The present invention relates to a particle drying system. Further, the present invention relates to a particle drying method, wherein a mass flow of particles is transported by gravitational forces through a tower, and a gaseous drying medium flows in the opposite direction of the particle transport direction.

Particle drying systems are known for example from GB-A-875,685 which discloses a vertical cooling tower for drying tobacco. The tobacco enters the tower from the top and is subjected to the cooling effect of a current of cold air drawn up through the tower. Within the tower inclined baffles or vanes are arranged. The tobacco falling under gravity in the tower is distributed by the vanes. The vanes also slow down the fall of the tobacco within the tower. However, depending on the size, moisture content and the amount of tobacco particles, the drying effect on the individual particles may vary, resulting in undesirable quality differences of the processed particles.

It is thus the object of the present invention to provide a particle drying system and method which is adapted to dry particles with consistent quality

The present invention provides a particle drying system comprising a tower through which the particles are moving substantially in a first direction and through which a flow of a gaseous drying medium is moving substantially in a second direction, wherein the first direction is substantially opposite of the second direction. According to the invention the particle drying system further comprises adjustment means adapted to change the flow characteristics within the tower. The adjustment means comprise at least one movable side wall of the tower. The at least one moveable side wall is moveable at least in a direction perpendicular to the longitudinal extension of the tower. This is an efficient and reliable manner to implement the variation of the cross-sectional area of the tower. The side walls adjacent to the moveable sides are preferably connected in a substantially gas-tight manner to the moveable side wall, for example with rubber seals. However, it is also possible to only provide a tight connection in between the at least one moveable side wall and the adjacent side walls. Alternatively, two opposing side walls may be moveable, or two adjacent side walls. By adapting the flow characteristics within the tower to different amounts and types of particles it can be achieved that the particles remain for a predetermined residence time in the tower while being subjected to the gaseous drying medium. This allows for a much more flexible drying system as compared to a drying system with fixed tower flow resistance. In particular, according to the invention it is possible to improve the adjustment of the residence time of the particles within the tower. This allows for a wide range of possibilities to influence the amount of moisture that is removed during the drying treatment of the particles.

Preferably, the particles move downward through the tower, accelerated by gravity. The downward movement is opposed by the upward current of the gaseous treatment medium. With a fixed flow resistance of the tower, the mass flow rate of the particles is substantially fixed. This means that the amount of moisture that may be removed from a given mass flow is predetermined, Additional parameters that influence the drying process are for example the temperature of the particles, the temperature of the gaseous drying medium and the difference in moisture of the particles and the drying medium.

The particle drying apparatus is particularly suitable for the drying of plant material such as for example tobacco particles such as for example tobacco cut filler, tobacco lamina, tobacco stems or tobacco shreds. Other plant material may be for example tea leafs, mint leaves, herbs or cloves.

The term "mass flow rate" of particles, for example tobacco particles, is used throughout the specification to indicate the correlation between the passage of a given mass of particles, in a first direction through the tower in a given time period, measured in kilograms per second.

Preferably, a substantially constant moisture level of the particles leaving the particle drying system may be obtained for a range of differing mass flows. Thus, in comparison to the state of the art systems, the system according to the invention allows for the selection of a wide range of adjustable operating conditions of the particle drying system according to the invention.

The control of the flow resistance independently of the mass flow rate of the particles has the additional advantage that the particles passing through the tower may be subjected to substantially constant frictional and fluidic forces during their residence time. This may prevent fluctuations in the moisture of the particles or other parameters of the particles within the particle drying system over time.

Preferably, the residence time of the particles inside the particle drying system is controlled. Preferably, the adjustment means are controlled to adjust the flow resistance of the tower such that a predetermined moisture content of the particles that leave the particle drying system is attainable for a range of different particle mass flows. In addition, potential degradation of the particles that may be caused by a high mass flow rate can be reduced or prevented. Degradation may lead for example to the reduction of the size of the particles and the creation of particle dust.

The longer the residence time of the particles is in the tower, the longer the particles are subjected to the gaseous drying medium. Thus, the drying time of the particles can be adjusted to one or more of the parameters of the mass flow rate of particles through the particle drying system, like for example particle size, moisture content of the particles before they enter into the particle drying system and the moisture content that is desired for the particles when the particles leave the particle drying system.

Preferably, the adjustment means are adapted to allow for the adjusting of the flow resistance by changing the cross section of the tower to a narrower or wider shape. Alternatively or in addition, the adjustment means adjust the flow resistance of the tower by adjusting the size or number of obstacles in the path of the particles or both. The adjustment means may be for example the combination of moving elements and actuators that move the moving elements. Moving elements may be for example walls of the tower or structures that may be introduced into or retracted from the tower.

Preferably, the volume flow rate of the gaseous drying medium is controllable. As the gaseous drying medium flows into the opposite direction of the mass flow of the particles, the volume flow rate of the gaseous drying medium may also be used to control the mass flow rate and residence time of the particles in the particle drying system. By adjusting the flow resistance of the tower and the volume flow of the gaseous drying medium, a variety of drying conditions in the tower can be selected.

Preferably, the adjustment means are adapted to alter the cross-sectional area of the tower.

Alternatively or in addition, the adjustment means may comprise adjustable vanes which extend into the tower. The vanes may serve two different purposes. They may constitute obstacles regarding the transportation of the particles through the tower, to control the free traveling distance of the particles and the drying time of the particles in the tower. Also, the vanes may control the flow of the gaseous drying medium, for example to produce swirl or turbulence. This may have a beneficial effect regarding the exposure of the individual particles to the gaseous drying medium, as the swirl and turbulent flow may separate the individual particles from each other.

In one embodiment, the vanes may be at least partially permeable to the gaseous drying medium. For example, the vanes may be at least partially made of meshed material. Alternatively, the vanes may be provided with nozzles or funnels, which direct the flow of gaseous drying medium in a desired direction or to locally increase the flow rate. The permeability of the vanes has the benefit that even the particles which are passing adjacent to the vanes are subjected to the flow of gaseous drying medium. Thus the gaseous drying medium may also be provided to parts of the tower which would be subjected only to little or no flow of gaseous drying medium if the vanes would not be permeable.

Preferably, the adjustable vanes are connected to the side walls of the tower by means of hinges. This allows an easy adjustment of the vanes. Preferably, the rotation axis of the hinges is in a substantially horizontal direction such that the inclination of the vanes with respect to a horizontal plane may be controlled. The hinges may also comprise actuators which are connected to individually or collectively control the vanes. In particular, the vanes may be provided with a pinion gear which engages in a tooth rack. The toothed rack is used to tilt the vanes around the hinge axes.

Preferably, two opposite side walls of the tower are provided with at least one but preferably a plurality of vanes respectively, like for example between 2 to about 50 vanes, preferably between 3 and 8 vanes. The vanes may extend substantially perpendicular to the extension direction of the tower along the width of the respective side walls. The vanes may be inclined towards the outside of the tower, such that the portion of the vanes which extends into the inside of the tower is lowest and the portion of the vanes which is connected to the wall of the tower is highest.

In one embodiment the vanes have a curved geometry. The curved geometry allows that the curvature of the vanes may be designed to be steeper in the areas towards the walls of the tower and less steep in the areas which are directed towards the centre of the tower. This has the benefit that the particles are gently directed to the center of the tower where the flow of gaseous drying medium rate is higher.

The vanes may be arranged in different heights at the opposite side walls. It is preferred that the vanes at one side wall are arranged at a height which is substantially in between the height of the two nearest vanes at the opposite side wall. In one embodiment the vanes are adapted to be titled in between a lowered position where they are closest to the side walls and a raised position where they are in their maximal extension into the tower. This feature increases the travelling time of the particles through the towers when the vanes are in a raised position and increases the flow of particles when the vanes are in their lowered position. Preferably the vanes overlap in their partly or fully raised positions with the vanes of opposite side walls, such that a zig-zag flow path is formed.

In one embodiment the vanes are adapted to vibrate to prevent that particles settle on any of the vanes. This has the advantage that the particles are constantly travelling through the tower such that all of the particles have a substantially similar average residence time in the tower. The vibration of the vanes may be actuated by the flow of gaseous drying medium or by actuators.

In one embodiment the cross-sectional area of the tower is varied along the longitudinal extension of the tower. Preferably, the tower has a conical form along its extension direction.

By implementing a tower which increases its cross-sectional area towards the top of the tower, the flow of gaseous drying medium velocity can be increased in the lower part of the tower, while at the top of the tower, the particle density is lowered. Alternatively, the cross- sectional area of the tower can also be decreased towards the top if a contrary effect is desired. In particular, the adjustment means may include a tiltable side wall to alter the local cross-sectional areas of the tower along the longitudinal extension of the tower.

In one embodiment at least one side wall of the tower is moveable in the longitudinal extension direction of the tower with respect to another side wall of the tower. Thus, by moving the side walls of the tower, at which for example the vanes are connected, the inner flow resistance of the tower may be varied and the flow characteristics for the gaseous drying medium and the particles may be changed.

In one embodiment at least one controllable additional nozzle is provided to locally introduce additional flow of gaseous drying medium within the tower. This nozzle may comprise ducts or fans which locally provide flow of gaseous drying medium within the towers. The nozzle may be disposed in the side walls of the tower and may provide flow which is directed towards the inside of the tower. The flow may also be directed in a substantially horizontal direction, or in a slanted direction. Preferably, the nozzle can be controlled to be actuated on demand. The additional air flow introduced into the tower may change the flow resistance of the tower by introducing countercurrents and turbulences.

In one embodiment the tower has a substantially circular cross-sectional area and the diameter of the tower can be adapted to change the flow characteristics of the tower. The variation of the circular cross-sectional area of the tower may be attained by manufacturing the tower of a flexible material which has a wavelike form in a circumferential direction thereof.

Another aspect of the invention is directed to a particle drying method of changing the flow characteristics in the tower by adjustment means. The flow characteristics in the tower are altered by movement of at least one side wall of the tower. This can be a transiational movement of the side wall in the longitudinal extension direction of the tower, or a transiational movement of the sidewall perpendicular to the extension direction of the tower. Further, the movement of at least one side wall of the tower may also comprise a rotational movement of the side wall, such as a tilting of the side wall around a horizontal axis. Of course, the different movements of the side wall of the tower may be combined.

Preferably, the flow characteristics in the tower are altered by movement of adjustable vanes provided in the tower. The adjustment of the vanes may comprise rotation around a horizontal axis which is extended substantially along a side wall of the tower. Also, it may comprise horizontal rotation of the vanes along a horizontal axis which is perpendicular to a side wall of the tower.

Preferably, the flow characteristics of the tower, the particle amount and the flow of gaseous drying medium are controlled such that the particles are subjected to the gaseous drying medium in the tower for a predetermined average drying time. Preferably, the adjustment means and the flow of the gaseous drying medium can be controlled by a control module which allows the variation of both the flow of the gaseous drying medium and the state of the adjustment means in accordance with the conditions of the particle drying measured by sensors within or outside the tower. Preferably, the control module comprises a feedback system to enable an online change of tower flow resistance responsive to the signals produced by the sensors.

The invention will be further described, by way of example only, with reference to the accompanying drawing in which:

Fig. 1 shows a cross-sectional schematic view of an embodiment of the particle drying system.

In Fig. 1 the particle drying system 1 is shown in a cross-sectional view, wherein the tower 2 extends in a vertical longitudinal extension direction 30.

The tower 2 comprises a left side wall 3 and a right side wall 4 which extend in a vertical direction parallel to each other. At each of the side walls 3, 4, a variety of vanes 5, 6 is provided. The vanes 5, 6 are disposed alternatively on the left side wall 3 and the right side wall 4 of the tower 2 in the longitudinal extension direction 30.

Additional to the left side wall 3 and the right side wall 4, the tower also comprises a front side wall and a rear side wall.

The right side wall 4 is moveable in a horizontal direction 40 to increase or decrease the cross-sectional area of the tower 2. Therefore, the right side wall 4 is in sliding engagement with the front side wall and the rear side wall. At the contact of the right side wall 4 with the front side wall, respectively the rear side wall, a sealing element, such as a rubber seal may be provided. The tower 2 is closed at its side walls 3, 4, and is open on the top and bottom.

Different possible positions of the right side wall 4 are indicated as dashed lines.

The adjustable vanes 5 are all connected hingedly to the left side wall 3, the adjustable vanes 6 are all connected to the right side wall 4 of the tower 2, and may be rotated around the horizontal angle 10 as indicated for the uppermost left vane 5. The adjustable vanes 5, 6 may be rotated together or individually. Different positions of the uppermost left vane 5 are indicated in dashed lines.

In one embodiment the adjustable vanes 5, 6 are provided with a pinion gear around their hinge connection to the side wall and a tooth rack is disposed along the side wall to adjust the angle of the vanes 5, 6 by being moved vertically.

Thus, by movement of the right side wall 4 and the vanes 5, 6 the flow resistance of the tower 2 can be altered and the flow characteristics of the tower 2 are changed.

The right side wall 4 and the adjustable vanes 5, 6 constitute the adjustment means in this embodiment. In the following a particle drying method according to the invention is described with respect to Figure 1.

After the cutting and optionally further processing steps to which tobacco is subjected, the tobacco particles are conveyed to the upper opening 7 of the tower 2 and released into same as indicated by the arrow 20.

The tower 2 is subjected to an upwards flow of gaseous drying medium as indicated by the arrow 50. The gaseous drying medium may comprise special ingredients for the drying of the tobacco as known in the art or may just consist of air of a certain moisture and temperature. The gaseous drying medium is preferably provided through a lower opening 8 of the tower 2 or through nozzles in the side walls 3, 4 near the lower opening 8 of the tower 2 or both.

The tobacco particles are transported by gravitational forces through the tower 2, wherein the flow of gaseous drying medium 50 and the flow resistance of the tower 2 determine how long the tobacco particles remain in the tower 2. Thus, the drying intensity of the particles in the tower 2 can be adjusted.

Thus, if the particle transport through the tower should be increased, either the cross- sectional area of the tower is increased or the vanes 5, 6 are lowered, such that a faster transport of the tobacco particles through the tower occurs. The transportation velocity of the particles through the tower can also be increased by lowering the velocity of the flow of gaseous drying medium 50.

To ensure that the particles remain a sufficient time in the tower while being subjected to treatment, the cross-sectional area of the tower 2 can be lowered by moving the side wall 4 towards the inside of the tower 2, or the adjustable vanes 5, 6 can be raised. Alternatively or additionally the flow 50 of gaseous drying medium in the counter direction of the transport direction of the particles can be increased, such that fluidic forces lower the transportation velocity of the tobacco particles through the tower 2. Thus, heavier particles, such as moist particles, which are fed into the tower 2, can be treated without traveling too fast.

By adjusting the flow of gaseous drying medium 50, the position of the side wall 4 and the adjustable vanes 5, 6, the drying time and intensity of the particles can be determined.

The drying process of the tobacco particles 20 can be constantly monitored, for example by measuring the particles to gas relation in the tower 2. Also the mass flow rate and the moisture content before and after the particle drying system can be measured. The measurements may be used for a feedback loop to control the position of the side walls, the position of the vanes or the temperature and volume flow rate of the gaseous drying medium.

The particles leave the tower at the lower opening 8 and are received by a conveyance means 9, such as a conveyor belt which transports the treated tobacco particles for further processing. The adjustability of the flow resistance of the tower 2 allows treating the particles in a gentle manner whilst allowing treating different types and amounts of particles and for example particles comprising different moisture levels.