Problems associated with microwave

Although RF and microwave dryers have been used by the textile industry, the market penetration of RF dryers is much greater. The cost of the equipment has been a barrier to the use of both types of dryers, and uniformity of treatment has been a problem associated with microwave drying. As RF... 

In many cases, the comparison of a reaction accelerated by microwave irradiation has been made with the same reaction in an oil bath at the same bulk temperature. Unfortunately, there have been quite a few reports in the chemical literature that have not been conducted with such proper control of conditions and consequently a fair comparison is not possible. Nevertheless, using this MW approach, the problems associated with waste disposal of solvents that are used several fold in chemical reactions, and excess usage of chemicals are avoided or minimized. The discussion pertaining to the preparation of supported reagents or catalysts has not been included in this chapter because numerous review articles are available on this theme [14—22],... 

Cycloaddition reactions often require the use of harsh conditions such as high temperatures and long reaction times. These conditions are not compatible with sensitive reagents or products such as natural products. The applicability of Diels-Alder cycloadditions is, moreover, limited by the reversibility of the reaction when a long reaction time is required. The short reaction times associated with microwave activation avoid the decomposition of reagents and products and this prevents polymerization of the diene or dienophile. All these problems have been conveniently solved by the rapid heating induced by microwave irradiation, a situation not accessible in most classical methods. With the aid of microwave irradiation, cydoaddition reactions have been performed with great success [9, 10]. 

In conclusion, there are three different approaches for microwave synthesis on a large scale (>100 mL volume). While some groups have employed larger batch-type multimode or single-mode reactors (< 1000 mL processing volume), others have used CF or SF techniques (multi- and single-mode cavities) to overcome the inherent problems associated with MAOS scale-up. 

Many problems associated with successfully identifying and simulating the flavors characteristic of conventionally baked foods have yet to be overcome in the development of new microwave products. This study addresses these problems by identifying compounds most important to the characteristic flavors of white cake batter, microwave and conventionally baked cake. Gas chromatography, mass spectrometry, and odor analysis by sniffing indicated that compounds such as diacetyl, C4-C10 aldehydes, C4-C10 alcohols, C8-C11 dienals, 3-octen-2-one, and 7-octen-4-ol were common to all three flavor systems. Conventional cake was found to contain higher levels of isopentenal and furfural than microwave cake. 

One of the major problems associated with the flavor development of microwave products results from the fact that the reduced time/temperature relationship during baking is not conducive to the formation of a crisp, outer crust or many of the Maillard compounds associated with a conventionally baked flavor. These microwave baked products thus have a different flavor character from conventionally baked products and are typically judged "inferior by consumers. 

Cycloaddition reactions have been performed with great success with the aid of microwave irradiation (Chapter 11). All the problems associated with these reactions have been conveniently solved by the rapid heating achieved with microwave irradiation, a situation not accessible by classical methods [4a, 6]. In some examples the selectivity of the reaction has also been modified. Langa described the cydoad-dition of N-methylazomethine ylides to C70 to give three regioisomers (83a-c) by attack at the 1-2, 5-6, and 7-21 bonds (Scheme 5.24) [68]. Under the action of conventional heating the 7-21 83c isomer was formed in only a low proportion... 

Despite the development of sophisticated instrumentation and techniques, the accurate determination of boron in biological materials is difficult at concentrations less than 1.0 mg B/kg. Problems associated with analysis of boron from biological sources include contamination from teflon vessels during microwave digestion losses due to freeze-drying variations in boron isotope ratios, standards preparation, and reagent backgrounds and instrumental interference. Inductively coupled plasma-mass spectrometry now... 

The continuous-flow microwave-assisted polymerization of 2-ethyl-2-oxazoline was reported by Paulus et al. [232]. The flow process overcomes the problems associated with scale-up of the batch process, including safety issues. 

Application of microwave irradiation in organic reactions has added a new dimension to solid phase synthesis. By the use of this technique, it is now possible to carry out reactions without the use of toxic or other solvents, which is one of the main problems associated with green synthesis. In these, the reactants are dissolved in a suitable solvent like water, alcohol, methylene chloride etc. and the solution stirred with a suitable adsorbent or solid support like silica gel, alumina or phyllosilicate (M" "-montomorillonite). After stirring. 

The problems associated with waste disposal of solvents and excess chemicals has been overcome by performing reaction without a solvent under microwave. The use of K2CO3 not only eliminate the need for external base to netralize HCl evolved but also enables aqueous workup (Scheme 90). ... 

The use of various pretreatments of the plastic wastes such as chemical soaking, heat treatments, microwave, and plasma treatments, etc. in conjunction with the pressurized method might be attractive areas for future research. Co-pyrolysis with other wastes such as food wastes is also plausible. Much work has been carried out on other pressurized carbonization methods such as biomass hydrothermal carbonization [111, 112]. If an industrial process is to emerge from the research, the combined use of various carbon sources would be attractive for economy-of-scale purposes. Producing porous carbons for further applications from plastic wastes would not only yield useful products from cheap precursors, but it would also help reduce the problems associated with the ever-growing plastic waste stream. 

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