METHOD OF OPERATING A PADDLE DRYER AND PADDLE DRYER

Abstract

 The invention relates to a method of operating a paddle dryer (1) comprising an inlet (2), a drying chamber (3) and an outlet (4), wherein product to be dried is fed through the inlet (2) into the drying chamber (3), in which drying chamber (3) the product is dried during a movement towards the outlet (4), through which outlet (4) the product leaves the drying chamber (3). According to the invention an additional inert medium, in particular water, is introduced into the drying chamber (3) at least temporarily in order to inert an atmosphere in the drying chamber (3).
Furthermore, the invention relates to a paddle dryer (1) comprising a drying chamber (3) with an inlet (2) and an outlet (4)

Description

[0001] The invention relates to a method of operating a paddle dryer comprising an inlet, a drying chamber and an outlet, wherein product to be dried is fed through the inlet into the drying chamber, in which drying chamber the product is dried during a movement towards the outlet, through which outlet the product leaves the drying chamber.

[0002] Furthermore, the invention relates to a paddle dryer comprising a drying chamber with an inlet and an outlet.

[0003] Paddle dryers are known from the state of the art in particular for drying industrial wet sludge of typically 15-30% dry solids, which have been mechanically dewatered beforehand, for example.

[0004] The mechanically dewatered sludge can be temporarily stored in a wet sludge silo. Normally the hold-up volume in these wet sludge silos is large enough to store 24 - 48 hours wet sludge feed flow towards the drying line. Exact storage volume depends on the local situation of the sludge logistics.

[0005] From the wet sludge storage silos, the wet sludge usually can be transported towards the paddle dryer in a constant flow rate by means of a pumping station. With this pumping station, the wet sludge is fed in a constant flow towards a wet sludge feeding nozzle on the paddle dryer, the inlet. This inlet is usually located at the top of a cover of the paddle dryer at one end of the dryer. Via this inlet, the wet sludge enters the drying chamber and falls down in a drying bed, which is also called a sludge bed.

[0006] In the sludge bed, usually two parallel paddle shafts are rotating in opposite direction, to create an intensive mixing and contact of the wet sludge with the heated surfaces of the paddle dryer. As a result of the intensive contact of the wet sludge with the heated surfaces of the paddle dryer, the majority of the water content of the sludge is evaporated, to create a relative dry end product.

[0007] Wet sludge is during normal operation continuously fed at the inlet side of the dryer and dry product is continuously discharged from the dryer via a dry product overflow weir at the other end of the dryer. Because of this continuous feed and discharge of product, a more or less plug flow is created in the sludge dryer during normal operation.

[0008] The dryer is commonly heated with thermal oil or saturated steam, which circulates through the paddle shafts, paddles and jacket of the trough. Consequently, the shafts, paddles and trough are considered as heat transfer area. Also, the cover of the dryer is heated with thermal oil or saturated steam via a heat tracing. The thermal oil is circulated with a thermal oil pump and re-heated in a thermal oil heater. In case steam heating is used, the saturated steam will condense inside the jacketed areas of the paddle dryer and will return to the boiler as condensate.

[0009] The dried sludge leaves the dryer usually via an overflow weir, falling into a discharge chute by gravity. The level of the overflow weir can often be adjusted to regulate the product bed level in the dryer accordingly. Adjustment of the overflow weir height is usually done manually from the outside of the paddle dryer.

[0010] Underneath the dry product discharge chute, usually a discharge screw conveyer extracts the dried product from the paddle dryer into a cooling screw. At the end of the screw conveyor, a dry product air-lock is maintained to prevent air of entering the drying chamber. The dried sludge is cooled down to enable the dried sludge safe handling and storage. The cooling screw conveyor is cooled with cooling water.

[0011] The water vapor coming from the dryer chamber is usually condensed in a wet scrubber system. In the scrubber, cooling water is circulating over the wet scrubber column to create condensation of the water vapor and washing out of dust, that is carried over from the dryer. In case of a desired heat recovery, a two stage condenser system can be applied.

[0012] The sludge reaches temperatures of more than 100 °C in the drying chamber during drying. This causes water contained in the sludge to evaporate, so that an atmosphere in the chamber is essentially filled with water vapour and is thus inert.

[0013] The inert atmosphere is usually kept while the paddle dryer is fully closed and air tight. An air-lock is often created at the product feed and product discharge.

[0014] Because an inert atmosphere is present, an explosive atmosphere will not be present during normal operation.

[0015] However, it is different during start up, shut down and idle mode, because then no new, wet sludge is fed into the paddle dryer, which leads to the atmosphere drying out. The product in the paddle dryer is then also not discharged from the drying chamber and has to be dried to avoid clogging. This heating of the product leads to a risk of explosion and/or smouldering of the product in the drying chamber.

[0016] Hence, it is an object of the invention to provide a method named at the outset, in which a risk of explosion and smouldering can also be avoided in an operation outside a normal operation, in particular during shutdown.

[0017] Furthermore, it is an object to provide a paddle dryer of the type mentioned above that can be operated without the risk of explosion even outside a normal operation, in particular during shutdown.

[0018] According to the invention, the first object is attained in that an additional inert medium, in particular water, is introduced into the drying chamber at least temporarily in order to inert an atmosphere in the drying chamber.

[0019] As part of the invention, it was found that by adding an inert medium, in particular water, a risk of explosion and smouldering can be easily avoided even if no new wet product is fed into the drying chamber. The insertion of an inert medium can compensate for the missing water vapour due to the drying out product in the atmosphere, so that oxygen penetration can be avoided in a simple way. Further, the inert medium can compensate for the volume that becomes free due to a volume decrease of the product during drying.

[0020] Preferably, water is added especially when no new product is fed to prevent the atmosphere from drying out and/or to ensure inertization.

[0021] Here, an inert medium is understood to be a medium that cannot react with the potential reaction partners in the paddle dryer, such as hydrocarbons and biomass in particular, under the conditions in the paddle dryer, for example in a burning reaction. Hence, water in particular is an inert medium within the meaning of this application, although other media are also quite possible for inerting the atmosphere.

[0022] Usually, water is used as the inert medium. This allows easy handling and safe inerting of the atmosphere. The quantity of water to be introduced into the drying chamber is usually enough to compensate a dryer evaporation rate during normal production.

[0023] As long as the water inside the paddle dryer evaporates, the product volume inside the paddle dryer is reduced and that means more empty volume available in the drying chamber that is usually filled with inert medium, in particular water vapour.

[0024] In principle, the inert medium can be introduced in anyway. Preferably, the inert medium is brought into the drying chamber in liquid form and evaporates in the drying chamber. Especially when the inert medium is water, this is usually easy to carry out, especially since a temperature of more than the boiling point of water usually prevails in the drying chamber in order to dry the product.

[0025] The dryer is preferably heated with thermal oil or saturated steam, which circulates through the paddle shafts, paddles and/or jacket of the trough. Hence, the shafts, paddles and trough can be considered as heat transfer areas. Usually, the cover of the dryer is heated with thermal oil or saturated steam via a heat tracing. In case thermal oil heating is used, the thermal oil is circulated with a thermal oil pump and re-heated in a thermal oil heater. In case steam heating is used, the saturated steam will condense inside the jacketed areas of the paddle dryer and will return to the boiler as condensate.

[0026] It has proven to be favorable that the inert medium is introduced into the drying chamber via spray nozzles. This ensures a good distribution of the medium in the drying chamber.

[0027] Usually, several spray nozzles are distributed over a length of the drying chamber to be able to introduce the medium evenly. This makes it easy to avoid an explosion at any point in the drying chamber. As a rule, it is intended that all the medium introduced into the drying chamber evaporates.

[0028] Preferably two to ten spray nozzles are provided.

[0029] It is favourable when the temperature is measured in the drying chamber and inert medium is introduced in the drying chamber, when a predefined temperature is exceeded. Usually, there is a risk of explosion or smouldering above a temperature that depends on the product, especially the composition of the product, which may have biomass, for example. If the medium is supplied to the drying chamber when the temperature is above this critical temperature, an explosion and smouldering risk is avoided. Furthermore, the supply can be stopped when the temperature falls below the critical temperature to avoid undesired wetting of the product.

[0030] In order to achieve a drying of the product at the same time as avoiding the risk of explosion, it is preferably that an amount of the inert medium is introduced that is small enough to dry the product while the medium is introduced. At the same time, the amount may be sufficient to compensate for missing water in the atmosphere. Precise control of the amount of water introduced is therefore advantageous. This corresponding quantity of the inert medium, in particular water, can easily be determined on the basis of a known mass of the product in the drying chamber and a temperature in the drying chamber, which is preferably measured continuously, so that an inert atmosphere is achieved by the introduced medium, which preferably evaporates in the drying chamber, but wetting of the product is avoided. The introduced amount of the inert medium is thus preferably just large enough to evaporate the inert medium completely, but not too large, so no condensation of the inert medium occurs in the drying chamber. Thus, no wet steam is usually achieved in the drying chamber, but the amount of evaporated water is large enough to prevent the formation of an explosive atmosphere in the drying chamber.

[0031] It is favorable when the inert medium is introduced in the drying chamber, when the oxygen concentration becomes higher than a limit oxygen concentration, in particular when the oxygen concentration becomes higher than 5 %, preferably more than 8 %. This easily prevents the formation of an explosive atmosphere. The process can thus be controlled by both temperature and oxygen content.

[0032] The determination of the oxygen concentration in the drying chamber can be done by one or more sensors and/or by calculation.

[0033] It is advantageous if the pressure in the drying chamber is lower than an ambient pressure, wherein a pressure difference between the ambient pressure and the pressure in the drying chamber is preferably 1 mbar to 10 mbar. This easily prevents toxic gases from escaping from the drying chamber into the environment.

[0034] Preferably, an evaporation rate of water in the product feed is measured and/or calculated. Particularly preferably, the amount of medium added is then determined based on this evaporation rate. Thus, as the amount of water introduced by the product decreases, especially during shutdown, an appropriate amount of the inert medium can be easily introduced to compensate for the missing amount of water and continuously maintain an inert atmosphere, even during non-normal operation such as a startup or shutdown.

[0035] A preferable provision is that the inert medium is introduced in the drying chamber, when the evaporation rate of water in the product supply falls below a threshold, in particular below 30 % of dryer evaporation rate during normal production, and a predefined temperature is exceeded. For example, the temperature could be that temperature at which there would be a risk of explosion given the amount of water in the atmosphere. The process respectively the amount of water introduced can thus be controlled also or only based on the evaporation rate or an evaporation rate deviating from an evaporation rate during normal operation.

[0036] In order to also prevent an explosion risk in an area downstream of the drying chamber, it is preferably provided that the product is conveyed from the outlet by means of an outlet conveyor, whereas a temperature at the outlet conveyor is measured and inert medium is introduced in the outlet conveyor, when a predefined temperature is exceeded. Preferably, separate spray nozzles are provided in the area of the conveyor for this purpose. Usually, water is also used as the inert medium for the outlet conveyor. The discharge conveyor can be embodied as a discharge screw conveyer or a cooling screw conveyor with a rotatable screw that moves the product in a desired direction.

[0037] It is advantageous if the method is carried out in that way that when a supply of material through the inlet is terminated or interrupted, a product in the drying chamber is heated to a temperature of at least 105 °C, while simultaneously inert medium is introduced into the chamber. This ensures that wet product is prevented from clogging in the drying chamber when the paddle dryer is shut down. At the same time, an explosion risk is avoided due to the introduced medium. Thus, a secure shut down is achieved.

[0038] Preferably, a pressure difference between the ambient pressure and the pressure in the drying chamber is preferably reduced, when the supply of material through the inlet is terminated or interrupted. This reduces an amount of oxygen that could flow through the seals, especially the air-locks, into the drying chamber and could lead to an explosion risk. For example, a pressure difference between the drying chamber and the environment of 4 mbar during normal operation can be reduced to 1 mbar during operation. The pressure in the drying chamber is usually lower than the pressure in the environment.

[0039] Usually, a volume of the product decreases due to drying in the drying chamber, whereby a volume in the drying chamber that becomes free as a result is filled up by gaseous, inert medium, usually water that is introduced via spray nozzles and evaporates in the drying chamber. This easily prevents the formation of an explosive atmosphere.

[0040] It is preferably that an amount of the inert medium is introduced that is small enough to dry the product while the medium is introduced. Then inert medium, in particular water, can be sprayed without wetting the product. An endless shutdown process can thus be avoided and at the same time a safe shutdown process and drying of the product during shutdown can be achieved.

[0041] The other object is obtained according to the invention in a paddle dryer named at the outset wherein at least one device for introducing an inert medium in the drying chamber is provided, in particular a spray nozzle. Thus, inert medium can be introduced in the drying chamber to avoid the formation of an explosive atmosphere. Usually, the medium is supplied in liquid form and then evaporates in the drying chamber.

[0042] Usually, the device for introducing an inert medium in the drying chamber is embodied as one or more spray nozzles that are supplied by water pipes.

[0043] The paddle dryer is preferably designed for carrying out a method according to the invention.

[0044] Usually, at least one shaft with paddles is rotatably arranged to dry sludge in the drying chamber. Preferably, two shafts are provided in the drying chamber.

[0045] The dryer is preferably heated with thermal oil or saturated steam, which circulates through the paddle shafts, paddles and/or jacket of the trough. Usually, the cover of the dryer is heated with thermal oil or saturated steam via a heat tracing.

[0046] It is advantageous if at least one temperature sensor is provided for determining a temperature of a product located in the drying chamber, wherein preferably several temperature sensors are provided for determining a temperature of the product at different positions in the drying chamber. A temperature of the product and thus a risk of smoldering or explosion can then be well assessed, preferably automatically. Based on the measured temperature a supply of medium can be initiated or terminated. Usually, this is also done automatically.

[0047] Furthermore, an oxygen sensor can be provided. An oxygen content in the atmosphere in the drying chamber can then also be taken into account when controlling a valve for the medium, preferably water, which is introduced via nozzles. However, the method according to the invention can of course also be carried out without an oxygen sensor.

[0048] Furthermore, one or more pressure sensors can be provided to measure a pressure in the drying chamber. The pressure of the atmosphere in the drying chamber can then also be taken into account when controlling a valve for the medium, preferably water, which is introduced via nozzles. However, the method according to the invention can of course also be carried out without measuring the pressure in the drying chamber.

[0049] Additional features, benefits and effect of the invention follow from the exemplary embodiment described below.

[0050] The drawings which are thereby referenced show the following:

Fig. 1 a side view of a paddle dryer;

Fig. 2 a top view of the paddle dryer of Fig. 1;

Fig. 3 a paddle dryer in front view;

Fig. 4 the paddle dryer of Fig. 3 in another view;

Fig. 5 a moisture profile;

Fig. 6 and 7 temperature profiles.

[0051] Fig. 1 and 2 show a paddle dryer 1 according to the invention in side view and top view. The paddle dryer 1 comprises a drying chamber 3 in which at least on shaft 5 with paddles 8 is rotatably arranged around rotation axis 6 to dry sludge. Usually two shafts 5 are provided in the drying chamber 3. The paddle dryer 1 is normally used to dry mechanically dewatered sludge of typically 15-30% dry solids to a final dryness of up to 95% dry.

[0052] The mechanically dewatered wet sludge is usually transported towards the paddle dryer 1 in a constant flow rate by means of a pumping station which is not shown. With this pumping station, the wet sludge is fed in a constant flow towards an inlet 2, which is here formed by a wet sludge feeding nozzle on the paddle dryer 1. This nozzle is located at the top of the cover of the paddle dryer 1 at one end of the dryer. Via this nozzle, the wet sludge enters the drying chamber 3 and falls down in a drying bed 9, which can also be referred to as the sludge bed.

[0053] In the sludge bed, a paddle 8 shaft 5 is rotating, to create an intensive mixing and contact of the wet sludge with the heated surfaces of the paddle dryer 1. As a result of the intensive contact of the wet sludge with the heated surfaces of the paddle dryer 1, the majority of the water content of the sludge is evaporated, to create a relative dry end product.

[0054] Wet sludge is continuously fed at the inlet 2 side of the dryer and dry product is continuously discharged from the dryer via an outlet 4, which is formed by a dry product overflow weir at the other end of the dryer. Because of this continuous feed and discharge of product, a more or less plug flow is created in the sludge dryer.

[0055] The product is moved in the drying chamber 3 along a transport direction 7 between inlet 2 and outlet 4. For such a movement, gravity is used, which is why the drying chamber 3 can have an inclination of, for example, 1°.

[0056] The paddle dryer 1 is heated with thermal oil or saturated steam, which circulates through the paddle 8 shafts 5, paddles 8 and jacket of the trough. Consequently, the shafts 5, paddles 8 and trough are considered as heat transfer area. Also, a cover of the dryer is heated with thermal oil or saturated steam via a heat tracing. The thermal oil is circulated with a thermal oil pump and re-heated in a thermal oil heater. In case steam heating is used, the saturated steam will condense inside the jacketed areas of the paddle dryer 1 and will return to the boiler as condensate.

[0057] The dried sludge leaves the paddle dryer 1 via an overflow weir, falling into a discharge chute by gravity through an outlet 4. The level of the overflow weir can be adjusted to regulate the product bed level in the dryer accordingly. Adjustment of the overflow weir height can be done manually from the outside of the paddle dryer 1.

[0058] Underneath the dry product discharge chute, a discharge screw conveyer 19 which is not shown in Fig. 1 and Fig. 2 extracts the dried product from the paddle 8 dryer 1 into a cooling screw. At the end of the screw conveyor, a dry product air-lock 20 is maintained to prevent air of entering the drying chamber 3. The dried sludge is cooled down to enable the dried sludge safe handling and storage. The cooling screw conveyor is usually cooled with cooling water.

[0059] The evaporated water from the drying chamber 3 is continuously extracted and afterwards condensed in a wet scrubber system, which is not shown. In the scrubber, cooling water is circulating over the wet scrubber column to create condensation of the water vapor and washing out of dust, that is carried over from the dryer. In case of a desired heat recovery, a two-stage condenser system can be applied.

[0060] In the drying chamber 3, there is usually a pressure of 4 mbar below ambient pressure and a temperature of 100 °C to 140 °C to dry the product during a movement from inlet 2 through the drying chamber 3 to the outlet 4. The product may contain flammable components, for example hydrocarbons. During normal operation, a proportion of water vapor in the atmosphere due to water evaporating from the product is usually high enough to prevent a risk of explosion. To prevent the occurrence of an explosive atmosphere in the drying chamber 3 even under non-normal conditions, spray nozzles 24 that are supplied by water pipes 10 are provided in the drying chamber 3 to increase a percentage of water vapor in the atmosphere. Machine safety is then ensured by evaporation of water that is additionally introduced via the water pipes 10 and the spray nozzles 24.

[0061] For example, if feed is stopped, no more water is coming in with new wet product. Hence without spraying water, the only water evaporation that would happen would be from the water that is already inside the paddle dryer 1. This would result in a relatively dry and explosive atmosphere since the product is then drying out. Thus, by supplying water as an inert medium through the spray nozzles 24 in this operating state, for example during a shutdown, the emergence of an explosive atmosphere is prevented.

[0062] When the feed of wet product is stopped, the amount of water inside the drying chamber 3 is known or can at least be calculated based on the temperature and the mass of product in the drying chamber 3. Although the feed of product is stopped, the drying chamber 3 is still heated to dry the product inside and avoid clogging. Hence, the content of water in the atmosphere of the drying chamber 3 can be calculated based on the temperature inside the drying chamber 3 and the time elapsed since the feed was stopped.

[0063] According to a preferred embodiment of the invention, only as much water is introduced via the spray nozzles 24 as is required to prevent an explosive. This prevents a condensation of water in the drying chamber 3 and wetting of the product. This ensures safe drying of the product even during a shutdown. Further, this avoids an endless shutdown during which the product cannot dry.

[0064] Based on a temperature of the product, which can be determined by temperature sensors 11 placed in the drying chamber 3, it is also possible to determine when the product is completely dry during shutdown. At this point, heating can then be ended. When the temperature in the drying chamber 3 has dropped below a critical value of, for example, 100 °C, the introduction of additional water via the spray nozzles 24 can also be ended.

[0065] Fig. 3 and 4 show a further embodiment of a paddle dryer 1 according to the invention with two parallel paddle 8 shafts 5 that are rotating in opposite direction around rotation axis 6. Fig. 3 shows the paddle dryer 1 in front view from e downstream end of the paddle dryer 1 and Fig. 4 shows the paddle dryer 1 from the side and slightly from above the paddle dryer 1.

[0066] In Fig. 3 a discharge screw conveyer 19 can be seen downstream of the drying chamber 3. The discharge screw conveyer 19 comprises a discharge screw 16 that is driven by a motor 17. The discharge screw 16 can be driven in two directions. In normal operation the discharge screw 16 runs in a first rotation direction and product is conveyed according to a normal product flow 14 into a cooling screw conveyor that is not shown. The dried sludge is here cooled down to enable the dried sludge safe handling and storage. The cooling screw conveyor is usually cooled with cooling water.

[0067] In case of problems at the dry sludge handling section or sludge cooling, the dried sludge can be diverted in the discharge screw conveyer 19 into the emergency outlet 18 of the discharge screw conveyer 19, which is equipped with a manual slide gate valve. For this purpose, the direction of rotation of the motor 17 driving the screw conveyor can be changed to run the discharge screw 16 in a second rotation direction. The product then flows along an emergency product flow 15 direction to an emergency outlet 18.

[0068] In the discharge screw 16 and the beginning of the cooling screw also a hazardous situation exists. A gravity fall of product, in combination with oxygen and, when paddle dryer 1 stands still, product smouldering can occur with dry product. Hence, serious fire hazard is present. An air-lock 20 at both ends of the discharge screw 16 is usually maintained in order to keep a possible fire limited to smouldering only. Furthermore, usually two temperature sensors 11 and water spray nozzles 24 are installed also in the cooling screw conveyor. If smouldering occurs in the cooling screw conveyor downstream of the drying chamber 3 countermeasures with water spraying can be activated.

[0069] As shown in Fig. 3 temperature sensors 11 and water spray nozzles 24 supplied by water pipes 10 are located in the area of the discharge screw conveyer 19. A fire in this area can thus be quickly detected in order to initiate countermeasures by activating the spray nozzles 24.

[0070] At the top of the drying chamber 3 a level sensor 13 is provided to be able to determine a volume of the product present in the drying chamber 3. In case of a shutdown, a volume of the product can then also be determined. The volume of the product is reduced during a shutdown due to the water that evaporates out of the product. Any space thus freed up is usually filled with water that is introduced into the drying chamber 3 via the spray nozzles 24.

[0071] As shown in Fig. 4, water pipes 10 lead into the drying chamber 3 at several positions along a transport direction 7 to supply water to spray nozzles 24 located inside the drying chamber 3 at corresponding positions.

[0072] Fig. 4 also shows positions where temperature sensors 11 are distributed in the drying chamber 3 along the transport direction 7. This allows the temperature of the product to be determined precisely at various positions in the drying chamber 3.

[0073] Furthermore, a pressure sensor 12 is shown, with which a pressure of the atmosphere in the drying chamber 3 can be determined.

[0074] The paddle dryer 1 shown in Fig. 3 and Fig. 4 has two rotatable shafts 5, on each of which blades are arranged. The shafts 5 can be rotated about two parallel axes of rotation.

[0075] This paddle dryer 1 shown in Fig. 3 and Fig. 4 also has an inlet 2 through which product can be fed downward into the paddle dryer 1. Furthermore, an outlet 4 is also provided here via which product can be discharged, preferably into an area below the drying chamber 3.

[0076] Fig. 5 shows a moisture content in the product depending on a position in the drying chamber 3 during normal operation along the length of the drying chamber 3 in transport direction 7. As can be seen, the product usually has a high moisture content of, for example, 80% in a region of the inlet 2 and a moisture content decreases to, for example, 9% up to the outlet 4. Thus, when the system is shut down, an area of the product near the outlet 4 dries out earlier, which means that this part of the product can no longer give off water. As a result, the atmosphere in the drying chamber 3 dries out during the shutdown process. The difference is compensated by the spray nozzles 24 by spraying water in the drying chamber 3. This water then evaporates immediately, as the temperature in the drying chamber 3 is more than 100 °C. Furthermore, during the shutdown process, a negative pressure difference of the drying chamber 3 compared to an environment of 4 mbar is reduced to 1 mbar. The pressure in the drying chamber 3 is then only usually only 1 mbar below ambient pressure.

[0077] Fig. 6 shows a first temperature profile 21 of the product during normal operation and a second temperature profile 22 of the product after drying during a shutdown. The temperature profiles 21, 22 show the temperature in the product in each case over a length of space along the transport direction 7 from the inlet 2 to the outlet 4.

[0078] As can be seen, the product is introduced into the drying chamber 3 at about 20 °C and heated up to about 105 °C during normal operation, evaporating the water from the product and thus drying the product in the drying chamber 3. The dry product at the outlet 4, which has, for example, a dry component content of 95%, therefore has a temperature of about 107 °C during normal operation. For example, a temperature of the surfaces in the drying chamber 3 may be about 140 °C to ensure efficient drying.

[0079] During normal operation, the product thus only reaches temperatures of 100 °C and more from about 50 % of a length of the drying chamber 3. During a shutdown, however, the product is also heated to more than 100 °C in an area of the inlet 2 to remove the water from the product. At the same time, the product in the area of the outlet 4 reaches a temperature of 135 °C as can be seen from the second temperature profile 22.

[0080] Since water is sprayed in via spray nozzles 24 and immediately evaporates, an inert atmosphere is ensured during the entire shutdown process, so that explosion or smouldering of the product, often biomass, is avoided even at the high temperatures near the outlet 4.

[0081] Continuous temperature measurement in the drying chamber 3 allows the product temperature to be determined precisely and as a function of a position in the drying chamber 3 as shown in Fig. 4. Thus, the product temperature can also be used to determine a moisture content in the product.

[0082] Further, this allows a quantity of water introduced by the drying product to be determined during a shutdown process. Thus, the reduced amount of water due to the shutdown process and the drying product compared to a normal operation can be easily determined and replaced by supplied water to achieve an inert atmosphere and at the same time ensure drying of the product.

[0083] Typically, water spraying is started when the evaporation rate of water in the product is less than 30 % of the evaporation rate during normal operation.

[0084] Furthermore, sensors for measuring an oxygen content and a pressure can of course be arranged in the drying chamber 3. This also allows to stop heating the drying chamber 3 when the product is dried.

[0085] Alternatively, the injection of water can also be made dependent on an oxygen content in the atmosphere reaching a defined threshold.

[0086] In the embodiment shown, six spray nozzles 24 are provided and water is introduced into the spray nozzles 24 at a pressure of 5 bar to ensure an inert atmosphere in the drying chamber 3.

[0087] Fig. 7 shows the second temperature profile 22 known from Fig. 6 and a third temperature profile 23 after heating of the drying chamber 3 is stopped. As can be seen, the temperature then drops, again reaching lower temperatures in an area near the inlet 2. If a maximum temperature of the product falls below a threshold value, in this case a temperature at the outlet 4 of 95 °C, a risk of smouldering or explosion is eliminated. The introduction of water via the spray nozzles 24 can then be stopped.

[0088] In the next step emptying valves can be opened in order to discharge the product. A discharge screw 16 can be provided to bring the dried product in direction of an emergency outlet 18. Subsequently the discharge screw conveyer 19, a vapor condensing system, a downstream equipment system, the paddle 8 shaft 5 rotation, lubricant units and the paddle dryer 1 are stopped.

[0089] With a method and a paddle dryer 1 according to the invention, safe operation of a paddle dryer 1 is possible, even outside normal conditions, whereby in particular O2 monitoring is not mandatory.

[0090] Thus, an explosive atmosphere in the paddle dryer 1 is avoided in a simple way by introducing an inert medium, which compensates for the lack of water in the atmosphere, for example during a shutdown. With the additionally introduced inert medium, usually water, an explosion risk is thus avoided in a simple and reliable way.

Claims

1. Method of operating a paddle dryer (1) comprising an inlet (2), a drying chamber (3) and an outlet (4), wherein product to be dried is fed through the inlet (2) into the drying chamber (3), in which drying chamber (3) the product is dried during a movement towards the outlet (4), through which outlet (4) the product leaves the drying chamber (3), characterized in that an additional inert medium, in particular water, is introduced into the drying chamber (3) at least temporarily in order to inert an atmosphere in the drying chamber (3).

2. Method according to claim 1, wherein the inert medium is brought into the drying chamber (3) in liquid form and evaporates in the drying chamber (3).

3. Method according to claim 1 or 2, wherein the temperature is measured in the drying chamber (3) and inert medium is introduced in the drying chamber (3), when a predefined temperature is exceeded.

4. Method according to one of claims 1 to 3, wherein the inert medium is introduced into the drying chamber (3) via spray nozzles (24).

5. Method according to one of claims 1 to 4, wherein an amount of the inert medium is introduced that is small enough to dry the product while the medium is introduced.

6. Method according to one of claims 1 to 5, wherein the inert medium is introduced in the drying chamber (3), when the oxygen concentration becomes higher than a limit oxygen concentration, in particular when the oxygen concentration becomes higher than 5 %, preferably more than 8 %.

7. Method according to one of claims 1 to 6, wherein the pressure in the drying chamber (3) is lower than an ambient pressure, wherein a pressure difference between the ambient pressure and the pressure in the drying chamber (3) is preferably 1 mbar to 10 mbar.

8. Method according to one of claims 1 to 7, wherein an evaporation rate of water in the product feed is measured and/or calculated.

9. Method according to claim 8, wherein the inert medium is introduced in the drying chamber (3), when the evaporation rate of water in the product supply falls below a threshold, in particular below 30 % of dryer evaporation rate during normal production, and a predefined temperature is exceeded.

10. Method according to one of claims 1 to 9, wherein the product is conveyed from the outlet (4) by means of an outlet (4) conveyor, whereas a temperature at the outlet (4) conveyor is measured and inert medium is introduced in the outlet (4) conveyor, when a predefined temperature is exceeded.

11. Method according to one of claims 1 to 10, wherein when a supply of material through the inlet (2) is terminated or interrupted, a product in the drying chamber (3) is heated to a temperature of at least 105 °C, while simultaneously inert medium is introduced into the chamber.

12. Method according to claim 11, wherein a pressure difference between the ambient pressure and the pressure in the drying chamber (3) is preferably reduced, when the supply of material through the inlet (2) is terminated or interrupted.

13. Method according to claims 11 or 12, wherein a volume of the product decreases due to drying in the drying chamber (3), whereby a volume in the drying chamber (3) that becomes free as a result is filled up by gaseous, inert medium.

14. Paddle dryer (1), in particular for carrying out a process according to one of claims 1 to 13, comprising a drying chamber (3) with an inlet (2) and an outlet (4), characterised in that at least one device for introducing an inert medium in the drying chamber (3) is provided, in particular a spray nozzle.

15. Paddle dryer (1) according to claim 14, wherein at least one temperature sensor (11) is provided for determining a temperature of a product located in the drying chamber (3), wherein preferably several temperature sensors (11) are provided for determining a temperature of the product at different positions in the drying chamber (3).

Drawing