Microwave electromagnetic field

Microwave catalysis is a catalytic process performed in the presence of a microwave (electromagnetic) field in which the catalyst acts as an energy convertor . It uses microwave irradiation to stimulate catalytic reactions. It is necessary to stress that any sort of electromagnetic or microwave radiation is not itself a catalyst, as has sometimes erroneously stated [1], Similarly, it is not correct to say that microwave irradiation catalyzes chemical reactions [1], The principles of microwave catalysis will be described in the following sections. 

To make interaction between the traveling microwave electromagnetic fields and the electron beam efficient, in a traveling wave tube, the phase velocity or propagation speed of microwaves in the tube is slowed down to approximately equal speed with the speed of electrons in the electron beam. In most... 

By design, in a traveling wave tube, the speed of electrons in the electron bean u (m/s) and the longitudinal phase velocity of microwave electromagnetic field in the slow-wave structure transmission line v (m/s) are made approximately equal. When an electron comes out of the electron gun, the speed of an electron is... 

Perkin, R.M., The heat and mass transfer characteristics of boiling point drying using radio frequency and microwave electromagnetic fields. Int J Heat Mass Transfer 23 687-695 (1980). 

Applications. Molecules couple to an electromagnetic field through their electric dipoles, so only those having a permanent dipole moment exhibit significant rotational spectra. For such species, microwave spectroscopy yields highly precise moments of inertia and details of centrifugal... 

Laboratory studies on the extraction of pectin from orange peels, pretreated in an electromagnetic field of hyper frequency, were carried out. The influence of intensity of microwave treatment (P ) and time on pectin yield and pectin quality was investigated. It was established that the increase of Pw and time lead to increase in the pectin yield with 180-240 % in comparison with the control. Apparently, the microwave treatment leads to a considerable increase in the soluble form of pectin, characterized by increase in the jelly strenght and in the polyuronic content. 

A particularly difficult problem in microwave processing is the correct measurement of the reaction temperature during the irradiation phase. Classical temperature sensors (thermometers, thermocouples) will fail since they will couple with the electromagnetic field. Temperature measurement can be achieved either by means of an immersed temperature probe (fiber-optic or gas-balloon thermometer) or on the outer surface of the reaction vessels by means of a remote IR sensor. Due to the volumetric character of microwave heating, the surface temperature of the reaction vessel will not always reflect the actual temperature inside the vessel [7]. 

Other microwave-assisted parallel processes, for example those involving solid-phase organic synthesis, are discussed in Section 7.1. In the majority of the cases described so far, domestic multimode microwave ovens were used as heating devices, without utilizing specialized reactor equipment. Since reactions in household multimode ovens are notoriously difficult to reproduce due to the lack of temperature and pressure control, pulsed irradiation, uneven electromagnetic field distribution, and the unpredictable formation of hotspots (Section 3.2), in most contemporary published methods dedicated commercially available multimode reactor systems for parallel processing are used. These multivessel rotor systems are described in detail in Section 3.4. 

If the effect of the temperature on reaction rate is well known, and is very easy to express, the problem is very different for effects of electromagnetic waves. What can be expected from the orienting action of electromagnetic fields at molecular levels Are electromagnetic fields able to enhance or modify collisions between reagents All these questions are raised by the use of microwaves energy in chemistry. 

The characteristic material response times for molecular reorientation are 10-12 s. Then, in the microwave band, electromagnetic fields lead to rotation of polar molecules or charge redistribution. The corresponding polarization processes are denoted orientation polarization. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

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