Electric heating – Microwave heating – Waveguide applicator
Reexamination Certificate
2002-10-11
2003-10-07
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Waveguide applicator
C219S696000, C219S750000, C219S746000, C219S761000
Reexamination Certificate
active
06630654
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a microwave heating apparatus comprising a microwave applicator, a microwave heating system including the microwave heating apparatus and a method of using the microwave heating system according to the preambles of the independent claims.
BACKGROUND OF THE INVENTION
Microwave heated system for carrying out chemical reactions, and particularly organic synthesis reaction, is an important and well-known technique. Using microwave heating makes it possible to increase the reaction rate of chemical reactions with order of magnitudes. The use of microwaves also often leads to higher yield and purity of the final product.
Microwave assisted chemistry has been used for many years. However, the apparatuses and methods have to a great extent been based upon conventional domestic microwave ovens. Domestic microwave ovens have a multimode cavity and the energy is applied at a fixed frequency at 2450 MHz. The use of single mode cavities have also been reported, see e.g. U.S. Pat No. 5,393,492 and U.S. Pat No. 4,681,740.
Recent developments have led towards apparatuses comprising a microwave generator, a separate applicator for holding the load (or sample) to be treated, and a waveguide leading the generated microwave radiation from the generator and coupling it into the applicator. Even if the system consists of a TE
10
waveguide using a 2450 MHz to which a magnetron generator is connected in one end and the sample container is in the other end, there is a need for a matching device in the form of at least a metal post or iris between the generator and load, in order to achieve a reasonable efficiency.
When coupling electromagnetic radiation such as microwaves from a source to an applicator, it is important to match the transmission line impedance and the applicator impedance (with load) in order to achieve a good transfer of power. However, the dielectric properties of the load may influence drastically upon the impedance of the applicator, as well as its electrical size, and the dielectric properties of the sample often change considerably with both temperature and applied frequency. Thus, an impedance mismatch between the source and the applicator will often occur and the coupling and thereby the heating process becomes less efficient and difficult to predict.
Below follows a short background description of different transmission modes used in a microwave applicator.
Consider a hollow waveguide with a given cross section that is uniform throughout its entire length. As a result of the application of these boundary conditions to the wave equation, it can be shown that only certain unique patterns for the distribution of the electric intensity E and the magnetic intensity H (taken together) can exist in the waveguide. Each unique pattern of the field distribution is called a mode. There are two types of modes possible in a hollow waveguide—one of them being the transverse electric (TE) mode and the other the transverse magnetic (TM) mode. In the TE mode the E has only a component transverse (that is perpendicular) to the direction of propagation, whereas the H has both transversal and longitudinal components.
In the TM mode the magnetic intensity has only a transverse component and the electric intensity has both components. Each type (TE or TM) of mode has an infinite number of sub-modes which have the common characteristics of the type to which they belong, but differs among themselves in the details of field distribution.
One of the most important characteristics of a TE or TM wave is that it has a cutoff wavelength for each mode of transmission. If the free-space wavelength is longer than the cutoff value, that particular mode cannot exist in the waveguide. For any given waveguide, the mode that has the longest cutoff wavelength is known as the dominant mode. By indexing the mode, e.g. TE
01
, this is indicated.
U.S. Pat. No. 4,392,039 relates to a dielectric heating applicator provided with a low-loss dielectric with a dielectric constant exceeding that of the object to be heated by microwaves. An internal resonance is excited in the applicator so that specific field pattern exists at and in the object.
According specific embodiments of the heating applicator of U.S. Pat No. 4,392,039 the dielectric is provided with an axial hole where the load can be heated (
FIGS. 10-12
of U.S. Pat No. 4,392,039).
U.S. Pat No. 3,848,106 discloses an apparatus for heating by microwave energy that includes a dielectric material having low losses and a dielectric constant exceeding the dielectric constant of air. In one embodiment of this known apparatus the dielectric body is shaped as a cylindrical lining in a metal tube (illustrated in
FIGS. 7 and 8
of U.S. Pat No. 3,848,106) intended for heating material with cylindrical cross-section being positioned in the tube in coaxial relationship to said tube.
The embodiment of the cylindrical dielectric material does not take into account any backwards interaction, i.e. load influence on the impedance matching conditions, or system matching.
The overall object of the microwave heating apparatus according to the present invention is to achieve a heating apparatus where the heating process is more efficient and easier to predict than in the prior art heating systems.
A further object of the present invention is to enable parallel processing of several microwave heating apparatuses arranged in a microwave heating system where each apparatus may be individually controlled with regard to temperature, time and frequency of the microwaves.
SUMMARY OF THE INVENTION
The above-mentioned objects are achieved by the present invention according to the independent claims.
Preferred embodiments are set forth in the dependent claims.
Thus, by providing a microwave applicator with the geometrical characteristics as stated in the independent claims the performance of the heating apparatus is increased.
The present invention is particularly advantageous for small load volumes. In this context a volume is considered small if it is less than 5 ml and in particular if it is less than 2 ml.
If the volume is more than about 2 ml, some technical advantages with ceramic applicator systems according to the present invention may no longer fully apply; a 2 ml load of a diameter of 10 mm becomes 25 mm high and this is so large that applicators without using a ceramic material may be used with acceptable results.
If the load volume is less than 0.4 ml it may be too difficult to control ambient influences such as cooling by a vial itself, and to measure the load temperature and pressure without disturbing it too much. A 0.5 ml load with Ø6 mm becomes 18 mm high, and another reasonably optimised load with regard to applicator design and associated sensors may have Ø9 mm, be 12 mm high and thus have a volume of about 0.75 mL.
The applicator is to be “small”, to facilitate the design of compact multi-applicator systems for parallel operation. Since the applicator with load must be resonant in order for the microwave heating efficiency (the percentage of input microwave power to the applicator which is actually absorbed by the load) to become high, the characteristic size (generally: the diameter) of the applicator must be of the order of between a half and a full wavelength. At 2450 MHz this wavelength is about 120 mm in free space, so an air-filled applicator will be at least about 60 mm in diameter. By using a ceramic applicator with high permittivity instead of air, the size can be reduced by a factor of approximately the square root of the ceramic permittivity.
As an example, using a microwave ceramic with permittivity 100, the applicator diameter becomes about 17 mm (as in the preferred embodiment described below).
Below follows an overview of some considerations taken into account when developing the present invention:
There are four degrees of freedom in cases where the microwave heating apparatus is intended for heating of liquids with 0.5-1.0 ml volume using 2450 MHz microwaves. The liquids have permittivities ranging fro
Fagrell Magnus
Risman Per Olov G.
Leung Philip H.
Personal Chemistry i Uppsala AB
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