Invert sugar from sugar cane molasses : a pilot plant study

摘要

An investigation was done into the recovery of invert sugar from sugar canemolasses. A pilot plant was designed and constructed to evaluate the clarificationand separation of molasses to produce invert sugar syrup. The aim of the pilotplant was to prove the process and deliver data so as to facilitate the design andprove the financial viability of a commercial plant. The pilot plant had to process300 kg/day of molasses.The clarification of molasses by centrifugal separation, a known desludgingprocess, did not produce a product of acceptable quality which could be used in achromatographic separator. The results were disappointing. The productobtained was also not suitable for dead end pressure filtration. The turbidityremaining after the centrifugal separation also did not respond to a secondflocculation process.Conventional settling clarification was investigated. Seven factors that couldinfluence the consolidation and settling of suspended solids in molasses wereidentified, namely: the age of the diluted molasses, the temperature of theflocculated mixture, the variations across various batches of flocculant, the effect ofreaction time of the phosphoric acid, the optimum flocculant dosing concentration,the optimum concentration of the molasses solution and the effect of increasing theacid dosage.The optimum conditions to clarify molasses through settling were found to be: freshmolasses, at 28 Brix and 60°C, allowing 10 min intervals between acidification with3,75 g (as 100%) phosphoric acid/ kg dry material (assumed equal to Brix) andneutralisation with 5 g (as 100%) caustic/kg dry material (assumed equal to Brix),flocculation with any batch of flocculant 6195, dosed as a 1000 ppm solution.Commercial equipment was evaluated. The pilot plant E-cat clarifier was operatedat 300 l/h and a thick sludge formed. The overflow was clear and it could befiltered. The molasses obtained was suitable for chromatographic separation.The recovery of sugars from molasses sludge has economic merit. From theevaluation of centrifugal separation and gravity separation it is clear that gravity separation again is the best method. The sweet-water obtained is consistent withinthe clarity requirement of 10 NTU/Brix and can be used to dilute raw molasses inthe upstream processing step. The clarification process that was developed ispatented. [Bekker, Stolz et.al. (2001)]A sugar recovery of 93.9 mass % at a purity of 99.7 mass % from molasses, wasachieved using a simulated moving bed, ion exclusion, pilot plant. The operatingconditions for this performance were: feed flow at 14 l/h and at a temperatureabove 60 °C; water flow at 63 l/h and at a temperature above 65 °C; extract flow at21 l/h; raffinate flow at 56 l/h; loop flow at 78 l/h and step time at 1326 seconds.This relates to the following bed volumes of the various separation zones: BedVolume Zone 1 = 0.694; Bed Volume Zone 2 = 0.591; Bed Volume Zone 3 =0.661; Bed Volume Zone 4 = 0.383. There is a trade-off between purity andrecovery and a reduction in water usage.A preliminary environmental impact assessment and conceptual mass balancewere done. The proposed plant integrates well into the existing Komati Mill of TSBand does not pose any significant environmental threat. The plant requires certainservices from the mill. The mass balance investigated the water and steamconsumption of the plant. Process integration was done so as to obtain theoptimum utility consumption. The utility consumption of the plant does not exceedthe capacity available at the mill. A small boiler is however required to producesteam during the annual mill maintenance period.Various techniques were used in a cost estimation for the invert sugar plant. Theinternal rate of return (IRR) is 42% for a fixed capital investment ofR94,270,000.00. The net return rate (NRR) for the project is 4%/year, the netpresent value (NPV) - discounted at a 30% cost-of-capital is R41,782,000.00. Thenet payout time (NPT) is 5.207 years. The project fulfils the financial requirementsset by TSB.It is now possible and viable to desugarize cane molasses.