EP1538879A1

EP1538879A1 - Microwave heating device

Info

Publication number: EP1538879A1
Application number: EP03104486A
Authority: EP
Inventors: Fredrik Stillesjö; Jan Johansson Barck-Holst
Original assignee: Personal Chemistry i Uppsala AB
Current assignee: Personal Chemistry i Uppsala AB
Priority date: 2003-12-02
Filing date: 2003-12-02
Publication date: 2005-06-08

Abstract

Microwave heating device comprising a microwave generator 4 and a hollow microwave waveguide 2 where standing wave microwave radiation 6 from the generator is adapted to be generated. The device comprises a passive microwave antenna 12 arranged to be inserted into the waveguide through an opening in a waveguide wall in order to absorb microwave energy and to transfer said energy out from said waveguide. In one embodiment is the microwave generator arranged in one end of the waveguide and a load applicator 10 adapted to receive a load to be heated is arranged in the other end of the waveguide.

Description

Field of the invention

The following invention is in the field of microwave engineering, especially microwave power engineering and heating applications where a gas, liquid or solid material is exposed to microwave radiation to change its thermal state.

Background of the invention

A standing wave applicator can be used as a simple, non-resonant microwave heating system with potentially very high heating rates for e.g. liquids and solids. It also has a good dynamic range regarding different load volumes and dielectric properties. However, a severe microwave power mismatch with the load will drastically reduce the lifetime of the microwave generator which is usually a magnetron or a semiconductor based generator.
The magnetron is a non-linear component that can withstand a limited amount of reflected microwave power. The magnetron can fail or brake in order of seconds when big amounts of reflected power hits the magnetron. Especially if internal resonances in the load are present that will change the phase of the magnetron bias under constant big mismatch.
Usually, a device called circulator is used to protect the magnetron in such cases. A circulator is a three-port device with an injection port, a load port and a port where the reflected power is dumped or redirected. Circulators can be purchased today in the power range watts to megawatts.
A directional coupler may also be used after the circulator to measure the microwave power in the non-resonant system. The complexity of the system increases and the benefits of the standing wave applicator as a very cheap system for mass production diminish therefore.
In US-6,614,010 a microwave heating apparatus is disclosed where a waveguide is provided with a circulator adapted to deflect at least a part of the electromagnetic waves reflected from an applicator towards a dummy load.
An object of the present invention is to achieve a microwave heating device provided with means to absorb a part of the electromagnetic energy in the waveguide that is less complex and cheaper compared to the prior art heating apparatus.
A further object of the present invention is to achieve a less complex and cheaper microwave heating device, compared to the prior art heating apparatus, where the amounts of reflected power that hits the magnetron is substantially decreased.
A still further object of the present invention is to achieve a microwave heating device that includes features arranged to provide a safe coupling of microwave energy to a second microwave heating device.

Summary of the invention

The above-mentioned objects are achieved by the present invention according to the independent claim.
Preferred embodiments are set forth in the dependent claims.
A standing wave applicator is designed to fit the phase of the microwave generator bias point and stabilised with a tuning element. Furthermore, and in accordance to the present invention, a passive and simple component in the form of a passive microwave antenna is introduced in the waveguide in order to absorb a portion of microwave power. The voltage standing wave ratio (VSWR) in the applicator then decreases and the magnetron lifetime increases. This energy can be used and transported to another microwave heating device or dumped in a dummy load.
Thus, the above-mentioned objects are achieved by mounting a passive microwave antenna in the applicator.
According to a first preferred embodiment the passive antenna is mounted in the applicator where the standing wave has a minimum for a load with a certain set of physical properties. The wave will propagate through the passive antenna and the power distributed in a dummy load or to another microwave heating device. The VSWR will therefore decrease for very small loads, which is more advantageous for the magnetron and its lifetime.
According to a second preferred embodiment the passive antenna is mounted such that the microwave energy is coupled to a another microwave heating device.

Short description of the appended drawings

Figure 1a shows a schematic illustration of a simple standing wave applicator according to a first preferred embodiment with a microwave generator, a load and the passive microwave antenna delivering its microwave power to another microwave system.
Figure 1b shows a similar system as in Fig. 1a, but the microwave power is dumped in a dummy load.
Figure 1c shows a schematic illustration of a simple standing wave applicator according to a second preferred embodiment with a microwave generator and the passive microwave antenna delivering its microwave power to another microwave system.
Figure 2 shows an end view of the embodiment shown in figure 1a.
Figure 3 shows a view from above of the embodiments shown in figures 1a or 1b.
Figures 4a-4c show first, second and third embodiments, respectively, of the preferred antenna cross sections.
Figure 5 shows an ordinary Rieke-diagram.

Detailed description of preferred embodiments of the invention

The microwave heating device and its function will now be described with references to figures 1-5. The device comprises a waveguide 2 provided with a waveguide wall enclosing a waveguide cavity with measures a and b (see figure 2) giving the fundamental TE₀₁-mode (generally both TE_mn or TM_mn -modes with n,m integers). A microwave generator 4, preferably a magnetron, generates microwave power and creates a standing wave 6 in the waveguide 2.
The waveguide 2 preferably has a rectangular cross-section. However, also a circular or elliptical cross-section may be used.
A rectangular waveguide should have a minimum length of 193 mm for a stable magnetron operation. Longer waveguides are possible, presumably in units of half a wavelength (λ/2=71.5 mm for 2450 MHz and a TB₁₀-mode).
The waveguide length must be chosen in such a way that the phase of the magnetron bias is the most favourable, i.e. optimises the waveguide length in a way that the magnetron works in the thermal region.
With references to figures 1a a first preferred embodiment is shown where a passive microwave antenna 12 is arranged in an opening through the waveguide wall in order to absorb microwave power from the waveguide 2 to deliver it to another microwave system (a second microwave device) 14a.
Figure 1b shows a similar system as in Fig. 1a, but the microwave power is dumped in a dummy load 14b.
A load is inserted in an applicator 10 in the waveguide, preferably close to the short circuit wall at the opposite end of the waveguide compared to where the microwave generator is arranged.
In the first preferred embodiment the passive microwave antenna 12 is placed in the waveguide cavity from below, above or from the side. The most efficient way, from below, is presented here. The actual position is then preferably chosen in such a way that the standing wave 6 has a minimum above the antenna. In this sense, one or two modes (TE₁₁ and/or TM₁₁) depending on the passive antenna diameter is excited and propagated in the antenna.
Figure 1c shows a schematic illustration of a second preferred embodiment the simple standing wave waveguide 2 with a magnetron and the passive antenna 12 acting as a component that couples electromagnetic energy from the waveguide to a second microwave device 14c. The passive antenna is placed in one of the short sides of the waveguide.
In the second preferred embodiment the antenna is specifically used to transmit electromagnetic energy from the non-resonant standing wave waveguide to another microwave system well separated from the previous one, e.g. a vacuum system, high pressure system or a system where a protection box must be used due to risks of explosion.
The passive antenna 12 may be used with any microwave generator, especially when using the antenna as described in the second preferred embodiment (figure 1c). The first preferred embodiment (figures 1a and 1b) are mainly adapted for use with a magnetron.
Figure 2 shows an end view of the embodiment shown in figure 1a.
Figure 3 shows a view from above of the embodiments shown in figures 1a or 1b.
For both preferred embodiments a tuning stub or metallic element 8 (e.g. a deflector) is preferably also included in the waveguide to match the impedance of the load with the magnetron and stabilize the phase of the wave also without any passive microwave antenna.
The shape and cross-section of the passive antenna may be changed or adapted to the applicator shape for the best performance. Especially when the antenna is mounted as in figure 1c.
Generally, the passive microwave antenna may be placed from below, from above, in one of the long sides or in one of the short sides (described above as the second embodiment in figure 1c).
Experiments (and simulations) have shown that the coupling of the microwave field to the antenna is most efficient when the passive antenna is placed and inserted from below (or above). The actual longitudinal position is preferably chosen in such a way that the standing wave 6 has a minimum above the antenna. For a 193 mm applicator, the antenna should then be placed 96 mm from the short circuit wall (load position). This has been experimentally (and by simulations) determined by moving the antenna in the longitudinal direction. One position may be found that works satisfactory for loads with volumes in the interval mention previously and different dielectric properties. It is therefore not necessary to have a moveable antenna in the longitudinal direction, even though it is possible. The diameter of the passive antenna is the most crucial parameter for a good performance and coupling to the microwave field. One or two modes (TE₁₁ and/or TM₁₁) depending on the passive antenna diameter is excited and propagated in the antenna.
When the antenna is placed horizontally in one of the short sides of a rectangular waveguide and further mounted in a separated system (second embodiment, figure 1c), the preference of either a maximum or minimum of the standing wave in the applicator is not important.
Moving the antenna in or out of the cavity may control the coupling of the wave to the antenna, i.e. the amount of power delivered to the absorber. The immediate problem with big VSWR for small loads in the thermal region and internal resonances in the load in the sink region that drastically influence the magnetron operation (called "moding") will decrease substantially when some of the power is drained to another system (figures 1a or 1c) or in a dummy load (figure 1b).
The permittivity and power loss in the antenna itself is crucial for the coupling efficiency. The dielectric properties is defined as: ε(ω) = ε'(ω)-jε(ω)"
Here, ε'(ω) is the frequency dependent permittivity and ε"(ω) the dielectric loss factor. For good performance, the ε' should be rather high (7-12) and a low dielectric loss factor (10^-2-10^-4).
The cross-section of the passive antenna may be circular according to a first embodiment in figure 4a, elliptical as in a second embodiment in figure 4b or rectangular as in a third embodiment in figure 4c. The geometric form chosen depends on the actual coupling to the standing wave wanted.
According to the first embodiment the antenna is circular cylindrical in shape with a diameter 25-30 mm and a varying length (e.g. 20-100 mm). The antenna is mounted through a hole or opening in the waveguide bottom wall (or roof wall) or from the sides and preferably fastened with e.g. a 5-10 mm high cylindrical ring (choke) welded to the waveguide wall with one or more threaded holes for fastening of the antenna. In this sense, the antenna may be moved axially in or out of the waveguide to find the optimal coupling efficiency (e.g. -5 to 5 mm with 0 as the bottom position). An option could be to have the antenna moveable, but a fixed axial distance into or out of the applicator bottom is the most practical arrangement.
The actual position must be determined for the actual waveguide, and in the 193 mm exemplary waveguide the passive antenna should be placed 96 mm from the short circuit wall (load position). The option of making the antenna movable in an adjustment slot in the waveguide wall or bottom may also be possible.
Its actual position in the waveguide is a function of the waveguide geometry chosen and the load properties. For a fixed waveguide length, the position of the passive antenna can be chosen in such a way that the VSWR decrease for a multitude of different loads.
The passive microwave antenna is preferably made of a solid material, e.g. a ceramic material such as aluminium oxide or similar or a composite material such as ceramoplastic materials sold under the name Mykroy/Mycalex by Spaulding Composites. It should be placed under or above the waveguide or placed on one of its sides.
The standing wave applicator has a great dynamic range, i.e. is able to heat both small and moderately large load volumes with different dielectric properties. The waveguide must therefore be designed to have a zero load system resonance around 2400 MHz for maximum performance.
The heating device works very well for load volumes in the interval 0.2-3 ml, presumably also satisfactory up to 5 ml with some risk for complications for loads with certain combination of dielectric properties.
However, there are loads of moderately large sizes that become internally resonant in this type of waveguide. This will drastically affect the magnetron if its bias point is in the sink-region, since the operating point will change under constant large VSWR. Extremely high VSWR can also be the result when small loads are used influencing the magnetron bias in the thermal region. Ratios (VSWR) above 30 may be the case.
In order to fully understand the invention a short discussion follows on bias and bias point.
A magnetron is a non-linear oscillator, which can interact both with a load and with its own impedance when the emitted wave is reflected back to the magnetron. The magnetron operation with a load is usually presented as a Rieke-diagram. An ordinary Rieke-diagram is presented in figure 5.
This diagram is a polar plot, where the magnetron frequency shifts and power lines are presented together with contours of constant VSWR (voltage standing wave region). The voltage standing wave minimum (VSW) towards the load is presented on the circular periphery. The reference plane, i.e. the antenna is 0 at the noon-position. The magnetron impedance during operation is characterised by a trajectory on this diagram. For a good performance and lifetime, the magnetron should be biased to a region with low VSWR. The magnetron should not pass the sink region (around 0.2-0.3 in VSW and high VSWR >7) under a constant high mismatch. This may in the end "kill" the magnetron due to a phenomenon called "moding". The thermal region (around 0-0.05 in VSW and high VSWR) is better adapted for high mismatch, but may imply problems with hot cathodes.
In the first preferred embodiment the microwave power picked up by the passive antenna is further transported to another microwave system 14 a in figure 1a or to a dummy load 14 b in figure 1b. Microwave power is then transported to a second heating system that cannot be connected with ordinary waveguide junctions, e.g. vacuum systems or high-pressure systems. This is possible since the passive antenna is made of solid materials such as composite materials, ceramics or metals, where ceramics and composite materials are preferably used.
In the second case the microwave power is preferably dumped in a dummy load, with circulating water or a specially designed specimen of a ceramic or composite material, e.g. silicon carbide. Silicon carbide is a material with good thermal properties with maximum allowed temperatures above 1000 °C.
A wire cage or water load instead of a silicon carbide load may also surround the antenna in the embodiment described in Fig. 1b.
However, a rather high dielectric constant and high dielectric losses in silicon carbide concentrates the microwave field in the vicinity of the passive antenna to a limited area of the load.
Silicon carbide has a high relative permittivity and also high losses. The microwave field will therefore be concentrated to a small region (due to the high electric permittivity) and with a high amount of the energy deposited in this small region (due to the high losses). Silicon carbide may be heated to high temperatures, but the antenna material properties and the surrounding applicator may be influenced negatively (changed dielectric properties and/or thermal expansion).
The invention also relates to the use of the above-described microwave heating device for carrying out organic chemical synthesis reactions. Chemical reactions that can be carried out by using the hereinabove described device are, for example, oxidation, nucleophilic substitution, addition, esterification, transesterification, acetalisation, transketalisation, amidation, hydrolyses, isomerisation, condensation, decarboxylation and elimination.
The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

Microwave heating device comprising a microwave generator (4) and a hollow microwave waveguide (2) where standing wave microwave radiation (6) from the generator (4) is adapted to be generated, characterized in that the device further comprises a passive microwave antenna (12) arranged to be inserted into the waveguide through an opening in a waveguide wall in order to absorb microwave energy and to transfer said energy out from said waveguide.
Microwave heating device according to claim 1, characterized in that a second microwave heating device (14a, 14c) is arranged to receive energy from said passive antenna (12), wherein a load is arranged to be heated in said second microwave heating device.
Microwave heating device according to claim 2, characterized in that said second microwave heating device is arranged inside a protective enclosure provided with an enclosure wall, wherein said passive microwave antenna couples the electromagnetic energy through said enclosure wall to the second microwave heating device.
Microwave heating device according to claim 1, characterized in that the passive microwave antenna is located in a plane perpendicular to the longitudinal axis of the waveguide, where the standing wave (6) has a minimum.
Microwave heating device according to claim 1, characterized in that the microwave generator is arranged in one end of the waveguide and a load applicator (10) adapted to receive a load to be heated is arranged in the other end of the waveguide.
Microwave heating device according to claim 1, characterized in that said microwave generator is a magnetron.
Microwave heating device according to claim 1, characterized in that microwave waveguide has a rectangular cross-section.
Microwave heating device according to claim 1, characterized in that the passive microwave antenna is arranged to be inserted a predetermined distance into the waveguide cavity in order to control the coupling of the microwave energy to the passive antenna.
Microwave heating device according to claim 1, characterized in that said passive antenna has an elongated cylindrical shape.
Microwave heating device according to claim 1, characterized in that said passive antenna has a circular cross-section.
Microwave heating device according to claim 1, characterized in that said passive antenna has a rectangular cross-section.
Microwave heating device according to claim 1, characterized in that said passive antenna has an elliptical cross-section.
Microwave heating device according to claim 1, characterized in that a dummy load (14b) is arranged to receive heat energy from said passive antenna.
Use of a microwave heating device according to any of claims 1-13 for chemical reactions and especially for organic chemical synthesis reactions.