Microwave heating industrial applications

Applications The broad industrial analytical applicability of microwave heating was mentioned before (see Section 3.4.4.2). The chemical industry requires extractions of additives (antioxidants, colorants, and slip agents) from plastic resins or vulcanised products. So far there have been relatively few publications on microwave-assisted solvent extraction from polymers (Table 3.5). As may be seen from Tables 3.27 and 3.28, most MAE work has concerned polyolefins. 

The conversion rate in aza-Baylis-Hillman reactions is generally low, which leads to extended reaction times [87]. Heating is normally used to increase the reaction speed however, it also promotes the formation of side products. Alternatively, microwave heating was successfully used as a way of promoting the reaction [92]. However, microwaves-promoted reactions are not easy to scale-up. Guided by this, the Stevens research group [89, 93] used the commercial CYTOS College System [18] to perform these reactions on a microscale in a continuous manner in order to improve the reaction rates and make it industrially more applicable. 

Microwave heating techniques have been widely used in textile chemistry. This paper presents a state-of-the-art review of microwave technologies and industrial applications. The characteristics of microwave interactions with textile materials are outlined together with microwave fundamentals in the heat-setting process. Further more, the limitations in current imderstanding are included as a guide for potential users and for future research and development activities. 

Microwave Power. Microwave heating has been examined with renewed interest since the development of ovens for the home and of high power, high efficiency industrial equipment. Microwave interaction has long been used to probe structural details of polymers (10) and rotational/vibrational spectra of small molecules. Higher power applications have become more prominent. 

The electric field is a crucial condition in microwave heating and the design of microwave ovens. If electric field distributions within empty microwave ovens are well known, the problem is totally different if loaded microwave ovens are considered. Perturbation theory can be used if the sample is very small. In fact, the magnitude of the perturbation is proportional to reactor-to-applicator volume ratio. The perturbation could be negligible if this ratio is close to 10 and most laboratory and industrial devices have higher ratios [120]. 

First, to avoid potential interference to communications and radar bands, the entire microwave spectrum is not readily available for chemistry. By international convention the frequencies 915 + 25 MHz, 2450 + 13 MHz, 5800 + 75 MHz, and 22125 + 125 MHz have been assigned for industrial and scientific microwave heating and drying applications. For synthetic chemistry, equipment operating at 2450 MHz, corresponding to a wavelength of 12.2 cm, is used almost exclusively. 

The key limiting factor is the penetration depth of microwave irradiation, which is only a few centimeters in most solvents at 2.45 GHz. An issue therefore arises in getting sufficient microwave power into the reaction mixture to achieve the desired heating effect. The core of a large reactor vessel will not receive any microwave radiation as it will all have been absorbed by the outer layers. As a result, the center is effectively conductively or convectively heated, and the potential benefits of microwave heating will be lost. Penetration depth does, however, vary with frequency. Only a limited number of Industrial, Scientific, and Medical (ISM) frequencies are allowed so as not to interfere with military and civil aviation frequencies and telecommunications. Alternative frequencies are used for other large-scale applications and thus may provide an alternative solution to the scale-up of micro-wave chemistry. ... 

Metaxas, A. C. Meredith, R. J. Industrial Applications and Economics. In Industrial Microwaves Heating-, Peter Peregrinus Ltd for the Institute of Electrical Engineers London, 1983 Chap. 11 pp 296—321 (reprinted 1993). 

Shorter reaction times, higher product yields, and enhanced selectivity are some of the advantages microwave heating has over conventional methods, causing its use to transition from a curiosity to mainstream, both in industrial and academic settings. Microwave Heating as a Tool for Sustainable Chemistry showcases the application of microwave heating in a number of areas of preparative chemistry as well as in the biosciences. 

The main disadvantage of electric heating is that it usually costs more per energy unit than gas heating. However, the total energy usage for electric furnaces may often be lower than for gas-fired furnaces. The cost issue is not normally a problem in a university laboratory, but it can be a major concern for industrial applications. Two other types of furnace that use electricity are induction furnaces and microwave furnaces. 

An industrial microwave heating system consists of a dc power supply, a microwave generator (magnetrons are available in 915- and 2450-MHz bands and klystrons at the higher-frequency bands) and an applicator. A microwave heater has only one electrode. On the other hand, an RF device requires two electrodes. The power can only be drawn from an RF generator when there is material present in the applicator therefore, the material is an essential electrical component of the circuit and affects the electrical characteristics [59]. 

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