Improved drying system and method

Abstract

A drying system for drying wet feed material, the drying system having a primary heating fluid source; a primary dryer operatively connected to the primary heating fluid source to receive heating fluid therefrom; a secondary dryer, being an indirect-heat dryer, operatively connected to a venting port of the primary dryer to receive waste steam as heating fluid, the waste steam resulting from drying of the wet feed material in the primary dryer; a compressed air source operatively connected to the secondary dryer to inject compressed air into the secondary dryer and thereby agitate feed material passing therethrough; and a feed material conveyor connecting the primary dryer and the secondary dryer, such that partially dried material feed material exiting the secondary dryer is conveyed into the primary dryer, or vice-versa.

Description

[0001] IMPROVED DRYING SYSTEM AND METHOD

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an improved drying system and method. More particularly, but not exclusively, it relates to an industrial drying system for the drying of wet bulk materials.

[0004] BACKGROUND OF THE INVENTION

[0005] Drying of bulk materials is a task which is important across many industries, and is accomplished by industrial dryer systems of various types. Common bulk materials requiring drying include minerals (e.g. ore or sand), polymers (e.g. plastic granules), agricultural products (e.g. grains, cereals, or coffee beans), and animal by-products (e.g. meat or bone meal). Industrial dryers are also used in the drying of other materials that might not necessarily be characterised as 'bulk', such as sludge. In general, industrial dryers are continuously or intermittently supplied with wet feed material, which is conveyed through the dryer and subsequently discharged with some amount of moisture content removed.

[0006] Industrial dryers generally utilise a heat source of some kind to transfer heat into the feed material and cause evaporation of the trapped moisture. However, there are various ways of transferring heat into the feed material. Industrial dryers can be broadly classified into direct- heat dryers, where a hot gas stream makes direct contact with the feed material, and indirect- heat dryers, where a hot fluid indirectly transfers heat to the feed material e.g. through the walls of a shell, discs, or some other structure. The two types of dryers each have certain advantages and may be better suited for different applications. For example, direct-heat dryers are typically more efficient, but indirect-heat dryers can avoid contaminating the feed material and may be better suited for heat-sensitive materials.

[0007] To give an example of a known direct-heat dryer, a superheated steam (SHS) dryer may use a combustor and a heat exchanger to continuously cycle a flow of superheated steam through the dryer shell to contact the feed material directly. To give an example of a known indirect-heat dryer, a disc dryer may use a steam boiler to supply steam into a series of hollow discs shaft-mounted within the dryer shell, the discs transferring heat into the feed material and discharging condensate. An SHS dryer is typically configured as a rotary dryer, where the shell is continuously rotated to convey the feed material towards the discharge end, whereas a disc dryer is typically configured as a rotary disc dryer where the discs are rotated to convey the feed material.

[0008] Dryer efficiency is generally measured in terms of energy input via the heat source, versus the latent heat of vaporisation for moisture removed from the feed material. Typical efficiencies from common types of dryers are 40-75%. Because the vapour coming off the feed material during drying still carries a portion of the heat energy transferred, if this heat can be recovered somehow then it is in fact possible for a drying system to achieve efficiency of greater than 100% under the above definition. Some drying systems utilising heat recovery techniques are known, and generally involve multiple dryers with some fluid connection. However, the efficiency of known drying systems that utilise heat recovery remains far from the theoretical maximum, and it is desirable to develop drying systems which achieve further improvements in efficiency. Given the scale at which such drying operations are conducted in industry, even relatively small efficiency improvements could provide considerable cost savings and substantially reduce the quantity of fossil fuels burned to heat the fluids used for drying.

[0009] It is an object of the present invention to provide a drying system which overcomes or at least partially ameliorates some of the abovementioned disadvantages or which at least provides the public with a useful choice.

[0010] BRIEF DESCRIPTION OF THE INVENTION

[0011] According to first aspect the invention broadly comprises a drying system for drying wet feed material, the drying system comprising: a primary heating fluid source; a primary dryer operatively connected to the primary heating fluid source to receive heating fluid therefrom; a secondary dryer, being an indirect-heat dryer, operatively connected to a venting port of the primary dryer to receive waste steam as heating fluid, the waste steam resulting from drying of the wet feed material in the primary dryer; a compressed air source operatively connected to the secondary dryer to inject compressed air into the secondary dryer and thereby agitate feed material passing therethrough; and a feed material conveyor connecting the primary dryer and the secondary dryer, such that partially dried material feed material exiting the secondary dryer is conveyed into the primary dryer, or vice-versa.

[0012] According to another aspect the drying system further comprises a heat exchanger connected between the compressed air source and the secondary dryer, and connected to a heating fluid outlet of the primary dryer, such that heat remaining in the heating fluid after exiting the primary dryer is transferred into the compressed air to be injected into the secondary dryer.

[0013] According to another aspect the secondary dryer comprises a body having a plurality of nozzles formed in an underside of the body, the plurality of nozzles being operatively connected to the compressed air source.

[0014] According to another aspect the partially dried material feed material exiting the secondary dryer is conveyed into the primary dryer, such that the secondary dryer is a predryer.

[0015] According to another aspect the secondary dryer is a rotary disc dryer.

[0016] According to another aspect the primary dryer is an indirect-heat dryer.

[0017] According to another aspect the primary dryer is a rotary disc dryer.

[0018] According to another aspect the primary dryer is a direct-heat dryer.

[0019] According to another aspect the primary dryer is a superheated steam dryer.

[0020] According to another aspect the heating fluid source is a steam boiler.

[0021] According to another aspect the heating fluid source is an indirectly-fired heat exchanger.

[0022] According to another aspect the compressed air source is an air compressor.

[0023] According to another aspect the drying system further comprises an air dryer connected to the compressed air source.

[0024] According to another aspect the drying system further comprises a mesh filter connected between the venting port of the primary dryer, and the secondary dryer.

[0025] According to another aspect the drying system further comprises a bag filter connected between the mesh filter and the secondary dryer.

[0026] According to another aspect the drying system further comprises a fan connected between the venting port of the primary dryer, and the secondary dryer. According to another aspect the drying system further comprises a condenser operatively connected to a venting port of the secondary dryer to condense moisture out of gas vented from the secondary dryer.

[0027] According to another aspect the drying system further comprises a cyclone separator operatively connected between the venting port of the secondary dryer and the condenser.

[0028] According to another aspect the primary dryer is also a cooker, and comprises an outlet for tallow released from the feed material.

[0029] According to another aspect the invention broadly comprises a method of drying wet feed material, the method comprising: a primary drying step comprising heating the wet feed material using a primary heating fluid; a secondary drying step comprising indirectly heating the wet feed material using waste heat extracted from the primary drying step; agitating the wet feed material with compressed air during the secondary drying step; and conveying partially dried feed material from the secondary drying step to the primary drying step, or vice-versa.

[0030] According to another aspect the method further comprises heating the compressed air using waste heat before using it to agitate the wet feed material.

[0031] According to another aspect the waste heat used to heat the compressed air is extracted from the primary drying step and transferred via condensate of the primary heating fluid.

[0032] According to another aspect heating in the primary drying step is indirect heating such that the primary heating fluid does not contact the wet feed material.

[0033] According to another aspect the primary heating fluid is steam which is introduced in the primary drying step at between 140-180°C.

[0034] According to another aspect heating in the primary drying step is direct heating such that the primary heating fluid contacts the wet feed material.

[0035] According to another aspect the primary heating fluid is superheated steam which is introduced in the primary drying step at between 200-400°C.

[0036] According to another aspect the method further comprises drying the compressed air before using it to agitate the wet feed material. According to another aspect the waste heat extracted from the primary drying step is transferred via steam vented from the primary drying step, at least part of which is evaporated from the wet feed material during the primary drying step.

[0037] According to another aspect the method further comprises separating solid particles out of the steam vented from the primary drying step.

[0038] According to another aspect the steam vented from the primary drying step is at 120-125°C.

[0039] According to another aspect the method further comprises condensing moisture out of gas exiting the secondary drying step.

[0040] According to another aspect the invention broadly comprises an indirect-heat dryer for drying wet feed material, the dryer comprising: a body within which wet feed material is to be indirectly heated by a heating fluid; a heating fluid inlet, a heating fluid outlet, and a heating fluid channel connected therebetween and configured to pass the heating fluid through or about the body without directly contacting the wet feed material; a wet material infeed for introducing the wet feed material into the body; a dried material outfeed; a conveying mechanism for conveying the wet feed material through the body as it dries, and subsequently out of the dried material outfeed; a plurality of nozzles formed in an underside of the body to allow an agitation gas to be injected into the body and thereby directly contact and agitate the wet feed material; a venting port for venting steam evaporated from the wet feed material, and agitation gas introduced through the plurality of nozzles.

[0041] According to another aspect the indirect-heat dryer is a rotary disc dryer such that the heating fluid channel is provided within a plurality of discs mounted within the body, and the discs act as the conveying mechanism.

[0042] According to another aspect the nozzles are provided on a modular nozzle plate removably attached to the underside of the body.

[0043] According to another aspect the nozzles are formed as a grating.

[0044] According to another aspect the nozzles are formed as individual spouts.

[0045] According to another aspect the nozzles are swivelable to create a swirling effect. Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

[0046] As used herein the term "and / or" means "and" or "or", or both.

[0047] As used herein "(s)" following a noun means the plural and / or singular forms of the noun.

[0048] The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner.

[0049] In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.

[0050] For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be chronologically ordered in that sequence, unless there is no other logical manner of interpreting the sequence.

[0051] BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The invention will now be described by way of example only and with reference to the drawings in which:

[0053] Figure 1 shows a schematic of a drying system wherein the primary dryer and the secondary dryer are both indirect-heat dryers;

[0054] Figure 2 shows a schematic of a drying system wherein the primary dryer is a direct- heat dryer instead of an indirect-heat dryer;

[0055] Figure 3 shows a schematic of the drying system of figure 1, wherein the compressed air is heated by waste heat from condensate; and

[0056] Figure 4 shows a simplified cross-section of an indirect-heat dryer having nozzles to introduce an agitation gas. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0057] According to various aspects of the various embodiments of the present invention as illustrated in figures 1-4, there is provided a drying system 100, and an associated method of drying wet feed material, which will now be described.

[0058] As shown in figure 1, the drying system 100 comprises a primary heating fluid source 102 and a primary dryer 104 operatively connected to the primary heating fluid source 102 to receive heating fluid therefrom. The primary dryer 104 may be any suitable industrial dryer as known in the art, but figure 1 shows an indirect-heat dryer in which the heating fluid does not contact the feed material. The specific example depicted is a rotary disc dryer. Other possibilities for the primary dryer 104 will subsequently be described.

[0059] The drying system 100 further comprises a secondary dryer 106, being an indirect- heat dryer, operatively connected to a venting port 108 of the primary dryer 104. The venting port 108 is used to vent waste steam that results from drying of the wet feed material in the primary dryer 104. The secondary dryer 106 thus receives waste steam from the primary dryer 104 as heating fluid. In this way, waste heat is extracted from the primary dryer 104, allowing the overall efficiency of the drying process to be improved.

[0060] The drying system 100 further comprises a compressed air source 110 operatively connected to the secondary dryer 106. Preferably the compressed air source 110 is an air compressor taking in ambient air (at e.g. 20°C). The compressed air source 110 is used to inject compressed air into the secondary dryer 106 and thereby agitate feed material passing therethrough. Preferably the compressed air is injected via a plurality of nozzles formed in the secondary dryer 106 - structure of the secondary dryer 106 will later be described in more detail. Preferably the injected air is dry air to avoid introducing moisture into the secondary dryer 106, which (dependent on ambient humidity) may require an air dryer as will subsequently be described.

[0061] Injection of compressed air into the secondary dryer 106 is preferably done in such a manner to fluidise the feed material, i.e. agitating it sufficiently to behave like a fluid. In order to achieve the desired effect, the compressed air is preferably introduced from underneath the secondary dryer 106, at a sufficiently high flow rate, and from a sufficient number of locations (e.g. nozzles). The ideal parameters will be dependent on application, in particular the properties of the wet feed material e.g. density, viscosity, particle size, pore size, and other such properties used in the characterisation of bulk / granular materials.

[0062] When compressed air is introduced in such a way, heat transfer to the wet feed material in the secondary dryer 106 is improved. Thus, more waste heat can be extracted from the primary dryer 104 via the waste steam. Given that the compressed air from the compressed air source 110 can be at ambient temperature and does not require heating, the energy consumed to supply the compressed air is relatively negligible compared to the increase in heat recovered in the drying process.

[0063] By combining the utilisation of waste steam with the injection of compressed air, overall efficiency of the drying system 100 (by the metrics as discussed in the background) can exceed 100%. The efficiency may be in the range of 130-150%.

[0064] The drying system 100 further comprises a feed material conveyor 112 connecting the primary dryer 104 and the secondary dryer 106. The feed material conveyor 112 conveys feed material between the two dryers, but the dryers may be arranged either way around. Figure 1 depicts the preferred arrangement wherein the partially dried feed material exiting the secondary dryer 106 is conveyed into the primary dryer 104, such that the secondary dryer 106 is a pre-dryer. This arrangement may be more efficient than the reverse, but an arrangement where the primary dryer 104 is the pre-dryer and the secondary dryer is still viable.

[0065] Without wishing to be bound by theory, the improved efficiency of the preferred arrangement of the dryers may be due to the temperature of the feed material being raised in the pre-dryer while the moisture content is still relatively high, with the primary heating fluid of the primary dryer 104 being at a higher temperature still. Thus, more heat is retained in the feed material as it is conveyed between the dryers, and the primary dryer 104 can remove the remaining moisture more efficiently. The feed material conveyor 112 depicted in figure 1 is a screw conveyor driven by a motor 114, which may be provided with a variable speed drive for process control. However, the feed material conveyor 112 could be of any other type which serves to transfer the material while preventing clogging . For example, the dryers could be placed in a stacked configuration where the feed material conveyor 112 is a rotary gate valve, preferably configured to prevent oxygen transfer into the primary dryer 104. Stacking the dryers may be especially useful where floor area is limited, for example when the secondary dryer 106 is being retrofitted to a primary dryer 104 which is part of an existing floor layout.

[0066] For the indirect-heat dryer depicted in figure 1, preferably the primary heating fluid source 102 is a steam boiler such that the heating fluid for the primary dryer 104 is steam. The steam boiler preferably provides steam to the primary dryer 104 at between 140-180°C and at high pressure, for example the steam boiler may operate at approximately 6 bar.

[0067] As the steam moves through the heating fluid channel of the primary dryer 104 and heat is indirectly transferred to the feed material, the steam may condense into water. Thus, the heating fluid which exits the primary dryer 104 may be condensate which is drained as waste.

[0068] However, it will be appreciated that because the heating fluid does not make contact with the feed material, the heating fluid could be another substance such as thermal oil, and it could be continuously reheated and cycled through the primary dryer 104 instead of being drained away after exiting. The ideal heating fluid may depend on the type of indirect-heat dryer.

[0069] The steam evaporated from the wet feed material will generally be at approximately 100°C and at atmospheric pressure. Thus, the waste steam vented from the primary dryer 104 and introduced to the secondary dryer 106 is at approximately 100°C.

[0070] In order to filter out solid matter that may exist in the waste steam vented from the primary dryer 104 a mesh filter 116 may be connected between the venting port 108 of the primary dryer 104, and the secondary dryer 106. A bag filter 118 may also be connected between the mesh filter 116 and the secondary dryer 106. However, various other filtering arrangements may be suitable to achieve the removal of particulates from the waste steam, and the ideal type may depend on factors such as the pressure and flow rate.

[0071] A fan 120 may be connected between the venting port 108 of the primary dryer 104, and the secondary dryer 106. The fan 120 may help to drive the waste steam into the secondary dryer 106. Preferably the fan 120 is connected after the mesh filter 116 and / or bag filter 118.

[0072] A condenser 122 may be operatively connected to a venting port 124 of the secondary dryer 106 to condense moisture out of gas vented from the secondary dryer 106, as the vented gas will be a mixture of waste steam and air. The resulting condensate can be drained away, and the air can be vented to atmosphere.

[0073] Preferably, a cyclone separator 126 is operatively connected between the venting port 124 of the secondary dryer 106, and the condenser 122, to separate out solids which may have entered the gas stream from the feed material. This can prevent clogging of the condenser 122.

[0074] Preferably, an air dryer 128 is connected to the compressed air source 110 to remove moisture from the air before it is injected into the secondary dryer 106. This can help prevent corrosion or other wear of system components, as well as potentially providing a slight increase in drying efficiency. Preferably it is connected in between the compressed air source 110 and the secondary dryer 106, but it might also be connected to dry air before it enters the compressed air source 110.

[0075] For some feed materials which are animal products, for example meat and bone meal, the primary dryer 104 may also be a cooker and cause the release of tallow from the feed material during drying. The primary dryer 104 may therefore comprise an outlet for discharging tallow, in addition to that for discharging the dried feed material.

[0076] As shown in figure 2, the primary dryer 104 of the drying system 100 may alternatively be a direct-heat dryer, in which the primary heating fluid directly contacts the feed material, rather than an indirect-heat dryer. The example shown is a rotary superheated steam dryer. The drying system 100 may be substantially unchanged compared to the arrangement of figure 1, except for select differences as outlined below.

[0077] When the primary dryer 104 is a rotary superheated steam dryer, steam is continuously reheated and cycled directly through a body of the dryer to dry the wet feed material, and the body of the dryer is rotated to convey the feed material therethrough. Thus, the primary heating fluid source 102 is an indirectly-fired heat exchanger. Part of the steam vented from the primary dryer 104, which will be a mix of the input steam and the steam evaporated from the feed material, is cycled back through the heat exchanger for reheating and reintroduced into the primary dryer 104. The excess steam, arising from the evaporation from the feed material, is the waste steam sent to the secondary dryer 106.

[0078] An additional cyclone separator 126 may be operatively connected to the venting port 108 of the primary dryer 104, to separate solids out of the vented steam before the steam is recycled and the waste steam is split off. An additional fan 120 may also be connected between that cyclone separator 126 and the indirectly-fired heat exchanger to help move steam through the system.

[0079] Preferably the indirectly-fired heat exchanger introduces superheated steam into the primary dryer 104 at between 200-400°C, for example at approximately 300°C, and at close to atmospheric pressure. Operation at different pressure may change the temperature range accordingly. Heat from the superheated steam is transferred to the wet feed material as the steam moves through the body, and when the steam is vented it is preferably at between 120- 125°C (it may be slightly hotter than when the primary dryer 104 is an indirect-heat dryer). This ensures no water particles are suspended in the steam i.e. the steam will not be wet steam. The waste steam introduced into the secondary dryer 106 may be at the same temperature or slightly lower due to heat losses, for example approximately 115°C. Flue gas exiting the indirectly-fired heat exchanger will be at a lower temperature than the superheated steam, for example approximately 200°C. The ideal parameters of the input superheated steam may depend on the amount of wet feed material being passed through the body of the primary dryer 104, as increased feed material throughput increases heat loading proportionally (if the same level of drying is to be achieved). Because the temperature of the vented steam is indicative of changes in loading conditions, the input parameters such as temperature and flow rate may be control variables in a closed loop control system where the temperature of the vented steam is the process variable to be maintained at a setpoint (this setpoint correlating to a desired level of drying). This type of control allows dynamic adjustment to different heat loading conditions.

[0080] Closed loop control of the primary heating fluid parameters can be provided by a controller which forms part of the drying system 100. A superheated steam dryer will generally exhibit a faster control response time than an indirect-heat dryer such as a disc dryer, because disc dryers carry large amounts of heat in the discs. However, closed loop control can nevertheless be applied to a disc dryer in a similar way. It will be appreciated that for either type of dryer, the controller may also receive input from various other sensors / input devices and perform other process control functions.

[0081] Although other types of direct-heat dryers may be suitable as the primary dryer 104, preferably the heating fluid remains steam. Although a different fluid could be used, the direct contact with the feed material means that the vented gas would then be a mix of steam and a different heating fluid, which may not be ideal for use in the secondary dryer 106. Heated air for example may nevertheless be suitable if the secondary dryer 106 is configured to receive a hot air and steam mixture.

[0082] The drying system 100 may be able to achieve greater overall efficiency with a direct- heat dryer as the primary dryer 104, given that they are typically more efficient than comparable indirect-heat dryers. However, an indirect-heat dryer may be preferable for its other advantages, at least with certain types of feed material.

[0083] As shown in figure 3, the compressed air injected into the secondary dryer 106 may optionally be heated prior to injection, instead of being injected at ambient temperature. Preferably, this is done using waste heat in order to further improve the overall efficiency of the drying system 100 - if a dedicated combustion process was used to heat the compressed air, there may be no benefit, or efficiency may even be decreased. The waste heat used could be taken from a source external to the drying system 100, for example waste heat from another industrial process in the same plant. However, the waste heat is preferably extracted from the primary dryer 104.

[0084] Preferably, the source of waste heat for heating the compressed air is heating fluid condensate exiting the primary dryer 104. Such condensate is generally only a by-product of certain types of indirect-heat drying. Thus, the drying system 100 may further comprise a heat exchanger 130 connected between the compressed air source 110 and the secondary dryer 106, and connected to a heating fluid outlet of the primary dryer 104. Thus, heat remaining in the heating fluid after exiting the primary dryer 104 is transferred into the compressed air to be injected into the secondary dryer 106. The heat exchanger 130 is preferably a liquid-to-air heat exchanger of any suitable type, where one flow is condensate and the other is compressed air.

[0085] It will be appreciated that the drying system 100 as described above also illustrates a method of drying wet feed material, corresponding to the operation of the drying system 100, that is not necessarily specific to any particular hardware mentioned. The corresponding method can be considered to comprise a primary drying step which corresponds to the function of the primary dryer 104, a secondary drying step which corresponds to the function of the secondary dryer 106, agitating the wet feed material with compressed air during the secondary drying step, and conveying partially dried feed material between the two drying steps in one direction or the other.

[0086] Other, optional steps of the corresponding method may be illustrated by other elements of the drying system 100 herein described.

[0087] Because the primary dryer 104 may be of a standard type, and thus already in use within a given plant that carries out industrial drying, the drying system 100 of the present invention may be retrofit into the plant by installation of the secondary dryer 106 in the manner described. Thus, the invention may also reside in a structure of the secondary dryer 106 which enables its use in the drying system 100.

[0088] As shown in figure 4, the secondary dryer 106 may comprise various features typical of known indirect-heat dryers. It comprises a body 132 within which wet feed material is to be indirectly heated by a heating fluid, a heating fluid inlet 134, a heating fluid outlet 136, and a heating fluid channel 138 connected therebetween and configured to pass the heating fluid through or about the body 132 without directly contacting the wet feed material.

[0089] The heating fluid inlet 134 is configured to receive waste steam, such that the secondary dryer 106 can be used to improve the overall efficiency of a drying system by extracting waste heat.

[0090] The heating fluid outlet 136 allows for condensate of the waste steam to be drained after heat is indirectly transferred into the wet feed material.

[0091] The secondary dryer 106 further comprises a wet material infeed 135 for introducing the wet feed material into the body 132, a dried material outfeed 137, and a conveying mechanism 138 for conveying the wet feed material through the body 132 as the feed material dries and subsequently out of the dried material outfeed 137. The infeed 135 and the outfeed 137 are preferably located at opposite ends of the body 132.

[0092] The conveying mechanism 138 preferably comprises a motor which rotates some part of the secondary dryer 106, but which part is rotated depends on the specific type of indirect-heat dryer. The motor may be provided with a variable speed drive for process control.

[0093] The wet material infeed 135 may comprise a motor-driven screw conveyor, which may also have a variable speed drive for process control.

[0094] The secondary dryer 106 further comprises a plurality of nozzles 140 formed in an underside of the body 132 to allow an agitation gas to be injected into the body 132 and thereby directly contact and agitate the wet feed material, and preferably to fluidise the material in the manner previously described.

[0095] It will be appreciated that the nozzles 140 will be capable of admitting many types of gases other than compressed air, many of which may also serve to agitate the wet feed material in the desired way. However, air is the generally preferred gas due to its ready availability at no cost. Thus, the nozzles 140 are preferably adapted to inject air to the extent that their design is adapted for any particular gas.

[0096] The secondary dryer 106 further comprises a venting port 124 for venting steam evaporated from the wet feed material and agitation gas introduced through the plurality of nozzles.

[0097] For the rotary disc dryer depicted in figure 4, the heating fluid channel 138 is provided within a plurality of discs 142 mounted within the body 132, and the discs 142 act as the conveying mechanism 139 in combination with the motor. The discs 142 are mounted on a hollow shaft 144, also forming part of the heating fluid channel 138, which is coupled to the motor and thereby rotated. The discs 142 comprise paddles 146 which serve to convey the feed material as the discs 142 rotate.

[0098] However, various other configurations (not shown) are possible - for example, the heating fluid channel 138 may be configured as a jacket about the body 132. The conveying mechanism 139 may therefore be independent of the heating fluid channel 138.

[0099] The term "nozzles" is meant in a broad sense, and the nozzles 140 may be formed as a grating or as individual spouts, which may or may not be convergent to increase the velocity of the agitation gas. When the secondary dryer 106 is a rotary disc dryer, the nozzles 140 may be spaced intermediate of the discs 142 to better agitate the feed material. They may be arranged in a grid format, such that at least one row of nozzles 140 is located between each disc 142. The nozzles may be swivelable, such that they can be actuated to create a swirling effect of the injected air and achieve better agitation. The ideal number and placement of nozzles 140 injecting agitation gas may depend on the feed material to be dried. The nozzles 140 may be configured to be selectively pluggable so as to adjust the number and placement of nozzles 140 which are actually used, allowing a given secondary dryer 106 to be adapted to different feed materials. Alternatively, the nozzles 140 may be provided on a modular nozzle plate removably attached to the underside of the body 132, such that different modular nozzle plates can be adapted to different feed materials and easily swapped as needed.

[0100] Although compressed air is the preferred agitation gas, in another embodiment of the invention the benefits of agitating the feed material to improve heat transfer may be realised with process steam itself. Thus, the primary dryer 104 or the secondary dryer 106 could be configured such that steam enters through nozzles in the underside and agitates the feed material as well as heating it.

[0101] For example, where the primary dryer 104 is a superheated steam dryer, it may be modified such that the superheated steam is introduced through a plurality of nozzles in the underside, instead of at a conventional inlet. Alternatively, the secondary dryer 102 may be configured as a direct-heat dryer rather than an indirect-heat dryer, with the waste steam introduced via nozzles in the underside.

[0102] It will be appreciated that the schematics as shown in the figures are simplified, and the drying system 100 may comprise various components including valving, regulators, gauges, safety devices, and the like which are not shown. However, such components would be well understood by a person skilled in the art and easily incorporated as necessary to work the invention.

[0103] Where components are referred to as 'connected to', 'connected between', or connecting' other components throughout the specification and in the appended claims, it will be understood that such terminology references connections which are not necessarily direct. There may be intermediate components which form part of any such connection, whether or not such intermediate components are explicitly referenced. To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Claims

CLAIMS1. A drying system for drying wet feed material, the drying system comprising: a primary heating fluid source; a primary dryer operatively connected to the primary heating fluid source to receive heating fluid therefrom; a secondary dryer, being an indirect-heat dryer, operatively connected to a venting port of the primary dryer to receive waste steam as heating fluid, the waste steam resulting from drying of the wet feed material in the primary dryer; a compressed air source operatively connected to the secondary dryer to inject compressed air into the secondary dryer and thereby agitate feed material passing therethrough; and a feed material conveyor connecting the primary dryer and the secondary dryer, such that partially dried material feed material exiting the secondary dryer is conveyed into the primary dryer, or vice-versa.

2. The drying system of claim 1, further comprising a heat exchanger connected between the compressed air source and the secondary dryer, and connected to a heating fluid outlet of the primary dryer, such that heat remaining in the heating fluid after exiting the primary dryer is transferred into the compressed air to be injected into the secondary dryer.

3. The drying system of claim 1 or 2, wherein the secondary dryer comprises a body having a plurality of nozzles formed in an underside of the body, the plurality of nozzles being operatively connected to the compressed air source.

4. The drying system of any one of the preceding claims, wherein the partially dried material feed material exiting the secondary dryer is conveyed into the primary dryer, such that the secondary dryer is a pre-dryer.

5. The drying system of any one of the preceding claims, wherein the secondary dryer is a rotary disc dryer.

6. The drying system of any one of the preceding claims, wherein the primary dryer is an indirect-heat dryer.

7. The drying system of claim 6, wherein the primary dryer is a rotary disc dryer.

8. The drying system of any one of claims 1 to 5, wherein the primary dryer is a direct- heat dryer.

9. The drying system of claim 8, wherein the primary dryer is a superheated steam dryer.

10. The drying system of any one of the preceding claims, wherein the heating fluid source is a steam boiler.

11. The drying system of any one of claims 1 to 9, wherein the heating fluid source is an indirectly-fired heat exchanger.

12. The drying system of any one the preceding claims, wherein the compressed air source is an air compressor.

13. The drying system of any one of the preceding claims, further comprising an air dryer connected to the compressed air source.

14. The drying system of any one of the preceding claims, further comprising a mesh filter connected between the venting port of the primary dryer, and the secondary dryer.

15. The drying system of claim 14, further comprising a bag filter connected between the mesh filter and the secondary dryer.

16. The drying system of any one of the preceding claims, further comprising a fan connected between the venting port of the primary dryer, and the secondary dryer.

17. The drying system of any one of the preceding claims, further comprising a condenser operatively connected to a venting port of the secondary dryer to condense moisture out of gas vented from the secondary dryer.

18. The drying system of claim 17, further comprising a cyclone separator operatively connected between the venting port of the secondary dryer and the condenser.

19. The drying system of any one of the preceding claims, wherein the primary dryer is also a cooker, and comprises an outlet for tallow released from the feed material.

20. A method of drying wet feed material, the method comprising: a primary drying step comprising heating the wet feed material using a primary heating fluid; a secondary drying step comprising indirectly heating the wet feed material using waste heat extracted from the primary drying step; agitating the wet feed material with compressed air during the secondary drying step; and conveying partially dried feed material from the secondary drying step to the primary drying step, or vice-versa.

21. The method of claim 20, further comprising heating the compressed air using waste heat before using it to agitate the wet feed material.

22. The method of claim 21, wherein the waste heat used to heat the compressed air is extracted from the primary drying step and transferred via condensate of the primary heating fluid.

23. The method of any one of claims 20 to 22, wherein heating in the primary drying step is indirect heating such that the primary heating fluid does not contact the wet feed material.

24. The method of claim 23, wherein the primary heating fluid is steam which is introduced in the primary drying step at between 140-180°C.

25. The method of any one of claims 20 to 22, wherein heating in the primary drying step is direct heating such that the primary heating fluid contacts the wet feed material.

26. The method of claim 23, wherein the primary heating fluid is superheated steam which is introduced in the primary drying step at between 200-400°C.

27. The method of any one of claims 20 to 26, further comprising drying the compressed air before using it to agitate the wet feed material.

28. The method of any one of claims 20 to 27, wherein the waste heat extracted from the primary drying step is transferred via steam vented from the primary drying step, at least part of which is evaporated from the wet feed material during the primary drying step.

29. The method of claim 28, further comprising separating solid particles out of the steam vented from the primary drying step.

30. The method of claim 28 or 29, wherein the steam vented from the primary drying step is at 120-125°C.

31. The method of any one of claims 20 to 30, further comprising condensing moisture out of gas exiting the secondary drying step.

32. An indirect-heat dryer for drying wet feed material, the dryer comprising: a body within which wet feed material is to be indirectly heated by a heating fluid; a heating fluid inlet, a heating fluid outlet, and a heating fluid channel connected therebetween and configured to pass the heating fluid through or about the body without directly contacting the wet feed material; a wet material infeed for introducing the wet feed material into the body; a dried material outfeed; a conveying mechanism for conveying the wet feed material through the body as it dries, and subsequently out of the dried material outfeed; a plurality of nozzles formed in an underside of the body to allow an agitation gas to be injected into the body and thereby directly contact and agitate the wet feed material; a venting port for venting steam evaporated from the wet feed material, and agitation gas introduced through the plurality of nozzles.

33. The indirect-heat dryer of claim 32, wherein the indirect-heat dryer is a rotary disc dryer such that the heating fluid channel is provided within a plurality of discs mounted within the body, and the discs act as the conveying mechanism.

34. The indirect-heat dryer of claim 32 or 33, wherein the nozzles are provided on a modular nozzle plate removably attached to the underside of the body.

35. The indirect-heat dryer of any one of claims 32 to 34, wherein the nozzles are formed as a grating.

36. The indirect-heat dryer of any one of claims 32 to 34, wherein the nozzles are formed as individual spouts.

37. The indirect-heat dryer of any one of claims 32 to 36, wherein the nozzles are swivelable to create a swirling effect.

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