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Factory Engineering

CONTINUOUS THICKENING AND ROTARY VACUUM FILTRATION OF FIRST CARBONATED CANE JUICE IN INDIA

N.N. Joshi & S.K. Goel*; and R.M. Rahul* * "Daurala Sugar Works, Daurala, U.P., India; and "Worr-Oliver (India),

Limited, Bombay, India

ABSTRACT

Continuous thickening and filtration of first carbonated can juice could not be adopted until 1970 because of various anticipated problems. Installation of the first large scale continuous thickening and filtration plants followed the successful trial of a semi-commercial scale plant at Mawana Sugar Works, Mawana, India. The equipment for handling 2500 tons of cane per day was installed in 1972 at Daurala Sugar Works, Daurala, and also at Mawana Sugar Works, Mawana. The installation comprised of 2 Dorr thickeners, type 444, and 3 Dorr-Oliver rotary vacuum belt filters ,at each of the above factories. The installations are now working quite satisfactorily and benefits including reduction in sugar loss, labour requirement, steam consumption and cost of filter cloth have been achieved and have justified the investment.

INTRODUCTION

The carbonation process for cane juice clarification was first adopted, in 1876, in Java. This process came to be followed in India on a large scale about four decades ago. The classical DeHaan process for first and second carbonation has been successfully used by as many as 35 Indian sugar factories. The growth of these factories which are mostly located in the northern part of the country has been encouraging and these factories have expanded their crushing capacity in the early stages of development.

In carbonation cane sugar factories, conventional plate and frame type filter presses are in use with cotton cloth as filtering media for the filtration of first carbonated juice. Such filter presses have draw-backs including high sugar loss in cake, requirement of a large labour force, discontinuous oper- ation and poor working conditions.

The continuous thickening and rotary vacuum filtration is free of the above drawbacks. The need for continuous filtration of juices was, therefore, long felt in the factories adopting the carbonation process. It has been a firm conviction that, for the success of continuous thickening and rotary vacuum filtration of first carbonated juice, it is imperative to have a contin- uous first carbonation system. The beet juice carbonation system had a smooth transition from a batch-wise first carbonation treatment to a contin-

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uous treatment at a very early stage of development. Continuous thicken- ing and rotary vacuum filtration of first carbonated beet juice was also adopted at an early stage. The cane sugar industry tried for a long time to develop continuous first carbonation, but cane juice characteristics, and the equipment designs, did not bring much success to their endeavours. Continuous first carbonation of cane juice has, at present, still t~ be firmly established. Nevertheless, in view ol the enormous advantages of vacuum filtration, tests were carried out on continuous thickening and vacuum filtration of first carbonated juice, with batch-wise first carbonation treatment, using gravity type thickeners and rotary vacuum filters fitted with a belt discharge arrangement.

MATERIALS AND METHODS

To a practical technologist, sedimentation generally means gravity set- tling or subsidence of solids suspended in liquids. The sedimentation unit as applied to first carbonation cane juice is termed the thickener instead of the clarifier, since the emphasis is on compacting the suspended solids into a dense mud. Thus the main objective of the carbonation thickener is to reduce the bulk and attain a sludge of about 10% consistency which is to be filtered through a rotary vacuum filter. Higher consistancy would be welcome, but longer detention of carbonated juice in contact with mud raises other problems such as destruction of sugar and reducing sugars resulting in purity drop and colour formation etc. The consistency of the thickened juice can also be increased by increasing the settling rate with the use of a flocculating agent. In India, howedkr, no proper flocculating agent has come into use, primarily on account of import restrictions and unavailability of suitable indigenous flocculants. As such, the technique of batch carbonation is all that is to be relied on for producing fast settling solids.

When reasonably uniform settling characteristics of solids in the feed had been ensured, a suitable thickener was selected. Earlier experience indicates that the multiple compartment tray thickeners provide maximum settling area with ~ninimum floor space and usually comprise no more than four compartments superin~posed on each other. This design is also most suitable from the angle of heat insulation. The multi-compartment type thickener has been developed to have unit or trayless thickener features in which each compartment of the thickener comprises a shallow, cylin- drical tank with a sloping bottom and a slowly rotating rakin mechanism, to sweep the settled solids to the central discharge opening. At the top of the compartment near its periphery an overflow manifold is placed to remove supernatant liquid. Juice in each conlpartment is independently fed through a central tube (see Fig. 1).

N.N. JOSHI, S.K. GOEL AND R.M. RAHUL 239 1

FIGURE 1 . General arrangement RapiDorr 444 cane juice clarifier.

Alter close study it was found that, under ideal conditions, 45 minutes detention in a thickener of this design can be used and, in most cases, not more than 60 minutes detention can be allowed.

The filtration unit used is a conventional rotary filter. The filtering medium, instead of being crimped on the drum and secured by a stainless steel wire, is in the form of a wide endless cloth belt whose ends are sewn together. The general arrangement oE the belt discharge filter is shawn in Fig. 2.

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FIGURE 2. General arrangement - Dorr Oliver Belt Filter.

BELT TRACK PO

BOWED PIPE

' TAKE L'F ROLL

OVERFLOW CONNECT

-- - SWING AGITA AGITATOR DR

Laboratory tests were conducted and ultimately a semi-commercial

scale plant comprisiilg one 3 metre dia. RapiDorr 444 type thickener with

three compartments and one 3 metre dia. x 4.3 metre face Dorr Oliver

rotary belt filter for thickening and filtration ol first carbonated juice were

commissioned in 1970 at Mawana Sugar Works, Mawana, U.P., India. The

general installation arrangement is show11 in Fig. 3.

Encouraged1 by the perforinance of this unit, it was decided to install

this system for a crushing capacity ol 2500 MTCD at both Daurala Sugar

Works and at Mawana Sugar Works (see Fig. 4).

N.N. JOSHI, S.K. OOEL AND R.M. RAHUL

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FIGURE 4. First carbonation, continuous thickening and filtration installation at Daurala Sugar Works, Daurala, India.

The following items of equipmnt were installed in each factory for the above capacity:

(a) Two 4.88 metres dia. thickeners, type RapiDorr 444, with four compartments, complete with accessories.

(b) Three 3.66 metres dia. x 6.1 metres. long Dorr Oliver rotary vacuum belt filters, with an approximate filtering area of 70 sq. metres, each complete with suitable accessories and auxiliaries.

Thickener and belt filter operation:

The first carbonated juice is fed at the centre of each compartment of the thickener and flows towards the periphery. In passing over this increasing area, the juice progressively loses velocity. A centrally located circular feedwell is designed to minimise short circuiting to the overflow manifold. Solids gradually settle on the tray, heavier ones near the centre, lighter towards the periphery. The settled solids are moved from all points on the tray towards the centre. This decreasing concentration of the settled solids towards the periphery reduces scraping load and brings about the formation of a denser and more easily filterable slurry.

The first carbonation of cane juice at high alkalinity, is sometimies accompanied by formation of loam. This ioam, in the flocculating chamber of the thickener, is swept by a set of revolving skimming blades into the foam canal. The supernatant juice and settled solids are withdrawn equally from each compartment by a gravity arrangement.

N.N. JOSHI, S.K. GOEL AND R.M. RAHUL 2395

To eliminate sugar loss due to bio-chemical activity it is necessary to clean the thickener after operation for about a week. In view of this, the provision of a standby thickener is essential.

The belt discharge arrangement of the Dorr-Oliver rotary belt filter is located at the front of the filter. The arrangement of filter cloth is such that it leaves the drum after filtration, cake washing and drying phases, in order to discharge the cake, after which the cloth is perfectly washed. It is thus possible to maintain the continuous filter rhythm for a very long period because there is no longer any additional clogging, but only one rotation clogging which is washed out with each rotation. Moreover, as the cloth is always unclogged and clean, its service life is increased.

After having left the filter drum, the cloth passes over the first 'bowed pipe' which provides support to the cake-laden cloth. It plays an important role in preparing the cake for discharge by breaking it.

FIGURE 5. Cake discharge from 12' dia x 20' face Dorr-Oliver belt Filter.

1 The cloth then passes over the second 'bowed pipe'. The sudden break in the circulation pattern of the cloth effects the discharge of the cake (see Fig. 5 ) . The pipe also serves to correct the seam line longitudinally. The belt discharge assembly is composed of two smaller diameter rolls in

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addition to the bowed pipes. The cloth is washed from both sides by a

series of flat jet spray nozzles generally supplied with hot water at a pres-

sure of 4 to 5 kg/sq.cm. Effective washing of the cloth belt is the most

important requirement ol a rotary belt filter. A periodic dilute acid (2.5% HCl) wash, generally three times a day, is given to the cloth for a short

period.

Cloth replacement in a belt iilter is very easy. The removal of the

cloth and fitting a new one on a belt filter takes no longer than 4 to 6

hours.

The belt filter technique requires use of a suitable cloth. For carbon-

ation mud synthetic nylon cloths are used. Satisfactory results from >plain

woven nylon fabrics were obtained.

RESULTS AND DISCUSSION

The semi-commercial unit' installed at Mawana Sugar Works gave

very encouraging results. It was possible to handle 700 TCD with a de-

tention time in the thickener not exceeding 60 minutes. Pol per cent filter

cake was only 0.6 to 0.8 on average, by comparison with 3 to 3.5 per

cent from filter presses. Cake wash water used on belt filters was only 6 T / I

per cent on cane as against 10 per cent for filter presses. No significant

destruction of sugar or reducing sugars was observed.

Encouraged by the above results, full commercial size units yere ins-

talled to handle 2500 TCD and they were commissioned in the crushing

skason 1972-73. The performance of the bigger units was initially not very

satisfactory due to various teething troubles. The capacity achieved was

only 60 to 65 per cent of the designed figure. Pol percent filter cake was

1.2, to 1.4 per cent with the use of 8 to 10 per cent wash water on cane.

There was also 1.0 to 1.5 unit drop in the purity of juice inside the thickener

as a result of longer detention. A number of modifications were carried out

on both process and equipment. Thereafter the workingeof the installations

has become quite satisfactory. The results being achieved now are compared

with the designed ones in Table I.

N.N. JOSHI, S.K. GOEL AND R.M. RAHUL

TABLE I. Results (designed and actually achieved) by using Dorr Oliver first carbo- nation juice thickening and filtration system.

S. Particulars Results no. Unit Designed Achieved

1 Capacity MT/hr 105 110

2 Pol in filter cake % 0.4310.1 0.7-0.8

3 Cake wash water consumption % cane 4*1 7-8

4 Retention time Minutes 60 50-55

5 Overall drop in sucrose purity of juice after thickening & filtration % nil 0.15-0.20

6 Reduction in reducing sugar content of juice % Bx nil 0.01-0.05

7 Drop in pH of juice PH nil 0.2-0.5

8 Mud solids in clear juice withdrawn from thickener % 0.3k0.1 0.25-0.3

9 Ratio of mud solids in thickened juice drawn from thickener and carbonated juice fed to thickener - 2.5 1.75-2.0

10 Colour content as determined by Lovibond tintometer in 2 . 5 4 cm cell (yellow and red constituents)

(a) Feed to thickener - - 19.5 Y, 0.9 R

(b) Supernatent overflow withdrawn from thickener - - 20 Y, 0.9 R

(c) Thickened underflow juice withdrawn from thickener - - 20 Y, 0.9 R

To improve the settling and filtration characteristics of the first car- bonated juice the following si'eps were taken:

1 . Thickened underflow juice withdrawn from the thickener was re-circulated to raw juice at a concentration of about 15 per cent of raw juice by volume.

2 . Intermediate alkalinity at first carbonation, before the addition of each dose of lime was maintained at a lower level of about 250 mg CaO/litre, while final alkalinity was kept at 400-450 mg CaO/ litre as usual2.

3. Centrifugal pumps were found to be spoiling the filtration characteristics of flocs and reciprocating pumps were therefore used for pumping the first carbonated juices.

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4. The diaphragm pump, originaily installed for the withdrawal of thickened juice, was observed to be causing too much disturbance inside the thickener due to its pulsating action, which resulted in poor settling of juice in the thickener. The mud pump was replaced by a mud box with gravity discharge of the thickened juice.

5 . It was observed that the first carbonated juice contained too much occluded air and there was gas formation in the thickener after 6-7 days use. Additional vent pipes were, therefore, provided.

6 . To improve and accelerate the flow of supernatant juice inside the thickener, the number of tappings in the manifold for the with drawal of overflow juicc were increased in each compartment of the thickener.

In addition to the above, the following steps were taken to improve the working of the rotary vacuum filters:

(a) Pressure of belt wash water at the ilozzles was increased to 4-5 kg/sq.cm. for efficient washing of the filter cloth.

(b) An additional bowed pipe was provided to improving the crack- ing and hence to give better discharge ol filter cake.

(c) Pressure and temperature of water at cake wash llozzles were increased to 4 kg/sq.cm:. and 60°C respectively to improve the removal of sugar from filter cake.

After taking the above steps, the'performance of units both a Daurala Sugar Works and Mawana Sugar Works became satisfactory. The following are the main advantages achieved after installing continuous thickeners and rotary vacuum filters:

- Reduction in sugar loss: Loss of sugar in lilter cake has been reduced by approximate 0.15% on cane which is quite a substantial gain.

- Reduction in labour: Labour requirement lor first filtration of juice has been reduced by about 80%.

- Reduction in steam consumption:

(a) Direct steam: The consumption of live steam required for drying has been reduced to nil. Previously for filter presses 1% steam on cane was used.

N.N. JOSHI, S.K. GOEL AND R.M. RAHUL 2399

(b) Reduction in wash water consumption: The wash water consumption for removal of sugar from filter calie has been reduced by about 2 per cent on cane, resulting in a saving of steain of about 0.5% on cane.

- Saving in the cost of filter cloth: The expenditure on cotton filter cloth with plate and frame type filter presses was quite high since the cloth had to be changed very frequently. Nylon filter cloth used on rotary vacuum filter workes for 30-35 days quite satisfactorily. The total expenditure on filter cloth has now been reduced by approximiately 45 per cent.

It may also be mentioned here that the following additional require- ments have arisen after the installation of rotary vacuum filters:

i) Water requirement: For washing the filter belts, approximately 70 000 litres of water per hour is required for a unit of 2500 TCD.

ii) Power requirement: Power load for operating 2500 TCD unit is 300 KW.

iii) Requirement of acid: Approximately 200 kg. of commercial hydrochloric acid is re- quired every day for the washing of nylon filter cloth.

The net saving due to the advantages has been quite substantial and the cost of the equipment installed can be recovered within three years of working.

CONCLUSIONS

The continuous RapiDorr type 444 thickeners and Dorr Oliver rotary vacuum belt filters are operating successfully at Daurala Sugar Works and Mawana Sugar Works. The earlier conception was that continuous thickening and vacuum filtration of first carbonated juice could be used successfully only by having continuous treatment of juice at the first carbonation stage. It has, however, been possible to use continuous thickening and rotary vacuum filtration, even with batch type first carbonation treatment, very successfully at Daurala and Mawana sugar plants. However, we feel that there is still scope for further improving the working of the units. The points that deserve consideration are:

(a) Production of faster settling juices to curtail detention time in thickener, either with or without the aid of flocculants. This is most important sitice it will not only further decrease purity drop in the thickener due to shorter detention time but will also give cake which will filter rapidly aqdf\wash better. Flocculants are now being used all over the world, both in the beet and the cane

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industry. In India efforts are being made to develop suitable indigenous flocculants and bring them into regular use. Further efforts in the direction of having continuous treatment of cane juice at the first carbonation stage are also desirable.

(b) Desanding of mixed juice and degritting of milk of lime are desirable. With filter presses, disposal of sand and grit along with other solids was no problem. But with the thickener and vacuum filter, the vessels of which have large enough detention time, the sand and grit settle down which requires periodic cleaning.

(c) For belt washing, about 90 litres water is needed per metre length of the filter, i.e. 550 litres per filter of 6.1 metre face length. This ,water consumption requires to be reduced.

REFERENCES

1. Singh, C.P., and Ciupta, R.K. (1971). Continuous filtration in a carbonation sugar factory. Proceedings of joint convention of STA, DSTA and SISTA.

2 . Shah, H.A., Joshi, N.N. and Goel, S.K. (1973). Studies made for improving settling rate of first carbonated iuice. Proceedines of 30th annual convention of STAI.~

- 3 . Joshi, N.N. et al. (1976). Observations on working of thickners and rosary vacuum

filters for filtration of first carbonated juice. Proceedings of all India seminar on rehabilitation and modernisation of sugar industry published by STAT, 153-162.

ESPESAMIENTO CONTINUO Y FILTRACION ROTATIVA AL VACIO DEL JUG0 DE CARA DE PRIMERA

CARBONATACION EN LA INDIA

N.N. Joshi, S.K. Goel y R.M. Rahul

RESUMEN

El espesamento continuo y la filtracion del jug0 de catia de pri- mera carbonatacidn no pudo ser adoptado hasta 1970 debido a varios problemas previstos. La instalaci6n de las primeras plantas de espe- samienta y filtracion continuas, en larga escala, vino despues del exito de una prueba de escala semi-comercial en la Compafiia Azu- carera Mawana de la India. Equipos para manejar 2500 tons de catia por dia fueron instalados en 1972 en la Compatiia Azucarera Daurala y tambien en la Compatiia Azucarera Mawana. La instala- ci6n estaba compuesta de 2 espesadores Dorr, tipo 444, y 3 filtros Oliver de rotacidn a1 vacio, en cada una de las fabricas menciona- das. Las instalaciones estan ahora trabajando muy satisfactoriamen- te y 10s beneficios obtenidos (como ~ d u c c i o n e s de perdidas de azucar, de requisitos de mano de obra, de consumo de vapor y de costos de tejido del filtro) justificaron le inversio~n.