DECANTER CENTRIFUGES FOR DE-LIQUORING SUBSIDER UNDERFLOW IN THE CANE SUGAR INDUSTRY

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1 DECANTER CENTRIFUGES FOR DE-LIQUORING SUBSIDER UNDERFLOW IN THE CANE SUGAR INDUSTRY J.J. Bhagat' & P.N. PastakiaZ ' Sugar Technology Mission, Apt. D-5, Qutab Hotel, New Mehrauli Road, New Delhi Pennwalt India Ltd., 507 Kakad Chambers, 132, Dr. Annie Besant Road, WorIi, Bombay Factory: Processing ABSTRACT The conventional rotary vacuum filters in the cane sugar industry suffer from various drawbacks including lower solid retention, high sugar loss and use of bagacillo as filter aid. Trials were therefore conducted with decanter centrifuges for de-liquoring the subsider underflow at Valsad Sugar Factory, Gujarat, India. A plant-scale primary decanter centrifuge along with a pilot-scale secondary decanter centrifuge and other ancillary equipment were installed for this purpose during the campaign. The trial established the commercial possibilities of replacing the conventional rotary vacuum filters w!th the decanter centrifuges to overcome the above drawbacks. The performance results indicate that it is possible to reduce sugar loss (up to 50% lower) in decanter cake and retain solids up to 98 % compared to 70 % with vacuum filters. Keywords: Sugarcane, de-liquoring, decantering, bagacillo, centrifuging, filtrate, India. INTRODUCTION Many cane sugar factories use vacuum filtration to de-sweeten the subsider (clarifier) underflow. The conventional system consists of a rotary vacuum drum filter fitted with perforated screens. The underflow from the clarfier to vacuum filters is mixed with fine bagacillo to aid filtration. The vacuum filtrate contains considerable insolubles and is therefore recycled with raw juice from the ex~action plant, This system of vacuum filtration has serious drawbacks such as low solid retention, high sugar loss in filter cake, recirculation of filtrate to process, and use of bagacillo which causes environmental pollution. Various alternatives were therefore reviewed with the intention of identifying reliable, efficient equipment for de-liquoring the subsider underflow (Sheih 1986). It was observed that industries such as the paper, chlor-alkali, fertilizer, petrochemical and other chemical industries use decanter centrifuges for solid-liquid separation. It was therefore decided to conduct trials with decanter centrifuges as a replacement for conventional rotary vacuum filters to de-sweeten clarifier underflow. Accordingly, a system consisting of decanter centrifuges was installed at Valsad Sugar Factory, Gujarat, India for hatidling the clarifier underflow at a crushing capacity of TCH during the campaign. SYSTEM DESCRIPTION The decanter (Fig. 1) is a continuous-scroll discharge centrifuge comprising a horizontal solid bowl spinning around a central axis. The solid bowl has a cylindro-conical collfiguration, within which rotates a helical screw conveyor of the same profile, but at a lower speed than the bowl. The differential speed between the bowl and the conveyor is determined by a two-stage epicyclic differential gear box. The feed slurry enters through a stationary feed tube into the feed zone of the conveyor and discharges onto the rotating bowl wall, subjecting it to a centrifugal force of > 2100 g. The denser solid particles settle at the rotating bowl under the action of centrifugal force, and the less dense liquid forms an inner concentric layer, which overflows as "centrate" (filtrate) over adjustable dam plates. The sedimented solids are contitiuously removed from the bowl by the helical screw conveyor and are plowed out of the "pond" and up the conical "beach." The centrifugal force compacts the solids, expels surplus liquor and discharges the cake from the rear end.

2 J.J. Bhagat & P.N. Pastakia I ' The system installed at Valsad Sugar Continuous Operation Factory comprised of the following main equipment: - A plant-scale Pennwalt Super-D-Canter centrifuge as primary unit - A pilot model Pennwalt Super-D- Canter centrifuge as secondary unit Solids Discl~argc Brief specifications of the Super-D- Canter centrifuges used for plant- and Figure 1. Decanter centrifuge. pilot-scale trials are given in Table 1. The purpose of installing a plant-scale primary unit was to ascertain the capacity requirements accurately when the equipment is run on a continuous basis. It also enabled an assessment of the equipment for commercial applications. In addition, the following ancillary Table 1. Specification of decanter centrifuges used in the trials. - - eauipment was installed to make up the m r y Secondary complete system. Machine super-d-canter, super-d-canter, - A HDPE-agitated tank for parameter ) plant scale pilot scale preparing polyelectrolyte stock solution Bowl speed rpm A agitated repulp tank Centrifugal force Xg A centrate receiving tank Beach angle A receiving-cum-repdp tank Drive motor kw A feed slurry tank Power at full load kw A set of metering pumps Construction material - A set of pipe lines, valves, flow Special features SS.3 16 SS.304 meters, etc. Differential Differential The general arrangement of the system is shown in Figure 2. speed.control speed control & rinse Figure 2. Trial arrangement.

3 Factory: Processkg METaODOLOGY The subsider underflow was drawn into a agitated receiving tank, from where it was pumped to the plantscale Super-D-Canter e dge. An in-line magnetic flow meter continuously monitored the feed rate to the machine. The cake discharge was repulped in a agitated tank and pumped to the pilot-scale secondary Super-D-canter centrihge. The pilot model machine was equipped with an effective beach wash arrangement to reduce sugar loss in cake. The centrate from both the primary and secondary Super-D-canters was mixed and pumped to the flash tank of the existing Dorr Clarifier. The cake from the secondary Super-D-canter was dumped for disposal. After meeting the requirement of the pilot centrifuges, the excess of repulped material from the tank was pumped to the existing vacuum filters for further de-sweetening before discharge. Different grades of polyelectrolyte (Elennet 1973) were used on a trialbasis at the decanters. All these were anionic (Hill 1972), with a medium- to high-anionic charge, and were obtained as solid powder from reputable manufacturers such as Allied Colloids, Sandoz, Dai-ichi Karkaria. The polyelectrolyte was dissolved in a HDPE tank to form stock solution (corn to 0.5 % w/w) and fed to the decanter through metering pumps with on-line dilution through a static mixer. Trials were conducted for a 3-mb period-from Feb until the end of the crushing campaign. Various performance data were collected during the trials (Table 2). Table 2. Performance results. Primary Plant Scale Super-D Canter at a Secondaty Pilot crush rate of Scale Super 100 TCH D-Canter Machine Parameters Bowl speed rpm Differential speed rpm Pool depth rpm Feed Underflow Repulp4 Rate m3/hr Isolubles % W/W Temperature " C Centrate Insolubles % W/W Bx % 14.2 Pol % 12.4 Purity 87.0 Cake Pol % Moisture % (estimated) Wet cake, rate t/hr Flocculant (Anionic) Concentration % W/W Rate t/hr Dosage Polymer (kg/t dry solids) Solid Recovery % Water (repulp & rinse): wet cake ratio

4 J.J. Bhagat & P.N. Pastakia RESULTS AND DISCUSSIONS Prior to commencing the trials it was thought that the primary decanter centrifuge might not be able to de-sweeten the clarifier underflow sufficiently when used as a stand-alone unit. Accordingly, pilot Super D-canters were installed as secondary units. Results gable 2) indicate that the pol % caki from the primary decanter centrifuge was ; from the secondary decanter, The moisture % cake in each case was %. The primary plant-scale centrifuge was able to handle the entire underflow from the existing clarifier at the rate of m3/h, corresponding to a crush rate of 100 TCH, with 98 % solids removal. The centrate insolubles were % wlw compared to the vacuum filtrate insolubles (ranging from 1.O-1.5 % wlw), making it technically possible to mix the centrate in the clarifier flash tank, unlike the vacuum filtrates which necessarily have to be recycled to the raw juice stage. The purity of centrate was 2-3 units higher than the vacuum filtrate. A comparison of the performance of decanter centrifuges with vacuum filters (Table 3) clearly indicated that the decanter centrifuges have an edge over the conventional vacuum filters in terms of high solid retention, lower sugar loss, better centrate quality and lower moisture in cake. In addition, the decanter centrifuges, unlike vacuum filters, do not need bagacillo as a filter aid, thereby avoiding air and process contamination. For effective centrifugation, however, polyelectrolytes are required at kglt dry solids. The process water requirement at the decanter centrifuge is also 15-40% higher than the cake wash water requirement at the vacuum filters. The water requirement can possibly be reduced through counter-current leaching and the beach wash arrangement shown in Figure 3. The space requirement for the decanter system is one-third that for a vacuum filter system of similar throughput. The power requirement was 40 % lower for equivalent solid retention. The trials also established the versatility of the decanter centrifuges in handling thin underflow with insolubles <6% wlw. The conventional vacuum filter system has an optimal performance at an insolubles solid concn. of 6 % WIW (Hugot 1972). Table 3. Performance comparison. Performance Centrifugal separation Vacuum filters parameter (2 nos super-d-canters) (2 nos 40 mz F. A. each) Feed rate (m3/h) Solids retent. (% wlw) Power consumption at rnax. load (kw) Polyelectrolyte addition (kglt dry mud) Bagacillo addition (kglt dry mud) % ret. 667 Filtrate or centrate primary Heavy Light Mixed Brix (%) Pol (%) purity Insolubles (% wlw) Cake discharge Pol (%) Moisture (%) Mud solids (%) Bagacillo (%) Wet cake (kglt cane) Water demand Waterlwet cake Water (kglt cane) Sugar loss (kglt cane) Space required (m3)

5 Factory: Processing Hot water I Subslder underflow (If required) Slgma Mlxer or Rlbbon Blender t + Rich centrate (To subslder 'lash tank) Repulp slurry Lean ~ehtrate (through inbuilt centripetal pump) 1 Figure 3. Counter current leaching and beach wash arrangement. Cake (To dump) n CONCLUSIONS The decanter centrifuge trials have convincingly demonstrated the possibility of economic replacement of the existing rotary vacuum filter system for handling clarifier underflow in a cane sugar plant. The decanter centrifuges are versatile and offer superior technical performance in terms of lower sugar loss, high solid removal and low power and space requirements. Their application will also eliminate the contaminating effects on the environment and process, arising from the use of bagacillo as.a filter aid in the conventional system. ACKNOWLEDGMENTS Our sincere thanks are due to the management and technical officer of MIS. Valsad SKUM Ltd. for the facilities provided for trials at their sugar plant. We also express our gratitude to the Secretary, Dept. of Science & Technology for his encouragement and financial support for conducting the trials. REFERENCES Bennet, M.C. (1973). Proceedings Jamaican Association of Sug. Technol. Hill, W.B. (1972). Filtration and Separation 9:681. Hugot, E. (1972). Handbook of cane sugar engineering. 2nd ed., Amsterdam, Elsevier, p 480. Shieh, -M.C. (1986). The continuous centrifugal clarifier. ISJ 88: 1049 p