Multimode cavities

The MARS-S is constituted of a multimode cavity very close to domestic oven with safety precautions (15 mL vessels up to 0.5 L round-bottomed flasks, magnetic stirring, temperature control). The magnitude of microwave power available is 300 W. The optical temperature sensor is immersed in the reaction vessel for quick response up to 250 °C. A ceiling mounted is available in order to make connection with a conventional reflux system located outside the cavity or to ensure addition of reactants. These ports are provided with a ground choke to prevent microwave leakage. It is also possible to use a turntable for small vessels with volumes close to 0.1 mL to 15 mL vessels (120 positions for 15 mL vessels). Pressure vessels are available (33 bar monitored, 20 controlled). 

In addition to packed catalyst bed, a fluidized bed irradiated by single and multi-mode microwave field, respectively, was also modeled by Roussy et al. [120]. It was proved that the equality of solid and gas temperatures could be accepted in the stationary state and during cooling in a single-mode system. The single-mode cavity eliminates the influence of particle movements on the electric field distribution. When the bed was irradiated in the multimode cavity, the model has failed. Never-... 

Several workers have employed monomodal cavities for microwave chemistry on the sub-gram scale. In some cases in which monomodal cavities have been used7, special benefits of so-called focussed microwaves have been claimed. As mentioned earlier, the dielectric properties of a sample can alter substantially with temperature and/or with changing chemical composition. Hence, regardless of whether multi-modal or unimodal cavities are employed, frequent tuning may be necessary if heating efficiency is to be retained. This aspect has often been overlooked by proponents of focussed microwaves. The nett result is that transfer of microwave conditions between monomodal to multi-modal cavities is usually facile. With the MBR (which had a tunable multimodal cavity), Cablewski et al. performed five reactions that had been conducted earlier on the gram scale or below with focussed microwaves (T. Cablewski, B. Heilman, P. Pilotti, J. Thorn, and C.R. Strauss, personal communication see also Ref. 117 for conference poster). These were scaled-up between 40- and 60-fold and reaction conditions... 

Figure 3.5 Reactor for batchwise organic synthesis. 1. Reaction vessel 2, top flange 3, cold finger 4, pressure meter 5, magnetron 6, forward/reverse power meters 7, magnetron power supply 8, magnetic stirrer 9, computer 10, optic fiber thermometer 11, load matching device 12, waveguide 13, multimodal cavity (applicator). (From Ref. 712, reproduced with permission.)...

Averaged overall properties of the crude oil obtained by microwave interaction are summarized in Table V. Although these published figures are not the best for some of the processes listed (2), they are presented for an approximate comparison. From these data, microwave-produced oil is shown to be a suitable liquid fuel precursor although differing from that produced from a variety of thermal processes. The recoveries from this prototype, multimode, cavity-retorting system are promising. 

The Ethos MR contains a multimode cavity very similar to that of a domestic oven but with safety precautions. It can use standard glass (420 mL up to 2.5 bar) or polymer reactors (375 mL up to 200 °C and 30 bar) with magnetic stirring. The microwave power available is 1 kW. The optical temperature sensor is immersed in the reaction vessel for rapid response up to 250 °C. An infrared sensor is also available. A ceiling mounting is available to enable connection with a conventional reflux system located outside the cavity or for addition of reactants. The Ethos CFR is illustrated in Eig. 2.17. It is a continuous-flow variant of the Ethos MR . 

Subirats M, Iskander MF, White MJ, Kiggans JO (1997) FDTD simulation of microwave sintering in large (500/4000 liter) multimode cavities. J Microw Powta- Electiomagn Energy 32 161-170... 

Iskander MF, Smith RL, Andrade AOM, Kimrey H, Walsh LM (1994) FDTD simulation of microwave sintering of ceramics in multimode cavities. TREE Trans Microw Theory Tech 42 793-800... 

Arnri A, Saidane A (2003) TLM simulation of microwave hybrid sintering of multiple samples in a multimode cavity. Int J Numer Model-Electron Netw Devices Fields 16 271-285... 

The applicator, where the sample is placed. It can be a multimode cavity where microwaves are randomly dispersed or the waveguide itself. In the latter case the sample vessel is placed directly inside to focus the microwave radiation onto the sample. 

The earliest microwave systems for analytical purposes were closed vessels with a multimode cavity. They usually allowed processing of several samples at the same (in a carousel) under pressure and temperature feedback control. Closed vessels exist basically in two different forms. One encompasses noninsulated, relatively thin, single-walled fluoro-polymer vessels. These vessels have minimal insulating characteristics and allow large amounts of heat to escape. The other type of closed vessel is a well-insulated container, usually of very thick-walled fluoropolymer, or one with a very thick outer layer or casing (or both). These vessels retain heat very efficiently, and so they do not allow rapid cooling when ambient air is forced over them within the microwave cavity. 

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