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Proceedings of T he South African Sugar Technologists' Association-March 1966 149

HO W TO MEASURE'A N D EXPRESS SUGAR MILLSEFFICIENCIESby T. H. FOURMOND

It is natural for the sugar industry to think andspeak in terms of sucrose: we purchase cane and sellsugar on the basis of sucrose content; through thedifferent phases of process, whether milling or boiling,we follow, analyse and check the sucrose to minimiselosses as much as possible.All the sucrose of the cane is not available forcrystallisation as there are two factors governingextraction and recovery of sucrose:(a) The fibre content of the cane(b) The purity of the juice.However efficient our milling and boiling tech-niques may be, some sucrose will be imm obilised andretained by the fibre of the cane and by the im puritiesof the juice.Bearing in mind that the aim of the milling processis to separate the sucrose from the fibre and that .o fthe boiling process is to separate and crystallise thesucrose from the non-sucrose of the juice, sugartechnologists have formulated yardsticks to measureand express efficiencies so that guidance is providedto all concerned. Such yardsticks a re based on practi-cal results.The choice of yardsticks should be based on theirmerits which means that they should measure andexpress efficiency, accurately and correctly, henceproviding guidance clearly to all concerned.

MillingFor the milling process, we have the choice of twoyardsticks :(a) The lost absolute juice % fibre;(b) The extraction ratio.Both yardsticks are based on the concept that theUnit of Fibre is the Unit surface of Adsorption whichshall immobilise and retain the juice and sucrose.Lost Absolute Juice % Fibre expresses the parts ofabsolute juice lost % fibre. Apart from the fact thatthe calcu lation of brix or solids in bagasse, hence lostabsolute juice, is based on the blind assumption thatthe purity of the residual juice is the same as that ofthe last expressed juice, th e term 'lost absolute juice'

, is fallacious for the following reasons :(a) Absolute juice lost is calculated by dividing thebrix or solids in bagasse by the brix % of the cane'sabsolute juice. W e should rem ember tha t the solids

of t h ~cane's absolute juice and those of theresidual juice in bagasse have totally differentpurities which means that they are of a totally

different nature, one containing a higher percent of sucrose than the other.Is it logical to use the brix % of a raw materialof a high purity to calculate the correspondingloss of such raw material from another rawmaterial of a much lower purity? It would appearthat this is quite a pertinent question bearing inmind that we are only interested in measuringand expressing the sucrose lost.

(b) As during the milling process the brix free wateris never extracted and remains attached to thefibre of the bagasse, we only extract undilutedand diluted juices. Therefore, how can we usean expression which is closely associated withbrix free water to m easure the loss of a juice whichdoes not contain brix free water?

For all these reasons it would appear that lostabsolute juice % fibre is not a reliable yardstickto measure accurately milling losses and does notexpress clearly and correctly milling efficiency.Extraction Ratio indicates the percentage of sucroselost in bagasse as a percentage ratio of the fibre in cane.From experience we know that the determination ofsucrose in mixed juice and bagasse is far m ore accu ratethan the determination of brix or solids in suchmaterials. Although the sucrose in bagasse is merelya Pol determination, the difference between Poland true sucrose in bagasse is so small as to benegligible. Bearing in mind that the aim of the millingprocess is to separate and extract the sucrose from thefibre, it would app-ar that such a yardstick is accurateand representative, and therefore gives better guidance.To illustrate how. lost absolute juice % fibre canbe confusing and misleading, let us take the case oftwo mills (A & B) crushing cane of the same quality:

Fibre % - 15.00Sucrose % = 14.00Solids % = 16.47Purity % = 85.00Mill A, being of smaller size, uses a higher imbibi-tion to achieve the same lost absolute juice as M ill B.From experience, we know that in any particularcane, the juices of higher purities are more readilyextractable and washable than the juices of lowerpurities as proved by the fact that in the millingprocess, sucrose extraction is always higher than brixextraction, which means that during the millingprocess, sucrose is more readily soluble or extractablethan non-sucrose. Therefore, we find that Mill A,

on account of a higher imbibition, has a lower purity oflast expressed juice than Mill B and the whole pictureis as follows:

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150 Proceedings of Tlre Soirtlr African Sugar Techn ologists' Association - Mar ch 1966Bagasse Analysis

MILL Moisture % Fibre % Solids % Sucrose % Purity last expressed

Calculating the corresp onding lost absolu te juice % fibre, extraction ratio, absolute juice and sucroseextractions, we arrive at the following figures:Bagasse Solids in Sucrose in Los t absolute Extr actio n Absolute juice Sucrose

MILL % Cane Bagasse Bagasse Juice % Fibre Ratio Extraction Extraction

and we find that although both mills show the samelost absolute juice, hence the same absolute juiceextraction, their corresponding extraction ratio andsucrose extraction differ somewhat: Mill A's Extrac-tion Ratio is 1.47 % lower than Mill B and its sucroseextraction higher by 0.22 %.A pertinent question would be the following one:

111 practice, does a higher imbibition lead to a highersucrose extraction than non-sucrose, hence a lowerpurity of last expressed juice or is it a mere assump-tion?The following figures taken from the summary ofLabo ratory, Reports period ending 30th O ctober,1965, will prove that it is no assumption but a plainfact.

Lost Abso- Imbibition Extractionlute Juice % % Fibre RatioFibre

Darnall 29.22 379 29.82Tongaat 29.2 6 224 31 -6 1Such practical findings prove that a higher im-bibition will extract sucrose more readily than non-sucrose, hence a lower extraction ratio. Theoretically,we should expect the same extraction ratio for thesame corresponding lost absolute juice as both arecorrelated to the unit of fibre. However, we find thatin practice it is not true.This illustrates clearly that l'ost absolute juice% fibre is not a reliable yardstick to .measure andexpress correctly milling efficiency.As the aim of the milling process is to separateand extract as much sucrose as possible from thefibre, it would appear that Extraction Ratio is a farbetter yardstick to measure and express millingefficiency.

Milling PerformanceFor the benefit of all concerned, it is preferable toexpress Efficiency as a per cent of what is availablein practice, and efficiency figures are usually related tostandards which have proved realisable in practice.Therefore, we suggest that a milling performancefigure be in troduce d to express milling efficiency.For instance, Natal Estates has proved that themilling process can achieve an Extraction Ra tio of 22.Is there any objection to creating a standar d of 20 asan incentive to the mill engineer's creative min d?

Should we agree to this, then the milling per-formance yardstick can be expressed by th e followingformula :Mill Extraction X 100

100-0.20 FWhere F = actual fibre of the cane.It is a simple and easy calculation and the meaningis so obvious that it would convey proper guidanceto all concerned as it expresses the sucrose extracted% sucrose available for extraction.As a matter of interest, Natal Estates MillingPerformance for last crushing season was 99.22%.

BoilingBoiling House Peuformance

So far, the most suitable and accurate yardstick tomeasure and express boiling efficiency is the BoilingHouse Performance as it expresses the crystallisablesucrose recovered in sugar % crystallisable sucroseavailable in mixed juice.

This yardstick is based upon the practical findingthat for every part of non-sucrose present in the mixedjuice a certain corresponding amount of sucrose willbe retained in the molasses when properly exhausted.The retention factor for calculating the non-cry-stallisable sucrose in mixed juice will vary accordingto the purity of the mixed juice as juices of lowerpurity (provided such low purities are not due tocane deterioration) are usually associated with ahigher reducing sugar ash ratio which, as we know,helps to achieve better exhaustion as salts have moreaffinity for reducing sugars than for sucrose.

Bearing in mind that the retention factors representthe averages obtained from practical results, the tableadjusting the retention factor according to the purityof mixed juice is as follows :Purity Mixed Juice Corresponding Reten tionFactor

0.4600.4700.4800.4890.4980.5070.5150.5230.530

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Proceedings of The South African Sugar Technologists' Association -March 1966 1 5 1The interpretation of this can easily lead to con-fusion, and be misleading. For instance one couldsay that as one part of non-sucrose will retain 0 1460part of sucrose, then the expected p urity of the final

molasses should be '460x loo= 31.5 which, as we1 ,460know, is not realisable in practice. This would be afallacious interpretation of the Boiling House Per-formance yardstick. The concept only means that forevery part of non-sucrose present in mixed juice wemust expect a certain corresponding amo unt of sucroseto be considered as non-crystallisable as such sucrosewill be found in the m olasses. However the same ra tioof non-sucrose-sucrose can not be expected in , themolasses for the simple reason that some 18% of thenon-sucrose is eliminated during clarification process.This means that for every 100 parts of non-sucroseavailable in mixed juice only 82 parts will be presentin the final molasses and this changes the whole pic-ture. It means that the expected purity of the Final

molasses will be .460 x 1001.280 = 35.9 which is morerealistic.

The following figures will clearly demonstrate thevast difference between the correct and incorrectinterpreta tion of Boiling House Performan ce for 82%recovery of non-sucrose in molasses.Correct In- Incorrect In-terpretation terpretationPurity for expec ted for expectedMixed Retent ion Molasses Molasses

Juice Factor purity purity-82 .460 35.9 31.583 .470 36.4 32 .O84 .480 36.9 32.485 .489 37.4 32.886 .498 37.8 33.287 .507 38.2 33.688 .515 38.6 34.089 .523 38.9 34.390 .530 39.2 34.6This is a totally different picture of Boiling HousePerformance as we know from practical experiencethat such corresponding purities for final molasses

are o btainable in practice from such purities of mixedjuice. In the light of such figures we can, therefore,conclude that Boiling House Performance representsthe best yardstick to measure and express BoilingHouse Efficiency.It could be said that sometimes such standardshave been surpassed. We should remember thatstandar ds are based on th e law of average. Any freshjuice associated with unusual high reducing sugarsand low ash contents is bound to show a higher boil-ing house performance than the one expected from th ecreated standard, and the application of DouwesDekker's formula for expected purity will confirm thefact and show the green light.It has also been found tha t after droughts, when thereducing sugars ash ratio is exceptionally high thro ugh

cane deterioration such juices do no t respond as above.This is quite in order b ecause crystallisation of sucroseand exhaustion of molasses are not the simple resultof a hypothetical chemical reaction where one partof non-sucrose will combine with so many parts ofsucrose but the physical properties of the juice(viscosity) will also play an impo rtant p art. Although ,viscosity does not prevent crystallisation, it, neverthe-less retards the rate of crystallisation tremendouslyand as the boiling house of any sugar mill is boundby the limitation of pans' and crystallisers' capacities,such deteriorated a nd viscous juices will lead t o po orboiling house performance, however high may be thereducing sugars ash ratio.

Obviously, unusual high losses in the filter cakeor in the undetermined losses, whether throughentrain ment o r inversion, will also lead to poor boilinghouse performance. Excessive destruction of the re-ducing sugars during clarification by a too high pHwould also lead to poor boiling house performance.

Overall PerformanceAt present, the only formula for expressing theoverall work performed by sugar mills is the overallrecovery. However, this yardstick is not a truereflection of a sugar mill efficiency as Mill Extractionand Boiling Hous e Recovery are closely related to thefibre content of the can e and t o the p urity of the juice.It is obvious that canes of low fibre content and ofhigh purity juice are bound to yield a higher overallrecovery than canes of high fibre content and of lowpurity juice.As the policy of the sugar industry is to recover asmuch sugar as possible from any quality of cane, wemust look forward to a yardstick which will give us atrue picture of ou r sugar m ills' efficiencies. If th atschool of thought is correct, then the overall per-formance of our sugar mills must come into thepicture.It is sometimes said that mill engineers are only in-terested in the performance of their mills and processmanagers in the performance of their boiling house.This is a fallacious school of thought because withoutclose co-operation between these two technical men,no team work can be expected to achieve the highestrecovery of sugar.But what about mill managers, general .managersand directors? We would imagine that their maininterests lie in the highest recovery of sugar and it isobvious that the only yardstick to express this is theOverall Performance.Therefore, it would appear that for the benefit ofboth technical an d financial control, a n overall picture,reflecting the tr ue efficiency of sugar m ills, appears t obe necessary.An overall performance yardstick cou ld be obtainedby multiplying M illing Performance by Boiling HousePerformance. It would indicate the quantity of crys-

tallisable sucrose recovered in the form of commercialsugar as a percentage ratio of the crystallisable sucroseavailable in cane.