WO1996034240A1

WO1996034240A1 - Method and device for heating and/or drying with an mhf-field of powdery, granular and/or pasty materials in a conical mixing vessel

Info

Publication number: WO1996034240A1
Application number: PCT/NL1996/000186
Authority: WO
Inventors: Antonius Maria Van Aken
Original assignee: Hosokawa Micron B.V.
Priority date: 1995-04-28
Filing date: 1996-04-27
Publication date: 1996-10-31
Also published as: JPH11504702A; NL1000248C2

Abstract

Method and device for heating and/or drying of powdery, grainy and/or pasty materials with an MHF-field in a conical mixing vessel (2) having a vertical axis, which vessel narrows in a downward direction, and in which the material is mixed by at least one mixing screw (3) making a rotary movement around its axis and a revolving movement parallel to the innerside of the mixing vessel (1), whereas the material to be heated and/or dried is irradiated by at least one HF-field. The invention is characterized by radiating an MHF- or RF-field substantially completely inside the material to be heated and/or dried, which field has a frequency of 10 to 50 MHz, preferably 27 MHz, and is rotated around the vertical symmetry axis of the vessel (2) with the same rotation speed as the revolving speed of the mixing screw (3). The impedance of the antenna (4) for the MHF-field is preferably automatically adapted to the alteration of the impedance of the material to be heated and/or dried.

Description

Method and device for heating and/or- drying with a MHF- field of powdery, granular and/or pasty materials in a conical mixing vessel

The invention relates to a method for heating and/or drying of powdery, grainy and/or pasty materials in a conical mixing vessel having a vertical axis, which narrows in a downward direction and in which the material is mixed by at least one mixing screw, which makes a rotating movement around its axis and a revolving movement parallel to the innerside of the vessel wall, whereas the material to be heated and/or dried is irradiated by at least a HF-field.

A method of this type is known from EP 0 306 563. Herewith one or more microwave generators or magnetrons are present on top of the lid of the conical mixing vessel. The antennae of these microwave generators extend into the vessel, but stay out of contact with the material to be dried. These antennae radiate from the lid of the vessel a UHF-field with a frequency of 900- 2450 MHz or more to the free upper surface of the material to be dried, which is held into movement by one or more mixing screws. The microwave radiation heats and evaporates the solvent which is present in the material, so that a quick drying is obtained.

The object of the invention is adapting this known method in such a way, that it is made suitable for drying of materials, which are very sensitive for microwave radiation and could easily change their characteristics in a undesirable way.

According to the invention this object is attained, by radiating a MHF- or RF-field substantially completely inside the material (M) to be heated and/or dried.

Such a MHF- or RF-field has a frequency of 3-150 MHz. According to a main embodiment of the invention the „„_jn 6/34240

MHF-field has a frequency of 10 through 50 MHz, preferentially 27 MHz.

The absorption of energy for drying is the result of the product of the dielectric constant of the material to be dried and the dielectric constant of the material to be heated and/or dried and the dissipation factor. This product is the loss factor and if it is greater than 0,05, the material can be heated dielectrically, such as for instance water and many alcohols. This MHF- or RF-field does not cause any damage of materials which are sensible for UHF radiation and allows the efficient heating and/or drying of the material. By the quick changes of the polarities heat is generated in the material to be heated and/or dried. According to a preferential embodiment of the invention the MHF- or RF-field is rotated around a vertical symmetry axis of the vessel with the same rotation velocity as the revolving velocity, which is connected with one or more electrically conductive antennae mounted inside the mixing vessel, which antennae are intended to extend substantially complete inside the material to be dried, whereas the conductors outside the mixing vessel are connected to the HF-generator. In this way a very good heating and/or drying of the material is attained.

_Special embodiments of the invention are described in the subclaims 6 through 31.

The invention will now be elucidated referring to the accompanying drawing of some non-limitating embodiments.

Fig. 1 shows a vertical sideview of the drier according to the invention.

Fig. 2 shows the drier of fig. 1 together with the electrical and the vacuum circuit. Fig. 3A and 3B show in cross-section and sideview a conical antenna with two symmetrical sectors.

Fig. 4 shows the electrical circuit of the impedance adaptor. Fig. 1 shows a heated mixer/drier according to the invention which is generally indicated with 1. It comprises a steel conical mixing vessel 2 having a vertical symmetry axis, and a mixing screw 3, which at the one hand rotates around its own axis and at the other hand revolves parallel to the sidewall of the vessel 2. The electrical motordrive 6 mounted on top of the concave lid 8 of the vessel is for driving the mixing screw 3 and thus for mixing of the material M to be dried. Vacuum is drawn on the mixing vessel 2 at the tube stub 9. The material M to be treated is supplied to the right tube stub on the mixing vessel having a releasable and mountable lid 8. At the lower side of the mixing vessel 2 there is an outlet having a valve 7 for the treated, that is to say heated and/or dried material M. According to the embodiment of fig. 1 and fig. 2 an isolated coaxial cylindrical electrical conductor 5 has been mounted in the central symmetry axis of the mixing vessel 2, which conductor has been connected inside the mixing vessel with an electrical conductive cylindrical HF antenna 4. The antenna 4 has been connected to a MHF- or RF-source outside the vessel 2.

Such a MHF- or RF-source has generally a frequency of 3-150 MHz. In this embodiment the frequency amounts to 10 through 50 MHz, preferably 27 MHz. The antenna 4 and the mixing vessel 2 form the metal plates of a capacitor between which the MHF- or RF-field caused by the MHF- or RF-source 11 is present. This causes the desired heating and/or drying of the material M which is held into movement by the mixing vessel 3, so that the treatment happens homogeneously and there is no local overheating.

Fig. 2 shows the antenna 4 in the vessel 2. The MHF-source 11 is at one hand connected to the antenna 4 through an impedance adaptor 10 and at the other hand to the vessel 2.

The impedance adaptor 10 serves to adjust the impedance according to the further heating and/or drying of the material M. In this connection fig. 2 shows a vacuum controller 16, which serves to control the level of the vacuum in the mixing vessel together with a computer PC.

At the device shown in fig. 1 and 2 the mixing screw 3 is constantly in the HF field. When this mixing screw is as usually from metal, a HF voltage will occur, which may raise until 6.000 Volt. When the MHF- or RF-field is strong and the mixing screw 3 is closed with a wall of the mixing vessel 2, there is danger for producing sparks from the outer edge of the helical edge along the mixing screw 3 to the vessel wall 2. This can be counteracted by making the mixing screw 3 from an isolating material, such as plastic or ceramics, or provide a metal mixing screw with a cover of plastic, ceramics or another isolated cover. Suitable ceramic materials are formed by aluminiumoxyde, porcelain, steatyte, and titaniumdioxyde. Suitable plastics are formed by polytetrafluortereftelate (TEFLON, trademark of DuPont), polyethylene or polypropylene.

Another phenomenon is, that as a result of the HF tension on the mixing screw 3 a HF current will flow via the operating arm of the mixing screw 3 through the bearings to the return conductor of the generator 11. This current can take values of some hundreds of amperes at high tension of the HF-field. Such a current will, when no special measures have been taken, be destructive for the bearings of the driving device of the mixing screw. This gives a limitation of the HF-tension which can possibly be used.

These problems are solved by the next embodiment of the invention. In fig. 3A one sees a coaxial tube 12,

13, 14 as electrical conductor. The innertube 12 extends through the isolation 13 to the antenna 4A and the outertube 14 to the antenna 4B. A and B are connected to the vessel 2 and the antenna 4 respectively. The outside of the outertube 14 is provided with isolation 15. The antenna 12, 14 is also provided with isolation 13 between the innertube 12 and the outertube

14. This isolation 13 extends somewhat further with regard to said outer isolation 15, to prevent sparking, especially in connection with the HF-tension which can reach values until 6.000 Volt.

In fig. 3A one sees also, that if the isolation 13 consists of parts, these are made interlocking, in order to prevent creaping paths. The isolation 13 between both tubes (the innertube can eventually be solid), is preferentially polyetherfluortereftelate (Er = 2,1), polyethylene (Er = 2,34) or polypropylene (Er = 2,4).

The sleeve 16 is mounted around the coaxial conductors 12, 14. This sleeve 16 is connected at the upperside with the outer tube 14 of the coaxial conductor 12, 14. At the lower side, close to the antenna connection A, B this sleeve has not been connected. When this outer sleeve 16 is brought into resonance at the frequency of the generator, then the impedance between this sleeve 16 and the outer tube 14 of the coaxial conductor 12, 14 will become infinitively high. This gives a substantially ideal balancing of the antenna device. Bringing the sleeve 16 into resonance can be achieved by giving it a length of a quarter wave length. The same cross-section as in fig. 3A is visible in fig. 4A. The outer sleeve 16 has a length L, which should be electrically a quarter wave length. The mechanical length L of this outersleeve 16 becomes at a frequency of 27 MHz equal to 2760 mm/Er. For polyethylene for instance, this length is 2760/2,34 = 1804 mm. Such a length is therefore dependent on the isolation.

In order to reduce the length a lower part of the isolation material 15 can be replaced by ceramic material, such as aluminiumoxyde, porcelain, steatite or titaniumoxyde, with still higher Er values.

A possible fine adjustment of the sleeve 16 can be realised very well by using a dielectric material having a high dielectic constant in the shape of the tube 15 between the sleeves 16 and the outer tube 14 of the coaxial conductor 12, 14.

The position of the dielectric tube influences the setting frequency of the sleeve.

This antenna device 4A, 4B has as object to reduce to a minimum the tension of the mixing screw. The antenna 4 therefore comprises a system of two antennae 4A, 4B which are symmetrically ranged with respect to the mixing screw 3. The tension on the two antennae 4A, 4B is equal, but in counterphase. Therefore no tension will build up on the mixing device and there is also no bearing current. In practice it has shown, that the surpression of the bearing current with respect to a non- symmetrical antenna device will amount to more than a factor 10, which results in a maximum supplied power of more than a hundred times in comparison with the non- balanced antenna. The disadvantages of the central antenna have therefore disappeared. A possible embodiment of such an antenna device is shown in fig. 2. The antenna 4A, 4B comprises two conical segments which are connected to the coaxial conductors 12 and 14. Furthermore in fig. 3A an embodiment is visible of a construction of the lead through of the outer tube 4 and the inner tube 12, with inner isolation 13 and outer isolation 15. According to fig. 3A, B the antennae 4A, B can have the shape of a cone or of hollow cone sectors. Preferably the cone angle in fig. 3A, B equals the cone angle of the mixing vessel 2. In this case the antenna 4 is rotary driven, for instance my means of a horizontal arm, on which the upper end of the mixing screw 3 is fastened. In this way at least one mixing screw 3 can be used between the antenna sectors 17.

As the antenna 4 is in a closed vessel 2 and the contents of this vessel being the material M with unknown characteristics for HF-fields, the impedance of the antenna is unknown. Furthermore the characteristics of the material will change and therefore also the impedance, during the proceding of the drying process. This alteration of the impedance can cover an important interval.

In order to ensure, that the HF energy is transferred to the material with a good efficiency, it is necessary to connect an antenna tuner between the generator and the antenna. This antenna tuner transforms the variable antenna impedance to a real value of 50 Ohm.

The antenna tuner or impedance adaptor comprises a circuit of a first variable capacitor 18, connected to the antenna line 17, a spool 19 and a second variable capacitor 20. The complete tuning procedure can be done by hand. It is also possible to have it done automatically.

To this end the impedance adaptor comprises a second circuit with a sensor circuit 21, which turns the first variable capacitor 18 of the first circuit by means of a control amplifier 22 and a motor 23. Also a second sensor circuit 24 is present, which controls a second motor 26 through a second control amplifier 25, whereas the motor 26 turns the second variable capacitor 20 of the first circuit 17, 18.

The transformation to 50 Ohm now takes place automatically.

Claims

C L A I S

1. Method for heating and/or drying of powdery, grainy and/or pasty materials in a conical mixing vessel having a vertical axis, which narrows in a downward direction and in which the material is mixed by at least one mixing screw, making a rotary movement around its axis and a revolving movement parallel to the innerside of the wall of the vessel, whereas the material (M) to be heated and/or dried is irradiated by at least a HF-field, characterised by radiating a MHF- or RF-field substantially completely inside the material (M) to be heated and/or dried.

2. Method according to claim 1, characterised by radiating a MHF- or RF-field with a frequency of 10- 50 MHz, preferably 27 MHz.

3. Method according to claim 1 or 2, characterised by rotating the MHF- or RF-field around the vertical symmetry axis of the vessel (2) with the same rotation velocity as the revolving velocity of the mixing screw.

4. Method according to claim 1, 2 or 3, characterised by adapting, preferably automatically, the impedance of the antenna for the MHF-field to the alteration of the impedance of the material (M) to be heated and/or dried.

5. Method according to claim 1 through 4, characterised by concurrently applying a vacuum V on the material (M) to be heated and/or dried by the MHF- or RF-field.

6. Device for performing the method according to one or more of claims 1 through 5, which device (1) comprises a conical mixing vessel (2) having a vertical axis, which narrows in a downward direction and in which at least one mixing screw (3) makes a rotating movement around its axis and a revolving movement parallel to the innerside of the sidewall of the vessel (1), in which a HF-generator (11) is present having an antenna (4) in the vessel (1) for radiation of a HF-field to the material (M) characterised in that one or more electrical coaxial conductors (5) extend through the central axis of the mixing vessel (1) into the mixing vessel (1) said conductors (5) being connected to electrically conductive antennae (4) inside the mixing vessel (1), said antennae (4) being adapted to extend substantially completely inside the material (M) to be heated and/or dried, whereas the conductors (5) are connected to a MHF-generator (11) outside the mixing vessel (1).

7. Device according to claim 6, characterised in that, the electrical conductors are inserted from the upperside of the mixing vessel (2).

8. Device according to claim 6 or 7, characterised in that an outlet having a valve (7) is present below the mixing screw (3) at the bottom of the mixing vessel (2).

9. Device according to one or more of claims 6, 7 or 8, characterised in that a vacuum connection (9) is present on the vessel (2) for applying a vacuum on the material (M) to be heated and/or dried.

10. Device according to one or more of claims 6 through 9, characterised in that, the one or more antennae (4) are cylindrically shaped.

11. Device according to one or more of claims 6-10, characterised in that, the antennae (4) have the shape of a number of hollow cone sectors (4A, B), which are symmetrically arranged with respect to the mixing screw (3) and are each connected separately, but each in counterphase, through two coaxial conductors (12, 14) on the MHF-generator (11), whereas the coaxial conductors (12, 14) are surrounded by a sleeve (16) which is connected with the outer tube 14 of the coaxial conductors.

12. Device according to one or more claims 6 through 10, characterised in that, the cone angle of the antennae (4A, B) being substantially equal to the cone angle of the mixing vessel (2).

13. Device according to one or more of claims 6 through 12, characterised in that, the antennae (4A, 4B) are rotary driven.

14. Device according to one or more of claims 6 through 13, characterised in that, the antennae (4A, 4B) are rotary driven by means of a horizontal arm (3B), on which the upper end of the mixing screw (3) is mounted, and revolve herewith.

15. Device according to one or more of claims 6 through 14, characterised in that at least one mixing screw (3) extends between the sectors of the antennae (4A, 4B), whereas the rotating movement of the antennae (4A, 4B) around the vertical axis of 12

the mixing vessel (2) being equal to the revolving movement of one or more mixing screws (3) around the vertical axis of the mixing vessel (2) and keeping an equal distance from it.

16. Device according to one or more of claims 6 through 15, characterised in that, the MHF-generator (1) is connected with the antennae (4A, 4B) into a circuit, which also comprises a tunable impedance adaptor (10), in which the impedance information of the antennae (4A, 4B) controls the output tension of the MHF-generator (11).

17. Device according to one or more of claims 6 through 16, characterised in that the impedance adaptor (10) comprises a circuit with a first variable capacitor (18) connected to the antenna line (17), a spool (19) and a second variable capacitor (20).

18. Device according to one or more of claims 6 through 17, characterised in that, the impedance adaptor (20) comprises a second circuit with a sensor circuit (21) which rotates the first varable capacitor (18) of the first circuit by means of a line amplifier (22) and a motor (23), whereas the second sensor circuit (24) controls a second motor (26) through a second line amplifier (25), which turns the second variable capacitor (20) of the first circuit.

19. Device according to one or more of claims 6 through 18, characterised in that, the impedance adaptor (10) sets a value of the impedance of substantially 50 Ohm.

20. Device according to one or more of claims 6 through 19, which is connected to a vacuum source (V) that is connected to the upper lid (9) of the mixing vessel (2), characterised in that, the impedance adaptor (11) is connected to, and delivers impedance information of the antenna, to a computer (PC) which in turn is connected to a vacuum controller (16) which controls the level of the vacuum in the mixing vessel 2 through the vacuum source (V).

21. Device according to one or more of claims 6 to 20, characterised in that at least one part of the mixing screw (3) extending into the mixing vessel (2) is covered with isolating material.

22. Device according to claim 21, characterised in that, the isolating material of the mixing screw (3) comprises a ceramic material.

23. Device according to claim 21, characterised in that, the isolating material of the mixing screw (3) comprises a plastic.

24. Device according to one or more of claims 6 through 23, characterised in that, the frequency of the MHF-generator (11) is 10 to 50 MHz, preferably 27 MHZ.

25. Device according to claim 23, characterised in that, the plastic is formed by polytetrafluortereftelate (TEFLON) .

26. Device according to claim 23, characterised in that, the plastic is formed by polyethylene.

27. Device according to claim 23, characterised in that, the plastic is formed by polypropylene.

28. Device according to claim 22, characterised in that, the ceramic material is formed by a1uminiumoxyde.

29. Device according to claim 22, characterised in that, the ceramic material is formed by porcelain.

30. Device according to claim 22, characterised in that, the ceramic material is formed by steatite.

31. Device according to claim 21, characterised in that, the ceramic material is formed by titaniumdioxyde.