WO1997036140A1

WO1997036140A1 - Method and apparatus for the drying of free-flowing granular solids

Info

Publication number: WO1997036140A1
Application number: PCT/DK1997/000122
Authority: WO
Inventors: Peter Kier
Original assignee: KIER MICROWAVE
Current assignee: KIER MICROWAVE
Priority date: 1996-03-21
Filing date: 1997-03-20
Publication date: 1997-10-02
Anticipated expiration: 1998-09-21
Also published as: EP0900353A1; DK172734B1; DK33296A; AU2152597A

Abstract

The invention is concerned with a method, an apparatus and an application of the method to the drying of free-flowing granular solids, such as grain, seed, or sawdust. The solids are conducted from an inlet into a cavity to an outlet from the cavity. While passing from the inlet to the outlet, the solids are exposed to electromagnetic waves. The electromagnetic waves are generated by magnetrons which are disposed in the vicinity of the cavity and equipped with a housing for a cooling medium. The cooling medium is conducted first past the magnetrons and then to a position in the cavity. The cooling medium conducted into the housing thus serves both to cool the magnetrons and to dry the solids.

Description

Method and apparatus for the drying of free-flowing granular solids

The present invention relates to a method for the drying of free-flowing granular solids, such as grain, seeds, and sawdust, whereby the solids are conveyed from a feed bin via an inlet to a cavity, and thence to an outlet from the cavity to discharge into a discharge bin, and wherein electromagnetic waves are generated by at least one magnetron and directed into the cavity, and wherein Lhe magnetrons are cooled by causing a cooling medium to pass through the magnetron.

The invention is further concerned with an apparatus operated according to the method.

US Patent 4,347,670 discloses an apparatus for drying a granular product. The apparatus comprises a drying vessel with a central unit consisting of a magnetron, a waveguide, and two drying columns disposed around the central unit. The drying columns are intended to direct the granular from an upper part to a lower part of the drying vessel while the product is subjected to electromagnetic waves. The drying columns may be equipped with spiral vanes to ensure that the product is uniformly exposed to electromagnetic radiation. A control rotor at the bottom end of each drying column controls the residence time of the granular product in the drying columns.

However, this apparatus has a number of drawbacks. The capacity of the apparatus is limited inasmuch as it includes only one magnetron, and this magnetron has to effect the drying of product in both drying columns. This implies that a limited quantity of granular product can be dried in a given time, thus limiting the capacity of the apparatus. Cooling of the magnetron, the waveguide, and the tubular window is effected by means of a fan which draws air first into a cavity formed by the tubular window, then past the waveguides, and then past the magnetron. Because of this indirect cooling of the waveguide-radiators and the magnetron, adequate cooling is not assured, a circumstance which can result in breakdowns. Moreover, drying by means of this apparatus requires the setting up of an additional step in association with the conveying of the product between different processing steps of the product, such as from a filling step to a storage step.

German patent DE 42.31.897 discloses a method for the thermal treatment of a product, preferably granular solids, by microwaves combined with convection. The product is fed through an inlet to a cavity and thence to an outlet from the cavity. Electromagnetic radiation is generated by magnetrons and directed into the cavity. A gas flow is directed past the magnetrons, where it takes up the heat formed in the generation of the microwaves. From there the gas is conducted into a zone ahead of the microwave radiation, where thermal treatment of the product takes place.

This apparatus also suffers from a number of drawbacks. The vessel of the apparatus can give rise to standing waves resulting in non-uniform exposure to radiation. The product, which is carried on a belt conveyor, undergoes no mixing, which again results in non-uniform radiation exposure. The gas flow passing through the vessel has a tendency to bypass the product, particularly through thin layers and less moist regions, another major problem which is generally known in air dryers, particularly flat dryers, and which results in uneven drying of the product.

German patent DE 39.07.248 discloses an apparatus for the drying of granular solids by microwave radiation. The apparatus comprises a vessel having walls which form a cavity. The vessel is equipped with an inlet and an outlet and with a mixing/conveying means extending from the inlet to the outlet. Adjacent to the walls of the vessel there is at least one magnetron, so arranged that electromagnetic radiation can be directed into the cavity.

This apparatus, too, has a number of drawbacks. The vessel of the apparatus can give rise to standing waves, resulting in local exposure and local non-exposure to radiation. The mixing/conveying means has no positive conveying motion from the inlet to the outlet, with the result that some product resides longer in the vessel than other product, causing uneven drying. The gas flow through the vessel has a tendency to bypass the product, particularly through the upper part of the vessel where there is no product, and this necessarily reduces the thermal effect.

It is the object of the present invention to achieve a method and an apparatus that eliminate the drawbacks of fhe prior art, so that, among other things, the capacity and efficiency of the apparatus are improved.

This object is achieved by a method which, according to the invention, is characterised in that the cooling medium, after passing over the magnetron(s), enters the cavity at a point where electromagnetic waves are generated.

The object is further achieved by an apparatus for the drying of granular solids, such as grain, seeds and sawdust, such apparatus to comprise a vessel having walls which form a cavity with an inlet and an outlet, a conveyor means extending from the inlet to the outlet, and at least one magnetron arranged adjacent to the vessel wall in such a way that electromagnetic waves can be directed into the cavity; the characteristic feature of the apparatus according to the invention being that a housing is arranged around the magnetron(s) and that a cooling device is provided adjacent to the housing and arranged so that cooling medium can flow from the cooling device to the magnetrons and thence into the cavity at a point where electromagnetic waves are generated.

Directing the magnetron cooling air into the cavity where the granular product is heated by the electromagnetic waves achieves the end that the heated cooling air contributes to producing a rapid heating of the granular product while at the same time ensuring effective through circulation, thus promptly carrying off the moisture liberated from the granular product by heating thereof, which altogether results in particularly rapid drying with high efficiency.

A preferred embodiment of the apparatus of the invention is characterised in that the cavity is located inside a cylinder, that the conveyor means is a screw disposed along a core shaft extending through the cylinder, that the magnetrons are located immediately adjacent to the cylinder, that the housing extends from an outer end as far as the cooling means, partially surrounds the cylinder and covers the magnetrons, and that the cooling means is designed to direct cooling medium from the outer end of the housing to the cooling means.

By the use of a screw conveyor and by a suitable design of the cylinder, the screw flights and the core shaft, an exactly calculated cavity can be formed which acts as a rotating waveguide for electromagnetic waves, whereby the problem of standing waves does not arise. The rotation of the screw causes effective mixing of the granular product whereby the moisture from the product is easily given up to the introduced cooling medium. The mixing further ensures that the product is subjected to uniform, controllable heating, which is of great importance particularly for e.g. the germination capacity of grain.

According to the invention the cooling device may consist of a first fan arranged between the inlet to the cavity and the magnetron or the first magnetron, and the housing may be provided with air slits for the entry of fresh air. By introducing the magnetron cooling air into the screw after the inlet and directed downstream along the conveying path, excellent and uniform air circulation through the product is obtained during the entire heating cycle until it reaches the outlet. In order to prevent the air flow from escaping from the cavity via the inlet, the apparatus may appropriately be designed as described in claim 5. The vaned rotor which serves as a reflector for air and for electromagnetic waves may if desired also be used for measuring of the product.

By designing the apparatus according to the invention as described in claim 6 one can eliminate the reflection of electromagnetic waves out of the cavity, whereby the electromagnetic energy is transferred to the product alone. The waveguides are suitably arranged so that the centre of the electromagnetic waves is directed towards the centre of gravity of the granular product and propagates over the entire length of the screw conveyor.

A housing is arranged around the magnetrons for safety shielding and also to permit the cooling of the magnetrons. Cooling ducts are fitted inside the housing to direct the cooling air through the magnetrons and then to an inlet to the cavity.

The system is designed on the basis of the material constants of the product and to obtain the most appropriate and optimal efficiency of the electromagnetic waves accordingly.

The apparatus of the invention possesses therfurther advantage that it is not necessary to set up additional steps in association with the drying of the product, because drying takes place in conjunction with the conveying of the product between various other process steps such as feeding and discharge.

The apparatus is further characterised by using, for preference, traditional equipment for the conveying of product, such traditional conveying equipment being modified to accommodate the drying of the product by means of electromagnetic waves. However, said modification is effected so that the conveying of the product between other process steps is not slowed down. The apparatus is further characterised in that it is quiet, economical of energy, and mobile.

The apparatus will be more particularly described below with reference to the accompanying drawings, where:

Fig. 1 is a schematic representation of a first embodiment of an apparatus according to the invention for operation by the method of the invention,

Fig. 2 is a schematic representation of a preferred embodiment of a conveyor and a preferred arrangement of magnetrons, housing, and waveguides,

Fig. 3 is a representation of a preferred embodiment of a waveguide,

Fig. 4 is a representation of a preferred embodiment of a discharge pipe and/or a feed pipe, and

Fig. 5 is a representation of another embodiment of an apparatus according to the invention, equipped with a vaned rotor.

Fig. 1 illustrates a preferred embodiment of an apparatus according to the invention. The apparatus is well suited to operation by the method of the invention. The apparatus comprises a feed bin 1 , a conveyor 2, and a discharge bin 3. The conveyor 2 is provided with a housing 4 partially surrounding the conveyor 2. Magnetrons 5 are arranged adjacent to the conveyor and under the housing. A first fan 6 is mounted between a feed pipe 13 and the magnetrons 5. A second fan 7 is mounted above the discharge bin 3. The feed bin 1 is provided with a funnel-shaped bottom 10. A feed pipe 13 is arranged between the feed bin 1 and an inlet 12 to the conveyor 2. The discharge bin 3 is likewise provided with a funnel-shaped bottom 14 having perforations 15. A discharge pipe is arranged between an outlet 16 from the conveyor 2 and the discharge bin 3. Granular product 20 to be dried is fed from the feed bin 1 through the feed pipe 13 to the inlet 12 into the conveyor 2. The product 20 is conveyed by the conveyor 2 from the inlet 12 to the outlet 16. During its passage from the inlet 12 to the outlet 16 the product is subjected to electromagnetic waves and dried thereby. The electromagnetic waves are generated by the magnetrons 5. After drying, the product 20 is fed from the outlet 16 through the discharge pipe 17 to the discharge bin 3.

The first fan 6, which is located between the inlet pipe 13 and the magnetrons 5, draws air through air slits 21 formed in the housing 4. The air drawn into the housing through the air slits 21 is directed through the magnetrons 5 and then to an air inlet 23 of the conveyor 2. In the process there is achieved, first, a cooling of the magnetrons 5 and, subsequently, a thermal treatment of the granular product 20 and a circulation of moist air through the granular product 20 in the conveyor. The air drawn in through the air slits 21 in the housing 4 thus serves both to cool the magnetrons 5 and to dry the granular product 20 and for air circulation. In other words, the drying of the granular product 20 is effected by a combination of electromagnetic waves generated by the magnetrons 5 and an air circulation system driven by the fan 6.

Air is drawn through the funnel-shaped bottom 14 with perforations 15 by the fan 7, and moist air is drawn out of the discharge pipe 17, also by the fan 7. The air thus cools the granular product 20 and removes steam from the drying of the granular product 20.

Fig. 2 illustrates a preferred design of conveyor 2 in an apparatus according to the invention. The conveyor 2 comprises a screw 25, such as an Archimedean screw, disposed along a core shaft 26 in a cylinder 27. The cylinder, the core shaft and the screw flights form a cavity 28 which acts as a rotating waveguide. The housing 4 is arranged to enclose half of the circumference of the cylinder 27. Magnetrons 5 are arranged inside the housing 4 and a waveguide 30 is arranged on the cylinder 27. Between the waveguide 30 and the cavity 28 there is arranged a shield 29 of plastic, which is transparent to electromagnetic waves and permits the electromagnetic waves to enter the cavity 28 but prevents the granular product from escaping from the cylinder 27 at this point.

The magnetron 5 is mounted on the waveguide 30. The waveguide 30 is so shaped and arranged that the electromagnetic waves cannot be directed towards the shaft 26 and reflected therefrom back into the waveguide. The window between the waveguide 30 and the cavity 28 is provided with an opening for the electromagnetic waves which is smaller than a cross-sectional area of the waveguide. This ensures that electromagnetic waves, after passing from the waveguide into the cavity, are not reflected back into the waveguide. The screw 25 is provided with flights which are of a size to ensure that the electromagnetic waves are not impeded thereby and are of special importance for controlling the propagation of electromagnetic waves from the inlet 12 to the outlet 16. The conveying of the granular product 20 produces a uniform distribution thereof around the screw 25 in the cavity 28 such that all of the product 20 is exposed to uniform electromagnetic radiation and uniform air circulation and thus attains the same degree of drying.

Fig. 3 illustrates a preferred embodiment of a waveguide 30 for use in an apparatus according to the invention as illustrated in Fig. 2. The waveguide has a trapezoidal cross-section when viewed in a direction parallel to the shaft 26 (see

Fig. 2). A first surface 31 of the waveguide is designed to run parallel to a plane P_j parallel to the shaft 26. A second surface 32 is designed to run parallel to a plane P (see Fig. 2) also parallel to the shaft 26. This design and arrangement of the waveguide 30 ensures that the electromagnetic waves do not strike and reflect off the shaft 26 back to the magnetron 5. It further ensures that the window between the waveguide and the cavity can be provided with an opening for electromagnetic waves which is smaller than the cross-sectional area of the waveguide. In an alternative embodiment the waveguide is reflected and rotated 180° so that the first surface is on the right and the second surface on the left in relation to the representation in the figure.

In a preferred embodiment of an apparatus according to the invention the waveguide 30 has a height of 34.5 mm and a square window with a width b of 80 mm. The angle θ between a lower flange 33 and the first surface 31 is 120' , and the angle β between the lower flange 33 and the second surface 32 is 100^* .

In the illustrated embodiment the waveguide is made of steel.

Fig. 4 illustrates a preferred embodiment of a discharge pipe 17 for an apparatus according to the invention. In the illustrated embodiment the pipe 17 is of square section and is interiorly provided with sheet elements 34 extending parallel to a longitudinal axis A of the pipe 17. The sheet elements 34 are designed to prevent electromagnetic waves from escaping from the discharge pipe 17.

The sheet elements 34 and the pipe 17 together form subsections 35 within the total cross-section L of the pipe. The total area of the subsections 35 is approximately equal to the area A of the cross-section L of the pipe 17. The capacity of the pipe 17 is thus not substantially changed by the sheet elements 34.

The embodiment of the apparatus of the invention shown in Fig. 5 is in many respects identical with the embodiment shown in Fig. 1. Thus, Fig. 5 shows a feed bin 1 which is connected via a feed pipe 13 to an inlet 12 to a conveyor 2, 27, wherefrom a discharge pipe 17 leads from an outlet 16 to a discharge bin 3. Unlike the embodiment in Fig. 1 , the fan 6 is attached directly to the cylinder 27 in the vicinity of the lower end thereof. Furthermore, in the feed pipe 13 a vaned rotor 40 is provided comprising reflectors for air and electromagnetic waves, which thus cannot escape through the feed pipe 13.

The description of Fig. 5 might be somewhat expanded with further indications of the particular advantages achieved by this embodiment.

The invention is described above with reference to a specific embodiment of an apparatus according to the invention and of specific parts of this apparatus. It is possible to develop other embodiments of the apparatus and of the various parts thereof. The design of the waveguide and of the feed and discharge pipes can be modified, and the entire apparatus can be composed of other or additional parts.

Method according to the apparatus.

Claims

1. A method for the drying of granular products, such as grain, seeds and sawdust, whereby the product (20) is fed from a feed bin (1) through an inlet (12) to a cavity (28) and through said cavity to an outlet (16) therefrom to discharge into a discharge bin (3), and wherein electromagnetic waves are generated by at least one magnetron (5) and caused to enter the cavity (28), and wherein the magnetrons (5) are cooled by conducting a cooling medium past the magnetrons, characterised in that the cooling medium, after being conducted past the magnetrons, is introduced to the cavity (28) at a point where electromagnetic waves are generated.

2. An apparatus for use in the drying of granular products, such as grain, seeds or sawdust, comprising a vessel (2, 27) with walls which form a cavity (28) having an inlet (12) and an outlet (16), a conveyor means (25) extending from the inlet (12) to the outlet (16), and at least one magnetron (5) arranged adjacent to the vessel wall (2) so that electromagnetic waves can be directed into the cavity (28), characterised in that a housing (4) is arranged around the magnetrons (5) and that a cooling device is provided adjacent to the housing so that cooling medium can be conducted from the cooling device to the magnetrons and then into the cavity (28) at a point where electromagnetic waves are generated.

3. An apparatus according to claim 2, characterised in that the cavity (28) is located inside a cylinder (27), that the conveyor means (25) is a screw extending along a core shaft (26) through the cylinder (27), that the magnetrons (5) are arranged immediately adjacent to the cylinder (27), that the housing (4) extends from an outer end as far as the cooling device, partially surrounding the cylinder (27), and covers the magnetrons (5), and that the cooling device is designed to conduct the cooling medium from the outer end of the housing to the cooling device.

4. An apparatus according to claim 3, characterised in that the cooling device consists of a first fan (6), which is located between the inlet (12) and the first magnetron (5), and that the housing (4) is provided with air slits (21) to admit fresh air.

5. An apparatus according to any of claims 2 to 4, characterised in that a feed bin (1) is provided with a feed pipe (13) which extends from the bottom of the feed bin (1) to the inlet (12) to the cavity (28), and that the feed pipe (13) is provided with a vaned rotor (40) comprising reflectors for air and electromagnetic waves.

6. An apparatus according to claim 3 or 4, characterised in that the cylinder (27) is of circular section, that the core shaft (26) extends concentrically through the cylinder (27), that the magnetrons (5) are provided with waveguides (30) so formed and arranged that the electromagnetic waves are directed around the core shaft (26).

7. An apparatus according to claim 6, characterised in that the waveguide (30) has a trapezoidal section viewed in a plane peφendicular to the shaft (26), and that one interior surface of the waveguide, disposed parallel to the shaft, lies in a plane that does not intersect the core.

8. An apparatus according to any of claims 2 to 6, characterised in that a discharge bin (3) is equipped with a discharge pipe (17) extending from the outlet (16) from the cavity (28) to the discharge bin (3), and that the discharge pipe (16) is equipped with sheet elements (34) constituting reflectors for electromagnetic waves.

9. An apparatus according to any of claims 2 to 8, characterised in that the discharge bin (3) is equipped with a second fan (7) which is disposed and arranged produce air circulation through, and cool, any granular product (20) in the discharge bin.