WO1989010678A1

WO1989010678A1 - Improved microwave treatment apparatus

Info

Publication number: WO1989010678A1
Application number: PCT/AU1989/000171
Authority: WO
Inventors: Van Nguyen Tran
Original assignee: Deakin University
Priority date: 1988-04-19
Filing date: 1989-04-19
Publication date: 1989-11-02
Also published as: EP0364571A4; EP0364571A1

Abstract

A microwave heating system comprising a multi-sided support (1; 9 to 13) supporting a multiplicity of microwave applicators (2 to 4; 14 to 18), a tube (8; 24a; 40) defining a treatment zone (T) for confining the material to be treated, generally concave reflectors (5 to 7; 19 to 23) supported in each corner of said support (1; 9 to 13) for reflecting microwave energy passing through said zone (T). The tube (24a) is supported for rotation and includes vanes (24c) for agitating the material in the tube (24a). The tube is surrounded by an outer tube (24a) and the space between the tubes is filled with a material having dissimilar dielectric properties to cause matching of the microwave applicators with the material to be treated. In another aspect, means for heating a moving strip (S) comprising one or more microwave applicators (32) in an over or underlying relationship to said strip (S) such that the long axis of the microwave pattern (usually elliptical) produced by at least said one applicator is in a direction transverse to the direction of travel of said strip (S).

Description

TITLE: IMPROVED MICROWAVE TREATMENT APPARATUS Field of the Invention: This invention relates to improvements in microwave treatment apparatus of the type described in our Australian Patent No. 565393 and corresponding United States Patent No. 4,631,380 Tran. Background of the Invention: In the above Australian and United States patents, we describe an improved system for the microwave treatment of materials in which the material to be treated is confined in a generally circular form and microwave sources are arranged around the material to be treated in a generally offset manner, the radiation emitted by said microwave sources being essentially non-coherent. Further research has shown that the offset nature of the microwave sources and the non-coherent nature of the radiation provides a particularly effective treatment system which avoids the problem of hot spots present in the known microwave treatment systems using coherent radiation. The odd-sided support for the microwave sources described in the above patents provides a convenient method of offsetting the sources to reduce coupling effects, with the opposed corners of the support acting as radiation reflectors for directing radiation which passes through the material to be treated. However, still further research has shown that such corner reflectors, while being convenient from a manufac uring point of view, do not provide the optimum reflection of the radiation passing through the material to be treated. Summary of Invention and 0bjects: It is therefore an object of the present invention to provide an improved microwave treatment system in which the above problems are at least ameliorated. Accordingly the invention in one aspect provides a system for the microwave treatment of materials comprising a treatment zone including means to confine the material to be treated in at least a partly cylindrical form, radiation means spaced from and outside said treatment zone for directing microwave radiation into said zone along at least three different axes of radiation, said radiation means being arranged in spaced relation around said zone with said axes of radiation being in spaced relation to each other at the radiation means, means for reflecting the microwave radiation directed along each axis back into^' the treatment zone after it passes through said zone, said microwave radiation being non-coherent, characterized in that said reflecting means has a generally concave curved form having at least one radius of curvature which lies within said treatment zone. By using a concave curved reflector, which is preferably part-circular in form, the radiation passing through the treatment zone is more effectively reflected back into the treatment zone, thereby improving the efficiency of the treatment system. The use of generally circular reflectors also enables the radiation means to be supported in closer proximity to the material to be treated in the treatment zone, thereby further improving the efficiency of operation of the system. The generally circular reflectors may be provided by a portion of a circular wall which serves to support the radiation means in spaced relation around the treatment zone containing the material to be treated. This form of the invention provides a particularly simple supporting structure which also provides the advantages of generally circular reflectors as discussed above. As described in our earlier patents, the treatment zone may be defined by a length of low-loss tubing which allows the material to be treated to be contained and transported through the treatment zone. The tubing may be supported for rotation about any desired axis and may include agitating means such as short angularly portioned vanes or auger means supported by the inner surface of the tube. In a further improvement embodying the present invention, the thickness and dielectric properties of the tubing may be selected to reduce reflection to thereby improve the matching of the microwave energy to the material to be treated. Al ernatively, the thickness of the tubing is selected so as to be sufficiently thin that the thickness of the tubing has little effect on microwave reflection. In a preferred form, the thickness of the material is selected to be one-half of the wavelength of the microwave radiation treating the material contained in the tubing. In a preferred arrangement, the tube may be a composite tube comprising two or more different materials, which may be separated by a further material, including air. By selecting the materials according to the dielectric constant of the material to be heated, the dielectric properties of the material to be heated may be matched to the microwave applicator supplying microwave energy to the composite tube. Depending on the effect desired, the dielectric properties of the different materials may increase from the outer tube to the inner tube or alternatively they may decrease from the outer tube to the inner tube. While the cheapest and most convenient material for the further material separating the inner and outer tubes is air, the material may be a liquid, such as a non-absorbing saline solution or oil, or may be a solid material such as polystyrene. In the treatment of strips or other generally planar lengths of material, or in the treatment of materials capable of being supported by a generally microwave transparent conveyor, it is common to pass the strip or conveyor under a support on which one or more microwave applicators are already entered such that the long axis of the radiation pattern (usually elliptical in shape) extends in the direction of travel of the strip or conveyor. This results in non-uniform heating of the strip or conveyed material such that undesirable hot spots or cold spots develop. While complex and expensive microwave heating arrangements which are claimed to uniformly heat material supported on a conveyor have been proposed, such arrangements are extremely expensive and are not economically viable when applied to the heating of many diff erent products . I t i s ano t her o b j e c t o f t h e p r e s en t i n v e n t i o n t o provide an improved microwave heating apparatus in which the above problems are ameliorated in a way which does not result in an expensive apparatus. According to another aspect, the invention provides a microwave heating system for a continuously moving strip or other generally planar object or for a conveyor supporting a material to be heated, comprising means for supporting one or more microwave applicators in an overlying and/or underlying relationship to said strip or conveyor, characterised in that at least said one microwave applicator is supported such that the long axis of the microwave energy pattern produced thereby is oriented in a direction which is transverse to the direction of travel of the material to be heated. In a preferred form of this aspect of the invention, one or more microwave heaters having a multi-sided microwave applicator supporting structure is arranged so that one part of the supporting structure is positioned on one side of the strip or conveyor belt while the other part of the supporting structure is positioned on the other side of the strip or conveyor belt, each microwave applicator being supported by one side of said support structure in a manner such that the axis of radiation of each applicator intersects the material to be heated before intersecting the axis of radiation of another microwave applicator. In one form, a multiplicity of spaced microwave applicators are supported along each side of the support structure, the applicators on adjacent sides of the supporting structure being offset with respect to preceding applicators by an amount equivalent to an appropriate fraction such as one- quarter of the wavelength of the microwave radiation generated by each microwave applicator. Where a series of microwave heating systems of the above type are positioned in tandem along the length of the strip or conveyor belt, the applicators of adjacent heaters may be offset by one-quarter of the wavelength of the radiation emitted by each applicator to ensure uniformity of heating. Brief Description of the Drawings: Presently preferred embodiments of the present invention are shown schematically in the accompanying drawings in which: Figure 1 is a sectional plan view of a three sided treatment system; Figure 2 is a sectional plan view of a five sided treatment system; Figure 3 is a sectional elevation of the treatment systems of Fig. 2 showing the method of transport of the material to be treated; Figures 4 to 7 are schematic perspective views showing various forms of microwave heating systems for heating strips or material carried by conveyors, and Figure 8 is a sectional end view showing a composite tube for the material to be heated. Referring firstly to Figure 1 of the drawings, the microwave treatment system shown schematically will be seen to comprise a three sided supporting structure 1 supporting three microwave applicators 2, 3 and 4, capable of emitting coherent or non-coherent microwave radiation into the structure 1." The supporting structure has three part- circular reflectors 5, 6 and 7 positioned opposite the applicators 2, 3 and 4 respectively. The reflectors may be integral parts of the wall or, as shown in Fig. 1, separate items. Each reflector 5, 6 and 7 has a radius of curvature r which has its centre located coincident with the centre of the treatment zone T, which in the present embodiment, is defined by a dielectric tube 8, although a radius of curvature having its centre located anywhere within the zone T will produce acceptable results. The tube 8 is made of low loss dielectric material such as glass, fused quartz, fibreglass, Teflon (registered trade mark), alumina and the like. As will be appreciated best from Figure 2 of the drawings, the part-circular reflectors 5, 6 and 7 operate to reflect any radiation passing through the treatment zone T directly back into the treatment zone, focusing at the centre of the treatment zone. In this way, interference between the microwave applicators is kept to a minimum. To reduce the reflection of radiation caused by the dielectric tube 8 containing the material to be treated in the treatment zone T, the thickness of the tube should be one-half of the wavelength of the radiation emitted by the applicator. A similar end may be achieved by applying pads of similar or different dielectric material to the outer surface of a thinner tube 8 to increase the thickness of the tube at the position of entry of the microwave radiation to one-half of the wavelength of the radiation. In this way, sources having different wavelengths may be accommodated by changing the pads. Referring now to Figure 2 of the drawings, one preferred embodiment of the system comprises a support structure having five sides 9 to 13, each of which centrally supports a microwave applicator 14 to 18. Part-circular reflectors 19 to 23 are positioned within the structure opposite the applicators 14 to 18, with the focus of each reflector being coincident with the centre of the treatment zone T defined by a tube 24 of low loss dielectric material such as glass, fused quartz, fibreglass, Teflon (registered trade mark), alumina and the like. To reduce the likelihood of hot spots developing in the material to be heated, and to transport the material through the treatment zone T, the tube 24 contains a further tube 24a having a multiplicity of angularly oriented vanes 24c attached to the inner surface of the tube 24a, The vanes 24c also assist in preventing agglomeration or packing of moist materials to be heated. An intermediate low-loss material 24b, which may be air or any one of the other materials described below in relation to Figure 8 of the drawings, is positioned between the tubes 24 and 24a to match the applicators 14 to 18 with the material to be treated in tube 24a, as described in greater detail in relation to Figure 8 of the drawings. As shown in Figure 3, the tube 24a is supported by rollers R rotatably mounted on a support frame F and engaging a bearing ring B attached to the tube 24a, and a drive collar C which is engaged by a belt drive or - 7 - gear drive (not shown) by means of which the tube 24a may be rotated in the direction shown in Figure 2 of the drawings. It will be noted that the vanes 24c are oriented angularly with respect to the axis of the tube 24a so that the material to be heated in the tube 24a is both circulated within the tube 24a and drawn into the tube 24a from a hopper H or the like, and discharged from the other end of the tube, the hopper may additionally have an auger (not shown) for assisting the feeding of certain materials, particularly moist materials such as soil and coal. By selecting the angle of orientation of the vanes 24c with respect to the axis of the tube 24a, and the speed of rotation of the tube 24a, a suitable residence time for the material to be heated may be achieved. The vanes 24c shown may be replaced by one or more continuous auger flights to achieve a similar results. The material from which the vanes 24c or auger flight are made is similar or identical to the material of the tube 24a, so as not to interfere with heating process. While the tube 24a is shown in Figure 3 to be generally horizontally positioned, the tube may be vertically oriented to allow gravity feed of the material, in which case the vanes need only perform a material circulating function. The space between the tubes may be used to cool the inner tube 24a by means of circulating air or some other suitable cooling liquid or gas such as nitrogen. Such an arrangement is suitable for the treatment of minerals at high temperatures, for example 1000°C, such as smelting operations, sintering materials such as SiC, ceramics or superconducting materials. Where the temperatures are likely to be higher, the space between the walls of the support structure and the tube(s) is filled with a suitable insulating material. In such an arrangement, the tube(s)/materia 1 s must be selected to allow for the additional material. Alternatively, the space may be evacuated to provide an insulating effect. In the modification of the arrangement shown in Figure 2, the vanes are removed to enable generally cylindrical objects, such as logs and the like, to be dried or heated for other purposes. In such an arrangement the log or the like will be pushed through the inner tube 24a to provide the required residence time. Rotation of the tubes is still desirable to ensure uniform heating. Such an arrangement may be used to initially dry a green log and to subsequently dry a log after chemical treatment. The use of such an apparatus has been found to provide a particularly efficient means of drying green and treated logs. Where the supporting structure for the microwave applicators is circular, the part-circular reflectors are provided by portions of the support. Again the material to be treated is confined within a low loss tube, which may be in the form shown in Fig. 1 or in Figs. 2 and 3. Where it is not desired that the radiation should be reflected in the manner described above, several alternative methods of causing absorption of the microwave radiation passing through the treatment zone T may be applied at each reflecting surface. The absorbing material may be solid, liquid, powder or gas and may be deployed permanently or in a removable manner when load conditions are expected to be discontinuous. The material is preferably selected so as to cause matching of the impedance of the applicator to the material to be heated, and making such a selection would be within the knowledge of a person skilled in the art. The positioning of absorbing material reduces the amount of reflection by the reflectors and thereby minimises damage to the magnetron of the microwave applicator caused by reflected radiation. Alternati ely, the reflectors may be constructed with passages into which liquid is pumped to increase the absorbing properties of the reflector, to achieve similar results. Referring now to Figure 8, a means of matching the microwave source and the material comprises a composite tube including outer and inner tubes 40, 41 of suitable low-loss material having dielectric properties or complex permiti ities c^ and £3, and a dissimilar material 42 of dielectric property . interposed therebetween. By appropriately selecting the thickness of the three materials corresponding to their dielectric properties, the composite tube will act as a matching device between the microwave energy applicator on the outside and the material to be heated inside the inner tube 40 to ensure that reflection is minimised and transmission of energy into the material to be heated is maximised. The composite tube should be designed for each material to be heated where efficiency is important. The composite tube therefore facilitates better transmission of the microwave energy and provides greater flexibility in the heating of different materials. The tubes 40 and 41 will for convenience usually be made of the same material, such as glass, Teflon, or any other of the low-loss materials described above, although the materials may be different to achieve different results. The material 42 is most conveniently (and cheaply) air, although the material may be polystyrene, Teflon, a non- absorbing liquid such as saline solution, oil or the like. If Teflon is used, the tube 41 could be of fused quartz, to match the tubes in the material to be heated. The inner and outer tubes 40 and 41 are usually thin (say 2 mm) while the material 42 is much thicker (say 16 mm). Where the complex permitivity of the inner and outer tubes is &^ = £3 = 2, and the material 42 has cT₂ = 4, the power transmission through the composite wall has been found to be about 90% at angles up to as high as ± 80°. This compares most favourably with a maximum angle of incidence of about ± 15° for arrangements in which a single tube is used to confine the material to be heated. The main advantages of the composite structure are therefore, high mechanical strength and low reflection at wide angles of incidence. To match the radiation from the applicator to the material to be heated having . _x , <_.\ has to be the geometrical average between the material inside the applicator (usually air = 1) and C_χ i.e. ε_χ =y£air £_x ~~ Jlϊ^ This method is usually restrictive: e.g. iZ_χ = 2.0 for Teflon to match a heated material having _χ = 4 - j 0.2 (0.2 being the loss factor, for a lossy material). The resulting thickness becomes, at 2450 MHz 12^2— __ §__X _{mm = 4>3 mm} 2 x /T ^~2 This is true for θ = 0°. For microwave radiation entering the tube at angles different from 0°, reflection will increase. To match a single tube with a particular microwave source, it can be shown from the general formulas for the complex reflection and transmission coefficients of the single wall tube, that the thickness of the wall is given for air to air by: d =

This is the 'half wave' thickness case in which θ is the angle that a microwave ray makes with the normal to the axis of the tube. Reflection is zero only at the design angle 0, when thickness = 0 (trivial case) and at Brewster's angle θg Θ_B = arctan ξ ) In general the reflection from the tube will be zero for a tube thickness equal to "j" or A, or any multiple thereof. It will be evident from the above that the function performed by the composite tube is to reduce the large step or increase in permitivity between the air or other material in the support structures such as 1 or 9 to 13 above between the applicators and the tube 8 or 24 into a series of smaller steps or a smooth or gradual increase to reduce reflection of the radiation where the material to be heated has a low dielectric constant, a single tube will usually be adequate, although a composite tube would produce superior results. However, when the material has a higher dielectric constant, the composite tube approach is far superior and may even involve more than three materials. Referring now to Figures 4 to 7 of the drawings, an alternative form of microwave heating system is shown in which a strip or other planar material to to be heated, or a conveyor belt carrying a material to be heated, is passed through slots formed in a series of microwave heaters of the type generally described above, modified to exclude the central tube. The conveyor is made from some suitable non- absorbing material such as cotton, Teflon, fibreglass, terylene or polypropylene to prevent reflection and to allow the lower portion of each heater to heat the material through the conveyor. Referring firstly to Figure 4, the strip or conveyor belt S passes between opposed halves 30, 31 of a series of supporting structures, similar to that shown in Figure 2 of the drawings, having a multiplicity of spaced microwave applicators 32 supported on each side 33 of each supporting structure half 30, 31. While the ends of the structures 30, 31 are shown in Figures 4 to 7 as being open, it will be appreciated that in practice the ends would be closed by suitable shielding material. Each microwave applicator 32 is supported by its supporting structure 30, 31 in such a manner that the long axis of the microwave energy pattern thereof (usually elliptical in shape) is directed transversely to the direction of travel of the strip or conveyor S. In this way, more uniform heating of the strip or material supported by the conveyor is achieved. To further improve uniformity of heating, the applicators 32 supported by adjacent sides, or by adjacent heaters are spaced by an amount equal to one-quarter of the wavelength of the radiation emitted by each applicator 32. This is shown most clearly in Figure 4 by the first microwave heater having its applicators 32 spaced by a distance d, while the next microwave heater has its applicators 32 spaced by a distance d + ~ζ, while the third heater has its applicators 32 spaced by a distance d - ~ζ. This arrangement ensures that the strip or material supported by the conveyor is uniformly heated across the entire width of the strip or conveyor. Although not shown in Figure 4 of the drawings, the corner of each supporting structure 30, 31 is preferably fitted with a part-circular reflector, or the type described above, preferably having its radius of curvature approximatel coincident with the material being heated. Although partial reflectors may be positioned where the strip S passes between the supports 30, 31, since the material to be heated partially occupies this position in any event, it is presently thought to be unnecessary to position partial reflectors at these locations. Modifications of the heating arrangement shown in Figure 4 will be seen in Figures 5 to 7 of the drawings. In Figure 5, the arrangement shown in Figure 4 is inverted; in Figure 6, the lower support structure is generally se i- circular, while in Figure 7, the upper support structure is generally semi-circular. While the arrangement shown in Figures 4 to 7 are preferred, it should be appreciated that acceptable results may be obtained by using the upper or lower support structure only to heat one surface of a travelling sheet or material supported by a conveyor. In this case, the transverse orientation of each applicator 32 and the preferred spacing between adjacent applicators provides a significant improvement in the uniformity of heating of the material to be heated. It will be appreciated that many different materials may be treated using the heaters shown in Figures 4 to 7 of the drawings. While the supporting structure for the conveyor or sheet S is not shown in the drawings, it is within the skill of the person of ordinary skill in the art to design a suitable support or conveyor. In the case of a strip or generally planar member such as a timber panel, the pairs of rollers would normally be used to feed the material through the heaters. Where a timber panel having some rigidity is to be heated, supporting rollers may be positioned at either end of the series of heaters and/or between the heaters, and the panel conveyed through the heaters by means of a pusher or puller of some suitable form. The tube containing the material to be treated may be partially evacuated for drying sensitive materials such as pharmaceutical products. Alternatively, the tube may be flushed with an inert gas to prevent combustion of inflammable material or may be flushed with oxygen or a fuel for material processing such as mineral smelting.

Claims

CLAIMS: 1. A system for the microwave treatment of materials comprising a treatment zone (T) including means (8, 24a, 40) to confine the material to be treated in at least a-'partly cylindrical form, radiation means (2 to 4; 14 to 18) spaced from and outside said treatment zone (T) for directing microwave radiation into said zone (T) along at least three different axes of radiation, said radiation means being arranged in spaced relation around said zone with said axes of radiation being in spaced relation to each other at the radiation means, means (5,6,7; 19 to 23) for reflecting the microwave radiation directed along each axis back into the treatment zone (T) after it passes through said zone (T), characterized in that said reflecting means has a generally concave curved form having at least one radius of curvature which lies within said treatment zone (T). 2. The system of claim 1, wherein said reflectors are defined by a portion of a circular wall serving to support the radiation means in spaced relation around the treatment zone containing the material to be treated. 3. The system of claim 1 or 2, wherein the treatment zone is defined by a length of low-loss tubing (8; 24a; 40) by means of which the material to be treated is contained and transported through the treatment zone (T). 4. The system of claim 3, wherein said tubing (24a) is supported for rotation within said treatment zone (T), said tubing being provided with agitating means (24c) formed on or attached to the inner surface of the tubing to agitate the material to be treated as it passes through the treatment zone (T). 5. The system of any preceding claim, wherein said tubing comprises a composite tube consisting of at least two tubes (24, 24a; 40, 41) of low-loss material having similar or different dielectric property separated by further material (24b; 42) having dissimilar dielectric properties. 6. The system of claim 5, wherein said tubes (24, 24a; 40, 41) are of similar thickness and said further material is of greater thickness. 7. A system for the microwave treatment of jaaterials comprising a treatment zone and radiation means (14 to 18) spaced from an outside said treatment zone for directing microwave radiation into said zone, said treatment zone being-defined by a composite tube including an outer tube (24; 40) and an inner tube (24a; 41) separated by a further material (24b; 41), the material of said outer tube, said inner tube and said further material being selected to match the radiation means to the material to be heated within said inner tube. 8. The system of claim 7, wherein said outer and inner tubes are of similar or dissimilar material having a predetermined thickness(es) and said further material is of dissimilar material having a thickness greater than said predetermined thickness. 9. A microwave treatment systems for a moving strip(s) or other generally planar object or a conveyor supporting material to be treated, comprising means (31, 32) for supporting one or more microwave applicators (32) in an overlying and/or underlying relationship to said strip(s) or conveyor, characterised in that at least said one microwave applicator (32) is supported such that the long axis of the microwave energy pattern produced thereby is oriented in a direction transverse to the direction of travel of said strip(s) or material to be treated. 10. The system of claim 9, wherein each supporting means (31, 32) has a plurality of faces supporting a spaced plurality of microwave applicators, the microwave applicators on one face being spaced by a predetermined distance, and the microwave applicators on adjacent faces being spaced by a distance which differs from said predetermined distance by an appropriate fraction such as one-quarter of the wavelength of the radiation emitted by each radiation means.