Microwave processing heat generation

High power microwaves are generated by vacuum tubes. The magnetron and klystron are the most commonly used tubes for the generation of continuous waves power for microwave processing. Power is normally launched from the microwave tube into a transmission line or waveguide, where it travels to a load or termination such an antenna or a microwave heating applicator. 

The domestic microwave oven is a serendipitous invention. Percy Spencer was working for Raytheon, a company heavily involved with radar during World War II, when he noticed the heat generated by a radar antenna. In 1947 an appliance called a Ra-darange appeared on the market for food processing. The first kitchen microwave oven was introduced by Tappan in 1955. Sales of inexpensive domestic ovens now represent a multibillion-dollar (euro) annual market. 

A very interesting approach to process intensification was recently presented by Jachuck and coworkers. The authors described the development and performance of an isothermal CF reactor to be used in a domestic microwave oven [81]. The small (270 p,L) CF reactor consists of two sections, a microwave transparent PTFE part for the reaction side and an alumina part for heat transfer (Fig. 23). The heat generated due to the activation by microwave irradiation was rapidly absorbed by the heat transfer liquid (H20) pumped through the alumina part. Inlet and outlet temperatures of both the reaction mixture and the heat transfer liquid were monitored using a PICO temperature recorder. 

Another area where microwave heating is nsed to heat carbon is in the regeneration of activated carbons. When spent activated carbon is snbject to a microwave field, the heat generated within the particles prodnces rapid temperatnre rises and the release of other componnds adsorbed on the carbon [19-23], In a similar process, carbon nsed as adsorbent to remove NO and SO from gas streams can be regenerated with microwaves prodncing CO2 and N2 as gases and elemental snlphnr [24],... 

Liu et al. (2004, 2005) examined a three-dimensional non-linear coupled auto-catalytic cure kinetic model and transient-heat-transfer model solved by finite-element methods to simulate the microwave cure process for underfill materials. Temperature and conversion inside the underfill during a microwave cure process were evaluated by solving the nonlinear anisotropic heat-conduction equation including internal heat generation produced by exothermic chemical reactions. 

When a dielectric is placed in an alternating electric field the dipoles attempt to maintain alignment with the field. This process requires a finite time that is different for each polarization mechanism. At the relaxation frequency the dipoles will only just be able to reorient themselves in time with the applied field. At this frequency the dielectic is lossy and energy is lost in the form of heat. The dielectric loss is at a maximum when the frequency of the external field coincides with the relaxation frequency of a given polarization mechanism. This is the principle behind the microwave oven. It operates at the relaxation frequency of water molecules and the heat generated warms the food. 

An intermolecular approach to carbazoles 273 was reported via an inverse electron-demand Diels-Alder reaction between 3-chloroindoles 271 and pyrones 272. This reaction proceeds through a thermally induced Diels-Alder, decarboxylation, elimination-domino process. Heating the two components under microwave irradiation generates carbazoles with methyl esters in the 3-position in a completely regioselective manner (140L1124). 

PNC developed a co-conversion technology utilizing the microwave heating direct denitration process (MH method) which converts plutonium nitrate and uranyl nitrate solution to MOX powder. Compared with the conventional method, it is a simple process and generates less liquid waste. 

In general, microwave systems consist of a microwave source (generator), a section of transmission line which delivers microwaves from the generator into applicator, and microwave applicator which can efficiently transfer the energy to heated materials. Also a reliable measuring system is necessary to monitor temperature of the process. For processes running at elevated pressures appropriate pressure and temperature control maybe required. 

Carlone and Palazzo (2008) developed a computational modelling of microwave assisted pultrusion. This model is based on an electromagnetic submodel, meant to evaluate the electric field distribution and the heat generation rate due to the microwave source and on a thermochemical submodel, used to determine the temperature and degree of cure distributions. The performed simulations revealed the relevance of design of the microwave cavity, the curing die, and the importance of the dielectric properties of the materials in microwave pultrusion process. 

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