WASHING SUGAR OUT OF CANE: A PROPOSED NEW HYDRODYNAMIC EXTRACTION PROCESS M. Rivigre Sucreria du Nord Est, Reunion ABSTRACT INTRODUCTION

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1 Factory Engineering WASHNG SUGAR OUT OF CANE: A PROPOSED NEW HYDRODYNAMC EXTRACTON PROCESS M. Rivigre Sucreria du Nord Est, Reunion ABSTRACT Modern preparation devices, by opening up to 92% of the iuice-containing cells of the cane make the extractable iuice readily available for leaching. Milling not only does not take advantage of this accessibility. The high pressure applied on the cane in the first mill separates most of the iuice, but at the same time fills the cells with air, thus irreversibly reducing the availability of the residual iuice in bagasse. No one is prepared to abandon such an efficient solid/liquid separator, although it accounts for the low efficiency of the following countercurrent imbibition process. From this point of view diffusion is sounder than milling in so far as it does not use pressure, except for the final dewatering. Nevertheless, stage efficiency in diffusion is no better than it is in milling and this low efficiency has to be compensated for by the multiplication of the number of stages, 12 to 18 being necessary when 5 to 7 are theoretically needed. Diffusion is a hydrostati; displacement process. A hydrodynamic process is needed to wash sugar out of cane efficiently. NTRODUCTON Extraction of iuice from sugar cane has long been a simple solid/liquid separation process. The amount of iuice extracted by dry crushi'ng in sugar cane mills was gradually improved, but pol extraction remained low because the residual iuice in final bagasse was nearly as rich as the original juice in cane. A great improvement was achieved with the introduction of the imbibition process. Water was used to dilute the residual juice, thus increasing pol extraction by reducing the amount of sugar lost in final bagasse. The more efficient the dilution process, the higher the pol extraction. mbibition is applied on cane in the diffusion process and on bagasse in the milling process. n both cases the dilution required cannot be achieved in one operation only. lrnbibition has to be a multistage countercurrent process. The number of stages to be used depends on the dilution efficiency as well as on the separation efficiency. Dilution efficiency (or stage efficiency) can be assessed as: Achieved drop in brix through each stage Achievable drop in brix Whether the drop in brix is achieved by mixing of the iuices (milling) or by displacement (diffusion) is immaterial. For the same efficiency a mill tandem, due to its higher separation efficiency (70%), will achieve a given pol extraction with less stages (3 to 6) than a diffuser, where the low separation efficiency (30%) has to be compensated for by the use of more stages (12 to 18). For a given separation efficiency, the number of stages needed to achieve a given pol extraction varies widely with the stage efficiency. f stage efficiency is 100% the number of stages theoretically needed to achieve 98% pol extraction is: 1 to 2 stages in milling and 5 to 7 stages in diffusion, depending on the rate of imbibition. The number of stages used in practice is 2 to 4 times higher, which gives a measure of practical stage efficiency. theoretical no. of stages needed Stage efficiency = = 20 to 50% actual no. of stages used

2 The basic reason for this very poor stage efficiency is that imbibition is practised when dilution is required, ie the mixing of two liquids. The first condition is to make the extractable juice accessible. n properly prepared cane up to 92% of the iuice can be made accessible by opening the cells using preparation devices such as heavy duty shredders or fiberisers. Further opening of the cells occurs in milling by the churning effect of the grooved mill rollers. n diffusion, scalding helps making the cell juice accessible by softening the cell walls, thus rendering them permeable. n bagasse, even if 100% of the cells are opened, juice is not readily accessible, because the high pressures applied to the cane particles in the first mill have converted them into small sponges. t has been known for a long time that it is!he presence of air inside the bagasse particles that makes the washing difficult. To avoid the air problem: (i) air can be prevented from entering the bagasse particle when it emerges from the back-roll ~f the mill (Hugot'). The idea is sound but difficult to implement (ii) air can be removed by applying imbibition under vacuum. This was done on an industrial scale at Quartier-Francais Factory in Reunion for three consecutive years in the mid seventies, using a process patented by FCB-Maxime RiviBre. Stage efficiency was increased from 35 to 95%, but only with difficulty (iii) apply imbibition to the shredded cane and not to bagasse, which is the only easy solution. The extractable iuice in cane must be diluted before squeezing it out un- \ der pressure. The "low pressure extraction system" developed in Brazil (Liebig2) is an improvement on conventional diffusion in so far that the separation efficiency is increased using low pressure rollers. These rollers must be set to the no void volume of cane in order not to allow air to enter. But stage efficiency in the LPE system is only slightly in excess of 50%. t involves only 9 stages instead of the 12 to 18 in conventional diffusers. But 4 stages should be enough. Stage efficiency still has to be improved. n diffusers, even when the liquid content of the mixture is almost great enough it is still a hydrostatic process in which no agitation is applied. HYDRODYNAMC EXTRACTON PROCESS A fully hydrodynamic process is already used in laboratories for assessing the sugar content of a sample of cane or bagasse. First, enough water is added to make a liquid mixture of 5% fibre content. This slurry is then agitated in a high speed wet disintegrator, the stage efficiency being 100%. On an industrial scale adding so much water is not practical because it has to be evaporated in the boiling house. So recirculation has to be resorted to. The rate of recirculation must be such that it reduces the fibre content of the slurry to not more than 7%. This slurry then has the hydrodynamic properties of a liquid. n other words it should be possible to pump it. Complete dilution of the extractable iuice is then easily obtained by agitation. NUMBER OF STAGES The number of stages needed, assuming 100% stage efficiency, depends on imbibition rate and separation efficiency. The effects of those two parameters are best shown by making use of the diagram shown in Fig 1. By increasing the imbibition rate from 2.2 f to 3.5 f saves one stage. Fig 2 shows the effect of separation efficiency on the theoretical number of stages required. n milling, if stage efficiency were TOO%, 95% extraction would be obtained with only 226

3 FACTORY ENGNEERNG FNAL BAGASSE 4 98 % EXTRACTON NUMBER OF STAGES MXED JUCE Figure 1. Ponchon Savarit diagram for conventional diffusion: theoretical number of stages needed dependent on imbibition rate /. /,.,/,,- /,,,, ---,, 1 / ->'A /'.. / -." DFFUSER LPE EXTRACTON SYSTEM MLLNG Figure 2. Ponchon Savarit diagram: theoretical number of stages needed dependent on separation efficiency 217

4 two stages using an imbibition rate of 2.2 f and dewatering final bagasse to 50% fibre. The low pressure extraction system would give 96.5 extraction. Under the same conditions conventional diffusion should give 96% extraction after five stages. Fig 3 shows the number of stages to be used with 100% stage efficiency in a "no pressure" hydrodynamic extraction system. This diagram is based on the actual milling con- 1 trol figures of Beaufonds sugar mill in After the first stage, if the material leaving i were dewatered to the moisture content of the bagasse delivered by a first mill, the pol content would be 5.81% and the first stage extraction 82.5%. 1 After seven stages, if the shredded cane were dewatered to 50% fibre content, overall extraction would be over 98%, using only one mill. 1 ilj N' of stages needed for a target extraction of 98 ): CANE /,',,,',,' --' #,, <.,.' - -, ,',',' ; Figure 3. Ponchon Savarit diagram: using Beaufonds mill control figures for 1988 crop: Cane pol 13.07, imbibition 2.2 f, Fibre STAGE EFFCENCY Pilot plant At Quartier-Francais factory in 1981 a pilot plant (Fig 4) was designed to simulate hydrodynamic washing of shredded cane, using diluted juice successively from mill numbers 2, 3, 4 and 5 and finally water. The juice in cane was diluted to a final residual l juice of less than 3.5 brix after only five stages. After dewatering in a static hydraulic press to a final moisture content of 55%, a pol extraction of 97% was obtained, so the 1 stage efficiency must have been close to 100%. This hydrodynamic process was patent- ed in First industrial scale experimentation (cane lixiviation) Quartier-Francais factory was closed down at the end of the 1982.crop, and it was not until the end of the 1986 crop that it was possible to experiment with cane lixiviation on an industrial scale at Beaufonds factory. i

5 AR AR l BAR Figure 4. Hydronamic cane lixiviation pilot pl rtier-francais 1981)

6 For 24 hours (the last day of the 1986 campaign), the first mill was by-passed and the shredded cane sent to the bagasse lixiviation cell in which iuice from mill No 3 was applied as in normal bagasse imbibition (Fig 5). Recirculation of the iuice was used to lower the fibre content of the mixture. The drop of brix through this cane lixiviation stage was 84% of the achievable drop, and stage efficiency was therefore 84%., ', CANE f, 1, STAGE EFFCENCY: 84% 35% 21 '% OVER ALL EXTRACTON 95% Figure 5. Beaufonds 1986 (3 mills): flow diagram of 3 mills tandem with: 1 cane lixiviation stage 2 bagasse imbibition station MBBTON EFFCENCY FRST STAGE SECOND STAGE THRD STAGE FOURTH STAGE n = 74.33% n = 34,965 n = 55.11% n = 44.6% BRlX EXTRACTON 80.27% 39.34% 42,11% 37.38% JUCE EXTRACTON 84.18% 74.9% 73.98% 74.67% Figure 6. mbibition Efficiency 230

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8 EXTRACCON QEL AZ~~CAR DE LA CANA POR LAVADO: UN PROCESO DE EXTRACCON NUEVO HDRODNAMCB M. Riviere Sucriere du Nord Est, Reunion RESUMEN Por procesos modernos que abren hasta 92% de las celulas que lo contienen el iugo extractible de la cafia es facilmente puesto en condiciones de lixiviacibn. Solamente 10s molinos no facilitan esta acessibilidad. La alta presi6n aplicada en el primer molino extrae la mayor parte per0 al m'ismd$iempo rellena de aire a las c6lulas, reduciendo de manera irreversible la dispo"ibil!idad Ael iugo residual del bagazo. No hay como substituir este eficiente separador s6lido-liquid0 cuya baja eficiencia es compensada con el uso de embibimiento en contra corriente. Desde este punto de vista la difusidn es mas perfecta que la molienda pues no usa presi6n a no ser para el secado final del bagazo. Sin embargo, la eficiencia de la difusibn no es mejor que la de 10s molinos y su baio desempefio tiene de ser compensado por una multiplicaci6n de escalones, 12 a 18 contra 5 a 7 teoricamente necesarias. L& difusi6n es un proceso de desplazamiento hidrostatico pero, para una eficiente extrdcci6n del azljcar'de la cafia es necesario un procedimiento hidrodinamico.