Sugar Cane Processing

Field of the Invention

The invention relates to the processing of sugar cane. In particular, the invention relates to a new method for the disintegration of sugar cane.

Background to the Invention

The initial stage in the manufacturing of sugar, after the cane has been harvested, is the so-called "preparation" phase. This invariably involves mechanical disintegration of the cane stalks. There have been numbers of continuous improvements in the machinery used in the preparation phase, see for example, US Patent 6435433.

The problem with a mechanical approach in this or indeed any other industry is that of wear and tear. This is exacerbated by the ubiquitous presence of sand or soil carried into the process by the cane stalks. Materials improvements and coating technologies continue to evolve, for example, to coat wear areas with a wear resistant layer. This is usually done using a welding process for regenerating surfaces or a surface treatment in the case of dimensionally sensitive components. Although such treatments are effective, they add to the direct costs as well as indirect costs because of down-time.

The presence of sand leads to further problems in the handling and use of the residual fibre or bagasse that is left after the sucrose has been extracted. This sand causes wear and abrasion in the boilers used to burn the bagasse.

It is obvious from the above considerations that a non-mechanical approach will offer significant reduction in maintenance as well as possible capital investment and that significant further advantages are gained by early removal of the sand.

Thus the inventors propose an alternative and superior solution which will eliminate the need for knives and hammers or other mechanical approaches used in the pulverisation of sugar cane and offer other process improvements that reduce the net cost of production of sugar.

The use of water jets is a well known alternative for accurate cutting or slicing of a range of materials that includes metals as well as agricultural products. Cutting or slicing of sugar beet by water jets is described in "Alternative cutting technologies for separating organic material" (Brϋser, C and Harms, HH (2005), Landtechnik, VoI 60, No.5, pp. 254-255) while the harvesting of sugar cane by water-jet slicing has been described elsewhere (WO 2006/077464 A1).However the preparation of sugar cane for extraction of sucrose requires disintegration and not fine cutting, a different approach to those described above.

The closest prior art to the current invention of which the applicant is aware describes a "Method and Device for Disintegrating Cellular Structures of Agricultural Products by Liquid Jets" (WO 2004/045813 A1 ). This approach has been developed to effect the crushing of sugar beet with the objective of extracting the juice. Sugar beet, unlike sugar cane, has a substantially homogenous texture and the outer coating or peel adds little mechanical strength to the body of the vegetable. Sugar cane, on the other hand, presents as a long and thin product with: • A thick "rind" that is substantially stronger than the pith within

• A strongly textured inner core consisting of pithy cells containing sucrose separated by stiff fibrous strands.

Accordingly, if water jets could be used to disintegrate sugar cane and stalks there may be process-related advantages. Indeed, as will be shown in the present invention, the use of water jets offers significant improvement in the

fraction of ruptured cells that results from a unit input of energy when compared with conventional mechanical disintegration.

Summary of the Invention

According to a first aspect of the invention, there is provided a method of pulverising or disintegrating sugar cane using a jet of liquid at a pressure of above 10 MPa.

The liquid may be water or an aqueous medium.

In this specification, the term "water" includes the term "aqueous stream" unless the context clearly indicates to the contrary.

The water may include dissolved solids and or suspended solids. These dissolved and/or suspended solids may originate partially or even totally from the pulverising and/or disintegrating of sugar cane or the further processing thereof.

The water may include additives such as surfactants. However, other additives may also be present in the water.

Typically the water is provided at a pressure of around 100 MPa to nozzles directed at the cane to be disintegrated.

The cane for disintegration may be arranged in a compact and rigid form so that the jets strike the cane and the dissipation of elastic energy by the cane is reduced and the energy is spent in disintegration of the cane. An example of this is shown in Figure 1.

The arranging of the cane may include passing the sugar cane between a pair of rollers or any other system that constrains the previously random and loose collection of cane stalks substantially into a planar arrangement. This is intended to ensure that the mean distance between cane and nozzle is within desired parameters as well as maximising the effective scattering cross section of the cane mat.

The typical increase in density of the cane may be from 160 kg/m ³ to 300 - 400 kg/m ³ when presented to the jets.

The method may include controlling the quantity of water used in the disintegration so that the resulting water mixture has a sugar concentration in a similar range to that of a secondary extraction process usually applied in conventional sugar cane extraction systems.

The method may include use of a mass of water equal to or greater than the mass of fibre in the cane and typically in the range of 2 to 3 times the mass of fibre.

The method may include the use of ambient temperatures in the jet of water.

The method may include the use of pre-heated water in the jet of water.

The method may include the use of recycled process water in the jet of water.

The method may use other liquids including liquefied gases.

The method may use chemicals added to the water or abrasive particles added to the water.

The method may include directing the jet or jets of water perpendicularly to the direction of displacement of the cane leading to finer fibres being present in the disintegrated cane relative to those produced by conventional mechanical disintegration.

However, the jet of water or some of the jets of water may be directed off- perpendicular to the direction of displacement of the cane resulting in less fine fibres being present in the disintegrated cane typically comparable to conventional mechanical processes. An off-perpendicular geometry typically uses less water and energy.

The method in the present invention may lead, for comparable energy inputs, to enhanced rupturing of the sucrose-containing cells. The enhancement is typically in the range of 10%. Unlike the conventional mechanical approach where the fibres shield and spread, and therefore average the forces imposed, the water jet removes the fibres, as well as previously damaged layers of rind sequentially before impinging directly on the cell surfaces.

Following preparation using conventional mechanical disintegration, one of two methods of extraction of the sucrose is typically employed; these are diffusion and milling-based approaches respectively ("A Comparison of Cane Diffusion and Milling", PW Rein, Proceedings of the South African Sugar Cane Technologists' Association, June 1995; "A Milling Review", A Wienese, Proceedings of the South African Sugar Cane Technologists' Association, June 1995). However the present invention produces feedstock that is finer and therefore allows for simpler and cheaper extraction technologies than either conventional diffusion or milling. Such approaches may include but are not limited to:

• Vacuum belt filters. This is a moving sieve with a negative-pressure applied to the lower surface to effect the extraction of liquid;

• Use of screw-press extractors;

• Use of Riviere extractors(United States Patent 5772775);

• Modified diffuser; and

• Low-pressure extractors.

The intermediate products produced by jet disintegration of sugar cane are also suitable for the production of ethanol by fermentation similar to the process described in "Solid-State Fermentation in Biotechnology : Fundamentals and Applications", (2001), Pandey, A, Soccol, CR, Rodriguez-Leon, JA and Nigam, P.

According to a second aspect of the invention, there is provided an apparatus for disintegrating sugar cane, said apparatus including: a liquid supply system capable of supplying liquid at a pressure of at least 10 MPa pressure for a direct jet or lower for a pulsed jet; an array of nozzles that spans the width of the mat fed into the path of the jets; one or more sugar cane presentation means for presenting sugar cane for disintegration by the jet of high pressure liquid; and slurry recovery means for recovering a mixture of liquid and disintegrated sugar cane for further processing.

The presentation means may include one or more rollers for presenting the sugar cane as a substantially orientated mat for disintegration by a jet of high pressure water. An example of this is shown in Figure 1.

A water supply system may include a high pressure pump capable of pressures of above 10 MPa, typically above 100 MPa.

The nozzles may be fixed jet nozzles.

The nozzles may be rotating nozzles.

The nozzles may be displaceable relative to the cane on which the apparatus is to be utilised. The distance between the nozzle and the top of the plane of the cane mat is typically between 20 cm and 30 cm.

The nozzles may be mounted on movable parts such that the nozzles can both be positioned relative to the cane flow but may also be moved or oscillated in directions within the plane of the movement of the cane during the disintegration process.

Different designs of nozzles may be used that produce different cross sections of jets or lead to circular or other rotation of the liquid within the jet.

The liquid jet may be pulsed using a range of possible waveforms or duty cycles in order to minimise the energy consumption of the pumps. The frequency of the pulses is typically below 50 kHz.

Specific Description of the Invention

Without limiting the scope of the invention, the invention will now be described on the hand of experimental data and a calculation.

Figure 2 shows a process using a water jet broadly in accordance with the invention.

Experimental Data

Assumptions: The following calculations related to the experiments are based on an assumption of a throughput of 100 ton per hour.

Example 1

In order to establish the lack of obviousness of the usefulness of a jet to the processing of sugar cane, an initial experiment was set up with a domestic "high- pressure" hand held jet.

Billeted cane was subjected to a single, rotating jet of water at a pressure of 12 MPa at a flow rate of 20 l/min. The water jet had no noticeable effect on the rind of the cane. In the view of the inventors, this may explain why water jets have not previously been considered for the preparation of sugar cane.

However, when the rind was mechanically fractured before subjecting it to the water jet, the jet effectively removed the pith from the centre of the cane billet.

Example 2

The test of Example 1 was repeated at 50 MPa and 27 l/min. The test was conducted with both a rotating nozzle and later with a fixed jet nozzle.

Fixed Nozzle

The fixed nozzle was held steadily over the cane billet and the application of a jet resulted in the cane stalk being cut through rather than disintegrated, as expected from the prior art.

When the jet was moved over the cane at a frequency of a few Hz, the result was to obtain a disintegrated mat similar to that obtained by a conventional hammer and knife process.

The changing of the distance of the wand to the billet had little effect with the same degree of disintegration being observed at both 200 mm and 500 mm.

Rotating Nozzle

The rotating nozzle effectively moves the jet but now at a frequency of about 30Hz so that similar results are expected to those obtained with an oscillating fixed nozzle. Indeed the cane was readily disintegrated resulting in a similar product to that obtained with conventional hammer and knife arrangements.

Example 3

More rigorous tests were carried out to determine the water and power consumption required to shred cane. Two pressures, namely 50 MPa and 100 MPa, were used. For the 50 MPa configuration, a single nozzle was used and for the 100 MPa configuration, three nozzles were used. The nozzle arrangement was mounted to direct the jet downward onto constrained sticks of cane. The sticks were driven past the jets and the amount of cane shredded was measured as a ratio of the mass of water used.

The nozzle chosen has a simple construction. It consists of a round chamber where the incoming water is caused to rotate or swirl around. A small diameter cylinder has one end constrained to direct the flow through the outlet. The other end follows the flow forming the conical flow pattern. The flow rate of the nozzle could be changed by replacing a nozzle kit inside the outer housing.

Experimental evaluation

Water consumption

The amount of water used by the process can be manipulated through the pressure used, the size and design of the nozzles and the speed of the cane conveyor.

Extraction tests

Cane was shredded in bundles of 2x2 or four stalks at a feed rate of 10 Hz with the nozzle at a height of 205 mm using 50 MPa. The resulting fibre was collected in two fractions: fine and coarse. Fine fibres were light enough to be entrained in the air and deposited away from the nozzle whereas the coarse fibres were deposited near to nozzle.

The cane pulp was placed in a press and compressed at 10 MPa and the juice collected. 300 mL water was added and the process repeated another three times.

The overall brix extraction was 95.14%. This needs to be compared with best result on the same equipment for conventionally prepared cane of 86%. In other words, the brix can be extracted more easily from the shredded cane of the invention.

A power consumption of 59.7 kW/Tfh was achieved which indicates that, with the correct combination of feed, pressure and flow rate the energy consumption may be better than 84 kW/Tfh, which is the industry average installed preparation capacity.

The inventors believe that the present process as illustrated will lead to possible advantages including: • reduced maintenance and capital costs at the preparation stage

• more efficient extraction of sucrose; and

• a reduction in maintenance costs associated with burning the bagasse because of the modified ash content properties that results from reduced sand inserted into the boilers.