Electric heating – Microwave heating – With control system
Reexamination Certificate
2001-06-27
2002-06-11
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
With control system
C219S679000, C219S697000, C219S746000, C204S157430, C422S021000
Reexamination Certificate
active
06403939
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/IB99/02021 which has an International filing date of Dec. 17, 1999, which designated the United States of America and was published in English.
The present invention relates to an apparatus for heating chemical reaction mixtures. In particular, the present invention relates to an apparatus applying one or more semiconductor based microwave generators making the apparatus suitable for parallel processing of chemical reaction mixtures. The invention further relates to methods for performing chemical reactions, e.g. methods for heating a plurality of samples simultaneously or sequentially, methods for monitoring a microwave heated chemical reaction and methods where the optimum conditions with respect to frequency and applied power can be determined.
One of the major obstacles for an organic chemist today is the time consuming search for efficient routes in organic synthesis. As an example, the average performance some ten years ago in the pharmaceutical industry was around 25-50 complete substances per chemist per year resulting in an equal amount of new chemical entities as potential new drug candidates. Today the figure is several 100's per year and will soon be expected to be in the region of 1000's per year.
Thus, the challenges for the pharmaceutical industries and the organic chemist include identification of ways of reducing time in drug development, identification of ways of creating chemical diversity, development of new synthesis routes and maybe reintroduction of old “impossible” synthetic routes. Also, it is a constant challenge to reach classes of totally new chemical entities.
As it will be apparent from the following, microwaves assisted chemistry offers a way to circumvent at least some of the above-mentioned problems, namely
speeding up the reaction time with several orders of magnitudes,
improving the yield of chemical reactions,
offering higher purity of the resulting product due to rapid heating and thereby avoiding impurities from side reactions, and
performing reactions which are not possible with conventional thermal heating techniques.
Microwave assisted chemistry has been used for many years. However, the apparatuses and methods have to a great extent been based on conventional domestic microwave ovens. Domestic microwave ovens have a multimode cavity and the energy is applied at a fixed frequency at 915 MHz or 2450 MHz (depending on country). 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.
The market for microwave generators is totally dominated by magnetrons. In some situations travelling wave tubes (TWT) are used to amplify a microwave signal. There are several disadvantages related to the conventional apparatuses. Some of these will be listed in the following:
It is a disadvantage that the energy distribution in conventional microwave ovens is non-uniform. This leads to a varying temperature in the sample depending on the position of the sample in the oven. Furthermore, the non-uniform energy distribution makes it difficult to obtain reproducible results. This effect is especially noticeable if an array of sample holders such as a microtiter plate (e.g. with 96 wells) is used. Rotation of the sample in the oven does not significantly improve the reproducibility.
In conventional systems the power provided to each sample in an array of samples can only be calculate as an average power per sample by dividing the measured input power with the total number of samples. Due to the non-uniform energy distribution in the cavity this calculation will only provide a rough estimate of the applied power to each sample.
One way of controlling the reaction is to monitor pressure and temperature in all individual wells. This may give information of the conditions in a specified well during a particular run. Changing the position will give a different result leading to poor reproducibility. An alternative way of trying to obtain a uniform energy distribution is to place a large load in the cavity in order to absorb energy more uniformly.
Single mode cavity resonators offer a possibility of high efficiency and controlled heating patterns in small loads. However, the dielectric properties of the load often change considerably with temperature, resulting in very large variations in power absorption since an essentially constant frequency microwave generator is used. Hence, the process becomes difficult to predict.
A further disadvantage of conventional system relates to the fact that magnetrons usually only provide a fixed frequency or a minor adjustment around the center frequency of the magnetron. Furthermore, magnetrons have an unpredictable behaviour and are extremely temperature sensitive, especially when the efficiency decreases, towards the end of its “life”.
TWT's have be used as variable frequency amplifiers. However,TWT's are rather expensive and often very complicated to use. Furthermore,TWT's require warm-up time before start meaning that TWT's cannot rapidly be switched on and off. In addition, wear out of TWT's is associated with high maintenance costs.
Both magnetrons and TWT's require a high voltage power supply, which is a disadvantage in view of complications and the risk.
In U.S. Pat. No. 5,521,360 a variable frequency heating apparatus for providing microwaves into a furnace cavity is described. The apparatus comprises a voltage controlled microwave generator, a voltage controlled pre-amplifier and a power amplifier. The power amplifier may be a TWT. The TWT is operationally connected to the furnace cavity. The power delivered to the furnace is determined by measuring the power reflected from the furnace using a power meter. Upon placing a sample in the cavity furnace, power may be coupled to the sample causing the temperature of the sample to change.
The system described in U.S. Pat. No. 5,521,360 suffers from the above-mentioned disadvantages relating to e.g. TWT's.
It is a further disadvantage of the apparatus described in U.S. Pat. No. 5,521,360 that it is restricted to be used with only one cavity furnace, i.e. parallel heating of a plurality of samples using different heating parameters is not possible.
It is another object of the present invention to provide an apparatus comprising a first semi-conductor based electromagnetic generator, and a first applicator for holding a sample, which apparatus are capable of performing a controlled heating of the sample.
It is another object of the present invention to provide an apparatus capable of performing parallel processing of many samples, with individually settings of process parameters such as frequency, power, temperature, pressure etc.
It is a further object of the present invention to provide an apparatus capable of monitoring many samples in parallel, with individually monitoring of process parameters such as frequency, power, temperature, pressure etc.
It is a still further object of the present invention to provide an apparatus capable of controlling many samples in parallel, with individually adjustments of process parameters such as frequency, power, temperature, pressure etc.
It is a still further object of the present invention to provide an apparatus in which samples can be evenly heated by using various applicators.
It is a still further object of the present invention to provide an apparatus in which the frequency of the applied energy can be varied.
It is a still further object of the present invention to provide an apparatus in which it is possible to evaluate and separate thermal and chemical effects on the electromagnetic absorption capability and behaviour of the sample.
It is a still further object of the present invention to provide an apparatus in which it is possible to measure the temperature in the reaction vessel by monitoring the change in resonance frequency of a second material introduced into the reaction chamber. This material could be a crystal, semicon
Harness & Dickey & Pierce P.L.C.
Leung Philip H.
Personal Chemistry I'Uppsala AB
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