Microwave-Assisted Drying Process

Before applications are dealt with, the main variables governing microwave-assisted processes and the parameters characterizing specific microwave treatments are examined. The applications discussed include not only microwave-assisted digestion and extraction — which are the two most widely implemented and hence those with the highest potential interest to readers — but also others of special significance to solid sample treatment such as microwave-assisted drying, distillation and protein hydrolysis. Finally, some safety recommendations on the use of microwave equipment are made. 

Typically, food products dried by microwave-assisted processes can display three types of structural change ... 

Hettenbach, K., David, J., Dias, E., Brenek, S., Laforte, C. and Barnett, S. Microwave assisted vacuum drying and endpoint determination using mass spectrometry, part 1. Org. Process Res. Dev. 8 2004. (Introduction). 

It Is difficult to forecast a realistic timescale for the commercial development of microwave assisted processing techniques. Some, such as drying, are already In operation In a number of countries, whilst other processes, such as slip casting, are expected to have been commercialised to a greater or lesser extent by the early 1990 s. For the Other applications, considerably more research and development is still required however, given the interest currently being shown, progress Is expected to be rapid. 

The process was also applicable to microwave-assisted reactions. Thus, 140a, 140b, and 140 (R1 = z-Pr, R4 = indol-3-ylmethyl) were prepared in a two-step, one-pot synthesis in yields of 55%, 39%, 20%, and with 70%, 73%, 50% ee, respectively. In the first step anthranilic acid was reacted with the appropriate A-BOC-protected amino acid (glycine, L-alanine, and L-valine, respectively) in the presence of P(OPh)3 and dry pyridine under irradiation at 150 °C for 140a or conventional heating at 55 °C for 140b and 140 (R1 = z-Pr, R4 = indol-3-ylmethyl). In the second step the resulting... 

Consequently, we consider that the industrial scale technological management of microwave assisted chemical reaction is no compatible with batch reactors coupled with multimode applicators. Some typical processes with a systematic decrease of the dielectric losses of the concerned reactant, such as filtration and drying of mineral or pharmaceutical powders are compatible with multimode applicators. To our knowledge, the only industrial batch microwave device is the microwave variant of the Turbosphere ( all in one solution mixer/granulator/dryer designed by Moritz... 

Loupy et al.7 demonstrated that the smaller of the two microwave systems could process sample sizes of30-40 g or 70 ml. For scale-up of microwave assisted dry media organic chemistry, a larger reactor was developed. Examples of reactions conducted on this instrument, some of which are presented in Table 9.1, included esterification75,... 

As noted earlier, not all open-vessel systems (viz. those that operate at atmospheric pressure) are of the focused type. A number of reported applications use a domestic multi-mode oven to process samples for analytical purposes, usually with a view to coupling the microwave treatment to some other step of the analytical process (generally the determination step). Below are described the most common on-line systems used so far, including domestic ovens (multi-mode systems) and open-vessel focused systems, which operate at atmospheric pressure and are thus much more flexible for coupling to subsequent steps of the analytical process. On the other hand, the increased flexibility of open-vessel systems has promoted the design of new microwave-assisted sample treatment units based on focused or multi-mode (domestic) ovens adapted to the particular purpose. Examples of these new units include the microwave-ultrasound combined extractor, the focused microwave-assisted Soxhlet extractor, the microwave-assisted drying system and the microwave-assisted distillation extractor, which are also dealt with in this section. Finally, the usefulness of the microwave-assisted sample treatment modules incorporated in robot stations is also commented on, albeit briefly as such devices are discussed in greater detail in Chapter 10. 

Scanning electron micrographs taken from control (non-treated) and leaves that were previously treated by the extraction processes mentioned above revealed significant differences between the microwave-assisted extracted leaves and all other mint leaves which had been extracted by means. This was true for dry, fresh, and re-hydrated leaves. In fact, it was clear that the process by which the extraction occurs when using microwaves was fundamentally different from any other extraction processes investigated at the time. Actually, to the best of our knowledge, this assertion still holds true. In summary, the conclusions drawn from the micrographs taken from mint leaves that were subjected to extraction in hexane were as follows [1-6] ... 

Microwave-assisted hydrolysis synthesis of (1, fi-arylated acetaldehyde (7b). The enol ether (6b) was placed in a Smith process vial together with 0.25 mL water, 0.25 mL concentrated HC1 and 1.0 mL toluene. The tube was sealed and positioned in a Smith synthesiser rack. After microwave irradiation at 180 °C for 2 minutes and subsequent cooling the reaction mixture was carefully neutralized with 2 M sodium hydroxide and extracted with diethyl ether. After drying with potassium carbonate (solid) and evaporation of the solvent the crude aldehyde (7b) (98%) was obtained as a yellow oil (> 95% pure by GC-MS). NMR and IR spectra were quickly recorded on this air and light sensitive compound. 

A survey of microwave activation in the chemistry of Hantzsch 1,4-dihydropyridines (1,4-DHP) was reported in 2003 [195]. The experimental method proposed more than a century ago remains the most widely used for synthesis of these heterocycles. Since 1992 the process has been adapted to microwave irradiation under a variety of conditions to reduce the reaction time and enhance the yield. Among these experiments, Zhang reported a solvent-free process starting from 3-aminocrotonate (20 mmol), methyl acetoacetate (20 mmol), and aromatic aldehydes (20 mmol) in a domestic oven [196]. Yields from 59 to 77% were reported for 10-min reaction. A variety of conditions (solution, dry media, solvent-free) has been used for microwave-assisted synthesis of Hantzsch 1,4-DHP. Only procedures involving solvent-free conditions under the action of microwave irradiation led to the aromatized pyridine derivatives. 

An essential oil (EO) is internationally defined as the product obtained by hydro-, steam-, or dry-distillation of a plant or of some of its parts, or by a suitable mechanical process without heating, as in the case of Citrus fruits (AFNOR, 1998 Council of Europe, 2010). Vacuum distUladon solvent extraction combined offline with distillation simultaneous distillation extraction supercritical fluid extraction microwave-assisted extraction and hydro-distiUation and static, dynamic, and high concentration capacity headspace sampling are other techniques used for extracting the volatile fraction from aromatic plants, although the products of these processes cannot be termed EOs (Faleiro and Miguel, 2013). 

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