ig ./$, assuming the level is atthe set point, the inflow to the system and outflow are balanced. *bviously nocontrol action is re!uired whilst this status !uo exists. Control action is onlynecessary when a difference or error exists between the set point and themeasured level. 0epending on whether this error is a positive or negative !uantity,the appropriate control correction will be made in an attempt to restore theprocess to the set point.

(enceforth, the error will always take the form of+Error 1 %et point - 2easured 3uantity

e 1 %& 2

Instrumentation Engineering

*4

1

" ./$

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Fig 6.1 Level Control Sy te!The control action will be either to vary the inflow or outflow from the system inorder to keep the level at the set point. 5et us consider the general format for

achieving these ob6ectives. 's can be seen from ig .7, the process can be represented by a closed loop.The system output "level$ is monitored by a process sensor and the measurementsignal is feedback to a comparator at the input of the system. The second inputto the comparator is the set point signal8 the comparator#s output being thedifference or error signal. The amplifier, a present 6ust a black box, will provide theappropriate correction to maintain the process at its set point despitedisturbances that may occur. It can be seen that if the system were beingoperated in manual control the feedback path would not be present. The operator would provide this feedback and apply the necessary correction to the system

whilst observing the effect on the controlled variable. This is termed open loopoperation.Fig 6."#

CLOSED LOOP PROCESS

6.1.1 Fee$%&'( Control

This concept 6ustifies the use of the word negative in three ways+

Instrumentation Engineering 2

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Rolta Academy :Engineering Design Services The negative aspect of feeding the measured signal backwards from the

output to the input of the system. "'ctual definition of negative feedbackcontrol$.

The control correction must be negative in that a correction rather than acompounding of error must occur.

The fact that an error must occur before a correction can take place, i.e.,retrospective or negative control action.

In the next section we will study in more detail the methods used to effect thenecessary control corrections.

6.1." Fee$ )or*&r$ ControlIf we wish to control our process without an error first occurring, we must baseour control on correction of the disturbances, which will eventually, cause aprocess error. This is termed feed forward control. eed forward control is rarelyif ever used on its own but is used in con6unction with feedback control to improvethe response of control to process disturbances.

6.1.+ S,!!&ry

Controlled 9ariable - output !uantity of system "5evel, Temperature, etc.$. 2anipulated 9ariable - means of maintaining controlled variable at the set

point.

Error signal - e!uals the difference between the set point and themeasurement. "e 1 %& - 2$. %et point - desired process level. "%&$ 2easurement - actual process level. "2$ Closed 5oop - automatic control. *pen 5oop - manual control. Feedback control is error correction following a disturbance. Feed forward control is control of disturbances, which could cause a process error.

6."ON-OFF CONTROL


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Fig 6 + Ty i'&l On-O)) Control S'/e!e

5et us consider our level control system in a little more practical detail. The valvein the inflow line to the system is an electrically operated solenoid valve."4emember an electrically operated solenoid valve has only two operatingpositions - fully open or fully closed.$ 'ssume that under initial conditions with ademand on the system the level will start to fall and 9 / will have to be opened toprovide an inflow. This can easily be achieved by mounting a differential pressureswitch, &/ at the bottom of the tank to operate when the level falls to 5 / . When thelevel is at 5 / the li!uid will be height h / above switch. The pressure at the switch

will be & / 1 :gh / .Where : - the mass density of the li!uid

g - the acceleration due to gravity

h / - the height of the li!uid

The resulting switch closure can energi;e the solenoid valve 9 / causing an inflowto the tank. 'ssuming the valve is correctly si;ed, this will cause a rise in thelevel back towards the set point.

In order to arrest the rise in level the built in differential feature of the switch canbe employed to de energi;e the solenoid valve when level 5 7 is reached. Thissystem will achieve a mean level in the tank about the desired set point. Thismethod is known as *

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Rolta Academy :Engineering Design Servicesas this will result in excessive cycling, and hence wear, of the valve. >sualpractice is to control with a dead band about the set point as shown in ig .?.

Fig 6. Ty i'&l ON-OFF Re on eThe sinusoidal cycling is typical of on=off control. on=off control can be used toadvantage on a sluggish system, i.e., where the periodic time is large. Typical

uses in C' units are electric heater controls in de aerator tanks and &(T%bleed condenser and pressuri;er. If fine control is re!uired a simple on=off controlsystem is inade!uate. We will discuss a method for achieving a finer control inthe next section.

6.".1 S,!!&ry@ *n=off control control signal is either AB or /AAB@ Control at set point not achievable, a dead band must be incorporated.@ >seful for large, sluggish systems particularly those incorporating electricheaters.

6.+ BASIC PROPORTIONAL CONTROLIn our example of on=off control it was seen that an all or nothing control correctionwas applied as the result of an error signal occurring. Clearly it would be to our advantage if the control signal were proportional to the magnitude of error. This isthe basis of proportional control and is the most fre!uently encountered controlmode. (ow can this control be achieved) 4eferring to ig . it can be seen that wecan modify our system to use a pneumatically operated control valve and a leveltransmitter with a 7A - /AA k&a pneumatic output.


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Fig 6. Level Control o) O en T&n(

If the outflow "3 o$ increases then the level in the tank will fall. The pressure sensed

by the level transmitter, which is representative of the level in the tank, will also fallcausing a decrease in the output signal from the level transmitter. This output signalis fed to the "air to close$ control valve "valve fully open with 7A k&a signal, fullyclosed with /AA k&a signal$. ' falling level will therefore cause the valve toprogressively open and hence raise the level in the tank. The system as shown issomewhat impractical as the initial set point conditions will need to be set by somemanual method and then ensuring that steady state conditions are achieved withthe valve at, say AB opening and a level transmitter output of A k&a " AB range$.

This simple system does illustrate however a ma6or disadvantage with proportionalcontrol.

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Rolta Academy :Engineering Design ServicesExample 7

'n alternative method of illustrating proportional control is by means of a simple floatsystem " ig . $. 'ssume the inflow and outflow are e!ual and the level is at the setpoint. If an increase in outflow occurs the level in the tank must fall. The float will also

fall as the level falls. This drop in float position will cause the valve on the inflow toopen more thus increasing the inflow. Eventually the fall in level will result in a valveopening, which will restore the mass balance between the in flow and the outflow

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6. ." Pr&'ti'&l Pro ortion&l Control ' more practical proportional control scheme can be achieved by inserting acontroller between the level transmitter and the control valve. This will eliminatethe setting up problems mentioned in the previous module "i.e., it will have a setpoint control$ and also introduce other advantages, which will be discussed inthis section.In a practical system one of the primary considerations is the failure mode of thevalve.In our example of an open tank with a valve on the inflow it would be reasonableto assume that the valve should close in the event of an air supply failure toprevent the tank overflowing, i.e., an air to open valve


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Fig 6.2 O en T&n( ControlTo achieve the necessary control action on, say, a falling tank level it is necessaryto convert the decreasing output of the level transmitter to an increasing inputsignal to the control valve. The level controller will perform this function and istermed an indirect or reverse acting "FG$ controller. It can be seen that if the valveaction had been chosen air to close, then this reversal would not have beenre!uired and a direct "FF$ acting controller could have been used. p to now we have only assumed proportionality constant or one, i.e., the controlsignal e!uals the input error. Is this always the best ratio) Consider the followinggraphs of input, output and level with respect to time+

Fig 6.3 Pro ortion&l Control Re on e C,rve

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Dy inspection it can be seen that a &D of /AAB is the same as a gain of one sincechange of input e!uals change in output. &D is the reciprocal of gain, expressedas a percentage. The general relationship is+

%mall values of &D "high gain$ are usually referred to as narrow proportional bandwhilst low gain is termed wide proportional band.

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Rolta Academy :Engineering Design ServicesFig 6.11 7 De'&y Re on e C,rve

With reference to ig ./A, consider a high gain system "say gain 1 A, &D 1 7B$.>nder steady state conditions with the process at the set point the inflow will have aconstant value. This is usually taken to be a control signal of AB for a proportional

controller with the process at the set point. In other words we have a AB controlcapability. With our high gain system it can be seen that the maximum control signalwill be achieved with an error of 1/B "control signal 1 gain x error$. This controlsignal will cause the valve to go fully open, the level will rise and the process willcross the set point. The error signal will now change sign and when the error againexceeds /B the resultant control signal will now cause the valve to fully close hencecompletely stopping the inflow. This process will be repeated continuously we havereverted to an on=off control situation with all the disadvantages previouslymentioned. *bviously there must be some optimum setting of &D which is a trade off between the highly stable but sluggish low gain system with large offset, and thefast acting, unstable on=off system with mean offset e!ual to ;ero. The accepted

optimum setting is one that causes the process to decay in a K decay method asshown in both ig ./A and .//.

The !uarter decay curves show that the process returns to a steady state conditionafter three cycles of damped oscillation. This optimi;ation will be discussed morefully in the section on controller tuning.

4ecall the output of a proportional controller is e!ual to+

m 1 ke where m 1 control signalk 1 controller gain 1 /AAB=&D

e 1 error signal 1 "%& 2$

Clearly if the error is ;ero the control signal will be ;ero, this is an undesirablesituation. Therefore for proportional control a constant term or bias must be added toprovide a steady state control signal when the error is ;ero.or the purposes of this course we will assume the steady state output of aproportional controller when at the set point to be AB. The e!uation for proportional control becomes+

m 1 ke b where b 1 bias "1 AB added to output signal$

C&l',l&tion o) o)) et

E8&! le# 'n air to open valve on the inflow controls level in the tank. When the process is atthe set point the valve opening is AB .'n increase in outflow results in the valveopening, increasing to a new steady state value of LAB. What is the resulting offsetif the controller &D is +

a$ ABb$ 7 B


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Rolta Academy :Engineering Design Services 'nswer+ to achieve correct control the controller will be reverse "FG$ acting.

a$ &D1 AB gain 17 Change in valve position 1LA A17AB This is the output change for the controller. Jain1 output= input 717AB= input

Input1/AB

%ince the controller is reverse acting measured variable must have been negativei.e.8 -/AB. This is e!ual to a error or a - offset

*ffset1 /AB below set point.

b$ &D17 B gain 1?

Input 1 B

*ffset 1 B below set point

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Fig 6.1" Re on e C,rve Pro ortion&l Control OnlyIf we wish to restore the process to the set point we must increase the inflow over and above that re!uired restoring a mass balance. The additional inflow mustreplace the lost volume and then revert to a mass balance situation to maintain thelevel at the set point. This is shown in ig ./ . This additional control signal mustbe present until the error signal is once again ;ero.

Fig 6.1+ A$$ition&l Control Sign&l Re tore Pro'e To Set ointThis additional control signal is known as 4eset action, it resets the process to theset point. 4eset action is always used in con6unction with proportional action.2athematically, reset action is the integration of the error signal to ;ero hence the

alternative nomenclature - Integral action.The combination of proportional plus reset action is usually referred to as &I control.The response of &I control is best considered in open loop form, i.e., the loop isopened 6ust before the final control element so that the control correction is not infact made. This is illustrated in ig ./?.

Fig.6.1 Pro ortion&l Pl, Re et O en Loo Re on e


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Rolta Academy :Engineering Design ServicesIt can be seen that proportional action will be e!ual to ke where k is the gain of thecontroller. 4eset action will cause a ramping of the output signal to provide thenecessary extra control action. 'fter time, say t, the reset action has repeated theoriginal proportional response8 this is the repeat time, the unit chosen for defining

reset action. It can be seen that increased reset action would increase the slope of the reset ramp.

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Rolta Academy :Engineering Design Services ' very slow reset rate will ramp the control signal up very slowly. Eventually theprocess will be returned to the set point. The control will be very sluggish and if thesystem is sub6ected to fre!uent disturbances the process may not ever be fullyrestored to the set pointM

If a very fast reset rate is used, the control signal will increase very !uickly. If we arecontrolling, say, a large volume tank, the level response of the tank may lag behindthe response of the controller.

The control signal will go to its limiting value "A or /AAB$ and the limiting controlsignal will eventually cause the process to cross the set point. The error signal willnow change its sign, and reset action will also reverse direction and !uickly ramp tothe other extreme.

This process will continue indefinitely, the control valve cycling, with resulting wear and tear, from one extreme to the other. The actual process level will cycle about

the set point. This cycling is known as reset windup and will occur if the process issub6ect to a sustained error and a too fast reset rate. The reset rate must bedecreased "reset time increased$.

The mathematical expression for & I control becomes+

&roportional control i.e., "proper sign of gain$ inputs a /NAO lag into the system "thecorrection must be opposite to the error$. 4eset action introduces a further lag. Thisfact must be taken into account when tuning the controller. "It follows proportionalaction$. The total lag must be increased and is now closer to AO. " AO lag meansthe feedback signal is now in phase with the input and adding to it the system isnow unstable.$ 4eset action causes the loop to be less stable.

6. .1 S,!!&ry

4eset action removes offset. It#s units are 4epeats per 2inute "4&2$ or 2inutes per 4epeat "2&4$


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The use of derivative control is limited. 't first glance, derivative control looksattractive. It should help reduce the time re!uired to stabili;e an error. (owever, itwill not remove offset. The control signal from derivative action ceases when theerror stops changing, which will not necessarily be at the set point.Its use, in practice, is also limited to slow acting processes. If used on a fast actingprocess, such as flow, control signals due to derivative action will often drive thecontrol valve to extremes following !uite small but steep "large de/dt $ changes ininput.Consider a simple flow control system, consisting of an orifice plate with flow

transmitter and s!uare root extractor plus direct acting controller and air to closevalve "refer to ig / $. This system is sub6ected to a small, but fast, processdisturbance. (ow will this control scheme perform under proportional and derivativecontrol modes)

Fig 6.16 Si! le Flo* Control Sy te!

To answer this !uestion let us consider the &0 response a fast change in process %ignal in an open loop system " ig /L$.


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Fig 6.12 O en Loo Re on e O) Pro ortion&l Pl, Deriv&tive :PD; A'tion toR& i$ly C/&nging Error Sign&l

The upper portion of ig /L shows a positive process excursion, 'D, from the ;eroerror condition, followed by an e!ual negative excursion, DC, which returns the error to ;ero.

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Rolta Academy :Engineering Design Services &roportional plus derivative "rate$ & 0. It is also possible to use a combination of all three control modes,

&roportional plus Integral plus 0erivative "& I 0$.

't a glance proportional only does not appear very attractive - we will get an offsetas the result of a disturbance and invariably we wish to control to a fixed set point.

'n application of proportional only control in a C' system is in the li!uid ;onelevel control system. The reason that straight proportional control can be used hereis that the controlled variable is not level but neutron flux. The manipulated variableis the water level8 therefore offset is not important as the level is manipulated toprovide the re!uired neutron flux.

In general it can be said that the vast ma6ority of control systems "probably greater than HAB$ will incorporate proportional plus integral modes. "We usually want tocontrol to a fixed set point.$ low control systems will invariably have & I control.

0erivative control will generally be limited to large sluggish systems with longinherent control time delays, "for example, that shown in ig /N.$. ' good generalexample is the heat exchanger. The thermal interchange process is often slow andthe temperature sensor is usually installed in a thermal well, which further slows thecontrol signal response. re!uently heat exchanger temperature controllers willincorporate three mode control "& I 0$ 6.3 T=PICAL NE>ATI5E FEEDBAC? CONTROL SCHE

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Rolta Academy :Engineering Design Servicesbe reverse acting and will usually have & I modes. The system is shown in ig /H

If it is necessary to mount the valve in the outflow, the best failure mode would

probably be to fail open "'=C$. This valve action would re!uire an increasing signal tohalt a falling tank level8 again a reverse acting "& I$ controller is necessary.

The same reasoning would apply to closed tank or bubbler systems, the onlydifference being in the sensing method employed. 4emember control modes use of derivative action on large, slow, systems.

Fig 6.14 O en T&n( Level ControlFlo* Control

Fig 6."0 Ty i'&l Flo* Control ' typical flow control system re!uires some form of restriction to provide a pressuredifferential proportional to flow "e.g. orifice plate$ plus a s!uare root extractor toprovide a linear signal. The controller action depends upon the choice of controlvalve. If an air to open valve is chosen then controller action should be reverse, asan increase in flow must be countered by a decrease in valve opening. or an air toclose valve the action must of course be direct. The general format is shown in ig 7A.

The control modes will be proportional plus integral "never use derivative on aflow control loop$.


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Rolta Academy :Engineering Design ServicesThe general problem with temperature control is the slowness of response. or this reason the use of derivative action is fairly standard. ig .7 shows arepresentative heat exchanger, which cools hot bleed with cold service water.The choice of control valve would probably be air to close, i.e., fail open, to give

maximum cooling in the event of a air supply failure to the valve.

Fig 6."+ Te! er&t,re Control o) He&t E8'/&nger

'n increase, say, in bleed temperature re!uires a larger valve opening, i.e.,smaller valve signal. ' reverse acting controller is re!uired. Three mode, & I 0, control is fairly usual.

6.4

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Rolta Academy :Engineering Design ServicesWhen tuning a control loop, it is important to take into account the presence of multivariable loops. The standard procedure is to tune the secondary loop beforetuning the primary loop because ad6ustments to the secondary loop impact theprimary loop. Tuning the primary loop will not impact the secondary loop tuning.

6.4.1 FEEDFOR@ARD CONTROLFeed forward control is a control system that anticipates load disturbances andcontrols them before they can impact the process variable. or feed forwardcontrol to work, the user must have a mathematical understanding of how themanipulated variables will impact the process variable. ig .7 shows a feedforward loop in which a flow transmitter opens or closes a hot steam valve basedon how much cold fluid passes through the flow sensor.

Fig 6." Fee$ For*&r$ Control 'n advantage of feed forward control is that error is prevented, rather thancorrected. (owever, it is difficult to account for all possible load disturbances in asystem through feed forward control. actors such as outside temperature,buildup in pipes, consistency of raw materials, humidity, and moisture contentcan all become load disturbances and cannot always be effectively accounted for in a feed forward system.In general, feed forward systems should be used in cases where the controlledvariable has the potential of being a ma6or load disturbance on the processvariable ultimately being controlled. The added complexity and expense of feedforward control may not be e!ual to the benefits of increased control in the case

of a variable that causes only a small load disturbance.

6.4." FEEDFOR@ARD PLUS FEEDBAC?Decause of the difficulty of accounting for every possible load disturbance in afeed forward system, feed forward systems are often combined with feedbacksystems. Controllers with summing functions are used in these combinedsystems to total the input from both the feed forward loop and the feedback loop,and send a unified signal to the final control element. ig .7 shows a feed


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Rolta Academy :Engineering Design Servicesforward plus feedback loop in which both a flow transmitter and a temperaturetransmitter provide information for controlling a hot steam valve.

Fig 6."6 Fee$ )or*&r$ l, Fee$%&'( Control Sy te!This module has discussed specific types of control loops, what components areused in them, and some of the applications "e.g., flow, pressure, temperature$they are applied to. In practice, however, many independent and interconnectedloops are combined to control the workings of a typical plant. This section willac!uaint you with some of the methods of control currently being used in processindustries.

6.4.+ CASCADE CONTROLCascade control is a control system in which a secondary "slave$ control loop isset up to control a variable that is a ma6or source of load disturbance for another primary "master$ control loop. The controller of the primary loop determines theset point of the summing controller in the secondary loop " ig .7L$.


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Rolta Academy :Engineering Design ServicesFig 6."2 C& '&$e Control

6.4. BATCH CONTROLBatch processes are those processes that are taken from start to finish inbatches. or example, mixing the ingredients for a 6uice drinks is often a batch

process. Typically, a limited amount of one flavor "e.g., orange drink or appledrink$ is mixed at a time. or these reasons, it is not practical to have acontinuous process running. Datch processes often involve getting the correctproportion of ingredients into the batch. 5evel, flow, pressure, temperature, andoften mass measurements are used at various stages of batch processes.

' disadvantage of batch control is that the process must be fre!uently restarted.%tart up presents control problems because, typically, all measurements in thesystem are below set point at start up. 'nother disadvantage is that as recipeschange, control instruments may need to be recalibrated.

6.4. RATIO CONTROL

Imagine a process in which an acid must be diluted with water in the proportiontwo parts water to one part acid. If a tank has an acid supply on one side of amixing vessel and a water supply on the other, a control system could bedeveloped to control the ratio of acid to water, even though the water supply itself may not be controlled. This type of control system is called ratio control ( ig.7N$. 4atio control is used in many applications and involves a controller thatreceives input from a flow measurement device on the unregulated "wild$ flow.The controller performs a ratio calculation and signals the appropriate set point toanother controller that sets the flow of the second fluid so that the proper

proportion of the second fluid can be added.4atio control might be used where a continuous process is going on and anadditive is being put into the flow "e.g., chlorination of water$.

Fig 6."3 R&tio Control

6.4.6 SELECTI5E CONTROLSelective control refers to a control system in which the more important of twovariables will be maintained. or example, in a boiler control system, if fuel flowoutpaces airflow, then uncombusted fuel can build up in the boiler and cause anexplosion. %elective control is used to allow for an air rich mixture, but never a


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Rolta Academy :Engineering Design Servicesfuel rich mixture. %elective control is most often used when e!uipment must beprotected or safety maintained, even at the cost of not maintaining an optimalprocess variable set point.

6.4.2 FU = CONTROL

Fuzzy control is a form of adaptive control in which the controller uses fu;;y logicto make decisions about ad6usting the process. Fuzzy logic is a form of computer logic where whether something is or is not included in a set is based on a gradingscale in which multiple factors are accounted for and rated by the computer. Theessential idea of fu;;y control is to create a kind of artificial intelligence that willaccount for numerous variables, formulate a theory of how to makeimprovements, ad6ust the process, and learn from the result. u;;y control is arelatively new technology. Decause a machine makes process control changeswithout consulting humans, fu;;y control removes from operators some of the

ability, but none of the responsibility, to control a process.

6.10 APPLICATION

Ai! &n$ o% e'tiveThe aim of these notes is to provide some basic ideas and rules that may beused to select a distillation control strategy. %eparate notes will discuss morecomplex mathematical techni!ues that may also be used as part of a PtoolboxP of methods that have evolved as aids in distillation control strategy selection.

Intro$,'tion

The effective operation of a binary distillation column is determined by the controlof many variables. Jenerally, the variables in table / need to be controlled.

T&%le 6.1 Ty i'&l 5&ri&%le t/&t /&ve to %e !&int&ine$ in & $i till&tionCol,!n

The two main disturbances that affect a column are+ eed flow rate, eed composition, ; f


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Rolta Academy :Engineering Design Services%o called Pmanipulated variablesP are ad6usted to counter act the effect of disturbances and ensure desired operation. Dut what are the manipulatedvariables)

4elationships between inputs "mvPs and dvPs$ and outputs "cvPs$ are !uantified bysteady state material and energy balances. To simplify preliminary discussionsconsider Qperfect control# of pressure "i.e. the energy balance e!uations are notconsidered$.

Ste&$y t&te !&teri&l %&l&n'e &ro,n$ & $i till&tion 'ol,!nThe following ig .7H is a material balance diagram for a typical distillationcolumn+

Fig 6."4 Ste&$y St&te

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Rolta Academy :Engineering Design Servicesor a binary column, the two independent overall balances are+

The total material balance+ 1 0 D The component balance+ ; f 1 0x 0 Dx D

Eliminating either D or 0 from these e!uations gives the following+

The e!uations " $ define the RcutS, i.e what percentage of the total feedflow exitsthe column as distillate and bottoms product for specified inlet and outletconcentrations.

rom e!uations " $ it is apparent that distillate "0$ and bottoms flow "D$ arerelated to top and bottom product compositions "x 0 and x D$ and are thereforepotential manipulated variables. 's expected, changes in and ; f will also affect

x0 and x D.

'round the condenser and accumulator assuming a total condenser, the materialbalances are #

The material balance+ 9 n 1 0 5 n " .7$ The component balance+ 9 nyn 1 0x 0 5 nx0 " . $

'nd around the reboiler, The material balance+ 5 m 1 9 b D " .?$ The component balance+ 5 mxm 1 9 byb Dx D " . $

or a li!uid feed 5 m 1 5 n

'ssuming that the molar flows of li!uid and vapour are constant through thecolumn "constant molal overflow$ then,5 1 5 n 1 5 n/ etc.9 1 9 n 1 9 n/ 1. etc.

Therefore+ 0 1 9 5 and D 1 5 9 " . $

E!uations " $ demonstrate that 0 and D may be used to regulated x 0 and x D,based upon the relationships "e!uation N$ it is obvious that 5 and 9 will also affectthe product compositions.

S,!!&ry# the potential manipulated variables for product compositions are D,B , L and 5 .

Col,!n 'ontrol tr&tegie :&n intro$,'tion; ' Qbottom - up# approach should be adopted whereby variables that are essentialto operation are regulated before !uality variables. In other words, pressure and

Instrumentation Engineering 3!

:6.1;

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Rolta Academy :Engineering Design Servicesthen level must be ade!uately controlled before attention is focused on control of composition.

Pressure control

4e!uired as a change in pressure will affect relative volatility "U$, the temperaturedifference across the reboiler and condenser as well as process safety. 'common pressure control loop is shown below+

Fig 6.+0. A 'o!!on re ,re 'ontrol loo :PC re ,re 'ontroller;.

(ere, pressure is regulated using the flow rate of coolant to the condenser.Increasing or decreasing the water flow rate will alter the temperature of thecondensing li!uid and hence the amount of vapor in the column. This, in turn,alters the pressure in the column. This will be a slow loop as the dynamics effectsof the cooling can be slow in comparison to simply venting the system by e.g.opening a valve the ig . A also shows this option, which may be re!uired as a

safety mechanism, in case a situation of excessive pressures arose$.Level control There will be two level loops on a distillation column as+

The column base level must be maintained at an acceptable value. The reflux drum level must be maintained at an acceptable value.

The possible schemes that may be employed to do this are summari;ed below+


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T&%le 6." Po i%le Level Loo Control

Re& on */y tr&tegy i not r&'ti'&l#

eed low would not be used to control reflux drum level. Dottoms flow would not be used to control reflux drum level. 9apor flow would not be used to control reflux drum level. This scheme violates the mass balance relationships therefore cannot be

used (the reason why will be explained later in the notes) . 0istillate flow rate would not be used to control level at the column

base.


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Rolta Academy :Engineering Design Services ' flow controller has been placed on the distillate line (to ensure steady

flow of product)"

Configuring a control strategy+ scheme III "a material balance control scheme$


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Ensure that the material balance is maintained around the column base.4ecall that 1 0 D so if, for a constant , 0 changes then there must bean e!ual and opposite change in D or the level in the base of the columnwill either drop or start to increase. To ensure that the level remainsconstant "and that an appropriate change is made to D$ a level controller

is re!uired with its manipulated variable being D.

This control scheme corresponds to scheme III and is one of the more popular control schemes. It is often referred to as a material balance control scheme .

E8&! le "# S, o e t/&t t/ere i & !&ll %otto! )lo* r&te :B; &n$ & l&rge$i till&te )lo* r&te :D;.

>sing this information the strategy may be developed as follows+ low control the bottoms flow "D$. Ensure that the material balance is maintained around the column, 1 0

D.

or constant , if D changes there must be an e!ual and opposite change in 0 or li!uid inventory will change "e.g. level may rise in the reflux drum, column base,or both$. To maintain constant inventory, a level controller is used to make anappropriate change to 0. @ ensure that the material balance is maintained aroundthe column base. 4ecall that at the column base 5 D 1 9, for a constant and 5, if D changes then there must be an e!ual and opposite change in 9 or thelevel in the base of the column will either drop or increase. To ensure that thelevel remains constant "and that an appropriate change is made to 9$ a levelcontroller is re!uired "the mv being 9$.

This control scheme corresponds to scheme II and it should be noted that thecontrol of level using 9 may have weird dynamic effects and therefore is not afavorite. 'gain, this control scheme is often referred to as a material balancecontrol scheme .

R,le o) t/,!% "# Qmaterial balance control scheme "III$ should be favored if there is a large reflux ratio, i.e. "5=0$ V # :i) L i l&rge in comparison to D t/enrel&tively !&ll '/&nge in L *ill en ,re goo$ level 'ontrol i.e. t/e ro'eg&in i l&rge;.

R,le o) t/,!% +# Qcontrol scheme "I$, often referred to as the energy balance

control scheme, should be favored if there is a small reflux ratio, i.e. "5=0$ /# :i) L i !&ll in comparison to D t/en rel&tively !&ll '/&nge in D *ill en ,regoo$ level 'ontrol i.e. t/e ro'e g&in i l&rge;.Co! o ition 'ontrol

*n line analy;ers are rarely used as the installed cost will normally be in therange of X/AA Y per instrument. Therefore composition is often regulatedindirectly using temperature "at constant pressure there is a direct relationship

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between temperature and composition for a binary mixture$. >sing a li!uidtemperature near the base of the column for bottom composition and a li!uidtemperature near the top of the column for top product composition, theremaining mv#s "i.e. those not used for the purposes of level and pressurecontrol$ may be used to regulate composition. This leads to the following

schemes+S'/e!e : I ;

Top product composition "through a li!uid temperature near the top of thecolumn$ is regulated by ad6usting reflux flow, 5.

Dottom product composition "through a li!uid temperature near the bottomof the column$ is regulated by ad6usting vapour flow, 9 "indirectly viasteam flow$.

This gives rise to an alternative name for this control strategy+ t/e L5'on)ig,r&tion .

Composition Control+ scheme / "the energy balance control scheme$

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Fig 6.+ S'/e!e :II; inventory 'o! o ition 'ontrol :PC re ,re'ontrol LC level 'ontrol FC Flo* 'ontrol &n$ TC te! er&t,re'ontrol;. T/i '/e!e i &l o (no*n & t/e LB 'on)ig,r&tion.

S'/e!e : III ;

Top product composition "through a li!uid temperature near the top of thecolumn$ is regulated by ad6usting distillate flow, 0. Dottom product composition "through a li!uid temperature near the bottom

of the column$ is regulated by ad6usting vapour flow, 9.This gives rise to an alternative name for this control strategy+ t/e D5'on)ig,r&tion .

Composition Control+ scheme III "a material balance control scheme$

Fig 6.+6 S'/e!e :III; inventory 'o! o ition 'ontrol :PC re ,re'ontrol LC level 'ontrol FC Flo* 'ontrol &n$ TC te! er&t,re'ontrol;. T/i '/e!e i &l o (no*n & t/e D5 'on)ig,r&tion.

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@or(e$ e8&! le# & !et/&nol - *&ter 'ol,!n.A = A wt B methanol = water mixture is to be separated in a /A stage column.The feed rate is kg=hr entering at stage . The ob6ective is to separate themixture into a top product of H wtB methanols and a bottom product of wt B.

The feed is li!uid at its boiling point. The condenser is a total condenser. Thereflux flow is kg=hr.a$ What are the materials flows through this system "external li!uid andinternal li!uid and vapor flows$)b$ %uggest a possible control strategy for this column.

4ules of thumb, common sense and a basic knowledge of chemical engineeringcan generally be used to specify an appropriate manipulated variables and hencethe control scheme of a distillation column. (owever, this basic knowledgeshould also be complemented by rigorous systems analysis. To do this it isnecessary to consider distillation column modeling in greater detail.

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