ITDG 
Interdev Agroalimentaire  

White crystal cane sugar processing using open pan sulphitation (OPS)

	Type de fiche	Techniques/Technologies
	Mise en œuvre	ITDG(Royaume Uni)
	
	Zone géographique	Asie; 
	Mots clés	PRODUCTION A PETITE ECHELLE; CANNE A SUCRE

	Echelle d’action	Groupe
	Public(s) cible(s)	Small- and medium-scale food processors

Contexte
White crystal sugar can be produced from sugar cane by one of two methods: the open pan sulphitation (OPS) or the vacuum pan (VP) process.  Each method involves six individual stages. The two processes are distinguished from each other by the technology used during processing and the scale of production. Vacuum pan production is larger scale than OPS and is described in a separate fiche.

OPS is a small-scale sugar processing technology developed in India in the 1950s for the production of white crystal sugar. The technology is ideally suited to processing between 100 and 500 tonnes of sugar cane per day with a recovery rate of between 5 and 8%.

Unlike large-scale factories, OPS plants do not usually have their own estates to supply cane, but rely on contractual agreements between local growers and the factory. 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.

The OPS technology is based on an upgrade of khandsari (a non-crystalline powdery sugar) production in India, using a mix of traditional and scaled down versions of modern sugar technologies.

Problématique / Objectifs
  The yield of juice from sugar cane depends mostly on the quality of the cane and on the efficiency of the extraction of juice. High quality cane has a good juice content with high sugar levels.  Poor quality cane, or cane that has been harvested too early may have a similar juice content, but the levels of sugar will be lower. 
  Sugar cane deteriorates rapidly after harvest and must be crushed within 24hours of being cut. After this time the sugars start to invert and the yield of sugar declines.
  Juice is extracted from the cane by a crushing unit consisting of two or three power roller mills of three rolls each. The pressure exerted by the mill on the cane is determined by the gap between the top roller and the bottom rollers. If the gap is too wide, poor extraction will result and if the gap is too small then the cane may not pass through or jam the rollers.
  Crushing efficiency is the most important factor in good sugar yields. This is achieved by correct setting of the rollers on the crushing unit. The correct setting is determined at the site of crushing as it depends on the cane variety, size and quality.
  Improved extraction can be achieved by hydraulically loading the pressure roller and by  slicing the cane along its length before crushing or chopping into small nodes. Using this type of system can achieve extraction of around 66% of the available juice form the cane.
  The juice is clarified to remove impurities that will inhibit the formation of crystals and discolour the final product. Chemical clarifying agents based on the modern cold lime sulphitation are commonly used. Adding lime also has the advantage of reducing the natural acidity of the juice, which limits the formation of invert sugars.

Description/Mise en oeuvre
The OPS sugar process involves six  basic steps;

  Extraction of juice
  Clarification of juice
  Boiling
  Crystallisation
  Centrifuging
  Drying and packaging.

Extraction of juice
Sugar cane should be crushed within 24 hours of being cut. It is extracted using a crushing unit that consists of two or three power rollers mills of three rolls each. The gap between the rollers should be adjusted so as to maximise juice extraction.

Clarification
Batches of juice are treated simultaneously with milk of lime (Calcium oxide (CaO)) and sulphur dioxide (SO2), 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 and takes most of the suspended impurities in the juice with it. The juice is then filtered and allowed to settle. The clear juice is decanted and transferred to the boiling furnaces.

Boiling
The filtered juice is boiled to evaporate of  water and reduce the juice to a concentrated form knows as massecuit. It is carried out using a series of four or five open pans, each pan boiling a successive concentration of juice at progressively higher temperatures. The massecuit is removed from the final boiling pan at a temperature of around 112 C and with a sugar concentration of 84 Brix. The pans are located above a furnace that is fuelled using bagasse (waste sugar cane fibre) that is often supplemented with 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 crystallisers.

Crystallisation
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 crystallisation in motion. Rotation promotes even cooling of the massecuit, which helps to achieve uniform crystal growth. Uniform crystal growth can also be encouraged by placing granulated massecuit from a previous batch into the crystalliser with the fresh massecuit.  This process is known as seeding. 

After this process, the massecuit is now a suspension of sugar crystals in molasses. It is transferred to a centrifuge for separation.

Centrifugation
The massecuit is centrifuged to separate out the crystals from the molasses. The centrifuge 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 drum to assist in the removal of the molasses.

The crystals are removed from the centrifuge and transferred for drying. The molasses are collected and can be re-boiled, crystallised and re-centrifuged to produce a second grade, lower quality crystal sugar known as number two or B-sugar.

Drying and packaging
The crystals can be dried in a number of ways, depending on the type of drier available and the local climate.  The simplest method is to dry them in the sun, but this is not necessarily the cleanest or most hygienic. Solar driers, rotary and hopper driers can all be used. The dried sugar is packaged into bags or bins for distribution.

Résultats/Eléments chiffrés
Using forced-draught shell furnaces it is possible to obtain a fuel balance for the boiling operation using bagasse. To operate the crushers, crystallisers, centrifuge and other powered equipment will require an extra energy source.  If electricity from the grid is not available, it will be necessary to generate power at the factory. This means the installation of diesel powered generators that require a reliable supply of diesel fuel and engine spares.
Enseignements
  Since the introduction of the technology in the 1950s, large numbers of OPS sugar plants have been built throughout India. It is estimated that there were several thousand units still in use by the late 1980s. Dissemination of the technology outside the Indian sub-continent has been limited to one successful OPS sugar factory in western Kenya. However, the potential for OPS production is considerable in countries that produce non-crystalline sugars such as gur, jaggery and panela, as they already have some of the necessary expertise.
  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. The success of an OPS plant is more closely related to the design and administration of this contract than to any other single factor.
  The main problem with sugar cane is its rapid deterioration after harvest, which can seriously affect the recovery rate of sugar. Extraction of juice should be completed within 24 hours of harvest, therefore it is very important that harvesting and processing are carefully co-ordinated so that just enough cane is available to keep the factory operating at full capacity.
  Sugar production, even at the small-scale is a complex business, therefore skilled people are required to manage and supervise the production from the collection of cane to the delivery of sugar to the market place.  If these skills are not available locally, they will have to be brought in to set up the business and to train staff.
  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 OPS factory in Kenya, estimated that it would cost US$ 0.5million  for the complete factory including buildings, tractors and import of sugar processing equipment.

Auteur : Sue Azam Ali
Saisie le : 06/12/2001
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