Batch Process Control Using a Digital Computer

Batch Process Control Using a Digital Computer

BATCH PROCESS CONTROL US ING A DIGITAL COMPUTER p. ALLEN C.L. PHILLIPS LECTURER Dept. of Chemical Engineering Loughborough University Loughborough ...

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BATCH PROCESS CONTROL US ING A DIGITAL COMPUTER

p. ALLEN

C.L. PHILLIPS

LECTURER Dept. of Chemical Engineering Loughborough University Loughborough ENGLAND

WORKS D!RECTOR Scott Bader & Co. Wellingborough

ABSTRACT The use of a digital computer for the control of batch reactors, used in the manufacture of unsaturated polyester resins, is described. The reason for the project, objectives and economic justification for a computer system are given . A reactor is fitted with packed distillation column and variable surface partial condenser for removal of wa·ter from the system . Heating is by means of hot oil circulated through internal heating coils. The processing cycle is discussed The control scheme and hardware necessary for its implementation are dealt with . Conculsions are made regarding hardware costs and the effects of thesystem, upon product quality and processing times.

of the batch so that saleable material is produced in the equipment provided . This kind of development does not seek to establish the causes of differencies brought about by differencies in scale between laboratory and plant size equipment, nor is meaningful data produced which could be used to improve plant design . Any proposal which sets out to deal with the situation described has to satisfy three main objectives . They are: A. Improve the consistency of the product. B . Ensure that the necessary supervision and control of batches is achieved by the most economical method . C. Collect data to increase knowledge concerning the processes, improve operating strategies and improve plant design. Three options were considered for improving plant operation: 1. Improve the quality of present instrumentation and increase the level of reactor supervision if necessary by assigning one person to each reactor for this purpose . 2. Provide normal controllers with a sequential device such as a punched card controller on all reactors. 3. Install a computer to supervise and control all reactors. A data logging exercise has revealed that it is unlikely that manual control of a reactor , following a strategy embodied in operating instructions is possible. In fac t gross departures from desirable conditions have been observed. Often these departures have occurred at critical stages in the batches history. The investigation has shown that meaningful information has to be derived from plant measurements if suitable strategies are to be operated in our situation. Finally it is imperative that the process is managed in accordance with operating instructions and that this fact is established beyond doubt. The technical decision is clearly in favour of a computer because of its power and flexibility as an instrument for control but also as a tool for process development. The economic justification has been made by comparing the cost of the three options listed above. Interest on capital is charged at 10 per cent per year and depreciation is c harged at 15 per cent per year. Certain assumptions have been made concerning the cost of

1 . INTRODUCTION Polyester resins of the type used in the manufacture of boats etc . are manufactured by batch processes . The reactants comprise one or more glycols and two acids. The process converts the reactants into a polymer which is dissolved in a special solvent . The properties of the resin result from the particular chemical species produced as well as the distribution of chain lenghts of the polymer. Characterisation of the polymer structure presents formideble difficulties and the effects of process variables on the structure cannot be determined quantitatively . The physical and chemical properties of a product produced by a customer is dependent upon the consistency of the resin system and the care exercised by the customer's employees when hand lay-up methods are used. The consistency of the resin system is all important when mechanised moulding equipment is used . In both cases there is an increasing demand for consistency and improved quality. Resin manufacturers often find that a significant proportion of the batches produced do not meet internal specifications. These batches are 'doctored' and/or released to selected customers who can use the material satisfactorily . Resin production is usually an attempt to produce on the plant scale the identical product to that made in glass ware in the laboratory. The process instructions are designed to tailor the batch formulation and the management

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professional advice maintenance contracts and human resources to be acquired. No credit is assumed for cash benefits resulting from improved consistency or quality, in the form of enlarged markets or market share .

start for a one-stage reaction and in the case of a two-stage reaction there is a second bulk addition at a suitable point in the processing cycle. 2.2. The processing cycle

CUMULATIVE CASH OUTFLOW Year

Option I

Option 2

Option 3

1 2 3 4 5

24,000 51,000 81,000 114,000 148,000

38,000 54,000 77,000 111,000 131,000

57,000 64,000 82,000 102,000 122,000

The table shows that the computer option becomes favourable financially after three years. The three options do not however offer the same long term benefits indeed it is true to say that the computer proposal is the only one which satistEs marketing technical and financial objectives simultaneously . The proposal offers a dramatic leap forward at a low cost relative to the value of process plant which is to be controlled . The initial outlay has been estimated to be £43,000 which is less than 10 per cent of the value of the plant which is required to control.

2. PROCESS DETAILS 2.1. '!he Plant The plant building houses six reactors which are similar to each other in layout but vary in size. Three reactors are of 5,000 Ibs. capacity and three of 20,000 Ibs. A typical unit is depicted in Fig . 1. The reaction vessel is equipped with an outside jacket, through which cooling water can be passed and an internal helcial wound heating coil. The heating medium being hot oil which flows through the coil with constant velocity but at a temperature that can be varied to give the required heat input. The contents of the reactor are well mixed by means of an agitator. A fractionating column is mounted directly on top of the reactor and the column is surmounted by a partial condenser. The partial condenser consists of a shell and tube heat exchange with the tubes in a vertical position. Cooling water is passed through the shell countercurrent to the vapour in the tubes, and the level of water in the shell can be varied. The vapour leaving the top of the partial condenser is totally condensed in a second shell and tube heat exchange mounted with its tubes horizontal. The condensate from this exchange being collected in a small measuring tank. The whole system can be operated either at atmospheric pressure or under vacuum and is continually purged with nitrogen gas. The feed to the reactor contains solid and liquid reactants. Liquid reactants are charged by first weighing out the quantity required, in a holding tank mounted on a weighing platform; and then feeding under gravity to the reactor. Solid material is manually charged by emptying an appropriate number of weighed bags into the reactor. There is no continuous feeding of reactants, the appropriate quantities are all charged at the

The chemical reaction takes place in two stages. First of all a half ester is formed which then reacts to produce the unsaturated polyester. In the second reaction water is formed which, unless it is removed from the reaction mixture, will prevent this reaction going to completion. The purpose of the fractionation column, described in 2 . 1 is to enable the water to be removed from the reaction system by distillation without loosing any other reactants. The chemical reactions are such that at one point they are exothermic and, further on in the cycle endothermic. Unsaturated polyester resins are produced by following one of two possible processing cycles. These are known as one stage and two stage processes. In a one stage process the reactants are all charged at the start. The temperature of the batch is then raised until distillation starts after which the batch temperature rises slowly as the reaction proceeds, until an upper limit is reached after which the heat input must be restricted. In a two stage process one of the reactants is added after the initial chemical reaction has taken place. In order to do this the reactor must be forcibly cooled, the reactant added and the temperature raised again. It is necessary to control the heat input to the reactor within narrow limits if the batch temperature is to follow a specified profile. In addition, for most of the time the relationship between heat input and batch temperature is non-linear . During some of the exthermic period's, no heat at all is necessary and it is imperative to detect the onset of an exothermic period. If this is not done quickly enough then the acceptable temperature for that part of the process cycle will be exceeded. The result of this being degradation of the final product and an ex·tended reaction time. 2.3. Operating conditions The following constraints are given for the operation of the plant. (i) batch temperature gradients up to the initial boiling point of the batch. A typical figure is 40 0 C per hour. (ii) maximum reaction temperature for each stage of the process. Typical figures for a two stage 0 0 process are 195 c for the first stage and 220 C for the second stage. (iii) temperature of the vapour leaving the partial condenser. Normally 102 C. (iv) maximum heat during the distillation period (v) conditions under which vacuum may be applied . (vi) termination conditions for the reaction. At the start the reactor temperature gradient is proportional to the heat input. Once the formation of the half ester begins there is an exothermic reaction taking place and the linear relationship no longer holds. Forgation of half esters can be significant at 40 C and last up to 0 135 C. The magnitude of the exotherm depends

306

COOLING

TOT ......

l.EVEl.

W,uu

COHQENSER

INDICATOR

f

~

~

OIL

~

~

IN

Z

0

V

Q ~

~

OIl. OUT

t--~:---- COOliNG

WATER: WATER ~ETUIO'N

Fi gure 1,

General Plant Layout .

PRXORAM

COMPU TER

Q~; FO Co

e

QO- Q'O

Pc

K,e + K2

ter

clp

OIL PUMP

Figure 2,

T2 )

ORIFICE & CELL

OIL VALVE

MAIN OIL LOOP

(1j -

FROM HEAT BALANC E

Oil Hea ti ng Control.

307

Q~ = Fo Co (T 2 - T l ) (3.4 . ) where:P is valve position at the 'n'th sampling point n Kl and K2 are controller constants

upon the type of resin being manufactured. Once the exotherm has subsided there is again a linear relationship between heat input and temperature rise. This holds until the total vapour pressure in the reactor reaches 760 mm. when boiling starts. When boiling of the half ester and unreacted glycols begin the second stage of the polymerisation proceeds and the concentration of unreacted glycols decreases . At this point variations in heat input causes variations in the quantity of material vapourised rather than varying batch temperature . As the conversion nears 100% the quantity of vapouri.sed material decreases until boiling ceases. It is at this point, that vacuum is applied in order to remove the last traces of water . When vacuum is applied, no further control is exercised over the temperature of vapour leaving the partial condenser. The end of the processing cycle is determined by sampling the contents of the reactor and carrying out a straightforward chemical analysis.

e

Q o 1

Q F

~

e

Q

n

1

o

- Q

o

n

is the temperature of oil leaving the coil .

TB

is maximum allowable reaction temperature.

TB

is current value of reaction temperature. o

is the value of heat input for a satisfactory boil-up rate.

3.2 . Control of the Partial Condenser The partial condenser, see fig . 3 has a heat transfer area that can be varied b y varying the water level on the shell side . It is vital that the level is such that the exit water temperature is as high as possible . This is to reduce the degree of sub-cooling of reflux returned to the packed column . If the level is too high , however, the exit vapour temperature oscillates. The control strategy is an automated version of a manual procedure. There are two control loops which must be brought into "automatic" by the computer each time distillation begins . An initial value for water level is obtained from previous experience c orrelating suitable values with a given heat input . The cooling water flow is then adjusted, within two levels of constraint , to control the vapour temperature, T . The flow diagram is given in fig . 4 Once ¥ reaches lOOoC, the value of heat input determines an initial value for water level and cooling water flow . When T has reached steady state then the water control loop is closed using the steady state value for T as its set point . This value 0 is moved towards ¥he desired value in 0.5 C steps but water flOW, ~w ' falls below 200 gals/hr . then water level, H, 1S decreased . If F rises above 1,000 gals./hr . then H is increased~ If Hand F

Using

and the required value for dT B

K2 ~ e

T2

Ql

dt

Kl en +

is the temperature of oil entering the coil .

1

a new value for Q is arrived at. If this is negative then theOoil valve is closed . Otherwise it is used as the set point for the feedback algorithm controlling the oil valve. The control loop for the oil valve, see fig. 2 , is based upon a conventional proportion plus integral action control algorithm. Pn

Tl

Le

It is reasonable to ignore heat loss at this stage since the reactor is well insulated . At any given time we can measure dT and Q so o B dt

this value for

is specific heat of the oil.

The oil flowrate is obtained by linearising the differential pressure from the orifice plant, Suitable values for Kl and K2 were obtained by 2 minimising after step changes in set point. (1). Once distillation is taking place then equation 3 . 1. is no longer valid. The value for Q is then fixed at a level that results in the mgximum allowable boil up rate, for the reactor in question, subject to the constraint that there i ~ a maximum permissible reaction temperature. Once this maximum is reached then Q is adjusted o by a pruportional action . l K(TB TB) + Qlo (3 . 5 . ) Q o where:K is a proportional constraint

(it

can be obtained .

is the actual heat input. is the oil flowrate through the heating coil.

C

0

A temperature gradient is specified for the start of the batch cycle when the batch temperature is raised from ambient to the point at which the reactor start boiling. This point being detected by the rapid rise in temperature at the top of the partial condenser. If the initial exotherm is enough to cause the temperature to rise too fast then the oil valve must close completely. For this initial period the following heat balance can be written MC dTB Q oil + Q reaction (3.1.) dt where:M is the mass of reactants C is the average specific heat is the rate at which heat enters the Q reactor from the heating coil. is the rate at which heat is released by the reaction. dT is the batch temperature gradient B

~

0

0

3 . THE CONTROL SCHEME 3.1. Heat input

therefore a value for

is the error signal at the 'n'th sampling point. is the required heat input .

n

(3.2. ) (3 . 3 )

w

308

Tv FROM OPERAltlRS 10 TOTAL

PAtJEL

- - c.oNDENSER

COOLING WAlER

VAPOUR FROM COLUMN

H FROM LEVEL Sl6R0UT1N£ Fi~re

3.

Partial Condenser Control System.

F H "100

T

w< 200

F

T TO Tv

CON"TROL

AU50RITHM

H=O

T

F" =0 F

To Tv Fip;ure 4.

AdjustJnent of Level in the Partial Condenser.

CONTRoL ALGoRITHM

INT1:RFAC£

Fi.'(ure S.

309

Block DiagT
T

SIGNAL 10 APPLY VAC.

becomes zero then the operator is instructed to apply full vacuum and the partial condenser control is by-passed . The vapour temperature and water flow control loops are standard types but level control is different. Since level is controlled by varying the size of outlet restriction a very high controller gain can be used . The algorithm is a two-stage one. Whilst the level is within + 5% of set point then a high gain proportional controller is used . If the error is greater than this then the outlet valve is fully opened, or shut, until the + 5% band is entered. 3.3.

armoured cables . A centralised constant current power supply is switched through one pair of wires to each thermometer as it is called, the potential drop across the thermometer being measured by connecting the digital voltmeter via the remaining pair of wires . A backing off voltage is applied and the voltmeter output scaled to degrees centigrade by means of the linearisation unit . Liquid flowrates are measured by means of orifice plates, the differential pressure across the plate being converted to an electrical signal . Whereas temperature measurement requires the digital voltmeter to operate on the 0 to lOOm V range, the signals such as differential pres~ require the 0 to 10v range. Range switching, accomplished via the pin board programming system, eliminates the need for amplifers . The interface between the logger and digital computer is achieved by using the computer manufacturers standard device i.nterface. A block diagram is given in fig. 5

Weighing system

At the present time the weighing out of raw materials is still done by operators. Since however, all the material can be in liquid form it would be a straightforward matter to use the computer to carry out weighing and charging . This would eliminate faulty weighing and, since the only number of loadcells plus on/off valves are required, it would be reasonably cheap to implement.

4 . 3 . Communicating with the system

The system is constructed round two main subsystems. These are a POP 11 computer (2) and a data logger (3)

There are three devices for communicating with the control system. 4 . 3. 1. plant operator's panel 4.3 . 2. 24" typewriter 4.3 . 3. ASR 33 teletype set In addition a detailed record of plant operating conditions is maintained via a paper tape punch.

4 . 1 . The PDP - 11 computer

4 . 3.1.

This machine is an example of a new class of 16bit digital computers that has only recently become available. As far as this project is concerned it is this type of machine that makes it feasible to use a digital computer in the control system. The features of these computers which we consider are important are low cost, powerful instruction set, adequate speed and quick delivery. Examples of costs are as follows . A central processor, rack mounted, with 4K of random access store and ASR 33 teletype may be bought for £5,730. Additional core store costs £1 . 770 each 4K block . There is a wide range of backing stores and input/output equipment that can be readily added to a basic system. Also a comprehensive range of sub-modules and constructional ironmongery for those capable of engineering their own interfacing equipment between plant transducers and the central processor .

So that only a minimum of knowledge is required before an operator can use the system, each of the six reactors has been provided with its controls wherever reasonable. Each reactor is allocated the following switches to give information to the computer, such as START/STOP processing, end of stage I, start stage 11, batch finished. The computer can signal to the operator via the following lights Which are also duplicated for each reactor. These are, apply vacuum, heating failure, oil pump on/off. Three sets of digital switches are used to insert the following constraints. The applicable reactor being indicated on another switch. Required vapour temperature, maximum reaction temperature in stage I, maximum reaction temperature in stage 11. A number of general lights can be turned on by the computer such as Computer system O.K., oil main temperature failure, water main failure etc.

4 . 2 . The Compa c t Series 2 Logger

4 .3 . 2 .

This was originally used to obtain data on plant operating conditions and identify the control problems but it can also be used as the analogue input system for a digital computer . This is essentially a conventional data logger features. These are temperature measurement, signal linearisation, pinboard programming system and good noise rejection . A limitation of the system is that it can only operate at speeds up to a maximum of 10 signals per second but in our case this was considered adequate . All temperatures are measured by means of resistance thermometers connected to the compact logger by 4 - wire

This is used for printing the working logsheet for the whole plant. A print out occurs for all reac tors, once every 10 minutes, and lists such information as heat input , batch temperature and temperature of vapour leaving the partial condenser .

4. COMPUTER HARDWARE AND PROCESS INSTRUMENTATION

4 .3. 3.

Plant operators panel

The

24~pewriter

ASR 33 teletype

This is intended as a device for change to parameters or requests information . For example it can changing c ontroller parameters .

310

making major for detailed be used for It is also used

to provide hard copy of what takes place via the opera tors pane 1.

NOMENCLATURE M

5.

SOFTWARE

At the present time there is no high-level process control language for this computer. Software is limited to normal compillers and on-line editing. To use this size of machine to the best advantage the final cQ)1.trol programme has to be written in machine code assembly language. This type of system is not suitable for project teams that do not coittain competant programmers. The "high level process control language" philosophy is out of place on a machine that is only fitted with an BK core store. A larger store could have been fitted but the size of project did not justify this. A useful piece of software that is availabl e with the machine is a variant of Dartmouth BASIC. Although it reduces the machine speed the fact that it is a high level language with on-line editing facilities and can call user generated subroutes that are in machine code makes it a valuable development tool. Obvious uses being checking out of hardware and control strategy.

mass of reactants average specific heat of reactants the rate at whi ch heat is entering the reactor from the heating coil. the rate at which heat is being released by the reaction. batch temperature temperature of oil entering the heating coil. temperature of oil leaving the heating coil. oil flowrate

C

o

specific heat of oil water level in partial condenser. cooling water flow in partial condenser temperature of c ooling water entering partial condenser.

1

T w temperature of cooling water leaving partial condenser. Tv temperature of vapour leaving the partial condenser.

ACKNOWLEDGEMENTS CONCLUSIONS

Our thanks are due to the board of directors of Scott Bader & Company Limited, Northampton, England for permission to publish details of the project. Mention must also be made of R. Barrick, S. Silsby, K. Tilley, S. Cooper, H. Peters, M. Elston, N. Bishop and A. Milne for their hard work.

The final comparison between operating with the computer system as opposed to manual control will be presented at Helsinki. We have shown that we can detect exotherms as they start, operate with a consistant heat input, maintain constant conditions in the partial condenser, reduce batch times and improve product quality. Some general comments can be made about using these new small computers for process control. It is important to recognise the limits of the machine particularly that final programmes require writing in the assembly language. The low hardware costs do mean, however, that online computers are no longer limited to major projects. Now that control processors and memories are cheap an equal reduction is required in the cost of linking plant transducers to the computer. We believe that using a data logger as an input device can be economic, In this case £3,000 was spent on the system for inputting 50 temperature points and 50 general signals which can be anywhere in the ranges 0 to 100 m V., 0 to I V. The cost of the analogue output system is still high being of the order of £4,700 to convert 32 channels from digital numbers to air pressures in the range 3 to 15 p.s.i. The analogue output system could be replaced by a purely digital system. This would require that the plant dynamics were such that control valves were simply shut or fully open. A variable mark to space ratio being used to provide variable process fluid flows. We propose in future to consider using such a technique on the oil circuits and partial condenser cooling water.

KEY WORDS Computer control, control of batch reactors, manufacture of unsaturated polyester resins.

REFERENCES

311

1.

p. ALLEN, D.C. FRESHWATER, H.W. KROPHOLLER and D.J. SPIKINS "A Self Adjusting Digital Controller" I.Chem.E. Symposium Series No.24

2.

Manufacturers literature, Digital Equipment Corporation, Reading, England.

3.

Manufacturers literature, Solartron Electronic Company, Farnborough, Hants., England.

Discussioo on Paper IX:5 by P. AlIen, C. L. Phi I lips

T. W. Oerlemans! Netherlands: For the design of the cootrol algorithn, it is crucial to know the response of the error e 00 the valve positioo P . Because of the calculaticns used in the system c presented, there are several paths of information coonecting e and Pc' nanely via TI and Q~dTvia T2 and Q~ and via the reactor dynamics and 2. dt Doesn't this lead to a noo-m1n1mum phase behaviour and therefore either unstable or slow control, if you use a normal P + I algorithm?

2. What was the software expense reductioo due to the macro-instructions mode available?

P. Allen: We have not noticed any instability up to now and the control we are getting is adequate at present. It is likely, however, that in the future we will need to use a more advanced cootrol algorithm when the present rather dramatic reductions in processing time are realized for the whole range of products and we are then seeking further improverrents.

3.

3. Have you looked at the possibility of defining t1me-dependent macro-instructicns? P. Allen: 1. en an internal costing basis, which simply takes into account the man-hours involved, we have spent approximately 2,000 pounds. 2. '!he approximate reduction in costs, due to macro-instructions, was 50%.

T. W. Oerlemans, Netherlands: In the system presented cootrol actions in any reactor will cause pressure changes in the main oil loop and therefore disturbances in the flow F. Might installing a flow controller for F not improve 00 this situation? P. Allen: 'Ih1s might alleviate the problem but we undoubtedly need to improve the cootrol system 00 the process heaters and the arrangement of oil control valves in the individual, reactor heating circuits. In particular, the existing 3-port valves are unsuitable for this particular application. A. M. Khan, Netherlands: '!he author has mentiooed at many points the good quality of the hardware supplied by Digital Equipment Corporation. Can the sane be said about their software? If so, is it not better to let the computer companies do the work on computer control instead of having develq:ment groups in every project? P. Allen: In our experience the software supplied by Digital is good, but it is not specific to process control. I think it is essential that the control work for a process be carried out by chemical engineers and control engineers whose prirre concern is to serve the interests of the process operating canpany. '!he computer company is solely concerned, and rightly so, with selling its own computers and does not have the expertise, or the desire, to engage in detailed work on process cootrol. F. de la Vall~e Poussin, Belgil.lll: 1. What were the expenses for software develq)rrent?

312

'!he answer to this question is no.