:1!MANU;FAOTURING SEOTION.

Dr. R. W. Harman, Ohainnan.

The Ohairman suggested that all four papers of the Symposium beintroduced consecutively, and discussed together. .

Symposium on Sugar Boiling.

The Control of the Boiling of Shipment Massecuites.*

by

L. 1. A. MICHELI,

Beseorch. Department, The Oolonial Sugar Refining Oompany, Limited, Sydney.

I. INTRODUCTION.

Instruments Used in the Oontrol of Sugar Boiling.

When a general survey is made of the instruments which have been usedfor the investigation and control of sugar boiling, it is observed that they fall

into two main elasses i-e-

(1) Those which determine a physical quantity which gives an in­dication of the concentration of the syrup in the massecuite,apart from its possible crystal content.

(2) Those instruments which measure some overall effect of the

massecuite itself..Typical of Class 1, are the Brasmoscope in any of its many forms, and the

Zeiss pan refractometer, and of Class 2, the conductivity method and massviscosity methods such as that developed by Mallez & Co. (Vereins Zeitsehrift,

1903, 251).

The Brasmoscope-The first attempts of importance to utilise the elevation of the boiling point

as a measure of the concentration of the mother liquor was due to Claassen withhis so-called Brasmoseope. With this' instrument, the concentration of themother liquor is determined from the rise in boiling point, this figure beingobtained from the temperature of the massecuite and the pressure of the vapourin equilibrium therewith. Earlier attempts had been made but they were notcarried through with the energy and precision which Claassen devoted to hiswork. To Claassen also, we are indebted in no small measure for the clarifica­tion of our ideas of supersaturation in relation to the rate of crystallization[IUd spontaneous graining. Claassen's term" Coefficient of Supersaturation" isstill widely used. He. found that when dealing with heet sugar products, the

* This paper is presented by the courtesy of The Colonial Sugar Refining Co.,whose Research Laboratory and Factories this work was carried out.

ofsu.crose.,work has shown that the opposite is.observeddn bane products, that is,;'the solubility of sucrose is less (due to the presence of reducing sugars) thanin a pure sugar solution. To the foregoing factis due the failure of Claassen's

..Brasmoscope when applied to cane sugar manufacture. In any case, the cir­,eulation in pans in use at that time was such that a true measure of the boiling,temperature of the masseeuite (that is, the temperature of that portion of the'massecuite in equilibrium with the vapour) was not giyen by the pan thermo­

meter. Also, corrections for barometric pressure were not always applied.

Thieme, in his recent work in Java, determined the saturation coefficients:, of cane products and obtained some success, with the use of the Brasmoscope."We have attempted to use Thieme's saturation coefficients for the control of, our own experimental pan without success. Using a pan thermometer in the'<Jonventionalposition, we found that the results were usually vitiated by in­Mcurate temperature determinations. However, determining the temperatureof the massecuite in the pan well is more .satisfaetory.

A recent apparatus, the "Micromax" of W. E. Smith, may be lookedupon as an electrical form of the Brasmoscope. The temperature differencebetween the mother liquor and a vessel of water boiling under the same vacuumis determined by two resi~tance thermometers and hence the boiling point eleva­tion obtained. The results of our experiments with the Micromaxwill be dealtwith in some detail later.

"rile Zeiss Pan Refractometer.This instrument is designed to make .use of the refractive index asa

measure of the concentration of the syrup in the massecuite. We have foundthat, except with freely circulating masseeuites (usually massecuites not too lowin grade and not too high in percent crystal), the syrup in contact with therefractometer prism is not representative of the bulk, of pan. Moreover, theprecision of the setting possible with low grade massecuites is usually not high.

i

(:onductivity Metlloll.A method based on electrical conductivity has recently been developed by

Honig and his co-workers in Java, by Spengler and his assistants in Germany,and in France by Courriere, It may be emphasised that, except before grain­ing, it is the conductivity of a massecuite that is determined and not that ofthe syrup in the massecuite, in contrast with the previous methods.

]Iassecuite Viscosity.In 1903, Mallez & Co. patented an apparatus for determining the viscosity

ofa masseeuite from the power 'required to drive a propellor immersed in themassecuite. In this connection, it is interesting to recall the experience ofworkers with, recent pan stirrers who have found the load a useful indicationof the massecuite condition. It will be evident that this load will depend bothupon the syrup <}qncentration (which, for a particular grade, determines itsviscosity), and the percent crystal in the.masaecuite.

The Problem of. Sugar Boiling Control.The main operations which a sugar boiler .Is called upon to carry. out

are:~

-~-ii'_

(1) Strikin.g a grain crop of the required number and regularity,and free from conglomerates.

(2) Checking the pan to prevent further addition of grain.(3) . Bringing the grain together into a "closed" massecuite.(4) Keeping the masseeuite in a condition free from secondary

crystallization (false grain) during the subsequent boiling on,while maintaining an economic rate of crystallization.

We have so far found control based on instruments to be possible only forstages 1 and 4. The control of the graining operation will depend,' when theright choice of material and temperature has been made, upon the supersatura­tion. Any of the instruments given in the preceding section should. ideally becapable of measuring the supersaturation or a figure determined thereby, class2 (those depending upon a masseeuite property) giving a measure of the super­saturation when there is no grain present.

The straight boiling. on of the syrup will involve maintaining within

required limits two factors s-e-

(1) The supersaturation of the syrup in the massecuite.

(2) The percent crystal in the massecuite.

It has been found that a certain amount of confusion is caused by theterms generally used to specify the condition of the massecuite, that is, "light"or "heavy," "close" or '.'open". We propose to use the term "heavy" or"light" when referring to the'concentrationof tho.syrup in the massecuite and"close" or "open" when referring to the percentage of crystal.

II. EXPERIMENTAL EQUIPMENT.

The investigations described in this section were carried out partly in theResearch Laboratory of the Colonial Sugar Refining Co. Ltd., partly at one ofits refineries and partly at a mill, For the Research Laboratory experimentsa 180-gallon pan was used. This was provided with four coils and a jacketgiving a heating surface of 39sq: feet. This pan, despite its size,was found tocirculate well.

The control instruments used include the W. E. Smith Micromax, the Zeisspan refractometer, the Brasmoscope, our own saturation. cell for supersaturationdeterminations, and a Leeds & Northrup sugar ash vbridge and milliammeterassembly for electrical resistance.

With the milliammeter arrangement used, small variations in the currentbetween the electrodes determined by the massecuite resistance have little effecton' the voltage across the electrodes. . However, if fluctuations in the rmeinvoltage occur, as may be frequent in factory work, an adjustment must be madeon a rheostat to keep the volt~ge across the ,electrodes constant when a reading

".0':."; .' ,"'--",",',,',:'

being made. Thi~i'epr~seIitsilliamperage is. taken as a measure between

. Wheatstone Bridge circuit overcomes such difficulties.. ·The most /Sltl,llSl.ltCLU.r·y

nstrument for this work would be a recording Wheatstone Bridge a'ange suitable for the masseeuite and the electrodes. In. all work of this-type

it would appear to be more convenient to record the resistance rather than theconductivity. between the electrodes, since the resistance changes in the. same

. direction as the other ,chief physical quantities with which We are concerned,£01' example, supersaturation, viscosity and "temperatl,ue difference".

CONTROL OF THE BOILING. OF SHIPMENTMASSECUITESBASED ON THE CONTROL OF THE MOTHER LIQUOR

CONCENTRATION.

The importance of the grain characteristics of the raw sugar is now gener-recognised. If it is to affine satisfactorily, it must be free from conglomerates

· hndaggregates, and of uniform grain size. Such a sugar can only be producedby efficient pan work. The use. of instruments to control the boiling of thistype- of masseeuite is of special. importance and presentsproblems not met with

··in the lower grade masseeuites. Some results obtained using W. E. Smith's. Micromax are given and serve to draw attention to some essential aspects of the· problem.

The pan was provided with a recording thermometer, a vacuum recorder,and the steam pressure was controlled by an "Arca" regulator and recorded

>by a steam pressure gauge. The Micromax resistance thermometer and the· recording thermometer were placed together in the centre of the pan well and at

the 1,OOO-gallon level of a 3,500-gallon pan. The general plan was to obtaina recorder chart from a manually controlled skip which" appeared to behavesatisfactorily, and from this to prepare a schedule setting out the desired tem­perature difference for successive heights of masseeuite in the pan. Attemptswere then made to apply automatic control as per schedule. "Continuous feed"was first tried, but proved unsatisfactory, so that "intermittent feed" withshort time intervals, was resorted to.

Charts 1 and 2 show the results of two mill "An massecuites, one of whichbehaved satisfactorily and the other unsatisfactorily.valthough the conditionsof boiling were apparently identical. With the unsatisfactory massecuite, itwas found that a point was reached towards the end of the skip at which itsuddenly became much too elose and a drink had to be given by-manual. control.It will be seen from the chart that the percent crystal in the massecuite at theclosing point had reached 55 per cent. as compared with 45 per cent. in thefinished "A," and moreover, the closing of the massecuite was accompaniedbya fall in supersaturation, so that the Microrriax reduced the feed when anincrease was really required. It was found that once closing commenced itproceeded at a progressively accelerated rate due apparently to the continuouslyincreasing ratio of available crystal surface to syrup. An examination of thecharts reveals that the temperature difference, supersaturation, and boiling

10

temperature,were maintained 'at approximately the same value in the twocases; in fact, the supersaturation was slightly lower in the unsatisfactory case,in which the rate of crystallization was apparently too high. -T'his result wouldappear to be due to slight differences in the crystallizing properties in the twocases.

~ O"O~ 2900 gallons. ~/$a(.;z. I~J~ %(;ystld

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kh! :princ(p/e. EXHI77,iJ/e arJ"MclJnlfa/,

For satisfactory control with the Micromax, the vacuum must be main­tained at a constant or predetermined value, for if this is not done the relation­ship between the temperature difference and the supersaturation will change.For efficient control, it is also necessary to adjust the speed of the control valvewithin fairly narrow limits and to maintain a steady density in the pan feed.

Controlling through the Micromax controls one thing and one thing only,'namely the concentration or supersaturation of the mother liquor. Continuouscontrol of the supersaturation controls the rate of crystallization through-.out the skip and hence indirectly the percent crystal at any stage. With a highgrademassecuite, equilibrium is unstable, so that once the masseeuite commences'

GJ EVA'T'/ON - ~

CHART/ye2

That is, the rate at which the water in the syrup is increasing is equal tothe rate at which water is being fed into the pan minus the. rate of evaporation.

(1)

(2)

40'0 e 10 Ie 20 .e 00 S&

I; ~I"O,PfK'd • 900 yo.lt£~

L-----'I nrlnJ- Iv l.<Jn~"'".0:00

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0 e 10 Ie 1/:00 0 . ,25 .30, SO 40

s« =" 7i1m~rab"""n"""",.~..,~) 2000 90.'(lons

( 1# OF. S, Is<un N#-. #01 Cryste t.syrup i;z massuea," €'.79·0· ~"<: 0,; ~11K.·

C A~ ltDn.troi SU tua /400 ~at(~... - i '

it doesstal surface. A in 'the syrup

ed is cut off and, if unattended; the pan would go solid.

80/#17,9 ,,1ti1r/"~ If MiUlt'l:ui/lP. t9~I71G0~~QI1lro/> <16'~ ~horl' o';,~,.:mill,,,/m-in/r

t?etl",ori/1/:ijJll. exampl, or /l'7a,14CI.II"Ie"CI061/1,g up i.

"The percent crystal in the masseeuite .depends upon equilibria whichdetermine the rates of crystallization and evaporation,

Considering evaporation for a massecuite at any stage, we have :­

Ws=Wf -Wewhere Ws =' the amount of water in the syrup in the massecuite,

W f = the amoUJ?-t of water which has been brought into the pan in the feed,

and We = the amount of water evaporated,

or, differentiating equation (1) with respect to time :-d Ws d Wf aw,ili= ili - dt

or,as,dt

(4)

where Ss = ~ucrose in the syrup,Se = sucrose present as crystal,Sf = sucrose entering in the feed.

Or, putting equation (4) into words, the rate of crystallization equals therate of sugar feed minus the rate 'of increase of sugar in the syrup. With arefinery granulated massecuite where the impurities are negligible, equations(2) and (4) 'will' determine the supersaturation of the syrup in the massecuite.With lower grades of massecuite, however, the effect of impurities has to beconsidered. These enter with the feed and accumulate in the syrup, and, ifeffective crystallization is being carried out, the ratio of sugar to impurities inthe syrup in the massecuite will fall progressively.

For a given material, the rate of crystallization will be determined by thepurity of the syrup, the supersaturation, the available crystal surface, and therate of circulation, while the rate of evaporation will depend upon the heatingsurface, the temperature gradient between the heating surface and the masse­cuite, '. and the coefficient of heat transmission, which, in turn, is largelyinfluenced. by the viscosity. Moreover, an increase. in the rate of evaporationwill, other things being equal, increase. also the rate-of circulation and hencethe rate of crystallization.

With automatic control the feed will be opened as soon as the syrupconcentration reaches a predetermined value; if, in the same time, sufficientcrystallization has not occurred, a continuous opening out of the massecuitewill result. This effect can be checked either by reducing the rate of evapora­tion through a rise in the boiling temperature, a. lowering of the steam pressure,or a reduction of the heating surface, or by applying a water feed and soincreasing' the .water to be evaporated. Raising the boiling temperature orusing a water feed has the auxiliary effect of increasing the rate of crystallizationthrough increased circulation, while reducing the steam pressure or heatingsurface' reduces circulation and hence the rate of crystallization. Consequently,a rise in boiling temperature or use of a water feed. is usually resorted to whenthe rate of crystallization is too low.

With a, shipment massecuite, the ainountof impurities present in thesyrup is not sufficient. to produce an appreciable fall in the syrup purity whenthe massecuite becomes too close du~ to a high crystallization rate, so that insuch cases, a closing up may occur until the massecuite becomes almost solid.This fact is well known and in practice it is found that a high grade massecuitecannot usually be left shut down in the pan for any considerable lengthtime.

included'shipment rila:ssetmites' only

impurities.

I MAS¥C<'lIll"-l I .c~VA~/ON __ Y

I·;'iiArM I •1&$ 'lfRAlUIe .l F_"'~N..J.? I ?6 ZIU' 8'S

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CHART 1'Y1l .;J.Reli'nl'IJ//l~~;'1N 1J76$6El'ui/e b"iled'i17Re6l'ilteh tJeplpm. 8hPwln,glh4' ~/lid,,1"

1IJ..'6 m~.~6l'Clli/i,.'e Pel'",.:mfr:9. . 'h;17 cllJSl'; x:. Qoiling,8/" ,t:017s!J/71" 5.wnr:81/vrAliol2 .

ChartNo. 3 gives the results obtained from the ,Research Department pan. Itwill be. observed that for about 11 hours the pan boiled steadily ata constanttemperature difference of 17!OF., almost without adjustment of. the control.A point, however,' was reached, when, without any alteration in the temperaturedifference, the pan closed up .rapidly towards the danger point and a suddenincrease in feed .had to be .given to '. open up the .masseeuite,

A silllilareffec'thas been observed when-attempting' to control with theZeiss pan refractometer. Both the Zeiss pan refractometer ~ and the Micromaxare capable of maintaining a syrup concentration such that the crystallizationrate is economic without secondary crystallization (or false grain formation)occurring. At the same time, the percent crystal can vary over very widelimits.

IV. CONTROL OF' THE PERCENT CRYSTAL.

It will thus be apparent that, with shipment and refinery massecuites,an instrument which maintains a predetermined value for the supersaturationof the syrup in the massecuite may permit wide variations 'in the percent crystal.The electrical resistance gives a figure which depends both upon the percentcrystal and the concentration of the syrup in the massecuite, and consequentlycan be used for control upon a different basis. Some difficulty is experiencedin separating the. effects of the percent crystal and the supersaturation on theresistance, which is necessary if the resistance is to serve the dual purpose ofcontrolling both of these factors. The work is simplified by the fact that, witha supersaturated syrup, the relatively small changes in supersaturation areaccompanied by a large change in the resistance, and consequently, if thepercent crystal remains within normal limits, the resistance affords a sensitivemeasure of the supersaturation. On the other hand, the massecuite cannotbecome dangerously close without the effect being detected by the resistance.Moreover, it may be recalled that the supersaturation of the syrup in a highgrade massecuite in a very close condition is usually not high. Taking theabove factors into consideration, it will be apparent that the electrical resistancecan be profitably used as an aid to the boiling of high grade massecuites.

It has been found that, when the resistance is used as a measure of thesupersaturation, a fairly simple relationship exists.' Taking a wide range ofsyrup purities from 90 to 50 and a temperature range of 20°C., the resistancefollows the supersaturation. The following example illustrates the point.Material of 90 true- purity was concentrated at 51°c. until the resistance was1,450 ohms. The supersaturation was found to be 1·03. The temperature wasthen raised and the syrup again concentrated .until the same resistance wasindicated. On measuring the supersaturation.it was again found to be 1·03.

Honig and Alewijn (Arch. Suik, Ned-Indie, Med. No. 21, page 825) haveattempted to differentiate between the effect of syrup concentration and percentcrystal by determining theconductiv'ityboth of the syrup in'the massecuite andof the massecuite itself; to make the method sufficiently rapid for control pur­poses involves technical difficulties, so that developments along these lines areawaited with interest.. .

GraphT; shows the effect of the percent crystal on the resistance of amixtnreof crystal sugar and syrup. The resistance of the syrup alone wasdetermined as follows :-Samplesof the syrup covering the" range of purities

It is apparent from the 'graph that a reasonable agreement exists betweenthe calculated and determined values for specific resistance of the syrup. Also,the specific resistance of the syrup runs almost parallel with' the specificresistance' of the mixture, so that the variations in the percent crystal taken,that is, from 22 per cent.-30per eent., have much less effect on the resistancethan the v:ariations in the supersaturation,

Lli/l?rel1r /blit/ Volumes.

EFFECT OF p6CRYSr"'ll.

OIY ELECTRIC"'IL RE$IS7JtIYCe

u) R"5/5/imu P/ Illlxllm,"!'~r,,1' end <rj'5sl.

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6000

resistance ofthese~yrups determin~d over th.ec·· .

ssary temperatures supersaturations. ,The resistance of thefup was also calculated from the resistance of the mixture from the formula:

, R ~ R m (100 - % crystal).s - 100

R, = specificresistance of the syrup,Rm = specific resistance of the massecuite.

This corresponds to the formula for conductivity used by Honig and

eRr/PH, /Y!?/

3&00"-.~-~

'i!'1200

V. THE SENSITIVITY OF. CONTROL NECESSARY FOR REFINERYGRANULA'l'ED AND SHIPMENT MASSECUITES.

An investigation 'of the sensitivity of control necessary for satisfactorilyboiling refinery granulated massecuites,has been, made-in theResearch Depart­ment. 'This should give 'lome idea of the relative' difficulties .likely to be encoun­tered in controlling different grades, irrespective 9f the instruments used.

Refinery Granulated.

Chart No. 4 shows the result of: boiling a refinery granuiateeexpress purpose of determining the margin between themassecuite is undersaturated and that at which false grain appears. This willdefine the extreme limits between which boiling must be carried out. On the

I

same chart is shown a record of the Micromax temperature difference, the

/ffI

o

,0.

o

5haw/ng ~i//l1renc~/Jerween unduss/ilrlfliirn",d nne S/"lin,PDinl.; /Dr,·,fi.linuj' Cr,mu/8/N1

~S5i'cvi~ Doilet:! /rr Research Oe/J4rtRrn.

electrical resistance and the temperature. It may be pointed out that therather rapid alternation of over and undersaturation made it difficult to controlthe pan temperature, so tl~at in consequence, some inaccuracy will be introduced .into the temperature difference results althoughthe resistance record. will belittle affected. Some little difficulty was experienced due to the delay in takingH proof and overshooting. the mark where fine grainapp(jared. This effect,

"',,, ,,,, 0'0' the extremefrom the will be seen that the pan is under-

saturated.. at a temperature difference of 14·5°F. and fine grain appeared atI 6·5°F.,that is 2°F. With the resistance, the margin appeared to be about1500~2400. The resistance figures, however, may be affected by variations inthe percent crystal. Next, an attempt was made to boil at a temperature differ­€nceof 15·5°F., thetmid-point iof' the range determined. It was found that,crystallization at this. temperature difference was so extremely slow that. thepan .was kept on- water for approximately two hours without the massecuitecorning together. Later the temperature difference was raised to about 16°F.and the crystallization became normal in speed for this type of massecuite, butat 16io -17°F. fine grain again appeared as in the last massecuite.

These results show that there is little more than iOF. temperature differ­ence for this class of massecuite between the point at which the rate ofboiling is fast enough 'to.be practicable,and the point at which fine grainappears. There is little possibility of false readings on the Micromax due topoor circulation,' as the pan throughout was free and moving vigorously. 'I'hedifficulty of controlling a refinery, granulated manually or by any controlinstrument will be appreciated when jt is realised that the difference in super"saturation between the point of reasonable work and that at which false grainappearscorrespondsto a change from 1·10 to 1·13 supersaturation. Such workwould require not only accurate control of the pan, the boiling temperature,andhence the vacuum, but also the temperature difference. The temperaturedifference has to be controlled to within iO]'..at constant temperature, which isoequivalentin supersaturation to approximately 5'1]'. in the boiling temperatureat constant temperaturedifference.. Moreover, with such control, the circulationwould have to be good enough to pre,;ent an error of iOF. in the temperaturedifference thermometer. It may be concluded that such control is beyond therange of any of the control instruments available and emphasises the extremedifficulty in boiling a refinery granulated massecuite without the introduction'Of false grain.

'lUlll "A" Massecuites.With a mill "A" massecuite the limits of work are a little wider than

with a refinery granulated. For example, about' 3°F. temperature differencewas found between .thapointof undersaturation and that at which fine grainappeared, .with. 1°F. between the point at which a reasonably rapid rate of

·crystallization took place and the point at which fine grain was evident. '

SUMMARY.(1) A survey is given of the instruments available, for the control of

sugar boiling. These fall into two main classes: those which determine the"concentration of the mother liquor, and those which .determine a property ofthe massecuite itself.

:...:..-~...:..:..:.-~~-~.-,--_.

(8

) Res.ultsare -given for shipment,a,lldrefinery .' granulat~~niassecuites,showing'thes@sitivity.of,controLneeesfiarYif.falsegrainis ,·tohe ,avoided, :Whilemaintaining an economicboilillg'l'ate; " '. ,

.' ' ." ">,,,,~i:'1V%1[oc~;~hgJi';~i~t~'%~~t::'HJ,~<,~9i~ip "..>.'.. ," ......•. '..).,..,",....,'." .'iy"',(•.:.G(7~'t~,;~1t;"'·gr[ln.foriiiatj(jll, andmailltl;iining.a.satj$factory. perGent,'crylltal:in,.theinass~~ti,ite.

.'i) .... ,.' ..,' '.... ..( .. '.... '..... .'ii'i: ..'i(',. (3) 'Thellseof resistance instead of conductivity isrecomml3Ilded,J~inee

the,formel' r.unsp.• araHeLw.it.h. both supersaturatiol1and viscosity, ''," .... " .. -, ... '. -", ' ,'",,'," . .

•(4) No appreciable changl3j~fillPersatllratio~",.occurs, over '. aFailg~,Qf ,2Qo O. if the, electrical' resista,nceo£asyrupisKl3Pt90nstant. ,K truepurjty

r·~fro~~?~6Qm..P~';J'iDYest~.~d.. . ...... •• •... •..• ". . ••...•. •.. •mA~"''"~ti" co~tro\.b~sc~ 0<inniD~t""",cnt Whio~~OternUn",onI.YAhesyr~pconceiltrl1tioncannotenfiurethatthe percent ci.'Ystal in a hlghgradeor ··in, '~'" refinerYl1J.assecuite is Illaintainedwithin .norp:1al '.limits. .

(6}A. high',grade1Ii~ssec1iitehehaves ELf> if itwer~.in, a.state ofnunf>tableequilibrium," foronce'itcommen'C8s ,to ."cometoget~er"it, doesf>oat a, pro"gref>f>ively accelerated riLte.duetothe,'resultantincrease inayailaJ.)iecrystal '

·snrface. ' ,', , . , .

(7)·.• Withahighg~ade. orwitliia' ,reflnerY 1nkssf3Cliite,.qirectt1iepercentcryst~lseeIlls:l1eCesfiary.,·· . '," .