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Membrane filtration for sugar manufacturing
Cleaner, healthier, sweeter

It’s as though shelves in department stores have undergone a major makeover. Regular brands have been replaced by ‘herbal’, ‘natural’ and ‘organic’ food. Even advertisements use the biggest font type to announce ‘pesticide-free’, and ‘100% natural’ food. Of all the items in the shopping list, sugar too has made consumers read labels carefully. ‘Mind you’, the savvy shopper will tell you, ‘it has to be sulphur-free’.

A membrane module

And now a sweet innovation
Though India is the largest manufacturer of plantation white sugar in the world (annual production is estimated to be about 26 million tonnes raw value), its high-sulphur content and high colour does not attract many takers in the export market. With the help of the STM (Sugar Technology Mission) and TIFAC (Technology Information Forecasting and Assessment Council), TERI’s path-breaking demonstration in a sugar mill has established that membrane filtration guarantees consistently better, clarified juice that leads to superior sugar quality.

Though many laboratory experiments and bench-scale trials with this technology have been conducted since the 1970s, TERI’s demonstration is one of the few done worldwide where polymeric membrane modules have been successfully employed for applications above 60 °C. The only other similar attempt in the sugar industry has been reported by Koch Membrane Systems, USA.

A side view of the membrane filtration system

Why membrane filtration?
The Indian sugar industry is continually exploring ways to enhance recovery, improve the quality of the final product, and also reduce energy consumption in the manufacture of plantation white sugar.

Differences in sugar cane variety, soil and growing conditions, weather patterns, and seasonal and periodic fluctuations in the manufacturing process itself have acted as speed breakers. Also, the conventional liming-sulphitation process does not remove colloidal and dissolved impurities, and thus lends a turbid, dark brown-yellow colour to the clarified juice. This high-colour sugar (>100 ICUMSA [International Commission for Uniform Methods of Sugar Analysis]) compares unfavourably with refined sugar (<50 ICUMSA) in the international market.

Further, there are concerns regarding the high sulphite content (up to 70 PPM [parts per million]) in plantation white sugar. Moreover, conventional refining of sugar is both capital- and energy-intensive.

Not just clear, crystal clear
Any change is difficult, especially one that uproots decades of well-established processes. The Indian sugar industry too has resisted any modification in its 70–80-year old manufacturing scheme.

Newer ideas like membrane filtration were met with a high degree of scepticism since it was seen as a ‘laboratory product’ incapable of operation in a sugar mill environment. 

A front view of the membrane filtration system

To convince the sugar industry about the applicability of membrane filtration for juice clarification, TERI demonstrated the technique through an industrial-scale pilot plant of 10 TPH (tonnes per hour) feed capacity employing indigenous polymeric membranes at the Simbhaoli Sugar Mills Limited, Simbhaoli, Uttar Pradesh. The trials confirmed that the filtrate was consistently superior to the clarified juice obtained by the conventional liming-sulphitation process.

Thanks to the semipermeable membranes that distinguish between molecules on the basis of size, and, to a lesser extent, the shape and chemical composition, as compared to conventionally-clarified juice, the membrane-filtered juice was sparkling clear, had purity higher (by 0.9 units), reduced juice colour (by about 50%), and a drop in the inorganic content (by 5%) .

From left: Retenate, feed, and filtrate

Beneficiaries
The membrane filtration technology has succeeded in generating interest among sugar mill owners. Through this demonstration, dissemination of findings, and one-to- one interactions with decision-makers, TERI has elicited interest in the progress of this technology among several sugar factories. In addition, the trials have provided valuable inputs for the improvement of the existing manufacturing process. The Simbhaoli Sugar Mills Limited, the venue of the pilot study, has adopted a full-scale juice pre-filtration system that was tested during pilot trials for the removal of suspended solids from clarified juice.

Applications/benefits
TERI is in the process of investigating membrane improvements to ensure the operational reliability of the system and also establish the economic implications of the process. This will lay the foundation for a scale-up and the subsequent commercial plant design.

The process has been jointly patented by TIFAC and TERI, and the work has won, twice in a row, the prestigious Noel Deer Gold Medal awarded by the Sugar Technologists Association of India.