BACKGROUND OF THE INVENTION Dryers employing superheated steam or hot gasses require the containment of the superheated steam of hot gasses. For example, dryers employing superheated steam will typically operate at elevated pressures.

The operating pressure of superheated steam may be 1.5 bar, higher than atmospheric pressure. Material entering and leaving the dryer, whether of the superheated steam type or hot gas type must therefore pass through some sort of gas containment device.

Containment devices may be of a type similar to a rotary valve.

A rotor of generally cylindrical form has a number of chambers formed-by radial extensions that have an outer surface in close contact with the inner bore of the valve housing. Material falls into the chamber and is carried around to the lower discharge point by rotation of the rotor. The presence of gasses such as steam in the pressurised chamber has been found to cause difficulties as the empty chamber carries steam to the entry point. The steam condenses and has been found to cause material to stick to the outer surface of the rotor extensions that are meant to form a seal. In hot gas dryers the egress of hot gas is also disadvantageous.

Another pressure or gas containment device is a screw feeder.

This device is suitable for vegetable fibres and pulp materials used in papermaking but has not been found suitable for the clumps of fibre that are typical of pulps produced by mechanical crushing of fibrous crops like sugar cane.

SUMMARY OF THE INVENTION We have now found a dryer configuration that is particularly suited to the dewatering of pulp as well as other products, preferably fibrous

products such as crushed plant materials. In accordance with one aspect of the present invention there is provided a drying apparatus comprising a drying chamber, a feed assembly and a discharge assembly wherein the respective assemblies each comprise a housing having a bore with radial inlet, and an outlet, a piston disposed within said bore for reciprocation therein, wherein the radial inlet is open for charging the bore with pulp when said piston is in a first position and wherein the radial inlet is closed by said piston whilst the piston is driven by said device towards the outlet whereby pulp disposed within the bore is urged out of the outlet and returned to the first position.

The drying apparatus of the present invention incorporates feed and discharge assemblies that facilitate the transport of material both into and out of the drying chamber. The feed assembly and discharge assembly utilise the low permeability of compressed material to form a seal against discharge of drying medium from the pressurised drying chamber.

The drying apparatus includes a drying chamber. The drying chamber may be of any convenient configuration, preferably the drying chamber includes the features described hereunder as the second and third aspects of the present invention.

The respective assemblies may be the same or different. Whilst identical assemblies may be used for both the feed and discharge, the characteristics of the material before and after drying may render it preferable for the feed and discharge assemblies to be designed to account for the different characteristics of the material whilst retaining the basic elements being a housing having a chamber with radial inlet, and an outlet, and a piston disposed within said bore for reciprocation therein.

Each assembly comprises a housing. The housing may be any convenient shape of sufficient dimensions to accommodate the bore and retain the configuration of the bore in use. The bore may have a circular, square, rectangular or any suitable form in cross-section. The piston will have the same form in cross-section with sufficient clearance to allow free movement in the bore.

The piston is disposed within the bore for reciprocation therein.

The piston and the bore may have a cylindrical, rectangular or any other

convenient shape. It is preferred that the piston be closely, but freely received in the bore. The piston is preferable sealed against the bore to prevent the egress of gas from a pressurised drying chamber.

We have found that improvements can be made to the piston and the inlet to reduce the impact loading on the piston, the reciprocating device and the chamber caused by the cutting through of material as the piston fully enters the chamber. In one example, the piston face is cut at an angle to the cross-section of the chamber. Alternatively the edge formed by the generally vertical wall of the inlet and the generally horizontal top part of the chamber may be caused to be at an angle to the cross-section of the chamber. In a further embodiment, the face of the piston that contacts the material may be angled back from the top edge or have an otherwise sharp insert to act as a cutting agent.

The piston may be connected to the device for reciprocating the piston by a piston-actuating rod. The rear end of the bore may have hydraulic or pneumatic ram seals for engagement with the piston-actuating rod.

The radial inlet allows the bore to be filled with material and may include a hopper or the like. The radial inlet is disposed on the bore adjacent the position of the head of the piston in its first position. In the first position the piston does not close the inlet and material may fill the bore. The piston head and chamber inlet may be shaped to reduce the impact loading as the piston head closes the inlet and urges the material towards the outlet. The piston head is preferably of sufficient length that the inlet is kept closed by the piston head during the reciprocating stroke of the piston until it returns to the first position.

The open end of the bore through which the piston urges the material preferably forms the outlet.

The device for reciprocating the piston may be any convenient device. Suitable devices include hydraulic rams and mechanical cranks The feed assembly preferably has an inlet of sufficient length when filled with material to effect sealing against the pressure in the drying chamber. Alternative sealing means may be employed to optimise the efficiency of the drying chamber.

In the feed assembly a plug of material is compressed by the reciprocating piston acting in an elongated, bore leading into the drying chamber.

Preferably a chopping mechanism is provided at the outlet of the bore to break up the material compressed by the piston within the bore. A suitable chopping mechanism may include a series of arms mounted on a rotating shaft. The use of a chopping mechanism allows the outlet of the bore to be disposed above the perforated plate and the material may enter the drying chamber from an inlet in the top of the drying chamber. Other forms of chopping mechanisms will be apparent to those skilled in the art.

The discharge assembly preferably seals the drying chamber from the egress of pressurised gas by the closing of the inlet in the closed condition and in the open condition by the plug of material in the bore.

Preferably the bore is of sufficient length for the compressed plug of material to form an effective seal. The discharge assembly preferably includes hydraulic or pneumatic ram seals on the piston-actuating rod to seal the rear of the bore. The bore of the discharge device may be open to and at the same pressure as the drying chamber.

Drying of the material may be by any convenient medium, such as hot inert gasses, superheated steam and the like. The material may be dried preferably by using superheated steam. The drying apparatus of the present invention may be used for drying a variety of materials. For example, in the process described in our co pending application, sugar cane is preserved by the steps of crushing the sugar cane to separate a juice portion and a pulp portion, thermally dewatering said pulp portion to form a dewatered pulp, concentrating said juice portion to form a syrup, and combining said dewatered pulp and said syrup to form a preserved sugar cane. The drying apparatus of the present invention may advantageously be used for the thermal dewatering of the crushed sugar cane (pulp).

Whilst the drying apparatus of the present invention is particularly suited to use in the preservation of sugar cane, we have found that it is also advantageously employed in the preservation of other fibrous plant materials including for example but not limited to, sweet sorghum, and

lucerne. Crushed plant materials that contain an appreciable proportion of fibre are amenable to removal of water by the drying apparatus of the present invention.

Crushed sugar cane will have a pulp portion that typically contains the fibre of the plant material although a small quantity of fibre may be extracted with the juice portion. The pulp portion will typically also contain residual water and solutes. Preferably the pulp portion has a moisture content of less than 55% by weight. Preferably the moisture content is in the range of from 46% to 52% by weight. We have found optimum moisture content in the pulp portion of crushed sugar cane to be about 48% by weight.

We have found that the use of superheated steam is particularly advantageous in the dewatering of the pulp portion. Steam drying offers environmental and thermal efficiency benefits compared to drying by hot gas.

In sugar processing the steam produced by evaporation of the water from the pulp portion can be used for heating in subsequent processes such as the evaporation of juice to syrup.

In order to effect efficient heat transfer from the superheated steam to the pulp, a bed of pulp may be fluidised whereby contact between the pulp and the superheated steam drying medium is achieved by upward flow of the steam through a bed of the pulp. The steam flow causes the material in the bed to be fluidised. Fluidisation is desirable as the movement of solid particles causes enhanced mixing of the drying steam and the pulp.

However, we have found that continuous fluidisation may result in overheating and unnecessarily high consumption of energy.

In a second aspect of the present invention there is provided a drying apparatus having a drying chamber, said drying chamber comprising a perforated plate for supporting a bed of material, said perforated plate disposed above a plenum containing superheated steam wherein superheated steam is intermittently passed through the perforated plate whereby the bed of material is intermittently fluidised, and further comprising a feed assembly and a discharge assembly wherein the respective assemblies each comprise a housing having a bore with radial inlet, and an outlet, a piston disposed within said bore for reciprocation therein, wherein

the radial inlet is open for charging the bore with pulp when said piston is in a first position and wherein the radial inlet is closed by said piston whilst the piston is driven by said device towards the outlet whereby pulp disposed within the bore is urged out of the outlet and returned to the first position.

The intermittent fluidisation of the bed of pulp may alternatively be referred to a pulse fluidisation and produces a differential flow of drying steam to portions of the bed in pulses.

In this second aspect the dryer for dewatering pulp includes a drying chamber. The drying chamber may be of any convenient configuration suitable for drying the pulp with superheated steam.

The drying chamber comprising a perforated plate for supporting a bed of pulp. The perforated plate may be of any convenient configuration for supporting the pulp. The perforations in the perforated plate are preferably sized so as to allow the superheated steam to readily pass through and prevent the pulp from passing therethrough.

The perforated plate disposed above a plenum. The plenum may be of any convenient configuration whereby superheated steam is contained therein for intermittent passing through the perforated sheet.

The superheated steam is intermittently passed through the perforated plate so as to cause the bed of pulp to be momentarily fluidised.

Pulses of superheated steam may be passed through the bed of pulp by a rotating baffle plate with openings generally of the same area as the area of the bed to be pulsed. The openings will normally be fixed and the speed of rotation changed to optimise the flow of superheated steam and still retains the benefits of fluidisation. A second baffle plate may be placed between the rotating baffle plate and the perforated plate to provide the differential flow to the wetter and drier portions of the bed. The second differential flow baffle plate would normally be fixed and have openings cut in the plate. If further control of flow of the drying steam either through change of the pulse affected area or change of the flow of drying steam to wetter and drier portions of the bed was needed, the openings could be made adjustable by various means including sliding plate sections that covered a variable part of the openings of the second baffle plate.

Pulses may also be caused by oscillating sections of a baffle plate. The sections are preferentially in a sector form. Each sector may be comprised of two sector pieces that may be adjusted to give a bigger or smaller gap between adjacent sector pairs thereby providing the means to achieve differential flow to wetter and drier portions of the bed and to vary the area of the bed to be momentarily pulsed. Varying the speed of oscillation provides a means for optimising the flow of drying steam while retaining the benefits of fluidisation. A further level of control of differential flow may be achieved by arranging multiples of sector pairs to be actuated by separate oscillating mechanisms at independently variable speeds.

We have found that the use of pulsed fluidisation of the bed of pulp allows the process of the present invention to be optimised to maximise drying effect at reduced steam flow rate. The flow of superheated steam to bring about fluidisation will be higher where the pulp is wet compared to where it is dry. Furthermore, the flow required for complete or continuous fluidisation may be higher than the optimum flow for drying. The optimum flow of drying steam for heat sensitive materials will be close to but higher than the flow at which the steam becomes saturated when it is in contact with the drying solids. One of the advantages of pulse fluidisation over full-bed, continuous fluidisation relates to the lower volume flow of the fluidising material. Velocity above the bed is lower in pulse fluidisation. Carry-over of fine particles is correspondingly less and so too is the need to have vertically extended disengaging space.

For bagasse-like solids, the rate of drying is controlled by the diffusion rate of liquid within the cellular structure. Little increase in drying rate is obtained by holding the particles in a constant stream of hot gas.

In the present invention, the benefits of pulse fluidisation with superheated steam as the drying medium have been realised for the first time. Heat transfer rates from steam to a cooler solid can be very high provided condensation on the surface is limited. In a continuously fluidised bed, the solid surface will either be under or over saturated by water, as steam flow rates must lie within a narrow fluidisation range. In pulse fluidisation, the period of most intense contact between superheated steam

and the solid can be adjusted to limit condensation on the surface. In the "rest"period between pulses, the water content in the particles will tend to equilibrate.

The pulp in the bed moves from the inlet to the discharge as it is dewatered. Pulp having large fibre pieces that tend to clump by mechanical entanglement such as pulps derived from crushing of sugar cane may not be reliably transported by fluidisation even with very high flows of the fluidizing medium and a highly expanded bed. We have found that the flow of dewatered pulp to the outlet may be greatly facilitated by the use of mechanical rake-like assemblies in combination with the use of a fluidised bed of pulp.

In a third aspect of the present invention there is provided a drying apparatus having a drying chamber comprising a bed of material, said drying chamber comprising an arm extending into said bed of material and moving from an inlet into said drying chamber to a discharge from said drying chamber, further comprising a feed assembly and a discharge assembly wherein the respective assemblies each comprise a housing having a bore with radial inlet, and an outlet, a piston disposed within said bore for reciprocation therein, wherein the radial inlet is open for charging the bore with pulp when said piston is in a first position and wherein the radial inlet is closed by said piston whilst the piston is driven by said device towards the outlet whereby pulp disposed within the bore is urged out of the outlet and returned to the first position.

In this third aspect the dryer includes a drying chamber. The drying chamber may be of any convenient configuration suitable for drying the pulp, preferably the drying chamber includes the features described herein as the second aspect of the present invention.

Preferably, the dryer of this aspect includes a multiplicity of arms disposed across the bed at one or more levels. The arms rotate or move in a direction that takes wet pulp from the inlet to the discharge. The lower arms close to a bed support screen will move more slowly than the arms higher up in the bed. During fluidisation, the partially dried pulp will migrate to the upper levels due to a lower density. Differential, faster movement of the drier layers

reduces the residence time during which the drier particles are subjected to higher temperature. The arms may be equipped with rake or plough-like protrusions to cause more effective transport and to assist mixing.

The dewatered pulp preferably has reduced moisture content in the range from 10% to 35% by weight. More preferably the moisture content of the dewatered pulp is in the range of 12% to 15%. The dewatered pulp material should have uniform moisture content, and the residual sugar content of sugar cane should not be adversely affected by colour derived from overheating.

We have found a dryer configuration that permits the separation of dried and partially dried materials. In a fourth aspect of the present invention there is provided a drying apparatus having a sealed drying chamber comprising a plurality of perforated plates for supporting a beds of material, said plurality of plates being disposed in a vertical array wherein each perforated plate is disposed above a manifold wherein drying medium from the drying chamber is drawn through the bed of material and through the perforated plate into the manifold, each perforated plate having an opening wherein partially dried material may pass through said opening onto the perforated plate below, the drying apparatus further comprising a feed assembly and a discharge assembly wherein the respective assemblies each comprise a housing having a bore with radial inlet, and an outlet, a piston disposed within said bore for reciprocation therein, wherein the radial inlet is open for charging the bore with pulp when said piston is in a first position and wherein the radial inlet is closed by said piston whilst the piston is driven by said device towards the outlet whereby pulp disposed within the bore is urged out of the outlet and returned to the first position.

The drying apparatus of this fourth aspect of the present intention is particularly suitable for use as a superheated steam dryer. The superheated steam as the drying medium may preferably enter the top of the drying chamber and be drawn out of the drying chamber through respective manifolds. The superheated steam may be allowed to pass around the periphery of the perforated plates such that only one inlet for superheated steam is required. The configuration of the drying chamber may also be such

that an open channel may be disposed substantially vertically in a wall of the drying chamber to allow the passage of steam to be respective layers. The openings through which partially dried material may pass between respective layers may also allow the passage of steam.

The respective manifolds may be connected to a single group exhaust, to a number of groups exhausts or to individual exhausts. Suitably the exhausts may be in the form of extractor fans that draw the used steam from the respective manifolds. In one embodiment, a single exhaust fan may be connected to all of the manifolds such that steam is drawn through each bed of material, each perforated plate and each manifold to the respective exhaust. This embodiment has the advantage of requiring only a single exhaust and thus the drying apparatus may be of simple construction. We have found that by providing multiple exhausts it is possible to better tune the removal of steam through each layer of the drying chamber. In some configurations it may be possible to group a number of layers by connecting selected manifolds to a single exhaust. The layers connected to a single exhaust may be adjacent layers or they may be spaced apart through the drying chamber. Whilst the optimum drying may be achieved by having an exhaust dedicated to each layer, the capital cost of installing multiple exhausts may not be warranted.

The drying apparatus of this fourth aspect may advantageously be used to separate partially dried material into various classifications. For example, by including additional discharge assemblies on selected layers partially dried material may be removed from the drying chamber. The moisture level of the material being determined by the extent of drying before removal. Generally the lower the layer in the chamber from which the material is removed the lower than moisture content.

A screen or screens may also be installed in the drying chamber between layers within the chamber. The screen or screens may be perforated with holes sized to retain partially dried material above a desired size and allow smaller materials to pass therethrough for further drying. The drying of plant materials for use as animal feed may usefully be sorted according to size. For example, feed for cattle preferably comprises relatively

large dried segments of plant material whereas feed for pigs preferably comprises only fine dried segments of plant material. Thus a screen disposed part way down the chamber having a discharge assembly associated therewith allows partially dried plant material suitable for use as cattle feed to be removed from the chamber. Finer partially dried material may pass through the screen for further drying prior to removal for use as pig feed. Material may be removed from the screen using a removal assembly as described above or using an auger or screw feed to discharge into a hopper external to the drying chamber. The hopper is mounted above the radial inlet of the discharge assembly as described above.

It is preferred that each of the respective layers includes a movable arm or a movable screen that facilitates the movement of partially dried material over the perforated screen to the respective opening to allow transfer of partially dried material to the next layer or to a discharge assembly associated with the layer. Suitable moving arms may be of the type described with reference to the third aspect of the present intention.

Alternatively, the perforated screens may be rotatable or be able to the vibrated to facilitate the movement of partially dried material.

In order that the invention may be more fully understood and put into practice, preferred embodiments thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional representation of a drying apparatus in accordance with the present invention.

Figure 2 is a perspective view of drying apparatus in accordance with the fourth embodiment of the present invention.

Figure 3 is cross-sectional view of the drying apparatus shown in figure 2.

Figure 4 use a cross-sectional view of the feeding assembly shown in figure 3.

Figure 5 is a cutaway view of a chopping mechanism shown in figure 3.

Figure 6 is a plan view of the perforated plate shown in the drying assembly of figure 3 Figure 1 is a cross-section of the steam dryer according to the present invention.

The feed (1) and discharge (2) utilises the low permeability of compressed bagasse to form a seal against discharge of steam from the pressurised drying chamber. A plug of bagasse is formed by a reciprocating piston (3) acting in an elongated, close fitting bore (4) leading into the drying chamber (5). The bore length can be adjusted to effect sealing against a range of pressure in the drying chamber.

The discharge, external to the pressurised drying chamber has conventional hydraulic or pneumatic ram seals (6) on the piston-actuating rod to seal the bore. The bore of the discharge device is open to and at the same pressure as the drying chamber.

The drying chamber employs a system of pulse fluidisation. The baffle plate (7) separating the bed of bagasse (8) from the underlying plenum chamber (9) has openings that can be optimised to maximise drying effect at reduced steam flow rate. The speed of rotation or oscillation of the baffle plate may also be varied to suit the characteristics of the material to be dried.

The action of the baffle plate is to cause pulses of fluidisation in the material to be dried as the openings pass under the bed.

Referring to figure 3 wet bagasse discharges from the outlet of the discharge chamber of the feed device (1) in semi-coherent compressed blocks. A decompaction device consisting of rotating arms disrupts the compacted material so that material falling to the bed support plate (11) in loose form. This plate has perforations suitable sized for distribution of the fluidising steam while retaining the bed solids. Bagasse transport arms (12) are arranged in one or more sets. The speed of rotation of the arms increases from the lowest to the highest set. The multiple set configuration provides for the more rapid movement of the drier and hence lighter surface layer to the discharge point.

Figures 2 and 3 show a drying apparatus 20. The drying apparatus 20 includes a sealed chamber 21. Within the sealed chamber 21

are a series of layers 22. Each layer 22 comprises a perforated plate 23 mounted above a manifold 24. Steam is circulated within the sealed chamber 21 by fans 29 through the heat exchanger 31. The steam is able to flow freely to each of the layers 22 within the chamber 21.

A bed of partially dried material 26 is disposed on each perforated plate 23. Rotating arms 27 are operated by the drive mechanisms 28 mounted on the wall of the drying chamber 21. The respective manifolds 24 are connected to the extraction fans 29 mounted on the wall of the drying chamber 21. Each of the respective manifolds 24 are connected to a single exhaust fan 29 that extracts used steam from the chamber 21. Steam from the chamber 21 passes through the bed 26, through the perforated plate 23 and into the manifold 24 from which the exhaust fan 29 extracts of the used steam. The steam extracted from the respective manifold's 24 passes through a longitudinal annular spaces disposed around the heat exchanger chamber 31. The annular spaces 32 may be divided to provide isolated transfer of steam from a respective manifold 24 to its associated exhaust fan 29.

Steam extracted from the manifolds 24 through annular spaces 32 is circulated by fans 29 that discharge into a common plenum chamber 33 the chamber 33 directs the circulating steam through tubes arranged in the form of a shell and through the tube head exchanger 31. The circulating steam is heated by the condensation of steam at a higher pressure or by hot flute gas on the shell side of the exchanger.

Material to be dried is fed into the drying apparatus 20 using feeding apparatus 35 and is shown in Figures 4 and 5. The feeding assembly 35 includes a chute 36 that leads to a radial inlet 37 within the bore 38 of the feeding assembly 35. A piston 39 reciprocates within the bore 38 and urges the material entering the bore 38 through the chute 36 towards the outlet at 40. The length of the head of the piston 41 is sufficient to close the radial inlet 37 whilst the head of the piston 42 urges the material within the bore 38 to the outlet 40.

At the end of the outlet 40 is a chopping mechanism 45 that can be bolted to the outlet 40. A series of arms 46 are mounted spirally to a shaft

47. The shaft 47 years rotated by drive 48. The arms 46 are rotated to break up the compressed material urged from the outlet 40 of the feeding assembly 35. It will be appreciated the other means for breaking up the plug of material forced through the outlet 40 will be apparent to those skilled in the art.

The dried material is removed from the bottom bed 26 by removal apparatus as shown in figure 1 at items 2 and 6.

Partially dried material is transferred from the bed 26 to the bed 26 immediately below it through a chute 30. The rotating arms 27 urge the partially dried material through an aperture in the perforated plate 23 and into the chute 30.

Figure 6 shows the perforated plate 23 mounted around annular spaces 32 and the heat exchanger 31. The perforated plate 23 has an opening 45 through which partially dried material may fall. Arms (not shown) rotate in a clockwise direction around the perforated plate 23 such that material is urged to the opening 45. Regional 46 shows where material from either the feeder assembly 35 is disposed or alternatively where material falling from the perforated plate above is a initially disposed. The arms move the material around the perforated plate 23 to the opening 45. In hashed relief is shown the position of the opening on the perforated plate below.

Persons skilled in the art will appreciate that the invention described above may be subject to improvements and modifications that will be apparent without departing from the spirit and scope of the invention described herein.