I Processing

DEVELOPMENT OF A CONTINUOUS CENTR1FUC;AL FOR WHITE SUGARS

D. Hoks

Research Department, Stork-Werkspoor Sugar B.V., Hengelo, The Netherlands

ABSTRACT

The working principle of the SWS-type patented continuous centrifugal for high-grade massecuites is explained and some construction details and data are given. Tests with an experimental machine have shown that the same sugar quality and the capacity of an automatically programmed discontinuous centrifugal can be reached. Some average results are given and compared with those of the batch machine.

In order to make such comparisons possible the concept of the specific curing effect number has been derived and its usefulness demonstrated.

From the experience with the experimental centrifugal it has been concluded that this new principle of continuously curing has a great potential for white sugar applications.

INTRODUCTION

About 20 years ago when the first continuous centrifugal of the conical type was successfully introduced into the sugar industry it was also hoped that, after gaining sufficient experience, all massecuites would be purged continuously, thus further increasing the number of continuous unit-ope- rations in the sugar production process, in which the crystallization and the curing-station are still forming the exceptions.

The well known advantages of continuous work are: maintenance and power saving, less buffer storage capacities, elimination of peaks in power consumption, less ndise due to braking and the action of the pneumatical systems, and a continuous flow of massecuite, run-off and sugar which enables an optimal process control, without interruptions and time losses.

Whereas after several technical improvements low and intermediate massecuites could be purged satisfactorily and yields and molasses purities became a~ceptablel~2~3~4~~*6~7~19~, high grade massecuites i.e. white and refined sugars could not. be handled till now, because of considerable crystal

.I

271 1

27 12 PROCESSING

breakage and other shortcomings in the curing process of the continuous centrifugal of the conical type.6~8~9J0J1 Technical alterations, as enlarging the diameter of the casing, building in flexible deflectors and brushes, in- troducing airstreams were made.8~'2~13~t4*15.16~17~18 The ballistic movement of single crystals leaving the rim of a rotating basket has been studied and mathematical models for conlputer simulation evolved, yet all applied measures could not eliminate crystal damage.20921p22

In order to solve the problem of labour saving and to reduce the pro- duction losses during discharge, steaming-out, filling, accelerating and braking as much as possible the batch type centrifugals were fully automated and optimalized. The power consunlption, however, could not be reduced, power peaks not eliminated and still these centrifugals are operating batchwise.

The more complicated additional systems-and programming equipment for operating the fully automatic discontinuous centrifugal ask for more expensive maintenance and larger financial investments.

The crystal wear and/or breakage in the conical type continuous ccn- trifugal is caused by the following two reasons:

1) After being fed iiito the centrifugal each crystal has to travel on the back of other crystals over the screen driven by the centrifugal force to the rim ol the basket.

During this spiral movement in the course of which the mother liquor is separated, the crystals are by the centrifugal force heavily pressed against each other and against the basket. This causes an abrasion of the crystals.

2 ) Damage is further caused when the crystals hit the wall of the casing after leaving the rim of the cone rotating with a high speed. I

When the kinetic energy of the crystal, which is proportional to the mass of the crystal and the ,circumferential speed of the rim of the basket, exceeds a certain minimum limit of the mechanical strength of the crystal, damage $f fhe crystal can take place during the collision with the wall, when this kinetic en&gy is transformed into mechanical energy and heat.

In trying to solve all problems of spinning white sugar in a continuous centrifugal the Research Department of Stork-Werkspoor Sugar (SWS) looked for other ways and made use of a different principle both in dis- tributing the massecuite, spiilning and discharging the sugar.

1 D. HOKS

WORKING PRINCIPLE

The batch centrifugal has been the basic starting-point for the design of the SWS-type continuous centrifugal:

1 ) Just as in the batch machine, the sugar crystals must lay motionless during spinning, so that no abrasion to the crys- tals due to any movements on the screen, can occur.

2) The same sequence of the successive periods in the total spinning cycle must be maintained. These are (Fig. 1 ) :

a ) Decantation and separation period, during which each crystal must be settled on the screen by the centrifugal force and the mother liquor separated.

b) Washing period for removing the rests of mother liquor from the crystal surface and from the inter-crystalline spaces by dilution and by heating-up.

c ) Drying period for separating the last rest of the mixture of water and mother liquor and to dry up some extent the sugar layer before discharging.

11 16 4) Discharge of the sugar from the basket.

FIGURE 1 . Working principle of the SWS continuous centrifugal for high grade massecuite. Top view.

Fllterlng period

Drytng pernod

Discharge

Sugar layer Centrifugal bsrkel

top view

27 14 PROCESSING

These considerations led to the design of the continuously operating SWS-centrifugal. It consists of a conical basket and an internal massecuite feed- and discharge system which is centrally located in the basket (Fig. 1 and 2).

FIGURE 2 . Working principle of the SWS continuous centrifugal. Vertical section. Massecuite inlet

u Driving shaft of basket

vertical section

The feed- and discharge system and the centrifugal basket are rotating coaxially with different speeds, so that there exists a relative motion between both in such a way that the feed- and discharge system is leading (Fig. 1 ) . This relative motion is derived from the main driving shaft and from an auxiliary shaft by means of a planetary gearbox.

A specially patented distribution pot, in which the massecuite enters centrally, has two outlet ports and 2 radial hollow arms facing each other at 1 80°.

On the front side ol each arm a patented scraping sliding system, extending itself over the whole height of the basket, for the discharge of the product, is mounted. ,

On the rear side of each arm, i.e. behind each scraper, special devices for the distribution of the massecuite over the whole height of the basket and for the building-up of the sugar layer is fitted.

D. HOKS 271.5

The massecuite entering centrally the distribution pot (Fig. 1 and 2) is divided into several branch currents of certain proportions.

These branch cur>ents are flowing separately of each other under the influence of the centrifugal force to both outlet ports of the pot and there- after through channels in the hollow arms into the respective compartments of the distribution/buiIding up device, which is provided with as many com- partments as there are branch currents.

With the layer-building-up device also the required thickness of the sugar layer can be adjusted.

The wash water nozzles are fitted on one or more pipes in series. Each pipe, mounted to the distribution pot, is provided with the required number of nozzles of different sizes in order to distribute the wash-water in proportion to the thickness of the sugar layer, over the entire height of the basket.

The centrifugal basket has such a conicity that the transversal com- ponent of the centrifugal force acting on the sugar layer is in equilibrium with the counter-acting friction force between the sugar layer and the screen

i in the centrifugal basket. The scrapers have a smooth surface in order to decrease the friction and, after lifting up the sugar layer from the screen, break the said equilibriuill ol forces suddenly. The product crystals are I

moving or sliding under influence of and in the direction of the said com- I

ponent of the centrifugal force and are discharged over the rim of the basket 1 from the centrifugal. The scrapers are acting as slides and are not carrying I out a scraping action on the sugar layer.

. As can be seen from Fig. 1 the centrifugal is divided into two symmetrical halves. Each hall is provided with a massecuite acceleration1 distribution system, a layer building-up system, washpipe(s) with nozzles and a discharge system consisting of a sliding scraper.

As in the discontinuous machine, the SWS continuous centrifugal has the same possibilities to adapt the operating conditions of the curing-process in the machine to the flow characteristics of the massecuite and its crystal size composition, which is important for the handling of bad strikes. In order to be able to handle strikes of various rheological properties the con- trollability of the retention or curing time by changing the relative speed between the feed discharge system and the basket, the speed of the basket in connection with the mass acceleration, the layer thickness and the washing has to be provided.

The feed to the centrifugal can be automatically controlled by the current intensity of the driving motor.

Advantages

Based upon the same quality and the capacity of the delivered product the following advantages of the SWS-continuous centrifugal in comparison

271 6 PROCESSING

with the well known automatically programmed discontinuous centrifugal have to be mentioned:

1) Continuous operation so that constant process streams and an optimal process control is obtained.

2) Low constant energy consumption with no peak loads. I 3 ) Electrical energy saving of abt. 35%. I1 4) High capacity related to the total volume of the machine and

of the additional equipment. I 5 ) Low qb'ikture content of the delivered sugar, resulting in a

considerable h ~ a t saving (abt. 50% ) in the sugar drying process.

This low moisture content is the consequence of the cir- cumstance that immediately after discharge each single crystal is coming into an intimate contact with the surrounding air in the casing and drying takes place. This is a quite different circumstance as in the batch machine, in which all crystals are discharged practically at. the same time and at a low speed of the machine so that a moist mass of sugar crystals leaves the centrifugal.

6 ) No complicated and vulnerable programming equipment and pneumatical systems with auxiliares are needed.

7) Lower noise level.

Compared with the other types of .coptinuous centrifugals as the flow cone, cone with screw, pushing cone and' vibrating cone, the 'SWS-continuous centrifugal has the following advantages: ' 4 ,, 4 4

; ,#$ sl$';iz

1) The crystals are not moving over the screen during spinning so that abrasion to the crystals cannot take'talace in the centrifugal itself. * p

2) Low mass acceleration or g-factor and consequently a low circumferential speed of the rim of the basket so that crystal breakage due to the collision with the wall of the casing is limited to a minimum.

3 ) The mass acceleration or g-factor can be adjusted and is not limiting the throughput seriously.

D. HOKS 2717

4 ) The throughput and the quality of the delivered sugar can be changed or corrected independently by: - adjusting the thickness 01 the sugar layer. - varying the retention time as said before.

5 ) The initial time and the wash-water quantity can be adapted with a view to an optimal quality of the delivered sugar.

6 ) Lumps in the entering massecuite do not disturb seriously the operation of the centrifugal.

EXPERIMENTAL CENTRIFUGAL AND ITS RESULTS

According to the described principle an experimental centrifugal (the so-called Multi-Purpose-Test-Centrifugal MPTC) has been built and tested extensively in a Sugar-Refinery during 1975 and 1976.

The MPTC is shown on Fig. 3 and Fig. 4.

FIGURE 3 . (left) Top view of the centrifugal basket with massecuite feed/discharge system; FIGURE 4 . (right) SWS Experimental centrifugal with control panel.

The technical data of it are the following:

Installed motor power : 30 kW Speed of basket : n, = 700-980 rpm

(adjustable) Mass acceleration or g-factor : 300-600 (adjustable) Retention time : 12-40 sec. Ranges of adjustable layer thickness: 18-24-30 mm Wash water consumption abt. : 1 . 5 - 3 . 0 (% on sugar)

D. HOKS 2719 ,

It will be clear that the inassecuite quantity handled by the cen- trifugal is related to the sugar mass by its crystal content (% on weight). '*

The specific gravity Y, of bulk sugar amounts as to our experience between 0.9 till 1.2, depending on the crystal size distribution; the wider the distribution the higher the specific gravity of the bulk sugar will be.

For the calculation of the sugar throughput the mean value of the layer thickness 6, and the mean diameter D,,, of the basket have to be used.

Experiments

In order to investigate the optimal curing conditions in terms of re- tention time, layer thickness and mass acceleration, in view of the handled throughput and the quality of the discharged sugar, series of tests without washing and with washing havc been carried out.

Into this investigation also comparative tests with a laboratory cen- trifugal and with an industrial automatic batch machine have been included. Reference in this respect is made to the work of Eichhorn et ~ 1 . ~ ~ 3 ~

In Fig. 5 a diagrammatical reproduction of the curing process with its intermediate periods, as mentioned before, is shown from the moment of feed till the moment of discharge as the function of the momentary ash content and the retention time of the s~gar.~3,%

The decrease of the momentary ash content represents in fact the ash reduction in the sugar layer during spinning. This curve, or the so called separation characteristic, which is also influenced by the crystal size dis- tribution and the viscosity of the mother liquor to be separated, consists of two functionally different parts:

- the decantation period, till all crystals have been settled on the basket (settling point) and during which the excess of mother liquor is separated. It will be clear that for several reasons the ash reduction per unit of time takes place faster than during the following filtration period.

- the filtration period during which the rest of the mother liquor has to be filtrated through a full sugar layer thickness. The ash reduction per unit of time is slower due to the higher filtration resistance of the layer than during the decantation period.

Theoretically the curve will show a bending point correlating with the settling point in which the decantation period is ending and where the filtration period is beginning.

OZLZ

D. HOKS

FIGURE 6. Ash content of sugar - without washing - versus retention time.

0 centrrf basket 600 1200

RETENTION PERIOD Tc (degrees) + Sugar discharge

In Fig. G an example of the separation characteristics with the same massecuite is shown at different speeds of the centrifugal basket.

For such a test the machine, after feeding with massecuite, was suddenly stopped within 15 seconds by electrically braking and sugar samples for ash analysis were taken out from the sugar layer covering the circumference and the whole height of the basket.

The local thickness of the sugar layer was measured and the retention time calculated for each spot of sampling. The crucial point of these tests was to investigate the interaction of the final lowest ash content of the sugar at the moment of discharging (point 3a in Fig. 5 ) with the afore-said adjusted layer thickness, mass-acceleration and retention time, in order to find an optimum in respect of the final ash content and the throughput of sugar. It will be clear that in order to achieve a low ash content for the washed white sugar from the centrifugal, meeting the required quality standards for same and combined with the highest possible throughput, the ash reduction without washing has to be optimal.

The higher this ash reduction is by the curing process itself, the less and simpler extra means, such as washing and wash water quantity, heating-up of the incoming, iliassecuite etc. are required for further ash removal. , .A<

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2722 PROCESSING

The investigation of the ash reduction without washing in the curing process under different operating conditions forms an important part of the total development work of the contitluous white sugar centrifugal. The optimal operating conditions of the machine are functions of the rheological characteristics of the massecuite, which itself depends on the viscosity of the mother liquor, the mean crystal size and the crystal size distribution as well as on the crystal content.

These investigations also included tests without washing with several I types of massecuite in a laboratory centrifugal and with automatically pro- I , grammed industrial batch centrifugals.

For the interpretation and the comparability of the results of the la- boratory and the batch centrifugal with the SWS continuous machine a special number, called the specific curing effect number, has been introduced.

This number, according to:

has been derived from the laminar decantation filtration theory according to the similarity with the filtration process. With this number in the near future a separate publication will deal more extensively.

Also Strawinski25~26 has indicated special numbers in relation to several types of centrifugals and other equipment for sedimentation and filtration, whereas Pause27~28 has used a curing effect relating to batch centrifugal processes.

On behalf of our investigations the specific curing effect numbers for the afore said three centrifugals have been calculated.

Concerning the laboratory and the discontinuous centrifugal mean vaIues for the mass acceleration during the acceleration and the braking period have to be taken. In the continuous centrifugal the local values lor the mass-acceleration, curing time and the layer thickness for each point on the basket have to be determined for the calculation of the specific curing effect number.

Plotting the ash contents of the sugars of a massecuite against its caI- culated specific curing effect numbers, a straight line is derived (Fig. 7). This line represents the relation between the ash reduction of the sugar from the moment the decantation of crystals took place, till the moment of discharge and the specific curing effect numbers (line I ) . This in- vestigation also shows that the ash reduction of the sugar of a given massecuite in a laboratory and in a discontinuous centrifugal occurs according to the same-straight line (see indication on line I) as with the continuous centrifugal. This means that for a given massecuite the ash re- duction of sugar in a continuous and in a discontinuous centrifugal se-

C. HOKS

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2724 PROCESSING

paration process is fully comparable in terms of specific curing effect numbers. In order to establish a comparison between the new and the existing batch type centrifugal the application of the useful method of specific curing effect numbers has been oI inestimable value.

The slope of the straight line expressed as tangens a (Fig. 7 ) is a function of the crystal size and its size distribution.

The level of the line (line 11, Fig. 7) depends on the viscosity of the mother liquor in the massecuite. The higher the viscosity is, the higher will be the level of this line.

Results

The experimental continuous centrifugal (SWS) was integrated into a production system for a white sugar and parallelly operated with a fully automatic di~c~ontinuous machine (DC). Tests with and without washing were carried out on both machines and samples of massecuite, run-offs and sugar have been taken.

Curing the same massecuite the following average figures resulted:

a ) Capacity: abt. 9.5 - 10 tons of sugar per hour

massecuite Brix = 92 Purity d = 97 Crystal content = 56.1/2% on weight

ci

b) Sugar Quality (in co4parison with the sugar fiom the discontin- uous centrifugal D.C. ) b'

- ash content of sugar (% ) :

SWS-Exp.: abt. 0.02 % (with washing) . 9 , 0.064% (without washing)

D.C. . 7 ) 0.01 6 % (with washing) . 3 ) 0.062% (without washing)

- colour (Icumsa-units)

SWS-Exp.: abt. 54 (with washing) . 9 - 1 18 (without washing)

D.C. . 7 7 61 (with washing) . 9 9 123 (without washing)

- moisture content in sugar (%) with washing:

SWS-Exp. : 0.23% (!) D.C. 1 .OO%

D. HOKS 2725

- crystal breakage in sugar (%) : Compared with the sugar from the laboratory centrifugal (crystal breakage = 0 % ) it has been found that the same in the discharged sugar from the SWS exp. machine is less than 2%.

1 Brix Purity

g,*;:r Laboratory centrifugal (without washing) 81.5 91.0 ,+ SWS-exp. (without washing) 82.0 91.4 SWS-exp. (with washing) 77.5 92.4 D.C. (with washing) 75.0 91.8

I d) Power consumption.

The consumed driving power of the continuous experimental centrifugal amounts to 27 kW constantly, which means a specific

I power consun~ption of abt. 2.7 kwh per 1 ton of sugar produced.

CONCLUSIONS

From the investigation on the SWS experimental continuous centrifugal and comparing the results with that of the discontinuous centrifugal it can be concluded that the new priniciple of continuous curing looks promising for the application on high grade massecuites for the continuous production of white sugars.

The introduction of the specific curing eifect number as a method for fcomparison with the batch curing process is a useful tool for finding the optimal curing conditions of this new concept of a continuous centrifugal as well as for providing design data for its further development and its in- dustrial application.

NOMENCLATURE

= mass acceleration (dimensionless) = local diameter of the conical basket .= mean diameter of the conical basket = height of the sugar layer on the basket z specific curing effect number z mass of the discharged sugar = a constant z retention time = relative speed = correction factor = speed of the centrifugal basket

(seconds) (m/sec. )

2726 PROCESSING

i = total gearing ratio of the driving unit for the relative motion

Y, = specific weight of bulk sugar (kg/dm3 8 = mean thickness of the sugar layer (mm)

ACKNOWLEDGEMENT

The author wishes to thank the Directors of Messrs. Tate & Lyle Ltd. for their kind permission to carry out tests with the new centrifugal at the Thames Refinely in London and to express his special indebtedness to Messrs. H. Wheeler, S.W. Copp and R. Lixenfield for their invaluable help and cooperation.

I 1 f .

REFERENCES

1 . Sockhill, B.D. (1964). Continuous Fugal Operations on Cane low-grade Mas- secuites. Proc. Q.S.S.C.T. 31: 271.

2 . de Saint Antoine, J.D. de R.. Wiehe, F. (1963). Untersuchungsergebnisse beim Schleudern von Nach~roduktfiillmassen in hochturigen Zentrifugen. Z. Zuckerind. -

13 (9) 511. 3 . Keller, A.G. Continuous Centrifugals Operating Problems (1964). Sugar J. (26)

11, 46. 4 . Anon: Symposium on Continuous Centrifugals. (1964). SIT. Proc. 180-191. 5 . Birkett, L.S. Continuous versus Batch Centrifugals for centrifuging Low Grade

Cane Sugar Massecuites (1966)4 Proc. Meeting, British West Indies Sug. Techn. NOV. 1966, 331-337.

6 . Weismann, B.J., Gilette, E.D. Continuous Centrifugal for Granulated Sugar (1967). SIT. Proc. Vol. XXVI ( I ) , 84-94.

7 . Kirby, L.K., Atherton, P.G. The performance of continuous centrifugals (1974). Proc. ISSCT XV Congr. Vol. 3, 1206.

8 . Hugot, E. (1972). Handbook of Cane Sugar Engineering (2""ed.) Elsevier Publ. Comp. Amsterda~niLondon/New York 748-757.

9 . Eastaughffe, G.F. (1972). An analysis of continuous centrifugation. Proc. Q.S.S.C.T. 39, 223.

10. Kirby, L.K. (1969). Continuous Centrifugals, Q.S.S.C.T., 36: 77-84. 1 1 . Siepe, W. (1964). Uber die Kornzerstorung in kontinuierlich arbeitenden

Stromungszentrifugen, Zucker 17 ( 1 ) 15. 12. Grimwood, G.F., Ferrier D.C. (1968). Improvements in or relating to the Dis-

charge of Solid Particles from Centrifugal Machines British Patent no. 1293761. 13. Buckau R. Wolf. (1969). Kontinuierlich arbeitende zent?ifuge German Patent

no. 1953965. 14. Soc. Fives Lille-Cail. (1963). Essoreuse centrifuge continue avec dispositif rC-

ducteur de la force B la sortie du panier French Patent no. 1375.219. 15. Soc. Fives Lille-Cail. (1974). Perfectionnements aux essoreuses centrifuges B

marche continue, French Patent no. 2.187.422. 16. Soc. Fives Lille-Cail. (1973). Dispositif de reception des pgrticules solides dans

un skarateur centrifuge B mardhe continue French Patent no. 2.184.502. 17. Soc. Fives Lille-Cail. (1974). Procede de ralentissement des particules solides

projeties hors du panier rotatif d'une essoreuse centrifuge B marche continue et essoreuse pour la mise en oeuvre de ce procedi, French Patent no. 2.186.298.

18. Halder, J. (1971 ) . Kontinuierlich arbeitende Zentrifuge, Austrian Patent no. 313825.

19. Ahari, D. Genotelle, J. (1975). Etude du comportement de centrifuges continues en bas-produits de sucrerie, Ind. Alim. Agr. (92), 837-842.

20. Niediek, E.A. (1971). Untersuchungen zur Zerkleinerung von Kristallzucker. Z. Zuckerind. 21 (9, lo ) , 432-492.

21. Norbury, R.J., White, E.T. (1973). Preliminary studies on the breakage of sugar crystals on impact; Proc. Q.S.S.C.T. 40, 171.

22. Swindells. R.J.. White. E.T. (1975). The motion of crystals leaving a continuous fugal; roc. Q.s.s.c.?.' 42, 283.

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23. Chapman, F.M. (1963). (Honig, P.) Principles of Sugar Technology Vol. 111, chapter 5 (223-260), Elsevier Publ. Company, Amsterdam.

24. ~okolow, W.J. ( 1971 ) . Moderne Industriezentrifugen, VEB Verlag Technik, Berlin.

25. Trawinski, H. (1 954). Zentrifugen, Trenngerate mit hochster Abscheidungswirkung, Chem.-1ng.-Techn. 4 (26), 189-201.

26. Trawinski, H. (1955). Versammlungsbericht Cheml-1ng.-Techn. 6 (27), 372. 27. Pause, K. (1961 ) . Zur Leistungsbeurteilung von Weisszuckerzentrifugen, Z. Zuc-

kerind. 6 (111, 328. 28. Pause, K. (1963). Die Zentrifugenentwicklung am Scheidewege. Z. Zuckerind. 3

(13), 138. 29. Eichhorn, H. (1966). Uber das Trennen von Kristall-Sirup-Gemischen mit Zen-

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Zucker in Zentrifugen Z. Zuckerind. 5 (17) 246.

DESARROLLO DE UNA CENTRIFUGA CONTINUA PARA AZUCARES BLANCOS

Ill D. Hoks

RESUMEN I

Este trabajo explica y aporta datos y algunos detalles de la construccion, incluyendo ademas el principio basico de trabajo de la centrifuga continua tip0 SWS, patentada, para purgar masas coci- das de alto grado. Las pruebas con una maquina experimental mues- tran que se puede obtener la misma calidad de azlicar y la misma capacidad de una centrifuga no-continua programada automatica-

I mente. Se dan algunos resultados promedio, comparandolos con 10s de una maquina por cargas.

Con el fin de hacer posible tales comparaciones, se ha esta- blecido el nlimero especifico de eficacia de la purga, demostrandose su utilidad.

Como resultado de la experiencia con la centrifuga experimen- tal se ha llegado a la conclusion que este nuevo principio sobre la purga continua tiene un gran potencial para aplicarlo al azllcar blanco.