ITDG 
Interdev Agroalimentaire  

White crystal cane sugar processing using the vacuum pan (VP) method

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

	Echelle d’action	Groupe
	Public(s) cible(s)	Medium and large-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.

Vacuum pan sugar technology is a large-scale technology that is common throughout the world. It is a high-cost, low labour process suited to processing upward of 1000 tonnes of sugar cane per day, with recovery rates of between 10-12%.

To ensure a reliable and consistent cane supply, most VP factories have their own estate. Although this is essentially a capital intensive, low labour industry, the shear size of the operation means that VP factories are major employers in the areas in which they are located.

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 using five or more hydraulically loaded, three-roll mills operating in tandem. The major difference between these and OPS mills is that water is sprayed into the bagasse before the final mill and the diluted juice is re-circulated to the previous mills. This system, known as imbibition, helps to wash out more of the sucrose with the juice.  Extraction rates of 75% of the available juice are common.
  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.
  Rotary of fluidised bed driers should be used as they produce a higher quality product.

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

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

Extraction of juice
The cane is shredded then crushed using five or more hydraulically loaded, three-roll mills in tandem.

Clarification
The extracted juice is treated simultaneously with milk of lime (Calcium oxide (CaO)) and sulphur dioxide (SO2), by air forced through a sulphur furnace. The juice is then 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 off water and reduce the juice to a concentrated form knows as massecuit.  The juice is boiled under vacuum inside closed vessels until it reaches about 70 Brix. Low-pressure steam is used to boil the juice, which circulates though tubes inside a closed vessel. The vapour driven off passes into a second, similar vessel, where it is used to heat more juice. In order to maintain suitable temperature differentials for heat transfer to occur, a partial vacuum is applied to each pan, effectively lowering the boiling temperature of the juice.

It is common for four vessels to be used in series, each subjected to a progressively higher vacuum. This system is capital intensive but is highly energy efficient and, more importantly, boils the juice at progressively lower temperatures from 103 to 50 C, reducing the effects of inversion, minimising discoloration and enhancing the formation of sugar crystals.

Final boiling to over 95 Brix takes place under vacuum in a single vessel, known as the vacuum pan, which is designed to handle the viscous massecuit. During this stage, crystallisation of the sugar begins, enabling the maximum yield of crystals to be developed before the massecuit is transferred to the crystallisers.

Crystallisation
The massecuit is cooled and the crystallisation process completed using large vessels that stir the massecuit continually for up to 48 hours.

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 are dried at low temperatures using rotary of fluidised bed driers. The sugar crystals are packed in large bins or bags for distribution.

Résultats/Eléments chiffrés
Bagasse is used to fuel highly efficient high-pressure steam boilers. The high-pressure steam is used to generate electricity to meet the VP plants needs and the low-pressure exhaust steam is then used to boil the juice in the vacuum pans.

Most VP factories are self-sufficient, producing their own electricity and heating for both the process and the factory as a whole. With careful management and using modern, highly efficient bagasse-fired boilers it is possible to produce more electricity than is required by the factory. It may be possible to sell this excess electricity to the grid or other users.

Enseignements
  It is usual for VP factories to have their own cane estate, providing the bulk of their requirements, and occasionally contracting larger farmers to produce certain amounts of cane for them. The estates can be fully owned by the factory or leased from the farmers in the area. In both cases the factory farms the land in accordance with its needs, providing all equipment, fertiliser, pesticides and transport.  Operating and managing an operation of this magnitude requires resources such as tractors, ploughs, harvesting equipment as well as a skilled workforce to use them.
  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 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.
  As a rough guide, a VP factory with a capacity of 3000 tonnes of cane per day would cost at least US$2.5 million to establish.  The cost of setting up a cane estate will easily double this figure.

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