INTERNATIONAL SUGAR JOURNAL

Mini sugar complexes for developing countries

By JEAN FORGERON

Because the processing capcity of the sugar factory is not directly related to its capital cost, the "economies of sale" ensure that the unit cost of sugar manufacture falls as throughput rises. It is generally accepted that, at the present time, the optimum plant size corresponds to about 5000 tonnes of cane per day, after which the cost of bringing a cane supply from further away raises raw material cost so that unit cost of sugar begins to rise.
Such a factory will produce perhaps 50,000 tonnes of sugar a season and this will justify considerable capital expenditure on roads and a transport system as well as other aspects of infrastructure. But in many developing countries such infrastruture does not exist, nor is the population large enough to justify such a volume of sugar production, and there is a lack of technical expertise and support. Furthermore, the enormous cost of such a project is well beyond the financial resources of many counties and it may not have the priority requiered for allocation of such funds as are available.
In the tropical countries of Africa, Asia and Latin America, poeple have often grown sugar cane for chewing and in some cases have developed small scale cottage industries with very low efficiences. The cane, grown in small plots, is often subject to disease and pest damage but the scale of specially suitable varieties. Nevertheless, the people are well aware of the importance of sugar in their diet and provide a ready, if limited, market for crystal sugar.
Without a commercial distribution system and sufficent qualified technical personnel, there is no way in which a large-scale sugar industry can be established ; however, in recent years it has become possible to introduce efficient technology, combined with limited production of sugar to match the needs and capabilities of the developing country. This system, known as the "Mini Sugar Complex" is offered by Agrotechnip of France, specialists in agro-industral complexes, especilly sugar.
Like all cane sugar projects, the Mini Sugar Complex is intwo parts - cane growing and cane processing. The land area to be planted is a function of local conditions and of the capacity of the mini-factory with, for instance, about 650 hectares needed to produce 6000 tonnes of sugar per year if irrigation is available or 900 hectares for rain-fed cane. Such a small area permits selection of land most suitable for cane growing, with consequent maximizing of yields. Generally, the heavier initial work of land clearing, levelling and ploughing is mechanized, but other operations in cultivation, stone picking, spraying, harvesting, etc., provide work of the local labour force.
Initial preliminary trials need to be carried out to identify suitable cane varieties to be introduced and deed cane should be obtained from diease-free stocks by way of a quarantine station or experiment station. It is advisable to provide a heat treatment unit to ensure freedom from ratoon stunting and other dieseases.
Assistance is readily availble to identify the most suitable cultivation methods, precautions against pests and diseases, and to provide irrigation for the cane where rainfall is inadequate. Various irrigation systems can be considered, depending on the local conditions, but for a mini sugar complex the one found to be is usually sprinkler irrigation.
The cane may be grown by individual farmers or by the complex itself and will usually be cut green,using manual cutters. The trash is separated and the clean stalks loaded manually into special containers ; these are hauled onto self-loading trailers when full and are towed to the mini-factory by tractors.
The cane processing part of the complex is especially adapted to the circumstances in wich it will function. It

Mini sugar complexes for developing countries

embodies specific processes suited to modular design and to unit processing capacities of 300, 450 and 600 tonnes of cane per day. The equipementis assembled in standard container modules which may be transported by regular haulage units to site. Préliminary work at the site consists only of pouring concrete for the foundations ; the modules are unloaded and connected to one another, the container frames forming the steel structure of the factory. In other containers are shipped auxiliary buildings such as the office, workshop, laboratory and even housing. Under normal conditions, erection requires less than three months, the modular construction eliminating most erection problems and reducing construction time considerably.

The cane diffuser

The standard processing plant provides everything necessary to obtain raw crystal sugar from the cane supply, while a further option is the manufacture and packaging of white sugar direct from the cane, and even the production of lump sugar.The cane is weighed and sent to a storage area in its container ; it is recaimed, on a first-in first-out basis to avoid deterioration through excessively long storage, and the container carried to a feeder table by means of a tractor-drawn self-loading trailer. Here thr cane is discharged onto a conveyor which carries it to a rotary drum-type cutter which chops the cane transversely into slices, preserving their cellular structure.
The slices pass through a diffuser against a countercurrent of water at 70°C which provides a reduced extraction of about 95% and a juice of purity higher than that achieved in a mill tandem under the same conditions. The wet bagasse, containing about 80% water, is dewatered in a twin-screw press and is then sufficiently dry to use as boiler fuel. The press water returned unfiltered to the diffuser.
The raw juice is heated, limed to neutral pH and passed to a decanter-clarifier. The settled muds are returned to the diffuser where their sugar content is removed and they are discharged with the bagasse. This eliminates the filtration stage conventionally employed in the large-scale factory. The juice is concentrated to 65° Bx in a triple-effect evaporator.
Should the white sugar option be incorported, the evaporator syrup is then subjected to a further purification by the Talodura method of Tate & Lyle Process Technology Ltd of England. In this, the syrup is treated by phosphatation, combined with areation in the presence of a special flocculant ; the scum is separated in a clarifier of short retention time while the clear liquor is further decolorized by sulphitation.
From either the standard or Talodura process, the syrup goes to the vacuum pan section where a three-boiling system is employed, the details of which depend on the quality of the cane and so of the syrup and on the desired sugar quality. The massecuites are discharged into crystallizers which serve as buffer storage before the centrifugals as well as to enhance molasses exhaustion. Continous centrifugals are employed since they offer simplicity of construction, low energy consumption and economical price. The sugar is dried in a dryer especially designed for small throughputs, cooled in a screw conveyor and stored in a silo. Sugar may be pack-aged in 1 Kg polyethyethylene sacks. Bagasse from the press is used as fuel for a boiler which generates superhearted steam at 20 bars pressure ( about 300 psi) and this is used to drive a high-efficiency steam engine coupled to a generator providing power for the whole of the complex. A stand-by diesel generator is used in the off-season. Exhaust steam is used for the heaters, evaporator and vacuum pans. Surplus bagasse is stored for use in case factory operation is interruped. The complex is thus self-sufficient in energy.
It is advisable to provide a water treatment unit to ensure suitable boiler feed quality and also if a potable water supply has to be provided for employees housing.
Creation of a mini sugar complex promotes local employment without disturbing the existing social balance. For instance, for the production of 6000 tonnes of sugar a year, some 350 permanent employees and 450 seasonal workers are required, with a further 150 workers if the cane area is irrigated. Other advantages for the mini sugar complex in a developing country include easier financing, short construction period (less than two years for the whole complex), high technical performance and competitive cost of production.

(Agrotechnip, Cedex 23, 92090 Paris- La Défense, France)