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Закрити книгу / close this bookGATE - 3/88 - Beekeeping (GTZ GATE; 1988; 44 pages)
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New approaches in sugar cane processing

Cane Separation and the Diversification of Conventional Sugar Making
by Helmut C. C. Bourzutschky

Sugar cane and auger mills seem to live in symbiosis, for how else can one explain the fact that, for thousands of years, men have never developed any other system to extract the auger in the auger cane stalk than that of crushing and squeezing the whole stalk. The exception in recent years has been the adaptation of the "diffuser process", a leaching process whereby water is used as a solvent to extract the auger from finely chopped auger cane stalks.

Unfortunately, water cannot distinguish between sugar and non sugar, with the result that whatever is water-soluble is extracted. The resulting juice, relatively highly diluted and impure, is almost identical to that produced by conventional milling, and a complex multistage recovery process is therefore necessary to obtain sugar in its crystallized form.

The cane sugar industry had no other choice but to abandon the idea of improving purity and remedying these shortcomings. Instead, we devoted our efforts to other areas of processing, from purification to crystallization. To rationalize and increase efficiency, sophisticated processes were developed and computers employed. The results of the work done by the modern cane sugar industry were impressive; over the years, the industry reached a standard where it could compete favorably with the ever-present economic challenge of the high-tech beet sugar industry.

Development of the cane separator

Unnoticed by the sugar i industry, a new phase began 25 years ago, when a Canadian on holiday in the Caribbean decided to look at alternative ways of preparing sugar cane. He was extremely impressed by the rind of the sugar cane, which seemed to him comparable to wood in its good physical properties. He knew of particle board made from bagasse, as well as of its disadvantages, and set his mind to finding a way of recovering the rind, uncrushed and in a cleaner, more usable form, from the cane stalk.

To cut a long story short, it was 1980 before satisfactory separator machines became available that were capable of providing a clean piece of sugar cane rind that was free of sugar pith cells on the inside and of its waxy layer on the outside. The Canadian's initial objective was achieved when full length, clean fibre shreds of sugar cane rind suitable for the manufacture of structural board could be produced by the separator. Apart from matters of economics and capacity, the question that still had to be answered was how best to extract the juice from the cleanly separated pith core of the plant stalk.

The working principle of the cane separator

All the cane separators at present available work according to the principle shown in Fig. 1. Sugar cane, harvested either burnt or green and stripped of trash, is chopped into billets of roughly the same length (200 - 300 mm). These are then cleaned, metered and fed into the separator via fast-moving conveyor belts. The billets, guided by two feed rollers, hit a wedge and are split into two halves along their longitudinal axis. The billet halves then travel to the next station, where high-speed cutters mill away the soft inner core of the plant stalk, fracturing over 95% of the pith cells in the process. Immediately after the pith removal station comes the dewaxing station, where the outer waxy covering is removed from the rind. Having been cleaned, the rind is transported to a shredder station, where it is sliced longitudinally into approx. 5 mm wide strips. The resulting material resembles long, wood-like strands.

The separator is driven by various electric motors. While the older designs use fewer motors, and therefore more belts and chains, to drive the various rollers and cutters, the newer models have more single drives with variable-speed motors. These allow one to vary individual parameters and determine optimum speeds. At the present stage of development, the advantage is that parameters can be varied to optimize performance and yield, and so improve the cost efficiency of the cane separator process.


Fig. 1: Working principle of cane separators at present available.

Drawing: Helmut C.C.Bourzutschky

Commercial utilization of the cane separator

At one ton per hour, the capacity of the first machines to be developed was rather low, and they were originally used for animal feed production. Owing to economic reasons, however, they were phased out after a short period of operation. To use the machine just to produce animal feed from fresh sugar cane was too expensive. The problem was that the machine had been developed by someone who, exclusively interested in producing rind strands for board manufacture, did not realize that sugar is the most valuable part of sugar cane - too valuable to be used as plain animal feed.
Eventually, the sugar-producing side of the problem was taken into consideration. A separator was installed in front of a conventional sugar mill and the pith processed in the usual way. The sugar factory did not really benefit, except that now the rind could be taken out of the system without wasting precious sugar. The overall calculation showed that less sugar was produced, due to the loss of the sugar left in the rind. It was no real advantage that the juice extracted from the pith was of a higher quality, given that the old production system and its cost could not be improved. Because the equipment had been designed for a certain low level of purity, the new, higher level of purity was of no advantage.

The project in Jamaica

Assisted by the Federal German government, a project was started in Jamaica which focusses on the cane separator machine and has the following objectives:

• to transfer and adapt alternative food technologies to process sugar cane pith in the most efficient way;

• to develop and introduce products which increase the number of ways in which sugar can be used;

• to increase substantially the energy efficiency of the various processes and to reduce the overall energy demand;

• to aim for a reduction in production costs;

• to demonstrate the viability of a small-scale (60,000 tons of cane per year) sugar plant, using a 10 t/h capacity cane separator;

• to develop this plant as a demonstration unit for interested users;

• to determine technical and economic parameters and develop a module system for each of the various products.

Description of the plant

The central unit is a 10 t/h cane separator, built in 1982 by Intercane Systems Inc. of Canada and given to SIRI (Sugar Industry Research Institute) by CIDA (Canadian International Development Agency). The unit is equipped with a feeding and preparation system and has been used over the past years for test purposes in the institute's pilot plant.

All the equipment necessary to make the following products was provided by the FRG:

• cane-juice beverage;
• table syrup;
• amorphous sugar: i.e. fine, crystallized sugar produced without any interim or final molasses.

On leaving the separator, the cleanly separated pith, about 78 per cent of the total weight of the cane, is conveyed to a horizontal, double spindle screw press, where the juice is extracted. The press is fitted with a variable-speed (frequency conversion) electric motor. In a single pressing, the juice is squeezed out of the pith cells and collected in a trough under the press. After preliminary tests, the addition of imbibition water would seem to be unnecessary. The result is a juice with a concentration of about 20 degrees Brix.

After the juice has been extracted, the pith falls onto a conveyor belt, is transported to a blower and transferred to the boiler fuel-storage bin. With a moisture level of less than 50 per cent, it can also be used as fuel, or as part of a fully-balanced animal feed mix, in which case replacement fuel will be needed for steam production.

The juice is screened twice, first through a DSM screen and later through a rotating brush. This is to remove as many solids as possible, as these would reduce the capacity of the mechanical juice purifier (a high-speed centrifugal separator).

Having been purified, the juice is immediately sent to a sterilization plant similar to that used in the dairy industry. The capacity of this plant corresponds with the amount of juice produced by the press.

If the product chosen is syrup or sugar, the juice is not sterilized but sent to the evaporator station.

This is a three-stage falling film plant, capable of concentrating the juice from 20 to 78 degrees Brix, using 1723 kg steam at an absolute pressure of 4 bar.


Fig. 2 Comparison of conventional cane sugar process and Process based on cane separation (Image)


Process based on cane separation

To increase heat transfer efficiency, the temperature is kept deliberately high. Theoretically, sugar destruction should be low, as it only remains in this type of evaporator for a short period of time. The temperature in the third vessel is in the region of 90°C, which again is much higher than in conventional evaporators.

The plant is completely automated, the flow of juice and steam to the station being controlled by the final concentration of the syrup.

Level controllers in the individual vessels ensure that exposure to high temperatures is kept to a minimum. Only vapor from the third evaporator vessel is used for bleeding; i.e. for the interim heaters, which are all of the plate heat exchanger type. Live steam is used only for the clear juice heater; the steam further heats the already boiling juice entering the evaporator and thus provides an additional flash evaporation effect. From the last vessel of the evaporation tank, the syrup is pumped directly to the 75 t capacity storage tank via a heat exchanger which reduces its temperature to approx. 30°C.

If the product desired is sugar, the syrup is immediately transferred from the evaporator station to the amorphous sugar plant. After further concentration in a single stage process, this transforms the supersaturated syrup into a solid, fine, crystallized product. When it leaves the plant, this product is a light brownish colour and has a temperature of approx. 70°C. It is air cooled as it is conveyed to the packing machine. Two different household-pack sizes are planned for the packing machine. Both are cushion type packs of transparent polyethylene/polyester combifilm.

To complete this description of the plant, mention should be made of the boiler plant for steam generation. This has a capacity of 5000 kg steam per hour, superheated at 6 bar absolute to the equivalent of 151°C.

The boiler will run either on liquid fuel (diesel or marine diesel) or on solid fuel. The solid-fuel furnace is a five-step grate with flue-gas re-circulation to pre-dry the fuel before combustion. The boiler has a rated efficiency of 80 per cent at a load factor of between 50 and 100 per cent.

A 500 kW electric diesel generator serves as a standby or permanent electricity generator to safeguard the proper operation of all the components in the plant.

It is hoped to convert the rind from the separator into charcoal. As the technology for this process is not as readily available as the various food technologies, it has been decided to perform preliminary tests and trials with indigenous Jamaican material in the FRG before making any decisions about equipment or processes.

The data collected so far seem to indicate that a technically and economically viable process is available. A final decision is expected soon, which would allow fulfilment of the original plan of having a charcoal component in operation by the third year of the project.

Charcoal made in the FRG from Jamaican material has already been sent back to Jamaica, where a market acceptance study has shown that it is well accepted by the population.

The potential of this waste biomass-charcoal as an energy saver (reducing the consumption of imported fuel such as kerosene) and as a factor in protecting the environment were the main objectives behind this particular approach.

Total energy demand is expected to be below 300 kg steam and 30 kW per ton of cane. Both figures have been carefully checked and verified.

Comparison of conventional cane sugar
process and process based on cane separation

The diagram in Fig. 2 demonstrates the differences between the individual steps of the two processes. It is clear where these processes are identical and where they differ. It is further apparent that more marketable products result from the separator process.

The key factor is the separator which, by removing the rind and its non-sugar components at the start of the sugar-making process, is not only a simple divider of rind and pith but also a "mechanical purifier". Conventionally, the rind can only be removed by the addition of chemicals.

Conclusion

The different composition of the sugar products from the cane separator made the development of new processing technologies necessary. In terms of sugar-processing technology, this product was of far greater value than conventionally extracted cane juice. After considering various options, it was decided to concentrate on the three sugar products mentioned above. Juice, syrup and sugar are produced at a ratio of 5:45:50 respectively.

Every year, some 7,200 tons of sugar products and 3,000 tons of charcoal will be produced from 57,500 tons of cane, which means that the incoming raw material is almost completely used up. The small size of the operation means that daily throughput is only about 200 tons of cane. However, with proper monitoring of cane quality, it is expected that the cropping season in Jamaica can be extended to 10 months from the presently usual five.

The R & D character of this first phase of the project is illustrated by the fact that the pilot plant is situated at the Sugar Industry Research Institute.

The plant is fully equipped with monitoring and measuring devices, allowing data for future decisions to be collected. It is planned to evaluate the various process steps and components as individual modules, thus allowing plants to be tailor made for other areas or countries.

The plant is open to anyone interested in seeing for himself how the equipment operates and performs.

In its second stage, it is hoped to transform the project plant into a commercially viable unit.

The plant should be running independently at the end of 1989, when the German personnel will leave after four years of technical assistance.

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