DRYER FOR HERBACEOUS MATERIAL WITH ACCESS HEATING

Abstract

The dryer comprises a dryer receptacle having an inner space for receiving herbaceous material, an access assembly providing access to the inner space of the dryer receptacle, and a heating system comprising at least one access heating element for actively heating the access assembly. The access assembly comprises at least one of an inlet conveyor for supplying herbaceous material into the inner space of the dryer receptacle and an outlet conveyor for removing herbaceous material from the inner space of the dryer receptacle. The at least one access heating element comprises a conveyor heating element incorporated into the conveyor.

Claims

1-15. (canceled)

16. A dryer for drying herbaceous material, comprising a dryer receptacle having an inner space for receiving herbaceous material; an access assembly providing access to the inner space of the dryer receptacle; and a heating system comprising at least one access heating element for actively heating the access assembly, wherein the access assembly comprises at least one of an inlet conveyor for supplying herbaceous material into the inner space of the dryer receptacle and an outlet conveyor for removing herbaceous material from the inner space of the dryer receptacle, and wherein the at least one access heating element comprises a conveyor heating element incorporated into the conveyor.

17. The dryer according to claim 16, wherein the at least one access heating element is incorporated into the access assembly.

18. The dryer according to claim 16, wherein the access assembly comprises a door provided at the dryer receptacle and the at least one access heating element comprises a door heating element incorporated into the door.

19. The dryer according to claim 16, wherein the at least one access heating element comprises an electrical resistance heating element or a heating fluid line.

20. The dryer according to claim 16, further comprising a temperature sensor configured to determine an access assembly temperature.

21. The dryer according to claim 16, further comprising a controller configured to control the at least one access heating element to maintain at least a predetermined minimum access assembly temperature.

22. The dryer according to claim 16, wherein the heating system further comprises at least one wall heating element being incorporated in a wall of the dryer receptacle.

23. The dryer according to claim 16, wherein the heating system further comprises a vane heating element for actively heating at least one vane extending from an inner surface of the dryer receptacle into the inner space of the dryer receptacle.

24. The dryer according to claim 16, wherein the at least one access heating element comprises a plurality of access heating elements, which are arranged to be independently controlled.

25. A method for drying herbaceous material, comprising heating herbaceous material received in an inner space of a dryer receptacle, wherein an inner surface of the dryer receptacle is heated such that the entire inner surface of the dryer receptacle is maintained above a condensation temperature of gases evaporated inside the dryer receptacle during heating of the herbaceous material, the method further comprising actively heating at least one of an inlet conveyor for supplying herbaceous material into the inner space of the dryer receptacle and an outlet conveyor for removing herbaceous material from the inner space of the dryer receptacle at least above the condensation temperature with a conveyor heating element incorporated into the at least one of the inlet conveyor and the outlet conveyor.

26. The method according to claim 25, further comprising actively heating at least one door of the dryer receptacle.

27. The method according to claim 25, wherein first and second doors of the dryer receptacle provided at opposing sides of the dryer receptacle are actively heated to different temperatures.

28. A method comprising using an access heating element for actively heating an access assembly providing access to the inner space of a dryer receptacle for receiving herbaceous material to prevent formation of lower temperature spots, wherein the access assembly comprises at least one of an inlet conveyor for supplying herbaceous material into the inner space of the dryer receptacle and an outlet conveyor for removing herbaceous material from the inner space of the dryer receptacle, and wherein the at least one access heating element comprises a conveyor heating element incorporated into the conveyor

Description

[0101] In the following, the invention is further described by describing embodiments of the invention with reference to the Figures.

[0102] FIG. 1 shows a schematic perspective view of a dryer for drying herbaceous material according to an embodiment of the invention;

[0103] FIG. 2 shows a schematic sectional view through the dryer according to the embodiment of the invention with the sectional plane being parallel to the rotation axis of the dryer receptacle;

[0104] FIG. 3A is a schematic sectional view showing an inner surface of the dryer receptacle and vanes extending from the inner surface of the dryer receptacle in a sectional plane perpendicular to the rotation axis of the dryer receptacle according to an embodiment of the invention;

[0105] FIG. 3B is a schematic sectional view showing an inner surface of the dryer receptacle and vanes extending from the inner surface of the dryer receptacle in a sectional plane perpendicular to the rotation axis of the dryer receptacle according to an alternative embodiment according to the invention;

[0106] FIG. 4 is a schematic perspective view of a channel-shaped collector and a part of a conveyor tube according to the embodiment of the invention;

[0107] FIG. 5 is a schematic view of an inner surface of a door of the dryer receptacle according to the embodiment of the invention in a view from an inside of the dryer receptacle, when the door is closed; and

[0108] FIG. 6 is a block diagram schematically showing a control scheme of the dryer according to the embodiment of the invention, in particular with respect to the heating system of the dryer.

[0109] FIG. 1 shows a schematic partial view of a rotary dryer 1 for drying herbaceous material, in particular tobacco material. The rotary dryer 1 comprises a dryer receptacle 3 having an inner space 5 for receiving the herbaceous material. The dryer receptacle 3 can be rotated about a rotation axis 10 of the dryer receptacle 3. The dryer receptacle 3 comprises a main body 7 extending from a first side 9 of the main body 7 (inlet side according to the present embodiment) to a second side 11 of the main body 7 (outlet side according to the present embodiment) along the rotation axis 10. In the illustrated embodiment, the main body 7 of the dryer receptacle 3 is substantially cylindrically shaped. However, also other shapes of the main body 7 are conceivable, such as a prismatic shape, for example. The dryer receptacle 3 further comprises a first door 13 provided at the first side 9 of the main body 7. Further, the dryer receptacle 3 comprises a second door 15 provided at the second side 11 of the main body 7. The doors 13, 15 can be opened to access the inner space 5 of the dryer receptacle 3 for maintenance or loading. When closed, the doors 13, 15 provide a substantially airtight seal together with the main body 7 of the dryer receptacle 3. The substantially airtight seal allows control over gases flowing into and out of the inner space 5 of the dryer receptacle 3. For example, the oxygen content in the inner space 5 of the dryer receptacle 3 may be controlled. According to an embodiment, there is an overpressure in the inner space 5 of the dryer receptacle 3 to avoid ambient atmosphere from entering the inner space 5 of the dryer receptacle 3 in an uncontrolled manner. When the dryer receptacle 3 is rotated, the main body 7 and the doors 13, 15 rotate together.

[0110] In some embodiments, the herbaceous material may be manually or automatically loaded into or removed from the dryer receptacle 3 through the opened doors 13, 15. In the illustrated embodiment, however, the herbaceous material is loaded into and withdrawn from the inner space 5 of the dryer receptacle 3 when the doors 13, 15 are closed. An inlet system 17 is provided at the first door 13 to feed herbaceous material into the dryer receptacle 3. The inlet system 17 comprises an inlet duct 19 extending through a central opening in the door 13. The inlet duct 19 is stationary and does not rotate together with the dryer receptacle 3. The inlet duct 19 is connected to the first door 13 via a substantially airtight rotation decoupling seal 21. Herbaceous material to be supplied to the inner space 5 of the dryer receptacle 3 is supplied to an inlet 23 of the inlet system 17. As illustrated in FIG. 2, an inlet conveyor 25 is provided inside the inlet duct 19. In the illustrated embodiment, the inlet conveyor 25 comprises a conveyor screw that is rotated by a drive assembly 27 about a rotation axis that is parallel and coaxial to the rotation axis 10 of the dryer receptacle 3. Alternatively, the inlet conveyor 25 may comprise a rotated spiral for feeding the herbaceous material into the inner space 5 of the dryer receptacle 3. As another alternative, the inlet conveyor 25 may comprise a scraper configured to move back and forth for feeding the herbaceous material into the inner space 5 of the dryer receptacle 3. If the inlet conveyor 25 comprises, for example, a conveyor screw, or a rotated spiral, or a scraper, the inlet conveyor 25 comprises an active conveying system. However, the inlet conveyor 25 could alternatively be configured as passive conveying system. In particular, the inlet conveyor 25 could comprise a chute for feeding the herbaceous material into the inner space 5 of the dryer receptacle 3. The inlet conveyor 25 could be configured to feed the herbaceous material into the inner space 5 of the dryer receptacle 3 without using any actively driven components. The inlet conveyor 25 conveys herbaceous material supplied to the inlet 23 of the inlet system 17 into the inner space 5 of the dryer receptacle 3.

[0111] Analogously, an outlet system 29 is provided at the second door 15 to withdraw herbaceous material from the inner space 5 of the dryer receptacle 3. The outlet system 29 comprises an outlet duct 31 extending through a central opening in the door 15. The outlet duct 31 is stationary and does not rotate together with the dryer receptacle 3. The outlet duct 31 is connected to the second door 15 via a substantially airtight rotation decoupling seal 33. As illustrated in FIG. 2, an outlet conveyor 37 is provided inside the outlet duct 31. The outlet conveyor 37 comprises a conveyor screw that is rotated by a drive assembly 39 about a rotation axis that is parallel and coaxial with the rotation axis 10 of the dryer receptacle 3. Alternatively, the outlet conveyor 37 may comprise a rotated spiral for removing the herbaceous material from the inner space 5 of the dryer receptacle 3. As another alternative, the outlet conveyor 37 may comprise a scraper configured to move back and forth for removing the herbaceous material from the inner space 5 of the dryer receptacle 3. If the outlet conveyor 37 comprises, for example, a conveyor screw, or a rotated spiral, or a scraper, the outlet conveyor 37 comprises an active conveying system. However, the outlet conveyor 37 could also be configured as passive conveying system. In particular, the outlet conveyor 37 could comprise a chute for removing the herbaceous material from the inner space 5 of the dryer receptacle 3. The outlet conveyor 37 could be configured to remove the herbaceous material from the inner space 5 of the dryer receptacle 3 without using any actively driven components. The outlet conveyor 37 conveys herbaceous material from the inner space 5 of the dryer receptacle to an outlet 35 of the outlet system 29.

[0112] As shown in FIG. 2, the dryer receptacle 3 may be mounted on a tilting device 41 for adjusting a tilting angle of the dryer receptacle 3 with respect to a horizontal plane. In FIG. 2, the dryer receptacle 3 is in a horizontal position, meaning that the tilting angle is zero. The tilting device 41 comprises an arm 43 carrying the dryer receptacle 3. The arm 43 can be tilted via a hinge 45 and a hydraulic cylinder 47, thereby tilting the dryer receptacle 3 by raising the inlet side 9 of the dryer receptacle 3 with respect to the outlet side 11. The tilting device 41 may be configured to establish a tilting angle between 0 degrees and 90 degrees, or between 0 degrees and 60 degrees, or between 0 degrees and 45 degrees, or between 0 degrees and 30 degrees, or between 0 degrees and 15 degrees, or between 0 degrees and 10 degrees for example.

[0113] The dryer 1 can be operated in two different operational modes. In a batch mode, a load of the herbaceous material is first loaded into the dryer receptacle 3, then dried in the dryer receptacle 3, and then removed from the dryer receptacle 3. During drying, the inlet conveyor 25 and the outlet conveyor 37 may be rotated to push material at the inlet and outlet sides 9, 11 back into the inner space 5.

[0114] In detail, in the batch mode, the herbaceous material to be dried may be introduced into the inner space 5 of the dryer receptacle 3 via the inlet system 17, while the inlet side 9 of the dryer receptacle 3 is raised with respect to the outlet side 11. Preferably, the dryer receptacle 3 is rotated during introduction of the herbaceous material. When all the material has been loaded into the inner space 5 of the dryer receptacle 3, the tilting device 41 lowers the inlet side 9 of the dryer receptacle 3 until the dryer receptacle 3 is horizontally aligned. The material is then processed for a desired amount of time, while the dryer receptacle 3 is rotated. During this time, the inlet conveyor 25 and the outlet conveyor 37 may be rotated to push material at the inlet side 9 and at the outlet side 11 back into the inner space 5. After expiry of the desired time, the inlet side 9 of the dryer receptacle 3 is again raised with respect to the outlet side 11, and the rotation direction of the outlet conveyor 37 is reversed so that the outlet conveyor 37 conveys the herbaceous material to the outlet 35. During this process, the dryer receptacle 3 may still rotate.

[0115] According to a continuous mode, herbaceous material is introduced into the inner space 5 of the dryer receptacle 3 and withdrawn from the inner space 5 of the dryer receptacle 3 continuously. The inlet conveyor 25 may continuously rotate to supply herbaceous material from the inlet 23 to the inner space 5 of the dryer receptacle 3, while the outlet conveyor 37 continuously rotates to remove herbaceous material from the inner space 5 of the dryer receptacle 3 to the outlet 35. Residence time of the herbaceous material in the inner space 5 of the dryer receptacle 3 can be regulated by appropriately setting the inclination of the dryer receptacle 3 via the tilting device 41. Additionally or alternatively, the speed of rotation of the dryer receptacle 3 may be regulated.

[0116] As stated above, the dryer receptacle 3 is provided to be substantially airtight. Preferably, drying of the herbaceous material in the dryer receptacle 3 is carried out under specific atmospheric conditions. This allows better control over the process. Further, yield of high-quality dried product may be improved by controlling the atmosphere in the dryer receptacle 3. The drying process can be carried out under inert gas atmosphere in the dryer receptacle 3. Inert gases in the inner space 5 of the dryer receptacle 3 may reduce the risk of fire. In particular, when processing tobacco material, it can be beneficial to carry out the drying process under nitrogen atmosphere. The nitrogen can, in particular, function as an inert gas. Also, other inert gases or mixtures of gases comprising inert gases could be used. In particular, the atmosphere in the dryer receptacle 3 could comprise noble gases. Nitrogen or another gas or a mixture of gases may, for example be provided to the dryer receptacle 3 via a gas inlet 62. In FIG. 2, the gas inlet 62 is illustrated at the first door 13 as an example. Gases from the inner space 5 of the dryer receptacle 3 may be withdrawn via a gas outlet 80. In FIG. 2, the gas outlet 80 is illustrated at the outlet tube 31 of the outlet system 29 as an example. The drying process could also be carried out under vacuum.

[0117] Volatiles evaporated in the dryer receptacle 3 during drying of the herbaceous material may be processed. Such volatiles may, for example, comprise flavor compounds evaporated during drying of the herbaceous material, in particular during drying of tobacco material. The volatiles may, for example, carry flavor extracted from the herbaceous material. The volatiles could, for example, comprise aromatic substances or oils. The volatiles could comprise, for example, alkaloids such as nicotine. The volatiles could also comprise pyrazines such as for example: 2-methylpyrazine; 2,5-dimethylpyrazine; 2,6-dimethylpyrazine; 2-ethylpyrazine; 2,3-dimethylpyrazine; 2-ethyl-5-methylpyrazine; 2-ethyl-6-methylpyrazine; 2,3,5-trimethyl Pyrazine; tetramethylpyrazine; 2-ethyl-3,6-dimethylpyrazine; or 2-ethyl-3,5-dimethylpyrazine. Other examples of volatiles include β-ionone; β-damascenone; or acetic acid.

[0118] To increase drying efficiency and quality of the resulting products, the herbaceous material inside the dryer receptacle 3 may be agitated during drying. This may be achieved with vanes 49 extending from an inner surface 51 of the dryer receptacle 3 into the inner space 5 of the dryer receptacle 3. Respective vanes 49 according to a first exemplary embodiment are illustrated in FIG. 3A, which shows the inner surface 51 of the dryer receptacle 3 and the vanes 49 in a sectional view with a sectional plane that is perpendicular to the rotation axis 10 of the dryer receptacle 3. FIG. 3B shows a corresponding view according to a second exemplary embodiment. In the first embodiment shown in FIG. 3A, the vanes 49 have a parallelepiped shape. The vanes 49 according to the second embodiment shown in FIG. 3B have curved shapes.

[0119] According to both embodiments, the vanes 49 are, in the cross-section with the sectional plane perpendicular to the rotation axis 10 of the dryer receptacle 3, inclined with respect to the radial direction, which is radial with respect to the rotation axis 10. The angle of inclination of the vanes 49 with respect to the radial direction is illustrated as angle 20 in the figures. To define the angle, the figures show radius lines 30 connecting, in the cross-section view, the rotation axis 10 of the dryer receptacle 3 with center points of base portions of the vanes 49, the base portions being portions of the vanes 49 where the vanes 49 meet the inner surface 51 of the dryer receptacle 3. Further, the figures illustrate extension lines 40 being lines that connect the center points of the base portions of the vanes 49 with center points of far end portions of the vanes 49, the far end portions of the vanes being the portions reaching farthest into the inner space 5 of the dryer receptacle 3.

[0120] In the illustrated embodiments, the angles 20 between the vanes 49 and the radial direction are the same for each vane 49. Preferably, the angle 20 is lower than 30 degrees. In particular, the angle 20 can lie between 5 degrees and 25 degrees or more preferably between 5 degrees and 15 degrees.

[0121] The arrows in FIGS. 3A and 3B illustrate the direction of rotation of the dryer receptacle 3. As illustrated, the inclination of the vanes 49 is such that in the sectional plane perpendicular to the rotation axis 10 of the dryer receptacle 3, angles between the vanes 49 and the inner surface 51 of one of the dryer receptacle 3 are larger when measured in the direction of rotation of the dryer receptacle 3 than when measured against the direction of the rotation of the dryer receptacle 3. The angles are in both cases measured starting from the respective vane 49 and ending at the inner surface 51 of the dryer receptacle 3. The larger angle measured in the direction of rotation of the dryer receptacle 3 is indicated as angle 50 in the figures, whereas the smaller angle measured against the direction of rotation of the dryer receptacle 3 is indicated as 60. Angles 50 and 60 are again defined referring to the extension lines 40 of the vanes 49. Generally, in case of nonlinear portions of the inner surface 51 of the dryer receptacle 3, the angles 50 and 60 may be measured with reference to tangent lines to the inner surface 51 of the dryer receptacle 3 at the center of the base portion of the respective vane 49.

[0122] As can be understood from FIGS. 3A and 3B, engagement surfaces 53 of the vanes 49 will engage herbaceous material received in the dryer receptacle 3, when the dryer receptacle 3 is rotated about the rotation axis 10 in the direction of rotation of the dryer receptacle 3. The inclination of the vanes 49 enables the vanes 49 to better engage the herbaceous material. Further, due to the inclination of the vanes 49, pockets 55 are formed between the vanes 49 and the inner surface 51 of the dryer receptacle 3. The pockets 55 can temporarily hold herbaceous material during rotation of the dryer receptacle 3. Due to the inclination of the vanes 49, the holding time of the herbaceous material in the pockets 55 is increased, thereby increasing overall agitation in the dryer receptacle 3. Further, if the vanes 49 are heated (see below description), the increased contact time between the inclined vanes 49 and the herbaceous material increases heating efficiency.

[0123] As the vanes 49 shown in FIG. 3B are curved in the direction of inclination of the vanes 49, large quantities of herbaceous material can be picked up by the vanes 49. The curvature of the vanes 49 may also increase contact time between the vanes 49 and the herbaceous material, as curved vanes 49 may let the herbaceous material slip later than straight vanes during rotation of the dryer receptacle 3.

[0124] Preferably, an arch distance between two adjacent vanes 49 with respect to the rotation axis 10 is equal to or greater than a height dimension of the vanes 49. This can ensure that adjacent vanes 49 do not too strongly interfere with picking up herbaceous material by the vane 49.

[0125] As shown in FIGS. 2 and 5, a collector 57 is provided in the inner space 5 of the dryer receptacle 3 at the outlet side 11. In the illustrated embodiment, the collector 57 is integrally formed with the outlet tube 31 of the outlet system 29. However, it is not required that the collector 57 and the outlet tube 31 are integrally formed. For example, the collector 57 could be fixed to the outlet tube 31 or another structure of the dryer 1. The collector 56 is stationary and does not rotate together with the dryer receptacle 3. FIG. 4 shows a schematic perspective view of the collector 57 and the outlet duct 31. The collector 57 is channel-shaped and is open in its upper portion. The collector 57 has at least two sections which are extending in different directions. The collector 57 can, for example, be essentially U-shaped or essentially V-shaped or essentially U/V-shaped. In particular, the collector 57 may have a center section 59 forming a bottom of the collector 57 and two side sections 61 upwardly extending from opposing ends of the center section 59. The distance between the side sections 61 can increase in a direction away from the center section 59 (upwards direction). When the dryer receptacle 3 is rotated about its rotation axis 10, herbaceous material inside the dryer receptacle 3 is agitated. During rotation of the dryer receptacle 3, herbaceous material may enter the collector 57. In particular, herbaceous material may be agitated and picked up by the vanes 49 and fall by gravity into the collector 57. The divergence of the side sections 61 of the collector 57 in the upwards direction leads to a funnel effect funneling herbaceous material falling due to gravity toward the center section 59 of the collector 57.

[0126] The collector 57 constitutes a simple and effective way of collecting herbaceous material in the inner space 5 of the dryer receptacle. In particular, the collector 57 can be constituted by or comprise a curved plate defining the center section 59 and the side sections 61.

[0127] According to the illustrated embodiment, at least a portion of an upper rim of the collector 57 defining the top opening of the collector 57 is slanted downwards. In particular, upper rims of the side sections 61 of the collector 57 may be slanted downwards. Herbaceous material falling onto the slanted rim from above may slide down the slanted rim instead of remaining at the rim or sticking to the rim.

[0128] FIG. 5 shows a view on the inner surface of the second door 15 from inside the dryer receptacle 3. As illustrated, the rotation axis 10 of the dryer receptacle 3 extends within the collector 57. In particular, the collector 57 extends in parallel to the rotation axis 10 of the dryer receptacle 3. The outlet conveyor 37 extends through the outlet duct 31 into the collector 57 to convey herbaceous material collected in the collector 57 towards the outlet 35.

[0129] In the illustrated embodiment, the collector 57 is asymmetrical with respect to a plane defined by the rotation axis 10 and a vertical direction. The asymmetric shape of the collector 57 may facilitate collecting herbaceous material during rotation of the dryer receptacle 3. In particular, one of the side sections 61 of the collector 57 may be longer than the other side section 61 of the collector 57. The shorter side section 61 may facilitate entry of the herbaceous material into the collector 57. The longer side section 61 may contribute to holding the herbaceous material in the collector 57. Preferably, the dryer receptacle 3 is rotated such that the shorter side section 61 is downstream of the longer side section 61 with respect to the rotation direction of the dryer receptacle 3.

[0130] The collector 57 could also be is asymmetrical with respect to the plane defined by the rotation axis 10 and a vertical direction in other ways. For example, the shapes or the dimensions or the shapes and the dimensions of the side sections 61 of the collector 57 could differ from each other. Also, the orientations of the side sections 61 of the collector 57 could differ from each other. Alternatively, the collector 57 could be symmetrical with respect to the plane defined by the rotation axis 10 and the vertical direction.

[0131] As illustrated in FIG. 2, a liquid dispersion assembly having two nozzles 61 is provided. The nozzles 61 are provided at the ends of the conveyor screw of the inlet conveyor 25 and the conveyor screw of the outlet conveyor 37, respectively. Inside the respective conveyor screw, a channel 63 is provided for supplying liquid to the nozzle 61. The nozzles 61 are configured to spray liquid inside the inner space 5 of the dryer receptacle 3. During drying of herbaceous material, liquids for treating the herbaceous material may be sprayed through the nozzles 61. Further, when the dryer receptacle 3 is to be cleaned between uses, cleaning liquid can be sprayed by the nozzles 61. As the nozzles 61 rotate, they can reach even spots that are usually difficult to reach for cleaning. The nozzles 61 may be set to rotate together with the respective conveyor screw. Alternatively, the nozzles 61 could be decoupled from the conveyor screws. For example, the nozzles 61 could be rotated by the liquid spraying out of the nozzles 61.

[0132] To facilitate drying of the herbaceous material, the dryer 1 comprises a heating system 65. The heating system 65 comprises multiple heating elements, which can comprise wall heating elements 67, vane heating elements 69, door heating elements 71, conveyor heating elements 73, and one or more collector heating elements 74. The wall heating elements 67 can be incorporated into the circumferential wall of the main body 7 of the dryer receptacle 3. The vane heating elements 69 can be incorporated into the vanes 49 protruding inside the inner space 5 of the dryer receptacle 3. The door heating elements 71 can be incorporated into the first and second doors 13, 15 of the dryer receptacle 3. The conveyor heating elements 73 can be incorporated into the inlet conveyor 25 and the outlet conveyor 37. The collector heating elements 74 may be incorporated into the collector 57. Wall heating elements 67, vane heating elements 69, and door heating elements 71 are illustrated in FIG. 2. The conveyor heating elements 73 and the collector heating elements 74 are not illustrated in FIG. 2 for the sake of clarity. Conveyor heating elements 73 could, for example, be incorporated into the conveyor screw of the inlet conveyor 25. Alternatively or additionally, conveyor heating elements 73 could, for example, be incorporated into the outlet conveyor 37. Conveyor heating elements 73 could also be incorporated into the inlet tube 19. Alternatively or additionally, conveyor heating elements 73 could also be incorporated into the outlet tube 31.

[0133] The first and second doors 13, 15 and the inlet and outlet conveyors 25, 37 are part of access assemblies providing access to the inner space 5 of the dryer receptacle 3. Heating such access assemblies by the door heating elements 71 and the conveyor heating elements 73 facilitates maintaining a certain temperature level in the dryer receptacle 3. If only wall heating elements 67 and vane heating elements 69 were present, parts of access assemblies, such as the doors 13, 15 or the conveyors 25, 37 might provide space for the formation of lower temperature spots. At lower temperature spots, gaseous material generated during drying of the herbaceous material in the dryer receptacle 3 could condensate, which could negatively influence drying efficiency and quality of the dried material.

[0134] Heating the vanes 49 with the vane heating elements 69 incorporated in the vanes 49 is highly effective, as the vanes 49 come in direct contact with large amounts of herbaceous material, when agitating the herbaceous material. Further, as the vanes 49 are inclined, the duration of contact between the herbaceous material and the vanes 49 is increased. That may increase heating efficiency.

[0135] Heating the collector 57 with the collector heating elements 74 may contribute to removing residual moisture before the herbaceous material leaves the inner space 5 of the dryer receptacle 3.

[0136] The block diagram shown in FIG. 6 shows a control scheme of the heating system 65. The heating elements 67, 69, 71, 73, 74 are grouped into heating sub-systems 75, which are controlled independently of each other by a controller 78.

[0137] According to the illustrated embodiment, five distinct main body heating sub-systems 75a are provided. Each of the main body heating sub-systems 75a comprises a plurality of wall heating elements 67 and a plurality of vane heating elements 69. An alternative would be to provide separately controlled wall heating sub-assemblies and vane heating sub-assemblies instead of providing main body heating sub-assemblies 75a comprising both wall heating elements 67 and vane heating elements 69. As shown in FIG. 2, there are five columns of wall heating elements 67 and five columns of vane heating elements 69 along the direction of extension of the rotation axis 10 of the dryer receptacle 3. Those correspond to the five main body heating sub-systems 75a. This means that, according to the illustrated embodiment, the wall heating elements 67 and the vane heating elements 69 are grouped into the main body heating sub-assemblies 75a by defining groups of heating elements 67, 69 that lie one behind the other along a direction parallel to the rotation direction A of the dryer receptacle 3. Independent control of the main body heating sub-assemblies 75a by the controller 78 allows establishing independently controlled heating zones along the extension direction of the rotation axis 10 of the dryer receptacle 3.

[0138] According to the illustrated embodiment, the heating system 65 further comprises two door heating sub-assemblies 75b. Each of the door heating sub-assemblies 75b comprises the door heating elements 71 incorporated into a corresponding one of the first and second doors 13, 15. Independent control of the two door heating sub-assemblies 75b by the controller 78 allows, for example, heating the first and second doors 13, 15 to different temperatures. Also, the first and second doors 13, 15 could be heated to the same target temperature, but with different feedback control parameters.

[0139] Further, according to the illustrated embodiment, the heating system 65 comprises two conveyor heating sub-systems 75c. The conveyor heating sub-systems 75c may comprise the conveyor heating elements 73 of a corresponding one of the inlet conveyor 25 and the outlet conveyor 37.

[0140] Further, according to the illustrated embodiment, the heating system 65 comprises a collector heating sub-system 75d. The collector heating sub-system 75d may comprise the one or more collector heating elements 74.

[0141] FIG. 6 schematically illustrates temperature sensors 77 distributed at appropriate places at the dryer 1 to measure temperatures corresponding to respective heating sub-systems 75. FIG. 6 illustrates ten temperature sensors 77, one for each of the heating sub-systems 75. The temperature sensors 77 are provided with wireless transmission devices 79 wirelessly transmitting the respective temperature sensor values to the controller 78. Alternatively, there could be a wired connection between the temperature sensors 77 and the controller 78. The controller 78 controls each of the heating sub-systems 75 based on an output of the corresponding temperature sensor 77. In the illustrated embodiment, the heating sub-systems 75 are all controlled independently of each other based on the sensing value from the respective temperature sensor 77. However, it would also be conceivable to group some or all of the heating sub-systems 75 to be controlled together or to at least be controlled based on an output of the same temperature sensor 77.

[0142] Having independently controlled heating sub-assemblies 75 provides a high level of control over the temperature distribution in the dryer receptacle 3 during drying of the herbaceous material. Therefore, the drying process can be accurately controlled and adjusted to obtain high-quality products. Depending on the herbaceous material to be treated and on the desired properties of the products to be obtained, different principles of operating the heating sub-systems 75 are conceivable. For example, the main body heating sub-assemblies 75a could be controlled to provide a temperature gradient in the inner space 5 of the dryer receptacle 3 along an extension direction of the rotation axis 10 of the dryer receptacle 3. This could, for example, be achieved by using different temperature target values for control of the different main body heating sub-assemblies 75a. For example, the temperature gradient could be such that a temperature is higher at the inlet side 9 of the dryer receptacle 3 and a temperature is lower at the outlet side 11 of the dryer receptacle 3. Alternatively, the temperature gradient could be established such that the temperature is lower at the inlet side 9 of the dryer receptacle 3 and higher at the outlet side 11 of the dryer receptacle 3. A respective temperature difference between the inlet and outlet sides 9 could be, at least 10 degree Celsius, at least 20 degree Celsius, at least 30 degree Celsius, at least 50 degree Celsius, at least 100 degree Celsius, or more than 100 degree Celsius, for example.

[0143] It would also be conceivable to use the same temperature target values for all main body heating sub-assemblies 75a, but to use the sensing values from the different temperature sensors 77 for the respective main body heating sub-assemblies 75a for independent control adapted to the characteristics of the main body heating sub-assemblies 75a, such as heat capacity, thereby achieving a highly even temperature throughout the longitudinal direction of the dryer receptacle 3.

[0144] The door heating sub-assemblies 75b and the conveyor heating sub-assemblies 75c are preferably controlled to maintain at least a predetermined minimum temperature, which is also called minimum access assembly temperature, at the doors 13, 15 or the inlet and outlet conveyors 25, 37, respectively. The minimum access assembly temperature could be selected such that the formation of lower temperature spots at the access assembly, in particular at the doors 13, 15 or the conveyors 25, 37, is prevented. Preventing lower temperature spots may prevent condensation of gaseous material generated during drying of the herbaceous material at such spots.

[0145] The door heating sub-assemblies 75b is preferably controlled based on different temperature target values for the first door 13 and the second door 15. This could, in particular, be done in combination with a temperature gradient established by appropriately controlling the main body heating sub-assemblies 75a.

[0146] Temperature target values for the respective heating sub-systems 75 could be entered by a user via an input device 81. Alternatively or additionally, temperature target values for the respective heating sub-systems 75 could be stored in a memory device 83.

[0147] FIG. 6 also illustrates an optional pressure sensor 82. The pressure sensor 82 may be configured to determine a pressure in the inner space 5 of the dryer receptacle 3. If superheated steam is used for heating the inner space 5 of the dryer receptacle 3, a temperature within the inner space 5 of the dryer receptacle may be deduced from the determined pressure. The pressure determined by the pressure sensor 82 may be wirelessly transmitted to the controller 78 and be used for controlling one or more of the heating sub-assemblies 75.

[0148] In FIG. 6, each of the heating sub-systems 75 comprises an actuator 85 that is controlled by the controller 78 to appropriately actuate the respective heating elements 67, 69, 71, 73, 74. The actuators may, for example, comprise electrical circuitry for supplying resistance heating elements with electrical power or pumps or valves for providing heating elements that are configured as heating fluid lines with appropriately heated fluid.

[0149] Further, the controller 78 may be configured for controlling a driving device 90 for rotating the dryer receptacle 3 about the rotation axis 10. Preferably, the controller 78 is configured to control the driving device 90 to rotate the dryer receptacle 3 exclusively in one direction of rotation. However, the controller 78 may also be configured to control the driving device 90 to change a rotation direction of the dryer receptacle 3. In particular, the rotation direction of the dryer receptacle 3 could be changed at intervals to improve distribution of the herbaceous material within the dryer receptacle 3. The controller 78 may also control the hydraulic cylinder 47 of the tilting device 41.