|Food Chain No. 17 - March 1996 (ITDG, 1996, 16 p.)|
Open-pan sulphitation (OPS) is a smallscale sugar processing technology developed in India in the 1950s for the production of whim crystal sugar. The technology is ideally suited to processing between 100 and 500 tonnes of sugar cane per day yielding 50 to 80kg of sugar per tonne of cane.
Unlike large-scale sugar factories (processing over 1000 tonnes of cane per day) OPS plants do not usually have their own estates to supply cane but rely instead on contractual agreements with local growers. This level of technology can be beneficial to rural communities by creating employment opportunities at the factory, and providing income for cane growers in the area.
Since the introduction of the technology, large numbers of OPS sugar plants have been built throughout India, with estimates of several thousand still in use by the late 1980s. Dissemination of the technology outside India has been limited to one successful OPS sugar factory in western Kenya. However, the potential for OPS is considerable in countries that pro duce non-crystalline sugars (jaggery, gur, panela, muscovado etc) as they already have some of the necessary expertise.
OPS SUGAR PRODUCTION PROSS
The technology is based on an upgrade of khandsari production (a non-crystalline powdery sugar) in India, using a mix of traditional and scaled-down versions of modern sugar technologies. The production process can be divided into six stages:
Extraction of juice from the cane
Clarification of the juice
Boiling of the juice
Drying and packaging
Juice is extracted from the cane by a crushing unit, consisting of two or three power roller mills of three rolls each. These crushers are similar to those found in large-scale factories. Improved extraction can be achieved by hydraulically loading the pressure roller and by slicing the cane along its length before crushing.
Chemical clarification, based on modern cold lima, sulphitation, is carried out to remove impurities which inhibit the formation a' the crystals and can discolour the final product. Lime also reduces the natural acidity of the cane juice.
Batches of juice are treated simultaneously with line (as a milky solution) and sulphur dioxide (by air forced through a sulphur furnace), after which the juice is transferred to an open boiling pan and quickly heated to 90°C or above. The lime and heat treatment form a heavy precipitant that flocculates, carrying with it most of the suspended impurities in the juice. The juice is then filtered and allowed to settle, the clear juice is decanted and transferred to the boiling furnaces.
The boiling operation is required to evaporate water and reduce the juice to a concentrated form usually called massecuit. The equipment used is based on traditional Indian technology for the production of khandsari and jaggery. A series of four or five open pans, each boiling a successive concentration of juice at progressively higher temperatures, is located above a furnace (see figure opposite). The massecuit is removed from the final boiling pan at about 84° Brix at a temperature of around 112°C. The term 'degrees Brix' (or more usually °Brix) is the sugar technologists measure of the concentration of dissolved solids in solution.
The heat required for boiling is provided by bagasse (the cane fibre remaining after juice extraction) and, depending on the design of furnace, is often supplemented by other fuels. The hot flue gases pass directly under the pans, heating the juice, before being exhausted to the atmosphere.
The transfer of juice from one pan to the next is controlled by valves through overflow pipes, or the juice can be ladled manually. The massecuit from the final pan is usually too viscous to flow easily so it is ladled into buckets and transferred to the crystallizers.
The massecuit is placed in U-shaped vessels where it is slowly rotated and allowed to cool for up to 48 hours. This technique is often referred to as crystallization in motion. Rotation promotes even cooling of the massecuit which helps to achieve uniform crystal growth. Seeding can also he carried out: that is, granulated massecuit from a crystallizer in which grains have already been developed are placed into the crystallizer before it is filled with fresh massecuit. This helps to promote uniform crystal growth
The massecuit, now consisting of crystals suspended in molasses, is transferred to the centrifuge.
The centrifuge, a scaled-down version of those used in large-scale factories, consists of a perforated inner drum located inside a larger drum. The perforated drum is rotated rapidly, forcing the molasses to separate from the crystals. Water is sprayed into the spinning drum to assist in the removal of the molasses.
The crystals of sugar are then removed from the centrifuge and transferred for drying. The molasses is collected and can be reboiled, crystallized and re-centrifuged to produce a second, lower quality, crystal sugar known as number two or B-sugar.
DRYING AND PACKAGING
The crystals can be dried in a number of ways: by placing them in the sun, by using simple solar driers, or by using rotary or hopper driers which require fuel to provide drying heal. The dried product can then be packed into suitable containers or bags for distribution.
NON-TECHNICAL FACTORS AFFECTING PRODUCTION
Although OPS is a low-cost option compared to large-scale production plants, it still requires substantial investment. A feasibility study, undertaken by ITDG in the late 1980s, for a 100 tonnes of cane per day (tpd) OPS factory in Kenya, estimated that it would cost US$0.5 million for the complete factory including buildings, tractors and imported sugar-processing equipment. The cost of a small 100tpd conventional vacuum pan plant is approximately $US3-5 million
Using forced draught shell furnaces it is possible to obtain a fuel balance for the (first sugar) boiling operation using only the bagasse. Therefore, operating the crushers crystallizers, centrifuge and other powered equipment will require an additional energy source (electricity, diesel etc.) all of which increase costs.
To ensure that enough cane is produced it is necessary for the OPS factory to enter into contracts with cane growers which will guarantee the supply of cane to the factory and income for the growers. Therefore the success of an OPS plant is more closely related to the design and administration of this contract than any other single factor. To transport the cane to the factory it is usual for the factory to supply the tractors and trailers and the farmer the labour.
Operation and management
Sugar production even at the small scale is a complex business so skilled people will be required to manage and supervise the production from the collection of cane to the delivery of the sugar to the market place. If these skills are not available locally then they will have to be brought in to set up the business and to train staff which will increase the cost of the project.
Kaplinsky R., et al, Cane Sugar - The small-scale processing option, IT Publications, London, 1989.
Kaplinsky R., Sugar Processtng - The development of a third-world technology, IT Publications, London, 1985
Garg, M.K., 'A case study on the development and extension of improved khandsari technology on open-pan sulphitation process', All India Improved Khandsari Sugar Manufacturers Association, India, uncrated.