1957_van der pol_some notes on the

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86SOME -NOTES ON THE EXTRACTION OF SUCROSE

FROM CANE BY DIFFUSIONBy C. VAN DER POL

By the term diffusion, as applied to the cane sugarindustry, we understand the recovery of sugars fromcane tissue 'by liquid extraction, as opposed tomechanical expression of the juices in conventionalmilling practice.The sugar juices in the cane tissue are enclosedin cells, which in live cane are not permeable to thesugars. In order to allow the sugars to diffuse throughthe cell wall, the latter has to be made partiallypermeable, which can be most easily achieved byheating to above 75C. At this temperature the

colloids, of which the cell wall ismade up, are precipitated and the true solutes of comparatively low molecular weight are free to move through the tissuetowards the surrounding extraction liquid. Inthis manner a juice free from impurities of highmolecular weight can be recovered since the cellwall acts as a molecular sieve. This is a decidedadvantage over the mechanical expression methodof extraction, where the cells are broken and thewhole contents are squeezed out.The driving force behind the movement of thesolute molecules from within the cane tissue to the

surrounding liquid is the difference in concentrationbetween the juice inside the tissue andl that surrounding the tissue. No useful diffusion can takeplace when these concentrations are equal. Themovement of molecules through the solution insidethe tissue is slow, and it is dear that the shorter thepath along which the molecules have to move, thequicker the process. High temperatures increasethe rate of movement of molecules and hence speedup the rate of diffusion. Also the larger the area ofcontact between the tissue and the surroundingliquid, the' quicker the concentration differencecan be equalised. Since diffusion stops when theconcentration gradient has disappeared, it is necessary for rapid diffusion that the concentration ofthe liquid surrounding the cane is always kept ata minimum value. Also rapid movement of thissolution past the exposed tissue surface minimisesthe possibility that a thin film of high concentrationis built up in the immediate vicinity of the diffusioninterface. Shortening the path of the diffusingmolecules and increasing the diffusion interfacialarea is simultaneously achieved by a fine preparationof the cane. This unfortunately cannot be accomplished without mechanical rupture of some juicecells which in tum allows the high molecular nonsugars to escape into the diffusion juice.The above considerations enable us to lay downcertain fundamental requirements for' both the

condition of the cane and the design of a plant andgive us the material with which to build up an equation which governs the diffusion process.Phint Requirements

(a) For preparation of theCaneAs will be shown later, the rate of diffusion isinversely proportional to the square of the lengthof the path along which diffusion takes' place.Hence not only should the cane dimensions be small

for rapid diffusion but the regularity of the sizemust also be controlled with great exactitude formaximum efficiency. Coupling to this, the furtherrequirement of minimum rupture of cells, it is notsurprising that the problem,of cane preparation fordiffusion is still a long way from solved.I t would appear as if the general attitude ofworkers in this field towards this problem is toforego the advantage of higher purity of juices andto concentrate on a high efficiency of sucrose extraction. I f this attitude is economically sound, a setof knives followed by a shredder of the type alreadyused in the preparation of cane for milling shouldfulfill the requirements adequately. Furthermoresince about 50 per cent of the juice in cane can berecovered in asingle three-roller mill with comparative ease, it will undoubtedly be an advantage toplace one mill in front of the diffuser. This wouldfurther assure even preparation and would reducethe load on the diffuser by 50 per cent, from whicha substantial increase in extract ion of sucrose canbe expected, as will be shown later.

(b) For the Diffuser and its AccessoriesIn order to kill any cells which have escapedrupture in the preparation of the cane for diffusion,the prepared cane must be brought to a temperatureof not less than 75C. There appears to be littlereason, other than steam economy, why this temperature could not be taken to 100C, since thestructure of carie fibre is not such that it will softenas does beet fibre at temperature around 100e.This scalding of the cane can be done in a varietyof ways, either inside or outside the diffuser.The requirement of maximum concentrationdifference between the juice inside the cane tissueand that surrounding it, is best fulfilled in continuous

countercurrent operation. The cane must be transported along the 'diffuser in such a manner thatfeed-back is prevented and also no channelling ofjuiceshould occur, since both these factors mitigate


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87against the existence of a maximum concentrationdifference at all points within the diffuser. The ca.nemust further be fully submerged in the diffusingliquid at all times to ensure maximum area of contact between the ca.ne and the surrounding juice.The volume of the diffuser must also be a minimumto ensure a maximum rate of movement of thediffusing liquid past the cane surfaces.

These requirements have already been fulfilled,partly or wholly, in diffusers in commercial use inthe beet sugar industry. Since cane fibre is an entirelydifferent material from beet pulp, the mechanicalproperties of fibre, such as its greater hardness andits tendency to felt, will have to be given specialconsideration when modifying a sugar beet diffuserdesign to the cane sugar industry. .

PILOT PLANT DIFFUSER

Top view of Diffuser showing Single Scroll Screw Conveyor

The extracted cane has a fibre content of aboutthe same order as the original cane and it will haveto be brought to the composition of bagasse beforeit can be used as fuel in the boilers. The most convenient way of achieveing this is to pass it througha three-roller mill, although other types of expressorsare available. The expressed liquid will containsome sugars in concentration varying with thedegree of efficiency obtained up to that point, andit can be returned to the diffuser at the appropriatepoint.

The Diffusion EquationFick's fundamental diffusion law states:

d - k ! A dc. dt,s - ' . , , ' . dxin which ds is the weight of sugar diffusing in thetime interval dt across an area A; de is the difference

in concentration across a diffusion path .of lengthdx; T is the absolute temperature, ." the viscosityof the soluti0!l and k a proportionality constant.Upon integration and substitution the equation canbe developed to:

S = K X '! X X (C1-Jl) - (C2-J2) ... (A)." d2 1 C1- J log. C2 - J2in which S=weight of sucrose diffused per unitweight of caneT= absolute temperature at which diffusion takes place.,,=viscosity at the diffusion temperature.t=time interval in minutes during whichdiffusion takes placed=average thickness in em. of the cane

chips in the direction of diffusionC=concentration of sucrose in juice in caneJ = concentration of sucrose in juice surrounding the cane

suffix 1= conditions at the cane inlet end of thediffusersuffix 2=conditions at the cane outlet end of thediffuser

K=proportionality constant independentof the other variables.Equation (A) can be further simplified and writtenin terms of variables more easily assessed than theabove. For counter-current conditions, adding purewater as the solvent,C _ n -1 . Co ... (B)n - n X [Antilog(KX.! X_!2 X(n-1)J-1

." d n'where Cn=Pol per cent extracted cane as it leavesthe diffuser

Co:.-Pol per cent canen= weight of diffusion juice

weight of extractable juice in cane=water added per cent cane

100-fibre per cent cane= draftand other symbols as before.

Cn '. hi hNow (1- Co) X 100= per cent extraction, w IC. from equation (B) can be completely. evaluatedfrom the draft n, and the physical variables T, 7],t and d, provided K is known.The constant K is extremely useful in diffusionwork in that it can be used to define the efficiencywith which a diffusion operation is carried out.Once an absolute value for K has been determined,that is, under perfectly ideal conditions, the deviation of any other value of K obtainedwith a particn-


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88

I t is obvious from the pol extraction data thatthe results obtained by diffusion only are considerablylower than one would expect from conventionalmilling. The question now arises whether a substantial increase in pol recovery could be expected if:

(a) cane preparation was improved;(b) the juice expressed in the mill from the diffu

sion residue were to be returned to the diffuser;(c) the design of the diffuser was improved.(It was already shown that partial crushing of thecane before diffusion will result in a substantial

improvement in pol extraction.)

I t did appear that the single scroll horizontalscrew conveyor is not an ideal design for transportation of cane through a diffuser. There was amarked tendency for the cane to be dragged withthe screw to the one side of the diffusion trough andchannelling of the extracting liquid could not beavoided. Due to the felting characteristic of thecane, this movement of the cane along the diffuserwas irregular and some.tendency towards back-feedwas observed.The drainage wheel whereby the extracted caneis removed from the diffuser, was of inadequatecapacity to cope with the rate of feed required tokeep the diffuser full, which resulted in a residue ofa much higher water content than would normallybe expected.In spite of these disadvantages, very useful

information on the diffusion process as applied tocane was obtained, which can be summarised asfollows:The results of the last 15 experiments, each ofabout 7 hours duration, gave the following averagevalues in the range indicated:

0.75- 2.13

Rangeper cent

80.9 - 92.70.04- 2.21

82.4 - 93.4

128 -14598 -17342 - 65

86.20.87

Averageper cent.

Per cent Pol Extrac-tion

Per cent Inversion ...Corrected Pol Extrac-tion 87.1

Pol per cent Extrac-ted 'cane 1. 38

Diffusion Residue percent cane 135

Juice per cent Cane r.. 118Residence time (min.) 53

lar diffusion process from this absolute value is ameasure of the inefficiency of the process.The equation also involves the term d, i.e. theaverage thickness of the cane chips in the directionof the diffusion. For shredded cane it will be im

possible to determine this variable directly and someinferential method will have to be found for itsevaluation. However, the equation can still beused to calculate the effect of changes in the othervariables on the diffusion process, provided d canbe assumed to remain constant.Laboratory Investigations

The aim of the laboratory investigations was toestablish the validi ty of the equation for diffusionin cane and also to determine an absolute value forthe constant K. Neither of these aims were fullyrealised. The difficulties encountered were twofold.Firstly the extremely high degree of accuracyrequired made reproducibili ty of results very difficult and secondly the role played by brix-free waterin determining the concept "extractable juice incane" has not been fully elucidated. However, thework will be continued and the results so far obtained do suggest the equation to be valid over awide range of extraction values. The value of Kwas found to be about 2X 10-5 Using this value ofK it was deduced that a cane chip thickness notexceeding 0.20 cm. would be required to reach96 per cent extraction in one hour, with a diffusionjuice rat io of 110 percent on cane, and 15 per centfibre in cane. (Australian workers have reported95 per cent extraction by diffusion at 180F for 60minutes with a slice thickness of -fir inch.)

I f now 50 per cent of the juice in cane is extractedby crushing prior to the diffusion process, 98.9 percent overall extraction could be expected, otherconditions being the same.pilot Plant Investigations

The plant used in these investigations was. theone designed and built by Mr. Saville of EntumeniSugar .Milling Co. This plant consists essentiallyof a single scroll horizontal screw conveyor and withshredded cane has a capacity of about 600 lb. caneper hour. . The first half of the trough is steamjacketted which allows .the whole of the contentsof the diffuser to be maintained at 95C.The cane was prepared by shredding' in a type offixed-blade hammer mill, since no other device wasavailable. The nature of the prepared cane variedconsiderably with the type of cane; .hard fibredcanes yielded a coarser product than soft-or 'lowfibred canes. It is estimated that on. the average at 'least 10 per cent of the feed to the diffuserconsisted of cane chips with a minimum dimensiongreater than half an inch.


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89

View of Feeding Box and Cane Shredder in the background

View of Residue Discharge Wheel and Hot Water Supply Tank

Furthermore, the efficiency of the hand mill wascomparatively low, moisture in the final bagassebeing as high as 63.5 per cent. I t is hence reasonableto expect that, when the bagasse is milled to 50per cent moisture, a proportionally, larger amountof pol will be recovered from the diffuser residue.(c) Design of the Diffuser

Whether or not changes in the design of the diffusercan be. expected to improve the efficiency of theprocess can only be deduced from the magnitudeof the diffusion constant K, when applying thediffusion equation to the process. Unfortunately d,the average length ofa cane chip, cannot bemeasureddirectly with shredded cane, and the equation cannotbe solved for K. Attempts were made to infer dfrom a measure of the percentage pol in openedcells, but more work will have to be done before anequation is established which relates the averageparticle size to the pol in opened cells, In theseinvestigations it was established that the shreddedcane used as diffuser feed had at least 70 per centof its cells ruptured. This introduces a furtheruncer tainty in applying the equation to shreddedcane.

The equation was developed and tested for diffu-sion from killed but unruptured cells, arid whereasleaching of juice is also a diffusional process, itneed not necessarily obey the same 'diffusion equa-tion. The value of the diffusion constant K willcertainly be different for the two processes. Henceoverall values of K for shredded cane cannot becompared with absolute values of K for cut canediscs to determine whether the process is efficientor not. Only a value of the diffusion constant deter-mined over that part of the process where leachinghas been completed could be used for this purpose.Thus unfortunately it is not yet possible to say

PILOT PLANT DIFFUSER

Purity82.5.0.6

(b) Return of Expressed JuicesA number of tests were carried out to determinethe pol distribution in the diffuser residue. Theresidue was passed through a . hand mill, afterallowing the surplus liquid to drain off. The followingaverage distribution of pol was found:

Per cent Per centon Pol on Polin Residue in Cane

(a) Cane PreparationOn analysis of the coarse and finer cane particlesin the extracted cane as it left the diffuser, the greatimportance of even preparation of the cane fed tothe .diffuser was forcibly demonstrated. . I t was

shown that 72 per cent of the pol in the residue waspresent in the coarse pieces. Assuming that thecoarse pieces made up 10 per cent of the cane fedto the diffuser, the pol extraction could have been.raised from 87 per cent to 96 per cent, if the coarsepieces were entirely eliminated, other conditionsbeing the same. 'PILOT PLANT DIFFUSER

Pol in drainingsPol in expressedjuices ... 61. 3 7.9 85.8Pol in final bagasse... 31.1 4.0 44.5

I t is clear that the return of both drainings andexpressed juices to the diffusion process will con-siderably improve the pol recovery. Using averagedata, the pol extraction could be increased from 87.1per cent to 96.0 per cent, other conditions remainingthe same.

The high purity of the expressed juice is not assurprising as it seems, when it is remembered thatthe bulk of the pol in residue is in the larger bits ofcane.


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whether improvements in the design of the diffusercan be expected to result in a substant ial improvement in pol extraction.General Conclusions

During the investigations a serious loss of sucroseby inversion was found, amounting to an averageof 0.87 per cent of pol in juice. This is notsurprisingsince no steps were taken to increase the naturalpH of the juice in cane. The pol extraction wascorrected for this lossof sucrose and hence inversiondoes not enter into the evaluation of the efficiencyof the process, as there is no reason to believe thatit can 'not be entirely eliminated by careful adjustment of the pH of the contents of the diffuser.As is to be expected from the very high percentageof ruptured cells in the cane fed to the diffuser, the

purity of th e diffusion juices was but little higherthan that which would have resulted if the canehad been milled in the usual way.This paper is but a brief summary of some aspectsof the work carried out on the diffusion process.A detailed report on this work is in preparation,

and will be presented once the laboratory investigations have been concluded. Even the latter reportis not expected to give the final answer to the question whether or not diffusion is a superior processto milling. The very most that can be said at thisstage is that diffusion as an adjunct to milling, asused at Entumeni at present, can be expected togive results superior to those generally obtainedby milling only, provided conditions are carefullycontrolled.

The Author wishes to express his appreciation tothe Management and Staff of Illovo Sugar Estatesfor facilities put at the disposal of the S.M.R.I.The experimental work was carried out with theassistance of Messrs. E. Beesley and C. Young,both of the S.M.R.. I.Dr. A. McMartin demonstrated diagrammatically

on the blackboard the construction of a plant cell.He pointed out in the s tructure of the mature cell ofsugarcane it often appeared as though the cell hadalready been ruptured or was altered anatomicallyand wondered if the high osmotic pressure due to thesucrose inside the cell had caused some alteration inthe structure. He asked if any work had been doneto find out if this was so.Dr-. C. Van del' Pol said that work had been donein Australia and it showed that heating to 75Cwasnecessary to kill the cell so that the sucrose coulddiffuse rapidly. He thought that Dr. McMartin'sremarks threw light on the behaviour of partiallycrushed cane when this is extracted with cold waterto determine the juice in ruptured and unrupturedcells respectively.

90Dr. K. Douwes Dekker said that it had been found

that S02 substances which had anaesthetisingeffect on human beings could kill j:he cell at lowtemperatures.Mr. Walsh commented upon the plant description

given in the paper which stated that a set of knivesfollowed by a shredder would be sufficient. Hequeried this because in America they were starting.to slice cane. He said that a beet sugar diffuserwould not be suitable for sugarcane. He had hadexperience in the past of using a beet sugar diffuserwhere the material was passed directly from cell tocell and the water introduced on the counter-currentprinciple. He had found that with a three-rollermill it was not possible to reduce the moisture downto 50 per cent in the final bagasse. The plant he hadused handled not 600 lb. of cane per hour but 60tons of cane per hour. He would have liked to haveseen various. types of diffuser tried. The passage ofthe cane through a diffuser must be positive and hethought it would be very difficult to do this with ascroll conveyor. He considered that the type ofdiffusion tried was dangerous because of the enormous amount of inversion which took place.Dr. Van del' Pol queried Mr. Walsh's statement

that Americans were trying out slicing of cane. Hesaid that he would have liked Mr. Walsh to haveenlarged upon his experiences, particularly the extraction, etc. At Entumeni where a similar plantwas operating on a big scale no trouble was obtainedin draining the product from the diffuser. The samething applied in Egypt. He may have been slightlyoptimistic in his statement that the pH of the juicecould be controlled to avoid inversion because as thejuice inside the cell was acid perhaps the damagecould be done before it could be neutralised by lime.Mr. Rault said that the experiments at MountEdgecombe were very costly and disappointing. Thetrouble was fundamentally that the plant was unsuitable and inadequate for the cane throughput.They had long delays and it was necessary to produce

sugar, so only a small part of the production was putthrough the diffusion plant. The company unwiselystarted off without having carried out preliminaryexperiments on a small scale.Mr. R. A-,Carter said that the Entumeni plant hadrun over a period of fifty-two hours only. Theprocess now used was to crush the cane through acrusher and then a mill with the result that big particles of cane were already broken and ruptured, thusenabling the sucrose to be extracted. Prior to theuse of the process where pre-liming took place in thediffuser, they had trouble with the filters, but thisdisappeared when using the diffusion process. Thelast mill, after the diffuser, had given no trouble, themoisture was reduced to 5 0 ~ 5 4 per cent despite thefact that cane carried over on the inter-carrier


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between Nos. 1 and 2 mills had to go into the lastmill with the diffusion product. This carry-over hadup to 10 per cent sucrose, but the extraction obtained by this process was as good as obtained bytheir mills and was from 90-92 per cent.In general he was extremely optimistic becausethe juice was good and so was the sugar, and theyhad no trouble in the filters.Mr. J.W. Main 'said that a tremendous amount of

e experience on cane diffusion was available from othercountries such as Egypt, Australia, etc. He himselfhad had some experience and he found a big problemwas to cool down diffused cane so that it could behandled by the mill. In many places where diffusionhad been tried it had eventually been discarded andhe wanted to know from Dr. Van der Pol what hadprompted the design of the present plant used byhim.Dr. Van derPol said that the Sugar MillingResearchInstitute had been asked to accumulate some dataondiffusion and as this plant was already availableit had been used. It was hoped to obtain other kindsof plant so the different performances could be compared. Cold water was used at the end of the diffuserat Entumeni and this resulted in heat economy asit was heated up by the hot residue from the diffuser.There was no difficulty in milling such diffusionresidue at Entumeni.Mr. C. A. Narbeth said there were many different

types of diffuser and there should not be muchtrouble with diffusion itself: The chief trouble wouldbe in the preparation of cane for diffusion. Recentlyin the beet sugar industry a tremendous amount ofwork had been done on preparat ion of beet slicesand great progress had been made.Dr. H. H. Dodds said that he was very interestedto see this paper presented as it described a processwhich might become eventually universal' in thefuture. He knew some years ago of some factories inJava which practised some system of diffusion andthey invariably obtained betterextraction than those

91factories using the conventional milling process. Hewondered if some modification of the process usedby Dr. Van der Pol could be reduced to laboratoryscale and then utilized for a direct determination ofsucrose in cane at the factories, for which there wasevidently much need as a routine method in thiscountry.Dr. C.VanderPolsaid that the diffusionprocess wasnot rapid and not really suitable for laboratory work.To speed up the process one had to reduce the sizeof the cane particle and one could hardly talk of it .as being diffusion any more, but rather as leachingor extraction. He pointed to other laboratory plant ,which could be more suitably used for the purposesuggested by Dr. Dodds.Mr. Leclezio said that it was obvious from Dr.Van der Pol's paper that the increased purity claimed

by others could not always be obtained by diffusion.He asked if the correction of pH by lime might notdelay diffusion.Dr.C. Van der Pol said that the purity of juiceobtained was a lit tle higher than one could expectfrom milling and he considered that this was due tosome extent to the heating which gave a partialdefecation of the juice. As far as the quantity ofwater required was concerned it had to be onehundred per cent of the juice in the cane but if thediffuser was preceded by a crusher then the amountof juice in cane would be so much lower that the

water requirement could perhaps be reduced.Mr. C. G.M.Perk said that the process Dr. Doddsreferred to, was known in Java as Nobel's HotMaceration. It simply consisted in pumping up juiceat a high temperature, several times, on to thebagasse. This resulted in bet ter extraction. Sincethe steam consumption was rather high, the application was limited to three or four mills. Like the hotmaceration process used in Australia, Nobel's hotmaceration may not be called diffusion, because thetime of contact between bagasse and juice is tooshort, for proper diffusion.

1957_van der pol_some notes on the

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