Continuous particle drying apparatus comprising a control loop

Abstract

The present invention relates to a dryer for drying particles comprising, • first and second circular plates mounted horizontally for rotation about an axis (Z), the surface of the plates being perforated and permeable to fluids, • a unit for blowing hot gas as a flow substantially parallel to the axis (Z) passing through the second plate before passing through the first plate, • first and second units for distributing the particles and first and second units for recovering the particles after a rotation of each plate, • a unit for transferring the harvested particles from the first plate (1a) to the second plate, • a processor controlling the rotational speeds of the plates (ωa, ωb) and the supply flow rate (dma / dt) of particles on the basis of an output control loop comprising a measurement of the final moisture content (H1b) of the particles measured in the second recovery unit.

Description

FIELD OF INVENTION

[0001] The invention relates to an industrial dryer for continuously drying particles, preferably organic particles, for example of agro-food origin, such as cereals, or waste used as fuel or construction materials such as wood chips or fibers, or other plant material. The dryer of the present invention makes it possible to dry to final moisture content values ​​(H1b) within a target range (H1t± e ) predefined in a constant and reproducible manner, particles whose initial moisture contents (HOa) can vary. This result is obtained by a first output control loop measuring the final output moisture values ​​(H1b) of the particles and modifying a series of dryer operating parameters including the rotation speeds ( oh has, ohb) the first and second trays as well as the feed rate (dma / dt) of the particles on the first tray (1a) so as to determine the values ​​of these parameters allowing the particles to be dried at rates within the target range (H1t± e ). The output control loop can be complemented by an input control loop measuring the initial humidity values ​​(HOa) of the particles and modifying only the feed rate (dma / dt) according to variations in these initial humidity values ​​(H0a), without modifying the rotation speeds ( oh has, oh b) first and second trays, so as to maintain the values ​​of the final moisture content (H1b) of the particles within the reference target range (H1t± e ), regardless of variations in the initial moisture contents (HOa) of the particles to be dried. TECHNOLOGICAL BACKGROUND

[0002] Many industrial processes require the drying of particles before their further use, whether before packaging granular food or industrial products, or before burning shredded waste used as fuel. Depending on the intended use, particles must be dried to achieve final moisture contents within target ranges (H1t± e) well-defined. For example, wood chips will need to be dried within different target ranges depending on whether they are intended for combustion, pellet production, bedding production or chipboard production. It is of course possible to dry the particles in batches by placing the particles on trays, preferably perforated to allow a hot gas to pass through and allow water and water vapor to escape. In some cases, a fluidized bed is formed by the particles suspended under the action of the hot gas flow. However, most industrial applications require flow rates that a batch drying process cannot achieve.For this reason, the same principle of placing the particles to be dried on a perforated support and exposing them to a flow of hot gas has been applied to devices allowing continuous drying, with a continuous source of the particles to be dried upstream of the dryer itself and a continuous discharge of the dried particles downstream of it.

[0003] In particular, a belt dryer comprises a continuous flexible perforated belt stretched between two motorized rollers forming a loop. Air or other hot gas is blown under the upper belt on which the particles to be dried are continuously deposited. The length of a belt dryer depends on the type of particles to be dried, their water load and the target range (H1t± e) to achieve. Thus, a belt can reach a length of 200 m which is very expensive and difficult to assemble / disassemble on the device. A belt dryer is therefore generally reserved for drying a single type of particles, because it would be uneconomical to change the belt to optimize the perforation type for a new type of particles. A belt dryer is very expensive and inefficient in terms of dimensions, since the particles are only dried over less than half the length of the belt.

[0004] There are also perforated tray dryers that resemble belt dryers, except that the belt is replaced by perforated trays coupled to each other to form a sort of caterpillar. The difference with a belt dryer is that the trays are articulated so that they present the same face whether they are on the upper or lower belt of the loop. This makes it possible to practically reduce the length of the dryer by half, since the particles are subjected twice to a flow of hot gas: once when they pass over the upper part of the loop and a second time when they pass in the opposite direction over the lower part. Although advantageous in this respect compared to a belt dryer, it is clear that the mechanics necessary for the movements of the trays are delicate and therefore expensive and fragile, especially when exposed to fine particles that seize the bearings.In addition, the openings created between two adjacent trays and, above all, the spaces opening in the tray transfer mechanism during each transfer of a tray from the upper portion to the lower portion of the track create as many preferential passages of least resistance for the flow of hot gas, which cause a significant drop in the efficiency of this type of dryer.

[0005] WO2013 / 139720 A1 describes a dryer according to the prior art. EP2503272 describes a dryer comprising a tube provided with trays in the form of circular segments distributed along the length of the tube with openings allowing passage from one tray to the next tray along the tube. The tube is enclosed in a cylindrical enclosure mounted to rotate relative to the tube. The inner surface of the tube comprises transfer elements which sweep the upper surface of the trays (3) when the cylindrical enclosure rotates relative to the tube. The rotation speed of the cylindrical enclosure is adjusted by observing the drying progress of the material being dried by measuring its moisture content and its temperature.

[0006] EP0197171 describes a dryer comprising several perforated, circular, superimposed trays rotatably mounted on a hollow central shaft. Each tray is enclosed in an individual cylindrical chamber provided with a roof and a floor which separate it from the other trays. Means for transferring the powder to be dried are provided between each adjacent tray. Each chamber is provided, on the one hand, with a first hot air introduction opening, in fluid communication with the cavity of the hollow central shaft, the first opening being positioned above the tray located in the corresponding chamber and, on the other hand, with a second discharge opening on the peripheral wall of the chamber in communication with the outside, the second opening being located below the corresponding tray. Hot air is blown into the cavity of the hollow shaft and is distributed in parallel in each chamber through the first hot air introduction opening.The hot air is forced to pass through the circular perforated tray before being discharged through the second opening located on the peripheral wall of each chamber. In reality, such a system is similar in principle to a belt dryer whose linear movement has been replaced by a circular movement distributed over several floors with means for transferring the powder from one tray to another. Certainly, such a rotary system has a considerable advantage in saving floor space compared to a linear belt dryer, but such a system lacks efficiency. Indeed, if the hot air having passed through the first trays loaded with very humid particles comes out relatively saturated with humidity, the hot air passing through the last trays loaded with particles already partially dried on the previous trays, comes out only slightly loaded with humidity, which represents a considerable waste of energy.

[0007] EP2828595 describes a dryer illustrated in Figure 1 , comprising first and second (or more) perforated trays (1a, 1b), superimposed and mounted to rotate about a vertical axis (Z). A ventilation system blows a hot gas vertically, first passing through the second tray (1b), before passing directly through the first tray (1a). As it is a dryer, the hot gas, after passing through the second tray (1b) then the first tray (1a) is either evacuated or recirculated, but on condition that it is dried and reheated before reinjecting it through the second tray.

[0008] The wet particles are distributed along a radius of the first tray (1a) by a first distribution unit (2a) and carried by the rotation of the first tray over an angular (or azimuthal) distance of slightly less than 360° before being collected by a first recovery unit (3a). During the rotation of the first tray (1a), the particles are exposed to the hot gas stream which has previously passed through the second tray where it has lost some of its heat energy and has taken on some moisture. The partially dried particles are transferred from the first recovery unit to a second distribution (2b) system (3a) which distributes the partially dried particles along a radius of the second tray (1b) which rotates around the vertical axis (Z) in the opposite direction to the first tray (1a).The partially dried particles are carried away by the rotation of the second tray (1b) (in the opposite direction to the first tray) over an angular (or azimuthal) distance of just under 360° before being collected by a second recovery unit (3b) and discharged. During the rotation of the second tray (1b), the particles are exposed to the hot gas stream directly from the ventilation system, where the hot gas has its maximum temperature and minimum moisture content.

[0009] As seen on the Figure 1, as the trays rotate in opposite directions, the hot gas that reaches the particles just after being deposited along the radius of the first tray, where they have their maximum moisture content (H0a / H0a = 100%) has the highest temperature and the lowest moisture content of all the hot gas that reaches the first tray, because it has previously passed through the practically dry particles (final moisture content H1b) just before being discharged with a moisture content that can be of the order (as an example) of H1b / H0a) = 12%, where H0a is the initial moisture content of the particles at the inlet of the first tray (1a) and H1b is the final content of the particles at the outlet of the second tray (1b).

[0010] The dryer described in EP2828595 is particularly efficient in terms of energy, use and occupation of floor space. In a stationary mode, with particles having a relatively constant initial moisture content (HOa), the dryer ensures that the final moisture content (H1b) will be constantly within the target range (H1t±ε). However, determining the dryer operating parameters necessary to achieve the stationary mode of operation can be time-consuming. Moreover, in some cases, the initial moisture content (HOa) can vary considerably within the same batch of particles. In particular, in the case where the particles are stored in the open air, and even exposed to the weather and picked up using a mechanical shovel which can once cap the top of the pile of particles and another time scrape the ground at the location where a puddle of water has formed.If the initial moisture contents (HOa) of the particles vary beyond a tolerance (±. d ) around a mean reference value (HOr) (i.e., H0a ≠ H0r± d ), then it becomes very difficult, if not impossible, to guarantee that the particles after drying have a final moisture content (H1b) within the target range (H1t± e ), (ie, if H0a ≠ H0r±δ) => H1b ≠ H1t± e ).

[0011] There therefore remains a need for an industrial dryer for continuous particle drying that is efficient, easy to maintain and that ensures particle drying to a final moisture content (H1b) within the target range (H1t± e ) regardless of the type of particles to be dried and variations in the initial moisture content (HOa) of the particles. The present invention provides such an industrial dryer. SUMMARY OF THE INVENTION

[0012] The present invention is defined in the independent claims. Preferred variants are defined in the dependent claims. In particular, the present invention relates to a dryer for drying particles having an initial moisture content (HOa) to a final moisture content (H1b) within a predefined target range centered around a target moisture value (H1t) (i.e., H1b = H1t + e ), the dryer comprising, (a) an enclosure comprising a substantially cylindrical wall extending along a vertical axis (Z) (b) a first circular plate mounted on the wall of said enclosure substantially normal to the vertical axis (Z), and rotating in a first direction about the vertical axis (Z) whose rotation is actuated by a first motor, the surface of the first plate being perforated and permeable to fluids such as air, water vapor and water, and (c) a second circular plate mounted at a certain distance from the first plate on the wall of said enclosure substantially normal to the vertical axis (Z) and rotating about said vertical axis (Z) whose rotation is actuated by a second motor (5b), the surface of the second plate being perforated and permeable to fluids such as air, water vapor and water.

[0013] The first and second trays are configured to, on the one hand, support the particles to be dried and, on the other hand, allow a hot gas blowing substantially parallel to the vertical axis (Z) to pass therethrough. In a preferred embodiment, the first tray is located below the second tray and the hot gas flows from top to bottom and is preferably hot air. In all cases, it is preferred that the second tray rotates in a direction opposite to the rotation of the first tray. The dryer further comprises, (d) a first unit for distributing the particles to be dried, configured to receive the particles to be dried from a feed unit and to distribute these particles before drying along a radius of the first tray, (e) a first recovery unit configured to recover the particles deposited on the first tray after a rotation of a given angle thereof, the first recovery unit being located downstream of, preferably adjacent to, the first distribution unit, (f) a unit for transferring the particles collected from the first tray by the first recovery unit to a second distribution unit, configured to distribute said particles along a radius of the second tray, (g) a second unit for recovering the particles deposited on the second tray after a rotation of a given angle thereof, the second recovery unit being located downstream of,preferably adjacent to the second distribution unit (2b) and being configured to discharge the particles after drying from the dryer by an evacuation system, (h) a hot gas blowing system following a flow substantially parallel to the vertical axis (Z), passing first through the perforated surface of the second tray before passing directly after through the perforated surface of the first tray. The hot gas is used to evacuate the moisture from the particles and thus dry them.

[0014] The present invention is distinguished in that the dryer comprises, (i) an outlet probe located at the second recovery unit or in the exhaust system and configured to measure the final moisture content (H1b) of the particles in or exiting the second recovery unit, (j) a processor coupled to the outlet probe, the first and second motors and the power unit and configured to optimize drying parameters as follows, extracting the final moisture content values ​​(H1b) of the particles measured by the outlet probe over time, and comparing them to the target moisture content (H1t), if the final moisture content values ​​(H1b) are not within the predefined target range (H1t ± e ), modify rotation speed values ​​( oh has, ohb) first and second trays and, optionally, a feed rate (dma / dt) of the particles by the feed unit, in order to obtain final moisture content values ​​(H1b) within the predefined target range (H1t ± e ).

[0015] In case the final moisture content values ​​(H1b) are higher than the predefined target range (i.e., H1b > H1t + e ), the processor can then be further configured to determine theoretical values ​​of one or more of the rotation speeds ( oh has, oh b) and the feed rate (dma / dt) allowing to obtain final moisture content values ​​(H1b) within the predefined target range (H1t ± e ). And for decrease the first rotation speed (|ωa|) of the first plate r to the corresponding theoretical value, decrease the second rotation speed (| ohb|) of the second plate to the corresponding theoretical value, for example, proportionally to the first rotation speed ( oh a) of the first tray, and optionally, reduce the feed rate (dma / dt) of particles distributed on the first tray to the corresponding theoretical value.

[0016] In case the final moisture content values ​​(H1b) are lower than the predefined target range (i.e., H1b < H1t + e ), the processor can then be further configured to determine theoretical values ​​of one or more of the rotation speeds ( oh has , oh b) and the feed rate (dma / dt) of the particles on the first tray allowing to obtain final moisture content values ​​(H1b) included in the predefined target range (H1t ± e ). And for increase the first rotation speed (| oha|) of the first plate (1a) to the corresponding theoretical value, increase the second rotation speed (| oh b|) of the second plate (1b) to the corresponding theoretical value, for example, proportionally to the first rotation speed ( oh a) of the first tray (1a), and optionally, increase the feed rate (dma / dt) of particles distributed on the first tray (1a) to the corresponding theoretical value.

[0017] In a preferred embodiment of the invention, the dryer also comprises an inlet probe placed at the feed unit or the first distribution unit and configured to measure an initial moisture content (HOa) of the particles entering the first distribution unit. The processor is configured to optimize the drying parameters only during a period during which the initial moisture content (HOa) has an initial value that is substantially constant, not varying by more than a predefined reference range (± d ) around a reference value (HOr) (i.e., H0a = H0r ± d ).

[0018] In a preferred example of this variant, the feed unit is preferably coupled to a source of particles to be dried. The processor is preferably configured to mitigate or eliminate variations over time in the final moisture content values ​​(H1b) due to variations over time in the initial moisture content values ​​(HOa) of the particles to be dried, as follows, extract the initial values ​​of the initial moisture contents (HOa) measured by the inlet probe over time, and compare whether the initial values ​​remain within the predefined reference range (± d ) around the reference value (HOr) (i.e., H0a = H0r ± d ), if the initial values ​​are not included in the predefined reference range (i.e., H0a ≠ H0r ± d ), modify the feed rate (dma / dt) of the particles by the feed unit, without modifying the rotation speeds ( oh has, ohb) first and second trays, in order to obtain final moisture content values ​​(H1b) within the target range (H1t ± e ) even in the case where the initial values ​​(HOa) are not included in the predefined reference range (i.e., H0a ≠ H0r ± d ), if the initial values ​​are within the predefined reference range (i.e., H0a = H0r ± d ), do not change the drying settings,

[0019] In the preferred example, if the initial values ​​of the initial moisture contents (HOa) are below the reference range (i.e., H0a < HOr - d ), then the processor is configured to determining a theoretical value of the feed rate (dma / dt) allowing to increase a thickness of a layer of particles distributed by the first distribution unit on the first tray, in order to ensure that an intermediate humidity content (H1a) of the particles adjacent to the first recovery unit is included in an intermediate range (i.e., H1a = H1i ± c ) and increase the power supply rate (dma / dt) of the power supply unit to the theoretical value.

[0020] Conversely, if the initial values ​​of the initial moisture contents (HOa) are higher than the reference range (i.e., H0a > HOr + d ), then the processor is configured to, determining a theoretical value of the feed rate (dma / dt) allowing to reduce a thickness of a layer of particles distributed by the first distribution unit on the first tray, in order to ensure that an intermediate humidity content (H1a) of the particles adjacent to the first recovery unit (3a) is included in an intermediate range (i.e., H1a = H1i ± c ) and decrease the power supply rate (dma / dt) of the power supply unit to the theoretical value.

[0021] The first and second trays preferably comprise a self-supporting rigid structure with high permeability of the grating type, on which is placed a filter layer comprising openings of size and density corresponding to the desired permeability according to the type and size of the particles to be dried.

[0022] The first and second units for distributing the particles to be dried on the first and second trays respectively, as well as the feed unit may each comprise at least one Archimedean screw extending along a radius of the first and second trays, respectively, said at least one Archimedean screw being enclosed in an enclosure provided with one or more openings extending along said radius of the trays.

[0023] Similarly, the first and second recovery units may each comprise at least one Archimedean screw extending along a radius of said plate which is enclosed in an enclosure provided with one or more openings extending along the radius of the first plate, said openings being connected to a scraper or brush capable of collecting and directing the particles brought by the rotation of the plate towards the Archimedean screw.

[0024] After the gas has passed through the perforated surface of the first tray, the hot gas blowing system (5) can either be configured to exhaust the gas from the enclosure, or include an air dryer to capture moisture present in the gas before reheating and recirculating the dried gas through the second and first trays, respectively.

[0025] To do this, the dryer may comprise a substantially hollow cylindrical chimney (6) centered around the vertical axis (Z) and the wall of which extends at least from the first tray to the last tray comprising one or more openings providing fluid access to the interior of the chimney for the gas having passed through the perforated surface of the first tray. The chimney may comprise either, one or more openings to the outside of the enclosure for discharging the gas from the enclosure, i.e. the dryer, and one or more openings configured to recirculate the gas after it has passed through the dryer, following the flow substantially parallel to the Z axis, first passing through the perforated surface of the second tray before passing directly after through the perforated surface of the first tray.

[0026] In a preferred variant, the feed unit (9) is connected upstream to a source of said particles to be dried, preferably a silo, the particles preferably comprising sawmill wood waste, construction material wood waste, paper or cardboard waste, agri-food products such as cereals, and may be in the form of powder, granules, chips, pellets, cakes, or pieces generally not exceeding 10 cm in length. BRÈVE DESCRIPTION DES FIGURES

[0027] For a better understanding of the nature of the present invention, reference is made to the following Figures, including; Figure 1 : illustrates a dryer according to EP2828595. Figure 2 : illustrates an alternative dryer according to the present invention comprising an output control loop provided with an output probe located at the second recovery unit and an input control loop provided with an input probe placed at the supply unit. Figure 3 : illustrates the output control loop with variations in rotation speeds ( oh has, oh b) and the particle distribution rate (dma / dt) on the first tray in response to final moisture content values ​​(H1b) measured by the outlet probe. Figure 4: : illustrates the inlet control loop with variations in particle distribution velocity (dma / dt) on the first tray in response to variations in the initial moisture content (HOa) measured by the inlet probe, allowing the final moisture content (H1b) of the particles to be stabilized. Figure 5(a) & 5(b) : illustrate an example of a distribution unit adapted to the present invention, (a) perspective view, (b) top view. Figure 6(a) & 6(b) : illustrate an example of a recovery unit adapted to the present invention, (a) top view, (d) cross section. Figure 6(c) & 6(d) : illustrate a second example of a recovery unit adapted to the present invention, (c) top view, (d) cross section. Figure 7 : illustrates in the form of a flowchart the output control loop, including the output probe for measuring the final moisture contents (H1b) at the outlet of the dryer. Figure 8 : illustrates the flowchart of the Figure 7, with an additional condition that to activate the output control loop, it must be ensured that the initial moisture contents (HOa) of the particles loaded into the dryer are relatively constant (i.e., H0a = H0r ± d ). Figure 9 : illustrates the flowchart of the Figure 8 , with in addition the input control loop including the measurement of the initial moisture content (HOa) of the particles. DESCRIPTION DÉTAILLÉE DE MODES DE RÉALISATION PARTICULIERS

[0028] The dryer according to the present invention is preferably a dryer of the type described in EP2828595, which is discussed in the “technological background” section above and illustrated in Figure 1 . It is not essential that the first and second trays rotate in opposite directions, but opposite rotations of the two trays is preferred as this increases the energy efficiency of the dryer.

[0029] The dryer of the present invention comprises an enclosure (10) comprising a substantially cylindrical wall extending along a vertical axis, Z. The enclosure encloses a first circular tray (1a) mounted on the wall of the enclosure substantially normal to the vertical axis, Z. The first tray (1a) is mounted for rotation in a first direction about the vertical axis, Z, the rotation of which is actuated by a first motor (5a). The surface of the first tray (1a) is perforated and permeable to fluids such as air and water vapor and water.

[0030] The enclosure (10) encloses a second circular plate (1b) mounted at a certain distance from the first plate on the wall of the enclosure substantially normal to the vertical axis, Z. The second plate is mounted for rotation about the vertical axis (Z) whose rotation is actuated by a second motor (5b). The second motor (5b) may be the first motor (5a) or may be a different motor from the first motor (5a). The rotation of the second plate may be in the same direction or in the opposite direction to the rotation of the first plate, and the rotation speeds (| oh a|), (| ohb|) (in absolute values) of the first and second plates may be equal or different and may vary over time, either independently of each other or, on the contrary, the rotation speed of one plate (= "master") imposing the rotation speed of the other plate (= "slave"). The surface of the second plate (1b) is perforated and permeable to fluids such as air and water vapor and water.

[0031] A first distribution unit (2a) for the particles to be dried extends along a radius of the first tray (1a) and is configured to receive the particles to be dried from a feed unit (9) and to distribute these particles along a radius of the first tray (1a) before drying. The feed unit makes it possible to control the feed or loading rate (dma / dt) of the particles to be dried on the first tray (1a).

[0032] A first recovery unit (3a) extends along a second radius of the first plate, located downstream of, preferably adjacent to, the first distribution unit (2a). The first recovery unit (3a) is configured to recover particles deposited on the first plate (1a) after rotation thereof by a given angle. The rotation angle is preferably at least 300°, preferably at least 320°, more preferably at least 340°, and preferably the largest angle allowing the first distribution unit (2a) and the first recovery unit (3a) to be accommodated along the respective radii of the first plate (1a). A large rotation angle allows the particles to be exposed to hot gases for a given rotation speed to be extended. An angle of practically 360° can be obtained by superimposing the first distribution unit (2a) above the first recovery unit (3a).

[0033] The enclosure (10) also encloses a transfer unit (4t) for the particles collected from the first tray (1a) by the first recovery unit (3a) to a second distribution unit (2b). The second distribution unit (2b) extends along a radius of the second tray (1b) and is configured to distribute the particles on the second tray (1b), along the radius of the second tray. A second recovery unit (3b) for the particles deposited on the second tray (1b) is located downstream of the second distribution unit (2b), preferably adjacent to the second distribution unit (2b), so that the particles reach it after rotating it through a given angle. The second recovery unit is configured to discharge the particles after drying out of the dryer by an evacuation system (4o).

[0034] The dryer comprises a hot gas blowing system (5) configured to form a flow of hot gas substantially parallel to the vertical axis (Z), passing first through the perforated surface of the second tray (1b) before passing directly thereafter through the perforated surface of the first tray (1a). It is the hot and dry gas which, upon contact with the wet particles, will (a) increase their temperature and (b) carry away some of their moisture. As a result, the temperature of the hot gas drops and its moisture content increases a first time when it passes through the second tray (1b), then a second time when it passes through the first tray (1a). The gas leaving the first tray therefore has a moisture content that is too high to be recirculated as is. In practice, the gas thus cooled and humidified is therefore either discharged outside the enclosure into the atmosphere or for another use such as a heat exchanger or a humidifier (cf.dotted arrows at the . Figure 1 discharging the gases out of the dryer upwards through a chimney (6) of the dryer), or recirculated after drying and heating. The blowing system may comprise one or, preferably, more fans. The fan(s) may be configured to suck in the hot gases by creating a vacuum downstream of the trays. In this variant, the fan(s) are positioned downstream of the first tray (1a). In an alternative variant, the fan(s) may be configured to blow the hot gas by creating a positive pressure upstream of the second tray (1b). In this variant, the fan(s) are positioned upstream of the second tray (1b). The terms “upstream” and “downstream” used to define the blowing system are defined in relation to the direction of movement of the gas through the second tray (1b) before passing through the first tray (1a).

[0035] The dryer of the present invention is distinguished from prior dryers in that it further comprises an outlet probe (7) located at the second recovery unit (3b) or in the exhaust system (4o) and configured to measure the final moisture content (H1b) of the particles in or exiting the second recovery unit (3b). A processor (11) is coupled to the outlet probe (7), the first and second motors (5a, 5b) and the power unit (9) and is configured to optimize drying parameters as follows, extract the final moisture content values ​​(H1b) of the particles measured by the output probe (7) over time, and compare them to the target moisture content (H1t), if the final moisture content values ​​(H1b) are not within the predefined target range (H1t ± e ), modify rotation speed values ​​( oh has, ohb) first and second trays (1a, 1b) and, optionally, a feed rate (dma / dt) of the particles by the feed unit (9), in order to obtain final moisture content values ​​(H1b) within the predefined target range (H1t ± e ). EXIT CONTROL BOUCH BASED ON EXIT PROBE MEASUREMENTS (7)

[0036] The dryer of the present invention is controlled by the processor (11) through a first control loop called the output control loop. The output control loop is based on the following principles.

[0037] For a constant particle layer thickness, a slower rotation speed of one of the trays increases the drying time of the particles on this tray and is therefore indicated in the case where the final moisture content values ​​(H1b) of the particles measured by the outlet probe (7) are higher than the target moisture content (H1t) (i.e., the particles are still too wet). Conversely, a faster rotation speed of one of the trays decreases the drying time of the particles and is indicated when the final moisture content values ​​(H1b) of the particles measured by the outlet probe (7) are lower than the target moisture content (H1t). If the particles are too dry or drier than necessary or permissible, this indicates that the dryer is wasting energy for nothing.

[0038] Preferably, the processor (11) controls the rotation speed of the second platter (1b) which acts as "master" by requiring that the first platter (1a) which acts as "slave" automatically adapts its rotation speed to that of the second "master" platter (1b). The first platter (1a) automatically adapts its speed to that of the second platter (1b) by rotating at a speed proportional to that of the second platter (1b) which is "master". The coefficient of proportionality ( oh has / oh b) between the rotational speeds of the first plate (1a) and the second plate (1b) is predefined and is preferably negative (i.e., the first plate (1a) rotates in the opposite direction to the second plate (1b)). In a preferred variant, the absolute value of the coefficient of proportionality ( oh has / ohb) is between 0.7 and 1.3, preferably between 0.8 and 1.05. Preferably, in absolute values, the rotational speed of the first plate (1a) is equal to the rotational speed of the second plate (1b). If the first plate (1a) rotates faster than the second plate (i.e., | oh has / oh b| > 1), the thickness of the particle layer on the second plate (1b) is greater than that of the first plate (1a). Conversely, if | oh has / oh b| < 1), the thickness of the particle layer on the second plate (1b) is less than that of the first plate (1a). Therefore, if | oh has / oh b| = 1, the thickness of the layer of particles on the second plate (1b) is substantially equal to that of the first plate (1a).

[0039] Finally, for a constant rotation speed of the first plate (1a), decreasing the feed rate (dma / dt) of the particles by the feed unit (9) has the effect of decreasing the thickness of the layer of particles formed on the first plate (1a). The particles of the first plate are thus dried more quickly by the hot gas flow. Such a decrease in the flow rate is therefore indicated when the final moisture content values ​​(H1b) measured by the outlet probe (7) are higher than the target moisture content (H1t) (i.e., the particles are still too wet). Conversely, increasing the feed rate (dma / dt) of the particles by the feed unit (9) has the effect of increasing the thickness of the layer of particles formed on the first plate (1a). A greater volume of particles of the first plate are thus dried by the hot gas flow.Such an increase in flow rate is therefore indicated when the final moisture content values ​​(H1b) measured by the outlet probe (7) are lower than the target moisture content (H1t) (i.e., the particles are even drier than necessary).

[0040] Taking into account the effects described above, the processor (11) is configured to control the rotation speeds ( oh has, oh b) first and second trays (1a, 1b) as well as the feed rate (dma / dt) of the particles by the feed unit (9) in the following manner illustrated in Figure 3 , 6 And 7 . There Figure 3 schematically illustrates variations in the final moisture content (H1b) of the particles measured by the outlet probe (7). The target moisture content (H1t) and the predefined target range (H1t ± e), are represented by a solid line centered between two dashed lines. Whenever the measured final moisture content (H1b) falls outside the predefined target range (H1t ± e ), the processor (11) can vary one or more of the rotation speeds ( oh has, oh b) first and second trays (1a, 1b) and the feed rate (dma / dt) so that the measured final moisture content (H1b) falls within the predefined target range (H1t ± e ). This allows you to define reference speeds ( oh ar, oh br) first and second trays (1a, 1b) and a reference feed rate (dmar / dt) for drying the particles within the predefined target range (H1t ± e ). There Figure 3illustrates constant variations in the final moisture content (H1b) of the particles to illustrate the possible parameter variations that the processor (11) can control. In reality, once a steady state is found, the reference values ​​( oh ar, oh br, dmar / dt) allowing particles to be dried within the predefined target range are recorded and maintained until a significant variation from the steady state occurs.

[0041] As illustrated in Figure 3 , And 6 (and 7 and 8), the output probe (7) measures the final moisture content (H1b) of the particles. If H1b is within the predefined target range (H1t ± e ), then the processor maintains the reference values ​​( oh ar, oh br, dmar / dt) (cf. Figure 3 , shaded areas, and Figure 7 , “yes” branch of the rhombus rhombus [H1b = H1t ± e ?]). If, on the other hand, H1b goes outside the predefined target range (H1t ± e), then the processor is configured to act in the following manner.

[0042] In a first case, where the final moisture content values ​​(H1b) are higher than the predefined target range (i.e., H1b > H1t + e ), then the processor (11) determines theoretical values ​​of the rotation speeds ( oh has , oh b) first and second trays (1a, 1b) and / or the feed rate (dma / dt) to obtain final moisture content values ​​(H1b) within the target range. As illustrated in Figure 3 (curve sections (H1b) above the dotted line “+ e ”) and 6 (“yes” branch of the diamond [H1b > H1t ± e ?)], then the processor is configured to decrease the second rotation speed (| oh b|) of the second plate (1b) to the corresponding theoretical value, reduce the first rotation speed (| oha|) of the first plate (1a) to the corresponding theoretical value; preferably the first plate is subject (= “slave”) to the second plate (1b) (= master”) by automatically adapting its rotation speed proportionally to that of the second plate, and / or reducing the feed rate (dma / dt) of particles distributed on the first plate (1a) to the corresponding theoretical value.

[0043] In a second case, where the final moisture content values ​​(H1b) are lower than the predefined target range (i.e., H1b < H1t - e ), then the processor (11) determines theoretical values ​​of the rotation speeds ( oh, oh b) and the feed rate (dma / dt) of the particles on the first tray (1a) allowing to obtain final moisture content values ​​(H1b) included in the predefined target range (H1t ± e). This allows (1) drying a given volume of particles to the predefined target range in less time and therefore less expensively. As illustrated in Figure 3 (curve sections (H1b) below the dotted line "- e ”) and 6 (“no” branch of the diamond [H1b > H1t ± e ?]), then the processor is configured to, increase the second rotation speed (| oh b|) of the second plate (1b) to the corresponding theoretical value, increase the first rotation speed (| oh a|) of the first plate (1a) to the corresponding theoretical value, for example, proportionally to the second rotation speed ( oh b) of the second tray (1b), and / or increasing the feed rate (dma / dt) of particles distributed on the first tray (1a) to the corresponding theoretical value. INPUT PROBE (8)

[0044] The output control loop based on measurement by the output probe (7) described above makes it possible to successfully determine reference rotation speeds (ωar, ωbr) of the trays (1a, 1b) and a reference feed rate (dmar / dt) allowing particles to be dried to a final moisture content (H1b) within the predefined target range provided that the initial moisture content (HOa) of the particles before drying is substantially constant, i.e., within a predefined reference range (± d ) around a reference mean value (HOr) (i.e., H0a = HOr ± d ).

[0045] If the initial moisture content (HOa) of the particles varies significantly (i.e. |dH0a / dt| > d) in a time range (t0), the outlet probe (7) will only detect such a variation after a delay (Δt) necessary for the particles deposited on the first tray (1a) in the time range (t0) to rotate on the first tray (1a), be transferred to the second tray (1b) and rotate on the second tray until reaching the second recovery unit (3b) and the outlet probe (7). The processor (11) will only be able to react to these humidity variations after the delay (Δt) which is much too long, and which will be useless if the initial moisture content (HOa) of the particles has again varied significantly during this delay (i.e., if t0 < Δt). It is therefore interesting to know the value of the initial moisture content (HOa) of the particles over time.

[0046] To overcome this problem, a preferred variant of the dryer of the present invention comprises, in addition to the outlet probe (7) discussed above, also an inlet probe (8) placed at the feed unit (9) or the first distribution unit (2a). The inlet probe (8) is configured to measure an initial moisture content (HOa) of the particles entering or located in the first distribution unit (2a).

[0047] In a preferred variant illustrated in the Figure 8 , the processor is configured to optimize the drying parameters based on the values ​​(H1b) measured by the outlet probe (7) only during a period during which the initial moisture content (HOa) has an initial value which is substantially constant, not varying by more than the predefined reference range (± d ) around the average reference value (HOr) (i.e., if IdHOa / dt| ≤ d , or if H0a = H0r ± d ) (cf. Figure 8, losange [H0a ≠ HOr ± d ,

[0048] By this configuration, the processor can determine the optimal reference values ​​of the rotation speeds (ωar,ωbr) of the trays (1a, 1b) and the feed rate (dmar / dt) to dry the particles in the desired range in a stationary operating mode, i.e., where the initial moisture content (HOa) of the particles remains substantially constant during the drying process. If the particles to be dried in a given batch have initial moisture contents (HOa) that vary substantially from one zone of the batch to another, two solutions are available to the person skilled in the art. A first option comprises a step of mixing the particles before drying in order to homogenize the initial moisture content (HOa) of the particles.A second option, which the dryer of the present invention allows to implement is to vary the thickness of the layer of particles deposited on the first tray (1a) according to the variations of the initial moisture content (HOa) of the particles. To do this, the processor (11) can comprise a second control loop, called input control loop, based on the measurements by the input probe (8). BOUCLE DE CONTROL D'ENTRÉE BASED ON MEASUREMENTS BY THE ENTRÉE PROBE (8)

[0049] As indicated above, in a preferred variant of the dryer of the present invention, the processor (11) comprises an input control loop, based on the measurements by the input probe (8). The Figure 9 is similar to the Figure 8with in addition the input control loop described in this section. In this variant, the supply unit (9) is coupled to a source (20) of particles to be dried, and the processor (11) is configured to attenuate or eliminate variations in time (dH1b / dt) of the final moisture content values ​​(H1b) due to variations in time (dH0a / dt) of the initial moisture content values ​​(HOa) of the particles to be dried. As illustrated in Figure 4 And 8 , the processor (11) is therefore configured to extract the initial values ​​of the initial moisture contents (HOa) measured by the input probe (8) over time, and compare whether the initial values ​​remain within the predefined reference range (± d ) around the reference mean value (HOr) (i.e., if |dH0a / dt| ≤ d , or if H0a = HOr ± d ) (cf. Figure 9 , “yes” branch of the diamond [H1b = H1t ± e?] followed by the diamond [H0a = HOr ± d ?]).

[0050] If the initial values ​​are not within the predefined reference range (i.e., if |dH0a / dt| > d or, equivalently, if H0a ≠ H0r ± d ), the processor is configured to modify the feed rate (dma / dt) of the particles by the feed unit (9), without modifying the reference rotation speeds ( oh ar, oh br) first and second trays (1a, 1b), in order to modify over time the thickness of the layer of particles deposited on the first tray (1a) and thus modify an intermediate humidity content (H1a) of the particles at the outlet of the first tray (1a), before being transferred to the second tray (cf. Figure 4“dma / dt” graph, in response to variations in the measured initial moisture content (HOa), and “da” and “H1b” graphs illustrating the consequences of variations in dma / dt on the thickness (da) of the layer of particles on the first tray (1a) and finally on the value of the final moisture content (H1b) which thus remains within the predefined target range.

[0051] It is clear that if the average value of the thickness of the layer deposited on the first plate (1a) varies from one revolution to another, the thickness of the layer formed on the second plate (1b) by the particles transferred from the first plate (1a) will vary according to these variations. However, unlike the first plate (1a), on which the thickness of the layer of particles can vary from one angular sector of the first plate (1a) to another, the thickness of the layer of particles on the second plate (1b) is substantially constant. Indeed, the transfer unit (4t) mixes and homogenizes the mass of particles collected by the first recovery unit (3a) and deposits the particles on the second plate (1b) at a constant flow rate, thus forming a layer of substantially constant thickness on the second plate (1b).

[0052] Thanks to the input control loop, it is possible to obtain final moisture content values ​​(H1b) within the target range (H1t ± e ) even in the case where the initial values ​​(HOa) vary significantly in the distribution time on the first tray (1a). With this input control loop, the thickness of the layer of particles deposited on the first tray (1a) can vary with the azimuthal angle, thus making it possible to respond almost instantaneously to the variations (dH0a / dt) of the initial moisture contents (HOa) in the distribution time. This makes it possible to obtain, after one revolution of the first tray (1a), a substantially constant intermediate moisture content (H1a) of the particles at the outlet of the first tray (1a), before being transferred to the second tray (1b), independently of the variations in the initial moisture contents (HOa) of the particles in the first distribution unit (2a).

[0053] The input control loop based on the inlet probe (8) is subordinate to the output probe (7) based loop. The output control loop based on the outlet probe (7) defines the basic operating parameters of the dryer, including the rotational speeds (ωa,ωb) of the first and second trays (1a, 1b) and the feed rate (dma / dt) for a given batch of particles. These basic parameters can remain constant as long as the properties of the batch of particles to be dried are substantially constant, such as initial moisture content (H0a), particle size and size distribution, etc. The output control loop is therefore a long-term control, while the input control loop is an instantaneous control based on the short term.

[0054] Indeed, the input control loop based on the input probe (8) allows to adapt these parameters to the instantaneous variations of the initial moisture content (HOa) of the particles in this batch. The input control loop allows to react very quickly to variations of the initial moisture content (HOa) of the particles by controlling the feed rate (dma / dt) according to the initial moisture content (HOa) measured by the input probe (8).

[0055] In particular, if the initial values ​​of the initial moisture contents (HOa) are below the reference range (i.e., H0a < H0r - d), then the processor (11) determines a theoretical value of the feed rate (dma / dt) allowing to increase the thickness of the layer of particles distributed by the first distribution unit (2a) on the first tray (1a). This makes it possible to ensure that the intermediate moisture content (H1a) of the particles adjacent to the first recovery unit (3a) is included in an intermediate range (i.e., H1a = H1i ± c ). The processor (11) then increases the power supply rate (dma / dt) of the power supply unit to the theoretical value.

[0056] Conversely, if the initial values ​​of the initial moisture contents (HOa) are higher than the reference range (i.e., H0a > HOr + d), then the processor (11) determines a theoretical value of the feed rate (dma / dt) allowing to reduce the thickness of the layer of particles distributed by the first distribution unit (2a) on the first tray (1a). This makes it possible to ensure that the intermediate moisture content (H1a) of the particles adjacent to the first recovery unit (3a) is included in the intermediate range (i.e., H1a = H1i ± c ). The processor then decreases the power supply rate (dma / dt) of the power supply unit to the theoretical value.

[0057] In its determination of the theoretical value of the feed rate (dma / dt) allowing to vary the thickness of the particle layer on the first tray (1a) in order to maintain the final moisture contents (H1b) within the predefined target range (i.e., H1b = H1t ± e ), with the first and second platters (1a, 1b) rotating at their reference speeds ( oh ar, ohbr) respective determined with the output control loop, the processor (11) takes into account the first rotation speed ( oh a) of the first tray (1a) and determines the thickness of the layer of particles making it possible to obtain an intermediate humidity content (H1a) of the particles which is substantially constant at the outlet of the first tray (1a), before being transferred to the second tray.

[0058] If the initial moisture content (HOa) values ​​of the particles vary to the point that the average reference value also varies, then varying the feed rate (dma / dt) of the particles on the first tray (1a) will certainly allow obtaining a constant value of the intermediate and final moisture contents (H1a, H1b) but it is possible that the value of the final moisture content (H1b) goes outside the predefined target range (H1t ± e). For this reason, the output control loop continues to measure the final moisture content (H1b) of the particles, and if this value goes outside the predefined target range, the controller (11) can determine a new value of the reference velocities ( oh ar, oh br) first and second trays (1a, 1b) for bringing the final moisture content (H1b) back into the predefined target range (H1t ± e ) (cf. Figure 9 , loop [measure H1b] - [measure H0a]). STRUCTURE DU SECHEUR - UNITÉ D'ALIMENTATION (9)

[0059] The feed unit (9) is coupled upstream to a source (20) of particles to be dried, for example stored in a silo, a container, a bin, etc. and, downstream, to the first distribution unit (2a). The feed unit (9) preferably makes it possible to precisely control and vary the particle feed rate to the first distribution unit (2a) in order to be able to vary the thickness (da) of the layer of particles deposited on the first tray by the first distribution unit (2a) in response to the output and / or input control loop.

[0060] Any feed unit allowing such control known to those skilled in the art may be used and the present invention is not limited to a particular type or model of feed unit. For example, the feed unit (9) may comprise one or more Archimedean screws whose rotational speed controls the feed rate (dma / dt) of the particles feeding the first distribution unit (2a). Alternatively, the feed unit may comprise a conveyor belt whose movement speed can be controlled in order to control the feed rate (dma / dt). DRYER STRUCTURE - 1 er< ET 2 nd< UNITÉS DE RÉPARTITION (2a, 2b)

[0061] The feed unit (9) is coupled downstream to the first distribution unit (2a) and is configured to feed the first distribution unit (2a) at a feed rate controlled by the processor. The first distribution unit (2a) of the particles to be dried on the first tray (1a) has the purpose of distributing the particles to be dried homogeneously along a radius of the first tray (1a). In general, the first distribution unit (2a) comprises, a structure extending from the outer periphery to the inner periphery of a tray, preferably along a radius thereof, means for transporting particles from the outer periphery to the inner periphery of the trays, and finally means for depositing said particles from the transport means to the trays.

[0062] Several solutions are possible. For example, the transport of the particles from the outer periphery to the inner periphery of the trays can be ensured by a conveyor belt, either perforated or transversely inclined so as to allow the particles to sprinkle the tray located below. To assist the dusting, the belt can be vibrated. In an alternative and preferred variant, the first distribution unit (2a) comprises at least one Archimedean screw extending along a radius of the first tray (1a), in order to transport the particles from the outer periphery to the inner periphery of the first tray (1a). Said at least one Archimedean screw is enclosed in an enclosure provided with one or more openings extending downwards and along said radius of the first tray (1a) in order to allow the dusting of the particles homogeneously along the radius of the first tray (1a).

[0063] In the case of an Archimedean screw, if the particles to be dried are discharged by the feed unit (9) at a first end of the Archimedean screw of the first distribution unit (1a), for example adjacent to the enclosure (10), there is a high risk that the thickness of the particle layer will decrease along the radius of the first plate (1a) as one approaches the center of the plate. Such a thickness gradient (d(da) / dR) is not advisable because it results in a gradient along the radius of the first plate (1a) in intermediate moisture contents (H1a) of the particles after one revolution on the first plate (1a). Worse still, if the layer becomes so thin that holes appear in the particle layer, this creates areas of low resistance to the flow of hot gas which will preferentially pass through these areas to the detriment of the particles to be dried.

[0064] To overcome this problem, the first distribution unit (2a) extending along a radius of the first plate (1a) may comprise, as illustrated in the Figures 5(a) and 5(b) , a distribution screw (21) and a recirculation screw (22), placed side by side and enclosed in a box (2h). The box (2h) comprises a feed opening coupled to an outlet (9o) of the feed unit (9). The feed opening is configured to deliver particles from the feed unit (9) to one end of the distribution screw (21). For example, the feed opening may be located above the distribution screw (21) to allow the particles to fall by gravity into the box (2h- and to be carried away by the rotation in a first direction of the distribution screw (21) along the radius of the first plate (1a).

[0065] A distribution opening (2o) extends along the length of a lower face of the box (2h), below the distribution screw (21) so that the particles can exit the box (2h) by gravity and fall onto the first tray (1a) along its radius. In order to prevent the particles from falling predominantly in a section adjacent to the feed opening (9o), the distribution screw (21) is only partially separated from the recirculation screw (22), allowing a surplus of particles to pass from the distribution screw (21) to the recirculation screw (22), which rotates in a second direction, opposite to the first direction of rotation of the distribution screw (21) so as to transport the particles thus transferred in the direction of the enclosure (10).At the end of the recirculation screw (22) adjacent to the enclosure, the recirculation screw (22) is provided with a vane (22s) which, by rotation of the recirculation screw (22), returns the particles to the distribution screw (21). A similar vane (21s) is arranged at the end of the distribution screw (21) close to the center of the dryer in order to transfer to the recirculation screw (22) the particles located at this end without falling onto the first tray (1a) through the distribution opening (2o). A first distribution unit (2a) of this type allows a homogeneous distribution of the particles along the radius of the first tray (1a), thus ensuring that the thickness (da) of the layer of particles deposited on the first tray (1a) is radially substantially constant.

[0066] The second distribution unit (2b) performs for the second tray (1b) the same functions as the first distribution unit (2a) for the first tray (1a), with the difference that it is not supplied upstream by a supply unit (9) but by the transfer unit (4t) discussed below. It may be different from the first distribution unit (2a), but the first and second distribution units (2a, 2b) are preferably similar and even preferably identical. The second distribution unit (2b) is preferably of the type discussed above with reference to Figures 5(a) and 5(b) . Even if similar, the first and second distribution units (2a, 2b) do not necessarily have to operate at the same flow rate, and the layers deposited on the first and second trays (1a, 1b) do not necessarily have to have the same thickness (da, db). STRUCTURE DU SECHEUR -1 er< ET 2 nd< UNITÉS DE RÉCUPÉRATION (3a, 3b) ET UNITÉ DE TRANSFERT (4t)

[0067] The first recovery unit (3a) of the first tray (1a) makes it possible to recover the particles deposited on the first tray (1a) after one rotation thereof. The first recovery unit (3a) is therefore positioned upstream of the first distribution unit, adjacent to it so that the particles having an initial moisture content (HOa) deposited on the first tray by the first distribution unit can rotate, preferably between 340 and 360°, or preferably between 345 and 355° before being collected and evacuated from the first tray (1a) with an intermediate moisture content (H1a) by the first recovery unit (3a).To maximize the rotation angle of the particles on the first tray (1a) between the first distribution unit (2a) and the first recovery unit (3a), they are preferably arranged next to each other, or even the first distribution unit (2a) can be arranged above the first recovery unit (3a).

[0068] As illustrated in the Figures 6(a) and 6(c), the first recovery unit (3a) preferably comprises at least one Archimedean screw (32v) extending along a radius of said trays which is enclosed in a box (3h) provided with one or more recovery openings (3i) extending along said radius of the corresponding tray. The openings are connected to a scraper (3r) or brush capable of collecting and directing the particles brought by the rotation of the first tray (1a) through the recovery opening (3i) into the box (3h) of the Archimedean screw (32v). By rotating, the Archimedean screw transports the particles thus collected towards a discharge opening (3o) which is connected to the transfer unit (4t). The first recovery unit (3a) is thus coupled downstream to the transfer unit (4t) configured to transfer the particles thus collected by the first recovery unit (3a) towards the second tray (1b).

[0069] THE Figures 6(b) and 6(d)illustrate another variant of the first recovery unit (3a), particularly suitable, but not exclusively, for cases where the first plate (1a) includes a raised circumferential rim requiring the Archimedes screw (32v) to be raised above this rim. As in the variant of the Figures 6(a) and 6(c), in the present variant, the first recovery unit (3a) comprises an Archimedean screw (32v) whose rotation makes it possible to transport the collected particles radially along a radius of the first plate (1a) towards the outside thereof and to discharge them towards the discharge opening (3o) connected to the transfer unit (4t). In the present variant, the first recovery unit (3a) further comprises a multi-blade mill (3s) arranged upstream of and parallel to the Archimedean screw (32v). The rotation of the multi-blade mill (3s) makes it possible to feed the Archimedean screw (32v) even if it is raised relative to the surface of the first plate. In all cases, the multi-blade mill (3s) ensures a reproducible and reliable supply of particles to the Archimedean screw (32v).

[0070] The transfer unit (4t) is coupled upstream to the first recovery unit (3a) of the first tray (1a) and downstream to the second distribution unit (2b) of the second tray (1b). The function of the transfer unit (4t) is therefore to transfer the partially dried particles from the first tray (1a) to the second tray (1b) to finalize the drying of the particles. The type of transfer unit (4t) of the particles from the first tray (1a) to the second tray (1b) depends on the configuration of the dryer. If the first tray (1a) is the upper tray, the transfer means can be a simple tube connecting the first recovery unit (3a) of the first tray (1a) to the second distribution unit (2b) of the second tray, into which the particles fall by gravity.If, on the contrary, the first tray (1a) is the lower tray, it is preferable that the transfer unit (4t) includes an Archimedes screw allowing the particles to be raised from the first lower tray (1a) to the second upper tray (1b). This configuration of the lower tray forming the first tray (1a) and the upper tray forming the second tray (1b) has the advantage of reducing the suspension of the finest particles, because in this configuration, the hot gas flows from the top to the bottom, crushing the particles against the respective trays.

[0071] The second recovery unit (3b) performs the same functions for the second tray (1b) as the first recovery unit (3a) for the first tray (1a), with the following differences, The particles collected by the second recovery unit (3b) have a final moisture content (H1b) which must be within the predefined target range (H1b = H1t ± ε ), after a first rotational turn on the first plate (1a) and a second rotational turn on the second plate (1b) exposed to a flow of hot gas passing through the second plate (1b) before passing through the first plate (1a), the second recovery unit (3b) is not coupled downstream to the transfer unit (4t) but is coupled by the recovery opening (3o) to an evacuation system (4o) which evacuates the particles from the dryer.

[0072] The second recovery unit (3b) may be different from the first recovery unit (3a), but the first and second recovery units (3a, 3b) are preferably similar and even preferably identical. FIRST AND SECOND PLATFORMS (1a, 1b)

[0073] The dryer according to the present invention is particularly advantageous because it can be used to dry particles of very different particle sizes ranging from fine particles such as sawdust, fine grains, ceramic, polymer or metal powders, to coarser particles, such as wood waste, chips, pellets, agricultural waste, corn husks, etc. by quickly and easily changing the diameter of the holes in the trays in the following manner. The first and second trays (1a, 1b) can thus comprise a self-supporting rigid structure with high permeability of the grating type, on which is placed a filter layer comprising openings of size and density corresponding to the desired permeability according to the type and particle size of the particles to be dried. The filter layer can be a perforated sheet, a sieve, a grid or a cloth.To facilitate the installation of such a filter layer, it can be cut into angular sectors, which can be placed and fixed side by side directly on the grating or other self-supporting structure with high permeability. This would be impossible in practice with belt or perforated tray dryers which are dedicated to drying particles of a single type of particle size.

[0074] The sequence of stacking the first and second trays (1a, 1b) depends on applications and preferences. For example, the first tray (1a) can be located above the second tray (1b) and the hot gas (e.g. hot air) flows from the bottom to the top. An advantage of this variant is that the transfer of the partially dried particles from the upper first tray (1a) to the lower second tray (1b) by the transfer unit (4t) is from the top to the bottom, assisted by gravity; a simple tube connecting the first recovery unit (3a) to the second distribution unit (2b) is sufficient. On the other hand, since the hot gas flow flows from the bottom to the top through the second and first trays, respectively, the particles can fly away and create dust.Slight fluidization of the particle layer can be beneficial for particle drying, but the formation of a fine dust cloud suspended in the air must be avoided. This configuration is therefore better suited to drying heavier particles that do not easily form a dust cloud.

[0075] For lighter or finer particles, the first plate (1a) can instead be located below the second plate (1b) and the hot gas thus circulates from top to bottom, as shown in Figures 1 to 7. In this configuration, the particles are pressed against the tray on which they are located, which compacts the layer of particles and considerably reduces the suspension of dust. The compaction of the layer of particles by a flow of hot gas from the top to the bottom risks forming temperature and humidity gradients in the thickness of the layer that are greater than in a layer slightly fluidized by a flow of hot gas from the bottom to the top. However, the particles having different temperatures and humidity contents are mixed during the recovery of the particles from the first tray (1a) and their transfer to the second tray (1b) by the transfer unit (4t) comprising, for example, an Archimedes screw.The mixing carried out in the transfer unit (4t) further increases the drying efficiency by remixing the particles, thus allowing a layer of particles of homogeneous temperature and humidity content to be deposited over the thickness of the layer.

[0076] THE Figures 1 And 2 illustrate dryers comprising two trays. However, to reduce the floor space occupied by the equipment, it is entirely possible to mount: at least one third circular tray mounted substantially horizontally at a certain distance from, and separated from, the first tray (1a) by the second tray (1b), rotating around the vertical axis (Z), in the opposite direction of rotation of the second tray, the surface of said tray being perforated and permeable to fluids such as air and water vapor and water, and a means for transferring the particles collected from the second tray (1b) by the recovery means (2b) to a third distribution means capable of distributing said particles along a radius of the third tray.

[0077] It is clear that as many parallel trays rotating around the Z axis as desired and according to the needs of a particular application can be mounted. However, a dryer comprising two trays (1a, 1b) is suitable for the majority of applications. The use of several stacked trays makes it possible to reduce the external diameter of the trays, but increases the production cost of the dryer.

[0078] The trays (1a, 1b) are enclosed in an outer enclosure of diameter corresponding to the diameter of the trays with enough margin to avoid friction, but as little as possible to allow sealing of the interface between the trays and the outer wall. Sealing can be ensured for example by a flexible skirt fixed to the outer wall and resting on a raised edge of the circumference of the trays. In this way, the layer of particles resting on a rotating tray is not in contact with the static skirt, thus ensuring good sealing and integrity of the layer of particles on the tray. This is not possible to achieve on a belt dryer, in which the sealing skirt is placed between the rolling belt and the particles located on the edges of the belt.So there is a fringe of particles in contact with the static skirt at each edge of the strip which does not move at the same speed as the particles in the middle of the strip.

[0079] As illustrated in the Figure 1, the central part of the trays is preferably hollow and included in a chimney (6) which is cylindrical, internal and centered on the axis of rotation (Z). Such a chimney (6) rising over practically the entire height of the dryer, in any case between the upper and lower trays, includes many advantages, which amply compensate for the loss in surface area available for drying. Indeed, if the external diameter of the trays is D1 and the diameter of the cylindrical chimney is D6 = nx D1, where n < 1, the loss in surface area available on each tray for drying between a solid tray and a tray including a chimney is D6 / D1 = n 2< . For example, if the chimney has a third of the diameter of the external enclosure, the loss in surface area available for drying is only 1 / 9 ≃ 11%. A chimney (6) firstly allows easy access by an operator to all the mechanical elements of the machine, such as bearings, geared motors, jacks, etc.It also facilitates the replacement of the flexible porous layers to be deposited and fixed on the gratings giving the trays their mechanical integrity. The chimney can also be used to house the motors (5a, 5b) driving the rotation of the trays, as well as the fans used to generate the flow of hot gas, with the advantage of a substantial reduction in the noise pollution generated by the dryer. In the case of a gas flow from top to bottom as shown in . Figure 1 , windows (6w) at the bottom of the chimney (6), located below the lower tray, allow the hot gas to be recovered and evacuated from the top inside the enclosure. Alternatively, the hot gases can be evacuated through a space defined in a double wall of the enclosure (10).

[0080] Furthermore, the chimney (6) makes it possible to fix the first and second distribution (2a, 2b) and recovery (3a, 3b) units at their two ends in order to avoid having to fix them cantilevered on the outer enclosure only. In addition, this frees up space at the inner ends of said means located side by side to accommodate their width. Finally, such a structure makes it possible to stiffen the surface between the chimney (6) and the outer enclosure (10), making it possible to maintain good flatness of the trays. This is important for cleaning and recovering particles by a scraper or a brush, which are only effective if the surface of the trays is perfectly flat.

[0081] Since the particle size distribution of particles of the same type can be wide, it is difficult to prevent the finest fraction of particles from passing through the perforations of the trays and falling onto the lower tray(s), then onto the floor of the enclosure enclosing the trays. In order to avoid excessive accumulation of particles on the floor and also to recover them, it is advantageous to provide the floor with an extraction opening for the finest particles that may have been deposited on the floor. In addition, a scraper or brush fixed integrally to the lower tray and capable of following its rotational movement is used to push the particles deposited on the floor towards said discharge opening. Since the scraper or brush is fixed to the lower tray, it is not necessary to motorize it individually. DRYING PRINCIPLE

[0082] The drying of the particles deposited on the first perforated tray (1a), transferred after a given rotation from the first tray to the second perforated and rotating tray (1b), is ensured by a hot gas blowing means (5g) following a flow substantially parallel to the vertical axis (Z), passing through the second tray (1b) before passing through the first tray (1a), thus defining a countercurrent drying system. It is important that the flow of hot and dry gas first passes through the second tray (1b), where the particles are already partially dried by their stay on the first tray (1a), which is itself reached by a flow of hot gas partially loaded with moisture after passing through the second tray (1b).

[0083] The particles are distributed on the first tray with their initial moisture content (HOa). The particles are then carried away by the rotation of the first tray (1a) before being recovered by the first recovery unit (3a) and transferred to the second tray (1b) by the transfer unit (4t). During the rotation of the first tray (1a), the particles are exposed to the hot gas flow exiting the second tray (1b), which is slightly cooler and more humid than the hot gas upstream of the second tray (1b). The humidity level of the particles on the first tray (1a) decreases under the action of the hot gas flow as the rotation of the first tray (1a) progresses, until reaching the first recovery unit (3a) with an intermediate humidity content (H1a) lower than the initial content (HOa) but still higher than the final content (H1b) (which must be within the predefined target range) (i.e., H1b < H1a < H0a).The particles transferred by the transfer unit (4t) arrive on the second tray (1b) partially dried with the intermediate moisture content (H0b = H1a) and begin a second rotation, preferably in the opposite direction, where the hot air flow finishes drying them until they reach their final moisture content (H1b). Drying remains optimal both on the first tray (1a) and on the second tray (1b) because the temperature (ΔT) and humidity (ΔH) gradients between the particles and the hot gases remain high on both trays. Indeed, with a higher temperature and a lower moisture content than the particles located on the first tray (1a), the particles on the second tray (1b) are exposed to hotter and drier gases than those on the first tray (1a).However, although the particles located on the first plate (1a) are exposed to gases that are less hot and more humid than those on the second plate, as they are more humid and less hot than the particles located on the second plate (1b), the temperature (ΔT) and humidity (ΔH) gradients between the particles and the hot gases remain high.

[0084] Counter-rotation of the first and second trays (1a, 1b) optimizes the drying process. The hot gas, for example hot air or any other gas from a combustion process, follows a path opposite to that of the particles. Since the moisture content of the particles on the second tray depends on their angular position, it follows that the moisture content of the air having passed through the second tray (1b) and carried away some of the moisture from the particles by transferring some of its heat, also depends on the angular position and will be higher where the particles affected by the flow have a higher moisture content, i.e. in the low angles of rotation of the second tray (1b). The hot gas downstream of the second tray (1b) is also the gas upstream of the first tray.

[0085] There Figure 1illustrates as an example the moisture contents of the particles according to their position on different angular sectors of the first and second trays (1a, 1b). It can be seen that after passing through the particles of the second tray (1b) adjacent to the second recovery unit (3b) which have the lowest final moisture content (H1b), the hot gas column reaches the particles freshly deposited on the first tray (1a) having the highest initial moisture content (HOa). Thus, the first contact of the particles on the first tray with a hot gas column is very effective in quickly removing a large amount of water.As the first tray rotates and the particles lose their initial moisture content, the hot gases are moderately moisture-laden but sufficiently hot and dry to lower the moisture content to the intermediate content (H1a) they must reach before being transferred to the second tray (1b).

Claims

1. Dryer for drying particles having an initial humidity level (HOa) until reaching a final humidity level (H1b) that lies in a predefined target range centred around a target humidity value (H1t) (i.e. H1b = H1t ± ε), the dryer comprising: (a) an enclosure comprising an essentially cylindrical wall extending along a vertical axis (Z) (b) a first circular plate mounted on the wall of said enclosure, substantially normal to the vertical axis (Z) and so as to rotate in a first direction about the vertical axis (Z), the rotation of which is actuated by a first motor configured to impart a first speed of rotation (ωa) to the first plate (1a), the surface of the first plate being perforated and permeable to fluids such as air, water vapour and water, and (c) a second circular plate mounted at a certain distance from the first plate on the wall of said enclosure, substantially normal to the vertical axis (Z) and so as to rotate about said vertical axis (Z), the rotation of which is actuated by a second motor (5b) configured to impart a second speed of rotation (ωb) to the second plate (1b), with a value different to the first speed of rotation (ωa) of the first plate (1a), the surface of the second plate being perforated and permeable to fluids such as air, water vapour and water, (d) a first distribution unit for distributing particles to be dried, which is configured to receive the particles to be dried from a supply unit and to distribute these particles before drying along a radius of the first plate, (e) a first recovery unit, which is configured to recover the particles deposited on the first plate after a rotation of the latter through a given angle, the first recovery unit being located downstream of, and preferably adjacent to, the first distribution unit, (f) a transfer unit for transferring particles gathered from the first plate by the first recovery unit to a second distribution unit, which is configured to distribute said particles along a radius of the second plate, (g) a second recovery unit for recovering particles deposited on the second plate after a rotation of the latter through a given angle, the second recovery unit being located downstream of, and preferably adjacent to, the second distribution unit (2b) and being configured to remove the particles from the dryer by a removal system after drying, (h) a system for blowing hot gas in a flow substantially parallel to the vertical axis (Z), passing first through the perforated surface of the second plate before passing directly after through the perforated surface of the first plate, characterized in that the dryer further comprises: (i) an exit sensor (7), which is located at the second recovery unit (3b) or in the removal system (4o) and is configured to measure the final humidity level (H1b) of the particles in or leaving the second recovery unit (3b), (j) a processor (11), which is coupled to the exit sensor (7), to the first and second motors (5a, 5b) and to the supply unit (9) and is configured to optimize drying parameters, including speeds of rotation (ωa, ωb) of the first and second plates (1a, 1b) and, optionally, a rate of supply (dma / dt) of the particles by the supply unit (9), in the following way: • extracting the final humidity level values (H1b) of the particles that are measured by the exit sensor (7) in the course of time and comparing them with the target humidity level (H1t), • if the final humidity level values (H1b) do not lie in the predefined target range (H1t ±ε), modifying the values of the speeds of rotation (ωa, ωb) of the first and second plates (1a, 1b) and, optionally, a rate of supply (dma / dt) of the particles by the supply unit (9), in order to obtain final humidity level values (H1b) lying in the predefined target range (H1t ±ε).

2. Dryer according to Claim 1, wherein if the final humidity level values (H1b) are greater than the predefined target range (i.e. H1b > H1t + ε), the processor (11) determines theoretical values of one or more of the speeds of rotation (ωa, ωb) and the rate of supply (dma / dt) that make it possible to obtain final humidity level values (H1b) lying in the predefined target range (H1t ±ε), and • reduces the first speed of rotation (|ωa|) of the first plate (1a) to the corresponding theoretical value, • reduces the second speed of rotation (|ωb|) of the second plate (1b) to the corresponding theoretical value, and • optionally, reduces the rate of supply (dma / dt) of particles distributed onto the first plate (1a) to the corresponding theoretical value.

3. Dryer according to Claim 1 or 2, wherein if the final humidity level values (H1b) are less than the predefined target range (i.e. H1b < H1t + ε), the processor (11) determines theoretical values of one or more of the speeds of rotation (ωa, ωb) and the rate of supply (dma / dt) of the particles onto the first plate (1a) that make it possible to obtain final humidity level values (H1b) lying in the predefined target range (H1t ±ε), and • increases the first speed of rotation (|ωa|) of the first plate (1a) to the corresponding theoretical value, • increases the second speed of rotation (|ωb|) of the second plate (1b) to the corresponding theoretical value, and • optionally, increases the rate of supply (dma / dt) of particles distributed onto the first plate (1a) to the corresponding theoretical value.

4. Dryer according to any one of the preceding claims, comprising an entry sensor (8), which is placed at the supply unit (9) or at the first distribution unit (2a) and is configured to measure an initial humidity level (HOa) of the particles entering the first distribution unit (2a), wherein the processor (11) is configured to optimize the drying parameters only during a period in which the initial humidity level (HOa) has an initial value that is substantially constant and does not vary by more than a predefined reference range (±δ) around a reference value (HOr) (i.e. H0a = H0r ±δ).

5. Dryer according to Claim 4, wherein the supply unit (9) is coupled to a source (20) of particles to be dried, and wherein the processor (11) is configured to attenuate or eliminate time variations of the final humidity level values (H1b) that are due to time variations of the initial humidity level values (HOa) of the particles to be dried, in the following way: • extracting the initial values of the initial humidity levels (HOa) that are measured by the entry sensor (8) in the course of time and comparing whether the initial values continue to lie in the predefined reference range (±δ) around the reference value (HOr) (i.e. H0a = H0r ±δ), • if the initial values do not lie in the predefined reference range (i.e. H0a ≠ H0r ± δ), modifying the rate of supply (dma / dt) of the particles by the supply unit (9) without modifying the speeds of rotation (ωa, ωb) of the first and second plates (1a, 1b), in order to obtain final humidity level values (H1b) lying in the target range (H1t ±ε) even in the event that the initial values (HOa) do not lie in the predefined reference range (i.e. H0a ≠ H0r ± δ).

6. Dryer according to Claim 5, wherein if the initial values of the initial humidity levels (HOa) are less than the reference range (i.e. H0a < H0r - δ), the processor (11) is configured to: • determine a theoretical value of the rate of supply (dma / dt) that makes it possible to increase a thickness of a layer of particles distributed by the first distribution unit (2a) onto the first plate (1a), in order to ensure that an intermediate humidity level (H1a) of the particles adjacent to the first recovery unit (3a) lies in an intermediate range (i.e. H1a = H1i ±γ), and • increase the rate of supply (dma / dt) of the supply unit to the theoretical value.

7. Dryer according to Claim 5 or 6, wherein if the initial values of the initial humidity levels (HOa) are greater than the reference range (i.e. H0a > H0r + δ), the processor (11) is configured to: • determine a theoretical value of the rate of supply (dma / dt) that makes it possible to reduce a thickness of a layer of particles distributed by the first distribution unit (2a) onto the first plate (1a), in order to ensure that an intermediate humidity level (H1a) of the particles adjacent to the first recovery unit (3a) lies in an intermediate range (i.e. H1a = H1i ±γ), and • reduce the rate of supply (dma / dt) of the supply unit to the theoretical value.

8. Dryer according to any one of the preceding claims, wherein the first plate (1a) is located below the second plate (1b), and wherein the hot gas circulates from the top downward and is preferably hot air.

9. Dryer according to any one of the preceding claims, wherein the second plate (1b) rotates in an opposite direction of rotation to the first plate (1a).

10. Dryer according to any one of the preceding claims, wherein the first and second plates (1a, 1b) comprise a rigid self-supporting structure with a high permeability of the grating type, on which a filtering layer is placed comprising openings with a size and density corresponding to the permeability desired according to the type and size of the particles to be dried.

11. Dryer according to any one of the preceding claims, wherein the first and second distribution systems (2a, 2b) for distributing particles to be dried onto the first and second plates (1a, 1b), respectively, as well as the supply unit (9), each comprise at least one Archimedes' screw extending along a radius of the first and second plates (1a, 1b), respectively, said at least one Archimedes' screw being contained in an enclosure provided with one or more openings extending along said radius of the plates (1a, 1b).

12. Dryer according to any one of the preceding claims, wherein the first and second recovery units (3a, 3b) each comprise at least one Archimedes' screw extending along a radius of said plate, which is contained in an enclosure provided with one or more openings extending along said radius of the first plate (1a), said openings being connected to a scraper or brush capable of gathering the particles conveyed by the rotation of the plate and directing them toward the Archimedes' screw.

13. Dryer according to any one of the preceding claims, wherein after the gas has passed through the perforated surface of the first plate (1a), the hot gas blowing system (5): • is either configured to remove the gas from the enclosure, or • to comprise an air dryer for capturing the moisture present in the gas before reheating the gas thus dried and recirculating it through the second and first plates (1b, 1a), respectively.

14. Dryer according to Claim 13, wherein the vertical axis (Z) is centred in an essentially cylindrical hollow shaft (6), the wall of which extends at least from the first plate (1a) to the last plate (1b, 1c) and comprises one or more openings offering fluidic access to the interior of the shaft (6) for the gas that has passed through the perforated surface of the first plate (1a), and wherein the shaft comprises either: • one or more openings toward the outside of the enclosure, making it possible to remove the gas from the enclosure, or • the dryer and one or more openings configured to recirculate the gas after it has passed through the dryer, in the flow substantially parallel to the axis Z, passing first through the perforated surface of the second plate (1b) before passing directly after through the perforated surface of the first plate (1a).

15. Dryer according to any one of the preceding claims, wherein the supply unit (9) is connected upstream to a source (20) of said particles to be dried, preferably a silo, said particles preferably comprising sawmill wood waste, construction material wood waste, paper or cardboard waste, food industry products such as cereals, and are in the form of powder, granulates, chips, pellets, cakes, or fragments generally not exceeding 10 cm in length.

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